Tectonophysics [T]

T43E   MCW:3004   Thursday  1340h

Dynamics of Orogenic Belts and Continental Plateaus V: Tibetan Margins and Plateau Growth

Presiding: L Schoenbohm Dr., Ohio State University; B Carrapa Dr., Universit�t Potsdam

T43E-01  

A New Kinematic Model for the Himalayan Development Based on Along-Strike Variations in Structural Geometry in the NW Indian Himalaya

* Webb, A G (awebb@ess.ucla.edu) , Dept. of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, UCLA, 595 Charles Young Dr. E., Los Angeles, CA 90095-1567, United States
Yin, A (yin@ess.ucla.edu) , Dept. of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, UCLA, 595 Charles Young Dr. E., Los Angeles, CA 90095-1567, United States
Harrison, T M (tmh@oro.ess.ucla.edu) , Dept. of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, UCLA, 595 Charles Young Dr. E., Los Angeles, CA 90095-1567, United States
Harrison, T M (tmh@oro.ess.ucla.edu) , Research School of Earth Sciences, Australia National University, Building 61, Mills Rd., Canberra, ACT 0200 Australia
Celerier, J (julien.celerier@anu.edu.au) , Research School of Earth Sciences, Australia National University, Building 61, Mills Rd., Canberra, ACT 0200 Australia
Burgess, W P (wburgess@ucla.edu) , Dept. of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, UCLA, 595 Charles Young Dr. E., Los Angeles, CA 90095-1567, United States
Gehrels, G E (ggehrels@email.arizona.edu) , Dept. of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, United States

Models for the development of the Himalayan orogen have focused on cross-sectional reconstructions based on the classic 3-layer division: the low-grade Lesser Himalayan Sequence (LHS) underthrust along the Main Central Thrust (MCT) beneath the high-grade Greater Himalayan Crystallines (GHC) and the low-grade Tethyan Himalayan Sequence (THS) overlie the GHC along the South Tibet Detachment (STD). Past research confirms this 3-layer paradigm throughout the central Himalaya but not across the western Himalaya where the GHC is discontinuously exposed. In the NW Indian Himalaya Late Proterozoic THS rocks can be traced continuously in places from the Indus-Tsangpo Suture in the north southward to the MCT. Our mapping along the uppermost Beas Valley demonstrates that the STD there is a gently north-dipping structure that is overturned to the south and merges with the MCT along a leading-edge branch line. The 3-layer paradigm persists to the north of this branch line, but to the south THS rocks are thrust directly over LHS rocks along the MCT. Current Himalayan models, e.g. wedge extrusion and channel flow, do not conform to this observed geometry. We propose a new kinematic model that includes the GHC as a thrust horse sandwiched between two decollements that merge in their up-dip directions. The dynamic evolution of the Himalaya implied by our model varies dramatically with previous models, which require focused erosion and the gradient of gravitational potential between Tibet and the Indian craton to play significant roles. Our kinematic model implies that the Himalaya grew via standard fold-and-thrust belt kinematics without the need of localized driving phenomena. To illuminate this model, we constructed a balanced cross-section across the NW Indian Himalaya. New analytical data, including U-Pb geochronology of igneous and detrital zircons help constrain the balanced cross-section. The geometry, kinematics, and geochronologic data reveal new correlations across the MCT discontinuity between both Early Proterozoic rocks and Late Proterozoic rocks, thus demonstrating that Himalayan Proterozoic rocks are likely derived from India.

T43E-02  

Late Miocene dismemberment of the Himalyan arc: Deformation of the Shillong Plateau, NE India

* Clark, M K (marinkc@umich.edu) , Dept. of Geological Sciences, University of Michigan, 2534 C. C. Little Building, 1100 N. University Ave., Ann Arbor, MI 48109, United States

A common feature of convergent plate boundaries is the self-organization of strain, exhumation and topography along discrete, arcuate boundaries. Deviations from this geometry can represent first-order changes in the static stress applied to the plate boundary that must affect how strain is partitioned within the interior of the orogen. The simplicity of the Himalyan fold-and-thrust belt seen along its central portions breaks down along the extremities of the arc, particularly in the east, where a 400 km long antiformal fold (Shillong Plateau) has developed in the Indian cratonic basement. It is arguably the largest active basement fold structure in the world, and is 5 to 10 times larger than its commonly-sited analogs found in the Laramide orogeny of the western US or the Sierras Pampeanas of the Andean orogen. New apatite (U-Th-Sm)/He data suggest deformation initiated in mid to late Miocene time, significantly earlier than was previously estimated from the sedimentary record alone. Helium ages collected within 150 meters depth from the plateau surface vary systematically with depth between 116 and 14 Ma and are suggestive of slow cooling during this time interval. Samples collected on a vertical transect along the southern limb of the anticline indicate rapid cooling between 14 and 8 Ma, which we associate with the initiation of fold growth. Geomorphic and fluvial analyses are used to assess the fault geometry that underlies the anticline. Fault geometry, combined with our helium data, allow us to estimate a horizontal shortening rate of 0.28 to 0.63 mm/yr. The timing of fold initiation suggests that deformation of the Shillong Plateau may be temporally linked to a number of kinematic changes within the Himalaya/Tibetan orogen, as well as along the eastern India/Burma- Sundaland plate boundary. These events include the onset of E-W extension in central Tibet, eastward expansion of high topography of the Tibetan Plateau, onset of rotation of crustal fragments in southeastern Tibet, and re- establishment of eastward subduction beneath the Indo-Burman ranges. We suggest that the coincidence of these tectonic events is related to the �dismemberment� of the eastern Himalayan arc, signifying a significant change in regional stress along the India-Eurasian-(Burma)-Sundaland plate boundaries that affects the deformation field more than 1000 km into to the orogen.

T43E-03  

Structural Evolution of the Ayishan Detachment: Implications for the Deformation of the Indus- Yalu Suture Zone

* Zhang, R (rzhang6@uh.edu) , Department of Geosciences, University of Houston, Room 312, Science & Research 1, Houston, TX 77204-5007, United States
Murphy, M A (mmurphy@mail.uh.edu) , Department of Geosciences, University of Houston, Room 312, Science & Research 1, Houston, TX 77204-5007, United States
Lapen, T J (tjlapen@uh.edu) , Department of Geosciences, University of Houston, Room 312, Science & Research 1, Houston, TX 77204-5007, United States
Sanchez, V (vsanchez2@uh.edu) , Department of Geosciences, University of Houston, Room 312, Science & Research 1, Houston, TX 77204-5007, United States

Geologic relationships exposed in the Ayishan in the west Tibet provide insight into the tectonic history of a region between the Tibetan plateau and west Himalaya. The Ayishan detachment mantles two large doubly plunging gneiss domes that are elongated towards the NW-SE. Our results show that development of the gneiss domes are associated with slip along northwest dipping low-angle normal faults and a thick (~300 m) top-to-the SE shear zone which juxtapose Cretaceous granitoids in its hanging wall against mylonitic, amphibolite facies orthogneisses and minor hornfels and schists in its footwall. Shear zone fabrics display ductilely deformed feldspar porphyroclasts indicating their formation at mid-crustal depths. The shear zone passes structurally downwards into weakly foliated granitoids. Mineral stretching lineations in the footwall rocks indicate the mean slip direction along the Ayishan detachment is S$60\deg$E. We correlate the hanging wall granitoids with the Gangdese batholith based on its composition and texture. The footwall orthogneisses contain large (2-7 cm) zoned feldspars, similar to some intrusive bodies exposed in the Gangdese batholith to the east of the Ayishan. The Ayishan detachment is broadly folded. On its east side the detachment merges and shares similar kinematics with the Karakoram fault. Moreover, the detachment forms the master fault of the Neogene Gar valley pull-apart basin. On the west side the detachment dips to the southwest structurally beneath the Great Counter thrust, a crustal-scale NE-directed thrust system. Field relations suggest that locally the Great Counter thrust and Ayishan detachment merge towards the west, placing rocks we correlate with the Gangdese batholith beneath the Tethyan sedimentary sequence as well as ophiolitic rocks within the Indus-Yalu suture zone. In order to explain these field relations we suggest the Ayishan detachment had an early phase of slip that together with the Great Counter thrust facilitated southwest emplacement of Gangdese batholithic rocks beneath the Tethyan sedimentary sequence. In this scenario the Ayishan detachment operates as a passive roof thrust. This model predicts the presence of a SW- directed thrust, possibly the Gangdese thrust, at structurally deeper levels. Timing constraints on the Great Counter thrust suggest this may have occurred during the Early to Middle Miocene. Subsequent development of the Karakoram fault in the Middle to Late Miocene reactivated the detachment and facilitated exhumation of the footwall rocks. This two-phase deformation history implies significantly more crustal thickening along the Indus-Yalu suture zone than has been previously estimated and indicates that rocks with an Asian affinity lie, at least locally, southwest of and beneath the surface trace of the suture zone. Recognition that the Ayishan detachment merges with the Karakoram fault system is more complex than has been previously interpreted and that slip estimates for it must take into account slip on the Ayishan detachment.

T43E-04  

Temporal Variation in Cenozoic Structural Style Along the Northern Margin of the Tibetan Plateau

* Darby, B J (bdarby@lsu.edu) , Department of Geology and Geophysics, Louisiana State University, E235 Howe-Russell Geoscience Complex, Baton Rouge, LA 70806, United States
Ritts, B D (britts@indiana.edu) , Department of Geological Sciences Indiana University, 1001 East 10th St., Bloomington, IN 47405, United States
Hourigan, J K (hourigan@ucsc.edu) , Department of Earth Sciences University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064, United States
Smith, J E (jaysmith@geol.lsu.edu) , Department of Geology and Geophysics, Louisiana State University, E235 Howe-Russell Geoscience Complex, Baton Rouge, LA 70806, United States
Bovet, P (pbovet@indiana.edu) , Department of Geological Sciences Indiana University, 1001 East 10th St., Bloomington, IN 47405, United States

Recent field work in Northern Qilian Shan (southeast of Yumen) and in the Hei Shan (50 km east of Yumen, 80 km beyond the commonly accepted end of the Altyn Tagh Fault) reveals a major mid-Cenozoic change in structural style. The Hei Shan contain a major ~east-striking, left-lateral fault system that bisects the range. This fault system juxtaposes strongly deformed Cambrian-Ordovician units with a Jurassic sedimentary cover (south) from Cretaceous strata (north). An excellent canyon exposure illustrates a very well-developed, sub-vertical, 80 meter wide, anastomosing fault zone with sub-horizontal striae and excellent left-lateral kinematic indicators. Approximately five kilometers to the east of the canyon exposure, the strike-slip fault intersects a Neogene sedimentary section which rests unconformably on disparate units both north and south of the fault zone. At this locale, the left-lateral fault consists of a single, poorly developed plane which has only been observed in the lower portion of the Neogene section. These cross-cutting relationships indicate large scale pre-Neogene displacement on the left-lateral fault system and limited post-Neogene slip. Based upon paleoflow indicators, Neogene conglomerate in the Hei Shan was derived from sources to the south, namely the Qilian Shan. We have begun a multidisciplinary (structural, sedimentologic, and thermochronologic) study of a NE-SW corridor in the Qilian Shan to determine timing and magnitude of Tertiary shortening. Geologic mapping from the range-front to ~80 km into the Qilian Shan reveals both north and south-directed thrust sheets that have accommodated approximately 30% Tertiary crustal shortening. Our current timing constraint for the initiation of shortening in the Northern Qilian Shan is onset of conglomerate deposition in the Miocene. Low- temperature thermochronologic transects have been collected from major thrust sheets and are currently being analyzed. Collectively, our data suggest that deformation along the northern margin of the Tibetan Plateau changed from large-scale, left-lateral faulting to crustal shortening and mountain building during the mid-Cenozoic.

T43E-05  

Steep Faults, Narrow Basins, and High Topography in the �Dulan-Chaka Plateau�: Observations and Implications for Growth in the Northern Tibetan Plateau

* Duvall, A (duvall@umich.edu) , Department of Geological Sciences, University of Michigan, 2534 C.C. Little Bldg., 1100 North University Ave., Ann Arbor, MI 48109-1005, United States
Clark, M (marinkc@umich.edu) , Department of Geological Sciences, University of Michigan, 2534 C.C. Little Bldg., 1100 North University Ave., Ann Arbor, MI 48109-1005, United States
Chuanyou, L (lchyou@hotmail.com) , Institute of Geology, China Earthquake Academy, Beijing, 100029 China
Dayem, K (Katherine.Dayem@colorado.edu) , Department of Geological Sciences, University of Colorado, Campus Box 399, 2200 Colorado Ave., Boulder, CO 80309-0399, United States
Farley, K (farley@gps.caltech.edu) , Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-25, Pasadena, CA 91125, United States

The importance of upper crustal shortening and its role in creating high topography in the Tibetan Plateau remains an important subject of debate concerning the dynamics of plateau evolution. Recently completed field work in the ``Dulan-Chaka Plateau'', the topographic highland east of the Qaidam basin and west of the Wenquan fault and Gonghe basin, yields several important constraints on upper crustal deformation along the northern Tibetan plateau margin. The Dulan-Chaka Plateau consists of several en echelon NW/SE trending mountain ranges that are bounded by steep, northeast-dipping reverse faults and separated by narrow Quaternary basins. These faults commonly place Triassic age arc-derived granitic, volcanic, and Paleozoic meta-sedimentary rocks over Neogene and Quaternary sediments. Patchy outcrops of Neogene mudstones and conglomerates are exposed throughout the Dulan-Chaka Plateau. Drainage patterns, conglomerate clast compositions, and structural geometry suggest that much of the preserved Neogene sediment is not syn-tectonic with range growth. The Neogene sediments, therefore, may represent remnants of a once continuous paleobasin that encompassed the modern Qaidam and Gonghe basins prior to dismemberment by the rise of the Dulan-Chaka Plateau. Lack of stacked thrust sheets, low angle faults, and flexural basins implies that the upper crust has not sustained significant horizontal shortening and crustal thickening; yet this region exists as a topographic highland with an average elevation of ~4500 m. Moreover, drainage pattern evolution and minimal range-basin relief support regional uplift over a lengthscale of several hundred kilometers as the main mechanism of topographic growth. One possible method of creating regionally high topography is by mid-lower crustal flow, a process evoked by other workers to explain both the eastern and southern Tibetan plateau boundaries. Alternatively, removal of a dense lithospheric root may explain the elevated landscape by buoyancy forces. The style of faulting and the topographic form of the Dulan-Chaka Plateau contrast with areas studied to the east. Thrust fault bounded ranges of the northeastern Tibetan Plateau create large, rhomb-shaped flexural basins. These basins contain several kilometers of sediment and often preserve growth strata as sediment was deposited during late Miocene and younger faulting episodes. Structural and depositional differences between the two regions are likely due to crustal anisotropies and show that the entire northern margin of the Tibetan plateau is not deforming in a uniform manner.

T43E-06  

Minimal Cenozoic Denudation of the Central Tibetan Plateau Revealed by low-Temperature Thermochronology

* Guynn, J (jhguynn@email.arizona.edu) , University of Arizona, Department of Geosciences Gould-Simpson Building #77, Tucson, AZ 85721, United States
Kapp, P (pkapp@geo.arizona.edu) , University of Arizona, Department of Geosciences Gould-Simpson Building #77, Tucson, AZ 85721, United States
Volkmer, J (jvolkmer@geo.arizona.edu) , University of Arizona, Department of Geosciences Gould-Simpson Building #77, Tucson, AZ 85721, United States
Carrapa, B (carrapa@geo.uni-potsdam.de) , University of Potsdam, Department of Geosciences, Golm, 14776 Germany
Heizler, M (matt@nmt.edu) , New Mexico Geochronological Research Laboratory, New Mexico Institute of Mining and Technology, Socorros, NM 87801, United States

Low-temperature thermochronologic data from crystalline rocks across central Tibet reveal mostly limited denudation and cooling since the Late Cretaceous - Early Eocene. Apatite fission-track analysis of the mid- Cretaceous Bange granite in the north-central Lhasa terrane indicates slow cooling since crystallization with a model age of ~62 Ma. Orthogneisses of the Amdo basement, located ~200 km northeast of the Bange granite along the Bangong suture, record pooled fission track ages of 72-74 Ma and coupled with K-feldspar Ar$^{40}$/Ar$^{39}$ data, suggest slow denudation since rapid exhumation during the Early Cretaceous Lhasa- Qiangtang collision. North of the Amdo basement ~100 km, an ~67 Ma granite from the Tanggula Shan in the central Qiangtang terrane yielded an apatite fission track age of ~53 Ma, despite being located in a region of relatively high relief (~1500 m) and active E-W rifting. These ages are similar to older fission track ages from the eastern Tibetan plateau, which fall in a range from 60-100 Ma. Cooling data from K-feldspar Ar$^{40}$/Ar$^{39}$ diffusion modeling from the northern Lhasa terrane in central and westernmost Tibet are also consistent, generally revealing Cretaceous episodes of rapid cooling followed by slow cooling in the Tertiary. Cretaceous- Eocene rapid cooling followed by slow cooling is also a characteristic of parts of the southern Lhasa terrane where there is a widely preserved angular unconformity beneath the Tertiary volcanic Linzizong Formation. Some areas of Tertiary upper-crustal exhumation are indicated by K-feldspar diffusion modeling and mid-Tertiary shortening and basin development in the north-central Lhasa terrane and the development of narrow, E-W trending Tertiary nonmarine basins in central Tibet. In general, however, the data suggest a relatively low-relief landscape characterized much of the present-day Tibetan Plateau prior to the Indo-Asian collision and shortening in Tibet in response to the Indo-Asian collision resulted in only minor exhumation. The lack of early-mid Tertiary rapid cooling in the interior of the plateau is evidence against the northward and eastward migration of a high- elevation margin, as the current margins show evidence of mid-late Tertiary cooling. The initiation of slow-cooling periods immediately follow episodes of relatively rapid cooling and major upper crustal shortening, which leads to the possibility that the crust was thick and the elevation high when the low-relief surface landscape developed.

T43E-07  

The neotectonic deformation and geomorphic features of the western Sichuan Basin

Jia, Q (jiaqiupeng2004@nju.org.cn) , Earth Sciences Departmen, Nanjing Unversity, Hankou Road 22#, Nanjing, js 210093 China
* Jia, D (djia@nju.edu.cn) , Earth Sciences Departmen, Nanjing Unversity, Hankou Road 22#, Nanjing, js 210093 China

Not only the high Tibetan Plateau topography is constructed after the India-Asia collision, the surrounding units are also involved into the rising Plateau or rejuvenate as the response to the collision. At the central eastern Plateau front (Longmen Shan), recent thermochonological studies display that the Longmen Shan has experienced a fast exhumation history from Late Miocene or Early Pliocene until today. However, the sharp topography and rare late Cenozoic sediments exposed in its foreland (the western Sichuan Basin) raise the argument that the crustal thickening in the eastern Plateau is mainly due to the lower crustal flow and the eastern Plateau topography is heavily influenced by the geometry of the western Sichuan basin. Thus, it is critical to examine the neotectonic deformation of the western Sichuan basin. We investigate the neotectonic deformation of the western Sichuan basin by means of geomorphic observations, digital elevation model, field mapping, seismic reflection profiles, well data, relocated microearthquakes, and paleoearthquake records. In the South Longmen Shan and western Sichuan Basin, the relocated microearthquakes align along the northeast-trending Longmen Shan thrust between Anxian and Dayi and diffuse into the basin south of Dayi. Correspondingly, to the south of Dayi, our preliminary works find several anticlines developing after Pleistocene. A NS-trending blind thrust has cut a Pleistocene alluvial fan near Qionglai, and the NE-trending antecedent structure is deformed by the fault too. Although we are still not sure whether the fault extends to the south of Xiongpo anticline or not, the length of surface trace is at least about 30 kilometers based on remote sensing data at the north of Xiongpo anticline. From the seismic reflection profiles, we also estimate the amplitude of the related fold is more than several hundreds meters. The NS-trending blind fault seems to attest for the seismic activity in the southwestern Sichuan basin, and to the east of this fault, the density of the relocated microearthquakes decreases apparently. Several smaller north-south striking anticlines also developed at the north of Qingyi River. To the south of Qingyi River, the structures are more complex. Most of the anticlines, except Emei anticline, strike north-south. The Qingyi River flows southeast without cutting through these folds except Emei anticline. Both the drainage pattern and the structure style of these folds suggest the folds should develop from the west to the east. Although the current investigations still do not permit us to identify the deformation time of these folds and the relationship between the folds and river, the strike of these folds raises the possibility that these structures also developed after Pleistocene. Summarizing the observations above, we suggest: (1) the structures trending north-south develop after Pleistocene and deform the antecedent northeast-trending structures which seem to be diachronous. (2) Thus, The western Sichuan basin does not behave as a stable craton ideally in the late Cenozoic, especially at the south of Dayi.

T43E-08  

Seismic Anisotropy in the Central Tien Shan and Geodynamic Implications

* Li, A (ali2@uh.edu) , University of Houston, 4800 Calhoun Rd., Houston, TX 77004, United States
Chen, C , University of Houston, 4800 Calhoun Rd., Houston, TX 77004, United States

The Tien Shan, located ~1000 km to the north of the India-Asia collision boundary, is the most active intraplate orogen on the earth. As a natural laboratory for studying mountain building processes, a variety of geological and geophysical studies have been conducted in the Tien Shan region. In this study, we have analyzed SKS splitting in the central Tien Shan and its vicinity. The data are from 17 teleseismic events recorded at 30 broadband seismic stations, 28 from a temporary network, the GHENGIS, and 2 from the Geoscope network. Fast polarization directions at most stations on the Tien Shan ranges are ENE-WSW, parallel to the mountain strikes. Such anisotropy can be largely interpreted as the coherent, strike-parallel displacement of the Tien Shan lithosphere in response to the NS shorting. Interestingly, anisotropy with NNE-SSW fast axes is observed at stations around the Issyk Lake and in the southern end of the central Tien Shan, where lithosphere is presumably strong and less deformed. The NNE-SSW fast direction is consistent with the surface motion of the central Tien Shan and is probably caused by the shear between the lithosphere and the asthenosphere on a regional scale, rather than due to small scale mantle convection or plume-related mantle flow as suggested in previous studies. In addition, there is no indication for the change of anisotropy across the Talasso-Fergana fault, a large strike-slip fault that separates the western and central parts of the Tien Shan. This observation, which is different from that in Tibet where anisotropy near major strike-slip faults tends to orient towards the fault strikes, suggests that the Talasso-Fergana fault is probably shallow and does not penetrate the lithosphere.