Bababudan - A Late Archaean Intracratonic Volcanosedimentary Basin, Karnataka, Southern India. Part II: Structure

The boundary between the Dharwar volcanosedimentary rocks and their basement (Peninsular Gneiss, c. 3100 Ma, with tracts and enclaves of the Sargur supracrustal association) on the south of the Bababudan basin is an unconformity, faulted and steepened in the southeast by inclined folds verging to the south. These folds pass east into a complex area of neulral folds and faults. Boundaries between the Dharwar cover and its basement on the west, north and east of the basin are steep or vertical faults. Basement gneisses within 2-3 km of the eastern and western boundary faults are intensely schistose or mylonitic with L and LS fabrics identical to those in the cover rocks. The S fabrics in the cover and basement in the east of Bababudan are steep or vertical and associated L fabrics have variable, gentle to moderate plunges to the north or south. Displacement on the eastern and western boundary faults appears to have been dominated by uplift of the basement and shortening at a high angle to the S. fabrics. Movements on the northern boundary fault gave rise to mylonites and tectonic melanges. in the basement and cover and took place before movements on the eastern and western boundary faults and folding in the northeast (Kaldurga syncline and the Nandi arch) and northwest (Lakkavalli syncline) of the basin. Further faulting (Tarikere Fault) related to basement uplift in the northeast occurred during or after folding in the cover.

The structure within the basin is dominated by steep faults and upright open folds with strongly curved hinge lines within steep axial surfaces. The Bababudan syncline is an open east-west fold which curves gently westwards into a tight cusp trending north parallel to the western boundary fault. East of the syncline the structure is dominated by the upright open arch and complex saddle zone of the Santaveri anticline. LS fabrics associated with the Bababudan syncline, the Santaveri anticline and other major folds within the basin have widely variable trends but their continuity from one structure to another suggests the folds, fabrics and faults formed contemporaneously with synchronous refolding that gave rise to the curved axial surfaces such as that of the Bababudan syncline. Synchronous refolding and rapid variations in style of small-scale folds are especially common in the multilayer complexes of banded ferruginous cherts. Whilst fabrics are mostly of LS type, L tectonites indicative of intense constrictional strains are prominent in the east. Mineral lineations and elongated clasts and amygdales forming the L fabric arc commonly parallel to fold axes.

The geometrical relations between upright folds, steep faults and LS fabrics and the effects of synchronous refolding suggest the Dharwar volcanosedimentary rocks deformed in response to constriction by segments of basement rising on all sides of the Bababudan basin, the rise beginning in the norlh. Uplift of basement segments with widespread, pervasive retrogression related flushing of fluids, including CO₂, through myriad cracks and narrow shear zones, especially north of the Bababudan basin, may have taken place as part of a regional thermal event manifested at the present level of outcrop by the Arsikere and Banavara granites to the east. The case for tectonic thickening by thrusts and nappe emplacement reported in Archaean terranes elsewhere has not been proved in the Bababudan basin.