GROWTH OF CONTINENTS AT EXTENSIONAL ARCS: A VIEW FROM THE UPPER CRUST

Convergent boundaries are a major contributor to the growth of continents today, but more attention has focused on “vertical” crustal growth (through melting processes in the lower crust and mantle) than on “horizontal” crustal growth. Horizontal growth may be accomplished by:

1. Tectonic accretion of a “hot” extensional oceanic arc. This mechanism allows an evolved upper crust to be detached along a weak, thick middle crustal tonalitic layer; meanwhile, the lower crust and mantle have been coupled, strengthened and densified by mafic rift intrusions, so they subduct. Examples include the 300 X 30 km Cretaceous Alisitos arc of Baja California, and the modern Izu arc in Japan (cf Busby et al., 2006, JVGR 149: 1-46).

2. Extremely efficient tectonic trapping of eruptive products in extensional continental and oceanic arcs. This has been widely under-recognized, because the modern Earth is biased toward compressional arcs (Busby, 1988, Geology 16:1121-1125; Busby, 2012, Tectonics of Sedimentary Basins, Wiley-Blackwell). In both oceanic and continental extensional arcs, eruptive products are more evolved, with unusually high production rates, and a high proportion of “instant sediment” in the form of silicic pyroclastic material; sedimentation rates may thus match very high subsidence rates of up to 10 km/myr, in belts 10’s to 100’s of km wide and 100’s to 1,000’s of km long. These conditions may persist unabated for many 10’s of myr in extensional continental arcs, while extensional oceanic arcs are more susceptible to localized closure events because they are associated with basins floored by thin basaltic crust. However, on a regional scale, large-scale horizontal continental growth is produced by alternating extension and compression within the upper plate of a single long-lived oceanic subduction zone that fringes a continent. One example is the western third of Mexico, which is composed of extensional oceanic arcs that fringed the Mexican continental margin (Centeno-Garcia et al., 2011, GSAB 123: 1776-1797).

Now that it is more widely understood that extensional arcs are common in the geologic record, geologists must work to decipher the primary strain regime of arc terranes of all ages, in order to evaluate the importance of extensional arcs in “horizontal” creation of continental crust.