Kinematic and thermochronological constraints on the Xincheng-Huangpi fault and Mesozoic two-phase extrusion of the Tongbai-Dabie Orogen Belt

摘要

The Tongbai-Dabie Orogenic Belt (DOB) is composed of several fault-bounded terranes with different metamorphic and deformation intensities. These tectonically stacked terranes were extruded along their boundary faults from depths of >120 km during the Mesozoic. Systematic structural studies on these boundary faults can provide important information for the exhumation processes of the orogen. In this paper, we focus on the Xincheng-Huangpi fault (XHF) which is located along the southern boundary of the central DOB ultrahigh-pressure complex unit (UC) Comprehensive macro- and micro-structural, U/Pb zircon and ~(40)Ar/~(39)Ar geochronological data are presented here to constraint the structural chronology of the XHF. The XHF represents the lower detachment surface formed during extrusion of the UHP slab and experienced two episodes of ductile shearing. The XHF experienced early southwest-ward thrusting around 234 Ma and was exhumed to middle crustal levels during 234-195 Ma with an average exhumation rates of ca. 1.5 mm/yr. The UHP rocks then exhumed to the surface by erosion. Later NW-SE trending dextral strike-slip shearing of the XHF occurred between 145 and 140 Ma and was generally contemporaraneous with sinistral-oblique slip of the Xiaotian-Mozitan fault along the northern margin of the UC. Coeval dextral and sinistral-oblique shearing along the southern and northern margins of the UC would have caused southeastern lateral extrusion. This lateral extrusion has no relationship with the UHP exhumation.