Possible detachment zone in Precambrian rocks of Kanjamalai Hills, Cauvery Suture Zone, Southern India: Implications to accretionary tectonics D.P. Mohanty a,b,⇑ , T.R.K. Chetty b a Department of Geology, University of Pune, Pune 411007, India b CSIR-National Geophysical Research Institute, Hyderabad 500007, India article info Article history: Received 8 July 2013 Received in revised form 24 February 2014 Accepted 25 February 2014 Available online 21 March 2014 Keywords: Detachment zone Cauvery Suture Zone Collision Granulites Southern India abstract Existence of a possible detachment zone at Elampillai region, NW margin of Kanjamalai Hills, located in the northern part of Cauvery Suture Zone (CSZ), Southern India, is reported here for the first time. Detailed structural mapping provides anatomy of the zone, which are rarely preserved in Precambrian high grade terranes. The detachment surface separates two distinct rock units of contrasting lithological and structural characters: the upper and lower units. The detachment zone is characterized by a variety of fold styles with the predominance of tight isoclinal folds with varied plunge directions, limb rotations and the hinge line variations often leading to lift-off fold like geometries and deformed sheath folds. Presence of parasitic folding and associated penetrative strains seem to be controlled by differences in mechanical stratigraphy, relative thicknesses of the competent and incompetent units, and the structural relief of the underlying basement. Our present study in conjunction with other available geological, geochemical and geochronological data from the region indicates that the structures of the detachment zone are genetically related to thrust tectonics forming a part of subduction–accretion–collision tectonic history of the Neoproterozoic Gondwana suture. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction In the past two decades, much emphasis has been paid to understanding the natural deformation of rocks in terms of shear zone systems (Ramsay, 1980). Localization of strain into high strain zones is a common feature in such shear zones, which are also described as ductile deformation zones (Mitra, 1978). High strain zones with typical structures like box folds, lift-off folds, and refolded sheath folds are specially described as detachment zones (Mitra, 2003). Detachment folds or fault zones provide direct infor- mation about the processes involved in transporting the thrust sheets to higher crustal levels (Gray, 1995). A detachment zone commonly develops when a layer of rock deforms above a bed- ding-parallel detachment surface or décollement, which are cored by incompetent, often disharmonically folded strata (Dahlstrom, 1970; Laubscher, 1977; Jamison, 1987). Recently, Mitra (2003) has defined that detachment folds are characterized by the termi- nation of a parallel fold at a basal detachment within a ductile unit. These structures form in stratigraphic packages with high compe- tency contrasts among units. The competent upper units exhibit parallel fold geometries, whereas the weak lower unit displays disharmonic folding and significant penetrative deformation. The major mechanism in detachment zones is the transportation of deep crustal materials to the shallow crustal levels, referred to as the tectonic unroofing of high grade terrains. The driving force in such cases has been widely reported to be from a variety of geodynamic settings. Detachment zones are rarely exposed in case of high grade metamorphic regions unlike in sedimentary terrains, where they are mostly well preserved. The best examples are in the Jura Moun- tains, associated with middle Triassic evaporates (Noack, 1995). It is well known that identifying a detachment zone in Precambrian high grade terranes is a challenging task because of multiple defor- mational events. During our recent regional geological traverses, we have come across a possible detachment zone at the NW margin of Kanjamalai Hills, located on the northern boundary of Cauvery Suture Zone (CSZ), Southern Granulite Terrane (SGT), Southern India. We have attempted a detailed structural analysis of partly exposed detachment zone and documented a well exposed structural transition from less deformed to intensely http://dx.doi.org/10.1016/j.jseaes.2014.02.028 1367-9120/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: Department of Geology, University of Pune, Pune 411007, India. Tel.: +91 9404859095; fax: +91 02025695373. E-mail address: dpmohanty@unipune.ac.in (D.P. Mohanty). Journal of Asian Earth Sciences 88 (2014) 50–61 Contents lists available at ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes