Geophysical potential field data interpretations to study continental construction processes of the Central Asia Orogenic Belt

The Central Asia Orogenic belt (CAOB) is the largest accretionary orogen in the world which forms one third of the continental crust of Asia. This vast region was accreted from Late Proterozoic to Permian during several major tectonic pulses. The CAOB consists of vast regions represented by oceanic crust, domains of typical ocean-continent transition, accretionary wedges, magmatic arcs, ophiolitic belts and continental fragments of various age and origin. We present a complete Bouguer anomaly map, different treatments on an airborn magnetic map and new gravity model of structure of the belt together with new interpretative geotectonic map of the CAOB based on modern structural investigations. The gravity surveys show that typical oceanic domains and ophiolitic belts are characterized by large regions of gravity lows, while continental fragments reveal presence of major gravity highs. The transitional passive margin areas and magmatic arcs show intermediate values between oceanic and continental domains. The crustal thickness depicted from seismic refraction data show that the crust is in general 45 km thick in the whole area except northern continental domain where it reaches 55 km. This indicates that gravity anomalies are developed in crustal levels. Magnetic anomalies delineate well the arcuate pattern of ophiolite belts, oceanic crust belts and major magmatic and the position of volcanic arcs lineaments. Gravity modeling shows that the oceanic crust domains consist of crust of anomalous density of dioritic average composition. These estimates are consistent with mafic to intermediate composition of garnet and opx granulite xenoliths in tertiary volcanic indicating crustal thickness of 35 - 40km. Based on geophysical and new geological data we propose a new model of continental construction in the area of CAOB. According to this model, continental fragments, oceanic zones and lineaments of magmatic arcs developed during Early Paleozoic times at high angle to the Siberian continental margin. This complex and vast system was progressively shortened during orthogonal Permo-Triassic shortening which leads to complex and arcuate structural pattern of the whole belt. It is a crustal buckling and reorientation of continental fragments in conjunction with thickening of passive margin areas which is responsible for development of structure and anomalous crustal thickness of the CAOB accretionary system.