Flow controlled inverted metamorphism in shear zones

Abstract

Numerous processes and models have been developed in order to explain inverted metamorphism in shear zones. This article demonstrates, by means of continuum mechanics, that the orientation of a sample profile across a shear zone before and after deformation can be used to quantify the conditions under which inverted metamorphism is likely to have developed. A thrusting shear zone deforming under ideal simple shear will develop an inverted metamorphic zonation if the angle between the thrust and isotherms is greater than 5–10°, even at only moderate strains. If the flow is general shear, this angle is considerably larger (i.e. >60° for a kinematic vorticity number of W k=0.5). Additionally, the stretch of the sample profile across the shear zone, which is not influenced by the flow type, bears important implications for the interpretation of petrological, geochronological and structural data. Although this kinematic model is clearly an end-member model, neglecting dissipative processes, it can easily account for the structural and petrological data of the inverted metamorphism observed throughout the Higher Himalayan Crystalline wedge in the Sutlej Valley (India).