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Advanced Field - Otay Mountain

Thesis Defense - Spring 2009 - Loren Wimmer

Scenario-Based Seismic Loss Estimation for San Diego County, Californiaspan>

Loren Wimmer
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Steven Day

Wednesday, May 22nd, 2008
CSL 422, 1:00pm



ABSTRACT
A study was done to estimate seismic losses to San Diego County, California, and test the sensitivity of those loss estimates to the choice of ground motion model. The scenario chosen was a magnitude 7.1 earthquake on the Rose Canyon Fault Zone. A rupture surface for this scenario was defined following the 2002 National Seismic Hazard Mapping Program. Two types of ground motion forecasts are used as input to the loss analysis. The first type uses empirical attenuation relationships. Nine different attenuation relationships are employed and compared, three of which are from the recent Next Generation Attenuation (NGA) project. The second type of ground motion input is broadband numerical simulations. For the broadband simulation methodology three rupture directions are considered, north-to-south, south-to-north and a bilateral rupture. Using the nine attenuation relationships and three broadband simulations, 12 different ground motion estimates for the M7.1 scenario were computed. Loss estimates were made by importing each scenario into the FEMA program HAZUS-MH MR3. The analysis estimates total economic loss to be between $9.1 and $26 billion and 15 - 412 fatalities. This study shows the choice of ground motion model to have a large effect on both loss estimates and the spatial distribution of those losses. In general, using the NGA attenuation relationships reduces loss estimates by 40 – 50% when compared with the loss estimates generated using pre-NGA attenuation relationships. Broadband numerical simulations on the other hand show a wide range of total loss estimates and impact the spatial distribution of losses.

Thesis Defense - Spring 2009 - Dylan Collins

Latitunal Controls on the Martian Crater Densities
Dylan Collins

B.S. Candidate
Department of Geological Sciences
San Diego State University
Dr. Brad Thomson
The Johns Hopkins Applied Physics Laboratory

Wednesday, May 13th, 2008
CSL 422, 9:40am



In order to gain new insight into Mars’ planetary dynamics, an examination of Martian crater densities as a function of latitude was completed. There are two complete crater catalogs of the Martian surface; one manually compiled by Nadine Barlow and one compiled by crater counting software created by Tomasz Stepinski and colleagues. After loading these two data sets into ArcMap 9.2, a pole-to-pole swath centered on 0˚ longitude was completed. Using equal-area trapezoids, representative size-frequency distributions (SFD) were constructed for every 5˚ in latitude. Upon comparison, the resulting two data sets had similar crater distributions, and both revealed a broader trend of thinning crater densities towards the poles. In addition, an absence of craters greater than 1.9 km was evident at the poles. From these new observations, it is concluded that either there is a deficit in impactor flux towards the poles of the planet or that there is a currently unrecognized crater modification process taking place at the poles. It is herein suggested that the Martian polar ice caps are responsible for modifying and erasing the craters near the poles. However, it would take a long time for large crater modification by ice cap transgression and regression to take place, and would require that Mars’ axial orientation has not changed for a geologically significant period of time.

Thesis Defense - Spring 2009 - Robert Edie

Using XRD Peak Broadening Analysis on Quartz to Identify Shatter Cones of the Santa Fe New Mexico Impact Structure

Robert Edie

B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Jared Morrow

Wednesday, May 13th, 2008
CSL 422, 9:00am



In addition to such large-scale features as craters and remote sensing anomalies, evidence for meteorite impact events is abundant at both micro- and mesoscopic scales. Micro-structural damage, planar deformation features, and planar fractures within quartz and other silicate mineral grains all give direct, supportive petrographic evidence for the existence of shock-metamorphic conditions associated with impact events. Planar deformation features and planar fractures can be difficult to identify. They can also be easily confused with other natural geologic patterns and misinterpreted. A diagnostic mesoscopic shock-metamorphic feature found at many impact sites are shatter cones, which are distinctive, meter-scale, horsetail-shaped, striated conical surfaces that develop at low to moderate shock pressures (i.e., ~5–25 GPa). This project investigates a new technique for identifying quartz grains along the surfaces of shatter cones that have been affected by shock metamorphism. As previously shown in laboratory and naturally shocked quartz-rich samples, peak broadening within quartz X-ray diffraction (XRD) spectra can give evidence for shock-driven sub-microscopic alteration of mineral crystal lattice structures. Well-developed shatter cones developed within quartz-rich crystalline host rocks collected from the Santa Fe Impact Structure in New Mexico were analyzed with XRD to detect evidence of shock-related spectral peak broadening. This evidence is identifiable and repeatable with the chosen data set of the project. Although highly variable, peak broadening was detected for all samples tested at specific crystal orientations. This peak broadening further supports the shock-metamorphic origin of the shatter cones. The XRD method may provide an important new tool for identifying the shock origin of possible shatter cones present at other suspected impact sites.

Thesis Defense - Spring 2009 - Justin Kerl

Possible relationship of the Iron Mountain metavolcanic assemblage to the El Cajon Mtn ring complex, San Diego Country, California

Justin Kerl

B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. David Kimbrough

Wednesday, May 13th, 2008
CSL 422, 8:40am



The Cretaceous Santiago Peak Volcanics are exposed along the western edge of the Peninsular Ranges batholith(PRB) and are interpreted as the volcanic cover of the batholitic intrusions. Ring dikes are also a common feature in the western PRB and were first described almost 70 years ago (Merriam, 1941). Ring dikes and ring complexes are interpreted as subvolcanic feeder systems that represent the transition from the plutonic realm to the volcanic realm in magmatic arcs (Johnston et al., 2002). However, there has been very little work on the petrology of the ring complexes in the PRB, and in particular how they might link the intrusive and extrusive history of the magmatic arc.

This study is focused on the El Cajon Mountain ring complex which was mapped by Todd (2004) and represents the possible roots of a central-vent volcano. The hypothesis tested in this study is that the El Cajon ring complex is the root of a volcano that fed surface volcanic deposits of the nearby Santiago Peak Volcanics at Iron Mountain. The Iron Mountain metavolcanics occur as a strongly foliated and steeply dipping screen completely surrounded by batholith wallrock. Zircon U-Pb ages from the ring dikes and metavolcanic rocks yield similar ages of ~120 Ma. Here, I present whole rock major and trace element concentration data from the dikes and volcanics to test if they could be closely related to one another as indicated by the similarity of the zircon ages.

Ten samples of Iron Mountain metavolcanic rocks and two samples of El Cajon Mountain ring dikes were analyzed in this study. Whole rock chemical analyses were obtained in the SDSU Geological Sciences laboratory by X-ray fluorescence analysis. Nine of the ten Iron Mountain samples are high silica rocks with SiO2 contents averaging 72.9±2.7 wt% (1 sigma sd) corresponding to a rhyolite composition for the volcanics. The two El Cajon Mountain ring dikes similarly yield high SiO2 contents of 74.5 and 77.7 wt%. A plot of SiO2 verse K2O plot indicates the rock are arc tholeiites or calc-alkaline in character consistent with a volcanic arc origin. A plot of immobile trace elements (Nb/Y vs. Zr/TiO2) also supports a possible close relationship of the metavolcanic rocks and ring dikes.

Johnson, S.E., Schmidt, K.L., and Tate, M.C., 2002, Ring complexes in the Peninsular Ranges Batholith, Mexico and the USA: magma plumbing systems in the middle and upper crust: Lithos, v. 61, p. 187- 208.
Merriam, R., 1941, A Southern California ring-dike: American Journal of Science, v. 239, p. 365-371.
Todd, 2004, Preliminary Geologic Map of the El Cajon 30´ x 60´ Quadrangle, Southern California, USGS Open-File Report 2004-1361, Detailed Description of Map Units, version 1.0.

Thesis Defense - Spring 2009 - Jerehme Acosta

Petrology of the Santiago Peak Volcanics, San Diego County, California

Jerehme Acosta

B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. David Kimbrough

Wednesday, May 13th, 2008
CSL 422, 8:20am



The Santiago Peak Volcanics (SPV) is exposed discontinuously in a belt along the western side of the Peninsular Ranges batholith in southern and Baja California. The SPV is intruded by ~125-100 Ma intrusions of the batholith and unconformably overlain by Late Cretaceous forearc basin strata of the Rosario Group. The SPV has been interpreted as the volcanic cover of the batholith and is one of the best places anywhere that genetically related volcanic and plutonic assemblages of a convergent margin arc can be studied together.
This study provides reconnaissance whole rock major and trace element data from SPV samples in areas of southern San Diego County where there is little or no data. The goal is to better constrain the nature of the volcanic arc represented by the SPV.

Twelve samples of SPV were collected from the La Mesa area to Otay Mountain near the US-Mexico border. The samples are volcanic flow rocks and lapilli tuff breccia typical of the SPV. Although original textures are visible in hand specimens, the rocks have experienced greenschist facies metamorphic overprinting.

Whole rock chemical analyses were obtained from fresh outcrop samples initially breaking up the rocks with the hydraulic RockLabs splitter/crusher to facilitate selection of approximately 90-100g of freshest homogeneous rock fragments. This material was then reduced to a fine powder using a tungsten carbide ring mill. The rock powder was used to make both a pressed pellet (for trace element analysis) and a fused disc (for major element analsis) which were analyzed using the Phillips XRF spectrometer in the Department of Geological Sciences.

The samples are strongly subalkaline and range in SiO2 from ~61 to 74 wt% which indicates they are andesite, dacite and rhyolite. The SiO2 verse K2O plot indicate the rock are arc tholeite or calc-alkaline in character consistent with a volcanic arc origin. The SiO2 versus FeOT/MgO also indicated calc-alkaline and tholeite affinities as well as the Nb/Y versus Zr/Ti plot. The composition and nature of the samples suggest derivation from nearby stratovolcanoes and/or possible caldera complexes.

Thesis Defense - Spring 2009 - Krista Muela

Stratigraphy and structure of the Miocene Bear Canyon Conglomerate, Imperial County SE California: Evidence for the episodic growth of the Chocolate Mountains anticlinorium

Krista Muela
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Gary Girty

Wednesday, May 13th, 2008
CSL 422, 8:00am



Between Indian Pass and Picacho State Recreation Area crystalline basement is overlain by early Miocene (~23 Ma) volcanic rocks. These units are in turn unconformably overlain by undated alluvial sedimentary rocks of the informally recognized Bear Canyon conglomerate. On the north limb of the anticlinorium, three steeply dipping faults and associated splays cut this sequence of rocks consistently off setting them in a dextral sense while a fourth fault at Picacho transects both limbs of the anticlinorium. Where exposed, fault surfaces contain sub-horizontal slickenlines and offset hingelines of E-W trending folds. The three westernmost faults appear to tip out into the core of the anticlinorium. Hence, we speculate that the anticlinorium may have taken up the slip on these faults through tightening and growth. Over an ~15 km EW distance, the cumulative dextral offset on the four faults is ~3.5 - 4 km.
At Picacho, the Bear Canyon conglomerate lies on the northern limb of the anticlinorium. There the lower member of the Bear Canyon dips ~21° northward, and is unconformably overlain by the ~15° northward dipping middle member. In contrast, the unconformably overlying upper member is not tilted.
Between Carrizo Wash and Indian Pass, the Bear Canyon conglomerate lies on both the southern and northern limbs of the anticlinorium. North of the intersection between Carrizo Wash and Gatuna Wash, the Bear Canyon conglomerate dips ~17 ° NE. Northwest of Carrizo Wash, it dips ~10° - 27 ° SW. Just NW of Indian Pass, the Bear Canyon dips about ~8° - 12° southward and is interstratified with the basalts of Black Mountain. The basalts ~2 km SE of Indian Pass are transected by a major dextral strike-slip fault and dip ~20° - 57 ° SW. Published K-Ar data suggest an age somewhere between ~9.4 and ~13.6 Ma for the basalts of Black Mountain. Hence, our data imply that the anticlinorium was growing after ~9-13 Ma ago, and that it, and the dextral strike-slip faults that we have mapped, are the likely record of the development of a segment of the Eastern California Shear Zone.

WEBINAR - 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

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.

Thesis Defense - Spring 2009 - Robert Gallardo

Seismic Anisotropy in the Julian Schist, San Diego CA

Robert Gallardo
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Robert Mellors

Wednesday, May 13th, 2008
CSL 422, 9:00am



ABSTRACT
P-wave velocity measurements of a Julian Schist rock sample collected in the Laguna Mountains region of East San Diego County show significant P-wave velocity anisotropy at near surface conditions. Multiple P-wave velocity measurements were taken from orthorhombic cut sample of Julian Schist using the Tico Ultrasonic Instrument with varying parameters of length. A maximum anisotropy of 22.1% was obtained from the P-wave measurements gathered in the X, Y, and Z axes of the Julian Schist sample. Cracks within orthorhombic schist sample demonstrate the need to take measurements with increased pressure parameters as measurements made within the same axis on different locations show p-wave velocity differences of up to 54.0%. This is based on observations of varying numbers of visible cracks on the external surface of rock sample within the same axis of measurement. Increasing metamorphic grade has been known to represent increasing anisotropy within rock units and is an important factor in identifying proximity to faults, general lithologic and structural descriptions of regional rock types, and could have an effect on the Southern California Communal Velocity Model (SCCVM).

Thesis Defense - Spring 2009 - Jared Warner

Biotic Response of Ostracodes to the Middle Eocene Climatic Optimum (MECO; 40.0 Ma) in the Southern Indian Ocean (ODP Site 748)

Jared Warner
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Stephen Schellenberg

Wednesday, May 13th, 2008
CSL 422, 11:30am



ABSTRACT
The long-term cooling trend from “greenhouse” to “icehouse” conditions through the middle-to-late Eocene (49.0 – 33.7 Ma) was interrupted by the middle-Eocene Climatic Optimum (MECO; ~40.0 Ma), a transient (~500 kyr) global warming of ~4-6 C° widely recorded in marine and terrestrial stratigraphic sections. The broad biotic response to the MECO is poorly known, with most published studies focused on planktonic protists such as foraminifera, calcareous nannoplankton, and dinoflagellates. To initiate a complementary benthic biotic perspective, we conducted a high-resolution ostracode faunal analysis (>150 mm size-fraction) through the MECO at Ocean Drilling Program Site 748 on the southern margin of the Kerguelen Plateau (~725 m paleodepth) in the southern Indian Ocean. Ostracodes are the only readily preserved deep-ocean metazoans and thereby provide a unique biotic perspective on this geographically vast ecosystem.
The ostracode faunal assemblages were assigned to three stratigraphic intervals (pre-MECO, MECO, and post-MECO) based on the nominal onset and termination of the oxygen-isotope excursion. Ostracode generic richness, Margalef’s (d), and Simpson’s Index (1/l) were all significantly lower (ANOVA; p <0.001) within the MECO interval, indicating a decrease in ostracode diversity. A Likelihood Ratio Test (LRT) was used to further evaluate the diversity loss via an ecological response versus a sampling artifact. Decline in ostracode diversity is also denoted by the disappearance of Bairdia during the MECO interval, a significant pattern revealed by the LRT. In addition, within the MECO interval, the whole-faunal percentage of smoother, less ornate genera was significantly lower while some genus-specific adult:juvenile valve ratios were significantly higher; both patterns are consistent with a hypothesized decrease in carbonate saturation during the MECO. The absence of faunal turnover and pronounced changes in faunal percentages through the MECO stands in stark contrast to ostracode assemblages previously documented during other intervals of rapid climate change (i.e., PETM)

Thesis Defense - Spring 2009 - Dale Burns

The Recent Prehistoric Geochemical Evolution of Summit Lavas From Kilauea Volcano, Hawaii

Dale Burns
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Aaron Pietruszka

Wednesday, May 13th, 2008
CSL 422, 1:45pm



ABSTRACT
Lavas from Kilauea Volcano display rapid geochemical and isotopic variations (e.g., 206Pb/204Pb or Nb/Y ratios) on a time scale of decades to centuries. The wall of Kilauea Caldera at Uwekahuna Bluff exposes a sequence of recent prehistoric, caldera-filling lavas (erupted mostly between AD 900-1400). Here we present a detailed geochemical study (major element abundances, and Pb, Sr, and Nd isotope ratios) for lavas from the lower portion of the Uwekahuna Bluff section as well as trace element abundances for the entire section. The 206Pb/204Pb ratios of the lavas from the lower Uwekahuna Bluff display small variations that merge with an isotopic excursion towards low 206Pb/204Pb ratios in the lavas from the upper portion of the section. At least four distinct source components within the Hawaiian mantle plume are required to explain the variations in Pb, Sr, and Nd isotope ratios. Two of these components were previously thought to be restricted to the neighboring active volcanoes, Mauna Loa and Loihi Seamount. The occurrence of Mauna Loa- and Loihi-like mantle sources in Kilauea’s melting region (in addition to Kilauea’s typical mantle source) suggests that (1) the Hawaiian plume contains three large-scale compositional heterogeneities and (2) all three heterogeneities extend into Kilauea’s melting region. However, the source region of Kilauea lavas must also be heterogeneous on a small scale to explain the rapid variations in the Pb, Sr, and Nd isotope ratios of the lavas. The fourth component appears to be common to all three of the active Hawaiian volcanoes, and thus, might represent the plume matrix. The Uwekahuna Bluff lavas also reveal a systematic temporal excursion in ratios of highly over moderately incompatible trace elements (e.g., Nb/Y) towards the lowest values yet observed at Kilauea. These low Nb/Y ratios (which correlate with the low 206Pb/204Pb ratios) are thought to result from relatively high degrees of partial melting of the plume matrix. Based on a correlation between high inferred degrees of partial melting and high eruption rates historically at Kilauea, the low Nb/Y ratios of the Uwekahuna Bluff lavas suggest that the eruption rate at Kilauea’s summit may have been unusually high in recent prehistoric times.

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.

Thesis Defense - Spring 2009 - Kean Bliss

Evaluation of XRD and Raman peak broadening in shock-metamorphosed carbonates from selected carbonate-target bolide impact structures

Kean Bliss
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Jared Morrow

Wednesday, May 13th, 2008
CSL 422, 2:30pm



ABSTRACT
Identification of shock metamorphism in minerals is an important line of evidence for confirming the origin of potential impact-generated structures and associated deposits. Unlike common quartz and feldspar occurring in crystalline target rocks, shock-metamorphosed carbonates are not readily identified petrographically. Studies have indicated that X-ray diffraction (XRD) and micro-Raman spectrometry (MRS) peak broadening is a reliable indicator of shock metamorphism in carbonates. Samples from seven confirmed carbonate-target impact structures and other high temperature-pressure regimes (i.e., carbonatite, marble, and fault breccia) were analyzed in order to determine whether peak broadening is unique to impact structures. In addition, the MRS method is further tested for comparability to XRD analyses, and for the effects of different sample preparation (i.e., thin section vs. powder).
XRD analyses reveal that microstructural effects (i.e., crystallite size and lattice strain) contributing to peak broadening in potentially shocked samples follow expected trends of decreased crystallite size and increased lattice strain. However, fault breccias and carbonatites show microstructural effects comparable to those recorded for shocked samples, indicating that peak broadening is not unique to impact-related deposits.
Although XRD and MRS sample trends are similar, machine-derived and sample-derived variability is high in MRS analyses, contributing to uncertainty and less accurate peak width measurements. Large discrepancies exist between thin section and powder MRS analyses of identical samples. Estimated peak shock pressures calculated from previously established calibration lines for dolomite are much higher than expected; indicating that calibration curves should be specific to each impact structure and sample preparation protocol.

Thesis Defense - Spring 2009 - Emily Allen

Physical and Chemical Characteristics of Unweathered Pulverized Rock along the San Andreas Fault, Little Rock, CA

Emily Allen
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Tom Rockwell
Wednesday, May 13th, 2008
CSL 422, 1:00pm



ABSTRACT
We present new observations on pulverized granitic rocks recovered from a shallow core adjacent to the San Andreas fault near Little Rock Creek. The site is characterized by extensive outcrops of granitic rocks with varying degrees of damage, at distances of up to a few hundreds of meters from the fault’s primary active strand. We used an auger drill-rig to recover a continuous core of pulverized rock to a depth of 42 meters. The core is composed mainly of pulverized Si-rich, Si-intermediate, and Si-poor granitoids. It crosses through several high clay content gouge zones, which correspond to secondary fault cores. Detailed results of particle size distribution (PSD) measured using a laser particle analyzer and standard sieving and pipette methods indicate that medium to coarse silt and fine sand (50-600 microns) are the dominant particle sizes for pulverized Si-rich granitoids recovered in the core. Very little clay size particles were observed in both surface and depth samples. Si-rich granitoid samples are comprised only of quartz, feldspar, white mica, and minor biotite with no pedogenic clay. Geochemical data indicate little if any chemical weathering. We conclude that the signature of pulverization results in shattering to silt and sand sized particles with essentially no component of weathering, which is similar to but slightly coarser than prior studies that were conducted on near surface samples at Tejon Pass and along the Garlock fault where a minor weathering component was observed.

Thesis Defense - Spring 2009 - Frank Forcino

Mytilus californianus as an environmental monitor: a case study from central California

Frank Forcino
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Stephen Schellenberg

Wednesday, May 13th, 2008
CSL 422, 10:45am



ABSTRACT
Intraskeletal oxygen stable-isotope ratios (δ18O) and minor element ratios (Mg/Ca) were examined in modern Mytilus californianus specimens to test the hypothesis that these variables record ambient seawater temperature, and therefore can be measured in fossil specimens as a means of reconstructing paleotemperature. Specimens representing a range of ontogenetic stages and two extreme intertidal positions were collected monthly for one year from Santa Cruz, CA. While general linear modeling (GLM) of various monitored environmental parameters revealed temperature as the primary explanatory variable, the best goodness-of-fit GLMs for δ18O and Mg/Ca explained only 37.1% and 28.5% of variation, respectively, with additional significant variance attributable to intertidal positions and specimen size. Thus, M. californianus appears to be an unreliable means to reconstruct paleotemperature given the large amount of unexplained variation in intraskeletal δ18O and Mg/Ca and the role of variables effectively unknowable in the fossil record (e.g., intertidal position). In addition, a proxy species of interest cannot be assumed to be a comparable indicator of environmental conditions to similar species in the same genera or that share ecological characteristics."