Paleogeographic Orientation of the Sino-Korean Plate Based on Evidence for a Prevailing Silurian Wind Field

摘要

The regional geology of southern Inner Mongolia records relationships between an upper Silurian coastline and a small island on what was formerly the continental shelf of the Sino-Korean Plate(North China Block).In both cases,rocky shorelines are defined by unconformities between strata of Ludlow age in the Xibiehe Formation and underlying igneous rocks of Ordovician age.The paleoisland occurs near Bater Obo about 50 km south of the Chinese-Mongolian border.The diorite core of this geological inlier is oriented on an axis N 55° E.A simple dichotomy of exposed windward versus sheltered leeward environments accounts for contrasting carbonate facies on the flanks of the paleoisland.Wider corroboration of a prevailing Silurian wind field is supported by evidence from other unconformities about 30 km to the southwest,where a linear paleoshore extends for 15 km on an average bearing of N 60deg E.Siliciclastic sandstone derived from a granodiorite source in the Gushan area now sits in close proximity to basaltic bedrock in the Baoerhantu area.Local transport of sand is the result of alongshore currents driven by winds that were consistent in direction with the facies pattern around the paleoisland.Global-scale interpretations of Silurian paleogeography typically place the Sino-Korean Plate in low latitudes north of the paleoequator,where it would receive trade winds out of the northeast.In terms of present geography,however,the Silurian winds of Inner Mongolia are aligned with a source out of the northwest.None of the published scenarios of Sino-Korean paleogeography accommodates the physical data from the Bater Obo,Baoerhantu,and Gushan areas of Inner Mongolia.Based on these relationships,the Sino-Korean Plate must be rotated 90deg clockwise from its present orientation in order to approximate its original Silurian orientation under a prevailing atmospheric system associated with northeasterly trade winds.