Ductile strain rate measurements document long-term strain localization in the continental crust

Abstract

Quantifi cation of strain localization in the continental lithosphere is hindered by the lack of reliable deformation rate measurements in the deep crust. Quartz-strain-rate-metry (QSR) is a convenient tool for performing such measurements once calibrated. We achieve this calibra- tion by identifying the best piezometer-rheological law pairs that yield a strain rate in agree- ment with that measured on the same outcrop by a more direct method taken as a reference. When applied to two major continental strike-slip shear zones, the Ailao Shan-Red River (ASRR; southwest China) and the Karakorum (northwest India), the calibrated QSR high- lights across-strike strain rate variations, from 1 ◊ 10 −13 s -1 in zones where it is localized. Strain rates integrated across the shear zones imply fast fault slip rates on the order of 1.1 cm yr -1 (Karakorum) and 4 cm yr -1 (ASRR), proving strong strain localization in these strike-slip continental shear zones.