UNRAVELING THE COMPLEX RECORD OF GROWTH AND DEFORMATION IN ASIA: DETAILS FROM THE EASTERN GOBI ALTAI, SOUTHWESTERN MONGOLIA

Much of the debate in Asian tectonics concerns the timing and nature of the collision of India with the Asian continent. How has more than 1500 km of Cenozoic collision been accommodated? What is the relative contribution of escape tectonics versus distributed crustal deformation? It has been recognized that in order to answer these questions we must study not only the Tibetan uplift, but also the ranges and structures further north. Determining the timing, extent and mechanisms of deformation of the ongoing collision with India and Asia requires understanding the details of the complex geologic history of southern Mongolia. Rocks of this region record much of the Phanerozoic growth and deformation of Central Asia. New ⁴⁰Ar-³⁹Ar dates combined with structural data and detailed mapping from the eastern Gobi Altai record details of this history.

Paleozoic strata contain evidence of southward continental growth within a long-lived volcanic arc setting. Angular unconformities and strongly-deformed rocks record deformation associated with this convergent margin and a newly-dated amphibolite is interpreted to represent subduction-related metamorphism occurring at ~295 Ma. Collision of southern Mongolia with northern China occurred near the end of the Permian and led to the late-stage emplacement of granites which yield a cooling age of ~230 Ma. Devonian volcanics, whose age of formation is constrained by overlying fossiliferous beds, produced an age of ~170 Ma. These rocks were reheated during a tectonic event, most likely due to Middle and Late Jurassic contraction associated with the collision of the Qiangtang block. Cross-cutting faults, cleavage and fold data from the region also point to multiple tectonic events. Mesozoic and Paleozoic formations are more deformed than Cenozoic formations, suggesting much deformation occurred prior to the Cenozoic. Regional seismicity suggests this area has experienced Cenozoic tectonic activity as well. Continued detailed mapping, geochemical and geochronolgoic studies such as this one are needed to document structural relationships and further constrain the Mesozoic and Cenozoic components of deformation.