T41E-1255 0800h
The role of dewatering in the progressive deformation of a sandy accretionary wedge: Constraints from direct imagings of fluid flow and void structure
Geological investigation of the deformation structures and sedimentary setting of the Emi Group, a Miocene sand-rich accretionary complex, central Japan, revealed a six stage-structural evolution during shallow level accretion in a subduction zone. The early deformation (stage 1) is characterized by independent particulate flow in layer parallel faults, scaly cleavages and web structures, and upward dewatering in dish-and-pillar structures and breccia injections, while later deformation (stages 2-6) involve mappable scale folding, meso- to macro-scopic thrusts and web structures with cataclastic flow. Based on microscopic analyses of these structures, the early faulting with independent particulate flow (stage 1 deformation) is associated with dilatancy and preferred orientation of void space, whereas the later faulting with cataclastic flow (stage 2 deformation) occurs with compaction and crude preferred orientation. The former features imply more permeable fluid migration pathways, supported by the permeability measurements and direct imaging of fluid flow by X-ray CT. On the other hand, the later fault zone has lower permeability and porosity than intact rock, and plays as fluid sealing. Thus, in the early stage (stages 1), fluid flow occurs as focused flow through dilatant fault zones with independent particulate flow or fluid migration by upward dewatering forming dish-and-pillar structures and breccia injections, whereas no evidence of fluid flow is recognized at the later stages (stages 2-6). Namely the fault zones focus fluid flow during primary accretion in shallow levels, and the fluid flow is strongly controlled by the deformation mechanism. Furthermore, the change of the deformation mechanism could be effected by progressive increment of the confining pressure, accompanied with accretion and lithification in the accretionary prism. In the shallow, dilatant-faulting regime where the deformation mechanism is independent particulate flow, focused flow dominates, whereas in the deep, cataclastic regime distributed flow may play a main conduit rather than the focused flow.
T41E-1256 0800h
Opal diagenesis and sediment properties in the Nankai Trough, Japan
We have measured the opal content and modeled opal diagenesis for sediment from Ocean Drilling Program (ODP) Sites 1173, 1174, and 1177 in the Nankai Trough, Japan. The porosity of the Upper Shikoku Basin facies at 1173 is nearly constant with depth from $\sim$102 to 344 mbsf. There is a step decrease in porosity across the Upper Shikoku Basin / Lower Shikoku Basin facies boundary at Site 1173; porosity decreases with depth normally in the Lower Shikoku Basin facies. Sediment physical properties suggest that the cementing of grain contacts within the Upper Shikoku Basin facies inhibits sediment consolidation, however, the nature of the cement has not been previously identified. In addition, it has been suggested that the Upper Shikoku Basin / Lower Shikoku Basin facies boundary is at least partially a diagenetic boundary. Opal comprises $<$2 wt% of the sediment at all of the sites. At Site 1173, the Upper Shikoku Basin sediment contains $\sim$1.25 wt% opal. The opal content drops abruptly to $<$0.5 wt% across the Upper Shikoku Basin / Lower Shikoku Basin facies boundary - corresponding with the drop in porosity and the return to normal consolidation behavior in the Lower Shikoku Basin facies. Secondary and back-scattered electron image analysis of sediments at this site reveals a low density, Si-rich phase that is probably opal. This material occurs as a pore-filling cement, coating grains and grain-to-grain contacts. It appears to be more common in the Upper Shikoku Basin facies than in the Lower facies. SEM analyses suggest the presence of authigenic clays in the Lower Shikoku Basin facies. We model sediment accumulation and the thermal evolution of the sediment columns for the sites. Then, we use the thermal history of the sediment and laboratory derived kinetics for the opal-to-quartz diagenetic reaction to model the opal content in the sediment column. Modeled opal content at Site 1173 decreases rapidly below 250 mbsf, where temperatures exceed 50$^\circ$C. The model results indicate that the reaction runs to completion (i.e. the last of the opal is removed) approximately at the Upper Shikoku Basin / Lower Shikoku Basin facies boundary. The model results are in good agreement with the opal measurements and suggest that opal cement may be holding open the pore space in the Upper Shikoku Basin section. It appears that during opal diagenesis the strength of the cement is lost and the pore space collapses. This apparent loss of strength is consistent with previously described loss of shear rigidity reflected in anomalous seismic velocities measured across this zone. Both this loss of strength and high dissolved silica content in sediment pore water in the Upper Shikoku Basin facies may result from the loss of opal without immediate conversion into quartz. A sharp decrease in dissolved silica across the facies bounadry may be related to the uptake of silica into authigenic clays in the Lower Shikoku Basin facies.
T41E-1257 0800h
Transition from rifted continental to oceanic crust at the southeastern Korean margin in the East Sea (Japan Sea)
The southeastern Korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the East Sea (Japan Sea). In this study, we present the transitional structure of the southeastern Korean margin and its formation process from rifted continental to 10 km thick normal oceanic crust. The two-dimensional P velocity model of the southeastern Korean margin was computed from ocean bottom seismometer data by tomographic inversion and distilled by iterative forward modeling. The crustal structure shows the emplacement of high-velocity ($>$7 km/s) lower crust under the continental shelf and slope area associated with a rapid transition from rifted continental to oceanic crust. The high-velocity lower crust is interpreted as magmatic underplating formed by voluminous igneous activity during rifting. Magnetic modeling confirms its primary correlation with a prominent magnetic anomaly along the edge of the southeastern Korean Peninsula that is assumed to represent volcanic extrusives and intrusives. The continental margin featuring a rapid transition from continental to oceanic crust exhibits a remarkable decrease in crustal thickness accompanied by shallowing of the Moho over a distance of about 50 km. It thus appears that the Korean margin experienced intense tectonism comprising crustal deformation and volcanism associated with the opening of the East Sea and consequently registered the early history of continental rifting and subsequent sea floor spreading. We suggest that the rifting and subsequent seafloor spreading at the Korean margin was significantly controlled by the supply of magma in a region of hotter than normal mantle temperature.
T41E-1258 0800h
Joint analysis of seismic, gravity, magnetism and seismological data for passive margin structure imaging
The eastern Gulf of Aden is a key place for investigating seafloor spreading processes and strain localisation, given its thin post-rift sedimentary strata, the good exposure of onshore and nearshore rift structures, the lack of salt deformation structures and its large distance away from the Afar plume. Further, exploratory well data exist for stratigraphic ties, and the two conjugate passive margins can be reconstructed within lateral errors smaller than 10 km. First, the 2000 ENCENS-SHEBA cruise has revealed the structural and geophysical framework using bathymetric swath mapping and underway geophysics (Leroy et al. 2004; d'Acremont et al. submitted). Second, the Dhofar Seismic network in 2004, on the onshore Northern margin (11 BB stations for receiver function and tomography studies) has improved our understanding of the rifting and the oceanic spreading processes in this area, as well as the transitional phase between them. A smaller deformation wavelength prevails on the northern margin, which is also steeper and narrower than the southern one. The southern-rifted domain is about twice as large as the northern one, while the crust is thinner in the northern margin. Besides the influence of rifting obliquity, this asymmetry of the structural pattern could be a consequence of inherited basins and faults associated with the Jurassic rifting episode that affected the southern domain. The transition between the thinned continental crust and the onset of oceanic seafloor spreading is characterized by an ocean-continent transition (OCT). Although its precise nature remains unknown, two possible origins can be proposed with respect to our data; either an exhumed mantle, or an ultra-thinned continental crust intruded by partial melt products from the underlying mantle. Between the Alula-Fartak and Socotra transform faults, the non-volcanic margins and the OCT are segmented by two transfer fault zones trending N027°E. These zones define three N110°E trending segments which evolve with time. The segmentation of the first oceanic spreading centre, which is dated at least 17.6 Ma by the magnetic anomaly (A5d) identification, seems to be directly related to the structural geometry of the margins.
T41E-1259 0800h
DHOFAR Seismic Experiment: First results to understand the breakup processes in a passive margin context
The process of strain localisation preceding the onset of seafloor spreading is still poorly understood, though extensively studied. The reason is the differences in lithospheric properties, proximity to hot spot(s) and melt generation and extraction that lead to a variety of structural styles with major differences. The eastern Gulf of Aden represents a natural laboratory to study passive continental margins for many reasons: post-rift sedimentary strata are relatively thin, both onshore and nearshore structures are well-exposed, and conjugate margins can be precisely reconstructed. A first cruise (ENCENS-SHEBA), in 2000, has established the structural and geophysical framework using bathymetric swath mapping and underway geophysics. Later on, the Dhofar seismic experiment consisted in the deployment of a network of 11 broadband seismic stations from March 2003 to March 2004 on the northern margin, in the Dhofar area, southern Oman. This experiment was dedicated to the detailed study of the crust and upper mantle beneath the northern passive margin. CMG40TD 3-components seismometers from SEIS-UK network were used. We have recorded hundreds of teleseismic events with a good azimuthal coverage. Three main studies are then attempted. First, a teleseismic image of the first 200 km depth will be established using the P- and PKP-phases. This will allow a 3D representation of crustal and upper mantle velocity structures. We present here the preliminary results from the study of the traveltime residuals. The image resolution shall be enhanced by combining gravity data. Second, a receiver function analysis will locally determine the depth of the main interfaces (eg, Moho boundary). Finally, events recorded within the 1000-6000 km distance range will improve the regional S-wave velocity structure in this area and will help to locate the main regional wide structures related to this extended area.
T41E-1260 0800h
Seismological Constraints on the Magmato-tectonic Behavior of the Asal-Ghoubbet Rift (Afar Depression, Republic of Djibouti) Since the Last 1978-Rifting Episode
The Asal-Ghoubbet rift was the locus of a seismic and volcanic crisis in 1978 followed by 8 years of rapid opening (60 mm/yr) before returning to its long-term opening rate of 16 mm/yr. We analyze the space-time evolution of the seismicity that occurred in the rift between 1979 and 2001. The data recorded by the Djibouti Observatory provide only hypocentral locations before 1995 and P and S-wave arrival times since 1996. Additional data acquired during a five months experiment in 2000-2001 allowed us to determine a 3D-velocity model of the rift, used to precisely relocate post 1996 events. The 2545 small-magnitude earthquakes (Md $\leq 3.2$) recorded in the rift since the 1978 crisis provide a negligible contribution to the total extension across the rift, which occurs essentially aseismically. The temporal evolution of the seismicity reveals two distinct phases consistent with those observed in the geodetic data. The post-crisis period (1979-1986) is characterized by large-magnitude earthquakes exclusively located below the northern rift shoulder. These events are associated with the contraction of the side of the rift resulting from the fast opening of the central dyke system. The subsequent period (1987-2001) corresponding to normal opening rate across the rift is characterized by a micro-seismicity essentially located below the major rift caldera (Fieale). Most recorded events during this period concentrate within the rift inner floor at the top of an aseismic, low velocity zone located below the Fiale caldera, which we interpret as hot material above the magma chamber. Outside from post-crisis periods, the seismicity tends to cluster in time in response to stress changes in the brittle layer induced by episodic magmatic movements.
T41E-1261 0800h
Rifting to spreading processes in the eastern Gulf of Aden, the ENCENS project
Existing academic and industry data from the eastern Gulf of Aden away from the Afar flood basalt province show that it is an ideal natural laboratory for seismic studies of passive continental margins because 1) the Gulf of Aden margins are largely free of salt deformation structures, and Oligocene-Recent sedimentary strata are relatively thin (< 4 km; Leroy et al. 2004; d'Acremont et al submitted); 2) the onshore and nearshore rift structures are well exposed, and have been mapped in details (e.g., Watchorn, 1998; d'Acremont et al., submitted); 3) Exploratory well data exist for stratigraphic ties; 4) Our study area is more than 1300 km from the proposed centre of the Late Oligocene Afar plume (e.g., Schilling et al., 1992), and isolated intrasedimentary volcanic occurrences should pose no problem to seismic imaging; 5) The Gulf of Aden is one of the few oceanic basins worldwide where the two conjugate passive margins can be reconstructed within relatively little uncertainty. A cruise, ENCENS II in 2006 (R/V L'Atalante), will be dedicated to multichannel seismic profiling and ocean bottom seismometers (OBS) deployment in order to image the crustal structure of the conjugate margins. To complete our view of this area, a detailed broadband seismic survey along continuations of the crustal-scale OBS seismic refraction/wide-angle reflection profiles in ENCENSII will take place onland in the Dhofar area (northern margin) and in the Socotra island (southern margin). This combination of methods will ensure we define crustal and upper mantle seismic velocity variations. We will then be able to constrain both the geometry of the Moho and that of the lithosphere-asthenosphere boundary, and yielding means to evaluate the current continental rifting and breakup models. We will integrate these unique seismic data with existing ENCENS-SHEBA offshore data and Dhofar Seismic Experiment data, bridging the gap between studies of young margins and extended continental rifts.
http://gdrmarges.lgs.jussieu.fr
T41E-1262 0800h
Crustal Structure From EAGLE Teleseismic and Gravity Studies Across the Northern Main Ethiopian Rift
We present new teleseismic earthquake and gravity data that forms part of the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE). 97 broadband seismometers were deployed along a profile across the northern Main Ethiopian Rift (EAGLE line 1) for two months at a nominal spacing of 5 km and the recorded waveforms are being analyzed for travel-time residuals, shear-wave splitting and crust / upper mantle P-to-S wave conversions using receiver functions. Teleseismic P-wave residuals have been calculated with respect to published Earth seismic velocity models and a consistent regional delay of 5 s is observed. Superimposed upon the regional delay are significant along-line residual variations of 1.0 to 1.5 s indicating a highly heterogeneous crust and upper mantle velocity structure. A smooth transition to relatively early arrivals is observed over 50 km of the southeast rift flank which could be explained by crustal thinning, elevated P-wave velocities or a combination of both. Analysis of SKS and SKKS arrivals indicates split shear wave delays of 1.0 to 2.5 s across the rift with maximum splitting occurring beneath the southeast flank. The orientation of the fast shear-wave is broadly rift-parallel but in detail it exhibits a systematic rotation northwards toward the rift valley from each end of the profile. There is significant variation of both the fast-to-slow shear-wave lag time and the orientation of the fast shear-wave over short length scales, suggesting that at least part of the cause of the variation in shear-wave polarizations is crustal. 72 new gravity stations provided Bouguer anomaly (BA) values with $\sim$5 km station spacing along the same profile. The stations were located using a differential GPS technique and accurately terrain corrected so that the final BA precision is less than 0.5 mgal. The most prominent feature observed is an asymmetric positive anomaly with maximum amplitude of 70 mgal near the rift axis. The latest EAGLE cross-rift seismic refraction model is used as a starting model for 2.5D gravity anomaly modelling that provides important constrains on the number and location of upper crustal mafic intrusive bodies, the requirement for magmatic underplating and changes in upper mantle density beneath the rift.
T41E-1263 0800h
Gravity Field of the Central Portion of the Main Ethiopian Rift
Recently, we have acquired new gravity data along the axis of the central portion of the Main Ethiopian rift. The survey was designed to have close station spacing, and high precision GPS measurements where made to insure positioning was precise. We combined our new data with previous data available for the region to produce an improved map of the gravity anomalies in the region. Our analysis shows that this portion of the Main Ethiopian rift is characterized by a broad gravity high that follows the physiographic riftt valley with localized short wavelength gravity highs in several places within the rift valley. These anomalies correlate with known volcanic centers such as the Aluto volcano (Geothermal field), the Gadamota caldera, Bora volcano, Corbetti caldera and Lake Shalla areas. Further to the northeast, the Gedemsa caldera and the spreading center known as the Wonji fault belt (WFB) are depicted as a gravity high trending northeast towards Afar. We have derived an axial gravity model that is constrained with seismic data from the EAGLE project and models for NW - SE profiles that cross the rift perpendicularly. Some filtering techniques have been applied to provide gravity anomalies of local and deep crustal origins. The eastern and western plateaus are characterized by relative gravity minima. However we have found that the eastern plateau has a more negative anomaly than the western plateau, and the difference is about 20 mGal. This may indicate that the eastern plateau has a thicker crust than the western plateau.
T41E-1264 0800h
Rift Geometry and Evolution Associated with the Break-up of Pangea
Diabase dikes related to the rifting of Pangea have been used to elucidate mechanisms by which the super-continent broke-up since the early 1970s. Subsequently, these dikes were related to each other and the massive sills and lava flows that make up the Central Atlantic Magmatic Province (CAMP). Our research focuses on the dikes and mechanisms of continental break-up in the southeastern United States. While, past studies indicate that the majority of Mesozoic diabase dikes within the Carolinas dominantly trend to the northwest, recent work in west-central South Carolina and the Piedmont of North Carolina has revealed numerous previously unmapped dikes with N and NE trends. These dikes have been related to the N and NE trending dikes located in the Northeastern United States via geochemical analyses. Cross-cutting relationships between dikes of different orientation has revealed an apparent pattern whereby the dikes were emplaced in the following order NW, N, and then NE-trending. Recent Ar39/Ar 40 dates suggest that these dikes were all intruded within a 2 million year window, indicating that the least compressive stress field (which would be perpendicular to dike orientation) rotated from NE-SW through E-W to NW-SE within this time period. Based on these observations, which are contrary to previous studies that attribute the northwest trending dikes in the Carolinas to a deep mantle plume, we constructed several finite element models to determine the source of the stress field change. These models were constructed to test the influence of geometry, rifting sequences, and location of force application on the orientation and evolution of stress fields in the Carolinas. Model results suggest that the direction from which rifting progressed has a strong effect on the stress field within the Carolinas and that it is unlikely that rifting began at the Blake Plateau as proposed by the plume hypothesis. Further, it is apparent that the geometry of the rifts themselves has a strong effect on the stress field within the continent, which suggests the need for further mapping of the final rifting events along the southeastern margin of the United States.
T41E-1265 0800h
Rift Basin Architecture Near the Collision Zone in the Northern end of Back-arc Rift of the Izu-Bonin Arc, Based on Airgun Profiles
Two concatenated basins, Hachijo and Mikura basins in northward order, are distributed in the northern part of the Izu-Bonin Arc, in collision with the NE Japan Arc. The Izu-Bonin Arc has long but punctuated narrow depression in back-arc area; the `back-arc rift'. The Hachijo basin is accepted as `Hachijo Rift'(e.g. Tamaki et al., 1981), whereas the Mikura basin remains uncertain, because the Hachijo basin is similar to the Sumisu Rift, well-investigated typical rift in the Izu-Bonin Arc, while the Mikura basin shows unclear boundary fault. The Hachijo basin is accordant to bathymetric trends in the Izu-Bonin Arc (Murakami, 1996): Rift basins tend to shallow northward. However, the Mikura basin is deeper (1600m) than the Hachijo basin (1100m). The Hachijo basin is 40$\times$60 km$^2$ NS-elongated basin, bounded the eastern and the northern sides by cliff to 500m deep bank and cliff to the Mikura basin respectively. The Mikura basin is 50km wide square basin, bounded the northern and the eastern sides by steep slope to 300m deep bank and slope to the 500m deep Kitakurose Bank respectively. Profiles show the Hachijo basin is thickly-deposited($\geq$1.2sec in two-way travel time) half-graben, bounded the eastern cliff by NS-strike normal fault. The Mikura basin is also filled with thick deposits($\geq$1.5sec) but bounded the northern slope by EW-strike normal faults partially covered with slump deposits. The reflectors in eastern slope of the basin continue to east of the basin, Kitakurose Bank. In this bank, the NS-strike W-dip normal fault is developed with downthrow offset more than 1.2sec. The deposits on the hanging wall are estimated at Pliocene (Yuasa, 1984). The Mikura basin might have been originally NS-elongated (half-)graben, bounded by this fault, and changed its spreading axis. Concludingly the Mikura basin shows tectonic margin, influenced with the collision to the NE Japan Arc as well as repeated rifting, whereas the Hachijo basin represents as the northern end of typical back-arc rift. The northern slope and EW normal fault of Mikura basin is situated at the boundary between northern shallow area rich in volcanogenic islands, and southern deep area rich in rifts, and might indicate the structural boundary.
T41E-1266 0800h
Cenozoic Tectonism of the Southeastern United States Continental Margin from Deep Seismic Reflection Data
A deep seismic reflection profile, SEISDATA 6 (SD 6), was collected in the state of Georgia (United States), in 1981, with the main purpose to study the crustal structure and tectonic evolution of the southeastern North American continental margin. The SD 6 profile, with a total length of approximately 430 km, was collected by Seisdata Services Inc. of Houston, Texas in conjunction with the USGS in two separate transects, the Blue Ridge and Piedmont Provinces (8 s) and the Atlantic Coastal Plain Province (6 s). The Piedmont Province consists of metamorphic and igneous rocks that record tectonic regional metamorphism with the intrusion of igneous plutons. The Piedmont is bounded by the Brevard fault zone to the NW and the Fall line to the SE. Exposed faults and Mesozoic rift basins and complex folding are present in the Piedmont. The Atlantic Coastal Plain (ACP) Province is bounded by the Fall line to the NW and the Atlantic Ocean to the SE. Marine and fluvial/deltaic sediments make up the strata and irregularly cover the Piedmont Province. While only limited parts of the SD 6 seismic profile had been processed in the past, the main target of this study is to reprocess the entire profile using more modern techniques in order to obtain a crustal-scale image of the southeastern U. S. continental margin, and evaluate evidence for, still disputable, (1) reactivation and sense of movement of preexisting basement faults in the Piedmont region, (2) recognition of potentially multiple stages of deformation on these faults in the Paleozoic, Mesozoic, and Cenozoic times, and (3) identification of the differences in rheology of the crystalline basement from west to east. Reprocessing of the SD 6 line has focused on reflection enhancement at large depths, static corrections from first break picking, velocity analysis, wavelet deconvolution, time-variant amplitude gain, and post-stacking migration. Preliminary results of the reprocessing show evidence for large offsets in the crystalline basement in the Piedmont, with reverse sense of movement, indicating basement faults that trend predominately toward the northeast. High amplitude crustal reflectors that slightly dip toward the SE are present in the Piedmont and in the ACP below the marine and fluvial/deltaic sediments. Coastal marine sediments reflectors are parallel to sub-parallel with a sharp high amplitude reflector between the coastal plain sediments and the basement. Reprocessing of the data has shown further evidence of Cenozoic movement on the preexisting basement faults. Although it is widely accepted that the Southeastern margin of the U.S. has been a passive boundary since the Cretaceous time, however, there is evidence of Cenozoic tectonism including regional scale uplift from these seismic data.
T41E-1267 0800h
Focused Deformation in the Lower Crust During Continental Extension: Fiordland, New Zealand
Lower crustal flow plays an important role in the large-scale rifting of continents, however the lack of preserved structures associated with lower crustal flow have made field studies difficult. Exposures located in Fiordland, New Zealand provide a unique opportunity to observe a portion of the lower crust that has been brought from depths of more than 40 km. These exposures contain extensional structures which formed 108 Ma during the initiation of continental rifting that led to the opening of the Tasman Sea (84 Ma). Field observations show that the upper-amphibolite facies Doubtful Sound Shear Zone (DSSZ) is a lower crustal extensional detachment that separates garnet granulites deformed at 14 kbar from calc-silicate and pelitic schists deformed at 7-9 kbar in a zone of deformation <1 km wide. Within the DSSZ strain was concentrated along an anastomosing network of shear bands. The DSSZ also appears to be part of a similar larger-scale network of shear zones which continue to higher structural levels. The DSSZ formed along the margin of a dioritic batholith which was emplaced 119-126 Ma during convergence and uplift that preceded the initiation of continental rifting. Deformation in the DSSZ is solid-state and appears to have occurred in the absence of large amounts of melt. There is no evidence for major magmatism in the region during extension. The network of shear zones is a possible mechanism for the transfer of strain to higher structural levels where core-complex-style extension was occurring simultaneously to movement on the DSSZ. These results provide information useful in creating a geometric model for behavior of the lower crust below active core complexes and in reconstructing a sequence of events that took place in the early stages of non-volcanic continental rifting.
T41E-1268 0800h
Deformation Kinematics of Rifted Continental Margin Lithosphere From Measured Bathymetry, Gravity and Upper Crustal Extension Using a New Model of Sea Floor Spreading Initiation
We apply inverse methods to a two-dimensional coupled fluid-flow thermal model of sea floor spreading initiation and rifted continental margin formation in order to determine the rifted margin lithosphere deformation history and structure. The model assumes that stretching of continental lithosphere leading to breakup and sea floor spreading initiation is generated by an upwelling divergent flow field within continental lithosphere and asthenosphere. This flow field is defined by the horizontal divergence velocity Vx and the vertical upwelling velocity Vz. The model outputs are crustal thickness, lithosphere temperature structure, bathymetry, basement subsidence history, crustal and lithospheric stretching factors, free-air gravity and top basement heat-flow. The new rifted model successfully predicts lithosphere depth-dependent stretching at both volcanic and non-volcanic margins and mantle exhumation at non-volcanic rifted margins. The forward problem is characterised by a non-linear relationship between parameters and data and a significant computational burden. We employ a multi-dimensional grid search method to systematically explore parameter space. We minimise the least squares misfit between predicted and observed bathymetry, free-air gravity and upper crustal thinning factors to recover the horizontal (Vx) and vertical (Vz) velocities of the upwelling divergent flow field and the initial pre-breakup lithospheric stretching factor ($\beta$). The method has been successfully tested using synthetic data generated by previous runs of the forward model. We present preliminary results from inversions of geophysical data measured on profiles across Atlantic continental margins including the Goban Spur, Iberian and Grand Banks margins. Using a combination of bathymetry, satellite-derived free-air gravity anomalies and upper crustal stretching factors, we extract parameter values which describe the deformation kinematics responsible for the formation of these margins and the resulting crustal thinning, lithosphere temperature structure and bathymetry. Our initial results using observed data from Atlantic margins are consistent with their inferred volcanic or non-volcanic histories.
T41E-1269 0800h
Seismic Structure of the Continental Margin Offshore French Guiana and North-Eastern Brazil
During November/December 2003 we carried out a marine geophysical survey of the continental margin offshore French Guiana and north-eastern Brazil onboard RRS Discovery. The survey comprised 1800 line km of coincident multi-channel seismic (MCS) reflection and wide-angle seismic refraction data which were acquired along 3 'transects' of the margin. In addition, more than 3000 line km of underway gravity and magnetic anomaly data were acquired. Preliminary processing of the MCS data has revealed the main stratigraphic units and the nature of the underlying basement at the margin. Up to 4 stratigraphic units have been identified, the uppermost of which is separated from lower units by an angular unconformity. We interpret the unconformity as of mid-Miocene age that corresponds to a major influx of sediment to the margin associated with uplift in the Bolivian Andes and the development of the Amazon deep-sea fan. The lower units beneath the fan offshore north-eastern Brazil comprise of gently landward-dipping reflectors that we interpret as post-rift sediments. Beneath the Demerara plateau offshore French Guinea, however, there is evidence of more steeply dipping reflectors that we attribute to Early Cretaceous and older syn-rift sediments. Basement has been imaged along each transect, most clearly beneath the middle and lower Amazon fan and the deep-water to the north of the Demerara plateau where we interpret it to be of oceanic-type. Preliminary processing of the wide-angle data recorded by ocean bottom instruments deployed at 10 km interval along each transect reveal high velocity arrivals at source-receiver offsets greater that 200 km. Ray-modelling indicate that primary intra-crustal first arrivals originate at major discontinuities within the stratigraphic column. Additional wide-angle land recording data contain arrivals at offsets greater than 400 km. We discuss here the results of the new seismic data, especially as they relate to the early opening history the Equatorial Atlantic and the structure and evolution of both its rift- and transform-type margins.
T41E-1270 0800h
Evidence for Early Crustal Thickening, Poly-phase Oligo-Miocene Extension, and Footwall Rotation at the Sierra Mazatan Metamorphic Core Complex, Sonora, Mexico
The Sierra Mazatan metamorphic core complex records large-magnitude extension in the Mexican portion of the southern Basin and Range. We present new geologic and thermochronologic evidence that documents the pre-extensional history and the tectonic exhumation of this core complex. The geological evolution of Sierra Mazatan provides important insights into the extensional history of northwestern Mexico as well as models of core complex formation. The shallowly (10-$15\deg$) west dipping detachment fault that unroofed the core complex extends $>$50 km north from Sierra Mazatan and exhumed the southern portion of the Aconchi batholith. The fault likely extends both further north and south, but its trace is covered and its full length is unknown. Rare schistose intervals in the eastern footwall contain kyanite overprinted by sillimanite, indicating that at least local crustal thickening, likely of Sevier age, occurred in Sonora and may have played an important role in localizing core complex-style extension. New $^{40}$Ar/$^{39}$Ar thermochronologic data reveal two distinct pulses of rapid footwall cooling that support a poly-phase unroofing history. Based on these data, an early pulse of slip on the detachment fault occurred from 25-23 Ma followed by later slip event from 21-15 Ma. $^{40}$Ar/$^{39}$Ar geochronology of moderately NE dipping volcanic flows interbedded with hanging wall sediments yield ages ranging from 17.7 to 15.3 Ma, supporting the timing of the last slip event and demonstrating syn-extensional basin development during that time. The unconformable deposition of a 12.4 Ma ignimbrite on the footwall brackets the end of major slip. Total slip on the detachment fault was likely 15-20 km based on the amount of rapid footwall cooling (likely 200-$300\deg$C) and the apparent offset of correlative hanging wall and footwall sequences. Multiple lines of evidence support a steep initial dip for the presently low-angle detachment fault. Thermochronologic data indicate a $>$$300\deg$C temperature difference across 15.5 km of the footwall in the slip direction at 21 Ma, implying 20-$45\deg$ of eastward footwall tilt depending on the assumed paleo-geothermal gradient (15-$30\deg$C/km). Geologic data also strongly support significant footwall tilt. Sedimentary sequences east of the granitic footwall consistently dip 40-$55\deg$ NE and footwall dikes generally dip 45-$75\deg$ WSW, suggesting $\sim$$45\deg$ of eastward footwall tilting. Accounting for this footwall tilt would restore the detachment fault to a steeper initial dip of 40-$60\deg$. This study demonstrates that at least some detachment faults related to core complexes formed at steep initial dips. Early crustal thickening, poly-phase extension, and steep initial dips for detachment faults may be common in the evolution of many Cordilleran metamorphic core complexes.
T41E-1271 0800h
The Effect of Temperature Dependent Rheology on a Kinematic Model of Continental Breakup and Rifted Continental Margin Formation
The effect of temperature dependent rheology has been examined for a model of continental lithosphere thinning by an upwelling divergent flow field within continental lithosphere and asthenosphere leading to continental breakup and rifted continental margin formation. The model uses a coupled FE fluid flow and thermal solution and is kinematically driven using a half divergence rate Vx and upwelling velocity Vz. Viscosity structure is modified by the evolving temperature field of the model through the temperature dependent Newtonian rheology. Continental lithosphere and asthenosphere material are advected by the fluid-flow field in order to predict crustal and mantle lithosphere thinning leading to rifted continental margin formation. The results of the temperature dependent rheology model are compared with those of a simple isoviscous model. The temperature dependent rheology model predicts continental lithosphere thinning and depth dependent stretching, similar to that predicted by the uniform viscosity model. However compared with the uniform viscosity model the temperature dependent rheology predicts greater amounts of thinning of the continental crust and lithospheric mantle than the isoviscous solutions. An important parameter within the kinematic model of continental lithosphere breakup and rifted continental margin development is the velocity ratio Vz/Vx. For non-volcanic margins, Vz/Vx is thought to be around unity. Applying a velocity ratio Vz/Vx of unity gives a diffuse ocean-continent transition and exhumation of continental lithospheric mantle. For volcanic margins, Vz/Vx is of order 10, falling to unity with a half-life of order 10 Ma, leading to a more sharply defined ocean-continent transition. While Vx during continental breakup may be estimated, Vz can only be inferred. FE fluid flow solutions, in which Vz is not imposed and without an initial buoyancy driven flow component, predict a velocity ratio Vz/Vx of around unity for both temperature dependent rheology and isovisous fluid-flow solutions. The effect of incorporating a lithology dependent continental lithosphere rheology (quartz-feldspar crust, olivine mantle) with temperature dependence is also being investigated. The work forms part of the Integrated Seismic Imaging and Modelling of Margins (iSIMM*) project. This work forms part of the NERC Margins iSIMM project. iSIMM investigators are from Liverpool and Cambridge Universities, Schlumberger Cambridge Research & Badley Geoscience, supported by the NERC, the DTI, Agip UK, BP, Amerada Hess Ltd, Anadarko, Conoco-Phillips, Shell, Statoil and WesternGeco. The iSIMM team comprises NJ Kusznir, RS White, AM Roberts, PAF Christie, R Spitzer, N Hurst, ZC Lunnon, CJ Parkin, AW Roberts, LK Smith, V Tymms & D. Healy.
T41E-1272 0800h
A Kinematic Fluid-flow Model of Continental Lithosphere Deformation Leading to Continental Lithosphere Breakup and Rifted Continental Margin Formation
The observations of depth dependent lithosphere stretching at rifted continental margins and the exhumation of continental lithosphere mantle at non-volcanic margins are not explained by depth-uniform lithosphere stretching. Depth dependent lithosphere stretching, in which stretching of the continental lower crust and lithospheric mantle greatly exceeds that of the upper crust or upper crustal extension is entirely absent, is observed at both volcanic and non-volcanic margins. The dominant process for thinning rifted continental margin lithosphere is not depth-uniform intra-continental extension. A kinematic fluid-flow model of continental lithosphere deformation leading to continental lithosphere breakup and sea-floor spreading initiation has been developed in which a divergent upwelling flow field is applied to continental lithosphere and asthenosphere. The model uses a iso-viscous stream-function corner-flow to predict the flow field which is used to advect continental lithosphere and asthenosphere material and their temperature fields. Flow is defined by Vx, the divergence half-velocity, and Vz, the vertical upwelling velocity. The distribution of thinning of the continental margin crust and lithospheric mantle and the evolution of the lithosphere and asthenosphere temperature field are particularly sensitive to the velocity ratio Vz/Vx. The model predicts depth dependent stretching of rifted continental margin lithosphere for both volcanic and non-volcanic margins, as observed. For non-volcanic margins (where Vz/Vx ~1) the model predicts exhumation of continental lithospheric mantle and a diffuse ocean-continent boundary with lateral dimension ~ 100 km. For volcanic margins (where initially Vz/Vx > 5 due to buoyancy assisted flow) the predicted ocean-continent transition is sharper and little or no exhumation of continental lithospheric mantle is predicted. The model predicts that most depth dependent stretching of continental margin lithosphere occurs before continental lithosphere rupture and the onset of sea-floor spreading. Predicted crustal thinning and lithosphere temperature may be used to predict the development of margin bathymetry, subsidence history, top basement heat-flow and gravity anomaly. The model has been successfully applied to volcanic and non-volcanic margin examples using observed bathymetry, gravity and sediment thickness data to invert for the kinematic parameters describing lithosphere breakup deformation. During the initiation of the divergent upwelling flow field within continental lithosphere and asthenosphere, the model predicts subsidence of the lithosphere surface above the developing flow-field with amplitude ~ 2km and width ~ 100-200 km, but with no associated stretching of the upper crust. Such behaviour is consistent with the observation of regional subsidence in the absence of upper crustal stretching as observed at the propagating tip of sea-floor spreading in the Woodlark and South China Sea Basins.
T41E-1273 0800h
Distribution of Post-Rift Sills on the Newfoundland Nonvolcanic Margin Around the ODP Leg 210 Transect From Waveform Inversions and Synthetic Seismograms
Nonvolcanic rifted margins commonly include a section of crust of uncertain affinity between normal oceanic crust and extended continental crust. The origin of this portion of the margin, often called the transition zone, is essential to establishing the symmetry of rifted margins and developing models for margin evolution. The Newfoundland-Iberia rifted margin pair is among the best-studied nonvolcanic conjugate margin pairs in the world. Drilling on the Newfoundland nonvolcanic margin during ODP Leg 210 at Site 1276 did not reach enigmatic transitional basement, but did reveal the presence of interlayered sills and sediments immediately above the inferred top of transitional crust. The presence of sills has implications for the appearance of underlying transitional basement in seismic reflection sections, the signature of this section of the margin in shiptrack magnetic data and late-stage margin evolution. Sills are estimated to be approximately 100 m.y., while rifting likely occurred ~125 Ma. Shipboard work suggests that sills encountered at Site 1276 correspond to bright reflections in the lowermost sedimentary section overlying transitional crust that are observed in coincident seismic reflection profiles collected during the SCREECH (Studies of Continental Rifting and Extension on the Eastern Canadian SHelf) experiment during 2000. These profiles also show that basement in the transition zone is essentially featureless in seismic reflection data except where it rises above these bright reflections, implying either that impedance contrasts in the deep section prevent signal transmission or that there is little contrast between interlayered sills and sediments and the underlying basement. Here, we strengthen the link between seismic reflection data and drilling data by creating synthetic seismograms from physical properties measurements made at sea aboard ODP Leg 210. Physical properties data were used in lieu of logging data, which could not be collected due to poor hole conditions. To extend the results from Site 1276 to rest of the grid of seismic lines in this region, waveform inversions were also carried out on select supergathers around the SCREECH Line 2 survey and compared with waveform inversions of data near Site 1276. This work reveals significant variability in the velocity contrasts required by deep reflections thought to be associated with sills in the transition zone, implying similar variability in the distribution of sills. Furthermore, a distinct difference is observed between bright reflections overlying transitional crust and reflections at similar depths further seaward over unambiguous oceanic crust.
T41E-1274 0800h
The 2003 Potrillo Volcanic Field Seismic Experiment, Southern Rio Grande Rift: New Refraction Results From a Classic Continental Rift
The Rio Grande Rift is a major Tertiary tectonic feature that profoundly modifies the lithospheric structure of the southern Rocky Mountains. Patterns of magmatism and extensional structures of the rift both crosscut and reoccupy older structures including those associated with Precambrian continental assembly, late Paleozoic Ancestral Rockies and late Mesozoic to Early Tertiary Laramide tectonism. Modern active source experiments allow us to illuminate the connection between mantle, crustal, and surfaces processes. In May of 2003, the Potrillo Volcanic Field (PVF) experiment was carried out to investigate the crustal structure of the southern Rio Grande Rift and relate it to xenoliths from within PVF. This experiment was comprised of 8 shots of 1000-2000 lbs., 793 seismic recorders (TEXANS) deployed at variable spacing of 100 m, 200m and 600m over 205 km. Near vertical reflection shot data image numerous intracrustal reflections, complex reflectivity at Moho near 11 s, particularly beneath the PVF, and reflected energy due to the conversion of P to S at the Moho. A tomographic velocity model shows a change in mid-crustal velocities from 5.5 - 6 km/s west of the PVF to 6 - 6.5 km/s to its east. The Moho may be as deep as 35 km and likely dips to the east. Here we present a new velocity model from ray-based modeling of secondary arrivals from within the crust and from the crust-mantle boundary, and interpret the model in light of xenolith data.