Geology 199RA - Independent Research
Local crustal structures of Cape Colbeck's offshore region in the eastern Ross Sea, West Antarctica, were modeled using a 2-D forward gravity modeling software to help determine tectonic constraints for this region of West Antarctica. Two survey lines (line 1 and line 2) along shiptracks across the Colbeck Trough were chosen from maps of bathymetry, free-air and Bouguer gravity anomalies. These two lines were used to forward model the sedimentary layer of the subsurface. An analysis of the model profiles does not reveal any distinct structural features. However, an approximation can given of the various thicknesses of the layers, that are probably sedimentary and have a density of 2.1 g/cm3. Background density is set to 2.7 g/cm3. The trough at line 1 has sediment layer thicknesses ranging from 1 to 2 km. The trough at line 2 has sedimentary layer thicknesses ranging from 50 to 750 m. There is a possibility that the Colbeck Trough, a 1000 m deep NNW trending feature is fault controlled; e.g. a normal fault up on the east. This may be a major structure defining the Edward VII Peninsula, the first expression of structural relief on the east side of the Ross Embayment.
National Science Foundation, NSF EAR 93-04130
This research concentrated on detailed volcanic facies mapping and description of the Late Jurassic (?) volcanic deposits, named the Temporal and Bathtub Formations, in the Santa Rita Mountains in southern Arizona. The volcanic deposits are interbedded with boulder conglomerates correlative with the Jura-Cretaceous Glance conglomerate which marks the onset of regional extension in the Bisbee basin. This extension has been hypothesized as being caused by either back arc spreading behind the Jurassic continental arc or by continental rifting exploiting the thermally weakened arc crust associated with the opening of the Gulf of Mexico. The Temporal-Bathtub Fms. were deposited during this time of transition in tectonic setting which was also possibly coeval with the strike-slip of the Mojave-Sonora megashear, of which the nearby Sawmill Canyon fault zone may have been a strand. Facies mapping indicates that deposition of the boulder conglomerates in the northern Santa Rita Mtns. occurred in a paleocanyon that must have had continuous subsidence throughout deposition in order to accumulate such a thick section of boulder conglomerates. This implies the paleocanyon was fault controlled, the most obvious candidate being the Sawmill Canyon fault zone. The presence of a lesser paleocanyon in the south may indicate a small half-graben basin geometry with the northern side being the more deeply downdropped side. Dacite lapilli tuffs were also funneled down the paleocanyon from a source external to the local basin or possibly along the Sawmill Canyon fault zone.
The pumiceous white tuffs and rhyolites of the Temporal-Bathtub Fms. are interpreted as plinian, phreato-plinian, and rhyolite dome deposits suggesting ongoing rhyolite doming as one style of local eruption within the basin. Andesite lava flows and cryptodome intrusions are another style of local eruption. The andesite flows appear to be preceded by the eruption of scoriaceous biotite-feldspar tuffs suggesting some sort of transition between rhyolitic and andesitic eruptions.
The presence of a red, welded tuff bed in the Temporal-Bathtub Fms. that is very similar to the red, welded tuffs of the underlying Mt. Wrightson Fm. suggests the continuation of an external volcanic source. This in turn implies that the deposition of the Temporal-Bathtub Fm. was closer in time to the Middle Jurassic Mt. Wrightson Fm. than previously indicated. This is also supported by the probable unlithified nature of the Mt. Wrightson Fm. quartz arenites at the time of Temporal-Bathtub Fms. deposition as indicated by the lack of quartz arenite boulders in the conglomerates.
National Science Foundation, NSF EAR 92-19739
This accomplishment-based renewal targeted well-exposed, little-deformed rocks that show the interrelations of grabens, calderas and basement structures in an extensional to transtensional arc rift setting. This was accomplished through basin analysis and U-Pb zircon geochronological studies of volcanic rocks in the basins. These studies show that craton-derived quartz sands were funneled along the Triassic to Middle Jurassic extensional arc graben-depression of Busby (Geology, 1988) for a time span of at least 40 my. Basin analysis studies during this project also support the 1988 extensional arc model, although these studies also document the importance of caldera collapse in preserving continental arc sequences. They have also demonstrated a Late Jurassic age for the base of regionally important unit, the McCoy Mountains Formation, and interpreted it to be the fill of a dominantly strike slip basin related to the Bisbee basin, a transtensional basin that is in turn related to extension of the Gulf of Mexico. Work on this project in the Bisbee basin was concentrated along the Sawmill Canyon fault zone, where abundant and distinctive, widespread, and dateable ignimbrites has permitted Busby to make correlations between fault blocks.
Texas A & M, 155-20842B
Under the Texas A&M ODP grant Cisowski has been measuring the magnetization of samples which have recorded several periods of anomalous behavior of the earth's magnetic field. One of these "excursions" of the field was first detected in a study of the paleolithic fire hearths from Australia, dating to about 30,000 bp. The extremely high sedimentation rates of the Amazon River fan deposits that he is studying are giving extremely detailed records of this and other geomagnetic excursions which have occurred over the past 100,000 years. ODP studies have also produced a detailed record of the fluctuations in the intensity of the geomagnetic field over the past 40,000 years. This intensity record can be used to determine the age of the Amazon fan sediments, and will aid in deciphering the climatic and botanical history of the Amazon basin since the last glacial period.
John Crowell, Professor Emeritus
Dr. Crowell has continued work on his memoir entitled Pre-Cenozoic Ice Ages and the Many Complex Controls of Climate. It reviews the long record of glaciation on Earth during the past three billion years as preserved in distinct strata at many far-flung places on the planet. He plans to have this comprehensive work completed within the next several months, and submitted to Geological Society of America for publication. In addition, he has served as Chairman of the Geology Section of the National Academy of Sciences. He has also conducted several field trips to the San Andreas fault and other areas in southern California, primarily for visitors.
University of Southern California, 1995 Summer Intern
This project was carried out by Southern California Earthquake Center (SCEC) summer intern, Windy Elliott. Windy correlated the very detailed dated stratigraphy in the Ocean Drilling Program core hole 893 to high resolution seismic reflection profiles in the Santa Barbara Channel. This allowed relatively recent fault displacement rates to be calculated on some faults in the central part of the Santa Barbara Channel. This brings a new level of analysis of fault displacement study to offshore faults which are largely inaccessible to direct examination and conventional trenching and dating techniques.
National Science Foundation, NSF EAR 95-06687
This project focuses on documenting the petrologic evolution of a region of the western United States that has undergone large-magnitude extension. The Northern Colorado River Extensional Corridor, located in Nevada, Arizona, and California, experienced >=100% extension over a relatively short period of time (15.12-13.0 Ma) based on the detailed structural and geochronological studies of Gans and his coworkers.
Pre-, syn- and postextensional basalts exposed in the northern Eldorado Mountains provide a record of how the source regions of basaltic magmas change during continental extension. Based on work in other highly extended areas, the source region for basalts apparently changes from lithospheric to asthenospheric during extension. By combining chemical and isotopic studies of these basalts with the well-known structural and geochronological history of the Eldorados, Gans and Bohrson will test the hypothesis of changing source region as well as place constraints on the rates at which this change occurs. Their results will allow them to evaluate the relative contributions that lithosphere and asthenosphere make in magmatism in regions of large-magnitude extension.
J. Scott Hornafius
UC Energy Institute, UCSB 08950920
The Institute for Crustal Studies conducted a marine survey of the dissolved natural gas content in the northern Santa Barbara Channel on September 25-27, 1995, in an area previously surveyed in 1981, located down current from an area of natural gas seepage near offshore oil platform Holly. The survey quantified the amount of dissolved methane, ethane, propane, helium, hydrogen, and carbon dioxide in the water, as well as the carbon isotope ratio of the dissolved carbon dioxide. The methane, ethane, and propane concentration near the seepage was found to be proportional to the seepage rate and inversely proportional to the current speed. The hydrocarbon concentrations were about three times higher in 1981 compared with 1995, which suggests that the seepage rate has diminished in the vicinity of the offshore oil platform (assuming current speeds were about the same during the two surveys). Helium concentrations in the ocean near Coal Oil Point were found to be up to 500 times higher than normal ocean values. The large helium concentrations did not occur in association with high methane contents, which suggests that the helium is entering the ocean along with ancient groundwater that is being expelled from the subsurface due to basinal dewatering. The high-helium waters appear to be associated with areas of natural oil seepage closer to shore. Elevated carbon dioxide and hydrogen contents down current from the offshore gas seeps are due to consumption of methane, ethane, and propane by bacteria. The rate of bacterial methanogenesis was calculated from the rate of decrease in flux of dissolved methane away from the gas seeps, and from the carbon isotope ratio of the dissolved carbon dioxide.
J. Scott Hornafius
University of Southern California, 572726
The North Channel fault is the western continuation of the San Cayetano-Red Mountain-Pitas Point fault trend that forms the northern edge of the rapidly deforming Ventura Basin. The Pitas Point - North Channel fault was responsible for the 1978 Santa Barbara earthquake and aftershock sequence. The fault is well imaged by seismic reflection data from the city of Santa Barbara to west of Goleta. In this area, a high amplitude fault plane reflector is imaged by both 2D and 3D seismic surveys with a wide range of acquisition parameters. The seismic reflection data indicate that the North Channel fault is a blind thrust that dips 20-40 degrees to the north. The fault tip occurs in the Pleistocene Pico Formation at a depth of 1.5 km subsea (1.5 seconds two-way time) and the fault plane steepens near its termination. A change in structural dip occurs at the fault tip. The fault plane is imaged to a depth of 3.7 km (2.5 seconds two-way time), at which depth there is significant offset of Miocene reflectors. In map view the fault plane bifurcates at two places: 119deg. 45' W and 119deg. 56' W. These locations correspond to the longitudes at which en echelon offsets occur in the trend of the hanging wall anticline above the North Channel fault. It is concluded that the North Channel fault is comprised of fault segments about 15 km in length near Santa Barbara. This conclusion is consistent with the observation that the aftershock sequence for the 1978 Santa Barbara earthquake extended for a distance of 12 km along a different segment of the North Channel fault.
Mapping of cross faults and analysis of earthquake aftershock hypocenters was done while involved in the Oak Ridge and North Channel fault studies. Kamerling is collaborating with Larry Gurrola, a graduate student of the Department of Geological Science at UCSB, who has been mapping these cross-structures onshore. The mostly northeast-southwest trending structures have been described locally in the past but their work has shown that they have much larger regional significance and probably influence the area of earthquake rupture zones. Also, several large earthquakes have been shown to originate at the intersection of the main fault and the cross faults. Thus a better understanding of these structures may help determine the magnitude of likely earthquakes along a particular fault.
The Hosgri fault system has been studied for some time but is still very controversial. It has been labeled a thrust fault and a strike-slip fault with widely varying ranges of slip. The northwest trending Hosgri fault terminates into the east-west trending faults and folds of the Transverse Ranges. This study seeks to determine how much of the strike-slip displacement on the Hosgri fault system is taken up by folding of rocks on either side of the fault near it's southern terminus. In order to accomplish this Kamerling and Sorlien created a structure map on a Pliocene age horizon using oil industry well and seismic reflection data. This structure map was then digitized and the surface was then unfolded or flattened by computer, to determine the amount of crustal shortening on either side of the Hosgri fault system.
University of Southern California, 572726
This project concerns the deformational history of active faults within the Ventura Basin and Santa Barbara Channel, with particular emphasis on the Oak Ridge fault system. Recent published models for the structural geometry of the Oak Ridge trend offshore in the Santa Barbara Channel [e.g., Shaw and Suppe, 1994] are not consistent with earlier published models for the Oak Ridge fault located farther east in the Ventura Basin. To the west, detailed cross sections constructed across the central Santa Barbara Channel, using deep drill-hole and multi-channel seismic (MCS) data [Kamerling and Nicholson, 1994], also show the Oak Ridge trend as a steeply south-dipping reverse-separation fault. In well P0231 #5, Monterey Formation is steeply-dipping and exhibits over 2600 m of vertical separation between repeated sections. The P0467 #2 well shows steep dips and repeated Monterey section with 1300 m of vertical separation. Dipmeter logs from several wells show increasing dip with depth and proximity to the Oak Ridge fault. This is particularly true for the Miocene and Oligocene sections. These data directly contradict the model proposed by Shaw and Suppe  that largely assumes constant dip panels with depth and only moderate-to-low dip angles for deep structure. Wells that penetrate gouge zones, repeated sections, and abnormally thick San Onofre Breccia, Vaqueros and Sespe formations also support the interpretation of a south-dipping fault and steeply-dipping to overturned strata. These steep dips are not imaged by the MCS data, making interpretations of this type of structure difficult if the images from the seismic reflection data are taken literally, or if the seismic data are interpreted without proper well control.
High-resolution seismic data in the Santa Barbara Channel clearly show that the Oak Ridge fault offsets shallow sediments and in places, a near sea-bottom unconformity. These data, plus recent earthquake hypocenters that align along a south-dipping structure (and which exhibit a high-angle south-dipping nodal plane), suggest that the Oak Ridge fault is an active fault and not simply an "active axial fold surface." Shaw and Suppe  interpreted these deep events to represent bedding-plane slip through an active axial surface; however, the earthquakes occur between depths of 7 and 15 km, where no such bedding-planes are known to exist.
These observations of an active south-dipping Oak Ridge fault are thus inconsistent with models which infer only growth folding above low-angle thrust faults that dip north [e.g., Shaw and Suppe, 1994]. If anything, the two-dimensional model for this type of structure can be more accurately modeled as a fault-propagation fold above a steeply south-dipping fault, rather than as a fault-bend fold. Models which rely on fault-bend fold theory may very well explain the component of compressive deformation in the shallow sedimentary section, but this does imply that these models can then be extrapolated to depth, or be used to infer the geometry of deep fault structure.
The reason that such models fail to adequately predict deep earth structure in the Santa Barbara Channel is because these models assume that (1) structure in the Santa Barbara Channel is 2-D, (2) that formation contacts are parallel--allowing near surface dips to be projected to depth as constant dip panels, (3) that strain is uniform and constant with time, and (4) that there is no strike-slip motion in or out of the plane of the cross section. These 2-D balanced cross section models are thus incompatible with the structure in Santa Barbara Channel because the geologic structure is inherently 3-D and exhibits considerable variation along strike, because strain--that includes a significant strike-slip component--has been partitioned between high-angle and low-angle structures, and because more recent faults and folds are strongly controlled by earlier normal-separation faults of Miocene age--which have been subsequently rotated and reactivated. The observations of an active, reverse-separation fault along the Oak Ridge trend in the Santa Barbara Channel could significantly alter the estimation of earthquake and tsunami hazards along the south coast of California.
National Science Foundation, NSF EAR 95-26016
For decades, undergraduate geology majors from several UC campuses have completed field geology courses in the Poleta folds area, Inyo Mountains, California. Looking to the east, these students had an excellent view of the geomorphically well-defined Deep Springs normal fault that bounds the east side of Deep Springs Valley. Little is known about the faulting and earthquake history of this fault. Jeff Lee has a NSF grant to undertake detailed geologic mapping, geomorphic, paleoseismic, structural and kinematic studies, and age determinations of offset Pliocene and younger units along the Deep Springs fault to document the number of prehistoric earthquakes, the magnitude and kinematics of slip, average slip rates, and the age of initial fault activity. This research is part of a larger project that includes field-based studies of the recent faulting and earthquake histories along part of the White Mountains fault zone and Towne Pass fault zone that will: (1) improve understanding of prehistoric earthquake activity and mechanisms of fault interactions that will better characterize seismic risk in the area, (2) document the space-time evolution of fault motions in this part of eastern California, and (3) improve understanding of the time-integrated contribution of this part of California to overall Pacific-North America plate motions.
During the fall of 1995 Lee mapped (1:24,000 and 1:6,000 scale) the Deep Springs fault and part of the White Mountains fault zone. Along the Deep Springs fault, Lee identified one Late Pliocene to Pleistocene sand and gravel deposit and three Holocene alluvial fan units. The oldest two alluvial fan deposits have been offset between 1-10 m across the fault; the youngest alluvial fan unit has been deposited across the fault escarpment. Ar/Ar and 14C geochronology of tephra, charcoal, and tree branch samples collected from all four units will provide constraints on the timing of faulting as well as on long term (last 1-2 million years) slip rates along this fault. Analyses of these samples should be completed by the end of this summer.
On the basis of Lee's mapping of the Deep Springs fault, three potential sites have been identified for paleoseismic (trenching) studies, which may provide a short term (last 50,000-100,000 years) slip rate and an estimate of the earthquake recurrence interval. At the beginning of this summer, Lee, along with undergraduate students from Central Washington University, created ~1:1000 scale topographic maps of these three areas using a Leica Total Station. The topographic maps were then used as a base for detailed geologic mapping. Paleoseismic studies of the best of these sites will commence during late August.
National Science Foundation, NSF EAR 95-26861
This proposal outlines detailed geologic mapping, structural, kinematic, geochronologic, and thermochronologic investigations to address the origin and nature of the Himalayan gneiss domes of southern Tibet. Investigations will characterize the nature, geometry, and kinematics of ductile and brittle deformation, characterize the nature of the gneissic core-metasedimentary cover contact, document the timing of pluton emplacement, and document the cooling of plutonic rocks and country rocks within these gneiss domes. These studies are designated to test three models proposed for the evolution of the gneiss domes: (1) they are metamorphic core complexes that may indicate a significant amount of NS-extension parallel to the regional shortening direction, (2) they are compressional structures directly related to NS-convergence between India and Asia, or (3) they formed as an outcome of diapiric rise of anatectic melts. It is most likely that the results of these investigations will indicate that these domes have complex histories and polygenetic origins. Therefore, these studies will provide a much clearer picture of the timing of unroofing and cooling, and an assessment of the relative importance of extension, shortening, and diapirism in the evolution of these gneiss domes. These data are critical in order to assess models proposed for the formation of overthickened crust and high elevations within the Tibetan plateau.
Louis Bartek, University of Alabama
National Science Foundation, NSF OPP 93-16712
This is a collaborative project between Bruce Luyendyk at UCSB and Louis Bartek at the University of Alabama. They conducted a study of the Cenozoic glacial and tectonic history of western Marie Byrd Land (MBL), West Antarctica, in the region of the eastern Ross Sea, Edward VII Peninsula, and the Ford Ranges, in winter, 1996. The study region is located at the eastern edge of the Ross Sea and Ice Shelf. The approach is an offshore geology and geophysics study of the continental margin here. This project follows on a 3 year onshore geological and geophysical investigation by Luyendyk and colleagues in the northern Ford Ranges and the Edward VII Peninsula. This work included mapping, petrology, geochronology, paleomagnetism, gravity, and preliminary observations on the glacial geology in western MBL. The offshore work used the icebreaker N.B. Palmer (cruise NBP-9601). No marine geophysical data are available from this large region. They also finished a site survey for the Cape Roberts stratigraphic drilling project in the western Ross Sea.
Luyendyk brought along 7 undergraduate students; six from UCSB (C. Alex, T. Garcia, E. Johnson, J. Sandlin, E. Vanek, K. Zellmer) and one from Univ. of British Columbia (R. Hamilton). These students were enrolled in a class given at sea. They also have individual responsibilities for independent projects using 9601 data. Prior to the cruise three students did REU projects. Also in Luyendyk's party was Chris Sorlien, a postdoctoral from UCSB and LDEO.
Early examination of NBP-96-1 seismic and Multibeam data reveals tectonic activity and glacial erosion of the shelf in the area offshore of the Keil Glacier on the west side of Edward VII Peninsula. These same data along with cores reveal that the inner shelf in the Marie Byrd Land region has been stripped of sediment by glacial erosion and that much of this sediment has been deposited near the shelf edge in the region. Cores reveal that a very different mode of glacial marine deposition is occurring in the inner shelf basins of the Marie Byrd Land region as compared to other regions of Antarctica. Multichannel seismic data (MCS) collected during the cruise from the central Ross Sea, where DSDP drill sites exist, provide an opportunity to correlate and provide relative ages to units to seismic stratigraphic units in the Colbeck Trough area. MCS data collected in the Eastern Basin will also augment ongoing studies of glacial seismic stratigraphy/facies relationships to lithofacies. They are also currently integrating new NBP-96-1 MCS data with existing seismic data from NBP-94-7 and NBP-93-8, and older PD-90 data to stochastically characterize that spatial and temporal nature of the facies variation.
Geophysical data map half grabens in the continental shelf of the eastern Ross Sea adjacent to Edward VII Peninsula, and in Sulzberger Bay offshore from the southern Ford Ranges. These structures trend subparallel to the ranges onshore. A 1000 meter high fault scarp trending NNW mapped on the east side of the Colbeck trough, defines the west side of Edward VII Peninsula and apparently routes the flow of outlet glaciers. For example, the Keil Glacier flows into the Colbeck trough. The trough is bare of sediments near the peninsula but farther offshore tills apparently bury or fill it. This structure is one of the most prominent features of the eastern rift shoulder found so far. Farther east offshore from the Ford Ranges, Sulzberger and Saunders Basin are fault-controlled structures that also may have been conduits for glaciers during previous glacial maxima. These faults have down-to-the-east displacement. Therefore, the gross structure of the Edward VII Peninsula is a horst.
Gravity data offshore show a positive free air anomaly of 20 to 40 mGals over the outer shelf of western Marie Byrd Land. The bouguer anomaly varies from +40 to 60 mGals but is generally lower in Sulzberger Bay than the eastern Ross Sea. Adjacent to the east side of Edward VII, the anomaly is strongly negative. These data suggest a thicker crust under the peninsula with a possible discontinuity to thinner crust at its east side.
Several high amplitude (1000 nT) dipolar magnetic anomalies were mapped on the shelf on both sides of Edward VII peninsula during cruise 9601. No obvious magnetic trend was detected. However, only a few magnetic lines were made in Sulzberger Bay due to dense sea ice conditions.
They also conducted a seismic, multibeam, and chirp sonar surveys in the area of the Cape Roberts Drilling Project area. The Cape Roberts data will assist them in correlating the results of drilling away from the sites. Also tested was a Jumbo Piston Corer at a site in Granite Harbor where "soupy" sediment that was more suitable for successful coring with this device had cored during another survey.
J. Scott Hornafius
UC Energy Institute, UCSB 08950921
Prolific natural gas seepage, a significant air pollution source in Santa Barbara County, occurs offshore from Coal Oil Point near Santa Barbara, above the South Ellwood Offshore Field. Luyendyk and colleagues have been studying this seepage field in order to determine the impacts of oil production on natural hydrocarbon seepage, and to develop a physical model that could be applied to other oil fields so that the effects of future oil production offshore from southern California can be predicted. Seepage rates can be quantified by measuring sonar reflections from gas bubbles rising through the water column. In 1995 they digitally recorded 3.5 kHz sonar data in the same location as a 1973 survey. Comparison of the data indicates a substantial reduction in seepage within one mile of Platform Holly since 1973. The physical mechanism that is responsible for the natural seepage is the flow of hydrocarbon fluids from the oil reservoir to the sea floor through natural fractures in the rock. The flow rate is controlled by the pressure difference between the oil reservoir and the sea floor; as oil has been produced the pressure has decreased. In order to develop an empirical data set on time variations, they are analyzing sonar data acquired in 1980, 1981, 1982, 1983, and 1984 in the vicinity of Platform Holly. This would enable the timing of seepage reduction relative to the decrease in reservoir pressure to be compared against model predictions. They also will analyze the composition of the seep gases in order to evaluate the air quality benefit of the reduction in natural hydrocarbon gas seepage.
University Navstar Consortium, 08950706
National Science Foundation, NSF OPP 94-23534
The U/Pb detrital zircon study of Proterozoic metaclastic rocks in Arctic Alaska, designed to provide additional constraints on the tectonic evolution of the Canada Basin, initiated with a 3 week summer field program in 1995. A total of 32 samples spanning the 1200 km strike length of the Brooks Range orogen were collected for detrital zircon analysis. All of the samples have been processed by 6 undergraduate students supported by the REU grant. In addition, the students were provided with instruction in mineral separation techniques as well as several informal seminars concerning geochronological methods. To date, approximately 50 single zircon grains have been analyzed from 3 samples collected from the Proterozoic Nerukpuk Formation in the northeastern Brooks Range. On the basis of the initial detrital zircon ages, which range from 1.0 to 3.1 Ga, it appears that Proterozoic clastic rocks in the Brooks Range received sediment from non-North American sources. Additional analyses from the remaining samples will further understanding of the paleogeographic origin of northern Alaska and tectonic evolution of the Canada Basin.
National Science Foundation, NSF EAR 94-05261
Recent studies using a wide range of geophysical techniques, including wide-angle seismic reflection, refraction and gravity, have inferred the presence of remnant fragments of subducted oceanic crust underneath the California continental margin. Additional geological and geophysical data have documented that the western Transverse Ranges (WTR) have rotated substantially since early-Miocene time and are continuing to rotate today. This rotation has been previously linked to the evolving Pacific-North American transform boundary and, recently, to large-scale extension and rifting of the inner California Continental Borderland. However, it has never been adequately explained as to why the WTR should accommodate such plate boundary deformation by tectonic rotation, nor why they should have developed when and where they did. Nicholson et al have developed a new tectonic model for the evolution of the plate boundary that explains many of these observed features of the California margin, including this WTR rotation [Nicholson et al., 1994]. Evolution of the Pacific-North America plate boundary can be explained largely by the process of microplate capture by the Pacific plate of remnant pieces of subducting Farallon plate before they were able to fully subduct. Because these remnant pieces extended well beneath the North America plate at the time of capture, this capture led to an eastward shift of Pacific plate motion down along the subduction interface, and necessarily implies that the initial transform geometry was a low-angle fault system. Microplate capture thus subjected parts of the overriding North American plate to distributed basal shear and crustal extension. This resulted in the rifting, rotation, and translation of the continental margin as various pieces of North America were transferred to the Pacific plate, including the large-scale (>90deg.) rotation of the WTR block in Neogene time. This model helps to explain the timing of initial WTR rotation and basin formation; the sudden appearance of widely distributed transform motion and enhanced crustal extension well inland of the margin in early Miocene time; and several other fundamental characteristics of central and southern California. The model also provides major constraints on Pacific-North America strike-slip motion, a more direct tie between the position through time of offshore oceanic plates with respect to onshore geology, and a general explanation for what may happen as a subduction zone evolves into a transform system.
This project is testing this new tectonic model by investigating the relationship and interaction of offshore oceanic plates with respect to the known geology of western North America. It utilizes a unique geological database already compiled for the Tectonic Map of North America by AAPG. This database includes regional onshore geology and positions of offshore microplates based on magnetic anomalies in the deep ocean. This database needs to be augmented, however, with the near-shore geology of the offshore California continental margin. This will be done using the results from previous investigations, a regional grid of MCS profiles, and available sea-floor geology. Additional stratigraphic control will be provided by ties to offshore and near-shore test wells. These data would be then used to construct quantitative palinspastic maps of the evolving Pacific-North America plate boundary since ~30 Ma. The reconstructions will help evaluate the tectonic validity of the new model, identify specific problems that the new model may have, and any necessary modifications that may help improve model accuracy. Specific onshore (and offshore) sites where further geological or geophysical tests can be performed can then be targeted for further investigation.
National Science Foundation, NSF EAR 94-05560
This year represented the third field season of this NSF-funded study of orogenic processes in the Brooks Range. The primary goals of the study include (1) documenting the thermo-mechanical development of the Brooks Range metamorphic core with emphasis on subduction/underplating and exhumation processes and (2) establishing the link between deformation in the metamorphic core and the more external unmetamorphosed fold-thrust belt. This summer in the field, Vogl and field assistant Eric Vanek mapped a series of moderately north-dipping normal faults in mildly metamorphosed rocks north of the metamorphic core. These normal faults, occurring over a distance of only ~6 kilometers, have led to the omission of up to 2 km of section. Together with a more prominent normal fault to the south, the faults may have a combined displacement of ~5-8 km and can explain much of the differential uplift between higher-grade rocks of the metamorphic core to the south and relatively unmetamorphosed rocks to the north. The mapping has shown that the area has undergone a complex history of north-directed contraction, south-directed contraction and north-south extension. Cooling and exhumation histories are currently being investigated through metamorphic and thermochronologic data collected in the laboratory. These data will allow them to estimate rates of exhumation and examine the roles played by contraction, erosion and extension in the exhumation of the once deep-seated metamorphic rocks.
With this integrated structural, metamorphic and thermochronologic approach they will enhance understanding of not only the poorly studied Brooks Range, but also of thermal and mechanical processes in collisional orogens.
USDI/US Geological Survey, USGS 1434-HQ96GR-02732
This research is to determine what proportion, if any, of the geodetically measured elastic horizontal strain in the Los Angeles and Ventura basins is being released vertically, slowly and quietly, without earthquakes. Such strain release is indicated by almost imperceptible folding, tilting, fault creep. Indeed, uplift at a few millimeters a year occurred in the Ventura Avenue anticline between 1978 and 1991 unaccompanied by recorded earthquakes. Sylvester and colleagues will resurvey leveling lines and compare with historic leveling data across three anticlines and one fault in the subject region. If they find that vertical deformation has occurred aseismically, then by boundary element modeling they shall estimate the proportion of anelastic strain to quantify a conclusion that impending earthquakes may be smaller or less frequent for the Los Angeles metropolitan area than recently and dramatically prognosticated by other investigators.
National Science Foundation, NSF EAR 93-03913
The objective of this research is to characterize the interseismic activity of one of the largest normal faults in the Basin and Range - the Teton fault - which is responsible for the uplift of the magnificent Teton Range of Grand Teton National Park, Wyoming. By comparing results of repeated precise leveling surveys of a 22 km-long line of 50 permanent bench marks across the fault, UCSB undergraduate students and Sylvester seek geodetic evidence of aseismic vertical slip or creep across the fault. This grant supported a resurvey in 1993 to compare with surveys in 1988, 1989, and 1991. Before 1991 the valley on the hanging wall of the fault rose 10mm aseismically relative to the footwall (Teton Range) in 1988-89, probably due to poroelastic effects caused by refilling Jackson Lake, and the valley tilted toward the mountains. Between 1991 and 1993, however, the valley tilted about 1 microradian eastward , away from the Teton Range and opposite to the long range tectonic tilt inferred from the slope of the valley floor and its subsurface strata. They postulated that the tilt was caused by nontectonic, asymmetric lowering of the water table engendered by a drought in the area. With supplementary funds awarded to this grant, they extended the line 7.8 km in 1994 to the mountains east of the valley. Thus future surveys of the line, now lengthened to 30 km, will monitor not only the behavior of the Teton fault relative to the adjacent valley, but also of the valley to nontectonic effects. In the event of a major earthquake on the fault, modeling of consequent displacements will reveal the geometry of the fault to a depth of about 15 km, a matter of great dispute, concern, and little data for major normal faults at present.
USDI/US Geological Survey, USGS 1434-93-G2290
This project conducted precise leveling of arrays established across active or potentially active faults by Sylvester and students under contracts or grants from the U.S. Geological Survey, or arrays established by the U. S. Geological Survey or the U. S. Coast and Geodetic Survey (now National Geodetic Survey - NGS) and resurveyed by them, also under contracts or grants from the U. S. Geological Survey. All of the arrays are located in the south half of California (Figs. 1 and 2).
Earthquakes did not occur during the contract period on those parts of the fault where they had existing geodetic arrays, and they did not measure any nearfield vertical displacements across faults that can be attributed to tectonism. Thus, vertical strain, if it is being released at the surface along the fault traces themselves, as is proven by the measurements of horizontal strain (Lisowski et al., 1991), is either too small and too slow to detect with precise leveling, or it is released episodically over time periods exceeding the time span of their surveys, or it is manifested at a areal scale beyond that at which they surveyed.
Tilt arrays in the Mammoth Lakes area yield a pattern and rate of radial tilt outward from the center of the Long Valley caldera that is consistent with that measured by permanent USGS tiltmeters.
University Navstar Consortium, 08950586
National Science Foundation, NSF OCE 92-96207
The aims of this project were to improve understanding of seismic velocity structure in the upper mantle of the Earth and to learn the role of hotspots and ridges in global Earth tectonics. Particular emphasis was placed on improving the resolution of structure using short period seismic surface waves. Tanimoto's approach has been to measure phase velocity of surface waves by waveform correlation, a method has been developed since 1987. Data from global digital networks, mainly dominated by US networks, have been analyzed. In total, approximately 18,000 seismograms were used to construct the global earth model.
The main discoveries are (1) slow seismic velocity anomalies found under many (proposed) hotspots, (2) comparatively shallower slow velocity peaks under ocean ridges and (3) good correlation of short period phase velocity maps with crustal structure. Discovery (1) indicates that many hotspots, proposed mainly because of ocean floor topography, are indeed active hotspots with distinct thermal anomalies below the lithosphere. Discovery (2) demonstrates that differences in upwelling mechanism exist between ridges and hotspots. The latter point has been in dispute with another research group, however, and further work is required to resolve the question. Discovery (3) may not be a surprising discovery because short period waves should be dominated by shallow crust structure. But it supports the soundness of the analysis and offers the possibility in the future that the global crustal structure is recoverable by extending this type of analysis.