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      Volcanic record of the arc-to-rift transition onshore of the Guaymas basin in the Santa Rosalía area, Gulf of California, Baja California

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          Abstract

          The Gulf of California is an archetype of continental rupture through transtensional rifting, and exploitation of a thermally weakened arc to produce a rift. Volcanic rocks of central Baja California record the transition from calcalkaline arc magmatism, due to subduction of the Farallon plate (ca. 24–12 Ma), to rift magmatism, related to the opening of the Gulf of California (<12 Ma). In addition, a suite of postsubduction rocks (<12 Ma), referred to as “bajaites,” are enriched in light rare-earth and other incompatible elements (e.g., Ba and Sr). These are further subdivided into high-magnesian andesite (with 50%–58% SiO2 and MgO >4%) and adakite (>56% SiO2 and MgO <3%). The bajaites correlate spatially with a fossil slab imaged under central Baja and are inferred to record postsubduction melting of the slab and subduction-modified mantle by asthenospheric upwelling associated with rifting or slab breakoff. We report on volcanic rocks of all three suites, which surround and underlie the Santa Rosalía sedimentary rift basin. This area represents the western margin of the Guaymas basin, the most magmatically robust segment of the Gulf of California rift, where seafloor spreading occurred in isolation for 3–4 m.y. (starting at 6 Ma) before transtensional pull-apart basins to the north and south ruptured the continental crust. Outcrops of the Santa Rosalía area thus offer the opportunity to understand the magmatic evolution of the Guaymas rift, which has been the focus of numerous oceanographic expeditions.

          We describe 21 distinct volcanic and hypabyssal map units in the Santa Rosalía area, using field characteristics, petrographic data, and major- and trace-element geochemical data, as well as zircon isotopic data and ten new 40Ar-39Ar ages. Lithofacies include lavas and lava domes, block-and-ash-flow tuffs, ignimbrites, and hypabyssal intrusions (plugs, dikes, and peperites). Calcalkaline volcanic rocks (13.81–10.11 Ma) pass conformably upsection, with no time gap, into volcanic rocks with rift transitional chemistry (9.69–8.84 Ma). The onset of rifting was marked by explosive eruption of silicic ignimbrite (tuff of El Morro), possibly from a caldera, similar to the onset of rifting or accelerated rifting in other parts of the Gulf of California. Epsilon Hf zircon data are consistent with a rift transitional setting for the tuff of El Morro. Arc and rift volcanic rocks were then juxtaposed by normal faults and tilted eastward toward a north-south fault that lay offshore, likely related to the north-south normal faults documented for the early history of the Guaymas basin, prior to the onset of northwest-southeast transtenional faulting. Magmatism in the Santa Rosalía area resumed with emplacement of high-magnesian andesite lavas and intrusions, at 6.06 Ma ± 0.27 Ma, coeval with the onset of seafloor spreading in the Guaymas basin at ca. 6 Ma.

          The 9.69–8.84 Ma rift transitional volcanic rocks underlying the Santa Rosalía sedimentary basin provide a maximum age on its basal fill. Evaporites in the Santa Rosalía sedimentary basin formed on the margin of the Guaymas basin, where thicker evaporites formed. Overlying coarse-grained clastic sedimentary fill of the Santa Rosalía basin and its stratiform Cu-Co-Zn-Mn sulfides may have accumulated rapidly, coeval with emplacement of 6.06 Ma high-magnesian andesite intrusions and the ca. 6 Ma onset of seafloor spreading in the Guaymas basin.

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          Author and article information

          Journal
          Geosphere
          Geological Society of America
          1553-040X
          May 26 2020
          May 26 2020
          Affiliations
          [1 ]Department of Earth and Planetary Sciences, University of California, Davis, 2119 Earth and Physical Sciences, One Shields Avenue, Davis, California 95616, USA
          [2 ]Department of Geosciences, University of Missouri–Kansas City, 5100 Rockhill Road, Kansas City, Missouri, USA
          [3 ]Depto. Geología, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California, México
          [4 ]Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78759, USA
          [5 ]Department of Geological Sciences, California State University, 800 N. State College Boulevard, Fullerton, California 91832, USA
          [6 ]Department of Geology, Occidental College, Los Angeles, California 90041, USA
          [7 ]Departamento de Ingenieria en Minas Metalurgia y Geología, Universidad de Guanajuato, Guanajuato, 36000, Mexico
          [8 ]Department of Geological Sciences, San Diego State University, San Diego, California 92182, USA
          [9 ]Department of Geology and Geography, Brooks Hall, West Virginia University, 93 Beechurst Avenue, Morgantown, West Virginia 26506, USA
          [10 ]West Virginia Geological and Economic Survey, 1 Mont Chateau Road, Morgantown, West Virginia 26508, USA
          Article
          10.1130/GES02094.1
          57fb26e1-0e9d-4b31-929d-c462a9e993be
          © 2020
          History

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