Postcollisional Eastward Extrusion and Tectonic Exhumation along the Eastern Tianshan Orogen, Central Asia: Constraints from Dextral Strike-Slip Motion and Ar-40/Ar-39 Geochronological Evidence

摘要

Postcollisional extrusion and tectonic evolution in the eastern Tianshan orogenic belt (ETOB) remains poorly known, especially the mechanism of dextral strike-slip motion and associated tectonic exhumation. To better constrain this development, a structural and Ar-40/Ar-39 geochronological study was carried out on a syndextral strike-slip intrusion the Jueluotag batholith-as well as on other granitic plutons in the ETOB. Ar-40/Ar-39 analyses of hornblende, biotite, K-feldspar, and plagioclase from quartz-mica diorite, granodiorite, and dioritic porphyry dykes were used to construct cooling histories of the ETOB. Hornblendes have cooling ages of 277-272 Ma, similar to the syntectonic granitic intrusions, but biotite ages are 261-254 Ma along the syndextral strike-slip pluton from east to west. The dextral strike-slip motion cuts through similar to 268-Ma dioritic porphyry dikes as well. From these data we conclude that dextral strike-slip motion occurred from similar to 270 to 245 Ma. Based on the syntectonic granitic intrusions, structural features, and cooling ages along or outside of the dextral strike-slip belt, we demonstrate that a positive flower structure is the main structural framework for the Paleozoic northern segment of the ETOB. Rapid cooling and tectonic exhumation occurred during similar to 240-220 Ma along the ETOB but did not occur in the western Tianshan orogen. The central Tianshan crystalline belt along the Gangou-Aqikekuduk fault zone was cut and offset southeastward by the dextral strike-slip motion. This suggests that dextral strike-slip motion occurred later than sinistral strike-slip along the southern margin of the ETOB. Geological features and age constraints suggest that the postcollisional eastward extrusion occurred at similar to 270-245 Ma with dextral strike-slip motion, syntectonic granitic intrusions, and synextrusion tectonic exhumation.