Strain Localization In The Midlle Crust: Constraints From Quartz Crystallographic Fabrics Along The Jinzhou Detachment Fault Zone

A sequence of tectonites are resulted from progressive detachment faulting during gradual exhumation of the lower plate of Jinzhou Fault Zone,North China.The tectonites transit from brittle cataclasites near the master fault surface,to brecciated mylonites,mylonites,mylonitic gneisses,and striped gneisses,and finally grade into undeformed metamorphic rocks in the lower plate.This variability is ordered in general but disordered in part.The fabric during deformation in brittle-ductile transition level were preserved in the exhumed Jinzhou detachment fault zone which come as a breakthrough to reveal strain localization as well as the extremely complex deformation mechanics of rock within mid-crust.The electron backscattering diffraction technique(EBSD)was used to analyze crystallographic preferred orientations(CPOs)of naturally deformed quartz aggregates.Strong quartz CPOs were developed during dislocation creep in the mylonite zone exposed in the lower wall of the Jinzhou Fault Zone.The CPOs have a consistent asymmetry indicating a high ratio of simple to pure shear strain,with a shear sense of dextral-up to the NW,consistent with the mesoscopic shear sense indicators and with slip on the active fault.There is a transition from Y-maxima and asymmetric single girdles within mylonitic gneisses and mylonites to cross-girdle fabrics within the brecciated mylonites closer to the brittle fault.The strong Y-maxima or single girdle CPOs are ascribed to prism <a> and rhomb <a> slip systems under amphibolite facies conditions.The cross-girdle patterns found in the brecciated mylonites are interpreted to represent single girdle CPOs development through lower temperature conditions where basal <a> slip is dominant at depth within the mid-crust.Sample bulk fabric are contributed by new recrystallized grain which possess orientations with c-axes preferentially near the Z-axis resulted from basal <a> slip combined with residual grain which remain the prism <a> system due to those grains with c-axis subparallel to Y-axis which are unsuitable for basal <a> slip under lower temperature conditions.The exhumed fabrics are preserved in latest increment deformation so that CPO record the fabric where rocks get rid of the affected scope of detachment fault during exhumation.The exhumed fabrics zone support the proposal that localization of shear strain must have occurred along the fault zone leading to reduction of affected area of shear strain as temperature decrease.