LITHOSPHERIC BUOYANCY AND COLLISIONAL OROGENESIS - SUBDUCTION OF OCEANIC PLATEAUS, CONTINENTAL MARGINS, ISLAND ARCS, SPREADING RIDGES, AND SEAMOUNTS

The sizes of continental blocks, basaltic oceanic plateaus, and island arcs that would cause collisional orogenesis when they enter a subduc tion zone are calculated in an analysis based upon the assumption of l ocal isostasy and the assumption that plate subduction is primarily dr iven by the negative buoyancy of the lithosphere. Buoyancy analysis in dicates that the bulk density contrast between 80-m.y.-old oceanic lit hosphere capped by a 7-km-thick basaltic crust and the less dense unde rlying asthenosphere is on the order of 0.04 gm/cm3. Oceanic lithosphe re that is approximately 10 m.y. old is the youngest that is more dens e than the asthenosphere and hence inherently susceptible to subductio n. Subduction zone metamorphism causes the crustal layer of basalt/gab bro to transform into more dense amphibolite and eclogite. Where eclog ite formation is extensive, the descending oceanic lithosphere increas es in bulk density by as much as 0.04 gm/CM3. Lithosphere that is 100 km thick with a 30-km-thick granitic continental crust resists subduct ion because it is approximately 0.09 gm/cm3 less dense than the asthen osphere. Contrasts in lithospheric bulk density (crust + mantle) of < 0.10 gm/cm3 are the difference between whether subduction is nearly in evitable (as for normal ocean crust) or greatly resisted (as for thick , ancient continents). Collisional orogenesis is defined as a plate in teraction of the sort that causes a rearrangement of plate motions, ge nerally with the initiation of a new subduction zone and the creation of mountains. Buoyancy analysis indicates that only bodies of continen tal and oceanic island arc crust that are > approximately 15 km thick make the lithosphere buoyant enough to jam a subduction zone. Oceanic island arc complexes built upon ocean crust typically must be active f or more than approximately 20 m.y. to attain crustal thicknesses so th at their attempted subduction causes collisional orogenesis. Oceanic p lateaus where basaltic crust as much as approximately 17 km thick caps 100-km-thick lithosphere are inherently subductable and actually less buoyant than normal oceanic lithosphere following subduction metamorp hism. Basaltic plateaus must have crustal thicknesses > approximately 30 km to typically cause collisional orogenesis during subduction. Sho rt subducting seamounts (<1-2 km tall) typically cause only temporary dents but taller seamounts locally cause permanent distortions as they bulldoze the front of the fore-arc block. The direct tectonic effect resulting from the subduction of most bathymetric highs is only a temp orary isostatic uplift of the fore-arc region of as much as several ki lometers, followed by subsidence to original elevations.