Mountain building processes in the Central Andes

False color composite images of the Thematic Mapper (TM) bands 5, 4, and 2 were examined to make visual interpretations of geological features. The use of the roam mode of image display with the International Imaging Systems (IIS) System 600 image processing package running on the IIS Model 75 was very useful. Several areas in which good comparisons with ground data existed, were examined in detail. Parallel to the visual approach, image processing methods are being developed which allow the complete use of the seven TM bands. The data was organized into easily accessible files and a visual cataloging of the quads (quarter TM scenes) with preliminary registration with the best available charts for the region. The catalog has proved to be a valuable tool for the rapid scanning of quads for a specific investigation. Integration of the data into a complete approach to the problems of uplift, deformation, and magnetism in relation to the Nazca-South American plate interaction is at an initial stage

Modelling gravitational instabilities: slab break-off and Rayleigh-Taylor diapirism

A non-standard new code to solve multiphase viscous thermo–mechanical problems applied to geophysics is presented. Two numerical methodologies employed in the code are described: A level set technique to track the position of the materials and an enrichment of the solution to allow the strain rate to be discontinuous across the interface. These techniques have low computational cost and can be used in standard desktop PCs. Examples of phase tracking with level set are presented in two and three dimensions to study slab detachment in subduction processes and Rayleigh–Taylor instabilities, respectively. The modelling of slab detachment processes includes realistic rheology with viscosity depending on temperature, pressure and strain rate; shear and adiabatic heating mechanisms; density including mineral phase changes and varying thermal conductivity. Detachment models show a first prolonged period of thermal diffusion until a fast necking of the subducting slab results in the break–off. The influence of several numerical and physical parameters on the detachment process is analyzed: The shear heating exerts a major influence accelerating the detachment process, reducing the onset time to one half and lubricating the sinking of the detached slab. The adiabatic heating term acts as a thermal stabilizer. If the mantle temperature follows an adiabatic gradient, neglecting this heating term must be included, otherwise all temperature contrasts are overestimated. As expected, the phase change at 410 km depth (olivine–spinel transition) facilitates the detachment process due to the increase in negative buoyancy. Finally, simple plume simulations are used to show how the presented numerical methodologies can be extended to three dimensions.Peer ReviewedPostprint (author’s final draft

Do trench sediments affect great earthquake occurrence in subduction zones?

Seismic energy release is dominated by the underthrusting earthquakes in subduction zones, and this energy release is further concentrated in a few subduction zones. While some subduction zones are characterized by the occurrence of great earthquakes, others are relatively aseismic. This variation in maximum earthquake size between subduction zones is one of the most important features of global seismicity. Previous work has shown that the variation in maximum earthquake size is correlated with the variation in two other subduction zone properties: age of the subducting lithosphere and convergence rate. These two properties do not explain all the variance in maximum earthquake size. I propose that a third subduction zone property, “trench sediments”, explains part of the remaining variance in maximum earthquake size. Subduction zones are divided into two groups: (1) those with excess trench sediments, and (2) those with horst and graben structure at the trench. Thirteen of the 19 largest subduction zone events, including the three largest, occur in zones with excess trench sediments. About half the zones with excess trench sediments are characterized by great earthquake occurrence. Most of the other zones with excess trench sediments but without great earthquakes are predicted to have small earthquakes by the age-rate correlation. Two notable exceptions are the Oregon-Washington and Middle America zones. Overall, the presence of excess trench sediments appears to enhance great earthquake occurrence. One speculative physical mechanism that connects trench sediments and earthquake size is that excess trench sediments are associated with the subduction of a coherent sedimentary layer, which at elevated temperature and pressure, forms a homogeneous and strong contact zone between the plates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43133/1/24_2004_Article_BF00874629.pd

Seismotectonics of thin- and thick-skinned deformation in the Andean foreland from local network data: evidence for a seismogenic lower crust

Local network data from San Juan, Argentina, provides new information about crustal seismicity in the Andean foreland above a horizontal segment of the subducted Nazca Plate. Two areas of foreland seismicity are found, one associated with the Sierras Pampeanas basement uplifts, and the other beneath, but not within, the Precordillera foreland fold-thrust belt. The Precordillera seismicity provides direct evidence for basement deformation beneath the sediments of the thrust belt and supports the idea that its eastern part is significantly modified by underlying basement deformation. The depth distribution suggests a brittle-ductile transition near 30-35 km and that the upper plate may have a geotherm similar to stable cratonic regions. A spatial correlation of upper plate seismicity and structures with subducted plate seismicity raises the possibility that a major lithospheric structure cutting across the strike of the foreland near 31°S may be related to subduction of the Juan Fernandez Ridge. -from Author

Crustal thickness variation in the Andean Foreland, Argentina, from converted waves

Local network three-component digital data from the San Juan area, Argentina, provide the first seismological images of the deep crustal structure in the Andean foreland above a horizontal segment of the subducted Nazca plate. S-to-P seismic phases converted on the Moho have been identified by analysis of seismograms formed by taking the product of the radial and vertical conponents (R*Z) from intermediate-depth earthquakes in the Benioff zone directly beneath the network. Under the Sierras Pampeanas, the Moho is estimated to be at a depth of about 52 km. Beneath the eastern Precordillera, the Moho deepens to 55/57 km and further west, beneath the central Precordillera, to 60 km. A 5° to 10° westward dip to the Moho under the Andean foreland is estimated. -from Author

Seismotectonics of Sierra Pie de Palo, a basement block uplift in the Andean foreland of Argentina

A digitally recording seismic network was operated from September 1987 to May 1988 in the San Juan Province of northwestern Argentina. The data provide a detailed view of the crustal seismicity of Sierra Pie de Palo, one of the most seismically active mountain blocks of the Sierra Pampeanas and the site of the destructive Ms 7.3 Caucete earthquake in 1977. The morphology of the range, seismicity pattern, and focal mechanisms strongly suggest Sierra Pie de Palo is cut into two blocks by a northeast-southwest striking fracture. In the northern block, seismicity occurs along a well-defined west-dipping seismic zone, while the southern block is characterized by east-dipping seismic layers. Both dipping zones connect to a subhorizontal mid-crustal detachment that we interpret as the lower boundary of the brittle failure domain. Local boundary effects within the Sierras Pampeanas Province partially control the shortening direction in the crust. -from Author