Characteristics and dynamic origin of the large-scale Jiaoluotage ductile compressional zone in the eastern Tianshan Mountains, China

The Jiaoluotage ductile compressional zone (JDCZ) in the eastern Tianshan Mountains, China, developed in volcanic and sedimentary rocks that were deposited in a Carboniferous back-arc basin situated between the Jungar plate and middle Tianshan terrane. The ductile compressional zone, ∼500 km long and 20–50 km wide, consists of a number of east-trending vertical foliation zones that display an augen geometry in plan view and a fan-like geometry in transverse profiles. Ductile compressional deformation has produced pervasive foliation, radial stretching lineations, contemporaneous folds, and various macroscopic to microscopic symmetric structures within the JDCZ. Rotations of most clasts and porphyroclasts are bidirectional. The preferred orientation patterns of the optical axes of calcite and quartz display axial or orthorhombic symmetry. Three kinds of strain patterns have been determined in the JDCZ: apparent flattening, plane strain and apparent constriction. The overall strain character of the JDCZ is ‘cream-cake’ style. The volcanic and sedimentary rocks within the JDCZ were metamorphosed into greenschist facies with temperature of 400–600 °C and pressure of 300–400 MPa. Development of the JDCZ is interpreted as due to N–S coaxial compression caused by the collision between the Tarim plate and the middle Tianshan arc–Jiaoluotage basin–Jungar plate system during the Permian between 270 and 290 Ma.