A naturally constrained stress profile through the middle crust in an extensional terrane

Abstract

We present a method in which paleopiezometry, Ti-in-quartz thermobarometry (TitaniQ), and 2-D thermal modeling are used to construct a naturally constrained stress profile through the middle crust in an area 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 ~20km 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 (BDT). Increasing strain localization and cooling during exhumation allowed earlier microstructures to be preserved, and rocks in the WMCC therefore represent several points in temperature–stress space (and by inference depth–stress space). We identify enough of these stress–depth points to construct a complete profile of the flow stress through the middle crust to a depth of ~20km, from which we derive regional estimates of the ambient stresses in the brittle upper crust, and the peak strength at the brittle–ductile transition in this region during Miocene extension.Maximum differential stress reached ~136MPa just below the brittle–ductile transition at a depth of ~9km. Stress levels are consistent with Byerlee's law in the upper crust assuming a vertical maximum principal stress and near-hydrostatic pore fluid pressures, and suggest a coefficient of friction on the 25°-dipping Whipple fault of ~0.4. Differential stress decreases to 10–20MPa at 20km depths and ~500°C. For strain rates typical of actively deforming regions (10−12 to 10−15/s), our stress profile is bracketed by the Hirth et al. (2001) flow law for wet quartzite, whereas the flow law of Rutter and Brodie (2004) overestimates the strength of this particular region.