Abstract

The sizes of continental blocks, basaltic oceanic plateaus, and island arcs that would cause collisional orogenesis when they enter a subduction zone are calculated in an analysis based upon the assumption of local isostasy and the assumption that plate subduction is primarily driven by the negative buoyancy of the lithosphere. Buoyancy analysis indicates that the bulk density contrast between 80-m.y.-old oceanic lithosphere capped by a 7-km-thick basaltic crust and the less dense underlying asthenosphere is on the order of 0.04 gm/cm3. Oceanic lithosphere that is ∼10 m.y. old is the youngest that is more dense than the asthenosphere and hence inherently susceptible to subduction. Subduction zone metamorphism causes the crustal layer of basalt/gabbro to transform into more dense amphibolite and eclogite. Where eclogite formation is extensive, the descending oceanic lithosphere increases 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 Subduction because it is ∼0.09 gm/cm3 less dense than the asthenosphere. Contrasts in lithospheric bulk density (crust + mantle) of <0.10 gm/cm3 are the difference between whether subduction is nearly inevitable (as for normal ocean crust) or greatly resisted (as for thick, ancient continents).

Collisional orogenesis is defined as a plate interaction of the sort that causes a rearrangement of plate motions, generally with the initiation of a new subduction zone and the creation of mountains. Buoyancy analysis indicates that only bodies of continental and oceanic island are crust that are > ∼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 for more than ∼20 m.y. to attain crustal thicknesses so that their attempted subduction causes collisional orogenesis. Oceanic plateaus where basaltic crust as much as ∼17 km thick caps 100-km-thick lithosphere are inherenty subductable and actually less buoyant than normal oceanic lithosphere following subduction metamorphism. Basaltic plateaus must have crustal thicknesses >∼30 km to typically cause collisional orogenesis during subduction. Short 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 temporary isostatic uplift of the fore-arc region of as much as several kilometers, followed by subsidence to original elevations.

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