13 research outputs found
Sources and Sinks of Diversification and Conservation Priorities for the Mexican Tropical Dry Forest
Elucidating the geographical history of diversification is critical for inferring where future diversification may occur and thus could be a valuable aid in determining conservation priorities. However, it has been difficult to recognize areas with a higher likelihood of promoting diversification. We reconstructed centres of origin of lineages and identified areas in the Mexican tropical dry forest that have been important centres of diversification (sources) and areas where species are maintained but where diversification is less likely to occur (diversity sinks). We used a molecular phylogeny of the genus Bursera, a dominant member of the forest, along with information on current species distributions. Results indicate that vast areas of the forest have historically functioned as diversity sinks, generating few or no extant Bursera lineages. Only a few areas have functioned as major engines of diversification. Long-term preservation of biodiversity may be promoted by incorporation of such knowledge in decision-making
Paleomagnetism of the Acatlan terrane, southern Mexico: evidence for terrane rotation
232 drill samples were collected from 33 sites from the Acatlan Terrane, southern Mexico. The medium-grade metamorphosed basement rocks (schists, granitoid, greenstones) showed magnetic directions that are randomly distributed and no meaningful results could be obtained. However, late Paleozoic red beds, the Ordovician (?) Totoltepec Granite and the early Paleozoic (?) Tecomate Limestone (metamorphosed in Acadian times) give coherent paleomagnetic directions. The samples responded well to thermal demagnetization but not to alternating field demagnetization. The ages of the magnetizations are reasonably bracketed between Carboniferous and Jurassic. Compared with data from the North American craton and from the Oaxaca terrane, conclusive evidence for major north-south displacements of the Acatlan terrane is not present, but significant clockwise rotations of the terrane with respect to the craton and the adjacent Oaxaca terrane are quite evident. These rotations occurred during the Jurassic or Early Cretaceous.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27809/1/0000214.pd
Intra-arc opening and closure of a marginal sea: The case of the Guerrero Terrane (SW Mexico)
The Laluzia armini (gen. et spec. nov.) ecosystem: understanding a deeper-water rudist lithosome from the Early Maastrichtian of Mexico
Water-rock interaction and mixing processes of complex urban groundwater flow system subject to intensive exploitation: The case of Mexico City
Revisión y análisis panbiogeográfico de las Hymenophyllaceae de las Yungas meridionales de Argentina (Selva Tucumano-Boliviana)
Magmatism and tectonics of the Sierra Madre Occidental and its relation with the evolution of the western margin of North America
The Sierra Madre Occidental is the result of Cretaceous-Cenozoic magmatic and tectonic episodes related to the subduction of the Farallon plate beneath North America
and to the opening of the Gulf of California. The stratigraphy of the Sierra Madre Occidental consists of fi ve main igneous complexes: (1) Late Cretaceous to Paleocene
plutonic and volcanic rocks; (2) Eocene andesites and lesser rhyolites, traditionally grouped into the so-called Lower Volcanic Complex; (3) silicic ignimbrites mainly
emplaced during two pulses in the Oligocene (ca. 32–28 Ma) and Early Miocene (ca. 24–20 Ma), and grouped into the “Upper Volcanic Supergroup”; (4) transitional
basaltic-andesitic lavas that erupted toward the end of, and after, each ignimbrite pulse, which have been correlated with the Southern Cordillera Basaltic Andesite
Province of the southwestern United States; and (5) postsubduction volcanism consisting of alkaline basalts and ignimbrites emplaced in the Late Miocene, Pliocene,
and Pleistocene, directly related to the separation of Baja California from the Mexican mainland. The products of all these magmatic episodes, partially overlapping in
space and time, cover a poorly exposed, heterogeneous basement with Precambrian to Paleozoic ages in the northern part (Sonora and Chihuahua) and Mesozoic ages
beneath the rest of the Sierra Madre Occidental.
The oldest intrusive rocks of the Lower Volcanic Complex (ca. 101 to ca. 89 Ma) in Sinaloa, and Maastrichtian volcanics of the Lower Volcanic Complex in central
Chihuahua, were affected by moderate contractile deformation during the Laramide orogeny. In the fi nal stages of this deformation cycle, during the Paleocene and Early Eocene, ~E-W to ENE-WSW–trending extensional structures formed within the Lower Volcanic Complex, along which the world-class porphyry copper deposits of
the Sierra Madre Occidental were emplaced. Extensional tectonics began as early as the Oligocene along the entire eastern half of the Sierra Madre Occidental, forming
grabens bounded by high-angle normal faults, which have traditionally been referred to as the southern (or Mexican) Basin and Range Province. In the Early to Middle
Miocene, extension migrated westward. In northern Sonora, the deformation was sufficiently intense to exhume lower crustal rocks, whereas in the rest of the Sierra
Madre Occidental, crustal extension did not exceed 20%. By the Late Miocene, extension became focused in the westernmost part of the Sierra Madre Occidental, adjacent
to the Gulf of California, where NNW-striking normal fault systems produced both ENE and WSW tilt domains separated by transverse accommodation zones. It is worth noting that most of the extension occurred when subduction of the Farallon plate was still active off Baja California.
Geochemical data show that the Sierra Madre Occidental rocks form a typical calc-alkaline rhyolite suite with intermediate to high K and relatively low Fe contents.
Late Eocene to Miocene volcanism is clearly bimodal, but silicic compositions are volumetrically dominant. Initial 87Sr/86Sr ratios mostly range between 0.7041 and 0.7070,
and initial εNd values are generally intermediate between crust and mantle values (+2.3 and −3.2). Based on isotopic data of volcanic rocks and crustal xenoliths from
a few sites in the Sierra Madre Occidental, contrasting models for the genesis of the silicic volcanism have been proposed. A considerable body of work led by Ken Cameron
and others considered the mid-Tertiary Sierra Madre Occidental silicic magmas to have formed by fractional crystallization of mantle-derived mafi c magmas with
little (<15%) or no crustal involvement. In contrast, other workers have suggested the rhyolites, taken to the extreme case, could be entirely the result of partial melting of the crust in response to thermal and material input from basaltic underplating. Several lines of evidence suggest that Sierra Madre Occidental ignimbrite petrogenesis involved large-scale mixing and assimilation-fractional crystallization processes of crustal and mantle-derived melts.
Geophysical data indicate that the crust in the unextended core of the northern Sierra Madre Occidental is ~55 km-thick, but thins to ~40 km to the east. The anomalous
thickness in the core of the Sierra Madre Occidental suggests that the lower crust was largely intruded by mafi c magmas. In the westernmost Sierra Madre Occidental
adjacent to the Gulf of California, crustal thickness is ~25 km, implying over 100% of extension. However, structures at the surface indicate no more than ~50% extension. The upper mantle beneath the Sierra Madre Occidental is characterized by a low velocity anomaly, typical of the asthenosphere, which also occurs beneath the Basin and Range Province of the western United States.
The review of the magmatic and tectonic history presented in this work suggests that the Sierra Madre Occidental has been strongly infl uenced by the Cretaceous-Cenozoic evolution of the western North America subduction system. In particular, the Oligo-Miocene Sierra Madre Occidental is viewed as a silicic large igneous province formed as the precursor to the opening of the Gulf of California during and immediately following the final stages of the subduction of the Farallon plate. The mechanism responsible for the generation of the ignimbrite pulses seems related to the removal of the Farallon plate from the base of the North American plate after the end of the Laramide orogeny. The rapid increase in the subduction angle due to slab roll-back and, possibly, the detachment of the deeper part of the subducted slab as younger and buoyant oceanic lithosphere arrived at the paleotrench, resulted in extension of the continental margin, eventually leading to direct interaction between the Pacific and North American plates