Cenozoic Tectonic History of the Hosgri fault zone,
Offshore California, from Seismic Imaging
and Stratigraphic Analysis
William Honjas, John N. Louie, and Sathish K. Pullammanappallil
Seismological Laboratory 174, Mackay School of Mines,
The University of Nevada, Reno, NV 89557-0141;
honjas@seismo.unr.edu; http://www.seismo.unr.edu
ABSTRACT
Performing post-stack and pre-stack migration of data obtained from seismic
reflection line RU-3 constrains the character, the nature of deformation and the downdip geometry of the Hosgri fault zone. RU-3 trends NE and crosses the fault zone in
San Luis Obispo Bay north of Point Sal. A suite of post-stack images were calculated at
different migration velocities were produced for analysis. Pre-stack migration utilized
the Kirchhoff summation method. This method takes into account strong lateral
changes in velocity across the fault zone, which are assumed not to exist in the stacking
process. The velocity model used in performing the Kirchhoff migration is derived from
a simulated- annealing Monte-Carlo inversion of RU-3 first arrival picks. This model
constrains shallow lateral velocity variations to 0.2 km/s or better accuracy, and is
consistent with the stacking velocities.
The Kirchhoff pre-stack images reveal at least three fault planes within the
Hosgri fault zone : 1) A central fault that shallows in dip from about 70 degrees near the
surface to about 60 degrees at depth; 2) A west branch that dips at about 60 degrees to
the east and intersects the central branch at depth; 3) and a near-vertical east branch
which intersects the central branch at about 1 km depth. The Hosgri fault zone appears
to have undergone reorientation from being a basin-bounding, southwest dipping
normal fault during the Miocene to a near-vertical, northeast dipping predominantly
strike-slip fault in the Cenozoic. Observation of the seismic images suggests ongoing,
plastic deformation or detachment at about 10 to 12 km depth. Vertical offset near the
surface since 5 Ma is limited to less than 300 m.
Through stratigraphic analysis, we reconstruct the tectonic history of the southcentral coast of California. Evidence indicates the Hosgri fault zone is currently a
structural boundary separating transpressional tectonics to the east from orthogonal
compression to the west.
INTRODUCTION
In this paper we perform post- and pre-stack migration imaging of seismic
reflection data from profile RU-3 to assess the down-dip geometry and deformational
history of the Hosgri fault zone along the south-central coast of California (figure 1).
Profile RU-3 trends northeast across the fault in San Luis Obispo Bay north of Point Sal.
In this area, the fault separates the offshore Santa Maria basin to the west from the
onshore Santa Maria basin to the east between the structurally northwest-trending
southern Coast Range province and the east-west trending Transverse Ranges province
(figure 2). The area is a significant hydrocarbon-producing coastal basin with almost 1
billion bbl of estimated ultimate reserves. The Santa Maria Basin, and adjacent coastal
basins of southern California, contain thick sections of sediments, such as the prolific
Monterey Formation, which display evidence of tectonic deformation during and after
their deposition. Seismicity within and around the Santa Maria basin, along with data
provided by geodetic measurements and deformed Quaternary deposits, testify to
ongoing tectonic activity (e.g., Namson and Davis, 1990; Lettis and Hanson, 1992).
The geometry and history of the Hosgri fault zone, and therefore its role in the
current development of the Santa Maria Basin, have been the subject of intense debate.
Models have been developed which postulate that the south-central coast of California
is dominated by compressional tectonics resulting in basement-involved, west directed
thrusting (Crouch et al., 1984; Namson and Davis, 1990; McIntosh et al., 1991). Other
analyses propose that the region is dominated primarily by transpressional tectonics
(PG&E, 1991). In the later case, the Hosgri fault zone is characterized as a steeply
dipping, predominantly strike-slip fault. Differing theories on the down-dip geometry
of the Hosgri fault zone, based on geophysical evidence, parallel the controversy
surrounding the tectonic assessment of the fault. Images produced from seismic
reflection profiles across the fault have led to interpretations of its dip and mechanics
ranging from near-vertical with normal motion to low-angle with reverse/thrust
motion. Each of these interpretations has a unique implication for the tectonic evolution
of the Hosgri fault zone. Images and time-stratigraphic measurements obtained during
this study suggest that the Hosgri fault has performed as a predominately strike-slip
fault during Neogene time.
The neotectonic history of the Hosgri fault zone is a subject of additional debate.
At issue is weather the Hosgri has been active during the Pliocene-Holocene, and if so,
if the motion has changed significantly since Miocene time. Other points of contention
are based on the validity of regional structural models and on the interpretation of
mapped geologic features along the fault zone. Properly interpreting the character of
the fault is important for assessing potential seismic hazard in the region and the
structural style and location of potential petroleum bearing traps within the Santa Maria
Basin.
This study supports the interpretation that the Hosgri fault zone evolved from a
basin-bounding normal fault during the Oligocene and mid Miocene, to a primarily
strike-slip fault representing a major structural boundary during the Neogene. Previous
interpretations of the Hosgri fault zone by Crouch et al. (1984), McIntosh et al. (1991),
Namson and Davis (1991), and Pacific Gas and Electric Co. (PG&E, 1991) did not utilize
time-stratigraphic information and data on fault geometry that are currently available
using pre-stack imaging of seismic data.
This study demonstrates that the use of new methods in processing seismic
reflection data can lead to improved understanding of fault systems. Conventional
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seismic reflection stacking analysis assumes that strong lateral variations in seismic
velocities do not exist. For this reason, post-stack migration of reflection data can
properly image horizontal to sub-horizontal stratigraphy and indirectly infer the
presence of vertical structures (Louie et al., 1988; Louie and Qin, 1991). Pre-stack
migration utilizing the Kirchhoff summation method will produce images of vertical
features with strong lateral variations in velocity, such as the plane of the Hosgri fault
zone. The results of post- and pre-stack migrations have been interpreted in
consideration of available geologic data and regional structure. The integrated model
addresses observed seismic stratigraphy, fault plane geometry, tectonic history, and
neotectonic motion within the Hosgri fault zone.
DATA PROCESSING
The principal source of data in this study is seismic reflection line RU-3, acquired
in the offshore Santa Maria Basin, north of Point Sal, California. In November 1986, over
400 km of deep-penetration seismic reflection data were acquired by Rice University
and the Houston Area Research Center (HARC) as a cooperative PG&E and EDGE deep
crustal experiment (figure 1). The RU-3 data were acquired with a 4.5 km long, 180
channel digital streamer.
Receiver stations are spaced at 25 meter intervals, correlating to the hydrophone
section lengths. The source was fired at 50 meter intervals offset toward the northeast,
and consisted of a large air gun array to optimize imaging of deep crustal structure
without sacrificing upper crustal data quality. The profiles are nominally 45-fold, and
were recorded to 16 s two-way travel time. Data were recorded at a sampling interval of
4 ms, with band-pass filtering between 3 and 80 Hz (Ewing and Talwani, 1991; Miller et
al., 1992). Our analysis utilizes a subset of the RU-3 data, down-sampled to 8 ms. The
subset consists of 180 shot records traversing 13.7 km of records from the northeastern
part of RU-3, which were selected to cover the tectonic and stratigraphic features across
the Hosgri fault zone (figure 2).
Since the purpose of this study is to interpret images of relatively shallow
structures, the records were "windowed" to a six second two-way travel time. The six
second cut-off was based on an analysis of first-arrival data, and consistent reflection
and diffraction patterns. Band-pass filtering of the records from 5-65 Hz was applied to
the data in order to suppress water column multiples, which obscured refractions and
other critical first arrival data increasingly toward the northeast end of the study area.
Since the frequency content of the diffractions, refractions, and water column multiples
were found to be similar, energy that constituted much of the noise within the first 3
seconds of the record was allowed to pass through the filtering process.
Therefore, two different muting processes were applied to the data to suppress
water column multiples, but preserve the diffraction patterns in the filtered records. A
permissive mute was applied to all records prior to pre- and post-stack migration in
order to enhance the imaging of basin sediments by allowing as much energy to pass as
possible. A second, more rigorous mute was designed to eliminate as much noise as
possible, allowing only the strongest diffraction patterns to pass. For the pre-stack
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migration only, mutes were incorporated into the imaging process so that they would
be increasingly rigorous from the southwest end of the study area toward the northeast
end, across the Hosgri fault zone. In this manner, the imaging of Neogene sediments
was enhanced from records 1 through 99 (progressing northeast), while pre-stack
imaging of vertical structures was enhanced from records 100 through 180, in the
vicinity of the Hosgri fault zone. Near-horizontal stratigraphy near the fault was image
by applying liberal mutes to the data prior to performing post-stack migration.
Post-Stack Migration
We conducted post-stack migration on the data to independently reproduce the
images of workers who had previously studied the Hosgri fault zone, and to obtain
detailed images of near-horizontal stratigraphy on both sides of the Hosgri fault zone.
Conventional velocity analysis was employed in order to create velocity models for use
in stacking. The analysis was an iterative process in which prominent, coherent events
were picked from a suite of constant velocity stacks. Midpoint traces on the constant
velocity stacks were picked at 200 m intervals, thus producing models with smooth
lateral velocity transitions (Honjas, 1993). Dix interval velocities were then calculated
between all velocity-depth points (Dix, 1965). Aberrant interval velocities were repicked based on re-examination of integrated CDP stacks and constant velocity stacks.
A stacked section was developed from the integrated velocity model. The stacked
section was then repeatedly Stolt-migrated utilizing a suite of velocities obtained by
measuring the slopes of diffraction tails from the stacked section (figure 3). The success
of the Stolt migration trials was enhanced because variation of velocity with depth had
been built into the velocity model through the conventional velocity analysis.
Pre-Stack Migration
A basic premise of the stacking process is that strong lateral variations in velocity
do not exist. Vertically oriented structures, such as the Hosgri fault zone and inclined
stratigraphy, violate this premise. The down-dip geometry of the Hosgri fault zone was
constrained by employing a Kirchhoff sum algorithm (KSA; Louie et al., 1988; Louie
and Qin, 1990) to form a migrated image of the multi-offset reflection data from the
study area of RU-3. This method takes lateral variations in velocity into account more
completely than the layer-stripping pre-stack migration of this data set by Lafond and
Levander (1993). The KSA requires the pre-calculation of an accurate interval velocity
model. To obtain the best possible model, Pullammanappallil and Louie (1994)
employed a non-linear optimization technique, a generalized simulated annealing, to
estimate velocities from RU-3 first-arrival time picks. Figure 4 shows the velocity model
they derived utilizing simulated annealing for our study area.
In the Kirchhoff pre-stack imaging procedure, unsorted seismogram traces are
mapped into a depth section by computing the travel time from the source to every
possible depth point and back to the receiver through the velocity model (figure 5). The
travel time calculation is based on Vidale's (1992) finite-difference solution of the
eikonal equation, including turning rays and diffractions, thus allowing for the imaging
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of structures having dips greater than 90 degrees (Louie and Qin, 1991). Once the travel
times down to and up from every point in the migration section were obtained, the
smoothed amplitude of the seismogram at that time was summed into the section at
that depth point. The tomographic summation of all trace amplitudes at the reflection
time allowed for the definition of reflecting points in the earth from all ellipses
calculated for the source-receiver pairs.
A complete Kirchhoff pre-stack migration image is displayed in figure 6. The
Neogene sediments of the Santa Maria Basin do not display relative amplitudes as large
as those observed in the post-stack migration. However, subtle characteristics of
stratigraphy with lateral variations in velocity, such as buttress unconformities and the
terminations of bedding at the crests of folds were imaged (figures 7 and 8). Since the
pre-stack migration method, coupled with Pullammanappallil and Louis (1994) accurate
velocity model, yields a true depth image, it is possible to make stratigraphic thickness
measurements directly. The pre-stack migration image (figures 6 and 8) also shows
multiple fault branches within the Hosgri fault zone (Honjas, 1993).
STRATIGRAPHIC ANALYSIS
Post-Stack Image Analysis
Post-stack analysis was performed on pre-processed data from the RU-3 study
area (Figure 1). Figure 7 illustrates the post-stack image of the RU-3 study area, Stoltmigrated at a constant velocity of 1.4 km/s. The image is vertically exaggerated by
approximately 2:1. The presence of a steeply dipping Hosgri fault zone with upward
diverging, multiple branches is shown by the dispersed diffraction column within the
Hosgri fault zone at about 11 km distance from the southwest side of the RU-3 study
area (Figure 1). A number of major acoustic unconformities that correlate with regional
stratigraphic horizons may be identified on the post-stack images, consistent with
previous work (Namson and Davis, 1990; PG&E, 1990). The image clearly displays the
interface between the acoustically incoherent Franciscan Complex basement and the
well-stratified Neogene sediments of the Santa Maria Basin. Growth of the Purisima
Structure deformed the early Neogene sediments, but is overlain by undeformed late
Neogene sediments showing that compressional tectonic deformation largely ceased
sometime during the earliest Pliocene along the northern part of the structure (figures 7
and 9).
Neogene sediments can be seen thinning abruptly across the Hosgri fault zone.
The overall sense of vertical separation on the Hosgri fault zone is up on the east. Most
of this deformation occurred prior to the late Miocene, as shown by stratal thickening of
basin sediments west of the Hosgri fault zone. Disturbance of horizontal stratigraphy
within the late Neogene sediments east of the Hosgri fault zone is indicative of recent
tectonic deformation. Truncations of middle to late Neogene sediments show that
significant vertical movement may have occurred as late as the Pliocene.
Pre-stack Analysis
5
Multiple fault planes within the Hosgri fault zone have been imaged through
application of the Kirchhoff pre-stack summation algorithm. Figure 8 is a close-up of
the RU-3 study area in the vicinity of the Hosgri fault zone. Compared to the post-stack
images (such as figure 7), the horizontal stratigraphy is not as prominent. However,
dipping beds and fault branches within the Hosgri fault zone have been imaged well.
At least three prominent fault branches are evident: (1) A central fault shallowing in dip
from about 70 degrees near the surface, to about 60 degrees at depth. The presence of
the Hosgri fault zone below a depth of 1.3 km depth is inferred by the truncation of
Neogene sediments against the acoustically incoherent Franciscan Formation; (2) A
west branch, which dips at about 60 degrees to the east, and appears to intersect the
central branch at depth; and (3) A near-vertical east branch, which intersects the central
branch at a depth of about one kilometer. The multiple fault branches are indicative of
strain partitioning due to strike-slip motion on a non-vertical fault (Lettis and Hanson,
1992).
The character of Neogene sediments east of the Hosgri fault zone show some
vertical displacement of the Franciscan basement. Vertical displacement is apparently
due to rigid-block uplift of acoustically transparent basement material. These
observations are consistent with the presence of crustal transpression to the east of the
Hosgri fault zone identified by previous workers (Lettis and Hanson, 1992).
Below a depth of 2.6 km, Miocene sediments appear to truncate against the east
side of the Purisima structure and the west side of the inferred trace of the Hosgri fault
zone (figure 8). Above 2.4 km depth, thinning and upwarping of sediments directly
west of the Hosgri fault zone zone are suggestive of compressional stress. Truncation
and acoustic transparency at about 2.5 km depth are suggest faulting and/or folding
within the sedimentary pile to the west of the Hosgri fault zone. The upwarped and
tectonically deformed Miocene sediments suggest that a southwest-northeast directed
element of compression has acted across the Hosgri fault zone, thickening the Miocene
sedimentary pile directly to its west by plastic deformation.
Acoustic boundaries which correlate with regional stratigraphic horizons have
been identified on the pre-stack images (figure 10). Observed pre-stack stratigraphic
boundaries are consistent with those identified in post-stack images produced for this
study, and correlate well with those identified by previous workers (PG&E, 1988;
Namson and Davis, 1990; McIntosh, 1991; Honjas, 1993).
Stratigraphy and Depositional Environment
Analysis of the seismic stratigraphy derived from observation of the post- and
pre-stack images provides information on the tectonic and depositional history of the
northeast margin of the offshore Santa Maria Basin. Combined with research by other
workers, the tectonic evolution of the south-central coast of California can be further
constrained. Figure 9 delineates the stratigraphic boundaries which have been
identified on the post and pre-stack images, and correlates them with offshore and
onshore stratigraphic columns constructed for the Santa Maria and adjacent basins
(Honjas, 1993).
6
Several workers have made good correlations of the stratigraphy from the
offshore to the onshore Santa Maria and adjacent basins (PG&E, 1990; Isaacs et al.,
1983). The only significant difference in interpretation between onshore and offshore
stratigraphy in the Santa Maria Basin appears to be in the age assignment of the Lospe
Formation, which workers from PG&E (1990) place in the early Oligocene, while
Namson and Davis (1990) assign it to the early Miocene. Since there is agreement that
the Lospe Formation is not present in the offshore Santa Maria Basin and other coeval
formations correlate well from the Oligocene to the early Miocene, the age assignment
of the Lospe Formation does not present an obstacle to extrapolating the tectonic history
of the Hosgri fault zone from stratigraphic boundaries observed in this study.
The Cretaceous acoustic basement unconformity defines the boundary between
highly deformed deposits of the Mesozoic Franciscan-Knoxville Complex and the upper
Mesozoic to Paleogene pre-Monterey sedimentary deposits (PG&E, 1990). The
Franciscan Complex is unconformably overlain by sedimentary rocks of the Knoxville
Formation (Miller et al., 1992), emplaced in a forearc basin setting. Pre-Monterey
sediments of the Santa Maria, Pismo and Husana Basins indicate increased subsidence
during the early Miocene (figure 9).
The mid-late Miocene acoustic unconformity correlates with the SisquocMonterey boundary. The Point Sal Formation underlies the Monterey Formation and
shows initiation of deep marine conditions in the Santa Maria Basin (Namson and
Davis, 1990). Transform faulting began to modify the California coastal margin during
emplacement of the Point Sal and Monterey Formations, preceded by the arrival of the
East Pacific rise at the North American trench during the Oligocene. In the initial phase
of transform faulting, right-lateral strike slip was accommodated by offshore faults such
as the Hosgri. At this time, the Pacific-North American transform boundary was a
diffuse zone of strike-slip deformation that included formation of pull-apart basins such
as the Santa Maria Basin (Miller et al., 1992).
The Late Miocene-Lower Pliocene Unconformity constitutes the division of the
upper and lower Sisquoc Formation (figure 9). The Sisquoc Formation ranges in age
from late-Miocene to mid-Pliocene and consists of fine-grained siliceous rocks and
deep-water siltstone and sandstone. During emplacement of the lower Sisquoc
Formation, the locus of right-slip motion between the North American and Pacific
plates was transferred to the San Andreas fault. In the early Pliocene, while the upper
Sisquoc was emplaced, a small change in relative North American-Pacific plate motion
introduced compression across the San Andreas plate boundary. This resulted in uplift
of the Coast Ranges onshore and uplifted basement blocks, and folded and faulted
basin sediments offshore (Miller et al., 1992).
The mid-Pliocene unconformity demarks the boundary between early and late
Pliocene rocks, dividing the upper Sisquoc Formation from the Foxen Mudstones and
younger formations (PG&E, 1990). It is distinguished by the truncation of events against
the unconformity, which are prominent against the flanks of large folds forming
buttress-like geometries. The Foxen and younger beds consist of shallow to non-marine
rocks and are indicative of Pliocene basin shoaling and erosion (figure 9).
7
Sedimentary Tectonic Analysis
The Kirchhoff pre-stack migration shows earth structure in true location versus
depth position. Therefore, it is possible to conduct a sedimentary tectonic analysis of
observed stratigraphic units with measurements taken directly from the pre-stack
seismic images.
Correlation of stratigraphic units from geologic mapping, geophysical
investigations and well data show significant differences in stratigraphic thickness
between the offshore and onshore Santa Maria basins (Figure 9). The thickness of
relative stratigraphic intervals have been measured from the geologic columns in figure
9 which correlate to the time-stratigraphic boundaries described by previous workers,
and identified in the pre-stack seismic images (figure 10). The relative thickness of each
stratigraphic interval from the geologic columns are expressed as a percentage of the
total thickness of the "reference column" (Table 1). The thickness of each stratigraphic
interval from Table 1 was then measured directly from the pre-stack seismic image
shown in figure 10 at points A through H. Figure 11 is a plot of measured sequence
thickness at points A through H expressed as a relative percentage of the total record of
Neogene sediments measured from the pre-stack image. The relative measurements are
derived from a section of the Neogene sediments from figure 10 from the top of the
Franciscan to the Quaternary, which coincides with the reference column described in
Table 1.
Established theory in stratigraphy and sedimentation allows for the
extrapolation of tectonic history by studying relative stratigraphic thicknesses. The
combination of tectonic elements present, their relative distribution and their degree of
activity all play a part in controlling the nature and thickness (i.e., geometry) of
accumulating sediments (Krumbein and Sloss, 1963 ). Process-response models for
sedimentary environments show relations between the properties of the sedimentary
environment and corresponding attributes of the sediments being formed. The model
consists basically of two parts, materials and geometry, which correspond to
depositional environment and tectonism. Environment and tectonism determine the
properties of accumulating sedimentary deposits. Sedimentary materials are acted upon
by local environmental processes, which in turn will dictate the lithologic properties
and areal variations of the sediments being formed. The boundary conditions of a
depositional environment are controlled by local tectonism. The boundary conditions
that exist at the time of deposition determines the geometry, or thickness of
sedimentary deposits. The materials variable of the process-response model for the
Santa Maria Basin, such as time-stratigraphic boundaries and associated lithology, have
been described by previous workers (PG&E, 1990; Isaacs et al., 1983). The sedimentary
responses to the tectonic and environmental processes through time are manifested in
the pre-stack seismic images, which show earth structure in true depth position.
The geometry of sedimentary units is determined by the time-transgression and
the style and rate of tectonic deformation during the depositional history of a basin. The
style and rate of tectonism defines the boundary conditions of the sedimentary
8
environment during and after the deposition of each stratigraphic unit. Boundary
conditions are dictated by the motion of major regional tectonic features. For the
northeast margin of the offshore Santa Maria Basin, the major tectonic feature affecting
the boundary conditions of the local depositional environment is the Hosgri fault zone.
Therefore, the character and history of tectonic motion on the Hosgri fault zone can be
assessed through sedimentary analysis utilizing relative thickness of time-stratigraphic
intervals measured at discrete points across the pre-stack seismic image and comparing
the results with the imaged tectonic features that transgress these points.
In this way the materials and environmental response aspects are known
quantities in the process-response model for the Santa Maria Basin. Examination of the
geometry, or relative stratigraphic thickness, across an imaged section, therefore will
reveal local perturbations in sedimentary thickness that have been imprinted on
previously deposited and contemporaneous sediments by local tectonic features. The
reconstruction of the material aspects in a depositional environment are more difficult
than the reconstruction of contemporaneous tectonism, since a degree of tectonism
ordinarily prevails for a long enough time to impress its characteristics on accumulating
sediments. During the same time, the material aspects of a sedimentary environment
may pass through many phases (Krumbein and Sloss, 1963).
Examination of figure 10 and Table 1 reveals significant differences between the
stratigraphic thickness of mid-late Miocene to upper Pliocene formations from the
offshore Santa Maria Basin to the onshore Santa Maria and adjacent basins. Rocks
constituting Sequence 1 increase in thickness from the offshore to the onshore Santa
Maria Basin, from occupying 35 percent of the Neogene stratigraphic record to
occupying over 85 percent. In the offshore Santa Maria Basin, rocks of Sequences 2 and
3 represent a combined total of 51 percent of the reference column. In the onshore Santa
Maria and adjacent basins, these Sequences represent about 8.5 percent of the reference
column. Sequence 4, representing the stratigraphic interval from the mid-Pliocene
unconformity to the top of the Pleistocene, is relatively consistent in relative thickness,
decreasing from 14 to 6 percent of the reference column from the offshore to onshore
Santa Maria Basin. Comparison of measurements of sequence interval thicknesses from
Table 1 and figure 9 to the plot of observed relative sequence thicknesses from figure 11
reveals that transition from offshore to onshore stratigraphic thicknesses occurs across
the Hosgri fault zone.
Examining figure 11, the relative thicknesses of Neogene sediments in Sequences
1 through 4 are relatively constant until point C. From point C, Sequence 1 thickens
eastward to about 80 percent of Neogene sediments adjacent to the Hosgri fault zone at
Point E. The thick sections of Oligocene to late Miocene sediments from points A-E
indicate rapid subsidence and sedimentation (Krumbein and Sloss, 1963). A change
relative motion between the Pacific and North American plates during the Mesozoic
resulted in the formation of a number of wrench fault basins, including the Santa Maria
Basin. Increased thickness of Sequence 1 sediments from points B to E, and the amount
of vertical separation demonstrated by progressive upward displacement of the
9
acoustic basement unconformity to the east of the Hosgri fault zone shows
development of the offshore Santa Maria Basin continued well into the late Miocene.
The most notable feature on figure 11 is the inverse relationship between relative
thicknesses of sediments in Sequence 1 and Sequence 2 from points B to E. Observation
of the post- and pre-stack images reveals little or no disturbance of Sequence 2
sediments due to growth of the Purisima Structure (figure 7 and 10). However,
observation of Sequence 2 sedimentary structure at points C to E reveals stratal thinning
due to upwarping of Sequence 1 sediments above 2.5 km depth west of the Hosgri fault
zone.
Stratal thinning of Sequence 2 sediments over thickened Sequence 1 sediments
indicate syn-depositional tectonic compression in the upper 2.5 km of the crust from
points C to E, east of the Purisima Structure through the Miocene.
Sequence 1 also gains thickness at the expense of Sequence 2 from points C to E
due to stratal thickening of Sequence 1 sediments below 2.5 km depth (figure 7 and 10).
Stratal thickening of Sequence 1 sediments at the expense of later sequences is further
evidence that an element of compression was occurring along the Hosgri fault zone
through the late Miocene and earliest Pliocene. Thickening of Sequence 1 sediments
relative to Sequence 3 and Sequence 4 indicates that compressional stress diminished
through the early Pliocene. Similarities in the character of Sequence 3 sediments relative
to Sequence 2 from points B to E indicate compression in the upper 2.5 km of the crust
adjacent to the Hosgri fault zone, ceasing by middle Pliocene. Cessation of significant
vertical separation across the Hosgri fault zone from the middle Pliocene to the present
is evidenced by the character of Sequence 4 sediments, which show little variation from
points A to H across the Hosgri fault zone, and less than 200 m of cumulative vertical
separation.
The character and time-stratigraphic relationships of the sedimentary sequences
show that compression reactivated the former southwest-dipping Hosgri fault zone and
reoriented its configuration in profile. The northeast-dipping axis of the fold observed
on the pre-stack image west of the Hosgri fault zone suggests that reorientation of the
fault resulted from a difference in cumulative compressional strain above and below
about 2.5 km depth. The fold axis is located at the top of Sequence 1 at point D on figure
10. Its projection increases in dip toward the northeast, between the sediments
truncating against the east side of the Purisima Structure and the inferred trace of the
Hosgri fault zone, to a point between E and F on figure 10. The geometry and nature of
the axis projection, and its relationship to observed stratigraphy suggests that the crust
below 2.5 to 3.0 km depth to the east of the Hosgri fault zone was directed toward an
easterly direction at a more constant rate than the upper 2.5 km, which has resulted in
the reorientation of the Hosgri fault zone from a southwest-dipping normal fault in the
early to middle Miocene to a northeast dipping from the middle Miocene to the early
Pliocene.
Sequence thicknesses fluctuate east of the Hosgri fault zone between points E
and G. Perturbations in Sequence 1 sediments east of the Hosgri fault zone relative to
Sequence 2 sediments from points E to G suggest rigid block uplift of the Franciscan
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basement. Between points E and F, a local decrease occurs in the thickness of Sequence
1 relative to Sequence 2. Observation of post-stack seismic images reveals a block of
Franciscan basement displaced upward into the Neogene sediments of Sequence 1, thus
decreasing its relative thickness (figure 7 and 10). From points F to G, the Franciscan
basement can be seen displaced downward once again, thus increasing the relative
thickness of Sequence 1 to Sequence 2.
Sequence 3 sediments follow the same characteristic trends of Sequence 2 east of
the Hosgri fault zone. The characteristics of Sequence 4 are independent of the
preceding sequences, showing that compressional deformation and significant vertical
displacement across the upper 2.5 km of the Hosgri fault zone ceased sometime during
the middle Pliocene, consistent with structural and stratigraphic observations west of
the Hosgri fault zone.
Our analysis of stratigraphic and structural characteristics of the study area,
combined with the differing character of tectonic features to the east and west of the
Hosgri fault zone, support the hypothesis that the fault has undergone counterclockwise reorientation from a southwest- dipping normal fault to a northeast-dipping
fault. Stratigraphic evidence indicates that the strain, and resulting reorientation,
occurred primarily during the late Miocene and early Pliocene. During the
reorientation, the Hosgri fault zone absorbed regionally directed compression from the
east as the tectonic environment of the south-central coast of California adjusted its
structural regime to accommodate the change in motion of the Pacific plate relative to
the North American plate. As onshore structures were established to compensate for
these changes in the tectonic regime, vertical motion diminished on the Hosgri fault
zone, being replaced predominantly by strike-slip motion transferred to it from the San
Gregorio-San Simeon fault system. Most of the compression and deformation on the
Hosgri fault zone ceased by the middle Pliocene. The pre-stack images show that the
area east of the Hosgri fault zone may be locally affected by rigid block uplift of the
Franciscan basement, consistent with the presence of a transpressional stress regime
(e.g., Lettis et al., 1994).
CENOZOIC DEVELOPMENT OF THE SANTA MARIA BASIN
This study reveals that the Hosgri fault zone plays a significant role in the
regional tectonic framework of the south-central coast of California. Certain aspects of
tectonic models for the region encompassing the south-central coast of California have
been found to be consistent with the observations and analysis of this research. We
present an interpretation of the development of the Hosgri fault zone based on the
results of this study, after consideration of previously published geological and
geophysical models of the south-central coast of California.
Mesozoic to late Miocene: The Hosgri fault zone developed as a southwestdipping normal fault, bounding the northeast end of the offshore Santa Maria Basin.
Normal faulting produced thick accumulations of Oligocene to late Miocene sediments
between acoustic basement and the middle to late Miocene unconformity (figures 9 and
10). Relative upward displacement of the acoustic basement unconformity across the
11
Hosgri fault zone shows that significant development of the wrench fault basin
occurred into the late Miocene.
Geological evidence supports the theory that the Santa Maria Basin was a rapidly
subsiding basin from the Oligocene into the late Miocene (figure 12A). The thick
accumulations of Monterey sediments west of the Hosgri fault zone are consistent with
models established by Krumbein and Sloss (1963), which postulate that thick
accumulations of rapidly buried sediments are formed when rates of subsidence and
detrital supply are both large. Rapid subsidence of the Santa Maria Basin from the
Oligocene into the late Miocene is also supported by the properties of the sediments.
Pre-Monterey sediments are indicative of increased subsidence during the early
Miocene, with rocks of the Point Sal and Monterey Formation signaling initiation of
deep marine conditions in the Santa Maria Basin in the late Miocene (Honjas, 1993).
The combined evidence lends support to the theory that the Santa Maria Basin was a
subsiding, transtentional, wrench fault basin during this period.
Late Miocene to upper Pliocene: During the late Miocene and early Pliocene, the
northeast margin of the Santa Maria Basin was affected by a change in azimuth of
movement of the Pacific plate to a more northerly direction. This change in relative
plate motion resulted in compression and crustal shortening oriented roughly
orthogonal to the plate boundary (figure 12B). Crustal shortening produced uplift of the
Coast Ranges (Diblee, 1982) and development of major offshore tectonic features, such
as the Purisima Structure. During this period, Miocene sediments continued to
accumulate on the flanks of the Purisima Structure. The apparent truncations of
Miocene sediments of Sequence 1 against the east side of the Purisima structure could
have formed as a result of anticlinal growth during continued deep water
sedimentation. The sediment truncations also could be due to the development of
antithetic faulting in response to normal motion on the Hosgri fault zone.
With the onset of regional compression, basin-forming transtension was replaced
by transpression along the Hosgri fault zone. Clockwise rotation of the Transverse
Ranges also occurred during this time period producing a component of northeastdirected crustal shortening east of the Hosgri fault zone (PG&E 1988, 1991). It was
during this period that the Hosgri fault rotated from a southwest-dipping normal fault
to steeply northeast-dipping, strike-slip fault.
Relation to Regional Tectonics
Analysis of the data in this study concurs with proposed regional models for
evolution of the south-central coast of California. Other workers have advocated
reorientation of the Hosgri fault zone to accommodate a change in direction of plate
motion during the late Miocene (Namson and Davis, 1990; McIntosh et al., 1991). The
Hosgri fault zone, acting as a weak structural boundary, absorbed most of the
transpressional strain being placed on it from the east allowing only a small component
of compression oriented orthogonal to the trace of the fault to be transferred to the west.
At the same time, dextral strike-slip motion was transferred to the Hosgri fault zone via
the San Gregorio-San Simeon fault system.
12
Localized compression west of the Hosgri fault zone is evidenced by the
Purisima Structure. A sedimentary tectonic analysis shows that the Purisima Structure
formed during late Miocene to the early Pliocene (figure 10 and 11). To the east of the
Hosgri fault zone, the Los Osos-Santa Maria Valley Domain formed during the early
Pliocene to accommodate initiation of the clockwise rotation of the Transverse Ranges
(PG&E, 1988). Structures within the Domain consist of a series of rigid crustal blocks
bounded by reverse faults which approach the Hosgri at oblique angles from the
southeast (figure 13). Lettis et al. (1994) proposed that crustal shortening in the Domain
is accommodated primarily by reverse faulting or subsidence of these rigid structural
blocks, with no significant folding within the blocks. Stratigraphic interpretation of the
seismic images reveals that vertical displacement east of the Hosgri fault zone is
controlled by rigid displacement of the Franciscan basement upward. Pliocene and
Quaternary vertical separation on the Hosgri fault zone is locally controlled controled
by spatial distribution of the adjacent uplifting and subsiding rigid blocks within the
Los Osos-Santa Maria Valley Domain.
Lack of significant vertical motion on the seismic images, combined with clear
images of multiple, steeply dipping fault branches into post-Pliocene sediments,
suggest that motion on the Hosgri during the Neogene has been predominantly strikeslip. Multiple branches imaged in figure 10 are interpreted to be "flower structures".
Based on the local trend at these fault branches, localized tensional and compressional
deformation occurs along the fault zone. The prominence of truncations on the east
side of the Purisima Structure shown in figure 8 also suggest that it's growth continued
into the early Pliocene.
Lateral motion along the Hosgri fault zone decreases southward as slip is
consumed by crustal shortening along the more easterly trending, oblique faults of the
Los Osos-Santa Maria Valley Domain. A number of workers have noted a southward
decreasing rate of lateral slip along the Hosgri fault zone, and related it to the clockwise
rotation of the Western Transverse Ranges (PG&E, 1988; Lettis et al., 1989). They
suggested that rotation of the Western Transverse Ranges' relatively thick crust
compresses the Los Osos-Santa Maria Valley Domain against the relatively stationary
Santa Lucia and San Rafael Ranges, which is underlain by a more stationary granite.
This results in north-northeast crustal shortening (figure 14).
We suggest that development of the northeast-directed regional strain field in the
Los Osos-Santa Maria domain resulted in reorientation of the Hosgri fault zone
primarily during the late Miocene to early Pliocene. Observations from this study are
consistent with northeast-crustal shortening as a result of the clockwise rotation of the
Transverse ranges. We speculate that greater crustal shortening is concentrated in the
upper 2.5 km of crust with lesser crustal shortening occurring in the crust below 2.5 km
depth. This resulted in passive reorientation of the Hosgri fault zone from an earlier
southwest dip toward its present northeast dip while absorbing the surrounding
stresses that were placed on it: strike-slip from the north and dip-slip from the east.
In the upper 2.5 kilometers, west of the Hosgri fault zone, a gentle up-warping of
sediments in the Santa Maria Basin occurs as a small compressional element of
13
southwest-northeast directed transpression is transferred to its west side. Most of the
transpression is absorbed by the Hosgri fault zone, with some lateral motion and minor
vertical displacement of the Franciscan basement directly to the east of the Hosgri fault
zone within the rigid blocks of the Los Osos-Santa Maria Valley Domain.
The differences in rate of northeast-directed crustal motion with depth are
consistent with a shallow horizontal velocity discontinuity from 2.5 to 3 km depth
(Miller et al., 1992; Lafond and Levander, 1993). Differential rates in movement of the
crust centered at about 2.5 km depth toward the northeast implies that the granitic root
of the Santa Lucia Range is not much deeper than 5 km. Franciscan crust of similar
velocity would be present above subducted oceanic crust, at about 10-12 km and below
the granites of the Santa Lucia Range, at about 5 km. This model fits observed
compressional features imaged directly west of the Hosgri fault zone, while allowing
more constant plastic deformation to take place at depth, where ductile motion is less
restricted by the presence of the shallower, higher velocity granites above 5 km depth.
Such differential rates of movement with depth would allow for the mapped surface
features within the Los Osos-Santa Maria Valley Domain, while being consistent with
regional and deep crustal models proposed by other workers (Lettis and Hanson, 1992;
Miller et al., 1992; Miller, 1993; Lafond and Levander, 1993).
Upper Pliocene to Recent: From the Upper Pliocene to Holocene, tectonic activity
along the Hosgri fault zone decreased as fault systems and tectonic domains formed in
the early Pliocene to accommodated relative Pacific-North American plate motion. This
is primarily evidenced by the undeformed nature of Sequence 4 rocks, which range in
age from upper Pliocene to Pleistocene (figure 11 and 12D)
Examination of the pre-stack images shows that several fault branches developed
on the Hosgri fault zone which truncate sediments as young as Sequence 4 in age
(figure 8). The west branch of the Hosgri fault zone seems to reach almost to the
seafloor. Some vertical separation is apparent across these branches of the Hosgri fault
zone in Sequence 1 and Sequence 2 rocks, and perhaps in Sequence 3. However, there is
no real evidence for vertical separation in rocks younger than late Pliocene. Our images
limit this displacement to no more than 200 m (figure 10). This indicates that motion
from the Upper Pliocene to Holocene has been predominantly strike-slip.
CONCLUSIONS
Sedimentary tectonic analysis of imaged basin structure suggest that the Hosgri
fault zone has been rotated rotated from a southwest dipping fault to a steeply
northeast dipping fault. This rotation occurred primarily between the late Miocene and
early Pliocene as a passive response to shear stress created by a change in plate motion
and transpressional deformation to the east of a weak Hosgri fault zone.
Transpressional deformation manifests itself in the Los Osos-Santa Maria Valley
Domain, which formed, in part, in response to clockwise rotation of the Transverse
Ranges. The Hosgri fault zone thus defines the southwest margin of the Los Osos-Santa
Maria Valley Domain and represents a major structural boundary (Lettis and Hanson,
1992).
14
Neotectonic motion on the Hosgri fault zone is primarily strike-slip, with a minor
component of vertical motion occurring on its east side as a result of uplift, subsidence
or tilting of rigid crustal blocks. Strike-slip motion is supported by the imaging of what
appear to be flower structures merging with the central branch of the Hosgri fault zone.
Flower structures commonly occur along strike-slip systems.
Vertical displacement of 200 m or less is apparent on the post- and pre-stack
seismic images from the upper Pliocene to upper Holocene. This is supported by
onshore mapping of the faults that bound these structural blocks (Lettis et al., 1989).
Much of the northeast-directed motion of the crust has been transferred, or dispersed to
other on-shore systems which have established themselves in order to accommodate
present-day plate-margin tectonics. This is similar, in theory, to dextral motion being
increasingly transferred to "in-board" fault systems, such as the San Andreas, from
offshore fault systems during the late Miocene and early Pliocene (Sedlock and
Hamilton, 1991).
The Hosgri fault zone is near vertical at the surface, but seems to shallow with
depth as it dips toward the northeast. It is possible that the fault zone is rooted in a
detachment east of the Santa Maria Antiform at about 10 to 12 km depth (Miller et al. ).
Tectonic features within the Los Osos/Santa Maria Valley Domain suggest detachment
or plastic flow at relatively shallow depth. Rigid blocks within the domain essentially
act as microplates which simply respond to regional stresses acting at their margins.
The presence of shallow detachments throughout south-central California would allow
for the rapid development of fault systems to accommodate the dynamic tectonic
environment. Microplates, or "flakes", less than 10 km in thickness have been suggested
in the Mojave region, south of the Garlock fault. Relative shifts and rotations between
sub-regions in south-central California could be taking place among flakes possibly less
than 10 km thick (Louie and Qin, 1991).
Differential rates of motion with depth, as evidenced by the rotation of the
Hosgri fault zone, suggest that the granitic mountains which define the northeast
margin of the Los Osos-Santa Maria Valley Domain, such as the Santa Lucia Range, are
rooted at about 5 km. This would allow for the formation of compressional features
above 5 km depth, while providing for a more constant, ductile creep to take place
below 5 km depth, which would result in the creation of a shear couple that would
rotate the fault zone. The images also imply that crust of similar density to the
Franciscan complex is present above the oceanic crust at 10-12 km depth, but below the
granitic roots of the Santa Lucia Range.
Since middle Miocene time, the Hosgri fault zone has mainly performed as a
passive, pre-existing weakness in the earth's crust, absorbing stresses that are acting on
its margins. It has restricted transpressional motion to its east, while allowing only a
small element of compression to its west.
REFERENCES CITED
15
Crouch, J. K., Bachman, S. B., and Shay, J. T., 1984, Post-Miocene Compressional
tectonics along the central California margin: in Crouch, J. K., and Bachman, S.
B., Eds., Tectonics and Sedimentation Along the California Margin, Pacific
Section, Society of Economic Paleontologists and Mineralogists, v. 38, p. 37-54.
Diblee, T.W., Jr., 1982, Geology of the Santa Ynez - Topatopa Mountains, southern
California: in Fife, D. L., and Minch, J. A., Eds., Geology and mineral wealth of
the California Transverse Ranges, Los Angeles, California, South Coast
Geological Society, Inc., p. 41-56.
Ewing, J. and Talwani, M., 1991, Marine deep seismic reflection profiles off central
California: Journal of Geophysical Research, v. 96, no. B4, p. 423-6,433.
Honjas, W., 1993, Results of post- and pre-stack migrations: imaging the Hosgri fault
zone, offshore Santa Maria basin, California: Master's Thesis, University of
Nevada-Reno, Reno, 109 p.
Isaacs, C. M., Keller, M.A. Gennai, V.A., Stewart, K. C., and Taggart, J. E., Jr.,1983
Preliminary evaluation of Miocene lithostratigraphy in Point Conception cost
well, OCS-CAL 78-6164 No., 1, off southern California: in Isaacs, C. M., Garrison,
R.F., Graham, S. A., and Jensky, W. A., Eds., Petroleum generation and
occurrence in Miocene Monterey Formation, California, Pacific Section, Society of
Economic Paleontologists and Mineralogists, 228 p.
Krumbein, W. C., Sloss, L. L., 1963, Stratigraphy and Sedimentation: San Francisco,
California, W. H. Freeman and Company, 660 p.
Lafond, C. F. and Levander, A. R., 1993, Migration moveout analysis and depth
focusing: Geophysics, v. 58, n. 1, p. 91-100.
Lettis, W. R., Hall, N. T., and Hamilton, P. H., 1989, Quaternary tectonics of southcentral coastal California, (abstract): 28th International Geological Congress, v. 2,
p. 2-285.
Lettis, W.R., and Hanson, K. L., 1992, Quaternary tectonic influence on coastal
morphology, south-central California: Quaternary International., v. 15-16, p. 135148.
Louie, J. N., Clayton, R. W., and Le Bras, R. J., 1988, Three dimensional Imaging of
steeply dipping structure near the San Andreas Fault, Parkfield, California:
Geophysics, no. 53, v. 2, p. 176-185.
16
Louie, J. N., and Qin, J., 1989, Structural imaging of the Garlock fault, Cantil Valley,
California: Journal of Geophysical Research, v. 96, p. 14,461-14,479.
McIntosh, K. D., Reed, D. L., Silver, E. A., and Meltzer, A. S., 1991, Deep structure and
structural inversion along the central California continental margin from EDGE
seismic profile RU-3: Journal of Geophysical Research, v. 96, p. 6,459-6,473.
Miller, K. C., Howie, J. M., and Rupert, S. D., 1992, Shortening within underplated
oceanic crust beneath the central California margin: Journal of Geophysical
Research, v. 97, B13, p. 19,961-19,980.
Miller, Kate, 1993, Crustal structure along the strike of the offshore Santa Maria basin,
California: Tectonophysics, v. 219, p. 57-69.
Namson, J., and Davis, T. L., 1990, Late Cenozoic fold and thrust belt of the southern
Coast Ranges and Santa Maria Basin, California: AAPG Bulletin, v. 74, p. 467-492.
Pacific Gas and Electric (PG&E), 1988, Final report of the Diablo Canyon long term
seismic program, U. S. Nuclear Regulatory Commission, Docket 50-275 and 50323, San Francisco, California, 680 p.
Pacific Gas and Electric (PG&E), 1990 (March), Question GSG 4: Diablo Canyon power
plant long term seismic program final report, San Francisco, California, 7 p.
Pacific Gas and Electric (PG&E) attachment GSG Q1-A, 1990 (March), Montage of
geophysical data and interpretations, Hosgri fault zone, eastern offshore Santa
Maria basin data base, interpretation procedures, and key observations: Diablo
Canyon power plant long term seismic program final report, San Francisco,
California, 59 p.
Pacific Gas and Electric (PG&E), 1991, Safety evaluation report related to the operation
of Diablo Canyon nuclear power plant, units 1 and 2, U. S. Nuclear Regulatory
Commission, Office of Nuclear Reactor Regulation: NUREG 06 75, Supplement
34, 257 p.
Pullammanappallil, S. K., and Louie, J. N., 1993, A generalized simulated-annealing
optimization for inversion of first-arrival times: Bulletin of the Seismological
Society of America, v. 84, No. 5, p. 1,397-1,409.
Sedlock, R. L., and Hamilton, W. B., 1991, Late Cenozoic tectonic evolution of
southwestern California: Journal of Geophysical Research, v. 96, p. 2,325-2,352.
17
FIGURE CAPTIONS
Figure 1. Index map for the seismic reflection surveys performed in the offshore Santa
Maria Basin. A section of northeast RU-3 is the subject of this study. SAF=San Andreas
Fault, RNF=Rinconada-Nacimiento Fault, SSSGF=San Simeon-San Gregorio Fault.
Figure 2. The configuration of seismic studies executed by Pacific Gas and Electric
(PG&E) and EDGE/Rice University (RU). In this study, we use data from NE RU-3.
Figure 3. Post-stack, Stolt migration of the integrated CDP stack. The image was
migrated at 1.7 km/sec. A suite of migration velocities were run on the integrated CDP
stack to test for best results. The depth axis on the migrated image was determined by
multiplying the migration velocity by the maximum time displayed on the vertical axis
of the CDP stack, which in this case is 3 seconds.
Figure 4. First-arrival optimization results on data collected along the EDGE-RU-3 line,
offshore Santa Maria Basin, CA. (a) Velocities obtained after the non-linear optimization
process on the first arrival picks. (b) A statistical analysis on a set of models near the
global minimum (having comparable least square errors) to obtain the standard
deviation of the velocities. After Pullammanappallil and Louie (1993).
Figure 5. We use a non-linear optimizing technique, or generalized simulated
annealing, to invert for velocities from travel-time picks (figure 4). Once the best model
has been obtained, travel times are calculated down to and up from each depth point.
The value of the seismogram at that time is then summed into the section at each depth
point: For each recorded receiver pair (S,g) over each point (X,Z) in some depth section,
a travel time is calculated. The amplitude of the recorded trace at that time is then
summed into the point of the depth section. Figure modified after Louie et al. (fig. 7, p
180, 1988).
Figure 6. Kirchhoff pre-stack migration, windowed from stations 254 to 548 in order to
show structural details in the vicinity of the Hosgri fault zone. The image is
uninterpreted in order to display the strong, coherent, negative amplitude reflections
within the Hosgri fault zone. The strongest, most coherent reflections define an image
of multiple fault planes within the Hosgri fault zone. The Miocene sediments west of
the fault zone are also been well imaged.
18
Figure 7. Post-stack image Stolt-migrated at a constant velocity of 1.4 km/sec. The
image is vertically exaggerated by a factor of 2. Sediments that have no significant
lateral variations in velocity, such as the Neogene sediments within the Santa Maria
Basin, are well imaged. Locations of vertical structures, such as the Hosgri fault zone,
are inferred by indirect evidence. The Purisima structure, which has been identified by
other workers as a Miocene/Pliocene fold (PG&E, GSG Q1-A, 1990), has also been
imaged.
Figure 8. Interpretation of image shown in figure 6. At least three fault branches have
been imaged within the Hosgri fault zone ; A central fault shallowing from about 70
degrees near the surface to about 60 degrees at depth, a west branch dipping at about 60
degrees to the east, and a near vertical, east branch which intersects the central branch
at about 1 km depth. The multiple branches have the character of "flower structures",
which form on strike-slip faults as a result of strain partitioning (PG&E, GSG Q1-A,
1990). The faults coincide in space with the dispersed diffractions, diffraction columns
and truncated sediments shown in figure 7. Thickened sediments can be seen at about
2.5 km depth directly west of the Hosgri fault zone. Seismically transparent zones and
bedding truncations within the Miocene sediments can be seen at label "A".
Figure 9. Correlation of geologic formations from the offshore Santa Maria Basin and
onshore Santa Maria and adjacent basins. The only difference in stratigraphy between
onshore and offshore geologic columns is in the age of the Lospe Formation, which
offshore is placed in the early Oligocene rather than the early Miocene, as displayed in
the figure. Stratigraphic divisions that can be identified on the seismic sections are
labeled by color. FR = Franciscan Complex, O = Oligocene, FY = Foxen and younger,
LP = lower Pliocene, EP = early Pliocene, M = Miocene, US = upper Sisquoc, LS = lower
Sisquoc, NM = non-marine. The offshore stratigraphic column is modified after PG&E
(Fig. GSG Q1-A.2, p 20, GSG Q-1A, 1990). Onshore stratigraphic columns are modified
from Namson and Davis (Fig. 5, p 471, 1990).
Figure 10. Image used to measure thickness of time-stratigraphic intervals, or
sequences. Measurements of each sequence was taken at points A-H, and plotted as a
percentage of the reference column in Figure 11. Sequence 1 correlates with the
stratigraphic-time interval from the acoustic basement to the late-mid Miocene
unconformity; Sequence 2 correlates with the interval from the late-mid Miocene
unconformity to the top-of Miocene unconformity; Sequence 3 correlates with the
interval from the top-of-Miocene unconformity to the middle Pliocene unconformity;
and Sequence 4 correlates with the time-stratigraphic interval from the middle Pliocene
unconformity to recent.
Figure 11. Thickness of each sequence expressed as a percent of the reference column as
measured in Figure 10. Comparison of Figure 11 with Table 1 shows that the Hosgri
19
fault zone represents the boundary between average offshore stratigraphic thickness
and average onshore stratigraphic thickness. Krumbein and Sloss (1963) have shown
that the properties and geometry, or thickness, of sediments are a function of the
tectonics affecting the depositional basin. Analysis of the relationship between relative
stratigraphic thicknesses and imaged structure reveals the history of motion in the
vicinity of the Hosgri fault zone.
Table 1. Thickness of stratigraphic intervals identified on the pre-stack images (figure
10) expressed as a percentage of the total reference column. The reference column is
defined as the time-stratigraphic interval from the top of the Franciscan Formation
(Paleogene-Eocene boundary), to the top of the Pasos Robles Formation (top of the
Pleistocene) as measured from figure 9. The table defines the thickness as a percentage
of the reference columns for offshore and onshore Santa Maria Basin.
Figure 12(a). Schematic off the evolution of the northeast margin of the Santa Maria
Basin. From the Oligocene to the middle Miocene, the Hosgri fault zone was a
southwest dipping normal fault bounding the northeast margin of the Santa Maria
Basin.
Figure 12(b). During the late Miocene and early Pliocene, an element of orthogonal
compression was introduced due to a shift in relative motion between the Pacific and
North American plates. Strike-slip motion is increasingly concentrated along major
dextral fault systems, such as the San Simeon-San Gregorio-Hosgri fault system.
Compressional structures begin to form adjacent to the Hosgri fault zone.
Figure 12(c). Rotation of the Transverse Ranges toward the northeast introduces
transpression east of the Hosgri fault zone, while allowing a small element of
orthogonal oriented compression west of the fault. The Hosgri fault zone commences a
counter-clockwise reorientation consistent with the northeast rotation of the Transverse
Ranges. Strain features reflect the stress caused by the new tectonic regime: A) The
Purisima Structure; B) Compressional thickening of Miocene sediments west of the
Hosgri fault zone; C) Truncation of sediments against the Purisima Structure due to
fold growth and compressional displacement of sediments west of the Hosgri fault
zone; D) Tilting, uplift and deformation of the Franciscan Basement and sediments east
of the Hosgri fault zone due to the onset of transpression; E) Strike-slip motion becomes
more dominant and flower structures begin to form.
Figure 12(d). Motion on the Hosgri fault zone decreases as other tectonic features
evolve to accommodate the relative change in Pacific-North American plate motion.
Reorientation of the Hosgri fault zone also decreases rapidly. The Purisima Structure
(A) ceases deformation prior to the mid Pliocene. Local deformation resulting from
compressional thickening of Miocene sediments causes local acoustic transparency (B).
Compressional deformation west of the Hosgri fault zone is also evidenced by
20
truncation of Miocene sediments against the Purisima Structure (C). Rigid block uplift,
subsidence and rotation continues east of the Hosgri fault zone within the Los
Osos/Santa Maria Valley Domain (figure 13) due to transpression, but to a lesser
degree. Pliocene-Holocene vertical motion is less than 200 meters (D). Strike-slip motion
becomes dominant, but decreases as lateral motion is transferred to other on-shore fault
systems (E). As the Hosgri fault zone is reoriented and becomes increasingly strike-slip,
motion may be transferred to the more vertical branches of the flower structures (F).
Figure 13. Map of the Los Osos/Santa Maria Valley Domain illustrating the general
distribution of major basement terranes. CAM = Casmalia block, SLP = San Luis/Pismo
block, SMV = Santa Maria Valley block, CAS = Casmalia block, VSY =Vandenburg/Santa Inez Valley block, WTR = Western Transverse Ranges, SRR = San
Rafael Range, SLR = Santa Lucia Range, OFSMB = offshore Santa Maria Basin, PH =
Purisima Hills block, SH = Solomon Hills block. After PG&E (Fig. 2.2, p. 2-6, 1991)
Figure 14. Distribution of lateral and vertical slip rate along the Hosgri fault zone,
assuming that lateral slip decays southward due to crustal shortening within the Los
Osos/Santa Maria Valley Domain. After PG&E GSG 4 (fig. GSG Q4-2, p 5, 1990).
21