Rheology of the middle crust under tectonic extension: Insights from the Jinzhou detachment fault zone of the Liaonan metamorphic core complex, eastern North China Craton

Abstract

Although exhumation of metamorphic core complexes (MCCs) have been traditionally attributed to collapse of orogenically overthickened crust, recent studies reveal that they can also result from extension of lithosphere with normal crustal thickness. For example, there is an ongoing debate about the mechanisms responsible for exhumation of the Liaonan MCC, which occurred during late Mesozoic lithospheric thinning of the North China Craton. This paper attempts to present rheological constraints on middle to upper crustal detachment faulting during the exhumation of the Liaonan MCC and, therefore, on the genesis of the MCC based on a comprehensive study of the microstructural and fabric characteristics of tectonites from the Jinzhou master detachment fault of the Liaonan MCC.The Jinzhou detachment fault zone comprises a thick sequence of fault rocks of middle to shallow crustal depths. Three types of mylonitic rocks characterize middle crustal deformation along the detachment fault during early Cretaceous lithospheric extension. Microstructural and fabric studies reveal that these fault rocks were formed via different mechanisms of crystal plastic deformation and dynamic recrystallization at temperatures from ca. 300 to 650°C. The flow stresses at different crustal depths were calculated using classical paleopiezometers. This study suggests that a pre-heated crust was responsible for the low flow stresses during the Jinzhou detachment faulting, which is somehow different from, e.g., the Whipple Mountain detachment faulting in the North American Cordillera. Based on inferences about the properties of middle to upper crustal flow associated with the Jinzhou detachment faulting, it is suggested that the Liaonan metamorphic core complex was formed by tectonic extension of cratonic lithosphere with a normal crustal thickness, instead of being a typical Cordilleran-type core complex that occurred in a setting with overthickened crust.