Deep crustal seismic reflection images from the Dharwar craton, Southern India-evidence for the Neoarchean subduction

Abstract

Major part of the Earth's continental crust is evolved during the Archean, however, the mechanism for its formation is controversial. It could have formed either through horizontal accretion similar to the modern plate tectonic processes or by vertical accretion by plume activity. Here, we present the results of a new deep crustal seismic reflection Profiling, the DHARSEIS experiment conducted along a 200 km long Perur-Chikmagalur profile across the Archean Dharwar craton, to understand the crustal evolutionary processes during the Neoarchean. The data were processed using the Common Reflection Surface (CRS) stack method. Seismic images show distinctly different reflectivity patterns in the Mesoarchean Western and Neoarchean Eastern Dharwar Cratons (WDC and EDC), the two crustal blocks of the composite Dharwar craton. The WDC consists of a simple structure with a major part of the crust from 6 to 28 km displaying a gently dipping reflection fabric and a subhorizontal reflection fabric from 28 to 40 km except beneath the Chitradurga schist belt. On the other hand the EDC displays a complex reflectivity pattern, contrary to the simple crustal structure suggested by various other studies. A dipping Moho, oppositely dipping reflection fabric and a thrust fault are the major crustal features in the EDC. The present seismic study imaged a west-dipping reflection fabric extending from 34 to 43 km in the EDC, which is interpreted to represent an upper-mantle subduction zone. During this process the EDC was thrusted obliquely against the pre-existing proto-continent WDC and accreted to it. Oppositely dipping reflection fabrics with a crustal root at the convergence boundary suggest accretion of WDC and EDC during the Neoarchean orogeny. The collisional boundary coincides with the location of ~2.5 Ga Closepet granite. Seismic images suggest the Moho as a detachment boundary. The present study reveals a likely two-stage subduction-accretion process for the evolution of continental crust during the Neoarchean. Plate tectonic processes were responsible for the Neoarchean crustal growth in the region.