Fission-track constraints on Jurassic folding and thrusting in southern Mongolia and their relationship to the Beishan thrust belt of northern China

Across much of southern Mongolia, thick sequences of Permian through Middle Jurassic strata are generally strongly faulted, folded, and/or tilted, whereas Upper Jurassic and younger strata are much less deformed. This is evidence for as yet poorly documented Jurassic deformation in the region. To better constrain the magnitude and timing of this deformation, we have applied apatite fission-track thermochronology methods at seven localities. Our principal study area is the Noyon Uul syncline, a large (&30 10 km), upright, tightly folded, east-west-trending structure that exposes 5400 m of Permian through lowermost Jurassic(?) strata. Fission-track ages and track length distributions are nearly constant through the entire exposed section, with an average age of 138 Ma and mean track length of 13.6 Mm. Simulation modeling of the fission-track data indicates major cooling in Late Jurassic time, with cooling below the 100 °C isotherm at 150 10 Ma shallow in the section and 145 10 Ma deep in the section. We infer that this cooling is related to a major episode of thrusting in the Beishan of northern China, &100 km south of Noyon Uul. The thin-skinned Beishan thrust system accommodated inferred displacements of 140 km at some time between Middle Jurassic and Cretaceous time, and erosional klippen of the thrust system are preserved within 50 km of Noyon Uul. Associated basement-involved shortening of much smaller total magnitude apparently affected Noyon Uul, collapsing a preexisting east-west-trending depositional trough, tightly folding the syncline, and tectonically burying the currently exposed land surface at least 4-5 km. Subsequent rapid erosion of the resulting elevated area unroofed and cooled the fission-track samples in Late Jurassic time. Limited data from the Toroyt and White Mountain localities, &200 km west and &170 km southeast of Noyon Uul, respectively, yield very similar Late Jurassic cooling histories, attesting to the regional nature of this shortening episode in southern Mongolia. In addition to the work at Noyon Uul, we collected fission-track samples near the Tost fault, a major east-northeast-trending structure that passes ∼100 km west of Noyon Uul. Recent work has documented 95-175 km of left-lateral slip on the fault; offset and overlapping units indicate that motion occurred at some time between the Early Permian and Late Cretaceous. Fission-track data from four locations close to the fault record major Middle Jurassic cooling, with cooling through the ∼100 °C isotherm at 180 ± 20 Ma, significantly older than the cooling at Noyon Uul. We tentatively infer that these data record major rapid uplift and erosion during Middle Jurassic transpression along the Tost fault, supporting interpretations that motion along the Tost fault was related to an early phase of shortening in the Beishan. Given the east-northeast strike of the Tost fault, very strong transpression, uplift, and erosion would be expected along the fault if its left-lateral slip was related to north-directed shortening in the Beishan.