T32A-01
Stable Isotopic Insights into Sedimentary Basin Evolution Along the Northern Tibetan Margin
Oxygen isotopic data from 14 sections collected along the northern margin of Tibet show two distinct trends indicative of northward growth of the plateau in the Paleogene followed by basin isolation and growth of local mountain ranges in the Neogene. Our isotopic study encompass many of the large basins found north of Tibet including the Tarim, Qaidam, and Junggar basins, the Hexi Corridor, and basins in the Altun Shan. The oxygen isotopic results from paleosol, lacustrine, fluvial, and alluvial carbonate preserved in sedimentary basins show a general increase from the mid-Miocene to Recent, which we attribute to increased aridity as a result of basin isolation beginning in the Miocene. In contrast, in the Paleogene oxygen isotope values decrease with decreasing age within sections located along the northern margin of the plateau. We suggest that these decreases result from northward growth of Tibet and rearrangement of drainage patterns in the Paleogene. These isotopic stratigraphies provide records of the growth of adjacent ranges and segmentation of the landscape with this growth. Isotopic transects of basins perpendicular to the margin of the Tibetan orogen most likely support the growth of topography northwards through time. Additionally, isotopic gradients change with time, suggesting that each basin underwent a similar progression of climatic changes associated with the growth of topography. Isotopic records also vary depending on lithology sampled, and thus isotopic trends dependent on lithology are identified as well as regional isotopic trends. This study emphasizes the challenges of interpreting isotopic records in locales of extreme continentality, where isotopic records during known periods of surface uplift behave differently than predicted by models of isotopic distillation with elevation. To address these challenges, we have attempted to conduct spatially and temporally comprehensive sampling in this region, and have interpreted oxygen and carbon isotope stratigraphies in the context of available structural and thermochronological information.
T32A-02
Local climate differences between the adjacent Linxia and Xunhua basins, NE Tibet reveal 11 Ma history of relief in the intervening Jishi Shan
The 3500-4000 m high Jishi Shan located on the boarder between Gansu and Qinghai Provinces along the northeast margin of the Tibetan Plateau stands as an orographic barrier to easterly derived summer rainfall. Comparison of stable isotope compositions of modern rainfall (δ18O and δ2H) and paleo-soil carbonate (δ18O and δ13C) from the leeward Xunhua basin and the windward Linxia basin provides a method for the interpretation of changes in local climate related to the formation of relief in the intervening Jishi Shan. Rayleigh distillation models suggest that a vapor mass experiencing orographic rainout should be relatively depleted in 18O on the lee side of the range. However, increased aridity in the rain shadow of the Jishi Shan results in a net 2‰ enrichment in the δ18O values of modern rainfall in the Xunhua basin due to evaporative enrichment of 18O. Using the stable isotope compositions of pedogenic and lacustrine carbonates in the Xunhua and Linxia basins as a proxy for paleoclimate, we find that the aridity difference between these basins has existed throughout at least the past 11 Ma, implying the presence of the Jishi Shan. These data indicate that intra- basin comparisons of the stable isotope composition of sedimentary carbonates can be used to assess the timing of emergence of basin-segmenting mountain ranges between the sub-basins of northeastern Tibet.
T32A-03 INVITED
Erosion histories of the northeastern Tibetan Plateau from low-temperature thermochronometry: Evidence for collision-age faulting followed by a kinematic shift in middle Miocene time
Low-temperature thermochronology from the West Qinling and South Qaidam faults indicates accelerated erosion at 45 and 35 Ma respectively. Coupled with a fault gouge age from the West Qinling fault (Duvall et al., this session) we conclude that faulting and accelerated erosion occurred simultaneously, and that fault activity on two of the longest and most continuous structures in northeastern Tibet initiated by Middle to Late Eocene time. Early Cenozoic fault ages and published evidence for foredeep deposition in Linxia Basin since 29 Ma suggest long-lived strain accumulation within the northeastern plateau margin, however, a significant change in the style of faulting occurs since ~15 Ma that is associated with a small degree of propagation of new faults to the north and east (100-200 km or about ~5-10% of the modern orogen length). Cooling ages from numerous reverse faults and transpressional ranges suggest that east-west directed thrusting and strike-slip faulting become dominant during this later deformation phase, including initiation of the Kunlun Fault. On average, the magnitude of erosion since 15 Ma in response to faulting is comparable to the early Cenozoic period of faulting, which may reflect relatively similar magnitudes of fault offset. A change in structural style after 15 Ma in the northeastern plateau correlates with a major reorganization of deformation that affects most, if not all, of the central and eastern plateau and eastern Himalayan thrust belt. However, we show that the areal extent of deformation in northeastern Tibet and the magnitude of erosion have not significantly changed since approximately the beginning of continental collision suggesting that the localization of faulting was relatively insensitive to changes in regional and global climate, increasing crustal thickness, and the major reorganization of faulting patterns across the central and eastern plateau.
T32A-04 INVITED
Progress on the Timing and Magnitude of Cenozoic Shortening Across the Qilian Shan, NW China
Substantial debate centers on the spatial extent and slip history of the Altyn Tagh Fault (ATF) and the role that it plays in accommodating strain due to the India-Asia collision. Two models - a continuum thickening model and a two-stage evolution model – comprise end-member hypotheses for Altyn Tagh Fault evolution. The former posits that ATF terminates at the Qilian Shan range front, while the latter argues for continuation of the ATF beyond the present day physiographic edge of the Tibetan Plateau. Both yield a distinct suite of structural, sedimentologic and thermochronologic predictions that can be readily tested with geological and thermochronologic observations. For instance, the continuum model predicts that all of the ~375 km of observed strike-slip deformation along the ATF is transferred into shortening with the Qilian Shan. In contrast, a two-stage model permits substantial slip to be transferred beyond the plateau via early rigid lateral extrusion requiring more limited shortening within the Qilian Shan later in its evolution. Clearly, the Cenozoic shortening history of the Qilian Shan is fundamental to this debate; however, the timing and magnitude remain poorly known. This set of ranges presents particular challenges given that they are largely made up of Precambrian and Paleozoic metamorphic rocks assembled during the late Paleozoic, leaving little in the way of young strata that aid in structural reconstructions. In this presentation, we outline progress made toward compiling estimates for the timing and magnitude of Cenozoic shortening in the western and central Qilian Shan. Our approach is multidisciplinary, integrating bedrock structural and thermochronologic observations with intramontane and foreland basin evolution studies. In particular, we utilize: (1) field mapping of Cretaceous and Neogene sedimentary units and low- relief surfaces to constrain thrust sheet geometries at the range scale; (2) structural analysis of exposed faults integrated with extant geophysical data to elucidate fault geometries; and (3) thermochronologic data to constrain the magnitude of exhumation – key to understanding the throw on faults where offset marker horizons are absent. We will present two detailed structural case studies from the Jaiyuguan and Sunan areas of the Qilian Shan to illustrate our structural approach. Further we will present preliminary (U-Th)/He data across a sampling transect (collected in Summer 2008) that spans from the Qaidam Basin to the Hexi Corridor.
T32A-05 INVITED
A Tale of Two Tibets: Oligo-Miocene Basin Development in the Bangong and Yarlung Suture Zones
Coeval Oligo-Miocene basins along the Bangong and Yarlung suture zones (BSZ and YSZ) in Tibet record drastically different environmental conditions, tectonic processes, and possibly paleoelevations. The Nima and Lunpola basins formed in a contractile tectonic setting during late Oligocene reactivation of the BSZ, and filled with alluvial, fluvial, and lacustrine facies that were strongly influenced by boreal or alpine dry climate and high paleoelevation (>4.6 km). At the same time, the Kailas basin developed along the YSZ approximately 450 km to the west-southwest. The Kailas Formation is >2500 m thick, and rests in buttress unconformity upon ca. 67 Ma silicic volcanic rocks, which are intruded by 55 Ma granite. U-Pb ages from tuffs and detrital zircons in the Kailas Formation indicate deposition at 26-23 Ma, possibly continuing into mid- Miocene time. Unlike the strongly oxidized, evaporitic, and paleosol carbonate-rich deposits of Nima basin, the Kailas basin is dominated by an upward fining succession of alluvial fan to lacustrine deltaic and offshore deposits. Individual lacustrine parasequences exceed 50 m in thickness, and contain laminated black shale that accumulated in profundal settings in very deep lakes. Nearshore facies contain evidence of large storm waves and combined currents, suggesting that Kailas lakes were large and deep. Organic material in Kailas profundal facies consists of amorphous kerogen, fungal spores, and palynomorphs with warm tropical affinities. Provenance data (detrital zircon and modal sandstone point-counts) indicate derivation of sediment almost exclusively from the Kailas magmatic complex and Gangdese magmatic arc. Although the southern flank of the Kailas basin outcrop belt is involved in early Miocene shortening, overall basin fill characteristics suggest accumulation in an extensional tectonic environment. The uppermost part of the Kailas succession shifts abruptly into redbed/marl/paleosol carbonate facies typical of the "high elevation" facies assemblage documented in Nima basin. Oxygen isotope values from paleosol carbonate in these rocks are in the range of --21.5 to --15.4‰ (PDB), suggesting that paleoelevation comparable to Nima basin developed only in the final stages of Kailas basin filling. The absence of paleosol carbonate and abundance of organic matter, woody plant fossils, and deep water lacustrine facies in the bulk of the Kailas section, coupled with evidence for extensional tectonics, suggests potentially much lower elevation in the YSZ during the Oligocene and earliest Miocene. Contrasts between basins in the YSZ and BSZ raise the prospect that mid-Tertiary paleogeography and tectonic processes in the two sutures were profoundly different.
T32A-06
Modern and Ancient Precipitation O-18/O-16 and D/H Gradients on the Tibetan Plateau
The hydrogen and oxygen isotopic composition of modern precipitation and surface waters on the Tibetan plateau increases from south to north. This enrichment is thought due both to a gradation from southern Indian monsoon moisture to a westerly moisture source and greater moisture recycling in the north. Both the northward extent of monsoon precipitation and westerly moisture are tied to the elevation of the plateau, therefore the evolution of this isotope gradient provides a means to evaluate how moisture sources, climate and topography have changed through the Cenozoic. We compare estimates for the isotopic composition of precipitation from a number of sites on the Tibetan plateau at several different time periods. These estimates are derived from oxygen isotope measurements on soil and lake carbonates along with hydrogen isotope analyses of plant-wax n-alkanes in a number of localities. This comparison shows that the modern δ18O and δD isotope gradient has been a persistent feature on the plateau since at least the early Miocene. Earlier evidence hints at a reduced gradient during the Eocene however there are fewer data for this time period. These results are interpreted in terms of the modern atmospheric circulation on the plateau and climate model simulations of the effects of topography and geography on this circulation.
T32A-07 INVITED
Stable isotopic records from Cenozoic basins in the SE Margin of the Tibetan Plateau, Yunnan Province, China: implications for regional paleoelevation
We report the initial results of an ongoing stable isotope study of Cenozoic sediments from the northern part of China's Yunnan Province. The aim of this study is to constrain paleoelevations in this area in order to determine which geodynamic and/or climatic factors are responsible for the enigmatically gentle topographic gradient of this margin of the Tibetan Plateau and the formation of deeply incised canyons since the late Miocene. This work includes sampling small catchment area (<100 km2) tributaries of the Mekong river in ~100 m vertical increments from 500 m (near the Vietnam border) to 4000 m elevation (near Daqen), which we use to constrain the modern gradient in δ18O with elevation in this area. We measured detailed stratigraphic sections and collected samples of authigenic carbonates from the Paleogene-lower Miocene Jianchuan and the Upper Miocene Lanping Basins. A smaller number of samples come from exposures of Pliocene age rocks near the city of Weishan and village of Jin Bao Shan. We rely almost exclusively on the age assignments from Chinese geologic maps. The majority of our δ18O data from the sediments have δ18O values between -14 to -10 per mil (VPDB) from the pre-Miocene through the Pliocene. When compared to the modern elevation- dependent gradient in δ18O these data suggest that there has been < 500 m of elevation change in this area since the at least the early Miocene. If no significant Neogene elevation change has occurred on the SE Margin of the plateau, then incision was most likely driven by some other mechanism, perhaps climatic in nature.
T32A-08
Minimal erosion in central Tibet since the Eocene and implications for plateau development
New apatite (U-Th)/He data from the northern Lhasa and Qiangtang terranes of central Tibet provide new insight into the thermal and tectonic evolution of the Tibetan Plateau. Samples collected away from major late Cenozoic rifts in the northern Lhasa terrane yielded apatite He ages between 54 and 49 Ma. This indicates that less than ~3 km of crust has been removed since ~50 Ma and very slow exhumation rates (~0.05 mm/yr). These findings complement recent K-feldspar 40Ar/39Ar and AFT results from the northern Lhasa terrane by J. Guynn et al. (unpublished data), indicating rapid exhumation during thrust belt development prior to ~50 Ma and minimal exhumation during the Indo-Asian collision. Findings of major Cretaceous-Eocene crustal shortening (>50%) and exhumation, followed by minimal subsequent denudation, raise the possibility of the establishment of a proto-plateau in central Tibet by Eocene time. Whereas the majority of low-temperature theromochronologic results from central Tibet record pre-Indo- Asian collision cooling, we note two exceptions. A granite in the hanging wall of a mapped Tertiary thrust fault north of the Bangong suture in the central Qiangtang terrane yielded an AHe age of 18.0 ± 0.8 Ma. One granite in the northern Lhasa terrane yielded an AFT age of 25 Ma (Guynn et al.). These results may indicate localized thrust belt reactivation during the mid-Tertiary and show the potential for future low- temperature thermochronologic studies to assess geodynamic models of the collision process. Collectively, the exhumation history of central Tibet, away from the influence of late Cenozoic rifts, contrasts sharply with that of the Lhasa region in the southern Lhasa terrane and near the modern margins of the plateau which show prominent signatures of the Indo-Asian collision. Any viable model of plateau development must explain these prominent spatial variations in exhumation history as well as the lack of a corresponding expression in the modern topography.