A CHINESE CONUNDRUM — CAN A TRIASSIC METAMORPHIC CORE COMPLEX IN NORTHERN CHINA BE BETTER EXPLAINED BY MID- PALEOZOIC THRUST FAULTING?: A NEW INTERPRETATION FOR THE WESTERN HILLS "METAMORPHIC CORE COMPLEX" OF BEIJING

An important Chinese conundrum is the existence — or not — of an Indosinian (Triassic) metamorphic core complex (mcc) in the Western Hills of Beijing. This area has a Mesoproterozoic to Cretaceous stratigraphy atop an Archean basement. Silurian and Devonian strata are missing, as they are throughout most of northern China. The Western Hills are widely interpreted as an extensional mcc based on several lines of evidence, among them that the entire section through the early Triassic has been metamorphosed and contains a variety of stretching-type lineations with WNW-ESE trends. In addition, the pre-Carboniferous stratigraphic section is said to be dramatically thinned and to contain detachment surfaces, subparallel to stratigraphic units, that cut out section.

What is missing in this "mcc" is a master detachment fault that overlies a lower plate of the proposed complex. A second complication is that the metamorphic grade of pre-Carboniferous units is much higher than that of Carboniferous rocks (coal, slate, phyllite, metasandstone and metaconglomerate). Lower Paleozoic and Proterozoic calc-mylonites and other ductilely deformed lithologies include kyanite- and garnet-staurolite- chloritoid-bearing rocks in the Neoproterozoic Qingbaikou Fm; PT conditions for the kyanite-bearing rocks are reported as >5.5 kbar and 450-550° C (Wang and Chen, 1996). This thermobarometry is incompatible with a Qingbaikou depth of burial beneath the overlying Cambrian through Triassic section of ≤4-5 km. It is suggested here that the Cambro-Ordovician and Proterozoic section lay within a ductile shear zone beneath a thick cover of Archean basement and cover nappes. An apparent thrust belt remnant, an allochthonous slice(s) of Qingbaikou through Ordovician units, lies below Late Paleozoic strata west of Nanjiao. Following nappe erosion, Carboniferous to Triassic strata were deposited, and then metamorphosed and deformed in response to terrane amalgamation along the northern margin of the North China craton. If this controversial hypothesis for late Early or mid-Paleozoic basement-involved thrust faulting within the North China craton is correct, it may be the first deformation of this age to be recognized. Ordovician-Silurian oceanic subduction has been documented along north and south margins of the North China craton.