Evidence for rapid recycling of subduction erosion forearc material into Cordilleran TTG batholiths:
Insight from the Peninsular Ranges of southern and Baja California
The La Posta TTG Flareup
The defining characteristic of the Peninsular Ranges of southern and Baja California is a petrologically distinctive, high Sr/Y plutonic suite of 95 & 3 Ma tonalite, trondhjemite, and granodiorite that is referred to as the La Posta TTG suite. These rocks are chemically similar to high-Al tonalite-trondhjemite-granodiorite gneiss terrains of Archean crust produced by high pressure melting of mafic source regions. La Posta belt magmatism records an eastward relocation of the Cretaceous arc accompanied by abrupt chemical change to TTG compositions. This shift was preceded by tectonic shortening and thickening of the crust along the locus of pluton emplacement. Intrusions occur as a series of large internallyzoned intrusive centers with outcrop areas that range up to at least 1400 km2. Field and thermobarometric data indicate emplacement at -3-6 kbar depths followed by rapid uplift and denudation at rates of -1-2 mmlyr. Zircon U-Pb ages (n=43) from throughout the -1300 km length of the batholith indicates the bulk of the La Posta rocks were emplaced in a narrow time frame from -98-92 Ma. Mainphase biotite k hornblende tonalite from individual zoned intrusive centers were emplaced over -3-4 m.y. time intervals followed by small volume peraluminous core zone intrusions. A series of distinctly earlier (c. 105-11 0 Ma) and smaller "precursor" intrusions parallel the western edge of the main locus of intrusion. The La Posta TTG belt is more deeply exhumed in the north (up to 25 km) relatively to south (-10 km) providing an oblique cross-section through arc crust. Zircon oxygen isotopic compositions indicate that high 6180 predominate in the deeper northern part of the La Posta TTG belt (7.6-12.2) while lower values characterize the shallower southern part (6.3-7.2). This pattern indicates the possibility of significant and previously unrecognized depth gradients in a180 in the PRB consistent with large input of supracrustal contaminants into deeper parts of the batholith relative to shallower parts. The implication is that vertical gradients in 6180 within the eastern zone La Posta belt may be as great as the wellestablished west to east surface gradients across the northern part of the batholith. Because the volume La Posta-type crust in the Peninsular Ranges may easily exceed a million cubic kilometers, simple mass balance considerations require supracrustal recycling on a massive scale. The fact that high a180 La Posta-type zircon are from rocks with Sri values mostly ~0.706co nstrains the nature of the supracrustal contaminant. Assimilation of high Sri Julian Schist-type metasedimentary wallrock to account for La Posta zircon oxygen isotope compositions is unrealistic on several counts. We speculate that large-scale sediment underplating of isotopically primitive accretionary prism material may have played an important role leading up to La Posta-type melt generation. This view is supported by a sediment deficit in adjacent forearc basin & accretionary prism belts of the California borderland.
Was deeply accreted Catalina Schist an important component of the La Posta source region?
Studies at oceanic convergent margins show unambiguously that large volumes of continental material are recycled into the mantle by sediment subduction and forearc subduction erosion. The fate of this subducted continental material is poorly known; mass balance consideration suggest that at least 95 percent of this material is recycled to deep mantle circulation. The Late Cretaceous La Posta-type TTG suite of the eastern Peninsular Ranges batholith however displays evidence that accretionary prism sediments were subducted and magmatically recycled into the Late Cretaceous the on a very short ( 4 0 Ma) timescale. The La Posta suite represents a tremendous flare-up of deep (garnet-involved) partial melting preceded by an episode of crustal shortening and presumed crustal thickening from ca. 98-92 Ma. Evidence from the Catalina Schist subduction complex terrane directly outboard of the PRB demonstrates that these rocks were being underthrust and metamorphosed synchronously with La Posta magma generation. Elevated 6180 values from La Posta plutons indicate substantial contributions of isotopically unevolved (initial Sr < 0.706) intermediate to mafic composition supracrustal materials into the melt souce regions consistent with contributions from the Catalina Schist accretionary complex. Further, inherited zircon in the La Posta rocks are dominated by a 120-100 Ma component that closely matches the age of the western PRB as well as the dominant detrital zircon component in Catalina Schist. These results are consistent with "conveyor belt-type" recycling of western PRB material into the forearc and then downward via underthrusting into the magma source region of La Posta melts.
INVESTIGATORS David Kimbrough and Marty Grove
October 2007
ShakeOut
1 Hz Ground Motion Simulations for the Southern San Andreas Fault
Objective
Compute 0-1 Hz ground motion from the M7.8 kinematic SoSAFE (Southern San Andreas Fault Evaluation) ShakeOut scenario.
ShakeOut Scenario
The M7.8 ShakeOut rupture scenario initiates near Bombay Beach by the Salton Sea and propagates unilaterally 300 km toward the northwest up to near Lake Hughes.
ShakeOut Velocity Model
A 600 km by 300 km by 80 km area from the CMU etree representation of the SCEC Community Velocity Model (CME) version 4 was extracted at a constant grid spacing of 100 m into 14.4 billion grid points. The lowest S-wave velocity was truncated at 500 m/s, and Qp and Qs values were based on the Qp-Vp and Qp-Vp regression formulas by Brocher (2006).
ShakeOut Source Description
The source description (Aagaard) contains slip that combines a long length scale background distribution with short length scale random variations, and a spatially-variable distribution of rake, rise time, peak slip rate, and rupture speed. The source is specified on non-planar fault geometry derived from the SCEC Community Fault Model (CFM) version 3 and mapped onto a regular grid.
Preliminary Results ShakeOut: 1 Hz Ground Motion Simulations for the Southern San Andreas Fault Location map for the ShakeOut simulations. The red rectangle depicts the simulation area (same as for the TeraShake simulations) , which is rotated 40 degrees clockwise from North. We have simulated 250 s 0-1 Hz ground motion in the SCEC CVM V4.0 for the SoSAFE ShakeOut V1.1 scenario in a 600 km by 300 km area of southern California. The wave propagation shows strong amplification effects in the basin areas, including the Salton Trough, Los Angeles and Ventura. The ground motions are strongly focused in the direction of rupture. Similarly to the results of the TeraShake simulations, this rupture directivity effect is dramatically modified by interactions with the chain of sedimentary basins (the San Bernardino, Chino, San Gabriel, and Los Angeles basins) running westward from the northern terminus of the rupture to downtown Los Angeles. This chain of basins forms a low-velocity structure that acts as a waveguide, trapping seismic energy along the southern edge of the San Bernardino and San Gabriel Mountains and channeling it into the Los Angeles region. Also similarly to the TeraShake results the Love waves are wrapped around the Puente Hills, channeling seismic energy not just into the Whittier Narrows area, but also into the southern part of the LA basin (see snapshots of the instantaneous velocity magnitude). This multi-pathing effect separates the seismic energy into two branches, with the (northern) wave guide through Whittier Narrows generating the strongest amplification. The maximum PGV (root-sum-of squares of the three components) away from the fault (365 cm/s) is generated by the northern wave guide, near Whittier Narrows. The maximum near-fault PGV occurs in the Coachella Valley area (546 cm/s).
Cumulative peak ground velocities for the ShakeOut simulation. The maximum PGV (root-sum-of squares of the three components) away from the fault (365 cm/s) is generated by the northern wave guide, near Whittier Narrows. The maximum near-fault PGV occurs in the Coachella Valley area (546 cm/s).
Investigators -Kim Bak Olsen, Steven Day, Yifeng Cui, Jing Zhu, Amit Chourasia, Gideon Juve, Phil Maechling
May 2007
Testing the use of GPR to detect
clandestine graves in a San Diego soil
The purpose of the experiment is to consider the usefulness of GPR in forensic investigations of clandestine graves in the San Diego region. Ground Penetrating Radar (GPR) is a common tool used to view objects within the subsurface.The penetration and resolution of a GPR unit can vary with frequency used and soil type. A 6.58 kg turkey and 4.54 kg ham were buried at a depths to bottom of .75 meters and 1.25 meters respectively. There is also a 1m diameter aqueduct pipe that traverses the site at approximately two meters deep. The soil type at location of survey was a poorly graded clayey silty sand common of the San Diego area. The 100MHz antenna revealed no significant data. The 500MHz antenna revealed the shallower ham and the aqueduct. The turkey was not obvious, but may become more apparent with further filtering.
Objective and Hypothesis To test the capabilities of Ground Penetrating Radars for forensic use in a common San Diego area soil Does experimental resolution match theoretical resolution? Resolution varies with depth, soil type , materials and shape of objects in the subsurface. A ham, turkey and aqueduct are located in the soil at depths of .75 meters, 1.25 meters and 2 meters respectively. The items vary in size and shape.
Procedure Knowing the depth and dimensions of the aqueduct, it was surveyed first. - Then the ham and turkey were buried and surveyed on a grid (Images 1-6 and figure 4-6). - Data was imported into Matlab and filtered (demeaned) and images created for analysis (Figure 1 and 2). - Using the known depth of the aqueduct, radar propagation velocity was determined and resolution calculated.
Calculations Theoretical resolution is approximately 0.5 wavelength (λ) 3 cm. This should provide us width the necessary resolution to reveal the buried items. Using ε= (c / ν)^2 , the dielectric constant is equivalent to 53. A dielectric constant of this means the soil is significantly saturated.
Conclusion The 500 MHz GPR can resolve objects as deep as 2 meters in a typical San Diego Soil. - Both the ham and the turkey were difficult to detect directly. The disturbed soil was detectable. - We determined the velocity within the soil to be 4.10x10^7 meters per second. Successful detection of small body parts would require a higher frequency GPR unit. We suspect the low contrast in dielectric constant between soil and poultry caused a poor signal.
Investigators -Luke Zimmerman and Rob Mellors
2nd Place Winner of the SDSU Undergraduate Research Symposium
March 2007
Impacts and Mass Extinction
An important outcome of research by impact and planetary geologists
is the realization that impact cratering is a continuing, active process that
has affected the long-term development of many Earth systems strongly,
including the biological. Increasing interest by professional
and amateur researchers, expanding subsurface exploration for
hydrocarbon resources, ongoing mapping of the ocean floor, advances in
satellite imaging, and the development of automated, digital crater-detection
programs have greatly increased the number of previously unrecognized,
potential impact sites. Comparison of published Earth crater
counts clearly demonstrates the growing number of possible identified
impact structures — compare early crater tabulations, for example, by
Dietz (1961; 14 structures identified) with such recent, continually updated,
web-based impact databases as those maintained by the Planetary
and Space Science Centre, University of New Brunswick (174 confirmed structures
now identified), and by D. Rajmon and the Impact Field Studies
Group, University of Tennessee (see the SEIS, Suspected Earth Impact
Sites. The SEIS database
now lists over 500 confirmed, probable, and possible impact sites. With
continuing interest in impact events, this number will continue to grow.
With regard to driving mass-extinction events, of course the question is not only one of ‘‘how many craters?’’ but also of ‘‘how large are they?’’ Impact-kill curves (Raup, 1992; Jansa, 1993; Poag, 1997), which postulate a relationship between crater diameter and percent biotic extinction, suggest that mass extinctions (i.e., the rapid loss of 50% or more species globally) require a crater size of 150 km or more in diameter. Thisand Popigai impacts preceded significant biotic turnover at the Eocene- Oligocene boundary by 1–2 m.y. (Poag, 1997). Similarly, multiple Late Devonian impacts preceded the stepped mid-Late Devonian (F-F) mass extinction by 0–6 m.y. (McGhee, 2001; Sandberg et al., 2002; Reimold et al., 2005). If, however, multiple sub-critical impacts are implicated as a killing mechanism, a model must be developed that allows the impactrelated environmental effects not only to be cumulative but also to be either very long lived or capable of working in concert with other, presumably telluric, causes.
Current Research at SDSU on Impact and Event Stratigraphy
Middle Paleozoic depositional history and event stratigraphy, Great Basin and Western Europe
Geologic record of bolide impacts, emphasizing the Late Devonian Alamo event, central Great Basin
Patterns and processes of the mid-Late Devonian (Frasnian-Famennian, F-F, Kellwasser) mass extinction, utilizing conodont-based event stratigraphy
NEW MOISTURE + DENSITY INDICATOR (M+DI)
Addition to the Grain Density Laboratory
New Moisture + Density Indicator (M+DI) addition to the grain density laboratory. The M+DI is a non-nuclear gauge that measures both volumetric water content and dry bulk density simultaneously in unconsolidated to loosely consolidated materials. Time Domain Reflectometry (TDR) is used to measure the travel time of an electromagnetic step pulse produced by the TDR pulse generator through four spikes inserted into the ground. The voltage signal is displayed on and analyzed by a PDA or computer running specially developed algorithms to determine apparent dielectric constant and the electrical conductivity of the host material. The software uses a set of equations to relate these two properties to volumetric water content and dry bulk density which are displayed on the screen.
With this new instrument, research will cross the artificial disciplinary lines separating soil science, geomorphology, and geology. Researchers and students have already begun using the M+DI to investigate volume strain relationships between saprolite, grus, and corestone at Santa Margarita Ecological Reserve, and mass transfer processes associated with the development of fault zones in the San Felipe Hills.
Example of data collection at the Santa Margarita Ecological Reserve
Investigator
Dr. Gary H. Girty, Sarah Johnson, Cameron Cambell, Aaron Hebler, Afton Van Zandt, Peter Winther, Matt Burgess, Chris Martinez, Zack Rayburn
January 2007
Lake Tahoe Tsunamis
Numerous submarine landslide deposits are located along the margin walls of Lake Tahoe. The lack of deltas at stream inputs and the lack of slides near the largest stream inputs indicate the majority of the slides are not the result of sediment overloading, while the close association between active faults and submarine landslides in the Tahoe basin indicate the slides are probably seismically induced. Lacustrine debris flows and turbidites that occur as the lake floor is disturbed by the slides affect basin sediments drastically and may be dated to determine the timing and frequency of strong shaking and landsliding in the basin. Landslide complexes in McKinney Bay, southeastern Lake Tahoe, and North Tahoe contain the largest slides; however several smaller slides line the basin walls. The McKinney Bay landslide is the largest slide complex in the lake with multiple generations of slides. The initial failure deposited ~6 km<sup>3</sup> of material on the lake floor in a ~40 m thick chaotic zone. Large, tilted, blocks seen in the bathymetry and seismic reflection profiles are rooted in this chaotic zone. In southeastern Tahoe, several large block slides and smaller slumps are found, along the eastern wall, away from major stream inputs. In North Tahoe, steep fault-bounded slopes have undergone multiple episodes of sliding with several deposits located between the blocks. Lacustrine debris flows and turbidites associated with the slides can be correlated in piston cores taken throughout the lake. Radiocarbon dating in one core reveals that the cores encompass the Holocene and latest Pleistocene. The combination of the lake-wide correlations and the radiocarbon dating results in a turbidite event frequency of one event per 1000 years. Turbidite events often show multiple source areas around the lake, indicative of strong shaking and subsequent landsliding in multiple areas concurrently. Strong earthquake shaking on the Tahoe basin faults and the Genoa Fault to the east of the basin has led to multiple generations of massive submarine landslides and major lake disruption throughout the Holocene.
Investigators
Gordon Seitz (SDSU), Graham Kent (SIO), Shane Smith (UNR), Bob Karlin (UNR)
December 2006
MT field site in Southern Alps, New Zealand
Seismogenic, Electrically Conductive, and Fluid Zones at Continental Plate Boundaries in New Zealand, Himalaya, and California
Evidence that fluid-related processes well-removed from the actual seismogenic crust may be critically important in the earthquake cycle has been bolstered by recent discoveries that nonvolcanic, episodic tremors originating 10s of km below correlate with increased seismic activity above. We explore the idea that fluid occurrence below the seismogenic zone plays an active role in the rupture process by examining how fluids spatially relate to seismicity at continental plate boundaries. To address this question we co-plot three-dimensional distributions of earthquake hypocenters projected onto magnetotelluric (MT) electrical resistivity cross-sections. Although MT is unique in its ability to detect conductive zones in the deep crust with very low fractions (<1%) of fluids, detection requires an interconnected network of fluid-filled porosity. Interconnection is facilitated by shearing, micro- and macrofracturing, and/or grain-edge wetting.
Underlying Assumption Magnetotelluric-detected crustal conductive zones are dominantly caused by interconnected fluid (aqueous or partial melt).
Underlying Question How do fluid zones below the brittle-ductile transition relate to seismicity at active plate boundaries, e.g., in New Zealand and Himalaya?
Investigators
George R. Jiracek, Victor M. Gonzalez, T. Grant Caldwell, Philip E. Wannamaker, and Debi Kilb
November 2006
Dr. Barry Hanan helping to recover a dredge filled with lava specimens
Chemical, Isotopic and Volatile Constraints on the Evolution of the Lau Basin
Dr. Barry Hanan and Aaron Pietruszka working with other scientists from Scripps Institution of Oceanography spent 8 days at sea aboard the RV mellville, dredging seafloor lavas to better understand the chemical, isotopic and volatile constraints on the evolution of the Lau Basin. The Lau Basin is an active volcanic back arc basin between the Fiji and Samoan Islands that formed due to the westward subduction of oceanic crust. The project is a comprehensive geochemical (major, trace and volatile elements) and isotopic (Sr, Nd, Hf, Pb, rare gases, and U-series) analytical program to determine the distinct mantle sources beneath the Lau Basin and to map their spatial distribution. This information will be used to constrain mantle flow and to investigate the geochemical processes operating in the supra-subduction zone (SSZ) mantle and its control on the evolution of the Lau back-arc basin. Our investigation builds on previous and existing information and sampling from 5 Scripps sampling cruises to the Lau Basin lead by J. Hawkins. The sample suite consists of about 400 lavas and glasses that represent a wide and extensive coverage of the Lau Basin with lavas from all the recognized spreading centers including the Northeastern Basin, the Northern Lau Spreading Center (which is rifting the part of the basin that may be influenced by the Samoan Hotspot/Plume), the Mangatolo Triple Junction, the Central Lau Spreading Center,
Dr. Aaron Pietruska preparing samples fresh off the ocean floor
the Eastern Lau Spreading Center, the Relay Zone between central and eastern spreading centers, and the Valu Fa Ridge. We also have samples from primitive seamounts such as the Zephyr Shoal and from old pre-existing crust (e.g., Donna Seamount and the western edge of the Peggy Ridge). The samples from the Mangatolo Triple Junction include those from one limb that impinges on crust proximal to the Tofua Arc and another limb that penetrates old OIB crust. With these samples we will investigate outstanding questions concerning the origin, nature, and evolution of the Lau Basin SSZ mantle. These questions include: (1) what is the spatial distribution and extent of Indian- and Pacific-type mantle in the basin; (2) can we map the spatial extent of Samoan plume material into the Lau Basin; (3) how does the proximity of the Tofua Arc affect mantle melting beneath the Lau Basin spreading centers, and (4) what proportion of volatile enrichment in the lavas is due to ridge-arc source interaction versus crustal assimilation.
The samples will be analyzed here at SDSU using the Nu 1700 mass spectrometer. Postdoctoral researcher Tyrone Rooney will be conducting analysis.
August 2006
New Nu Plasma 1700 Lab
The Department of Geological Sciences at San Diego State University Geological has just completed the installation of the Nu Plasma 1700 mass spectrometer. The Nu 1700 is a high-resolution mass spectrometer used to identify the proportions of isotopes of an element, i.e., atoms with the same number of protons but differing numbers of neutrons. Distinctive ratios of isotopes can be used as fingerprints or tracers to indicate the origin and evolution of the rock or earth material from which the element came.
The Nu 1700 is the second of its kind in the world. The first instrument was installed in Zurich in March of 2002. Our instrument was manufactured in Wrexam, Wales, UK by Nu Instruments LTD., the only company making instruments like the 1700. The Nu 1700 took three years to build and test. It is very large, occupying about 400 sq ft of lab space. The magnet alone weighs 5 tons. For vibration interference protection, the instrument was installed in a specially built laboratory on a concrete and steel pedestal, sunk into the earth beneath the GMCS building.
Barry Hanan, the administrator for the Isotope Geochemistry Laboratory for the past 16 years, secured through a combination of SDSU internal funding, National Science Foundation grant money, and corporate cost sharing, the funds to acquire the $1.2 million instrument.
The SDSU geochemistry labs serve as a world-class center for isotope geochemical research. Researchers from as far away as Japan, Germany, France, and Turkey come to SDSU for collaborative projects involving exotic places like Iceland, Hawaii, the Pacific-Antarctic Ridge, the Galapagos Islands, and many other places of geologic importance. We believe that the interaction between visiting world class scientists and SDSU students and colleges fosters an intellectually challenging climate focused on identifying global problems in the Earth sciences and their resolution through isotope geochemistry.
Special Thanks Ethan A. Singer - Associate Vice President for Academic Affairs, Thomas R. Scott - Vice President for Research, Patrick J. Papin - Associate Dean for College of Sciences, Carl Schneider - Planning & Design Physical Plant, Carl J. Carrano - Department Chair for Chemistry, and Gary H. Girty - Department Chair for Geological Sciences.
June 2006
Effects of Predation on the Morphology of Pennsylvanian Bellerophontid Gastropods
Predation is an important process affecting the morphology of mid-Paleozoic (Signor and Brett 1984) through modern marine organisms significantly (Vermeij 1977). Crushing predation is one of the few cases in which we have direct evidence of biotic interactions in the fossil record. While changes in repair frequency (RF) may be the best proxy for analyzing crushing predation, their interpretation is ambiguous (Leighton 2002). For instance, an increase in RF can be caused by: 1) an increase in the number of predators, 2) an improvement in prey defenses, 3) a preservational bias (Vermeij 1987) or 4) a predator’s change in prey preference (Leighton 2002). Leighton (2002) suggests combining the analysis of RF with size-class data to enhance the study of predation. The purpose of this project is to use Leighton’s method to analyze predation on bellerophontid gastropods from the upper-Pennsylvanian of North-Central Texas. The following hypotheses will be tested: 1) size refugia exist among Pennsylvanian bellerophontid gastropods, 2) bellerophontid morphotypes vary in ability to deter crushing predation, and 3) predation intensity increases throughout the interval studied.
Creep measurements and depth of slip along the Superstition Hills faults as observed by InSAR
Data from 80 ERS-1 and ERS-2 interferograms (descending, track 356, frame 2943) covering Southern California in the Western Salton Trough and spanning a time period from 1992 to 2000 are analyzed to measure surface deformation along the Superstition Hills fault. InSAR, Interferometric Synthetic Aperture Radar, is a radar remote sensing technique that may be used to measure relative movement of the surface of the earth; movement is measured by change in phase in a time interval for a precise area. InSAR was used to model the near-fault (within 5 km) deformation along the Superstition Hills fault using a 2D analytic model of a vertical strike-slip fault in a homogeneous media. The relationship of earthquake seismicity, fault movement, and depth of fault movement are explored; in addition, we test to see if slip occurs at the depth in which earthquakes occur. All the observed signal is assumed to be due to shallow slip (above the seismic zone). Using data from cross-sectional profiles of interferograms across the fault, we estimate an average rate of slip of 7.7 ± 2.2 mm/yr and the depth of the slip zone at 3.1 ± 2.3 km.
TeraShake 2: Simulating Earthquakes for Science and Society
Earthquakes are a fact of life in California. The southern part of the major San Andreas fault, however, has not seen a major earthquake since about 1690, and the accumulated movement may now amount to as much as six meters—setting the stage for an earthquake as large as magnitude 7.7—the “big one.”
To understand the basic science of earthquakes and to help engineers better prepare for such an event, scientists want to identify which regions are likely to experience the most intense shaking, particularly in the populated sediment-filled basin of Los Angeles and similar areas in Southern California and northern Mexico. This understanding can be used to improve building codes in high-risk areas and to help engineers design safer structures, potentially saving lives and property.
But the challenges in modeling earthquakes are daunting. Accurate simulations must span an enormous range of scales, from meters near the earthquake source to hundreds of kilometers across the entire region, and time scales from hundredths of a second—to capture the higher frequencies which have greatest impact on buildings—to hundreds of seconds for the full event. Adding to the challenge, ground motion from earthquake waves is strongly influenced by the complex 3-D subsurface structure of the soil, which is not fully known and scientists can observe only indirectly.
Now, based on previous simulations at the San Diego Supercomputer Center (SDSC), earthquake scientists from the Southern California Earthquake Center/Community Modeling Environment (SCEC/CME) have run enhanced simulations at SDSC using the improved TeraShake 2 earthquake model. The new simulations, which used the Anelastic Wave Model (AWM), a fourth-order finite difference code developed by Kim Olsen, associate professor of geological sciences at San Diego State University (SDSU), are the most realistic yet of where the most intense ground motion may occur in Southern California during a magnitude 7.7 San Andreas Fault earthquake. More........
Dr. Gary Girty will begin a five year research plan focused on establishing the geologic framework of the four SDSU Field Stations beginning with the San Margarita Ecological Reserve in 2006. The field station will help with Student support, thin sections, chemical analyses, grain size distributions, and bulk and grain density analysis of selected soil and rock samples. The studies will result in several student thesis and set the ground work for future independent research projects that can be incorporated into the department's course curriculum.
Potentially Pulverized Granites along the Garlock Fault:
An analysis into their Physical and Chemical Properties
We collected samples of pulverized granitic rock along three trench exposures across the Garlock fault, Tejon Ranch, CA. Our primary purpose is to assess the roles of mechanical fragmentation versus chemical weathering in the reduction of grain size displayed by samples collected during this study. In each transect, granitic rocks are finer-grained immediately adjacent to the active trace of the fault. Feldspar and quartz grains range from 1-2 mm in size, and material along the fault is so powdered that it was difficult to collect large, intact samples. Quartz and feldspar grains smear to powder when pressed between the fingers.
Laboratory work with newly aquired department equipment includes bulk and grain density, porosity (Micrometerics Accupyc and Geopyc), mineral XRD, XRF major and trace element chemistry (Philips X’Pert Pro and XRF - Philips Magix Pro), thin sections (Buehler Petrothin), grain size (Camsizer and Horiba Laser LA 930), and SEM. Resulting data will resolve whether chemical weathering has played a significant role in the observed reduction of grain size. To date, we have completed major and trace element analyses of 11 samples. Resulting Chemical Index of Alteration (CIA) values range from 46-53, and are like values displayed by unweathered granite in the Peninsular Ranges. This result indicates that the finely-powdered granitic material is not distinguishable from fresh, unweathered granite, and that there is no indication of significant chemical weathering. Initial particle size distribution analysis from the department new shows grain sizes <0.2 μm. These preliminary observations are consistent with the idea that collected samples are derived solely from mechanical (dynamic) fragmentation during slip along the Garlock fault.
InvestigatorsMatthew Sisk, Tom Rockwell, and Gary Girty, San Diego State University; O. Dor, and Y. Ben-Zion, University of Southern California
The vertical motion database for Southern California is a compilation of geologic data, reference frame information, and processing tools to determine vertical crustal motions at 104 – 106 year time-scales. All original data, reference frames, and processing information are encapsulated within a PostgreSQL object-relational database. Querying data proceeds interactively via a web interface to the database through three steps: (1) select data points, optionally filtered by location and data type, (2) select one of the appropriate reference frames for each data type in the selected set, and (3) process the data points into vertical motion rates. Data compilation efforts are complete for marine terraces from central California to the border with Mexico. The majority of these data are for terraces formed 80 – 120 ka near the present coastline, with a few older points inland. Thermochronology data available for the Transverse Ranges have been compiled to provide exhumation rates (a proxy for uplift rates) at million-year time scales. River terrace and aquifer elevation data have also been added and include: Ventura River terraces, Los Angeles River terraces (along the Elysian Park anticline), Santa Ana River terraces (Yorba Linda terrace, Grand Terrace, as well as the Santiago, San Timoteo and Reche Creek terraces), and the San Gabriel River terraces. Efforts are ongoing to incorporate compiled stratigraphic horizon information into the database, and challenges remain in bridging reference frames between the coastal and interior basins. The web interface for obtaining and processing information from the vertical motion database is available at http://geomorph.geosci.unc.edu/vertical. Results may presently be viewed online in table format, downloaded as a GIS-compatible file, or browsed via the Google Maps web service.
InvestigatorsDr. Tom Rockwell (SDSU) , Danielle Verdugo (SDSU), M. Oskin (UNC), and N. Niemi (Caltech)
September 2005
Sharing the Land: Building Tribal Earth Science Expertise
A program of the Indigenous Earth Sciences Project http://rohan.sdsu.edu/~eriggs/IESP/
With new support from the National Science Foundation’s Opportunities for Enhancing Diversity in the Geosciences program, the SDSU Indigenous Earth Sciences Project has been able to launch a new collaborative venture called Sharing the Land. Directed by Dr. Eric Riggs and Dr. Eleanora Robbins from the Department of Geological Sciences, this project is a community-wide effort designed to increase the number of Native Americans who have access to a well-rounded, place-based education in the earth sciences at SDSU and collaborating institutions.
The program seeks to increase the number of local American Indian students who gain formal education and work experience in the geosciences with the long-range goal of improving the workforce available for tribal resource management in the coming years. Partners in this collaboration include San Diego State University, many regional Native American reservation communities, the Young Native Scholars bridge program, and the Explorer’s Clubs outdoor education program. Together these programs have combined into a complete and culturally-responsive recruitment and retention pipeline to grow the tribal environmental leaders of the coming generation.
The Grain Analysis Laboratory is an analytical facility designed specifically to address geologic problems in the field of grain studies. State-of-the-art analytical equipment, combined with the basic principles and techniques, provide a full range of advanced grain characterization and data collection. Some of the departments facilities include:
AccuPyc
The AccuPyc determines density and volume of grains by measuring the pressure change of helium in a calibrated volume.
The GeoPyc determines Envelope Density Analyzer rapidly measures the envelope density of porous objects of irregular size and shape by using a unique displacement measurement technique.
GeoPyc
Camsizer
The Camsizer, using digital image processing, measures 30um-30mm, while also providing shape measurement. The high-resolution analysis provides greater information about the sample material and an increased understanding of the behavior of the material.
The LA-930 is a full-range analyzer, measuring 0.02-2000µm, in a fully automated system allowing one-button operation. Offers unparalleled accuracy and precision, with the widest range of sampling accessories.
LA 930
June 2005
Dynamics of Localized Currents and Eddy Variability in the Adriatic (DOLCEVITA)
DOLCEVITA is an international project focusing on the Adriatic Sea sponsored by the Office of Naval Research (ONR). It involves several oceanographers, both experimentalists and modelers, of the United States and Europe. Clive Dorman participated in an oceanographic experiment in the northern Adriatic where an intensive field program was conducted in January and February 2003. Scientists from Croatia, Slovenia, Italy, Austria and the U.S. participated in this project named Dynamics of Localized Currents and Eddy Variability in the Adriatic (DOLCEVITA). The most important Adriatic winter meteorological event is the “bora”, a strong cold wind from the NW that blows off Croatia. The main objective of the DOLCEVITA project is to quantify the kinematic and dynamic properties of the northern and middle Adriatic (NMA) Sea.
Recent Publication Northern Adriatic Response to a Wintertime Bora Wind Event, Volume 86, Number 16, 19 April 2005. EOS, Transactions, American Geophysical Union.
Investigators Lee, C. M., F. Askari, J. Brook, S. Carniel, B. Cushman-Rosin, C. Dorman, J. Doyle, P. Flament, C. K. Harris, B. H. Jones, M. Kuzmic, P. Martin, A. Ogston, M. Orlic, H. Perkins, P.-M. Poulain, J. Pullen, A. Russo, C. Sherwood, R. P. Signell and D. Thaler Detweiler
Show me the future: Enriching and measuring spatial reasoning in teacher education
As the primary supplier of K-12 teachers in San Diego and Imperial Counties, California, San Diego State University (SDSU) plays an important role training the future teachers of this culturally diverse region. We plan to develop and assess a novel set of inquiry based modules for use in an introductory earth systems science class commonly included in the pre-service teacher training. Historically, pre-service teachers comprise approximately 40% of the total enrollment of several hundred per year in this class. These modules will be based on NASA material and other ongoing NASA education efforts (such as the GLOBE program). In parallel with the development of these modules, considerable efforts will be made in exploring the role of spatial learning and strategies. The assessments will be designed for application to large (> 200) classes and will include a significant online component, allowing them to be scaled to a variety of class sizes easily. Future teachers in the class will have had almost daily experience with web-based NASA STEM materials and have the potential to impact thousands of students in the next few years. The assessment materials and techniques will be useful for all NASA related educational activities and we anticipate that information about spatial reasoning will help create a new generation of highly effective educational modules.
High-resolution paleoseismology in Southern California: Investigation of segment controls on the rupture history of the San Jacinto fault
The San Jacinto fault is one of the primary structural elements of the San Andreas system in southern California, and with a Holocene slip rate of about 15 mm/yr, accommodates about 30% of the total plate margin motion. The fault has produced at least 10 earthquakes over M6 in the past 110 years, although only a few are known to have produced surface rupture.
Paleoseismic studies were performed at seven sites along the central and southern San Jacinto fault zone to understand the relationship between pre- historical and historical ruptures across the segment boundary between the Coyote Creek (CCF) and Superstition Mountain (SMF) faults, and to understand their relationship to the timing of large ruptures along the Clark fault and other large faults of the San Andreas system.
The central main strand of the SJF is the Clark fault. Excavations at Hog Lake near Anza show repeated ruptures during the late Holocene, with presumably large displacement based on the average recurrence interval and the late Holocene slip rate of >15+3 mm/yr, as determined from trenching. We identify evidence for 15 surface ruptures in the past 3200 years, with the most re3cent event about 250 years ago (ca 1760 AD). The average recurrence interval is about 215 years, so it appears that the Anza seismicity gap is rip for failure. Offset of a distinctive gravel indicates greater than 28m of slip averaged over the past seven events, suggesting greater than 4m of slip per event. Thus, when it’s time to rock and roll, expect at least an M7.
Trench Log at Hog Lake (click for larger image)
InvestigatorsTom Rockwell, Daniel Ragona, Gordon Seitz, Tim Dawson, Geoffrey Faneros, and Orgil Altangerel.
February 2005
Functional morphology of chonetidine (Brachiopoda) spines:
Biomechanical tests of a potential key innovation.
Chonetidine brachiopods are a significant component of Paleozoic marine life. Chonetidines are defined by the presence of tubular spines projecting posteriorly from the hingeline; these spines have been inferred to have been a key innovation facilitating the adaptive radiation of the group. Although the function of these spines has been the subject of much speculation, no biomechanical experiments have been conducted. The present study performs such a biomechanical experiment, using specimens of Neochonetes granulifera with and without artificial spines on a sandy substrate in a recirculating flume, to determine if the spines may have inhibited entrainment in higher energy settings, and thus possibly facilitated an adaptive radiation of the clade into previously unoccupied habitats. The results indicate that (a) chonetidines with spines were less likely to be overturned or transported than chonetidines without spines at the same free-stream velocity; (b) chonetidines with spines were overturned or transported at significantly higher free-stream velocities than specimens without spines; (c) these results were improved by the addition of another pair of spines; and (d) chonetidines with spines consistently reoriented such that their commissures faced into the current. The study demonstrates that chonetidines could have survived in higher-energy environments with shifting substrates, even if the brachiopods were libero-sessile, i.e. not physically attached to the substrate.
Comparison of free-stream flow velocity at end of trials
Investigators Brandon Mills and Dr. Lindsey R. Leighton
January 2005
Radial Volcanic Migrations above Continental Hotspots:
Arabia and the Pacific Northwest
The ongoing debate on the nature of hotspots has led many to consider alternative, tectonic models for the origin of hotspot tracks. These linear volcanic migrations, thought by most to form above supposed plume tails, have received much attention in the geological literature. In contrast, radial volcanic migrations forming above supposed plume heads have gone largely unrecognized. Two such examples are described here in the harrat volcanic province of Yemen and Saudi Arabia, and in the Columbia River Basalt (CRB) Province of the western U.S. Although most models of plume impingement predict a period of thermal uplift followed by basalt volcanism, the opposite appears to be true at the Afar triple junction, where the peak of flood basalt eruption, from ~13-19 Ma, was followed by uplift and exhumation which began at ~20 Ma, but accelerated at ~14 Ma. Uplift here was contemporaneous with the eruption of widely scattered basaltic lava fields which form the Miocene-to-Holocene harrat province in Yemen and Saudi Arabia. The Yemeni harrats become progressively younger and more alkalic away from the Afar region, prompting Orihashi et al. (1998) to suggest that they formed by the outward dispersion of a mantle plume, consistent with high 3He/4He ratios (Ra. 21.6) from the easternmost of the harrats. Age-equivalent harrats in Saudi Arabia erupted in a similar fashion, from linear vent systems that become progressively younger away from the Afar triple junction. As a group, the Arabian vent systems form a fan-shaped radial pattern consistent with the outward progression of a hot mantle source. One striking similarity between the Arabian harrats and the CRB Province is that both erupted from vents located on a basement of accreted oceanic terranes, adjacent to an older cratonic margin. Like the Arabian harrats, the CRB Province also erupted from a radial system of dikes concentrated along three, age-progressive trends - the Chief Joseph, Steens-Picture Gorge, and Northern Nevada Rift trends. These three trends emanate from a focal point in southeastern Oregon which is thought to be the Miocene site of plume impingement associated with the Yellowstone hotspot. Two younger volcanic migrations emanate from the same region - the eastward-younging Snake River Plain and the westward-younging Oregon High Lava Plains. The former is thought to have formed as a hotspot track above the plume tail, and the latter by asthenospheric drag of the plume head after it was sheared off against the westward moving cratonic margin. The recognition of radial, age-progressive volcanic migrations adds support to the argument that giant radiating dike systems propagate outward with advancing time. Such spatial and temporal volcanic and plutonic trends are consistent with a mantle plume origin, but difficult to reconcile by nonplume alternatives. Vic Camp (SDSU), Orihashi, Y (University of Tokyo), Ross, M E (Northeastern University)
December 2004
TeraShake
SCEC/SDSC
TeraShake, a Southern California Earthquake Center Community Modeling Environment (SCEC/CME) compute and storage intensive simulation, models a large earthquake occurring on the southern San Andreas Fault in California. There is significant interest in modeling earthquakes in Southern California and Northern Mexico. The simulation is based on the Anelastic Wave Propagation Model code, developed by Kim B. Olsen from San Diego State University. The code, enhanced during the TeraShake effort, is now available to the earthquake community.
The simulation, modeled on a rectangular 3000 x 1500 x 400 mesh with 1.8 billion points, will run 20,000 time steps at the San Diego Supercomputer Center (SDSC) DataStar machine. Each step can produce a 21.6GByte snapshot containing ground motion velocity vectors. A 4D wavefield containing 2,000 snapshots, amounting to 43TBytes of data, will be generated at SDSC. Surface data will be archived every step for synthetic seismogram engineering analysis, totaling 1 Tbyte.
SDSU Team Members Dr. Steve Day and Dr. Kim Bak Olsen
Other Team Members Jean-Bernard Minster (University of California, San Diego), Reagan Moore (San Diego Supercomputer Center), Philip Maechling (University of Southern California), Thomas H. Jordan (University of Southern California), Marcio Faerman (San Diego Supercomputer Center), Yifeng Cui (San Diego Supercomputer Center), Geoffrey Ely (University of California, San Diego), Yuanfang Hu (San Diego Supercomputer Center), Boris Shkoller (University of California, San Diego), Carey Marcinkovich (ExxonMobil Exploration Company), Jacobo Bielak (Carnegie Mellon University), David Okaya (University of Southern California), Ralph Archuleta (University of California, Santa Barbara), Nancy Wilkins-Diehr (San Diego Supercomputer Center), Steve Cutchin (San Diego Supercomputer Center), Amit Chourasia (San Diego Supercomputer Center), George Kremenek (San Diego Supercomputer Center), Arun Jagatheesan (San Diego Supercomputer Center), Leesa Brieger (San Diego Supercomputer Center), Amit Majumdar (San Diego Supercomputer Center), Giridhar Chukkapalli (San Diego Supercomputer Center), Qiao Xin (San Diego Supercomputer Center), Richard Moore (San Diego Supercomputer Center), Bryan Banister (San Diego Supercomputer Center), Donald Thorp (San Diego Supercomputer Center), Patricia Kovatch (San Diego Supercomputer Center), Larry Diegel (San Diego Supercomputer Center), Tom Sherwin (San Diego Supercomputer Center), Christopher Jordan (San Diego Supercomputer Center), Marcus Thiebaux (Information Sciences Institute), Julio Lopez (Carnegie Mellon University)
This work, in collaboration with Marty Grove (UCLA) and others, funded by the National Science Foundation, focuses on the fundamental question of how the exhumation history of orogenic systems is preserved in the ancient stratigraphic record, and with what fidelity this history can be extracted from derivative sedimentary successions. Specifically, we are testing the utility of integrating multiple isotopic techniques (detrital zircon U-Pb geochronology, whole rock Nd analyses, and detrital K-feldspar & muscovite 40Ar/39Ar thermochronology) with traditional sedimentologic analyses to assess the dynamic linkage between orogenesis, basin subsidence and resultant sediment dispersal and accumulation in continental margin arc systems. The combination of techniques focused on here is particularly sensitive to analysis of relatively deeply exhumed continental margin orogens that are often characterized by pulsed, episodic and/or multistage denudation histories. InvestigatorDr. Dave Kimbrough
October 2004
Production of high-purity 229Th for analyses of U- and Th-series isotopes in geological materials
The short-lived intermediate daughter isotopes of the U- and Th-decay series have proven to be important tracers of the nature and time scales of recent Earth processes. Current applications of these isotopes range widely from Quaternary geochronology to groundwater hydrology to ocean chemistry to magmatic processes beneath active volcanoes. One of the most important laboratory tools for these measurements is a high-purity Th-229 isotopic tracer, which is used both to determine the abundance of Th and, often, the Th isotope ratio of a sample by mass spectrometry. Unfortunately, there is no commercially available supply of high-purity Th-229 that will meet the current and future demands of the U- and Th-series isotope research community. This project will secure a long-term source of high-purity Th-229 through collaboration with the scientific staff of the Oak Ridge National Laboratory. The research conducted using this new high-purity Th-229 tracer will include diverse topics of unambiguous benefit to society, such as understanding climate and sea level change during the late Quaternary, deciphering magmatic processes at active volcanoes, evaluating potential sites for long-term storage of radioactive waste, and characterizing the transport of radionuclides in groundwater. The purity of the Th-229 tracer used by these researchers has a profound impact upon data quality, and thus, the outcome of projects such as these. This is an issue of particular importance for the future potential of measurements using plasma ionization mass spectrometry, which makes it possible to push the limits of accuracy and precision for U- and Th- series isotopic analyses of geological materials. Ultimately, this project will also promote the ability of these researchers to teach and train undergraduate and, especially, graduate students in the field of geochemistry. InvestigatorAaron Pietruszka
5500 Campanile Dr • 237 Geology Mathematics and Computer Science Building • San Diego • CA 92182-1020 • (619) 594-5586
