Paleozoic tectonic evolution of the Yili Block, Western Chinese Tianshan

Abstract : The Yili block is a triangular area bordered by sutures and fault zones in the western Chinese Tianshan belt. It is often considered as a part of the Central Tianshan micro-continent with Proterozoic basement extending westward into Kazakhstan and Kirgizstan, but this interpretation is questionable. This paper aims to synthesize the available data, discusses the meaning of the tectonic boundaries and proposes a model for the Paleozoic evolution of the Yili block. Alike the entire Tianshan belt, the Yili block underwent a polyphase evolution including subduction of oceanic crust and collision with micro-continents and volcanic arcs. The southern boundary of the Yili block is formed of Proterozoic basement and Early Paleozoic platform sediments, tectonically overlain by oceanic high-pressure metamorphic rocks and ophiolite. It has been involved in a south-dipping subduction associated with the closure of the Tianshan Ocean and the subsequent collision with a micro-continent correlated with Central Tianshan. This tectonic event resulted in top-to-the-north ductile thrusting observed in oceanic HP metamorphic rocks and Proterozoic basement as well. During the Late Paleozoic, the northern boundary of the Yili block was an active continental margin related to the southward subduction of the North Tianshan oceanic basin, this boundary is represented by Late Carboniferous turbidite and ophiolitic mélange. The southern and northern boundaries have been both reworked by Permian strike-slip faults.