Extension of Achankovil shear zone, Southern Granulite Terrain, South
India in to Madagascar
Abstract
The Achankovil Shear Zone (AKSZ) is a NW-SE trending ductile shear zone
passing through the Southern Granulite Terrain (SGT) has remained
enigmatic among scientific community for the perspective of shear zone
status and extension. While some consider the AKSZ to represent a Pan
African terrain boundary between the Trivandrum block (TB) in the Kerala
Khondalite Belt (KKB) and Madurai block (MB) of the SGT (Drury et al.
1984; Santosh, 1987; Harris and Santosh 1994; Santosh et al. 1992, 2005,
2006; Guru Rajesh and Chetty, 2006; Sreejith and Ravindra Kumar, 2013)
that can be traced in to now- dispersed crustal fragments of Madagascar
and Sri Lanka (Braun and Kriegsman, 2003; Guru Rajesh and Chetty, 2006;
Santosh et al. 2009, Sreejith and Ravindra Kumar, 2013). Others consider
MB and TB evolved coherently and AKSZ to be a high strain zone (MB is
the continuation of TB) within the SGT (Ghosh et al 2004). Detailed
structural field work across the SGT suggests that MB and TB are two
different blocks and the lineament occurs between these two blocks is
the Achankovil Shear Zone (AKSZ). Tracing the paleo-sutures across the
rifted crustal fragments is an important method for the reconstruction
of the pre-drift crustal assembly. For tracing paleo-sutures detailed
structural evolutionary studies are necessary. Available data suggest
that the middle and central crustal domains of the present day landmass
of Madagascar was with India vis-à-vis East Gondwanaland (comprising
India, Australia and Antarctica) that accreted with the West
Gondwanaland (comprising Africa), during Late Neoproterozoic and Early
Paleozoic (Tucker et al., 1999; Sommer et al., 2003; Collins, 2006).
Geologists correlated the AKSZ of India with different major and minor
shear zones present in the southern Madagascar, for example Ranotsara
shear zone (RSZ), Tranomaro shear zone (TSZ), Angavo shear zone (ASZ)
etc. (Tucker et al., 1999; Raharimahefa and Kusky, 2006, 2010; Sajeev et
al., 2009; Tucker et al., 2011a, b). Monazites analyzed from the
Antananarivo block maintained Neoproterozoic age of 796Ma (Cenki Tok et
al, 2016) and also the Imorona–Itsindro Suite of central Madagascar
represents 850–750 Ma magmatism (Zhou et al, 2015; Archibald et al,
2016) which is similar to the chronology of Madurai block, SGT. Age of
Androyan group (Southern Madagascar) ranges from 560-530 Ma (Jons &
Schenk, 2011) obtained from monazites which is similar to the chronology
of Trivandrum block, SGT. Taking the arguments a step further, we argue
the extension of AKSZ of SGT towards the RSZ of Madagascar which is
consistant with the suggestion by Windley et al. 1994; Markl et al.
2000; Cenki et al. 2004; Jons & Schenk, 2011; Zhou et al, 2015;
Archibald et al, 2016 based on structural studies and chronology.