EARTHSCOPE: A VISION FOR UNDERSTANDING CONTINENTAL PLATE TECTONICS

Oceanic plate tectonics is well established; the generation and subduction of oceanic plates is understood, at least in first order. In contrast, continental plate tectonics is still elusive. What is the nature of continental plates, especially their mantle component? How are continental plates connected to both shallow and deep mantle flow? What is the relationship between crust and mantle deformation during orogeny? The Earthscope program offers the opportunity for a quantum leap in understanding continental plate tectonics by combining the tools of seismology, petrology, geochronology, and structural geology applied to the North American continent. This discussion focuses on western North America and points out some of the key problems that can be addressed by an integrated Earthscope science. The North American continent displays a large variety of active and recent tectonic systems covering the spectrum of displacements from transpression to transtension. Incipient transtension is taking place in Baja California and the Eastern Sierra region, and more evolved transtension is preserved in the Basin and Range province. Continental transpression is active in the San Andreas region and is preserved in the Meso-Cenozoic tectonic systems of western North America to Alaska. The North American Cordillera offers an example of an orogenic wedge in which the foreland and the metamorphic core complexes represent the contraction and orogenic extension/collapse. For each of these tectonic systems, a wealth of geologic data exists from surface observations, but data on the structure of the mantle lithosphere are sparse. Perhaps the most promising vision of the Earthscope program is to link mantle flow to present-day surface displacements and also to crustal structure and tectonic history of Meso-Cenozoic orogenic zones. As more experimental data become available, seismic attenuation and anisotropy (based on olivine fabrics) are poised to become established parameters to determine the thermal state and structural geology of mantle lithosphere. Based on crustal geology, hypotheses can be formulated about mantle flow and tested against the seismic signature. These types of integrated studies will advance our understanding of continental tectonics by linking flow/deformation at depth (mantle, deep crust) with surface motions.