Eocene stabilization and Miocene incision of Tibetan Plateau Remnants in the Northwest Himalaya
Abstract
We have recently described Eocene Tibetan Plateau remnants that are preserved as relict high-elevation, low-relief landscapes within the north-west Himalayan syntaxis (van der Beek et al., Nature Geosci., 2, 364-368, 2009). The recognition of these remnants is based on common morphologic characteristics and similar histories of long-term slow exhumation since at least 35 Myr ago. Here we extend our morphologic analysis to show that the Babusar area (Gamugah surface) and parts of the Northern Karakorum could constitute similar remnant Tibetan landscape elements. We also report additional thermochronology data from the Indus and Astor valleys that record the more recent incision history of the Deosai plateau borders (Northern Pakistan). Apatite fission-track and (U-Th)/He ages from the valleys are significantly younger (<10 Ma) than those from the plateau surface. Thermochronological age-elevation profiles sampled up the Indus valley escarpment of the plateau cannot be interpreted in a straightforward manner because they cover a region with strong lateral contrasts in topography and denudation rate. We use an inverse modeling approach with the thermo-kinematic model Pecube in order to constrain the incision history of the plateau and find that incision of the Indus Valley most probably started around 26 Ma in this region, consistent with independent sedimentological evidence for drainage reorientation in the Northwest Himalaya (Sinclair and Jaffey, J. Geol. Soc. London, 158, 151-162, 2001). In order to fit the age difference between the Indus Valley and the Deosai Plateau samples, relative rock uplift of the valley with respect to the plateau is required, implying either tilting of the plateau or, more probably, significant isostatic rebound in response to valley incision. The inferred histories of constant slow denudation on the Deosai Plateau and other plateau surfaces contradict the hypothesis that widespread low-relief surfaces in the northwest Himalaya result from efficient, km-scale glacial erosion during Quaternary times; such erosion would have been recorded as a phase of rapid recent denudation that is not observed in the data. Slow continuous denudation since Eocene times, i.e. only 15-20 Myr after the onset of India-Asia collision, implies that the surfaces developed early in the Himalayan history and limits the phase of orogenic relief growth in the Ladakh-Kohistan arc to the early Paleogene. Finally, our data record late stage tilting of the Deosai Plateau toward the west, associated with uplift and exhumation of the Nanga Parbat - Haramosh massif and toward the north, due to isostatic rebound. Inferred very low flexural rigidities for the region (effective elastic thickness ~3 km) are consistent with an overthickened fluid-bearing crust and with lower crustal flow from underneath the plateau toward the Nanga Parbat tectonic aneurism.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T41D..04V
- Keywords:
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- 1140 GEOCHRONOLOGY / Thermochronology;
- 8102 TECTONOPHYSICS / Continental contractional orogenic belts and inversion tectonics;
- 8159 TECTONOPHYSICS / Rheology: crust and lithosphere;
- 8175 TECTONOPHYSICS / Tectonics and landscape evolution