Major ignimbrites and volcanic centers of the Copper Canyon area: A view into the core of Mexico’s Sierra Madre Occidental

Eric R. Swanson* Department of Earth and Environmental Science, The University of Texas at San Antonio, San Antonio, Texas 78249, USA Kirt A. Kempter 2623 Via Caballero del Norte, Santa Fe, New Mexico 87505, USA Fred W. McDowell Department of Geological Sciences, The University of Texas at Austin, Austin, Texas 78712, USA William C. McIntosh New Mexico Geochronology Research Laboratory, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA

ABSTRACT ing picture is one of a dramatically thickened (see Nieto-Samaniego et al., 1999; Ferrari et and stratigraphically complex volcanic sec- al., 2002), the Durango-Mazatlán highway in Reconnaissance mapping along Copper tion related to numerous overlapping caldera the central part of the range (see McDowell and Canyon highway has established ignimbrite complexes, much like that documented for Clabaugh, 1979), and the El Paso–Chihuahua stratigraphic relationships over a relatively the core of the San Juan Mountains volcanic and Chihuahua-Hermosillo highways of the large area in the central part of the Sierra fi eld, Colorado. northern Sierra Madre Occidental (see McDow- Madre Occidental volcanic fi eld in western ell and Mauger, 1994; Swanson and McDowell, Chihuahua, Mexico. The oldest ignimbrites Keywords: calderas, ignimbrite, Sierra 1985; Cochemé and Demant, 1991; McDowell are found in the central part of the area, and Madre Occidental, volcanism, Mexico. et al., 1997). they include units previously mapped from These studies, and others, have shown that north of the study area, in and around the INTRODUCTION the Sierra Madre Occidental volcanic fi eld con- Tomóchic volcanic complex. Copper Canyon, sists of a relatively unfaulted central core of at the southern end of the study area, exposes The Sierra Madre Occidental Volcanic Field Tertiary volcanic rock, framed by NNW-trend- younger units, including the intracaldera tuff ing normal faults that enter Mexico from the of the Copper Canyon caldera and fi ve over- The Sierra Madre Occidental volcanic fi eld Basin and Range Province of the southwest- lying ignimbrites. Well-exposed calderas are (Fig. 1), from the United States–Mexico border ern United States, diverge around the core of found near San Juanito, in the central part to its intersection with the younger Mexican the Sierra Madre Occidental, and recombine at of the map area, and at Sierra Manzanita, to volcanic belt, covers at least 296,000 km2 of the southern end of the mountain range (Henry the far north. Stratigraphic evidence for yet western Mexico and is composed of lava and and Aranda-Gómez, 1992; Ferrari et al., 2002). another caldera in the northern part of the ignimbrite related to an estimated 350 major Several periods of faulting have been identi- area is found in the Sierra El Comanche. The calderas (Swanson and McDowell, 1984). If fi ed (Nieto-Samaniego et al., 1999; Henry and stratigraphic and limited available isotopic exposures of equivalent age in southern Mex- Aranda-Gómez, 2000; Ferrari et al., 2002). age data suggest that volcanism was particu- ico are considered, the volcanic cover grows Studies, mostly from the west side of the larly active ~30 m.y. ago. This reconnaissance to ~393,000 km2 (Aguirre-Díaz and Labarthe- Sierra Madre Occidental, reveal that western survey also documented lava-fl ow litholo- Hernández, 2003). Although the general aerial Mexico has experienced a lengthy, subduction- gies consistent with previous observations extent and dominantly silicic composition of the related magmatic history (see Roldán-Quintana from Tomóchic that intermediate lavas have Sierra Madre Occidental volcanic fi eld has been et al., 2003; Ferrari et al., 2005). This includes erupted throughout that area’s volcanic his- known for more than a century (Ordóñez, 1896), the emplacement of Cretaceous to early Tertiary tory and that basaltic andesite became par- detailed geologic maps showing ignimbrite cool- batholithic rocks and the widespread eruption ticularly abundant as felsic volcanism waned. ing units and calderas did not appear until the of the coeval, but relatively unstudied, andesite The combined Copper Canyon–Tomóchic 1970s (Swanson et al., 1978). Since that time, and rhyolite informally known as the lower vol- area gives the fi rst view into the core of the a series of “discovery-phase” mapping projects canic complex (McDowell and Keizer, 1977). giant Sierra Madre Occidental volcanic fi eld, have progressed, mainly along the major access Subduction-related volcanism culminated with expanding that offered by earlier reports, routes crossing and fl anking the area. Notably, massive outpourings of mid-Tertiary volcanic mostly from peripheral regions. The emerg- these are the Guadalajara-Zacatecas and Gua- rocks, mostly ignimbrite (informally the upper dalajara-Fresnillo highways and connecting volcanic supergroup of McDowell and Keizer, *E-mail: [email protected]. roads in the southern Sierra Madre Occidental 1977), which were emplaced toward the end

Geosphere; May 2006; v. 2; no. 3; p. 125–141; doi: 10.1130/GES00042.1; 8 fi gures, 2 tables, Data Repository 2006119.

For permission to copy, contact [email protected] 125 © 2006 Geological Society of America

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W 110° W 100° CA AZ NM TX

El Paso

N 30°

Hermosillo Figure 1. Major exposures of Chihuahua Tertiary volcanic rocks of the Tomóchic Sierra Madre Occidental volca- Study nic fi eld and of adjacent regions Creel Area in Baja California, Arizona, New Mexico, Texas, and in Sierra Madre Occidental southwestern Colorado (inset volcanic field Hidalgo at the same scale). Outcrop Los del Parral pattern is adapted from Swan- Mochis son and McDowell (1984), and modifi ed by information from Durango Ferrari et al. (2002). CO Zacatecas San Juan Mazatlan UT volcanic field

km Guadalajara N 20° NM 0 400

of Farallon–North America plate convergence. exposure of the overriding plate to hotter asthe- Western Chihuahua Ignimbrite volcanism in the core of the volca- nospheric mantle (Ferrari et al., 2002). nic fi eld began as early as 38 Ma (Wark et al., All current tectonic and petrogenetic interpre- Mexican national highway 16 (here called the 1990), and volcanism appears to have become tations rely on the “discovery-phase” mapping Chihuahua-Hermosillo highway) has previously extremely voluminous just prior to abruptly ter- begun in the 1970s. While much progress has served as the locus for a series of mapping proj- minating over most the Sierra Madre Occidental been made toward understanding the origin of ects across the northern part of the Sierra Madre volcanic fi eld at 28 Ma. Younger ignimbrites rocks of the Sierra Madre Occidental, we esti- Occidental volcanic fi eld. These include the (ca. 24–21 Ma), however, are found in a belt mate that more than 90% of this great volcanic Tomóchic area (Fig. 2) in the core of the range, along the western fl ank of the volcanic fi eld that fi eld remains unmapped and that fewer than 10% where two overlapping calderas and six major expands southward to cover much of the south- of its calderas have been identifi ed. Mapped ignimbrite formations with K-Ar ages ranging ern Sierra Madre Occidental (see Ferrari et al., areas in the central Sierra Madre Occidental are in age from 38 to 29 Ma are found (Swanson 2002; Roldán-Quintana et al., 2003). Basaltic particularly sparse, and for over 1000 km along and McDowell, 1985; Kempter, 1986; Wark et andesite lava fl ows, the Southern Cordilleran the length of the fi eld, except for very narrow al., 1990). Wark (1991) also studied the pet- Basaltic Andesite, or SCORBA, of Cameron et transects along the Durango-Mazatlán highway, rogenesis of Tomóchic volcanic center rocks, al. (1989), appear to be widespread throughout the Chihuahua-Hermosillo highway, and along emphasizing a genetic relationship between the range and are commonly found intercalated highways north of Guadalajara, the core of the the large-volume rhyolite ignimbrites and more with, or overlying, each area’s youngest ignim- Sierra Madre Occidental volcanic fi eld is virtu- mafi c lithologies, the dominate role played by brites. Mechanisms explaining the timing, pet- ally unknown. This paper presents new infor- crystal fractionation, and the temporal relation- rogenesis, and extreme volume of the Sierra mation on ignimbrites and their source calderas ship between rhyolite volcanism and the waning Madre Occidental volcanic fi eld have focused for a large area in the heart of the Sierra Madre stages of Farallon plate subduction. A study of on a change in stress regime from compression Occidental, giving a view into the geologic core the isotopic composition of Tomóchic volcanic to one of extension (Wark et al., 1990) and on of this great volcanic fi eld and providing infor- rocks (McDowell et al., 1999) indicated that a transient thermal event possibly triggered by mation needed for more detailed studies of all Laurentian basement, like that in the south- foundering of the Farallon slab with resulting descriptions. western United States, does not extend under

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W108°00’ W107°45’ W107°30’ 2 16

Tomóchic Rio Papigochic 127

1 Pachera

Sierra N28°15’ Gasachic Rio Terrero 3 Rio Sierra Tomóchic El Comanche Sierra Manzanita La Canoa Rancho Pescaditos Cueva Blanco del Toro 4 Road to El Piloncillo Basaseachic Alamito

River Cordon Cumbre Alta Unpaved road Rio Terrero Figure 2. Location map for the Paved highway N28°00’ Copper Canyon and Tomóchic Train tracks 5 San Juanito areas with numbers (1–6) marking the names and loca- Study Areas tions of the various calderas Northern Section listed in the legend. Sierras El Comanche and Manzanita Central Section Road to Divisadero - San Juanito Carichí Southern Section Divisadero - Creel Bocoyna Previous studies

Caldera boundary (approximate) Calderas Creel N27°45’ 1. Tomóchic 2. Las Varas ?? Panalachi 3. El Comanche 4. Manzanita 5. San Juanito Rio 6. Copper Canyon Conchos 23

N Rio ?? Oteros 6 ?? Divisadero COPPER San CANYON 015 km Rio Road to Rafael Urique Batopilas

the area. Results from a regional isotopic study below the Gulf of California and recorded in Chihuahua-Hermosillo highway, passes 50 km (Housh and McDowell, 2005) are compatible Texas has indicated a crustal thickness of 55 km south of the Tomóchic volcanic center, and with the region being underlain by a Proterozoic for the northern part of the central core of the extends southwestward to the rim of Copper basement and Paleozoic arc sequence accreted Sierra Madre Occidental volcanic fi eld (Bonner Canyon (Fig. 2). Yet another highway is being to the southern margin of the North American and Herrin, 1999). completed between Creel and Batopilas, the site craton during Ouachita convergence. A sur- Chihuahua highway 127 (here called the of the nearest mapped area to the south (Bagby, face-wave study using seismic waves generated Copper Canyon highway) diverges from the 1979). As a result, a large area of the core of

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the volcanic fi eld has become open to geologic Tomóchic areas, and they also reveal the newly (2–4 mm), embayed and broken grains of quartz investigation. Our reconnaissance mapping discovered San Juanito caldera (Fig. 2). The (2–3 mm), biotite (to 1.5 mm), and a minor identifi ed well-preserved calderas in the Sierra rugged northeastern third of the Copper Can- amount of hornblende. The rock is generally Manzanita and near San Juanito (Fig. 2). Evi- yon area contains the Sierras El Comanche and altered, and the pyroclastic nature is not always dence for two other calderas was also found, Manzanita (Fig. 2); each range is dominated immediately apparent at the outcrop. The upper and the area’s ignimbrite stratigraphy has been by a single major ignimbrite overlain by com- 200 m is composed of a white to lavender ignim- correlated with that at Tomóchic. The combined plexly interlayered sedimentary rock and lava. brite locally containing abundant, white, spheru- Tomóchic–Copper Canyon area reveals that the From south to north, therefore, the study area is litically devitrifi ed pumice, some lithic fragments, core of the Sierra Madre Occidental contains naturally segmented into southern (Divisadero- and distinctly fewer total phenocrysts. a stratigraphic section that is typically thicker Creel), central (Creel–San Juanito), and north- Observations from the canyon rim and from and more complex than those known from the ern (Comanche-Manzanita) sections, and the helicopter reconnaissance show that the Copper periphery of the volcanic fi eld. regional stratigraphy is discussed in terms of Canyon tuff with its prominent white top can be these subareas (Fig. 2). The various calderas seen throughout the canyon. Although indicators STRATIGRAPHY will be discussed in a later section. such as caldera collapse megabreccias and moat sedimentary sections are not seen at Mogotabo, Introduction Southern Section: Divisadero to Creel the exposed thickness of 1 km strongly sug- gests that the Copper Canyon tuff lies within its The Sierra Madre Occidental is commonly Divisadero, on the southern side of the nar- source caldera. The Rio Urique, it seems, has cut described as an elevated volcanic plateau, and row divide between the Rio Oteros and the Rio through younger ignimbrites from various other much of the range is plateau-like in form as a Urique (Fig. 4), overlooks Copper Canyon and sources to become entrenched into the intra- consequence of ignimbrite volcanism. The the Rio Urique nearly 1400 m below. Divisadero caldera ignimbrite of what we designate as the sierra plateau, however, does contain signifi cant is also the site of the only previous work in the Copper Canyon caldera (Fig. 2). As commonly local relief. The Copper Canyon area, for exam- area, that of Achim Albrecht, who examined and seen with intracaldera ignimbrites (see Lipman, ple, exhibits extensive plateaus at elevations sampled Copper Canyon rocks as part of a geo- 1984), the densely welded, lower part of the between 2200 and 2400 m, with mountains chemical study aimed at understanding basement unit displays prominent, closely spaced, vertical rising 400–700 m above, and canyons cutting rocks of northwestern Mexico (Al brecht and joints. These generally strike WNW at nearly 1 km or more below these plateaus. Given such Brookins, 1989; Albrecht, 1990; Albrecht et al., right angles to Copper Canyon at Divisadero, relief, and considering that 1 km is a commonly 1990; Albrecht and Goldstein, 2000). Albrecht and it seems probable that this joint system is reported thickness for the silicic volcanic cover, sampled rocks along a trail just outside our map the structural control for the distinctive right- sections exhibiting good stratigraphic informa- area to the south, but the region’s ignimbrite angle turn of the Rio Urique seen just upstream tion might reasonably be expected. Such sec- units are much better exposed along a trail down from Divisadero (Fig. 2). tions, however, are rare. Mountain masses ris- from Mesa Mogotabo (Figs. 3 and 4). ing above the plateau are typically composed of Six distinctive major ignimbrites have been Unit 2 lava sequences or a lava dome complex, but a identifi ed in the 1400-m-thick Mogotabo strati- The Copper Canyon tuff is overlain at the few consist of massive intracaldera ignimbrite. graphic section (Figs. 3 and 4). The basal 1025 m Mogotabo section by a thin (24 m), densely Canyons that dissect the mesas also tend to consists of an extraordinarily thick ignimbrite welded ignimbrite informally called unit 2 reveal only single thick ignimbrites or volcani- informally named the Copper Canyon tuff. Five (Figs. 3 and 4). A basal vitrophyre, ~1 m in clastic sequences, possibly fi lling older caldera different conformable ignimbrite units with thick- thickness, passes upward to a red, eutaxitic structures. Sections exposing multiple, diverse nesses typical of outfl ow units constitute most of ignimbrite with a moderate percentage (~25%) ignimbrite outfl ow units are sparse in the Cop- the upper 375 m of the section. These units are of phenocrysts, which consist of relatively large per Canyon area. It may be that this is typical of exposed near Divisadero as two relatively thin plagioclase (2–3 mm), fairly abundant clino- the central axis of the Sierra Madre Occidental, layers, two relatively thick overlying units, and pyroxene, and minor hornblende phenocrysts but it is unlike fringing regions such as Durango a densely welded capping tuff, of which only a enclosed in reddish shards. Lithic fragments and (Swanson et al., 1978) or central Chihuahua (see thin remnant remains along the mesa rim (Fig. 3). small pumice are fairly common, and the rock McDowell and Mauger, 1994), where extensive The Mogotabo section is described from the base remains densely welded to its top. regions display “layer-cake” stratigraphy. The upward in the following sections. most diverse stratigraphic sections currently Unit 3 known from the core of the northern sierra are Unit 1—Copper Canyon Tuff The third ignimbrite up from the base of the along the eastern topographic margin of the The Copper Canyon tuff is presently known Mogotabo section is ~60 m thick and grades Tomóchic caldera (Swanson and McDowell, only from the depths of Copper Canyon, where from a densely welded, reddish brown, cliff- 1985; Wark et al., 1990) and near Divisadero, an 800 m thickness (base not exposed) of dark, forming rock to a soft, white top seen forming were Copper Canyon slices 1.4 km into the cliff-forming ignimbrite grades upward to a thick, a slope between hard ignimbrite ledges (Fig. 3). sierra plateau (Fig. 3). whitish top, forming a total exposed thickness of The rock contains ~20% phenocrysts, mostly From Divisadero’s excellent exposures north- ~1 km (Fig. 3). Although indications of layer- plagioclase (to ~3 mm) and biotite (to ~2 mm), ward to the village of San Juanito, two-thirds of ing and cooling breaks can be seen in cliff-face set in distinctively reddish shards. Small pumice the distance across the Copper Canyon area, little exposures, most observations support this simple and lithic fragments are common. but the upper part of the stratigraphic sequence relationship. A densely welded sample obtained can be seen. Exposures in the central part of the from the trail south of Divisadero is dark red and Unit 4 area, however, do shed light on the stratigraphic extremely crystal-rich (approaching 50%). Its Unit 4 at Mogotabo was measured at 86 m relationship between the Copper Canyon and phenocryst assemblage consists of plagioclase thick. Rock low in the unit is light reddish brown

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E 237238 239 240 241 242 243 244 245 246 247

River Tfl Tst N 3074000 Unpaved road 2400 m Tdt 3073000 Paved highway N E 228 229 230 231 232 233 236000 N 3072000 Creel Train tracks (Chepe) Rio Oteros Valle 3071000 Tfl de Los Monjes <5° Strike and dip 3070000 2200 m MCR-1 3070000 UTM coordinates SJ-42 Valle de Los 3069000 Hongos MCR-1 Age sample 2501 m Estación 2482 m Arroyo Sanchez 3068000 Sonó 2300 m Laguna 2501 m Elevation (in meters above MSL) Arareco 3067000 Tdt Lava phenocryst symbols Arroyo p, b, hbl Puerto San Ignacio p = plagioclase, b = biotite, hbl = hornblende 3066000 Blanco Rio <5° Oteros Ojo del 05 km3065000 Buey <5° Tfl 3064000 p+b 217 218 219 220 221 222 223 224 225 to Cerro Rio 3063000 los Hojitos San Ignacio Cusarare, <5° 2645 m Arroyo Batopilas El Lazo 2435 m El Molino 2000 m 3062000 Tst Recowata p + b +hbl 3061000 <5°

3060000 Pitorreal 7° Tst Arroyo 2000 m 2400 m del 3059000 Ochavado Rio Nacaybo Tdtl Arroyo San Ignacio 3058000 del Lazo Tml Geologic Explanation 3057000 Tst Cerro Tdtu Pelon Mesa Tdtu Tml Fault with bar and ball on downthrown side 3056000 Ahuérabo

Arroyo Tdt Divisadero tuff, undifferentiated 3055000 Tdtu Tst del Lazo Tdtu Upper Divisadero tuff 3054000 Tdtl Magimachi Tst Poorly welded tuffs and sediments 2300 m Tm3 2288 m 3053000 Tfl Tararecua Canyon Tdtl Lower Divisadero tuff Tdtu 3052000 Cerro 1800 m Tfl Felsic lavas Huerarachi 2559 m Tml Mafic lavas 3051000 p + b <5° Tm5 Arroyo Tm5 Tm4 Rurahuachi Unit 5 at Mogotabo - Quemada tuff 3050000 Tm1 2300 m Tm3 Tm4 Unit 4 at Mogotabo 3049000 Divisadero Tm3 Unit 3 at Mogotabo Mesa opper 2307 m 2300 m Mogotabo 1100 m C C 3048000 Trail a Tm2 Unit 2 at Mogotabo Sample CC-03 Tm2 1000 m n collected 2 km south near Río y o Tm1 Unit 1 at Mogotabo - Copper Canyon tuff 3047000 bottom of south trail Urique n

Figure 4. Geologic map of the southern section from Divisadero to Creel. MSL—mean sea level.

to pink, and eutaxitic with blue-gray streaks. rich ignimbrite. Unit 5 is typically pale orange the bright rims to contain fi ne-scale, spherulitic As with unit 3, a poorly welded, white top is or reddish brown (more strongly colored where devitrifi cation surrounding darker pumice cores preserved. The rock has relatively thin, clear densely welded) and grades upward to a poorly that exhibit more coarsely crystalline spherulitic shards, and a low to moderate percentage of welded, light-gray or white top. The rock is or granophyric devitrifi cation. Elsewhere, entire phenocrysts (~20%), consisting mostly of pla- characterized by a low phenocryst content pumice fragments may be either light or dark, gioclase (to 4 mm) and pyroxene (1 mm), with (~10%), consisting of plagioclase and biotite perhaps refl ecting compositional differences. an occasional grain of biotite. Units 3, 4, and 5 set in a groundmass of rather large, thick, clear Unit 5 is overlain at the Mogotabo section by also appear somewhat up-canyon along tributar- shards. Lithic fragments are rare. Where devoid ~90 m of volcaniclastic and pyroclastic rock ies leading toward Copper Canyon (Fig. 4). of pumice, unit 5 is a homogeneous, massive, containing clasts similar to that of the lava fl ow reddish-orange ignimbrite that tends to break dome at nearby Cerro Huerarachi (Fig. 4). Unit 5 along smooth, curved fractures. Pumice, how- Mogotabo’s unit 5 is ~80 m in thickness, and ever, is locally abundant in unit 5, and distinc- Divisadero Tuff: Divisadero to Creel this simple cooling unit is separated from unit tively bright rims commonly outline these pum- The 10 m thickness of Divisadero tuff capping 4 below by ~30 m of soft, underlying, biotite- ice fragments. Thin-section examination reveals the Mogotabo section is but a thin, erosional

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remnant of an extremely widespread unit of great contain a small amount of titanite (typically ing the previously mentioned mesas between local thickness and complexity. A more typical about one grain per thin section), distinguishing Divisadero and Creel, a total road distance of thickness of the ignimbrite along this part of the between the Vista and Divisadero tuffs in the 75 km. It is the region’s most widespread unit, canyon rim is exposed at Divisadero, where the fi eld can pose a problem. At Bocoyna, however, and it will be discussed in more detail later. unit takes its name. There, the Divisadero tuff the Divisadero tuff is clearly seen to overly both Reconnaissance traverses tens of kilometers consists of two ignimbrite cooling units, whitish the Vista and Rio Verde tuffs, establishing it as a southeastward on side roads from Creel toward to pink in color, in which relatively large pumice younger, separate ignimbrite (Fig. 5). Batopilas, from Bocoyna toward Panalachi, and and lithic fragments (including plutonic rocks) Kempter (1986) and Wark et al. (1990) between San Juanito and Carichi (Fig. 2) indi- are found in abundance. The ignimbrite contains described the Rio Verde tuff from the Tomóchic cate that multiple cooling units of crystal-rich ~40% phenocrysts, which consist mostly of siz- area as consisting of four similar-looking mem- ignimbrite form an extensive plateau throughout able (to 4 mm) grains of plagioclase and deeply bers that are typically thin, highly welded and the upper Rio Urique and the upper Rio Con- embayed and broken quartz (to 4 mm), while the with basal vitrophyres. The second-oldest mem- chos river valleys. The unit can be exceptionally hydrous mafi c minerals, biotite and hornblende, ber of the formation was distinguished by its thick. At least fi ve separate cooling units can are also common. plagioclase-pyroxene phenocryst mineralogy, be observed from the mesa overlooking Creel The Divisadero tuff at Mogotabo thins north- its reddish color, and especially by its distinc- to the bottom of the nearby Rio Conchos to the ward and overlies rocks of the nearby Cerro tively vuggy nature. A single cooling unit of Rio south, a vertical drop of 300 m (Fig. 4). The Huerarachi rhyolite fl ow dome. The unit then Verde tuff crops out above the Vista tuff north of Divisadero tuff locally contains intercalated lava increases in thickness and complexity north- Bocoyna, and samples from Bocoyna are indis- or sedimentary rocks. These rocks and the unit’s east toward Creel, as crystal-rich cooling units tinguishable from the second member of the Rio radical local thickness variations show that the both overlie and underlie local felsic fl ow-dome Verde Tuff at Tomóchic. Rio Verde ignimbrite various cooling units were erupted over a sur- rhyolite, minor volcaniclastic sequences, some at Bocoyna, as at Tomóchic, was deposited over face of signifi cant topographic relief caused by poorly welded tuffs, and mafi c lava. Except a surface of signifi cant paleorelief. This is seen erosion, lava-dome emplacement, and probably for these local disruptions, the Divisadero tuff along the Rio de Bocoyna, for example, as the by prior caldera formation as well. Regionally, forms a widespread resistant cap to the ignim- unit thins dramatically southward, pinching out this ignimbrite package thickens dramatically brite section from Divisadero to Creel and in the direction of a felsic fl ow-dome complex eastward from the Copper Canyon highway beyond (Fig. 4). just south of Bocoyna (Fig. 5). into the Conchos and Urique drainage basins (Fig. 2). Central Section: Creel to San Juanito Mogotabo Units 3 and 5 The Divisadero tuff as mapped in this recon- Mogotabo units 3 and 5 are found in the naissance study consists of units sharing a The Copper Canyon highway from Creel to southern moat of the San Juanito caldera. crystal-rich nature and high stratigraphic posi- San Juanito continues the pattern of extensive Unit 3 there has a thickness similar to that at tion. Although we found it impossible to sepa- mesas formed on Divisadero tuff, interrupted Mogotabo, but it displays a thin basal vitrophyre rate them at a reconnaissance level, the vari- locally by rugged topographic highs developed in exposures along the southern fl ank of the San ous Divisadero cooling units display suffi cient on felsic fl ow-dome complexes (Fig. 5). The San Juanito caldera’s resurgent dome. Exposures of variations to suggest that genetically different Felipe, Vista, and Rio Verde tuffs, units known unit 5 are somewhat thinner, less welded, and units may well have been included. Individual from mapping of the Tomóchic area (Kempter, less eutaxitic than at Mogotabo, but they main- Divisadero cooling units, for example, display 1986), are exposed northwest of the San Juanito tain their distinctive low percentage of biotite somewhat different phenocryst minerals, and our caldera (Fig. 5). Exposures, however, along the and plagioclase phenocrysts, and locally their samples fall into three types. Ignimbrite exposed upper reaches of the Rio Conchos (locally called light- and dark-color pumice and pumice with at Divisadero is quartz-rich and contains the the Rio de Bocoyna) are particularly instructive distinctively bright pink rims. Exposures of the mafi c minerals biotite and hornblende, as does because they shed light on the stratigraphic rela- unit are particularly good on Mesa Quemada Divisadero tuff in the San Juanito–Bocoyna tionship between the Copper Canyon and the (Fig. 5), and the unit is henceforth referred to as area. The Divisadero tuff capping the extensive Tomóchic areas (Fig. 5). the Quemada tuff. mesa in the southern section between Creel and There are obvious stratigraphic complexities Arroyo Puerto Blanco (Fig. 4) is similar, except Vista and Rio Verde Tuffs in the Creel–San Juanito area. The Divisadero that pyroxene joins the mafi c mineralogy. This The Vista tuff and overlying Rio Verde tuff tuff, for example, can variously be found Divisadero cooling unit overlies a third phase in described from the Tomóchic area by Swan- directly overlying either the Quemada, the Rio which quartz is not easily recognizable in hand son and McDowell (1985) were interpreted as Verde, or the Vista tuffs. Also, neither the Cop- specimen, biotite is the only mafi c mineral, and having erupted from the Tomóchic volcanic per Canyon tuff nor Mogotabo unit 2 or 4 has the total phenocryst content tends to be lower. complex at 34.1 and ca. 31.5 Ma, respectively been identifi ed in the area, and there is a large Even this type, however, is locally crystal-rich. (Wark et al., 1990). Both units are locally area west of El Ranchito (Fig. 5) that contains a Quartz-poor Divisadero is widely exposed along exposed northwest of the San Juanito caldera very thick ignimbrite that remains uncorrelated the broad mesas around Pitorreal and in Arroyo and along the Rio Bocoyna below yet another with any known units. The Divisadero tuff also Puerto Blanco (Fig. 4), where it and the overly- mesa capped by Divisadero tuff (Fig. 5). The shows considerable internal variations, here and ing quartz-rich variety dip toward and thicken Vista tuff, with its readily visible quartz, abun- throughout its known exposures. dramatically in the direction of Copper Canyon. dant feldspar, biotite, and hornblende, is nearly Some Divisadero tuff, as at San Juanito, is rela- identical in appearance to the Divisadero tuff as Divisadero Tuff tively devoid of lithic fragments and pumice, seen at Copper Canyon and at Bocoyna. While The phenocryst-rich Divisadero tuff forms a while pumice is common elsewhere, and lithics, thin-section examination reveals that some Vista resistant cap for nearly 20 km between Bocoyna including granitoid rocks, can be found to 0.5 m cooling units carry sanidine and that all units and San Juanito (Fig. 5), as well as underly- in diameter.

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Tvt Arroyo Arroyo Tst1 Arroyo Cerro Trvt Lagunita La Luna Bajío Tsft Potrero Blanco E 238 239240 241 242 243 244 245 246 247 248 N3115000 Tfl2 La Til2 Arroyo Pescaditos 3114000 Canoa <5° p+s+ b+ti Tml2 3113000 Rio Arroyo Cueva Tomóchic Pescaditos Cueva del Toro 3112000 del Toro Basoneachic <5° Rio Tct La Luisiana 3111000 Tml dike La Luisiana 3110000 San Pablo Tst Tst1 de la Sierra 3109000 Calaveras Tfl1 Arroyo 3108000 <5° Bajio El Redondo Unpaved Highway Alamito 3107000 to Basaseachic Tst1 Tst1 p + hbl 3106000 Arroyo Meguachic Las or La Cienega Chinacas p + cpx 3105000 Arroyo 2860 m >10° Telefono Til1 3104000 Cordon Cumbre Alta Arroyo Tst Tml2 Machavique Arroyo Tst El Oso 3103000 Tfl1 Arroyo Arroyo Los 3102000 Rio Mascaritas Magueyes San Vicente Tst Tsft Arroyo 3101000 N-63 Yerbanis p + b 3100000 Arroyo Arroyo road to El Ranchito Carichí Yerbanis Setiápachi 3099000 <5° Arroyo El Ochenta Tst del Zorro Tst1 3098000 Tm3 Tml dikes 3097000 Tml1 San Juanito 3096000 Tst4 Tml2 El El Retiro 2600 m Cuervo Arroyo Tst2 Tm5 3095000 El Cuervo Tst3 Tfl1 Rio de Cerro El Ranchito Sitúriachi 2455 m 3094000 Nechúpiachi

Tut Tst3 Arroyo 3093000 San Juanito p + cpx <5° Tm3 3092000 Tm3 2700 m Trvt 3091000 Lagunita Rio de Bocoyna Tst1 3090000 Rio Oteros Mesa del Oso (2600 m) <5° 220221 222 228 229230 231 232 233 234 <5° 223 224 225 226 227 235 3088000 Tdt Surrounding Area Stratigraphy River 3087000 Tml1 Mafic lavas Unpaved road Tfl1 Felsic lavas 3086000 Paved highway Arroyo de 2390 m Tst1 Poorly welded tuffs and sediments Rechagachi 3085000 Trvt Tml2 Mafic lavas Train tracks Sierra de Tdt Divisadero tuff <5° Strike and dip 3084000 Choguita Tvt Tst2 Poorly welded tuffs and sediments Tml1 N-63 Age sample 3083000 Tm5 Quemada tuff (Mogotabo Unit 5) 2600 m Tct Comanche tuff 311 UTM coordinates 3082000 Bocoyna Tm3 Mogotabo Unit 3 Lava phenocryst 2339 m p + hbl 3081000 Tst3 Poorly welded tuffs and sediments symbols Arroyo Tut Unknown tuff Choguita p = plagioclase, ti = titanite, 3080000 Tst4 Sediments and poorly welded tuffs <5° hbl = hornblende, s = sanidine Tst1 road to Trvt Rio Verde tuff b = biotite, cpx = clinopyroxene 3079000 Panalachi, 2479 m Carichí Tvt Vista tuff Structural 3078000 p + b Tfl2 Felsic lavas resurgent dome Intermediate lavas 3077000 Til2 Proposed 2439 m Tfl1 caldera margin(s) Cerro Nechipachi Caldera-Related Stratigraphy 3076000 p + hbl de Choguita <5° Tfl1 Felsic lavas N 3075000 + cpx Tfl1 Rio Conchos Tst Poorly welded tuffs and sediments 3074000 Til1 Intermediate lavas 2400 m 2200 m Tsft San Felipe intracaldera tuff 0 5 km 3073000 Tdt 236000 Tsft San Felipe tuff Creel

Figure 5. Geologic map of the central section from Creel to San Juanito, showing the San Juanito caldera centered on Cordon Cumbre Alta, 15 km northwest of the town of San Juanito.

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These differences aside, Divisadero cooling and with detailed stratigraphic relationships Northern Section: Sierra Manzanita and units typically grade from densely welded red obscured by profound lateral thickness varia- Sierra El Comanche to purple (or lavender) bases to less welded, tions form a resistant ignimbrite cap to mesas brownish or whitish tops. White tops, if pre- from Divisadero to San Juanito (Figs. 4 and 5) The Sierras Manzanita and El Comanche served, are typically massive and poorly veg- and eastward across wide regions of the upper constitute the northeastern limit of the study etated, while brownish tops tend to weather Rio Conchos and Urique drainage basins. Lithic area (Fig. 6), and they may also mark the east- into hoodoo forms like those seen in the bluffs blocks near Copper Canyon locally reach 0.5 m ern margin of the relatively undisrupted struc- around Creel and at the nearby tourist attraction in diameter, and as the area’s youngest major tural core of the Sierra Madre Occidental vol- of the Valle de los Monjes (Valley of the Monks) ignimbrite sequence, its caldera source(s) might canic fi eld. Exposures farther east have been overlooking the Rio Conchos (Fig. 4). be expected to be among the most obvious, but strongly affected by Basin and Range faulting. In summary, multiple phenocryst-rich units we did not discover evidence for that caldera in The lack of ignimbrite diversity has frustrated exhibiting a range of phenocryst mineralogy the study area. efforts toward making regional stratigraphic

E 245 246 247 248 249 250 251 252 253 254255 256 257 258 259 260 261 262 263 264265 266 267 268 269 270

N 3127000 Sierra Gasachic Terrero 2900 m 3126000 San Miguel de Temeychic 3125000 ?? Arroyo <5° 2200 m Yeguachic Rio Rio 3124000 Tfl2 Arroyo Terrero Papigochic Ancho Arroyo Ancho Arroyo Hondo 3123000 Talayotes Javier 3122000 Rojo Gomez

Arroyo 3121000 Bachurichic <5° Temeychic <5° <5° 3120000 Tct 2800 m <5° Arroyo Tst 3119000 El Salto Arroyo Echi 3118000 >5° Sierra Tfl1 El Comanche Tif2 2200 m Arroyo Tfl1 3117000 Arroyo Recubichic Rancho El Muerto Blanco Piloncillo

3116000 Tat Arroyo 3111000 p + s + Hondo Arroyo q + b 3115000 Sierra Majochic Tfl2 Manzanita 2700 m Arroyo Cebolla Tst 3114000 Baragomachic 2685 m Tct Tqt Arroyo Tif1 Ranchito 3113000 El Salto <5° Arroyo Arroyo Ramina Piloncillo Pichachic Arroyo El Alamito El Piloncillo 3112000 Cueva del Toro Tif2 Arroyo Arroyo La Marina 2600 m p + cpx <5° Huahuitare 2460 m 3111000 Arroyo Tif2 Primero El Nogal Tif1 3110000 Arroyo Tst Tif2 Yorachic Arroyo 3109000 El Nogal <5° Rio Terrero El Arroyo Mezteño Tfl1 2600 m Arroyo Las Minas Jarilla 3108000 Rejoyochic 3104000 UTM coordinates Tfl1 Felsic lavas ?? 3107000 Tst Sediments and minor tuffs Arroyo Rivers and arroyos Rejoyochic Unpaved road Til1 Intermediate/felsic lavas N 3106000 Paved highway Tqt Intracaldera Quemada tuff Train tracks Tfl2 Felsic lavas 3105000 Resurgent dome Tif2 Intermediate/felsic lavas Ataros Arroyo Caldera margin Tct El Comanche tuff El Arco 3104000 Strike and dip Tat Alamito tuff <5° 05 km p = plagioclase, s = sanidine, 3103000 p + cpx Lava phenocrysts cpx = clinopyroxene, q = quartz, b = biotite

Figure 6. Geologic map of the northern section covering Sierra El Comanche and the Sierra Manzanita, showing the Manzanita caldera.

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correlations for the Sierra El Comanche, but not TABLE 1. GEOCHRONOLOGY: COPPER CANYON AREA VOLCANIC ROCKS for the Sierra Manzanita. That mountain mass Unit Sample %K %40Ar* 40Ar* Age (Ma) mostly consists of a reddish, vitric ignimbrite (10–6 scc/g) (±1σ) containing the sparse plagioclase-biotite min- K-Ar analyses eralogy, the large, clear shards, and distinctive Felsic lava N 63 6.939 64.9 7.961 29.4 ± 0.5 pumice of the Quemada tuff (Mogotabo’s unit San Juanito caldera rim 59.5 8.010 5). The thickness of the Quemada tuff in the Divisadero tuff SJ 42 6.515 63.2 7.510 29.9 ± 0.7 Sierra Manzanita, together with other evidence 6.494 65.2 7.737 for a caldera (discussed later), identifi es this Divisadero tuff M CR1B 6.94 70 8.08 29.8 ± 0.5 7.05 72 8.27 as its intracaldera facies. The larger and loftier Sierra Comanche contains two ignimbrites of less certain origin, which are informally named Unit Mineral Analysis N MSWD K/Ca Age (Ma) (±2σ) (±2σ) the Alamito and El Comanche tuffs. 40Ar/ 39Ar analyses Alamito and El Comanche Tuffs Copper biotite Plateau 11 1.3 0.1 29.62 ± 0.32 Canyon tuff, Sample 03 hornblende Mean 6 3.6 30.4 ± 53.0 28.84 ± 0.58 The Alamito tuff is the oldest ignimbrite recognized in the Sierra El Comanche. It was Note: Sample locations are shown on area geologic maps (Figs. 4 and 5). K-Ar ages were obtained at the University of Texas at Austin using conventional techniques; Ar was analyzed by isotope-dilution found at two locations, a small exposure along mass spectrometry, and K was analyzed by fl ame photometry or inductively coupled plasma–mass the highway near El Alamito, and an exposure spectrometry (ICP-MS). 40Ar/39Ar ages were obtained at the New Mexico Geochronology Laboratory. See of several kilometers length along an arroyo appendix (see footnote 1) for details. *Radiogenic argon. just upstream from El Alamito (Fig. 6). The MSWD—mean square of weighted deviates; scc—standard cubic centimeters. Alamito tuff is a red to orange-red, eutaxitic, densely welded ignimbrite containing ~25% phenocrysts, including fairly large plagioclase the San Juanito caldera. The Copper Canyon heating of single grains. Initial low-temperature (to 4 mm) and relatively abundant clinopyrox- tuff is the basal unit of six ignimbrite formations steps preferentially degassed atmospheric Ar, so ene, but no biotite. The exposure along the Cop- exposed at Copper Canyon below Divisadero. the high-temperature steps had better yields of per Canyon highway shows the highly distorted A sample from along the south trail down from 39Ar and radiogenic 40Ar (22%–87%). High-tem- foliation typically attributed to postemplace- Divisadero (Fig. 4) was dated by 40Ar/39Ar perature steps from 6 of the 12 analyzed grains ment, downslope rheomorphic fl ow. method at the New Mexico Geochronology with the highest yields of 39Ar and radiogenic The Alamito tuff is overlain by El Comanche Research Laboratory. Samples of Divisadero 40Ar (70%–87%) have a weighted mean age of tuff, which passes upward from a thick basal tuff, the unit which caps the stratigraphic sec- 29.62 ± 0.32 Ma. Although the weighted mean vitrophyre to a brick-red, extremely eutaxitic tion at Copper Canyon and which forms ignim- ages for the hornblende and biotite separates ignimbrite. Thin sections of El Comanche tuff brite-capped mesas across much of the study overlap, we consider the biotite age to be the show distinctively thin shards and fewer pheno- area, were collected near Creel (Fig. 4). The best estimate for the eruption age of the Copper crysts (~20%) than the Alamito, but its mineral- Divisadero samples along with lava collected Canyon tuff, given the apparent alteration of the ogy (plagioclase + clinopyroxene) is the same. El from the San Juanito caldera (Fig. 5) were dated hornblende. However, we note that the biotite Comanche tuff also displays rheomorphic folds. by K-Ar at the University of Texas at Austin. radiogenic yields also indicate some alteration; The Copper Canyon highway from Rancho Age data are summarized in Table 1, and the more precise and accurate 40Ar/39Ar dating of the Blanco to El Nogal (Fig. 6) traverses the southern analytical details are provided in Appendix 1.1 Copper Canyon tuff will require further work. periphery of Sierra El Comanche where it passes Both biotite and hornblende separates from Conventional K-Ar dating was used for bio- through the purple, less-welded, upper part of the Copper Canyon tuff sample were dated by tite separated from the other three samples. El Comanche tuff, which contains compressed 40Ar/39Ar methods. The age spectrum obtained Two ages for the Divisadero tuff were 29.9 pumice fragments to 0.5 m in length. The ignim- by resistance-furnace incremental heating of the ± 0.7 and 29.8 ± 0.5 Ma (±1σ errors). Both of brite dips gently away from Sierra El Comanche hornblende is relatively fl at, but radiogenic yields these ages are in statistical agreement with the to pass beneath a sequence of lavas (rhyolite to are low (7%–32%), and apparent ages of indi- 40Ar/39Ar biotite age. The results suggest that a andesite), various soft tuffs, and volcaniclastic vidual steps are imprecise, probably refl ecting 1500-m-thick section, which includes six major sedimentary rocks fl anking the southeastern incipient chloritic alteration of the hornblende. ignimbrite formations, was emplaced from 29 side of the range. Northwest, into the Sierra El Eleven of the 12 temperature steps, compris- to 30 Ma in the Copper Canyon area. The age Comanche, the unit’s exposed thickness is sev- ing 90% of the total gas, yield a weighted mean of biotite separated from a lava exposed across eral hundred meters, and it is overlain along the plateau age of 28.84 ± 0.58 Ma (±2σ error). The the margin of the San Juanito caldera indicates northern fl ank of the range by an equally thick biotite separate was analyzed by two-step laser that the caldera must have formed prior to 29.4 sequence of lava rock. The thickness of the ± 0.5 Ma, and we present evidence in the next Comanche tuff in this area is suggestive of an section suggesting that the caldera may have 1GSA Data Repository item 2006119, a table show- intracaldera setting (discussed later). ing 40Ar/ 39Ar analytical data for Copper Canyon tuff formed signifi cantly earlier. sample 03 hornblende and biotite, a plot of the incre- Isotopic Age Information mental heating age spectrum for the hornblende, and Area Ignimbrite Correlations and an age probability spectrum for the biotite laser- fusion Stratigraphic Relationships analyses, is available online at www.geosociety. Isotopic age information is reported for org/pubs/ft2006.htm, or on request from editing@ two area ignimbrites, the Copper Canyon and geosociety.org or Documents Secretary, GSA, P.O. At least a dozen ignimbrite formations are Divisadero tuffs, and for a lava fl ow from within Box 9140, Boulder, CO 80301-9140, USA. found in the Tomóchic–Copper Canyon region

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(Table 2). The oldest identifi ed outfl ow ignim- TABLE 2. AREA IGNIMBRITE UNITS brites in the Copper Canyon area are the San Unit Age and minerals dated Phenocrysts* Felipe (36.5 Ma), Vista (34.1 Ma), and Rio Description (Ma) Verde (31.7 Ma) tuffs, previously known from Cueva tuff 29.0 ± 0.8 b, p 5–15% p, s, q, b the Tomóchic area (Fig. 7). The Vista and Poorly welded, light-colored, pumice-bearing, multiple cooling units form massive deposits near Tomóchic. Rio Verde are found unconformably below the Divisadero tuff (and others) in the central Heredia tuff N.D. ~15% p, cpx section (Fig. 5). Divisadero tuff also caps the Strongly welded, red, two cooling units with basal vitrophyres, ~50 m thick, lithics, large phenocrysts, rheomorphic. thick Mogotabo section in the southern sec- tion at Copper Canyon, where neither Vista Divisadero tuff 29.8 ± 0.5 b to 40% p, q, b, hbl, cpx nor Rio Verde tuffs are exposed (Fig. 7). The Strongly welded, red to brown, multiple cooling units, highly variable thickness to more than 300 m, lithics and pumice locally abundant, may lack q and/or cpx, poorly welded top commonly forms hoodoo. Mogotabo section is composed of six con- formable ignimbrites, nearly all of which have Quemada tuff N.D. ~10% p, b preserved, poorly welded tops and gener- Strongly welded, orange or red grading upward to gray, 80 m thick at Mogotabo (Mogotabo unit 5), may show pumice in two colors or pumice with bright rims, large clear shards, erupted from Manzanita caldera. ally lack interlayered sedimentary rocks. The stratigraphic evidence at Mogotabo indicates Mogotabo Unit 4 N.D. ~20% p, cpx, minor b rapid emplacement of units with little interven- Strongly welded, eutaxitic with red-brown and blue-gray streaks, 86 m thick at Mogotabo, thin section shows thin clear shards, resembles the thick, rheomorphic El Comanche tuff at Sierra El Comanche. ing time for either erosion or deposition. The limited available age information supports this Mogotabo Unit 3 N.D. ~20% p, b Densely welded, red-brown grading upward to white, 60 m thick at Mogotabo, distinctively red shards, interpretation and also indicates that the entire lithics and pumice common, basal vitrophyre near the San Juanito caldera. Mogotabo section postdates the Vista and Rio Verde tuffs (Fig. 7). Mogotabo Unit 2 N.D. ~25% p, cpx, minor hbl Strongly welded, red, 24 m thick at Mogotabo, eutaxitic with reddish shards and a basal vitrophyre, lithics The thick Copper Canyon tuff at the base and pumice common, resembles the locally rheomorphic Alamito tuff of Sierra El Comanche. of the Mogotabo section has not been identi- fi ed elsewhere in the area. Biotite-bearing, Copper Canyon tuff 29.62 ± 0.32 b to 50% p, q, b, hbl Strongly welded, red grading upward to prominent white top, some lithics and pumice, known only from Mogotabo units 3 and 5 are both found in the Copper Canyon where a >1 km thickness suggests its source caldera. central section, within the moat of the San Juan- ito caldera, where unit 5 is recognized as the Rio Verde tuff 31.7 ± 0.7 p ~15% p, cpx, b Strongly welded, red, commonly displays two cooling units, ~50 m total thickness, lithophysal cavities and Quemada tuff (Fig. 7). The source for unit 3 is basal vitrophyres are common, previous work suggests Tomóchic caldera as the source. not known, but the Quemada tuff’s intracaldera Vista tuff 34.1 ± 0.9 b, s, p to 50% p, q, b, hbl, s, ti facies is located in the northern section’s Sierra Moderately welded and light gray but can be strongly welded and red, multiple cooling units recognized Manzanita (Fig. 7). The Quemada tuff, then, is locally, rare basal vitrophyre, outfl ow thickness 50–400 m, erupted from Las Varas caldera. the only unit known from all three sections of San Felipe tuff 36.5 ± 1.6 b, p 5%–10% p, b the Copper Canyon strip. Strongly welded, streaky lavender and white, to multiple cooling units and 300 m thick, basal vitrophyre Mogotabo units 2 and 4 each possess a pla- common, extremely eutaxitic, some lithics, resembles the intracaldera tuff of San Juanito caldera. gioclase-pyroxene phenocryst mineralogy, as do Cascabel tuff 38.2 ± 0.6 b to 35% p, s, q, b, cpx the Alamito and El Comanche tuffs of the north- Poorly to moderately welded, lavender, at least 70 m thick with base not exposed, andesitic lithics ern section. The only other known plagioclase- common. pyroxene ignimbrites in the region are some Note: Ignimbrite units are arranged stratigraphically according to present state of knowledge. cooling units of the Rio Verde tuff and the pre- *p—plagioclase, q—quartz, b—biotite, s—sanidine, cpx—clinopyroxene, hbl—hornblende, ti—titanite. viously unmentioned Heredia tuff. Petrographi- N.D.—not determined. cally, the Rio Verde tuff is a poor match for either Mogotabo ignimbrite, and it can also be dis- missed on age considerations. The Heredia tuff, Area Lava Rock of the San Juanito caldera as well as at various informally named by Kempter (1986), is found places within the Manzanita caldera and Sierra near the top of the stratigraphic section in the Lavas ranging in composition from mafi c to El Comanche (Figs. 5 and 6). All of these fel- Tomóchic area, but can also be dismissed on the felsic cover ~30% of the map area (Figs. 4, 5, sic lavas have intersertal textures, and although petrographic grounds that its distinctively large and 6), and reconnaissance east of San Juanito almost all are sparsely populated with phe- plagioclase phenocrysts (to 7 mm) easily exceed indicates that mafi c lavas cover an even higher nocrysts of plagioclase and biotite, a few also those found in units 2 and 4 at Mogotabo. The percentage of the area there. While much of this contain clinopyroxene. Phenocryst-rich, quartz Alamito and El Comanche tuffs, however, are lava clearly fi ts within the framework of known and/or sanidine-bearing lavas are found capping reasonable matches for Mogotabo units 2 and or postulated calderas, older caldera structures the Cerro de la Luna immediately north of the 4, respectively, in terms of phenocryst size and are likely obscured by these late-stage lavas San Juanito caldera, and near the Copper Can- mineralogy and in terms of shard size and color and related pyroclastic deposits. The road to yon highway southwest of Rancho Blanco, both characteristics. In making this tentative correla- Copper Canyon crosses felsic lava domes and of which are located on the fl anks of the Sierra tion, it must be noted that Mogotabo units 2 and related pyroclastic deposits north of San Juan- El Comanche (Figs. 5 and 6). Cerro Neychupia- 4 are separated by a relatively thin ignimbrite of ito, south of Bocoyna, in and around Creel, at chi (Fig. 5), rising some 230 m immediately east plagioclase-biotite mineralogy (unit 3), and that Cerro Los Hojitos, and as it approaches Copper of San Juanito, is composed of less felsic-look- no such unit is presently known from the Sierra Canyon at Divisadero (Figs. 4 and 5). Similar ing, pyroxene-plagioclase–bearing lava erupted El Comanche (Fig. 7). rock is found on the eastern and western sides over the Divisadero tuff. Similar-looking lava is

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Rio

Conchos 127 0 ?? W 107°30’ 16 San Juanito Creel Northern Area ?? Area Alamito Tuff Area Central Quemada Tuff Location Map Comanche Tuff Southern Northern Area ?? Caldera boundary River Highway Train tracks Area see Figure 2 for more detail (Manzanita - Gasachic) N Tomóchic ignimbrite units exposed in each area and ignimbrite units exposed in each area ern area and the northern area’s Alamito and and the northern area’s ern area W 108°00’ N 28°15’ N 27°45’ N 28°00’ ?? ?? Vista Tuff Quemada Tuff Rio Verde Tuff San Felipe Tuff Divisadero Tuff Central Area Mogotabo Unit 3 (Creel - San Juanito) 29.62 ± 0.32 Ma 31 ± 0.7 Ma 36.5 ± 1.6 Ma 29.8 ± 0.5 Ma 38.2 ± 0.6 Ma 29 ± 0.8 Ma 34.1 ± 0.9 Ma Heredia Tuff Mogotabo Unit 5 Mogotabo Unit 4 Mogotabo Unit 3 Mogotabo Unit 2 Tomóchic Area Southern Area (Divisadero - Creel) Cueva Tuff Vista Tuff Rio Verde Tuff Cascabel Tuff San Felipe Tuff Copper Canyon Tuff Divisadero Tuff Figure 7. Regional ignimbrite correlation diagram showing the relative geographic position of the various areas discussed, the geographic position of the various areas diagram showing the relative 7. Regional ignimbrite correlation Figure between units 2 and 4 of the south as discussed in the text. Note tentative correlation correlations the suggested regional El Comanche tuff.

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abundantly found high in the section throughout that its resurgent dome is completely mantled San Juanito (Figs. 5 and 8), the town from which the Sierra Comanche. by younger, probably resurgence-related volca- the caldera takes its name, had been suspected Hornblende-bearing andesite forms a partial nic rocks. With no exposed intracaldera ignim- from earlier remote-sensing observations and cap to the San Juanito caldera’s resurgent dome. brite, the identity of the erupted ignimbrite has reconnaissance work (see Swanson and Wark, Some pyroxene andesite is also found there, and remained problematic. 1988). This study confi rms those observations. precaldera, clinopyroxene-plagioclase andesite All previous published work has related The caldera’s resurgent dome forms the continen- covers a wide area along the northern rim of Tomóchic caldera formation to eruption of the tal divide in this part of the Sierra Madre Occiden- the San Juanito caldera. The situation seems 31.7 Ma Rio Verde tuff. This correlation is based tal, and precipitation there nourishes three major similar to that at Tomóchic, where Wark (1991) primarily on the Rio Verde tuff being the young- river systems (Fig. 2). Rio Tomóchic drains the found that intermediate magmas had been pres- est ignimbrite of any consequence exposed along caldera’s northern moat, fl ows northward through ent throughout volcanic activity at the Tomóchic the caldera’s topographic rim. Yet, caldera resur- the moat of the Tomóchic caldera, then follows volcanic complex. gent doming and ring-fracture volcanism are a torturous path westward to discharge into the Mafi c lava, similar in appearance and strati- dated at ca. 29–30 Ma (Swanson and McDowell, Gulf of California as the Rio Yaqui. Streams graphic position to Southern Cordilleran Basal- 1985; Wark et al., 1990). The only unit overlying draining the caldera’s southern side feed into the tic Andesite (SCORBA) as defi ned by Cameron the Rio Verde tuff near Tomóchic is the infor- Rio Conchos and ultimately discharge into the et al. (1989), is common throughout the Cop- mally named Cueva tuff. While its ca. 29 Ma Gulf of Mexico via the Rio Grande. The calde- per Canyon area (Figs. 4 and 5). Its occurrence age is appropriate for eruption from Tomóchic ra’s western moat is drained by the Rio Oteros, there is like that previously noted at Tomóchic caldera, it was discounted by Swanson (1977) which ultimately joins water fl owing from Cop- (Wark et al., 1990; Wark, 1991), where mafi c as relatively insignifi cant and also by Wark et per Canyon’s Rio Urique to fl ow into the Gulf of lava was found to be particularly abundant al. (1990), who considered it correlative with the California as the Rio Fuerte. high in the stratigraphic section yet still inter- Cascada tuff, which was believed to have erupted The central dome of the San Juanito caldera layered with ignimbrite or rhyolite lava. A few from the Ocampo caldera centered 50 km west rises to an elevation of 2860 m above sea level dikes of similar composition are found in the (Swanson, 1977; Bockoven, 1980). Kempter and is composed mostly of lithic-rich ignim- Copper Canyon area (Fig. 4). Again, this is (1986), however, noted that although poorly brite, the base of which is not exposed (Fig. 5). similar to the Tomóchic area, where NW- to welded, the Cueva tuff could be considered a The rock contains ~20% phenocrysts of plagio- NNE-trending dikes, trends similar to younger signifi cant ignimbrite by virtue of its thickness clase and altered biotite set in a groundmass of normal faults, suggest eruption during regional and widespread distribution. The Cueva tuff, in reddish-brown shards, which have been twisted extension. A reconnaissance traverse from San fact, may exist in the Copper Canyon area, but and contorted by an abundance of included rock Juanito eastward to Carichi (70 km by road) we have not attempted to distinguish Cueva tuff fragments. Lithic fragments up to 6 m in length showed that much of the intervening area of the from among this area’s many similar-looking, are noted, and they appear to increase in size sierra is covered by basaltic andesite, including punky, stratigraphically high ignimbrites. and abundance toward the caldera’s margin. The thick accumulations of olivine-bearing lava, The Heredia tuff could also probably be intracaldera tuff is locally overlain along the as in Cerro Rumurachi rising 500 m above the considered a candidate for eruption from the dome’s crest and fl anks by lava-fl ow rock, gen- plateau surface. Certainly this late, thick lava Tomóchic caldera, and our study of the Copper erally of intermediate composition (Fig. 5). covering makes it diffi cult to recognize older Canyon area reveals a number of other units of The northern half of the caldera’s resurgent caldera structures in this region and possibly appropriate age. The Divisadero tuff, for example dome is surrounded by the typical moat litholo- over vast regions in the core of the Sierra Madre (ca. 30 Ma), also corresponds well with ages from gies of sedimentary rock and poorly indurated Occidental volcanic fi eld. within the Tomóchic caldera. The Divisadero tuff. The southern fl ank of the resurgent dome is tuff is far from being a minor unit, and its high overlain by Mogotabo’s unit 3 and the Quemada CALDERAS OF THE COPPER CANYON stratigraphic position fi ts well with the relatively tuff, which dip gently away from the central AREA pristine condition of the Tomóchic caldera. It dome. Typical moat lithologies, therefore, are is also interesting that the Divisadero tuff is so not exposed around the southern half of the cal- Previous Work—The Tomóchic Volcanic strikingly similar in appearance and mineralogy dera, making the position of the caldera’s south- Center to the Vista tuff, the fi rst ignimbrite that erupted ern margin somewhat speculative. The caldera’s from the Tomóchic volcanic center. A major topographic margin is well displayed around the Although calderas have been previously pro- drawback is that the nearest known exposure of northern side of the caldera, where moat sedi- posed for several locations in the northern part of Divisadero tuff is at San Juanito, 25 km southeast mentary rocks adjoin older ignimbrite and inter- the sierra (see Swanson and McDowell, 1984), of the Tomóchic caldera. While the solution to mediate lava of the caldera’s topographic rim the best-documented calderas are those of the this problem will fall to future workers, we can (Fig. 5). A caldera, ~20 km in diameter, is sug- Tomóchic volcanic center (Fig. 2) (Swanson and at least eliminate those ignimbrite units erupted gested by a projection of the caldera’s rim, but McDowell, 1985; Wark et al., 1990; Wark, 1991; from other caldera sources. The following sec- complexities and uncertainties remain because McDowell et al., 1999). The Tomóchic volcanic tion describes the well-preserved Manzanita and the southern part of the caldera is covered by center includes the Tomóchic and Las Varas San Juanito calderas and discusses stratigraphic younger ignimbrites. calderas. Las Varas caldera formed ca. 34 Ma evidence indicating a Copper Canyon caldera and Because petrographic differences imposed during eruption of the Vista tuff, and its exposed hinting at yet another in El Sierra Comanche. by differences in depositional settings between intracaldera facies is cut by the northern struc- intrac aldera and outfl ow ignimbrites can be sig- tural margin of the Tomóchic caldera. With its San Juanito Caldera nifi cant, we place particular emphasis on pheno- prominent central dome, surrounding moat, and cryst mineralogy and shard characteristics in ring-fracture rhyolite, Tomóchic caldera neatly The existence of a resurgent caldera centered making our correlations. Of the various area units, fi ts the classic resurgent caldera model except on Cordon Cumbre Alta, 15 km northwest of the San Juanito’s intracaldera tuff most closely

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resembles Mogotabo unit 3 and the San Felipe (Mogotabo unit 5). Outfl ow exposures of this work, however, will be needed to determine if tuff. Mogotabo unit 3, however, is interpreted unit throughout the study area share the same the syncline is related to an underlying caldera to lie within San Juanito caldera’s moat. The low crystal content, phenocryst mineralogy, and structure, perhaps as a moat-fi lling sequence. San Felipe tuff, dated at 36.5 Ma (Wark et al., general appearance in terms of color, shard, and Divisadero tuff cooling units exposed along the 1990), is the oldest ignimbrite exposed along pumice characteristics, but they lack the abun- highway between Pitorreal and Creel (Fig. 4) the topographic rim of the Tomóchic caldera. dant lithic fragments. increase drastically and abruptly in thickness At Tomóchic, it consists of three, thick, similar- The caldera’s resurgent dome is overlain and in the direction of Copper Canyon. Again, it is looking cooling units that thicken considerably nearly surrounded by the typical moat litholo- tempting to call upon an underlying structural in the direction of the San Juanito caldera and gies of sedimentary and poorly indurated volca- control. This limited stratigraphic information that match that caldera’s intracaldera ignimbrite nic rocks (Figs. 6 and 8). Lavas of intermediate suggests that Copper Canyon exposes the thick, in terms of phenocryst mineralogy. A felsic to silicic composition, however, dominate in intracaldera facies of a caldera that has served fl ow-dome lava exposed across the San Juanito the southern moat (Fig. 6). The Copper Canyon as a trap for subsequent volcanism, allowing caldera’s western margin (Fig. 5) was sampled highway passes between the Sierras Manzanita the local preservation of one of the region’s best for age determination in the hope that its age and El Comanche (Figs. 2 and 6), and from stratigraphic sections. might strictly constrain both the caldera’s age Arroyo Ancho to Rancho Blanco, it traverses and erupted unit. An age of ca. 36.5 Ma, for the Manzanita caldera’s western moat. The A Comanche Caldera? example, would have strongly supported the San Manzanita caldera’s topographic rim can be Juanito caldera as the source for the San Felipe traced in an unbroken arc west of the highway as The central part of the Sierra El Comanche tuff. While the rock sampled is in a structural it cuts the eastern fl ank of Sierra El Comanche. contains a great thickness of ignimbrite of pla- position appropriate for ring-fracture volcanism, Also, Manzanita moat sedimentary rock con- gioclase-pyroxene mineralogy overlain in places the lava exposures do not extend for any great tains clasts of Comanche tuff, indicating that the by hundreds of meters of intermediate lava rock distance around the caldera margin but do con- Manzanita caldera and the Quemada tuff post- (Fig. 6). Reconnaissance traverses along fl anks tinue for many kilometers southwest of the cal- date the Comanche tuff and any potential caldera of the range encounter lava and sedimentary dera (Fig. 5). Also, similar-looking felsic fl ows immediately to the west. Viewed from the Sierra layers, typical of moat lithologies, dipping east of the caldera appear to extend across the El Comanche, the Manzanita caldera’s central radially outward from the core of the Sierra El caldera boundary to where they overlie mafi c dome gives the appearance of being buried to Comanche. Although extremely thick, a basal lava known to be post-Divisadero in age (Fig. 5). a relatively high level by moat sedimentary and vitrophyre of El Comanche tuff can be observed, While the age of 29.4 Ma (Table 1) provides a volcanic rocks (Fig. 8). Based upon known rim- something unexpected of an intracaldera ignim- minimum age for San Juanito caldera formation, to-dome distances and reconnaissance mapping, brite. Still, cooling breaks with vitric horizons the petrographic similarity of the intracaldera the Manzanita caldera appears to be a relatively have been described from other intracaldera tuff to the 36.6 Ma San Felipe tuff argues for a modest 15 km in diameter, but only its western tuffs, as in the San Juan volcanic fi eld’s Bachelor signifi cantly older age. rim has been located. caldera, where vitrophyric zones occur adjacent to caldera-collapse breccias (Lipman, 2000). Manzanita Caldera Copper Canyon Caldera The known distribution of presumed resurgent dome and moat lithologies suggests a caldera of The Manzanita caldera, unlike those at A 1-km-thick ignimbrite (base not exposed) ~25 km in diameter. Because the circular form Tomóchic and San Juanito, is not obvious on constitutes the lower two-thirds of the exposed of the adjacent Manzanita caldera moat cuts into space photos or imagery. Its presence was sus- stratigraphic section at Divisadero. While this Sierra El Comanche, and blocks of Comanche pected by the arcuate geometry of area streams extraordinary thickness certainly suggests accu- tuff are found in its moat sedimentary rock, the as seen on topographic maps and by the massive, mulation within a caldera, very little else is Comanche tuff and any related caldera must monolithic appearance of the Sierra Manzanita known about the proposed structure, and we can predate formation of the Manzanita caldera. (Fig. 8). It was confi rmed as a caldera when geo- discern no physiographic expression of a Cop- logic mapping showed the Sierra Manzanita to per Canyon caldera on space images. SUMMARY AND DISCUSSION consist of over 400 m of very lithic-rich ignim- The Copper Canyon tuff, as viewed from brite (base not exposed) surrounded by typical Divisadero, appears to extend in a fairly hori- The Copper Canyon Area moat lithologies. zontal manner over great distances across the The intracaldera ignimbrite exposed in the canyon and upriver. Rock layers a few kilo- Copper Canyon and the Mogotabo section at resurgent dome contains ~5%–10% phenocrysts meters downstream from Divisadero, however, the southern limit of the map near Divisadero is of plagioclase and biotite set in a groundmass are seen to dip steeply in a downstream direc- interpreted as exposing an intracaldera ignim- of large, clear, chaotically foliated shards and tion. It is not known if this might represent the brite from a caldera that subsequently served as pumice. Lithic fragments, typically rhyolite southern fl ank of a resurgent dome or if they are a trap for fi ve younger ignimbrites from other lava, are found in abundance, and they range in caused by some other structural control. North sources. The concordant nature of the units, the size from microscopic fragments to megablocks of Divisadero, the various volcanic units above preservation of unwelded tops, the general lack many tens of meters in diameter. The abundance the Copper Canyon tuff do dip gently eastward of interlayered sedimentary beds, and the avail- of fragments produces a chaotic orientation of away from the Copper Canyon highway (Fig. 4), able age information suggest rapid emplace- shards, as seen at the thin section level. In the becoming approximately horizontal near the Rio ment relatively late in the volcanic history of fi eld, eutaxitic foliation is locally seen to wrap Urique, and then they rise slightly further to the the region. The Mogotabo section is capped in all directions around included, building-size east. The Rio Urique may have followed the axis by the Divisadero tuff, which laterally forms lithic fragments. The intracaldera tuff of the of this syncline prior to cutting into the hard, a widespread ignimbrite cap across much of Manzanita is correlated with the Quemada tuff vertically jointed Copper Canyon tuff. More the southern and central map sections, as well

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as throughout the upper Rio Urique and Rio unknown, would have provided a ca. 30 Ma, overlapping caldera-forming volcanism has pro- Conchos drainage systems. The Vista and Rio rapid-fi re climax to caldera-related volcanism duced a region where virtually every area may Verde tuffs, previously known from Tomóchic, in the region. ultimately be found to contain at least one cal- are exposed in the central part of the map area, dera and in which an abnormally thick, strati- unconformably below the Divisadero tuff. The Discussion graphically complex silicic volcanic section is central section also contains the San Juanito the rule. While recurrent stoping and assimila- caldera, the southern moat of which is buried In a review of calderas of the Sierra Madre tion from below combined with resurgent uplift by some units exposed in the Mogotabo section. Occidental, Swanson and McDowell (1984) and erosion from above would have prevented Lava exposed across the San Juanito caldera’s called attention to the general similarities the total core-area ignimbrite thickness from western rim returned an age of 29.4 Ma, but the between the well-known San Juan volcanic approaching total subsidence, clearly the com- structure’s intracaldera ignimbrite resembles fi eld and volcanic rocks of the Sierra Madre monly reported 1 km average ignimbrite section the 36.5 Ma San Felipe tuff, known from expo- Occidental. Volcanism in both areas was noted thickness for sierra ignimbrites does not apply sures north of the caldera and at Tomóchic. to have progressed from dominantly interme- to the core of the volcanic fi eld and will have The northern map area contains the Manzanita diate, stratovolcano-related eruptions to domi- to be adjusted upward, as will estimates of total caldera, source for the Quemada tuff. The Que- nantly silicic volcanism of the “great ignimbrite sierra ignimbrite volume. mada tuff is one of the units that fl owed into the fl are-up.” The view provided by the combined With the San Juan volcanic fi eld as a reference San Juanito caldera, and it crops out at Copper Tomóchic–Copper Canyon area into the heart standard, Swanson and McDowell (1984) esti- Canyon as Mogotabo unit 5. A pre-Manzanita of the Sierra Madre Occidental volcanic fi eld mated that the volume of the Sierra Madre vol- caldera may exist in the Sierra El Comanche, extends those earlier comparisons to the core of canic fi eld might require 350 calderas. It might where units similar to Mogotabo units 2 and 4 each volcanic fi eld. be surprising then that so few are “glaringly are exposed. Lipman (2000) reported that exposures of obvious” on space photographs and images. early stratovolcano-related rock (Conejos For- Similar observations have been used to sup- The Copper Canyon–Tomóchic Region mation) in the central San Juan volcanic fi eld port the hypothesis that fi ssure-vent eruptions are rare and occur mostly as surviving pre- related to episodes of Basin and Range faulting The region’s oldest known ignimbrites are ignimbrite topographic highs exposed along are the dominant source for Sierra Madre Occi- the Cascabel and San Felipe tuffs (Table 2) caldera walls. Similarly, we have yet to identify dental ignimbrites (Aguirre-Díaz and Labarthe- described from the Tomóchic area and dated any pre-ignimbrite volcanic sequence in the Hernández, 2003). Our fi eld investigations at 38.2 and 36.5 Ma (Wark et al., 1990). The Copper Canyon–Tomóchic region, although a show, however, that ignimbrite volcanism was source for the Cascabel is unknown, but our thinner cover exposes Laramide-age plutonic most active in that part of the volcanic fi eld least study suggests the San Juanito caldera as the and volcanic rocks in Batopilas canyon to the affected by extensional faulting, that complete source for the San Felipe tuff. If so, known south (Bagby, 1979). Lipman (2000) reported and fragmented calderas occur in abundance regional caldera activity then shifted from San that intermediate-composition volcanism con- in the Tomóchic-Copper Canyon region, that Juanito (36.5 Ma), northward to the Tomóchic tinued during the period of ignimbrite eruption these calderas generally conform to the resur- volcanic complex, where the Vista tuff was and caldera formation. Andesitic volcanism in gent caldera model that has evolved since the erupted at 34.1 Ma during formation of the the Copper Canyon–Tomóchic region predates seminal work of Smith and Bailey (1968), and Las Varas caldera (Fig. 2). Although Tomóchic and postdates caldera formation at the Tomóchic that similar calderas and caldera complexes can ultimately experienced a second major erup- volcanic complex, the San Juanito caldera, the be expected throughout the core of the Sierra tion during formation of the Tomóchic caldera, Manzanita caldera, as well as ignimbrite in the Madre Occidental. the ca. 30 Ma ages on rocks so closely related Sierra El Comanche. Andesitic lavas, therefore, to the Tomóchic caldera cycle (see Wark et al., are found throughout the ignimbrite stratigraphic ACKNOWLEDGMENTS 1990) cast doubt on the caldera as the source sections in core regions of both the Copper Can- The research was supported by National Science for the Rio Verde tuff. Alternatively, a num- yon and San Juan volcanic fi elds, suggesting Foundation grant EAR 99-09417 to Swanson. Formal ber of ignimbrite formations of more appro- that intermediate-composition magmas played reviews by Luca Ferrari and Eric H. Christiansen pro- priate age, e.g., Cueva, Heredia, Divisadero, a role in the ignimbrite genesis for both regions vided extensive comments that considerably improved and Mogotabo unit 3, and without recognized (Wark, 1991; Lipman, 2000). the manuscript. source calderas are now known. Tomóchic is The silicic components of both volcanic REFERENCES CITED the region’s most obvious caldera with a pris- fi elds thicken from a fringe populated by iso- tine condition, which is also more consistent lated sources (see Swanson and McDowell, Aguirre-Díaz, G., and Labarthe-Hernández, G., 2003, Fis- with it being the source for one of the region’s 1984; Lipman, 2000) to a central core consisting sure ignimbrites: Fissure-source origin for voluminous younger units. of an extremely thick ignimbrite section erupted ignimbrites of the Sierra Madre Occidental and its relationship with Basin and Range faulting: Geology, Following activity at San Juanito (36.5 Ma) from a cluster of overlapping calderas. Nine v. 31, no. 9, p. 773–776, doi: 10.1130/G19665.1. and at Las Varas (34.1 Ma), a tentative sequence major ignimbrites and seven exposed calderas Albrecht, A., 1990, The geochemistry of the mid-Tertiary for caldera formation consistent with strati- are known from the core of the San Juan volca- volcanic rocks of the northern Sierra Madre Occiden- tal, Chihuahua, Mexico: Indicators of basement varia- graphic relationships across the Copper Can- nic fi eld, and six successive stacked calderas are tions [Ph.D. thesis]: Albuquerque, University of New yon area would be the formation of the Copper indicated for the Creede caldera area, represent- Mexico, 161 p. Albrecht, A., and Brookins, D.G., 1989, Mid-Tertiary sili- Canyon caldera, the proposed Sierra Coman- ing a potential cumulative subsidence exceeding ceous igneous activity above cratonic and accreted base- che calderas, the Manzanita, and the Tomóchic 15 km (Lipman, 2000). Incomplete though it ment in northern Mexico: Comparison of two localities: calderas. The emplacement of ignimbrites is, the image emerging from the Copper Can- Geofísica Internacional, v. 28, no. 5, p. 813–850. Albrecht, A., and Goldstein, S.L., 2000, Effects of base- from these major sources, together with the yon–Tomóchic region is similar to that of the ment composition and age on silicic magmas across an eruption of interlayered units from sources yet central San Juan volcanic fi eld, where repeated, accreted terrane–Precambrian crust boundary, Sierra

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Madre Occidental, Mexico: Journal of South American Nourse, J.A., McKee, J.W., and Steiner, M.B., eds., Ordóñez, E., 1896, Rocas eruptivas en bosquejo geologíco Earth Sciences, v. 13, p. 255–273, doi: 10.1016/S0895- The Mojave-Sonora megashear hypothesis: Develop- de Mexico: Boletín del Instituto de Geología, México, 9811(00)00014-6. ment, assessment, and alternatives: Geological Society v. 4, 5, 6, p. 251–270. Albrecht, A., Ward, D.W., Bufl er, R., Dobbelstein, A., and of America Special Paper 393, p. 671–692. Roldán-Quintana, J., Calmus, T., McDowell, F.W., Amaya- Brookins, D.G., 1990, Application of the Rb-Sr dating Kempter, K., 1986, Mid-Tertiary volcanic history of the Martinez, R., and Bellon, H., 2003, Geology of north- technique to Tertiary siliceous igneous rocks, northern Tomóchic region, northern Sierra Madre Occidental, ern Sierra Madre Occidental, eastern Sonora and west- Sierra Madre Occidental, Chihuahua, Mexico: Iso- Chihuahua, Mexico [M.A. thesis]: Austin, The Univer- ern Chihuahua, Mexico, in Geologic transects across chron/West, no. 55, p. 6–9. sity of Texas, 134 p. Cordilleran Mexico, Guidebook for the 99th Cordil- Bagby, W.C., 1979, Geology, geochemistry and geochronol- Lipman, P., 1984, The roots of ash fl ow calderas in west- leran Geological Society of America Annual Meeting: ogy of the Batopilas quadrangle, Sierra Madre Occi- ern North America: Windows into the tops of granitic Instituto de Geología, Universidad Nacional Autónoma dental, Chihuahua, Mexico [Ph.D. thesis]: Santa Cruz, batholiths: Journal of Geophysical Research, v. 89, de México, Publicación Especial 1, p. 71–122. University of California at Santa Cruz, 271 p. p. 8801–8841. Smith, R.L., and Bailey, R.A., 1968, Resurgent cauldrons, Bockoven, N.T., 1980, Reconnaissance geology of the Lipman, P., 2000, Central San Juan caldera cluster: Regional in Coates, R.R., Hay, R.L., and Anderson, C.A., eds., Yécora-Ocampo area, Sonora and Chihuahua, Mexico volcanic framework, in Bethke, P., and Hay, R., eds., Studies in volcanology: Geological Society of America [Ph.D. thesis]: Austin, The University of Texas, 197 p. Ancient Lake Creede: Its volcano-tectonic setting, his- Memoir 116, p. 613–662. Bonner, J.L., and Herrin, E.T., 1999, Surface wave study of tory of sedimentation, and relation to mineralization Swanson, E.R., 1977, Reconnaissance geology of the the Sierra Madre Occidental of northern Mexico: Bul- in the Creede mining district: Geological Society of Tomóchic-Ocampo area, Sierra Madre Occidental, letin of the Seismological Society of America, v. 89, American Special Paper 346, p. 9–69. Chihuahua, Mexico [Ph.D. thesis]: Austin, The Uni- p. 1323–1337. McDowell, F.W., and Clabaugh, S.E., 1979, Ignimbrites of versity of Texas, 123 p. Cameron, K.L., Nimz, G.J., Kuentz, D., Niemeyer, S., and the Sierra Madre Occidental and their relation to the Swanson, E.R., and McDowell, F.W., 1984, Calderas of the Gunn, S., 1989, The Southern Cordilleran Basaltic tectonic history of western Mexico, in Chapin, C.E., Sierra Madre Occidental volcanic fi eld, western Mex- Andesite Suite, southern Chihuahua, Mexico: A link and Elston, W.E., eds., Ash-fl ow tuffs: Geological ico: Journal of Geophysical Research, v. 89, no. B10, between Tertiary continental arc and fl ood basalt mag- Society of America Special Paper 180, p. 113–124. p. 8787–8799. matism in North America: Journal of Geophysical McDowell, F.W., and Keizer, R.P., 1977, Timing of mid- Swanson, E.R., and McDowell, F.W., 1985, Geology and Research, v. 94, p. 7817–7840. Tertiary volcanism in the Sierra Madre Occidental geochronology of the Tomóchic caldera, Chihuahua, Cochemé, J.J., and Demant, A., 1991, Geology of the Yécora between Durango City and Mazatlán, Mexico: Geo- Mexico: Geological Society of America Bulletin, v. 96, area, northern Sierra Madre Occidental, Mexico, in logical Society of America Bulletin, v. 88, p. 1479– p. 1477–1482, doi: 10.1130/0016-7606(1985)96<1477: Pérez-Segura, E., and Jacques-Ayala, C., eds., Studies 1487, doi: 10.1130/0016-7606(1977)88<1479: GAGOTT>2.0.CO;2. of Sonoran geology: Geological Society of America TOMVIT>2.0.CO;2. Swanson, E.R., and Wark, D., 1988, Mid-Tertiary silicic vol- Special Paper 254, p. 81–94. McDowell, F.W., and Mauger, R.L., 1994, K-Ar and U-Pb zircon canism in Chihuahua, Mexico, in Clark, K., Goodell, Ferrari, L., López-Martínez, M., and Rosas-Elguera, J., chronology of Late Cretaceous and Tertiary magmatism in P.C., and Hoffer, J.M., eds., Stratigraphy, tectonics and 2002, Ignimbrite fl are-up and deformation in the central Chihuahua State, Mexico: Geological Society of resources of parts of Sierra Madre Occidental province, southern Sierra Madre Occidental, western Mexico: America Bulletin, v. 106, p. 118–132, doi: 10.1130/0016- Mexico: El Paso Geological Society, 22nd Field Con- Implications for the late subduction history of the 7606(1994)106<0118:KAAUPZ>2.3.CO;2. ference Guidebook, p. 229–239. Farallon plate: Tectonics, v. 21, no. 4, 1035, doi: McDowell, F.W., Roldán-Quintana, J., and Amaya-Mar- Swanson, E.R., Keizer, R.P., Lyons, J.I., and Clabaugh, 10.1029/2001TC001302. tínez, R., 1997, Interrelationship of sedimentary and S.E., 1978, Tertiary volcanism and caldera develop- Ferrari, L., Martín, V., and Scott, B., 2005, Magmatismo y volcanic deposits associated with Tertiary extension ment near Durango City, Sierra Madre Occidental, tectónica en la Sierra Madre Occidental y su relación in Sonora, Mexico: Geological Society of America Mexico: Geological Society of America Bulletin, v. 89, con la evolución de la margen occidental de Nortea- Bulletin, v. 109, p. 1349–1360, doi: 10.1130/0016- p. 1000–1012, doi: 10.1130/0016-7606(1978)89<1000: mérica: Boletín de la Geología Mexicana, v. 57, no. 3, 7606(1997)109<1349:IOSAVD>2.3.CO;2. TVACDN>2.0.CO;2. p. 343–378. McDowell, F.W., Housh, T.B., and Wark, D.A., 1999, Wark, D.A., 1991, Oligocene ash fl ow volcanism, northern Henry, C.D., and Aranda-Gómez, J.J., 1992, The real south- Nature of the crust beneath west-central Chihua- Sierra Madre Occidental: Role of mafi c and intermedi- ern Basin and Range: Mid- to late Cenozoic extension in hua, Mexico, based upon Sr, Nd, and Pb isotopic ate-composition magmas in rhyolite genesis: Journal Mexico: Geology, v. 20, p. 701–704, doi: 10.1130/0091- compositions at the Tomóchic volcanic center: Geo- of Geophysical Research, v. 96, p. 13,389–13,411. 7613(1992)020<0701:TRSBAR>2.3.CO;2. logical Society of America Bulletin, v. 111, no. 6, Wark, D.A., Kempter, K.A., and McDowell, F.W., 1990, Henry, C.D., and Aranda-Gómez, J.J., 2000, Plate interac- p. 823–830, doi: 10.1130/0016-7606(1999)111<0823: Evolution of waning, subduction-related magmatism, tions control middle-late Miocene, proto-Gulf and NOTCBW>2.3.CO;2. northern Sierra Madre Occidental, Mexico: Geological Basin and Range extension in the southern Basin and Nieto-Samaniego, A.F., Ferrari, L., Alaniz-Alvarez, S.A., Society of America Bulletin, v. 102, no. 11, p. 1555– Range: Tectonophysics, v. 318, p. 1–26, doi: 10.1016/ Labarthe-Hernández, G., and Rosas-Elguera, J., 1564, doi: 10.1130/0016-7606(1990)102<1555: S0040-1951(99)00304-2. 1999, Variation of Cenozoic extension and volcanism EOWSRM>2.3.CO;2. Housh, T.D., and McDowell, F.W., 2005, Isotope provinces across the southern Sierra Madre Occidental volca- in Laramide and mid-Tertiary igneous rocks of north- nic province, Mexico: Geological Society of America MANUSCRIPT RECEIVED BY THE SOCIETY 10 NOVEMBER 2005 western Mexico (Chihuahua and Sonora) and their Bulletin, v. 111, p. 347–363, doi: 10.1130/0016- REVISED MANUSCRIPT RECEIVED 2 MARCH 2006 relation to basement confi guration, in Anderson, T.H., 7606(1999)111<0347:VOCEAV>2.3.CO;2. MANUSCRIPT ACCEPTED 7 MARCH 2006

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