IS MAGMATIC ADDITION A SIGNIFICANT CRUSTAL GROWTH MECHANISM IN CORDILLERAN SYSTEMS?

The role of magmatic processes (specifically in situ continental arcs) has a long history of being invoked or dismissed as a significant mechanism for contributing to the evolution of high elevation plateaus in cordilleran systems. One current prevailing view is that tectonic shorting (with minor magmatic addition) is sufficient to account for nearly the entire estimated crustal thickness imaged in the Central Andes and by corollary, provides the dominant mechanism for plateau grown. Foundering of the lithosphere may augment the elevation history of cordilleran plateaus, but the initiation of synorogenic foundering requires a crustal thickening mechanism. We suggest that recent independently measured values (e.g. seismic tomography, geomorphic modeling, geological mapping and dating) of plutonic to volcanic ratios (P:V) and magmatic addition rates (MAR) along the American Cordillera strongly argue for an increasingly significant role of magmatic addition as a crustal thickening mechanism than is currently accepted.

The contribution of magmatic addition as a significant mechanism for high plateau formation remains a lingering question because of the inherent difficulty in quantifying P:V ratios and MAR. Although estimates of P:V ratios and MAR still have uncertainties associated with them, the general agreement between larger P:V ratios and MAR measured from recent independent approaches is a provocative topic for research. It is our contention that integrating advances in crustal-scale seismic imaging with increased geological and geochemical sampling of volcanic deposits can further elucidate the enigmatic plutonic to volcanic relationship. The focus of our work presented here tries to address the following question: Are the recent P:V ratios and MAR a function of the non-uniform sampling imposed by our ability to measure them in only a few select regions of the American Cordillera or can they be generalized to represent type behavior in cordilleran systems? While not yet being able to answer this question definitively, we present both geophysical and geological evidence that show promise in resolving this interesting problem of plateau growth in cordilleran systems.