Seismic crustal structure and evolution of the Coast Mountains Batholith

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The Coast Mountain Batholith (CMB) of British Columbia is the largest batholith in the North American Cordillera. A batholith is a very large volume of previously molten rock in the deep subsurface that worked its way upward and solidified. The CMB is an ideal natural laboratory in which to address an outstanding problem in Earth Sciences. The average composition of the continental crust is represented by andesite [a volcanic rock intermediate between silicic (silicon oxides) and mafic (iron/magnesium oxides)]. However, melts derived from the mantle in all tectonic regimes are basaltic [a volcanic rock of mafic composition]. This discrepancy poses a fundamental problem for Earth Sciences: What processes generate an andesitic crust from basaltic rocks? We expect that the lighter silicic components fractionate and rise up, leaving a heavier ultramafic residue in the upper mantle. The residue may be so dense that it sank deeper into the mantle, or it may lay "hidden" below the crust-mantle boundary as a dense root. The BATHOLITHS project, primarily funded from U.S. sources ($4.6M over 5 years), investigates this problem through an integrated geological and geophysical study of the CMB. Seismic studies are among the most effective techniques for determining rock types in the deep crust and mantle. With NSERC funds of $286,500 over 3 years, Canadian scientists will participate as full partners in a major crustal-scale refraction / wide-angle-reflection (R/WAR) seismic program across the CMB. This contribution will bring nearly double our requested amount in U.S. funds to run the experiment, will provide access to the huge U.S. pool of seismometers (>2500 instruments), and will allow us to interact with U.S. and Canadian scientists from many Earth Science disciplines. We will share the significant discoveries to be made and promote the training of undergrad and graduate students who will become Earth scientists in industry, government and academia.