Structural and geochemical evidence for a failed rift crustal evolution model in Western Dharwar Craton, South India

Abstract

Detailed geological mapping along with structural and geochemical analysis were carried out in the Kibbanahalli Schist Belt (KSB) of the Bababudan Group in the Western Dharwar Craton (WDC). Repeated sequences of sedimentary rocks and volcanic rocks with conformable contact relations are the major lithological association in KSB. Sedimentary structures and field relations indicate that the Bababudan Group begins with basal conglomerate composed of debris (talus) and alluvial fan deposits or fluvial deposits. Volcanic rocks preserve features of explosive volcanism in an aqueous environment.Based on the field and microstructural observations five stages of deformations (D0, D1, D2, D3, and D4) are characterized from the study area. Among these deformation events, D2 is the major regional-scale event. East dipping NW–SE trending thrust faults and F2 parallel folds that are upright and isoclinal mark the D2 event. These F2 folds are sandwiched between D2 thrust faults. D2 structures suggest the temporal evolution of a fold-and-thrust belt by inversion of an intracratonic basin.Trace element geochemical analysis of the mafic to intermediate rocks from KSB and Chitradurga Schist Belt (CSB) shows three types of patterns. Type 1 has Nb–Ta negative anomalies, Type 2 is relatively flat trend and Type 3 has Zr–Hf positive anomalies. Type 1 has slightly enriched LREE trend while relatively flat LREE trend in Type 2 and 3. HREE trend is relatively flat in all samples, except for one sample from Type 3. The geochemical variations are related to the degree of crustal contamination experienced during their eruption in connection with the intracratonic extension.A pair of thrust faults and folded sequences of volcanic, volcanoclastic and sedimentary rocks summarize the major structural association in the study area. This association is identical to an inverted failed rift structure. Geochemical evidence also points to an intracratonic mafic to intermediate volcanism. Our study thus provides a well-preserved example of Archean failed rift. We thus propose the post-3.0 Ga crustal growth throughout the Western Dharwar Craton is therefore initiated by similar failed rifting events.