Passive seismic study of a magma-dominated rift: the Salton Trough

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Non-technical explanationA well-accepted paradigm explains the architecture of structurally dominated continental rifts (that is, regions of the continents that are stretching or have been stretched, by extension on earthquake-faults) in terms of crustal thickness and heat flow. No equivalent paradigm explains the evolution of magmatically dominated rifts (stretched provinces in which volcanoes and their underlying igneous intrusions force the amount of extension), largely because we lack the observational data-sets to test numerical models. Understanding the formation and evolution of such rift basins is societally important because they contain much of Earth?s hydrocarbon reserve. NSF?s Geophysics program through this project is teaming with NSF?s MARGINS and Earthscope-Science programs to provide a detailed set of such observations for a rift in which the key features in the crust and upper mantle are buried beneath a deep basin, the Salton Trough in southernmost California. The Salton Trough is located across the southern end of the San Andreas transform system that has high risk of producing a devastating earthquake, and all new information obtained in this project will become a part of the community Southern California velocity model that is the basis for assessing strong ground motion and earthquake hazard throughout that areaTechnical descriptionThe Salton Trough is a magma-dominated rift linking the Gulf of California to the San Andreas fault system. Because the rift is buried beneath a thick pile of Colorado River sediments, surprisingly little is currently known about the total volume of intrusion into the crust and the magma distribution within and beyond the rift margins. We are determining these distributions and volumes with velocity measurements in the lower lithosphere. Wavespeeds are largely controlled within the crust by composition, and within the uppermost mantle by temperature (and fluid content). Lithology of the lower crust is best determined by combining P-wave velocities that are most reliably obtained from controlled-source refraction profiles with S-wave velocities that are typically better constrained by the joint surface-wave/receiver-function inversions that we are carrying out. We are deploying 40 passive seismometers across the Salton Trough, at nominal 5-km spacing, collinear with the NSF(MARGINS-Earthscope)-USGS controlled-source seismic profile. The NSF-USGS refraction/reflection profile is providing structural constraints, including Moho travel-time, and a vastly improved P-wave velocity model. Previous surface-wave studies carried out using permanent broadband stations across southernmost California will provide the phase-velocities for joint inversions with new receiver functions calculated from the data acquired here. Additional structural constraints (based on the P-wave reflection and refraction profiling) can be imposed on the joint inversions to obtain one of the best-determined Vp-Vs-lithologic profiles across a magmatic rift. The combination of dense receiver functions and well-determined velocity profiles allow a new test of whether the plate-bounding (San Andreas) strike-slip faults offset the Moho or disappear into a ductile middle crust.The Salton Trough is the northernmost part of the Gulf of California extensional province that rifted the North American continent and transferred Baja California and the Peninsular Ranges to the Pacific plate. Despite similar total extension along the province, very different extensional structures have been produced, from seafloor spreading in the southern Gulf to 14-20 km thick continental or transitional crust in the northern Gulf and Salton Trough. The new profile across the Salton Trough provide a comparative section to those recently determined across the Gulf, to enhance our knowledge of early ocean rifting and its propagation into a continent, and to understand the causes of along-strike variation in magmatic activity.