THE CHENGDE ALLOCHTHON, NORTHEASTERN HEBEI PROVINCE, CHINA: ONE FAULT OR TWO?

The Yanshan fold-thrust belt is a Mesozoic belt of deformation trending east-west in northern Hebei Province, China. Two east-west trending thrust faults, located north-south of each other in this thrust belt, delineate the northern and southern limbs of a 20 km wide, westward-plunging synform. These two thrusts have been interpreted as the northern and southern outcrop terminations of a 20 km wide, synformally folded klippe formed by a single, north-directed thrust that roots far to the south (the Chengde thrust; Davis et. al., 1998). Displacement on the Chengde thrust would be at least 35-50 km.

A key test of this interpretation is whether or not the two faults converge at the exposed eastern hinge of the synform. In this region, the basal Proterozoic stratigraphic sequence that elsewhere forms the upper plate of both faults rests unconformably upon Archean basement and is apparently unbroken by any large fault linking the northern and southern thrusts. Such a fault, if present, would locally juxtapose folded Proterozoic strata in its upper and lower plates in proper stratigraphic sequence and with no dip discordance. Interpretation of the northern and southern faults as separate thrusts with opposing vergence allows the stratigraphically continuous Proterozoic sequence that occupies the hinge of the synform to remain unaffected.

This interpretation, if correct, has implications for the magnitude of shortening experienced by the Yanshan fold-thrust belt in Jurassic time. Displacement along these two faults is not constrained, but was certainly far less than would be indicated by displacement along a single folded thrust.