CORDILLERAN VERSUS TETHYAN OPHIOLITES IN OROGENY

The distinction between Cordilleran vs. Tethyan ophiolites (Moores 1982) based on their emplacement mechanisms is a first-order tectonic problem in studying the evolution of orogenic belts. Cordilleran ophiolites (CO) in accretionary-type orogenic belts structurally overlie subduction-accretion complexes and are incorporated into active continental margins via progressive underthrusting of oceanic material and/or through ridge-trench interactions. Tethyan ophiolites (TO) structurally overlie passive continental margins, microcontinents or island arcs, and are emplaced via collision of these buoyant crustal entities with trenches followed by their partial subduction. This process may lead to induced subduction initiation in the region (via subduction jump and/or flip) and precedes terminal continental collisions, which create intra-continental mountain belts (Alps, Himalayas). CO are commonly polygenetic, developed on and across a deformed, heterogeneous oceanic basement and may include fully developed island arc sequences having IAT to calcalkaline affinities, pyroclastic rocks, and felsic differentiates. Prolonged history of subduction with variable polarity and kinematics may generate nested Cordilleran ophiolites with different ages & chemical compositions that may have been affected by orogen-parallel wrench faulting due to oblique convergence. TO include Ligurian-type ophiolites with Hess-type oceanic crust and Mediterranean-type ophiolites with Penrose-type oceanic crust, and may contain well-developed sheeted dyke complexes developed due to robust magmatic extension beneath narrow rift zones during their seafloor spreading history. Igneous accretion of typical Tethyan ophiolites may have been facilitated by slab rollback and mantle flow that resulted in upper plate extension and further melting of previously depleted asthenosphere, showing a progressive evolution from MORB-like to IAT to boninitic proto-arc assemblages. Rift-drift sequences recording early phases of basin opening may be preserved as mélange units beneath Tethyan ophiolites. Recognition of Cordilleran vs. Tethyan ophiolites in ancient mountain belts (Precambrian) is a useful tool to distinguish between accretionary and collisional orogens and their mode of continental growth.