Subduction, Orogeny, and Mainstream Mantle Convection: ABSTRACT

Abstract

Orogenic belts are the fossil sites of lithospheric plate subduction. Subduction along continental edges forms huge compressive drag folds or rotation zones which are expressed as fan-shaped arrangements of schistosity, folds, and dip-slip faults. Associated thrust belts are small, and are antithetic to the subduction. Large thrust belts synthetic to the subduction are typical of subduction zones which have oceanic crust in the upper plate, but may also develop as late stages of extreme rotation in continental upper plates. Most mountain systems are composed of several converged subduction zones, and most subduction zones have flipped or reversed their direction of subduction. On the basis of the difference in crustal response to westward versus eastward underthrusting along active subduction zones, we postulate that an eastward-flowing mainstream is present within the earth's upper mantle. Coupled to convection cells beneath the oceanic ridges, the mainstream forms a pattern of migrating zones of primary and secondary upwelling which, in a kinematic model, indicates (1) that a form of asymmetric sea-floor spreading has created the small ocean basins behind island arcs, (2) that the depth to the base of convection increases systematically eastward from ridge to ridge, (3) that secondary zones of mantle upwelling underlie such features as the East African rifts, and (4) that plates descend into the mantle along subduction zones in response to the removal of aterial from below. End_of_Article - Last_Page 361------------