Structure of central and southern Mexico from velocity and attenuation tomography

Properties of the subduction system in Mexico

In part I, the 3D velocity and attenuation structure of the Cocos subduction zone in Mexico is imaged using earthquakes recorded by two temporary seismic arrays and local stations. Inversion results reveal low-attenuation and high-velocity Cocos slab. The slab dip angle increases from almost flat in central Mexico near Mexico City to about 30 degrees in southern Mexico near the Isthmus of Tehuantepec. High attenuation and low velocity in the crust beneath the Trans-Mexico Volcanic Belt correlate with low resistivity, and are probably related to dehydration and melting process. The most pronounced high-attenuation, low-Vp and high-Vp/Vs anomaly is found in the crust beneath the Veracruz Basin. A high-velocity structure dipping southward from the Gulf of Mexico near the Isthmus of Tehuantepec coincides with a discontinuity from a receiver functions study, and provides an evidence for the collision between the Yucatan Block and Mexico in the Miocene. In part II, we show that a model of small repeating earthquakes based on laboratory-derived rate and state friction laws reproduces the observed scaling between the recurrence time and seismic moment. In the model, a small fault patch governed by velocity-weakening friction is surrounded by a much larger velocity-strengthening region. For a fixed set of friction parameters, the observed scaling is reproduced by varying the size of the velocity-weakening patch. We further investigate the behavior of small repeating earthquakes in related models under different scenarios, including several forms of the state evolution equations in rate- and state-dependent friction laws, rectangular velocity-weakening patch geometries, quasi-dynamic vs. fully dynamic representation of inertial effects, and 2D vs. 3D simulations. We find that the simulated scalings between the recurrence time and seismic moment for these different scenarios are similar while differences do exist. We propose a theoretical model for the scaling between the recurrence time and seismic moment of small repeating earthquakes. The obtained theoretical insight is used to find the combinations of fault properties that allow the model to fit the observed scaling and range of the seismic moment and recurrence time.

Distribution of hydrous minerals in the subduction system beneath Mexico

Central and southern Mexico represents a strategic place to understand the dynamics of Pangaea break-up and its influences on the evolution of the Pacific margin of North America. Lower–Middle Jurassic volcano-sedimentary successions, and scarce magmatic rocks, crop out discontinuously across this region and have been interpreted either as a vestige of a continental arc or as several deposits of syn-rift magmatism. At present, their origin is controversial. Available geochemical data on these igneous rocks suggest that they represent almost pure crustal melts produced in a rift environment rather than in an arc. In fact, the studied rocks exhibit the high silica contents and moderate to strong peraluminous character typical of sediment melts. The enriched isotopic composition (high86Sr/87Sr and low143Nd/144Nd) and the age distributions of inherited zircon grains readily identify the widespread Upper Triassic metasedimentary sequences presently exposed in southwestern and central Mexico as the most likely crustal source of these Jurassic igneous rocks. Accordingly, we argue that these Early–Middle Jurassic magmas originated in a syn-rift igneous province associated with extensional-driven crustal attenuation in the context of Pangaea fragmentation. Our findings also constrain post-Pangaea subduction initiation to be younger than Middle Jurassic time in central and southern Mexico.

Biomass burning from grassland, forests, and agricultural waste results in large amounts of gases and particles emitted to the atmosphere, which affect air quality, population health, crop development, and natural vegetation. Regional atmospheric circulations can transport those plumes of pollutants over hundreds of kilometers, affecting vulnerable environments such as those considered protected natural areas (PNAs). This study evaluates the spatiotemporal distribution of active fires detected, and associated emissions, in central and southern Mexico from satellite data between March and June 2017, to assess the impact of the smoke plumes on protected ecosystems. The arrival of smoke plumes to selected PNAs (both near large urban centers and in remote areas) is assessed using airmass forward trajectories from selected emission sources. The spatial distribution of the remotely derived aerosol optical depth confirms the regional impact of particle emissions from the observed fires on PNAs, particularly in central Mexico. The identified areas of high fire density are also associated with large coarse particle concentrations at the surface. Moreover, there is a significant contribution of organic carbon to the total coarse particle mass, 60% on average. Finally, while most of the impact in ambient pollution is observed in PNAs located close to the regions with active fires in southern Mexico and Central America, the long-range transport of smoke plumes reaching the USA was also confirmed.

This volume furthers our understanding of key basins in central and southern Mexico, and establishes links to exhumed sediment source areas in a plausible paleogeographic framework. Authors present new data and models on the relations between Mexican terranes and the assembly and breakup of western equatorial Pangea, plate-tectonic and terrane reconstructions, uplift and exhumation of source areas, the influence of magmatism on sedimentary systems, and the provenance and delivery of sediment to Mesozoic and Cenozoic basins. Additionally, authors establish relationships between basement regions (sediment source) in the areas that supplied sediment to Mesozoic rift basins, Late Cretaceous foreland systems, and Cenozoic basins developed in response to Cordilleran events.

In Mexico, ultramafic complexes are present in different regions from the northwest (Baja California Norte) to the southeast (Chiapas). In this paper, we present the results of the exploration of three ultramafic (serpentine) habitats in central and southern Mexico: Cuicatlán–Concepción Pápalo (Oaxaca), Tehuitzingo–Tecomatlán (Puebla), and San Juan de Otates (Guanajuato). Previous geology studies showed that these complexes are mainly made up of serpentinized peridotites. Soil analyses demonstrated typical ultramafic characteristics such as high content of Mg in relation to Ca, and high concentrations of Fe, Cr, Co, and Ni. Soil samples from Oaxaca and Puebla had similar Ni contents around 2300 mg kg−1, while samples of Guanajuato showed the lowest Ni levels with an average of 200 mg kg−1 as well as for other metals such as Co, Cr, Mn, and Zn. During this study, 83 plant specimens were collected, of which 52 were identified at genus level and 40 at species level. The collected plants belong to 19 different families such as Anacardiaceae, Fabaceae, Acanthaceae, Asteraceae, Sterculiaceae, and Verbenaceae which are also widely present in other ultramafic areas in Iran, Brazil, Sri Lanka, and Costa Rica. Only two Mexican endemic species are included in the collection. Ni hyperaccumulators were not detected at any of the studied sites. Therefore, hyperaccumulation, as a tolerance mechanism of the flora in response to ultramafic geochemical stress, does not seem to be developed in Central Mexico, as observed in the close Costa Rican site of Santa Elena.

In southern Mexico, the subducting Cocos slab drastically changes its geometry: from a flat slab in central Mexico to a ∼45° dip angle beneath Chiapas. Also, the currently active volcanic arc, the modern Chiapanecan volcanic arc, is oblique and situated far inland from the Middle America trench, where the slab depth is ∼200 km. In contrast, the Central America volcanic arc is parallel to the Middle America trench, and the slab depth is ∼100 km. A two-dimensional steady-state thermomechanical model explains the calc-alkaline volcanism by high temperature (∼1300 °C) in the mantle wedge just beneath the Central America volcanic arc and the strong dehydration (∼5 wt%) of the Cocos slab. In contrast, the thermal model for the modern Chiapanecan volcanic arc shows high P-T conditions beneath the coast where the extinct Miocene Chiapanecan arc is present, and is therefore unable to offer a reasonable explanation for the origin of the modern Chiapanecan volcanic arc. We propose a model in which the origin of the modern Chiapanecan volcanic arc is related to the space-time evolution of the Cocos slab in central Mexico. The initiation of flat subduction in central Mexico in the middle Miocene would have generated a hot mantle wedge inflow from NW to SE, generating the new modern Chiapanecan volcanic arc. Because of the contact between the hot mantle wedge beneath Chiapas and the proximity of a newly formed cold, flat slab, the previous hot mantle wedge in Chiapas became colder in time, finally leading to the extinction of the Miocene Chiapanecan volcanic arc. The position and the distinct K-alkaline volcanism at El Chichon volcano are proposed to be related to the arrival of the highly serpentinized Tehuantepec Ridge beneath the modern Chiapanecan volcanic arc. The deserpentinization of Tehuantepec Ridge would have released significant amounts of water into the overlying mantle, therefore favoring vigorous melting of the mantle wedge and probably of the slab.

The fine‐scale seismic structure of the central Mexico subduction zone is studied using moderate‐sized (M4‐6) intraslab earthquakes. Regional waveforms from the Mapping the Rivera Subduction Zone (MARS) seismic array are complicated and contain detailed information about the subduction zone structure, including evidence of lateral heterogeneity. This waveform information is used to model the structure of the subducted plates, particularly along the transition from flat to normal subduction, where recent studies have shown evidence for possible slab tearing along the eastern projection of the Orozco Fracture Zone (OFZ). The lateral extent of a thin ultra‐slow velocity layer (USL) imaged atop the Cocos slab in recent studies along the Meso America Subduction Experiment array is examined here using MARS waveforms. We find an edge to this USL which is coincident with the western boundary of the projected OFZ region. Forward modeling of the 2D structure of the subducted Rivera and Cocos plates using a finite difference algorithm provides constraints on the velocity and geometry of each slab's seismic structure in this region and confirms the location of the USL edge. We propose that the Cocos slab is currently fragmenting into a North Cocos plate and a South Cocos plate along the projection of the OFZ, in agreement with observations of variable Cocos plate motion on either side of the OFZ. This tearing event may be a young analogy to the 10 Ma Rivera‐Cocos plate boundary, and may be related to the slab rollback in central Mexico.

This chapter provides a glimpse of the native experience in central Mexico since independence. Before independence, many legal and historical documents relating to central Mexico were still written in Nahuatl, and native litigants could present their cases in their own languages. Between independence and the present, most native peoples were culturally absorbed into a more Europeanized, Spanish-speaking nation. At the time of independence, many native people in both central and southern Mexico were members of former Indian republics or native pueblos, with their own land base and separate administrative structures. The social structure of native pueblos, throughout Mesoamerica, is usually depicted as a closed corporate community. Going back even before Mexican independence, the struggle of native peoples for land has been intrinsically related to legal battles in the courts, ideological debates, and armed rebellion. The logic of native political participation takes on a different form during times of relative political stability on the national level.

The Euphorbia adiantoides complex is here considered to consist of four species. This group is readily distinguished from other New World Euphorbia by the combination of two unusual features: entire styles with capitate stigmas and dichasial bracts with relatively long, filiform stipules. Euphorbia sonorae is reduced to a synonym of Euphorbia adiantoides, a taxon disjunctly distributed between Mexico and western South America. The other species of the complex are all restricted to Mexico. Two of these are described as new: E. zamudioi, an endemic to the Sierra Madre Oriental, and E. breedlovei, which is widespread in central and southern Mexico. A key to distinguish the species is provided, as too are data concerning their morphology, distribution, habitat, phenology, common names, and uses. Phylogenetic analyses were conducted using the nuclear ITS and the chloroplast psbA-trnH regions and including multiple samples of each species. The phylogenetic results are not always congruent with morphology, and of the four species herein recognized, only Euphorbia zamudioi is suggested to form an exclusive, well-supported lineage. This species is nested within E. breedlovei, and two collections of E. breedlovei from central Mexico are more closely related to E. zamudioi than they are to other E. breedlovei from southern Mexico. We hypothesize that E. zamudioi arose through peripatric speciation, in which a northern population of E. breedlovei became reproductively isolated and morphologically differentiated from the remainder of the populations of E. breedlovei.

A major provenance change in sandstones from the Tezoatlán basin, southern Mexico, controlled by Jurassic, sinistral normal motion along the Salado River fault: Implications for the reconstruction of Pangea

Seed and endocarp traits as markers of the biodiversity of regional sources of germplasm of tejocote ( Crataegus spp.) from Central and Southern Mexico

Eight local earthquakes were recorded during the operation of a small‐aperture seismic array at Pinyon Flat, California. The site was chosen for its homogeneous granitic geology and its planar topography. Amplitude spectral ratios for the same signal measured at different stations had average values of less than 2 and maximum values of 7. Magnitude‐squared coherences were estimated for all station pairs. These estimates were highest for the P wave arrivals on the vertical component and lowest for the P wave recorded on the transverse component. At 500 m station separation the P and S waves were incoherent above 15 Hz and 10 Hz, respectively. Coherence for both the P and S waves decrease as frequency increases and as distance increases. The coherence of signals from borehole sensors located at 300 and 150 m depth displays higher average coherence than equally spaced sites located on the surface. The results here suggest that even for sites that appear to be very similar, that is, those which are located on a planar surface within a few meters of granite bedrock, the measured seismic wavefield can be distorted substantially over scale lengths of 500 m. Coherence properties were calculated from synthetic seismograms which were computed for velocity models with exponential and self similar distribution perturbations. Standard deviations of 10% are not sufficient for the random velocity distributions to approximate the results from the small‐aperture array.

Imaging fluid-related subduction processes beneath Central Java (Indonesia) using seismic attenuation tomography