LINKAGES BETWEEN LITHOSPHERIC SHORTENING, MAGMATISM, AND CONTINENTAL PLATEAU FORMATION

Recent work in fold-thrust belts adjacent to modern and ancient orogenic plateaus indicates that horizontal shortening in the upper crust commonly exceeds several hundred kilometers. Structural balance requires that an equal length of continental foreland lithosphere be disposed of beneath the adjacent magmatic arc or continental plate. In either case, melt-fertile mid to lower crustal foreland-derived material may contribute significantly to upper plate magmatism. Radiogenic isotopic data from the North American arcs show that more than 80% of the mass of Cordilleran batholiths is upper plate (North-American) derived. In addition, development of eclogitic arc roots may promote delamination/dripping of lower crust and lithosphere into the mantle. In turn, this process of mass removal will relieve the room problem caused by underthrusting, thus allowing further, perhaps increasingly rapid, underthrusting of foreland-derived crust and lithosphere. In contrast, flattening of the subducting plate would exacerbate the room problem perhaps leading to migration of deformation into the foreland. Replacement of cold lithosphere with hot asthenosphere, either by upper plate delamination or rollback of a flat slab, may initiate widespread crustal melting and provide thermal support for orogenic plateaus. A cyclic process may be envisaged in which underthrusting causes crustal and lithospheric thickening, followed by magmatic flare-up, accompanied perhaps by eclogitization and removal of large amounts of lithosphere, which in turn would promote further rapid underthrusting and renewal of the cycle. The temporal relationship between these processes may be decipherable from the kinematic history of the fold-thrust belt and its foreland basin.