Pure Appl. Geophys. Ó 2010 Springer Basel AG DOI 10.1007/s00024-010-0204-z Pure and Applied Geophysics Volcanic Markers of the Post-Subduction Evolution of Baja California and Sonora, Mexico: Slab Tearing Versus Lithospheric Rupture of the Gulf of California 1 1,2,3 2,3 4 2,3 THIERRY CALMUS, CARLOS PALLARES, RENE´ C. MAURY, ALFREDO AGUILLO´ N-ROBLES, HERVE´ BELLON, 5 6 MATHIEU BENOIT, and FRANC¸ OIS MICHAUD Abstract—The study of the geochemical compositions and extending from the Pacific coast of Baja California to coastal K-Ar or Ar-Ar ages of ca. 350 Neogene and Quaternary lavas from Sonora. Tholeiitic, transitional and alkali basalts of subslab origin Baja California, the Gulf of California and Sonora allows us to ascended through this window, and adakites derived from the discuss the nature of their mantle or crustal sources, the conditions partial melting of its upper lip, relatively close to the trench. Calc- of their melting and the tectonic regime prevailing during their alkaline lavas, magnesian andesites and niobium-enriched basalts genesis and emplacement. Nine petrographic/geochemical formed from hydrous melting of the supraslab mantle triggered by groups are distinguished: ‘‘regular’’ calc-alkaline lavas; adakites; the uprise of hot Pacific asthenosphere through the window. During magnesian andesites and related basalts and basaltic andesites; the Plio-Quaternary, the ‘‘no-slab’’ regime following the sinking of niobium-enriched basalts; alkali basalts and trachybasalts; oceanic the old part of the Farallon plate within the deep mantle allowed the (MORB-type) basalts; tholeiitic/transitional basalts and basaltic emplacement of alkali and tholeiitic/transitional basalts of deep andesites; peralkaline rhyolites (comendites); and icelandites. We asthenospheric origin in Baja California and Sonora. The litho- show that the spatial and temporal distribution of these lava types spheric rupture connected with the opening of the Gulf of provides constraints on their sources and the geodynamic setting California generated a high thermal regime associated to astheno- controlling their partial melting. Three successive stages are dis- spheric uprise and emplaced Quaternary depleted MORB-type tinguished. Between 23 and 13 Ma, calc-alkaline lavas linked to the tholeiites. This thermal regime also induced partial melting of the subduction of the Pacific-Farallon plate formed the Comondu´ and thinned lithospheric mantle of the Gulf area, generating calc- central coast of the Sonora volcanic arc. In the extensional domain alkaline lavas as well as adakites derived from slivers of oceanic of western Sonora, lithospheric mantle-derived tholeiitic to tran- crust incorporated within this mantle. sitional basalts and basaltic andesites were emplaced within the southern extension of the Basin and Range province. The end of the Key words: Slab tearing, slab melting, ridge-trench collision, Farallon subduction was marked by the emplacement of much adakite, basalt, comendite, magnesian andesite, asthenospheric more complex Middle to Late Miocene volcanic associations, window, basin and range, Gulf of California, Baja California, between 13 and 7 Ma. Calc-alkaline activity became sporadic and Sonora, Me´xico. was replaced by unusual post-subduction magma types including adakites, niobium-enriched basalts, magnesian andesites, comen- dites and icelandites. The spatial and temporal distribution of these lavas is consistent with the development of a slab tear, evolving into a 200-km-wide slab window sub-parallel to the trench, and 1. Introduction The geochemical (major, trace elements and iso- topic) compositions of fresh magmatic rocks are 1 Estacio´n Regional del Noroeste, Instituto de Geologı´a, Universidad Nacional Auto´noma de Me´xico, C.P 83000 Hermo- mostly inherited from those of their source materials sillo, Sonora, Mexico. E-mail: [email protected] during partial melting, although they may have been 2 Universite´ Europe´enne de Bretagne, Universite´ de Brest, modified later by intracrustal petrogenetic processes Brest Cedex 3, France. 3 CNRS, UMR 6538 Domaines Oce´aniques, Institut Uni- such as fractional crystallization coupled or not with versitaire Europe´en de la Mer, Place N. Copernic, 29280 Plouzane´, assimilation of host rocks, or magma mixing. On one France. hand, experimental studies allow the petrologist to 4 Instituto de Geologı´a, UASLP, Av. Dr. Manuel Nava no. 5, Zona Universitaria, San Luis Potosı´, S.L.P, C.P 78250 Mexico, take into account the geochemical effects linked to Mexico. variable source mineralogy, temperature, pressure 5 UMR 5562, OMP, Universite´ Paul Sabatier, 14 Avenue and melting rate on the composition of the melts. On Edouard Belin, 31400 Toulouse, France. the other hand, the presence of a given source at 6 UMR 6526, Ge´osciences Azur, Universite´ Pierre et Marie Curie, 06235 Villefranche sur Mer, France. depth and the physical conditions governing its T. Calmus et al. Pure Appl. Geophys. partial melting are controlled by the regional geody- continental crust. These authors claim that the partial namic setting. Magmatic rocks are thus potential melting of these sources was due to the thermal markers of the tectonic regime prevailing during their rebound following the end of subduction, and that the emplacement. temporal and spatial distribution of post-subduction The Neogene and Quaternary geological history of lavas resulted from local tectonic features like the Baja California, Sonora and the Gulf of California has stress field and the tensile strength of the Baja been marked by the almost continuous emplacement California crustal rocks. TILL et al.(2009)considerallthe of volcanic rocks showing an exceptional geochemical Miocene volcanism in Sonora as subduction-related diversity (GASTIL et al., 1979;SAWLAN, 1991;BENOIT (continental arc type), and find only subtle geo- et al., 2002). Mafic lavas encompass the whole range chemical changes (slight variations of incompatible of basaltic compositions, from depleted mid-oceanic element ratios, e.g., La/Nb) concomitant with the ridge basalts (MORB) to plume-type alkali basalts, ridge-trench collision off Baja California at 12.5 Ma. through various kinds of tholeiitic, transitional and They suggest that the subduction signature of the sub- calc-alkaline basalts and the very rare niobium-enri- arc Sonoran mantle was not erased 4 m.y. after the ched basalts (NEB: AGUILLO´ N-ROBLES et al., 2001). end of subduction, and thus that what they call Intermediate and evolved lavas are also highly diver- ‘‘petrotectonic modeling’’ is a perilous exercise. sified. In addition to the types commonly found in Numerous good quality geochemical analyses of calc-alkaline series, they include unusual rocks such as K-Ar and/or Ar-Ar dated lavas from Baja California magnesian andesites (SAUNDERS et al., 1987;CALMUS Peninsula, the Gulf of California islands and coastal et al., 2003), adakites (AGUILLO´ N-ROBLES et al., 2001; Sonora have been published during the last 10 years. CALMUS et al., 2008), icelandites and peralkaline rhy- The purpose of this paper is to review them, to dis- olites (VIDAL-SOLANO et al., 2008a, b). cuss their implications on the nature of the magmatic A majority of authors have considered this geo- sources at depth and finally to examine critically the chemical diversity as resulting from the partial melting constraints that they may provide on the tectonic of contrasted mantle and crustal sources, during the evolution of the Pacific margin of northwestern complex tectonic evolution of the Pacific margin, Me´xico. which followed the end of the subduction of the Farallon oceanic plate around 12.5 Ma. In Baja California, the wide range of erupted magmas is 2. Tectonic Framework generally attributed to the opening of an astheno- spheric window, although the details of the process are The Middle Miocene to Recent tectonic and debated: for instance, the source of adakites is thought magmatic evolution of northwestern Mexico is closely to be either the subducted Farallon crust or the mafic related to the transition between subduction regime base of the continental crust (see PALLARES et al., 2007, and the opening of the Gulf of California. After the 2008;CASTILLO, 2008, 2009;MAURY et al., 2009, and Pacific-Farallon ridge entered the trench at the latitude references therein). In Sonora, the association of tho- of present-day Los Angeles, the Rivera triple junction leiitic to transitional basalts (temporally evolving migrated progressively to the south, until the east- towards alkali basalts) with icelandites and peralkaline wards subduction of the Farallon plate and subsequent rhyolites is linked to the transition from a typical Basin microplates below the North America plate ended and Range regime to rift opening in the nearby Gulf of between 12.5 and 12.3 Ma with the capture of mi- California (VIDAL-SOLANO et al., 2008a, b). croplates by the Pacific plate (LONSDALE, 1991). This is However, a rather different point of view has been the case north of the Shirley transform fault, when the developed in two recent articles. NEGRETE-ARANDA seafloor spreading between the Pacific and Farallon and CAN˜ O´ N-TAPIA (2008) consider that a stalled plates stopped after Chron 5AB (LONSDALE, 1991) and Farallon slab is still present beneath Baja California, more precisely during the younger part of Chron 5A and that the post-subduction magmas originated from (DYMENT, 2003) along the Guadalupe ridge. South of sources located in the mantle wedge or the overlying the Shirley transform