The role of footwall deformation and denudation in controlling cooling age patterns of detachment systems: An application to the Kongur Shan extensional system in the Eastern Pamir, China

Abstract

Determining the relationship between deformation and exhumation history in the footwall rocks of major low-angle normal faults (i.e., detachment faults) provides a way to quantify different models of continental extension. Although pressure–temperature–time ( P–T–t) datasets are commonly used to infer magnitude and rates of slip across detachment faults, correct use of these datasets requires a sound understanding of coupling effects between footwall deformation and advection of isotherms on the distribution of cooling ages. In this study we present thermo-kinematic modeling results which examine the relationship between different footwall kinematic models, erosion, and the spatial distribution of muscovite and biotite 40Ar/ 39Ar cooling ages. Our results show that large magnitudes of footwall erosion during normal faulting significantly affect cooling age patterns as footwall rocks at different distances from the fault surface pass through different thermal gradients. We apply our modeling results to simulating muscovite and biotite cooling-age patterns from the footwall of the Cenozoic Kongur Shan normal fault system in the Pamir. Previous studies had interpreted a documented ~5-fold increase in cooling rate at ~ 2 Ma to indicate an increase in exhumation rate during the Quaternary. However, our results show that the observed cooling age patterns and increase in cooling rate can be produced by a constant exhumation rate over the last 7 Ma due to the effect of changes in the rate of isotherm advection during exhumation.