IMAGING ACCRETED TERRANES AT THE CASCADIA MARGIN: THE KLAMATH MOUNTAINS PROVINCE OF CALIFORNIA AND OREGON

Phanerozoic growth of the North American craton was primarily by accretion of tectonostratigraphic terranes, a process that may have been important back to Late Archean time. Therefore, understanding continental growth requires a thorough understanding of accretionary processes. Description of terrane accretion in the 4-D framework of EarthScope requires detailed study of a “pristine” example of accretion in the continental United States. This example is the Klamath Mountains province (KMp) of NW California and SW Oregon, which records ~400 m.y. of subduction-related terrane accretion in a series of shallowly dipping, thrust fault-bounded slices. These terranes lack significant post-accretion metamorphic overprints, preserving important information about conditions of accretion. Even though the basic geology is well known, many aspects of crustal architecture, accretionary events, exhumation, and terrane modification remain unknown. Important questions include: What is the middle and deep crustal architecture of the province? How far do individual terranes extend in the subsurface? How did crustal architecture change through time? What was the nature, tempo and episodicity of deformation, magmatism, sedimentation, burial and exhumation? Does the KMp reflect a specific, repeated “style” of accretionary process or were individual accretion events distinctive? The research will involve field analysis (structural and chronologic history of terrane-bounding fault zones, of Neogene erosional and depositional surfaces, and of high-angle normal faults), laboratory analysis (geochronology, thermochronology, DEM analysis, isotopic analysis and limited geochemistry), and a 350-km-long E–W seismic transect from the Pacific Ocean across the Klamath Mountains to the High Cascade Range. The seismic experiment will consist of a controlled-source refraction survey, within which will be embedded a 150-km-long reflection survey focused on the Neogene Condrey Mountain dome, an enigmatic, out-of-sequence structural window into the middle crust. Our results will be integrated with data from the densified passive array to obtain images of the crust and upper mantle, to distinguish structures associated with Mesozoic and younger terrane accretion, and to evaluate effects of exhumation and uplift.