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1 School of Earth Sciences, Stanford University, Stanford, California 94305, USA
2 Department of Geology, University of Montana, Missoula, Montana 59812, USA
3 School of Earth Sciences, Stanford University, Stanford, California 94305, USA
4 Center of Paleontology, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351
5 Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351
6 Shell International Exploration and Production, Box 60, 2280 AB Rijzwijk-ZH, Netherlands
7 Brigham Exploration, Inc., Austin, Texas 78736, USA
8 Center of Paleontology, Mongolian Academy of Sciences, Peace Avenue 63, Ulaanbaatar, Mongolia 210351
9 Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA
10 Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA
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. Extension is especially well recorded in the structure and stratigraphy of the Unegt and Zuunbayan subbasins southwest of Saynshand, Mongolia, where outcrop and subsurface relationships permit recognition of prerift, synrift, and postrift Mesozoic stratigraphic megasequences. Within the synrift megasequence, three sequences developed in response to climatic and rift-related structural controls on sedimentation. Where best exposed along the northern margin of the Unegt subbasin, each of the synrift sequences is bounded by unconformities and generally fines upward from basal alluvial and fluvial conglomerate to fluvial and lacustrine sandstone and mudstone. Resedimented ashes and basalt flows punctuate the synrift megasequence. Rifting began in the Unegt subbasin 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, dominated 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. A brief period of compressional and/or transpressional basin inversion occurred at the end of the Early Cretaceous, prior to deposition of a widespread Upper Cretaceous overlap sequence.
The driver(s) of Late Jurassic–Early Cretaceous extension remain uncertain because southeast Mongolia occupied an intraplate position by the beginning of the Cretaceous. Extension in the East Gobi basin was coeval with collapse and extension of early Mesozoic contractional orogenic belts along the northern and southern borders of Mongolia and probably was a linked phenomenon. Strike-slip faulting associated with collisions on the southern Asian and Mongol- Okhotsk margins likely also played a role in late Mesozoic deformation of the East Gobi region, perhaps partitioning the Gobi from apparently coeval large-magnitude contractional deformation in the Yinshan- Yanshan orogenic belt south of the study area in Inner Mongolia.
Key Words: Asia • Mesozoic • Mongolia • rifting • sedimentary basins • tectonics
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