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

Abstract

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/Ar 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/Ar 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 ∼268-Ma dioritic porphyry dikes as well. From these data we conclude that dextral strike-slip motion occurred from ∼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 ∼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 ∼270–245 Ma with dextral strike-slip motion, syntectonic granitic intrusions, and synextrusion tectonic exhumation.