T41A-0350
Young Conjugate Margins Laboratory in the Gulf of Aden : the YOCMAL project
A comprehensive multi-disciplinary study of the eastern part of the Gulf of Aden has been undertaken recently in the framework of the French margins program (Encens project), with the long-term objective to integrate in a consistent model of evolution field observations where the margin crops out, results of marine geophysical survey where the margins are submerged, seismological observations of deep interior structure and geodetic measurements. First results show the potentiality of such an approach in this area to understand the relationships between the dynamics of rifting and spreading, the observed structures and the vertical motions of a margin in its early stages. Among the main results, we evidence the structural and sedimentologic characteristics of the continental and oceanic domain of the margin, especially their segmentation, related to the opening obliquity, and their consequences on the oceanic spreading, the importance of the structural inheritance at both lithospheric and crustal scale. We also describe the presence of a ocean-continent transition zone with high heat flow values, of a deep thermal anomaly (170 km) and of a sub-active volcano in the deep margin, that relaunch debate on the importance of the volcanism in the margins formation. The results also emphasize that both the area of investigation and the methods should be extended to be able to describe and understand the variability and the evolution in space and time of the studied structures, notably related to the Afar hotspot. This project intends therefore to study the Gulf of Aden as a natural laboratory for the study of rifting and oceanic spreading onset processes. The extended further ongoing investigations onland in the eastern area (Oman) and its conjugate margins in the South (Socotra island) and in the volcanic margin in the West (Yemen) allow to reach the objectives concerning: - The deep structure and thermal regime (seismic tomography, OBS, heat-flow). - The crustal structure (tectonics-sedimentation relationships, MCS, Ocean Bottom Seismometer, receiver functions on land, gravity and magnetic anomalies, dredging on the ocean-continent transition, basalts geochemistry). - The vertical motions throughout the Cainozoic (topography-bathymetry analysis, sedimentology and stratigraphy field work, apatite fission tracks and U/Th/He thermochronology) and more recent deformation (concentrations of cosmogenic elements) - The present-day opening (GPS network monitoring and extension of the network) - The thermo-mechanical modelling (numerical and analogue models). The overall goal of this project is to build 3D images of the deep (mantle), intermediate (crust), and superficial (sediments) structures of the Gulf of Aden basin from its rifting to its present-day oceanic spreading in order to understand the mechanisms of continental margins formation.
T41A-0351
Evidence of partial melting beneath the passive margin of the Gulf of Aden from a joint analysis of gravity and seismology
Rifting processes though extensively studied are still not well known. Nevertheless geophysical studies can provide new insight into the mechanisms of continental opening. The Gulf of Aden is a young narrow and obliquely opening oceanic basin formed during the Olig-Miocene. Its conjugate margins are well preserved beneath a thin post-rift sedimentary cover.It thus makes it an ideal region to study the processes of rifting and continental lithospheric break-up. In 2003 and 2005, numerous teleseismic earthquakes were recorded at twenty-nine broadband seismic stations in Dhofar (Oman) in order to study the northern margin of the Gulf of Aden. In this work, we used a joint inversion of teleseismic P-wave delay times and Bouguer anomaly. We obtain velocity ans density models that shows (1) crustal heterogeneities that match to the main geological features at the surface, (2) the presence of two low velocity anomalies in the continuation of Socotra and Alula Fartak fracture zones that appear at 60 km depth and may extend to at least 200 km, (3) gravity edge effect on the margin. The S wave tomography results are consistent with the joint inversion ones, and evidence partial melting within the two deep velocity anomalies. These results which suggest that the Afar hotspot has an influence up to the Dhofar will be discussed.
T41A-0352
Crustal Geometry of a First Order Segment in the Northeastern Gulf of Aden Margin From Seismic Reflection (Offshore Oman)
The Gulf of Aden is an oceanic basin separating Arabia from Somalia. The rifting started 35 Ma ago followed by oceanic spreading from 17.6 Ma. The gulf orientation (N75°E) and the kinematics (about N30°E divergence) mark an oblique rifting where normal faults striking between N70°E (rift axis parallel) and N110°E (perpendicular to the divergence), are due to an extension direction probably evolving from N20°E to N160°E. The accurate 3D structure of the margins and the influence of structural inheritance or thermal and rheological evolution need to be better constrained. In order to answer this question, we mapped the 3D architecture of sedimentary units and tectonic features in the first-order segment between Alula-Fartak and Socotra Fracture Zones of the eastern Gulf of Aden continental margin. During the Encens cruise (Leroy et al., 2006) conducted in this area where the syn-rift structures are well exposed and covered by thin post-rift sediments onshore, multibeam bathymetry, 360 channels seismic reflection (10 km spaced profiles), gravity and magnetism data were gathered. Furthermore two reflection seismic profiles were processed with a pre-stack migration method. This excellent-quality dataset will permit us to image the structure of the margin and to reconstruct the 3D evolution from rifting to the onset of oceanic spreading. These results complement the field work realized onshore on conjugate margins (Oman and Socotra). The style of deposit seems completely different in the proximal and in distal parts of the margin. Indeed fault controlled syn-rift carbonate systems, well developed onshore, are not really well expressed offshore. The paleohigh seems to be preserved at the end of the rifting and filled by a thick syn-OCT and/or post-rift series. A late rifting or coeval to the OCT uplift seemingly sets going on the margin edge. Several seaward dipping normal faults cut the main escarpment of the last continental block, filled by small sedimentary wedges. The post-rift sequence covers all these features and the oceanic crust, also cut by several southward dipping normal faults. Refraction and seismological studies, providing MOHO depth, will constrain future lithospheric analogue models of oblique rifting.
T41A-0353
Deep Structure of the Northeastern Gulf of Aden Margin From Wide-Angle Seismic Network
The Gulf of Aden is an oblique rift system with spreading segments connected by several transform faults. Rifting in the gulf began ca. 35 Ma ago when the Afar hotspot activity takes place. Onset of spreading recorded by the first magnetic anomaly (A5d) is dated at least 17.6 Ma. We describe first results from the Encens seismic experiment in the northeastern Gulf of Aden and present crustal-scale images across three second-order segments between Alula-Fartak and Socotra fracture zones. Each of these segments was instrumented with 10-15 km, evenly spaced ocean-bottom seismometers complemented onshore by an array of seismometers. 8000 shots from the 8410 in3 ( ~ 138 l) source on board R/V L"Atalante (Ifremer) have been recorded, providing an extensive dataset, with offsets up to 180 km. Excellent-quality data from this dense source and receiver coverage enable the modeling of the P-waves across and along the strike of the ocean-continent transition (OCT) and continental margin for the different rift segments. Coincident multichannel seismic data is used to better constrain the structure down to the basement, and the crustal structure is eventually validated through the modeling of synthetic gravity anomalies. Our results indicate that a single morphology and geometry of the continental margins does not exist in the eastern Gulf of Aden. Indeed, distinct styles of rifting occur within the first-order segmentation of the gulf, which may be related to pre-rift history and to the syn or post-rift magmatism. The continental crust has a maximum of 35 km thickness in the west and 32 km in the east, with velocities ranging from 6 to 7.3 km/s. The base of the oceanic crust has relatively low P-wave velocities ranging from 6.6 to 7.5 km/s, and a thickness of about 5 km in the west and 7 km in the east. A 130 km long gradual thinning of the crust is observed while the OCT width ranges from 20 to 30 km. The transitional crust is around 8 km thick and the P-wave velocities are upper than 6 km/s. The mantle has P-wave velocities lower than 8 km/s, probably implying serpentinization of the mantle and/or hot mantle. Thermomechanical models contribute to understand the first order parameters that influence passive margins evolution from rifting to present.
T41A-0354
Segment linkage in Afar via magma intrusion: the birth of a transform fault?
Both continental and oceanic rifts are segmented along their length, but the relation between transfer faults and transform faults linking segments remains unclear. How and when do transform faults initiate to link rift segments? Does magma intrusion achieve some of the strain transfer between segments? A temporary seismic array in the volcanically and seismically active Afar rift of Ethiopia provides insights into these two fundamental questions. We analyze the spatial and temporal patterns of earthquakes, and compare these to patterns in high-resolution satellite imagery and space geodetic data from the ongoing seismo-volcanic episode that began in 2005. We integrate these results to understand how stresses are transferred between ridge segments and how this possibly relates to the initiation of transform faults. Earthquake swarms from October 2005 to March 2006 form narrow bands coinciding with NW-SE striking fault zones linking the active Erta' Ale and Tat `Ale magmatic rift segments, and the Dabbahu and Alayta magmatic segments. Step over distances are ~15 km and ~20 km respectively. The time and spatial distribution of these seismic events as well as the correlation of events with magmatic centers suggests that earthquakes are triggered by magma intrusion. These patterns offer insight to magma accommodation along faults and between rift segments, suggesting magma intrusion facilitates transform fault initiation. We compare and contrast active structures during the 2005- 2006 episode with segment linkage patterns preserved in the rock record to understand transform evolution.
T41A-0355
Continued dyking in the Dabbuhu Rift segment, Afar, Ethiopia, from radar interferometry.
The 60-km-long Dabbahu segment of the Nubia-Arabia plate boundary lies in the Northern Ethiopian region of Afar. In September 2005 a major rifting episode resulted in the injection of a 60-km-long dyke with a maximum thickness of ~8m (Wright et al., 2006). Subsidence observed at Dabbahu and Gabho volcanoes implied that some of the magma was sourced from shallow reservoirs beneath the volcanoes. Using a combination of satellite interferometry, GPS and seismicity we aim to monitor the ongoing deformation in the Dabbahu rift segment. Interferograms in the following months show only isolated deformation around the volcanoes and around the centre of the Dabbahu rift segment. However, in June 2006, intense deformation is observed along a rift parallel, 10 km long zone extending north of Ado"Ale, which is a volcanic complex located in the middle of the rift segment. Modelling of radar interferometry (InSAR) data indicates the injection of a ~2 m thick, ~10 km long dyke, with deformation spatially coincident with seismicity (Keir et al., this session). Since June 2006 a further 5 dyke intrusions have been detected using InSAR - in July, September and December of 2006 and in January and August of 2007. Elastic modelling of InSAR data suggests that a ~2 m thick, ~9 km long dyke was intruded in July 2006 followed by a ~2 m thick, ~8 km long dyke in September, neither dyke reached the surface and both dykes were confined to the upper 10 -12 km of the crust. Preliminary results suggest similar styles of intrusion for the December 2006 and January 2007 dykes. A fissural basaltic eruption occurred with the August 2007 deformation showing that the dyke reached the surface. . All dykes since June 2006 are south of the Ado Ale complex - a rifted silicic centre in the middle of the rift segment. InSAR and GPS show no deflation at either of the volcanoes at the northern end of the segment implying an alternate source is feeding the ongoing intrusions. We do not observe surface subsidence associated with any source region, and thus require a deep source (> ~ 8 km) and suggest that this is located near the centre of the rift segment. The apparent migration sequences seen so far appear to be similar to those seen in Iceland during the 1975-1984 Krafla rifting episode, and we anticipate further dyking episodes and eruptions over the next few years.
T41A-0356
An Estimation of the Electrical Resistivity of the Crust in Northern Baja California, Mexico, Based on Magnetotelluric Data
In this work we show an electrical resistivity model of the crust, obtained from a magnetotelluric transect through Sierra San Pedro Martir (SPM), in the northern Baja California, Mexico. The profile has a length of ~110 km and consists of 26 magnetotelluric (MT) sites located across the main tectonic structures occurring in Baja California crust. We used a set of magnetotelluric invariant impedances and a regularized inversion technique to estimate a 2-D resistivity model of the crust. The resulting resistivity model reveals a high conductivity anomaly dipping towards the east, possibly associated with a shear zone developed during a major accretion episode in Cretaceous time. In addition, the model provides information about the vertical extension of several plutonic bodies mapped in the surface. On the other hand, it is observed an increase in the conductivity at a depth of about 20 km, which could be associated to a weakness zone inferred by existent rheological models. Enhancement of electrical conductivity in the rocks of the upper crust highly depends on the presence of fluids and/or conductive minerals. Hence, the assessment of subsurface conductive anomalies may contribute to understand the tectonic evolution of Baja California peninsula.
T41A-0357
Late Quaternary Faulting along the San Juan de los Planes Fault Zone, Baja California Sur, Mexico
As a result of continued distributed deformation in the Gulf Extensional Province along an oblique-divergent plate margin, active normal faulting is well manifest in southeastern Baja California. By characterizing normal-fault related deformation along the San Juan de los Planes fault zone (SJPFZ) southwest of La Paz, Baja California Sur we contribute to understanding the patterns and rates of faulting along the southwest gulf-margin fault system. The geometry, history, and rate of faulting provide constraints on the relative significance of gulf-margin deformation as compared to axial system deformation. The SJPFZ is a major north-trending structure in the southern Baja margin along which we focused our field efforts. These investigations included: a detailed strip map of the active fault zone, including delineation of active scarp traces and geomorphic surfaces on the hanging wall and footwall; fault scarp profiles; analysis of bedrock structures to better understand how the pattern and rate of strain varied during the development of this fault zone; and a gravity survey across the San Juan de los Planes basin to determine basin geometry and fault behavior. The map covers a N-S swath from the Gulf of California in the north to San Antonio in the south, an area ~45km long and ~1-4km wide. Bedrock along the SJPFZ varies from Cretaceous Las Cruces Granite in the north to Cretaceous Buena Mujer Tonalite in the south and is scarred by shear zones and brittle faults. The active scarp-forming fault juxtaposes bedrock in the footwall against Late Quaternary sandstone-conglomerate. This ~20m wide zone is highly fractured bedrock infused with carbonate. The northern ~12km of the SJPFZ, trending 200°, preserves discontinuous scarps 1-2km long and 1-3m high in Quaternary units. The scarps are separated by stretches of bedrock embayed by hundreds of meters-wide tongues of Quaternary sandstone-conglomerate, implying low Quaternary slip rate. Further south, ~2 km north of the Los Planes highway, the fault steps to the right 2km with no overlap. The fault is inactive until ~3km south of the Los Planes highway where scarp heights in the Quaternary sediments rise to ~3-11m for ~11km with an average trend of 160°, implying increasing slip rate. The fault then steps left 2km with no overlap, trending 145°. Scarp heights range from 3-6m in the step. The southernmost 9km of the fault zone, trending 200°, is marked by discontinuous scarps and embayed bedrock, reflecting diminished fault activity. The footwall landscape in this area is characterized by a broad, gently-sloping, low-relief pediment surface with thin Quaternary cover, disrupted by inselberg-like hills. The young scarp-forming fault appears to have reactivated older faults to rupture this pediment, reflecting the episodic nature of slip along this fault zone. Preliminary OSL ages of the youngest faulted deposit imply a Late Pleistocene-Holocene slip rate of 0.1-1mm/yr. The SJPFZ is thus characterized by reactivation of pre-existing faults to rupture a pre-existing low relief erosional landscape. Whereas the entire region might have experienced the quiescent period that allowed for development of the low- relief, stable surface along the SJPFZ, we speculate that while the SJPFZ was dormant, other faults within the gulf-margin system were actively accommodating strain.
T41A-0358
Late Pleistocene-Holocene Faulting History Along the Northern El Carrizal Fault, Baja California Sur, Mexico: Earthquake Recurrence at a Persistently Active Rifted Margin
The El Carrizal fault is a NW striking, east dipping normal fault located 25 km west of the city of La Paz, Baja California Sur, Mexico and is the westernmost bounding fault of the gulf-margin system at this latitude. The fault is ~70 km long onshore and ~50 km long offshore to the north in La Paz Bay. As many as three Quaternary geomorphic surfaces formed on the footwall and were identified on the basis of mapping and topographic profiling. In the north, the El Carrizal fault splays into multiple strands and exhibits a pattern of alternating N-S and NW-trending segments. Results from geologic mapping, paleoseismic investigations, and preliminary optically stimulated luminescence (OSL) geochronology provide some of the first numerical constraints on late Pleistocene-Holocene faulting along the El Carrizal fault. A 20 m long, 2-3 m deep trench (Trench 28) was excavated across the fault 23 km south of La Paz Bay. The trench was photographed, hand logged, and sampled for OSL dating. The trench revealed a succession of fluvial and channel deposits of sands, gravels, and cobbles. The main fault zone is manifested by a 0.5 m thick wedge-shaped deposit that consists of silty-sand and also contains rotated blocks of caliche- cemented gravels. Preliminary OSL ages from a silty-sand unit offset 2 m by the fault average latest Pleistocene. A trench 4 km south of Trench 28 (Cuadradito Trench) was also documented and sampled for OSL analysis. Preliminary OSL ages from a fluvial sand unit deposited against faulted bedrock range from mid to late Holocene. Sedimentary comparisons and surficial mapping suggest that the Holocene unit at Cuadradito Trench may be correlative to sediment that overlies faulted units from Trench 28. Such a correlation would constrain the timing of the 2 m offset at Trench 28 to be between latest Pleistocene and mid Holocene. A quarry 10 km north of Trench 28 exposes Quaternary sand and gravels buttressed against a 5-10 m wide bedrock shear zone. Here, the sediments appear to be faulted by 3 to 4 earthquakes. The two most recent events may have experienced 1-2 m of offset each. Map relationships suggest that this faulted unit is correlative to a fluvial unit near Bonfil that yields an optical age of early Holocene. This would imply that the two most recent events at the quarry are Holocene in age. Based on preliminary OSL data and total measured offset at trench sites, the slip rates are estimated to be from 0.1 to 0.5 mm/yr. In summary, earthquake recurrence and slip rate may increase towards the north along the El Carrizal fault, consistent with footwall geologic mapping. If the El Carrizal fault abides by the rules of normal fault geometry and fault offset, it is likely that the fault is at a maximum total offset offshore of San Juan de la Costa, a prediction to be tested in the summer of 2008 by offshore imaging using Compressed High Intensity Radar Pulse (CHIRP). Improving the earthquake record along the El Carrizal fault will prove beneficial for understanding the seismic hazards to the city of La Paz. Furthermore, quantifying faulting rates throughout the southwestern margin of the Gulf of California will improve our understanding of the rift-to-drift process and how that process has operated along an oblique-divergent plate margin.
T41A-0359
Crustal Structure of the Southern Rio Grande Rift Defined by Wide Angle Reflection Data
A new model of crustal velocity structure in the southern Rio Grande Rift across the Potrillo Volcanic Field (PVF) provides new information on the crustal evolution of the region from Laramide to Recent times. The model results from analysis of seismic refraction/wide-angle reflection data acquired from a 205-km-long profile, comprised of 8 shots and 793 receivers. Together, the data and model exhibit 1) a new detailed cross-section of the modern Basin and Range structure of southern New Mexico and far West Texas; 2) evidence for a number of high velocity bodies between 5 and 10 km depth that underlie Basin and Range structure; 3) a dramatic step up in a mid-crustal interface from 15 to 11 km depth that leads to thickening of the mid-crust below the PVF; 4) increased seismic reflectivity within the crust and at the Moho interface concentrated below the PVF and; 5) evidence for thicker crust (~30-31 km) than previously determined. The velocity structure in the upper 3 to 5 km defines the geometry of the late Tertiary basins and ranges. The basin fill has velocities from 2.5 to 4.5 km/s. In the ranges, velocities are 4.7 to 5.3 km/s and represent uplifted Paleozoic sedimentary rock. By contrast, we interpret high velocity bodies and complex lateral velocity variations at 5 to 10 km depth to reflect the signature of Laramide basement uplifts. The transition between the upper and middle crust is defined by an interface that steps up abruptly from a depth 15 km to 11 km beneath the PVF. Velocities range from 6.28 to 6.4 km/s below this transition to velocities of 6.7 to 7.1 km/s at the base of the crust. Crustal thickness varies from 35 km at the western end of the profile to as little as ~30 km beneath the El Paso area. Upper mantle velocities range from 7.75 to 7.9 km/s, which is consistent with a warm upper mantle and high heat flow values of 85 to 125 mWm2 associated with the southern Rio Grande Rift. Near-vertical incidence records show a complicated reflectivity pattern at the Moho, which may be related to recent volcanism. Oligocene-age (ca. 27 Ma) crustal xenoliths from Potrillo Maar crystallized at depth similar to that of the top of the thickened mid-crust. Petrographic features of the xenoliths indicate the presence of a compositionally-extended plutonic complex that underwent ductile shear during and after emplacement. The age and deformation of the xenoliths, velocity structure, and character of seismic reflectivity near the Potrillo Volcanic Field are strongly reminiscent of metamorphic core complexes in southern Arizona.
T41A-0360
New Constraints on plate motions in the Woodlark Basin, Papua New Guinea: Can Euler pole kinematics be used to predict continental extension?
The Woodlark Basin is one of the only active rift systems in the world where the amount of strain can be predicted through direct observations of brittle extension, subsidence, and motion around a well-constrained Euler pole. Work on rift systems worldwide has shown that in many instances the amount of brittle extension falls well short of the extension predicted by subsidence. A recent interpretation of marine seismic reflection data from the Woodlark basin has shown that at the rifting-to-spreading transition the estimates of extension predicted from subsidence and brittle extension are in close agreement if multiple phases of faulting and sub-resolution faulting are taken into account. However, extension predicted by both methods is approximately a factor of two less than that predicted by Euler pole kinematics. In an attempt to thoroughly catalogue this discrepancy, we re-evaluate the estimate of extension calculated using Euler pole kinematics. Due to a lack of data close to the margins in the eastern half of the basin, previous reconstructions of the basin opening history used two distinct Euler poles and assumed that the earliest was valid back to 6 Ma, the approximate time at which seafloor spreading started in the basin. In 2004 a marine geophysical survey mapped the remainder of the basin, including fracture zone traces from the spreading center to the margins and all identifiable magnetic chrons. Using bootstrap re-sampling constrained by all available data, Euler pole locations and rates were derived for each magnetization chron. The results show that the Euler poles from present to chron 2A are very well constrained. Prior to this time the errors in the location and rate of the Euler poles become far greater. However, there is little reason to believe that the location of the Euler pole changes dramatically. Though the amount of extension predicted using this method still exceeds that predicted through detailed interpretations of the reflection seismic data and estimates of extension predicted by subsidence, potential mechanisms to close this gap include the accommodation of strain by metamorphic core complex emplacement and magmatic arc additions.
T41A-0361
Thinning Factors and Crustal Thicknesses at the Propagating Tip of Sea-floor Spreading in the Woodlark Basin
Understanding how the continental crust and lithosphere thins at the propagating tip of sea-floor spreading is the key to understanding the continental breakup process. The Woodlark Basin, a young ocean basin located in the Western Pacific to the east of Papua New Guinea, commenced formation at approximately 8.4Ma and is propagating westwards at a rate of approximately 140km/Myr. Immediately to the west of the most recent segment of sea-floor spreading propagation, in the vicinity of the Moresby Seamount, evidence from bathymetry, subsidence and seismic Moho depth suggests that continental lithosphere is being thinned. In this study we have determined lithosphere thinning in the vicinity of the Moresby Seamount at the level of the whole lithosphere, the whole crust and the upper crust. Whole lithosphere thinning factors have been determined from subsidence analysis; whole continental crustal thinning factors have been determined from gravity inversions and upper crustal thinning factors have been determined from fault analysis. Three 2D seismic profiles surrounding the Moresby Seamount have been flexurally backstripped to the base of the syn-rift sediments to determine the water loaded subsidence. Using the McKenzie lithosphere extension model, modified to include volcanic addition at high thinning factors, whole thinning factors for the lithosphere have been determined from the water loaded subsidence. Results show that thermal subsidence alone cannot account for the observed subsidence, and that an additional initial subsidence is needed. Whole lithosphere thinning factors increase from an average of 0.5 to 0.8 across the Moresby Seamount eastwards towards the propagating tip. A satellite gravity inversion incorporating a lithosphere thermal gravity anomaly correction has been used to determine Moho depth, crustal thickness and thinning factors for the propagating tip in the Woodlark Basin. Moho depths are consistent with depths obtained from receiver function analysis (Ferris et al. 2006). Crustal thickness estimates do not include a correction for sediment thickness and are upper bounds. Crustal thinning factors in the vicinity of the Moresby Seamount are similar to those observed for the whole lithosphere. Fault analysis of the three 2D profiles have been used to determine upper crustal thinning factors. Upper crustal thinning factors between 0.1 to 0.2 are observed for the vicinity of the Moresby Seamount, substantially lower than thinning factors predicted for the whole lithosphere and continental crust, suggesting depth-dependent lithosphere thinning. Crustal thicknesses predicted from gravity inversion immediately to the east of the Moresby Seamount are substantially greater than would be expected for oceanic lithosphere in this region, while highly thinned, has not completely ruptured.
T41A-0362
New Observations on the Complex Interplay Between Normal Faulting and Metamorphic Core Complex Formation at the Rifting-to-Spreading Transition in the Woodlark Basin, Papua New Guinea
The rifting-to-spreading transition in the Woodlark Basin is tectonically very complex. Both low- and high-angle faults and intense magmatic intrusions co-exist in a small area immediately along strike of a region of active metamorphic core complex exhumation that began at about 4 Ma, when the spreading center was still more than 500 km to the east. Though questions still remain with regard to the mode of formation of the core complexes, it seems that isostatic exhumation of less dense felsic crust through a denser overlying ophiolite complex may play an important role. Six-channel marine seismic reflection profiles with coincident HAWAII-MR1 interferometric bathymetry and sidescan data were collected in 1993 in the vicinity of the core complexes and the rifting-to-spreading transition. Re-processing of this extensive dataset has resulted in the removal of artifacts related to poor airgun tuning and revealed a complicated relationship between core complex exhumation and the normal faults associated with rifting ahead of the spreading tip. Three time sections are used as the primary focus of this study. The area directly offshore of the Prevost Range's northern flank is characterized by large back-tilted fault blocks above north-dipping normal faults. The absence of sediment fill in the half graben implies that these structures are relatively recent. Further north, a major graben is bounded by these faults on the south side and south- dipping normal faults on the north side. Seafloor morphology and the location of earthquake epicenters indicate that this graben is the current focus of rifting in the basin. The offshore continuation of the core complex is imaged directly to the west of the Prevost Range. On the north side of the core complex, a north-dipping low-angle normal fault is imaged that is consistent with previous interpretations that the core complex was exhumed along a north- dipping deep crustal detachment. However, at the surface this detachment surface has been broken up by more recent normal faulting, presenting the possibility that it is no longer active. Further south, the core complex is in the hanging wall of a major north-dipping normal fault. The footwall of the same fault is broken up by north- dipping normal faults that cut the seafloor. An interpretation of this is that the normal fault bounding the north side of the core complex has jumped to the south. Whether this is indeed the surface extension of a deep crustal detachment remains to be seen.
T41A-0363
Structural style and Basin Formation in Deep-water Area of Northern South China Sea
In the deep-water area of northern South China Sea (SCS) developed a series of sedimentary basins. Active exploration for deep-water hydrocarbon has begun in these areas since this century. The well LW3-1-1 at water depth of 1480m in the BaiYun Sag (BYS) of the Pearl River Mouth Basin in 2006 discovered 56m layer of pure gas, demonstrated the good hydrocarbon potential of the area. Wide-angle seismic profiling has verified the transitional type of crust in the slope areas. The Moho surface shoals step-by-step from 30-29km under the shelf, ~15 km under the slope, and ~12km under the abyssal plain. Moho also rises beneath depocenters, mirroring the shape of sedimentary basement. The crustal thickness at the center of the BYS is <7km. Lower crustal high velocity layer is found in the eastern and central portions of the northern SCS. The pre-Cenozoic basement in northern SCS is the extension of the inland basement and consists of mainly metamorphosed Paleozoic and Mesozoic marine and continental strata, complicated by Yanshanian (J-K) intrusive and extrusive rocks. From geophysical data we inferred that a SW-NE Mesozoic trench-arc system exists beneath the Cenozoic sediments in the northeastern SCS, related to the subduction of the Paleo-Pacific Ocean towards the East Eurasian margin. The stress field in the East Eurasian margin changed abruptly in Late Cretaceous. Rifting started in the entire margin and eventually led to the opening of the SCS in late Early Oligocene. Large sedimentary basins developed in the margins of the SCS. Paleogene lacustrine sediments contain hydrocarbon sources, while traps are mostly found in Neogene marine strata. The structure of the northern SCS shows clear W-E variation, divided into NE-, NEE-, and NE-trending segments by two major NW-SE transfer faults. The Southern Depression of the Qiongdongnan Basin to the west is characterized by NE-trending half grabens. The BYS at the central segment is characterized by NEE-trending composite grabens and down warps with relatively small offset of boundary faults. To the east the Chaoshan Depression is composed of Mesozoic strata under very thin (<1km) Cenozoic cover. The origin of such a W-E variation might be related to the existence of Mesozoic subduction system in the east, which influenced not only the formation of Cenozoic sedimentary basins and the variation of sedimentary facies, but also the thermo- rheological structure of the underlying lithosphere. The BYS from the bottom upwards includes 3 layers, a layer of rifts, a layer of faulted down-warps, and a layer of down-warps. Compared with the 2-layer bull-head structure of the Zhu 1 depression in the shelf, the BYS has one more layer of faulted down-warps. This might indicate that after a short period of brittle rifting the relatively hot lithosphere in the slope has undergone a period of ductile extension. The post-extension sequence in BYS is much thicker than that predicted by thermal subsidence theory. We suspect that in a passive margin the formation mechanism of deep-water basins is different from that of shallow-water basins. A study is ongoing to explore the basin formation mechanism, taking into account of the factors of abnormal lithosphere rheology, active mantle underplating and magmatic heating, lower crust flow, as well as the superposition of later extensional events. The study is supported by NSFC grants 40576027 and 40238060.
T41A-0364
Character of the lower crust and upper mantle below the Baikal Rift Based on the Seismic Wave Field Pattern along Profile 1 from BEST Project
The Baikal Rift Zone (BRZ) is located on the largest continental plate on the Earth, the Euro-Asiatic plate, between the Siberian platform to NE and the region of Paleozoic deformation belt to the SE. The BRZ is an active rift of late Cenozoic age. The aim of the BEST (Baikal Explosion Seismic Transects) project was to study the structure of the crust and upper mantle below Baikal Rift and surrounding areas. Experiment measurements were performed along two deep seismic profiles number 1 and 2. We concentrate on the profile 1 crossing BRZ from west to east direction. Along this profile 10 land shot points are located as well as air gun shots in the Baikal lake, recorded by the three arrays located in two sides of lake bank. Tectonically, the first 6 shot points of the profile 1 are located on the Siberian Craton and last 4 are on the Palaeozoic fold belt. Between these tectonic units is the BRZ (lake Baikal). We modeled the structure using different kinematics and dynamic programs based on raytracing and tomographic methods. We present velocity models across rift from NW to SE direction. We present optimal kinematic and dynamic model along the profile 1 with relation to recorded wave pattern. We discuss significant differences between tectonic units derived from variations of the wavefield pattern and character, studying in particular the arrivals from the transition between crust and upper mantle. For the understanding of the structure of the lower part of crust and upper mantle and differences between BRZ, Craton and Paleozoic fold belt, we present dynamic interpretation of wave pattern which suggest significant differences between the above-mentioned blocks. On the both sides of the BRZ, the Moho depth varies not so much and is located between 35 to 42 km with slight deepening towards the rift zone. Below the rift zone we localize reflection/refraction boundary on the depth 47-52 km with high velocity about 8.6 km/s (Pnrift?). The character of the dynamic properties of the Moho reflections is significantly different for Siberian Craton and for Palaeozoic foredeep region.
T41A-0365
Residual depth anomalies on the Iberian, Newfoundland, Labrador and Nova Scotian margins; implications for their lithosphere mass and density distribution
The break-up of the Iberian and Newfoundland margins has resulted in large scale asymmetry between these conjugate margins. One documented asymmetry is that the oldest oceanic crust on the Newfoundland margin is elevated with respect to that of the Iberian margin. This work employs residual depth anomalies (RDA) to investigate this asymmetry. Observed bathymetry has been compared to the global oceanic bathymetry-age models of Parsons and Sclater (1977) and Stein and Stein (1992) in order to calculate RDA. The oceanic crust on the Nova Scotia and Labrador margins have also been included in this study in order to explore the lithosphere mass and density distribution across the wider region. Seismically derived cross-sections have been flexurally backstripped to correct the RDA for sediments. A correction has also been made to the RDA for seismically observed variation in oceanic crustal thickness, about the global mean thickness, using local isostasy. Key observations of the corrected RDA are: (1) oceanic crust on the Labrador and the Newfoundland margins are over a kilometre shallower than predicted by the global bathymetry-age models; (2) oceanic crust on the Newfoundland margin is elevated by 700 to 900m relative to its conjugate Iberian margin; (3) the RDA decrease oceanwards; (4) the RDA decrease southwards on both sides of the North Atlantic. Potential sources of the observed RDA have been investigated, including flexural coupling of oceanic and continental lithosphere, mantle plume uplift, serpentinization, magmatic intrusions and lithospheric mantle geochemical heterogeneity. Modelling shows flexural coupling of oceanic and continental lithosphere, during thermal subsidence of the oceanic lithosphere, may be the sole source of the ocean-wards decrease in RDA. However, the elevation of the Newfoundland margin relative to the Iberian margin can not be explained by flexural isostatic coupling. The Iceland plume might be the source of the southerly decrease in RDA; however, this is unlikely, due to its large distance from the margins. Moreover, the Iceland and Azores plumes are unlikely to be the source of the elevation of the Newfoundland margin relative to the Iberian margin, since the plumes are approximately the same distance from the margins. A possible explanation for the elevation of the Newfoundland margin relative to the Iberian margin, is a compositional mass deficiency within the ocean-continent transition (OCT) of the Newfoundland margin. Possible compositional deficiencies include gabbroic intrusions, serpentinized lithospheric mantle and lithospheric mantle depletion. The quantities of these compositional deficiencies, required to generate the observed positive RDA, has been estimated using local isostasy. If all the observed positive RDA is caused by a compositional mass deficiency then the required thicknesses of gabbroic intrusions, fully serpentinized mantle or partially serpentinized (50 to 60%) mantle are 4 - 10 km; 2 - 5 km or 3.5 - 9 km respectively. Partial depletion of the entire lithospheric mantle, within the OCT of the Newfoundland margin is an alternative explanation for its elevation relative to the Iberian margin. If mantle depletion is the sole source of the positive RDA of the Galicia Bank (northern Iberia) and Flemish Cap (northern Newfoundland) margins, then the entire lithospheric mantle within their OCT must be depleted by 11 and 23% respectively. These depletion levels are consistent with the measured depletions of mantle rocks from the two margins (Muntener and Manatschal, 2006).
T41A-0366
Insights on the Deep Structure of the Iberia Abyssal Plain and Galicia Bank Southern Edge From new MCS and Wide Angle Data
In the scope of the Portuguese Continental Shelf Extension Program, 1600 km of 2D MCS reflection lines were acquired over the Iberia Abyssal Plain and the southern edge of the Galicia Bank. The survey ties in with other academic and industry surveys. It comprised four E-W lines, crossing from continental to oceanic crust along the Zone of Exhumed Continental Mantle, and two N-S lines crossing the Iberia abyssal plain from the Galicia Bank to the Tore seamount and the Tagus Abyssal plain. Additionally nine OBSs were deployed along the MCS IB02 profile (roughly at 41°N). The MCS acquisition layout consisted in a streamer 8 km long and an array of bolt long- life air guns (5720 cu.in.). In order to record deep reflections the record length was set to 18 s. The E-W lines were Kirchoff pre-stack time (PSTM) and depth migrated (PSDM). Velocity model for the depth migration was obtained on the sedimentary cover by using a grid based tomography algorithm from Paradigm's Geodepth software, and constrained, from the basement to deeper sections, using IB02 wide angle results and other published refraction studies on the area. Results show a clear improvement of the PSDM lines with respect to the original time migrated lines, especially in the deeper sections. A good agreement is verified between the wide-angle IB02 velocity model and the coincident depth migrated MCS line. The basement topography is highly variable across the studied lines and suggests that the tectonic segmentation pattern of the margin is complex in this sector. Mid to High amplitude intra-basement reflectors are often observed: i) forming wedges internally defined by sets of sub-parallel reflectors, sometimes disrupted by high angle faults (pre-rift sequences?); ii) Consisting on several low angle intra-basement reflectors, interpretable as detachments faults, dipping both westward and eastward and reaching, in some cases, at least a depth of 20 km. We discuss the implications of our interpretations, from these enhanced PSDM images, to the existing tectonic models for this segment of the West Iberian Margin.
T41A-0367
Predicting OCT Location and Continental Extension for North Atlantic Rifted Margins Using Gravity Inversion
Gravity inversion incorporating a lithosphere thermal gravity anomaly correction has been used to determine Moho depth, crustal basement thickness and lithosphere thinning factor for ~50 2D regional profiles across North Atlantic rifted margins including Labrador Sea, Baffin Bay, Iberian – Newfoundland and Norwegian – Greenland conjugate margins. Sediment thickness derived from seismic refraction/reflection data has been included in the gravity inversion: sediment density is assumed to be compaction controlled. Gravity inversion has been used to estimate the location of the ocean-continent transition (OCT) and to determine lithosphere extension across the rifted margins. The lithosphere thermal model used to predict the lithosphere thermal gravity anomaly correction may be conditioned using plate reconstruction models to provide the age and location of oceanic lithosphere. OCT locations predicted by plate reconstruction models may be unreliable as a consequence a gravity inversion method has been used in which the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly correction assuming that the entire region is continental lithosphere with a constant breakup age applicable to the margin being investigated. A correction to the predicted thinning factor from gravity inversion is made for the addition of continental volcanic material produced by decompression melting during breakup, lithosphere thinning and seafloor spreading. This method, in which isochrons are not used, provides an independent check for the location of the OCT. Comparing crustal thickness obtained from the gravity inversion with seismic estimates allows us to constrain the age of continental breakup. Lithosphere extension estimates across the margins can be used to refine plate reconstruction models.
T41A-0368
Crustal Thickness and Continental Lithosphere Thinning on the N. Angolan Rifted Margins Predicted Using Gravity Inversion
Crustal thickness and continental lithosphere thinning have been determined for the N. Angolan Margin using gravity inversion; a margin in which seismic imaging of the ocean-continent transition is obscured by large thicknesses of allochthonous salt. The gravity inversion is carried out in the 3D spectral domain and includes a correction for the lithosphere thermal gravity anomaly generated by elevated geothermal gradients within stretched continental margin and adjacent ocean basin lithosphere. Sediment thicknesses used to determine the sediment gravity anomaly contribution are derived from the P7+11 and P3 seismic cross-sections of Contrucci et al. (2004). A compaction-controlled sediment density is assumed for non-salt lithologies. Moho depth predicted by the gravity inversion are in good agreement with the Contrucci et al. (2004) seismic estimates. Continental lithosphere thinning along the P3 and P7+11 profiles have been determined from crustal basement thicknesses predicted by gravity inversion and include a correction for volcanic addition generated by decompression melting during continental breakup. The sensitivity of Moho depth, crustal thickness and continental lithosphere thinning to sub-salt sediment thickness, breakup age and volcanic addition has been investigated. If the N. Angola margin is magma poor, gravity inversion predicts a region, for lines P3 and P7+11, up to 180 km wide of highly thinned pre-breakup continental crust between 15 and 6 km thick. If the N. Angolan margin has "normal" volcanic addition, then gravity inversion predicts the width of the highly thinned continental crust to be reduced to approximately 100km width. It is unclear whether the ocean-continent transition zone beneath the allochthonous salt is underlain by exhumed mantle.
T41A-0369
Characterization of the U reflection on the Newfoundland Margin: Evidence for Wide-Spread Early Postrift Magmatism on ‘Magma-Poor' Rifted Margin
Drilling during ODP Leg 210 on the magma-poor Newfoundland margin revealed evidence for varying amounts of both syn- and early post-rift magmatism. Here, we focus on post-rift magmatism that appears to have affected the deep margin ~10-15 m.y. after the onset of seafloor spreading. Two postrift sills (105.3 and 97.8 Ma) were encountered in the deep sediments overlying ‘transitional' basement at Site 1276. These sills have been linked to bright reflections in seismic reflection data by means of synthetic seismograms. The shallower of the two sills is at least locally coincident with the U reflection which is observed throughout the Newfoundland Basin and which has been interpreted by some authors to mark final breakup of continental lithosphere. However, the lateral extent of the sills in the Newfoundland Basin and their relation to the U reflection away from Site 1276 has been uncertain. Likewise, the cause of the magmatism and the relationship between the sills and other contemporaneous magmatic features is unknown. Our study investigates the magnitude and nature of the post-rift magmatic event by mapping the U reflection and associated bright reflections throughout the deep Newfoundland margin. U is characterized by relatively high amplitudes and good continuity. It is generally flat and limited to the zone landward of magnetic anomaly M3, pinching out on basement both continent-ward and northward. The reflection is often underlain by reflections with comparable or higher amplitudes that are interpreted to represent sills. Disruptions are often observed in the sub-U stratigraphic sequence, and these may represent pathways of magma injection. To complement the seismic facies mapping we have also created a series of synthetic seismograms to explore variations in sill properties that might account for observed variations in seismic reflection characteristics. Our results indicate that the level of the U reflection was the preferred depth at which sills were intruded (at least for initial injection), possibly because it was a level of hydrostatic equilibrium and/or a stratigraphic level of weakness. The widespread distribution of high-amplitude reflections associated with sill-like geometries implies that sills are ubiquitous throughout the Newfoundland Basin. The restriction of major post-rift magmatism to the Newfoundland margin may indicate that it was associated with passage over the Madeira and Canary hotspots, which affected the Iberia plate only at its southern margin and only beginning at c.70 Ma.
T41A-0370
The Galicia Bank Influence on the Southern Iberia Abyssal Plain Drilling Transect
The Iberia nonvolcanic rifted margin has been studied primarily within the context of two distinct segments: Galicia Bank (GB) and the Southern Iberia Abyssal Plain (SIAP). The bathymetric contrast between GB (2-4 km water depth) and the SIAP (~5 km water depth) differentiates the two segments, in addition to distinctions in the widths of extended continental crust (GB: ~260 km; SIAP: ~80 km) and the zone of exhumed continental mantle (ZECM; GB: 0-60 km; SIAP: ~170 km). We have speculated that crustal-scale mass wasting may have altered the original transform segment boundary between GB and the SIAP. Our interpretation proposes transport of extended continental crust and overlying pre-rift sediments southward from GB onto exhumed, serpentinized peridotite of the SIAP. We reexamine the ODP Legs 149/173 drilling transect in the context of this hypothesis. Shallow-water Tithonian-Berriasian sediments sampled at sites 901 and 1065 atop basement highs of interpreted 4-6 km-thick continental crust could have been originally deposited atop GB, slumping with upper crustal blocks to the south sometime during the Tithonian to Valanginian. In this interpretation, serpentinized peridotite sampled at site 1068 represents the pre-existing SIAP ZECM onto which the slump was deposited. We suggest that intra-crustal faulting caused by east-west extension may have weakened the upper crust of GB sufficiently to allow gravitational collapse. Our interpreted slump then represents the latest faulting of a polyphase rift evolution. Examination of the Iberia margin bathymetry hints at a concave fault-scarp at the crown of our proposed slump; we use the bathymetry to resolve other potential locations of slumping, as well as regions that appear unaffected by mass wasting.
T41A-0371
Brasilian and Angolan Passive Margins: the kinematic constraints.
The thinning of continental passive margins is usually explained by conservative models using stretching and/or simple shear. Nevertheless, those models imply hypothetical extensional structures and large horizontal movements between the two homologous margins (more than 250 km for the Brazilian and Angolan Margins). Therefore, the consequences induced by a pre-break-up kinematic reconstruction are tremendous on the genesis of the continental passives margins that marked the break-up area. Several authors have already addressed the problem of the pre-opening reconstruction in the South Atlantic Ocean, in the past. Nevertheless, the more recent entire reconstructions of the break-up present numerous unexplained misfits (gaps, overlaps and misalignments) that invalidated the fit. We present here a new reconstruction based on new interpretation of magnetic data, satellite altimetry and oceanic and continental geological constraints. The Zaiango refraction/reflection data together with this new closest pre-opening fit show that the Angolan-Campos system presents a 200 km wide thinned basin which is, according to the shallow deposed salt layer, in high position all along its genesis and at least until the break-up. This basin cannot be explained by further horizontal movement: vertical motions prevail compared to horizontal motions in the formation on the huge thinned Angolan-Brazilian basin. Thus, middle and/or inferior crusts have to be involved in processes as: "flowing" in the first accreting process (for the volcanic margin), or denudation processes to create the first "proto-oceanic crust" and/or flowing laterally along different margin segments, or even mixing with the upper mantle underneath.
T41A-0372
Implications of a 3D Density and Magnetic Model of the Møre Volcanic Margin Offshore Norway: An Example of a Non-Volcanic Margin Becoming Volcanic.
The Møre volcanic margin as part of the mid-Norwegian margin is one of the world's most extensively studied continental margins. Volcanic margins are considered an end-member of rifted margins and are characterized by transient, voluminous basaltic volcanism that impedes imaging of deeper structures, a feature generally missing on non-volcanic margins. A 3D density and magnetics model constrained by petrophysical and seismic data was constructed to map the main regional structures of the Møre margin. The general trend of the crustal thickness is from about 30 km at the coastline and gets as thin as 2 km in the basin, thickens again below the marginal high and finally tapers off to about 8 km beyond the Continent-Ocean Boundary (COB). The thinnest crust in the basin is spatially correlated to an anomalously dense body in the lower crust. About 90% of the crust immediately overlying this body is thinner than 10 km. This implies a serpentinized origin of the body as observed on the Iberia and Galicia margins. Spatial correlation of thin crust and proposed deep seated sill complexes is also noted in the model. The sill complexes are further correlated to shallower saucer shaped sills and extrusives landward of the marginal high. This implies an igneous origin of the anomalously dense body. We propose an interaction of two main processes to explain the opposing observations on the model: The margin was a non-volcanic margin that experienced considerable amount of thinning in Jurassic-Early Cretaceous times where mantle serpentinization occurred. In Tertiary times continental breakup occurred further to the west and accompanied temperature elevation resulted in melt production at the thinnest crust. We therefore argue that the Møre margin was a non-volcanic margin until the onset of drifting when it acquired the characteristics of a volcanic margin.
T41A-0373
Extreme Crustal Thinning in a Transtensional Environment: the Example of Bay of Biscay - Western Pyrenees
The Bay of Biscay-Western Pyrenees is the ideal natural laboratory to study extreme crustal thinning within a transtensional environment. This area underwent several rift phases with a major phase occurring in late Jurassic to early Cretaceous time, responsible for the opening of the Bay of Biscay and the formation of smaller rift basins, the Parentis and Mauléon basins, ahead of the propagating ocean. The Parentis basin shows localized crustal thinning to less than 10 km, an important asymmetry of the basin and only little evidence for normal faulting. To the north the basin is characterized by a sag geometry whereas to the south it is floored by a strong reflection. This reflection truncates tilted blocks that are associated with Lower Cretaceous sediments showing evidence for growth structures. Two wells drilled across the strong reflection and penetrated into tectonized Paleozoic meta-sediments. Based on these observations we interpret the strong reflection as a detachment structure and the overlying blocks as extensional allochthons. About 30 km further to the east, the strong reflection approaches the surface near massif du Labourd. This massif is formed by granulite facies lower crustal rocks that are separated, along a mylonitic shear zone, from upper crustal rocks including Paleozoic meta-sediments. The top of the basement is tectonized and covered by tectono-sedimentary breccias, which are sealed by Lower Cretaceous sediments. In analogy to similar observations in the Alps and Iberia, we interpret the tectono-sedimentary breccias as remnants of a top basement detachment fault that truncated an older mylonitic shear zone separating lower and upper crustal rocks. This structure might be important for the extreme crustal thinning observed in the basin. The Mauléon basin was partly reactivated during the Pyrenean compression, which has the advantage that deeper parts of the basin are directly accessible and can be studied in the field. A key observation is that serpentinized mantle peridotites, associated with ophicalcites, are overlain by sedimentary breccias composed of mantle rock, lower crust rocks and mylonites. This suggests that the mantle and lower crustal rocks were locally unroofed and reworked within the Mauléon basin. In such a context, the massive pre-rift sediments forming the mountain ridges within the Mauléon basin may be interpreted as extensional allochthons rather than remnants of classical fault bounded rift-basins, an idea that need to be tested. The stratigraphic record of extreme crustal thinning is documented by up to 900 m of Albian conglomerates reworking Paleozoic metasediments, indicating rapid subsidence in the basin simultaneous with uplift at the southern margin of the basin. Our preliminary results show evidence for extreme crustal thinning in the Parentis and Mauléon basins forming ahead of a propagating oceanic system. In contrast to orthogonal rift systems, the deformation is more localized and characteristic structures such as the H-block are not observed. Instead, complex relationships with syn- sedimentary lower Cretaceous transform faults are observed and need to be taken into account to describe the overall strain distribution. The results of this work, if confirmed, will not only allow to understand extreme crustal thinning in a 3D transtensional setting, but will also result in a reinterpretation of the Pyrenees and ultimately of the kinematics of the Iberian plate with major consequences for the tectonic evolution of the North Atlantic and Alpine realms.
T41A-0374
Unraveling the Interaction Between Mantle Processes and the Tectono-Sedimentary Evolution During Final Rifting Based on the Study of Remnants of the Alpine Tethys Rifted Margins Exposed in the Alps
The tectonic, sedimentary and isostatic evolution of distal rifted margins are poorly constrained and the available data from present-day magma-poor rifted margins, such as the Iberia-Newfoundland or the Southern Atlantic margins suggest that its evolution is complex and very different from that of proximal margins. In contrast to present-day rifted margins, where rift structures are covered by sediments and are at abyssal depth, remnants of ancient margins preserved in collisional orogens bear, if not overprinted by later deformation, important information on the stratigraphic, tectonic and mantle evolution during rifting. This is particularly true for the Adriatic and parts of the European margins exposed in the Alps in Central Europe. From these margins remnants of the first oceanic crust, the subcontinental mantle, from lower crustal rocks, detachment systems, remnants of the distal and proximal margins and the stratigraphic record of rifting, including pre-, syn- and post-rift sediments are preserved. A paleogeographic reconstruction of all these structures including the associated stratigraphy and the underlying basement represents a unique opportunity to study the relations between shallow crustal and mantle processes during rifting. Previous studies suggested that the margins in the Alps resulted from a complex poly-phase evolution that initiated with distributed stretching (220 to 190 Ma), continued with localized thinning (around 180 Ma) and terminated with exhumation of mantle rocks and first MOR-type magmatism (at 160 Ma). Thus, rifting leading to breakup and opening of the Alpine Tethys was shown to be the result of strain localization and to include a transition from decoupled to coupled deformation in which detachment faulting played an important role. How crustal thinning is linked in detail with strain localization, uplift of distal domains and melt infiltration in the rising mantle during crustal thinning is, however, not yet understood. We will present preliminary results from the study of the most distal Adriatic (Canavese/Err/Bernina domains) the conjugate European margin (Briançonnais domain), and the Sesia/Lanzo and the Ivrea/Balmuccia zones, representing deep crustal and mantle portions. These units bear the information of how mantle, lower crustal and upper crustal domains evolved during final rifting in Middle Jurassic time (180 to 160 Ma) and how their evolution is recorded in the stratigraphic record of the Alpine Tethys margins.
T41A-0375
Guiana's Basin, A Unique Setting with Two Break-up and Drift Unconformities and a Feature Hypothesized to be a Southern North Atlantic, Iceland Equivalent Atlantis
In the Guiana's Basin, two distinct Break-up/Drift Unconformities associated with thermal doming, rifting, cooling, compressing and drifting of plates were identified and mapped. The earliest unconformity 1) Mid-Jurassic-aged separation of North/Central America from Western Europe/North Western Africa and the latest unconformity 2) Albian-aged separation of Central West Africa from Central Northern South America, exist here. What complicates the interpretation of the plate motion is the amount of overburden deposited post Albian time on both sides of the ridge separating the African and the South American plates. To the east, the Niger River pushed out 5-20 km of sediments on top of the drift unconformity. In the Guiana's Basin the sedimentation is only about 5-10 km of sedimentation therefore it is easier to image and interpret the geologic history. Through a synthetic well to seismic tie, a correlation of new biostratigraphic data from a well spud in Nov. 1977 and TD'ed in June of 1978 by Esso, was incorporated into the regional geologic interpretation. The interpretation based on this data was that the timing of the opening of the South Atlantic was synchronous with that of the North Atlantic. The feature of interest closely resembles a buried volcanic landmass. It just happens to be located in the area hypothesized as the Mid Jurassic spreading center related to a hotspot, after Pindell and Kendall, March 2005. Interpretation and conclusions for this study are based on 1999 and 2004 vintages of 2D seismic data and a new interpretation of the biostratigraphy. Additionally, gravity and magnetic data along with regional subsidence modeling and a tectonic evaluation study supports these conclusions. On several 2D seismic lines from the area, a deep volcanic feature can be seen and interpreted to exist in Mid Jurassic time. This feature is similar in portion to Iceland the volcanic island currently residing in the middle of the North Atlantic Ocean. Today in the Guiana's Basin the feature identified on seismic is now deeply buried. We would content that this paleo volcanic "Island" produced some 180 mybp has since disappeared from the face of the Earth through normal subsidence processes. Therefore, what had evolved in a similar way to the Iceland we now know, has since disappeared much the same as the fictional city of Atlantis. What makes this area unique is the presence of two distinct unconformities and our ability to image them with conventional 2D seismic data.
T41A-0376
Heat-Flow at the edges of continental lithosphere and implications for the evolution of extensional margins
Heat-Flow variations across continental rifted margins are difficult to obtain for methodological reasons: direct measurements are not possible below a certain water depth and values derived from oil exploration are often biased by perturbations on temperature records and unreliable conductivity estimates. We have developed recently a methodology that provides better estimates of thermal conductivity in oil exploration wells, based on neural networks linking this physical property to geophysical well logs. The method has been applied systematically on a large number of wells on Atlantic and Australian margins, providing almost 1,000 new heat- flow estimates. In all cases, the mantle heat-flow below the margins is comparable to that of oceanic domain, and in some cases higher. These conclusions arise from old margins (>50 Ma), but measurements on young margins (e.g. Red Sea, Aden) show unexpected high values. This is interpreted as a consequence of temperature differences at depth between continental and oceanic lithospheres. Several 2D numerical experiments show that such anomalies are likely to develop with variable amplitude and pattern depending on the temperature regime of the continental lithosphere, rheology of the mantle and geometry of the interface. It seems that such anomalies can appear rapidly after the break-up of continents and maintain permanently. This changes significantly the subsidence evolution and the relations with the pre-existing thermal regime of the continent.
T41A-0377
Rift tectonics in the Amundsen Sea Embayment: Stepwise break-up of New Zealand from West Antarctica
The Amundsen Sea Embayment of West Antarctica is in a prominent location for a series of tectonic and magmatic events from Paleozoic to Cenozoic times. It played a central role in the rifting and break-up of greater New Zealand from West Antarctica as it is the location where the junction of Chatham Rise and Campbell Plateau (New Zealand) lies conjugate to the West Antarctic margin. New seismic, magnetic and gravity data from the Amundsen Sea Embayment and Pine Island Bay reveal the crustal thickness and tectonic lineations. The Moho is 24-22 km deep on the shelf. NE-SW trending magnetic and gravity anomalies and the thin crust indicate a former rift zone that was active during or in the run-up to breakup between Chatham Rise and West Antarctica before or at 90 Ma. NW-SE trending gravity and magnetic anomalies, following a prolongation of Peacock Sound between Thurston Island and Ellsworth Land, indicate the extensional southern boundary to the Bellingshausen Plate which was active between 79 and 61 Ma. However, both lineation trends, NE-SW and NW-SE, seem to be observed over broad regions. This infers stepwise and multiple rift and extension phases over a wide period of time before, during and after the break-up between New Zealand and West Antarctica.
T41A-0378
Geological and Tectonic Evidence for the Formation and Extensional Collapse of the West Antarctic Plateau: Implications for the Formation of the West Antarctic Rift System and the Transantarctic Mountains
The Transantarctic Mountains (TAM), the world's longest and highest non-contractional intracontinental mountain belt, define the western boundary of the West Antarctic rift system (WARS). The WARS is a broad region of extended continental lithosphere, ca. 750-1000 km wide, lying dominantly below sea-level. A new model (Bialas et al., 2007), proposes that a region of thickened continental crust and high-standing topography, the "West Antarctic Plateau", underwent extensional collapse to leave a remnant edge representing the proto-TAM. Tectonic and paleogeographic reconstructions indicate the plateau formed inboard of a continental arc along the paleo- Pacific margin of Antarctica, active throughout the Paleozoic until the late Mesozoic. This high-standing region was responsible for confining sediments (Beacon Supergroup) to elongate basins along the length of the TAM. Much of the present region of the WARS has been correlated with the Lachlan Fold belt of southeastern Australia. This belt formed from the Ordovician to Carboniferous during back-arc basin formation associated with slab roll- back with short periods of compression. Convergence along the paleo-Pacific margin, perhaps enhanced by subduction of more buoyant oceanic lithosphere as the Phoenix-Pacific ridge was obliquely subducted, resulted in crustal thickening and formation of high-standing terrain (the plateau). Extensional collapse of the plateau most likely began in the Jurassic during initial rifting between East and West Antarctica, but was mainly accomplished during distributed rifting in the Cretaceous (ca. 105-85) following subduction of the Phoenix-Pacific ridge and prior to the separation of New Zealand from Marie Byrd Land. Continued formation of the TAM continued in the Cenozoic concomitant with extension in the WARS that was localized along its western margin adjacent to the TAM. Glacial erosion in the Oligocene and early-Miocene enhanced peak height in the TAM. In this presentation we discuss the diverse geological, geophysical, thermochronological and tectonic evidence for the West Antarctic Plateau and the implications for the formation of the Transantarctic Mountains.
T41A-0379
Observations From Fieldwork and (U-Th)/He Thermochronologic Study of the Central Arabian Flank of the Red Sea Rift System
Improvement in our modeling of continental lithosphere rupturing and rifting dynamics requires an absolute understanding of the temporal and spatial strain distribution throughout an entire rift system. This statement holds especially true for our understanding of Red Sea rift system dynamics. However, critical geologic data used to determine the tectonic evolution of the Red Sea rift system come mainly from the Gulf of Suez and the Egyptian and Yemeni margins of the Red Sea while the rift flanks in Sudan and Saudi Arabia have remained largely unstudied. This study aims to fill that information gap by focusing on the development of extensional structures and rift-related Tertiary basaltic volcanism along the central Saudi Arabian rift flank. Traditional structural analyses, coupled with thermochronometric techniques, are used to elucidate the temporal and spatial evolution of strain markers manifested by structurally-controlled extensional basins that parallel the trend of the main Red Sea rift. Constraints on the dynamics of rift flank deformation are achieved through the collection of long-baseline thermochronometric transects that traverse the entire Arabian shield and short-baseline transects that resolve the thermal evolution of individual normal faults that bound inland basins. The variation in sampling resolution is a comprehensive method that allows tectonic study at different scales; long-baseline transects aim to resolve the timing and kinematics of rift flank uplift and exhumation while short-baseline transects address issues well inboard from the modern rift margin such as structural control on pre- and syn-rift stratigraphy. Structural field analyses have shown that the NW-SE trend of inland basin-bounding faults is strongly coincident with the trend of pre-existing crustal fabrics produced by the Late Proterozoic Najd Fault System. Furthermore, the magnitude of normal fault activation along these heterogeneities controlled the geometry of inland basins. Preliminary (U- Th)/He apatite analysis of samples from both short- and long-baseline transects reveal a temporally dichotomous and spatially complex cooling history for the central rift flank that includes active footwall exhumation approximately 130 km inland from the rift margin during the Middle Miocene.
T41A-0380
Lithosphere-Asthenosphere Boundary from S-receiver functions in Ethiopia
Seismological investigations have been carried out in the past in the Main Ethiopian Rift to study crust and upper mantle structures. More constraints on the thickness of the lithosphere and the nature of the lithosphere- asthenosphere boundary (LAB) are needed to better understand the tectonics of the Main Ethiopian Rift and adjacent plateaux. To tackle this, we use the S-receiver function technique to identify the LAB as already successfully performed by other authors in other areas. We computed S-receiver functions from data recorded by the Ethiopian Broadband Seismic Experiment (Nyblade & Langston, 2002, Eos Trans. AGU, 83, 405–410) in central Ethiopia. We used S phases from 20 earthquakes with magnitudes greater than 5.5 (mb) at epicentral distances between 65°–85° and with good azimuthal distribution in order to detect the presence of the (LAB). Preliminary observation of the data shows a clear coherent negative signal following the Moho phase that can most probably be interpreted as the conversion at the LAB. This phase precedes the S-onset by 8-12 seconds suggesting lithospheric thickness of 80 to 120 km.
T41A-0381
Joint Inversion of Receiver Functions and Surface Waves in the Main Ethiopian Rift
We jointly invert receiver functions and surface wave dispersion measurements from 10 to 175 sec to study the crustal and upper mantle structure beneath the Main Ethiopian Rift and the surrounding rift plateaus. These models provide a 1-D velocity profile beneath broadband stations from the 2001-2003 EAGLE experiment (Ethiopia-Afar Geoscientific Lithospheric Experiment), extending our knowledge of crustal structure beneath these stations beyond our previous knowledge of Moho depth. Thirty stations were deployed in Phase I of EAGLE for 16 months, with 50 additional stations deployed for 4 months in Phase II over a smaller region focused on the rift valley (Stuart et al., 2006, Geol Soc Lond Spec Pub 259). Our models help constrain the deep crustal structure of the Ethiopian Plateau and provide information on crustal modification away from the active source profiles.
T41A-0382
Comparison of the Lithospheric Structure Beneath Kenya and Ethiopia From Joint Inversion of Receiver Functions and Rayleigh Wave Dispersion Velocities
Shear-wave velocity structure of the crust and upper mantle beneath Kenya has been investigated using joint inversion of receiver functions, and Rayleigh wave group and phase velocities. Most of the data for this study come from the Kenya broadband seismic experiment, conducted between 2001 and 2002. Shear velocity models obtained from the joint inversion show crustal thicknesses of 37 to 42 km beneath the East African Plateau in Kenya and near the edge of the Kenya Rift, and a crustal thickness of about 30 km beneath the Kenya Rift. These crustal parameters are consistent with crustal thicknesses published previously by different authors. A comparison has been made between the lithosphere under Kenya and other parts of the East African Plateau in Tanzania. A comparison between the lithosphere under Kenya and that under Ethiopia has also been made, specifically between the lithosphere under the Ethiopian Plateau and the Kenya Plateau, and between the lithosphere beneath the Main Ethiopian Rift (MER) and the Kenya (Gregory) Rift. The lithospheric mantle beneath the East African Plateau in Kenya has a maximum shear wave velocity of about 4.6 km/s, similar to the value obtained under the East African Plateau in Tanzania. Beneath the Kenya Rift, the lithosphere extends to a depth of at most ~75 km. The average velocity of the mantle lithosphere under the East African Plateau in Kenya appears to be similar to the lithosphere under Tanzania away from the East African Rift System. The lithosphere under the Kenya Plateau is not perturbed as compared to the highly perturbed lithosphere beneath the Ethiopian Plateau. The lithosphere under the Kenya Rift is perturbed as compared to the rest of the region but is not as perturbed as that under the Main Ethiopian Rift or the Afar. Though Kenya and Ethiopia have similar uplift, volcanism and rifting at the surface, they have different lithospheric structures at the bottom. The Afar Flood Basalt Volcanism (AFB) may be the cause of this striking difference in the two lithosphere.
T41A-0383
Structural and Stratigraphic Evolution of the Corinth Rift: Providing Constraints for Early Syn- Rift Deformation Modeling
Observations of the style of extension and strain distribution during the initiation and early stages of rifting can be used to test predictions of syn-rift subsidence and stratigraphy generated by numerical rift models. Plausible syn- rift deformation models must consider: the role of pure shear vs. simple shear; the nature and distribution of border faults; the duration and rate of slip on these faults; and the activation and propagation of the rift system. The young, <5 Ma Gulf of Corinth rift is an ideal place to study these syn-rift processes. The EW trending rift is actively extending at a rate of up to ~15 mm/yr in a NS direction, overprinting a relatively simple pre-rift geology. Using a combination of new high resolution MCS and swath bathymetry data in the western Gulf, together with archived and published datasets we produce a comprehensive fault map for the Gulf of Corinth rift system and document spatial and temporal changes to its basin geometry. In addition to the well-studied N- dipping faults on the southern margin we observe a system of offshore S-dipping faults that define the northern boundary of the major sediment depocenter. The switching dominance of these fault systems through time and space is responsible for the complex basin geometry of the Corinth rift. A major apparently basin-wide unconformity, with an estimated age of ~0.4 Ma separates syn-rift stratigraphy into two main units. In the western part of the Gulf, pre ~0.4 Ma, the geometry of sediment packages indicates that rift geometry was controlled by S-dipping faults on the northern margin. Toward the center and eastern part of the rift, the activity of N- and S-dipping faults was more equivalent, producing a symmetrical graben morphology. Post ~0.4 Ma stratigraphy tilts and thickens southward in the central and eastern Gulf with N-dipping faults having structural control at this time. In the western Gulf S-dipping faults retain overall control and horizons dip north. Sequence stratigraphic interpretation in some parts of the rift has enabled slip rate estimation for major fault systems. Multiple fault dislocation modeling has been used in an attempt to recreate basement structure using the fault slip rate estimates as derived from stratigraphic analysis and uplifted marine terraces. The complex spatially and temporally varying style of syn-rift extension in the Corinth rift must be considered in future rift models that attempt to accurately recreate the mechanical, thermal and sedimentary histories of extensional basins.
T41A-0384
Geology of Northeast Part of Fukue Island, Goto Islands, Nagasaki Prefecture
The Goto islands situated in the western part of Japan expose after Lower to Middle Miocene sedimentary and igneous rocks. The geologic evidences in this area are very important to identify relation between the north west of Kyushu and the Asian continent. We examined stratigraphy and geological structure of the Fukue Island of the southwest part of the Goto islands. Stritigraphy: In The Fukue Island, three lithofacies, Goto Group, Goto granite intrusion and Fukue Rhyolite are observed. Lower Goto Group consists of green colored volcaniclastics and massive tuff, based on the petrographic observation, there are very rounded quartz, siltstone listhic fragments and volcanic rock fragments. Upper Goto Group consists of alternation of sandstone and mudstone with upward grading structure, which is considered fluvial-lacustrine. And these were intruded by granitic dike. The Fukue Rhyolite is reported that the age is 12.4¥pm0.6Ma. Deformation: In center of the Fukue Island, there are about 10-20 degree plunges into north NE-SW trending fold, which is called "Goto central fold". In the northwest side of the fold hinge, high angle dipping beds and asymmetric fold are observed. On the other hand, in the southeast side of the fold hinge, dipping to east gentle bedding structure is observed. 　In the Fukue Island, the following three structures can be recognized. D1-a; NE-SW trending normal fault and fault related fold; The Goto center fold, which is one of the main structure in the deformation. D1-b; NE-SW trending west dipping normal fault and right lateral strike-slip fault. In the southwest region, the high angle bed formed by D1-a is bended from NE-SW trend to N-S by D1-b. D2; NW-SE trending normal fault and left lateral strike-slip fault. The trend of this structure is vertical to NE-SE trending of The Goto islands. Because of the cross cutting relationship, we consider that D1-a and D1-b were formed from 15 to 7 Ma, and D2 was formed in later 7Ma. Based on age data and lithology, we suggest that the Goto Group was deposited in the rifting area of the southwest end of the Japan sea. In the near northwest off the Goto islands, there is NE-SW trending tectonic line that is west end fault of the Japan sea opening, called the Tsushima-Goto TL. The early stage volcanic activity which formed the lower Goto Group and D1-a might be related to the TL movement. The trend of D2 is parallel to the Goto submarine canyon at the south of the Fukue Island, we consider that D2 possibly related to the opening of the north Okinawa trough.
T41A-0385
Tectonic evolution of the northernmost Okinawa Trough
Over the last decades, mechanism of backarc opening has been the subject of controversy. Okinawa Trough has been noticed as an incipient continental backarc basin. So far studies on the Okinawa Trough have been conducted mainly in its southern and middle part. Nevertheless, only a few studies have been conducted on the relationship between geologic structures and the tectonic evolution of northernmost Okinawa Trough. Our main object is to consider the tectonic evolution of the Okinawa Trough by means of structural geology, not only in ocean but also land area. We executed seismic reflection survey in northernmost Okinawa trough and investigated fault systems on Koshikijima islands located on northeastern edge of the Okinawa Trough. Then we compared geologic structures on land and sea floor. First of all, we interpreted seismic reflection data acquired by hydraulic department of Japan Coast Guard in northernmost Okinawa Trough in 1975. Furthermore, we executed multi-channel seismic reflection survey in this area (KT06-03, KT07-03 cruise). NNE and ENE trending fault systems were recognized. It is clear that Northernmost Okinawa Trough has been subsided by activation of NNE trending fault system at the east edge of trough basin with several hundred meters of displacement. The ENE trending fault system has displacement in the surface sediment, which means they are active faults. On the other hand, we held on-land field survey in northern part of the Koshikijima Islands. Fault system, which can be related to the Okinawa Trough, is reported in this area (Inoue et al., 1982). So we focused on that NNE trending normal fault system (F2 fault system). On the basis of crosscut relationship and K-Ar dating of fault rocks and dikes, F2 fault system was formed after late Miocene and likely to have been formed in a shallow underground. F2 fault system on Koshikijima islands and NNE trending fault systems in the Okinawa Trough have similar strike, displaced direction and lineaments. It is likely that they both had been formed in one event, which may be rifting of northernmost Okinawa Trough. As another noteworthy result of multi-channel seismic reflection data, strike slip fault and normal faults were observed almost same depth in shallow (approximately 750 m) sediments. It shows that they have been activated at the same time. Both Strike-slip and normal faults are accommodated by oblique rifting (Withjack and Jamison, 1986). We support that the Okinawa Trough rifting proceeded as an oblique rift system in recent time. This result is consistent with southward migration of Ryukyu arc derived from Earthquake slip vectors (Fournier et al., 2001), GPS survey (Nishimura et al., 2004).
T41A-0386
Backarc Oceanic Core Complexes Formed During Initial Spreading in the Southern Shikoku Basin
Seafloor spreading occurs in two distinct geodynamic environments, major ocean basins and backarc basins. Unusual magma-poor seafloor spreading has been identified at slow- and intermediate-rate spreading centers in major ocean basins, e.g., Mid-Atlantic Ridge, Southwest Indian Ridge, and Australia-Antarctica Discordance. Some of these spreading centers are characterized by corrugated bathymetry known as megamullions, and some by chaotic bathymetry. Serpentinized peridotite and altered gabbro have been sampled from megamullions, and the three-dimensional geological structures that form megamullions are known as oceanic core complexes. Oceanic core complexes have also been identified at extinct backarc spreading centers, e.g., Parece Vela Basin and Shikoku Basin. The Shikoku Basin formed in conjunction with subduction along the Izu- Bonin arc at the eastern edge of the Philippine Sea plate. Although the general spreading history of the basin is known from identification of magnetic lineations, the early tectonic history of Proto-Izu-Bonin arc breakup and subsequent initial backarc spreading is uncertain. We identify, describe, and interpret oceanic core complexes amid chaotic bathymetry of the southern Shikoku Basin just east of the Kyushu-Palau Ridge, the remnant arc of the Proto-Izu-Bonin arc, on the basis of marine geological and geophysical data including multichannel seismic reflection, seismic refraction, swath bathymetry, and gravity. Just west of the core complexes, the Kyushu-Palau Ridge has been dated as Oligocene in age (~25 Ma), and just to the east lies magnetic anomaly 6B (~23 Ma). Crustal structure derived from seismic and gravity data indicates that anomalously thin -less than 5 km thick- crust is located in the arc-ocean transition between the central Kyushu-Palau Ridge and southern Shikoku Basin, which suggests rift-related crustal thinning and low magma productivity during backarc spreading initiation. Near the core complexes, seamount fragments or small ridges strike perpendicular to the spreading axis, extending from the central Kyushu-Palau Ridge to the southern Shikoku Basin. These features suggest that magma-poor spreading forming the oceanic core complexes coexisted with the Proto-Izu-Bonin arc magmatism forming seamount fragments during initial, late Oligocene spreading in the southern Shikoku Basin. To explain the complicated initial backarc spreading of the southern Shikoku Basin, we compare and contrast the southern Shikoku Basin oceanic core complexes with those formed at other spreading systems, and discuss the implications of the oceanic core complexes for the tectonic evolution of the southern Shikoku Basin in the broad context of generic backarc spreading systems.
T41A-0387
Rifted Structure of the Vietnam Continental Margin Near the South China Sea Spreading Center
The extinct spreading center of the South China Sea intersects the continental margin off Vietnam, providing an excellent opportunity to study the interaction of these two features. As part of a collaborative project between the Geological Survey of Denmark and Greenland, the University of Copenhagen and the Vietnam Petroleum Institute, the crustal structure of this area has been investigated by the use of seismic reflection profiles, to provide control on the sedimentary and basement structure, combined with modelling of gravity data from global satellite altimetry, to constrain the crustal thickness. A complex pattern of rifting is seen, which may be ascribed to the complex stress fields of the propagating rift axis, together with an apparent progression in structure. In the more oceanic area, the rifting is relatively sharp, with fairly rapid crustal thnning of about 10 km. Towards the continent, in the region of the tip of the rift axis, the crustal thinning is less, around 5-7 km, and takes place over a greater distance. In the absence of data on the deep crustal structure it is not possible to determine the absolute crustal thickness with certainty, but the gravity modelling suggests that the pre-existing crust was no more than 20 km thick, having been thinned in earlier stages of formation of the South China Sea. A preliminary analysis of the isostatic balance along the various transects was inconclusive but suggests that the sedimentary sequences are largely isostatically compensated, rather than being supported by lithospheric rigidity. Detailed modelling of the rifting and subsidence may provide further insight into the processes that occur when an oceanic spreading center intersects and propagates into a continental margin.
T41A-0388
NW-SE Quaternary Extension and the emplacement of the Apan-Tezontepec Monogenetic Field, Northeast of Mexico Basin
The eastern portion of the Mexico basin, located within the Trans-Mexican Volcanic Belt (TMVB), is transected by right-stepping variably dipping NE-SW normal faults. The Apan-Tlaloc fault is a major discontinuity that divides the region into two contrasting areas with different structural and volcanic styles. a) The western area is characterized by horst-graben geometry with widespread Quaternary monogenetic volcanism and scattered outcrops of Miocene and Pliocene rocks. b) The eastern area is dominated by tilted horsts with a domino-like geometry with widespread Miocene and Pliocene rocks and scattered Quaternary monogenetic volcanism. Gravity data suggest that this structural geometry continues into the calcareous Mesozoic basement. Normal faulting was active since the Pliocene times with three stage of extension, with a high dilatational activity during the Pliocene late-Pleistocene, according with the age of the Apan-Tezontepec Volcanic Field. Statistics analysis of cone elongation, instability of volcanic cone , besides with kinematics analysis of faults show during the Pliocene late-Pleistocene was emplacement the ATVF under a stretching oriented NW-SE The activity in some portions of the Apan Tlaloc-fault system continues today as indicated by earthquake swarms recorded in 1992 and 1996 and deformed soil horizons.