Silurian-Devonian Lithospheric Thinning and Thermally Softening Along the Northern Margin of the Tarim Craton: Geological Mapping, Petro-Structural Analysis and Geochronological Constraints

While the western part of northern Tarim Craton has long been considered as a Paleozoic passive margin, a pronounced Silurian-Devonian magmatism developed on eastern part of this margin may indicate different but active margin setting. In this contribution, detailed structural mapping, petro-structural analysis, and geochronological investigations were conducted in the Korla area, eastern part of northern Tarim Craton. Three main generations of fabrics were recognized. The earliest pervasive fabric is an originally sub-horizontal metamorphic S1 foliation that is in part associated with migmatization characterized by high temperature/low pressure metamorphic mineral assemblages, interpreted as reflecting crustal extension. S1 foliation was affected by D2 contraction forming regional-scale F2 upright folds associated with sub-vertical axial planar foliation S2. D3 is marked by development of NW-SE oriented dextral fault, asymmetric mega-folding of S2 and spaced NW-SE-striking S3 foliation, likely in response to dextral transpression. Geochronological data indicate that D1 extension occurred from ca. 420 to 410 Ma, D2 contraction started around 410 Ma and lasted till 400 Ma or later, and D3 transpression was ongoing around similar to 370 Ma. Integrated with regional data, an updated geodynamic model is proposed by interpreting the Central Tianshan, South Tianshan and NE Tarim Craton as an early Paleozoic supra-subduction system. We suggest that the Silurian-Devonian event reflects thermal softening and horizontal stretching of the supra-subduction crust, resulting in drifting of the Central Tianshan continental arc from the proto Tarim Craton in association with opening of the South Tianshan back-arc basin in-between.