Crustal Structure and Tectonics

Personnel: C. Alex, R. Archuleta, A. Blythe, J. Bartek, J. Beitzel, K. Bassett, L. Blikra, C. Busby, S. Cisowski, S. Critelli, B. Dinklage, W. Elliott, B. Fackler-Adams, P. Gans, J. S. Hornafius, M. Kamerling, M. Kohler, J. Lee, B. Luyendyk*, W. McClelland, E. McWayne, C. Nicholson, B. Patrick, N. Pinter, J. Sandlin, C. Sorlien, A. Sylvester, A. Till, T. Tanimoto, E. Vanek, J. Vogl, K. Zellmer (*Agenda coordinator)

Development of Basins and Calderas in Transtensional Arc Settings (NSF EAR92-19739 ) (Cathy J. Busby)

This project targets well-exposed, little-altered and barely-deformed rocks that show the interrelations of grabens, calderas and basement structures in a transtensional arc setting. We are working along the Sawmill Canyon fault zone in southern Arizona to determine the nature and timing of faulting and basin formation contemporaneous with the opening of the Gulf of Mexico in Jurassic time. We are testing the hypothesis that the Sawmill Canyon fault zone is a sinistral transtensional fault system related to the Mojave-Sonora megashear system. As part of this work, we are analyzing the fault-subsidence styles of graben systems immediately before and during caldera development, and examining ways in which caldera collapse and filling are influenced by regional structures. Strike slip dismemberment is a common process in modern arcs, and the study area in southern Arizona provides a view of basins, calderas and fault zones at a variety of structural levels. This research currently involves post-doctoral researcher Kari Bassett.

Facies Modeling and Process Volcanological Studies in an Oceanic Arc Terrane (NSF EAR93-04130) (Cathy J. Busby, Benjamin Fackler-Adams)

Volcanic arcs are the surface manifestation of magmatism at convergent plate margins, and are important for their contribution to the growth of the earth's crust. Our observations of the growth of oceanic arcs is poorer than it is for arcs formed on continental crust, partly because of the logistical difficulties and expense of making submarine observations in modern oceans. Also, the uplifting of an oceanic arc into the subaerial realm, where it can be more easily studied, commonly involves tectonic accretion to a continental margin, processes of which tend to obscure primary features. The Early Cretaceous oceanic arc of the Peninsular Ranges in Baja California provides an excellent opportunity to develop a better understanding of oceanic arc volcanic and sedimentary processes, basin evolution and facies architecture. The terrane is large enough to permit direct comparison with the stratigraphy and structure of modern systems, and it is very well exposed and well preserved. We are currently working to:(1) characterize the response of sedimentary systems to explosive volcanism, (2) establish criteria for distinguishing subaerial vs. subaqueous environments of eruption, transport and deposition of, pyroclastic material, and (3) determine the controls of tectonic uplift and subsidence events on facies architecture in oceanic arcs.

This research involves Ph.D. students Ben Adams and Paul Brown, and postdoctoral researchers Salvatore Critelli and Lars Blikra.

Anomalous Behavior of the Earth's Magnetic Field during "Excursions" (Ocean Drilling Program) (USSSP-108) ( Stanley M. Cisowski)

Under the Texas A&M ODP grant I have 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 I am studying are giving extremely detailed records of this and other geomagnetic excursions which have occurred over the past 100,000 years. My 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.

Seismic Hazards in the Santa Barbara Channel Using High Resolution Seismic RefLection Data and Dated Horizons From ODP 893 (SCEC USC 572726) (Windy Elliott -1995 Intern, Marc J. Kamerling )

A grid of high-resolution seismic reflection data has been correlated to horizons identified in ODP 893A drilled in the Santa Barbara Channel. These horizons have been accurately dated using oxygen and carbon isotopes (Kennett, 1995). The age of the oldest sediment penetrated by the core sample is late Pleistocene. Several younger horizons can be correlated with coherent reflections identifiable in the seismic data. Any near-seafloor deformation that affects these horizons must be considered potentially active. This dated seismic stratigraphy can be used to estimate the rate of slip on faults, the relative ages of submarine slides and unconformities, and the development of growth strata associated with fault-related folding. This information can be used to improve understanding of local tectonic processes and seismic hazard estimation in the channel. Based on the core-hole stratigraphy, a strong continuous reflection that was mapped throughout much of the channel, is dated at approximately 120,000 years. Measurable time separations of this horizon have been identified across several possible fault(?) structures and were converted to depth using velocity-depth relations for P-waves in silt-clay and turbidites (Hamilton, 1979).

Seismic Mapping of the North Channel Fault near Santa Barbara, CA. (SCEC USC 572726) (J. Scott Hornafius, Marc J. Kamerling, Bruce P. Luyendyk )

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.

Offshore Oak Ridge Fault, Ventura Basin, (SCEC USC 572726) (Marc J. Kamerling)

The Oak Ridge fault in the Ventura Basin and eastern Santa Barbara Channel is a continuation of the structure which was responsible for the M 6.7 Northridge earthquake. Offshore, in the Santa Barbara Channel, this structure has been modeled as a fold created by a ramp in a sub-horizontal thrust fault. However, investigation of this structure offshore in the Santa Barbara Channel using exploratory oil well and seismic reflection data indicates that the structure is instead created by a steeply south dipping fault. This interpretation is consistent with the structure observed onshore, and results in a very different assessment of the seismic hazard posed by the Oak Ridge fault compared to the hypothesized fault-bend-fold origin of the structure.

Fault Geometry of Blind Thrusts Along the Oak Ridge Trend in the Santa Barbara Channel (SCEC USC 572726) (Marc J. Kamerling , Craig Nicholson)

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 [e.g., Yeats et al., 1994]. 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 [1994] 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 [Suppe and Medwedeff, 1990].

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 [1994] 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, well below the sedimentary structure where such bedding-planes do not 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 [Suppe and Medwedeff, 1990] above a steeply south-dipping fault, rather than as a fault-bend fold [Shaw and Suppe, 1994]. 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 we believe 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.

Integrated Study of Seismic and Well Data in the Ventura Basin and the San Fernando Valley for Active Subsurface Faults (NSF EAR-9416194) (Marc J. Kamerling , Craig Nicholson )

The 17 January 1994 M6.7 Northridge earthquake occurred on a blind, south-dipping fault beneath the San Fernando Valley. This structure is believed to be associated with the same Oak Ridge fault trend that extends farther west into the Ventura Basin and out into the Santa Barbara Channel. Similar active blind faults represent a significant seismic hazard to many other communities in California, as indicated by the devastating earthquakes near Santa Barbara in 1925, near Coalinga in 1983, and near Whittier in 1987, yet this seismic hazard is still not well resolved. The use of high-quality seismic reflection data, however, can provide tremendous insight into the nature of such subsurface fault systems through the analysis and interpretation of near-surface deformation that occurs in response to deep crustal faulting. The major problem is that such high-quality seismic data is extremely expensive to acquire, and, in many cases, areas of particular interest are situated in regions where it is physically impossible to collect such data today, at no matter what the cost. This is because these areas are now either highly populated or because new environmental regulations would preclude such data acquisition activities today.

As a result of a recent bankruptcy settlement, a unique and irreplaceable dataset of seismic reflection profiles is now being made available at a fraction of the original cost it took to collect the data. This is the California dataset from GTS Corporation. The dataset consists of seismic reflection lines shot with dynamite in areas before many of the current restrictions were put in place, and includes lines in the Ventura Basin, the San Fernando Valley, and the Los Angeles Basin, as well as additional profiles throughout the Great Valley and other parts of the Transverse Ranges. Although these lines were limited to regions thought to have hydrocarbon potential, because folds above active faults have proven to be excellent hydrocarbon traps, many of these seismic lines were shot directly above active subsurface faults in those areas. This project consists of two parts: 1) purchase of the GTS California dataset; and 2) analysis of a subset of the available data for active subsurface structures in the Ventura Basin and San Fernando Valley that are directly related to the Northridge event. ICS will act as the purchasing agent for the GTS data, although the data license will be held by the Southern California Earthquake Center. This will allow other member institutions to use the data under a single-purchase license. Purchase of the data will thus provide a valuable asset to the scientific community that is far greater than the specific aspect of the data that is relevant to the Northridge event, and at far less cost than the data subset related to Northridge could be duplicated today by any other means. In addition, because of the amount of data coverage, this dataset will be invaluable to the generic problem of investigating blind faults in other areas of California and to assessing their seismic hazard potential.

ICS has successfully acquired the GTS data in both analog and digital form, and is now conducting an integrated investigation of those seismic lines in the Ventura Basin. Preliminary analysis reveals that the GTS data need to be carefully tied to subsurface structure and stratigraphy by using available drill-hole data. We are currently negotiating the acquisition of such data files that can provide the necessary subsurface control. The 2-D subsurface structure models [e.g., Yeats et al., 1994] previously identified for the Ventura Basin and San Fernando Valley will be used as a starting point for our subsurface analysis. These results will then be compared with the pattern of active subsurface faults defined by recent earthquake activity. These studies will be combined with on-going investigations of faulting in the offshore Santa Barbara Channel, that tie directly to the onshore structures we will investigate using the GTS data. This will help provide a basis for understanding the geometry, tectonic development, and seismic hazard potential along strikes of such major fault and fold systems as the Oak Ridge trend.

Quaternary Evolution of the E. California Shear Zone Between Latitudes 36 Degrees and 39 Degrees (NSF EAR95-26016) (Jeff Lee)

An integrated geological investigation is proposed to test and refine a new kinematic model for the Quaternary evolution of eastern California shear zone (ECSZ) north of the Garlock Fault. Field-based investigations during 1995 and 1996 will focus primarily on a series of little studied northeast-striking, northwest-dipping normal faults that link the dominantly right lateral Owens Valley and Hunter Mountain-Panamint Valley fault zones with the right lateral northern Death Valley-Furnace Creek fault zone. Integrated investigations including geologic mapping, geomorphic, paleoseismic, structural and kinematic studies, and age determination of offset Late Pliocene to Holocene units will be performed along these displacement transfer normal faults to document the number of prehistoric earthquakes, the magnitude and kinematics of slip, average slip rates and the age of initial fault activity. The results from the proposed studies of this zone will dramatically improve understanding of the fundamental mechanisms that control fault interactions and slip partitioning between parallel strike-slip faults and connecting normal faults. An improved understanding of these processes will have significant implications for seismic risk assessment.

Glacial Marine Stratigraphy in the Eastern Ross Sea and Western Marie Byrd Land, and Shallow Structure of the West Antarctic Rift (NSF OPP-9316712) (Bruce P. Luyendyk)

There are two goals of this project; 1) to map structures in the continental margin of western Marie Byrd Land resulting from both Gondwana and Late Tertiary rifting; and 2) to map the glacial marine stratigraphy on the continental shelf of western Marie Byrd Land. Objective (2) is primarily the responsibility of Prof. Louis Bartek from the University of Alabama who is co-PI on the project with Luyendyk. The field work for this project will take place along the Saunders Coast during the Antarctic Summer of January and February, 1996, using the icebreaker N. B. Palmer. Seven undergraduate students from Geological Sciences will be participating.

Work to date has been to develop background materials in preparation for field work. We made a preliminary bathymetric map with data from the three cruises in the Saunders Coast region; Deep Freeze 1961/62; Deep Freeze 1983; and Polar Queen 92/93 (GANOVEX VII). These reveal an overdeepened margin typical of Antarctica. Also, several 700 meter-deep linear troughs were located that appear to be of glacial origin. Linear banks and troughs are possibly fault-controlled. This mapping is continuing with supplemental data in the surrounding region. These data include a bathymetric compilation for the Ross Sea by Fred Davey; we are joining his data to the Saunders Coast data. We are also preparing Bouguer gravity maps using onshore gravity data collected in 1966/67 by John Beitzel and 1992/93 by Luyendyk. These mapping projects are being pursued by REU interns Kirsten Zellmer and Carmen Alex. Jill Sandlin is assisting in this work.

The historical sea ice cover offshore western Marie Byrd Land is of critical importance in our planning. Luyendyk and REU intern Erik Vanek are studying this with satellite imagery. We have viewed both passive microwave and AVHRR images. Vanek constructed an animation of daily ice cover for the 1992/93, 1993/94 and 1994/95 seasons from microwave data. These clearly show the retreat of ice along the Saunders Coast in mid January. The retreat is slow compared to a sudden refreezing seen in early March of all years. The retreat is variable for different years, with the 1994/95 retreat the most pronounced. We are now generating a Quicktime movie of the animation so that it may be freely viewed by colleagues. The ice cover is highly variable nonetheless, and we are investigating whether phenomena such as El Niño year intensity might be predicting parameters.

Luyendyk also investigated seismic processing system alternatives that could be useful on the Palmer which has a 48 channel OYO DAS One recording system. Seismic Processing Workshop (SPW) from Parallel Geophysics was selected for purchase by OPP.

Thrust Ramps and Detachment Faults In The Western Transverse Ranges (SCEC USC 572726) (Bruce P. Luyendyk, Ralph J. Archuleta)

This SCEC workshop dealt with the seismic hazard in the western Transverse Ranges in the light of proposals that this region is underlain by large low angle fault planes. The thick-skinned (reverse faults) vs. thin-skinned (ramp and flat) interpretations of seismogenic faults lead to very different assessments of seismic hazard. The workshop demonstrated that first order questions exist concerning thick skin versus thin skin tectonics for the region. Perhaps the most important question is, what direct evidence exists for thrust ramps in the western Transverse Ranges? Have they been penetrated by wells? How are ramps and detachments best imaged? The LARSE experiment, if it is successful in imaging low-angle faults directly, is our best hope in terms of an entirely new data set that images these faults.

Other issues were raised at the workshop including: Is a simultaneous rupture on the San Andreas and western Transverse Ranges thrust faults a reasonable scenario; has it happened in the past; what is the evidence? The 1957 Gobi Altai earthquake in Mongolia may be a model for this possibility. Scenarios in the LA Basin area might include the eastern segment of the Elysian Park along with the Whittier fault, or the San Andreas (San Bernardino segment) along with the Cucamonga and/or Sierra Madre thrusts. What is the history of activity on faults within the western Transverse Ranges; what are the paleoseismic records for such faults as the Mission Ridge-Arroyo Parida, Santa Ynez, Santa Cruz Island, etc.? What is the relation between proposed thrust ramps and reverse faults and older (?) Miocene detachment and normal faults? What is the relation to basement structures and geology? Did Miocene extension create anisotropy in the crust, and is this significant in the initiation of thrust ramps? Where is shortening occurring in the Santa Barbara Channel; what faults are absorbing this? How does the E-W trending high-velocity mantle Vp anomaly relate to the tectonics of the western Transverse Ranges and particularly any thrust faulting?

Review of earthquake data at the workshop suggested that the moment-release rate for the entire western Transverse Ranges as a whole is similar to the observed geodetic strain rate. Thus, we would not necessarily expect any more (larger or more frequent) earthquakes than we have had over the last 200 years or so, since these rates are approximately the same.

Microplate Capture, Large-Scale Rotations, and Initiation of the San Andreas Transform: Test of a New Tectonic Model (NSF EAR94-05261) (Craig Nicholson, Chris Sorlien, Marc J. Kamerling)

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. We 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 American 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 America 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 will test this new tectonic model by investigating the relationship and interaction of offshore oceanic plates with respect to the known geology of western North America. We will utilize a unique geological database already compiled for the Tectonic Map of North America by AAPG [Muehlberger, 1992]. 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 our 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.

Late Neogene Deformation in the Santa Barbara Channel and Ventura Basin (SCEC USC 572726) (Chris Sorlien)

Reprocessing of USGS-808 was completed by Erick McWayne. This NNE-SSW seismic reflection profile crosses Santa Barbara Channel from western Santa Rosa Island to west of Campus Point at UCSB. A depth-converted section was created. An important result is the documentation of gentle dip. Very little contraction could have occurred during the last few million years along this profile. Another important result is the mapping of the Santa Cruz Island fault for 50 km through the southwest part of Santa Barbara Channel. This fault is continuous, south-dipping, and south-side-up on Quaternary sediments in this area.

A depth map of a 6 Ma horizon beneath Santa Barbara Channel was restored to an unfolded state with J. Scott Hornafius and Bruce P. Luyendyk. The software and technique of J.-P. Gratier was used to flatten folded surfaces within fault blocks. The unfolded surfaces were then fit together across faults. Comparing the present day surface to the restored surface gives the displacement with respect to a fixed block. Sorlien combined these results with the restoration of onshore Ventura basin by Gratier. The combined results indicate that left-lateral motion has occurred along the NE-striking segments of the Oak Ridge fault. The average post-6 Ma rate of shortening across the 30 km width of the restored area is near zero in the west and 1 mm/yr in the east.

Sorlien worked on the ICS-LLNL-MMS evaluation of seismic hazard near Santa Barbara Channel. An outgrowth of that study was evaluation of models for blind faulting and related folds. A new model was developed with researchers at Lamont-Doherty Earth Observatory and with Nicholas Pinter.

High Resolution Global Body Wave Waveform Inversion for the Mantle and Core (NSF EAR-9296218) (Toshiro Tanimoto)

Research supported by this grant sought to improve our understanding of the global, three-dimensional structure in the mantle and core. In particular, efforts to understand the structure near the Core-Mantle boundary, both above and below the boundary, were the focused target. Understanding this region is a key to understanding geomagnetism as well as geodynamics. A new model of mantle and uppermost core was obtained with the collaboration of Monica Kohler at Caltech (now at UCLA). Resolution tests as well as other examinations were performed and various problems were understood. Although the model is not final and future improvements are possible with additional data, a path to better understanding in the future was clearly indicated by the study.

Large-scale Oceanic Upper Mantle Structure and Constraints on Ridges and Hotspots (NSF-OCE9296207) (Toshiro Tanimoto)

In order to understand geodynamics in the lithosphere and asthenosphere, it is essential to obtain seismic velocity structure in the upper 200 km of the Earth. This project focused on retrieving shallow (0-200 km) mantle structure, aiming to retrieve hotspot and ridge signatures in seismic maps. Long wavelength features, down to the wavelength of 800-1000 km, were obtained and its tectonic implications were explored using various physical models.

Evolving Thermal Regimes During Collisional Orogenesis (NSF EAR-9405560) (James Vogl, William McClelland, Phil Gans, Brian Patrick)

This year represented the second field season of our three year NSF-funded study of orogenic processes in the Brooks Range, Alaska. 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, we completed a structural transect in which we were able to document increases in amount of strain and intensity of fabric development between the fold-thrust belt and the metamorphic core during the dominant north-directed phase of deformation. This work has also documented widespread south-vergent deformation which may have also affected the metamorphic core. Thermochronologic studies were initiated and combined with similar studies in the southern part of the metamorphic core (B. Dinklage, B. Patrick and A. Till) will provide a detailed cooling/exhumation history for the core. With our integrated structural, metamorphic and thermochronologic approach we should enhance our understanding of not only the poorly-studied Brooks Range, but also of processes in collisional orogens.

Aseismic Creep and Crustal Deformation of Large Normal Faults: Precise Leveling Across Teton Fault, Wyoming (EAR 9303913) (Arthur Sylvester)

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, my undergraduate students and I 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 10 mm aseismically relative to the footwall (Teton Range) in 1988-89, probably due to poro-elastic effects caused by refilling Jackson Lake, and the valley tilted toward the mountains. Between 1991 and 1993, however and to our surprise, 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. We postulated that the tilt was caused by non tectonic, asymmetric lowering of the water table engendered by the current drought in the area.

With supplementary funds awarded to this grant, we 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 non tectonic 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.

Nearfield Geodetic Investigations of Crustal Movements in Southern California (USGS 1434-93-G2290) (Arthur Sylvester)

The long-term, fixed purpose of this investigation is to search for and monitor the spatial and temporal nature of nearfield displacement across active and potentially active faults.

The surveying array measurements yield data on the amount of surficial preseismic, coseismic and post-seismic horizontal displacement at a scale intermediate between that obtained by GPS and by existing USGS geodimeter networks and what might be gained from study of offset tire tracks, stream gulches, and similar imprecise and ephemeral markers at the time of the earthquake. Leveled alignment array give detailed information on the distribution of horizontal and vertical displacement, which, in turn, provides information on fault slip. We survey and maintain 63 short leveling arrays in California ranging in length from 250m to 7000m and ranging in geometry from straight lines to L-, Z-, W-, and box-shapes.

We concentrated on four main tasks during 1994: 1) establish and survey new leveling lines across the Sierra Madre frontal fault; 2) resurvey the CDMG geodetic array across the Cucamonga fault at the mouth of Day Canyon; 3) establish and survey a leveling line across a growing fold in Quaternary gravel above an area of abundant aftershocks of the 28 June 1992 Landers 1992 earthquake (M=7.3); and 4) resurvey of all of our trilateration arrays established across surface ruptures related to the 28 June 1992 Landers earthquake to search for continued afterslip. We also resurveyed 20 leveling arrays and 5 trilateration arrays, and we spent several days in the 1994 Northridge earthquake area searching for surface ruptures of tectonic origin that might yield measurements of post seismic slip.

Earthquakes did not occur during the contract period on those parts of the San Andreas fault where we had existing geodetic arrays, and we did not measure any nearfield vertical displacements across faults that we can attribute to tectonism, most particularly at the southern end of the San Andreas fault in the Salton Trough since the 1992 Landers earthquake sequence. 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 our surveys, or it is manifested at an areal scale beyond that at which we survey.

Thus, most of the vertical relief, which is so prevalent along parts of California strike-slip faults, probably forms coseismically as it did in some California earthquakes involving strike-slip (1940 and 1979 Imperial Valley; 1992 Landers) and thrusting (1971 San Fernando) rather than by vertical creep. It may also mean that the shortening component of transpressive strain manifests itself neither as vertical displacement at a strike-slip fault, nor as bending near the fault. Other authors believe the shortening component may be spread diffusely across a zone as much as 100 km wide across a strike-slip fault, well beyond the limited range of our nearfield arrays.