SEDIMENTARY, STRUCTURAL AND THERMOCHRONOLOGICAL RESPONSE TO PULSED MIOCENE MOUNTAIN RANGE GROWTH IN NORTHEASTERN TIBET

Growth of mountain ranges is manifested by deformation and slip on faults, uplift and cooling of rock, erosion of rock, and deposition of sediments. A coordinated study is needed to test the fidelity of these proxies to illuminate the dynamics of the climatic-geodynamic system. We explore the Miocene-to-recent history of range growth in a focused area of the northeastern Tibetan Plateau (36°N, 103°E) by examining (i) thermochronological transects from contractional ranges in combination with (ii) magnetostratigraphic successions from adjacent foreland basins and (iii) balanced structural cross-sections. The north-vergent Laji Shan thrust started at ca. 22 Ma and the east-vergent Jishi Shan at ca.13 Ma. Within the adjacent Xunhua-Linxia basin complex, sequential Late Cenozoic range growth is expressed by punctuated, 2- to 3-fold increases in sedimentation and basin subsidence rates that suggest periods of enhanced flexural loading due to thrust faulting. The most rapid accumulation rates occur within the finest-grained strata, consistent with a starved basin during the interval of most intense deformation. Additionally, the introduction of new detrital zircon provenance signatures within basin fill heralds the erosion of newly-emergent Laji Shan and Jishi Shan basement terranes at ca. 21 and ca. 13 Ma. Progradation of conglomerate into Xunhua basin starts >5 Myr after Laji Shan range growth commenced. Conglomerate progradation coincides with a 3-fold decrease in basin subsidence rates and an order of magnitude deceleration in cooling rates, suggesting cessation of major Laji Shan thrusting and loading. Thus, conglomerate deposition in our study area appears to be post-tectonic. Finally, variations in the direction and timing of range growth in northeastern Tibet demonstrate how India-Asia convergence was manifest on the margin of the Tibetan Plateau throughout the Cenozoic. Our observations from mountains and basins document a middle Miocene change in the kinematic style of plateau deformation from contraction along a trajectory that mimicked the India-Asia collision to the onset of east-west contraction. Cross-section restorations suggest that more than half of all Cenozoic shortening in this area occurred since middle Miocene time, as northeastern Tibetan crust began moving eastward.