THERMOCHRONOLOGIC CONSTRAINTS FOR THE LARGEST TRIASSIC DEPOCENTER IN THE NW MONGOL-OKHOTSK BELT

The Mongol-Okhotsk Ocean started to close during the Permian and formed the Mongol-Okhotsk Belt, a Triassic to Jurassic collisional belt. The cooling record related to the scissor-like closure of the Mongol-Okhotsk Ocean is poorly constrained near the western and older end of the Mongol-Okhotsk orogenic belt. This contribution focuses on investigating the cooling record near a Triassic depocenter located north of a regional orogen-parallel left-lateral fault. This depocenter is crosscut by a series of faults expected to be Late Triassic to Jurassic. Previous apatite fission track data for Triassic basin samples show cooling between ca. 150 and 90 Ma, which corresponds to a time of regional extension during the collapse of the orogen. The goal of this study is to investigate the local magnitude of the Cretaceous heating and cooling event and if the Triassic to Jurassic orogenic signature is preserved within the Mongol-Okhotsk orogen. In order to meet our goal we analyzed basement samples collected near regional structures within the Triassic depocenter and used zircon (U-Th)/He (ZHe) thermochronology and HeFTy to identify 1D time-temperature histories. Results for two samples show that the ZHe cooling record was heterogeneous across the study area. ZHe results for a Carboniferous sandstone in the hanging wall of a thrust fault show cooling between 200 and 180 Ma before staying at a temperature below ca. 140 ℃ for the last ca. 180 Ma. This cooling episode is interpreted to record the timing of erosion and exhumation controlled by shortening during the growth of the orogen. ZHe results for the Paleozoic sample next to a high-angle fault of unknown kinematics show individual zircon grain ages between ca. 358 - 280 Ma. Thermal modeling suggests that the sample was partially reset and experienced heating to temperatures between ca. 150-115 ℃ during the Devonian (ca. 380-360 Ma) and stayed at temperatures below ca. 140 ℃ since ca. 360 Ma. This cooling record 1) indicates that the Cretaceous heating event was locally not strong enough to fully reset the ZHe system in the Paleozoic basement, and 2) provides a limit for the heating and cooling both during the growth and collapse of the orogen. Future work will further constrain the cooling record of these samples using apatite fission track thermochronology.