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