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.