TIME SCALES OF FLUID FLOW AND THERMAL RELAXATION DURING HIGH-STRESS DEFORMATION IN EXTENSIONAL DETACHMENT MYLONITE

The time scales of ductile deformation, thermal relaxation and associated syntectonic fluid flow in mylonites provide key elements in reconstructing the kinematic and tectonic history of orogenic belts once the orogen starts to collapse. Low-angle brittle and ductile normal fault systems, termed detachments, delimit the metamorphic core complexes of the North American Cordillera and accommodated most of the exhumation related strain during uplift of high grade metamorphic core rocks. We have investigated the relationships between recrystallization, meteoric fluid flow, and oxygen and argon isotope exchange in mylonitic quartzite from the eastern detachment of the Shuswap metamorphic core complex (British Columbia). Oxygen isotope compositions of discrete muscovite-quartz pairs from a strain gradient across the detachment yield a systematic fractionation pattern that increases with distance from the hanging wall. Calculated oxygen isotope temperatures decrease from 480 °C at the contact with the hanging wall to 380 °C at 500 m below the fault contact. This systematic variation correlates with ⁴⁰Ar/³⁹Ar muscovite crystallization ages that show along the same profile decrease from 49.0 Ma to 47.9 Ma. The results of oxygen isotope thermometry and in situ ⁴⁰Ar/³⁹Ar geochronology suggest that deformation temperatures in the detachment were transient in space and time and ductile deformation in the mylonitic quartzite was localized in progressively deeper levels of the footwall over 1.1 ±0.2 m.y. at cooling rates on the order of 75 to 125 °C/m.y. Rapid cooling rates in the mylonitic quartzite are most likely to result from rapid uplift and exhumation of the footwall mylonite. This conclusion is supported by the ubiquitous presence of high-stress microstructures in quartz and the lack of pervasive recrystallization microstructures in the mylonite. The variable depletion in ¹⁸O in the mylonite compared to the undeformed and isotopically homogeneous core quartzite suggests that meteoric fluid flow was localized and variably affected the δ¹⁸O values in the mylonite due to the very strong anisotropy of the deformation fabric. This implies that prominent mylonitic fabrics commonly associated with extensional detachments provide a crustal-scale anisotropy that can act as conduit for localized fluid and heat transport.