CONVERGENT MARGINS, GROWING AND SHRINKING CONTINENTS, AND THE WILSON CYCLE

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? 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 0.14 km²/yr of areal loss, a value dominated by the Alpine-Himalayan collision zone. Converting this present-day estimate to a long-term, Wilson-Cycle-average yields 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 or subtly growing 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 erosion of the thickened and uplifted continental crust and 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 and sediment volume delivered to continental margins, but requires that the resulting continental sediment and underlying oceanic crust, termed “quasi-continent”, be interpreted as true continent. During each Wilson Cycle, up to 25% of all continental area should be 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 Myr. Quasi-continent formation has a profound effect on continental evolution - comparable to or exceeding by a significant factor the areal contribution of island arc accretion. Quasi-continent becomes a progressively larger fraction of total continental area over time at the expense of ancient continent formed by purely igneous processes.