Constraints from rocks in the Taiwan orogen on crustal stress levels and rheology

Abstract

Taiwan's Hsüehshan range experienced penetrative coaxial deformation within and near the brittle-plastic transition between ∼6.5 and 3 Ma. This recent and short-lasting deformation in an active, well-studied orogen makes it an ideal natural laboratory for studying crustal rheology. Recrystallized grain size piezometry in quartz and Ti-in-quartz thermobarometry yield peak differential stresses of ∼200 MPa at 250–300°C that taper off to ∼80 MPa at ∼350°C and ∼14 MPa at ∼400–500°C. Stress results do not vary with lithology: recrystallized quartz veins in slates and metasiltstones yield equivalent stresses as recrystallized grains in quartzites. A minimum strain rate of 2.9 × 10^(−15) s^(−1) associated with this deformation is calculated by dividing a strain measurement (axial strain ∼0.3) in a strongly deformed quartzite by the available 3.5 m.y. deformation interval. We estimate a maximum strain rate of 7.0 × 10^(−14) s^(−1) by distributing the geodetic convergence rate throughout a region homogeneously deformed under horizontal compression. These stress, strain rate and temperature estimates are consistent with the predictions of widely applied dislocation creep flow laws for quartzite. The samples record stress levels at the brittle-plastic transition, indicating a coefficient of friction (μ) of 0.37 in the upper crust consistent with results based on critical taper. Integrated crustal strength of the Hsüehshan range amounts to 1.7 × 10^(12) N/m based on our analysis, consistent with potential energy constraints based on topography. Other strength profiles are considered, however high crustal stresses (>300 MPa) conflict with our analysis. The study supports the use of the recrystallized grain size piezometer in quartz as a quick and inexpensive method for resolving stress histories in greenschist facies rocks. For consistency with the independent constraints presented here, we find it accurate to within +20%/−40%, significantly better than previously recognized.