Precambrian continental strain and shear zone patterns: South Indian case

Abstract : This study addresses the tectonic and mechanical significance of crustal-scale strain and shear zone patterns in the South Indian Precambrian lithosphere. It is based on a tectonic map of the Dharwar craton derived from LANDSAT imagery, existing documentation, and our own field observations. We document the contrasted responses of the two main parts of the craton to late Archean (2.56­2.51 Ga) shortening. The old (>3 Ga) lithosphere of the Western Dharwar craton stabilized at 2.61 Ga underwent moderate shortening and strain localization along spaced shear zones. The Eastern Dharwar craton, rejuvenated by late Archean juvenile magmatic accretion, responded to shortening by flowing laterally against the Western Dharwar craton. Shortening operated without significant thickening because of the high buoyancy of the juvenile crust and the very low strength of its mantle lithosphere. The late Archean crustal-scale shear zone network did not accommodate large displacements and only contributed to smoothing out of strain heterogeneities during the latest stages of flow. The shear zone pattern only reflects the symmetry of the regional finite strain field of a wide hot orogen. It does not result from terrain amalgamation but accompanies crustal flow and does not compare with those of modern active margins, which accommodate unidirectional transpression. Our analysis further suggests that the shear zones bounding the craton did not record large horizontal displacements during final assembly of Gondwana but rather transverse shortening and vertical extrusion.