A SEISMIC SEARCH FOR THE HIDDEN ROOTS OF SOME MAJOR NORTH AMERICAN BACKBONE BATHOLITHS

A major tectonic element of the North American Backbone is the line of large-volume granitoid batholiths of Mesozoic and Cenozoic age that stitch together the western margin of the continent from Alaska to Mexico. The emplacement, unroofing, and uplift of these giant batholiths are an intimate part of the tectonic story of the Cordillera, and have important implications for the growth and compositional evolution of continental crust in general. A hidden part of this story involves the formation and potential foundering of a dense root to the batholith that is formed by the ultramafic residue of the melting process. Unanswered questions include: Do all major batholiths develop an ultramafic residue? What other kinds of residues are formed? What is the fate of heavy, ultramafic roots; do all such roots eventually founder, or can some remain in place for geologically long periods? How does root removal manifest itself at the surface? In this presentation, we illustrate how broadband (passive-source) seismology can be used to investigate some of these questions by analyzing data from the Sierra Nevada, Mojave-Salinian Block, and the Peninsular Ranges of California.

Much of both the southern Sierra and northern Peninsular Ranges batholiths are characterized, at depth, by large amplitude P-to-s conversions marking a sharp Moho. We suggest that this signal characterizes a batholith whose root has foundered, leaving behind a sharp boundary between the remaining granitoid body and newly emplaced mantle. Differently, much of the western portion of the central Sierra Nevada exhibits almost no Moho conversion. The lack of a Moho signal may result from a transitional increase in wavespeeds with depth reflecting a compositional gradient and an in-place ultramafic root. Comparing observed P-to-s conversions for data recorded at various batholiths to those predicted for a range of compositional columns will help to constrain the presence of a root and the composition of the remaining body.