Using geoid anomalies determined from satellite observations of the South American plate, we demonstrate the existence of a lithospheric flexural forebulge east of the High Andes. Using the planform and location of the geoid anomalies and accounting for the curvature of the Andean orogen, we can successfully model plate flexure using a uniform elastic thickness. Topography above 3 km elevation between −5° and −30° latitude in the Andes loads the margin of the western side of the Precambrian shield of the continental plate and drives bending of the cratonic plate. Removal of horizontal wavelengths greater than 4500 km from the geoid anomaly reveals a 5–7 m positive anomaly paralleling the trend of the orogen some 400 km east of the mountain front. We interpret the secondary geoid high as a flexural forebulge that developed in response to topographic loading of the South American plate by the Andes. While the topographic expression of this forebulge is hidden by the alluvium shed from the Andes and the vegetative cover of the Amazon jungle, our filtered geoid anomalies and a three-dimensional, single-plate flexural model in spherical geometry are both well fit by a single model with ~50 km effective elastic thickness.
Optical microscopy characterization of thin sections from corehole U-15n#12 is part of a larger material characterization effort for the Source Physics Experiment (SPE). The SPE program was conducted in Nevada with a series of explosive tests designed to study the generation and propagation of seismic waves inside Stock quartz monzonite. Optical microscopy analysis includes the following: 1) imaging of full thin sections (scans and mosaic maps); 2) high magnification imaging of petrographic texture (grain size, foliations, fractures, etc.); and 3) measurement of microfracture density.
The late Cenozoic extension in the Rio Grande rift of north-central New Mexico was predominantly accommodated by the north-south–trending Pajarito and Sangre de Cristo normal faults and the intervening east-northeast–striking predominantly strike-slip Embudo fault. Using this segment of the rift as our primary example, we have analyzed a series of three-dimensional nonlinear elastic-plastic finite-element models to assess the role of mechanical interactions between pairs of en echelon rift-scale listric normal faults in the evolution of intervening relay zones. The model results demonstrate that under orthogonal extension and an overall plane-strain deformation, relay zones may evolve in a three-dimensional strain field and along non-coaxial strain paths. The extent of non-plane strain and non-coaxial deformation depends on the fault overlap to spacing ratio, the relative orientations of the bounding faults, and the structural position within the relay zone. The model-derived minimum compressive stress vectors within the relay zone are oblique to the regional extension direction throughout the deformation.
Abstract The application of palaeomagnetism in fold and thrust belts is a unique way to obtain kinematic information regarding the evolution of these systems. However, since many potential problems can affect the reliability of palaeomagnetic datasets and their interpretations, such data should be used with caution. In this paper, we thoroughly review the sources of error from palaeomagnetism with a particular focus on deciphering vertical-axis rotations and the assumptions behind the method. Recent investigations have demonstrated that the age of the magnetization and syn-folding results from the fold test must also be carefully examined: factors such as internal deformation, deficient isolation of components (i.e. overlapping) or incorrect restoration procedures may produce apparent syn-folding results. In fact, the restoration procedure used to return the palaeomagnetic signal to the palaeogeographic coordinate system may itself inhibit accurate estimations of vertical-axis rotations when complex deformation histories induce different, non-coaxial, deformation axes. We recommend the auxiliary use of the inclination v. dip diagram as an efficient tool for identifying many errors. Finally, to determine accurate vertical axis rotations, the reference direction should honour standard reliability criteria and would ideally be measured within the undeformed foreland of the thrust system. In this paper, we review five decades of palaeomagnetic research in fold and thrust belts by concentrating on maximizing standard reliability criteria procedures to reduce uncertainty and increase confidence when applying palaeomagnetic data to unravel the tectonic evolution of fold and thrust belts.
The condition of emplacement of internal thrust sheets in the Sevier fold-and-thrust belt can be characterized by detailed studies on deformation of quartzites within these sheets. The Sheeprock and Canyon Range thrust sheets in north-central Utah both carry Proterozoic Caddy Canyon and Mutual quartzites in their hanging walls. The Caddy Canyon is a medium to coarse-grained quartzite with subrounded grains and some grain-overgrowth. The Mutual is typically coarse-grained to pebbly, has subrounded to well rounded grains with overgrowths, and is characterized by clasts showing an older metamorphic (in some cases, mylonitic) fabric. In the Sheeprock sheet both quartzites show evidence for considerable crystal-plastic deformation, including deformation lamellae, undulose extinction, and development of subgrains; the Caddy Canyon also shows considerable pressure solution. Late stage cataclasis, characterized by the development of cemented cataclasites, is also evident. In the Canyon Range, both quartzites show less plastic deformation, although the Caddy Canyon shows large amounts of pressure solution. The late stage cataclasites are coherent but not cemented, and there is locally strong plastic deformation adjacent to the cataclasite zones. The microstructures suggest an overall shallower level of deformation for the Canyon Range sheet. Strains in the quartzites are quantified using the Fry technique, andmore » the strains are partitioned into plastic and diffusional components using deformed rutile needles within quartz grains and pressure-solved grain shapes.« less
This Research is about Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies which presents a preliminary model of the three-dimensional seismic structure of the Iran region.
