Paleo-Asian oceanic crust trapped by irregular continental margin contributes to continental growth: Evidence from airborne gravity and magnetic imaging across the Alxa tectonic belt, Central Asia

Abstract

Unsubducted, and trapped or accreted oceanic crust contributes greatly to continental growth. The Alxa tectonic belt (ATB) is located at the critical intersection of the Central Asian Orogenic Belt (CAOB) and the North China Craton. However, little attention has been paid to the influence of the evolution of the Beidashan-Langshan arcuate tectonic belt in the ATB on the closure of the Paleo-Asian Ocean, the formation of the CAOB, and the growth of continental crust. Here we report new integrated airborne gravity and magnetic surveys in the ATB that image a high-amplitude magnetic anomaly zone distributed across the Quagan Qulu ophiolite belt. The three-dimensional magnetic structure inversion reveals that the Quagan Qulu magnetic anomaly zone consists of mid-lower crustal high-value magnetic susceptibility. Gravity-magnetic forward modeling reveals that the Quagan Qulu magnetic anomaly zone may have been generated by a crustal mafic pluton characterized by high magnetism and density, approximately same to the magnetic susceptibility and density of the exposed Late Paleozoic gabbros measured in the field, which are consistent with those of the Yueyashan-Xichangjing ophiolite belt in the Beishan orogenic belt adjacent to the ATB. We argue that the mafic pluton beneath the Quagan Qulu zone may be the trapped oceanic crust derived from the Late Paleozoic-Triassic Paleo-Asian Ocean (i.e., the Quagan Qulu Ocean). In conjunction with the integrated petrologic, chronologic and geochemical observations, we argue that the Beidashan-Langshan arcuate tectonic belt dominated the differential subduction of the Paleo-Asian Ocean and finally trapped partial Quagan Qulu oceanic crust. Therefore, we propose that, in addition to oroclines, the irregular continental margins could alternatively provide tectonic space for trapping oceanic crust, contributing to the lateral growth of the continental crust and serving as an important means of continental accretion in the CAOB.