Download Abstract of the Dissertation-JNL

The application of active-source seismic imaging techniques to transtensional
structures in the Salton Trough and Walker Lane
Abstract of the Dissertation
The plate margin in the western United States is an active tectonic region that contains
the integrated deformation between the North American and Pacific plates. Nearly
focused plate motion between the North American and Pacific plates within the northern
Gulf of California gives way north of the Salton Trough to more diffuse deformation. In
particular a large fraction of the slip along the southernmost San Andreas fault ultimately
bleeds eastward, including about 20% of the total plate motion budget that finds its way
through the transtensional Walker Lane Deformation Belt just east of the Sierra Nevada
mountain range. Fault-bounded ranges combined with intervening low-lying basins
characterize this region; the down-dropped features are often filled with water, which
present opportunities for seismic imaging at unprecedented scales. Here I present activesource seismic imaging from the Salton Sea and Walker Lane Deformation Belt,
including both marine applications in lakes and shallow seas, and more conventional
land-based techniques along the Carson range front.
The complex fault network beneath the Salton Trough in eastern California is the on-land
continuation of the Gulf of California rift system, where North American–Pacific plate
motion is accommodated by a series of long transform faults, separated by small pullapart, transtensional basins; the right-lateral San Andreas fault bounds this system to the
north where it carries, on average, about 50% of total plate motion. The Salton Sea
resides within the most youthful and northerly “spreading center” in this several
thousand-kilometer-long rift system. The Sea provides an ideal environment for the use
of high-data-density marine seismic techniques. Two active-source seismic campaigns in
2010 and 2011 show progression of the development of the Salton pull-apart sub-basin
and the northerly propagation of the Imperial–San Andreas system through time at
varying resolutions. High fidelity seismic imagery documents the timing of strain transfer
from the Imperial fault onto the San Andreas fault through the application of sequence
stratigraphy. Evidence shows that the formation of the Salton and Mesquite sub-basins
and the associated change of strain partitioning occurred within the last 20-40 k.y.,
essentially modifying a broader zone of transtension bounding the Imperial and San
Andreas faults into two smaller zones of focused extension. The sedimentary infill within
the Salton sub-basin has been modified through hydrothermal alteration and magmatism
providing new insight into the formation of crust within a basin characterized by thick
sediments and high heat flow. Modern earthquakes beneath the Salton Sea proper appear
to nucleate at times within this altered sedimentary pile, providing constraints on the
degree of metamorphism of lake sediments that now comprise “basement.” Stratigraphic
relationships within the northern Salton Sea also provide evidence that the southern San
Andreas fault locally dips to the southwest and this geometry has persisted for at least the
past 200-300 k.y.
The north-central Walker Lane contains a diffuse network of both strike-slip and normal
faults, with some degree of strain partitioning characterized by normal faulting to the
west along the eastern edge of the Sierra Nevada mountain range, and strike-slip faults to
the east that define a diffuse boundary against the Basin and Range proper. A seismic
study across the Mount Rose fault zone, bounding the Carson Range near Reno, Nevada,
was carried out to investigate slip across a potential low-angle normal fault. A hammer
seismic reflection and refraction profile combined with airborne LiDAR (light detection
and ranging) imagery highlights fault scarp modification through minor
slumping/landslides, providing a better understanding of the nature of slip on this fault.
The seismic data were processed using both a linear and a non-linear velocity inversion
scheme; both techniques indicate a shallow low velocity zone suggestive of a landslide
section. The combination prestack depth migration (PSDM) reflectivity and LiDAR
imagery suggests that a high-angle normal fault scarp could have been modified at the
surface by at least one landslide event, allowing doubt that slip was isolated at the top of
the Hunter Lake Formation (an ~33° dipping plane).
The northeastern margin of the Walker Lane is a region where both “Basin and Range”
style normal faults and dextral strike-slip faults contribute to the northward propagation
of the Walker Lane (essentially parallel to an equivalent northward propagation of the
Mendocino triple junction). Near this intersection lies Pyramid Lake, bounded to the
southwest by the dextral Pyramid Lake fault and to the northeast by the normal Lake
Range fault. A high-resolution (sub-meter) seismic CHIRP survey collected in 2010
shows intriguing relationships into fault architecture beneath Pyramid Lake. Over 500
line-km of seismic data reveal a polarity flip in basin structure as down-to-the-east
motion at the northern end of the Pyramid Lake fault rapidly gives way to down-to-thewest normal motion along the Lake Range fault. Alternating patterns of asymmetric and
symmetric stratal patterns west of the Lake Range fault provides some evidence for
segmentation of total slip along this large normal fault. Using dated sediment cores, slip
rate for the Lake Range fault was found to be approximately 1 mm/yr during the
Holocene. A complex zone of transtenstion was also observed in seismic CHIRP data in
the northwest quadrant of the lake, where short, discontinuous faults hint at the
development of a nascent shear zone trending to the northwest.
Introduction
The western United States (Figure 1) contains a dynamic range of fault structures
beginning in southeastern California where the Gulf of California (GOC) rift system
hands off the majority of transform motion between the Pacific and North American
plates onto the San Andreas system. The on-land progression of the GOC rift, including
the Cerro Prieto and Imperial faults, truncates into the Salton Trough (Atwater and
Stock, 1998). This larger Imperial–San Andreas step-over within the Salton Trough is
the northern-most extension of the GOC spreading centers (Persaud et al., 2003;
Wesnousky, 2005; Brothers et al., 2009). Northward, a large fraction of slip along the
southernmost San Andreas fault (SSAF) is bled off through a series of left-lateral fault
systems (e.g., Pinto Mountain fault) into the Eastern California Shear Zone, Walker Lane
and Basin and Range proper (Savage et al., 2001; Peltzer et al., 2001). The San Andreas
fault (SAF) then regains some of this slip deficit at the merger of the San Jacinto and San
Andreas fault systems, near San Bernardino, California.
To the north, in the Walker Lane (dashed lines in Fig. 1), the fault dynamics continue
with a transtensional regime (Wesnousky, 2005; Unruh et al., 2003). Beginning north
of the Mojave Block, the fault configuration accommodates a combination of normal
fault motion and dextral shear with minor amounts of northeast-directed sinistral shear
(Dokka and Travis, 1990). Global Positioning System, GPS-derived geodetic data show
that ~20% of the motion between the Pacific and North American Plates is accounted for
within the 100 km wide Walker Lane Deformation Belt (Bennett et al., 2003; Hammond
and Thatcher, 2004; Thatcher et al., 1999; Unruh et al., 2003). Plate kinematic data also
suggest that over time, the Walker Lane will continue to propagate northward, acquiring
more of the North American–Pacific plate motion and ultimately evolve into a system
mimicking the GOC (Faulds et al., 2005).
The combination of study areas, one in a region that is in the process of active rifting
within a heavily sedimented basin with high heat flow, and one in an incipient form,
offers a view of the time development of rifting. Active-source seismic data presented in
this dissertation give insights to how strain is transferred between faults beneath the
Salton Sea, California and Pyramid Lake, Nevada. Land-based seismic data, combined
with LiDAR imagery, help to constrain the nature of slip on a range-bounding normal
fault within the Mount Rose fault zone, near Reno, Nevada to test the applicability of this
strain-transfer model.
Summary of Dissertation Chapters
The first of two Salton Sea manuscripts presented in this thesis highlights seismic
stratigraphic analysis of two large-scale marine campaigns to collect high quality
multichannel seismic data from the Salton Sea (labeled in Fig. 1). The first deployment
acquired approximately 1.2-kilometer-deep records using a 24-channel multi-channel
seismic streamer. Instruments were deployed from a 24-ft (7.5-m) SeaArk vessel owned
and operated by U.S. Geological Survey (USGS), Salt Lake City. The seismic equipment
was contracted from SubSea Systems, Inc. and included a 24-channel GeoEel streamer
(coiled in Fig. 2), the “sparker” source (Fig. 2), and all onboard equipment used for shot
timing, source power and data recording. The 350+ line-kilometers of data were collected
over a two-week period in May of 2010. Recording equipment remained in the cabin of
the vessel, allowing a small crew to operate the survey. During much of the deployment,
3-4 investigators were on board including Mike Barth (SubSea Systems Inc.), Robert
Baskin (USGS Salt Lake City) and Annie Kell (Univ. of Nevada Reno, dissertation
author) with Principal Investigators (Graham Kent, Neal Driscoll and Alistair Harding)
attending when possible.
This marine seismic deployment allowed a relatively small crew to collect a large number
of survey lines along and across this roughly 45-km-long by 10-km-wide body of water.
The streamer (coiled in Fig. 2) was fed off the back of the boat by a single person with an
additional person attaching steamer-leveling “birds” (orange objects in Fig. 2) used to
control the streamer elevation in the water while in tow (ideally 2-3 m below the surface).
The seismic source (1.6 kJ three-tip “sparker,” Fig. 2) was towed at a similar depth
between the boat and the streamer. Once the equipment was deployed, surveying required
only that the recording be monitored and that the boat was steered to maintain the ship
track through interactive GPS-based navigation. While in tow, the entire receiver array
(hydrophone groups spaced 3.125 m apart) is moved for each source signal (by the same
3.125 m) allowing for a high density of shot/receiver pairs and making long surveys
covering a large area relatively easy.
The second field acquisition occurred in April 2011 as part of the Salton Seismic Imaging
Project (SSIP). The SSIP was an NSF-funded collaborative project (Margins Program)
between a number of research institutions (Virginia Tech, Caltech, USGS, UCSD/Scripps
and UNR) to study the crustal structure beneath the Salton Trough using various seismicarray and source pairs, ranging from 2000 lb (900 kg) explosive shots to thousands of
Texan geophone recorders, and 210 cu. in. (3.4 l) GI-gun sources to 38 Ocean-Bottom
Seismometers (OBSs) . This second manuscript presents the marine portion of the
experiment, which utilized a 48-channel multichannel-seismic (MCS) streamer towed
behind a large barge and 38 OBSs deployed in 78 unique locations. This deployment
used a generator-injector air-gun source fired at a 1-minute repetition rate.
In the marine seismic campaign of 2011, instruments were deployed from a 100-ft (30-m)
barge (shown in Figure 3) that was towed by a 55-ft (17-m) motor vessel. The barge
served as the equipment deployment and tow point, provided room for the recording and
source equipment, and storage of excess racks of equipment. Mobility in this survey was
limited when compared to the 2010 campaign due to the logistics of towing such a large
barge. In contrast to the 3-man-operation in the 2010 survey, the 2011 campaign required
about a dozen skilled workers, including science investigators, OBS field technicians and
boat operators. The longer MCS streamer (300-m-long streamer) made retrieval by a
single person infeasible, and the size of the source signal required a relatively large aircompressor that necessitated additional skilled personnel capable of servicing such
equipment.
The ~175 line-km of MCS profiles collected in 2011 provided reflection cross sections to
~4 km depth after applying a similar processing as implemented for the 2010 survey.
Critical lines from these 2 surveys are shown in Chapter 2. These data give constraints to
the timing of propagation and strain partitioning within the larger Imperial–San Andreas
pull-apart system. The reflection images show the depth extent of the deformation in the
seismic stratigraphy and give constraints on the timing of basin development through the
application of sequence stratigraphy.
Processing of the MCS reflection records included common mid-point (CMP) stacking,
filtering and spectral deconvolution. In stacking, all similar location points in the
subsurface (i.e., CMP geometry) are combined to produce a single cross section that
shows subsurface structure. Some regions in the cross section show negative effects of
gas (potentially biogenic in origin) where signal is effectively wiped below the seafloor
or lower stratigraphic unit. In these environments, spectral whitening and deconvolution
help to resolve horizons that would otherwise be too attenuated to interpret.
The OBS array recorded continuously during the SSIP. Research groups that were
working on land are presently using both land-sourced and marine refraction records to
study the deepest sections of the crust (to ~20 km depth) beneath the Salton Trough to
investigate crustal thinning associated with extension. Chapter 3 presents my results
using OBS receiver gathers generated from the air-gun source array. P-wave (first) arrival
times were picked from the receiver gathers and inverted to create a series of 2–D
velocity cross sections co-located with the prominent reflection profiles. The methods of
Van Avendonk et al. (2001) were implemented to calculate travel time arrivals between
source and receiver through an initial starting model. Misfit in travel time was used to
invert for changes or perturbation in the velocity model in a linearized fashion (with a
roughness penalty) to reduce the χ2 misfit error between the picks and predicted arrival
times within an updated velocity model. A series of MATLAB modules were used in
combination with shell scripts to create the travel time and error calculation for each
model iteration.
The P-wave velocity inversion for line 1 is shown in Chapter 2, which addresses the time
evolution of strain between the Imperial fault and SAF. Inversions from a handful of
profiles in the southern Salton sub-basin were studied independently in Chapter 3 to
analyze the velocity structure in this most actively deforming portion of the Salton
Trough, which provides insight into the hydrothermal alteration of sediments within this
high heat flow region. These “cooked” and likely magmatically intruded sedimentary
rocks have high P-wave velocities of around 4-5 km/s, and host significant earthquake
nucleation activity highlighting the degree of alteration.
Chapter 4 presents cross sections from the Mount Rose fault zone using land seismic
methods (location shown in Fig. 1; survey shown in Fig. 4). While marine methods move
the receiver array in conjunction with the source signal, land techniques require that the
receiver array remain stationary while the source is moved along the profile. While the
logistics of land-based operations may reduce survey complications, the data collection
process is much more labor intensive for an even reduced survey length compared to
marine acquisition.
For the survey presented in Chapter 4, 48 active channels recorded input from an array of
6 geophones planted for each channel in an alignment that reduces the effect of seismic
waves traveling along the surface of the ground. The seismic signal was created using a
16-lb (7-kg) sledgehammer on a steel plate. Each source location was the summation of
10 hammer strikes, and this survey accomplished 218 source locations moved along the
survey line at varying distances. To accomplish the 250-m-long profile, the geophone
array had to be moved in a leapfrog manner and some source points re-shot accordingly.
These land based reflection/ refraction data were used for two velocity analysis schemes
and then advanced migration/imaging techniques were applied to produce a reflectivity
cross section for each velocity section. The velocity cross section can be interpreted
independently from the migrated reflectivity section, or combined, to identify numerous
parts of the sub-surface structure such as sedimentary horizons and landslides.
Chapter 4 also uses new LiDAR images to complement the seismic cross sections. The
LiDAR data were shared for academic purposes by Washoe County, Nevada and were
collected to update high resolution, orthorectified photography of the Truckee
Meadows and outlying areas. Results from the Mount Rose fault study reveal details
of the nature of slip on this range-bounding fault in the Walker Lane.
Lastly, high-resolution Compressed High-Intensity Radar Pulse (CHIRP) data were
collected in 2010 on Pyramid Lake, Nevada (location shown in Fig. 1). The twoweek-long data campaign deployed the self-contained Edgetech Subscan CHIRP unit
off the previously mentioned 24-foot-long (7.5-m) USGS-owned and operated vessel
(shown with CHIRP in Fig. 5) and provided 500+ line-kilometers of high-resolution
(sub-meter) data. Chapter 5 shows portions of this study and focuses on basin fault
architecture, changes in fault polarity, fault segmentation and slip-rate calculation.
The following chapters go into the specifics of these studies including the processing
methods and interpretations. The chapters are drafts of multi-authored manuscripts,
which form the basis of my dissertation. As such, this dissertation presents only the work
where I led the data collection, processing and interpretations (Chapters 2, 3 & 4) or
where I co-led an experiment, as was the case at Pyramid Lake(Chapter 5), which was a
shared project with M.S. graduate student Amy Eisses.
Additional work during the course of my dissertation involves co-authorship on
additional studies throughout the study region including Fallen Leaf Lake (Maloney
et al., 2013), Cascade Lake, Lake Tahoe, Walker Lake, more work in the Salton Sea
and the Cascadia Subduction Margin in the Pacific Ocean. Fieldwork in Fallen Leaf
Lake involved collection of sediment cores (field rig shown in Fig. 6), multi-beam
bathymetry and CHIRP. Lake Tahoe, Cascade Lake and Walker Lake involved
participation in CHIRP data collection, while the 2-week excursion in the Pacific
Ocean acquired industry-quality deep marine MCS data. I was also involved with
land acquisition of critical multichannel seismic data in the Reno basin, to map faults
within this populated region.
Conclusions
The combination of active-source seismic data from the Salton Sea to the Walker Lane
provides insight into the way that strain partitions itself within transtensional systems and
how fault motion captures these forces through seismic images of stratigraphy. The
insights gained in the Salton Sea offers new information on the progression of the
Imperial–San Andreas pull-apart system, through the initiation of the Brawley fault that
focused a diffuse pull-apart system into two smaller basins (i.e., Salton and Mesquite). Pwave velocity tomographic images, along with coincident reflectivity images also
highlight the high-degree of alteration to the shallow-most crust that occurs when
sediments thermally insulate a rapidly deforming region of high heat flow.
The land seismic study on the Mount Rose fault brings further understanding to how slip
occurs on the densely faulted Carson range frontal system near Reno, Nevada. Seismic
CHIRP data in Pyramid Lake highlights the complication of fault stepovers where normal
movement is inherited at the ends of strike-slip faults when transtension is present. These
remarkable subsurface images also provide a unique view into basin architecture that
includes polarity flips, fault segmentation and development of a nascent shear zone. The
largest fault, the Lake Range fault, has slip-rate of ~ 1.0 mm/yr and is capable of M7
ruptures.
Figure 1: Map of the western United States with study locations highlighted in red boxes.
Chapters 2 and 3 are focused in the Salton Sea near the US/Mexico border. Chapter 4 is
at the base of the Carson Range (shown in chapter 4 figures) near Reno, NV. Chapter 5
presents work from Pyramid Lake, NV. The purple dashed lines show the border of the
Walker Lane Deformation Belt. Abbreviations: IF, Imperial fault; SAF, San Andreas
fault; GOC, Gulf of California.
Figure 2: Multi-channel seismic streamer equipment on board the boat during transport.
The 24-channel GeoEel Streamer is uncoiled and fed off the stern of the boat, with the
leveling birds attached at the end and halfway along the streamer. The “Sparker” source
is also tied off the stern between the boat and the closest streamer channel.
Figure 3: The 100-ft (30-m) barge in the Salton Sea that served as the equipment
deployment point and a floating laboratory. It was used to store equipment when not
surveying, to re-set OBSs for re-deployment and to carry all instruments necessary for the
survey.
Figure 4: The land-based hammer seismic survey within the pediment of the Carson
Range. The survey equipment is highlighted including the 100-Hz geophones, the steel
plate and the 16-lb (7 kg) sledge hammer used as the seismic signal.
Figure 5: The CHIRP system used in a series of shallow marine seismic surveys is easily
transportable on the 24-ft (7.5-m) SeaArk.
Figure 6: Other fieldwork opportunities included coring on Fallen Leaf Lake. The coring
system included construction of a barge platform and hydraulic lift system to drop the
gravity core. These core data were used to correlate horizons from CHIRP cross sections
in Fallen Leaf Lake.