T33B-2041
Transition from potassic to sodic volcanism at ~12 Ma in the south-north striking graben: cessation of break-off and extension in south Tibet?
It has been recognized that ultra-potassic and potassic igneous rocks were dominant from 13 to 25 Ma in south Tibet, which is generally interpreted that break-off of the northward subducted Indian lithosphere or delamination of the thickened lithosphere resulted in eruption (or intrusion) of these ultra-potassic and potassic magmas (Miller et al., 1999; Maheo et al., 2002; Williams et al., 2001; Chung et al., 2005), as well as occurrence of the N-S striking extension faults ( at least before 13.5-14 Ma) on the Tibetan plateau (Coleman and Hodges, 1995; Blisniuk et al., 2001) and main elevation of the plateau in this period (e.g., 13- 18 Ma). Here we firstly report post-collisional Na-rich peralkaline volcanic rocks erupted at 12.3 (0.5) Ma within a south-north striking graben in south Tibet. These Na-rich rocks are phonolites with actual nepheline and aegirine-pyroxene minerals and significantly high Na2O/K2O ratios (higher than 1). Os-Nd isotopic and elemental compositions coupled with mineral composition analyses reveal that the Na-rich volcanic rocks cannot be generated by differentiation of the associated potassic or ultra-potassic magma; instead they were derived from a mantle source distinct from that of the latter, hinting that the mantle-derived volcanism in south Tibet experienced a temporary composition variation from potassic to sodic at about 12 Ma. Moreover, some Na-rich volcanic rocks erupted in 10-12 Ma were also found in eastern part of the Lhasa block in south Tibet. Furthermore, it has not been observed for a mantle-derived igneous rock younger than 10 Ma in south Tibet (Chung et al., 2005). Therefore, the observed transition from dominantly potassic to Na-rich magmatism represents an important tectonic event in south Tibet. We suggest that transition at about 12 Ma most possibly implies end of the break-off or delamination event, as well as temporary cession of E-W striking extension and the related uplifting process on the plateau. Thus, the south Tibetan plateau had possibly risen to its highest level before 12 Ma rather than at 2-3 Ma as suggested by other studies (Power, 1986; Li, 1991; Zheng et al., 2000). References: Blisniuk, et al., Nature, v. 412, 628-632 (2001) Chung, S.L. et al. Earth Sci. Rev., v. 68, 173-196 (2005) Coleman, M. and Hodges, K , Nature, v. 374, 49-52 (1995) Li, J., Quat. Sci. Rev., v. 10, 479-483 (1991) Maheo, G. et al.. Earth Planet. Sci. Lett., v. 195, 45-58 (2002) Miller, C.et al., Journal of Petrology, v. 40, 1399-1424 (1999) Power, C. M, Earth Planet. Sci. Lett., v. 81, 79-94 (1986) Williams, H., et al., Geology, v. 29, 339-342 (2001) Zheng, H.B. et al., Geology, v. 28, 715-718 (2000).
T33B-2042
Evidence for Cenozoic Multiple-Phase Tectonic Evolution of the Northern Tibetan Plateau from the Northern Qaidam Basin and Hexi Corridor, NW China
The Qaidam basin and Hexi Corridor, two major basins lying within and adjacent to the northern Tibetan Plateau margin, are separated by the Qilian Shan fold-thrust belt and bounded by the Altyn Tagh fault (ATF) to the northwest. Understanding the formation and evolution of these basins provides insight into the tectonic history of their bounding tectonic features and the northern Tibetan Plateau. Here we present detailed basin analyses on eight Cenozoic sedimentary sections in these two basins (six in the Qaidam basin and two in the Hexi Corridor). Four sections from the northern Qaidam basin contain the most continuous and oldest Cenozoic sediments. Lithostratigraphically, the sedimentary succession of these four sections can be divided into four distinct units and each unit corresponds to a unique tectonic phase of northern Tibetan Plateau. The conglomeratic Unit 1 spanning the Paleocene (?)-Eocene is characterized by the strongly southwest-directed paleocurrent pattern and framework modes of sandstone showing recycled orogen provenance. We interpret this unit to document the early uplift in the south Qilian Shan, synchronous with, or immediately after, the Indo-Asian collision. The fluvio-lacustrine fine-grained Unit 2 from the Oligocene through the early Miocene suggests the most extensive lacustrine deposition in the Qaidam basin. This is synchronous with previously documented initiation of large-scale strike-slip faulting of the ATF, which may have ponded lacustrine systems in the Qaidam basin and prevented outflow toward the south Tarim basin. The Middle Miocene to the Late Miocene sandstone and conglomerate (Unit 3) and the Pliocene conglomerate (Unit 4) comprise a coarsening-up succession. Together with south-directed paleocurrent indicators and clast composition which matches source areas in the Qilian Shan, Units 3 and 4 indicate the proximal strong uplift and exhumation of the Qilian Shan. This sequence of tectonic events is also supported by data from the northeastern Qaidam basin and Hexi Corridor. Cenozoic deposition there initiated after the Middle Miocene. In those areas, Miocene sedimentary rocks are separated from the pre-Cenozoic rocks by thick paleosols with well-developed calcite nodules, indicating long periods of exposure on the basal unconformity, and are characterized by coarsening upward synorogenic packages derived from the Qilian Shan. These sedimentary records match very well with the independently defined change in slip rate on the ATF from fast to slower in the mid-Miocene. In particular, these results show that steepening of drainage basins and exhumation in the ranges around the Qaidam basin and Hexi Corridor occurred at approximately the change to slower slip rates on the ATF, consistent with a hypothesized change from earlier plate-like behavior to crustal thickening, topographic uplift and rock exhumation. Together, these emerging results demonstrate three distinct phases of tectonic evolution in the northern Tibetan Plateau: (1) the Paleocene (?)-Eocene early uplift in the south Qilian Shan near the ATF; (2) large-scale strike-slip faulting of the Altyn Tagh fault from the Oligocene through the early Miocene, and (3) extensive northward and southward- directed crustal shortening in the Qilian Shan since the Middle Miocene.
T33B-2043
Statistical Analyses of d18O in Meteoric Waters From the Western US and East Asia: Implications for Paleoaltimetry
Questions on the timing of Tibetan Plateau uplift and its associated influence on the development of the Indian and Asian monsoons are best addressed through accurate determinations of regional paleoelevation. Previous determinations of paleoaltimetry utilized the stable isotopic composition of paleo-meteoric waters as recorded in various proxies (authigenic minerals, fossils, etc.), in combination with empirically and model determined elevation isotopic lapse rates. However, the applicability of these lapse rates, derived principally from orogenic settings, to high continental plateaus remains uncertain. Our research aims to gain a better understanding of the potential controls on the δ18O composition of meteoric waters over continental plateaus through a principal component analysis (PCA) of modern waters from eastern Asia and the western US. In particular, we investigate how various environmental parameters (elevation, latitude, longitude, MAP, and MAT) influence the δ18O composition of these waters. First, these analyses reveal that elevation and latitude are the primary controls on isotopic composition in all regions investigated, as expected. Second, PCA results yield elevation lapse rates from orogenic settings (i.e. Sierra Nevada, Himalaya) of ~ -3‰/km, in strong agreement with both empirical and Rayleigh distillation model derived lapse rates. The Great Plains of the US, although not an orogenic setting, represents a monotonic topographic rise, and is also characterized by a ~ -3‰/km lapse rate. In high, arid plateau regions (Basin and Range, Tibet), however, elevation lapse rates are ~ -1.5‰/km, half that of orogenic settings. An empirically derived lapse rate from small source area springs collected over a 2 km elevation change from a single mountain range in the Basin and Range yields an identical rate. One clue as to the source of this lowered lapse rate is eastern China, which also displays an elevation lapse rate of ~ -1.5‰/km, despite being a relatively low elevation, humid region. All three regions of lowered lapse rates are dominated by convective storms, which violate basic assumptions of simple Rayleigh distillation. The similarity of lapse rates between these regions suggests that convective storm systems may result in a predictable change in elevation lapse rates. Third, the effect of latitude changes on isotopic composition should be considered in major orogenic systems. In the western US, best-fit linear models reveal latitude lapse rates of ~ -0.5‰/°N, thus significant northward or southward tectonic translations may be misinterpreted as elevation changes. The mixing of multiple moisture sources over eastern Asia appears to result in a polynomial function for latitude lapse rate. The determination of the effects of this latitude lapse rate on paleoelevation histories is ongoing. Finally, comparison of PCA models of modern isotopic composition with actual meteoric water values offers an opportunity to assess the accuracy of paleoelevation estimates. Predictive capabilites of our derived models are significantly better in orogenic settings (± ~950m 2σ) than over continental plateaus (± ~1950m 2σ). These statistical models enhance our understanding, and the predictive capability, of stable isotopes over high, arid plateaus. In particular, they point to the controlling effect of convective storms on elevation lapse rates, and thus the potential effect of the growth of the Tibetan Plateau, and onset of monsoonal climate conditions, in driving time-dependent elevation isotopic lapse rates.
T33B-2044
Timing and Magnitude of Upper Crustal Shortening in the Gonghe Basin Region of the Northeastern Tibetan Plateau
Characterizing the space-time patterns of the growth of high topography in Asia is an important step toward a deeper understanding of the mechanics of intracontinental deformation and its influence on global climate. In northeastern Tibet, there is emerging evidence that a number of ranges around the margins of the plateau experienced a pulse of deformation in the Late Miocene (ca. 12-8 Ma). It remains uncertain, however, whether this event was confined to the margins of the plateau, or whether interior regions deformed synchronously. Here we present a preliminary assessment of the timing and magnitude of upper crustal shortening along the margins of the Gonghe-Tongde basin complex. The Gonghe basin is located at the boundary between the high plateau of central Tibet and the southern flank of the Qilian Shan, and as such it is well-suited as a site to begin reconstructing patterns of plateau growth. The basin is overthrust by two regionally-extensive fault systems, the Qinghai Nan Shan (QNS) fault system on the north side and the Gonghe Nan Shan (GNS) fault system on the south side. Both fault systems are associated with deformation of Tertiary strata; variations in dip, sedimentary facies, and provenance are used to interpret the onset of growth along the margins of the Gonghe basin. A combination of the architecture of pre- and syntectonic basin strata, field measurements of fault dip, fault plane solutions, and topographic analysis of fold backlimbs for the GNS and QNS leads us to infer that the fault systems are a) trishear fault propagation style thrust faults and b) south vergent, with ~30 degree fault ramps soleing into a gently dipping decollement. Reconstructions of fold evolution suggest that the area has experienced > 5 km of upper crustal shortening in the late Cenozoic. A combination of magnetostratigraphy, biostratigraphy and cosmogenic burial ages provides preliminary age control. South of the GNS, a 250 m thick package of growth related strata are found to be 3.4 - 0.5 Ma. A 500 m thick exposure of growth strata on the north side of the range is also interpreted to be Plio-Quaternary in age. At present, however, we can only place a minimum bound on the onset of deformation of ca. 4-5 Ma. In light of a companion study interpreting the onset of deformation along the QNS at >= 5-7 Ma (Zhang H., in review), deformation of the Gonghe region appears to be slightly more recent than at the plateau margins. Historic seismicity and deformation of late-Quaternary alluvial surfaces on both fault systems indicate that these structures have remained active into the Pleistocene.
T33B-2045
Structural Evidence for the Leading Edge of the Greater Himalayan Crystalline Complex in the Kathmandu Region, Central Nepal Himalaya
Current models for the tectonic evolution of the Himalaya - wedge extrusion, channel flow, and tectonic wedging - make divergent predictions for the terminations of major faults and tectonic units. To test these models, field mapping was conducted along the northern margin of the Kathmandu Nappe in central Nepal. This work reveals the leading edge of the Greater Himalayan Crystalline complex, i.e., the high grade crystalline core of the Himalaya. The Kathmandu Nappe is dominated by three sequences that are folded in a syncline and bounded below by the Main Central thrust. These sequences are: (1) a southward tapering wedge of kyanite-bearing gneisses overlain by (2) garnet-bearing pelitic and psammitic metamorphic rocks succeeded along a sedimentary contact by (3) unmetamorphosed Lower Paleozoic Tethyan strata. Structural mapping along the contact between the kyanite-bearing gneisses and the garnet-bearing metamorphic rocks reveals a dominantly top-north shear zone (featuring S-C mylonite, extensional crenulation cleavage, and shear band fabrics) that intersects the Main Central thrust to the south. This shear zone is interpreted as the southern extension of the South Tibet detachment. It follows that (1) the kyanite gneiss wedge represents the Greater Himalayan Crystalline complex and (2) the intersection line of the South Tibet detachment with the Main Central thrust represents the leading edge of this unit. The intersection line has also been recently identified in the western Himalaya, so this line appears subparallel to the arc of the orogen. As the South Tibet detachment is generally sub-horizontal to north-dipping and its southern tip (the intersection line) is locally preserved in the central and western Himalaya, this structure was not exposed during its active motion. Only tectonic wedging models can accommodate the 3-D geometry of the Greater Himalayan Crystalline complex in the central and western Himalaya.
T33B-2046
EASTERN HIMALAYAN-TIBETAN ISOSTASY
We estimate the variation in strength of the Eastern Himalayan- Tibetan lithosphere from 2D gravity and topography coherence functions calculated with the Thomson-Slepian Multitaper technique. The coherence shows evidence of structural weakness of the plate in the N-S direction, aligned with the direction of maximum compression. The Tibetan plateau is characterized by an Effective Elastic Thickness (Te) that is much lower than would be expected from its large crustal thickness and the additional strength from a coherently folded mantle inferred from shearwave splitting. The seismogenic thickness indicates that only the upper crust is contributing to the elastic strength and not the mantle. The predominant weakness of the lithosphere in the N-S direction revealed from the anisotropy in the coherence function suggests that the mantle may not be directly coupled to crustal deformation beneath the Tibetan plateau
T33B-2047
Himalayan Hinterland-Verging Upper Crustal Folding During Foreland-Directed Mid-crustal Plastic Flow: Insights from Centrifuge Analog Modeling
The orogenic superstructure (SS) and infrastructure (IS), corresponding respectively to the upper and middle crust, constitute two levels in a mountain belt that may portray contrasting tectonic histories. In the Nepal Himalaya, N-verging back folds opposite to the orogenic vergence dominate the SS. Three end-member models for this hinterland-directed back folding have been proposed: (1) gravity-induced sliding and drag folding along a N-dipping normal fault; (2) early shortening of the SS overprinted by detachment faulting and IS ductile stretching; and (3) crustal thickening and drag folding due to coeval ductile extrusion of the IS. The viability of these competing models is tested using dynamically scaled analog centrifuge modeling. Models consist of two main units with similar density: low viscosity silicones simulate IS partial melt and leucogranites, and silicone-modeling clay microlaminates simulate the SS. A foreland erosional front is created to simulate the steep Indian topographic front. Our modeling suggests that SS folding can only occur during bulk shortening accompanied by IS thickening prior to or early in IS flow. Focused erosion then instigates lateral flow and stretching of the IS, accompanied by decoupling of the SS, and transposition of SS lower structural level into the detachment zone. Decoupling occurs at the IS-SS interface, and separates a SS dominated by older folds, and an IS characterized by younger horizontal transposition and stretching. Lateral flow of the IS locally modifies fold vergence in the SS. The fold asymmetry is thus controlled by the efficiency of coupling between IS and SS; a low viscosity at the IS-SS interface favors complete decoupling and hinders modification of fold vergence, whereas a higher viscosity IS-SS interface favors fold vergence modification. Modeling suggests that Himalayan hinterland- verging folds preserved in Nepal are the product of early shortening of the SS followed by local modification of fold geometry when the IS subsequently stretches and flows as a response to focused denudation and melt-enhanced mid-crust weakening.
T33B-2048
Petrology and structure of the Renbu gneiss dome, southeast Tibet
The Renbu granitic gneiss dome is the most northerly and one of the easternmost domes in the chain of North Himalayan gneiss domes. The Renbu dome comprises upper amphibolite-facies metasedimentary rocks of the lower Tethyan Himalaya Sequence intruded by a large, relatively undeformed leucogranite body. Paragneisses and schists have the assemblage Qtz + Kfs + Pl + Bt ± Ms ± Grt ± St ± Hbl ± Gr ± Chl and lack migmatite; leucogranite dikes locally intrude the country rock along a sharp contact between country rock and the leucogranite body. Structural data were collected from the northeastern exposure of the dome: the mean pole to foliation trends 203° and plunges 57°; stretching lineations define a great circle and are folded about an axis trending 292° and plunging 47°. The Renbu gneiss dome is likely an upwelling of a ductile mid-crustal channel that conveys partial melts of Indian crust beneath the Tibetan plateau to the topographic/erosional front of the Himalaya further south, in the Greater Himalayan Sequence. U-Pb SHRIMP dating of zircon from the Renbu leucogranite yields the youngest ages reported for Himalayan granites at ~7 Ma; this young granite age may constrain the timing for the end of active channel flow in the latest Miocene or suggest channel flow is modern, ongoing process.
T33B-2049
U-Pb and Trace Element Data From the Renbu Gneiss Dome, Southeast Tibet
The Renbu gneiss dome is located approximately 20 km south of the Yarlung-Zangbo suture zone just west of the Yadong-Gulu Rift system in southeast Tibet, and is one of the eastern most gneiss domes in the north Himalayan gneiss dome chain. It consists of a large, relatively undeformed leucogranite body at the core of the dome with a small number of leucogranite dikes intruding the country rock gneiss. Leucogranites intrude metasedimentary rocks of the Tethyan Himalayan sequence that have been metamorphosed to upper amphibolite-facies. U-Pb SHRIMP dating of zircon yield concordant ages ranging from Late Archean to Early Paleozoic for metasedimentary rocks (2.5 Ga to 480 Ma) and Late Miocene ages for granitic intrusions (11 to 7 Ma); these Late Miocene granites are the youngest reported Himalayan granites to date. Late Archean to Early Paleozoic ages are from zircon domains with consistently low U contents (70 to 800 ppm), while the U contents of Late Miocene ages range from 120 to 23000 ppm. Trace element data for metasedimentary samples have a flatter HREE pattern than granitoid samples, indicating that these zircon formed in the presence of garnet. Late Miocene ages for Renbu granitoids correspond to the younger ages of similar leucogranite bodies exposed throughout the Himalaya.
T33B-2050
Regional Tectonic And Climatic controls On The Evolution Of The Song Hong-Yinggehai And Qiongdongnan Sedimentary Basins, South China Sea.
Much of the sediment eroded from eastern Tibet has been preserved in the Song Hong-Yinggehai (SH-Y) and Qiongdongnan (Qi) sedimentary basins of the NW South China Sea which together form one of the largest sedimentary basins in SE Asia. Sediments preserved in these basins can be used to reconstruct erosion and in turn constrain the timing of regional tectonism and plateau uplift as well as for the history of intensification of the Asia Monsoon. Here we present some new results of the SH-Y and Qi Basins based on recently released 2D, multichannel seismic data. The SH-Y Basin is situated in along the SE extension of the Red River Strike-slip Fault (RRSF). It is a pull- apart basin and is bounded by a series of transtensional faults, especially the RRSF on the southwestern side and by the No.1 Fault to the northeast. The maximum sedimentary thickness in the basin centre is estimated to be ~20 km. Cross-sections through the basin show that rapid subsidence occurred from the Eocene until ~21 Ma. Two basin inversion phases are indicated by strong erosion, deformation and reverse faulting signatures observed on seismic profiles: the first event likely occurred at ~31 Ma and was triggered by the motion of the RRSF while the later Mid-Miocene uplift (~15 Ma) correlates with end of motion on the RRSF, the cessation of seafloor spreading and the generation of the Deep Regional Unconformity in the southern South China Sea. The Qi Basin developed on the northern rifted margin of South China Sea. The maximum sedimentary thickness of this basin was estimated to be ~7 km. Within the Qi Basin, a relatively large Late Miocene canyon is observed, which developed in a NE-SW direction, widening and deepening towards the SW. However, the mechanism for formation of this canyon at this time is still uncertain. Isodepth contoured maps show that most of sediment delivered from the Red River has been trapped in the central and northern SH-Y Basin until the middle Pliocene. The aggradation and progradation pattern of sediment in the Qi Basin during the middle Pliocene-Present demonstrates more sediment delivered into the Qi Basin during this time. This anomalous increase in sedimentation rate was possibly triggered by a pulse of uplift and erosion on Hainan Island and by over-spilling of the sediment from the SH-Y Basin.
T33B-2051
A high-resolution record of late Neogene environmental changes in southern Tibet
Elucidating the role of Tibetan uplift in controlling regional climate requires datasets documenting the spatial and temporal patterns of environmental changes in and around the plateau. Here we present high-resolution oxygen isotope data spanning the time period from 7.2 Ma to 3.2 Ma from the Gyirong Basin in southern Tibet. The data reveal three major catchment-scale events in the late Miocene and Pliocene that significantly altered the regional hydrological regime and environment. The 1st event occurred right before 7.2 Ma that initiated a paleo-lake system. The 2nd event at ~5.4 Ma, is indicated by an abrupt decrease in δ 18O values of lacustrine carbonates and a change in sediment mineralogy. The 3nd event happened around 3.2 Ma, signified by the disappearance of the paleo-lake in the basin. We suggest that these environmental events reflect tectonic events in this part of the Himalayas during the late Miocene and Pliocene. Paleotemperature estimates for the latest Miocene in the area, based on the δ 18O values of fossil enamel and bones, are significantly higher than the present-day mean annual temperature in the basin, implying that the basin was at a lower elevation in the latest Miocene than today.
T33B-2052
Drainage Reorganization and its Effect on Paleoaltimetry Studies
For more than a decade, stable isotopes have been used to reconstruct the paleoaltimetry of orogens throughout the world, including that of the Tibetan Plateau. Such studies use the oxygen or hydrogen isotope composition of authigenic minerals as a proxy for past altitudes, and often attribute isotopic shifts in these minerals over time to the growth of local topography. Though such studies generally account for factors such as evaporation, temperature, and diagenesis that can influence isotopic composition of authigenic minerals, the synorogenic reorganization of regional drainage patterns is rarely if ever considered. Using strontium, oxygen and carbon isotope data, this study demonstrates that rearrangement of drainages accompanying the rise of large continental plateaux can have strong effects on the isotopic composition of waters in distal basins. Although oxygen and hydrogen isotopes record changes in the hypsometric mean elevation of the waters flowing into intermontane basins, the elevation of the basins themselves may remain constant. Moreover, the boundaries of the drained area may evolve to include or exclude waters from different atmospheric source regions or waters that have undergone evaporation. As such, isotopic studies interpreting the rise of Tibet, for example, should incorporate data from multiple isotopic systems and sample localities in order to distinguish signals of regional surface uplift from hydrologic ones.
T33B-2053
Sequence Stratigraphy and Frequency Analysis of the Zhada Basin, SW Tibet
Zhada basin is a large late Miocene – Pleistocene extensional sag basin in the Tethyan Himalaya of southwestern Tibet. Sequence stratigraphy in the basin reveals a long-term tectonic signal in the formation and filling of Zhada basin. The sedimentological record of the Zhada Formation also archives higher frequency cyclical expansion and contraction of a large paleolake. Expansion and contraction of lakes and wetlands has been linked to variability in the strength of the Asian monsoon and thought to be modulated by orbital cyclicity. In order to determine the forcing mechanism for Zhada paleolake expansion and contraction we created a wave form by assigning numerical values to the various depositional environments and applied spectral analysis to this record. Depositional environments in the South Zhada measured section were identified at 0.5 m increments or where the depositional environment changed. The result is a clipped waveform with uneven sample spacing and temporal resolution better than 4,000 yrs. In addition to a peak at 91.7 kyr (95% confidence level), spectral analysis reveals a peak at 22 kyr (85% confidence level). These are within 1/2 bandwidth (6 dB bandwidth = 2.4) of the eccentricity and precession frequencies. The record of Milankovitch cycles in Zhada basin implies that global climate drove lake and wetland expansion and contraction in the southern Tibetan Plateau from the late Miocene to the Pleistocene.
T33B-2054
Pre-35 Ma Na-rich peraluminous leucogranites in the Yardoi gneiss dome, southern Tibet
The Northern Himalayan Gneiss Dome (NHGD) consists of a series of semi-continuous east-west trending gneiss domes within the Tethyan Himalayan Belt. The Yardoi gneiss dome, the easternmost among these domes, consists of garnet two-mica gneiss, garnet amphibolite,Cenozoic two-mica granite and leucogranite. New SHRIMP zircon U/Pb dating show that the Yardoi leucogranite formed at 35.5±1.1Ma, which is older than those similar leucogranites to the west (commonly with ages < 25 Ma). We have performed bulk chemical and Sr and Nd isotope analyses on a suite of leucogranites and the wall-rocks to characterize their geochemistry and evaluate their formation mechanism. These data show that: (1) a majority of the Yardoi leucogranites are of Na-rich peraluminous granite; (2) similar to the wall-rock gneisses, the Yardoi leucogranites are enriched in LILE (K, Sr, Rb, Ba, and Th), but highly depleted in Ti, Y, Yb, Sc, and Cr; (3) as compared to those either in the gneisses or in the amphibolites, the Yardoi leucogranites are depleted both in LREE and in HREE. However, they are enriched in LREE relative to HREE;(4) the Yardoi leucogranites have initial Sr isotope compositions (0.71195~0.71934), similar to those of the amphibolites, whereas their ƒÕNd(i) values (-8.9~-15.0) are between those in the amphibolites and the gneisses. Simple mixing calculations indicate that partial melting of a source mixed of garnet amphibolite with subordinate pelitic gneiss could account for the formation of the Yardoi Na-rich peraluminous melts. Though both H2O-fluxed melting of metapelite at high pressures (~10 kbar) and amphibolite parting melting can produce Na-rich peraluminous granitic melts, the ~35 Ma anatectic event in the Yardoi gneiss dome was dominated by amphibolite partial melting. This event might be a major factor led to transition from compressional to extensional deformation in the overthickened Himalayan belt.
T33B-2055
Coupled Low-temperature Thermochronometry and Ar Dating of Fault Gouge: New Evidence of Early Onset (~45 Ma) of Deformation Along the Northeastern Margin of the Tibetan Plateau
Understanding complex interactions among climate, erosion and tectonics is hampered by a lack of independent measure of faulting, erosion, and elevation change related to mountain building. The most common approach to measure erosion rates in compressional settings is by use of low-temperature thermochronometry; however correlation of erosion events with fault motion is complicated by climatic forcing. A separate technique, Ar/Ar dating of clays in fault gouge, provides direct timing of fault activity, though whether this age represents fault onset, end of fault motion, or some stage in between is poorly understood. We present He cooling and fault gouge ages from a single locality in northeastern Tibet in order to relate timing of fault motion with the erosion record and thermal conditions of hanging wall rocks. The West Qinling reverse fault is one of the longest, most continuous faults in northeastern Tibet. Apatite (U-Th)/He cooling ages spanning ~3 km of structural depth within the hanging wall of this fault show old ages/little exhumation in the Jurassic and early Cenozoic followed by an increase in apparent erosion rate at ~45 Ma that was sustained until at least 13 Ma. Timing of brittle faulting along this structure is determined by dating several size fractions of clay from fault gouge that represent variable populations of two illite polytypes: detrital clay (2M1) derived from wall rock and authigenic clay (1Md) formed in the fault zone during faulting. Preliminary results show that the purely authigenic component formed at ~50-45 Ma and that the purely detrital component formed at ~230 Ma; thus suggesting Middle Eocene faulting and a Middle Triassic age of the source area of wall rocks. Coupled results from the two datasets supports the interpretation that 1) the W. Qinling fault initiated at 45 Ma and continued until at least Middle Miocene time and 2) formation of authigenic clay in the fault zone occurred at temperatures of ~150° C. Lack of overprinting of younger clay ages at this site likely indicates that rocks were out of the thermal window for authigenic clay formation during later faulting episodes. Paired together, thermochronometry and clay dating in fault gouge provide an unambiguous interpretation of fault history difficult to infer from the erosion record or fault age alone. Results from this study emphasize the value of this new approach and provide the first direct evidence of Middle Eocene compressional fault motion in northeastern Tibet, which we relate to onset of Indo-Asian collision.
T33B-2056
Late Miocene - Pliocene exhumation of the Qinghai Nan Shan (Northeastern Tibetan Plateau) constrained by adjacent Chaka basin magnetostratigraphic architecture
Magnetostratigraphy from the Chaka basin in northeastern Tibet suggests that sedimentation began near 10 Ma, and sedimentary facies suggest that the adjacent Qinghai Nan Shan immediately north developed at that time, tens of millions of years after India and Eurasia collided. A fining-upward sequence and provenance analysis indicate a growing transport distance for southeastward flowing paleo-drainages from ~10 Ma to 6.4 Ma. The disappearance of granite gravel at 7.4 Ma, eroded from the southern Qinghai Nan Shan to the north, indicates that the paleo-Qinghai Nan Shan temporarily became isolated from the Chaka basin, potentially by burial or partial burial of the lacustrine sediment. A change in paleocurrent direction, from south-southeastward to southwestward flow, along the mapped section at 6.4 Ma agrees with the reappearance of granite clasts provided by the southern Qinghai Nan Shan. Following this re-exhumation signal within the basin deposits, a coarsening-upward architecture, together with enhanced granite gravel input into the Chaka basin, indicates the decrease in transport distance from 6.4 Ma to ~3.0 Ma, which might represent increased relief and/or horizontal shortening during that period, even until present. Finally, by preliminary comparison of landscape characteristics for four nearby basin sections in northeastern Tibet, we suggest that the temporal variations of conglomerate deposition might reflect slightly different responses to regional climate change due to differences of transport distances and/or in relief. The onsets of conglomerate deposition at 4.2 Ma in Chaka basin, 3.6 Ma in Guide basin, and at 3.6 - 4.5 Ma in Linxia basin precede that at 2.5 Ma in Qaidam basin perhaps due to shorter transportation distances between ranges and the first three basins and higher local relief than those for the much wider Qaidam basin.
T33B-2057
Late Quaternary strike-slip features along the western segment of Haiyuan-Qilianshan fault, NE Tibetan Plateau
Slip rates of major active strike-slip faults within and around the Tibetan plateau provide constraints for understanding the dynamics of continental deformation in general, because large slip rates can be taken to imply localized deformation between rigid blocks, and low slip rates on faults are more consistent with distributed deformation. As one of important active strike-slip faults in northeastern margin of the Tibetan Plateau, the Haiyuan-Qilianshan active fault has been argued to exhibit fast slip rates more than 10 mm/yr along much of its length, especially along the middle-eastern segment of Lenglongling and Maomaoshan faults, and plays a central role in models for eastward extrusion of Tibetan lithosphere. But, no one has attempted to study the strike-slip features and determine the slip rates of the western segment of Haiyuan- Qilianshan fault, about 260 km length of the Halahu fault segment, because of its remoteness and high elevation. Our recent investigate indicate that this fault segment consists of six subparallel fault strands in a left-stepping or right-stepping en echelon arrangement. The late Quaternary activity on the fault has been recognized by a series of left-laterally offset geomorphic features, and by obvious fault scarps along the fault. Combining the terrace riser offsets with terrace ages dated by 14C and OSL techniques, we determine the strike slip rates along the Halahu fault, the main fault segment from Ebo, Qilian to Jiangcang to be ~3-4 mm/yr, and at the western termination of the fault from Muli to Halahu, the rate drops to only about 1.0¡À0.2 mm/yr. These data mean that the slip rate along the fault does not remain uniform but instead reveal gradients in displacement similar to those expected at fault tips. The low slip rate of Halahu fault is consistent with the slip rate of ~4.5¡À1.0 mm/yr on the Haiyuan fault, the eastern segment of Haiyuan-Qilianshan fault. The relatively low slip rate suggests this fault does not transfer a significant portion of the convergence between India and Asia out of India's path into Eurasia, but merely redistributes crustal thickening.
T33B-2058
Extrustion vs. convergence? Two-phase late Tertiary growth of the Laji-Jishi Shan, NE Tibet
The northeastern margin of the Tibetan Plateau near Xining, Qinghai Province, China, has a preponderance of curvilinear mountain ranges like the Laji-Jishi Shan where an east-trending segment bends into south- trending segment (or vice-versa). It is debatable whether these curved ranges were created solely by one tectonic regime or are composite structures. Utilizing low-temperature thermochronology to decipher temporal variations in the nature and vergence of range growth provides a time-series of how sustained, far-field Indo-Asian convergence was manifest in northeastern Tibet throughout the Cenozoic. Ostensibly, the Laji- Jishi Shan has the appearance of a pop-up structure at the scale of the entire range. But a closer look at the corner of the range reveals north- and south-vergent thrust faults systematically cut by east- and west- vergent thrust faults. Apatite (U-Th)/He data vary from elevation-invariant ages of ~22 Ma at the northeast corner to elevation-invariant ages of ~8 Ma along the south trending portion of the range. The difference in ages can be explained by initial north-verging compression during early Miocene time that is overprinted by later east- and west- vergent thrusting by at least late Miocene time. Alternatively, the difference in ages reflects greater or longer-lived exhumation along the southern portion of the range compared to the north. This suggests that initial growth of the Laji-Jishi Shan was perhaps one of the last major expressions in this region of a middle Tertiary phase of predominantly unidirectional deformation that mimicked India-Asia convergence in sense. Later, east-vergent growth of the Laji-Jishi Shan occurred in the Late Miocene, potentially due to lateral extrusion and expansion of the plateau towards the east inducing localized shortening and exhumation.
T33B-2059
A 3-D finite element model for lithospheric deformation during the Indo-Asian collision
The Indo-Asian collision in the past 50-70 Ma has led to strong lithospheric deformation in the Himalayan-
Tibetan plateau and the surrounding regions. Previous models using the viscous thin sheet approximation
has illustrated some of the basic physics of the collisional tectonics, but the two-dimensional representation
and the lack of major fault zones in the viscous thin sheet model have also left many important details of the
collisonal tectonics uncertain. We have developed a fully three-dimensional (3-D) finite element model to
calculate the finite strain of the lithospheric deformation during the Indo-Asian collision. Treating the
deforming lithosphere as a 3-D viscous layer, our model considers both vertical and lateral variations of the
lithospheric rheology. The model also includes major faults, which are simulated as narrow weak zones. We
conducted a suite of forward models to explore the impacts of the 3-D rheological structure, tectonics
boundary conditions, and the fault zones on the tectonic evolution of the Himalayan-Tibetan Plateau. We
found that 1) the collision was mainly accommodated by crustal thickening during the early stage of the
collision, but lateral crustal flow and escaping of the Asian continental blocks have gradually become more
important as the plateau rises; 2) the vertical coherence of lithospheric deformation within the Tibetan
Plateau results mainly from the tectonic boundary conditions, hence is not indicative of whether the crust is
mechanically coupled with the mantle or not; and 3) the boundary faults around the Tibetan Plateau could be
the natural products of lateral contrasts of lithospheric rheology during the lithospheric deformation.
http://www.missouri.edu/~yangyo
T33B-2060
Paleoaltimetry of the Early Miocene-Pliocene Oiyug basin, southern Tibet
The stable isotope composition of carbonate and organic samples from the Oiyug basin in southern Tibet allows for model calculations of the Miocene-Pliocene paleoelevation of the Tibetan Plateau. We measured the oxygen isotope composition of pedogenic and lacustrine carbonates, and hydrogen isotope composition of n-alkanes from epicuticular plant waxes to reconstruct the δ18O and δD of Oiyug basin paleometeoric waters. Calculated δ18O and δD values from Oiyug carbonate and organic samples, respectively, are in close agreement, suggesting the preservation of an unaltered paleometeoric water isotopic signal for both archives. Calculated early-middle Miocene (~20-15 Ma) paleoelevation estimates for the Oiyug basin range from 3000-5400 m with an average of ~4700 m. Individual and average elevation estimates are identical within uncertainty to paleoelevation estimates based on Oiyug basin fossil floral physiognomy from the same interval. This is the first time that three independent paleoaltimeters have been directly compared and are in accord. Calculated late Miocene-Pliocene (~5 Ma) paleoelevation estimates range from 5300-6400 m with an average of ~5600 m. Given that the modern hypsometric mean elevation of the Oiyug basin is ~5000 m, our results allow for a possible decrease in the average elevation of the southern part of the Tibetan Plateau since the late Miocene, compatible with widespread east-west extension in Tibet since at least that time. The results of our study further demonstrate the utility of lipid-based estimates of paleoelevation and the value of a multiple-proxy approach of testing the fidelity of isotope-based paleoelevation records.