Synextensional Pliocene±Pleistocene eruptive activity in the Camargo volcanic ®eld, Chihuahua, MeÂxico

Jose Jorge Aranda-GoÂmez² Centro de Geociencias, Universidad Nacional AutoÂnoma de MeÂxico, Campus Juriquilla, P.O. Box 1-742, QuereÂtaro, QuereÂtaro 76001, MeÂxico James F. Luhr³ Department of Mineral Sciences, NHB-119, Smithsonian Institution, Washington, D.C. 20560, USA Todd B. Housh§ Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712, USA Charles B. Connor# Department of Geology, University of South Florida, Tampa, Florida 33620, USA Tim Becker²² Berkeley Geochronology Center, Berkeley, California 94709, USA Christopher D. Henry³³ Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557-0088, USA

ABSTRACT accommodation zone with west-dipping 1974). Despite this broad association between faults and east-tilted blocks to the north extensional faulting and volcanic activity, the The Camargo volcanic ®eld is the largest and east-dipping faults and west-tilted detailed interplay between these two phenom- ma®c alkalic volcanic ®eld in the Mexican blocks to the south. These faults are ex- ena can be highly variable and equally highly Basin and Range province, and the rela- pressed in the volcanic ®eld by a N30؇W- debated. This dichotomy is most notable for tionship between volcanism and normal trending graben with scarps up to ϳ100 m the Basin and Range province. In the eastern faulting is especially strong. The Camargo high through its central part. Volcanism Great Basin, Gans et al. (1989) noted a cor- volcanic ®eld lies in the northern part of the and faulting were at least in part coeval, relation between the peak of felsic ash-¯ow province, midway between the Sierra Ma- and younger volcanic products commonly tuff eruptions and the abrupt onset of large- dre Occidental and Trans-Pecos Texas. It is drape fault scarps that cut earlier lavas. magnitude extension. According to their mod- formed by Pliocene±Pleistocene (4.7±0.09 Normal faulting is bracketed between 4.7 el, eruption of ma®c magmas followed as Ma) intraplate ma®c alkalic volcanic rocks, and 2.1 Ma and may have also migrated slower, broadly distributed extension set in. In some of which contain peridotite, pyroxe- northeastward. Estimated vertical slip rates contrast, Axen et al. (1993) concluded that the nite, and granulite xenoliths. The volcanic on four Pliocene faults range from 0.03 onset of extension preceded volcanism in ®eld covers ϳ3000 km2 and has an esti- mm/yr, a likely long-term rate, to 1.67 mm/ some areas, was coincident with volcanism in mated volume of ϳ120 km3 erupted from yr, interpreted as a short-term rate opera- others, and postdated volcanism in still other Ͼ300 recognized vents. Twenty-six new tive during periods of active faulting. parts of the eastern Great Basin. Glazner and 40Ar/39Ar age determinations for the Ca- Northwest-striking normal faults that cut Bartley (1994) offered an even more contrary margo volcanic ®eld and its environs show alluvial-fan deposits and Pleistocene lavas position against a close association between that volcanic activity began in the south- in the northern Camargo volcanic ®eld and extension and alkalic magmatism in their in- west part of the ®eld and shifted toward the geomorphic evidence for recent uplift to the terpretation that ma®c alkalic volcanic rocks northeast at ϳ15 mm/yr. The average mag- south of the volcanic ®eld suggest that the with peridotite xenoliths from the Mojave De- matic eruption rate during growth of the region is still extending. sert erupted during contractional or transpres- ®eld was ϳ0.026 km3/k.y. sional deformation. Keywords: Basin and Range province, Chi- The Camargo volcanic ®eld lies within an The Oligocene ``ignimbrite ¯are-up'' of the huahua, Mexico, extension faults, intra- Sierra Madre Occidental of Mexico, the larg- plate, Pliocene, slip rates. ²E-mail: [email protected]. est continuous rhyolite province in the world ³E-mail: luhr@volcano.si.edu. (Swanson et al., 1978), coincided in time and §E-mail: [email protected]. INTRODUCTION #E-mail: [email protected]. space with brief pulses of rapid extension. ²²E-mail: [email protected]. Continental rift zones are associated world- This activity marked the transition from east- ³³E-mail: [email protected]. wide with alkalic volcanic rocks (Bailey, northeast compression, related to eastward

GSA Bulletin; March 2003; v. 115; no. 3; p. 298±313; 9 ®gures; 2 tables; Data Repository item 2003033.

For permission to copy, contact [email protected] 298 ᭧ 2003 Geological Society of America SYNEXTENSIONAL PLIOCENE±PLEISTOCENE CAMARGO VOLCANIC FIELD, CHIHUAHUA, MEÂ XICO

faults. Furthermore, regional structures and faulting appear to have determined the loca- tion of the Camargo volcanic ®eld, which lies within an antithetic transfer zone (Peacock and Sanderson, 1997) between two basin-and- range structures where the transfer zone co- incides with the regional San Marcos fault (Fig. 1).

REGIONAL TECTONIC SETTING

The Camargo volcanic ®eld is located in southeastern Chihuahua State, east of the Si- erra Madre Occidental, at the juncture of sev- eral Proterozoic to Mesozoic basement struc- tures that in¯uence its location (Figs. 1 and 2). Scattered outcrops and well data in north- ern Chihuahua demonstrate the presence of Grenvillian basement, presumably part of the North American craton. At two locations in southeastern Chihuahua, Grenvillian basement is overlain by Paleozoic rocks. These may be part of the Ouachita belt, which can be traced on the surface in west Texas and disappears Figure 1. Location of the Camargo volcanic ®eld and its relationship to major tectonic and magmatic features of the region. A belt of Cenozoic volcanic rocks extends between south of the MeÂxico±United States border the Sierra Madre Occidental and Trans-Pecos Texas. The trace of the San Marcos fault (Fig. 1, inset), under a thick cover of younger (solid line) was taken from McKee et al. (1984, 1990), and its inferred extension (dashed sedimentary and volcanic rocks. The edge of line) was taken from Padilla y SaÂnchez (1986). Inset shows the Ouachita orogenic belt in the ``nonaccreted continental rocks'' proposed the south-central United States and its probable continuation in Chihuahua (simpli®ed by Coney and Campa (1987) and the extrap- after Sedlock et al., 1993). olated Ouachita thrust front lie a short distance north of the Camargo volcanic ®eld. Thus, the southern edge of autochthonous Proterozoic subduction of the Farallon plate, to the east- ®eld with scarps up to a few tens of meters North America and the late Paleozoic suture northeast extension that has since dominated high, and Quaternary vent alignments parallel between Laurentia and Gondwana are thought the Mexican Basin and Range province (Mc- some faults (Aranda-GoÂmez et al., 1997). In to be near the Camargo volcanic ®eld (Cam- Dowell and Keizer, 1977; Aguirre-DõÂaz and the other Quaternary volcanic ®elds we have eron and Jones, 1993). McDowell, 1991; Nieto-Samaniego et al., investigated, however, no clear relationship is The Camargo volcanic ®eld is located near 1999; Luhr et al., 2001). This felsic volcanism evident on the surface between volcanism and the San Marcos fault, which is a major struc- and coeval faulting are consistent with the faulting (Ventura and Santo Domingo: Luhr et ture that can be traced on the surface for ϳ300 model of Gans et al. (1989), as is the subse- al., 1989; San QuintõÂn: Luhr et al., 1995b). km (Fig. 1) across the state of Coahuila. At quent Miocene±Quaternary ma®c alkalic mag- However, geophysical data suggest that San the Chihuahua-Coahuila state line, the fault matism. Among the major ma®c alkalic vol- QuintõÂn is located in a transtensional basin trace appears to end abruptly against the east- canic ®elds of the Mexican Basin and Range (Almeida-Vega et al., 2000). ern branch of the BolsoÂn de MapimõÂ, a major province, we have investigated three Miocene In this paper we describe the geology, age, Basin and Range structure (Fig. 2). Padilla y and four Quaternary examples. In the 24 Ma and structure of the Pliocene±Pleistocene Ca- SaÂnchez (1986) and Grajales-Nishimura et al. Rodeo ®eld of Durango State (Fig. 1), hawai- margo volcanic ®eld of Chihuahua, the most (1992) interpreted that the San Marcos fault ites are interbedded with graben-®ll gravel and voluminous ma®c alkalic volcanic ®eld in the may be traced another 250 km northwestward are cut by the Rodeo fault, demonstrating their Mexican Basin and Range province. We also through and past the Camargo volcanic ®eld synextensional nature (Luhr et al., 2001). seek to answer the question, How did volcanic (Fig. 1). The San Marcos fault has a complex South of Rodeo, the ca. 12 Ma Metates ha- activity and normal faulting interplay during history (McKee and Jones, 1979; McKee et waiites (Fig. 1) are similarly underlain by grav- the evolution of the Camargo volcanic ®eld? al., 1984, 1990), and it appears that since the els and displaced up to 60 m by normal faulting Volcanism and extension are distinctly evident Jurassic, it has been reactivated during each of in the RõÂo Chico graben (Aranda-GoÂmez et al., in the central part of the volcanic ®eld, where the major pulses of deformation in the region. 1997; Henry and Aranda-GoÂmez, 2000). the lava plains and individual vents are cut by The topography of the Camargo volcanic Among the Quaternary volcanic ®elds, only numerous faults (Fig. 2). These structures are, ®eld region re¯ects both Laramide and Basin the relatively large and voluminous Durango in turn, partly covered by younger volcanic and Range deformation. Ranges adjacent to volcanic ®eld (Fig. 1) shows evidence of syn- rocks. Ages for the syneruptive normal fault- the Camargo volcanic ®eld are occupied by chronous faulting and eruptive activity. Sev- ing can be bracketed in many localities, and folded Jurassic and Cretaceous rocks that are eral northwest-striking normal faults cut the vertical slip rates can be estimated for various overlain by a variety of faulted and tilted Ter-

Geological Society of America Bulletin, March 2003 299 ARANDA-GOÂ MEZ et al.

Figure 2. Regional setting of the Camargo volcanic ®eld (CVF). Near the southern end of Sierra El Diablo are exposed Paleozoic volcanic rocks. The San Marcos fault, as documented by McKee et al. (1984, 1990), ends at the eastern branch of BolsoÂn de MapimõÂ. Playa lakes: LLÐEl Llano; EGÐEl Gigante; LAÐLas Arenosas; ERÐEl Remolino.

tiary rocks. Regionally, Laramide folds near km-wide, east-northeast±trending belt of mid- tithetic transfer zone between basin-and-range the volcanic ®eld trend from north-northwest dle Cenozoic volcanic outcrops that extends structures. South of the ®eld, the Sierras San to west-northwest (Sociedad GeoloÂgica Mex- from the Sierra Madre Occidental into west Francisco and El Diablo are bounded to the icana, 1985; Tarango-Ontiveros, 1993). Tilted Texas (Fig. 1). Lavas of the Camargo volcanic east by approximately north-striking, down- and faulted Eocene conglomerate and gravel ®eld rest atop Cretaceous marine sedimentary to-the-east normal faults (Fig. 3). These faults are exposed adjacent to some of the folded rocks, middle Tertiary volcanic rocks, or, lo- abruptly change strike to N30ЊW and project limestone ranges (McKee et al., 1984, 1990; cally, gravel composed of fragments of lime- into the central Camargo volcanic ®eld. North Bartolino, 1992). Subduction-related volcanic stone and volcanic rocks. An andesitic sill of the volcanic ®eld and partly buried by its rocks (andesite-rhyolite) crop out extensively (Table 1: 40Ar/39Ar ϭ 13.97 Ϯ 0.08 Ma) that lavas, the Sierra Agua de Mayo is bounded by in the northeastern Camargo volcanic ®eld intruded gravel near Sierra Aguachile provides a N10ЊW, down-to-the-southwest, right-stepping, (Figs. 2 and 3). These Tertiary volcanic rocks a minimum age for the Tertiary arc-related en echelon system of normal faults. The op- (K-Ar: 40±31 Ma; Smith, 1993; Smith et al., magmatism (Fig. 3). posing Agua de Mayo and San Francisco±El 1996) lie in the southernmost part of a 150- The Camargo volcanic ®eld overlaps an an- Diablo fault systems meet beneath the central

300 Geological Society of America Bulletin, March 2003 SYNEXTENSIONAL PLIOCENE±PLEISTOCENE CAMARGO VOLCANIC FIELD, CHIHUAHUA, MEÂ XICO

Figure 3. Generalized geologic map of the Camargo volcanic ®eld. The box in the central part of the volcanic ®eld shows the location of Figure 4.

Geological Society of America Bulletin, March 2003 301 ARANDA-GOÂ MEZ et al.

TABLE 1. SUMMARY OF40Ar/39Ar AGES OBTAINED FOR ROCKS OF THE CAMARGO VOLCANIC FIELD irons, and abrupt low-sinuosity mountain AND ITS SURROUNDINGS, AND MINIMUM VERTICAL SLIP RATES FOR FAULTS IN THE CENTRAL GRABEN OF THE CAMARGO VOLCANIC FIELD fronts. Active extension in southern Chihua- hua is also supported by the observed seis- Sample Type Latitude Longitude Age Ϯ2␴ ⌬ age² Throw³ Slip rate (N) (W) (Ma) (m.y.) (m) (mm/yr)§ micity, together with the length, focal depth, and rupture complexity associated with the Samples collected in the surroundings of the Camargo volcanic ®eld Parral earthquake (1928, M ϭ 6.5), which are CHI-56 Sill 27Њ 51.181Ј 104Њ37.260Ј 40 to 41³³ CHI-103 Neck 27Њ 59.558Ј 104Њ01.529Ј 29.50 Ϯ 0.12 similar to the values that characterize events CHI-111 Sill 27Њ 55.350Ј 104Њ36.831Ј 13.97 Ϯ 0.08 of comparable magnitude elsewhere in the Ba- Samples not related to faulting from the Camargo volcanic ®eld sin and Range province (Doser and RodrõÂ- CHI-160B Lava# 27Њ33.278Ј 104Њ40.432Ј 4.69 Ϯ 0.06 CHI-43 Neck 27Њ29.089Ј 104Њ33.186Ј 4.11 Ϯ 0.04 guez, 1992). CHI-171 Cone 27Њ23.189Ј 104Њ41.672Ј 3.91 Ϯ 0.03 CHI-84 Cone 27Њ45.766Ј 104Њ28.402Ј 2.27 Ϯ 0.04 DOMAINS OF THE CAMARGO CHI-33 Cone 27Њ30.635Ј 104Њ18.168Ј 2.36 Ϯ 0.10 CHI-80 Cone 27Њ51.248Ј 104Њ22.255Ј 1.35 Ϯ 0.10 VOLCANIC FIELD CHI-62 Cone 27Њ53.743Ј 104Њ16.006Ј 1.66 Ϯ 0.10 CHI-57 Cone 28Њ02.182Ј 104Њ17.624Ј 0.09 Ϯ 0.04 El Milagro fault The lava plateau of the Camargo volcanic CHI-36 Pre-fault 27Њ33.914Ј 104Њ28.542Ј 4.73 Ϯ 0.04 ®eld is cut by a central graben, which divides CHI-37 Post-fault 27Њ34.161Ј 104Њ28.927Ј 3.05 Ϯ 0.12 1.84 Ͼ50 Ͼ0.03 the ®eld into three large volcano-tectonic do- Las Borregas fault at Cerro Mojoneras mains. The domains are bounded by normal CHI-92 Pre-fault 27Њ36.906Ј 104Њ28.356Ј 2.30 Ϯ 0.06 CHI-91A Post-fault 27Њ36.803Ј 104Њ28.520Ј 2.35 Ϯ 0.06 0.07 Ͼ50 Ͼ0.71 faults or fault systems and characterized by CHI-91B Post-fault# 27Њ36.803Ј 104Њ28.520Ј 2.37 Ϯ 0.04 0.03 Ͼ50 Ͼ1.67²² different ages of volcanism and fault density Las Borregas fault at Cerro Lamojino (Fig. 3). We term these domains La Loba ²² CHI-53 Post-fault 27Њ41.035Ј 104Њ29.514Ј 2.45 Ϯ 0.10 See text Ͼ40 See text (southwestern), El Venado (central), and Mar- CHI-54 Post-fault# 27Њ41.035Ј 104Њ29.514Ј 2.54 Ϯ 0.06 Las Hormigas fault avillas (northeastern). CHI-93 Pre-fault 27Њ36.157Ј 104Њ24.543Ј 2.25 Ϯ 0.04 0 Ͼ10 See text CHI-94 Post-fault 27Њ36.229Ј 104Њ24.032Ј 2.37 Ϯ 0.08 La Loba Domain Northern end of Las Borregas CHI-85 Pre-fault 27Њ46.251Ј 104Њ30.446Ј 2.94 Ϯ 0.02 CHI-87 Post-fault 27Њ45.756Ј 104Њ30.288Ј 2.45 Ϯ 0.04 0.55 Ͼ90 Ͼ0.16 The southwestern part of the Camargo vol- El Venado fault canic ®eld is a relatively unfaulted lava plain CHI-99 Pre-fault 27Њ37.123Ј 104Њ23.725Ј 2.24 Ϯ 0.02 with abundant remnants of deeply eroded cin- CHI-98A Post-fault 27Њ36.921Ј 104Њ23.501Ј 2.14 Ϯ 0.22 0.34 Ͼ10 Ͼ0.03 El Espejo fault der cones. Consequent streams, roughly per- CHI-83 Pre-fault 27Њ46.049Ј 104Њ28.569Ј 2.19 Ϯ 0.02 pendicular to the abrupt, southwestern bound- CHI-82 Post-fault 27Њ45.992Ј 104Њ29.099Ј 2.18 Ϯ 0.06 0.09 Ͼ10 Ͼ0.11 ary of the lava plain, incise deep canyons ²See Table DR1. where thick lava-¯ow stacks are exposed. ³Degraded scarp heights. Vent locations in the southwestern part of the §See discussion in text. #Age obtained from feldspar megacrysts. domain are marked by low rounded hills with- ²²To calculate slip rates we used the age obtained from groundmass separates. out vestiges of craters. The centers of these ³³ See Figure 5. scoria mounds are commonly marked by small plug-like lava necks with near-vertical ¯ow foliation and platy joints. Randomly oriented part of the Camargo volcanic ®eld to produce Ma (CHI- 57); all reported uncertainties are dikes radiate from these necks. Volcanic necks the central graben. two standard deviations (2␴, Table 1). The also occur in the eastern part of the La Loba best-known locality in the volcanic ®eld is La domain, as well as some breached volcanoes GEOLOGIC OVERVIEW OF THE Olivina, a cinder cone (Table 1: 40Ar/39Ar ϭ characterized by broad shallow craters with CAMARGO VOLCANIC FIELD 1.66 Ϯ 0.10 Ma) where spinel lherzolite man- gentle inner walls. The outer slopes of the tle xenoliths are abundant (Nimz et al., 1995). cones commonly exceed 33Њ and may be as The Camargo volcanic ®eld is the largest La Olivina also yields middle- to lower-crustal steep as 43Њ. These unusually steep slopes are and most voluminous of the xenolith-bearing feldspathic granulites (Cameron et al., 1983, products of erosion, which exposes resistant basaltic volcanic ®elds in the southern Basin 1992; Nimz et al., 1986; Rudnick and Cam- beds of volcanic agglutinate. and Range province (Fig. 1), and its relation- eron, 1991) and two distinct types of subcrust- La Loba domain is bounded on the west and ship to late Cenozoic faulting is the most ev- al pyroxenites, one regarded as comagmatic east by normal faults. It abruptly ends on the ident. The volcanic ®eld is formed by Ͼ300 with the host lavas and the other as comag- west along the Lagunetas fault (Fig. 3). The recognized vents and extensive lava ®elds, matic with the middle Tertiary basalts of the fault trace between CanÄada La Lazada and which cover an area of ϳ3000 km2. As dis- area (Nimz et al., 1993). Small (Ͻ3 cm) man- CanÄada Carrasco cuts and displaces the lava cussed in this paper, a rough estimate of the tle and crustal xenoliths are also found ¯ows (Fig. 3). Near the northern and southern average thickness of the lava plateau is ϳ40 throughout the Camargo volcanic ®eld. ends of the Lagunetas fault, the fault trace is m, yielding an estimated lava volume of ϳ120 Ongoing faulting and block uplift in the covered by distal lava ¯ows from younger La km3. volcanic ®eld are evidenced by features com- Loba volcanoes to the east. Therefore, activity Twenty-three new 40Ar/39Ar ages of volca- monly associated with these processes (Keller, along the Lagunetas fault probably ended be- nic rocks of the Camargo volcanic ®eld range 1986), such as offsets in alluvial fans, fanhead fore the onset of volcanism and faulting in the from 4.73 Ϯ 0.04 Ma (CHI-36) to 0.09 Ϯ 0.04 deposition near the apex of alluvial fans, ¯at- younger Venado domain to the east. The Las

302 Geological Society of America Bulletin, March 2003 SYNEXTENSIONAL PLIOCENE±PLEISTOCENE CAMARGO VOLCANIC FIELD, CHIHUAHUA, MEÂ XICO

Figure 4. Geologic map of the El Venado domain.

Borregas and El Milagro fault systems sepa- of eruptions along the structure. Contact re- up to ϳ100 m high and small intervening pla- rate the La Loba and El Venado domains. The lationships around Cerro Mojoneras are com- ya lakes. Approximately one half of the vents fact that the trace of the Las Borregas system plex and are best explained by several succes- are low hills with necks and radial dikes, and is locally covered by younger volcanoes (e.g., sive pulses of faulting and volcanism. the rest are younger breached cones with dis- Cerros Lamojino and Las Mojoneras: Fig. 4; tinct craters (Fig. 4). Short individual lava Cetenal, 1974) indicates that it acted as a mag- El Venado Domain ¯ows that caused partial collapse of the cones matic conduit. The elliptical shape of Cerro by rafting are common. These younger vol- Lamojino, having its long axis parallel to the The central El Venado domain is character- canic features are scattered over an older lava fault, suggests that it was formed by a series ized by numerous normal faults with scarps plain. Alignment of vents along faults indi-

Geological Society of America Bulletin, March 2003 303 ARANDA-GOÂ MEZ et al. cates that the faults acted as magma conduits; in the El Venado domain, and the distribution bles 1 and DR-11; Figs. 5±7) by using the an- younger cinder cones cover some fault traces. of lakes and their association with fault scarps alytical methods described by Sharp et al. The en echelon Agua de Mayo fault systemÐ indicate that they were caused by slight tilting (1996). Materials dated included 20 ground- formed by El Alamito, El Parapeto, La Sier- associated with normal faulting (Fig. 4). The mass separates and 3 feldspar megacrysts rita, San MartõÂn, and El CarretoÂn faults (Figs. largest basin in the El Venado domain is oc- from the Camargo volcanic ®eld, plus 3 2±3)Ðmarks the eastern limit of the domain. cupied by the El Milagro playa lake, which groundmass separates from ma®c volcanic Aerial-photographic analysis and ®eld obser- contains a well-developed fan delta near the rocks older than the Camargo volcanic ®eld. vations indicate that the north-northwest± trace of the Las Borregas and El Milagro fault Our goals were to (1) establish the duration of striking, down-to-the-southwest Agua de systems (Fig. 4). Slight tilting, coupled with activity for the Camargo volcanic ®eld by dat- Mayo fault system (Fig. 2) gradually trans- the modest observed displacement in the cen- ing vents with different estimated geomorpho- forms southward, beneath the Camargo vol- tral graben of the Camargo volcanic ®eld, in- logic ages (Noyola-Medrano, 1995), (2) eval- canic ®eld, into the northwest-striking, down- dicates that Pliocene±Pleistocene extension in uate whether feldspar megacrysts, which may to-the-northeast San Francisco fault. Farther the area is low. be xenocrystic (e.g., Luhr et al., 1995a), can south this structure abruptly changes to a provide reliable ages for their host lavas, (3) north trend and has a larger displacement than determine the ages of other ma®c volcanic Maravillas Domain shown where it cuts the volcanic ®eld. Thus, rocks near the Camargo volcanic ®eld, and (4) displacement on both the Agua de Mayo sys- set limits on the vertical slip rates for some of tem from the north and the San Francisco fault The northeastern Camargo volcanic ®eld the faults in the central graben. from the south decreases as they approach the domain, named Maravillas, is relatively un- All samples gave readily interpretable re- volcanic ®eld. However, Mesozoic sedimen- faulted and covered by extensive lava ¯ows; sults. Plateau and inverse isochron ages agree tary rocks along the San Francisco fault and vents are particularly abundant west of La Oli- within analytical uncertainties for all but one middle Tertiary volcanic rocks along the Agua vina (Fig. 3). Morphologically, the vast ma- very young sample. Sample CHI-57 gave im- de Mayo system show the greatest displace- jority of the Maravillas vents are breached precise plateau, 0.50 Ϯ 0.30 Ma, and isochron ment, indicating signi®cant pre±Camargo vol- cones characterized by distinct craters with ages, 0.09 Ϯ 0.04 Ma, not surprising given its canic ®eld motion along these structures. Ac- relatively steep inner walls and outer cone very young age. We choose the isochron age tive uplift of the east-central Sierra San slopes as steep as 22Њ. These features are in- because the isochron data indicate only a Francisco is suggested by ¯atirons, low sinu- terpreted as indicating only moderate degrees small amount of excess argon. The quality of osity of the mountain front, and fanhead de- of erosion, lower than that found in the other the isochron result suggests that this is the position at the apex of alluvial fans. Occur- two domains, and consistent with young 40Ar- most accurate re¯ection of true geologic age. rence of a small, internally drained basin and 39Ar ages discussed in the next section. Cones a playa lake (Laguna El Remolino: Fig. 2) im- with steep outer slopes (Ͼ33Њ) and exposed Northeastward Migration of Volcanism mediately east of the mountain front and south agglutinate beds are rare compared with the of Cerros Prietos indicates southwestward tilt- other domains, and necks are absent, except Sample ages for the Camargo volcanic ®eld ing of the hanging-wall block. at the northeastern edge of the Maravillas do- range between 4.73 Ϯ 0.04 (CHI-36) and 0.09 In the central graben of the El Venado do- main; however, we determined that these Ϯ 0.04 Ma (CHI-57, Table 1) and decrease main, the timing of movement along different necks are middle Tertiary in age (29.5 Ma). from southwest to northeast (Figs. 2±4). The 40 39 faults can be bracketed by Ar/ Ar dating of The presence of isolated outcrops of such old- four oldest samples are from the southwestern associated lava ¯ows, as discussed subse- er rocks, partially to completely surrounded part of the volcanic ®eld, either within or ad- quently. Relationships are clearest for faults by ma®c lavas of the Camargo volcanic ®eld, jacent to the La Loba domain. CHI-171 (3.91 with relatively small throws, where we could indicates that the Pliocene topography of the Ϯ 0.03 Ma) is from an outlier vent located collect and date a lava ¯ow cut by the fault area was irregular. The northeastern and ϳ15 km south-southwest of the southwestern and another draping the fault. Such cases are southeastern boundaries of the Maravillas do- corner of the contiguous Camargo volcanic discussed for the faults El Espejo, El Milagro, main are very irregular because relatively vo- ®eld (Fig. 2). CHI-160B (4.69 Ϯ 0.06 Ma) is and El Venado (Fig. 4). Extensive talus de- luminous lava ¯ows moved down prevolcanic a feldspar megacryst (Ca43Na53K4) from a lava posits that bury the fault traces hinder detailed stream beds and formed elongated lobes. Near ¯ow exposed at the base of the La Loba vol- interpretation of the geologic history at many the northeastern end of the domain, the Hon- canic sequence in CanÄada Carrasco (Fig. 3). localities. It can also be extremely dif®cult to orato fault (Fig. 3) cuts and displaces alluvial- CHI-36 (4.73 Ϯ 0.04 Ma) came from a lava establish whether a cinder cone was partly de- fan deposits and Pleistocene lava ¯ows of the ¯ow at the base of the sequence that is cut by stroyed by a younger faulting episode or Camargo volcanic ®eld and is partly buried by the El Milagro fault (Figs. 3±4). Vent loca- whether it erupted very close to an older fault younger alluvial deposits. Compared with oth- tions for these two samples are unknown. scarp, built part of its cone above the footwall er faults at the boundaries between the Ca- CHI-43 (4.11 Ϯ 0.04 Ma) is from a volcanic of the structure, and later was destroyed by margo volcanic ®eld domains, the Honorato neck near the southwest part of the La Loba mass-wasting and/or ¯uvial erosion. fault has small displacement, just a few me- domain (Fig. 3). Closed basins are common in the Camargo ters, but its trace is quite clear. The youngest rocks in the dated set are volcanic ®eld region. Regional normal fault- from the Maravillas domain, in the northeast ing associated with late Cenozoic basin-and- range extension controlled the locations of rel- AGE OF VOLCANISM AND FAULTING 1GSA Data Repository item 2003033, 40Ar/39Ar an- atively large playa lakes such as El Gigante, alytical data, is available on the Web at http://www. 40 39 Las Arenosas, El Llano, and El Remolino In total, 26 new Ar/ Ar ages were deter- geosociety.org/pubs/ft2003.htm. Requests may also (Fig. 2). Internally drained basins are common mined at Berkeley Geochronology Center (Ta- be sent to [email protected].

304 Geological Society of America Bulletin, March 2003 SYNEXTENSIONAL PLIOCENE±PLEISTOCENE CAMARGO VOLCANIC FIELD, CHIHUAHUA, MEÂ XICO

Figure 5. 40Ar/39Ar incremental heating spectra and inverse iso- chron data for samples unrelated to faulting. Uncertainties for all ages and ratios are ؎2 standard deviations. Vertical height of each increment in spectra is 2 standard deviations. The preferred age for each sample is in boldface type. Subscript tr denotes the initial or ``trapped'' component of the 40Ar/36Ar ratio.

Geological Society of America Bulletin, March 2003 305 ARANDA-GOÂ MEZ et al.

306 Geological Society of America Bulletin, March 2003 SYNEXTENSIONAL PLIOCENE±PLEISTOCENE CAMARGO VOLCANIC FIELD, CHIHUAHUA, MEÂ XICO part of the Camargo volcanic ®eld (Fig. 3). Eruption Rate (foreign) or cognate with the host magma. These include a youthful-looking lava ¯ow Megacrysts are commonly released from the with vestigial pahoehoe ropes from the mod- A conservative estimate of the total lava lava or tephra deposits by differential weath- erately eroded cinder cone Cerro Colorado volume of the Camargo volcanic ®eld is ϳ120 ering and can easily be collected as unaltered (CHI-57, 0.09 Ϯ 0.04 Ma), a lava ¯ow that km3, based on an area of ϳ3000 km2 and a crystals on the surface of the ¯ows. Regard- overlies the San MartõÂn fault near the CanÄoÂn roughly estimated average thickness of 40 m less of their origin, if these feldspar mega- Obscuro Ranch (CHI-80, 1.35 Ϯ 0.10 Ma), for the contiguous ®eld. Polymictic gravels crysts were at a temperature in excess of the and a bomb from La Olivina (CHI-62, 1.66 Ϯ are exposed beneath lava ¯ows in several Ar blocking temperature prior to eruption, or 0.10 Ma). places around the border of the volcanic pla- completely degassed during transport, they All 15 dated groundmass samples and feld- teau and along the Las Borregas and El Car- could be used directly to date the eruption. spars of intermediate age (2.94 Ϯ 0.02 [CHI- retoÂn fault scarps (Fig. 4). Our local estimates Feldspar megacrysts CHI-91B (An22Ab72Or6)

85] to 2.14 Ϯ 0.22 Ma [CHI-98A]) come from of the thickness of the volcanic pile above the and CHI-54 (An27Ab67Or6) were collected at the central domain, El Venado, or just to its gravels vary between 10 and 50 m. At the the same sites as volcanic rocks CHI-91A and west (Figs. 3 and 4). The 2.36 Ϯ 0.10 Ma age western edge of the Camargo volcanic ®eld, CHI-53, respectively, whose groundmasses of Cerros Prietos (CHI-33), an isolated cinder- in the barrancas (gullies) that drain into the El were also dated. In both cases the megacryst cone complex south of the El Venado domain, Llano playa (Fig. 2), it is evident that lava and groundmass ages are indistinguishable at agrees with ages of rocks within the domain stacks may locally reach 100 m in areas with the 2␴ level (Table 1). Although some workers (Fig. 3). relatively high vent density, such as the cen- have reported problems in obtaining reliable The 40Ar/39Ar ages demonstrate that volca- tral part of the La Loba and Maravillas do- 40Ar/39Ar ages from feldspar (e.g., Foland, nism migrated northeastward (Fig. 8B) at an mains. Therefore, we infer that an average of 1974; Harrison, 1990), our results indicate that average rate of ϳ15 mm/yr. This estimate uses 40 m for the lava plateau is conservative. Fur- feldspar megacrysts can yield reliable eruption only samples from identi®able vents (Fig. thermore, the fact that most volcanoes in the ages for host volcanic rocks. 8A), thereby excluding lava-plain samples La Loba and El Venado domains are moder- whose source-vent locations are unknown and ately to deeply eroded, as shown by the geo- Ages of Other Volcanic Rocks near the potentially distant from the sampling sites. morphologic analysis performed by Noyola- Camargo Volcanic Field Vent ages were projected onto a N60ЊE-trending Medrano (1995), makes a more precise line (Fig. 3), which is approximately perpen- estimate very dif®cult. The three older dated volcanic rocks near dicular to the Pliocene±Pleistocene tectonic If the ages obtained for the oldest (CHI-36, the Camargo volcanic ®eld (Fig. 3) are all grain de®ned by the trends of synvolcanic nor- 4.73 Ϯ 0.04 Ma) and youngest (CHI-57, 0.09 calc-alkalic and compositionally distinct from mal fault scarps. The total distance (ϳ60 km) Ϯ 0.04 Ma) samples represent the life span of the ma®c alkalic rocks of the Camargo vol- between the oldest vent in the southwest part the volcanic ®eld (4.72 Ma at the 2␴ level), canic ®eld. Two of these are from the north- of the ®eld and the youngest volcano was di- the long-term eruption rate is ϳ0.026 km3/k.y. west margin of the volcanic ®eld. CHI-56 vided by the life span of the ®eld (ϳ4 m.y.), This is similar to eruption rates reported for (41±40 Ma) is a hypersthene-normative basalt inferred from the 40Ar/39Ar ages of known other intraplate continental volcanic ®elds, from the thick, columnar-jointed Cerro Prieto vents. It is not possible from the available data such as the Ocate (New Mexico), Lunar Crater sill (Smith, 1993). This age is similar to those to establish whether the shift in volcanic ac- (Nevada), and Durango (MeÂxico) ®elds (Table reported by Smith et al. (1996) for the oldest tivity was continuous or episodic. Ages shown 2), but considerably lower than rates calculat- middle Tertiary volcanic rocks in this region. in Figure 8A appear to cluster in intervals old- ed for the intraplate Springerville ®eld and the CHI-111 (13.97 Ϯ 0.08 Ma) is a quartz- er than 4, approximately 3, 2.5±2.2, 1.6±1.3, subduction-related MichoacaÂn-Guanajuato normative trachyandesite, collected from an- and 0.09 Ma. This clustering may re¯ect an volcanic ®eld. All these are dwarfed by erup- other columnar-jointed sill (Smith, 1993) ϳ6 episodic phenomenon or incomplete sampling. tion rates for the world's largest intraplate, km to the north-northeast. This sill is ϳ20 m Furthermore, although the oldest volcanoes are oceanic volcanoes, such as Kilauea and thick and overlies ϳ15 m of poorly sorted, generally in the southwest part of the ®eld and Mauna Loa. poorly consolidated, coarse-grained, polymic- the youngest in the northeast, as seen in Figure tic sandstones that are partly conglomeratic. 4, there is signi®cant overlap between the areas Age of Feldspar Megacrysts These sediments appear to correlate with the with morphologically old and intermediate-age gravels that elsewhere underlie the ma®c al- volcanoes. Migration of alkali olivine basalt Feldspar megacrysts are common at many kalic rocks of the Camargo volcanic ®eld, in- vents has also been documented in the San localities in the Camargo volcanic ®eld, in dicating that the gravels are older than 14 Ma. Francisco volcanic ®eld of Arizona (29 Ϯ 3 other late Cenozoic ma®c alkalic volcanic The third of the older dated rocks is from the mm/yr toward 93ЊϮ5Њ; Tanaka et al., 1986) ®elds in the Basin and Range province, and in northeast margin of the Camargo volcanic and the Springerville ®eld of Arizona (29 Ϯ ma®c alkalic volcanic rocks from continental ®eld; CHI-103 (29.50 Ϯ 0.12 Ma) is a quartz- 11 mm/yr toward 93ЊϮ22Њ; Condit et al., rift zones worldwide (e.g., Binns, 1969; Ir- normative trachybasalt, collected from an ap- 1989b). These displacements have been inter- ving, 1974; Aspen et al., 1990). These inclu- parent volcanic neck northeast of Honorato de preted as the result of migration of the North sions range in size from a few millimeters to Abajo (Fig. 3). Its age is slightly younger than American plate over ®xed mantle hotspots. several centimeters and are either xenocrystic ages obtained by Smith et al. (1996) for the

Figure 6. 40Ar/39Ar incremental heating spectra and inverse isochron data for sample pairs related to faulting. Left-column samples are determined to be prefaulting. Right-column samples are corresponding samples determined to be postfaulting. See notes for Figure 5. N

Geological Society of America Bulletin, March 2003 307 ARANDA-GOÂ MEZ et al.

Figure 7. 40Ar/39Ar incremental heating spectra and inverse iso- chron data for sample pairs related to movement on the Las Bor- regas fault. Left-column samples are determined to be prefaulting. Right-column samples are corresponding samples determined to be postfaulting. See notes for Figure 5.

308 Geological Society of America Bulletin, March 2003 SYNEXTENSIONAL PLIOCENE±PLEISTOCENE CAMARGO VOLCANIC FIELD, CHIHUAHUA, MEÂ XICO

(Figure 8. Summary of 40Ar/39Ar results. (A) Ages of dated volcanoes (excluding outliers) projected onto a N60؇E-trending line. (B Isochron map of the Camargo volcanic ®eld using the 40Ar/39Ar results with intervals of 1, 2, and 3 Ma. (C) Ages of dated faults projected onto a N60؇E-trending line. The Lagunetas and Honorato faults were not dated and are positioned schematically on the basis of ®eld relationships that indicate their relative ages with respect to faults in the central graben. (D) Plot of elapsed time between delimiting ages vs. minimum vertical slip rates in the central part of the Camargo volcanic ®eld.

middle Tertiary Agua de Mayo volcanic group thus the Lagunetas fault was active between eral faults of the Camargo volcanic ®eld. Fol- in this region. 4.7 and 3.05 Ma. The easternmost structure of lowing the work of McCalpin (1995), we as- the La Loba domain (El Milagro) is older than sumed that the degraded scarp heights are Timing of Faulting four faults from the Venado domain. The latter equivalent to the minimum vertical compo- domain appears to have formed during a rel- nent, because the top of the downthrown block Our new 40Ar/39Ar ages also set limits on atively short (ϳ0.8 m.y.) period of synchro- is invariably buried under alluvium, lake beds, the timing of faulting in the Camargo volcanic nous volcanism and faulting. The Honorato and/or talus deposits of unknown thickness. ®eld and provide some of the ®rst vertical slip fault, located in the northeast part of the ®eld, Additional uncertainties in the estimated slip rates for faults in the Mexican part of the Ba- was not dated in this study, but it cuts allu- rates arise from (1) errors in estimating the sin and Range province. Plotting the ages of vium and lavas younger than those in the Ven- fault-scarp heights from topographic maps dated faults along the N60ЊE-trending line ado region and thus appears to be among the with 10 m contour intervals, (2) reported un- (Fig. 8C) suggests that the faulting, or at least youngest faults in the Camargo volcanic ®eld. certainties in the isotopic ages of the two sam- the cessation of faulting, may also have mi- ples used to bracket the age of the fault (in grated from southwest to northeast. The ob- CALCULATION OF SLIP RATES FOR order to calculate minimum slip rates we used servation that the Lagunetas fault cuts some NORMAL FAULTS IN THE CAMARGO maximum age differences at the 2␴ level; Ta- La Loba lavas but is buried by others erupted VOLCANIC FIELD ble 1), and (3) the assumption that total strain in the eastern part of the domain indicates that accumulated over the entire time period lim- the fault last slipped before volcanism and The 40Ar/39Ar ages (Figs. 5±7) were used to ited by the dated samples. faulting in the Venado domain commenced; calculate minimum vertical slip rates for sev- Three different types of sample pairs are

Geological Society of America Bulletin, March 2003 309 ARANDA-GOÂ MEZ et al.

TABLE 2. CALCULATED ERUPTION RATES FOR THE CVF AND OTHER VOLCANIC FIELDS ro Lamojino, a cinder cone that drapes the Las Locality Duration Area Volume Rate Rate/Area Borregas fault (Fig. 4). CHI-85 and CHI±36 (Ma) (km) (km3) (km3/ka) (km3/ka-km) are both samples of the lava plain exposed in Camargo volcanic ®eld² 4.64 3000 120 0.026 8.6E±6 the high scarps of the Las Borregas and El Servilleta basalt³ 1.0 200 0.2 Milagro fault systems. Bracketing the age of Ocate volcanic ®eld§ Pulse 1 2.6 95 0.9 0.002 2.1E±5 the Las Borregas fault with samples CHI-53 Pulse 2 0.8 420 31 0.04 9.5E±5 and CHI-85 and considering a minimum Pulse 3 0.2 320 23 0.12 3.8E±4 throw of 40 m yields a minimum slip rate of Pulse 4 0.2 360 28 0.14 3.9E±4 Pulse 5 0.6 185 4.0 0.007 3.8E±5 ϳ0.07 mm/yr. Calculating the time interval Total 4.4 1380 89.9 0.02 1.5E±5 from the pair CHI-53 and CHI-36 by using Springerville volcanic ®eld# 1.8 3000 300 0.17 5.6E±5 Durango volcanic ®eld 1.64³³ 2200 33²² 0.02 9.1E±6 the same 40 m throw yields a minimum slip MichoacaÂn-Guanajuato volcanic ®eld§§ 0.04 15,000 31 0.8 5.3E±5 rate of only ϳ0.02 mm/yr, which is three Lunar Crater volcanic ®eld## 5.7 100 0.018 times lower. Mount Etna## 0.000439 3.6 8.2 Kilauea## 0.000185 3.3 17.8 Mauna Loa## 0.5 42,500 85 DISCUSSION ²This paper. ³Dungan et al. (1986). Faulting, Volcanism, and Regional Stress §Nielson and Dungan (1985). #Condit et al. (1989a). ²²Smith (1989). Basaltic magma erupted as scoria cones and ³³McIntosh, W.C. (written comm., 1998). lava ¯ows along active faults in the Camargo §§Hasenaka and Carmichael (1985). ##Crisp (1984). volcanic ®eld (e.g., Fig. 4) because dikes abandon vertical ascent paths perpendicular to

the least principal stress direction (␴3) in favor recognized (Table 1). For the ®rst type, the pair of samples collected on the Las Hormigas of more energy-ef®cient paths along preexist- two samples have ages that are well resolvable fault (Figs. 4±5; location in Fig. 3), where the ing joints or faults (Conway et al., 1997). The 40 39 by Ar/ Ar dating; these samples are associ- median value of the closely similar ages is orientation of a fault relative to ␴3 plays a key ated with the El Milagro, El Venado, and El nonetheless the reverse of what was expected role in determining whether a fault zone is Espejo faults, and the northern end of the Las from ®eld relationships (Fig. 8C), and the likely to dilate in response to dike injection Borregas fault (Table DR-1). For these faults, maximum age difference at the 2␴ level is (Connor and Conway, 2000). Given the syn- minimum heights divided by the maximum zero (Table DR-1). It is clearly not possible to chronous nature of faulting and volcanism, age differences yield minimum slip rates av- estimate the slip rate from these data. most of the active normal faults in the Ca- eraged over those time intervals. The vertical The implication of the ages for the second margo volcanic ®eld, especially in the El Ven- displacement for the Borregas fault is Ͼ90 m, and third types of sample pairs is that the two ado domain, were perpendicular to ␴3 and corresponding to a slip rate of Ͼ0.16 mm/yr. bracketing eruptions and the intervening fault- were able to provide low-energy pathways for The minimum throw for the El Milagro fault ing occurred in very narrow time intervals. ascending dikes. is 50 m, corresponding to an estimated slip Therefore, these slip rates cannot be extrapo- Parsons and Thompson (1991) suggested rate of Ͼ0.03 mm/yr. In the case of the El lated to the regional context, and certainly that the topography related to normal faulting Venado fault, the maximum age difference be- they are not representative of long-term is suppressed near volcanic ®elds because tween the dated samples is 0.34 m.y., and the deformation. stress is accommodated by dilation of the scarp height of Ͼ10 m yields a slip rate of A plot of the age differences for the sample crust during dike injection rather than by fault Ͼ0.03 mm/yr. For the El Espejo fault, the pairs versus the minimum slip rates (Fig. 8D) slip. This mechanism can explain much of the maximum age difference between CHI-83 and shows that the average slip rate decreases dra- variation in structure of volcanic ®elds. For CHI-84 is 0.09 m.y., and the scarp of 10 m matically as the age difference increases. example, cinder-cone alignments are common gives a slip rate of Ͼ0.11 mm/yr. Probably the high values of 0.71±1.67 mm/yr in low-volume, low-density volcanic ®elds For the second type of sample pairs, rele- obtained for the Las Borregas fault at Cerro (e.g., Big Pine, California). In larger-volume vant to two different points along the same Mojoneras are short-term rates operative dur- basaltic ®elds, like the Springerville (Condit fault scarp (Las Borregas fault at Cerro La- ing periods of active faulting. The larger age and Connor, 1996) and MichoacaÂn-Guanajuato mojino and at Cerro Mojoneras, Fig. 4), the differences between samples for the other ®elds (Connor, 1990), mapped faults are rare, 40Ar/39Ar ages overlap at the 2␴ level. Slip- faults are somewhat arbitrary, dictated as they and cinder-cone alignments are less pervasive. rate calculations can be still made. In the case are by the age of the uppermost plateau lava In the Big Pine case, rates of dike injection of the Las Borregas fault at Cerro Mojoneras, at that site, which probably had no direct re- were not suf®cient to fully accommodate the maximum age difference between CHI-92 lationship to the age of the faulting, and by crustal stress. As a result, faults continued to and CHI-91A at the 2␴ level is [(2.30 ϩ 0.06) the fact that displacement probably occurred slip, and dikes tended to parallel or inject into ϫ 106 yr] Ϫ [(2.35 Ϫ 0.06) ϫ 106 yr] ϭ 0.07 during rapid clusters of paleoearthquakes sep- these faults. In contrast, rates of dike injection ϫ 106 yr. Thus, dividing the minimum scarp arated by longer aseismic intervals (Mc- were suf®cient to completely accommodate height (ϳ50 m) by this maximum age differ- Calpin, 1995). Independence of the lava-plain regional tectonic stresses within the larger ence yields a slip rate of Ͼ0.71 mm/yr. Treat- age from the age of faulting can be illustrated Springerville and MichoacaÂn-Guanajuato vol- ing the pair CHI-92 and CHI-91B in the same if slip rates of the Las Borregas fault are com- canic ®elds. This interpretation is also consis- way, a slip rate of Ͼ1.67 mm/yr is obtained. puted with samples CHI-53, CHI-85, and tent with the antithetical spatial relationship The third type is represented by only one CHI-36. Sample CHI-53 was collected at Cer- between faults and plutons documented by

310 Geological Society of America Bulletin, March 2003 SYNEXTENSIONAL PLIOCENE±PLEISTOCENE CAMARGO VOLCANIC FIELD, CHIHUAHUA, MEÂ XICO

able, we assume that extension in Chihuahua- Coahuila occurred in several distinct pulses separated by quasi-quiescent periods and that several pulses of deformation likely affected a given area. Thus, we infer that basin-and- range structures in the Camargo volcanic ®eld were developed in several pulses of activity that cumulatively produced the present-day physiography. The total vertical fault displacements in the Camargo volcanic ®eld, inferred from the height of the degraded scarps in the central graben and the subtle tilting marked by the playa lakes, indicate low-magnitude Pliocene± Quaternary extension. The lower values of the calculated vertical slip rates (0.03 mm/yr) ap- pear to be consistent with this scenario, be- cause continuous extension since the early Pli- ocene at this rate would produce vertical displacements of the same order of magnitude as those observed in the central graben (ϳ100 m). On the other hand, the higher slip-rate es- timates (0.67±1.67 mm/yr) operating for 3±4 m.y. would have caused unrealistically high vertical displacements on these faults. Thus, we infer that the long-term vertical slip rates in the area must be near the low end of the Figure 9. Vent and fault distributions in the Camargo volcanic ®eld. Contour lines are values obtained. These data are consistent vent densities obtained as follows: The number of vents around each volcano was counted with what little is known about slip rates in in a circular area of 25 km2. These values were used in the contouring process. The line the southern Basin and Range province (e.g., that represents one volcano is not shown. Shaded area represents the El Venado domain, Collins et al., 1996). Clear evidence of a close as shown in Figure 4. Heavy black lines are normal faults cutting the lava ®elds. relationship between faulting and volcanic ac- tivity, together with the well-documented northeast shift of volcanism in the Camargo volcanic ®eld, suggests that both volcanism Paterson and Schmidt (1999) and Schmidt and Regional Versus Local Extension, and the and faulting may have occurred in relatively Paterson (2000). Possibility of Active Faulting in Southeast short time spans of rapid and ephemeral activ- Regionally, volcanism in the Camargo vol- Chihuahua ity. Therefore, the high slip rates obtained for canic ®eld may account for the decreases in some of the Camargo volcanic ®eld faults displacement along the San Francisco and Our data set on late Cenozoic normal fault- with very tight time brackets may re¯ect short Agua de Mayo faults as they approach the vol- ing is limited to the Camargo volcanic ®eld intervals with high rates of local down drop- canic ®eld (Fig. 2). In both the La Loba and its immediate surroundings. Previous geo- ping, but do not indicate long-term (southwestern) domain and the Maravillas logic studies of eastern Chihuahua and adja- deformation. (northeastern) domain, rates of dike injection cent Coahuila have indicated that the region A belt of active faulting has been reported appear to have been suf®cient to equalize, or belongs either to the Rio Grande rift (Gries, along the Rio Grande, from the region west of nearly equalize, the magnitudes of principal 1979; Seager and Morgan, 1979; Smith and El Paso (Nakata et al., 1982) to the Big Bend horizontal stresses. Thus, vent alignments are Jones, 1979) or to the Basin and Range prov- area (Muehlberger et al., 1978; Henry et al., less common in these domains, and fault slip ince (e.g., papers on Chihuahua in Goodell 1985; Collins et al., 1996). Seismicity in Chi- is less dramatic or absent. In contrast, vent and Waters, 1981; Henry and Aranda-GoÂmez, huahua and surrounding regions and the oc- density is generally lower in the El Venado 1992, 2000). However, there are no detailed currence of at least three large earthquakes of (central) domain (Fig. 9), fault slip is more investigations on the general characteristics M Ͼ 6.3 (Doser and RodrõÂguez, 1992) indi- pronounced, and vent alignments are more ob- and evolution of Cenozoic extension in the re- cate that extension is occurring in the region. vious. In this area, crustal stresses do not ap- gion. Extension may have begun before 45 Ma Therefore, we interpret (1) the youthful geo- pear to have been completely accommodated in Chihuahua (e.g., Mauger, 1981; Capps, morphologic features in Sierra San Francisco by dike-related strain. Thus, synextensional 1981), was de®nitely active in the Oligocene± of the southern El Venado domain, and (2) volcanism in the Camargo volcanic ®eld ap- Miocene (Bartolino, 1992) and Pliocene (this faults that displace alluvial deposits and late pears to nicely demonstrate the complemen- paper), and may continue to the present (e.g., Pleistocene volcanic rocks in the northern tary roles of dike injection and faulting in re- Doser and RodrõÂguez, 1992). By comparison Maravillas domain as consistent with the in- sponse to regional stress in the southern Basin with other areas in northern and central MeÂx- terpretation that extension continues at a low and Range. ico where more detailed information is avail- rate in the region.

Geological Society of America Bulletin, March 2003 311 ARANDA-GOÂ MEZ et al.

SUMMARY AND CONCLUSIONS fully thank them for their assistance. We also thank La Olivina: Journal of Geophysical Research, v. 97, Luca Ferrari, Max Suter, John Geissman, and John p. 17,353±17,376. Bartley for the critical comments on the ®rst version Capps, R.C., 1981, Geology of the Rancho El Papalote area, The Camargo volcanic ®eld formed in the of this manuscript, and Gerardo Aguirre-Diaz, Hugo Chihuahua, Mexico, in Goodell, P.C., and Waters, A.C., Pliocene±Pleistocene (4.73±0.09 Ma) through eds., Uranium in volcanic and volcaniclastic rocks: Tul- Delgado-Granados, and Lang Farmer for insightful sa, Oklahoma, American Association of Petroleum Ge- hundreds of eruptions of ma®c alkalic mag- reviews of the revised version. Financial support for ologists Studies in Geology 13, p. 243±264. mas, some of which carried xenoliths of upper- this investigation was provided by Consejo Nacion- Comision de Estudios del Territorio Nacional (Cetenal), mantle peridotite and deep-crustal granulite. al de Ciencia y TecnologõÂa (MeÂxico) grant 3657PT 1974, El Milagro G13B22: MeÂxico, D.F., SecretarõÂa (to Aranda), the Smithsonian Institution's Scholarly de la Presidencia, ComisioÂn de Estudios del Territorio Compared with other volcanic ®elds of similar Studies Program (to Luhr), and the U.S. Nuclear Nacional, Topographic quadrangle, scale 1:50,000, 1 age and composition in the Mexican Basin Regulatory Commission (NRC) under contract sheet. NRC-02-97-009 to the Center for Nuclear Waste Collins, E.W., Raney, J.A., Machette, M., Haller, K.M., and and Range province, the Camargo volcanic Dart, R.L., 1996, Map and data for Quaternary faults ®eld is unusually large and voluminous, cov- Regulatory Analyses (to Connor). This work does in West Texas and adjacent parts of Mexico: U.S. not necessarily re¯ect the views or regulatory po- ering ϳ3000 km2 with a volume of ϳ120 km3. Geological Survey Open-File Report 96±002, 7 p. sition of the NRC. Condit, C.D., and Connor, C.B., 1996, Recurrence rates of The long-term magmatic eruption rate is es- volcanism in basaltic volcanic ®elds: An example timated at 0.026 km3/k.y., typical of other maf- from the Springerville volcanic ®eld, Arizona: Geo- REFERENCES CITED logical Society of America Bulletin, v. 108, ic alkalic volcanic ®elds in the Basin and p. 1225±1241. Range province. Condit, C.D., Crumpler, L.S., and Aubele, J.C., 1989a, The Camargo volcanic ®eld developed Aguirre-DõÂaz, G.J., and McDowell, F.W., 1991, The vol- Fourth-day ®eld trip: Springerville volcanic ®eld, in canic section at Nazas, Durango, MeÂxico, and the pos- Chapin, C.E., and Zedik, J., eds., Field excursions to where the regional San Marcos fault coincides sibility of widespread Eocene volcanism within the volcanic terranes in the Western United States, Vol- with an antithetic transfer zone between the Sierra Madre Occidental: Journal of Geophysical Re- ume I: Southern Rocky Mountain region: New Mex- opposing San Francisco and Sierra Agua de search, v. 96, p. 13,373±13,388. ico Bureau of Mines and Mineral Resources Memoir Almeida-Vega, M., Espinosa-CardenÄa, J.M., and Ledesma- 4, p. 33±41. Mayo fault systems. Volcanism and normal VaÂzquez, J., 2000, Structure of the San QuintõÂn coastal Condit, C.D., Crumpler, L.S., Aubele, J.C., and Elston, faulting at the Camargo volcanic ®eld were at valley, B.C., from aeromagnetic data analysis, in Me- W.E., 1989b, Patterns of volcanism along the southern least in part contemporaneous, and normal morias, V ReunioÂn internacional sobre la geologõÂa de margin of the Colorado Plateau: The Springerville la penõÂnsula da Baja California: Loreto, Baja Califor- ®eld: Journal of Geophysical Research, v. 94, faults commonly acted as magmatic conduits nia Sur, MeÂxico, Sociedad GeoloÂgica Peninsular, p. 64. p. 7975±7986. in the central graben. Eruptive activity mi- Aranda-GoÂmez, J.J., Henry, C.D., Luhr, J.F., and McDow- Coney, P.J., and Campa, M.F., 1987, Lithotectonic terrane ell, F.W., 1997, Cenozoic volcanism and tectonics in map of MeÂxico (west of the 91st meridian): U.S. Geo- grated northeastward at ϳ15 mm/yr. A similar northwest Mexico: A transect across the Sierra Madre logical Survey Miscellaneous Field Studies Map MF- northeast shift may also have occurred in the Occidental volcanic ®eld and observations on extension 1874-D, scale 1:2,500,000. locus or cessation of Camargo volcanic ®eld related magmatism in the southern Basin and Range and Connor, C.B., 1990, Cinder cone clustering in the Gulf of California tectonic provinces, in Aguirre-DõÂaz, TransMexican volcanic belt: Implications for structur- faulting. G.J., Aranda-GoÂmez, J.J., Carrasco-NunÄez, G., and Fer- al and petrologic models: Journal of Geophysical Re- Minimum vertical slip rates were calculated rari, L., eds., Magmatism and tectonics in the central search, v. 95, p. 19395±19405. for four faults in the central graben and range and northwestern Mexico: A selection of the 1997 Connor, C.B., and Conway, F.M., 2000, Basaltic volcanic IAVCEI General Assembly excursions: MeÂxico, D.F., ®elds, in Sigurdsson, H., ed., Volcano encyclopedia: between 0.03 and 1.67 mm/yr. The average Universidad Nacional AutoÂnoma de MeÂxico, Instituto New York, Academic Press, p. 331±343. slip rate decreases as the age difference be- de GeologõÂa, ExcursioÂn 11, p. 41±84. Conway, F.M., Ferrill, D.A., Hall, C.M., Morris, A.P., Sta- Aspen, J., Upton, B.G.J., and Dickin, A.P., 1990, Anorth- matakos, J.A., Connor, C.B., Halliday, A.N., and Con- tween the bracketing samples increases. The oclase, sanidine and associated megacrysts in Scottish dit, C.D., 1997, Timing of basaltic volcanism along larger estimates may approximate slip rates alkali basalts: High-pressure syenitic debris from up- the Mesa Butte fault in the San Francisco volcanic during main faulting intervals, whereas the per mantle sources?: European Journal of Mineralogy, ®eld, Arizona from 40Ar/39Ar dates: Implications for v. 2, p. 503±517. the longevity of cinder cone alignments: Journal of smaller values better re¯ect longer-term rates Axen, G.J., Taylor, W.J., and Bartley, J.M., 1993, Space- Geophysical Research, v. 102, p. 815±824. that include long intervals without fault move- time patterns and tectonic controls of Tertiary exten- Crisp, J.A., 1984, Rates of magma emplacement and vol- ments. It is likely that the calculated slip rates sion and magmatism in the Great Basin of the western canic output: Journal of Volcanology and Geothermal United States: Geological Society of America Bulle- Research, v. 20, p. 177±211. underestimate the vertical component of the tin, v. 105, p. 56±76. Doser, D.I., and RodrõÂguez, J., 1992, The seismicity of Chi- regional extension rate because volcanic activ- Bailey, D.K., 1974, Continental rifting and alkaline mag- huahua, MeÂxico and surrounding regions: Guidebook ity tends to suppress normal faulting (Parsons matism, in Sorensen, H., ed., The alkaline rocks: New for the 1992 Field Conference, El Paso Geological York, John Wiley and Sons, p. 148±159. Society, p. 99±105. and Thompson, 1991) and the transfer zones Bartolino, J.R., 1992, Modi®ed Basin and Range topography Dungan, M.A., Lindstrom, M.M., McMillan, N.J., Moor- or bends in a segmented normal fault coincide in the BolsoÂn de Mapimi, Durango and Chihuahua, bath, S., Hoefs, J., and Haskin, L.A., 1986, Open sys- MeÂxico: The Texas Journal of Science, v. 44, no. 3, tem magmatic evolution of the Taos Plateau volcanic with displacement minima in the system (Pea- p. 295±300. ®eld, northern New Mexico: 1. The petrology and cock and Sanderson, 1997). Binns, R.A., 1969, High-pressure megacrysts in basanitic geochemistry of the Servilleta basalt: Journal of Geo- Two different feldspar megacrysts yielded lavas near Armidale, New South Wales: American physical Research, v. 91, p. 5999±6028. 40 39 Journal of Science, v. 267-A, p. 33±49. Foland, K.A., 1974, Ar 40 diffusion in homogeneous or- Ar/ Ar ages identical to groundmass frac- Cameron, K., and Jones, N., 1993, A reconnaissance Nd-Sr thoclase and an interpretation of Ar diffusion in K- tions from the same eruptions, demonstrating isotopic study of pre-Cenozoic igneous and metaig- feldspars: Geochimica et Cosmochimica Acta, v. 38, that such megacrysts can be used to determine neous rocks of the Coahuila terrane, northeastern Mex- p. 151±166. ico, in Ortega-GutieÂrrez, F., Coney, P.J.,Centeno-GarcõÂa, Gans, P.B., Mahood, G.A., and Schermer, E., 1989, Synex- reliable eruption ages for their host magmas. E., and GoÂmez-Caballero, A., eds., First Circum-Paci®c tensional magmatism in the Basin and Range province: and Circum-Atlantic Terrane Conference, Proceedings: A case study from the eastern Great Basin: Geological ACKNOWLEDGMENTS Guanajuato, MeÂxico, Universidad Nacional AutoÂnoma Society of America Special Paper 233, p. 1±53. de MeÂxico, Instituto de GeologõÂa, p. 24±27. Glazner, A.F., and Bartley, J.M., 1994, Eruption of alkali Initial reconnaissance work and compilation of a Cameron, K.L., Clark, L.F., and Cameron, M., 1983, A pre- basalts during crustal shortening in southern Califor- photogeologic map of the Camargo volcanic ®eld liminary report on the nature of the lower crust and nia: Tectonics, v. 13, p. 493±498. was performed by MarõÂa Cristina Noyola-Medrano upper mantle beneath southeastern Chihuahua: El Goodell, P.C., and Waters, A.C., 1981, Uranium in volcanic Paso Geological Society Guidebook, p. 102±107. and volcaniclastic rocks: American Association of Pe- and Marco Antonio Rojas-BeltraÂn. Gerardo Aguirre, Cameron, K.L., Robinson, J.V., Niemeyer, S., Nimz, G.J., troleum Geologists Studies in Geology 3, 331 p. John Stamatakos, Marco Antonio, and Paul Kim- Kuentz, D.C., Harmon, R.S., Bohlen, S.R., and Coll- Grajales-Nishimura, J.M., Terrell, D.J., and Damon, P.E., berly assisted in the ®eld work. Gustavo Tolson read erson, K.D., 1992, Contrasting styles of pre-Ceno- 1992, Evidencias de la prolongacioÂn del arco mag- and commented on an earlier draft of the paper. Ig- zoic and mid-Tertiary crustal evolution in northern maÂtico cordillerano del TriaÂsico±JuraÂsico en Chihua- nacio Navarro helped to prepare Figure 9. We grate- MeÂxico: Evidence from deep crustal xenoliths from hua, Durango y Coahuila: BoletõÂn de la AsociacioÂn

312 Geological Society of America Bulletin, March 2003 SYNEXTENSIONAL PLIOCENE±PLEISTOCENE CAMARGO VOLCANIC FIELD, CHIHUAHUA, MEÂ XICO

Mexicana de GeoloÂgos Petroleros, v. XLII, no. 2, McDowell, F.W., and Keizer, R.P., 1977, Timing of mid- Evolution of geological structures in micro- to mac- p. 1±18. Tertiary volcanism in the Sierra Madre Occidental be- roscales: London, Chapman and Hall, p. 27±46. Gries, J.C., 1979, Problems of delineation of the Rio tween Durango City and Mazatlan, MeÂxico: Geolog- Rudnick, R.L., and Cameron, K.L., 1991, Age diversity of Grande rift into the Chihuahua tectonic belt of north- ical Society of America Bulletin, v. 88, p. 1479±1486. the deep crust in northern MeÂxico: Geology, v. 19, eastern Mexico, in Riecker, R.E., ed., The Rio Grande McKee, J.W., and Jones, N.W., 1979, A large Mesozoic p. 1197±1200. riftÐTectonics and magmatism: Washington, D.C., fault in Coahuila, Mexico: Geological Society of Schmidt, K.L., and Paterson, S.R., 2000, Analyses fail to American Geophysical Union, p. 107±113. America Abstracts with Programs, v. 11, p. 476. ®nd coupling between deformation and magmatism: Harrison, T.M., 1990, Some observations on the interpre- McKee, J.W., Jones, N.W., and Long, L.E., 1984, History Eos (Transactions, American Geophysical Union), tation of feldspar 40Ar-39Ar results: Chemical Geology, of recurrent activity along a major fault in northeast- v. 81, p. 197±203. v. 80, p. 219±229. ern MeÂxico: Geology, v. 12, p. 103±107. Seager, W.R., and Morgan, P., 1979, Rio Grande rift in Hasenaka, T., and Carmichael, I.S.E., 1985, The cinder McKee, J.W., Jones, N.W., and Long, L.E., 1990, Stratig- southern New Mexico, west Texas, and northern Chi- cones of MichoacaÂn-Guanajuato, central MeÂxico: raphy and provenance of strata along the San Marcos huahua, in Riecker, R.E., ed., The Rio Grande riftÐ Their age, volume and distribution, and magma dis- fault, central Coahuila, MeÂxico: Geological Society of Tectonics and magmatism: Washington, D.C., Ameri- charge rate: Journal of Volcanology and Geothermal America Bulletin, v. 102, p. 593±614. can Geophysical Union, p. 87±106. Research, v. 25, p. 105±124. Muehlberger, W.R., Belcher, R.C., and Goetz, L.K., 1978, Sedlock, R.L., Ortega-GutieÂrrez, F., and Speed, R.C., 1993, Henry, C.D., and Aranda-GoÂmez, J.J., 1992, The real Quaternary faulting in Trans-Pecos Texas: Geology, Tectonostratigraphic terranes and tectonic evolution of southern Basin and Range: Mid to late Cenozoic ex- v. 6, p. 337±340. MeÂxico: Geological Society of America Special Paper, tension in Mexico: Geology, v. 20, p. 701±704. Nakata, J.K., Wentworth, C.M., and Machette, M.N., 1982, v. 278, 153 p. Henry, C.D., and Aranda-GoÂmez, J.J., 2000, Plate interac- Quaternary fault map of the Basin and Range and Rio Sharp, W.D., Turrin, B.D., Renne, P.R., and Lanphere, tions control middle±late Miocene, proto-Gulf and Ba- Grande rift provinces, western United States: U.S. M.A., 1996, The 40Ar/39Ar and K/Ar dating of lavas sin and Range extension in the southern Basin and Geological Survey Open-File Report 82±579, scale 1: from the Hilo 1-km core hole, Hawaii Scienti®c Dril- Range: Tectonophysics, v. 318, p. 1±26. 2,500,000. ling Project: Journal of Geophysical Research, v. 101, Henry, C.D., Gluck, J.K., and Bockoven, N.T., 1985, Tec- Nielson, R.L., and Dungan, M.A., 1985, The petrology and p. 11,607±11,616. tonic map of the Basin and Range province of Texas geochemistry of the Ocate volcanic ®eld, north-central Smith, J.A., 1989, Extension-related magmatism of the Du- and adjacent Mexico: Austin, University of Texas, Bu- New Mexico: Geological Society of America Bulletin, rango volcanic ®eld, Durango, MeÂxico [M.A. thesis]: reau of Economic Geology, Miscellaneous Map 36, v. 96, p. 296±312. St. Louis, Missouri, Washington University, 102 p. scale 1:500,000. Nieto-Samaniego, A.F., Ferrari, L., Alaniz-Alvarez, S.A., Smith, R.D., 1993, The Agua de Mayo mid-Cenozoic vol- Irving, A.J., 1974, Megacrysts from the Newer Basalts Labarthe-HernaÂndez, G., and Rosas-Elguera, J.G., canic group and related xenoliths from La Olivina and other basaltic rocks from southeastern Australia: 1999, Variation of Cenozoic extension and volcanism southeast Chihuahua, MeÂxico [M.S. thesis]: Santa Geological Society of America Bulletin, v. 85, across the southern Sierra Madre Occidental volcanic Cruz, University of California, 112 p. p. 1503±1514. province, MeÂxico: Geological Society of America Smith, D.L., and Jones, R.L., 1979, Thermal anomaly in Keller, E.A., 1986, Investigation of active tectonics: Use of Bulletin, v. 111, p. 347±363. northern Mexico: An extension of the Rio Grande sur®cial earth processes, in Active tectonics: Wash- Nimz, G.J., Cameron, K.L., Cameron, M., and Morris, S.L., rift?, in Riecker, R.E., ed., The Rio Grande riftÐTec- ington, D.C., National Academy Press, p. 136±147. 1986, The petrology of the lower crust and upper tonics and magmatism: Washington, D.C., American Luhr, J.F., Aranda-GoÂmez, J.J., and Pier, J.G., 1989, Spinel- mantle beneath southeastern Chihuahua, MeÂxico: A Geophysical Union, p. 269±278. lherzolite-bearing, Quaternary volcanic centers in San progress report: GeofõÂsica Internacional, v. 25, no. 1, Smith, R.D., Cameron, K.L., McDowell, F.W., Niemeyer, Luis PotosõÂ, MeÂxico: I. Geology, mineralogy, and pe- p. 85±116. S., and Sampson, D.E., 1996, Generation of volumi- trology: Journal of Geophysical Research, v. 94, Nimz, G.J., Cameron, K.L., and Niemeyer, S., 1993, The nous silicic magmas and formation of mid-Cenozoic p. 7916±7940. La Olivina pyroxenite suite and the isotopic compo- crust beneath north-central MeÂxico: Evidence from ig- Luhr, J.F., Pier, J.G., Aranda-GoÂmez, J.J., and Podosek, F., sitions of mantle basalts parental to mid-Cenozoic arc nimbrites, associated lavas, deep crustal granulites, 1995a, Crustal contamination in early Basin-and- volcanism of northern MeÂxico: Journal of Geophysical and mantle pyroxenites: Contributions to Mineralogy Range hawaiites of the Los Encinos volcanic ®eld, Research, v. 98, p. 6489±6509. and Petrology, v. 123, p. 375±389. central MeÂxico: Contributions to Mineralogy and Pe- Nimz, G.J., Cameron, K.L., and Niemeyer, S., 1995, For- Sociedad GeoloÂgica Mexicana A.C., D.C., 1985, Plano geo- trology, v. 118, p. 321±339. mation of mantle lithosphere beneath northern MeÂxi- loÂgico minero Chihuahua, MeÂxico: Consejo de Re- Luhr, J.F., Aranda-GoÂmez, J.J., and Housh, T.B., 1995b, co: Chemical and Sr-Nd-Pb isotopic systematics of pe- cursos Minerales, scale 1:500,000, 1 sheet. The San QuintõÂn volcanic ®eld, Baja California Norte, ridotite xenoliths from La Olivina: Journal of Swanson, E.R., Keizer, R.P., Lyons, J.I., and Clabaugh, MeÂxico: Geology, petrology, and geochemistry: Jour- Geophysical Research, v. 100, p. 4181±4196. S.E., 1978, Tertiary volcanism and caldera develop- nal of Geophysical Research, v. 100, p. 10353±10380. Noyola-Medrano, M.C., 1995, Estudio comparativo de la ment near Durango City, Sierra Madre Occidental, Luhr, J.F., Henry, C.D., Housh, T.B., Aranda-GoÂmez, J.J., geologõÂa y morfologõÂa de algunos conos cinerõÂticos en MeÂxico: Geological Society of America Bulletin, and McIntosh, W.C., 2001, Early extension and as- los Campos VolcaÂnicos de Camargo, Chih. y San v. 89, p. 1000±1012. sociated ma®c alkalic volcanism from the southern QuintõÂn, B.C., [B.S. thesis]: San Luis PotosõÂ, MeÂxico, Tanaka, K.L., Shoemaker, E.M., Ulrich, G.E., and Wolfe, Basin and Range province: Geology and petrology of Universidad AutoÂnoma de San Luis PotosõÂ, 91 p. E.W., 1986, Migration of volcanism in the San Fran- the Rodeo and Nazas volcanic ®elds, Durango (Mex- Padilla y SaÂnchez, R.J., 1986, Post-Paleozoic tectonics of cisco volcanic ®eld, Arizona: Geological Society of ico): Geological Society of America Bulletin, v. 113, northeast Mexico and its role in the evolution of the America Bulletin, v. 97, p. 129±141. p. 760±773. Gulf of Mexico: GeofõÂsica Internacional, v. 25, no. 1, Tarango-Ontiveros, G., 1993, Bosquejo geoloÂgico de la Mauger, R.L., 1981, Geology and petrology of the central p. 157±206. porcioÂn surcentral del Estado de Chihuahua, in III part of the Calera±Del Nido block, Chihuahua, Mex- Parsons, T., and Thompson, G.A., 1991, The role of magma ExcursioÂn geoloÂgica al Mesozoico de Chihuahua ico, in Goodell, P.C., and Waters, A.C., eds., Uranium overpressure in suppressing earthquakes and topog- (Chihuahua-Parral-JimeÂnez-Chihuahua): Chihuahua, in volcanic and volcaniclastic rocks: American Asso- raphy: Worldwide examples: Science, v. 253, Sociedad GeoloÂgica Mexicana, DelegacioÂn Chihua- ciation of Petroleum Geologists Studies in Geology p. 1399±1402. hua, p. 2±10. 13, p. 205±242. Paterson, S.R., and Schmidt, K.L., 1999, Is there a close MANUSCRIPT RECEIVED BY THE SOCIETY 28 JUNE 2001 McCalpin, J.P., 1995, Frequency distribution of geological- spatial relationship between faults and plutons?: Jour- REVISED MANUSCRIPT RECEIVED 1MAY 2002 ly determined slip rates for normal faults in the west- nal of Structural Geology, v. 21, p. 1131±1142. MANUSCRIPT ACCEPTED 9JULY 2002 ern United States: Bulletin of the Seismological So- Peacock, D.C.P., and Sanderson, D.J., 1997, Geometry and ciety of America, v. 85, p. 1867±1872. development of normal faults, in Sengupta, S., ed., Printed in the USA

Geological Society of America Bulletin, March 2003 313