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Slab-Tearing Following Ridge-Trench Collision: Evidence from Miocene Volcanism in Baja California, México ⁎ Carlos Pallares A, , René C

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Slab-Tearing Following Ridge-Trench Collision: Evidence from Miocene Volcanism in Baja California, México ⁎ Carlos Pallares A, , René C Journal of Volcanology and Geothermal Research 161 (2007) 95–117 www.elsevier.com/locate/jvolgeores Slab-tearing following ridge-trench collision: Evidence from Miocene volcanism in Baja California, México ⁎ Carlos Pallares a, , René C. Maury a, Hervé Bellon b, Jean-Yves Royer a, Thierry Calmus c, Alfredo Aguillón-Robles d, Joseph Cotten b, Mathieu Benoit a, François Michaud e, Jacques Bourgois f a UMR 6538, Domaines Océaniques, IUEM, Université de Bretagne Occidentale, Place Nicolas Copernic, F-29280 Plouzané, France b UMR 6538, Domaines Océaniques, IUEM, Université de Bretagne Occidentale, 6, Av. Le Gorgeu, C.S. 93837, F-29238 Brest Cedex 3, France c Estación Regional del Noroeste, Instituto de Geología, UNAM, Hermosillo, Son., C.P. 83000, México d Instituto de Geología, UASLP, Av. Dr. Manuel Nava no. 5, Zona Universitaria, San Luis Potosí, S.L.P., C.P. 78250, México e UMR 6526, Géosciences Azur, Université Pierre et Marie Curie, F-06235 Villefranche sur Mer, France f IRD and CNRS, UMR 6526, Escuela Politecnica Nacional (EPN), Departamento de Geología y Riesgos Naturales (DGRN), Andalucia n/s, P.O. Box 17-01-2759, Quito, Ecuador Received 15 June 2006; received in revised form 3 November 2006; accepted 12 November 2006 Available online 5 January 2007 Abstract Neogene magmatic activity in Central Baja California underwent a major change at ca. 12.5 Ma, when the Pacific–Farallon active oceanic ridge collided with the trench east of Vizcaíno Peninsula. The calc-alkaline magmatism which built the Comondú volcanic arc vanished and was replaced by unusual volcanic associations, which were erupted within six Late Miocene to Quaternary volcanic fields (Jaraguay, San Borja, San Ignacio, Santa Rosalía, Santa Clara, La Purísima), delineating a 600 km array along the Baja California Peninsula. New fieldwork, K–Ar datings and geochemical analyses on Jaraguay and San Borja lavas, combined with previous data, allow us to show that these associations include: (1) adakites emplaced between 12.5 and 8.2 Ma in the Santa Clara, Santa Rosalía and Jaraguay volcanic fields; (2) niobium-enriched basalts (NEB) in Santa Clara and Santa Rosalía (11.2 to 7.4 Ma); (3) tholeiitic basalts and basaltic andesites (11.3 to 7.2 Ma) displaying a very weak subduction imprint in La Purísima and San Ignacio; (4) alkali trachybasalts in Mesas San Carlos and Santa Catarina, northwest of Jaraguay (9.3 to 7.5 Ma), and finally (5) basalts and associated magnesian basaltic andesites and andesites. Locally referred to as “bajaites”, the latter lavas display very specific geochemical characteristics, including very high Sr and Ba contents. They were emplaced in all the above-mentioned volcanic fields between 14.6 and 5.3 Ma. The origin of these Late Miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended (tholeiites and alkali trachybasalts), or to the melting of its edges due to thermal erosion (adakites) and the subsequent reaction between adakitic melts and the supraslab mantle (NEB and “bajaites”). However, the identification of the remnants of the Pacific–Farallon fossil ridge south of Vizcaíno Peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window. We propose that the emplacement of slab window-related volcanic rocks between 13 and 7 Ma from ca. 25°N to 30°N, was due to a process of slab-tearing, which started when the active ridge collided with the trench east of Vizcaíno Peninsula. The oldest part of the downgoing plate sunk into the mantle, leading to the opening of a ⁎ Corresponding author. Tel.: +33 298498746; fax: +33 298498760. E-mail address: [email protected] (C. Pallares). 0377-0273/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2006.11.002 96 C. Pallares et al. / Journal of Volcanology and Geothermal Research 161 (2007) 95–117 tear-in-the-slab, which likely started from the southern limit of the already existing Southern California slab window. It propagated parallel to the trench over about 600 km and developed into an asthenospheric window. © 2006 Elsevier B.V. All rights reserved. Keywords: slab-tearing; slab melting; ridge-trench collision; magnesian andesite; asthenospheric window; Baja California; México 1. Introduction slab windows within which magma genesis was trig- gered by decompression melting of upwelling mantle The Cenozoic magmatic history of the western (Dickinson and Snyder, 1979; Dickinson, 1997; Wilson margin of North America has been largely controlled et al., 2005). For instance, the evolution of the edges of by the subduction of the Farallon oceanic lithosphere, the Southern California slab window has been recon- generating abundant calc-alkaline volcanism. However, structed at 18.9, 17.5, 16.0 and 14.5 Ma (Wilson et al., it was also characterized by the eruption of magmatic 2005). types uncommon in a subduction context, e.g. alkali Late Oligocene to Miocene calc-alkaline magmatism basalts and related rocks displaying Ocean Island Basalt in Baja California led to the emplacement of the (OIB) geochemical signatures in California (Dickinson, Comondú arc all along the Peninsula from ca. 25 to 1997) and México (Ferrari, 2004), and magnesian an- 16–11 Ma (Gastil et al., 1979; Martín et al., 2000). desites of adakitic affinity (“bajaites”) in Baja California Although calc-alkaline volcanism continued sporadical- (Rogers et al., 1985; Calmus et al., 2003). The emplace- ly until the Quaternary, it was largely replaced during ment of these unusual volcanic rocks has been ascribed the Upper Miocene by the emplacement of a complex to the opening of asthenospheric windows in the sub- magmatic association including adakites and associated ducting Farallon plate (Rogers et al., 1985; Saunders niobium-enriched basalts (NEB: Aguillón-Robles et al., et al., 1987; Hole et al., 1991; Cole and Basu, 1995; 2001), magnesian andesites, and tholeiitic basaltic an- Dickinson, 1997; Ferrari, 2004). desites showing only a weak subduction imprint (Benoit The tectonic evolution of the North American margin et al., 2002). The emplacement of this association has has been marked by the approach of several oceanic been ascribed to the opening of an asthenospheric win- spreading ridges, which have either collided with it or dow due to the subduction of a spreading ridge (Benoit been subducted beneath it (Atwater, 1970; Dickinson et al., 2002; Bourgois and Michaud, 2002). However, and Snyder, 1979). These interactions lead to the open- this hypothesis seems inconsistent with the occurrence, ing of asthenospheric windows following either ridge east of the Vizcaíno Peninsula and Baja California Sur, subduction (Thorkelson, 1996) or slab-tearing due to the of abandoned spreading centers along the Magdalena detachment of the deep part of the subducted plate after Rise (Michaud et al., 2004, 2005, 2006) as originally ridge-trench collision (van den Beukel, 1990; Yoshioka hypothesized by Mammerickx and Klitgord (1982) and and Wortel, 1995). More specifically, during the Tertiary Lonsdale (1991). Thus, another hypothesis than ridge subduction of the Farallon plate beneath the North subduction has to be found to explain the unusual com- American margin, the active Pacific–Farallon ridge ap- position of Late Miocene volcanics from Baja California. proached the trench and the corresponding triple junc- In this paper, we present new field, petrologic and tion migrated towards the south during the Miocene geochronometric (40K–40Ar) evidence for the occur- (Atwater, 1970; Mammerickx and Klitgord, 1982; rence in the Jaraguay and San Borja volcanic fields Atwater, 1989). Therefore, the subduction of this active (Baja California State) of Late Miocene adakites, alkali ridge below Southern California and northernmost Baja trachybasalts and magnesian andesites, emplaced since California at ca. 20 Ma caused the formation of a ridge- 11 Ma. Combining our results with previously published related asthenospheric window (Dickinson and Snyder, data on similar rocks and tholeiites from Baja California 1979; Thorkelson and Taylor, 1989; Dickinson, 1997; Sur (Aguillón-Robles et al., 2001; Benoit et al., 2002; Atwater and Stock, 1998), the size of which increased Calmus et al., 2003; Conly et al., 2005; Bellon et al., progressively through thermal erosion of its edges 2006), we are able to trace the location and extent of an (Dickinson and Snyder, 1979; Severinghaus and asthenospheric window which developed below the Atwater, 1990). The spatial distribution and ages of Peninsula between ca. 13 and 7 Ma. Then we discuss the magmatic rocks unrelated to “regular” calc-alkaline origin of this window taking into account recent data on series have been used to constrain the geometry of the the structure and ages of the fossil ridges abandoned subducting slabs, and especially to locate slab tears or west of the Peninsula, and their incidence on the Late C. Pallares et al. / Journal of Volcanology and Geothermal Research 161 (2007) 95–117 97 Miocene history of the Guadalupe and Magdalena plates Swath bathymetry magnetic and seismic profiles (Michaud et al., 2006). We will show that a model of collected off Baja California Sur during the Famex cruise slab-tearing followed by the detachment of the deep part (R/V L’Atalante, April 2002) clearly evidence several of the subducted oceanic plate accounts for the abandoned spreading centers along the Pacific–Magda- magmatic successions on land (Calmus et al., 2003; lena Ridge (Michaud et al., 2004, 2005, 2006). Prior to Bellon et al., 2006) as well as for the kinematic history their extinction, which may have occurred as late as 8– of the Pacific–Farallon plate. 7 Ma, these spreading segments accommodated part of the transcurrent motion between the Pacific and North 2. Geological setting and magma types American plates. Then, this motion was progressively transferred to the Tosco–Abreojos dextral strike-slip 2.1. Tectonic evolution of Baja California during the fault (Spencer and Normark, 1989), which propagated Neogene southwards (Michaud et al., 2006).
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