Three-dimensional field perspective on deformation, flow, and growth of the lower continental crust (Dharwar craton, India)

The study of fabric development and juvenile batholith emplacement across the tilted crustal section of the Eastern Dharwar craton shows that horizontal, constrictional deformation affected large volumes of the midcrust and lower crust at the time of regional partial melting and magmatic accretion. Constriction is achieved by a combination of coeval shallow and steep planar fabrics sharing a common horizontal elongation direction, two sets of conjugate strike-slip shears, and extensional shear zones. The Eastern Dharwar craton illustrates an end-member deformation mode by which a particularly weakened lithosphere responds to shortening by developing distributed, horizontal plane strain on a crustal scale, resulting from the combination of crustal shortening and lateral gravity-driven flow. Thinning accompanying constrictional deformation is interpreted as compensating for juvenile magmatic accretion and thickening of greenstone belts and as acting to maintain a stable crustal thickness. Such a midcrustal to lower crustal deformation process may provide a resolution of the batholithic room problem in a softened crust submitted to lateral shortening and may explain nearly isobaric retrograde pressure-temperaturetime paths of high temperature - low pressure high-grade terrains.