ACCRETION TECTONICS OF THE CENTRAL ASIAN OROGENIC BELT

The 1.0 Ga-250 Ma Central Asian Orogenic Belt (CAOB) contains all the main components of an extant mid-oceanic ridge-trench-arc system, making it (following Japan, the most appropriate, 500 Ma-present, modern analogue), the world’s best-documented accretionary orogen, Viz: marginal continental rift, limestone-quartzite shelf-type passive margin, ridge-trench transition of ocean plate stratigraphy-OPS (MORB basalt, bedded red radiolarian chert, hemi-pelagic mudstone, trench shale-sandstone turbidite-sandstone-conglomerate, and carbonate reef cap), oceanic plateaus, ophiolites, island arcs (with diagnostic mineralization), active continental margin arcs, seamounts, plume-generated OIB-type basaltic lavas, and accreted microcontinents, plus post-accretion crustal-melt granites and sedimentary basins. Also prominent are HT/UHT metamorphism and HP/UHP blueschists and eclogites exhumed by wedge extrusion including the UHP Kokchetav block with a diamond-coesite isograd. Recent geochemical and isotopic evidence has accrued for ridge subduction: coeval high-temperature, high-Mg diorite dykes (=sanukitoids), adakites, Nb-enriched basalts, and boninites interpreted as a result of enhanced heat flow and asthenospheric upwelling through a hot-torch slab window created by ridge subduction. Structures characteristic of modern accreted orogens include thrust-repeated imbricated units especially of the ocean plate stratigraphy. There are several sutures, including the internal Mongol-Okhotsk suture, and the prominent terminal Tianshan-Solonker suture. Although not yet documented in detail, the ubiquitous presence of anonymously thin units of e.g. arcs, ophiolites, plateaus imply that subduction-erosion was über-active at the trench during accretion of this orogen.