ALBERT

Description: Abstract The Cenozoic convergence between India and Asia has created Earth's thickest crust in the Pamir‐Tibet Plateau by extreme crustal shortening. Here we study the crustal structure of the Pamir and western Tian Shan, the adjacent margins of the Tajik, Tarim, and Ferghana Basins, and the Hindu Kush, using data collected by temporary seismic experiments. We derive, compare, and combine independent observations from P and S receiver functions. The obtained Moho depth varies from ~40 km below the basins to a double‐normal thickness of 65–75 km underneath the Pamir and Hindu Kush. A Moho doublet—with the deeper interface down to a depth of ~90 km—coincides with the arc of intermediate‐depth seismicity underneath the Pamir, where Asian continental lower crust delaminates and rolls back. The crust beneath most of the Central and South Pamir has a low Vp/Vs ratio (〈1.70), suggesting a dominantly felsic composition, probably a result of delamination/foundering of the mafic rocks of the lower crust. Beneath the Cenozoic gneiss domes of the Central and South Pamir, which represent extensional core complexes, the Vp/Vs ratios are moderate to high (~1.75), consistent with the previously observed, midcrustal low‐velocity zones, implying the presence of crustal partial melts. Even higher crustal average Vp/Vs ratios up to 1.90 are found in the sedimentary basins and along the Main Pamir Thrust. The ratios along the latter—the active thrust front of the Pamir—may reflect fluid accumulations within a strongly fractured fault system.

Description: [1]  We present new seismicity images based on a two-year seismic deployment in the Pamir and SW Tien Shan. 9,532 earthquakes were detected, located and rigorously assessed in a multistage automatic procedure utilizing state-of-the-art picking algorithms, waveform cross-correlation and multi-event relocation. The obtained catalog provides new information on crustal seismicity and reveals the geometry and internal structure of the Pamir-Hindu Kush intermediate-depth seismic zone with improved detail and resolution. The relocated seismicity clearly defines at least two distinct planes, one beneath the Pamir, the other beneath the Hindu Kush, separated by a gap across which strike and dip directions change abruptly. The Pamir seismic zone forms a thin (ca.10 km width), curviplanar arc that strikes east–west and dips south at its eastern end, then progressively turns by 90 degrees to reach a north–south strike and a due eastward dip at its southwestern termination. Pamir deep seismicity outlines several streaks at depths between 70 and 240 km, with the deepest events occurring at its southwestern end. Intermediate-depth earthquakes are clearly separated from shallow crustal seismicity, which is confined to the uppermost 20–25 km. The Hindu Kush seismic zone extends from 40 to 250 km depth and generally strikes east–west, yet bends northeast, towards the Pamir, at its eastern end. It may be divided vertically into an upper and lower part separated by a gap at approx. 150 km depth. In the upper part, events form a plane that is 15–25 km thick in cross-section and dips sub-vertically north to northwest. Seismic activity is more virile in the lower part, where several distinct clusters form a complex pattern of sub-parallel planes. The observed geometry could be reconciled either with a model of two-sided subduction of Eurasian and previously underthrusted Indian continental lithosphere or by a purely Eurasian origin of both Pamir and Hindu Kush seismic zones, which necessitatesa contortion and oversteepening of the latter.

Description: : [1]  We use ambient-noise tomography to map regional differences in crustal Rayleigh-wave group velocities with periods of 8-40 s across north Tibet using the INDEPTH IV arrays (132 stations, deployed for 10-24 months). For periods of 8-24 s (sensitive to mid-crustal depths of ~5-30 km), we observe striking velocity changes across theBangong-Nujiang and Jinsha suture zonesaswell as the Kunlun-Qaidam boundary. From south to north, we see higher velocities beneath the Lhasa terrane, lower velocities beneath the Qiangtang, higher velocities in the Songpan-Ganzi and Kunlun terranes, and the lowest velocities beneath the Qaidam Basin. Maps at periods of 34 and 40 s (sensitive to the middle and lower crust at depths of ~30-60 km) do not show evidence of changes across those boundaries. Any differences between the Tibetan terrane lower crusts that were present at accretion have been erased or displaced by Cenozoic processes and replaced almost ubiquitously by uniformly low velocities.

Description: SUMMARY From the S- wave data collected along a 270-km-long profile spanning the Kunlun mountains in NE Tibet, 14595 Sg phase arrivals and 21 SmS phase arrivals were utilized to derive a whole-crustal S velocity model and, together with a previously derived P velocity model, a Poisson's ratio (σ) model beneath the profile. The final tomogram for the upper 10–15 km of the crust reveals the lower velocities associated with the predominantly Neogene-Quaternary sediments of the Qaidam basin to the north and the higher velocities associated with the predominantly Palaeozoic and Mesozoic upper crustal sequences of the Songpan-Ganzi terrane and Kunlun mountains to the south. This study finds no evidence that the Kunlun mountains are involved in large-scale northward overriding of the Qaidam basin along a shallow south-dipping thrust. The σ in the upper 10–15 km of the crust are often lower than 0.25, indicating a preponderance of quartz-rich rocks in the upper crust beneath the profile. Below 10–15 km depth, the remainder of the crust down to the Moho has an average σ of 0.24 beneath the Songpan-Ganzi terrane and Kunlun mountains and 0.25 below the Qaidam basin. These low σ are similar to other low σ found along other profiles in the northeastern part of the plateau. Assuming an isotropic situation and no significant variation in σ between 10–15 km depth and the Moho, then the lower crust between 25–30 km depth below sea level and the Moho with P velocities varying from 6.6 km s −1 at the top to around 6.9 km s −1 at the base and σ of 0.24–0.25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part. As the pre-Cenozoic Qaidam basin crust has probably not lost any of its lower crust during the present Himalayan orogenic cycle in the Cenozoic and only has a σ of 0.245–0.25, then it appears that the pre-Cenozoic Qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average σ of 0.265–0.27 and the Tarim and Sichuan basin crusts. This situation then probably facilitates the collision and promotes the formation of new high plateau crust at the NE margin of Tibet. South of the Qaidam basin, the crust of the Songpan-Ganzi terrane and Kunlun mountains has an even lower average crustal σ of 0.23–0.24 and is thus presumably even weaker and more easily deformable than the crust beneath the Qaidam basin. This then supports the hypothesis of Karplus et al. that ‘the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust'.

Description: SUMMARY Utilizing seismic refraction/wide-angle reflection data from 11 approximately in-line earthquakes, 2-D P- and S -velocity models and a Poisson's ratio model of the crust and uppermost mantle beneath the southern Tien Shan and the Pamir have been derived along the 400-km long main profile of the TIPAGE (TIen shan—PAmir GEodynamic program) project. These models show that the crustal thickness varies from about 65.5 km close to the southern end of the profile beneath the South Pamir through about 73.6 km under Lake Karakul in the North Pamir, to about 57.7 km, 50 km south of the northern end of the profile in the southern Tien Shan. Average crustal P velocities are low with respect to the global average, varying from 6.26 to 6.30 km s −1 . The average crustal S velocity varies from 3.54 to 3.70 km s −1 along the profile and thus average crustal Poisson's ratio (σ) varies from 0.23 beneath the central Pamir in the south central part of the profile to 0.265 towards the northern end of the profile beneath the southern Tien Shan. The main layer of the upper crust extending from about 2 km below the Earth's surface to 27 km depth below sea level (b.s.l.) has average P velocities of about 6.05–6.1 km s −1 , except beneath the south central part of the profile where they decrease to around 5.95 km s −1 . This is in contrast to the S velocities which range from 3.4 to 3.6 km s −1 and exhibit the highest values of 3.55–3.6 km s −1 where the P velocity is lowest. Thus, σ for the main layer of the upper crust is 0.26 beneath the profile except beneath the south central part of the profile where it decreases to 0.22. The low value of 0.22 for σ under the central Pamir, the along-strike equivalent of the Qiangtang terrane in Tibet, is similar to that within the corresponding layer beneath the northern Lhasa and southern Qiangtang terranes in central Tibet and is indicative of felsic rocks rich in quartz in the α state. The lower crust below 27 km b.s.l. has P velocities ranging from 6.1 km s −1 at the top to 7.1 km s −1 at the base. Further, σ for this layer is 0.27–0.28 towards the northern end of the profile but is low at about 0.24 beneath the central and southern parts of the profile, which is similar to the situation found in the northeast Tibetan plateau. The low values can be explained by felsic schists and gneisses in the upper part of the lower crust transitioning to granulite-facies and possibly also eclogite-facies metapelites in the lower part. Within the uppermost mantle, the average P velocity is about 8.10–8.15 km s −1 and σ is about 0.26. Assuming an isotropic situation, then a relatively cool (700–800°C) uppermost mantle beneath the profile is indicated. This would in turn indicate an intact mantle lid beneath the profile. An upper mantle reflector dipping from 104 km b.s.l., 120 km from the southern end of the profile to 86 km b.s.l., 155 km from the northern end of the profile has also been identified. The preferred model presented here for the crustal and lithospheric mantle structure beneath the Pamir calls for nearly horizontal underthrusting of relatively cool Indian mantle lithosphere, the leading edge of which is outlined by the Pamir seismic zone. This cool Indian mantle lithosphere is overlain by significantly shortening, warm Asian crust. The Moho trough that is a feature seen beneath some other orogenic belts, for example the Alps and the Urals, beneath the northern Pamir may mark the southern tip of the actively underthrusting Tien Shan crust along the Main Pamir thrust.

Description: The International Deep Profiling of Tibet and the Himalaya Phase IV (INDEPTH IV) active source seismic profile in northeast Tibet extends 270 km roughly north-south across the Songpan-Ganzi terrane, the predominantly strike-slip North Kunlun Fault (along the Kunlun suture), the East Kunlun Mountains, and the south Qaidam Basin. Refraction, reflection, and gravity modeling provide constraints on the velocity and density structure down to the Moho. The central Qaidam Basin resembles average continental crust, whereas the Songpan-Ganzi terrane and East Kunlun Mountains exhibit thickened, lower-velocity crust also characteristic of southern Tibet. The crustal thickness changes from 70 km beneath the Songpan-Ganzi terrane and East Kunlun Mountains to 50 km beneath the Qaidam Basin. This jump in crustal thickness is located ∼100 km north of the North Kunlun Fault and ∼45 km north of the southern Kunlun-Qaidam boundary, farther north than previously suggested, ruling out a Moho step caused by a crustal-penetrating North Kunlun Fault. The Qaidam Moho is underlain by crustal velocity material (6.8–7.1 km/s) for ∼45 km near the crustal thickness transition. The southernmost 10 km of the Qaidam Moho are underlain by a 70 km reflector that continues to the south as the Tibetan Moho. The apparently overlapping crustal material may represent Songpan-Ganzi lower crust underthrusting or flowing northward beneath the Qaidam Basin Moho. Thus the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust.

Description: Seismic data from central Tibet have been combined to image the subsurface structure and understand the evolution of the collision of India and Eurasia. The 410- and 660-kilometer mantle discontinuities are sharply defined, implying a lack of a subducting slab beneath the plateau. The discontinuities appear slightly deeper beneath northern Tibet, implying that the average temperature of the mantle above the transition zone is about 300 degrees C hotter in the north than in the south. There is a prominent south-dipping converter in the uppermost mantle beneath northern Tibet that might represent the top of the Eurasian mantle lithosphere underthrusting the northern margin of the plateau.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Kind, R -- Yuan, X -- Saul, J -- Nelson, D -- Sobolev, S V -- Mechie, J -- Zhao, W -- Kosarev, G -- Ni, J -- Achauer, U -- Jiang, M -- New York, N.Y. -- Science. 2002 Nov 8;298(5596):1219-21.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany. kind@gfz-potsdam.de〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/12424374" target="_blank"〉PubMed〈/a〉

Description: INDEPTH geophysical and geological observations imply that a partially molten midcrustal layer exists beneath southern Tibet. This partially molten layer has been produced by crustal thickening and behaves as a fluid on the time scale of Himalayan deformation. It is confined on the south by the structurally imbricated Indian crust underlying the Tethyan and High Himalaya and is underlain, apparently, by a stiff Indian mantle lid. The results suggest that during Neogene time the underthrusting Indian crust has acted as a plunger, displacing the molten middle crust to the north while at the same time contributing to this layer by melting and ductile flow. Viewed broadly, the Neogene evolution of the Himalaya is essentially a record of the southward extrusion of the partially molten middle crust underlying southern Tibet.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Nelson -- Zhao -- Brown -- Kuo -- Che -- Liu -- Klemperer -- Makovsky -- Meissner -- Mechie -- Kind -- Wenzel -- Ni -- Nabelek -- Leshou -- Tan -- Wei -- Jones -- Booker -- Unsworth -- Kidd -- Hauck -- Alsdorf -- Ross -- Cogan -- Wu -- Sandvol -- Edwards -- New York, N.Y. -- Science. 1996 Dec 6;274(5293):1684-8.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉K. D. Nelson, M. Cogan, C. Wu, Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USA. W. Zhao, J. Che, X. Liu, Chinese Academy of Geological Sciences, Beijing 100037, China. L. D. Brown, M. Hauck, D. Alsdorf, A. Ross, Institute for the Study of the Continents, Cornell University, Ithaca, NY 14853, USA. J. Kuo, Lamont Doherty Geological Observatory, Palisades, NY, 10964, USA. S. L. Klemperer and Y. Makovsky, Department of Geophysics, Stanford University, Stanford, CA 94305, USA. R. Meissner, Institut fur Geophysik, Christian-Albrechts-Universitaet zu Kiel, 24098 Kiel, Germany. J. Mechie and R. Kind, GeoForschungsZentrum Potsdam (GFZ), 14473 Potsdam, Germany. F. Wenzel, Geophysikalisches Institut, Universitaet Karlsruhe, 76187 Karlsruhe, Germany. J. Ni and E. Sandvol, Department of Physics, New Mexico State University, Las Cruces, NM 88003, USA. J. Nabelek, College of Oceanography, Oregon State University, Corvallis, OR 97331, USA. L. Chen, H. Tan, W. Wei, China University of Geosciences, Beijing, China. A. G. Jones, Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Ontario, Canada. J. Booker and M. Unsworth, Geophysics Program, University of Washington, Seattle, WA 98195, USA. W. S. F. Kidd and M. Edwards, Department of Geosciences, SUNY-Albany, Albany, NY 12222, USA〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/8939851" target="_blank"〉PubMed〈/a〉

Description: Knowledge of the rock types and pressure-temperature conditions at crustal depths in an active orogeny is key to understanding the mechanism of mountain building and its associated modern deformation, erosion and earthquakes. Seismic-wave velocities by themselves generally do not have the sensitivity to discriminate one rock type from another or to decipher the P-T conditions at which they exist. But laboratory-measured ratios of velocities of P to S waves (Vp/Vs) have been shown to be effective. Results of 3-D Vp and Vp/Vs tomographic imaging based on dense seismic arrays in the highly seismic environment of Taiwan provides the first detailed Vp/Vs structures of the orogen. The sharp reduction in the observed Vp/Vs ratio in the felsic core of the mountain belts implies that the α-β quartz transition temperature is reached at a mean depth of 24 ± 3 km. The transition temperature is estimated to be 750 ± 25°C at this depth, yielding an average thermal gradient of 30 ± 3°C/km.