Structural and rheological analysis of the crustal-scale Erguna detachment fault zone and implications for the Late Mesozoic crustal extension in continental Sino-Russia-Mongolia border tract

Abstract:

Detailed structural and rheological analysis of an exhumed ductile shear zone may provide insight into the processes and geodynamics of the crustal extension. A well-exposed crustal-scale shear zone in the footwall of the mylonitic Erguna detachment fault zone provides an ideal setting to investigate crustal evolution during Late Mesozoic large-scale extension in continental Sino-Russia-Mongolia border tract. This study reports on the geometry, kinematic indicators, rheological parameters, and ⁴⁰Ar/³⁹Ar biotite ages of the granitic mylonites to constrain the deformation across the detachment shear zone. Samples are collected from various structural levels of the shear zone in the footwall of the detachment, where a NW-dipping foliation and NW-trending stretching lineation reflect top-to-the-NW extension. Logarithmic Flinn parameters (1.18–2.35) indicate elongate strain which approximates to plane strain. Kinematic vorticity numbers (0.42–0.94) suggest that the shear zone formed under a combination of simple and pure shear. Mineral deformation behavior, quartz c-axis fabrics, quartz grain-size-frequency, and Kruhl thermometer reveal that the mylonites were formed by different mechanisms of crystal plastic deformation and dynamic recrystallization at temperatures from ca. 300 to 700 °C. The flow stresses at different crustal depths calculated using classical paleopiezometers show that the shear zone is subjected to slow deformation and is similar to the Whipple Mountain detachment faulting in the North American Cordillera. The ⁴⁰Ar/³⁹Ar plateau ages of biotite from the granitic mylonites are 121–106 Ma, which constrain the timing of low-temperature uplift and cooling but are younger than the ages of metamorphic core complexes in the Transbaikalia–northeast Mongolia region (ca. 130–120 Ma). The Late Mesozoic asymmetric collapse of the Mongol–Okhotsk orogenic belt caused the large-scale extension of the overthickened crust. The progressive west-to-east deformation triggered the extension to migrate from Transbaikalia–NE Mongolia to the Great Xing’an Range of NE China.