Journal of Volcanology and Geothermal Research 149 (2006) 1–46 www.elsevier.com/locate/jvolgeores View of an intact oceanic arc, from surficial to mesozonal levels: Cretaceous Alisitos arc, Baja California Cathy Busby a,*, Benjamin Fackler Adams b, James Mattinson c, Stephen Deoreo d a Department of Geological Sciences, University of California, Santa Barbara CA 93101, USA b Interdisciplinary Sciences, Skagit Valley College, 2405 E College Way, Mt Vernon, WA 98273, USA c Department of Geological Sciences, University of California, Santa Barbara CA 93101, USA d Department of Geological Sciences, University of California, Santa Barbara, CA 93106, USA Received 29 January 2005; received in revised form 13 June 2005; accepted 19 June 2005 Abstract The Alisitos arc is an approximately 300Â30 km oceanic arc terrane that lies in the western wall of the Peninsular Ranges batholith south of the modern Agua Blanca fault zone in Baja California. We have completed detailed mapping and dating of a 50Â30 km segment of this terrane in the El Rosario to Mission San Fernando areas, as well as reconnaissance mapping and dating in the next 50Â30 km segment to the north, in the San Quintin area. We recognize two evolutionary phases in this part of the arc terrane: (I) extensional oceanic arc, characterized by intermediate to silicic explosive and effusive volcanism, culminating in caldera-forming silicic ignimbrite eruptions at the onset of arc rifting, and (II) rifted oceanic arc, characterized by mafic effusive and hydroclastic rocks and abundant dike swarms. Two types of units are widespread enough to permit tentative stratigraphic correlation across much of this 100-km-long segment of the arc: a welded dacite ignimbrite (tuff of Aguajito), and a deepwater debris-avalanche deposit. New U–Pb zircon data from the volcanic and plutonic rocks of both phases indicate that the entire 4000-m-thick section accumulated in about 1.5 MY, at 111–110 MY. Southwestern North American sources for two zircon grains with Proterozoic 206Pb/ 207Pb ages support the interpretation that the oceanic arc fringed North America rather than representing an exotic terrane. The excellent preservation and exposure of the Alistos arc terrane makes it ideal for three-dimensional study of the structural, stratigraphic and intrusive history of an oceanic arc terrane. The segment mapped and dated in detail has a central major subaerial edifice, flanked by a down-faulted deepwater marine basin to the north, and a volcano-bounded shallow-water marine basin to the south. The rugged down-faulted flank of the edifice produced mass wasting, plumbed large-volume eruptions to the surface, and caused pyroclastic flows to disintegrate into turbulent suspensions that mixed completely with water. In contrast, gentler slopes on the opposite flank allowed pyroclastic flows to enter the sea with integrity, and supported extensive buildups of bioherms. Caldera collapse on the major subaerial edifice ponded the tuff of Aguajito to a thickness of at least 3 km. The outflow ignimbrite forms a marker in nonmarine to shallow marine sections, and in deepwater sections it occurs as blocks up to 150 m long in a debris-avalanche deposit. These welded ignimbrite blocks were deposited hot enough to deform * Corresponding author. Tel.: +1 805 893 3471. E-mail address: [email protected] (C. Busby). 0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2005.06.009 2 C. Busby et al. / Journal of Volcanology and Geothermal Research 149 (2006) 1–46 plastically and form peperite with the debris-avalanche matrix. The debris avalanche was likely triggered by injection of feeder dikes along the basin-bounding fault zone during the caldera-forming eruption. Intra-arc extension controlled very high subsidence rates, followed shortly thereafter by accretion through back-arc basin closure by 105 Ma. Accretion of the oceanic arc may have been accomplished by detachment of the upper crust along a still hot, thick middle crustal tonalitic layer, during subduction of mafic–ultramafic substrate. D 2005 Elsevier B.V. All rights reserved. Keywords: oceanic arc; geology; petrography; lithofacies; tectonostratigraphy; U–Pb zircon dating; Alisitos arc; Baja California 1. Introduction Baja California, south of the modern Agua Blanca fault zone (Fig. 1). A full discussion of the geolo- Recent years have seen major advances in under- gic setting of this oceanic arc terrane is given by standing of the tectonic, volcanic and sedimentary cha- Busby (2004). Geochemical and isotopic data sup- racter of modern oceanic arc systems, through the use port the oceanic arc interpretation, but several dif- of swath-mapping sonar surveys, submersible studies, ferent models have been proposed for the numbers magnetic and seismic surveys, dredging, and DSDP/ and polarities of subduction zones involved in its ODP coring (e.g., Bloomer et al., 1989; Taylor et al., generation and subsequent accretion to the Mexican 1990, 1991; Nishimura et al., 1992; Klaus et al., 1992; continental margin (see references in Busby, 2004). Taylor, 1992; Cambray et al., 1995; Clift and ODP Leg Most models for Mesozoic oceanic arc terranes of 135 Scientific Party, 1995; Fiske et al., 1995; Kokelaar western Mexico fall into two broad categories: (1) and Romagnoli, 1995; Yuasa, 1995; Fryer, 1996; Mur- the exotic arc model, where western Mexico grew akami, 1996; Wright, 1996; Clift and Lee, 1998; Fryer through accretion of exotic island arcs by the con- et al., 1998; Takahashi et al., 1998; Yamazaki and sumption of entire ocean basins at multiple subduc- Murakami, 1998; Izasa et al., 1999; Wright and Gam- tion zones with varying polarities, and (2) the ble, 1999; Worthington et al., 1999; Glasby et al., 2000; fringing arc model, where extensional processes in Bloomer et al., 2001; Wright et al., 2003; Yuasa and the upper plate of an east-dipping subduction zone Kano, 2003). These studies give a largely two-dimen- produced arc-related basins, some rifted off the sional view of oceanic arcs. Drill holes and geophysical continental margin and others formed of new ocea- studies yield some insights into the third dimension, but nic lithosphere that largely lay within reach of these are widely spaced. For this reason, we targeted a North American turbidite fans (Centeno-Garcia, segment of an ancient oceanic arc terrane for detailed 2005 and pers. comm.). In the fringing arc model, three-dimensional outcrop study (Fig. 1). This terrane, later phases of east-dipping subduction juxtaposed the Alisitos arc, provides one of the best exposed and these terranes through transtensional, transpressional best-preserved outcrop examples of an oceanic arc or compressional tectonics (Busby, 2004). Prelimin- reported in the literature to date (Fackler Adams and ary zircon provenance data support the fringing arc Busby, 1998). The segment we have mapped in detail model for at least parts of the Alisitos arc (Schmidt (Figs. 1–5) contains no postdepositional faults, and et al., 2002; preliminary data presented below). To subgreenschist metamorphism allows recognition of summarize, we interpret the Alisitos arc to be a primary microtextures. This makes it ideal for three- oceanic arc that fringed the continental margin dimensional study of the structural, stratigraphic and along an east-dipping subduction zone; it formed intrusive history of an oceanic arc terrane. Our results in an extensional strain regime, with well-preserved can be used to extrapolate into the third dimension, syndepositional normal faults and high rates of with greater detail, than is possible using the limited subsidence in both the arc region and the forearc sample base of modern oceanic arc systems. region (Busby, 2004). The Alisitos arc is a large (approximately In this paper we present the results of detailed 300Â30 km) oceanic arc terrane that lies in the (1:10,000 scale) mapping, petrographic analysis and western wall of the Peninsular Ranges batholith in dating of a 50Â30 km segment of the Alisitos arc ter- C. Busby et al. / Journal of Volcanology and Geothermal Research 149 (2006) 1–46 3 GEOLOGIC MAP OF THE PENINSULAR RANGES IN NORTHWEST BAJA CALIFORNIA, MEXICO (simplified from Gastil et al., 1971) USA MEXICO 0 50 km 32°°00' - CA AZ USA MEXICO North BAJ A Gul CA f LIF o OR Ca N li I A fo r n ia Pa cific Ocean San Quentín Tertiary volcanics segment (Fig. 10) SanSan QuintinQuintin X - Late Cretaceous to ° Tertiary ElE116° l RosarioRosario sedimentary rocks X Rosario segment (Fig. 2) Cretaceous plutons (Early Cretaceous in west, Late Cretaceous in east) 29°°30' - - 29°°30' - ° 115° Western belt: Eastern belt: Early Cretaceous Paleozoic oceanic arc to Mesozoic (Alisitos Group) continental margin metasedimentary rocks Fig. 1. Geologic setting of the Alisitos arc, western Peninsular Ranges, Baja California, Mexico. 4 C. Busby et al. / Journal of Volcanology and Geothermal Research 149 (2006) 1–46 Fig. 2. Geologic map of the Rosario segment of the Alisitos arc (locality shown on Fig. 1). C. Busby et al. / Journal of Volcanology and Geothermal Research 149 (2006) 1–46 5 B Explanation Qal/Qoa - Alluvium & older alluvium Quat. L. Cret.- KTrg - Rosario Group - sedimentary rocks, undifferentiated Tertiary Volcaniclastic and Sedimentary Rocks Kcvad - Dacitic/rhyolitic and andesitic pyroclastic rocks, largely Kvm - Marine tuffaceous volcaniclastic rocks, including subaqueous pyroclastic flow deposits nonmarine and tuff turbidites Recessive-weathering, grey to tan & brown outcrops