Role of strain-rate partitioning in aseismic strain localization:
implications for the Karakoram Shear Zone, India
Abstract
Shear zones in the earth’s crust commonly deform by aseismic creep
processes which involve localization of strain at varying rates across
the shears. Therefore, it is of key importance to quantify the order of
strain-rate variation to understand whether deformation was accommodated
solely by aseismic or by simultaneous seismic and aseismic slip. The
present study demonstrates partitioning of strain-rate between
coarse-grained quartz rich layers (CGQL) and amphibole-biotite rich
layers (AMBIORL) due to mineralogical and consequently rheological
heterogeneities within the mylonitized amphibolites of the Karakoram
Shear Zone (KSZ), India. Variations in modal proportion of quartz across
the layers resulted in a lower strain rate per unit area accommodated by
quartz, in the CGQL than in the AMBIORL that deformed a rate, 2.34-3.43
times higher than the CGQL. Therefore, a considerably higher proportion
of dynamically recrystallized quartz nucleated in the AMBIORL. Combined
analyses of the proportion of strain-rate partitioning with previously
calculated bulk strain-rates suggest that both the CGQL and the AMBIORL
deformed by aseismic creep. Thus, strain-rate partitioning due to
mineralogical or lithological heterogeneity is a feasible mechanism for
strain localization during aseismic creep. Calculated proportion of
strain-rate partitioning, when extrapolated to two adjacent
lithologically distinct hypothetical layers (at a regional scale),
yields differences of 100-200 kilometers, in displacement accommodated
by the two layers. Thus, this study suggests that variation in offsets
of streams and marker beds, along widely separated segments of the KSZ
might well be the manifestations of strain-rate partitioning in the
middle-lower crust.