An Wang

Timing and rate of exhumation along the Litang fault system, implication for fault reorganization in Southeast Tibet

The Litang fault system that crosses the Litang Plateau, a low relief surface at high elevation (~4200–4800 m above sea level) that is not affected by regional incision, provides the opportunity to study exhumation related to tectonics in the SE Tibetan Plateau independently of regional erosion. Combining apatite and zircon fission track with apatite (U-Th)/He thermochronologic data, we constrain the cooling history of the Litang fault system footwall along two transects. Apatite fission track ages range from 4 to 16 Ma, AHe ages from 2 to 6 Ma, and one zircon fission track age is ~99 Ma. These data imply a tectonic quiet period sustained since at least 100 Ma with a slow denudation rate of ~0.03 km/Ma, interrupted at 7 to 5 Ma by exhumation at a rate between 0.59 and 0.99 km/Ma. We relate that faster exhumation to the onset of motion along the left-lateral/normal Litang fault system. That onset is linked to a Lower Miocene important kinematic reorganization between the Xianshuihe and the Red River faults, with the eastward propagation of the Xianshuihe fault along the Xiaojiang fault system and the formation of the Zhongdian fault. Such strike-slip faults allow the sliding to the east of a wide continental block, with the Litang fault system accommodating differential motion between rigid blocks. The regional evolution appears to be guided by the strike-slip faults, with different phases of deformation, which appears more in agreement with an “hidden plate-tectonic” model rather than with a “lower channel flow” model.

Fission Track Geochronology of Xiaonanchuan Pluton and the Morphotectonic Evolution of Eastern Kunlun since Late Miocene

ABSTRACT Apatite fission track (AFT) thermochronology of seven samples from the Xiaonanchuan ( ) pluton in the Kunlun ( ) pass area was carried out, for the purpose of determining the timing of cooling and the relation between the exhumation and the morphotectonic processes. The AFT ages yield low denudation rates of 0.020–0.035 mm/a during the late Miocene, which correspond to a stable geomorphic and weak tectonic uplifting environment. The low denudation rates can be considered as the approximate tectonic uplifting rates. The AFT geochronology shows paroxysmally rapid cooling since the Pliocene and an apparent material unroofing of more than 3 km in the Xiaonanchuan area. This was not the result of simple denudation. The rapid cooling was coupled with the intensive orogeny since the Pliocene, which was driven by tectonic uplifting. The accelerated relief building was accompanied by a series of faulting, which caused the basin and the valley formation and sinking. The space pattern of the AFT ages also shows differential uplifting, which decreases northwardly. This trend is supported by the regional AFT data, which indicate that the exhumation decreases northwardly in eastern Kunlun. This trend also exists in east-west orientation from the western Kunlun range to the eastern. The uplifting trend is also supported by geomorphic characteristics including the elevation and the relief differences as well as the distribution of the Late Cenozoic volcanism.

On the Geodynamic Mechanism of Episodic Uplift of the Tibetan Plateau during the Cenozoic Era

: Multi-stage uplift of the Tibetan Plateau during the Cenozoic implies a complex geodynamic process. In this paper, we review main geodynamic models for the uplift of the plateau, and, in particular, analyze the spatio-temporal framework of the Cenozoic deformation structures, which are closely related to the deep geodynamic mechanism for the plateau uplift. From this perspective, significant change of the deformation regime over the Tibetan Plateau occurred by the middle-late Miocene, while thrust and thrust-folding system under NS compression was succeded by extension or stress-relaxation. Meanwhile, a series of large-scale strike-slip faults commenced or was kinemtically reversed. Based on a systematic synthesis of the structure deformation, magmatism, geomorphological process and geophysical exploration, we propose a periodical model of alternating crustal compression and extension for episodic uplift of the Tibetan Plateau.

Controls on Cenozoic exhumation of the Tethyan Himalaya from fission‐track thermochronology and detrital zircon U‐Pb geochronology in the Gyirong basin area, southern Tibet

The Gyirong basin, southern Tibet, contains the record of Miocene-Pliocene exhumation, drainage development, and sedimentation along the northern flank of the Himalaya. The tectonic controls on basin formation and their potential link to the South Tibetan Detachment System (STDS) are not well understood. We use detrital zircon (ZFT) and apatite (AFT) fission-track analysis, together with detrital zircon U-Pb dating to decipher the provenance of Gyirong basin sediments and the exhumation history of the source areas. Results are presented for nine detrital samples of Gyirong basin sediments (AFT, ZFT, and U-Pb), two modern river-sediment samples (ZFT and AFT), and six bedrock samples (ZFT) from transect across the Gyirong fault bounding the basin to the east. The combination of detrital zircon U-Pb and fission-track data demonstrates that the Gyirong basin sediments were sourced locally from the Tethyan Sedimentary Sequence. This provenance pattern indicates that deposition was controlled by the Gyirong fault, active since ~10 Ma, whose vertical throw was probably < ~5000 m, rather than being controlled by normal faults associated with the STDS. The detrital thermochronology data contain two prominent age groups at ~37–41 and 15–18 Ma, suggesting rapid exhumation at these times. A 15–18 Ma phase of rapid exhumation has been recorded widely in both southern Tibet and the Himalaya. A possible interpretation for such a major regional exhumation event might be detachment of the subducting Indian plate slab during the middle Miocene, inducing dynamic uplift of the Indian plate overriding its own slab.