DIAPIRISM, BULK HOMOGENEOUS SHORTENING AND TRANSCURRENT SHEARING IN THE ARCHEAN DHARWAR CRATON - THE HOLENARSIPUR AREA, SOUTHERN INDIA

Qualitative strain data are spatially integrated in order to define Archaean foliation trajectories, and variations in strain intensity and ellipsoid type, in the Holenarsipur area, Dharwar craton, southern India. The study provides constraints on the bulk geometry, the progressive deformation and the kinematic history of this region of Archaean continental crust. Two major structural events have shaped the study area: (i) a regional diapiric event contemporaneous with a magmatic event, and (ii) a later sinistral transcurrent ductile shear event which has locally reoriented both foliations and stretching lineations. The entire Holenarsipur supracrustal belt underwent a common tectono-metamorphic history that included two metamorphic events. Each event corresponds temporally to a distinct strain field. The first, a regional amphibolite facies event, was contemporaneous with the main tectonic event and probably with the intrusion of Halekote trondhjemites. The second occurred under greenschist-facies conditions and was restricted to a late transcurrent shear zone at the eastern boundary of the greenstone belt. The strain pattern prior to transcurrent shearing cannot be explained by a thrusting event unless all trace of this event had been erased by the diapirism. We interpret all structures older than the transcurrent shearing to be the direct result of vertical movements linked to body forces, augmented by minor horizontal crustal shortening.