Structural architecture and geological relationships in the southern part of Chitradurga Schist Belt, Dharwar craton, South India

摘要

Detailed structural mapping in the southern part of Chitradurga Schist Belt (3.0-2.5 Ga) (CSB) distributed around the Chikkanayakanahalli-Kibbanahalli area was carried out. Sargur Group, Basement Gneiss, Bababudan Group, Chitradurga Group, and Hiriyur Group of rocks are well preserved in the investigated area. Unconformable relation between Basement Gneiss-Sargur Group and Bababudan Group is defined by oligomict conglomerate with quartzite clast and occasionally preserve granite clast. A polymictic conglomerate separates Bababudan and Chitradurga Groups; similarly, Chitradurga and Hiriyur Groups are also separated by a polymictic conglomerate. A new zone, Akkanahalli Zone, in the eastern margin of the study area is proposed which is belonging to Sargur Group. Zircon grains in the metatuff sample from this zone provide an age of 3313 ± 6 Ma. Six stages of deformation events are recognized in the study area. General trend and megascopic structures in the mapped area have resulted from the earlier two stages of deformations (D_2 and D_3). The D_2 stage structure is distinctly characterized by a fold-and-thrust belt consisting of a NNW-SSE trending fold zone sandwiched between a pair of NNW-SSE trending thrust faults dipping east. Deformation during the D_3 stage resulted in regional-scale sinistral shear zones, such as N-S striking Gadag-Mandya Shear Zone, and narrow N-S and NW trending sinistral 'echelon' shear zones. Based on our structural and field relationship it is proposed that CSB developed in an immature or failed rift setting where shallow marine sequence and shelf deposits are predominant. Sediments and volcanic rocks were unconformably deposited horizontally above Basement Gneiss and later got deformed together in a sinistral transpression setting.