Global Age Distribution of Detrital Zircons, the Supercontinent Cycle, and Subduction Flux Through Time

Abstract:

The global age distribution of detrital zircons (DZ) tracks the supercontinent cycle. Abundance of zircon ages in modern sands fluctuates through an order of magnitude, with maxima at ca. 2.7, 1.9, 1.0, and 0.45 Ga and minima at ca. 2.3, 1.6, 0.9, 0.7, 0.40, and 0.21 Ga. The age distribution is shaped by differences in zircon production at rifts, arcs, and collisional orogens; by preservation, exhumation, destruction, and recycling of zircons; and by methodological and regional sampling biases. The first-order age maxima and minima have been explained by two largely incompatible models. Model 1 holds that global subduction flux (area subducted per unit time) is constant, that DZ minima reflect destruction of intraoceanic arcs by subduction erosion, and that maxima correspond to times of greater preservation of zircon sources leading up to and during collision. In contrast, Model 2 holds that stepwise changes in global subduction flux—and thus in zircon production at convergent boundaries—are intrinsic to supercontinent cycles. This follows because convergent boundaries are extinguished during collision; hence supercontinent tenures correspond to zircon minima.

Examination of >80 geologic secular trends reveals little empirical evidence bearing on these alternatives. The most telling evidence is provided by reconstructed sea levels. Model 1 implies no particular changes in sea level during supercontinent assembly. If plate motion were to cease across a collisional orogen, the same amount of convergence would begin elsewhere and the world population of ridges and ridge volume would be unaffected. In contrast, Model 2 predicts a drop in sea level triggered by supercontinent assembly because death of a collisional plate boundary would mean death of the corresponding ridge and consequent increase in room for seawater in the ocean basins. Reconstructions of global sea level show a major low centered in the Triassic, one that is not linked to glaciation. The sea level low overlaps with the 0.21 Ga DZ minimum and with Pangea's final assembly when convergence ceased across 1000s of kms of plate boundary. Sea level trends thus are consistent with Model 2. A carefully chosen global DZ dataset will eventually yield a quantitative record of global subduction flux, subject to proper accounting for other effects.