Imaging the crustal and upper mantle structures of the Junggar terrain in northwestern China: implications for subduction-collision-accretion processes in the past

The Junggar terrain of northwestern China is an ideal place to study the complex subduction collision-accretion processes in the Central Asian Orogenic Belts. Currently, the evolutionary history of the Junggar terrain is still under hot debate, especially the west Junggar region where multiple Paleozoic subduction systems are proposed to explain the tectonic evolution. Previous studies on the Junggar terrain are mostly based on geochemistry, geochronology and geomagnetism, a fine-scale velocity structure model of the Junggar terrain, which can provide vital constraints on the tectonic evolution, is still lacking. In this thesis, by employing receiver function analysis, ambient noise tomography, two planesurface wave tomography and teleseismic body wave finite-frequency tomography, I construct high-resolution 3D velocity models of the crust and upper mantle structure of the Junggar terrain using data from two seismic arrays deployed in the Junggar terrain. Because most of the subsurface features in the Junggar terrain are well-preserved since the late Paleozoic, our models help to unveil past tectonic processes in the Junggar region. From the receiver function analysis, I find that significant Moho variations are observed across the study region and a pronounced Moho offset is imaged between the Zaire mountains and the western Junggar basin. From the seismic tomographic models, I image a pronounced high velocity anomaly beneath the Junggar basin at the middle/lower crust and the uppermost mantle. This high velocity body slightly dips northwestward under the west Junggar to ~150 km depth, then appears at ~400 km depth. This high velocity anomaly in the middle/lower crust and uppermost mantle is interpreted as the remnant of fossil subduction slabs by examining its speed and temperature, while the high velocity near the mantle transition zone beneath the west Junggar is considered as either the detached slab or part of the mantle convection system in the Junggar region. Based on these interpretations, I propose a failed northwestward subduction model with the fossil oceanic lithosphere still trapped beneath the Junggar basin and the western part of Junggar to explain the latest tectonic activities in the west Junggar. The identification of the fossil slab in the Junggar region is significant because most ancient oceanic slabs in continental collision zones are not well preserved. The fossil slab in the Junggar region not only gives important clues on the evolutionary processes in this region but also provides a nature laboratory to study the ancient subduction systems through the Earth's history.