Seismogenic, Electrically Conductive, and Fluid Zones at Continental

George R. Jiracek, Victor M. Gonzalez, T. Grant Caldwell,
Philip E. Wannamaker, and Debi Kilb

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 three continental plate boundaries: South Island of New Zealand, the Himalaya, and San Andreas fault, USA. With this objective, we project earthquake hypocenters onto magnetotelluric (MT) electrical resistivity cross-sections. MT detection of conductive zones in the crust containing low fractions of fluids (less than 1%) requires an interconnected network of fluid-filled porosity facilitated by shearing, fracturing, and/or grain-edge wetting. Mechanisms promoting fluid reservoirs in the ductile crust include: 1) stalling of upward propagating porosity waves, 2) tectonically induced neutral buoyancy, and 3) development of ductile shear zones. Distinct conductive horizons are detected at depth in the ductile crust in New Zealand and the Himalaya where the tectonic convergence is high. In the Parkfield segment of the San Andreas fault, where convergence is low, there is high conductivity in the ductile crust but it forms a sub-vertical corridor to the surface with no distinct top. The tops of sub-horizontal conductive zones are ~20 km depth in New Zealand and ~25–40 km in the Himalaya where the seismogenic crust extends only to 12 and 25 km depth, respectively. The deep conductive layer in New Zealand may have originated as a “water sill” facilitating water-weakening, localized deformation, and eventually becoming a water-rich, anisotropic, mylonized, ductile shear zone. Fluid exchange through the active Alpine fault may initiate or be initiated by fault rupture. Localized, unstable flow in deep fluidized zones detected by MT may trigger earthquakes above.

Jiracek, G. R., Gonzalez, V. M., Caldwell, T. G., Wannamaker, P. E., and Kilb, D., 2007, Seismogenic, electrically conductive, and fluid zones at continental plate boundaries in New Zealand, Himalaya, and California, USA: in D. Okaya, T. Stern, and F. Davey, eds., A continental plate boundary: Tectonics at South Island, New Zealand, Geophys. Mono. Ser. 175, AGU., Washington, DC, 347-369.


A Continental Plate Boundary: Tectonics at South Island, New Zealand

David Okaya, Tim Stern, Fred Davey, Editors

A Continental Plate Boundary offers in one place the most comprehensive, up-to-date knowledge for researchers and students to learn about the tectonics and plate dynamics of the Pacific-Australian continental plate boundary in South Island and about the application of modern geological and geophysical methods. It examines what happens when convergence and translation occur at a plate boundary by

• describing the geological and geophysical signature of a continental transform fault;

• identifying the diverse vertical and lateral patterns of deformation at the plate boundary;

• assessing an apparent seismicity gap on the plate boundary fault and fast-moving plate motions;

• comparing this plate boundary to other global convergent continental strike-slip plate boundaries;

• documenting the utility of the double-sided onshore–offshore seismic method for exploration of a narrow continental island; and

• Providing additional papers presenting previously unpublished results.

This volume will prove invaluable for seismologists, tectonophysicists, geodesists and potential-field geophysicists, geologists, geodynamicists, and students of the deformation of tectonic plates.

Geophysical Monograph Series, Volume 175, 350 pages, hardbound, 2007, ISBN: 978-0-87590-440-5, AGU Code GM1754405

Dr. Gary Peterson - National Geographic Consultant

Dr. Gary Peterson, expert on North American stratigraphy, provides his knowledge as a Science Consultant for the National Geographic’s new giant-screen film SEA MONSTERS: A Prehistoric Adventure. This film submerges audiences in an 80-million-year-old scientific mystery. Produced by National Geographic, narrated by Tony Award-winning actor Liev Schreiber and featuring original music by Peter Gabriel, SEA MONSTERS: A Prehistoric Adventure brings to life the most bizarre, ferocious and fascinating creatures to ever inhabit Earth’s oceans. From the giraffe-necked Styxosaurus and 20-foot “bulldog” fish Xiphactinus to the T-Rex of the ocean — the 40-foot super-predator Tylosaurus — these wondrous beasts defy imagination. The film combines dynamic, state-of-the-art animation with exciting paleontological discoveries around the world to take audiences on a remarkable journey into the unexplored world of ocean reptiles that lived during Earth’s Cretaceous Period. Now playing at the Reuben H. Fleet Science Center IMAX® Dome Theater

New Publication - Barry Hanan

Yellowstone plume–continental lithosphere interaction beneath the Snake River Plain

Barry B. Hanan1, John W. Shervais2, and Scott K. Vetter3

1. Department of Geological Sciences, San Diego State University, San Diego, California 92182-1020, USA,

2. Department of Geology, Utah State University, Logan, Utah 84322-4505, USA,

3. Department of Geology, Centenary College, Shreveport, Louisiana 71134, USA

ABSTRACT
The Snake River Plain represents 17 m.y. of volcanic activity that took place as the North American continent migrated over a relatively fixed magma source, or hotspot. The identification of a clear seismic image of a plume beneath Yellowstone is compelling evidence that the Miocene to recent volcanism associated with the Columbia Plateau, Oregon High Lava Plains, Snake River Plain, Northern Nevada Rift and Yellowstone Plateau represents a single magmatic system related to a mantle plume. A remaining enigma is, why do radiogenic isotope signatures from basalts erupted over the Mesozoic–Paleozoic accreted terrains suggest a plume source while basalts erupted across the Proterozoic–Archean craton margin indicate an ancient subcontinental mantle lithosphere source? We show that ancient cratonic lithosphere like that of the Wyoming province superimposes its inherent isotopic composition on sublithospheric plume and/or asthenospheric melts. The results show that Yellowstone plume could have a radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt Group and that the plume source composition has persisted to the present day.

Hanan BB, Shervais JW, Vetter SK (2008) Yellowstone plume–continental lithosphere interaction beneath the Snake River Plain. Geology: Vol. 36, No. 1 pp. 51–54

2007 Honored Alumnus - Gary Peterson

2007 Honored Alumnus

Gary Peterson

The SDSU Geology Alumni Association and the Department of Geological Sciences are pleased to announce that Dr. Gary L. Peterson will be the recipient of the Baylor Brooks Honored Alumni Award at the annual banquet, tentatively scheduled for Feb 22, 2008. Dr. Peterson is very deserving as he has influenced more than 44 years worth of SDSU geology students (now alumni!). Congratulations, Gary!

December Crossword Puzzle - Meteorites

Java Interactive Puzzle


Printable Puzzle (pdf)

Fall 2007 Thesis Defenses

Hydrological investigations of the Santo Domingo Basin, New Mexico using electromagnetic soundings

Josh Woodworth

Friday, December 7th - CSL 422, 9:00am
Advisor - Dr. George Jiracek
B.S. Geological Sciences

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Testing the application and precision of optically stimulated luminescence on dating lacustrine shorelines in the Imperial Valley, southern California

Caitlin Lippincott

Friday, December 7th - CSL 422, 10:00am
Advisor - Dr. Tom Rockwell
M.S. Geological Sciences

Radiocarbon dating has proved ineffective to precisely date the timing of the last lake Cahuilla highstand. In contrast, Optically Stimulated Luminescence (OSL)
dating is an evolving technique with a different set of limitations and this method has demonstrated results with high precision that may allow for dating of deposits in this problematic period. Numerous explorers trekked through southern California beginning in the mid 1500’s. Diaries and maps were used in this study to further determine when a lake Cahuilla might have been present, mostly notably those of Father Kino in the late 1600 early 1700 and de Anza expeditions in 1774-1776. This study employs the use of OSL to try to precisely date the age of deposition of lake berms, fluvial sands and lake sands to better constrain the timing of the last Lake Cahuilla highstand. The initial OSL results show that there may be a significant partial bleaching problem with some samples.

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Geochemistry and U/Pb geochronology of the eastern-most Peninsular Ranges batholith of southern California and northern Baja California; implications for magmatism and tectonics at the onset of the Laramide orogeny

Rob Moniz

Friday, December 7th - CSL 422, 11:00am
Advisor - Dr. Dave Kimbrough
M.S. Geological Sciences

There are numerous models explaining the widely debated Laramide orogenic event that occurred in western North America from ~80-40 Ma. The shallowing of the subduction slab, or more likely a section of the slab, may have created the compressional regime that was expressed well inboard of the trench. Geologic evidence of this contraction has been documented from Canada down into Sonora, Mexico and includes the under-thrusting of the amphibolite facies Rand-Orocopia-Pelona schists, up­­lift of the continental interior and the “migration” of magmatism reaching ~1000 km inboard of the arc. Despite the abundant geologic data for Laramide and pre-Laramide orogenies, the initial magmatic units remain poorly understood. The characterization of these units may bring a better understanding of the transition to shallow slab subduction.
Using zircon U/Pb geochronology, a new suite of latest Cretaceous early Tertiary (i.e. 86-70 Ma) granitic intrusives has been characterized in the eastern-most Peninsular Ranges batholith. Previously, a small group of granitoids in the eastern Santa Rosa Mountains in southern California were the only characterized units of this age found in the Jurassic-Cretaceous Peninsular Ranges. This suite is sparsely exposed for at least 300 km along strike and likely comprises >200 km2 in surface outcrop exposure. The units lie within three geographically distinct areas. The youngest truly Laramide in age units (i.e. 80-70 Ma) were found in the Sierra Cucapas just south of the California-Baja California border whereas “transitional” aged units (i.e. 86-81 Ma) were identified in the Sierra Cucapa and further south in the Sierra San Felipe. Transitional aged units were also confirmed in the Santa Rosa Mountains of central southern California. Both the Laramide and transitional units in Mexico are found to be in cross-cutting relationship with the older La Posta-type plutonic rocks that comprise the main phase (100-90 Ma) of the eastern Peninsular Ranges batholith. The transitional units in the Santa Rosa Mountains appear to be distinctly to the east of La Posta aged units which are separated by the eastern Peninsular Ranges mylonite zone.
Major and minor elemental geochemistry from 54 samples indicate that although the majority of granitics in the eastern Peninsular Ranges are, as previously documented, part of the La Posta suite, many do not follow the typical documented deep crustal root signature defined by high Sr/Y ratios. The Laramide and transitional aged samples were found to overlap almost completely with the La Posta aged suite of this study on a regional and local scale, making characterization difficult.
The eastern batholith has been described as a “migrating” arc representing the initial stage of inboard migration of Laramide Cordilleran magmatism. The new data however, suggests that following the voluminous La Posta magmatic flare-up at 100-90 Ma, magmatism stalled but continued intermittently until ~70 Ma in the eastern Peninsular Ranges batholith. With numerous examples of Late Cretaceous magmatism as far inland as Colorado and New Mexico, it now appears that magmatism did not simply “migrate” inboard and that current models of shallow subduction inadequately account for the proximal Laramide intrusions of this study. This suite therefore provides a unique area that records the tectonics and magmatism associated with this major transition in Cordilleran geology.

New Publication - Lindsey Leighton & Stephen Schellenberg

Featured PALAIOS Article - January 2008

INFLUENCE OF SPATIOTEMPORAL SCALE ON THE INTERPRETATION OF PALEOCOMMUNITY STRUCTURE: LATERAL VARIATION IN THE IMPERIAL FORMATION OF CALIFORNIA

CORY M. REDMAN1, LINDSEY R. LEIGHTON*,2, STEPHEN A. SCHELLENBERG2, CHRISTOPHER N. GALE2, JENNIFER L. NIELSEN2, DONALD L. DRESSLER2 and MARY K. KLINGER2

1 Department of Biology, San Diego State University, San Diego, California 92182-4614, USA; Current address: Department of Geology and Geophysics, Texas A&M University, 3115 TAMU, College Station, Texas 77843-3115, USA
2 Department of Geological Science, Allison Center for Earth System History, San Diego State University, San Diego, California 92182-1020, USA

Numerous paleocommunity studies of marine ecosystems have demonstrated that water depth was the primary factor structuring paleocommunities. In contrast, many ecological studies find other factors play a greater role in delineating communities; this difference in results may be owing to the spatiotemporal scale at which the study is performed. To explore this hypothesis, the present study examines a set of Imperial Formation (Pliocene, California) paleocommunities at a scale potentially fine enough to exclude depth as a control over the communities, thus facilitating recognition of fine-scale ecological and environmental processes operating at this scale. Twenty-six taxa from 21 samples were collected in situ from an 8.5-m-thick interval within a 0.32 km2 area. Cluster, Bray-Curtis (polar) ordination, and detrended correspondence analyses were used to infer community structure. Cluster and ordination analyses produced similar results independent of choice of distance measure. To test whether depth would be the primary control, even at fine scales, the scores from the first ordination axis for each taxon were compared with their modern mean depth; no strong correlation exists between depth and ordination scores. Cluster and ordination results indicate that life mode (as determined by modern relatives) and to a lesser extent, grain size, were the primary factors influencing paleocommunity structure at this scale. Bivalve taxa were grouped by life mode: quasi-infaunal, shallow-burrowing infaunal, byssate, and cemented. Relative to paleoecological studies conducted on much broader spatiotemporal scales, this study highlights the potential utility of embedding fine-scale studies within broader-scale studies to capture and investigate additional sources of ecological and environmental variation.

Abstract . Full Text . PDF (1.31M)

PALAIOS; December 2007; v. 22; no. 6; p. 630-641; DOI: 10.2110/palo.2006.p06-044r