This research contributes to the understanding of how continents deform in response to major tectonic events such as continent--continent collisions. The continent of Asia is a natural laboratory to address questions such as: How has the interior of Asia deformed in response to the collision of India and Asia? How and why might the modes of deformation have changed over time? How does the sedimentary record reflect such events? This project employs tools of structural geology, geochronology, and sedimentary basin analysis to reconstruct displacement history and sedimentary basin evolution along the East Gobi Fault Zone, Mongolia. Preliminary data suggest that this major intracontinental shear zone was reactivated at least four times since the late Paleozoic; this work documents two of the least-constrained episodes of left-lateral slip (early Mesozoic and early Cenozoic). Due to paucity of data from southeastern Mongolia, the East Gobi Fault Zone is essentially played as a wildcard against models of evolution of the Asian tectonic "collage." For example, published models estimate left-lateral offset ranging from approximately 200 km of early Mesozoic slip to approximately 400 km of purely Cenozoic slip, citing regional map relations that have not been field-tested. The latter interpretation implies a pre-Miocene linkage between the Altyn Tagh fault and hypothesized--but as yet largely undocumented--strike-slip faults extending through southeastern Mongolia to the Sea of Okhotsk. This study tests whether this kinematic linkage is permissible via mapping near the China-Mongolia border zone, southwest from control sites where Cenozoic strike-slip faulting has been established in the East Gobi Fault Zone, and provides the first field and analytical constraints on the timing and magnitude of slip. These results bear further on mechanisms favoring initiation and reactivation of shear zones by testing the hypothesis that the East Gobi Fault Zone reactivates a Paleozoic suture. Geochronologic data are collected to evaluate the timing of intraplate deformation in southeastern Mongolia as a function of far field forces and evolving boundary conditions. Thus, this research is relevant to models of continental block extrusion and continuum shortening during Asia's history of collisions from Mesozoic-Recent. The great variety of structural levels exposed and lithologies involved in deformation in the East Gobi Fault Zone provide an excellent opportunity to investigate rheologic controls on shear zone/fault zone development in the upper crust and at depth. Furthermore, investigation of the relationship between deformational phases of the East Gobi Fault Zone and sedimentary rocks in the East Gobi region constrain mechanisms of initiation, stratigraphic development, and subsequent dismembering of "poly-history" sedimentary basins in wrench-tectonic settings. The multidisciplinary nature of the collaboration highlights the power of integration of diverse geologic datasets to address a complex set of questions. Results of this study contribute to the greater understanding of an understudied and mineral resource-rich region of our planet. The international nature of this work fosters collaboration between Mongolian and American geologists, including two early-career female investigators. Additionally, two Ph.D. students and undergraduate students from Utah and Syracuse participate in all aspects of the study and are trained in the use of technology to solve scientific problems.
Support for the project is provided by the Tectonics Program and by the East Asia and Pacific Group of the Office of International Science and Engineering.
