THE ROLE OF DENSITY SORTING IN THE GEOCHEMICAL AND GEOPHYSICAL EVOLUTION OF ARC CRUST

We investigate the influence of density-sorting processes on the geochemical and geophysical evolution of arc crust. Specifically, we compare two key processes: (1) delamination of dense arc lower crust and subsequent foundering into the underlying mantle, and (2) relamination of buoyant subducted material that detaches from the descending slab, ascends through the mantle wedge and accumulates within the arc crust. We use instability calculations derived for a density interface between two layers with non-Newtonian, temperature-dependent viscosity to quantify the time-scale and rates over which delamination and relamination may occur.

In the case of delamination, densities of negatively buoyant lower crustal rocks are constrained from the Talkeetna and Kohistan crustal sections, and arc Moho temperatures are estimated from a combination of geothermometry and seismic constraints. For relamination, we evaluate densities for a range of potential buoyant compositions (volcanics, plutonics, sediments) at pressures and temperatures appropriate for the top of the subducting slab.

The calculations show that delamination is an episodic process, with a periodicity of 0.5–5 Myr that may be regulated by the evolving thermal structure of the arc. In general each foundering episode removes a small fraction of the arc crust (e.g., <10–20% of the total arc thickness), after which the arc must re-thicken before the next foundering event can occur. By contrast, relamination likely occurs on a less regular basis, but each episode is more efficient, involving much larger volumes of crustal material.

Finally, we consider the role of each process on the evolution of arc crust to continental crust and show that a combination of delamination and relamination may be required to produce the observed geochemical (major and trace element) and geophysical (Vp and Vs) properties of the lower continental crust.