Age and stratigraphic relationships of pre- and syn-rift volcanic deposits in the northern Puertecitos Volcanic Province, Baja California, Mexico

Journal of Volcanology and Geothermal Research 93 Ž1999. 1–30 www.elsevier.comrlocaterjvolgeores

Age and stratigraphic relationships of pre- and syn-rift volcanic deposits in the northern Puertecitos Volcanic Province, Baja California, Mexico Elizabeth A. Nagy a

a,)

, Marty Grove b, Joann M. Stock

a

DiÕision of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA b Department of Earth and Space Sciences, UniÕersity of California, Los Angeles, CA, USA Received 6 October 1997; accepted 20 August 1998

Abstract Geologic mapping of volcanic strata of the northern Puertecitos Volcanic Province ŽPVP. in northeastern Baja California, Mexico, performed in conjunction with 40Arr39Ar analysis and petrochemical study, documents the Miocene geologic history of a well-preserved volcanic succession within the northern Sierra Santa Isabel and its relationship to the evolving Pacific–North America plate boundary. Subduction-related volcanic deposits, well-exposed in profile along the northern margin of the PVP in the informally named Santa Isabel Wash region, span pre-17 to 15 Ma. Minor rift-related volcanism occurred at ; 12.5 and ; 9 Ma, prior to voluminous PVP-forming volcanism at ; 6–6.5 Ma. Isochron ages typically exhibit precision Ž1 s . for plagioclase of "2–7% and for anorthoclase and sanidine of "2–5%, and replicate analysis of an internal anorthoclase standard indicate ; 1–2% reproducibility within a given irradiation and ; 2.5% for samples irradiated separately. Improved local correlations made possible by the rich stratigraphic section preserved in Santa Isabel Wash help constrain the relationships of several widespread pyroclastic flow deposits in northeastern Baja California. These correlations are important for both paleomagnetic studies within the region and for establishing geologic ties across the Gulf of California. The combined mapping and age results imply that most extensional deformation in the study area is post-6 Ma, although some earlier faulting and the development of the pre-6 Ma Matomı´ accommodation zone are also documented. Results support a transitional plate boundary model which implies that much of the Pacific–North America relative plate motion north of Delfın ´ basin Ži.e., the northernmost Gulf of California. is accommodated on N- to NNW-striking faults developed during Late Miocene ENE-directed extension. The model predicts a zone of divergence east of the PVP which provides a structural mechanism for the positions and jumps of nearby Gulf of California spreading centers ŽUpper and Lower Tiburon ´ and Delfın ´ basins. since 6 Ma, and relates major pulses of PVP volcanism at ; 6 and ; 3 Ma to these offshore spreading center adjustments. Results also imply that most extensional deformation in Santa Isabel Wash is the result of incorporation of the PVP into the Gulf Extensional Province ; 2–3 Ma due to northwestward propagation of the

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Corresponding author. Laboratoire de Geochronologie, Universite´ Paris 7, 2 Place Jussieu, tour 24-25, 1er etage, 75251 Paris, Cedex 05, ´ ´ France. Tel.: q33-01-4427-2823; fax: q33-01-4427-8148; E-mail: [email protected] 0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 8 0 - 3

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Guaymas fracture zone. Rotational deformation north of the PVP may have begun contemporaneously with this adjustment along the Gulf Extensional Province rift margin. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Puertecitos Volcanic Province; pyroclastic flow deposit

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Arr39Ar geochronology; Gulf Extensional Province; volcaniclastic deposit; lava flow;

1. Introduction The age and stratigraphic relationships of volcanic rocks produced throughout Baja California, Mexico, from Miocene through Recent time are important for understanding the Late Cenozoic evolution of the Pacific ŽPAC. –North America ŽNAM. plate boundary in this region. For example, volcanics of the Miocene–Pliocene Puertecitos Volcanic Province ŽPVP. ŽGastil et al., 1975. in northeastern Baja California ŽFig. 1. erupted in association with the developing Gulf Extensional Province ŽGEP. Žafter Gastil et al., 1975., a highly extended region bordering the Gulf of California ŽFig. 1, inset., and can be used to constrain the timing and amount of motion along rift-related structures since the time of deposition. Furthermore, documentation of the stratigraphic and age relationships of the PVP volcanics provides a basis for geologic ties constraining the amount of Neogene extension produced across the Gulf of California. To date, well-constrained geologic tie-points across the Gulf of California Že.g., Gastil et al., 1973. are extremely rare with estimates of post-Middle Miocene offset ranging from 300 km Že.g., Gastil et al., 1973; Curray and Moore, 1984. to 500 km Že.g., Gans, 1997.. Neogene volcanism along the Baja California peninsula is primarily associated with the changing tectonic setting of the evolving Pacific–North America plate boundary. Eastward subduction of the Farallon plate beneath Baja California Že.g., Atwater, 1970. produced an Early to Middle Miocene andesitic arc preserved on the eastern margin of Baja

California, on islands within the Gulf of California, and on the west coast of mainland Mexico ŽBarnard, 1968; Rossetter, 1973; Gastil and Krummenacher, 1977; Gastil et al., 1979; Hausback, 1984; Neuhaus, 1989; Neuhaus et al., 1988; Stock, 1989; Stock et al., 1991; Sawlan, 1991; Dorsey and Burns, 1994; Martın-Barajas et al., 1995; Lee et al., 1996; Lewis, ´ 1996.. Arc-related volcanism ceased when subduction ended 20–12.5 Ma along the peninsula ŽAtwater, 1970, 1989; Mammerickx and Klitgord, 1982; Spencer and Normark, 1989; Stock and Lee, 1994.. Between 12.5 and 5.5 Ma, accommodation of Pacific–North America motion was most likely partitioned between large, dextral, offshore transform faults, such as the Tosco–Abreojos and San Benito faults ŽFig. 1, inset; Kraus, 1965; Spencer and Normark, 1979., and ‘‘proto-Gulf’’ structures to the east accommodating ENE-directed extension ŽStock and Hodges, 1989., although this scenario is controversial Že.g., Gans, 1997.. Extension within the GEP produced Late Miocene to Pliocene silicic volcanism, including large rhyolitic volcanic provinces such as the PVP, at various locations along the length of the peninsula ŽGastil et al., 1975, 1979; Dokka and Merriam, 1982; Bryant, 1986; Stock, 1989, 1993; Stock et al., 1991; Martın-Barajas et al., 1995; Lee et ´ al., 1996; Lewis, 1996.. Oceanic crust with identifiable magnetic anomalies began forming between the PAC and NAM plates in the mouth of the Gulf of California ; 3.5 Ma ŽAnomaly 2A. ŽLarson et al., 1968; Curray and Moore, 1984; Lonsdale, 1989; DeMets, 1995.. Pliocene to Quaternary mafic volcanic rocks along the margins of the Gulf of Califor-

Fig. 1. Simplified geologic map of a portion of northeastern Baja California, Mexico Žmodified from Gastil et al., 1975. showing location of the Santa Isabel Wash study area ŽFig. 2.. The San Pedro Martir ´ fault ŽSPMf. marks the western edge of the Gulf Extensional Province X X Y north of the Puertecitos Volcanic Province. The pre-6 Ma Matomı´ accommodation zone wA–A after Stock Žin press. and A –A after Nagy Ž1997; in review.x separates a region of greater extension to the north from a less extended area to the south. CNf ŽCuervo Negro fault.; MC ŽMesa Cuadrada.; MT ŽMesa El Tabano .; SIf ŽSanta Isabel fault.; SSFf ŽSierra San Felipe fault.; VSFf ŽValle de San Felipe fault.. Inset: ´ GEP ŽGulf Extensional Province.; MGE ŽMain Gulf Escarpment.; SBf ŽSan Benito fault.; SJ ŽSierra Juarez ´ .; SLT ŽSierra Las Tinajas.; T–Af ŽTosco–Abreojos fault..

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nia, such as the 2.6 " 0.1 Ma Volcan Prieto ŽFig. 1; Martın-Barajas et al., 1995., are probably associated ´ with the present-day divergent plate boundary.

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Below we present new stratigraphy and 40Arr39Ar age results from the northern margin of the PVP ŽFig. 1.. The study is based upon geologic mapping

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Ž1:20 000., petrographic and petrochemical analyses of rock samples, and 40Arr39Ar measurements Žsee Nagy Ž1997. for details.. The results provide timing constraints on the initiation and development of extensional deformation in this portion of the PVP, revise and improve regional lithologic correlations between the northern PVP and nearby areas, and support ash-flow tuff correlations used in local paleomagnetic studies ŽNagy, 1997, in review; Nagy et al., 1995; Lewis and Stock, 1998a; Stock et al., 1999-this issue.. Furthermore, results have prompted a new model for the Late Miocene–Pliocene evolution of the Pacific–North America plate boundary in northeastern Baja California and the northernmost Gulf of California which involves a structurally transitional plate boundary zone that may have controlled the magmatic and tectonic evolution of the PVP as well as adjustments in the offshore ridge and transform fault system ŽNagy and Stock, 1998, in review..

2. Geologic setting of the Puertecitos Volcanic Province The subaerial part of the GEP has a sharp western boundary ŽMain Gulf Escarpment after Gastil et al. Ž1975.. represented north of the PVP by the 100km-long, E-dipping, San Pedro Martir ´ fault ŽFig. 1.. Although up to 5 km of normal separation occurs on this fault ŽGastil et al., 1975; Dokka and Merriam, 1982., displacement decreases southward to a maximum of 800 m in southern Valle Chico ŽStock and Hodges, 1990.. At the latitude of the northern PVP the San Pedro Martir ´ fault cannot be recognized; the western edge of the GEP is thus poorly defined. Several workers ŽDokka and Merriam, 1982; Nagy, 1997, in review; Stock, 1993, in press; Stock and Hodges, 1990; Axen, 1995. interpret the apparent southern termination of the San Pedro Martir ´ fault to mark the location of a pre-6 Ma, W- to NW-striking accommodation zone Žthe ‘‘Matomı´ accommodation zone’’ after Dokka and Merriam Ž1982... The first local geologic analysis of the PVP and surrounding areas was presented by Dokka and Merriam Ž1982.. Details of the stratigraphy and structure of the PVP and nearby regions ŽBryant, 1986; Stock,

1989, 1993; Stock et al., 1991; Martın-Barajas et al., ´ 1995; Lewis, 1996; Lewis and Stock, 1998b; Nagy, 1997, in review; Dorsey and Burns, 1994. have subsequently confirmed significant structural differences north and south of the pre-6 Ma Matomı´ accommodation zone. Extension east of the San Pedro Martir fault Ži.e., north of the accommodation ´ zone. has produced Basin-and-Range topography controlled by major E-dipping normal faults such as the San Pedro Martir ´ and Sierra San Felipe faults. This contrasts with the less extended PVP to the south where more closely spaced, E- and W-dipping normal faults exhibit smaller offsets. A longer period of extension north of the Matomı´ accommodation zone relative to the south Že.g., Dokka and Merriam, 1982. has contributed to these local structural differences. A more recent structural event involves ; 308 post-6 Ma, possibly post-3 Ma, vertical-axis clockwise rotation of the Sierra San Fermın ´ and Santa Rosa basin relative to Mesa Cuadrada in the southern Sierra San Felipe ŽFig. 1; Lewis and Stock, 1998a. and the northern PVP ŽNagy, 1997, in review; Nagy et al., 1995; Stock et al., 1999-this issue.. The northern Sierra Santa Isabel ŽFig. 1. exposes volcanic deposits and extensional structures related to GEP development Že.g., Gastil et al., 1975; CETENAL, 1977; Dokka and Merriam, 1982; Stock and Hodges, 1990; Axen, 1995.. The area of the present study in the northern PVP ŽFig. 2. includes two large arroyos informally named Santa Isabel Wash and Arroyo Oculto, and the entire study area is referred to as the Santa Isabel Wash region. Quaternary drainage patterns suggest that Santa Isabel Wash is a recently developed topographic low controlled by Nto NW-striking normal faults that deflected NE-directed drainages to the southeast along the structurally controlled margin of the wash Že.g., F4 in Fig. 2. ŽNagy, 1997, in review.. The pattern of faulting and erosion thus exposes up to ; 700 m of the volcanic succession along the northern margin of the PVP, providing a window into the pre-Late Miocene geology. Elsewhere, the PVP is a volcanic tableland of limited topographic relief Ž; 50–100 m. which rarely exposes underlying units. As detailed elsewhere ŽNagy, 1997, in review. it appears that the pre-6 Ma Matomı´ accommodation zone projects through the northeastern part of the study area and that the Cuervo Negro and Santa Isabel fault systems

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Fig. 2. Simplified geologic map Žafter Nagy, 1997. of Santa Isabel Wash in the northern Sierra Santa Isabel Žlocation in Fig. 1.. Informal names given here include Santa Isabel Wash, Arroyo Oculto, and Cuervo Negro and Santa Isabel faults. Stratigraphic column includes 40Arr39Ar ages determined in this study Žsee Tables 1 and 2.. The labels for the Miocene units indicate whether the rock is classified as a rhyolite, dacite, andesite, basalt, or volcaniclastic sediment by an r, d, a, b, or Õs, respectively, following the T ŽTertiary. and m ŽMiocene.. Subscripts are abbreviations of informal unit names. One anomalously young sample age for Tmrsiw , interpreted to be the result of glass contamination, is not listed. The stratigraphic relationship between group 7 lava flows is not apparent in the field except that Tmrgem clearly overlies Tmagem . Fault symbols: dashed faults are approximately located, dotted faults are concealed Žinferred., and faults with the letter ‘‘S’’ along them are features in the Quaternary alluvium interpreted to be X Y X Y fault scarps. A –A and B –B mark the positions of two NE-facing paleo-topographic slopes across which ;6–6.5 Ma tuffs Žgroup 6. thicken considerably from SW to NE. The paleo-topographic slopes are interpreted to be fault-controlled and associated with the southwestern margin of the pre-6 Ma Matomı´ accommodation zone.

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are major bounding structures accommodating post-6 Ma extension ŽFigs. 1 and 2..

3. Stratigraphy in Santa Isabel Wash 3.1. OÕerÕiew Twenty-one rock units mapped and defined in Santa Isabel Wash on the basis of lithology, stratigraphic position, and age are combined into seven lithologic groups ŽFig. 2.. Miocene units Žgroups 2–7. unconformably overlie pre-Miocene metasedimentary rocks Ž Pz . and granites Ž Mzg . comprising group Ž1.. From oldest to youngest the Miocene units are: group Ž2. volcaniclastic breccias and sedimentary rocks ŽTmÕs . and a local pyroclastic flow deposit ŽTmr bio ., group Ž3. intermediate to mafic lava flows and associated epiclastic breccias ŽTmb kc , Tmb lol , Tmd tomb ., group Ž4. a pyroclastic flow deposit ŽTmrsf ., group Ž5. intermediate lava flows ŽTmatoro ., group Ž6. a series of pyroclastic flow deposits Žoldest to youngest: Tmrsiw , Tmr3, Tmr4, Tmrao , Tmrec , Tmr bs , Tmrfp . and a local mafic lava flow ŽTmbnew ., and group Ž7. intermediate and felsic lava flows ŽTmagem , Tmrcan , Tma ugl , Tmahem .. Outcrops of undated granitic and metasedimentary rocks of group 1 are restricted to the northwest corner of Santa Isabel Wash. The metasedimentary rocks are likely related to lithologically similar metamorphosed Latest Precambrian to Paleozoic miogeoclinal to deep-water deposits of eastern Baja California ŽGastil, 1993.. The intrusive rocks are representative of the Mesozoic Peninsular Ranges Batholith Že.g., Silver and Chappell, 1988.. The oldest volcanic deposits Žgroups 2 and 3. are exposed in profile along north-facing topographic slopes, at the head of Arroyo Oculto, and in the footwalls of major normal faults. The 500 q m of stratified, poorly sorted, volcaniclastic breccias and sedimentary debris flows of group 2 are attributed to catastrophic sedimentation from the high-standing, unstable slopes of the Early to Middle Miocene, subduction-related andesitic arc. Laterally discontinuous lava flows and pyroclastic flow deposits occur within the breccias. Voluminous dacitic, and less abundant mafic Žbasaltic andesitic?., lava flows and associated collapse breccias Žgroup 3. overlie the

volcaniclastic breccias of group 2 and are also attributed to subduction-related volcanism based upon their Middle Miocene age Žsee Section 5 below.. Up to 450 m of outflow sheets of pyroclastic flow deposits Žgroups 4 and 6. and minor andesitic lava flows Žgroup 5. overlie group 2 and 3 rocks and are probably associated with development of the GEP Že.g., Martın-Barajas et al., 1995.. The ash-flow tuffs ´ of group 6 filled irregular, possibly fault-controlled, topography to produce the plateau-like, upper surface of the PVP. An ; 120-m-thick mafic lava flow is also preserved within the series of pyroclastic flow deposits. Andesite and rhyolite lava flows Žgroup 7. overlie these units north and south of Arroyo Oculto, where multiple flows forming deposits up to 500 m thick underlie Picacho Canelo Želev. ; 765 m. and Pico Los Heme Želev. ; 1040 m.. In addition to modern arroyo sedimentation in Santa Isabel Wash and Arroyo Oculto, Recent and older, highstanding alluvium occurs along the margins of the washes and surrounds isolated hills in the northern part of the map area. Fine-grained, modern, playa deposits lie within closed basins up to 2 km in diameter in the washes as well as on the volcanic plateau Žnot differentiated in Fig. 2.. 3.2. Detailed lithology Key features of each lithologic group are summarized below. In the absence of bulk chemical analyses, rock classification is based upon phenocryst assemblages and textures. Details such as specific location, size, and thickness of deposits, nature of contacts, outcrop and hand sample appearance, and mineralogical and petrographic information are in Nagy Ž1997.. Abbreviated field and petrographic descriptions of each unit ŽAppendix A. and ancillary electron microprobe results ŽAppendix B. are available from http:rroro.ess.ucla.edurPVPrsanta_ isabel.html. Appendix A also includes detailed information regarding stratigraphic relationships that supplement the simplified stratigraphic column shown in Fig. 2. Group 1 — Pz, Mzg. Basement rocks of group 1 consist of Mesozoic Ž?. granite Ž Mzg . and Paleozoic Ž?. metasedimentary rocks Ž Pz . pervasively intruded by 1-mm- to 4-cm-wide, ductilely deformed aplite

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dikes. Mzg is a medium-grained, unfoliated, garnetbearing, two-mica granite, and Pz consists of foliated garnet-epidote paragneisses interspersed with thin layers of diopside marble. Schists and weakly recrystallized chert, marble, and slate are also present. Group 2 — TmÕs, Tmr bio . A thick sequence of stratified, poorly sorted breccia to conglomerate lithofacies, designated elsewhere by Stock Ž1989. as volcaniclastic sediment ŽTmÕs ., occurs throughout Santa Isabel Wash and is interpreted as catastrophic debris flow deposits. Up to 500 q m thick, the deposits form steep, resistant cliffs with subhorizontal bedding planes discernible several kilometers away. Outcrops typically weather to pale blue or pink. Both monolithologic and heterolithologic deposits occur and include lithic fragments such as hornblende-phyric andesite, porphyritic rhyolite, and dark, fine-grained volcanic fragments. Basement clasts from group 1 were not found within TmÕs, similar to correlative deposits elsewhere Že.g., Stock, 1989; Lewis, 1996.. Interbedded, fluvial sandstones are rare. Thin Ž2–4 m., interbedded lava flows of intermediate composition and cross-cutting andesitic to basaltic dikes have been mapped with TmÕs. Significant relief is evident on the upper surface of TmÕs prior to angular unconformable deposition of subsequent units, including the other group 2 map unit, the Biotite tuff ŽTmr bio ., a non-welded, weakly indurated, crystal-rich, pumice- and lithic-lapilli pyroclastic flow deposit. Tmr bio is up to 80 m thick ŽE12 in Fig. 2. and appears to have been deposited within a narrow Ž- 100 m., steep-sided Žfault-controlled?. topographic low, such as a paleochannel or paleocanyon, whose axis trends roughly east–west over a distance of about 4 km. Tmr bio contains feldspar, quartz, and biotite phenocrysts in the matrix, amphibole- and biotite-bearing pumice pieces, and lithic fragments such as hornblende-phyric andesite and granite. Group 3 — Tmb kc , Tmb lol , Tmd tomb . A clinopyroxene – olivine – plagioclase basalt containing strongly resorbed quartz, the Klondike Canyon basalt ŽTmb kc ., and an olivine–pyroxene–plagioclase basalt with minor hornblende phenocrysts, the Land of the Lost basalt ŽTmb lol ., are small Ž- 0.5 km2 in map view. lava flows and dikes which intrude and unconformably overlie group 2 rocks

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ŽTmÕs .. Tmb kc is up to 140 m thick at the head of Santa Isabel Wash ŽD13 in Fig. 2., and Tmb lol is a maximum of 60 m thick ŽI13 in Fig. 2.. Tmb kc is unconformably overlain by the other group 3 unit ŽTmd tomb .. The latter is the Tombstone dacite, one of the most voluminous rock types in Santa Isabel Wash, which is a bronzite–hornblende–plagioclasephyric dacite with minor quartz phenocrysts. Solid, plug-like lavas and associated breccias present within Tmd tomb average 100–200 m in height and typically form continuous outcrops over distances of several km Že.g., L11 and F7 in Fig. 2.. Primary collapse– breccia structures suggest that these are proximal deposits. Although vent features have not been identified, the massive plugs present within Tmd tomb exhibit aspect ratios typical of intermediate to siliceous lava domes, e.g., ; 0.5 to 1.0 ŽBlake, 1990., and indicate that the flows were likely produced from 15–20 coalescing domes distributed over the 140 km2 area. Fresh hand samples range from pale pink to deep brick-red or black and contain phenocrysts of plagioclase, orthopyroxene, and hornblende. Granitic and gneissic xenoliths are common and typically rounded with black reaction rims. A few basalt flows up to 70 m thick below Tmd tomb are mapped with it and could be related to Tmb kc and Tmb lol volcanism. In some localities, 1 to 2 m of fluvial volcanogenic deposits occur between Tmd tomb and underlying TmÕs. Group 4 — Tmrsf . The Tuff of San Felipe ŽTmrsf . ŽStock et al., 1999. is a crystal-rich, strongly indurated, lithic-lapilli pyroclastic flow deposit up to 40 m thick restricted to the west side of the map area Že.g., B7–C6 in Fig. 2.. A basal vitrophyre characteristically grades upwards from 20–30 cm of brown glass bearing black lithophysae to a zone of reddish brown glass to 1 m of red and black glass. Spherulites increase upwards in size and abundance. In some places the basal vitrophyre overlies up to 50 cm of grey lithic ash. Stony, densely welded, cliff-forming Tmrsf above the vitrophyric base is devitrified, pale reddish purple, and contains large K-feldspar phenocrysts. Flattened lithophysae define a prominent foliation. Lithic fragments include trachytic and porphyritic volcanic rocks such as hornblende-phyric andesite. The devitrified top of Tmrsf is 3–5 m thick, less densely welded than underlying portions, and weathers to sky-blue. Hand samples are porcelaneous

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and lack the abundant lithophysae that characterize the lower sections. Group 5 — Tmatoro . The Pico del Toro andesite ŽTmatoro . is a hornblende-phyric andesite which crops out in two localities ŽF11–G10 and K12 in Fig. 2.. The larger lava flow covers ; 0.25 km2 in map view and is up to 200 m thick. Samples have a light brown to greyish-brown drusy matrix with dark brown hornblende phenocrysts. Group 6 — Tmrsiw , Tmbnew , Tmr3, Tmr4, Tmrao , Tmrec , Tmr bs , Tmrfp . The colorful, crystal-rich, pumice- and lithic-lapilli pyroclastic flow deposits comprising the Tuffs of Santa Isabel Wash ŽTmrsiw . are weakly indurated, distal outflow sheets up to ; 140 m thick that filled topographic lows. In general, deposits are thicker in the northern regions relative to the south where they apparently banked against a north-facing topographic slope. Tmrsiw can be vitric or devitrified with weathered surfaces characteristically grading upwards from yellowrorange to redrpink. Prominent eutaxitic foliation is common where the base overlies irregular topography. In some localities at least five cooling units can be recognized ŽAppendix A descriptions available from http:rroro.ess.ucla.edurPVPrsanta_isabel.html.. In general Tmrsiw contains pumice lapilli, feldspar phenocrysts, sub-rounded obsidian, and sub-angular lithic fragments of fine-grained, pale purple and pale blue–grey volcanic rocks Žjuvenile clasts?., porphyritic dacite ŽTmd tomb?., and granite Ž Mzg?.. The presence of disequilibrium olivine phenocrysts ŽAppendix A available from http:rroro.ess.ucla.edur PVPrsanta_isabel.html. distinguishes Tmrsiw from the other group 6 tuffs, and the general absence of hornblende, biotite, and quartz in Tmrsiw suggests a more anhydrous Žhotter?. parent magma than adjacent tuffs bearing hydrous phases. Paleomagnetic analysis shows that four of the cooling units preserve approximately the same remanent magnetization direction ŽNagy, 1997, in review. thus suggesting a relatively short period of time for deposition. Overlying New Year’s Mountain basalt ŽTmbnew . is a glomeroporphyritic olivine–pyroxene–plagioclase basalt with a 2–4-m-thick, scoriaceous basal breccia overlain by up to 120 m of stony lava ŽN10 in Fig. 2.. The steel grey to black matrix is dense and stony, can be highly vesiculated, and has glomerocrysts of plagioclase and pyroxene.

At least four cooling units are included in crystal-rich, pumice- and lithic-lapilli pyroclastic flow deposit rhyolite a3 ŽTmr3 . ŽStock, 1989.. Tmr3 deposits are 60–100 m thick, fill topography, and retain a fairly planar upper surface. The four devitrified cooling units look very similar in the field ŽAppendix A descriptions available from http:rr oro.ess.ucla.edurPVPrsanta_isabel.html., however the lowest cooling unit Ždesignated type I . preserves a significantly different paleomagnetic vector direction from the upper three units Ždesignated type II . ŽNagy, 1997, in review.. Tmr3 has a non-welded to slightly welded pale-colored matrix with fibrous silver–grey to white pumice up to 50 cm in length. Anorthoclase phenocrysts occur in both the matrix and pumice pieces and were used as an internal standard for the 40Arr39Ar geochronology in this study ŽINF-94-53.. Lithic fragments include finegrained volcanic rocks and granite. Rhyolite a4 ŽTmr4 . ŽStock, 1989. is a crystalpoor, weakly indurated, pyroclastic flow deposit. The characteristic base consists of 1 m of dark brown vitrophyre with orange, welded pumice which grades upwards to an orange then bright red, partially welded tuff. Spherulitic lithophysae are present in some outcrops and can be pink, blue, purple, or orange Žgenerally a different color from the groundmass.. Uppermost Tmr4 is purple and densely welded with similar lithophysae. In most of the map area Tmr4 is 3–5 m thick, however in Arroyo Oculto ŽFig. 2. it is up to 70 m thick and forms benches at its upper contact below more recessive units. These thicker exposures have a 2–4-m-thick, dark brown spherulitic base. Rare feldspar phenocrysts and volcanic lithic fragments are present. An overlying crystal-poor, weakly indurated, pumice- and lithic-lapilli pyroclastic flow deposit, the Arroyo Oculto Tuff ŽTmrao ., is 1–30 m thick. As with Tmr4, thicker exposures are restricted to the Arroyo Oculto region. Tmrao occurs either glassy or devitrified, unwelded to densely welded, and has a bright white weathered surface. In thick sections, an interior divitrified zone contains lithophysae with vapor phase crystallization. Feldspar and orthopyroxene phenocrysts are rare. Pumice and sub-rounded dacite clasts are present, as are granitic clasts in exposures around Arroyo Oculto. The Tuff of El Canelo ŽTmrec . is a densely welded, strongly indurated, crystal-rich, lithic-lapilli

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pyroclastic flow deposit which caps the mesa tops of the PVP in the Santa Isabel Wash region. Stock et al. Ž1991. and Martın-Barajas et al. Ž1995. define the ´ Tuff of El Canelo as a composite unit with at least six cooling units; the deposit described here is inferred to correlate with one or two of the cooling units from these nearby areas Žsee Section 5.2 below.. In most of Santa Isabel Wash, including the upper surface of the PVP plateau, Tmrec is 1–2 m thick with a devitrified groundmass, feldspar phenocrysts, and prominent eutaxitic foliation formed by subhorizontal, dark-colored fiamme. Near Arroyo Oculto ŽR7 in Fig. 2. Tmrec is up to 80 m thick and contains steeply dipping foliation planes with down-dip lineations suggesting syn- or post-compaction flow. These and other kinematic indicators, such as folded fiamme and tension cracks, suggest down-dip movement to the NNE. Topographic variations Ž1–3 m. in the upper surface of the thicker exposures impart a hummocky appearance to the top of the unit, while basal portions of the thick sections are densely welded with long, white, sub-horizontal fiamme giving outcrops a striped appearance and weathering in a stairstep fashion. A densely welded, upper cooling unit is up to 40 m thick at S6 in Fig. 2. The Bighorn Sheep Tuff ŽTmr bs . is a devitrified, weakly indurated, crystal-rich, lithic-lapilli pyroclastic flow deposit 1–15 m thick restricted to the Arroyo Oculto region. The unit contains feldspar phenocrysts, devitrified pumice, and minor lithic fragments including flow-banded volcanic rocks, glassy obsidian, and altered volcanic rock. The densely welded, strongly indurated Flag Pole Tuff ŽTmrfp . is a crystal-rich, lithic-lapilli pyroclastic flow deposit also restricted to the Arroyo Oculto region. Deposits are up to 7 m thick Žcommonly - 0.5 m., devitrified, and contain abundant, flattened lithophysae which have weathered to give the deposit a pock-marked appearance. Group 7 — Tmagem , Tmrcan , Tma ugl , Tmahem . The Pico de Los Gemelos andesite ŽTmagem . is a diopside–bronzite–plagioclase-phyric andesite ŽS5 in Fig. 2.. Its base consists of a series of red, 1–2-m-thick breccia layers that alternate with 1–2-m-thick layers of grey, foliated Žflattened vugs., stony lava. Hand samples have a fine-grained, purplish-grey to black matrix with flattened, sub-parallel vesicles, and small phenocrysts of pyroxene and plagioclase. Locally

9

Tmagem is underlain by a separate unit up to 2 m thick of black, mafic agglutinate or spatter with features such as lineations on clasts suggestive of fire-fountaining. The overlying Picacho Canelo rhyolite ŽTmrcan . is a series of plagioclase-phyric rhyolite flows which form the local topographic peak Picacho Canelo ŽFig. 2.. A well-exposed basal section ŽT3 in Fig. 2. consists of ; 1 m laminated airfall of rhyolite lithic fragments, pumice, and ash, overlain by 2–3 m of an obsidian-rich, perlitized, matrix-supported breccia. The perlite boulders average 50–80 cm within a yellow ashy matrix. The overlying flow-banded lava flows are up to 460 m thick and consist of a partially to completely devitrified matrix with plagioclase phenocrysts. Some portions contain small Ž- 1 cm. spherulites. A distinct feature is the presence of juvenile Ž?. and gabbroic xenolithic clasts. Folded flow structures developed around these clasts indicate transport to the northwest. Tmrcan extends beyond the map area to the north and east. A glomeroporphyritic orthopyroxene–plagioclase andesite, the Ugly Mountain andesite ŽTma ugl . ŽU6 in Fig. 2., has a 5–6-m-thick red Žoxidized. basal breccia below the main flow. Fresh surfaces exhibit a drusy texture and consist of red Žaltered. glass with feldspar phenocrysts and aggregates of altered mafic minerals. Weathered surfaces are brick red to orange, and weathered calcite Ž?. veins are ubiquitous throughout the rock. The Los Heme andesite ŽTmahem . is a diopside–bronzite– plagioclase-phyric andesite petrographically identical to Tmagem . The lava flow forms the local peak Pico Los Heme ŽFig. 2. and extends beyond the map area to the southeast. Large plagioclase and pyroxene phenocrysts are clearly visible in hand specimens. 4. 40Ar r39Ar results Details of the 40Arr39Ar procedures followed in this study are summarized in Appendix A and discussed in detail in Nagy Ž1997.. All reported ages result from inverse isochron analysis Že.g., McDougall and Harrison, 1988. using a weighted regression routine described in Mahon Ž1996.. All ages are calculated using Fish Canyon Tuff sanidine flux monitor ŽFCT-1. with an assumed age of 27.8 Ma ŽCebula et al., 1986.. Our quoted analytical uncer-

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10

tainties Ž"2 s . are based strictly upon analytical errors relevant to individual measurements. Accordingly, they dominantly reflect errors associated with measured peak heights, line blanks, and mass discrimination and do not incorporate errors in J-factors or other irradiation parameters that are only relevant to interstudy comparisons. We do emphasize, however, that recent estimates of the K–Ar age of FCT-1 vary considerably Že.g., 27.9 Ma, Renne et al., 1994; 27.6 Ma, Lanphere and Baadsgaard, 1997. and that an uncertainty of 0.3 Ma in the flux monitor age corresponds to 1% overall error in the calculated age of Miocene samples. Tabulated 40Arr39Ar results for

individual analyses, including model ages for each measurement, are presented in Appendix C Žavailable from http:rroro.ess.ucla.edurPVPrsanta_ isabel.html.. Note that model ages for individual analyses Žand weighted mean ages calculated from them. are unreliable for relatively unradiogenic Ž10%., low 39Ar yield Ž- 10y1 5 mol 39Ar. analyses because they are highly susceptible to errors in correction factors for mass discrimination and mass 36 background. Statistical results for our internal standard ŽINF94-53. are summarized in Table 1 while results for all other samples appear in Table 2. Corresponding

Table 1 40 Arr39Ar results from internal standard ŽTmr3: INF-94-53a anorthoclase. from Santa Isabel Wash Sample

J-factor

a of aliquots included in calculation over totala

Inverse Isochron Age ŽMa. Ž"2 s .

40 Arr39Ar Ž"1 s .

40 Arr36Ar Ž"1 s .

MSWD

95% critical MSWD range Žafter Mahon, 1996.

MIT INF-94-53b,c

0.003480

10r10

6.4 " 0.2

1.02 " 0.02

294 " 6

1.71

0.27–2.19

UCLA UofMa75 (tube 1) INF-94-53A INF-94-53B INF-94-53C INF-94-53g c

0.0004178 0.0004215 0.0004174 0.0004191

6r6 5r5 5r5 4r4

6.3 " 0.3 6.2 " 0.3 6.2 " 0.3 6.2 " 0.3

8.37 " 0.07 8.23 " 0.06 8.29 " 0.06 8.22 " 0.07

293 " 2 291 " 1 292 " 3 292 " 2

4.80 1.00 3.19 1.12

0.12–2.78 0.07–3.12 0.07–3.12 0.03–3.69

UofMa81 (tube 1) INF-94-53Ac 0.0008170 INF-94-53B c 0.0008275 INF-94-53C c 0.0008059

4r6 5r5 5r5

6.4 " 0.6 6.7 " 0.4 6.5 " 0.3

4.38 " 0.18 4.50 " 0.11 4.50 " 0.05

287 " 4 283 " 5 287 " 2

1.29 2.27 0.33

0.03–3.69 0.07–3.12 0.07–3.12

(tube 2) INF-94-53D c INF-94-53E c INF-94-53F c

4r4 5r5 2r2

6.5 " 0.3 6.4 " 0.3 6.4 " 0.3

4.48 " 0.07 4.37 " 0.05 4.50 " 0.05

287 " 2 289 " 2 288 " 2

0.73 0.18 NrA

0.03–3.69 0.07–3.12 NrA

UofMa75 d UofMa81d Overall a

0.0008066 0.0008097 0.0007928

a of duplicate slits included in calculation over total a

Weighted mean age ŽMa. Ž"2 s .

MSWD

95% critical MSWD range Žafter Mahon, 1996.

4r4 6r6 10r10

6.24 " 0.08 6.46 " 0.24 6.31 " 0.32

0.56 1.02 3.50

0.03–3.69 0.12–2.78 0.27–2.19

UTM coordinates of Sample Location Žnorth, east.: 33 62 905 mN 07 01410 mE. MIT Results recalculated for consistency using Fish Canyon sanidine age of 27.8 and isochron regression routines of Mahon Ž1996.. c Sample irradiated with g-radiation. d Regressions performed only for individual splits of INF-94-53 because of variations in J-factors. Mean ages provided for each irradiation. b

Table 2 40 Arr39Ar results from feldspars from Santa Isabel Wash Location in UTM coordinates Žnorth, east.

Sample

Mineral c

Bulk d J-factor e CarK

7 7 6 6 6 6 6 6 5 4 3 3 3 3 3 3 3 3 3 2

33

CAN-95-11 CAN-95-12 LCR-94-110 INF-94-75 INF-94-47 INF-94-33 CHO-94-45 SUN-94-30 INF-94-68 LCS-94-72 DIN-94-46 INF-94-52 INN-94-73 WIN-94-32 PHO-94-10 LCR-95-08 PHO-94-86 INF-95-24 KC-95-19 KC-95-17

Oligoclase Oligoclase Oligoclase f Anorthoclase g Oligoclase f Oligoclase Oligoclase Oligoclase Labradorite Anorthoclase Andesine Andesine Andesine Andesine Andesine Andesine Andesine Andesine Andesine Andesine

4.0 4.9 2.1 0.52 3.9 4.5 4.5 5.2 43 0.17 14 5.7 17 14 13 29 19 15 14 16

a

Tmrcan Tmrcan Tmrec Tmrao Tmrsiw Tmrsiw Tmrsiw Tmrsiw Tmatoro Tmrsf Tmd tomb Tmd tomb Tmd tomb Tmd tomb Tmd tomb Tmd tomb Tmd tomb Tmb lol Tmb kc Tmr bio

67 803 mN 67 705 mN 33 66 006 mN 33 62 808 mN 33 62 507 mN 33 65 004 mN 33 65 307 mN 33 65501 mN 33 62 507 mN 33 65 807 mN 33 62 807 mN 33 63 300 mN 33 64 403 mN 33 65 004 mN 33 67 102 mN 33 65 305 mN 33 66 607 mN 33 806 61 mN 33 62 309 mN 33 62 004 mN 33

07

09 990 mE 09 350 mE 07 04 410 mE 07 230 01 mE 07 03 520 mE 06 98 680 mE 07 00 800 mE 06 99 430 mE 07 03 080 mE 06 97 890 mE 07 03 590 mE 07 560 01 mE 06 98 280 mE 06 99 240 mE 07 02 350 mE 07 03740 mE 07 02 180 mE 07 02 040 mE 06 99 240 mE 06 98 530 mE 07

0.0008020 b 0.0008040 b 0.0008087 b 0.0004178 a 0.0004191a 0.0004184 a 0.0004179 a 0.0008084b 0.0004208 a 0.0008102 b 0.0004218 a 0.0004202 a 0.0004211a 0.0004220 a 0.0004218 a 0.0008298 b 0.0008298 b 0.0008120 b 0.0008102 b 0.0008203 b

40 a of aliquots Inverse Arr 39Ar included in isochron Ž"1 s . calculation age ŽMa. over total a Ž"2 s .

40 Arr 36Ar MSWD 95% critical Ž"1 s . MSWD range Žafter Mahon, 1996.

17r18 6r7 6r9 10r13 9r9 8r8 8r8 16r18 13r13 8r8 8r8 12r12 8r8 8r8 8r8 5r5 13r13 14r15 15r18 13r13

268"15 287"9 286"4 287"1 293"5 289"1 281"4 285"2 288"5 285"11 287"3 286"2 289"3 280"4 267"11 277"12 271"8 298"7 273"7 287"2

6.0"0.4 5.9"0.8 6.1"0.5 6.4"0.3 6.5"0.3 6.6"0.5 6.7"0.3 5.7"0.4 9.3"0.8 12.7"0.5 15.5"0.7 15.9"0.7 15.9"1.2 16.1"0.8 16.2"0.8 16.7"1.0 16.4"1.4 16.3"1.0 17.1"2.2 17.1"2.4

4.15"0.12 4.07"0.28 4.20"0.15 8.45"0.04 8.59"0.12 8.81"0.29 8.94"0.09 3.92"0.12 12.24"0.50 8.69"0.07 20.48"0.25 21.09"0.15 21.03"0.64 21.30"0.28 21.32"0.25 11.21"0.25 11.02"0.42 11.18"0.28 11.77"0.73 11.63"0.79

1.21 1.89 0.45 7.77 1.94 0.56 0.98 2.39 4.29 0.45 2.92 1.24 1.01 0.74 5.02 0.64 1.36 1.96 2.11 1.18

Irradiation UofMa75. Irradiation UofMa81. c Based on representative electron microprobe analysis. d Combined electron microprobe and calculated bulk CarK from the 40Arr39Ar analysis used to determine the relative purity of the mineral concentrates. e Calculated assuming 27.8 Ma for Fish Canyon sanidine. f Anorthoclase also present. g Plagioclase also present. b

0.42–1.83 0.12–2.78 0.12–2.78 0.27–2.19 0.24–2.29 0.21–2.40 0.21–2.40 0.40–1.86 0.35–1.99 0.21–2.40 0.21–2.40 0.33–2.05 0.21–2.40 0.21–2.40 0.21–2.40 0.07–3.12 0.35–1.99 0.37–1.94 0.39–1.90 0.35–1.99

E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30

Group Map unit

11

12

E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30

isochron plots are displayed in Fig. 3. Note that the 95% ranges of critical MSWD ŽMean Square Weighted Deviate. values discussed in Mahon Ž1996. are also included in Tables 1 and 2. Individual analyses were excluded from the isochron data only in instances in which either the results could be statistically identified as outliers or when the amount of sample gas evolved was sufficiently small Ž10y1 6 mol 39Ar. that the analysis was highly susceptible to line blank uncertainties. In some instances, MSWD values fell outside the range of ‘‘critical MSWD values’’ even after exclusion of data of the type described above. For these samples it appears likely that either a heterogeneous crystal population exists, andror analytical uncertainties are underestimated. Isochron ages in this study typically exhibit precision Ž1 s . for plagioclase Ž"2–7%. and anorthoclase and sanidine Ž"2–5%. that agrees well with values reported from similar studies of young, K-rich and K-poor volcanic phenocrysts Že.g., Cheilletz et al., 1992; Pringle et al., 1992; Martın-Barajas ´ et al., 1995; Zumbo et al., 1995; Spell et al., 1996.. Replicate analysis of Tmr3 ŽINF-94-53. indicate that our ability to reproduce results from separate aliquots of an apparently homogeneous, Late Miocene anorthoclase is ; 1–2% within a given irradiation and ; 2.5% for samples irradiated separately. For example, results from 10 different splits of INF94-53 representing two separate irradiations ŽUofMa75 and UofMa81. are displayed in Table 1. Normal polarity magnetization preserved in Tmr3 ŽNagy, 1997, in review; Lewis and Stock, 1998a. constrains the age of deposition to Subchron C3An.2n Ž6.269–6.567 Ma; Cande and Kent Ž1995... The weighted mean of the replicate isochron results for all UCLA analyses of INF-94-53 is 6.31 " 0.32 Ma Ž2 s uncertainty; MSWD s 3.5.. A comparable isochron age Ž6.4 " 0.2 Ma; 2 s uncertainty; MSWD s 1.7. for INF-94-53 anorthoclase was independently obtained from the MIT Cambridge Laboratory for Argon Isotopic Research ŽAppendix C available from http:rroro.ess.ucla.edurPVPrsanta_isabel. html.. Note that in the above comparison, the latter result has been recalculated for consistency with our use of 27.8 " 0.3 Ma for FCT-1 after Cebula et al. Ž1986.. As mentioned above, recent K–Ar results presented by Lanphere and Baadsgaard Ž1997. indicate that the K–Ar age of FCT-1 sanidine may in

fact be somewhat younger Ž27.55 " 0.10 Ma.; use of this younger age would generally reduce our calculated model ages by less than 0.15 Ma. Finally, we note that splits of INF-94-53 that were exposed to a 137 Cs g source to aid mineral separation prior to neutron irradiation ŽAppendix A. yield statistically identical results to untreated samples ŽTable 1.. For many samples, apparent ages determined from low yield 39Ar analyses are statistically younger than those obtained for fusion steps in which normal 39Ar yields were obtained ŽAppendix C available from http:rroro.ess.ucla.edurPVPrsanta_isabel.html.. This is particularly characteristic of the initial steps obtained from incrementally heated samples. This relationship results in isochron plots indicating 40 Arr36Ar values that are lower than modern atmosphere Ž40Arr36ArAT M s 295.5.. Renne and Basu Ž1991. and Renne et al. Ž1992. found similar lowtemperature discordances for plagioclase samples using an incremental laser heating technique. They attribute their low apparent ages to argon loss through minor reheating ŽF 1508C. or alteration of the volcanic rocks. We believe that either minor surface alteration andror undetected hydrocarbons contributing to mass 36 measurements are the most likely reason for the anomalously young ages and subatmosphere 40Arr36Ar ratios observed for our samples. However, isotopic mass fractionation of atmospheric argon prior to, or even after, eruption of the rock could conceivably produce a trapped component with a 40Arr36Ar ratio lower than the presentday, atmospheric value Že.g., Krummenacher, 1970; Kaneoka, 1980; Lippolt et al., 1990.. As indicated in Table 2, nearly 75% of the samples yield isochrons with MSWD values within the 95% confidence interval outlined by Mahon Ž1996.. Seven samples, however, yield MSWD values beyond this range. Although deleting additional analyses would lower the MSWD to acceptable values for most samples, we choose to consider all reasonable results in an attempt to avoid possible bias. Consistency of ages with stratigraphic order and agreement with results from other dated samples from correlative lithologic units indicate that isochron results for samples INF-94-75, INF-94-68, INF-94-46, PHO94-10, INF-95-24, and KC-95-19 are reasonable. Difficulties with samples INN-94-33 and KC-95-17 are likely due to extremely low radiogenic yields

E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30

13

14

E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30

E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30

15

Fig. 3. Isochron plots of 21 rock samples from Santa Isabel Wash dated by 40Arr39Ar geochronology. Error bars on individual data points are "1 s . Isochron ages Ž"2 s . are also given. Samples labeled ‘‘2W’’ were subjected to a low temperature fusion step Žsee Appendix A. and those labeled ‘‘gamma’’ were exposed to a 137Cs g-radiation source to facilitate mineral separation. Letters in plots Žq. and Žr. refer to different splits of our INF-94-53 anorthoclase internal standard that were distributed throughout sample tubes for both irradiations Žsee Table 1.. Sample labeled ‘‘MIT’’ Žq. is the same anorthoclase separate ŽINF-94-53. measured independently at the MIT Cambridge Laboratory for Argon Isotopic Research. All samples of the internal standard from irradiation UofMa81 and the MIT analysis were also exposed to g-radiation prior to neutron bombardment.

Žaverage 40ArU of 9% and 4%, respectively; Appendix C available from http:rroro.ess.ucla.edur PVPrsanta_isabel.html.. Both samples, however, have well-constrained isochrons ŽFig. 3a and o., and

their ages are consistent with the local stratigraphy. Sample SUN-94-30 Žaverage 40ArU of 22%. is more problematic given that its age Ž5.7 " 0.4 Ma. is anomalously young relative to the results from three

16

E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30

other Tmrsiw samples Ž6.5 " 0.3 to 6.7 " 0.3 Ma. and 40Arr39Ar ages of four overlying units ŽFig. 2.. Sample SUN-94-30 was taken from the base of densely welded Tmrsiw , while the other three Tmrsiw samples were collected higher in the section. It is possible that glass matrix adhering to the phenocrysts was incompletely removed, thus providing material more susceptible to radiogenic argon loss due to hydration and submicroscopic devitrification than the phenocrysts ŽMcDougall and Harrison, 1988..

5. Regional lithologic correlations Six of the Miocene units mapped in Santa Isabel Wash ŽTmÕs, Tmrsf , Tmr3, Tmr4, Tmrec , Tmrcan . are correlated to units defined previously in nearby localities ŽStock, 1989, unpub. mapping; Stock et al., 1991, 1999-this issue; Martın-Barajas et al., 1995; ´ Lewis, 1996.. The following discussion summarizes these, and other more tentative, lithologic correlations made on the basis of outcrop and hand sample descriptions, stratigraphic position, mineralogy, geochronology, and electron microprobe determinations of phenocryst compositions ŽAppendix B available from http:rroro.ess.ucla.edurPVPrsanta_ isabel.html.. Unless stated otherwise, correlations between lava flows assume that the various lava flows are coeval but not necessarily the exact same deposit. In contrast, correlations between pyroclastic flow deposits are meant to imply that they are the same depositional unit Žan exception is unit Tmr3; see below.. Correlations are discussed between deposits in Santa Isabel Wash and other parts of northeastern Baja California, as well as on the previously adjacent west coast of mainland Mexico ŽSonora. and islands in the northern Gulf of California. Diagrams summarizing these correlations are presented in Figs. 4 and 5 with corresponding references Žwhich are consequently omitted from the following discussion for simplicity.. 5.1. Early to Late Miocene deposits (Fig. 4) Group 2. An 18–22 Ma subduction-related hornblende andesite province has been mapped along the east side of the Baja California peninsula, on islands

in the Gulf of California, and in northwest Sonora ŽGastil et al., 1979.. Volcaniclastic deposits derived from this andesite belt that are correlative to TmÕs in Santa Isabel Wash occur locally ŽFig. 1. in southern Valle Chico, the Sierra San Felipe, the Sierra San Fermın, ´ and ; 60 km south of Puertecitos. Slightly further from the PVP, additional deposits have been noted in the Sierra Juarez, Isla Tiburon, ´ ´ and Sonora ŽFig. 1, inset.. Although not dated in this study, the ages of overlying units constrain TmÕs deposition in Santa Isabel Wash to predate 17 " 2 Ma. For example, plagioclase from overlying Tmr bio yields an isochron age of 17.1 " 2.4 Ma ŽTable 2; Fig. 3a.. TmÕs deposition was replaced by near-source-facies volcanism ŽTmd tomb . around 17 Ma in Santa Isabel Wash, in contrast to regions north of the PVP where TmÕs deposition continued until about 12 Ma ŽStock, 1989; Lewis, 1996.. Nearby andesitic vent facies Ži.e., potential source rocks for the volcaniclastic material; not included in Fig. 4. include 17–20 Ma deposits ; 10 km northwest of Santa Isabel Wash ŽStock et al., 1991. and Miocene Pico Matomı´ ŽGastil et al., 1971, 1975. ; 20 km to the west. Still others may have been eroded andror buried by later volcanism. Overlying Tmr bio is petrographically similar to one of the biotite-bearing Tuffs of Toronja Hill ŽMtt. in Arroyo Matomı´ south of the Sierra San Felipe. Mtt is undated but overlies a 17.0 " 0.3 Ma basalt ŽMb3.. Group 3. The Early to Middle Miocene age and intermediate to mafic composition of group 3 proximal lava flows suggests that they are subduction-related deposits Že.g., Sawlan, 1991.. Nearby basalt flows which could be contemporaneous with ; 16– 17 Ma Tmb kc ŽTable 2; Fig. 3b. and Tmb lol ŽTable 2; Fig. 3c. include Mbsu Žundated. and Mb4 Ž14.5 " 0.2 Ma. in southern Valle Chico and Tmb2 Žundated. in the Sierra San Fermın. ´ The voluminous lava flows of 15.5–16.5 Ma Tmd tomb ŽTable 2; Fig. 3d–j. do not appear to have correlatives in southern Valle Chico, Sierra San Fermın, ´ or southern Sierra San Felipe. Evidently these lava flows were extruded synchronously with TmÕs deposition in the north, illustrating a lateral facies change between more proximal ŽTmd tomb . and distal ŽTmÕs . deposits. An ; 16 Ma, 4-km-diameter, andesite and dacite complex ŽTma. in Arroyo Los Heme ŽFig. 1. is potentially coeval with Tmd tomb volcanism, as are Early to

E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30 Fig. 4. Stratigraphic columns showing preferred correlations between group 2, 3, 4, and 5 rocks defined in Santa Isabel Wash and those described by other workers in nearby localities. Stratigraphy from other studies has been simplified by listing only the lithologic units relevant to the discussion. All ages are "2 s . Solid lines indicate preferred correlations; dashed lines show unresolved ambiguities.

17

18 E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30

Fig. 5. Stratigraphic columns showing preferred correlations between group 6 and 7 rocks defined in Santa Isabel Wash and those described by other workers in nearby localities. The study area of Martın-Barajas et al. Ž1995. includes the region between Arroyo Matomı´ and Arroyo Los Heme from the coast to ;10 km west ŽFig. 1.. In some cases the ´ stratigraphy from other studies has been simplified by listing only the lithologic units relevant to the discussion. Ages are "2 s . The date with the ‘‘U ’’ is from the same Tmr3 mineral separate analyzed here measured in a different 40Arr39Ar laboratory ŽAppendix C available from http:rroro.ess.ucla.edurPVPrsanta_isabel.html; see also Nagy, 1997.. Solid lines indicate preferred correlations; dashed lines show unresolved ambiguities. The overall stratigraphic order of the various cooling units in the Tmr3 position from different areas is unclear; however, paleomagnetic studies suggest that deposits in Santa Isabel Wash are not the same as those from the identical stratigraphic position in the nearby regions. Note that units t3, t4, t2u, and t2l in Arroyo Matomı´ are also defined as Mpru by Stock Ž1989; 1993.. Stratigraphic order of some group 7 units in Santa Isabel Wash is ambiguous.

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Middle Miocene, mafic to intermediate vent-related deposits in the southern Sierra Juarez and Middle ´ Miocene hornblende andesites in Sonora. Group 4. Middle Miocene Tmrsf Žrenamed the Tuff of San Felipe by Stock et al. Ž1999.. was previously referred to as Tmr1 or Mr1 in the Santa Rosa basin, southern Valle Chico, southern Sierra San Felipe, and the Sierra San Fermın. ´ The source region of Tmrsf may lie below modern alluvium north of the Sierra San Fermın ´ in the eastern Sierra San Felipe ŽFig. 1; Lewis, 1996; Stock et al., 1999this issue.. Structural observations suggest that Tmrsf is a syn-rift deposit whose vent lay at the western margin of the rift at the time of eruption ŽStock et al., 1999-this issue. and thus represents some of the earliest extension-related volcanism within the GEP. Its unique age Ž12.7 " 0.5 Ma; Table 2; Fig. 3k. and widespread occurrence also make Tmrsf a regionally important ash-flow tuff. Detailed correlation criteria and seven age determinations Žincluded in Fig. 4. are summarized by Nagy Ž1997. and Stock et al. Ž1999this issue.. Lithologic correlations are further supported by an unusual low inclination, reversed polarity magnetization preserved in the Sierra San Fermın, ´ Sierra San Felipe ŽLewis and Stock, 1998a., Santa Rosa basin, and Santa Isabel Wash ŽStock et al., 1999-this issue.. A rhyolite ignimbrite on Isla Tiburon ´ and adjacent coastal Sonora is potentially correlative with Tmrsf ŽM. Oskin, pers. commun., 1998.. Should this correlation hold true, the stratigraphic tie would provide an important constraint on pre-extension paleogeography. Group 5. The ; 9 Ma age of Tmatoro ŽTable 2; Fig. 3l., as well as its association with silicic rocks and its isolated, small-volume outcrop nature, suggest that it represents post-subduction, rather than subduction-related, volcanism Žsee discussion of such criteria by Sawlan Ž1991... This hornblende-phyric andesite is in the same stratigraphic position as a mineralogically similar, yet older Ž11.8 " 0.7 Ma; whole rock, 40Arr39Ar. andesite in the Sierra San Fermın ´ ŽTma. as well as an undated andesite from southern Valle Chico ŽMa2.. Other potentially contemporaneous andesites include an 8.9 " 0.6 Ma andesite in the northern Sierra Pinta Ž; 140 km north of the PVP., a 10.9 " 2.3 Ma Žor 15.3 " 1.3 Ma. andesite andror a 9.9 " 1.3 Ma andesite on Isla Tiburon, ´ an 11.3 " 1.2 Ma andesite in western

19

Sonora, and a 12.3 " 2.9 Ma andesite northwest of Rancho Buenas Noches in Sonora. 5.2. Latest Miocene to Pliocene deposits (Fig. 5) Preferred correlations for group 6 and 7 rocks are shown in Fig. 5. Due to the completeness of the stratigraphic section preserved in Santa Isabel Wash, the stratigraphic order of several ; 6–6.5 Ma pyroclastic flow deposits in the PVP region is now clearly established. The source vents, or calderas, for group 6 pyroclastic flow deposits are inferred to be submerged beneath the Gulf of California, or buried beneath ; 3 Ma volcanic rocks in the northeastern PVP. In addition to geochronology summarized here, normal polarity magnetization in several group 6 deposits ŽTmrsiw , Tmr3 Ž type I and II ., Tmr3 – 4 , Tmr4, and Tmrao . ŽNagy, 1997, in review. and their correlatives ŽTmr3b and Tmr4. ŽLewis and Stock, 1998a. further constrains the age of deposition to Subchron C3An.2n Ž6.269–6.567 Ma; Cande and Kent Ž1995... Group 6. Tmrsiw cooling units are believed to be unique to the northern PVP in Santa Isabel Wash. Although they are in the same stratigraphic position as the Tuffs of Matomı´ ŽMmt; further divided into Tmmt and Tmr3a in some studies. in southern Valle Chico, the southern Sierra San Felipe ŽMesa Cuadrada., and the Sierra San Fermın ´ ŽFig. 1., field descriptions, petrography, and paleomagnetic studies suggest that Mmt and Tmr3a are not correlative with Tmrsiw . For example, Tmrsiw lacks biotite, epidote and hornblende that are present in Mmt and Tmr3a, and instead contains olivine and orthopyroxene. Tmr3 Ž type I . may be equivalent to a portion of Mmt, or may also be unique to Santa Isabel Wash. Paleomagnetic data indicate that Tmr3 Ž type I . is a distinct cooling unit from the Tmr3a units. Eruption ages for Mmt and Tmr3a have not been determined. Isochron results from Tmrsiw indicate that the tuffs are ; 6.6 Ma ŽFig. 5; Table 2.. Specifically, a composite plagioclase and anorthoclase-bearing concentrate from unwelded Tmrsiw yields an isochron age of 6.5 " 0.3 Ma ŽFig. 3m., plagioclase from the upper purple ash of Tmrsiw yields an isochron age of 6.7 " 0.3 Ma ŽFig. 3n., and plagioclase from densely welded Tmrsiw at two locations yields ages of 6.6 " 0.5 Ma ŽFig. 3o. and 5.7 " 0.4 Ma ŽFig. 3p.. As previously

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discussed the anomalously young age is attributed to contamination from the glassy matrix of the tuff. Field appearance, electron microprobe data, and petrography strongly support correlations among the three Tmr3 Ž type II . cooling units in Santa Isabel Wash and Mr3 Žrenamed Tmr3b in some studies. in southern Valle Chico, Mesa Cuadrada, and the Sierra San Fermın. ´ Individual isochron ages yielded by anorthoclase from Tmr3 Ž type II . are consistent with the overall value of 6.31 " 0.32 Ma ŽTable 1; Fig. 3q–r., which is slightly younger than that reported for Tmr3b and marginally older than that of Mr3. Exact lithologic correlations are not consistent with paleomagnetic studies, however, which suggest that there are at least six different cooling units in this particular stratigraphic position which are very similar in mineralogy, appearance and age ŽNagy, 1997, in review.. These include the four Tmr3 cooling units defined in Santa Isabel Wash and the two cooling units sampled for paleomagnetic analysis in the Sierra San Felipe and Sierra San Fermın ´ ŽLewis and Stock, 1998a.. Any lithologic correlations with Tmr3 units between Santa Isabel Wash and these other areas results in unacceptable amounts and senses of rotations based upon the paleomagnetic data from underlying and overlying units. Consequently, exact correlations between Santa Isabel Wash deposits and those from nearby regions are not made in Fig. 5 for cooling units in the Tmr3 position. The unit which caps the various Tmr3 units, labeled Tmr4 in these studies, does, in fact, appear to be the same pyroclastic flow deposit throughout the region. Field appearance and petrography strongly suggest that Tmr4 is equivalent to Mr4 in southern Valle Chico and Mesa Cuadrada and Tmr4 in the Sierra San Fermın, ´ although biotite phenocrysts identified in these studies were not observed in Santa Isabel Wash deposits. Tmr4 is also correlated to t2u in Arroyo Matomı´ on the basis of descriptions in previous studies as well as petrographic study of t2u samples collected for this study. This correlation is further supported by outcrop, hand sample, and petrographic examination of the unit which underlies t2u Žt2l. which is correlated to a 1–2-m-thick deposit which underlies Tmr4 in Santa Isabel Wash ŽTmr3 – 4 .. Correlations with the Arroyo Matomı´ region imply that the southern end of the Sierra San Felipe fault

ŽFig. 1., against which the unfaulted stratigraphic package containing t2u and t2l is deposited ŽMpru of Stock, 1989, 1993; Stock et al., 1991., has not experienced post-6 Ma displacement. This contrasts with post-6 Ma escarpment development inferred along the Sierra San Felipe fault to the north Že.g., Lewis and Stock, 1998b.. The 6.1 " 0.5 Ma deposits of Tmrec ŽFig. 3s; Table 2. are correlated to t3 in Arroyo Matomı´ and with deposits previously identified as the Tuff of El Canelo, including t12 and t9 of Mesa El Tabano ´ ŽFig. 1. and Tmc6 Žthe uppermost of six cooling units. in the northeastern PVP. The presence of an additional upper cooling unit in Santa Isabel Wash is consistent with the multiple flows seen in other localities Že.g., Stock et al., 1991.. Lewis Ž1996. suggests that the Tuffs of Dead Battery Canyon in the Sierra San Fermın ´ ŽTmr5–Tmr8. may be correlative to the Tuff of El Canelo, which is supported by geochronology and microprobe identification of oligoclase phenocrysts in Tmr7 and Tmrec ŽLewis, 1996; Nagy, 1997; see also Appendix B available from http:rroro.ess.ucla.edurPVPrsanta_isabel. html.. The vent for the Tuff of El Canelo may have been located south of the Sierra San Fermın ´ near Arroyo El Canelo Ž3–4 km south of Arroyo Matomı´. ŽMartın-Barajas et al., 1995; Lewis, 1996.. Tmrec ´ correlations are supported by similarities in field appearance, mineralogy, and stratigraphic position of underlying units ŽTmrao in Santa Isabel Wash, t14 Žalso defined as part of the Tuff of El Canelo. on Mesa El Tabano, Tmc5 in the northeastern PVP, and ´ Tmr5 in the Sierra San Fermın ´ .. Moreover, an age of 6.4 " 0.3 Ma for Tmrao ŽFig. 3t; Table 2. is similar to that reported for Tmc units in the northeastern PVP. Group 7. The outcrop appearance and mineralogy of Tmrcan lava flows are similar to foliated, devitrified 6–8 Ma rhyolite flows ; 10 km northeast of Picacho Canelo in the Sierra San Fermın ´ ŽTmr9. and a series of 5.8 " 0.1 Ma rhyolite lava flows ; 10 km east of Picacho Canelo in the northeastern PVP ŽTmru.. Isochron ages of ; 6 Ma were obtained for Tmrcan rhyolites in this study ŽFig. 3u–v; Table 2.. All three lava flows overlie ash-flow tuffs interpreted to be equivalent to Tmrec . Lewis Ž1996. interprets Tmr9 to be coeval with rhyoliterdacite flows designated f4 in Arroyo Matomı. ´ The f4 lava flows also

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overlie Tmrec and differ only in the presence of trace hornblende from Tmrcan , Tmr9, and Tmru. Unpublished mapping ŽStock; see also Stock et al., 1991. south of Arroyo Matomı´ indicates that they are present within ; 1.5 km east-northeast of Picacho Canelo, thus Tmrcan lavas are most likely continuous with f4 deposits. All of these lava flows ŽTmrcan , Tmr9, f4, Tmru. are considered to be part of a field of Latest Miocene rhyolite domes in the northeastern PVP as previously noted by Martın-Barajas et al. ´ Ž1995.. Tmagem overlies Tmrec and underlies Tmrcan and is thus constrained to be ; 6 Ma. There are no dated andesites of similar age in the region. The ages of Tma ugl and Tmahem are not constrained by overlying units, and although they are petrographically similar to Tmagem , it is possible that they are younger than 6 Ma. The proximity and similar nature of Tma ugl and Tmagem in the field suggests that they are probably related; in contrast, Tmahem is several km to the south and could represent a younger pulse of volcanism. If so, Tmahem could be coeval with a 5.1 " 0.3 Ma, mineralogically similar andesitic dike and associated basaltic andesite lava flow in the northeastern PVP ŽTba. andror with several hundred meters of andesite flows in the southeastern Sierra San Fermın ´ ŽTpa.. Tpa, which differs mineralogically from Tmahem by the presence of alkali feldspar and biotite, yields a whole-rock 40Arr39Ar age of 5.7 " 0.2 Ma; however, field relationships in southeastern Sierra San Fermın ´ indicate that Tpa overlies the ; 3 Ma Tuffs of Mesa El Tabano. Lewis Ž1996. thus inter´ prets the age to be incorrect and suggests that Tpa is coeval with an olivine-bearing basalt or basaltic andesite in Arroyo Matomı´ ŽMb6 of Stock et al. Ž1991... 6. Geologic and Pacific–North America plate boundary implications Integrated field, petrographic, and geochronologic study described here from the Santa Isabel Wash area, combined with structural analysis and magnetostratigraphic data from the area presented elsewhere ŽNagy, 1997, in review; Nagy et al., 1995., form the basis for a proposal ŽNagy, 1997; Nagy and Stock, 1998, in review. that an important structural transition in the Pacific–North America plate boundary in the northernmost Gulf of California Žthe Wag-

21

ner Transition Zone. controlled the magmatic and tectonic evolution of the PVP and regions to the north. Below we summarize key results from this study and explain how they constrain this model for the Late Miocene–Pliocene evolution of northeastern Baja California. The most voluminous volcanism in Santa Isabel Wash occurred ; 15–17 Ma and ; 6–6.5 Ma, with minor pulses around 12.5 Ma and 9 Ma. Good stratigraphic control supports the geochronology. For example, an erroneous age from one of the group 6 tuffs ŽTmrsiw . was easily identified on the basis of stratigraphic position and ages of overlying units. Results also aid in identification of anomalous ages from previous geochronology studies for regionally extensive units Že.g., Tmrsf in Fig. 4. and thus help to converge towards the actual crystallization ages of these deposits. Determining the correct age of Tmrsf is especially important because it provides a maximum age for the initiation of extension in this part of the GEP Že.g., Stock and Hodges, 1989; Stock et al., 1999.. Geologic study in Santa Isabel Wash elucidates the Middle to Late Miocene geologic history in the northern PVP and, placed within the context of previously documented local and regional geology Že.g., Figs. 4 and 5., augments our understanding of this portion of northeastern Baja California. For example, a Middle Miocene lateral facies change is evident between the northern PVP Ži.e., Santa Isabel Wash. and regions a few km to the north. Deposition of distal volcaniclastic sediments from subductionrelated andesites ŽTmÕs . continued north of the PVP, such as in the Sierra San Fermın ´ and southern Sierra San Felipe, until perhaps 12 Ma ŽStock, 1989; Lewis, 1996., whereas voluminous near-source facies volcanism ŽTmd tomb . replaced deposition of distal deposits in Santa Isabel Wash ; 15–17 Ma. Contemporaneous mafic to intermediate composition vent facies also occur in the northeastern PVP ŽMartın´ Barajas and Stock, 1993; Martın-Barajas et al., 1995. ´ and in other localities in northeastern Baja California such as the Sierra Juarez ´ and Sierra Las Tinajas ŽFig. 1, inset; Gastil et al., 1979; Lee et al., 1996; Axen and Fletcher, 1998.. The regional distribution of subduction-related facies thus suggests that the Sierra San Felipe and Sierra San Fermın ´ areas were somewhat anomalous within northeastern Baja California

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in their absence of vent facies volcanism ; 15–17 Ma. As another example, careful documentation of the Late Miocene Žgroup 6. tuffs has significantly benefited regional correlations. The sequence of several pyroclastic flow deposits whose stratigraphic relationships are ambiguous elsewhere is clearly laid out in the rich stratigraphic section preserved in Santa Isabel Wash. In particular, the stratigraphic relationship of the Tuff of El Canelo ŽTmrec . relative to other regionally distributed tuffs such as Tmr3 and Tmr4 is now well established ŽFig. 5.. In contrast to previous interpretations, lithologic correlations between Santa Isabel Wash and Arroyo Matomı´ suggest that the southernmost extension of the Sierra San Felipe fault did not experience post-6 Ma displacement. Clearly, a well-documented stratigraphic section preserved in the PVP region is important for lithologic correlations with regions in Sonora, Sinaloa, and intervening islands in the Gulf of California. Detailed geologic study of these remote areas is currently underway Že.g., McDowell et al., 1997; Mora-Alvarez and McDowell, in press; L. Delgado, pers. comm., 1998; M. Oskin, pers. comm., 1998.. Reliable correlations between volcanic cooling units is crucial for paleomagnetic studies in which characteristic remanent magnetization ŽChRM. vectors are compared between distant localities. Because volcanic deposits record an instantaneous field direction, in contrast to sediments, for example, which smooth out effects of secular variation of the earth’s magnetic field, RM directions should only be compared between identical volcanic cooling units. When such units cannot be traced continuously in the field, correlations depend upon details such as those presented in this study, including rock unit descriptions, stratigraphic order, petrography, and geochronology. Nagy Ž1997. details the difficulties with using this

approach for paleomagnetic studies in the PVP region, and concludes that only the deposit labeled Tmr4 in the group 6 series of tuffs in Santa Isabel Wash can be confidently correlated to cooling units sampled in the Sierra San Fermın ´ and Sierra San Felipe ŽLewis and Stock, 1998a.. Results imply that at least parts of the Sierra San Fermın ´ have rotated ; 308 clockwise about a vertical axis relative to Santa Isabel Wash since Tmr4 deposition ; 6 Ma ŽNagy, 1997, in review; Lewis and Stock, 1998a.. Paleomagnetic study of ; 12.5 Ma Tmrsf in Santa Isabel Wash, Santa Rosa basin ŽStock et al., 1999-this issue., the Sierra San Fermın, ´ and Sierra San Felipe ŽLewis and Stock, 1998a. agrees with the amount and sense of relative rotation recorded in the overlying ; 6 Ma tuff. These studies thus imply Ž1. no relative rotations between the southern Sierra San Felipe Ži.e., Mesa Cuadrada. and Santa Isabel Wash and Ž2. ; 308 clockwise rotation of the Sierra San Fermın ´ and Santa Rosa basin relative to Mesa Cuadrada and Santa Isabel Wash Žlocations in Fig. 1.. Although additional sampling is needed to identify the limits of the rotated region, results suggest that the Pacific–North America plate boundary in this part of the GEP has experienced distributed dextral shear partially accommodated by vertical-axis rotations since the Latest Miocene ŽLewis and Stock, 1998a.. Deformation in Santa Isabel Wash is illustrated schematically in Fig. 6 for pre-6 and post-6 Ma. The earlier phase of deformation is probably related to the pre-6 Ma Matomı´ accommodation zone ŽNagy, 1997, in review.. Several of the group 6 tuffs ŽTmr4, Tmrao , and Tmrec . thicken considerably in the Arroyo Oculto region ŽFig. 2., and the uppermost units ŽTmr bs and Tmrfp . appear to be restricted to this region. The thickening occurs across two NE-facing,

Fig. 6. Schematic block models of pre-6 and post-6 Ma deformation in Santa Isabel Wash in the northern PVP. Top: Prior to 6 Ma most of the northern Sierra Santa Isabel was located southwest of the NW-striking Matomı´ accommodation zone Žsimplified; see Fig. 2 for segmentation. which separated the Gulf Extensional Province to the northeast from the unextended regions to the southwest. Elevation differences across the accommodation zone of various lithologic contacts imply 300–350 m NE-side-down displacement. A strike–slip component of motion along the zone is not evident in this area. NE- to ENE-directed extension is inferred from the orientation of the Matomı´ accommodation zone and its structural relationship with the San Pedro Martir ´ fault Žsee Nagy Žin review. for details.. Bottom: Most faults in the study area displace all mapped units and are thus post-6 Ma features. About 500 m of E-side-down displacement of ; 12.5 Ma Žgroup 4. and 6.3–6.6 Ma Žgroup 6. pyroclastic flow deposits is evident across the Cuervo Negro and Santa Isabel faults. Slickenlines and fault orientations suggest ENE- to E-directed extension. Individual 6.3–6.6 Ma Žgroup 6. tuffs thicken from 1–2 m southwest of the Matomı´ accommodation zone to 40–80 m to the northeast.

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paleo-topographic slopes ŽAX –AY and BX –BY in Fig. 2. interpreted to be the southwestern margin of the Matomı´ accommodation zone Žsimplified in Fig. 6.. The orientation of the Matomı´ accommodation zone may have developed as a result of NE- to ENE-directed extension in a zone of weakness separating two en echelon portions of the NNW-striking GEP margin ŽNagy, 1997, in review; Stock, in press.. As much as 350 m of pre-6 Ma NE-side-down normal separation is estimated across the zone on the basis of present-day elevation of contacts within the series of 6.3–6.6 Ma tuffs ŽNagy, in review.. Additional minor, pre-6 Ma N- to NE-side-down deformation in Santa Isabel Wash includes faults which cut group 2 and 3 rocks, and in one locality group 4 and 5 rocks, but not group 6 rocks, constraining deformation to be post-9 Ma in some places but possibly as old as 15 Ma in others. This pre-6 Ma N- to NE-side-down deformation is probably the result of its proximity to the Matomı´ accommodation zone. Conformable contacts between the ; 12.5 and 6.3–6.6 Ma tuffs in Santa Isabel Wash, in contrast to angular unconformable contacts between them north of the Matomı´ accommodation zone Že.g., Stock, 1989, 1993; Lewis, 1996., further suggest that the northern PVP did not experience significant pre-6 Ma deformation associated with the developing GEP. Most deformation in Santa Isabel Wash is post-6 Ma. Numerous, high-angle normal faults offset the ; 6–6.5 Ma Žgroup 6. tuffs ŽFigs. 2 and 6., including major N- to NNW-striking structures presently accommodating extension ŽSanta Isabel and Cuervo Negro faults.. About 500 m of E-side-down displacement of the ; 12.5 and ; 6–6.5 Ma tuffs occurs across these major E-dipping normal faults. Offset drainages and fresh fault scarps suggest that the Santa Isabel fault is an active structure. Field evidence for extension direction is scarce; however, slickenlines preserved on the Santa Isabel fault plane indicate dip–slip with a small component of sinistral slip ŽNagy, 1997, in review.. Reconstructed crosssections across Santa Isabel Wash suggest a maximum of 4% post-6 Ma extension in an ENE-direction, which may have occurred as recently as 2–3 Ma when dextral strike–slip motion along the Guaymas fracture zone replaced similar motion along the Tiburon ´ fracture zone, thereby enlarging the GEP margin westward to include the PVP region ŽNagy,

1997; 1998, Nagy and Stock, in review; Stock, in press.. The results summarized above have suggested to us a new structural interpretation for the evolution of the Pacific–North America plate boundary in the northernmost Gulf of California ŽNagy, 1997; Nagy and Stock, 1998, in review.. Present-day Pacific– North America relative plate motion is ; NW–SEdirected Že.g., Stock and Hodges, 1989. and discretely accommodated throughout most of the Gulf of California along NW-striking transform faults and NE-striking spreading centersrdivergent basins Že.g., Lonsdale, 1989.. However, the zone of plate boundary deformation becomes more diffuse in the northernmost Gulf of California, as evidenced by ongoing E- to NE-directed extension in northeastern Baja California Že.g., Barnard, 1968; McEldowney, 1970; Rossetter, 1973; Gastil et al., 1975; Dokka and Merriam, 1982; Bryant, 1986; Stock, 1989; Stock and Hodges, 1990; Martın-Barajas et al., 1995; Lee et al., ´ 1996; Nagy, 1997, in review; Lewis and Stock, 1998b. and vertical-axis clockwise rotations bounded by N-striking normal faults and NE-striking sinistral strike–slip faults in the hanging wall of the San Pedro Martir ´ fault ŽLewis and Stock, 1998a; Stock et al., 1999-this issue.. The E- to NE-directed extension may be occurring on structures remnant from earlier ŽLate Miocene. ENE-directed extension Že.g., Stock and Hodges, 1989.. Active N- to NNW-striking normal faults in northeastern Baja California, such as the San Pedro Martir ´ fault, and N–S-trending bathymetric features in the northernmost Gulf of California Že.g., Dauphin and Ness, 1991., suggest a plate boundary scenario in which N- to NNW-striking Late Miocene faults have not yet been overprinted by more favorably oriented structures such as NWstriking transform faults and NE-striking extensional basins. Relationships outlined above are readily accounted for by the existence of a broad, structurally transitional region Žthe Wagner Transition Zone. characterized by coupled E- to NE-directed extension and dextral shear between oceanic Žto the south. and continental Žto the north. portions of the Pacific–North America plate boundary ŽFig. 7; Nagy, 1997; Nagy and Stock, 1998, in review.. The northwestern boundary of the Wagner Transition Zone merges with a complex portion of the plate boundary

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Fig. 7. Present-day Pacific-North America plate boundary in the northern Gulf of California region. Solidrstriped patterns represent activerextinct spreading centers. Plate motion direction after Atwater and Stock Ž1998.. The Wagner Transition Zone is proposed as a region of diffuse plate boundary deformation which includes ENE-directed extension Ždouble-headed arrow., clockwise vertical-axis rotations, and dextral, perhaps oblique, strike–slip motion along N- to NNW-striking faults Ždepicted schematically through Wagner Basin.. Boundaries Ždashed lines. are oriented perpendicular to Pacific-North America relative plate motion. A zone of divergence created at the southeast margin of the Wagner Transition Zone offers one explanation for the positions and jumps of nearby spreading centers ŽUpper and Lower Tiburon ´ and Delfın ´ basins. since ; 6 Ma and for the timing of incorporation of the Puertecitos Volcanic Province into the Gulf Extensional Province in Pliocene time. Santa Isabel Wash is presently within the GEP Žopen circle.. See Nagy and Stock Žin review. for structural evolution of the Wagner Transition Zone since the late Miocene.

consisting of continental transform faults and regions of local extension, and the southeastern boundary coincides roughly with the southern end of the PVP. Plate motion is partitioned along Late Miocene structures within the Wagner Transition Zone between Ž1. dip–slip faults Žslight sinistral component. in the west such as the Santa Isabel fault, Ž2. dextral shear along the coast manifested as vertical-axis rotations, and Ž3. SSE-directed slip along inferred dextral-oblique faults submerged in the northernmost Gulf of California ŽNagy and Stock, in review.. A transitional plate boundary zone in the northernmost Gulf of California is corroborated by heat flow, gravity, and seismic refraction data which indicate lower heat flow and a greater depth to the MOHO in the north

relative to the central and southern Gulf of California, as well as an absence of gravity lows in the north such as those associated with extensional basins further south Že.g., Henyey and Bischoff, 1973; Gastil et al., 1975; Couch et al., 1991.. A NE-striking zone of divergence is created as a consequence of the structural change from diffuse deformation at the southeastern boundary of the Wagner Transition Zone to more localized slip on the transform faults of the central Gulf of California. According to the Wagner Transition Zone model, this zone of divergence developed into the Upper and Lower Tiburon, ´ and subsequently Delfın, ´ basins ŽNagy, 1997; Nagy and Stock, 1998, in review.. The ; 6–6.5 Ma pulse of volcanism recorded in the

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PVP, as well as a ; 3 Ma pulse preserved in the eastern PVP Že.g., Stock et al., 1991., can be related to the history of development of these offshore extensional basins. Continued extension in the Wagner Transition Zone explains the greater width ŽENE to WSW. of the northernmost Gulf of California relative to regions further south. N- to NNW-striking Late Miocene structures control local topographic and bathymetric trends, thus explaining the 358 bend in the coastline and western edge of the Gulf Extensional Province north and south of the Puertecitos Volcanic Province.

7. Conclusions Geologic mapping, 40Arr39Ar geochronology, electron microprobe analysis, and petrography have been combined to define 21 Miocene volcanic units along the northern margin of the Miocene–Pliocene Puertecitos Volcanic Province ŽPVP.. The deposits overlie pre-Miocene batholithic and pre-batholithic basement rocks, and consist of stratified, poorly sorted, breccia lithofacies, dacitic, andesitic, and basaltic lava flows and associated breccias, and outflow sheets of rhyolitic pyroclastic flow deposits. The stratigraphic order of several ; 6–6.5 Ma pyroclastic flow deposits whose depositional relationships are ambiguous elsewhere is clearly laid out in the rich stratigraphic section preserved in Santa Isabel Wash. In particular, the stratigraphic relationship of the Tuff of El Canelo ŽTmrec . relative to other regionally distributed tuffs such as Tmr3 and Tmr4 is now well established. Results confirm lithologic correlations made in several paleomagnetic studies which document vertical-axis rotational deformation in the region ŽNagy, 1997, in review; Lewis and Stock, 1998a; Stock et al., 1999-this issue. and provide an important foundation for geologic tie-points across the Gulf of California. Twenty-one rock samples from 11 lithologic units yield 40Arr39Ar ages spanning ; 17–6 Ma. Major pulses of volcanism occurred at ; 15–17 Ma Žsubduction-related. and ; 6–6.5 Ma Žextension-related; PVP-forming. with minor pulses at ; 12.5 Ma Žcommencement of GEP extension to the north. and ; 9 Ma. Three of the tuffs dated in this study ŽTmrsf , Tmr3, and Tmrec . yield isochron ages in

good agreement with previous dates ŽKrAr and 40 Arr39Ar. from other locations. Only one of the 21 ages is stratigraphically inconsistent within the volcanic package. Relationships between extensional structures in Santa Isabel Wash and the 6.3–6.6 Ma pyroclastic flow deposits Žgroup 6. constrain two phases of extensional deformation to pre-6 and post-6 Ma. The NW-striking Matomı´ accommodation zone is a pre-6 Ma feature which accommodated NE- to ENE-directed extension at the GEP margin during the Late Miocene. Up to 350 m of NE-side-down separation is evident across the zone. Major post-6 Ma structures include the N- to NNW-striking Santa Isabel and Cuervo Negro fault systems, across which ; 500 m of E-side-down displacement has occurred. A transitional plate boundary model ŽWagner Transition Zone of Nagy, 1997; also Nagy and Stock, 1998, in review. relating PVP volcanism and deformation to the evolution of the Gulf of California spreading center system suggests that most of the post-6 Ma deformation in Santa Isabel Wash is related to incorporation of the PVP region into the GEP 2–3 Ma. According to the model deformation in the Wagner Transition Zone occurs along N- to NNW-striking Late Miocene structures, such as the San Pedro Martir fault, which have not yet been ´ overprinted by more favorably oriented structures such as NW-striking transform faults and NE-striking extensional basins. A NE-striking zone of divergence, created as a consequence of the structural change from diffuse deformation at the southeastern boundary of the Wagner Transition Zone to more localized slip on the transform faults of the central Gulf of California, may have localized the positions and jumps of nearby Gulf of California spreading centers ŽUpper and Lower Tiburon ´ and Delfın ´ basins. since 6 Ma.

Acknowledgements E.A.N. thanks M.A. House, A.J.R. Kent, C.J. Lewis, and X.Y. Quidelleur for helpful discussions and reviews of early versions of this manuscript. K. Holt, K. Rostedt, T. Tyndall, and K. Wertz are thanked for invaluable field assistance, and P. Koke-

E.A. Nagy et al.r Journal of Volcanology and Geothermal Research 93 (1999) 1–30

laar and M. Howells are thanked for their enlightening visit to the field area. X.Y. Quidelleur and K.D. Mahon are also acknowledged for providing software employed to analyze the 40Arr39Ar results. G. Axen is thanked for providing a preprint ŽAxen and Fletcher, 1998. and C.J. Lewis is thanked for providing assistance with Fig. 1. We appreciate helpful reviews by G. Axen, C. Devey, and M. Heizler. This work was supported by NSF grants EAR-9218381, -9296102, and -9614674. This is contribution 6220 from the Division of Geological and Planetary Sciences of the California Institute of Technology. Appendix A. 40Ar r39Ar analytical methods Feldspars were concentrated using conventional magnetic and density techniques following crushing and sizing to 30–40 mesh. Grains analyzed were hand-selected after ultrasonic cleaning in 10% HCl. In selected instances, exposure to a 137Cs source for 4–5 days Žreceiving 1.06 Mradrday of g-radiation. was employed to aid in identifying plagioclase, alkali feldspar and quartz contamination Žsee Rose et al., 1994.. Most samples consisted exclusively of plagioclase with the exceptions of Tmrsf Žanorthoclase and sanidine., Tmr3 Žanorthoclase., and Tmrsiw , Tmrao , and Tmrec Žplagioclase and anorthoclase.. See Nagy Ž1997. for detailed mineral separation techniques. Feldspar concentrates were wrapped in Al foil and sealed under vacuum in quartz ampoules with aliquots of Fish Canyon Tuff sanidine ŽFCT-1; 27.8 Ma; Cebula et al. Ž1986.., and an internal standard Žanorthoclase from Tmr3 . interspersed at 1 cm intervals. Samples were irradiated in either one of two sessions in the L67 position of the Ford Reactor ŽUniversity of Michigan.. Samples from the first irradiation Ždesignated UofMa75 . received a three hour dose while those from the subsequent irradiation Ždesignated UofMa81. were subjected to a five hour dose to enhance 39Ar yield. All samples including flux monitors were fused with a continuous 5W Coherent Ar-Ion laser. Although single crystal analysis was possible for sanidine and some anorthoclase samples, individual plagioclase analyses required 10-30 grains to obtain a measurable signal and most anorthoclase analyses consisted of 3 to 6 grains. In some instances incre-

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mental heating accomplished by defocusing the beam and reducing the power setting was employed to develop spread on isochron plots. Typically plagioclases were preheated with a defocused, 2 W beam and measured prior to fusion with a focused, 5 W beam Žsee Appendix C available from http:rr oro.ess.ucla.edurPVPrsanta_isabel.html.. Because the plagioclase crystals are spread on the tray in a sheet, instead of within individual wells, it is likely that some crystals did not contribute to both analyses. Pringle et al. Ž1992. found this kind of ‘‘low temperature cleaning’’ useful for removing non-radiogenic argon contamination in Quaternary plagioclases from the Taupo volcanic zone in New Zealand. Following laser heating, sample gas was purified using two SAES ST101 alloy getter pumps operated at 2508C Ž50 lrs. and 4008C Ž10 lrs.. Gas transfer was via expansion with the total quantity of 39Ar entering the mass spectrometer recalculated for 100% delivery. Argon ion intensities were measured using a VG3600 Rare Gas mass spectrometer fitted with a Daly electron multiplier operated at an effective 40Ar sensitivity of 1.85 = 10y1 7 molrmV. Atmospheric Ar measurements performed throughout the analyses indicated a mass discrimination value of 0.994 " 0.001 per amu. Representative total system blanks for a typical procedure involving 1–3 min laser heating of ; 0.5–1.5 mg aliquots of plagioclase and 5–10 min gettering were: 5 = 10y1 6 moles, 2 = 10y1 8 moles, 1 = 10y1 8 moles, 1 = 10y1 7 moles, and 7 = 10y1 8 moles for mass 40 through 36, respectively.

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