Northern Sierra Madre Occidental Research Articles

Population genetics and species distribution modeling highlight conservation needs of the endemic trout from the Northern Sierra Madre Occidental

Population genetics and species distribution modeling highlight conservation needs of the endemic trout from the Northern Sierra Madre Occidental

Morphology of two native catfishes from the Sierra Madre Occidental, México: I. pricei and Ictalurus sp. (Siluriformes: Ictaluridae)

Introduction: Morphotypes of native catfish of the genus Ictalurus (Siluriformes: Ictaluridae) are known to occur in allopatry in the northern Sierra Madre Occidental of Mexico, with only the Yaqui catfish (Ictalurus pricei) taxonomically described. Recent genetic analysis of these morphotypes has revealed the monophyly of the I. pricei complex, which indicates Ictalurus sp. from the Culiacán River and San Lorenzo River basins as its nearest genetic relative and recognizes as an evolutionarily significant unit the Culiacán River and San Lorenzo River morphotypes. Objective: To compare the meristic and morphometric characteristics of the catfish of the Culiacán River basin with its nearest genetic relative, the Yaqui catfish, in order to determine the presence of distinctive morphological characters that support genetic evidence previously reported for these morphotypes. Methods: Catfish specimens were collected during various field expeditions (1990-2012) to remote sites of the Sierra Madre Occidental and conducted in the Yaqui River and Culiacán River basins with the purpose of morphological comparison. Forty-five morphological characters (40 morphometric and five meristic) were examined in 76 adult specimens – 52 Ictalurus sp. and 24 Ictalurus pricei. Three groups were subject to a discriminant function analysis (DFA), including two Ictalurus sp. groups from the Humaya River and Tamazula River sub-basins, representing the Culiacán River basin, and one I. pricei group representing the Yaqui River basin. The standardized measurements and meristic data of the catfish morphotypes were compared by means of DFA. Results: The DFA revealed 12 characters to be significantly different (P < 0.01) among the groups compared. The morphological characters separating the Ictalurus sp. (Culiacán River basin) from the Yaqui catfish were associated with lower anal, pelvic and pectoral fin ray numbers, shorter head and predorsal lengths, shorter longest lateral barbel and longest dorsal ray lengths and a narrower premaxilar dentary plate; and finally longer distances in Ictalurus sp. for dorsal-fin origin to last anal-fin ray base and dorsal-fin origin to posterior end of the adipose fin base. The standardized coefficients for canonical variables 1 and 2 accounted for 85.6 % and 14.4 % of the total variation, respectively. Conclusions: The distinctive morphological characters of the Ictalurus sp. found in the Culiacán River basin, combined with the known mitochondrial evidence for this morphotype, identify it as an evolutionarily significant unit that requires description as a new species based on taxonomical protocols.

Patterns and drivers of long‐term changes in breeding bird communities in a global biodiversity hotspot in Mexico

AbstractAimTo evaluate changes in breeding bird communities and assess implications for conservation.LocationMadrean Sky Islands and northern Sierra Madre Occidental, Mexico.MethodsI compared observations from recent fieldwork (2009–2012) with an extensive historical data set (1887–1954) and used modelling and multivariate techniques to assess spatiotemporal changes in species occurrence, richness and assemblage composition, and associations with climate, land use and landscape factors.ResultsBreeding species richness peaked in larger ranges often in the south and east, and regional beta diversity was attributable much more to turnover (0.80) than nestedness (0.07). Although richness increased across time, spatial assemblage heterogeneity declined due only to changes in nestedness, with temporal shifts in beta diversity equally attributable to variation in nestedness and turnover. Community change was associated with variation in climate, land use and landscape factors in ways that depended on species’ traits. Major gains in Madrean and lowland Neotropical species from the south contrasted little change in Nearctic species, and there was some evidence lowland species expanded eastward into the higher‐elevation interior, suggesting poleward and up‐elevation shifts now occurring globally. Some such patterns were associated with increasing temperature and summer–fall precipitation typical of the south suggesting climate forcing. Despite regional gains in pine‐dependent species linked to post‐logging forest recovery, losses and turnover were greater in smaller more arid ranges due likely to climate‐mediated habitat loss. High regional losses of cavity‐nesting species in forest were greater in ranges subjected to past logging indicating effects of historical habitat degradation persist today.Main conclusionDespite ongoing forest recovery and potential for northward range expansion to offset climate‐mediated losses of montane species, further assemblage homogenization seems likely. Active forest restoration that promotes large old trees, snags and mature forest conditions combined with increasing the capacity of local communities to implement best management practices will enhance conservation.

Convective-Permitting Hindcast Simulations during the North American Monsoon GPS Transect Experiment 2013: Establishing Baseline Model Performance without Data Assimilation

AbstractDuring the North American monsoon global positioning system (GPS) Transect Experiment 2013, daily convective-permitting WRF simulations are performed in northwestern Mexico and the southern Arizona border region using the operational Global Forecast System (GFS) and North American Mesoscale Forecast System (NAM) models as lateral boundary forcing and initial conditions. Compared to GPS precipitable water vapor (PWV), the WRF simulations display a consistent moist bias in the initial specification of PWV leading to convection beginning 3–6 h early. Given appreciable observed rainfall, days are classified as strongly and weakly forced based only on the presence of an inverted trough (IV); gulf surges did not noticeably impact the development of mesoscale convective systems (MCSs) and related convection in northwestern Mexico. Strongly forced days display higher modeled precipitation forecast skill than weakly forced days in the slopes of the northern Sierra Madre Occidental (SMO) away from the crest, especially toward the west where MCSs account for the greatest proportion of all monsoon-related precipitation. A case study spanning 8–10 July 2013 illustrates two consecutive days when nearly identical MCSs evolved over northern Sonora. Although a salient MCS is simulated on the strongly forced day (9–10 July 2013) when an IV is approaching the core monsoon region, a simulated MCS is basically nonexistent on the weakly forced day (8–9 July 2013) when the IV is farther away. The greater sensitivity to the initial specification of PWV in the weakly forced day suggests that assimilation of GPS-derived PWV for these types of days may be of greatest value in improving model precipitation forecasts.

Geology of the Selene perlite deposit in the northern Sierra Madre Occidental, northeastern Sonora, Mexico

El yacimiento mineral de perlita Selene del Noreste de Sonora, Mexico, se encuentra en el limite sur de un extenso paquete de rocas volcanicas Oligoceno temprano-Mioceno. Este ensamble volcanico, limitado por fallas y basculado hacia el Este (85 x 35 km), es de composicion variable y fue emplazado sobre un basamento Cretacico. Este paquete volcanico se encuentra localizado dentro de una region tectonicamente extendida en la porcion norte de la Gran Provincia ignea Silicica de la Sierra Madre Occidental (SMO). El mapeo geologico realizado en el area de estudio revela una sucesion volcanica de tipo bimodal y de edad Oligoceno temprano, que ha sido fallada y levantada. La secuencia contiene abundantes vestigios perliticos y es cortada por fallas de a lo mas una edad Mioceno temprano, que promueven el desarrollo de un semi-graben adyacente, rellenado por una unidad volcaniclastica. La secuencia volcanica bimodal comprende, a la base, tanto un derrame, como una toba de composicion riolitica, que son cubiertos por un deposito ignimbritico de edad Eoceno temprano. Un domo-colada de composicion riolitica en la parte intermedia, hospeda a la perlita Selene y, un basalto en la parte superior cubre discordantemente a la secuencia. La unidad volcaniclastica, considerada Mioceno temprano, ocurre en discordancia angular sobre la secuencia volcanica bimodal. Esta unidad presenta, a la base, brechas de falla con clastos de volcanicas locales, una capa previamente no identificada de cuarzo-arenita y, en la parte superior, un potente conglomerado polimictico, que contienen clastos de perlita, riolita fluidal, andesita y basalto. Las fuentes potenciales para los miembros maficos y felsicos del volcanismo bimodal se consideran puntos de emision de tipo fisural y distal, y/o distal-proximal, que fueron controlados por fallas ligadas a la Provincia Extensional del Basin and Range. Esto

Diet of New Mexico Ridge-nosed Rattlesnake (Crotalus willardi obscurus) in the Sierra San Luis and Sierra Pan Duro, México

Abstract We examined the diet of New Mexico Ridge-nosed Rattlesnakes (Crotalus willardi obscurus) from the Sierra San Luis and the Sierra Pan Duro in the northern Sierra Madre Occidental, Mexico. All snakes included in this study were encountered during August, September, and October in 2003 and 2004. Including recaptures of previously captured individuals, 107 of 199 snakes contained at least one prey item. We examined 103 prey items from 92 of these snakes; some snakes contained multiple prey items. Approximately two-thirds of prey items were ectotherms (54.4% lizards and 13.6% scolopendromorph centipedes); birds (21.4%) and mammals (10.7%) were also taken. The diet of juvenile snakes (n = 32) consisted primarily of lizards (62.5%) and centipedes (25.8%), although large juveniles also consumed mammals (6.3%) and passerine birds (6.3%). Adult snakes (n = 71) fed primarily on lizards (50.7%) and passerine birds (28.2%) but also consumed mammals (12.7%) and centipedes (8.4%). Crotalus willardi in the Sierr...

Ocelot (Leopardus pardalis): An Addition to the Mammals of Chihuahua, Mexico

Abstract. We present the first documented record of an ocelot (Leopardus pardalis) for the Mexican state of Chihuahua. The animal was camera-trapped in open oak woodland. This finding increases the number of mammal species present in the northern Sierra Madre Occidental and reinforces the need for continuing surveys in the barrancas region of Chihuahua.

Extension and magmatism in the Cerocahui basin, northern Sierra Madre Occidental, western Chihuahua, Mexico

The Sierra Madre Occidental of northwestern Mexico is the biggest silicic large igneous province of the Cenozoic, yet very little is known about its geology due to difficulties of access to much of this region. This study presents geologic maps and two new U-Pb zircon laser ablation inductively coupled plasma mass spectrometry ages from the Cerocahui basin, a previously unmapped and undated ~25 km-long by ~12 km-wide half-graben along the western edge of the relatively unextended core of the northern Sierra Madre Occidental silicic large igneous province. Five stratigraphic units are defined in the study area: (1) undated welded to non-welded silicic ignimbrites that underlie the rocks of the Cerocahui basin, likely correlative to Oligocene-age ignimbrites to the east and west; (2) the ca. 27.5–26 Ma Bahuichivo volcanics, comprising mafic-intermediate lavas and subvolcanic intrusions in the Cerocahui basin; (3) alluvial fan deposits and interbedded distal non-welded silicic ignimbrites of the Cerocahui clastic unit; (4) basalt lavas erupted into the Cerocahui basin following alluvial deposition; and (5) silicic hypabyssal intrusions emplaced along the eastern margin of the basin and to a lesser degree within the basin deposits.The main geologic structures in the Cerocahui basin and surrounding region are NNW-trending normal faults, with the basin bounded on the east by the syndepositional W-dipping Bahuichivo–Bachamichi and Pañales faults. Evidence of syndepositional extension in the half-graben (e.g. fanning dips, unconformities, coarsening of clastic deposits toward basin-bounding faults) indicates that normal faulting was active during deposition in the Cerocahui basin (Bahuichivo volcanics, Cerocahui clastic unit, and basalt lavas), and may have been active earlier based on regional correlations.The rocks in the Cerocahui basin and adjacent areas record: (1) the eruption of silicic outflow ignimbrite sheets, likely erupted from caldera sources to the east during the early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up, mostly prior to synextensional deposition in the Cerocahui basin (pre-27.5 Ma); (2) synextensional late Oligocene mafic-intermediate composition magmatism and alluvial fan sedimentation (ca. 27.5–24.5 Ma), which occurred during the lull between the Early Oligocene and early Miocene pulses of the ignimbrite flare-up; and (3) post-extensional emplacement of silicic hypabyssal intrusions along pre-existing normal faults, likely during the early Miocene pulse of the ignimbrite flare-up (younger than ca. 24.5 Ma). The timing of extensional faulting and magmatism in the Cerocahui basin and surrounding area generally coincides with previous models of regional-scale middle Eocene to early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental controlled by post-late Eocene (ca. 40 Ma) rollback/fallback of the subducted Farallon slab.

Resurrection and redescription of the pocket gopherThomomys sheldonifrom the Sierra Madre Occidental of Mexico

Thomomys sheldoni Bailey, 1915 is resurrected to recognize a genetically divergent clade of smooth-toothed pocket gophers that inhabits the high-elevation pine–oak forests of the Sierra Madre Occidental in Mexico from northwestern Chihuahua through Durango to western Zacatecas and northeastern Nayarit. Analyses of DNA sequences from 8 genes and genotype assignment tests for 21 allozyme loci establish this clade as a genetically isolated taxon within the T. umbrinus (sensu lato) complex. In addition to the T. sheldoni clade (diploid number of 2n ¼ 76), 3 genetic clades within T. umbrinus (2n ¼ 78) and 1 newly discovered clade (2n ¼ 76) also were recovered in the phylogenetic analyses. Individuals of T. sheldoni and T. umbrinus occur in close proximity with no evidence of gene flow and an average cytochrome-b genetic divergence of 15.6%. Analyses of cranial morphology reveal that skulls of T. sheldoni are, on average, broader and longer than skulls of T. umbrinus and T. atrovarius. We recognize and provide synonymies for 2 subspecies within T. sheldoni based on a weak genetic and morphological break that is coincident with a large, north–south gap in its distribution: T. s. sheldoni in the southern Sierra Madre Occidental, which includes specimens from the type locality and populations in Durango, Zacatecas, and Nayarit; and T. s. chihuahuae in the northern Sierra Madre Occidental, which is known only from Chihuahua.

Synvolcanic crustal extension during the mid-Cenozoic ignimbrite flare-up in the northern Sierra Madre Occidental, Mexico: Evidence from the Guazapares Mining District region, western Chihuahua

The timing and spatial extent of mid-Cenozoic ignimbrite flare-up volcanism of the Sierra Madre Occidental silicic large igneous province of Mexico in relation to crustal extension is relatively unknown. Extension in the Sierra Madre Occidental has been variably interpreted to have preceded, postdated, or begun during Early Oligocene flare-up volcanism of the silicic large igneous province. New geologic mapping, zircon U-Pb laser ablation–inductively coupled plasma–mass spectrometry dating, modal analysis, and geochemical data from the Guazapares Mining District region along the western edge of the northern Sierra Madre Occidental silicic large igneous province have identified three informal synextensional formations. The ca. 27.5 Ma Parajes formation is an ∼1-km-thick succession composed primarily of welded to nonwelded silicic outflow ignimbrite sheets erupted from distant sources. The 27–24.5 Ma Temoris formation is interpreted as an andesitic volcanic center composed of locally erupted mafic to intermediate composition lavas and associated intrusions, with interbedded andesite-clast fluvial and debris flow deposits, and an upper section of thin distal silicic outflow ignimbrites. The 24.5–23 Ma Sierra Guazapares formation is composed of silicic vent facies ignimbrites to proximal ignimbrites, lavas, plugs, dome-collapse deposits, and fluvially or debris flow–reworked equivalents. These three formations record (1) the accumulation of outflow ignimbrite sheets, presumably erupted from calderas mapped ∼50–100 km east of the study area that were active during the Early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up; (2) development of an andesitic volcanic field in the study area, likely related to rocks of the Southern Cordillera basaltic andesite province that were intermittently erupted across all of the northern Sierra Madre Occidental toward the end of and following the Early Oligocene ignimbrite pulse; and (3) the initiation of explosive and effusive silicic fissure magmatism in the study area during the Early Miocene pulse of the mid-Cenozoic ignimbrite flare-up. The main geologic structures identified in the Guazapares Mining District region are NNW–trending normal faults, with an estimated minimum of 20% total horizontal extension. Normal faults were active during deposition of all three formations (Parajes, Temoris, and Sierra Guazapares), and bound half-graben basins that show evidence of synvolcanic extension (e.g., growth strata) during deposition. Normal faulting began by ca. 27.5 Ma during deposition of the youngest ignimbrites of the Parajes formation, concurrent with the end of the Early Oligocene silicic ignimbrite pulse to the east and before magmatism began in the study area. In addition, preexisting normal faults localized andesitic volcanic vents of the Temoris formation and silicic vents of the Sierra Guazapares formation, and some faults were reactivated during, as well as after, deposition of these formations. We interpret extensional faulting and magmatism in the Guazapares Mining District region to be part of a regional-scale Middle Eocene to Early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental. We show that extension accompanied silicic volcanism in the Guazapares region, and overlapped with the peak of mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental; this supports the interpretation that there is a relationship between lithospheric extension and silicic large igneous province magmatism.

Mechanisms for Precipitation Enhancement in a North American Monsoon Upper-Tropospheric Trough

Abstract Tropical upper-tropospheric troughs (TUTTs), transient summertime disturbances over the Pacific and Atlantic Oceans, are also observed to frequently occur in the North American monsoon (NAM) region. However, unlike TUTTs over the Pacific and Atlantic, which feature a predominance of precipitation on the eastern flank of the disturbances, TUTTs in the NAM region have been shown to enhance precipitation on their western flank as they pass over the mountains of northern Mexico. To investigate this phenomenon, convection-permitting simulations are performed over the core NAM region for the 12–14 July 2004 TUTT event that occurred during the North American Monsoon Experiment (NAME). The effects of the TUTT are isolated using an approach that removes the vorticity anomaly associated with it. Six simulations, three with the TUTT and three without it, are then executed. It is found that the mean of the TUTT simulations has increased surface–500-hPa and 700–400-hPa shear and that there is incrementally more convective available potential energy (CAPE) along the Sierra Madre Occidental (SMO). These differences lead to convective changes, with the TUTT simulations having more convection, larger maximum updraft velocities, and more precipitation in lower elevations. Overall, the TUTT simulation mean has about 15% more precipitation during the primary period of TUTT interaction with the northern SMO. There are also slight simulated microphysical differences that agree with nearby polarimetric radar observations. Finally, TUTT removal impacts the gulf surge event on 13 July via the convective modifications.

Fomitiporia cupressicola sp. nov., a parasite on Cupressus arizonica, and additional unnamed clades in the southern USA and northern Mexico, determined by multilocus phylogenetic analyses

Fomitiporia cupressicola sp. nov., found in living Cupressus arizonica, is described on the basis of several collections originating from a high altitude forest in the northern Sierra Madre Occidental, Mexico. The species forms a monophyletic clade, basal to a larger lineage comprising species originating mainly from temperate to Mediterranean areas of the northern hemisphere. The phylogenetic approach in Fomitiporia also revealed multiple unnamed clades within the F. robusta complex in the southern USA and northern Mexico, representing potential species. The status of the F. robusta complex in North America is discussed briefly.

Timing of intense magmatic episodes in the northern and central Sierra Madre Occidental, western Mexico

The ∼400,000 km 2 calc-alkalic Sierra Madre Occidental volcanic field is the largest component of the Tertiary ignimbrite flare-up of western North America. Laser-fusion 40 Ar/ 39 Ar geochronology of sanidine and anorthoclase from ignimbrites sampled within three areas of concentrated geologic mapping in the Mexican states of Chihuahua and Durango provides the first examination of fine-scale temporal fluctuations within this significant area of the ignimbrite flare-up. Overall, the 112 40 Ar/ 39 Ar ages range from 28 to 46 Ma, with a separate group near 24 Ma. The oldest ages (40–46 Ma, with a strong cluster at 44–46 Ma) are found only along the eastern margin of the field. The predominant age concentration (28–36 Ma) is found across two east-west traverses of the volcanic field that are separated by 500 km. Within this interval, there are strong concentrations at 28–30 Ma, ca. 33 Ma, and 35–36 Ma in the northern traverse, and at 30–31 Ma and at 31.5 Ma in the southern traverse. A cluster of ages near 24 Ma is present in the western part of both traverses. Ignimbrite sequences of similar age are oriented in belts generally parallel to the axis of the Sierra Madre Occidental volcanic field. The belts partly overlap but are progressively younger toward the west. This geographic pattern fits with numerous studies that relate the position of arc magmatism to the delamination/rollback of the subducted Farallon slab. However, a more difficult challenge is to reconcile a pattern of discrete, brief (∼2 m.y.) and intense episodes of magmatism to behavior of the Farallon plate.

Ephemeral Pleistocene woodlands connect the dots for highland rattlesnakes of the Crotalus intermedius group

Aim To test how Pleistocene climatic changes affected diversification of the Crotalus intermedius species complex. Location Highlands of Mexico and the south-western United States (Arizona). Methods We synthesize the matrilineal genealogy based on 2406 base pairs of mitochondrial DNA sequences, fossil-calibrated molecular dating, reconstruction of ancestral geographic ranges, and climate-based modelling of species distributions to evaluate the history of female dispersion. Results The presently fragmented distribution of the C. intermedius group is the result of both Neogene vicariance and Pleistocene pine–oak habitat fragmentation. Most lineages appear to have a Quaternary origin. The Sierra Madre del Sur and northern Sierra Madre Oriental are likely to have been colonized during this time. Species distribution models for the Last Glacial Maximum predict expansions of suitable habitat for taxa in the southern Sierra Madre Occidental and northern Sierra Madre Oriental. Main conclusions Lineage diversification in the C. intermedius group is a consequence of Pleistocene climate cycling. Distribution models for two sister taxa in the northern and southern Sierra Madre Occidental and northern Sierra Madre Oriental during the Last Glacial Maximum provide evidence for the expansion of pine–oak habitat across the Central Mexican Plateau. Downward displacement and subsequent expansions of highland vegetation across Mexico during cooler glacial cycles may have allowed dispersal between highlands, which resulted in contact between previously isolated taxa and the colonization of new habitats.

ARCHITECTURAL PATTERNS ALONG THE RIO TARAISES, NORTHERN SIERRA MADRE OCCIDENTAL, SONORA

The prehistory of the northern Sierra Madre Occidental of Mexico is poorly understood. This article reviews previous research in the region and describes the archaeology of the Rio Taraises (Lister 1958). My principal interest is the cliff-dwellings Cueva Bringas and Cueva el Aguaje, which were the subject of a recent archaeological project. Architectural data, particularly the size and shape of doorways, and the construction techniques used to build walls, is used to place these two sites and the peoples who built them in a broader regional context particularly in regard to social or ethnic group membership. Finally, I argue that these two cliff-dwellings are more representative of the majority of archaeological sites in northern Mexico, than the larger and more elaborate site of Paquimé.

Moving and Stationary Mesoscale Convective Systems over Northwest Mexico during the Southwest Area Monsoon Project

Abstract Some characteristics of the life cycle and motion of Mesoscale Convective Systems (MCSs), which formed over the northern Sierra Madre Occidental during the Southwest Area Monsoon Project (SWAMP), are examined. The motion characteristics of MCSs and presence of residual circulations (mesoscale vortices) are determined using Geostationary Operational Environmental Satellite infrared and visible satellite imagery. Composite wind fields associated with moving and stationary systems were constructed using soundings from the operational upper-air network and from special pilot balloon stations that were operated in Mexico as part of the SWAMP experiment. The MCSs were classified as moving or stationary based on the observed displacement during a 6-h period. Moving and stationary systems occurred with about the same frequency. The moving systems were distributed uniformly along the length of the Sierra Madre, while the stationary systems were primarily confined to the southern Gulf of California. The st...

BIOGEOGRAPHIC PATTERNS OF MONTANE MEXICAN AND CENTRAL AMERICAN CARABIDAE (COLEOPTERA)

AbstractCarabid taxa inhabiting the montane forests of México and Central America are analyzed using cladistic biogeographic methods. Taxa exhibiting the Nearctic, Meso-American Montane, and Paleo-American distributional patterns are included in the analysis. Widespread taxa are assumed to indicate that the areas of endemism they inhabit are closely related, and the general pattern of area relationships is determined using parsimony analysis. Of the nine areas of endemism recognized in the analysis, the Sierra Madre Oriental is considered the most distinct. The northern Sierra Madre Occidental, Arizona Mountains, and Sonoran Desert comprise one monophyletic set of areas, as do the more southerly areas of the Sierra Transvolcanica, Sierra Madre del Sur, Chiapan–Guatemalan highlands, and Talamancan Cordillera. The southern Sierra Madre Occidental area of endemism is equivocally related to both areas to the north and areas to the south. Biogeographic information derived from widespread species is as congruent with the general area cladogram as information derived from groups with precinctive species, indicating that the same biogeographic barriers often limit distributions of both widespread species and more inclusive clades comprising precinctive species. Taxa exhibiting the Meso-American Montane Pattern are more congruent with the general area cladogram than those representing the Nearctic Pattern, even though they exhibit less fidelity to highland habitats.

Oligocene ash flow volcanism, northern Sierra Madre Occidental: Role of mafic and intermediate‐composition magmas in rhyolite genesis

Field, geochemical, and isotopic data from the Tomochic volcanic center in Chihuahua, Mexico, are interpreted to indicate a genetic relationship between large‐volume rhyolite ash‐flow tuffs and associated more mafic lithologies. These lithologies include (1) porphyritic, two‐pyroxene andesite (>35 Ma) that was extruded mostly before ash‐flow volcanism, and (2) crystal‐poor basaltic andesite that was erupted mostly after ash‐flow activity (∼30 Ma) but which was also extruded earlier (∼34 Ma) with hybrid intracaldera lavas. Major silicic units at Tomochic include the Vista (∼34 Ma) and Rio Verde (∼32 Ma) rhyolite ash‐flow tuffs; also present are ash‐flow tuffs (∼38, 36, and 29 Ma) erupted from other sources. A model of rhyolite genesis by closed‐system crystal fractionation of andesite is consistent with geochemical and isotopic data. The least evolved Vista rhyolite was formed by fractionation of ∼65% the original mass of andesite; an additional ∼55% fractionation of plagioclase, alkali feldspar, quartz, biotite, hornblende, FeTi oxides, and sphene generated the most evolved Vista sample. Rio Verde rhyolites were generated from andesite by ∼50% mass fractionation of an assemblage dominated by plagioclase, pyroxene, and FeTi oxides. Initial Nd and Sr isotope ratios of andesite‐dacite lavas (εNd = −2.3 to −5.2; 87Sr/86Sr = 0.7060 to 0.7089) and of rhyolites (εNd = +0.5 to −2.7; 87Sr/86Sr = 0.7053 to 0.7066) partly overlap and extend from values near the mantle array toward values typical of old continental crust on an εNd‐87Sr/86Sr diagram. These isotope ratios, which do not correlate with indices of differentiation, are interpreted to indicate that parental andesite already contained a crustal component (possibly >20%) before fractionation to rhyolite. The isotopic and geochemical signatures of andesites apparently reflect the incorporation of crust by subduction‐related, mafic melts represented by (but more primitive than) exposed basaltic andesites, which have isotope ratios (εNd = +1.0 to −0.1; 87Sr/86Sr = 0.7044 to 0.7053) near “bulk earth”. The pattern of volcanic evolution at the Tomochic center, specifically the transition from andesitic to rhyolite dominated, with late extrusion of basaltic andesite, also occurred in other parts of the volcanic field, and roughly coincided with a sharp decrease in the rate of Farallon plate subduction. This change in subduction rate apparently resulted in a decreased flux of mafic melts into the crust from below, and was associated with the onset of crustal extension and hence, shorter residence times for mafic melts formerly ponded in the deep crust These, in turn, resulted in (1) the change from andesitic to rhyolite‐dominated volcanism as ascending intermediate‐composition magmas stalled, coalesced, and differentiated to produce rhyolite, (2) extrusion of basaltic andesite upon brittle failure of the shallow crust, and (3) subsequent termination of calc‐alkalic volcanism throughout the Sierra Madre Occidental.

Evolution of waning, subduction-related magmatism, northern Sierra Madre Occidental, Mexico

The stratigraphic record at the Tomochic volcanic center, Chihuahua, documents a change from andesitic to rhyolite-dominated volcanism, with late extrusion of mafic lavas, during the final ∼10 m.y. of subduction-related activity in the northern Sierra Madre Occidental. Andesitic volcanism, which started before 38 Ma and continued until ∼35 Ma, was followed by eruption of large-volume rhyolite ash flows from the Las Varas (34.1 Ma) and Tomochic (31.8-31.4 Ma) calderas. Basaltic andesite, the most mafic rock type in the area, was extruded near the end of calcalkalic volcanism at ∼30 Ma. The change in style of volcanism at the Tomochic volcanic center also occurred in other parts of the northern Sierra Madre Occidental and is interpreted to reflect the evolution of waning, subduction-related magmatism, as convergence between the North American and Farallon plates slowed before it eventually ceased. Slowed plate convergence resulted in a decreased flux of mafic melts from the mantle to the crust, leading, in turn, to the stagnation and coalescence of intermediate-composition magmas similar to those that fed early andesitic eruptions. Subsequent differentiation led to growth of zoned magma reservoirs from which the rhyolite ash flows were extruded. Slowed convergence was accompanied by the onset of crustal extension, which possibly aided the coalescence of andesitic magmas and which allowed the ascent and eruption of mafic melts near the end of silicic volcanic activity. Although minor differences exist, the history of mid-Tertiary magmatism and tectonism in the northern Sierra Madre Occidental was broadly similar to that in parts of the south-western United States.

A New Species of Perityle (Asteraceae) from Southwestern Chihuahua, Mexico

Perityle vigilans is described from the Sierra Madre Occidental of southwestern Chihuahua. It differs from other Perityle by the combination of its white ligules, sparsely setose- hispid achenes, finely grayish-villosulous tomentum, and absence of pappus bristles. The new species is believed to be most closely related to P. rosei and P. trichodonta, which occur about 800 km to the south. A key is presented that distinguishes the 12 known taxa of Perityle occurring in the northern Sierra Madre Occidental, an area of high diversity in the genus.