Lateral constrictional flow of hot orogenic crust: Insights from the Neoarchean of south India, geological and geophysical implications for orogenic plateaux

Abstract

[1] This study provides an in situ geological perspective on fabrics produced by synconvergence lateral crustal flow of hot orogens. It is based on the example of the Neoarchean orogen of the Dharwar craton (India) and combines structural analysis and ion microprobe U-Pb zircon geochronology. We document a pervasive, three-dimensional flow mode of the lower crust, called lateral constrictional flow (LCF), which combines orogen-normal shortening, lateral constrictional stretching, and transtension. LCF achieves gravity-driven flow, lateral escape, and 3-D mass redistribution in a viscous lower crust submitted to convergence. LCF tends to mechanically and thermally homogenize the lower crust and efficiently compensates topographic relief at a shallow level in the crust. Three type-geodynamic contexts are envisaged for LCF: plateau interiors, inner parts of collisional crustal wedges or plateau edges, and throughout wide ultrahot orogens such as the Neaoarchean orogen of south India. LCF makes the lower crust act as a strain gauge between lateral gravitational collapse or tectonic thickening of the upper crust, thrust stacking in the lowermost crust (collisional crustal wedge case), crustal shortening, and/or lateral flow of the upper mantle. In the case of plateau interiors or ultrahot orogens, LCF of a thick lower crust enables coupling of upper crustal deformation with upper mantle flow through a hot and thin lithosphere being shortened coherently. LCF generates a subhorizontal lamination that should produce the strong seismic reflectivity and lateral anisotropy of the Tibetan Plateau and a variety of hot orogens.

SHRIMP U–Pb zircon ages of granitoids adjacent to Chitradurga shear zone, Dharwar craton, South India and its tectonic implications

“…The evolution of Chitradurga shear zone is still unanswered in the tectonic context of Dharwar craton. Recently, Chardon et al (2011) verified that the Chitradurga shear zone was obviously active between 2.56-2.50 Ga. All the three dated granitoids in the current study are older that this period, it can be suggested that the Chitradurga shear zone must be developed after the magmatic activity in the study area. Furthermore, field observations such as the development of sheared amphibolites and granitoids, solid state shear fabrics, layer-parallel/intrafolial folds etc.…”

“…All the data clearly suggest that Peninsular Gneiss was formed between 3.4-3.1 Ga and the older granitoid activity in the study area is contemporaneous with the formation of Peninsular Gneiss of TTG composition. The geochronological data in and around Chitradurga are in the age range of 2650-2550 Ma (Chadwick et al, 2007;Chardon et al, 2011;Jayananda et al, 2006;Taylor et al, 1984;Ram Mohan et al, 2014;Sarma et al, 2011), suggestive of a protracted magmatic episode in this region (Fig. 1a).…”

“…Other published ages in nearby areas are 3080 ± 110 Ma and 3175 ± 45 Ma (Taylor et al, 1984 by Rb-Sr method), 3204 ± 30 Ma (Bhaskar Rao et al, 1991 by Rb-Sr method), and 3328 ± 10 to 3200 ± 20 Ma (Peucat et al, 1993 by Pb-Pb method). Recently, Chardon et al (2011) presented U-Pb ages for the large remnants of older crust of 3.32 Ga in the eastern part of the study area. All the data clearly suggest that Peninsular Gneiss was formed between 3.4-3.1 Ga and the older granitoid activity in the study area is contemporaneous with the formation of Peninsular Gneiss of TTG composition.…”

“…This boundary marks a drop in crustal thickness from 42-51 km for the WDC and 34-39 km for the EDC (e.g., Chardon et al, 2008). The Chitradurga shear zone has been considered as a sinistral transpressive shear zone (Chadwick et al, 1989(Chadwick et al, , 2000 or a zone of lateral constrictional flow of hot crust (Chardon et al, 2008(Chardon et al, , 2011.…”

“…The tectonic context of the continental growth of the Dharwar craton is a subject of debate. Both the uniformitarian and non-uniformitarian models have been proposed to explain the geological framework of the craton (e.g., Balakrishnan et al, 1999;Chadwick et al, 1997Chadwick et al, , 2000Chadwick et al, , 2007Chardon et al, 2008Chardon et al, , 2011. In almost all the existing models, a North-South trending crustal scale shear zone known as Chitradurga shear zone is considered as the boundary between two juxtaposed subcratonic blocks namely the Eastern Dharwar craton (EDC) and the Western Dharwar craton (WDC).…”