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)
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.