DRAWING THE CURTAIN ON SLAB WINDOWS IN SOUTHWESTERN NORTH AMERICA

The concept of slab windows, “holes” that form in the subducted slab when a mid-ocean ridge encounters a subduction zone, is commonly used to explain aspects of magmatism at subduction zones. The slab window concept predicts that basaltic magmas produced by decompression melting erupt above the slab window as asthenosphere wells up into the void. This concept was first proposed in southwestern North America, and magmatism there was used to justify it. The availability of NAVDAT (www.navdat.org), a much larger dataset than used in the original studies, allows for a more rigorous look. Surprisingly, this analysis finds little or no support for such magmatism.

We examined the space-time distribution of about 29,000 Cenozoic igneous rocks from the western United States. Points were color-coded by composition and displayed as animations on an unrestored base map with migrating plate boundaries added. The slab window should have started to form ca. 30 Ma when the Pacific plate first came in contact with the North American plate, forming the San Andreas transform and two triple junctions. The slab window concept predicts that a region of basaltic volcanism should have grown progressively larger with time as the slab window grew inland. Our analysis finds essentially no magmatism that can be tied to such a window; there was no inland-enlarging area of magmatism, nor was magmatism within the area of the window predominantly basaltic. Volcanic rocks near the California coast could be related to a slab window, but placing them in their proper position requires restoring large offsets on coastal faults. Although widespread late Cenozoic basaltic volcanism in the Southwest is sometimes ascribed to a slab window, such volcanism occurred all over western North America and has been especially abundant in the last 8 m.y.

Whereas opening of a slab window seems a necessary consequence of ridge-trench collision, there is no evidence that a slab window triggered melting in upwelling sublithospheric mantle or controlled the distribution of magmatism through time in the western U.S. As with the ancestral Cascades, which are also essentially missing in the geologic record, it is time to reevaluate some of the fundamental concepts about magmatism in western North America.