Structural characteristics and formation mechanism of the Kalaqin metamorphic core complex in the Yanshan area, China

Abstract

Based on new detailed field observations and magmatic zircon dating, we reanalyzed deformation characteristics and tectonic evolution of the Kalaqin metamorphic core complex in the northern Yanshan tectonic belt. We propose that two stages of extension occurred in formation of the core complex in Late Jurassic and Early Cretaceous, rather than only one in Early Cretaceous as previously suggested. During Late Jurassic (~156-150 Ma), the core complex originated from a shallow detachment ductile shear zone with a top-to-the-NE shear sense under NE-SW extensional stress. The footwall was ductilely deformed, and shear zone development was associated with magmatic activity and formation of rift basins. Ductile shearing was followed by limited isostatic uplift ~149-145 Ma. During Early Cretaceous (~141-100 Ma), the extensional direction switched to NW-SE, leading to development of two NE- striking major normal faults with opposite dip directions along the eastern and western edges of the core complex. Early Cretaceous half-graben basins developed on the hanging walls of both major faults, as their shared footwall rose in an extensional dome. This doming resulted in exhumation of the Late Jurassic ductile extensional structures to shallow levels. The Kalaqin metamorphic core complex was overprinted by earlier ductile deformation belts and later brittle normal faults during the extensional doming of Early Cretaceous, the deformation shows characteristics of strain localization. Our new structural evidence allows determination of the evolution and formation mechanism of the Kalaqin metamorphic core complex, and confirms extensional deformation in both Late Jurassic and Early Cretaceous in the Yanshan tectonic belt. The Early Cretaceous extension is intense and widespread, and represents the peak destruction of the eastern North China Craton.