Dextral Pan‐African Shear Along the Southwestern Edge of the Achankovil Shear Belt, South India: Constraints on Gondwana Reconstructions: A Discussion

Abstract

Achankovil shear zone (ACSZ) (figure 1a), intering charnockites, garnet-biotite gneiss, biotite gneiss, calc-gneiss and granite, as revealed in the preted as a major NW-trending strike-slip shear zone of probable Proterozoic age (Drury et al. 1984), lithological mapping carried out as part of this study (figure 2). A preliminary structural assesshas prominently figured in Gondwana correlative studies (e.g., Windley et al. 1994). Although it is ment reveals the presence of a distinctive assemblage of asymmetric structures in the gneissic exclear that there is a major lithologic change across ACSZ (figure 1b), detailed studies indicating the posures indicating a consistent sinistral sense of shear. Gneissosity, forming the secondary compoprobable sense of shear are lacking across this 10– 20 km wide and .100 km long shear zone. Resitional banding, strikes WNW-ESE to NW-SE dipping moderately toward SW (with relatively rare cently, Sacks et al. (1997) reported field observations bearing on the probable sense of shear of the NE dip) and is conformable in all the rock types of the area, in conformity with the general NW-SE ACSZ. After combining field criteria and kinematic analysis of asymmetric features along the SW trend of the ACSZ. Structurally, the terrain has undergone four phases of deformation (summarized in edge of the ACSZ (known as the Tenmala shear zone), they concluded that the ACSZ indicated priRajesh and Santosh 1997). Open to isoclinal intrafolial folds encountered in marily dextral shear, with a minor component of reverse shear. The authors illustrate their discusthe study area are invariably asymmetric. Fold limbs are zones of transposed gneissosity that consion with examples of kinematic indicators, including shear bands, asymmetric folds, stretched and tain indicators of a probable sinistral sense of shear and well-developed mineral elongation lineation asymmetric garnet, feldspar and quartz porphyroclasts, hook folds, and lineations. Thus they appardefined by quartz ribbons, trails of xenoblastic garnet grains, and the alignment of the long axes of Kently question the correlation of the dextral Achankovil shear zone (rather Tenmala shear zone) with feldspar augen. Pinch-and-swell structures and boudins commonly show rotation with respect to the sinistral Bongolava-Ranotsara shear zone (BRSZ) in the India-Madagascar Gondwana enfoliation. Tight ‘‘S’’ asymmetric F2 folds and synkinematic pegmatite veins are seen in the western semble. We focus the present discussion on the probable part of the study area. The development of a refold structure by superposed buckling is illustrated in sense of shear of ACSZ in the light of new data. Our study area falls around Punalur and its adjoinfigure 3a. There are smaller asymmetric Sand Zfolds congruent with the larger F1, with one limb ing areas within the Quilon district, southern Kerala, South India and is located between longitudes striking E-W while the other strikes N50°E, showing negative extension. During the development of 76°55′ and 76°59′ E and latitudes 9°1′ and 9°3′ N (figure 2), represented in the Survey of India topo F2 on the traces of preserved axial planar cleavage the simple shear is dextral on the left limb and sisheet No. 58C/16. Outcrops within the study area expose a strongly deformed suite of rocks comprisnistral on the right limb. Asymmetrically folded veins (figure 3b), the bulk asymmetry of which represents a sinistral sense of shear, have been found 1 Manuscript received May 14, 1997. in a few locations. Elongate quartz lenticles and 2 Centre for Earth Science Studies, PB 7250, Trivandrum 695 031, India. garnet and biotite aggregates define penetrative lin-