Maximiliano Bezada

Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains

The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere ∼400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large-scale lithospheric loss in a mildly contractional orogen.

Synchronous interhemispheric Holocene climate trends in the tropical Andes

Holocene variations of tropical moisture balance have been ascribed to orbitally forced changes in solar insolation. If this model is correct, millennial-scale climate evolution should be antiphased between the northern and southern hemispheres, producing humid intervals in one hemisphere matched to aridity in the other. Here we show that Holocene climate trends were largely synchronous and in the same direction in the northern and southern hemisphere outer-tropical Andes, providing little support for the dominant role of insolation forcing in these regions. Today, sea-surface temperatures in the equatorial Pacific Ocean modulate rainfall variability in the outer tropical Andes of both hemispheres, and we suggest that this mechanism was pervasive throughout the Holocene. Our findings imply that oceanic forcing plays a larger role in regional South American climate than previously suspected, and that Pacific sea-surface temperatures have the capacity to induce abrupt and sustained shifts in Andean climate.

The History and Disappearance of Glaciers in Venezuela

The disappearance of glaciers is Venezuela is chronicled through an extensive compilation and discussion of the scientific literature, maps, and historical photographs. Climate data analysis and GPS mapping in 2009 and 2011 provide the modern context. 20th-century glacier recession in Venezuela is similar to elsewhere in South America and consistent with the expected impacts of regional climate change. In 2011, one glacier with an area of about 0.1 km2 remained in Venezuela and it will likely disappear within this decade. However, the lack of local glacier and climate data limits our ability to appreciate the history of glaciers in Venezuela and a comprehensive monitoring program is urgently needed before Venezuela becomes an ice-free country.

GEOPHAGY OF HOLSTEIN HYBRID CATTLE IN THE NORTHERN ANDES, VENEZUELA

During the course of a Quaternary stratigraphic reconnaissance of the northern Venezuelan Andes, W. C. Mahaney and M. Bezada observed Holstein hybrid cattle consuming reddish-brown soil formed in residual regolith from bedrock outcrops (Figure 1). In all cases, the source sites of the eaten soils were located above 3,000 m a.s.l. in the Venezuelan paramo. While there are a number of such sites, only one was sampled to characterize mineral composition, particle size, and geochemistry, in order to determine whether analysis of soil quality would point to an explanation for the geophagic behavior of these cattle. While turf destruction by cattle in the Andes is documented (Perez, 1993), the actual mining of soil by cattle to offset nutritional/dietary deficiencies or to obtain a pharmaceutical benefit is reported here for the first time.

Representing anisotropic subduction zones with isotropic velocity models: A characterization of the problem and some steps on a possible path forward

Despite the widely known fact that mantle flow in and around subduction zones produces the development of considerable seismic anisotropy, most P-wave tomography efforts still rely on the assumption of isotropy. In this study, we explore the potential effects of erroneous assumption on tomographic images and explore an alternative approach. We conduct a series of synthetic tomography tests based on a geodynamic simulation of subduction and rollback. The simulation results provide a self-consistent distribution of isotropic (thermal) anomalies and seismic anisotropy which we use to calculate synthetic delay times for a number of realistic and hypothetical event distributions. We find that anisotropy-induced artifacts are abundant and significant for teleseismic, local and mixed event distributions. The occurrence of artifacts is not reduced, and indeed can be exacerbated, by increasing richness in ray-path azimuths and incidence angles. The artifacts that we observe are, in all cases, important enough to significantly impact the interpretation of the images. We test an approach based on prescribing the anisotropy field as an a priori constraint and find that even coarse approximations to the true anisotropy field produce useful results. Using approximate anisotropy, fields can result in reduced RMS misfit to the travel time delays and reduced abundance and severity of imaging artifacts. We propose that the use of anisotropy fields derived from geodynamic modeling and constrained by seismic observables may constitute a viable alternative to isotropic tomography that does not require the inversion for anisotropy parameters in each node of the model.

Natural Hazards and Human-Induced Disasters Triggered by Intense and Episodic Tropical Rains in the Venezuelan Mountains

In Venezuela, more than 50% of the population lives in the coastal-mountain region because most of the country’s economic activities are concentrated in these areas. Presently, hundreds of thousands of people live in very vulnerable sites, and it seems impossible to avoid potential catastrophe, especially in the poorest neighborhoods, which are located in intermountain areas of the main valleys and in the foothills of the northern Caribbean Mountains. In these areas, urban expansion since 1950 can be traced very clearly, as the result of population growth close to the center of the main economic activities. This chapter analyzes some of the most remarkable disasters that have recently affected Venezuela. The impacts of disorganized urban growth on alluvial fans and the catastrophic flood on the north face of the Caribbean Mountains of Venezuela are presented here in historical perspective. Because of the magnitude of the event, details are offered on the disaster in 1999 at Vargas, which killed more than 15,000 people as a result of debris flows on the coastal alluvial fans. The torrential avalanche of the Limon River in the south side of the north central Caribbean Mountains and other recent human and natural disasters caused by catastrophic floods in the Venezuelan Andes are discussed. On the basis of all the disasters we have reviewed, we can conclude that the catastrophes were induced mainly by human mistakes produced by poverty and by the lack of governmental policies on territorial planning of new urban zoning. Urban expansion was undertaken without taking into account the natural dynamics of the geomorphic systems.

The Waveform Browser, an Interactive and Intuitive Tool for Exploring Seismic Event Data

Online Material: Technical details on running the Waveform Browser and format of the input variables, with Matlab, GUI, and sample dataset files. Seismic waveforms carry much information about the source that generated them and the material properties of the media through which they propagated. Systematic changes in waveform character offer clues that can help us understand earthquake rupture processes as well as heterogeneous velocity structure. To find these systematic changes, record sections are often constructed. However, the character of the waveforms normally changes as a function of many variables, including epicentral distance, azimuth, exit point on the focal sphere, and recording station site effects, among others, all acting simultaneously. It is therefore useful to view the waveforms in their geographic/geologic context. Recognizing this, many recent publications dealing with waveform character plot waveforms distributed over a map in the location of the station that recorded them (examples are abundant; a recent one that uses this type of figure extensively is Sun and Helmberger, 2011). This is a very useful representation, but it becomes problematic if station spacing is small, when it becomes difficult to separate the waveform plots. In addition, by necessity, the plotted waveforms must be small, because they must all fit inside the map. This makes it difficult to assess details and more subtle effects when trying to analyze the data. An intimately related issue is the increase of data availability in recent years. For a single seismic event, we may now have access to hundreds of records. Such large volumes of data can reveal complex, but predictable, behavior of the waveforms, which would otherwise be obscured by the sparseness of the observations. An abundance of data provides many opportunities, but organizing, aggregating, and visualizing large volumes of data in a meaningful and useful way poses its own challenges …