Fossil oceanic subduction zone beneath the western margin of the Trans-North China Orogen: Magnetotelluric evidence from the Lüliang Complex

Abstract

The Trans-North China Orogen (TNCO) is a major collisional belt along which the Western and Eastern Blocks of the North China Craton were amalgamated through subduction–accretion–collision tectonics during Paleoproterozoic. The Lüliang Complex along the western margin of the TNCO is thought to be the trace of an ancient suture zone. Here we employ magnetotelluric array to build a lithosphere-scale high-resolution 3-D resistivity model for this region. Between 36°N and 38°N, E-W trending slices of the resistivity model show relatively high-conductivity (∼10–100Ωm) lithospheric mantle of the Western Block that extends eastward beneath the relatively high-resistivity lithosphere of the TNCO at depths from 80 to 180km. We interpret this east-dipping upper mantle conductivity contrast as the lithosphere-scale suture between the Western Block and TNCO that formed through eastward subduction. The enhanced conductivity of the fossil oceanic subduction zone might be a reflection of the reduction in grain size of olivine grains in mantle peridotite by the localized shearing stress associated with the subduction, combined with the interconnecting high-conductivity minerals along grain boundaries formed through metasomatism by slab-derived fluids. Similar subduction-related boundaries are absent in the resistivity models beneath the northern Lüliang Complex near 39°N, where prominent high-conductivity anomalies indicating asthenosphere upwelling and lower crustal underplating are seen. Such mantle architecture could result from the subduction of an oceanic ridge or slab window, as also proposed for the origin of the subduction-related Luyashan charnockite suite in this region. Our data also suggest that the Phanerozoic tectonic and magmatic events in this region only locally modified the Precambrian lithospheric tectonic framework without total destruction. We propose a 3-D model to illustrate the contrasting Paleoproterozoic tectonic evolution between the northern and southern domains of the Lüliang Complex and the TNCO during the transition stage between the termination of oceanic subduction and the subsequent continental collision.