Understanding the mechanisms of strain localization is the key to our understanding of the transition from steady-state flow to unstable flow in the middle crust. In this paper, studies on deformation of gneisses sheared at mid-crustal level along the Jinzhou detachment fault zone, Liaodong peninsula, North China, reveal that biotite grains, as pre-existing weak-phase, have important influences on deformation of middle-crustal rocks. High phase strength contrasts between biotite grains and other mineral phases resulted in stress concentrations during shearing and occurrences of microcracks at the tips of biotite grains. Consequently, microcracks are formed either along contacts between high strength mineral grains or propagate into the mineral grains. The microcracks filled with biotite grains and fine-grained feldspar aggregates continue to nucleate, propagate, and coalesce in the rocks, while basal plane slip and grain boundary sliding (GBS) operate in biotite grains and fine-grained feldspar aggregates, respectively. These processes lead to a transition from load-bearing framework (i.e., coarse plagioclase grains) to interconnected weak phase (i.e., biotite grains and fine-grained feldspar aggregates), and the formation of incipient strain localization zones (SLZs). With the propagation and linkage of the SLZs, high stress concentrations at the tips of SLZs lead to nucleation of fractures. At the same time, there occurs an abrupt increase in strain rates that result in the transition from dislocation creep and GBS (velocity strengthening) to unstable slip (velocity weakening). The processes are accompanied by occurrence of mid-crustal earthquakes, and formation of pseudotachylite veins along with SLZs.

To better understand the mechanisms of crustal exhumation related to tectonic extension, we report on the progressive doming and detachment faulting of the Cretaceous Liaonan metamorphic core complex (MCC). The detachment fault zone of Liaonan MCC is comprised of two branches, i.e., the Jinzhou detachment fault zone (JDFZ) and the poorly-researched Dongjiagou shear zone (DSZ). Thus, integrated structural, microstructural, quartz c-axis fabrics, and fluid inclusion analysis, and U-Pb on zircon dating were performed on mylonites along the DSZ. In contrast with the JDFZ that possesses characteristics of detachment fault zone, the DSZ encompasses Archean gneisses and Neoproterozoic meta-sedimentary rocks, between which exists an obvious metamorphic contrast forming a tectonic discontinuity contact (TDC). However, rocks from both sides of the TDC possess structures and fabrics for identical geometries and kinematics that are consistent with those along the JDFZ. Thermometric analysis of fluid inclusions from syn-tectonic quartz veins (630 °C, 470 °C, 350 °C) and quartz c-axis fabric from mylonites along the DSZ show that the shearing penetrates throughout the Archean to Neoproterozoic rocks. Dating of zircons from syn-kinematic granitic dikes from DSZ yields an age ca. 134 Ma, which is similar to the ages of early shearing along the JDFZ (ca. 133~134 Ma). The results imply that the shearing initiated in both JDFZ and DSZ at an early stage, then progressive shearing continued, and finally developed the detachment faulting along the JDFZ. Based on the timing and processes of the regional extension, a geodynamic model of MCC’s is proposed.