|M. Meghan Miller1, J. Kim McLean1, Kanatbek Abdrakhmatov2, Ray J. Weldon II3, and Stephen C. Thompson4|
ABSTRACT: The Bayabache-Kara Too-Naryn series of anticlines separate the Naryn and At-Bashi basins in the central Tien Shan and produce about 9 km of structural relief on an early Cenozoic erosion surface. Structural and stratigraphic studies indicate that the folds and associated faults constitute a single, greater than 100-km long and 30-km wide structure that formed in the center of a Miocene to Pliocene intermontane basin during the past 3 m.y.
In cross section from NNW to SSE, the structure consists of (1) a 20-30 degree NNW-dipping , (2) NNW steeply-dipping reverse faults, (3) a 50-60 degree SSE-dipping, 5-10 km-wide forelimb locally broken by reverse faults, and (4) a NNW-dipping, partially blind thrust fault that has accommodated most of the recent shortening. The overall structure appears to have initiated as a SSE-vergent, basement-involved blind thrust within the "foreland" of a larger NNW-vergent structure. The initial fault-related folding accommodated 50-80% of the shortening seen at the surface. Subsequent en echelon reverse faults that formed as anticlinal breakouts uplifted the pre-Cenozoic basement core. Both NNW- and SSE-dipping reverse faults within the forelimb locally displace several hundred meters to more than a kilometer of Neogene cover sediments.
The SSE-vergent thrust and associated fault-propagation fold that defines much of the SSE margin of the structure has accommodated about 1 mm/yr of shortening during Holocene time. This rate is less than the 3 m.y.-long shortening rate for the larger Bayabache-Kara Too-Naryn structure by a factor of 2 to 5. The Bayabache-Kara Too-Naryn anticlines represent the clearest example in the Tien Shan of basement-involved shortening within an intermontane basin. The structural geometry and evolution documented here serve as a template for regional cross-section interpretation and preliminary measurements of total N-S late Cenozoic shortening across the Kyrgyz central Tien Shan.
1Department of Geological Sciences, Central Washington University, Ellensburg, Washington
2 National Academy of Sciences, Institute of Seismology, Bishkek, Kyrgyzstan
3Department of Geological Sciences, University of Oregon, Eugene, Oregon
4Department of Geological Sciences, University of Washington, Seattle, Washington
|GSA 2000 poster (58 MB)|