Thesis Defense - Spring 2009 - Peter Winther

Magnetotelluric Investigation of the Santo Domingo Basin, Rio Grande Rift, New Mexico

Peter Winther
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. George Jiracek

Wednesday, May 13th, 2008
CSL 422, 10:00am



ABSTRACT
Magnetotelluric (MT) investigations of the Santo Domingo Basin were made in 2000-2007 to assess the deep stratigraphy and tectonics of an intra rift basin. The studies were conducted during the SAGE (Summer of Applied Geophysical Experience) program to provide ‘hands-on’ learning experiences for students and to evaluate the local hydrologic regime. Most MT soundings were accomplished using Quantec Geoscience’s Titan 24 MT system. The Titan system is a distributed array MT instrument that allows the collection of up to 24 MT soundings simultaneously with a station spacing of 100 m. Sixty-five Titan MT soundings were collected along a 6.4 km-long profile where a petroleum exploration seismic survey was recorded during the 1970’s. The MT data were modeled using a smooth, two-dimensional (2-D) inversion code developed by Geosystem, Inc. Modeled resistivity values vary from 2-1000 ohm-m in the 2 - 4.5 km-thick sedimentary section. Resistivities exceed 1000 ohm-m in the basement interpreted to underlie the sediments. The sedimentary sequence indicates three distinct regimes: (1) an upper, unsaturated and freshwater saturated, basin-fill that ranges from 350 - 800 m-deep with resistivities of 8 - 85 ohm-m, (2) a deeper, mostly conductive region with higher brine and/or clay concentrations approximately 3 km thick with resistivities varying between 2 - 100 ohm-m and (3) a resistive sedimentary cover over the crystalline basement that ranges from 100 - 1000 ohm-m. Tectonic interpretations of the 2-D MT inversion profile indicate the locations of several buried, unmapped faults within the basin which agree with recent U. S. Geological Survey aeromagnetic interpretations. A geologic structure model for the area includes two grabens with the western graben down dropped with respect to the western graben.

Thesis Defense - Spring 2009 - Matthew Sisk

Chemical and Physical Characteristics of Pulverized Tejon Lookout Granite Adjacent to the San Andreas and Garlock Faults: Implications for Earthquake Physics

Matthew Sisk
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Thomas Rockwell
Friday, May 8th, 2008
CSL 422, 3:00pm

ABSTRACT
We present new detailed analysis of pulverized Tejon Lookout granite from sections adjacent to the San Andreas and Garlock faults in southern California. The granite is pulverized in all exposures within about 100 m from both faults. Chemical analyses indicate essentially no or little weathering in the bulk of the rock, although XRD analysis shows the presence of smectite. Illite, and minor kaolinite in the clay-size fraction. Weathering products may dominate in the less than 1 micron fraction. The average grain size in all samples of pulverized granite range between about 26 and 208 microns (silt to fine sand), with the size distribution in part a function of proximity to the primary slip zone. The San Andreas fault samples are generally finer than those collected from adjacent to the Garlock fault. The particle size distribution for each sample follows a pseudo power law with a continuously changing exponent, which suggests that pulverization is not simply a consequence of direct shear. This average particle size is quite coarser than previous reports, which we attribute to possible measurement errors in prior work. Our data and observations suggest that dynamic fracturing in the wall rock of these two major faults only accounts for about 1% or less of the earthquake energy budget.

WEBINAR - Shawn Wright

TIR spectroscopy of shocked Deccan basalt: Implications for Mars and Martian meteorites

Shawn Wright
Institute of Meteoritics
Department of Earth and Planetary Sciences
University of New Mexico
Hundreds of thousands of impact craters dominate the surfaces of the Moon, Mercury, and Mars. There exists much geomorphic and spectral evidence for basalt on those surfaces, so basaltic target rocks are most likely common. However, little work has been done on the thermal infrared (TIR) spectroscopy of shocked basalt metamorphosed by meteorite impact. This will have a direct application to the large amount of TIR data currently being returned from Mars orbiters and Rovers. The relationship between these TIR data collected remotely and laboratory data of samples is of much interest given that our only samples of Mars are shocked basalts. Results of research involving field work and samples from Lonar Crater, India, the only known terrestrial impact site emplaced in basalt, are described. The Deccan “Traps” flood basalts have been labeled as an excellent compositional and spectral analog for plagioclase-rich basalt on Mars identified from various orbiter and Rover instruments. Petrography provides details on the approximate range of shock pressure each class of shocked basalt has been subjected to, and field work has yielded an “ejecta stratigraphy” that displays where each class of shocked basalt is located. A comparison of the TIR spectroscopy of the unshocked Deccan basalt to its shocked equivalent aids in quantifying the changes due to shock. An application of this research has provided constraints on possible source craters/regions of certain shergottite meteorites, the shocked basalts from Mars.