THE LOUZIDIAN NORMAL FAULT NEAR CHIFENG, INNER MONGOLIA: MASTER FAULT OF A PSEUDO-METAMORPHIC CORE COMPLEX

A pseudo-metamorphic core complex underlain by a low-angle strike-slip ductile shear zone is present near Chifeng in eastern Inner Mongolia of northern China. The geology of the study area bears similarity to that of several Cordilleran metamorphic core complexes, but contrasts in significant ways as well. The Luozidian fault strikes NNE for at least 100 km, and typically dips east-southeastward at angles between 10 to 65 degrees. This major SE-dipping normal fault separates a crystalline footwall from a non-metamorphic hanging wall that is distended by normal faults. The footwall consists of the Archean gneisses (ca 2500 Ma, U-Pb zircon, Wang, C., 1989; Wang, S. et al., 1994) and Mesozoic granitic plutons (142 to 192 Ma, K-Ar, Wang, S. et al., 1994). The hanging wall is dominated by Jurassic and Cretaceous coal-bearing strata. The Louzidian fault contains a number of structural elements that resemble those of the Cordilleran metamorphic core complexes: (1) a >100-km long NNE-trending antiformal dome; (2) a major, shallow-dipping normal fault along the SE flank of the dome; (3) footwall mylonitic gneisses up to 3 km thick that lie parallel to the fault along ca 40 km of its length; (4) chloritic breccias (retrograded mylonitic rocks) up to 100 m thick in the footwall of the fault and directly below it; (5) microbreccias up to 40-50 cm directly below the fault at some localities; and (6) a hanging wall assemblage of unmetamorphosed supracrustal strata and their Archean basement. However, in marked contrast to the geology of Cordilleran core complexes, the shear sense indicators in the footwall mylonitic rocks exhibit strike-slip rather than normal faulting. The strike-slip shearing predates younger dip-slip normal faulting. That appears to have been controlled by the mylonitic fabrics and produced chloritic breccias, microbreccias, and gouge. Two illite-rich gouge samples yield the latest Cretaceous ages of 65.3 and 71.1 Ma, respectively. This is in good agreement with the fault truncation of Late Jurassic-Early Cretaceous terrestrial strata in the hanging wall and Triassic plutons in the footwall.