Monazite as a monitor of shear strain in orogenic crust

The Ramsay-Graham heterogeneous simple shear zone model revolutionized our understanding of finite strain variation in orogenic belts. Many deeply exhumed (>20 km depths) shear zones in orogenic continental crust are a consequence of sub-simple shear strain accumulation during contraction and displacement at high temperature. Here we explore the micro-scale record of this process using syn-kinematic monazite in two shear zones in a >20,000 km2 terrane of exhumed lower continental crust. Field observations combined with high-spatial resolution X-ray mapping, Th–U-Pb electron microprobe petrochronology, and electron backscatter diffraction analysis demonstrate that monazite grains can provide an important textural and temporal record of shear strain in mylonite and ultramylonite gneisses. Monazite grains exhibit aspect ratios of 1.7–4.6 parallel to the stretching lineation and parallel or oblique (<30°) to the penetrative foliation. Monazite EBSD data for grain orientation spread, i.e., average misorientation relative to the mean, are consistent with no significant dislocation-accommodated internal deformation. We conclude that dissolution precipitation creep of monazite produced compositionally-defined geometries compatible with the dextral sense of shear observed in outcrop and thin section. Syn-kinematic rims on monazite grains that develop in low strain domains during accumulation of shear strain can provide absolute ages for μm-to km-scale kinematics in continental orogens.