Imaging Radar Observations Of Normal Faults In Tibet

NASA/JPL has an ongoing program to study land processes that will lead to a better understanding of the geologic evolution of the continents and the history of global climate change. Northwestern China and Tibet are key areas for these investigations because arid regions such as these retain the record of climate change and a wide variety of large-scale structures are present throughout the area as a result of the continuing collision between the Indian and Asian plates. During the Shuttle Imaging Radar Missions (SIR-A and SIR-B) a number of image swaths were acquired over the People's Republic of China that show interesting landforms indicative of climate change and large-scale faulting in the arid regions of Tibet and northwestern China. One of the 50-km wide swaths (data-take 32-33) passed from northeastern India, through Lhasa and southern Tibet, to the Karakorum Himalaya and beyond. This swath provides a view of several active faults related to the collision between India and Asia, in particular, normal faults in the Yangbajain Valley, northeast of Lhasa. The scarps are oriented N12°E±16° and most of them dip to the west, which is generally within a few degrees of being parallel to the illumination direction of SIR-A. Since conventional interpretation of the interaction of radar signals with steep scarps predicts that they will not be visible if the illumination azimuth is nearly parallel to the scarp strike because of a lack of the "highlighting" that occurs when a scarp face is oriented normal to the incoming illumination, it is surprising they show up in such light tones on the SIR-A image. The most likely reason for the high radar returns from the scarp faces is that their surfaces are rougher than the smooth, grass-covered valley floor. Height, slope-angle, and surface-roughness measurements were obtained on several scarps and we found that scarps which were visible on the SIR-A image are higher than 5 m and are rough at the decimeter to meter scale. This result is significant since orbital radar sensors can obtain high-resolution images of large areas that are difficult to access, and it appears that they may provide an efficient means with which to survey large areas, extrapolate local observations, and derive quantitative estimates of rates of tectonic processes

A multidisciplinary study of a syntectonic pluton close to a major lithospheric-scale fault: relationships between the Montmarault granitic massif and the Sillon Houiller Fault in the Variscan French Massif Central. Part II: Gravity, aeromagnetic investigations and 3D geologic modeling.

International audienceNew gravity and aeromagnetic investigations have been carried out to understand the emplacement mechanisms of a granitic pluton and the relationships with a nearby lithospheric-scale fault. This paper concerns the second part of a methodological multidisciplinary study and complements previous geochronologic and Anisotropy of Magnetic Susceptibility (AMS) studies on the same pluton. In the northern part of the Variscan French Massif Central (FMC), the Montmarault massif crops out along the Sillon Houiller Fault (SHF). Bouguer and aeromagnetic anomaly maps imply thickening of the pluton along the SHF and suggest laccolitic spreading northwestwards. Based on petrophysical measurements, direct 2D joint gravity and magnetic modeling has been performed along 10 cross-sections. In order to quantitatively constrain the 3D pluton geometry and its relationships with surrounding geologic units, these geophysical cross-sections, new structural information (field and AMS measurements) and petrophysical data have been integrated into a regional 3D geological and geophysical model. Altogether, the results obtained from geochronology, petro-magnetic fabrics (Part I), gravity and aeromagnetic investigations as well as 3D modeling (Part II), demonstrate that the Montmarault pluton was emplaced during the Namurian period along the SHF. Our results further show that, at that time, in response to a NW-SE regional extension, if the SHF existed, it behaved as a normal fault. Mylonites attesting for synmagmatic normal motion on the northeastern part of the Montmarault pluton strengthen this tectonic scheme. During the Late Carboniferous, the FMC experienced NE-SW extension along the SHF by 80 km of brittle left lateral wrench offset. This second tectonic event is well recorded in the Stephanian coal basins which were formed along NW- SE listric brittle faults and constrain the present-day shape of the Montmarault pluton

About the lithospheric structure of central Tibet based on seismic data from the INDEPTH III profile

Signals from 11 shots and 8 earthquakes, and numerous teleseismic events were recorded along the 400-km seismic line INDEPTH III in central Tibet and interpreted together with previous seismic and tectonic data. The abnormal behavior of various mantle phases reveals a complex Moho-transition zone, especially in the northern part of the line, in the Changtang Block, where the lower crust and the mantle show unusually low velocities, a shingled appearance of Pn and no low-velocity layer in the upper crust. The strong east-west anisotropy in the Changtang Block is related to an easterly escape movement of the whole lithosphere, facilitated by the warm and weak layers in the lower crust and the upper mantle, bounded apparently by two prominent west-east running fault zones

Imaging Radar Observations Of Normal Faults In Tibet

NASA/JPL has an ongoing program to study land processes that will lead to a better understanding of the geologic evolution of the continents and the history of global climate change. Northwestern China and Tibet are key areas for these investigations because arid regions such as these retain the record of climate change and a wide variety of large-scale structures are present throughout the area as a result of the continuing collision between the Indian and Asian plates. During the Shuttle Imaging Radar Missions (SIR-A and SIR-B) a number of image swaths were acquired over the People's Republic of China that show interesting landforms indicative of climate change and large-scale faulting in the arid regions of Tibet and northwestern China. One of the 50-km wide swaths (data-take 32-33) passed from northeastern India, through Lhasa and southern Tibet, to the Karakorum Himalaya and beyond. This swath provides a view of several active faults related to the collision between India and Asia, in particular, normal faults in the Yangbajain Valley, northeast of Lhasa. The scarps are oriented N12°E±16° and most of them dip to the west, which is generally within a few degrees of being parallel to the illumination direction of SIR-A. Since conventional interpretation of the interaction of radar signals with steep scarps predicts that they will not be visible if the illumination azimuth is nearly parallel to the scarp strike because of a lack of the "highlighting" that occurs when a scarp face is oriented normal to the incoming illumination, it is surprising they show up in such light tones on the SIR-A image. The most likely reason for the high radar returns from the scarp faces is that their surfaces are rougher than the smooth, grass-covered valley floor. Height, slope-angle, and surface-roughness measurements were obtained on several scarps and we found that scarps which were visible on the SIR-A image are higher than 5 m and are rough at the decimeter to meter scale. This result is significant since orbital radar sensors can obtain high-resolution images of large areas that are difficult to access, and it appears that they may provide an efficient means with which to survey large areas, extrapolate local observations, and derive quantitative estimates of rates of tectonic processes

Loading rate dependence of permeability evolution in porous aeolian sandstones

Mechanical properties of rocks are characterized by their notable dependence on the applied deformation rate. However, little is known about the strain rate dependence of fluid flow properties since most laboratory tests are conducted using a single, high strain rate. We have investigated the effect of loading rate on the permeability of porous sandstones by carrying out triaxial compression tests at four different temperatures and strain rates with continuous monitoring of permeability, acoustic emission (AE), and pore fluid chemistry. All tests are characterized by an initial permeability decrease due to inferred compaction of favorably oriented cracks. The amount of initial permeability reduction increases with decreasing strain rate, thus implying a more efficient initial compaction at slower strain rates. At a later stage of loading, permeability correlates with stress, ion concentration, or AE damage depending on the strain rate used. High strain rate tests are characterized by a positive power law or logarithmic correlation between permeability and AE damage. At slow strain rates, permeabilities decrease exponentially with mean effective stress and axial strain for the Locharbriggs sandstone. The Clashach sandstone exhibits a linear correlation between permeability and exit pore fluid concentrations (Si, Mg, Fe, Al) if a slow strain rate is used. These observations have important implications for the applicability of room temperature, high strain rate laboratory data to the conditions that prevail in the Earth's crust