MULTIPLE DEFORMATION AND TECTONIC EVOLUTION OF XING’AN- MONGOLIA OROGENIC BELT

Based mainly on field geological observation and geochronologic data, an early-middle Paleozoic convergent orogenic system (COS), composed of a Northern Orogenic Belt (NOB) and Southern Orogenic Belt (SOB) that comprise fold area, mélange, arc-pluton rocks, and foreland basin, has been recognized from Xing’an- Mongolia orogenic belt (XMOB) in north China. The COS was followed by the forming and closure of extensional basins (EB) during late Paleozoic to early Mesozoic extension and compress. Multiple deformations have been observed in the NOB, SOB and EB, which is characterized by different types, time constraints, and regional distribution.

The first stage of deformation occurred in fold area, mélange, arc-pluton rocks of the NOB and SOB, which can be divided into two events: the first event is characterized by centimeter-sized rootless folds that are not the main ones responsible for the regional structure. The second structural event is marked by folds that are typically developed at the outcrop scale. Regionally, they are recognized by a widespread foliation underlined by mesericites, mica or quartz aggregates. The deformation in foreland basin is characterized by open folds with middle to high angel axial-plane cleavages that cut bedding. Regionally, these folds and axial-plane cleavages are widespread and easily observed, indicating the continuous north or south-inverted fold style. These deformations suggest the formation of the XMOB during the Paleozoic.

The second stage of deformation developed in the EB, which represented by fold and ductile thrust or transform shear zones. They result in the closure of the EB and cut both Triassic cover and older basement. These deformations display a regional compress and transform after extensional tectonic setting from the late Paleozoic to early Mesozoic in the XMOB.