The Cambrian collisional suture of Gondwana in southern India: A geophysical appraisal

摘要

The southern Indian crustal fragment occupied a central position within the Late-Neoproterozoic-Cambrian Gondwana supercontinent assembly. Here we synthesize the available geophysical data that includes gravity, seismic tomography, deep seismic sounding (DSS) and magnetotellurics (MT) from the Palghat-Cauvery Suture Zone (PCSZ), which is considered as a trace of the Gondwana-forming suture in southern India, as well as the surrounding regions to delineate the crustal architecture and tectonic history of the region. An increased crustal thickness immediately north of the PCSZ is correlated to crustal thickening associated with the subduction-collision processes during continental amalgamation. A prominent gravity low of about -45 m gal beneath Kodaikanal in the central Madurai Block, south of the PCSZ might suggest the deep root of a thick magmatic arc. Deep seismic studies in and around Chennimalai at the central domain of the PCSZ indicate the presence of ca. 10 km thick low velocity (6.0 km/s) layer at mid-crustal depths. The gravity model indicates a high density (2.80 gm/cm~3) layer corresponding to these depths. Two-dimensional MT model shows highly resistive (>20,000 Ω-m) felsic upper crust down to 15-16 km all along the profile. The resistivity of the mid-crust is more than 10,000 Ω-m and the resistivity of the lower crustal domains is in the range of 500-3000 Ω-m. The MT model and revised gravity model, constrained by MT, show a southward dipping low resistive zone (<100 Ω-m) and a high density region at a depth range of 15-45 km beneath the Chennimalai dome within the PCSZ. The interpretation of magnetotelluric and revised gravity model confirm the PCSZ to be the trace of a major suture zone, and correlate with a plate tectonic model of subduction-collision-accretion tectonics along this zone related to the final amalgamation of the Gondwana supercontinent.