SEDIMENTARY RECORD AND TECTONIC IMPLICATIONS OF LATE MESOZOIC EXTENSION, SOUTHERN MONGOLIA

The East Gobi basin of Mongolia is a poorly described Late Jurassic-Early Cretaceous extensional province that holds great importance for reconstructions of Mesozoic tectonics and paleogeography of eastern Asia. Late Jurassic-Early Cretaceous extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southeast of Saynshand, Mongolia, where outcrop and subsurface relations permit recognition of pre-rift, syn-rift, and post-rift Mesozoic stratigraphic megasequences. Within the syn-rift megasequence along the well exposed, inverted, northern margin of the Unegt subbasin, each of three successive syn-rift sequences is bounded by unconformities and generally fines upward from basal alluvial/fluvial conglomeratic units to fluvial/lacustrine sandstone and mudstone successions. Resedimented ashes and basalt flows punctuate the syn-rift megasequence. Rifting began in the Unegt and Zuunbayan subbasins prior to 155 Ma with coarse alluvial filling of local fault depressions. Subsidence generally outstripped sediment supply, and fresh to saline lacustrine environments, expanding southward with time, characterized the Unegt-Zuunbayan landscape for much of latest Jurassic-Early Cretaceous time. Episodic faulting and volcanism characterized the basin system for the balance of the Early Cretaceous, and a brief period of compressional/transpressional basin inversion occurred at the end of the Early Cretaceous. SE Mongolia occupied an intraplate position by the beginning of the Cretaceous, and the driver(s) of Late Jurassic-Early Cretaceous extension are uncertain. Extension in the East Gobi basin was coeval with collapse and extreme extension of early Mesozoic contractile orogenic belts to the north and south, and thus likely was a linked phenomenon. Strike-slip strain partitioning associated with collisions on the southern Asian and Mongol-Okhotsk margins also may have played a role in late Mesozoic deformation of the East Gobi region.