The deep thermochemical structure of the Dabie orogenic belt from multi-observable probabilistic inversion

Abstract

The Dabie orogenic belt (DOB) was formed by a continent-continent collision between the North China Craton (NCC) and Yangtze Block (YB) in the Middle to Late Triassic. It is thought to have experienced a large-scale orogenic collapse in the Early Cretaceous and significant modification/metasomatism during subsequent tectonic events. However, the deep thermochemical structure of the DOB and its surroundings remains poorly known. Here, we jointly invert multiple geophysical data sets with a probabilistic inversion method to obtain a 3D model of composition, bulk density and temperature beneath the DOB and surrounding areas. Our 3D model reveals high-temperature anomalies in the form of “hot fingers” beneath the eastern DOB (particularly under the Tanlu fault zone), indicating the existence of small-scale sublithospheric convection and a complex interaction between the sublithospheric mantle and the overlying lithosphere. Our imaged Mg# reveals a new NNW-trending boundary crossing the DOB, separating two distinct mantle compositional domains. The eastern DOB is characterized by a relatively thin lithosphere and lower Mg# (fertile), indicating that it has been more severely altered by the orogenic collapse and subsequent tectonic activity than the western DOB, where we observe a relatively thicker and more depleted lithospheric mantle. Finally, we combine this information with electrical conductivity data from a recent magnetotelluric profile to estimate the water content of the upper mantle. We find that the conductivity of the lithospheric mantle beneath the NCC, western DOB, and YB is controlled by water content rather than by temperature and composition. The higher water content of the lithospheric mantle beneath the NCC and YB indicates that water may have been introduced in the mantle before or during the Mesozoic and Cenozoic tectonism.