Crustal redistribution, crust-mantle recycling and Phanerozoic evolution of the continental crust

We here attempt a global scale mass balance of the continental crust during the Phanerozoic and especially the Cenozoic (65 Ma). Continental crust is mostly recycled back into the mantle as a result of the subduction of sediment in trenches (1.65 km3/a), by the subduction of eroded forearc basement (1.3 km3/a) and by the delamination of lower crustal material from orogenic plateaus (ca. 1.1 km3/a). Subduction of rifted crust in continent–continent collision zones (0.4 km3/a), and dissolved materials fixed into the oceanic crust (ca. 0.4 km3/a) are less important crustal sinks. At these rates the entire continental crust could be reworked in around 1.8 Ga. Nd isotope data indicate that ca. 80% of the subducted continental crust is not recycled by melting at shallow levels back into arcs, but is subducted to depth into the upper mantle. Continent–continent collision zones do not generally form new crust, but rather cause crustal loss by subduction and as a result of their physical erosion, which exports crust from the orogen to ocean basins where it may be subducted. Regional sedimentation rates suggest that most orogens have their topography eliminated within 100–200 million years. We estimate that during the Cenozoic the global rivers exported an average of 1.8 km3/a to the oceans, approximately balancing the subducted loss. Accretion of sediment to active continental margins is a small contribution to crustal construction (ca. 0.3 km3/a). Similarly, continental large igneous provinces (flood basalts) represent construction of only around 0.12 km3/a, even after accounting for their intrusive roots. If oceanic plateaus are accreted to continental margins then they would average construction rates of 1.1 km3/a, meaning that to keep constant crustal volumes, arc magmatism would have to maintain production of around 3.8 km3/a (or 94 km3/Ma/km of trench). This slightly exceeds the rates derived from sparse seismic experiments in oceanic arc systems. Although the crust appears to be in a state of rough equilibrium during the Phanerozoic, 200–300 million years cycles in sealevel may be governed in part by periods of crustal growth and destruction. During the Cenozoic the crustal volume may be running a long term loss of < 1.8 km3/a, meaning that arc production rates could be as low at 2.0 km3/a (50 km3/Ma/km), if sealevel fall approaches 175 m since 65 Ma. Periods of orogeny cause crustal thickening and enhanced loss via subduction and delamination, effectively increasing the size of the ocean basins and thus freeboard.