CLOSE CORRESPONDENCE OF CENOZOIC AND RECONSTRUCTED PALEOZOIC SHORTENING DIRECTIONS IN THE CENTRAL TIEN SHAN, KYRGYZSTAN

The terranes forming the Tien Shan of Central Asia were accreted to the Kazakh platform in a sequence of collisions dated to the late Paleozoic. Structures in exposed Paleozoic bedrock in the Cenozoic ranges of the Central Tien Shan define principal strain directions during the assemblage of this portion of the continent. The close correspondence between Paleozoic strain axes and Cenozoic shortening further supports arguments that inherited structures control the ongoing deformation.

This reconstruction is possible due to the persistence of a well-preserved, widely exposed, regional-scale, late Mesozoic erosion surface, originally horizontal. Rotation of structural data from Paleozoic rocks beneath the erosion surface about horizontal axes can restore these structures to their original orientations. We assume the assembled blocks have not rotated about vertical axes since Paleozoic assemblage.

In the Molda-Too Range, unambiguous slickenlines on the range-frontal thrust fault indicate the direction of movement and maximum shortening S2E, in close agreement with GPS measurements. Open folding of an uninterrupted 6 km segment of the exposed erosion surface produced Cenozoic shortening of only 3% in this block. Two analyses of major folds below the erosion surface give maximum Paleozoic shortening directions of N5E, with 28% shortening, and N14W, with 40% shortening.

In the Kara-Too and Baybiche-Too Ranges, steeply plunging striations on the high-angle basin-forming reverse faults suggest a mean slip direction of S36E, while folding of the erosion surface indicates a shortening direction of N12W. Cenozoic shortening on two transects measures only 6-8% and 10%. Reconstructing Paleozoic strain along these transects gives 43% shortening oriented N15W, and 30% shortening oriented N42W.