Lithospheric Stresses and Fracture Mechanics Posters
Presiding: G Karner, Lamont-Doherty Earth Observatory; S Grandi, Massachusetts Institute of Technology, Earth Resources Laboratory
T43C-01 1330h
Resolving Accelerations of Earth's Tectonic Plates
The fact that the Pacific Plate has recently been documented (Bowin, 2004) to have had two periods of acceleration during the past 68 million years means that conservation of angular momentum must apply to Plate Tectonics. The angular momentum lost when the Pacific plate shifted (at about 46 my) from underthrusting the North American Plate at Sakahlin to underthrusting the Eurasia and Philippine Plates, was at least in part conserved by a renewal, at about 46 my, of suduction of the Australian Plate beneath the Indonesian arc /trench. These results raise the probability that large roller-type thermal convection cells are NOT active in the Earth, and that spreading centers are passive reaction features. The fact that the plate accelerations, numerically, are about 10-8 times smaller than their velocities explains why plate accelerations have been so elusive. However low these acceleration rates are, they, nevertheless, confirm that it is impulse forces that produce the mountain building observed today and in the geologic record. In this paper we summarize our efforts to improve resolving the acceleration history of the Pacific Plate, and also the acceleration histories of other plates comprising the Earth's surface. Our principal new methodology utilizes filtering the quaternions of Euler pole series, and the recalculation of their stage poles.
http://www.whoi.edu/scc (No.12)
T43C-02 1330h
In Situ Stress Field From Borehole Measurements and Plate Tectonic Models
We obtain in situ stress information based on two methods. The first method consists of estimating stress directions from the orientation of breakouts in a borehole. Stress magnitudes are interpreted from elastic models of stress distribution around the borehole. The second method is applied where the borehole is not deformed and a crossover in the dispersions of polarized flexural waves is observed. Data rotation to principal axes provides stress orientation. Magnitudes are inferred from the velocity anisotropy measured at low frequencies. For a particular field dataset we observe that the greatest stress is horizontal in the NNW-SSE direction (SHmax). Both methods agree in the estimation of stress orientation and magnitude. The relative magnitudes of principal stresses are on average SHmax ~eq 1.1 Sv (Sv: vertical stress) and Shmin ~eq 0.9 Sv (Shmin: minimum horizontal stress), suggesting predominantly strike slip tectonics. The estimated stress orientation is in agreement with a regional stress field calculated from the relative motion of the Caribbean and South America plates.
T43C-03 1330h
Volume Based Curvature Attributes Illuminate Stress Effects in Contiguous Fault Blocks, Central Basin Platform, West Texas
We compute curvatures for 3-D seismic volumes covering 200+ mi2 of the Central Basin Platform in West Texas and find that these attributes illumination lineations not seen on other displays of the seismic data. We analyze the preferred orientations of these lineations defined by well imaged faults and fault zones and find that the patterns vary according to the nature of the faults bounding the blocks, mostly strike-slip, high angle reverse, or oblique slip. We perform the analysis in the pre-Mississippian section which is decoupled from the overburden by a Permian age unconformity. Our technique differs from that of previous workers in that we compute curvatures on each sample of a seismic volume using a moving subvolume rather than along surfaces interpreted from the data. In this way, we minimize high frequency variations in the results that arise from picking errors in the interpretation or noise in the data. We are able to extract and display values of curvature along time or depth slices, along horizon slices, and along poorly imaged horizons.
T43C-04 1330h
Effects of the Specimen Geometry and Rock Texture on the Fracture Mechanics Properties of Rock Materials
The application of linear elastic fracture mechanics principles has proven to be an effective approach in rock engineering and geoscience fields. The main objectives of this study are to standardize the mode I fracture toughness values measured by various specimen geometries, and to investigate the effects of the test environments and rock texture on fracture toughness measurement of rock materials. Fracture toughness of rock materials is generally measured by the standard test method suggested by the ISRM. Rock fracture toughness is generally affected by the specimen geometry, like other rock properties, and the toughness values measured employing the attractive NBD, SECRBB, SCB, and MHT specimen geometries are substantially different from those measured by the standard test method. Hence, the main content of this research is to compare the fracture toughness values of several rock types measured by various test methods with the standard values, aiming to standardize the test geometries. It is expected that the direct comparison of rock fracture toughness values measured by different test methods and the selection of proper test geometry will be possible by the result of this research. Many test results have been published concerning the effects of test environments, such as the water content, loading rate, temperature, on the fracture toughness measurement of rock material. However, the effect of the rock texture, that is, the mineralogical composition, bonding characteristics of individual grains, and distribution of microcracks, has not been fully investigated. Theoretical fracture toughness values of typical minerals has been given, but the relationship is still unknown. Moreover, fracture toughness of rock materials is presumably affected by the bonding characteristics. Therefore, another important objectives of this study are to quantify these effects, and to introduce a new method to evaluate the fracture toughness of rock materials from the microstructural observations of rock textures. Also, effects of the rock texture on various rock properties can be verified quantitatively. This will lead to wider application of the fracture mechanics principles to rock engineering fields.