Structural Evolution of the Bayabache-Kare Too-Naryn Series of Anticlines and Shortening Across the Intermontane Tien Shan, Kyrgyzstan

PANGA M. Meghan Miller1, J. Kim McLean1, Kanatbek Abdrakhmatov2, Ray J. Weldon II3, and Stephen C. Thompson4

Abstract

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)