Age and stratigraphic relationships of pre

Age and stratigraphic relationships of pre- and syn-rift
volcanic deposits in the northern Puertecitos Volcanic Province,
Baja California, Mexico
Elizabeth A. Nagya‡, Marty Groveb, and Joann M. Stocka*
a
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena
b
Department of Earth and Space Sciences, University of California, Los Angeles
* Corresponding author: [email protected]
‡
now at: Laboratoire de Géochronologie, Université Paris 7, 2 Place Jussieu, tour 24-25, 1er étage, 75251 Paris, Cedex 05,
France
Manuscript submitted September 29, 1997 to Journal of Volcanology and Geothermal Research (Special Issue on
Rift-Related Volcanism: geology, geochemistry, and geophysics)
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Abstract. Geologic mapping of volcanic strata of the northern Puertecitos Volcanic Province (PVP) in northeastern
Baja California, Mexico, performed in conjunction with
40
Ar/ 39Ar analysis and petrochemical study, document 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, wellexposed in profile along the northern margin of the PVP in the informally named Santa Isabel Wash region, span pre17 to 15 Ma. Minor rift-related volcanism occurred at ~ 12.5 and ~ 9 Ma, prior to voluminous PVP-related volcanism at
6 Ma. Improved local correlations made possible by the rich stratigraphic section preserved within Santa Isabel Wash
help constrain the relationships of several widespread volcanic 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. Moreover, the combined mapping and age results allow us to conclude that most extensional
deformation in the study area is post-6 Ma, although some earlier faulting and the development of the pre-6 Ma
Matomí accommodation zone are also documented. Finally, results support a structural model (presented elsewhere)
relating major pulses of PVP volcanism to adjustments in the offshore spreading center system.
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1. Introduction
Knowledge of the timing and stratigraphic relationships of volcanic rocks produced throughout Baja
California, Mexico, from Miocene through Recent time is vital to understanding the Late Cenozoic evolution of the
Pacific (PAC)-North America (NAM) plate boundary in this region. Volcanic deposits of the Miocene-Pliocene
Puertecitos Volcanic Province (PVP) (Gastil et al., 1975) in northeastern Baja California (Figure 1) are particularly
important in this regard in that they constrain the age of prominent rift-related structures of the Gulf Extensional
Province (GEP). Moreover, documentation of the stratigraphic and age relationships of the PVP volcanics are needed
to establish geologic ties across the Gulf of California. The latter provide the basis for determining the amount of
Neogene extension produced by development of the PAC-NAM plate boundary. To date, well-constrained geologic
tie-points across the Gulf of California (e.g., Gastil et al., 1973) are extremely rare with estimates of post-Middle
Miocene offset ranging from 300 kilometers (e.g., Gastil et al., 1973; Curray and Moore, 1984) to 500 kilometers (e.g.,
Gans, 1997) depending on the area considered.
Neogene volcanism along the peninsula of Baja California is primarily associated with the evolving PACNAM plate boundary. Eastward subduction of the Farallon plate beneath Baja California (e.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 (Barnard, 1968; Rossetter, 1973; Gastil and
Krummenacher, 1977; Gastil et al., 1979; Hausback, 1984; Neuhaus et al., 1988; Neuhaus, 1989; Stock, 1989; Sawlan,
1991; Stock et al., 1991; Dorsey and Burns, 1994; Martín-Barajas et al., 1995; Lee et al., 1996; Lewis, 1994, 1996). Arcrelated volcanism ceased when subduction ended 20-12.5 Ma along the peninsula (Atwater, 1970; Mammerickx and
Klitgord, 1982; Spencer and Normark, 1989; Atwater, 1989; Stock and Lee, 1994). Between 12.5 and 5.5 Ma
accommodation of PAC-NAM motion was most likely partitioned between large, dextral, offshore transform faults,
such as the Tosco-Abreojos and San Benito faults (Kraus, 1965; Spencer and Normark, 1979), and “proto-Gulf”
extensional structures to the east (Stock and Hodges, 1989). The GEP (after Gastil et al., 1975) is a highly extended
region that borders the Gulf of California. Continuing extension within the GEP produced Late Miocene to Pliocene
silicic volcanism, including large rhyolitic volcanic provinces, at various locations along the length of the peninsula
(Ga stil et al., 1975, 1979; Dokka and Merriam, 1982; Bryant, 1986; Stock, 1989, 1993; Stock et al., 1991; Martín-Barajas
et al., 1995; Lee et al., 1996; Lewis, 1996). Oceanic crust with identifiable magnetic anomalies began forming between
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the PAC and NAM plates in the mouth of the Gulf of California 3.5 Ma (Larson et al., 1968; Curray and Moore, 1984;
Lonsdale, 1989). Late Miocene to Quaternary mafic volcanic rocks along the margins of the Gulf of California (e.g.,
Martín-Barajas et al., 1995) are probably associated with this developing spreading center system which marks the
present-day PAC-NAM plate boundary.
Below we present new stratigraphy and 40Ar/ 39Ar age results from the northern margin of the PVP (Figure 1).
The study is based upon geologic mapping (1:20000), petrochemical analyses of collected rock samples, and
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Ar/ 39Ar measurements (see Nagy (1997) for additional details ). We have determined that in this locality, subduction-
related deposits span 17-15 Ma while rift-related volcanism occurred from ~ 12.5 Ma to less than 6 Ma. The results
provide additional 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 (Lewis, 1994; Lewis and Stock, in review in J. Geophys.
Res.; Nagy, 1997, in prep. for J. Geophys. Res.; Stock et al., in prep. for J. Volc. Geotherm. Res.).
2. Geologic setting of the Puertecitos Volcanic Province
A simplified geologic map shown in Figure 1 differentiates the Mesozoic and Paleozoic lithologic units
(batholithic and prebatholithic metasedimentary rocks) from Cenozoic deposits (volcanic and sedimentary rocks) in
the northern PVP. The subaerially exposed portion of the GEP has a sharp western boundary (Main Gulf Escarpment)
defined north of the PVP by the 100-km-long, E-dipping, San Pedro Mártir fault. Although up to 5 kilometers of
normal separation occurs on this fault (Gastil et al., 1975), displacement decreases southwards to a maximum of 800
meters in southernmost Valle Chico (Stock and Hodges, 1990). At the latitude of the northern PVP the San Pedro
Mártir fault cannot be recognized. The western edge of the GEP is thus poorly defined at the latitude of the study
area. Recent work (Nagy, 1997; Nagy and Stock, in prep. for J. Geophys. Res.) suggests that the Cuervo Negro and
Santa Isabel fault systems (Figure 2) are major bounding structures accommodating extension at this latitude. Several
workers (Dokka and Merriam, 1982; Stock and Hodges, 1990; Stock, 1993; Axen, 1995; Stock, in press) interpret the
southern termination of the Main Gulf Escarpment to mark the location of a pre-6 Ma, W- to NW-striking
accommodation zone (the “Matomí accommodation zone” after Dokka and Merriam (1982)). The limits of the Matomí
accommodation zone are indicated in Figure 1 (A-A’; after Stock, in press). As described elsewhere (Nagy, 1997;
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Nagy and Stock, in prep. for J. Geophys. Res.) the southeastward extension of the Matomí accommodation zone
projects through the northern part of the study area.
Differences in structural style are apparent north and south of the Matomí accommodation zone. Extension
in the hanging wall of the San Pedro Mártir fault has produced Basin and Range-type topography characterized by Edipping normal faults. This contrasts with the less extended nature of the PVP to the south where more closely
spaced, E- and W-dipping normal faults exhibiting smaller amounts of offset prevail (Dokka and Merriam, 1982; Nagy
et al., 1995; Nagy, 1997). Additionally, post-6 Ma, vertical-axis rotational deformation documented for the Sierra San
Fermín relative to the Sierra San Felipe (Figure 1; Lewis, 1994; Lewis and Stock, in review in J. Geophys. Res.) has not
been identified in the northern PVP (Nagy et al., 1995; Nagy, 1997, in prep. for J. Geophys. Res.; Stock et al., in prep.
for J. Volc. Geotherm. Res.).
The first detailed geologic analysis of the PVP and surrounding areas was presented by Dokka and Merriam
(1982) on the basis of earlier work (Hamilton, 1971; Sommer and Garcia, 1970; Gastil et al., 1975) as well as ground
studies, air photo interpretations, and observations from low-flying aircraft. Since these early studies, details of the
volcanic deposits and deformational history within and near the PVP have gradually emerged. In particular, more
detailed studies of the region covered in Figure 1 have focused upon the Santa Rosa Basin (Bryant, 1986), southern
Valle Chico (Stock, 1989, 1993) and the region south of it within the PVP (Stock, Lewis, Salton, and Holt, unpub.
mapping), the region bordering Arroyo Matomí (Stock et al., 1991; Stock, unpub. mapping), the Sierra San Fermín and
southern Sierra San Felipe (Lewis, 1994, 1996; Lewis and Stock, in review in J. Struct. Geol.), the eastern PVP adjacent
to the Gulf of California between Arroyo Los Heme and Arroyo Matomí (Martín-Barajas et al., 1995), and scattered
localities west and south of the PVP (Dorsey and Burns, 1994).
The northern Sierra Santa Isabel (Figure 1) has received attention both because of its well-developed
volcanic and extensional structures, and because of its relationship to GEP development (e.g., Gastil et al., 1975;
Dokka and Merriam, 1982; Stock and Hodges, 1990; Axen, 1995). Despite the obvious importance of the area, it had
not been previously examined in detail with the exception of field checks for a 1:50000 geologic map compiled from air
surveillance (CETENAL, 1977). No prior lithologic descriptions or geochronology exists for the units described here.
The area mapped in the present study (Figure 2) covers ~140 km2 along the north-central PVP. We informally refer to
the two large arroyos as Santa Isabel Wash and Arroyo Oculto, and to the entire study area as the Santa Isabel
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Wash region. Quaternary drainage patterns suggest that Santa Isabel Wash is a structurally-controlled, topographic
low created by ENE- to NNE-side-down normal faults which have deflected northeast-directed drainages to the
southeast along the structurally controlled margin of the wash (e.g., F4 in Figure 2) (Nagy, 1997). These features
provide excellent exposure of the volcanic succession along the northern margin of the PVP. In most other areas the
PVP is a flat, plateau-like, volcanic tableland of limited topography on the order of 50-100 meters. However, the ~ 700
meters of topographic relief in Santa Isabel expose deeper units that provide a window into the pre-Late Miocene
geologic history.
3. Preserved geologic record in Santa Isabel Wash
3.1 Overview
Twenty-one lithologic units defined in Santa Isabel Wash on the basis of lithology, stratigraphic position,
and age have been combined into seven groups (Figure 2). The Miocene units (Groups 2-7) unconformably overlie
pre-Miocene metasedimentary rocks (Pz) and granites (Mzg) which constitute Group 1. From oldest to youngest the
Miocene units are: Group 2) volcaniclastic breccias, sedimentary rocks, and a local pyroclastic flow deposit (Tmvs,
Tmrbio ), Group 3) intermediate to mafic lava flows and associated epiclastic breccias (Tmb kc, Tmb lol , Tmd tomb ),
Group 4) a pyroclastic flow deposit (Tmrsf), Group 5) intermediate lava flows (Tma toro ), Group 6) a series of
pyroclastic flow deposits (oldest to youngest: Tmrsiw, Tmr3, Tmr4, Tmrao , Tmrec, Tmrbs, Tmrfp ) and a local mafic
lava flow (Tmb new) between Tmrsiw and Tmr3, and Group 7) intermediate and felsic lava flows (Tma gem, Tmrcan ,
Tma ugl , Tma hem).
Basement outcrops of undated granites and metasedimentary rocks comprising Group 1 are exposed
exclusively in the northwest corner of Santa Isabel Wash (Figure 2). The metasedimentary rocks are likely related to
lithologically similar metamorphosed Latest Precambrian to Paleozoic miogeoclinal to deep water facies deposits
found elsewhere along the eastern side of peninsular Baja California (Gastil, 1993). The intrusive rocks are
representatives of the Mesozoic Peninsular Ranges Batholith (PRB) (e.g., Silver and Chappell, 1988).
The oldest volcanic deposits (Groups 2 and 3) are exp osed in profile along north-facing topographic slopes,
at the head of Arroyo Oculto, and in the footwalls of major normal faults (Figure 2). The 500+ meters of stratified,
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poorly sorted, volcaniclastic breccias and sedimentary debris flows found in Group 2 are attributed to catastrophic
sedimentation from the high-standing, unstable slopes of the Early to Middle Miocene andesitic arc. Laterally
discontinuous lava flows and pyroclastic flow deposits occur within the breccias. Voluminous dacitic, and less
abundant mafic (basaltic andesitic?), lava flows and associated collapse breccias (Group 3) overlie the volcaniclastic
breccias of Group 2 and are also interpreted to represent products of subduction-related volcanism.
Up to 300 meters of distal outflow sheets of pyroclastic flow deposits (Groups 4 and 6) and minor lava flows
(Group 5) overlie Group 2 and 3 rocks and are believed to be associated with the development of the GEP. The ashflow tuffs of Group 6 filled irregular topography to produce the plateau-like, upper surface of the PVP. An ~120-mthick basaltic lava flow is also preserved within the series of pyroclastic flow deposits. Andesite and rhyolite lava
flows (Group 7) overlie these units north and south of Arroyo Oculto, and multiple flows forming deposits up to 500
meters thick make up local topographic peaks Picacho Canelo (elev. ~765 meters) and Pico Los Heme (elev. ~1040
meters; Figure 2). Although not explicitly represented in Figure 2, Quaternary alluvial deposits surround isolated hills
in the northern part of the map area and are the product of modern arroyo sedimentation. In some places older
alluvium at slightly higher elevations is present. Fine-grained, modern, playa deposits lie within closed basins up to 2
kilometers in diameter.
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. Additional details given by Nagy (1997) include
specific location, size, and thickness of deposits, nature of contacts, outcrop and hand sample appearance,
mineralogical and petrographic information, and a 1:20000 scale geologic map with cross-sections. Abbreviated
petrographic descriptions of each unit (Appendix A) and ancillary electron microprobe results (appendix B) are
available from http://oro/PVP/santa_isabel.html. Appendix A also includes detailed information regarding
stratigraphic relationships that supplement the simplified stratigraphic column shown in Figure 2.
Group 1 - Pz, Mzg. Basement rocks of Group 1 consist of Mesozoic (?) granite (Mzg) and Paleozoic (?)
metasedimentary rocks (Pz) pervasively intruded by 1-mm- to 4-cm-wide, ductilely deformed aplite dikes. Mzg is a
medium-grained, unfoliated, garnet-bearing two-mica granite, and Pz consists of foliated garnet-epidote paragneisses
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interspersed with thin layers of diopside marble. Schists and more feebly recrystallized chert, marble, and slate are
also present.
Groups 2 - Tmvs, Tmrbio . A thick sequence of stratified, poorly sorted breccia to conglomerate lithofacies,
designated elsewhere by Stock (1989) as volcaniclastic sediment (Tmvs), occurs throughout Santa Isabel Wash and
is interpreted as catastrophic debris flows. Deposits up to 500+ meters thick form steep, resistant cliffs with
subhorizontal bedding planes discernible several kilometers away. Outcrops typically weather to pale blue or pink.
The cryptocrystalline ashy matrix typically includes 20-35% phenocrysts. Both monolithologic and heterolithologic
deposits occur. Lithic fragments (10-35%), typically 3-10 cm in dimension (max. several meters), include hornblendephyric andesite, red- to orange-weathering porphyritic rhyolite, and dark, fine-grained volcanic fragments. Clasts from
Group 1 were not found within Tmvs which agrees with observations made in correlative deposits elsewhere.
Interbedded, fluvial sandstones are rare. Significant relief is evident on the upper surface of Tmvs prior to
unconformable deposition of subsequent units, which include the other Group 2 unit (Tmrbio ).
Thin (2-4 meters), interbedded lava flows of intermediate composition and cross-cutting andesitic to basaltic
dikes have been mapped with Tmvs. However, a non-welded, weakly indurated, crystal-rich, pumice- and lithic-lapilli
pyroclastic flow deposit, the Biotite tuff (Tmrbio ), has been recognized as a separate map unit. Tmrbio is up to 80
meters thick (E12 in Figure 2), unconformably overlies Tmvs, and is in turn unconformably overlain by younger units.
Tmrbio has distinct amphibole- and biotite-bearing pumice pieces (~ 15%) which average 1-2 cm (max. 5 cm).
Feldspar, quartz, and biotite phenocrysts are also visible in the matrix. Fine-grained, porphyritic, volcanic lithic
fragments (5-15%) are < 1-12 cm (ave. 3-5 cm).
Groups 3 - Tmb kc, Tmb lol , Tmd tomb . A clinopyroxene-olivine-plagioclase basalt containing up to 4%
strongly resorbed quartz, the Klondike Canyon Basalt (Tmb kc), and an olivine-pyroxene-plagioclase basalt with
minor hornblende phenocrysts, the Land of the Lost Basalt (Tmb lol ), are small (< 0.5 km2 in map view) lava flows and
dikes which intrude and unconformably overlie Group 2 rocks (Tmvs). Tmb kc is up to 140 meters thick at the head of
Santa Isabel Wash (D13 in Figure 2), and Tmb lol is a maximum of 60 meters thick (I13 in Figure 2). Both basalts are
locally unconformably overlain by Group 6 rocks while Tmb kc is also unconformably overlain by the other Group 3
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unit (Tmd tomb ). The latter is the Tombstone Dacite, one of the most voluminous rock types in Santa Isabel Wash
which consists of a bronzite-hornblende-plagioclase-phyric dacite with minor amounts of quartz phenocrysts. Solid,
plug-like lavas and associated breccias present within Tmd tomb average 100-200 meters in height and typically form
continuous outcrops over distances of several kilometers (e.g., L11 and F7 in Figure 2). Structures indicating that the
rocks represent collapse-breccia suggest that these are primary 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 (Blake, 1990) and indicate that the flows were likely produced from 15-20 coalescing
domes distributed over a 140 km2 area. Fresh hand samples are pale pink to deep brick-red to black. Plagioclase
phenocrysts (5-10%) averaging 3-5 mm generally occur in hand samples, while green orthopyroxene (max. 2 mm) and
black, altered hornblende (max. 2.5 mm) phenocrysts may also be present. Granitic and gneissic xenoliths (1-2%) that
average 3-4 cm (max. 20 cm) are commonly rounded with black reaction rims. A few basalt flows up to 70 meters 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
meters of fluvial sedimentary deposits occur between Tmd tomb and underlying Tmvs.
Group 4 - Tmrsf. The Tuff of San Felipe (Tmrsf) (designated by Stock and others (1996, 1997)) is a crystalrich, strongly indurated, lithic-lapilli pyroclastic flow deposit up to 40 meters thick restricted to the west side of the
map area (e.g., B7-C6 in Figure 2). A basal vitrophyre characteristically grades upwards from 20-30 cm of brown glass
with black lithophysae to a small zone of reddish brown glass to 1 meter of red and black glass. There is an upward
increase in size and abundance of spherulites. The latter are red/orange and average 0.5 cm. 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 (3-6%). Lithophysae define a
prominent eutaxitic foliation. Millimeter-size (max. 2 cm) lithic fragments (1-3%) include trachytic and porphyritic
volcanic rocks such as hornblende-phyric andesite. The devitrified top of Tmrsf is 3-5 meters thick, less densely
welded than underlying portions, and weathers to a distinctive sky-blue. Hand samples are porcelaneous and lack the
abundant lithophysae that characterize the lower sections.
Group 5 - Tma toro . The Pico del Toro Andesite (Tma toro ) is a hornblende-phyric andesite which crops out
in two localities (F11-G10 and K12 in Figure 2). The larger lava flow covers ~0.25 km2 in map view and is up to 200
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meters thick. Samples have a light brown to greyish-brown drusy matrix with dark brown acicular hornblende
phenocrysts up to 3 mm.
Group 6 - Tmrsiw, Tmb new, Tmr3, Tmr4, Tmrao , Tmrec, Tmrbs, Tmrfp . The colorful, crystal-rich, pumiceand lithic-lapilli pyroclastic flow deposits comprising the Tuffs of Santa Isabel Wash (Tmrsiw) are weakly indurated,
distal outflow sheets up to ~130 meters thick that filled topographic lows. Tmrsiw is either vitric or devitrified with
weathered surfaces characteristically grading upwards from yellow/orange to red/pink. Prominent eutaxitic foliation is
common where the base overlies irregular topography. In some localities at least 5 cooling units can be recognized
(designated t1 to t5 from bottom to top). A discontinuous, white, pumice-flow deposit at the base (t1) is up to 30
meters thick. It often exhibits cross-bedded surge deposits or plinian (fallout) deposits that appear reworked in some
locations. The basal t1 unit is overlain by a cliff-forming, yellow-to-orange, unwelded to densely welded, pumiceous
lithic-lapilli tuff (t2) up to 15 meters thick. This subunit is in turn overlain by a less resistant, pink to pale orange,
unwelded pumiceous lithic-lapilli tuff (t 3) up to 20 meters thick with common vertical gas-escape structures.
Overlying t4 is a grey (brick-red-weathering), densely welded cooling unit up to 10 meters thick with a distinct lithicrich (= 30%) horizon in the center. The uppermost unit is a purple- to pink- weathering, unwelded to densely welded,
lithic-lapilli tuff (t5) that is up to 50 meters thick and usually includes a red and black, densely welded zone (1-5
meters thick) near its base with well-developed eutaxitic foliation and spherulites. Knobby weathered surfaces have a
perlitic (devitrified) texture and fiamme are up to 10 cm in length. The uppermost portion of t5 may exhibit a devitrified
purple matrix with red, glassy lithophysae forming bands 2-8 mm thick. Alternatively the unit has been observed to
grade upwards into a 1.5-m-thick purple ash. The t1-t5 sequence is capped by 0.5-5 meters of an orange, non-welded,
bedded pumice-flow deposit. Tmrsiw contains 5-15% pumice lapilli (1-15 cm), 1-8% feldspar phenocrysts, 1-2% subrounded obsidian (1 mm to 6 cm), and 7-20% sub-angular lithic fragments (1 mm to 5 cm) of fine-grained, pale purple
and pale blue-grey volcanic rocks (juvenile clasts?), porphyritic dacite (Tmd tomb ?), and granite. The presence of
disequilibrium olivine phenocrysts (Appendix A) distinguishes Tmrsiw from the other Group 6 tuffs, and the general
absence of hornblende, biotite, and quartz in Tmrsiw suggests a more anhydrous (hotter?) parent magma than
adjacent tuffs bearing hydrous phases. Paleomagnetic analysis shows that cooling units t2 through t5 preserve the
same remanent magnetization direction (Nagy, 1997, in prep. for J. Geophys. Res.), suggesting a relatively short
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period of time for deposition. Overlying New Year’s Mountain Basalt (Tmb new) is a glomeroporphyritic olivinepyroxene-plagioclase basalt with a 2-4-m-thick, scoriaceous basal breccia overlain by up to 120 meters of stony lava
(in the vicinity of N10 in Figure 2). The steel grey to black matrix is dense and stony, can be highly vesiculated, and
has 10% green and white glomerocrysts (max. 5 mm) and small, euhedral feldspar phenocrysts.
At least four cooling units are included in crystal-rich, pumice- and lithic-lapilli pyroclastic flow deposit
Rhyolite #3 (Tmr3). The label is taken from Stock (1989). Tmr3 deposits are generally > 100 m thick, fill topography,
and retain a fairly planar upper surface. The four devitrified cooling units look very similar in the field, however the
lowest cooling unit (designated type I) preserves a significantly different paleomagnetic vector direction from the
upper three (type II) (Nagy, 1997, in prep. for J. Geophys. Res.). Tmr3 (type I) has a non-welded, pink to light brown
matrix, fibrous silver-grey pumice averaging 5-10 cm in length (max. > 40 cm) and wisps of obsidian. Anorthoclase
phenocrysts make up 5% of the rock, and lithic fragments (2-5%) include red and grey fine-grained volcanic rocks.
The second, or overlying, Tmr3 cooling unit (type II) is a slightly welded, tan to brown, cliff-forming tuff with
distinctly fibrous, pale buff pumice (5-10%) averaging < 1 cm in some exposures but much larger in others (max. > 50
cm). Anorthoclase crystals (1-5%) separated from one large pumice piece were used as an internal standard for the
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Ar/ 39Ar geochronology in this study (INF-94-53). Red and grey volcanic lithic fragments (3%) average 1 cm (max. 10
cm). The third cooling unit from the bottom (type II) has a bright white, orange-weathering ashy matrix and contains
white pumice (5-7%) averaging 1-2 cm. Dark red and grey, angular, fine-grained volcanic lithic fragments (10-12%) are
distinct within the white tuff and average 1-3 cm (max. 30 cm). Other lithic fragments include yellow ash-flow tuff and
granite. Large (> 5 mm) anorthoclase phenocrysts (3-4%) are clearly visible. The moderately welded uppermost
cooling unit (type II) has a pink, ashy matrix, weathers to white or orange, and contains 1-2% feldspar phenocrysts, 710% silvery-grey, devitrified, spherulitic elongate pumice, and 2-3% lithic fragments averaging 1-2 cm and including
granite, red and grey, fine-grained volcanic rocks, and a white ash-flow tuff.
Rhyolite #4 (Tmr4) is a crystal-poor (<1%), weakly indurated, pyroclastic flow deposit. The label is taken
from Stock (1989). The characteristic base consists of up to 1 meter of dark brown vitrophyre with 1-2% orange,
welded pumice (max. length: 1 cm). This grades upwards to an orange then bright red, partially welded tuff.
Spherulitic lithophysae (< 1 cm to > 10 cm) are present in some outcrops and can be pink, blue, purple, or orange
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(generally 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 meters thick, however in Arroyo Oculto it is up to 70 meters thick
and forms distinct benches at the upper contact below more recessive units. The thicker exposures can have a 2-4-mthick spherulitic base. Rare (< 1%) feldspar phenocrysts < 3 mm and volcanic lithic fragments < 1 cm are present. An
overlying crystal-poor (<1%), weakly indurated, pumice- and lithic-lapilli pyroclastic flow deposit, the Arroyo Oculto
Tuff (Tmrao ), is 1-30 meters 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. Pumice (5%) and < 1 cm
sub-rounded dacite clasts (1-2%) are present within Tmrao . Granitic clasts are also present in exposures around
Arroyo Oculto. Phenocrysts are rare and cannot easily be seen without a hand lens.
The Tuff of El Canelo (Tmrec) is a densely welded, strongly indurated, crystal-rich, lithic-lapilli pyroclastic
flow deposit which generally caps the mesa tops of the PVP in the Santa Isabel Wash region. The name is taken from
Stock and others (1991) and Martín-Barajas and others (1995) who 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 at least two of those cooling
units (see correlation section below). In most of Santa Isabel Wash, including the upper surface of the PVP plateau,
Tmrec is 1-2 meters thick with a devitrified groundmass and prominent eutaxitic foliation forming sub-horizontal, darkcolored fiamme, but near Arroyo Oculto (in the vicinity of R7 in Figure 2) it is up to 80 meters thick and contains
steeply dipping foliation planes with down-dip lineations suggesting syn- or post-compaction flow. Other kinematic
indicators such as rotated (folded?) fiamme and tension cracks also suggest down-dip movement to the northwest.
Topographic variations (1-3 meters) in the upper surface of these relatively thick exposures impart a hummocky
appearance to the top of the unit. At the base of the thick deposits in the Arroyo Oculto region, Tmrec is brick red on
fresh surfaces with long, thin, white, continuous, sub-horizontal fiamme. The latter are up to a few mm in width and
account for 5-10% of the rock which results in a distinct striped appearance. A densely welded, upper flow unit up to
40 meters thick occurs in the vicinity of S6 (Figure 2). Feldspar phenocrysts (~5%) are visible in hand samples. The
Bighorn Sheep Tuff (Tmrbs) is a devitrified, weakly indurated, crystal-rich, lithic-lapilli pyroclastic flow deposit 1-15
meters thick restricted to the Arroyo Oculto region. Devitrified pumice (10-15%) average 1-3 cm. Feldspar
12
phenocrysts (1-5%) are not always visible in hand samples. Lithic fragments (1-2%) average 2-3 cm in length (max. 30
cm) and include flow-banded volcanic rocks, glassy obsidian, and highly altered volcanic rock (dark grey with a red,
altered matrix). The densely welded, strongly indurated Flag Pole Tuff (Tmrfp ) is a crystal-rich, lithic-lapilli
pyroclastic flow deposit also restricted to the Arroyo Oculto region. Deposits are up to 7 meters thick (commonly <
0.5 m), devitrified, and contain abundant, flattened lithophysae which have weathered to give the deposit a pockmarked appearance.
Group 7 - Tma gem, Tmrcan, Tma ugl, Tma hem. The Pico de Los Gemelos Andesite (Tma gem) is a
diopside-bronzite-plagioclase-phyric andesite (S5 in Figure 2). Its base is formed by a series of red, 1-2-m-thick
breccia layers that alternate with 1-2-m-thick layers of grey, foliated (flattened vugs), stony lava. Hand samples have
a fine-grained, purplish-grey to black matrix with flattened, sub-parallel vesicles (5 mm). Green pyroxene phenocrysts
(1-2%) are barely visible and smaller plagioclase phenocrysts (1-2%) can be seen with a hand lens. Locally Tma gem is
underlain by a separate unit, 1-2 meters thick, of black, mafic agglutinate or spatter with features such as lineations on
clasts suggestive of fire-fountaining. The overlying Picacho Canelo Rhyolite (Tmrcan) is a series of plagioclasephyric rhyolite flows which form the local topographic peak Picacho Canelo (Figure 2). A well-exposed basal section
consists of ~1 meter laminated airfall of rhyolite lithic fragments, pumice, and ash, overlain by 2-3 meters 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 of Tmrcan are up to a total of 460 meters thick and consist of a partially to
completely devitrified matrix which is red and powdery-white with 3-4% plagioclase phenocrysts. Some portions of
Tmrcan contain small (<1 cm) spherulites. A distinct feature is the presence of 2-3% juvenile (?) and 1-20 cm
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 (Tma ugl), (U6 in Figure 2) has a 5-6-m-thick red (oxidized) basal breccia below the main flow.
Fresh surfaces exhibit a drusy texture and consist of red (altered) glass with feldspar phenocrysts and aggregates of
altered mafic minerals (1-2%). Weathered surfaces are brick red to orange, and weathered calcite (?) veins are
ubiquitous throughout the rock. The Los Heme Andesite (Tma hem) is a diopside-bronzite-plagioclase-phyric
andesite petrographically identical to Tma gem. The lava flow forms the local peak Pico Los Heme (Figure 2) and
13
extends beyond the map area to the southeast. Plagioclase and pyroxene phenocrysts (to 5 mm) are clearly visible in
hand specimens.
4. 40Ar/39Ar results
Details of the
40
Ar/ 39Ar procedures followed in this study are summarized in Appendix 1 and discussed in
detail in Nagy (1997). All reported ages result from inverse isochron analysis (e.g., McDougall and Harrison, 1988)
using a weighted regression routine described in Mahon (1996). Quoted uncertainties represent ± 2σ standard
deviations. Tabulated
40
Ar/ 39Ar results for individual analyses are presented in Appendix C (available from
http://oro/PVP/santa_isabel.html). A summary of statis tical results for our internal standard (INF-94-53) is given in
Table 1 while results for all other samples appear in Table 2. Corresponding isochron plots are displayed in Figure 3.
Note that the 95% ranges of critical MSWD value discussed in Mahon (1996) 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 (<10-16 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 data of the type described above had been excluded. For these samples it
appears likely that either a heterogeneous crystal population exists, and/or analytical uncertainties are
underestimated. Isochron ages in this study typically exhibit precision (1σ) for plagioclase (± 2-7%) and anorthoclase
and sanidine (± 2-5%) that agrees well with that reported from studies of similar young, K-poor materials (Pringle et
al., 1992; Zumbo et al., 1995; Martín-Barajas et al., 1995; Spell et al., 1996).
Replicate analysis of Tmr3 (INF-94-53) indicate that our ability in this study 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 INF-94-53 representing two
separate irradiations (UofM#75 and UofM#81) are displayed in Table 1. Normal polarity magnetization preserved in
Tmr3 (Lewis, 1994; Nagy et al., 1995; Nagy, 1997, in prep. for J. Geophys. Res.; Lewis and Stock, in review in J.
Geophys. Res.) constrains the age of deposition to Subchron C3An.2n (6.269-6.567 Ma; Cande and Kent, (1995)). The
weighted mean of the replicate isochron results for all UCLA analyses of INF-94-53 is 6.31 ± 0.32 Ma (2σ standard
deviation; MSWD = 3.5). A comparable isochron age (6.4 ± 0.2 Ma; 2σ standard deviation; MSWD = 1.7) for INF-94-
14
53 anorthoclase was independently obtained from the MIT Cambridge Laboratory for Argon Isotopic Research
(Appendix C available from http://oro/PVP/santa_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 the Fish Canyon sanidine flux monitor (Cebula et
al., 1986). More recent K-Ar results for FCT-1 sanidine presented by Lanphere and Baadsgaard (1997) indicate that
the K-Ar age of Fish Canyon sanidine may in fact be somewhat younger (27.55 ± 0.10 Ma). Use of the Lanphere and
Baadsgaard (1997) K-Ar age for Fish Canyon sanidine 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 137Cs γ source to aid mineral separation prior
to neutron irradiation (Appendix 1) yield statistically identical results to untreated samples (Table 1).
For many of our samples, apparent ages determined from low yield 39Ar analyses are statistically younger
than those obtained for fusion steps in which normal
39
Ar yields were obtained (Appendix C available from
http://oro/PVP/santa_isabel.html). This is particularly characteristic of the initial steps obtained from incrementally
heated samples. This relationship results in isochron plots indicating
40
Ar/ 36Ar values that are lower than modern
atmosphere (40Ar/ 36ArATM = 295.5). Renne and Basu (1991) and Renne and others (1992) found similar low-temperature
discordances for plagioclase samples using an incremental laser heating technique. They attribute their low apparent
ages to argon loss through minor reheating (= 150°C) or alteration of the volcanic rocks. We believe that either minor
surface alteration and/or undetected hydrocarbons contributing to m/e 36 measurements are the most likely reason
for the anomalously young ages and sub-atmosphere 40Ar/ 36Ar ratios observed for our samples. However, isotopic
mass fractionation of atmospheric argon prior to, or even after, eruption of the rock could conceivably have produced
a trapped component with a 40Ar/ 36Ar ratio lower than the present-day, atmospheric value (e.g., Krummenacher, 1970;
Kaneoka, 1980; Lippolt et al., 1990).
As indicated in Table 2, nearly 75% of our samples yield isochrons characterized by MSWD values within
the 95% confidence interval outlined by Mahon (1996). 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 the same lithologic unit indicate that isochron results
for samples INF-94-75, INF-94-68, INF-94-46, PHO-94-10, 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 (average
40
Ar* of 9% and 4%,
15
respectively; see also Figure 3 and Appendix C available from http://oro/PVP/santa_isabel.html). Both samples,
however, have well-constrained isochrons, and their ages are consistent with the local stratigraphy. Sample SUN-9430 (average 40Ar* of 22%) is more problematic given that its age (5.7 ± 0.4 Ma) is anomalously young relative to the
results from three other Tmrsiw samples (6.5 ± 0.3 to 6.7 ± 0.3 Ma) and 40Ar/ 39Ar ages of five overlying units (Figure
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 (McDougall and Harrison, 1988).
5. Regional lithologic correlations
Six of the Miocene units mapped in Santa Isabel Wash (Tmvs, Tmrsf, Tmr3, Tmr4, Tmrec, Tmrcan ) are
correlated to units defined previously in nearby localities (Stock, 1989, unpub. mapping; Martín-Barajas et al., 1995;
Lewis, 1994, 1996; Stock et al., 1991, 1996, 1997). 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 mineral compositions. 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 (Sonora) and islands in the northern Gulf of California. Diagrams
summarizing these correlations are presented in Figures 4 and 5.
5.1 Early to Late Miocene deposits (Figure 4)
Group 2. An 18-22 Ma subduction-related hornblende andesite province has been mapped along the entire
east side of the Baja California peninsula, on islands in the Gulf of California, and in northwest Sonora (Gastil et al.,
1979). Volcaniclastic deposits derived from this andesite belt that are correlative to Tmvs in Santa Isabel Wash occur
locally in southern Valle Chico (Stock, 1989), the Sierra San Fermín (Lewis, 1996), and ~60 kilometers south of
Puertecitos (Dorsey and Burns, 1994) (Figures 1 and 4). Slightly further from the PVP additional deposits have been
noted in the Sierra Juarez (Lee et al., 1996), Isla Tiburón (Gastil and Krummenacher, 1977; Neuhaus, 1989), and Sonora
16
(Gastil and Krummenacher, 1977). In Santa Isabel Wash Tmvs deposition was replaced by near-source-facies
volcanism (Tmd tomb ) around 17 Ma. Although Tmvs was not dated in our study, the ages of overlying units
constrain deposition of Tmvs to be older than 17 ± 2 Ma. For example, plagioclase from Tmrbio yields an isochron age
of 17.1 ± 2.4 Ma (Table 2; Figure 3a). This relationship contrasts with that observed for regions further north of the
PVP where deposition of distally-derived units may have continued until about 12 Ma (Stock, 1989; Lewis, 1996). The
closest andesitic vent facies (17-20 Ma) are ~10 kilometers northwest of Santa Isabel Wash (Stock et al., 1991) (vent
facies not included in Figure 4). Another nearby source of Miocene andesite, Pico Matomí (Gastil et al., 1971, 1975), is
~20 kilometers to the west, and still others may have been eroded and/or buried by later volcanism. The ~ 17 Ma
Biotite Tuff (Tmrbio ) is petrographically similar to one of the biotite-bearing Tuffs of Toronja Hill (Mtt) in Arroyo
Matomí south of the Sierra San Felipe (Stock, 1989). Mtt is undated but overlies a basalt which yielded an age of 17.0
± 0.3 Ma (Mb3 of Stock, 1989).
Group 3. Nearby basalt flows which could be contemporaneous with the ~ 16-17 Ma Klondike Canyon
Basalt (Tmb kc in Table 2; Figure 3b) and Land of the Lost Basalt (Tmb lol in Table 2; Figure 3c) include Mbsu
(undated) and Mb4 (14.5 ± 0.2 Ma) in southern Valle Chico (Stock, 1989) and Tmb2 (undated) in the Sierra San Fermín
(Lewis, 1996). The voluminous 15.5-16.5 Ma Tombstone Dacite lava flows (Tmd tomb in Table 2; Figures 3d-j) do not
appear to have correlatives in southern Valle Chico, Sierra San Fermín, or southern Sierra San Felipe. Evidently these
lava flows were extruded synchronously with Tmvs deposition in the north, illustrating a lateral facies change
between more proximal (Tmd tomb ) and distal (Tmvs) deposits. An ~ 16 Ma, 4-km-diameter, andesite and dacite
complex (Tma of Martín-Barajas and Stock (1993) and Martín-Barajas and others (1995)) in the northeastern PVP
(Arroyo Los Heme; see Figure 1) is potentially coeval with Tmd tomb volcanism.
Group 4. The Middle Miocene Tuff of San Felipe (Tmrsf) (renamed by Stock and others (1996, 1997)) was
previously referred to as Tmr1 or Mr1 in the Santa Rosa basin (Bryant, 1986), southern Valle Chico, southern Sierra
San Felipe (Stock, 1989; Stock and Hodges, 1990), and the Sierra San Fermín (Lewis, 1996) (Figure 4). The source
region of Tmrsf may lie below modern alluvium between the northern Sierra San Fermín and southern Sierra San Felipe
(Lewis, 1996). Its unique age (~12.7 Ma; Tmrsf in Table 2; Figure 3k) and widespread occurrence make Tmrsf a
regionally important ash-flow tuff. Detailed correlation criteria and seven age determinations (also given in Figure 4)
17
are summarized by Nagy (1997) and Stock and others (in prep. for J. Volc. Geotherm. Res.). Lithologic correlations are
further supported by a low inclination, reversed polarity magnetization preserved in the Sierra San Fermín, Sierra San
Felipe (Lewis, 1994; Lewis and Stock, in review in J. Geophys. Res.), Santa Rosa Basin, and Santa Isabel Wash (Nagy,
unpublished data; Stock et al., in prep. for J. Volc. Geotherm. Res.). An 11.2 ± 1.3 Ma rhyolite ignimbrite on southwest
Isla Tiburón (Gastil and Krummenacher, 1977; Unit 8 of Neuhaus, 1989) is potentially correlative with Tmrsf. Should
this correlation hold true, the stratigraphic tie would provide an important constraint on pre-extension
paleogeography. Only the presence of biotite distinguishes the Unit 8 rhyolite ignimbrite of Neuhaus (1989) from
Tmrsf in other areas.
Group 5. The ~ 9 Ma age of the Pico del Toro Andesite (Tma toro in Table 2; Figure 3l), as well as its
association with silicic rocks and its isolated, small-volume outcrop nature, suggest that it represents postsubduction, rather than subduction-related, volcanism (see dis cussion of such criteria by Sawlan (1991)). This
hornblende-phyric andesite is in the same stratigraphic position as a mineralogically similar, slightly older (11.8 ± 0.7
Ma; whole rock, 40Ar/ 39Ar) andesite in the Sierra San Fermín (Tma of Lewis, 1996) as well as an undated andesite from
southern Valle Chico (Ma2 of Stock, 1989). Other potentially contemporaneous andesites include an 8.9 ± 0.6 Ma
andesite in the northern Sierra Pinta (~140 km north of the PVP) (McEldowney, 1970; Gastil et al., 1979), a 10.9 ± 2.3
Ma (or 15.3 ± 1.3 Ma) andesite and/or a 9.9 ± 1.3 Ma andesite on Isla Tiburón, an 11.3 ± 1.2 Ma andesite in western
Sonora, and a 12.3 ± 2.9 Ma andesite northwest of Rancho Buenas Noches in Sonora (T4 of Gastil and
Krummenacher, 1977).
5.2 Latest Miocene to Pliocene deposits (Figure 5)
Preferred correlations between Group 6 and 7 rocks and deposits from nearby regions are shown in Figure 5.
Due to the completeness of the stratigraphic section preserved in Santa Isabel Wash, the relative stratigraphic order
of several ~6 Ma pyroclastic flow deposits in the PVP region is now clearly established. Correlations with Group 7
lava flows are based upon the assumption that the various lava flows are coeval but not necessarily the exact same
deposit, whereas correlations with Group 6 tuffs imply that they are the same pyroclastic flow deposit. The source
18
vents, or calderas, for Group 6 pyroclastic flow deposits are generally inferred to be submerged beneath the Gulf of
California, or buried beneath 3 Ma volcanic rocks in the northeastern PVP.
Group 6. The Tuffs of Santa Isabel Wash (Tmrsiw) are a distinctive assemblage of pyroclastic flow deposits
that 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 Matomí (Mmt) in southern Valle Chico and Mesa Cuadrada (Figures 1 and 5) (Stock, 1989)
and Tmr3a in the Sierra San Fermín and on Mesa Cuadrada (Lewis, 1994; 1996; Lewis and Stock, in review in J.
Geophys. Res.), field descriptions, petrography, and paleomagnetic studies (Lewis, 1994; Nagy, 1997; Lewis and
Stock, in review in J. Geophys. Res.; Nagy, in prep. for J. Geophys. Res.) 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. As indicated in Figure 5, Tmr3 (type I) in Santa Isabel Wash
may be equivalent to a portion of Mmt, or may also be unique to Santa Isabel Wash. Paleomagnetic data (Nagy, 1997;
in prep. for J. Geophys. Res.) also indicate that Tmr3 (type I) is distinct from the Tmr3a unit in adjacent areas.
Eruption ages for Mmt and Tmr3a have not been determined. Isochron results from Tmrsiw indicate that the tuffs are
~6.6 Ma (Figure 5; Table 2). For example, a composite plagioclase and anorthoclase-bearing concentrate from Tmrsiw
(cooling unit t2) yields an isochron age of 6.5 ± 0.3 Ma (Figure 3m), plagioclase from the upper purple ash of cooling
unit t5 yield an isochron age of 6.7 ± 0.3 Ma (Figure 3n), and plagioclases from densely welded Tmrsiw at two
locations yield ages of 6.6 ± 0.5 Ma (Figure 3o) and 5.7 ± 0.4 Ma (Figure 3p). As previously discussed this last result
is problematic given the agreement between the other three ages and the ages of overlying deposits.
Field appearance, electron microprobe data, and petrography strongly support correlations between the
three Tmr3 (type II) cooling units in Santa Isabel Wash, Mr3 in southern Valle Chico and Mesa Cuadrada, and Tmr3b
in the Sierra San Fermín and Mesa Cuadrada. As previously discussed isochron ages yielded by anorthoclase from
Tmr3 (type II) are consistent with 6.36 ± 0.10 Ma (Table 1; Figure 3q-r). This result is slightly younger than that
reported for the Tmr3b unit and marginally older than that provided for Mr3 (Figure 5). Field appearance and
petrography also strongly suggest that Tmr4 is equivalent to Mr4 in southern Valle Chico and Mesa Cuadrada and
Tmr4 in the Sierra San Fermín. Note however that biotite phenocrysts identified in these studies were not observed in
Santa Isabel Wash deposits. Tmr4 is also correlated to t2u ~ 8 kilometers north of Santa Isabel Wash (just south of
19
Arroyo Matomí) on the basis of descriptions in previous studies (Stock et al., 1991; Stock, unpub. mapping) as well
as petrographic study of t2u samples collected in this study. This correlation is further supported by outcrop, hand
sample, and petrographic examination of the unit which underlies t2u (labeled t2l) near Arroyo Matomí which is
correlated to a thin deposit which underlies Tmr4 in Santa Isabel Wash (Tmr3-4 of Nagy (1997)). Normal polarity
magnetization in Tmr3 (type II) , Tmr3b, Tmr3-4 , and Tmr4 (Lewis, 1994; Nagy, 1997; Lewis and Stock, in review in J.
Geophys. Res.; Nagy, in prep. for J. Geophys. Res.) further constrains the age of deposition of these tuffs to
Subchron C3An.2n (6.269-6.567 Ma; Cande and Kent, (1995)).
The Tuff of El Canelo (Tmrec) dated here at 6.1 ± 0.5 Ma (Figure 3s; Table 2) is correlated here to t3 of
Arroyo Matomí (Stock et al., 1991) and with other deposits previously identified as the Tuff of El Canelo, including
t12 and t9 of Mesa El Tábano (Stock et al., 1991) and Tmc6 (the uppermost of six cooling units) in the northeastern
PVP (Martín-Barajas et al., 1995). The presence of an additional upper cooling unit in Tmrec in Santa Isabel Wash is
consistent with the multiple flows seen in other localities (Stock et al., 1991). Lewis (1996) suggests that the Tuffs of
Dead Battery Canyon in the Sierra San Fermín (Tmr5-Tmr8) may be correlative to the Tuff of El Canelo, which is
supported by geochronology and microprobe results of Tmr7 and Tmrec from this study. Field observations suggest
that the vent for the Tuff of El Canelo may have been located south of the Sierra San Fermín in the vicinity of Arroyo
Matomí and Arroyo El Canelo (3-4 kilometers south of Arroyo Matomí) (Martín-Barajas et al., 1995; Lewis, 1996).
Tmrec correlations are supported by similarities in field appearance, mineralogy, and stratigraphic position of
underlying units (Arroyo Oculto Tuff (Tmrao ), t14 (also defined as part of the Tuff of El Canelo on Mesa El Tábano),
Tmc5 in the northeastern PVP, and Tmr5 in the Sierra San Fermín). Moreover 40Ar/ 39Ar results of 6.4 ± 0.3 Ma from
Tmrao (Figure 3t; Table 2) are similar to that reported for Tmc units in the northeastern PVP (Figure 5).
Group 7. The outcrop appearance and mineralogy of the Picacho Canelo Rhyolite lava flows (Tmrcan ) are
similar to foliated, devitrified 6-8 Ma rhyolite flows ~10 kilometers northeast of Picacho Canelo (Tmr9 of Lewis, 1994,
1996) and a series of 5.8 ± 0.1 Ma rhyolite lava flows ~10 kilometers east of Picacho Canelo (Tmru of Martín-Barajas
et al., 1995). Isochron ages of ~6 Ma were obtained for Tmrcan rhyolites in the present study (Figure 3u-v; Table 2).
All three lava flows overlie ash-flow tuffs interpreted to be equivalent to Tmrec. Lewis (1996) interprets Tmr9 to be
20
coeval with rhyolite/dacite flows designated f4 in Arroyo Matomí (Stock et al., 1991). The latter differ only in the
presence of trace hornblende from the lavas described above. The f4 flows also overlie Tmrec. Unpublished mapping
(Stock; see also Stock et al., 1991) south of Arroyo Matomí indicates that f4 flows are present within ~ 1.5 kilometers
east-northeast of Picacho Canelo, thus Tmrcan flows are most likely continuous with f4. All of these lava flows
(Tmrcan , Tmr9, f4, Tmru) are considered to be part of a field of Latest Miocene rhyolite domes in the northeastern
PVP as previously noted by Martín-Barajas and others (1995).
The Pico de Los Gemelos Andesite (Tma gem) is stratigraphically constrained to be ~ 6 Ma. There are no
dated andesites of similar age in the region. The ages of the Ugly Mountain Andesite (Tma ugl ) and the Los Heme
Andesite (Tma hem) are not constrained by overlying units, and although they are petrographically similar to
Tma gem, it is possible that they are younger than 6 Ma. The proximity and similar nature of Tma ugl and Tma gem in
the field suggests that they are probably related; in contrast, Tma hem is several kilometers to the south of these lava
flows and could represent a younger pulse of volcanism. If so, Tma hem could be coeval with a 5.1 ± 0.3 Ma,
mineralogically similar andesitic dike and associated basaltic andesite lava flow in Arroyo Los Heme (Tba of MartínBarajas and others (1995)), and/or with several hundred meters of andesite flows in the southeastern Sierra San
Fermín (Tpa of Lewis (1996)). Tpa, which differs mineralogically from Tma hem by the presence of alkali feldspar and
biotite (Lewis, 1994), yields a whole rock 40Ar/ 39Ar age of 5.7 ± 0.2 Ma (Lewis, 1996); however, field relationships in
southeastern Sierra San Fermín indicate that Tpa overlies the 3 Ma Tuffs of Mesa El Tábano. Lewis (1996) interprets
Tpa to be coeval with a 0.9 ± 0.6 Ma olivine-bearing basaltic andesite in Arroyo Matomí (Mb6 of Stock and others
(1991); see also Martín-Barajas and Stock (1993) and Martín-Barajas and others (1995)).
6. Timing of volcanism and extensional deformation
The stratigraphic and 40Ar/ 39Ar results from Santa Isabel Wash provide important constraints on the timing
and distribution of volcanism and extensional deformation in the northern PVP related to the opening of the Gulf of
California. Numerous N-S striking, high-angle normal faults offset Group 6 tuffs (Figure 2) and thus constrain most
deformation to post-6 Ma time. Some evidence exists for minor pre-6 Ma deformation, however, such as faults which
cut Group 2 and 3 rocks, and in one locality Group 4 and 5 rocks, but do not offset Group 6 units. These structures
21
constrain minor, north-side-down deformation (Nagy, 1997) to be post-9 Ma in some places but possibly as old as 15
Ma in others. Additionally, some of the Group 6 tuffs (Tmr4, Tmrao , and Tmrec) thicken considerably in the Arroyo
Oculto region (Figure 2), and others (Tmrbs and Tmrfp ) are in fact restricted to this region. The thickening occurs
across a northeast-facing, paleo-topographic slope (O7-T14 in Figure 2) interpreted to be the southeastward
continuation of the Matomí accommodation zone (A-A’ in Figure 1); thus the development of this structure must
have been prior to 6 Ma.
These results have led to new models for the Middle to Late Miocene development of the Matomí
accommodation zone and the Pliocene history of this portion of the GEP margin (Nagy, 1997; Nagy and Stock, in
prep. for J. Geophys. Res.). Specifically, the orientation of the Matomí accommodation zone may have developed as a
result of ENE-directed extension in a zone of weakness separating two en echelon portions of the GEP margin. Post-6
Ma extensional deformation in Santa Isabel Wash is interpreted to be the result of incorporation of this portion of the
PVP into the GEP at about 2-3 Ma when dextral strike-slip motion along the Guaymas fracture zone replaced similar
motion along the Tiburón fracture zone (Nagy, 1997; Stock, in press; Nagy and Stock, in prep. for J. Geophys. Res.).
The 6 Ma pulse of volcanism recorded in Santa Isabel Wash and elsewhere in the region, as well as a 3 Ma pulse
recorded further east (e.g., Stock et al., 1991), are probably related to the formation of offshore spreading centers at
the margin of a proposed transition zone within the PAC-NAM boundary (southeastern edge of the “Wagner
Transition Zone” of Nagy (1997)).
Paleomagnetic analyses of ~6 Ma Tmr4 imply that at least parts of the Sierra San Fermín (Figure 1) have
rotated ~30° CW about a vertical axis relative to Santa Isabel Wash since Tmr4 deposition (Nagy, 1997; Lewis and
Stock, in review for J. Geophys. Res.; Nagy, in prep. for J. Geophys. Res.). New paleomagnetic results from
underlying ~12.5 Ma Tmrsf in Santa Isabel Wash and Santa Rosa Basin (Nagy, unpub. data, in prep. for J. Geophys.
Res.; Stock et al., in prep. for J. Volcan. Geotherm. Res.) and previous studies in the Sierra San Fermín and Sierra San
Felipe (Lewis, 1994; Lewis and Stock, in review for J. Geophys. Res.) imply (1) no relative rotations between the
southern Sierra San Felipe (i.e., Mesa Cuadrada) and Santa Isabel Wash and (2) ~30° CW rotation of the Sierra San
Fermín and Santa Rosa Basin relative to Mesa Cuadrada and Santa Isabel Wash. These results suggest that the PAC-
22
NAM plate boundary in this part of the GEP experienced distributed dextral shear partially accommodated by verticalaxis rotations since Latest Miocene time (Lewis, 1994; Lewis and Stock, in review in J. Struct. Geol.).
Conclusions
1. Geologic mapping,
40
Ar/ 39Ar geochronology, electron microprobe analysis, paleomagnetic study, and
petrography have been combined to define 21 Miocene volcanic units along the northern margin of the MiocenePliocene Puertecitos Volcanic Province (PVP). 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 distal outflow sheets of rhyolitic pyroclastic flow deposits. The stratigraphic order of
several ~6 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 (Tmrec) relative to other regionally distributed tuffs such as Tmr3 and Tmr4 is now well established.
2. Twenty-one rock samples from 11 lithologic units yield 40Ar/ 39Ar ages spanning ~17-6 Ma. Major pulses
of volcanism occurred at ~15-17 Ma (subduction-related) and ~6-7 Ma (extension-related; PVP-forming) with minor
pulses at ~12.5 Ma (commencement of extension) and ~9 Ma. Three of the tuffs dated in this study (Tmrsf, Tmr3, and
Tmrec) yield isochron ages in good agreement with previous dates (K/Ar and 40Ar/ 39Ar) from other locations. One
sample date out of stratigraphic order is interpreted to be the result of incomplete removal of the glass matrix adhering
to the phenocrysts, thereby providing material more susceptible to radiogenic argon loss due to hydration and
submicroscopic devitrification than the phenocrysts (McDougall and Harrison, 1988).
3. Detailed observations and descriptions such as presented here are important for understanding the
regional geologic history of the PVP and its relationship to the evolving PAC-NAM plate boundary, and as a basis
for geologic tie-points across the Gulf of California. Geochronology from Santa Isabel Wash constrains ENE-directed
extensional deformation associated with the GEP to be post-6 Ma in age and supports the notion that the Matomí
accommodation zone is a pre-6 Ma feature. This work has provided the basis for several structural models relating
PVP deformation to the evolution of the Gulf of California spreading center system (Nagy, 1997; Nagy and Stock, in
prep. for J. Geophys. Res.). Additionally, the results confirm lithologic correlations made in several paleomagnetic
23
studies which document vertical-axis rotational deformation in the region (Lewis, 1994; Nagy, 1997; Lewis and Stock,
in review for J. Geophys. Res.; Nagy, unpub. data, in prep. for J. Geophys. Res.; Stock et al., in prep. for J. Volcan.
Ge otherm. Res.).
Acknowledgments
E. A. N. thanks M. A. House, A. J. R. Kent, C. J. Lewis, and X. Y. Quidelleur for helpful discussions and reviews of
this manuscript. X. Y. Quidelleur and K. D. Mahon are also acknowledged for providing software employed to
analyze the 40Ar/ 39Ar results. C. J. Lewis is thanked for providing assistance with Figure 1. 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.
24
Appendix 1. 40Ar/39Ar 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
Cs source for 4-5 days (receiving 1.06 Mrad/day of γ-radiation) was employed to aid in identifying
137
plagioclase, alkali feldspar and quartz contamination (see Rose and others, 1994). Most samples consisted exclusively
of plagioclase with the exceptions of Tmrsf (anorthoclase and sanidine), Tmr3 (anorthoclase), and Tmrsiw, Tmrao ,
and Tmrec (plagioclase and anorthoclase). See Nagy (1997) for detailed mineral separation techniques.
Feldspar concentrates were wrapped in Al foil and sealed under vacuum in quartz ampoules with aliquots of
FCT-1 Fish Canyon Tuff sanidine (27.8 Ma; Cebula and others, (1986)), and an internal standard (anorthoclase from
Tmr3) interspersed at 1 cm intervals . Samples were irradiated in either one of two sessions in the L67 position of the
Ford Reactor (University of Michigan). Samples from the first irradiation (designated UofM#75) received a three hour
dose while those from the subsequent irradiation (designated UofM#81) 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 anorthoclase, individual plagioclase analyses generally required 10-30
grains to obtain a measurable signal. In some instances incremental 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 prior to fusion with a focused, 5 W beam (see Appendix available from
http://oro/PVP/santa_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 and others (1992)
found this kind of “low temperature cleaning” useful for removing nonradiogenic 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
250°C (50 l/s) and 400°C (10 l/s). Gas transfer was via expansion with the total quantity of
39
Ar 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
40
Ar sensitivity of 1.85 x 10-17 mol/mV.
Atmospheric Ar measurements performed throughout the analyses indicated a mass discrimination value of 0.994 ±
25
0.001 per amu. Representative total system blanks for a typical procedure involving 1-3 minutes laser heating of ~0.51.5 mg aliquots of plagioclase and 5-10 minutes gettering were: 5 x 10-16 moles, 2 x 10-18 moles, 1 x 10-18 moles, 1 x 10-17
moles, and 7 x 10-18 moles for m/e 40 through 36, respectively.
26
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31
Figure Captions
Figure 1. Simplified geologic map of a portion of northeastern Baja California, Mexico, (modified from Gastil et al.,
1975) showing location of study area (dashed box labeled Figure 2) in the northern Sierra Santa Isabel. The San Pedro
Mártir fault (SPMf) marks the western edge of the Gulf Extensional Province north of the Puertecitos Volcanic
Province (PVP). The pre-6 Ma Matomí accommodation zone (A-A’ after Stock (in press)) separates a region of greater
extension to the north from the less extended area to the south. VSFf (Valle de San Felipe fault); SSFf (Sierra San
Felipe fault); SIf (Santa Isabel fault); MC (Mesa Cuadrada); MT (Mesa El Tábano).
Figure 2. Simplified geologic map of the study area (informally named Santa Isabel Wash) in the northern Sierra Santa
Isabel (location in Figure 1). See Nagy (1997) for 1:20000 scale geologic map and cross-sections. Informal names
given here include the Santa Isabel Wash, Arroyo Oculto, and the Cuervo Negro and Santa Isabel faults.
Stratigraphic column includes 40Ar/ 39Ar ages determined in this study (see Tables 1 and 2). One anomalously young
sample age for Tmrsiw, interpreted to be the result of glass contamination, is not shown. The stratigraphic
relationship between Group 7 lava flows is not apparent in the field except that Tmrcan clearly overlies Tma gem. Fault
symbols: dashed faults are approximately located, dotted faults are concealed (inferred), and faults with the letter “s”
along them are features in the Quaternary alluvium interpreted to be fault scarps.
Figure 3. Isochron plots of all samples (a-v) from 40Ar/ 39Ar geochronology. Error bars on individual data points are ±
1σ. Isochron ages (± 2σ) are also given. Letters in plots (q) and (r) (internal standard) refer to different splits of our
INF-94-53 anorthoclase internal standard that were distributed throughout both irradiations (see Table 1). Samples
labeled “gamma” were exposed to a 137Cs γ-radiation source to facilitate mineral separation. All samples of the internal
standard from irradiation UofM#81 were also exposed to γ-radiation prior to neutron bombardment.
Figure 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. Ages are ± 2σ. Solid lines indicate preferred
correlations; dashed lines show unresolved ambiguities.
32
Figure 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. Locations are identified in Figure 1; the study area
of Martín-Barajas and others (1995; labeled NE PVP) includes the region between Arroyo Matomí and Arroyo Los
Heme from the coast to ~10 kilometers west. In some cases the stratigraphy from other studies has been simplified by
listing only the lithologic units relevant to the discussion. Stratigraphic order is preserved in all cases. Ages are ± 2σ.
The date with the “*” is from the same Tmr3 mineral separate analyzed here measured in a different
40
Ar/ 39Ar
laboratory (Appendix C available from http://oro/PVP/santa_isabel.html; see also Nagy, 1997). Solid lines indicate
preferred correlations; dashed lines show unresolved ambiguities. Note that units t3, t4, t2u, and t2l in Arroyo
Matomí are defined as Mpru by Stock (1989, 1993). Stratigraphic order of some Group 7 units is ambiguous.
33
Table Captions
Table 1. 40Ar/ 39Ar results from an internal standard (INF-94-53 anorthoclase from Tmr3). All ages are calculated using
Fish Canyon sanidine with an assumed age of 27.8 Ma (see text for additional details). With the exception of the
result obtained from MIT, inverse isochron parameters were calculated using routines discussed in Mahon (1996).
The expected range for MSWD values applies to the 95% confidence level (see Mahon, 1996). Corresponding
isochron plots are shown in Figure 3 (q and r). Note that regressions were performed only for individual splits of INF94-53 because of variations in J-factors. Mean ages are provided for each irradiation. Note that results from the INF94-53 split exposed to 137Cs γ-radiation to aid in mineral separation prior to irradiation UofM#75 are statistically similar
to those obtained from adjacent samples.
Table 2. 40Ar/ 39Ar results obtained from feldspars from the Santa Isabel Wash area. See Table 1 caption for details on
inverse isochron analysis . Corresponding isochron plots are shown in Figure 3. Note that combined electron probe
and calculated bulk Ca/K from the
mineral concentrates.
40
Ar/ 39Ar analysis have been employed to determine the relative purity of the
Table 1. 40Ar/ 39Ar results from internal standard (Tmr3: INF-94-531 anorthoclase) from Santa Isabel Wash
Sample
J-Factor
# of aliquots
included in
calculation
over total #
Inverse
Isochron
Age (Ma)
(± 2σ)
0.003480
10/10
UCLA
UofM#75
(tube 1)
INF-94-53A
0.0004178
INF-94-53B
INF-94-53C
MIT
INF-942,3
53
INF-94-53γ
3
UofM#81
(tube 1)
INF-943
53A
INF-943
53B
INF-943
53C
(tube 2)
INF-943
53D
INF-943
53E
3
INF-94-53F
40
39
Ar/ Ar
40
36
Ar/ Ar
MSWD
95% critical
MSWD range
(after Mahon
(1996))
(± 1σ)
(± 1σ)
6.4 ± 0.2
1.02± 0.02
294 ± 6
1.71
0.27-2.19
6/6
6.3 ± 0.3
293 ± 2
4.80
0.12-2.78
0.0004215
5/5
6.2 ± 0.3
291 ± 1
1.00
0.07-3.12
0.0004174
5/5
6.2 ± 0.3
292 ± 3
3.19
0.07-3.12
0.0004191
4/4
6.2 ± 0.3
8.37 ±
0.07
8.23 ±
0.06
8.29 ±
0.06
8.22 ±
0.07
292 ± 2
1.12
0.03-3.69
0.0008170
4/6
6.4 ± 0.6
287 ± 4
1.29
0.03-3.69
0.0008275
5/5
6.7 ± 0.4
283 ± 5
2.27
0.07-3.12
0.0008059
5/5
6.5 ± 0.3
4.38 ±
0.18
4.50 ±
0.11
4.50 ±
0.05
287 ± 2
0.33
0.07-3.12
0.0008066
4/4
6.5 ± 0.3
287 ± 2
0.73
0.03-3.69
0.0008097
5/5
6.4 ± 0.3
289 ± 2
0.18
0.07-3.12
0.0007928
2/2
6.4 ± 0.3
4.48 ±
0.07
4.37 ±
0.05
4.50 ±
0.05
288 ± 2
N/A
N/A
4
UofM#75
4
UofM#81
Overall
# of duplicate
splits included
in calculation
over total #
Weighted
Mean Age
(Ma)
(± 2σ)
MSWD
95% critical
MSWD range
(after Mahon
(1996))
4/4
6/6
10/10
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
1
UTM coordinates of Sample Location (north, east): 3362 905mN 0701 410mE
2
MIT Results recalculated for consistency using Fish Canyon sanidine age of 27.8 and isochron regression routines of Mahon (1996)
3
Sample irradiated with γ radiation
4
Regressions performed only for individual splits of INF-94-53 because of variations in J-factors. Mean ages provided for each irradiation.
Table 2. 40Ar/ 39Ar results from feldspars from Santa Isabel Wash
Group
Map
Unit
Location in
UTM coordinates
Sample
3
4
Mineral
Bulk
Ca/K
J-Factor
5
(north, east)
7
Tmrcan
3367 803mN
0709 990mE
CAN-95-11
Oligoclase
4.0
0.0008020
7
Tmrcan
3367 705mN
0709 350mE
CAN-95-12
Oligoclase
4.9
0.0008040
3366 006mN
0704 410mE
LCR-94-110
3362 808mN
0701 230mE
INF-94-75
6
6
Tmrao
2.1
7
Anorthoclase
6
0.52
0.0008087
0.0004178
Tmrsiw
3362 507mN
0703 520mE
INF-94-47
Oligoclase
3.9
0.0004191
6
Tmrsiw
3365 004mN
0698 680mE
INF-94-33
Oligoclase
4.5
0.0004184
3365 307mN
0700 800mE
CHO-94-45
3365 501mN
0699 430mE
SUN-94-30
6
Tmrsiw
Tmrsiw
Oligoclase
Oligoclase
4.5
5.2
0.0004179
0.0008084
5
Tma toro
3362 507mN
0703 080mE
INF-94-68
Labradorite
43
0.0004208
4
Tmrsf
3365 807mN
0697 890mE
LCS-94-72
Anorthoclase
0.17
0.0008102
3362 807mN
0703 590mE
DIN-94-46
3363 300mN
0701 560mE
INF-94-52
3
3
Tmdtomb
Tmdtomb
Andesine
Andesine
14
5.7
0.0004218
0.0004202
3
Tmdtomb
3364 403mN
0698 280mE
INN-94-73
Andesine
17
0.0004211
3
Tmdtomb
3365 004mN
0699 240mE
WIN-94-32
Andesine
14
0.0004220
3367 102mN
0702 350mE
PHO-94-10
3365 305mN
0703 740mE
LCR-95-08
3
3
Tmdtomb
Tmdtomb
Andesine
Andesine
13
29
0.0004218
0.0008298
3
Tmdtomb
3366 607mN
0702 180mE
PHO-94-86
Andesine
19
0.0008298
3
Tmblol
3361 806mN
0702 040mE
INF-95-24
Andesine
15
0.0008120
3362 309mN
0699 240mE
KC-95-19
3362 004mN
0698 530mE
KC-95-17
3
2
Tmbkc
Tmrbio
Irradiation UofM#75
Irradiation UofM#81
3
Based on representative electron microprobe analysis
2
Oligoclase
6
6
1
Tmrec
6
Andesine
Andesine
14
16
0.0008102
0.0008203
2
2
2
1
1
1
1
2
1
2
1
1
1
1
1
2
2
2
2
2
# of
aliquots
included in
calculation
over total #
Inverse
Isochron
Age (Ma)
(± 2σ)
17/18
6/7
40
39
Ar/ Ar
40
36
Ar/ Ar
MSWD
95% critical
MSWD
range (after
Mahon
(1996))
(± 1σ)
(± 1σ)
6.0 ± 0.4
4.15 ± 0.12
268 ± 15
1.21
0.42-1.83
5.9 ± 0.8
4.07 ± 0.28
287 ± 9
1.89
0.12-2.78
6/9
6.1 ± 0.5
4.20 ± 0.15
286 ± 4
0.45
0.12-2.78
10/13
6.4 ± 0.3
8.45 ± 0.04
287 ± 1
7.77
0.27-2.19
9/9
6.5 ± 0.3
8.59 ± 0.12
293 ± 5
1.94
0.24-2.29
8/8
6.6 ± 0.5
8.81 ± 0.29
289 ± 1
0.56
0.21-2.40
8/8
6.7 ± 0.3
8.94 ± 0.09
281 ± 4
0.98
0.21-2.40
16/18
5.7 ± 0.4
3.92 ± 0.12
285 ± 2
2.39
0.40-1.86
13/13
9.3 ± 0.8
12.24 ± 0.50
288 ± 5
4.29
0.35-1.99
8/8
12.7 ± 0.5
8.69 ± 0.07
285 ± 11
0.45
0.21-2.40
8/8
15.5 ± 0.7
20.48 ± 0.25
287 ± 3
2.92
0.21-2.40
12/12
15.9 ± 0.7
21.09 ± 0.15
286 ± 2
1.24
0.33-2.05
8/8
15.9 ± 1.2
21.03 ± 0.64
289 ± 3
1.01
0.21-2.40
8/8
16.1 ± 0.8
21.30 ± 0.28
280 ± 4
0.74
0.21-2.40
8/8
16.2 ± 0.8
21.32 ± 0.25
267 ± 11
5.02
0.21-2.40
5/5
16.7 ± 1.0
11.21 ± 0.25
277 ± 12
0.64
0.07-3.12
13/13
16.4 ± 1.4
11.02 ± 0.42
271 ± 8
1.36
0.35-1.99
14/15
16.3 ± 1.0
11.18 ± 0.28
298 ± 7
1.96
0.37-1.94
15/18
17.1 ± 2.2
11.77 ± 0.73
273 ± 7
2.11
0.39-1.90
13/13
17.1 ± 2.4
11.63 ± 0.79
287 ± 2
1.18
0.35-1.99
4
40
39
Combined electron microprobe and calculated bulk Ca/K from the Ar/ Ar analysis used to determine the relative purity of the mineral concentrates
5
Calculated assuming 27.8 Ma for Fish Canyon sanidine
6
Anorthoclase also present
7
Plagioclase also present
115° 00' W
Santa Rosa
Basin
0
10
kilometers
SIERRA SAN
FELIPE
A
SIERRA SAN
FERMIN
MC
30° 30' N
MT
A'
Figure 2
SIf
CNf
Arroyo
Matomi
A"
Puertecitos
Arroyo Los Heme
Volcan Prieto
pre-Cenozoic Tertiary
Quaternary
Puertecitos Volcanic Province
MGE
Alluvium
and gravel
AREA
OF
MAP
GEP
SJ
SLT
Sonora
Post-batholithic
volcanic and
sedimentary rocks
Isla
Tiburón
Batholithic rocks
and prebatholithic
metasedimentary
rocks
Sinaloa
Figure 1
20
N
Geologic Map of Santa Isabel Wash
A
B
D
E
F
G
6
5
7
6
5
4
I
J
K
L
M
N
Tmrfp
Tmrbs
Tmrec
Tmrao
Tmr4
Tmr3
Tmbnew
Tmrsiw
Tmatoro
O
A'
P
Q
R
S
T
U
V
1
P
ca
icho
Can
oel
s
3
s
4
s
Tmrcan
5.9±0.8, 6.0±0.4
Tmagem,(Tmaugl,Tmahem)
s
s
5
6
s
s s
6.1±0.5
s
7
s
8
1
0
6.4±0.3
9
?
4
6.2±0.1, 6.5±0.2
6.5±0.3-6.7±0.3
B"
10
11
12
4
9.3±0.8
13
Tmrsf
12.7±0.5
Tmdtomb
15.5±0.7-16.7±1.0
Tmblol,Tmbkc
16.3±1.0,
2
Tmrbio
Tmvs
17.1±2.4
1
Pz,Mzg
3
H
2
B'
σ Ma
Age (±2 ) in
Lithologic
Group:
C
17.1±2.2
14
Cuervo
Negro
Fault
Santa
Isabel
Fault
A"
coi
P
LosHeme
15
16
17
18
19
N
20
1km
Figure 2
Early to Late Miocene stratigraphic correlations
Ma
8.9 ± 0.6h
Sierra Pinta
[McEldowney, 1970;
Gastil et al., 1979]
Group5
Tmr1
14.2 ± 0.9w
12.5 ± .02* Group4
Santa Rosa Basin
[Gastil et al.,
1979; Bryant, 1986;
*from Stock et al., 1996]
Tma
15.9 ± 0.1
16.3 ± 0.1
Arroyo Los Heme
[Martín-Barajas
et al., 1995]
Tmatoro 9.3 ± 0.8
Tmrsf
12.7 ± 0.5
15.5 ± 0.7 to
Tmdtomb 16.7 ± 1.0
Ma2
Mr1
12.0 ± 0.5a
(10.9 ± 0.3)
11.8 ± 0.7
Tmr1
(10.6 ± 0.1)
13.3 ± 0.5
T4
9.9 ± 1.3h
T4
10.9 ± 2.3p
15.3 ± 1.3h
unnamed
ignimbrite#
T4
11.3 ± 1.2p
T4
12.3 ± 2.9p
unnamed
ignimbrite#
La Morita
deposits
Tmblol
16.3 ± 1.0
Mbsu,
Mb4
Tmbkc
17.1 ± 2.2
Mtt
Tmrbio
17.1 ± 2.4
Mb3
17.0 ± 0.3w
pre-17 Ma
Mvs
~17-13b,w
Tmvs
Tma
14.5 ± 0.2w
Tmb2
~21-14p,b
T2
Santa Isabel Wash
[Nagy, 1997; this study]
Tmvs
~21-11
Sierra San Fermín
[Lewis, 1994, 1996]
Southern Valle Chico
and Sierra San Felipe
[Stock, 1989]
La Noche
~17-16
dacite
Comondú
T2
60 km S of Puertecitos
[Dorsey and Burns, 1994]
Sierra Juarez
[Lee et al., 1996]
~23-15p,h
Sonora
[Gastil and Krummenacher,
1977; # Stock et al., this volume]
Isla Tiburón
[Gastil and Krummenacher,
1977; # Stock et al., this volume]
Geochronology: dates listed in parentheses considered unreliable by associated authors; hK/Ar (hornblende), wK/Ar (whole rock), aK/Ar (anorthoclase), pK/Ar (plagioclase), bK/Ar (biotite);
all others 40Ar/39Ar.
Figure 4
Latest Miocene to Pliocene stratigraphic correlations
Tmahem
Tmrcan
5.9 ± 0.8, 6.0 ± 0.4
Tmagem
Tmaugl
Tba
f4
t3
t4
t2u
t2l
Tpa
(5.7 ± 0.2)
Tpet
3.1 ± 0.5
Tmr9
6-8
5.1 ± 0.3
Tmrfp
Tmr8
Tmru
Tmrbs
t9
Tmrec
6.1 ± 0.5
t12
Tmrao
6.4 ± 0.3
t14
5.8 ± 0.1
Tmr7
6.4 ± 0.3
Tmc(1-6)6.4 ± 0.1
Tmr6
Tmr5
Tmr4
Tmr4
Tmr4
Tmr3-4
Tmr3b 6.5 ± 0.2, 6.7 ± 0.2
Tmr3 (II)
Tmr3a
Tmr3 (II)
Tmr3b
Tmr3a
Mr4
Tmr3 (II) 6.2 ± 0.1, 6.5 ± 0.2, *6.4 ± 0.2
Mr3
Tmr3 (I)
Mmt
Tmmt
6.2 ± 0.2a, 6.1 ± 0.2a, (4.3 ± 0.1)a
Tmbnew
Tmrsiw
Arroyo Matomí
[Stock et al., 1991]
(5.7 ± 0.4), 6.5 ± 0.3, 6.6 ± 0.5, 6.7 ± 0.3
Santa Isabel Wash
[Nagy, 1997; this study]
Mesa El Tábano
[Stock et al., 1991]
Northeastern PVP
[Martín-Barajas
et al., 1995]
Sierra San Fermín
[Lewis, 1994, 1996]
Geochronology: dates listed in parantheses considered unreliable by associated authors; aK/Ar (anorthoclase); all others 40Ar/39Ar.
Figure 5
Southern Valle Chico
and Mesa Cuadrada
Mesa Cuadrada
[Stock, 1989]
[Lewis and Stock, 1998a]
Pre-6 Ma
NE- to ENE-directed extension
southwestern margin of
Matomí accommodation zone
N
A'
~ 12.5 Ma tuff
relatively
undeformed
region
withinGEP
regionof
deformation
A"
?
?
Post-6 Ma
ENE- to E-directed extension
W-dipping volcanic rocks
in hanging wall of
major E-dipping faults
N
Present-day
Santa Isabel Wash
~ 6-6.5 Ma tuffs
~ 12.5 Ma tuff
slickenlines
with 60-80° rake
A"
?
Synthetic and antithetic
structures in hanging wall
of major E-dipping faults
Figure 6
pre-6MaMatomí
accommodationzone