Structural and strain analyses of the middle part of the Talassian Alatau ridge (Middle Asia, Kirgizstan)

Abstract

The middle part of the Talassian Alatau ridge is composed of Upper Precambrian-Lower Paleozoic greenschists (the Usunakhmatian sheet), which were overthrust on unmetamorphosed rocks of nearly the same age (the Talassian and Kumishtagian sheets). Three phases of deformation can be recognized in the Usunakhmatian sheet, the most intensive of which was the Late Silurian-Early Devonian D 2 deformation. In the Usunakhmatian sheet, the axis of maximum extension is subhorizontal and parallel to F 2 fold axes. R xz varies from 2.4 to 7.5 and k from 0.03 to 1.75. Flattening is widespread, but the strain of many samples is nearly plane strain. In the Talassian sheet, R xz varies from less than 1.8. to 4.4 and k from 0 to 0.35. R xz and k values increase near large faults. R xz values determined by quartz and chert grains were found to be linearly related. A three-stage strain history is related to D 2 deformation. The earliest was simple shear parallel to the axial surfaces of folds. Following this, the fold and thrust system was transformed by pure shear with an orogen-parallel axis of extension. Some minor increments of strain are locally represented by pressure shadows around pyrite crystals.