46534-G8
Nucleation and Growth of Basement-Cored Uplifts Around Strike-Slip faults: Case Study of the Kungey-Zailiskey Ranges, Tien Shan, Central Asia
With PRF starter-grant funding I was able to undertake two inter-related pilot studies of shortening in the Tian Shan of Central Asia. The first study, of the growth and rates of shortening of the Kungey and Zailiskey ranges, resulted in a completed M.S. thesis and a publication in preparation. This study was largely funded with PRF support. The second study, of the Naryn Basin in the central Tian Shan, will comprise two Ph.D. theses when completed. PRF support allowed my research group to gather critical data in support of an NSF proposal. Both of these studies involved fieldwork by myself and my graduate student, Jacob Selander. PRF starter-grant funding was primarily used to support travel, field-support costs, and a fellowship for Jacob.
The Kungey and Zailieky ranges are among the youngest basement-cored uplifts of the northern margin of the Tian Shan. The objective of our study of these ranges was to understand how the Tian Shan orogen propagates, and in particular what role of the Chon-Kemin fault, an inherited Paleozoic strike-slip fault, played in this process. We undertook structural, stratigraphic and neotectonic investigations to develop a comprehensive picture of the evolution of these ranges. Our stratigraphic investigations found a 180° reversal in paleocurrent directions and a sharp lithologic transition indicative of sediments derived from the uplift of the Kungey Range. This finding confirmed our hypothesis that that the Kungey Range was formed much later than other ranges of the Tian Shan. From our structural mapping of the Kungey Range we found that reverse faulting was intimately related to this inherited strike-slip fault. Folding above these faults reveals steepening of fault dips with depth. Overall the Kungey-Zailiskey system resembles a mega-flower structure, with curviplanar reverse faults rooting into the Chon-Kemin fault zone.
For the neotectonic component of our study we quantified late Quaternary fault slip rates on the margins of the Kungey and Zailiskey ranges. We measured faulting and folding of a sequence of late Quaternary alluvial fan surfaces emplaced by rivers draining these ranges. A series of well-preserved alluvial fan surfaces were dated using in-situ accumulation of cosmogenic 10Be. From the deformation and ages of these terraces we were able to derive slip rates for many of the active faults responsible for range uplift. We discovered that superposed on the overall flower-structure of the Kungey-Zailiskey range system is a second pattern evident in the neotectonic rates: along-strike shortening-rate gradients. Along the southern edge of the Kungey Range shortening rates increase westward. Conversely, along the northern margin of the Zailiskey range shortening rates increase eastward. These opposing gradients suggest that rather than acting as a transform fault, the NE-striking sinistral Chon-Kemin fault must rotate clockwise during shortening. This rotation acts to transfer shortening strain from the NE Zailiskey range to the SW Kungey Range. Overall from our combined structural and neotectonic study a picture emerges of a complex, 3-D strain partitioning system. We found that this strain partitioning was consistent with a coulomb model of faulting above a reactivated, steeply dipping shear zone at depth.
The second part of my PRF-supported research was to initiate a structural and neotectonic study of the Naryn Basin of the central Tian Shan. The objective of this study is to understand the role of basin filling and evacuation in the partitioning of shortening across the Tian Shan. The Naryn Basin is a 50 km x 200 km intramontane basin with a 2-stage shortening history. Early shortening took place on the margins of a closed Naryn basin that filled with lacustrine sediments. More recent shortening appears to have commenced as the basin became externally drained and rapidly eroded by the Naryn river system. Presently, the Naryn basin absorbs 25% of the total shortening rate across the Tian Shan. The active convergent structures are localized along the rivers at the topographically lowest parts of the basin. This shift in the locus of deformation suggests that unloading of the basin through erosion localizes the presently high rate of shortening here.
