T31B-0463
Evidence for Vertical Coherent Deformation in Eastern Tibet from Splitting of Crustal S Phases
Regional shear wave phases recorded by stations in Yunnan Province and Eastern Tibet surrounding the Eastern Himalayan syntaxis are used to constrain seismic anisotropy at crustal depths which range from 45 km to 65 km, respectively. Shear wave splitting measurements were obtained using regional direct S phases recorded at stations operated by Carnegie Institution of Washington, Lehigh University, and MIT, with source depths that originate from 20 km to 55 km. S to P converted energy is avoided considering events with free surface incidence angles less than 37{°} and using records with low P energy. We apply a low pass filter with a corner frequency at 2.5 Hz to all records and correct for surface reflections using a free surface transform from previous methods. Anisotropy is small but well resolved for 14 records at 8 stations and indicate that delay times obtained for all stations are less than 0.4 {±} 0.05s. In Yunnan province the average delay time is 0.16 s but is twice this value in eastern Tibet at 0.31 s. The fast directions for all stations in Yunnan Province including KMI and CHTO (GSN) are consistent with fast directions for previous SKS studies, but with smaller delay times of 0.4s or less (where SKS are 1.0 – 1.5s). Stations in eastern Tibet display a {~}N-S azimuth roughly orthogonal to SKS studies in this area and may indicate sensitivity to cracks which form roughly parallel to the compressive stress direction. A N-S azimuth of anisotropy is also consistent with surface waves results for periods below 40s. By comparison with SKS splitting studies, we find that anisotropy in the crust is very small and contributes only about 15% to the total SKS splitting measurements, requiring about 1.0 – 1.2s of splitting to come from subcrustal depths. Such strong anisotropic fabric possibly present in the lithospheric mantle is inconsistent with a lower crustal flow model which predicts decoupling above the Moho. Consistently low splitting times for events with crustal depths which vary from 20 to 55 km suggest a shallow source possibly from cracks or other forms of SPO in the upper 20 km. The consistency between surface deformation studies of topography, structural geology, GPS observation and SKS fast direction suggests that crust and mantle flow around the Himalayan syntaxis is vertically coherent with clear evidence for lithospheric or sublithospheric LPO but indicating deformation of crustal material may not align in a preferred orientation as suggested in previous laboratory and seismic studies.
T31B-0464
Testing the Presence of Fluids/Crustal Melts in the India-Asia Collision Zone Using Rayleigh Wave Dispersion Analysis
A network of 15 broadband seismographs in an approximately 500 km long, N-S array recorded 12 months of data in 2002-2003 (Rai, S S, et al., Configuration of the Indian Moho beneath the NW Himalaya and Ladakh, GRL 33). The array traverses the NW Himalaya, from the Indian plain in the south, across the Indus-Tsangpo suture and the Tso Morari Dome, to the southern flank of the Karakoram in the north. Magnetotelluric (MT) studies in this region reveal low-resistivity zones which may be indicative of fluids, graphite, or partial melts in the mid-crust. We test this hypothesis by creating 1-D models of crustal velocity structure, which should contain low-velocity zones if partial melts or fluids are present. Our models are obtained by inverting group and phase velocity dispersion curves of fundamental mode Rayleigh waves in the period range of roughly 8 to 30 s. Numerous magnitude 4 events, several magnitude 5 events, and one magnitude 6 event occurred 900 km or less from the array. Current results reveal, as expected, demonstrably different crustal structure in the Indian shield and collisional zone, and preliminary results suggest that a low-velocity zone is present in the collisional zone.
T31B-0465
Vertical axis rotations and gravity-driven flow in southeastern Tibet
On the southeastern margin of the Tibetan Plateau lies a large region which seismicity and GPS data shows to be actively deforming. We describe the active faulting in the region, and suggest how this faulting accommodates the velocity field observed with GPS. We find that in places the velocity field is accommodated by rotations about vertical axes. We also compare the GPS velocities to velocities calculated using a model for thermomechanically coupled gravity-driven flow. For likely rheologies (estimated from experimentally derived mineral flow-laws) there is found to be good agreement between the model and GPS velocities. Two populations of normal faulting earthquakes are present in this area, and have mechanisms which are easily explained by gravity-driven deformation.
T31B-0466
Changes in the kinematics of deformation and lithospheric structure revealed from seismic anisotropy in southeastern Tibetan plateau
We combine all the available SKS splitting observations in southeastern Tibet to provide a more detailed analysis of the anisotropic seismic structure of the lithosphere as well as a better understanding of the complex regional deformation of the mantle beneath the plateau. The data display evidence for seismic anisotropy across the study region. Fresnel zone analysis suggests that the main source of anisotropy resides in the mantle lithosphere. The presence of small-scale and regional lateral variations in seismic anisotropy is observed within individual tectonic domains such as the Lhasa terrane and across major sutures and tectonic domains. In the Lhasa terrane, we observed a sharp change in fast direction resulting from a change in mode of deformation from orogen perpendicular extension in central Tibet to orogen parallel strike-slip in the eastern syntaxis. A recent tomographic study suggested that such a change may correlate with the disappearance of the underthrusting Indian lithosphere beneath the Eastern syntaxis. In this eastern portion of the Lhasa terrane, a few stations display a lack of anisotropy over a range of backazimuths indicating either that the medium is isotropic (or weakly anisotropic) or transversely isotropic with a vertical symmetry axis. Shear-wave splitting measurements also show lateral variations in seismic anisotropy across the Tsangpo and the Bangong sutures indicating that these sutures separate major lithospheric domains. Anisotropic changes across the Tsangpo suture may characterize a change from a more rigid Indian mantle to a more ductile Eurasian mantle while across the Bangong suture the anisotropic variations may reveal a change in ductile/viscous behavior as revealed by previous geophysical data. Such a behavior can explain the observed increase in delay times beneath the Qiangtang province. The reassessment of previously published splitting data in southeastern Tibet highlights the importance of obtaining well constrained splitting measurements with good azimuthal coverage to observe possible complex anisotropic structure.
T31B-0467
Present-day lithosphere structure underneath the Tibetan Plateau inferred from potential fields: Influence of mantle dynamics on the topographic evolution
The Tibetan Plateau is the product of crustal thickening caused by collision between India and Asia. Plate tectonic reconstructions suggest continuous northward movement of the Indian plate relative to stable Eurasia at nearly 50 mm/yr for the last 50 My. The plateau is now at ~5 km elevation with steep topographic gradients across the southern and northern margins. These gradients are also associated with large lateral variations in geoid and gravity anomalies. Uplift late in the tectonic evolution of the plateau, the widespread extension, and the associated magmatism have been attributed to removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere. Here we present a two-dimensional lithospheric thermal and density model of the present day structure and numerical modeling of the evolution of the Tibetan Plateau. The two-dimensional lithospheric model is along a transect from the Indian plate to Asia, crossing the Himalaya front and the Tibetan Plateau. The model is based on the assumption of local isostatic equilibrium, and is constrained by the topography, gravity and geoid anomalies and by thermal data within the crust. Our results suggest that the height of the Tibetan Plateau is compensated by thick crust in the south and by hot upper mantle to the north. The Tibetan Plateau as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the northern Plateau is required to explain the high topography, gravity, geoid and crustal temperatures. We also investigate numerically the long-term (50 My) evolution of crustal and lithospheric thickness, thermal structure, topography, and strain-rate of the Tibetan plateau through time, using a planform viscous approach. This suggests that lithospheric mantle must have been removed from beneath Tibet, and that the crust must have been warmed and weakened by an increase of radiogenic heat production at depth due to crustal thickening.
T31B-0468
The Petrogenesis of Cretaceous A-type Granites, SE Tibet: Geochemical and Nd Isotopic Constraints
Northward subduction of the Neo-Tethyan oceanic lithosphere beneath South Asia gave rise to an Andean-type convergent margin marked with arc magmatism starting from the early Jurassic (≥190Ma) and lasting until the Eocene (~45Ma). The Cretaceous to Paleocene volcano-plutonic belt thus formed in Bomi to Chayu areas, SE Tibet, consists of three major batholiths, namely, Achakung, Temula, and Chayu, respectively. While all granitoids and volcanics from this region are characterized by depletions in HFSE, enrichment in LILE and LREE, thus showing incompatible trace element patterns similar to those of typical arc magmas from the active continental margins, in the Temula batholith A-type granites with diagnostic features including high alkalis, Fe2O3T/MgO, Ga/Al, Rb, and HREE, large Eu negative anomalies, and low Ba and Sr contents were identified. The A-type granites, emplaced at 122±2Ma, exhibit crustal type Nd isotopic signatures [ε Nd(T)~-12]. To account for the petrogenesis of the Cretaceous A-type granites and associated arc rocks in this part of the Neo-Tethyan subduction zone, we propose a two-stage magma differentiation model involving (1) deep differentiation and crustal contamination of mantle-derived mafic liquids that intrude/underplate in the lower part of the continental crust around the Moho and (2) additional differentiation and upper crustal contamination of such differentiated liquids as they rise and install in shallow-level magma chambers. The shallow intrusion, moreover, may have triggered low-pressure (P≤4kbar) partial melting of calc-alkaline basement rocks, e.g., those atop the magma chambers, and hence produced the Temula A-type granites.
T31B-0469
Zircon U-Pb geochronology of the eastern Transhimalayan batholiths, SE Tibet
Before the India-Asia collision, northward subduction of the Neo-Tethyan oceanic lithosphere beneath South Asia produced an Andean-type convergent margin marked with arc magmatism starting from the early Jurassic (≥190 Ma) and lasting until the Eocene (~45 Ma). The magmatic arc is now exposed as widespread intrusive bodies or the so-called Transhimalayan batholiths in the Lhasa terrane of southern Tibet that have been divided into two suites, i.e., the northern plutonic belt and the southern Gangdese belt. However, the temporal distribution of the batholiths, in particular their eastern/southeastern extension, remains poorly constrained. We therefore conducted a field excursion from Chayu to Basu areas (~95.5-97.5°E and ~28.5- 30°N), SE Tibet, from which intrusive rocks were collected for a combined geochronological and geochemical analysis. Our preliminary results of zircon U-Pb dating show that these granotoids were emplaced between the Cretaceous (~133-110 Ma) and Paleocene (~60 Ma). They all are characterized by depletions in the high field strength elements (HFSE), enrichments in the large ion lithophile elements (LILE) and light rare earth elements (LREE), and thus show spidergram patterns similar to those of arc magmas from the subduction zones. These granitoids have heterogeneous Sr and Nd isotope ratios, with eNd(T) values varying from -1.0 to -13.0, which suggest the continental crust to have played a significant role in the petrogenesis. Furthermore analyses on the age, geochemistry and Sr-Nd isotopes of the samples will be performed and combined with the above data set to comprehend our understanding of the temporal and spatial distribution of the Transhimalayan batholiths and their tectonic significance.
T31B-0470
Quaternary volcanic rocks from Central Burma: Geochemical characteristics and petrogenesis
Burma is located in the eastern margin of the India-Asia collision zone. The most significant geologic feature in the region is arguably the Sagaing Fault representing a dextral strike-slip fault system that links the eastern Himalayan Syntaxis in the north and the Andaman Sea in the south. This region is situated in a unique or ¡§transitional¡¨ position between contractional (Himalayan) and extensional (Andaman Sea) tectonic settings, and furthermore characterized by the eruption of a series of Middle Miocene to Quaternary volcanoes along the Sagaing Fault. Here we report geochemical and Sr-Nd isotopic analyses of Quaternary basalts and basaltic andesites recovered from Mt. Popa and Monywa areas, Central Burma. Major element data of the Burmese rocks show a potash-rich nature, with most of the Mt. Popa samples (8 out of 12 analyses) plotting in the high-K calc-alkaline suite and the Monywa samples (5 out of 7 analyses) in the shoshonitic suite. The latter may be further specified as absarokite based on their mineral constituents and trace element characteristics. All these rocks display significant depletions in the high field strength elements (HFSE; e.g., Nb, Ta and Ti), enrichments in the large ion lithophile elements (LILE; e.g., Cs, Rb, Ba, Th, U) and light rare earth elements. Thus, the overall incompatible trace element distribution patterns are similar to those of arc magmas formed in the subduction zone. Moreover, the Burmese rocks show high Nd and low Sr isotopic ratios, with εNd = +1 to +4 and 87Sr/86Sr~ 0.7045. Consequently, we propose that the magma source of the high-K calc-alkaline rocks from Mt. Popa is a juvenile mantle wedge in the region despite the Indian Ocean slab beneath this part of the Asian continent has already ceased its subduction. Under this framework, the Monywa absarokites represent small-degree melting products of a phlogopite-bearing peridotite source in the mantle wedge.
T31B-0471
Three-component Seismic Observations Within the INDEPTH IV Transect: NE Tibetan Plateau to Qaidam Basin
As part of the INDEPTH IV controlled-source experiment in June 2007, 20 broadband and 29 short-period three- component seismographs at 5-6 km station spacing recorded 5 large shots (1000-2000 kg) and 100 small shots (80 kg) along a 270 km long profile across the Kunlun mountains in northeast Tibet. Following the controlled-source experiment, 50 broadband seismographs (35 from GIPP, Germany and 15 from SEIS-UK) were deployed for a period of one year along two profiles across the Kunlun mountains and the Jinsha river suture in northeast Tibet. The aims of the project are to determine the crust and upper mantle structure beneath northeast Tibet, detect the sharpness of any steps in major crustal boundaries (e.g. Moho) and detect how deep major faults penetrate in order to examine the viability of the crustal flow hypothesis. A description of the field experiment and data examples from both the controlled-source and passive-source components of the project will be presented.
T31B-0472
Tomographic Imaging of the Crust and Upper Mantle Beneath the Western Tien Shan
We combine P and S wave arrival times from the recent MANAS deployment of broad band sensors with the prior GENGHIS deployment and analogue observations from the Kyrgyz Institute of Seismology to generate a high resolution (5-20 km) image of elastic wavespeeds in the crust and upper mantle beneath the western Tien Shan. The total data set consists of 29,006 P and 21,491 S arrivals from 2176 local events along with 27,196 P arrivals from 2631 teleseismic events recorded at 144 stations. Near surface structure is constrained by a combination of arrival times from local and regional events, Moho depths determined from receiver functions, and travel times from the 2007 MANAS active source profile. Our principal finding is a large high wavespeed region in the mantle beneath most of western Tien Shan dipping to the north to depths as great as 600 km. This region appears to be continuous to shallow depths and surfaces at the southern range front near where the Tarim Basin is being overthrust by the Kokshal range. This result suggests that the Tarim Basin continues to actively subduct beneath the Tien Shan. We do not see any similar feature associated with underthrusting of the Kazach shield to the south. We also inverted a subset of regionally located events for Pn wavespeed and anisotropy. Similar to the larger scale results, Pn wavespeeds in the uppermost mantle are 3-4 percent lower than normal under most of the Tien Shan. The fast direction of Pn anisotropy is predominantly north-south in agreement with the sparser sampling of previous SKS studies. Taken with the tomographic image, we suggest that the anisotropy is most likely due to flow in the asthenosphere induced by the subduction of the Tarim basin.
T31B-0473
Three-dimensional velocity structure of the crust beneath the central Tien Shan, Kyrgyzstan: Implications for large- and small-scale mountain buildings
A temporary broadband seismic network was installed during 1997–2000 in the central part of the Kyrgyz Tien Shan. Three-dimensional P- and S-wave velocity structure of the crust in the central Tien Shan was obtained by applying a tomographic method to arrival-time data from shallow local earthquakes recorded by the seismic network of the Tien Shan. The central part of the Tien Shan is upthrusted between the Kazakh Shield to the north and the Tarim Basin to the south and represents a crust-scale ramp with main thrusts at its edges. This active region is under north-south compression due to the India-Eurasia collision. Accurate and precise tomographic imaging sheds light upon dynamics of the mountain building, interaction of these tectonic blocks in this complicated region. The study area selected is an area enclosed by 39.5-43.5°N and 73.5- 80.5°E. The used arrival-time data from about 1500 local earthquakes recorded by the network, which consists of 42 stations of KNET, GHENGIS, KazNet and Geoscope spanned over the central Kyrgyz Tien Shan. The tomography method by Zhao et al. (1992) has been used in this study. The spacing between grid nodes is about 30-50 km in horizontal direction and 5-15 km in vertical direction. The total number of grid nodes is about 1500. Obtained tomographic images at depths of 0-5 km show a good correlation with subsurface geology. Low-velocity (low-V) zones are distributed beneath the basins, whereas high-velocity (high-V) zones exist beneath the mountain ranges at the surface layer. The low-V zones beneath the basins are probably due to the Cenozoic deposition accumulated with a thickness more than 4 km. Paleozoic crystalline basement rocks may be responsible for the high-V zones in the mountain belts. South-dipping and north-dipping low-V zones within the crust can be seen on both north and south sides of the central Tien Shan, respectively. These zones spatially correspond to the north boundary fault and the south boundary fault, where the Kazakh Shield and the Tarim Basin underthrust the central Tien Shan, respectively. These low-V zones probably originate from a prominent low-V zone explicitly imaged in the lower crust beneath a high-elevated region, where a south-dipping mantle upwelling flow reaches the Moho. Fluids conveyed through the upwelling flow might have penetrated to the lower crust and caused the north-dipping low-V anomaly. Then laterally intruded fluids and/or melt seen as a low-V zone within the low- to the mid-crust might have caused the blurred south-dipping anomaly on the opposite side. This observation confirms a crust-scale ramp model of the entire Tien Shan by means of the seismic tomography. High-V zones distributed beneath basins and low-V zones beneath encompassing mountains at depths of about 20-50 km that gradually replaced by low-V and high-V regions at the shallow depths, respectively, are found in the most parts of the Tien Shan. It seems that along with north-south compressional stress due to the India-Eurasian collision, the small-scale convection or mantle upwelling is an additional driving force of the mountain building in this region which might have played a key role in the basin-mountain development within the Tien Shan.
T31B-0474
Geodynamics of the Carpathian Bend Zone: interconnection between surface and deep processes
The Carpathian Bend Zone is a key segment of the Alpine-Himalyan Belt and represent the home of a very particular seismic activity: regular very strong earthquakes are being generated in a very narrow, deep volume of a finger-shape, vertically distributed. The nature of processes involved is instill not yet understood and could be eventually related to several main hypothesis, like: (1) subduction of an oceanic lithosphere (and eventually a remnant of this is just now detaching from the continental lithosphere of the East European and Moesian Platforms, (2) the oceanic slab subduction ended sometimes in (late) Miocene and then a part of the continental lithosphere of the mentioned platforms has been delaminated. Various models of the lithosphere – astenosphere system take into account for example the seismic attenuation and share wave splitting. The intricate geometry of the lithosphere – astenosphere system as well as the mantle flow around the "slab" generates intriguing questions. We add another constraint to the various methodological approaches by taking into account the kinematics of the crustal blocks as suggested by robust outcomes of an extended and long-term investigated satellite geodesy network consisting of permanent and campaign stations. We will integrate results of GPS investigations with other, complementary results from other investigations techniques in order to test the deep feedback to the Tethyan closure, including the astenosphere – lithosphere system, trying to explain the very unusual high Carpathian seismicity and the geometry of the high velocity body beneath the Carpathian Bend Zone and the mantle flow around it and related surface processes.
T31B-0475
Descending Lithosphere Beneath the SE-Carpathians: An Insight From the Past
Early Miocene subduction beneath the Carpathian arc and the subsequent gentle continental collision transported cold and dense lithospheric material into the hotter mantle. Mantle heterogeneities imaged by seismic tomography in the SE-Carpathians contain information on the present thermal state of the mantle. We develop a model of the present crustal and mantle temperature beneath the region based on P-wave seismic velocity anomalies and constrained by heat flow data. The present model temperatures are assimilated into the geological past using the information on the history of the regional movement in the Early and Middle Miocene. Prominent thermal states of the lithospheric slab descending in the region are restored from its diffuse present state. In Miocene times the slab geometry clearly shows two portions of the sinking body. The northwest- southeast oriented portion of the body is located in the vicinity of the boundary between the East European and Scythian platforms, and this portion of the sinking body may be a relic of cold lithosphere that has traveled eastward. Another portion has a northeast-southwest orientation and is related to the present descending slab. Above a depth of 60 km the slab had a concave thermal shape, confirming the curvature of the Carpathian arc, and a convex surface below that depth. The slab maintained its convex shape until it split into two parts at a depth of about 220 km. We propose that this change in the slab geometry, which is likely to be preserved until the present, can cause stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region.
T31B-0476
The Age of the intra-Danubian Suture (Southern Carpathians, Romania)
The Southern Carpathians, as an Alpine chain are formed of two domains, namely the Getic Domain (GD) and the Danubian Domain (DD). The basement of DD is represented by two terranes, named Dragsan and Lainici- Paius sutured through Tisovita-Iuti ophiolitic complex. The two terranes were invaded by large granitic plutons, some of them being dated as Late Proterozoic by U/Pb method. Yet, along the inferred suture there are four granitic bodies whose ages have been only assumed by their geological relations. From North to South the four bodies sampled for LA-ICP-MS zircon U/Pb dating are: Muntele Mic, Sfardinu, Cherbelezu and Ogradena. The previously CL imaged zircon crystals were ablated at the China's University of Geosciences facilities in Wuhan. The zircons from all samples showed quite complex structures, with many inherited cores or affected by lead loss processes. In order to get a mean age for every pluton, we used the weighted average plots by projecting the 206Pb/238U apparent ages. The crystallization age of the Poiana Marului pluton is around 326.7±7 Ma (MSWD 1.6). A set of sixteen apparent ages ranging between 400 to 648 Ma together with other tree points indicating 897, 1353 and 1693 Ma, represent inheritances. There was no observable lead loss process in this data set. The Sfardinu granite crystallized at 310±7.9 Ma (MSWD 4.8). The inheritances found in this sample are at 427 and 723 Ma, but an important lead loss process occurred later, as indicated by ten apparent ages between 240-292 Ma. Cherbelezu granite gave a crystallization mean age of 326.9±4.9 Ma (MSWD 1.9). A single inherited core appears at 502 Ma, yet eight apparent ages ranging between 239-295 Ma signalize again an important lead loss process. Other two ages at 176 and 193 Ma confirm the presence of this process. The Ogradena pluton zircons display two possible crystallization ages for the outer zones of zircon grains, at 356.6±7.8 Ma (twelve apparent ages, MSWD 12) and 314.1±7.8 Ma (nine apparent ages, MSWD 13) respectively. It is difficult to explain this situation, and we suppose the second figure as the probable crystallization age. Inheritances are represented by a number of 37 206Pb/238U apparent ages scattered between 400 and 612 Ma, only one apparent age of 265 Ma possibly proving the presence of lead loss. The geochemical parameters ASI, Fe-number and MALI calculated from the major oxides analyses showed that all four plutons are metaluminous and magnezian. Both Cherbelezu and Ogradena have a calcalkaline character near to alkalicalcic while Muntele Mic and Sfardinu are alkalicalcic. Geochemically, all four plutons are closed to Cordilleran type, main portion or a little inboard of it. Out of these data several conclusions can be drawn. The age of the intra-Danubian suture is late Visean to Bashkirian or late Variscan. Muntele Mic and Ogradena granites exhibit abundant Cadomian inheritances, while Cherbelezu and Sfardinu plutons seem affected by lead loss processes due to an important thermotectonic event happened during 250 to 290 Ma interval. The emplacement of the four plutons probably reflects a subductional process. Finally, the inheritances suggest a Gondwanan source for the anatectic material.
T31B-0477
The Magnetic Signature of Zones of Continental Collision
Near-surface and satellite maps of the crustal component of the magnetic field can be interpreted in terms of thermal conditions at depth because the magnetic properties of rocks depend on their temperature. Observations related to continental deformation at diffuse plate boundaries are often considered in relation to three length scales: the thickness of the seismogenic upper crust, the entire continental crust, and the mechanical lithosphere. The lower boundary of the magnetic crust coincides with the Moho, or in the presence of an elevated geotherm, with the Curie isotherm. New global perspectives on the magnetic signature of zones of continental collision are afforded by the recently published Magnetic Anomaly Map of the World (Purucker, 2007, EOS, 88, 263), the MF-5 satellite magnetic field (Maus et al., 2007, Gcubed), and NASA's ST-5 constellation mission in 2006. The thickness of the magnetic crust can be estimated by integrating the MF-5 satellite magnetic field into the 3SMAC compositional and thermal model of the lithosphere, and a minimum estimate of the magnetization can be estimated using a Greens function approach. We compare our magnetic maps with the diffuse plate boundary maps of Gordon (1998) and Dumoulin et al. (1998). The diffuse plate boundary zones exhibit intermediate (22-31 km) magnetic thicknessses, significantly less than those of the adjacent stable plate. The diffuse NE Asia plate boundary zone, from the Lena River delta to the Sea of Okhotsk, is especially well- expressed in both satellite and near-surface magnetic maps.
T31B-0478
Recent faulting and active shortening of the Middle Atlas Mountains, Morocco, within the diffuse African-Eurasian plate boundary
The NE-SW trending Middle Atlas Mountains are an active intracontinental mountain belt within the diffuse African – Eurasian plate boundary. The mountain belt is obliquely oriented to the NNW-SSE direction of Late Cenozoic plate convergence. Both shear and compressional features are exhibited with apparent slip partitioning: Folding and thrusting is concentrated in the Folded Middle Atlas, whereas strike-slip dominates in the Tabular Middle Atlas. In the central part of the Folded Middle Atlas, fault scarps of Quaternary alluvium, including a 4.5 meter (probably composite) scarp and a 1 meter (possibly single event) scarp, attest to recent faulting along the mountain front. Detailed topographic mapping of the scarps provides a basis for geomorphic analysis and degradation modeling. Furthermore, the reconstruction of longitudinal stream terrace profiles helps constrain a long term deformation history. Radiocarbon and pending cosmogenic dates provide age constraints on the faulted surfaces and the multiple stream terraces in the area. To place these active tectonic observations in a larger context, the fault and fold geometry has been assessed by completing a 10 km structural transect across the frontal thrust, providing basis for the construction of a balanced cross-section. By combining the structural geometry with the uplift rate, a minimum estimate of the rate of horizontal shortening in the Middle Atlas can be evaluated. Preliminary results suggest the Middle Atlas may accommodate 5 – 10 percent of the total 4.5 mm/yr convergence between the African and Eurasian plates. These results demonstrate that the Middle Atlas Mountains are a integral part of the diffuse plate boundary, as well as suggesting a modest level of earthquake hazard in the region.
T31B-0479
Seismic Reflection Transect across the Central Iberian Zone (Iberian Massif): The ALCUDIA project
The lithosphere of the Central Iberian Zone (CIZ) differs from that of the southwestern Iberian Massif. They are limited by a suture zone. The seismic reflection profile IBERSEIS suggested that the activity of a Carboniferous mantle plume resulted in abundant intrusions of mafic magmas in the mid-to-lower crust which resulted in a singular crustal evolution. The current knowledge of the area based mostly in surface geological mapping suggests that basic magmatism continues further towards the north, indicating that the mantle plume may have affected a bigger area up to the Tajo depression. Furthermore, the existence of the Almadén mine, one of the largest mercury mine in the world within the CIZ, favour that the crust in this area is the result of anomalous lithospheric processes. Accordingly, the ALCUDIA project has been lauched aiming to study the structure and nature of the lithosphere of the CIZ. It includes the acquisition of a deep high resolution seismic reflection transect, detailed geological mapping, kinematic, petrologic and geochemical studies, and other geophysical studies (potential field methods). This new profile extends the previous IBERSEIS Transect towards the northeast, completing almost 600 km of deep seismic profiles, crossing the southern half of the Iberian Variscides. The transect crosses some important structures, such as the Toledo fault, Santa Elena Fault, Alcudia anticline, Almadén syncline, and some major magnetic anomalies. The preliminary results reveal that the crust is 30 km thick in average, with a horizontal Moho, a highly reflective mid-to-lower crust with a few mantle reflectors and well defined features in the upper crust with the indication of detachments zones that might link to the mid- crustal reflective zone.
T31B-0480
Post-Collisional Magmatism Across an Accreted Terrane Assemblage in South-Central Alaska: Connections Between Mantle Sources, Deformation, and Plate Margin Dynamics
Paleocene and Eocene volcanic and plutonic rocks in the Talkeetna Mountains (TM) of south central Alaska were emplaced across the Wrangellia composite terrane (WCT) and its suture zone during and after collision with southern Alaska. Past studies infer that these volcanic rocks were formed as part of a regional arc system. Our new Nd-Sr isotope and geochemical data along with previously described field and radiometric age data reveal that they instead represent a unique episode of continental margin magmatism. TM magmatism began at ca. 62 Ma with adakitic granitic plutons and then continued, through Eocene time, with granitic and shallow adakitic intrusions and tholeiitic to high-K basalt-andesite-rhyolite volcanism when there was a hiatus in arc magmatism across southern Alaska. Trace element and Nd- and Sr-isotopic data reveal a systematic trend among the TM volcanic rocks in which basalts are most depleted in the forearc region at the southern edge of the WCT (εNd(t) = 8.41 to 10.87; 87Sr/86Sr(t) = 0.702778 to 0.703193) and are more enriched at the northern, inboard, edge of the WCT (in the terrane suture zone) (εNd(t) = 7.24 to 1.98; 87Sr/86Sr(t) = 0.703725 to 0.704755). Intermediate to acidic rock compositions have more enriched isotopic ratios than the basalts in each region. A systematic south-to-north (continentward) increase in La/Yb, Nb/Zr, and Ba/Zr is consistent with the isotope data. The composition of crustal rocks in the accreted terrane assemblage changes from oceanic arc affinity in the south to granitic and sedimentary rocks of continental affinity in the north. These changes in basement composition are, in part, responsible for the range of Nd and Sr isotope compositions among the TM volcanic rocks. The southern TM rocks had a depleted source of basaltic magma consistent with derivation by adiabatic melting of a sub-oceanic MORB-like mantle reservoir. This mantle was emplaced beneath southern Alaska through a slab window following Late Paleocene ridge subduction. Farther inboard, the northern TM basaltic magmas had a more enriched mantle source and/or assimilated more enriched crustal rocks. In this region, high heat flow through a slab window could have induced partial melting of pre-existing sub-continental mantle and also produced crustal melts to form rhyolites. TM rocks with adakite characteristics are consistent with a slab window model; partial melting of the slab edges and/or melting of garnet-bearing crustal rocks of the WCT above a slab window could have produced adakitic magmas. TM magmatism occurred in a zone of crustal extension that was oblique to the continental margin. Extension can be attributed to oroclinal rotation of western Alaska and/or transtension associated with regional strike-slip faults. Subsequent deformation of parts of the TM volcanic fields (broad folding and small-scale brittle faults) is consistent with dextral simple shear along adjacent strike-slip faults. The TM rocks, therefore, record the magmatic response to terrane accretion and ridge subduction and the kinematics of margin-parallel transport of an accreted terrane assemblage after it was sutured to the continental margin. In addition, these rocks reveal how the composition of collisional crustal elements can influence subsequent continental margin magmatism. Overall, the TM volcanic rocks provide a unique record of crust-mantle interactions during the plate kinematics that shaped the southern margin of Alaska.
T31B-0481
Quantifying Strain in the Mantle Across a Paleotransform Fault, Bogota Peninsula, New Caledonia
The Massif du Sud in New Caledonia exposes the mantle section of an Eocene-age ophiolite thrust sheet. Field foliations over most of the Massif du Sud are horizontal to shallowly dipping to the SW with horizontal NS-trending lineations. The Bogota Peninsula along the northwest edge of the thrust sheet, in contrast, records moderately dipping to sub-vertical fabrics over a 50-km wide region, with the strongest mylonitic fabrics confined to a narrow 3-km wide zone. Field foliations rotate systematically and steepen across the shear zone; field lineations remain shallowly plunging but follow foliation rotations. Pyroxenite and dunite dikes, interpreted as early intrusions, record similar rotations and many become nearly transposed in the center of the high-strain zone. In contrast, late-stage diabase dikes have consistent NE-SW strikes and dip steeply in both directions at all locations across the shear zone. To quantify strain recorded across the Bogota Peninsula shear zone, we subdivided the region into four zones based on regional fabric consistency. We generated incremental forward models of inclined transpression and transtension and compared model results to the orientations of field foliation, lineation, and pyroxenite dikes within each zone. The triclinic fabrics in the Massif du Sud are consistent with sinistral inclined transpression and interpreted to form during oblique spreading along the south flank of an ~EW-trending ridge. Fabrics in the Bogota Peninsula shear zone, in contrast, have monoclinic symmetry and are interpreted to record dextral transpression along an oceanic transform fault linking two mid-ocean ridge segments. When combined, the modeling results from all four zones can be used to estimate the total fault offset and predict plate motion rates for this former oceanic plate boundary.