ACCRETIONARY OROGENS IN SOUTH ASIA AND ALASKA COMPARED AND CONTRASTED

Both the Himalaya-Tibet and South Central Alaska have been locations of net continental growth spanning much of the Mesozoic and especially Cenozoic. In both locations largely intact oceanic island arc complexes have been accreted to an active continental margin which appear to have been largely in a state of tectonic erosion prior to collision and which had the same polarity of subduction as the oceanic arc. In Alaska, accretion of the Talkeetna Arc is followed by a switch to accretion of sediment in the trench, probably because collision resulted in increased sediment flux to the trench eroded from the continental interior and from the newly uplifted collisional range. In the Himalaya this does not seem to have occurred either because arc accretion was more gentle, or because the drainage systems sent sediment northward, away from the trench, much like the modern Andes. No major accretionary prism is developed anywhere in the Indus Suture Zone. Zones of mélange up to a few km thick represent the subduction channel, but the only major accretion occurs when the Indian passive margin collided with the trench to be offscraped and preserved as the Tethyan Himalaya. Accretion in southern Alaska continues to the present day, fed by steady tectonic compression, uplift of sources and an erosive climate supplying the trench. Although accretion has ceased in the Indus Suture it has continued along strike in the Andaman-Nicobar Ridge where initial accretion of Paleogene-Lower Miocene turbidites derived from the adjacent arc was replaced by relatively inefficient offscraping of Himalayan sediments transported laterally from the collision zone and deposited in the Bengal Fan.