A NATURALLY CONSTRAINED PROFILE OF THE STRENGTH OF THE MIDDLE CRUST NEAR THE BRITTLE-DUCTILE TRANSITION

Electron backscatter diffraction (EBSD), Ti-in-quartz (Titani-Q) thermometry, and paleopiezometry allow us to construct naturally constrained profiles of crustal strength in areas of exhumed mid-crustal rocks. As an example, we examine the footwall of the Whipple Mountains metamorphic core complex (WMCC). Rocks in the WMCC were initially deformed at 15 – 20 km depth by distributed ductile shear, and were then progressively overprinted by localized ductile shear zones and eventually by discrete brittle fracture as the footwall was cooled and exhumed toward the brittle-ductile transition. Increasing localization and cooling during exhumation allowed earlier distributed fabrics to be preserved, and rocks in the WMCC therefore represent several ‘points’ in temperature-stress space (and by inference depth-stress space). We collected mylonitic granitoids from four transects near the Whipple detachment fault and applied EBSD, Titani-Q and the experimental piezometer of Stipp et al. (2002) to individual quartz-rich domains. To relate the temperature measurements from each sample with depth, we use a finite-element code (Milamin) to solve the 2D heat-transfer equation for the temperature distribution below the detachment, using published cooling rates and standard material parameters. Some samples have quartz ribbons with a consistent dynamically recrystallized grain size and uniform temperature distributions and thus represent a single point in stress-depth space. Others are ‘composite’ in that a progression from distributed high-T low-stress deformation to low-T high stress deformation can be tracked at the field- and micro-scale. The overprinting history can also be inferred from quartz crystallographic preferred orientation patterns, which indicate prism <a> slip in higher temperature regions and larger grain sizes, and rhomb and basal <a> slip with decreasing recrystallized grain size. Some samples have highly localized shear zones that change from brittle-to-ductile along their length, and recrystallized grain sizes from these give a maximum strength at the BDT. The largest uncertainties stem from establishing the initial geothermal gradient, but these can be reduced by thermochronological constraints of the timing of each point on the stress-depth path and the associated cooling rates.