TECTONIC REDUCTION OF CONTINENTAL AREA: IMPLICATIONS FOR CONTINENTAL EVOLUTION

The Wilson Cycle of continental collision and breakup alters continental area through deformation. Area is lost during the collisional phase, but is gained during breakup, as well as during extensional events such as the Basin and Range. We address a basic question. Does the deformation associated with the Wilson Cycle on average conserve continental area, or change it? To address this question we first estimated the instantaneous, deformation-induced change in continental area by integrating the areal strain-rate field inferred from geodesy, after correcting for recoverable elastic strain. We find 0.18 km²/yr of areal loss, a value dominated by the Alpine-Himalayan collision zone. Using the 500 my history of continental deformation, including the contribution from continental breakup, we have converted this instantaneous estimate into a long-term Wilson-Cycle-average value of approximately 0.10 km²/yr of areal loss. This represents dramatic continental shrinkage, equivalent to a 25% loss of continental area over a single Wilson Cycle. Since independent evidence suggests that total continental area has remained roughly unchanged since the Archean, this shrinkage is likely balanced by another process. Crust formation by island arc accretion, the best-known process for making new continent, is insufficient to balance this shrinkage. Instead, we find that the erosion of thickened, uplifted continental crust, and the deposition of continental sediment onto oceanic crust at continental margins, is the likely dominant balancing process. This inference is supported by present-day erosion rates, but requires that the resulting continental-sediment-on-oceanic-crust “quasi-continent” be interpreted as true continent, and that during each Wilson Cycle, 25% of all continental area consist of newly-formed “quasi-continent”. We identify ophiolite-bearing (outcropping of the oceanic-crust component) accretionary complexes as deformed ‘quasi-continent', and note that these complexes indeed represent roughly 25% of continental area over the last 500 my. Quasi-continent formation has a profound effect on continental evolution, since, over time, quasi-continents become a progressively larger fraction of total continental area, at the expense of ancient continent formed by purely igneous processes.