The Department of Earth Science

invites you to attend 

Jessica Thompson’s Ph.D. defense presentation:

"Neogene Tectonic Evolution of the NE Pamir Margin, NW China

Monday, August 5th at 10am

Webb Hall 1006C, Main Office Conference Room

ABSTRACT:

The Pamir and Tian Shan orogens in NW China are a delayed response to the Indo-Eurasian collision. During the Neogene, both orogens have experienced several pulses of deformation, propagating their deformation fronts into the western Tarim Basin, a foreland trapped between the two orogens. As the collision progressed, the two orogens impinged on each other and are gradually zippering the western Tarim Basin shut. To illuminate the timing, rates, and styles of deposition and deformation, a synthesis of new structural mapping, stratigraphic sections, magnetostratigraphy, cosmogenic burial and depth profile dating, optically stimulated luminescence dating, and topographic surveys of deformed Quaternary surfaces serves as the foundation to understand the tectonic evolution over timescales of millions to thousands of years.

To determine Late Quaternary shortening rates, terraces were dated using optically stimulated luminescence (OSL) and cosmogenic depth profile dating. Results suggest the frequently-used silt fraction in OSL dating tends to overestimate depositional ages of terraces by up to an order of magnitude, whereas the fine sand fraction, when combined with the appropriate age model, yields stratigraphically consistent ages within error. This difference in OSL ages from different grain sizes is likely related to the mode of transport of the grains in the fluvial system. Times of terrace abandonment were also successfully dated using cosmogenic 10Be depth profiles and yielded ages in agreement with the fine sand OSL ages.

Eight new 26Al/10Be cosmogenic burial ages on syntectonic growth strata and basin fills, in conjunction with magnetostratigraphic sections, provide new initiation ages and shortening rates for structures on the Tian Shan and Pamir margins. Models of the effect of post-burial nuclide production, which may occur due to Late Quaternary incision of rivers or prolonged exposure during sediment burial, suggest burial ages may be perturbed by as much as 60%. The new ages and shortening rates, when combined with detailed structural and stratigraphic mapping, document the Late Neogene outward growth and eastward propagation of deformation on both the Pamir and Tian Shan orogens.

A record of piggyback basin sediments in the hanging wall of the frontal fault of the Pamir shows pulsed and synchronous deformation and tectonically and climatically controlled deposition of the basin since ~10 Ma. During the Neogene, deformation propagated northward and eastward into the Tarim Basin as the collision with the Tian Shan progressed. Moreover, back-stepping and deformation on hinterland structures appears to play an important role in the growth of the Pamir. Our data suggest a change in the regional kinematics of the Pamir-Tian Shan region around the Miocene-Pliocene boundary ~5-6 Ma, consistent with increased coupling between the Pamir and Tarim plates, rapid exhumation of the NE Pamir, and accelerated deformation along the southern Tian Shan front. During the Late Quaternary, much of the deformation is accommodated on the frontal fault system, with shortening rates of 6-8 mm/a, consistent with the current geodetic rate.