T22C-01
Lithospheric structure of the Arabia-Eurasia collision zone in Eastern Anatolia from magnetotelluric exploration
Continent-continent collisions are a fundamental tectonic process that has shaped the evolution of the Earth. A series of these collisions have occurred in the Alpine-Himalaya Mountain belt and produced high elevation plateau. Eastern Anatolia is the location of a relatively young (~13 Ma) continent-continent collision between the Arabian and Eurasian Plates. In contrast to the Tibetan Plateau, limited geophysical data is available to constrain the mode of deformation in Eastern Anatolia. The first long-period magnetotelluric (MT) data were collected in Eastern Anatolia in 2005 on profiles extending from the Arabian plate to the Black Sea. Geoelectric strike directions were mostly east-west, rather than parallel to the major strike-slip faults that define the boundaries of the tectonic blocks. Two dimensional MT inversions were used to obtain resistivity sections along the profiles. The Anatolian block that is being extruded to the west is characterized by a low resistivity (fluid rich?) lower crust underlain by normal upper mantle structure. In contrast, the Anatolian plateau is underlain by an upper mantle with an anomalously low electrical resistivity that could be accounted for by a few percent partial melt. The lower crust beneath the Anatolian plateau exhibits an abnormally low resistivity that also requires an elevated fluid content. This could have originated by under-thrusting of the Eastern Anatolian Accretionary Complex. The presence of fluids in both the crust and upper mantle may weaken the lithosphere sufficiently to permit lateral flow, and may also allow a decoupling of the upper and lower portions of the lithosphere. The lithospheric structure of the Anatolian Plateau is quite similar to that of the Tibetan Plateau, with widespread zones of elevated fluid content and weak rheology. This is despite the fact that the two areas have undergone quite different styles of tectonic evolution.
T22C-02
Large-Scale, Long-Lived Subduction of Ultrahigh-Pressure Terranes: Western Gneiss Region, Norway
Recent Lu-Hf and Sm-Nd ages of garnets and a U-Pb age of zircon of eclogites from the Western Gneiss Region (WGR) ultrahigh-pressure (UHP) terrane, Norway, demonstrate that eclogite-facies metamorphism occurred over a large area (60,000 km2) for an unexpectedly long time. This observation stands in stark contrast to the general belief that continental subduction, and attendant (U)HP metamorphism, occurs over short timescales. Four HP eclogites (~700-800°C, ~2.0-2.5 GPa) from the central WGR yielded equivalent Lu-Hf ages of ~416 Ma; three of these samples gave Sm-Nd ages of ~400 Ma. Given the distribution coefficients for Lu and Sm, the older Lu-Hf ages reflect prograde growth, but are younger than the initiation of garnet crystallization. The younger Sm-Nd ages represent either eclogite-facies cooling through the blocking temperature of the Sm-Nd system or an ‘average' age of garnet growth. Both cases imply >16 m.y. of eclogite- facies conditions. Two UHP eclogites (~750-850°C, ~3 GPa) from the same region yielded significantly younger, but equivalent Lu-Hf and Sm-Nd ages of ~380 Ma, which likely indicate passage through the blocking temperature of both systems up to 20 m.y. after the HP eclogites had passed through the blocking temperature. Because these eclogites are unretrogressed, their ages are the youngest known for eclogite stability in the WGR. An eclogite from the northern WGR yielded a Sm-Nd age of 413.9 ± 3.7 Ma. This could represent a different HP history than that of the central WGR: U/Pb ages in the north are also ~15 m.y. older. Two HP (~650°C, ~2 GPa) eclogites from the southern WGR yielded Lu-Hf ages of 410.2 ± 3.1 and 427.5 ± 7.7 Ma, indicating a similar garnet growth history to the central WGR eclogites. A retrogressed eclogite from the undated eastern portion of the WGR gave a 206Pb/238U age of 408.0 ± 1.7 Ma. Thermal models mirror results from similar studies (Roselle et al., 2002), and confirm that slow subduction likely produced the P-T-t record observed in the WGR. For subduction of a continent into a formerly Andean-style subduction zone, Calculated temperatures of ~700-800°C are reached at 100 km depth only at burial rates slower than 4 mm/yr. These same temperatures are achieved with slow burial (4-7 mm/yr) during intracontinental subduction.
T22C-03
Proterozoic mantle lithosphere beneath the Tariat Depression and Dariganga Plateau, Mongolia: in situ Re-Os evidence
The Os isotope compositions of sulfides in mantle peridotites from the Tariat Depression (TD) in central Mongolia and the Dariganga Plateau (DP) in southeast Mongolia reveal the presence of Proterozoic lithospheric mantle beneath these regions. The least-disturbed sulfides, with 187Re/188Os<0.07, yield TMA model ages of 1.5±0.3, 1.1±0.1, 0.7±0.1 and 0.48±0.08 Ga (2σ) for TD and 2.0±0.2 and 1.4±0.4 Ga (2σ) for DP. Beside these low Re/Os sulfides, TRD model ages of other sulfides without later introduction/loss of Os, can still provide minimum estimates for the age of lithospheric mantle and record later metasomatic events. Ten TD Sulfides have TRD model ages ranging from 2.6 to 0.5 Ga, with peaks around 1.6 and 0.8-0.6 Ga; two DP sulfides have TRD model ages of 1.8 and 1.2 Ga. These old depletion/melting events are consistent with those from Nd model ages of peridotites from the same region (~2 Ga for TD [1]; >1 Ga [2] and 1.6 Ga [3] for DP). In the Tariat region, these mantle events are consistent with those known in the overlying crust as recorded by Mesoproterozoic Nd model ages (TDM = 1.4-1.1 Ga) of Paleozoic and Mesozoic granitoids from the Khangai microcontinental block, where the Tariat Depression is located, and similar Nd model ages of 1.5-1.0 Ga for crustal granulite xenoliths from the Shavaryn-Tsaram volcano in the Tariat Depression [4]. Younger sulfide Os ages (1.1, 0.7 and ~0.5 Ga) may date mantle thermal events that also affected the overlying crust, marking the beginning of the Central Asia Orogeny in Neoproterozoic time. Although the South Mongolia domain, where the Dariganga Plateau is located, is believed to consist of late Paleozoic accretionary complexes and arc terrains [5], the presence of Precambrian zircon xenocrysts in magmatic rocks and ancient detrital zircons in arc-derived sediments [6], and Proterozoic Nd model ages of basement rocks in the Xilinhot region (B. Chen, pers. comm.) suggest substantial reworking of old crust. The presence of a Precambrian crustal terrain in the Dariganga region, corresponding to the underlying Mesoproterozoic lithospheric mantle indicated by the DP sulfide Os data, appears probable. References [1] Stosch, H.-G. et al. (1986) GCA 50, 2601-2614; [2] Wiechert, U. et al. (1997) CMP 126, 345-364; [3] Deng, F.-L. et al. (1992) Nature 360, 333-336; [4] Kovalenko V.I. et al. (2004) JAES 23, 605-627; [5] Windley, B.F. et al. (2007) JGS 164, 31-47; [6] Kroner, A. et al. (2007) GASM, in press.
T22C-04
New Paleomagnetic Results From the Alxa Region of China: Implications for the Long-Term Slip History of the Altyn Tagh Fault and Strain Accommodation During the Indo-Asian Collision
The Altyn Tagh Fault (ATF) on the northern margin of the Tibetan Plateau is a key structural feature in all tectonic models of Asia. Mechanical models for strain accommodation within the Asian lithosphere largely fall into two end members: distributed crustal thickening and lateral extrusion. Geodetic and structural studies indicate that the northern margin of the Tibetan Plateau currently deforms via distributed crustal thickening, but recent piercing point studies demonstrate two distinct phases of slip on the ATF: fast, high-magnitude Late Oligocene-Early Miocene slip and slow, low-magnitude post-Early Miocene slip. These long-term slip-rate results suggest a coherent model that reconciles the two opposing mechanical models. First, the ATF accommodated plate-like lateral extrusion during the Oligocene and Early Miocene with ~310 km of left-lateral displacement and slip rates in excess of 20 mm/year. Second, since the Early Miocene, slip on the ATF has slowed to <10 mm/yr (~the present rate), accumulated only ~65 km of offset, and has accommodated distributed shortening within the northern Tibetan Plateau. An Oligocene-Early Miocene phase of lateral extrusion requires structures for accommodating lateral extrusion north of the Qilian Shan, suggesting transfer of left-lateral slip into the Alxa-East Mongolia fault system (AEMFS), a system of strike-slip faults posited to extend from the northeast end of the ATF to the Sea of Okhostk. This proposed fault system is supported by geological data from southern Mongolia and a few geophysical studies. Furthermore, existing paleomagnetic results from Tarim and the North China Block are compatible with the existence of a major post-Cretaceous tectonic discontinuity between these two lithospheric units. Recent field mapping in the Alxa region confirms the existence of a system of E-NE striking strike-slip faults with clear post-Cretaceous offsets of 10s to possibly ~150 km but limited post- Miocene offsets. Here we present new paleomagnetic results from Mesozoic and Cenozoic sedimentary rocks from the Alxa region (Yabrai Shan in the NE: 5 sites, 59 samples; Hei Shan in the SW: 10 sites, 121 samples). All samples were thermally demagnetized to isolate characteristic remanent magnetizations (ChRM). Similar ChRM directions carried by magnetite and hematite, and positive conglomerate tests suggest the ChRMs are ancient. Preliminary differential vertical axis rotation studies from fault-bound blocks and from coeval rocks from opposing sides of the mapped Alxa faults suggest significant (>15° of cw or ccw rotation) post-Mesozoic shear, with limited-to-negligible post-Miocene shear. These results are compatible with the hypothesis of a two-phase evolution of the ATF and the northern margin of the Tibetan plateau, in which the Asian lithosphere deformed via lateral extrusion along the ATF and AEMFS from Late Oligocene to Early Miocene time and subsequently by distributed crustal thickening within the northern margin of the Tibetan Plateau.
T22C-05
Metamorphism and deformation of the lower crust and crust-mantle interface at the eastern syntaxis of Tibet derived from converted seismic waves
In this study we examine the internal structure of the crust at the northeastern corner of the Indian-Asian collisional zone. This region marks a fundamental tectonic change, transitioning from collisional to escape tectonics, as it is influenced by the corner of the indenting Indian plate. Using detailed 3-d receiver function images the geometry and velocity structure of the crust is constrained through finite difference wave-form inversion. Our two fundamental observations are a dramatic change in lower crustal reflectivity across the end of the collisional zone and an anticlinal fold on the crust-mantle boundary that coincides with the Namche- Barwa/Gyala Peri antiformal basement massifs at the end of Himalayan arc. The observed change in lower crustal reflectivity coincides with the transition from classical central plateau collisional tectonics to the escape tectonics of the east. Associated with the along strike collision, a high velocity lower crustal layer with a P-velocity of 7.8 km/s is modeled beneath the southern Lhasa block. This layer extends to the north beneath the southern Lhasa block and terminates abruptly to the east about 100km to the west of the Namche-Barwa/Gyala Peri massifs. This layer is interpreted to be Indian lower crust that has subducted beneath Asian crust north of the Tsangpo suture and metamorphosed into eclogite. Further to the east beneath the Namche – Barwa/Gyala Peri massifs we image a north-south running 50 km wide, 10 km vertical step in the Moho. By modeling various crust- mantle geometries, sensitivity analysis was performed to investigate whether this imaged step was generated by a crust-mantle interface with a discrete vertical step or a continuous folded structure. This feature can best be explained as a north-south trending, asymmetric, anticlinal fold on the crust mantle boundary. This fold on the crust-mantle interface correlates with a region on the surface that has undergone large amounts of uplift and exhumation.
T22C-06
Tomographic Evidence for a Possible Link Between Mantle Lithosphere Delamination and North-south Trending Rifts in Southeast Tibet
The Tibetan plateau is a complex tectonic region resulted from the India-Eurasia collision, which has deformed the southeastern Asia through a combination of thrust, extension and strikeslip faultings. The north-south and northwest-southeast trending rifts on the surface of the plateau, which are manifested by the east-west extension, are mainly distributed in south and central Tibetan plateau and their origins still remain enigmatic. Some previous studies attributed these structures to gravitational spreading of the over-thickened crust, caused by an abrupt rise of the plateau due to convective removal of the lower part of the mantle lithosphere. Other studies have suggested that the north-south rift zones in the Tibetan plateau are shallow features formed by the eastward motion of the shallow crust that are decoupled from the mantle lithosphere by a low-viscosity lower crust. In this study, we present evidence suggesting that these features are a surface manifestation of a coherent deformation throughout the entire crust and mantle lithosphere and that the mantle lithospheric delamination plays a key role in the process. We performed finite-frequency tomography to investigate the crustal and upper mantle structures beneath the region of the Eastern Himalayan Syntaxis in southeastern Tibet. Our dataset comes from the Namche Barwa seismic experiment, which deployed a 50 stations broadband array and a 20 element short period array during 2003-2004 in southeastern Tibet. We also used data from the Global Seismic Network (GSN) station LSA, and 5 broadband stations from the Bhutan experiment, which overlapped temporarily with the Namche Barwa seismic experiment in 2003. Using a multi-channel waveform crosscorrelation technique, we measured P and S differential travel times between different stations in three frequency ranges : 0.03-0.1 Hz, 0.1- 0.5 Hz and 0.5-2.0 Hz for P waves, and, 0.02-0.05 Hz, 0.05-0.1 Hz and 0.1-0.5 Hz for S waves. The measured P and S delay times are then utilized to invert for spatial variations in P and S-wave speed perturbations according to 3-D finite frequency kernel formulation. Our results show the presence of a low-velocity anomaly in the crust and upper mantle to ~ 300 km depth beneath a north-south trending rift zone in southeast Tibet. This low- velocity anomaly is situated above a tabular, high-dipping-angle, high-velocity anomaly that extends into the upper mantle transition zone. The Vp/Vs ratio of this high-velocity anomaly suggests that temperature variations are not the only cause and a highly melt-depleted mantle is required. These observations provide clear evidence for the delamination of a thickened Eurasian mantle lithosphere and its causal relationship to the formation of the north- south trending rift in southeastern Tibet.
T22C-07
Deep Orogen-parallel Electrically Conductive Troughs And The Relationship To Surface Deformation In Eastern Tibet And Indochina
While it is accepted that the Cenozoic collision of India and Eurasia created the world's&plargest plateau in Tibet through a range of processes including crustal thickening, delamination and extrusion, the debate continues about which tectonic processes contribute to the overall mass balance of this continent- continent collision. Competing models explaining the post-collisional deformation in Himalaya and Tibet advocate eastward extrusion of rigid blocks between slip-surfaces (Tapponier et al. 2001) or ductile crustal flow decoupling the upper and lower parts of the lithosphere (Clark and Royden, 2000; Beaumont et al., 2001). Yet, the crustal and upper mantle structure in the southeastern Himalaya-Tibet regions remains poorly understood. It is pivotal for constraining the nature of lateral extrusion or ductile flow. We recently initiated a major regional study of the eastern Himalayan syntaxis (e.g. Sun et al. 2003). Here, we present the electrical conductivity images of the lithosphere across the eastern Tibetan Plateau and off- plateau in Indochina (lat. 25-32 deg. N, long. 97-105 deg. E) which suggest the presence of a coherent and geometrically similar orogen-parallel trough-like features of low electrical resistivity in the region. We compared our images to those published by other workers (Wei et al., 2001; Unsworth et al.2004) for the regions to the west and found the troughs to be a common regional feature. We find a continuous 10-30 km thick, electrically conductive crustal layer with two pronounced ca. 150-km-wide localised downwarps that are about 30-60 km deep and can be traced from the Tibetan plateau eastwards into Indochina. The lateral terminations of these low- resistivity troughs are marked on the surface by major E-W trending (Tibet) and N-S trending (Indochina) strike- slip faults. Their axes are parallel to the dominant surface structural trend suggesting coupling between upper and lower crust. Also, zones of Cenozoic volcanism appear to be spatially related to the crustal conductive layer suggesting a link between lower crustal conductivity heterogeneity and surface deformation in and off the Tibetan plateau. We suggest that the troughs are pre-existing fold structures or buried basins and that they probably influenced crustal melt distribution and deformation in the region. Our folding interpretation is shown to be consistent with inferences from gravity and topographical data (Jin et al.,1994; Braitenberg et al. 2000). References: Beaumont, C., R. A. Jamieson, M. H. Nguyen and B. Lee, Nature, 414, 738-742, 2001. Braitenberg, C., Zado, M., Fang, J., Wang, Y. & Hsu, H.T., J. Geodynamics 30, 489-505, 2000. Clark, M. K., L. H. Royden, Geology, 28, 703-706, 2000. Jin, Y., McNutt, M. & Zhu, Y. , Nature 371, 669-674, 1994. Sun, J., G. Jin, D. Bai, and L. Wang, Science in China, 46, 243-253, 2003. Unsworth M., Wei Wenbo, Jones A., et al., JGR, 109, B02403, doi:10.1029/2002JB002305, 2004. Tapponnier P., Xu Z., Roger F., Meyer B., Arnaud N., Wittlinger G., Yang J., Science, 294 (5547), 1671-1677, 2001. Wei W., Unsworth M., Jones A., et al., Science, 292 (5517), 716-719, 2001. Yin, A., T. M. Harrison, Annual Rev. Earth Planet. Sci., 28, 211-280, 2000.
T22C-08
Upper mantle earthquakes in the Himalayan collision: Flexure of the continental lithosphere?
Earthquake locations from the Himalaya Nepal-Tibet seismic experiment (HIMNT) during 2001-2003 suggest that the uppermost mantle in the Indian lithosphere deforms by brittle processes. We found clusters of upper mantle earthquakes beneath the transition from the Ganges Plains to the southernmost Himalaya, and beneath the southern Tibetan Plateau. Previous studies (e.g. Chen and Yang, 2004) have also noted the presence of upper mantle earthquakes beneath the southern Tibetan Plateau. Using a finite element model, we establish a link between upper mantle seismicity and flexural bending of the Indian lithosphere. The goal is to determine the steady-state stress field in the Indian plate in response to loading. Our models assume plane strain and use Cartesian coordinates on a representative arc-normal cross section of the Indian plate at the Himalayan collision zone. We use a time-dependent viscoelastic regime in a vertically layered lithosphere with a non-Newtonian rheology, overlying an asthenosphere with a constant and Newtonian viscosity. The crust is assumed to be composed by quartzite in its upper part and diabase in its lower portion; for the upper mantle we use olivine rheologies. The modeled stress field reveals that the highest differential stresses occur in the uppermost mantle, with magnitudes of around 1 GPa, concentrating roughly at the regions where upper mantle earthquakes originate. The predicted orientations of principal stresses just below the model crust-mantle boundary are consistent with previously determined focal mechanisms of near-Moho earthquakes in southern Tibet, with the maximum compressive stresses oriented in a nearly horizontal Himalayan arc-normal direction, and the maximum extensional stress being nearly horizontal and arc-parallel. The finite element modeling suggests that flexural bending of the Indian plate generates sufficiently large background stresses to put upper mantle rocks in near-brittle failure conditions.