DIFFERENTIATING BETWEEN ACCRETED ARC FRAGMENTS AND RIDGE-SUBDUCTION RELATED MAGMATIC BELTS IN FOREARC TERRANES: EXAMPLES FROM ALASKA, ALTAIDS, AND ARCHEAN

Accretionary orogens such as the Southern Alaska margin, the Altaids, and many Archean greenstone/granodiorite terranes are characterized by alternating belts of structurally complex turbidites, mélange, ophiolites, exotic blocks, gneiss, and suites of mafic-felsic magmatic rocks of uncertain significance. Some of these magmatic belts may be remnants of arcs that intruded the wedge during progradation of the wedge, and characterized by magmatic fronts that are virtually the same age everywhere along strike. Some other magmatic belts may be accreted oceanic arc sequences. Still other magmatic belts may be induced by passage of a ridge -trench -trench (or transform) triple junction, with magma intruding the forearc as a result of magmas from the slab window partially melting and mixing with shallow level accretionary prism material. These magmatic belts are characterized by diachronous migration of many features, including active magmatism, anomalous structural deformation, heating, mineralization, sedimentation, exhumation, and a possible change in kinematics of the wedge before and after ridge subduction, as different plates with different plate convergence vectors are interacting before and after ridge subduction. Ridge subduction is recognized as an important process in the generation of the Sanak-Baranof belt in Alaska, in aspects of the evolution of the Tuva-Mongolia and Kipchak arc belts in the Altaids, and in some Archean terranes such as the North China craton. These process should be more common than reported however, and we challenge workers in large mountain belts such as the Altaids and in Archean belts to re-examine some of the magmatic belts in that orogen to test if some may be ridge-subduction related, and not separate accreted arc sequences. Recognition of some of these belts as ridge-subduction related could dramatically change the tectonic models for the evolution of these orogens and for models for the growth of continental crust.