Paper No. 91-8
Presentation Time: 9:55 AM
QUANTIFYING STRAIN DISTRIBUTION IN SHEAR ZONES USING CRYSTAL PREFERRED ORIENTATIONS
Determination of quantitative strain distributions within orogen-scale ductile shear zones is of fundamental importance for understanding strain localisation processes and the macroscopic architecture of fault zones. However, most crustal-scale shear zones lack sufficient strain markers. We show that in such circumstances a suitable strain proxy can be provided by the strength of crystal preferred orientations (CPO) using an eigenvalue-based intensity parameter. This method is widely applicable and can provide strain profiles at the resolution of sampling density. Quartz c-axis CPO intensity (I) transects across the dextral Karakoram fault zone, NW India, provide rare quantitative assessments of strain distribution across such a large scale shear zone. Strain is distributed across multiple km-scale subparallel shear zones (I < 1.6) which cross-cut c. 16 Ma granitoids and separate less deformed lenses (I < 0.2). Quartz c-axis Y-maxima CPO indicate deformation dominated by prism-a slip and, along with deformation microstructures, suggest temperatures of c. 500°C. Al-in-hornblende geobarometry on a cross-cut 17 Ma monzogranite gives 449 ± 72 MPa corresponding to an emplacement depth of c. 16.6 ± 3.7 km, thus providing a maximum depth constraint for the formation of the strain profiles. These strain profiles provide an important insight into the distribution of deformation in the region between localised brittle deformation in the seismogenic upper crust and broadly distributed deformation in the partially molten mid-crust.