Overview of the Salton Trough and Northern Gulf of California What are the key scientific issues that can be addressed? Gary Axen New Mexico Tech Tectonics Program N. Gulf of CA & Salton Trough Cascadia Walker Lane Mojave Salton Trough Northern Gulf Tectonic Setting New “transitional” mafic/sedimentary crust in N Gulf, thick sedimentary layer New ocean crust in S Gulf New magnetically lineated ocean crust in Gulf mouth Key Questions • Extent, thickness, and distribution of new lower crust - badly known in S Salton Trough and N Gulf • Effects of thick sediments on melt extraction and thermal structure in rift • Interactions between magmatic and mechanical rifting • Active strain field and strain partitioning • Are LANFs still active in Laguna Salada? • Seismic hazards to El Centro, Mexicali, etc. • Geothermal resources Gulf Extensional Province (Transpression) Strain Partitioning: Transtension, Transpression, CW Vertical-Axis Rotation, Folding Salton Trough 5 Northern Gulf 4 3 2 1 Comparison of Crustal Structure Southern Gulf of California 3. Guaymas Northern Gulf - Salton Trough 5. Salton Trough ~20 km ~200 km 2. Alarcón Fuis, USGS website 1. Cabo - Puerto Vallarta 4. Delfín Tiburón ? ? González-Fernández et al., 2005 New crust Lizarralde et al. in review Present Area of New Crust Extent of new crust from refraction lines, gravity, and 2 wells into “granitic basement” (but wells within new basement area have hit juvenile granite…) New crust: forming in Brawley and Mexicali seismic zones; nonorthogonal spreading centers, asymmetric? Southern extent poorly known New Crust at 2 Ma New Crust at 4 Ma If most plate motion is accommodated magmatically, then new crust may be only ~ 4 Ma. LANFs on west side probably much older (8? Ma) Salton Trough Basement Sediments stripped off Precambrian basement Metasedimentary basement New mafic Fuis, USGS website lower crust • Does not incorporate folds, western LANFs Pre-San Jacinto, Elsinore • Could we improve upon these models? • Southern extent of new crust • Interaction of WSDS and magmatically accommodated rifting • Vibroseis surveys • Marine MCS in Salton Sea • Noisy, but mostly rural Obique view southeast from the shuttle shows bend in Main Gulf Escarpment near the south end of the cloud line Less Oblique Than Southern Gulf About 15° difference in trends of rift axis and western rift margin Well-constrained offset of ~ 320 km across Gulf (Oskin et al., 2001), but less extension than inferred farther south (~ 450 km) from seimsic surveys Low-Angle Normal Faults Present in N. Gulf West Salton Detachment ~8-4 to 1.1 Ma Cañada David Detachment ~12-8? To 0 Ma Las Cuevitas & Santa Rosa Detachments(?) ~12-8 to 4-2 Ma Active Low-Angle Normal Fault? Elsinore - Laguna Salada Flt Laguna Salada Cañada David Detachment Cañada David Detachment Detachment-related fault scarps & graben View ~ NNW (Axen et al., 1999; Spelz et al., in prep.) Cañada David Detachment Quaternary scarps follow detachment Many antithetic scarps Heave:throw ratio yields detachment dip equivalent to outcrop measurements Gravity & magnetic survey extends low dip ~ 2 km If all scarps formed in single EQ: ~10 m slip event! Axen et al., 1999 Conclusions • Northern Gulf & Salton Trough provide significant, attainable ties between EarthScope and MARGINS-RCL goals • Opportunities to study: • Unusual crustal structure (EarthScope) • Complex strain partitioning (PBO) • Fault mechanics (SAFOD)