THE MESOZOIC FENGNING-LONGHUA AND JIAOQIER FAULT ZONES, NORTH CHINA: NEW INTERPRETATIONS OF CONTROVERSIAL STRUCTURES

Field studies in North China cast new light on the origin of two major E-W-trending fault zones the Fengning-Longhua zone of northern Hebei province, and the Jiaoqier fault of Inner Mongolia. The former steep zone of faulting has a strike length of ~200 km and may continue westward another 160 km as the Shangyi-Chicheng fault. The outcrop pattern of the Fengning-Longhua fault zone in the Mesozoic Yanshan fold and thrust belt is quite puzzling. For about 70 km, near Longhua, the zone contains a very narrow belt (< 0.5 km) of subvertical Proterozoic strata bordered on both sides by Archean crystalline rocks. Steep faults commonly border both margins of the Proterozoic strata, but at several localities the southern vertical contact is a nonconformity. The northern contact is now a north-side up, steep reverse fault, except where locally reactivated by normal faults. We propose the following deformational sequence: (1) Late Triassic or E. Jurassic north vergent thrusting of Archean basement and Proterozoic strata, perhaps as a back-thrust to major south directed thrusting of similar rocks 60 km to the south; the anticlinally folded hanging wall of the north vergent fault contained vertical, north-facing Proterozoic strata; (2) southward steep reverse faulting across higher levels of the Proterozic section, placing Archean rocks against it; (3) Cretaceous formation of several half grabens along both sides of the vertical Proterozoic section. Contrary to earlier workers, we have found little evidence for major Mesozoic strike-slip displacement along the zone.

The south dipping Jiaoqier thrust(Wang et al., 1995) has an exposed length of >140 km, but is not a north-directed thrust at all. South of Sonid Zuoqi it is a shallow dipping extensional detachment fault with a thick (up to 1 km) footwall mylonitic zone and a SSE-ward sense of shear. Mylonitic foliation has variable southward dips from 20-45 degrees. Hanging wall rocks are predominantly low-grade Proterozoic and Paleozoic strata. Footwall mylonitic rocks with Triassic ages of 224 Ma (Ar/Ar) and 247 Ma (K/Ar) overprint a muscovite-bearing Permian granite (252 Ma, U-Pb; Li et al., 1997). Lower to Middle Jurassic coal-bearing strata sit unconformably on the footwall granite and indicate that this metamorphic core complex is considerably older than kinematically and geometrically similar Cretaceous core complexes in Yanshan (Yunmeng Shan), Daqing Shan (Huhhot), and South Gobi(Yagan-Onch Hayran) areas to the southeast, south, and west respectively. The regional extent and tectonic significance of this early Mesozoic intraplate extension has not yet been defined.