IDENTIFYING CRETACEOUS BURIAL AND EXHUMATION IN MONGOLIAN WALLED BASINS USING LOW TEMPERATURE THERMOCHRONOLOGY

The Dzereg and Dariv sedimentary basins are walled basins along the east flank of the Altai Mountains of western Mongolia. These basins preserve ~ 1.5 – 2.0 km of Jurassic through Pleistocene strata exposed along both intrabasin and basin bounding structures. This study quantifies the magnitude of burial and the timing of exhumation throughout Mesozoic-present multi-phase tectonic cycles of extension, compression, and transpression to better understand the relationship between intrabasinal deformation and bounding contractional uplift in walled basins. Apatite fission track (AFT) and (U-Th-Sm)/He thermochronology data from strata in both basins is used to reconstruct past burial timing and magnitude. Samples that have been thermally reset by burial record the time of post-burial exhumation. Initial AFT data collection focused on a ~1.9 km thick Mesozoic section in the Dariv Basin. Here, Jurassic strata are thermally reset, whereas Cretaceous and younger strata are unreset. The base of the exposed Jurassic has a reset AFT cooling age of 109 ±11 Ma, whereas 1.3 km up-section, just below the Jurassic/Cretaceous contact the reset cooling age is 126 ±19 Ma. The base of the Cretaceous is unreset at 241 ±37 Ma. The maximum burial threshold and the recorded cooling ages suggest that Jurassic strata must have been buried to a depth of at least 3 kilometers under a greater thickness of Mesozoic strata than is currently preserved, then exhumed during the Early Cretaceous. AFT analyses from our stratigraphic section match new AFT data from low elevation granitoid samples along the basin-bounding ridges (n=2), which also report Cretaceous cooling ages (130 ± 15 Ma and 114 ± 19 Ma). These Early Cretaceous AFT cooling signatures match a regional trend reported across central Asia, although a mechanism driving this deformation has not been agreed upon.