The evolution of a Gondwanan collisional orogen: A structural and geochronological appraisal from the Southern Granulite Terrane, South India

Abstract

Gondwana amalgamated along a suite of Himalayan-scale collisional orogens, the roots of which lace the continents of Africa, South America, and Antarctica. The Southern Granulite Terrane of India is a generally well-exposed, exhumed, Gondwana-forming orogen that preserves a record of the tectonic evolution of the eastern margin of the East African Orogen during the Ediacaran-Cambrian (circa 600-500 Ma) as central Gondwana formed. The deformation associated with the closure of the Mozambique Ocean and collision of the Indian and East African/Madagascan cratonic domains is believed to have taken place along the southern margin of the Salem Block (the Palghat-Cauvery Shear System, PCSS) in the Southern Granulite Terrane. Investigation of the structural fabrics and the geochronology of the high-grade shear zones within the PCSS system shows that the Moyar-Salem-Attur shear zone to the north of the PCSS system is early Paleoproterozoic in age and associated with dextral strike-slip motion, while the Cauvery shear zone (CSZ) to the south of the PCSS system can be loosely constrained to circa 740-550 Ma and is associated with dip-slip dextral transpression and north side-up motion. To the south of the proposed suture zone (the Cauvery shear zone), the structural fabrics of the Northern Madurai Block suggest four deformational events (D 1 -D 4 ), some of which are likely to be contemporaneous. The timing of high pressure-ultrahigh temperature metamorphism and deformation (D 1 -D 3 ) in the Madurai Block (here interpreted as the southern extension of Azania) is constrained to circa 550-500 Ma and interpreted as representing collisional orogeny and subsequent orogenic collapse of the eastern margin of the East African Orogen. The disparity in the nature of the structural fabrics and the timing of the deformation in the Salem and the Madurai Blocks suggest that the two experienced distinct tectonothermal events prior to their amalgamation along the Cauvery shear zone during the Ediacaran/Cambrian.

Phanerozoic surface history of southern Peninsular India from apatite (U‐Th‐Sm)/He data

“…Hence, we suggest that the enhanced erosion in the Palghat Gap could correlate with the timing of monsoon intensification over south Asia [ Dettman et al ., ; Molnar et al ., ; Zhisheng et al ., ]. Accelerated erosion can be attributed to the relatively rapid retreat of the Western Ghat escarpment facilitated by the escarpment perpendicular weak lithological backbone of the Precambrian Palghat‐Cauvery shear zone [ Ghosh et al ., ; Plavsa et al ., ], as it has been proposed earlier [ D'Cruz et al ., ; Gunnell and Harbor , ]. Our interpretation suggests a possible association between timing of gap formation, monsoon intensification over south Asia and elevated sediment fluxes on the Arabian Sea margin.…”

“…A significant result is the presence of AHe ages younger than 90 Ma along the southern flank of the Nilgiri Plateau and particularly in the Palghat Gap region. The ∼30 km wide E‐W trending topographic breach (mean elevation = ∼140 m) through the Western Ghat Mountains is superimposed on the Precambrian Palghat‐Cauvery shear zone interpreted to represent a strand of Mozambique Ocean that closed at circa 550–500 Ma [e.g., Collins et al ., ; Plavsa et al ., ; Santosh et al ., ]. Jacob and Narayanaswami [] and Arogyaswamy [] consider that the gap originated from structurally controlled fluvial and marine erosion.…”

“…Basement rocks of the southern SGT were overprinted by Ediacaran to Cambrian tectonics (ca. 620–540 Ma) related to collision and the formation of Pannotia (Gondwana) [ Collins et al ., , and references therein; Ghosh et al ., ; Plavsa et al ., ].…”

“…For southern Peninsular India, no constraints are available concerning the temperature from which samples might have cooled during the Mesozoic‐Cenozoic. The thermal history simulations begin at 550 Ma, when the geological constraints indicate peak metamorphic temperatures up to 900–1050°C in the SGT [ Collins et al ., ; Plavsa et al ., ]. Therefore, we assumed that prior to 550 Ma the rocks were at temperatures too high for retention of any radiation damage in apatite.…”

“…(Figure ). The Archean Dharwar Craton [ Jayananda et al ., ; Meert et al ., ] and the Mesoarchean‐Cambrian Southern Granulite Terrane [ Collins et al ., ; Plavsa et al ., ; Santosh et al ., ] make up the core of southern Peninsular India. It consists of metamorphic and igneous basement rocks (Figure ) [ Naqvi and Rogers , ; Ramakrishnan and Vaidyanadhan , ].…”