OBSERVATION-BASED RATES OF CREATING AND RECYCLING SOLID MATERIAL AT CONVERGENT MARGINS--IMPACT ON THE LONG-TERM PRESERVATION AND GROWTH OF CONTINENTAL CRUST

CONTINENTAL CRUST RECYCLING AT OCEAN MARGINS SUBDUCTION ZONES (OMZs): Offshore geophysical and geological investigations document that large volumes of rock and sediment are missing from the submerged forearcs of OMZs. Onshore studies establish that their structural and lithofacies fabric is truncated near the trench and commonly in conjunction with a history of inboard migration of the arc magmatic front. The trace-element signature of arc lavas reveal incorporated ocean floor sediment and forearc crustal rock. These observations are interpreted to mean that (1) lower plate sediment is transported within the subduction channel to reach mantle depths and that (2) the crust of the overlying forearc is thinned and truncated and this material is also transported to the mantle.

OMZ FLUXES: The solid-volume flux of recycling crust is estimated to be globally ~2.5 km3/yr (i.e., 2.5 Armstrong units or AU). The corresponding rate of forearc truncation is a sluggish 2-3 km/Myr, but over long periods of geologic time a vast volume of continental material is recycled at OMZs. During the past 2.5 billion years, when, arguably, subduction has been ongoing, a quantity of continental crust roughly equal to the standing volume has been tectonically erased from its surface. The amount of crust destroyed is so large that OMZ recycling must have been a major factor creating the rock pattern and age-fabric of continental crust.

CONTINENTAL CRUST RECYCLING AT CONTINENTAL/ARC COLLISION ZONES (CCZs): The rate at which arc magmatism globally adds juvenile crust to OMZs is generally estimated at ~1 AU. But new geophysical and dating information imply that the addition rate is at least ~5 AU (~125 km3/Myr/km of arc). Accepting Armstrong's posit that crustal additions are matched by recycling losses, then a setting for an additional 2-3 AU of crustal losses must be found. Because exhumed masses of ultra-high pressure metamorphic rock occur at continental collision zones (CCZ), we target CCZs as the setting of additional crustal losses, e.g. the 10,000-km-long Alpine-Himalaya CCZ. Recycling at CCZs is presumably effected by tectonic erosion of both plates and crustal delamination beneath thickened orogenic welts. Alternatively, the Earth's volume of continental crust has long been steadily growing and the Armstrong assumption is wrong.