Upper-crustal structure, compositions and tectonic settings obtained from Perur–Chikmagalur 3-C seismic profile of Archean Dharwar Province, southern India

Abstract

We present upper-crustal P- and S-wave velocity models (\({V}_{P}\) and \({V}_{S}\)) of the Archean Dharwar Province in southern India using both refraction and reflection phases of the 3-C seismic data along NE-SW-oriented 200-km long Perur–Chikmagalur profile. The velocity models reveal alternate horst and graben structures at shallow depth (0.5–2.5 km) filled with weathered volcano–sedimentary rocks toward Western Dharwar Craton (WDC) having low \({V}_{P}\) (5.20–5.58 km/s), \({V}_{S}\) (3.20–3.32 km/s), \({V}_{P}/{V}_{S}\) (1.63–1.68) and Poisson’s ratio \(\sigma\) (0.20–0.25) as compared to exposed granite and gneissic rocks of increased velocity along Eastern Dharwar Craton (EDC). The steeply dipping Chitradurga Shear Zone (CSZ) imaged extends to 6–8 km depth with anomalously high \({V}_{P}\) (6.85 km/s), \({V}_{S}\) (3.80 km/s), \({V}_{P}/{V}_{S}\) (1.80) and \(\sigma\)(0.28) comprising of deep crustal rocks impounded at shallow level. The complex suturing and oblique convergence occurred along CSZ with distinct compositions because of the shearing and transpression of EDC and WDC followed by compression and inter-wedging of the two blocks. The compositions of Neoarchean EDC are mainly felsic granites having relatively low \({V}_{P}\)(6.25–6.30 km/s) and \({V}_{S}\) (3.53–3.62 km/s). On the other hand, the Mesoarchean WDC is dominated by gneisses and green schists mainly corresponds to mafic and ultramafic compositions having comparatively higher \({V}_{P}\) (6.30–6.85 km/s) and \({V}_{S}\) (3.55–3.80 km/s) with corresponding variations of \({V}_{P}/{V}_{S}\) (1.74–1.77), \(\sigma\) (0.26–0.27) for EDC and \({V}_{P}/{V}_{S}\) (1.73–1.80), \(\sigma\) (0.25–0.28) for WDC. A distinct zone of detachment imaged at 3–11 km depth acts as a major unconformity having eastward-dipping low-velocity-layer (LVL) sandwiching Dharwar schist belts and Archean gneisses within the upper crust forming a complex Archean Province of southern India.