The Nartyn granite in eastern Mongolia: a particular mode of syn-extensional emplacement
Résumé
Hot orogens, as best exemplified by Archean and Proterozoic accretionnary orogens, show several specific characteristics that we review in this paper. At lithospheric scale, hot orogens combine horizontal flow of a thick and hot ductile crust with strain concentration along steeply dipping deformation zones along which pieces of upper crust are buried. These steeply dipping deformation zones may simply develop as transfer zones accommodating differential flow of lower crustal rock masses. They may also reflect overall transpressive boundary conditions to the considered orogen. Consistently, associated principal stretch directions may vary from dominantly down-dip to subhorizontal from one orogen to another. In contrast, lower crustal domains where horizontal flow dominates are characterized by flat-lying fabrics and principal stretch directions at low angle to the strike of the orogen and high-strain steep deformation zones. Field examples and analogue models outline that the spatial and temporal distribution of domains dominated by horizontal flow or vertical motions depends on the coupling between the viscous lower crust and the plastic upper crust. Coupling is achieved by an attachment layer that accommodates the contrasted kinematic responses of the upper plastic crust and the lower viscous crust to convergence, vertical mass transfers and longitudinal flow. Overall, hot orogens are marked by rather distributed strains compared to those involving cold lithospheres with a stiff and localising mantle lithosphere and thick brittle crust, allowing strong strain localisation in lithospheric-scale shear zones. At crustal scale, hot orogens are characterized by a thick partially molten lower crust subjected to very high temperatures, near isothermal conditions, extensive melting, and juvenile mantle magmatism. The viscous crust may overly a stiff granulitic lower crust, which itself overlies a mantle lid formed at the end of orogeny by the residue of melting and eclogitized mantle melts. The crystallization of such a mantle lid leads to the cratonization / quenching of the orogen. The formation of the stiff granulitic lower crust would be crucial in defining a transient orogenic flow mode by activation of an attachment layer between the mantle and the crust until the mantle lid ultimatly crystallizes. Distributed deformation at crustal scale has important implications on strain rates throughout hot orogens, their topography and relief, and specific surface mass transfers (erosion and sedimentation). Modern orogens that involve initially cold lithosphere may acquire, after maturation, comparable deformation patterns as a result of thickening- and/or mantle magmatism-induced crustal weakening. High plateaux in Cordilleran and collision belts would be Phanerozoic analogues of ancient exhumed hot orogens.