Partitioning of intermontane basins by thrust-related folding, Tien Shan, Kyrgyzstan

Well-preserved, actively deforming folds in the Tien Shan of Kyrgyzstan pro vide a natural laboratory for the study of the evolution of thrust-related folds. The uplifted limbs of these folds comprise weakly indurated Cenozoic strata that mantle well-lithified Palaeozoic bedrock. Their contact is a r egionally extensive unconformity that provides a persistent and readily tra ceable marker horizon. Based on the deformation of this marker, preserved f old geometries support simple geometric models for along-strike gradients i n fold amplitude and displacement along the underlying faults, linkage amon g multiple structures, transfer of displacement among folds and evolution o f the folds as geomorphic entities. Subsequent to initial uplift and warpin g of the unconformity surface, steeply dipping reverse faults cut the forel imbs of many of these folds. Wind gaps, water gaps, recent faulting and pro gressive stripping of the more readily eroded Cenozoic strata indicate the ongoing lateral propagation and vertical growth of fault-related folds. The defeat of formerly antecedent rivers coincides in several places with mark ed increases in erosional resistance where their incising channels first en countered Palaeozoic bedrock. Persistent dip angles on the backlimbs of fol ds indicate strikingly uniform geometries of the underlying faults as they propagate both laterally and vertically through the crust. Deformation swit ches irregularly forward and backward in both time and space among multiple active faults and folds with no systematic pattern to the migration of def ormation. This distributed deformation appears characteristic of the entire Kyrgyz Tien Shan.