Tectonic framework of Qilian orogen: reveal from an aeromagnetic anomaly feature

Abstract

The Qilian Orogenic belt is one of the typical orogenic belts globally and a natural laboratory for studying plate tectonics. Many researchers have studied the ophiolite and high pressure and ultra-high pressure metamorphic rocks in the Qilian orogen and obtained valuable achievements. However, a hot debate exists on the basement property, the distribution of ophiolite, and the boundaries of tectonic units. Large-scale high-precision aeromagnetic surveys have recently been conducted in the Qilian Orogenic belt and adjacent areas. In this study, we are trying to analysis the tectonic framework of the Qilian Orogen using 1:500, 000 aeromagnetic data. The results provide geophysical perspectives for studying the structural framework and deformation of this area. According to the aeromagnetic ΔT anomaly map, the central and Southern Qilian have the same magnetic anomaly feature that noticeably differs from the North Qilian Orogenic belt and the Qaidam Block. This result indicates that the central and Southern Qilian have a unified magnetic basement and differ from the North Qilian orogenic belt and Qaidam Block. The map shows the distribution of ophiolite in the North Qilian orogenic belt. Linear magnetic anomalies represent the ophiolites because the mafic–ultramafic rocks usually have high magnetic susceptibility. The ophiolite belts are continuously distributed in the western part of North Qilian orogenic belt and have a large scale. However, the scale of the ophiolite belt and the outcropping of mafic–ultramafic rocks reduces when they pass through Qilian County to the east. The results indicate differences in the evolution process between the eastern and western parts of North Qilian, with Qilian County as the transition zone. This study also systematically defines the geophysical boundaries of the Qaidam Block, Qilian Block, North Qilian Orogenic belt, and Alxa block. It is proposed that the sinistral displacement of the Altun Fault is adjusted and absorbed by the series of NE-trending faults in the Qilian orogen and merge into the Longshoushan–Gushi Fault. The extension of the North Qilian Orogenic belt is strengthened by the neotectonics movement along the shearing direction, which separated the North Qilian Orogenic belt into several segments and formed a series of northeast-trending faults.