Geology of the Alarcon Rise, Southern Gulf of California

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2018-03, Vol.19 (3), p.807-837
• Main Authors: Clague, David A., Caress, David W., Dreyer, Brian M., Lundsten, Lonny, Paduan, Jennifer B., Portner, Ryan A., Spelz‐Madero, Ronald, Bowles, Julie A., Castillo, Paterno R., Guardado‐France, Rigoberto, Le Saout, Morgane, Martin, Julie F., Santa Rosa‐del Río, Miguel A., Zierenberg, Robert A.
• Format: Article
• Language: English
• Subjects: Andesite; Autonomous underwater vehicles; Basalt; Bathymetry; Continental crust; Cores; Crystallization; Faults; Fractionation; Geology; Hills; hydrothermal vent; Lava; Lava flows; Magma; Mapping; mid‐ocean ridge; MORB petrology; Ocean floor; Oceans; rhyolite; Rhyolites; Ridges; Sampling; Seafloor spreading; Sediment; Sediments; Sequencing; Silica; sill; Sills; transform fault; Underwater vehicles; Unmanned vehicles
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2017GC007348

Meter‐scale AUV bathymetric mapping and ROV sampling of the entire 47 km‐long Alarcon Rise between the Pescadero and Tamayo transforms show that the shallowest inflated portion of the segment hosts all four active hydrothermal vent fields and the youngest, hottest, and highest effusion rate lava flows. This shallowest inflated part is located ∼1/3 of the way between the Tamayo and Pescadero transforms and is paved by a 16 km2 channelized flow that erupted from 9 km of en echelon fissures and is larger than historic flows on the East Pacific Rise or on the Gorda and Juan de Fuca Ridges. Starting ∼5 km south of the Pescadero transform, 6.5 km of the Alarcon Rise is characterized by faulted ridges and domes of fractionated lavas ranging from basaltic andesite to rhyolite with up to 77.3 wt % SiO2. These are the first known rhyolites from the submarine global mid‐ocean ridge system. Silicic lavas range from >11.7 ka, to as young as 1.1 ka. A basalt‐to‐basaltic andesite sequence and an andesite‐to‐dacite‐to‐rhyolite sequence are consistent with crystal fractionation but some intermediate basaltic andesite and andesite formed by mixing basalt with dacite or rhyolite. Magmatism occurred along the bounding Tamayo and Pescadero transforms as extensive channelized flows. The flows erupted from ring faults surrounding uplifted sediment hills inferred to overlie sills. The transforms are transtensional to accommodate magma migration from the adjacent Alarcon Rise. Plain Language Summary This study combines 1 m resolution bathymetry collected using an autonomous underwater vehicle, with chemical compositions of precisely located lava samples and ages of lava flows determined from short sediment cores collected using a remotely operated vehicle. The objective was to determine the history of an entire 47 km long segment of the global mid‐ocean ridge system. The ridge segment studied is named the Alarcon Rise and is located at the mouth of the Gulf of California. The Rise is bounded to the north and south by strike‐slip faults that offset the Rise from adjacent segments of the spreading ridge system. Such faults are usually thought to be parallel to the direction of seafloor spreading, but these have an oblique component to their movement that makes space for magma to be injected along the faults where it uplifts hills of sediment and sometimes erupts. Most lavas erupted along mid‐ocean ridges are basalts, but some highly unusual silica‐rich lavas were identified by their