Paleo−Pacific plate subduction on the eastern Asian margin: Insights from the Jurassic foreland system of the overriding plate

The subduction of the paleo−Pacific plate beneath the North China block has been well documented in terms of magmatic activity, geophysical investigations, and numerical modeling, but the timing of its onset along the eastern Asian margin and the tectonic processes involved remain poorly understood. We have now reconstructed the structural evolution and sedimentation of the Shihuiyao−Gangzidian−Yuantai region of the southern Liaodong Peninsula, on the overriding plate at the eastern Asian margin, using geologic mapping, borehole data sets, field structural studies, and zircon geochronology. In the Shihuiyao area, based on geologic mapping and drilling, top-to-the-NW thrust faults truncated earlier top-to-the-NE reverse faults and then were subsequently overprinted by normal faults. In the Gangzidian area, geologic mapping and 40 boreholes allowed us to construct eight cross sections revealing the top-to-the-WNW/W thrust faults from the surface to a depth of at least 0.5 km; the sections show that the thrust faults extend to the basement and that subsequent normal faults dip subvertically. In the Yuantai thrust system, the top-to-the-NW thrusts, expressed as four tectonic windows and a duplex on the map scale, were intruded by late porphyry and diabase sills. Integration of the data from the three study areas allowed us to identify one angular unconformity and at least two phases of later deformation (D1 and D2). The pre-D1 unconformity is marked by a foreland basin that was filled with Middle Jurassic clastic rocks that unconformably overlie the Neoproterozoic and Cambrian basement. The D1 deformation is represented by NE-SW−striking thrust faults that displaced Neoproterozoic or Cambrian strata onto the Middle Jurassic strata. The D2 deformation is defined by kilometer-scale, high-angle normal faults with variable dips and strikes. Although three samples from the Middle Jurassic clastic rocks did not yield ideal maximum deposition ages (MDAs; ca. 246 Ma), a porphyry dike and a diabase sill that intruded the Yuantai thrust system and remain undeformed yielded zircon U-Pb ages of ca. 124 and 117 Ma, respectively. Detrital zircon geochronology yielded a MDA of ca. 129 Ma for Cretaceous deposits in graben structures with hanging walls formed by D2 normal faults. Thus, the D1 and D2 deformation events can be constrained to the Late Jurassic and Early Cretaceous, respectively (D1 between ca. 174 and 124 Ma, and D2 after ca. 129 Ma). The provenance of the detritus in the Middle Jurassic Wafangdian Formation suggests that the deposits in the retroarc foreland basin had a source in a thrust sheet of the Paleoproterozoic basement, but the source of deposits in a minigraben (D2) was possibly the nearby Neoproterozoic rocks. Therefore, we reconstructed the retroarc foreland basin and fold-and-thrust belt of the southern Liaodong Peninsula in terms of a subduction margin and constrained a Toarcian−Aptian (ca. 174−124 Ma) age for the onset of paleo−Pacific plate subduction. We interpret the foreland basin system and the subsequent synconvergent extension to have been the result of slab flattening and rollback during episodic subduction of the paleo−Pacific plate.