Southern Sierra Madre Occidental Research Articles

Aeromagnetic anomalies and magnetic domains of the Jalisco Block, western Mexico

The Jalisco Block (JB) in western Mexico has been considered as a margin terrane being detached from the North American plate along the Tepic-Zacoalco rift and Colima graben. In this study, an aeromagnetic analysis of the JB and surrounding areas is performed applying a series of geophysical filters and VOXI inverse model. The JB is characterized by two aeromagnetic domains (AMD). AMD-Ia located throughout the south-central section that borders the continental margin is characterized by positive magnetic anomalies extending from the Armería river to Banderas Bay. The AMD-Ia corresponds to the Puerto Vallarta batholith emplaced in the late Cretaceous. The AMD-IIa in the northern section is marked by negative magnetic anomalies over volcanic and volcano-sedimentary rocks, extending to the southern Sierra Madre Occidental. We propose that JB tectonic boundaries correspond to the aeromagnetic AMD-Ia eastern limit and AMD-IIa northern limit. Estimated depths from the radially average power spectrum for deep magnetic sources in the AMD-Ia are in the range of 5.4 km, and for shallow sources from 0.8 to 1.2 km. For AMD-IIa depths are 10.45 km for deep sources and 0.13 km for shallow sources. Depths for deep sources from the Euler deconvolution for the AMD-Ia range from 3.3 to 4.5 km and from 3.5 to 4.9 km for the AMD-IIa. Shallow source depths in the AMD-Ia are 0.2–0.3 km and near surface for the AMD-IIa. The integrated analysis shows five aeromagnetic domains, two associated with the JB and three with the surrounding areas. The VOXI inverse model allows to estimate the dimensions and location of magnetic sources in the reduced to the pole and upward continuations.

Deciphering origins and pathways of low-enthalpy geothermal waters in the unconventional geothermal system of Juchipila graben (Central Mexico)

This work presents hydrochemical results for groundwater and dissolved gas samples collected from a thermal and cold aquifer in the Juchipila Basin, in southern Sierra Madre Occidental, central Mexico. Thermal springs in the Juchipila Basin reach temperatures of 60 °C, these manifestations are not related to recent or active volcanism as are all the known geothermal fields in Mexico. The thermal waters (>32 °C) are Na-HCO3 and Na-SO4 type, with an anomalous concentration of F, B, Li, and As. Their chemistry likely results from water-rock interaction processes. The cold waters (<32 °C) have a Ca-HCO3 composition typical of recent infiltration and shallow flow, but they have an anomalous concentration of NO3. The δ2H and δ18O indicate a common meteoric source for the warm and cold water plotting along an evaporation line. The waters have higher CO2 and He concentrations than the air-saturated water. The helium composition is mainly atmospheric and terrigenous with a mantle helium contribution of up to 14%. This suggests that faults affecting the region are deeply rooted, permitting mantle helium uprise. Geothermometry gives mean reservoir temperatures of 58–102 °C. Based on these results, we propose a model of hydrothermal circulation in the Juchipila Basin, in which rainwater infiltrates deeply through the graben edges fault system, dissolves ions and crustal helium, incorporates mantle helium, while heated by the geothermal gradient, and eventually surges and mixes with the cold, shallow aquifer along faults cutting the whole succession within the graben.

&lt;p&gt;&lt;strong&gt;A re-evaluation of taxonomy in &lt;em&gt;Quercus&lt;/em&gt; section &lt;em&gt;Lobatae&lt;/em&gt; subsection &lt;em&gt;Racemiflorae &lt;/em&gt;(Fagaceae), resurrection of the name &lt;em&gt;Q. pennivenia&lt;/em&gt; and description of a new taxon, &lt;em&gt;Q. huicholensis&lt;/em&gt;&lt;/strong&gt;&lt;/p&gt;

Changes to the taxonomy of Quercus section Lobatae subsection Racemiflorae occurring in western Mexico are here made to reflect new evidence regarding species relationships of two lineages occurring in the southern Sierra Madre Occidental. The name Q. pennivenia is resurrected to recognize plants previously included under the name Q. urbanii. A new taxon, Q. huicholensis, is described to represent a distinct lineage segregated from Q. conzattii. With these nomenclatural changes we recognize the subsection Racemiflorae to consist of six taxa and we provide a key for their determination.

Emplacement dynamics and hydrothermal alteration of the Atengo ignimbrite, southern Sierra Madre Occidental, northwestern Mexico

The Sierra Madre Occidental is a thick continental arc related to the subduction of the Farallon plate beneath North America resulting in a very intense and widespread Cretaceous to Cenozoic magmatic and tectonic activity. The 28 My old Atengo ignimbrite outcrops in the southern Sierra Madre Occidental, northwestern Mexico. From 12 sites that belong to various pyroclastic and lava flows emplaced during two pulses in the Oligocene (ca. 32–28 Ma) and Early Miocene (ca. 24–20 Ma), 97 rock specimens were drilled. The mineralogical and rock magnetic properties of the Atengo ignimbrite are compared with the surrounding volcanic rocks to identify the eruption mechanism, and with the El Castillo Ignimbrite, Veracruz, Mexico, to understand the depositional conditions. The comparisons reveal that the Atengo ignimbrite erupted from a single source, but less violently than the El Castillo ignimbrite, and cooled rapidly, inhibiting the formation of subhedral grains. The source of the Atengo Ignimbrite was a Plinian-type eruption, and the characteristic mineralogical and textural properties of each flow are related to different stages of the Plinian-type eruption. Further more, hydrothermal fluids were active during the last stages of volcanism, and caused moderate to intense alteration, especially in the ignimbrites, where high permeability aided the movement of hydrothermal fluids.

Mammals of the San Pedro-Mezquital River Basin, Durango-Nayarit, Mexico

The San Pedro–Mezquital River Basin is located in the southern Sierra Madre Occidental, at the Nearctic– Neotropical transition. The river traverses the Sierra through a canyon that reaches over 1000 m in depth. Based on examination of museum specimens, literature records, and our own collections, we documented the occurrence of 120 species (24.6% of the Mexican terrestrial mammals), 24 endemic to Mexico. Richness was comparable with other megadiverse areas of Mexico, and higher than any other Nearctic–Neotropical transition area, moreover species richness is likely to rise as survey continues. Contrary to expectation, distribution of mammals across the basin not only reflected the Nearctic–Neotropical divide, but a third fauna that is a mixture of tropical, temperate and desert species was identifiable at the canyon. Anthropogenic threats including damming of the river, uncontrolled cattle grazing, and pollution from domestic sources, call for effective management strategies to preserve one of the most biodiverse areas of Mexico.

Timing of rifting in the southern Gulf of California and its conjugate margins: Insights from the plutonic record

The Gulf of California is a young example of crustal stretching and transtensional shearing leading to the birth of a new oceanic basin at a formerly convergent margin. Previous studies focused along the southwestern rifted margin in Baja California indicated rifting was initiated after subduction and related magmatism ceased at ca. 14–12.5 Ma. However, the geologic record on the Mexico mainland (Sinaloa and Nayarit States) indicates crustal stretching in the region began as early as late Oligocene. The timing of cooling and exhumation of pre- and synrift plutonic rocks can provide constraints on the timing and rate of rifting. Here, we present results of a regional study on intrusive rocks in the southern Gulf of California sampled along the conjugate Baja California and Nayarit-Sinaloa rift margins, as well as plutonic rocks now exposed on submerged rifted blocks inside the gulf. Forty-one samples were dated via U/Pb zircon and 40Ar/39Ar mineral ages, providing emplacement age and thermochronological constraints on timing and rate of cooling. We found an extensive suite of early and middle Miocene plutons emplaced at shallow depths within the basement Cretaceous–Paleocene Peninsular Range and Sinaloa-Jalisco Batholiths. Early Miocene granitoids occur in an elongated WNW-ESE belt crossing the entire southern gulf from southern Baja California to Nayarit and Sinaloa. Most have an intermediate composition ( 75 SiO2 wt%) was emplaced 20.1–18.3 Ma, near the end of the early Miocene. Age span and chemical composition of the early Miocene silicic plutons essentially overlap ignimbrites and domes exposed in the southern Sierra Madre Occidental and in southern Baja California, suggesting that eruptive sources for the early Miocene ignimbrite flare-up may also have been located within the southern Gulf of California. Early Miocene plutons cooled below the 40Ar-39Ar biotite closure temperature (350–400 °C) in less than 2.5 m.y., which we interpret as evidence of a regional extensional event leading to the opening of the Gulf of California. A less widely distributed suite of intermediate-composition, middle Miocene granitoids (15–13 Ma) was sampled from the central-western part of the gulf, west of the Pescadero Basin, and these correspond to an episode of scarce volcanism recorded by the middle and upper members of the onshore Comondu Group in Baja California. Our widely spaced sampling of the generally sediment-covered igneous crust suggests that middle Miocene primary volcanic rocks are much less abundant than implied by previous models in which the gulf was the site of a robust Comondu arc. Thermobarometry data also indicate a very shallow depth (<5 km) of emplacement for the middle Miocene plutonic rocks. Some of these rocks also show a distinctive inequigranular texture indicative of at least two crystallization stages at different pressure. Early and middle Miocene granitoids away from the gulf axis yielded 40Ar-39Ar cooling ages very close to U-Pb zircon ages, demonstrating rapid cooling to <350 °C, which we attribute to their shallow emplacement and, possibly, to exhumation soon after intrusion. Since Comondu-age and middle Miocene magmatism in the gulf region coincided with rapid cooling of young plutons that predate the end of subduction, we suggest that intense crustal stretching controlled the pattern and timing of Comondu-age magmatism, rather than the middle Miocene magmatism controlling the locus of <12 Ma extension.

Evolution and phylogeography of theThomomys umbrinusspecies complex (Rodentia: Geomyidae)

The southern pocket gopher, Thomomys umbrinus, currently contains 17 subspecies distributed from southeastern Arizona and southwestern New Mexico southward into the Trans-Mexico Volcanic Belt (TMVB) of central Mexico. Here we reevaluate relationships within this species, which, based on previous studies, is composed of 3 genetically divergent clades. Multilocus phylogenetic analyses uphold these clades (Northern Desert, Central Plateau, and TMVB) as well as the monophyly of T. umbrinus with respect to the 3 other species in the T. umbrinus complex (T. atrovarius, T. nayarensis, and T. sheldoni). Reanalysis of published allozyme data shows a pattern of isolation by distance consistent with the hypothesis of gene flow among the 3 T. umbrinus clades. Species tree analyses of these clades reveal 4 divergent lineages (2 within the TMVB clade), which we recognize herein at the subspecies level. Cranial morphometrics show a moderate level of morphological separation among these clades in multivariate space. The present center of diversity of the T. umbrinus species group is in the southern Sierra Madre Occidental, which also appears to be the center of origin of the T. umbrinus complex. A synonymy of T. umbrinus is provided.

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.

Relationships of &lt;I&gt;Asanthus&lt;/I&gt; (Asteraceae, Eupatorieae)

Abstract Analysis of DNA sequence data was used to clarify the circumscription and relationships of the small Mexican genus Asanthus. Results from non-coding regions, including the nuclear ITS and the chloroplast psbA-trnH spacer showed that Asanthus is clearly monophyletic and distinct from Steviopsis and Brickellia, two genera with which it is sometimes lumped, as well as from the closely related Brickelliastrum and Carminatia. The DNA sequence data also agree with morphological observations in showing that Asanthus is composed of three species, A. thyrsiflorus from the Sierra Madre Oriental and A. squamulosus and A. solidaginifolius from the Sierra Madre Occidental. Samples from the southern Sierra Madre Occidental that have been identified as A. thyrsiflorus are suggested by the presence of an additive pattern of polymorphisms in the nuclear ITS region to be of interspecific hybrid origin between A. thyrsiflorus and A. solidaginifolius, which is consistent with reports of their morphological intergrad...

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.

Syn-extensional intra-plate trachydacite-rhyolitic dome volcanism of the Mesa Central, southern Sierra Madre Occidental volcanic province, Mexico

Oligocene dome complexes of trachydacitic to rhyolitic composition are common in the southern portion of the Mesa Central physiographic province, which forms part of the southern Basin and Range extensional province as well as of the southern Sierra Madre Occidental volcanic province. Generally, dome complexes occur aligned with regional fault systems, mostly associated with the southern Basin and Range province, and thus suggesting that faults controlled the felsic magmas that formed these domes. Two distribution patterns are evident, one aligned NE–SW and another aligned NNE. The set of domes were emplaced at 33–28 Ma. Emplacement of domes occurred in three continuous phases starting with those of trachydacite affinity at 33–32 Ma, to trachydacite–rhyolitic at 32–31 Ma, and finally to those with rhyolitic composition at 31–28 Ma. Felsic magmas that originated the domes were apparently generated by partial melting at the base of the continental crust. Contrary to previous hypothesis, our evidence suggest that these magmas in these particular areas of the Mesa Central were not accumulated in large magma reservoirs emplaced at shallow levels in the crust, but crossed the continental crust directly. Since continental crust in this region is relatively thin (30–33 km), we propose that an intense extensional episode favored the direct ascension of these magmas through the brittle crust, with little interaction with the country rock during ascent to the surface, to end up forming aligned dome chains or complexes. Geochemical data favors this model, as the felsic rocks show no depletions in Nb and Th but instead relatively enrichment in these elements. REE show flat or concave up patterns, suggesting that the magmas involved enriched (fertile), metasomatized lithospheric fluids that generated partial melting at the base of the continental crust. Based upon these data, we infer an intra-plate tectonic setting for these rocks.

Tectono-volcanic control of fissure type vents for the 28 Ma Panalillo ignimbrite in the Villa de Reyes Graben, San Luis Potosí, México.

The volcano-tectonic events at the Villa de Reyes Graben (VRG), in the southern Sierra Madre Occidental, Mexico, include 1) a regional NNE fault system developed before 32 Ma, 2) this pre-32 Ma faulting controlled the emplacement of 31.5 Ma dacitic domes, 3) NE faulting at 28 Ma that displaced the 31.5 Ma dacitic domes and formed the VRG, as well as the oblique grabens of Bledos and Enramadas oriented NW, 4) emplacement of Panalillo ignimbrite at 28 Ma filling the VRG and erupting from fissures related to the oblique grabens, and eruption of Placa basalt apparently also from fault-controlled vents.

A survey for the Sierra Madre Sparrow ( Xenospiza baileyi ), with its rediscovery in the state of Durango, Mexico

The Sierra Madre Sparrow (Xenospiza baileyi) is a highly endangered and endemic species of the highlands of south-central Mexico, where it is resident in bunchgrass (Gramineae) and adjacent marshy habitats in the southern Sierra Madre Occidental (Jalisco and Durango states) and in the mountains around the Valley of Mexico (Distrito Federal and the states of Morelos and Mexico). This species was first collected in the southern part of this range on 23 April 1945 at La Cima, D.F., where its persistence has been documented essentially continuously since 1951. The first specimens of the taxon were collected in the Sierra de Bolanos of extreme northern Jalisco on 3–10 March 1889, including the type from which the genus and species were described in 1931. Two populations have been found in southern Durango: one 30 miles (48 km) south-west of the City of Durango on 22 March 1931, and the other 5 miles (8 km) west of El Salto on 16–17 June 1951. The Sierra Madre Sparrow has not been otherwise confirmed in the northern part of its range, which in July 2004 led us to conduct an extensive search for it in these areas of Durango and Jalisco as well as south-western Zacatecas. Here we present the findings from that search, during which several sites were intensively surveyed and a single population of this sparrow was located – a new one between the city of Durango and El Salto, Durango. This rediscovery increases possibilities for understanding the biogeography, ecology and basic requirements of the Sierra Madre Sparrow, information of fundamental importance for proposing measures that promote its conservation in any of its remaining populations.

Model Climatology of the North American Monsoon Onset Period during 1980–2001

Abstract In this study, the seasonal development of the North American monsoon system (NAMS), as simulated by a mesoscale model during a 22-yr simulation from 1980 through 2001, is assessed. Comparison between model simulations and observations shows that the model simulation reproduces the precipitation, skin temperature, and wind field patterns in the seasonal development (May–July) of the NAMS reasonably well and that the mesoscale features and spatial heterogeneity of the NAMS are described correctly. The onset of the monsoon in the central and southern Sierra Madre Occidental (SMO) in Mexico occurs on 20 June, about 2 weeks earlier than the onset in Sonora, Mexico (6 July), the Sonoran Desert, and central Arizona and New Mexico (8 July). The temperature in Mexico is highest after the onset of the monsoon and then decreases with the increasing monsoon rainfall. However, the temperature in the Sonoran Desert and central Arizona and New Mexico is highest just prior to the onset of the monsoon, and high ...

Counterclockwise Rotation of the Michoacan Block: Implications for the Tectonics of Western Mexico

Subduction of the Farallon plate beneath North America resulted in formation of the Rivera and Cocos oceanic plates, the extensive magmatic arcs of the Sierra Madre Occidental (SMO), and the Trans-Mexican Volcanic Belt (TMVB). Southern Mexico consists of crustal blocks separated by a regional extensional structural system; the latter, called the Guadalajara triple junction, is defined by the Tepic-Zacoalco (TZR), Colima (CR), and Chapala (CHR) rifts. TZR and CHR separate the SMO from the Jalisco and Michoacan blocks, whereas CR is the boundary between the Jalisco and Michoacan blocks. In this study, we carried out combined radiometric and paleomagnetic analyses in the Michoacan block. Radiometric dates of 31.60 to 8.39 Ma confirm both the southern extension of the Sierra Madre Occidental and the early mafic TMVB succession into the Michoacan block. The Oligocene age agrees well with the radiometric dating reported for the southern SMO and the Tertiary volcanic fields of the Sierra Madre del Sur. Paleomagnetic data indicate a counterclockwise rotation of ∼24° about a vertical axis for the Michoacan block. Several plate models suggest either dextral or sinistral oblique convergence of the Cocos plate relative to North America. Our new results help to constrain these different models. These data demostrate that deformation in the Michoacan block is as old as late Miocene, and is related to sinistral oblique convergence of the Cocos plate relative to North America—inducing the southeast relative motion of the Michoacan block. The structural trends along both CHR and CR are thereby explained. On the other hand, right-lateral transtension along the TZR is related to the westward motion of the Jalisco block because of oblique convergence of the Rivera plate.

AGES OF EPITHERMAL DEPOSITS IN MEXICO: REGIONAL SIGNIFICANCE AND LINKS WITH THE EVOLUTION OF TERTIARY VOLCANISM

We present an analysis leading to a new space-time classification of the Mexican epithermal deposits, based on new data and interpretations from seven deposits in central Mexico, and a new age for the La Guitarra deposit, Temascaltepec district. From the latter, adularia from a barren stage of mineralization and from quartz bands of the main ore stage, together with orthoclase phenocrysts from host monzogranites (with a known late Laramide age) were analyzed by the 40Ar/39Ar method. The ages obtained from adularia samples are 32.9 ± 0.1 Ma and 33.3 ± 0.1 Ma, corresponding to the middle part of Lower Oligocene. These ages are slightly younger than that of the ignimbrite succession exposed next to the study area, and they suggest that the deposit is related to the crystallization of comagmatic intrusive rocks. Contrary to previous assumptions of a Miocene age for several epithermal deposits in southern Mexico, the ages from La Guitarra show that, south of the Trans-Mexican volcanic belt, these deposits may be Oligocene or older. In addition, the interpretation of a genetic link between intrusive rocks and epithermal mineralization is consistent with previous work on fluid geochemistry of the deposit. To evaluate the obtained ages in a regional context, we analyzed the geographic distribution and age of Tertiary epithermal deposits in central Mexico. Our study shows that mineral deposits coincide with the main volcanic pulses of the Sierra Madre Occidental. The distribution of epithermal deposits can be thus divided into three main groups: (1) between ~48 and ~40 Ma, (2) between ~36 and ~27 Ma, and (3) between ~23 and ~18 Ma. Within the first group are the Batopilas and Topia deposits that record the oldest ages found for epithermal deposits in Mexico. Deposits of this age are hosted by the Lower Volcanic Supergroup of the Sierra Madre Occidental, and are related to the Laramide magmatism of the Sierra Madre Occidental. Many of the epithermal deposits in Mexico occur within the second time span in a northwest-southeast belt from Chihuahua to the Mexico and Guerrero states. These deposits are related to the main episode of ignimbrite flare-up of the Sierra Madre Occidental. The third group includes the Bolanos, San Martin de Bolanos, and Pachuca-Real del Monte deposits, plus, possibly, seven other epithermal deposits analyzed in this work. These deposits are related to the last ignimbrite flare up of the Sierra Madre Occidental with a general west-northwest–east-southeast distribution, recently documented in the southern Sierra Madre Occidental.

Post-Oligocene river incision, southern Sierra Madre Occidental, Mexico

The Sierra Madre Occidental of western Mexico consists of a granitic basement covered by Oligocene ignimbrites that define a reference surface from which to estimate late Cenozoic river incision. A 90-m-grid digital elevation model was used to characterize contemporary topography and interpolate the Late Oligocene surface of the ignimbrite plateau from a surface fit to the highest points in the relatively undissected uplands between major river valleys. Long-term river incision rates calculated from the difference between this reference surface and longitudinal profiles of 11 rivers that flow toward the Tepic-Zacoalco rift zone range from about 0.01 to 0.2 mm year −1. River profiles of this region also show evidence of river capture driven by flexural uplift along the flank of the rift zone. River profile concavity values ( θ) in the Sierra Madre Occidental range from 0.22 to 0.63, a range similar to that reported previously for a wide range of environments. In contrast, the empirically constrained ratio of exponents in the stream power model of river incision ( m/ n) ranges from 0.44 to 0.52, close to the expected theoretical value of 0.5. The wider range of observed θ values may illustrate how θ can differ from the driving values of m/ n in non-steady-state bedrock river systems.

Ignimbrite flare‐up and deformation in the southern Sierra Madre Occidental, western Mexico: Implications for the late subduction history of the Farallon plate

The Sierra Madre Occidental (SMO) of western Mexico is one of the largest silicic volcanic provinces on Earth, but the mechanism for the generation of such a large volume of ignimbrites has never been clearly defined. We present new 40Ar/39Ar ages, geologic mapping, and structural data for the southern part of the SMO demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare‐up in Oligocene (31.5–28 Ma) and early Miocene (23.5–20 Ma) time, and that extensional deformation occurred mostly before the transfer of Baja California to the Pacific plate. Extensive ignimbrite successions, with 40Ar/39Ar ages clustering at ∼23 and ∼21 Ma, cover most of the southern SMO, thus correlating in age with ignimbrites exposed in southern Baja California and central Mexico. Grabens with a 020° to N‐S orientation developed in the east almost concurrently with this volcanic episode. Half grabens and NNW striking listric normal fault systems formed at the end of middle Miocene as far as 150 km from the present coast. A belt of left‐lateral transpressional structures formed along the southern boundary of the SMO during the same period. We link these magmatic and tectonic events to the evolution and dynamics of the Farallon and North America plates during the Miocene. Particularly, we propose that a first detachment of the lower part of the Farallon plate in early Miocene time produced a transient thermal event and partial melting of the crust via mafic underplating. Middle Miocene extension would be related to a second detachment event, resulting from the slowing subduction that preceded the final capture of the Magdalena microplate by the Pacific plate at 12.5 Ma. Transpression at the southernmost end of the SMO occurred along the inland projection of the Magdalena‐Cocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the Magdalena plate.

Cladistic biogeography of the Mexican transition zone

AbstractBiogeographic relationships among nine montane areas of endemism across the transition zone between North and South America are analysed cladistically based on phylogenetic hypotheses of thirty‐three resident monophyletic taxa of insects, fish, reptiles, and plants.Areas of endemism include the Arizona mountains (AZ), Sonoran Desert (SD), Sierra Madre Occidental (OCC), southern Sierra Madre Occidental (SOC), Sierra Madre Oriental (ORI), Sierra Transvolcanica (TRAN), Sierra Madre del Sur (SUR), Chiapan‐Guatemalan Highlands (CGH), and Talamancan Cordillera (TC).Area relationships are summarized using Brooks Parsimony Analysis and Assumption 0, with the former resulting in more defensible biogeographic hypotheses.Areas of endemism are dividable into two monophyletic groups; a northern group including AZ, SD, OCC, and ORI, and a southern group consisting of TC, CGH, TRAN, SUR, and the isolated southern regions of the Sierra Madre Occidental (SOC).The northern set of areas are characterized by recent, probably Pleistocene, isolation and prevalent widespread species, whereas the southerly areas probably diverged after Pliocene closure of the Panamanian isthmus.The southern areas are redundantly represented on many of the taxon‐area cladograms by endemic species, indicative of much higher levels of endemism in the Sierra Transvolcanica and further south.Use of a general area cladogram in such a transition zone permits explicit exploration of biogeographic patterns and establishes a predictive framework for taxonomy and conservation prioritization.

Variation of Cenozoic extension and volcanism across the southern Sierra Madre Occidental volcanic province, Mexico

The middle to late Cenozoic tectonic-magmatic evolution of the Sierra Madre Occidental volcanic province south of the Tropic of Cancer is summarized and analyzed for the first time, based on new geologic and structural work and published information. In the eastern part of the study region (Mesa central physiographic province) silicic volcanism occurred in a short-lived episode culminating at ca. 30 Ma and was followed by crustal-scale extension between 30 and 27 Ma. In the western part of the study area (Sierra Madre Occidental physiographic province) a voluminous episode of ignimbrite volcanism at 24‐21 Ma was succeeded by east-west extension that produced regularly spaced grabens affecting only the upper crust. In the westernmost part of the study region, an andesitic to rhyolitic arc, formed between 17 and 12 Ma, was affected by crustal-scale, north-northwest‐trending, extensional faulting, leading to the formation of the Gulf of California. In the Mesa central the maximum extension was oriented approximately east-west and amounted to ~20%. In the eastern Sierra Madre Occidental physiographic province extension was only 8% and oriented approximately east-west. We observe that trenchward shifting of the climax of subduction volcanism and extension occurred during late Oligocene, early Miocene, and late Miocene time. Comparison with the offshore tectonics indicates that the first two tectonic-magmatic pulses coincide with periods of fast spreading at the Pacific-Farallon boundary, south of the Shirley fracture zone. We propose that increases in the spreading rate are related to periods of high subduction rate, which in turn correspond to episodes of retreating subduction. A retreating slab may have generated a flux of hotter asthenospheric material into the mantle wedge, producing widespread melting at the base of the crust as well as intraarc extension in the overriding plate. Boundary conditions (i.e., plate tectonics) ultimately determined timing, magnitude, and orientation of extension, whereas volcanic and tectonic styles are controlled by the internal structure of crustal blocks and by the gravitational and thermal effects of magmatism.