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Crustal structure along the strike of the offshore Santa Maria basin, California
Tectonophysics, 219 (1993) 57-69
57
Elsevier Science Publishers B.V., Amsterdam
Crustal structure along the strike of the offshore Santa Maria basin, California Kate C. Miller Department of Geophysics, Stanford Uniuersity, Stanford, CA, USA
(Received November 18, 1991; revised version accepted April 30, 1992)
ABSTRACT Miller, K.C., 1993. Crustal structure along the strike of the offshore Santa Maria Basin, California. In: A.G. Green, A. Kriiner, H.-J. G&e and N. Pavlenkova (Editors), Plate Tectonic Signatures in the Continental Lithosphere. Tectonophysics, 219: 57-69.
The geology of the central California offshore region contains the record of the Early Miocene transition of the western North American continental margin from a site of subduction to one of transform motion. The fundamental elements of crustal structure and composition within the margin are defined by the seismic reflection image on the marine profile, PG&E-2. The profile shows that the crust in the offshore region is composed of three layers: (a) l-2 km of sediments in the offshore Santa Maria basin; (b) lo-12 km of Franciscan basement rocks; (c) a 7-lo-km thickness of oceanic crust. Seismic sequence analysis provides a foundation for interpreting the evolution of the offshore Santa Maria basin. Results indicate that the basin fill is predominantly Miocene to Pliocene in age and that subsidence began in Early Miocene times. A Late Miocene to Early Pliocene shortening event led to faulting and folding of basin strata and to uplift of basement blocks. The Franciscan Complex, an accretionary wedge assemblage which floors the basin, reveals little internal seismic structure. However, reflections from the Franciscan-oceanic crust boundary at 12-15 km depth supply evidence for the existence of an antiform with 2-3 km relief within the oceanic crustal layer. This antiform probably formed during Early Miocene shortening of the oceanic crust. Alternatively, the antiform could represent primary oceanic-crustal topography such as seamounts or a fracture zone.
Introduction The geology of the central California margin records the transition of the North American continental margin from a site of Mesozoic to early Tertiary subduction of the Farallon plate to one of transform motion between the Pacific and North American plates during Late Oligocene and Early Miocene times. The geologic evolution of this boundary is well studied onshore, but the offshore portion has remained relatively inaccessible. With the advent of offshore oil exploration, a database of seismic reflection profiles and well
Correspondence to: K.C. Miller, Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 799680555, USA.
0040-1951/93/$06.00
logs has accumulated in recent years. Early work with some of these data by Hoskins and Griffith (1971) documented the occurrence of shallow basins floored by Mesozoic rocks and filled with Miocene to Recent sediments. Later studies (Page et al., 1979; Crain et al., 1985; McCulloch, 1987; Clark et al., 1991) established a more detailed stratigraphic and structural framework particularly for the offshore Santa Maria basin (Fig. 1). Subsidence during a period of Miocene deepmarine sedimentation followed by terrigenous sediment input in the Plio-Pleistocene created the basin. Folds and faults representing shortening of Late Miocene to Recent age have reactivated basement structures and deformed the basin fill. The structure and composition of crustal rocks beneath the offshore basins has remained largely unknown, since exploration wells have rarely pen-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
58
ICC. MILLER
to-noise ratios for offsets of up to 150 km (Howie et al., 1992). This paper focuses on crustal structure as it is imaged in the 50-km long profile, PG&E-2 (Fig. 1). The transect lies along the axis of the offshore Santa Maria basin, 5 km west of the basin-bounding Hosgri fault. The PG&E-2 record is unique among the near-vertical-incidence seismic profiles collected in the experiment in that it provides a continuous image of a horizon at 12-15 km (5.545 s) beneath the offshore Santa Maria basin. The reflections probably correspond to a layer of oceanic crust that floors the crust beneath the margin (Putzig, 1988; Howie et al., 1992). In addition, PG&E-2 furnishes an excellent image of the shallow crust. Study of shallow structure and depositional geometry along the profile reveals the principal factors affecting the
etrated basement and industry reflection profiles have only imaged the shallow basins. To explore the deep crust of the west coast of central California, an onshore/offshore nearvertical-incidence and wide-angle reflection experiment was conducted in 1986 by Pacific Gas and Electric Company (PG&E) and the EDGE consortium. During the experiment, eight marine multi-channel and two wide-angle seismic reflection profiles were shot across the central California margin (Fig. 1). To ensure the best possible image of the lower crust, the seismic data were acquired with a large airgun array and were recorded to a two-way travel-time (twtt) of 16 s. Data-processing parameters were carefully chosen to enhance events from the deep crust. Likewise, the wide-angle data were also acquired with a large airgun array that resulted in good signal-
36
Ii&l
Fig. 1. Location
map for PG&E/EDGE
Filled circles represent for Pacific Wide-angle corridors.
ILL
the location
Gas and Electric reflection
profile.
RU profiles
(solid lines) were
These data are treated
Fault is the Santa
survey.
of the PG&E-2
Company.
transects
seismic
in other papers
I&I
Open
circles designate
to Morro
were collected.
for Rice University
in onshore/offshore
(see text). Thin lines are mapped
Lucia Bank fault. The thick line is the California
the single line parallel
near-vertical-incidence
A total of eight profiles
were collected acquired
.L”
in conjunction
experiments
along
line that crosses
that bound
profile
profiles
the PC& E-l
consortium. and PG&E-3
faults are labeled.
the Davidson
the Monterey
locations.
were collected
with the EDGE
faults. Major strike-slip
coast. The double
Ridge mark the dead ridge and transform
seismic S&E
seamount
Plate fragment.
SLB and
CRUSTAL
STRUCTURE
geologic evolution basin. Regional
ALONG
THE OFFSHORE
of the offshore
SANTA
MARIA
BASIN,
Pacific Rise at the North American trench (Atwater, 1970; Stock and Molnar, 1988; Atwater, 1989), the Farallon plate broke into smaller fragments. One of these fragments, called the Monterey plate, was trapped by the Pacific plate and now lies in the central California offshore (Fig. 1) (Lonsdale, 1992). As the transform boundary lengthened in the Early Miocene, a zone of diffuse deformation evolved along the margin. Within this zone, crustal blocks rotated and basins, such as the offshore Santa Maria basin, are thought to have formed (Atwater, 1989). The Santa Maria basin is the principal shallow-crustal feature of offshore central California. It is an elongate coast-parallel basin bounded on the northeast by the Hosgri fault and on the southwest by the Santa Lucia Bank fault (Fig. 11. The Hosgri fault is often cited as a possible Early Miocene locus for PacificNorth America transform motion, before the transform stepped inland to the San Andreas fault (Graham and Dickinson, 1978; Hall, 1975). In the last 5 to 6 Ma, a widespread shortening event has resulted in basin inversion and uplift of
Santa Maria
geology
Mesozoic to early Tertiary subduction of the Farallon plate followed by Late Oligocene to Recent transform motion are the primary plate tectonic processes that helped build the presentday California margin. During subduction, the large accretionary wedge known as the Franciscan Complex (Berkland et al., 1972) accumulated on top of downgoing oceanic crust. The Franciscan Complex is the primary basement rock for coastal and offshore central California. Evidence from this study and other work (e.g., Putzig, 1988; Ewing and Talwani, 1991; Meltzer and Levander, 1991; Howie et al., 1992) resulting from the PG & E/EDGE experiment suggest that oceanic crust has been tectonically underplated to the base of the Franciscan Complex. When the tectonic setting at the California margin changed from subduction to translation in the Late Oligocene (30-28 Ma) with the arrival of the East NW
59
CALIFORNIA
SE
Dishnce (km)
ffig-1
!‘G$Z-3
12.0
12.c
Fig. 2. Seismic divergence. offshore
reflection
Hyperbolic
profile
events
PC&E-2.
inside
Data
the boxes,
Santa Maria basin (O-2 s), a non-reflective reflective
have
A and
been
mid-crust
lower crust (6-8.5
dip filtered
B, are out-of-plane (2-5.5
s), interpreted
and
amplitudes
events.
s), interpreted
have
The principal
been features
corrected
to be made up of Franciscan
to be underplated
oceanic
crust.
for spherical
on the profile
are the
rocks, and a more
60
K.C. MILLER
8
mountains at the margin. The shortening has been associated with a clockwise shift in relative plate motion between the Pacific and North America plates (Cox and Engebretson, 1985; Harbert and Cox, 1989). Data acquisition and processing The PG&E-2 profile lies parallel to the offshore Santa Maria basin margin, 15 km offshore of Point Buchon. The data are 180-channel, 45fold airgun data acquired with a 6000-cubic inch (0.098 m3) array fired at source intervals of 50 m. Receivers were located every 25 m along a 4.5-km cable with a maximum source to receiver offset of approximately 4.7 km. The data were recorded with a 4-ms sample rate to 16 s twtt (two-way travel-time). Preliminary processing of the data by the acquisition contractor, Digicon, included gain recovery, signature deconvolution, velocity analysis, predictive deconvolution, common-depth-point stacking, filtering and migration. The data were reprocessed at the facilities of the Stanford Exploration Project (SEP) using Cogniseis’ DISCO processing software, GECO’s SKS processing software, as well as SEP software. Reprocessing efforts were directed toward evaluating and improving the signal returned from the deep crust (twtt of 5-8 s). A variety of data analysis techniques was used to assess the quality and reliability of the deep-crustal reflections. Subsequent application of time-dependent gain, dip filtering, migration and depth conversion improved the image of the deep crust. Out-of-plane reflections
The complex, laterally varying geologic structure of offshore central California makes it a potential site for numerous out-of-plane reflections. Before a geologic interpretation was assigned to the profile, these reflections had to be isolated from in-plane events. A reflection or diffraction with normal moveout velocity significantly lower than reasonable stacking velocities originates from out of the plane. Using this criterion, two sets of out-of-plane reflections have been identified on the PG&E-2 transect. The
7 t...,.,,l.......l..“.,.I.,...,,I,..,..~,.....,l,.,....l,,.,..~ -8 -6 -4 -2 Distan?e (km 5
Fig. 3. Theoretical hyperbolas Figure event
B near
kilometer velocities.
equation:
curves>
along
kilometer
6
and
8
observed
reflections
seen in
25 and 8.5 s twtt; (b)
15 and 4.5 s twtt. The solid curves moveout
along hyperbolas
They were calculated
t2 = t,’ + x’/
t, is the vertical
and 1’ is velocity. moveout
A near
the theoretical
ent stacking reflector,
(solid
(stars) for the two out-of-plane
2: (a) event
represent time
hyperbolas
4
L.’ where incidence
The theoretical
observed
hyperbola
has a moveout
In Figure
3b, the observed
at differ-
using the travel
t is the twtt to the
travel
curves
time,
x is offset
are used to bracket
events.
In Figure
3a, the observed
velocity
between
1.5 and 2.0 km/s.
hyperbola
has a moveout
velocity
between 3.0 and 4.0 km/s.
first occurs at kilometer 25 and 8.5 s (label A, Fig. 2) of the profile. It is of unusually high amplitude and has easily distinguishable diffraction tails. Comparison of the moveout along the diffraction with theoretical curves for hyperbolas originating at 8.5 s (Fig. 3a) shows that the event best matches a moveout curve with a velocity in the range of 1.5 to 2.0 km/s. These velocities are more typical of the water column and uppermost crust than of the deep crust. The second set of out-of-plane reflections lies between kilometers 15 and 25 at 4-6 s (label B, Fig. 2). This energy cannot be unequivocally distinguished from in-plane reflections based on analysis of the normal moveout of diffractions. The energy has a moveout velocity between 3.5 and 4.0 km/s (Fig. 3b), which is only slightly less than the stacking velocity for this travel-time.
CRUSTAL
STRUC TURE
ALONG
THE
OFFSHORE
SANTA
MARIA
BASIN,
61
CALIFORNIA
Distance (km) 12.0
16.0
20.0
24.0
6.0
Fig. 4. Dipping reflection
B (Fig. 3) migrated at 5.5 km/s. Note crossing relationship sub-horizontal reflections at 5.3 s.
However, examination of a record migrated at 5.5 km/s, the expected interval velocity of rocks at this travel-time (Fig. 4), does suggest that these reflections are from out of the plane. After migration, the dipping reflections from event B still cross the sub-horizontal reflections that lie at 5.2 s. ~though geologically unreasonable, this crossing geometry typically occurs in 2-D migrated sections in which sideswipe is present (French, 1974). Since both sets of out-of-plane reflections occur at the transition in depth to the top of oceanic crust, the possiblity arises that these events are somehow related to this structural transition. It seems likely that event A originates somewhere in the upper crust, since a very slow moveout velocity is associated with it. However, event B is associated with moveout velocities much closer to stacking velocity appropriate for this depth. Thus it may indeed represent out-of-plane structural complications associated with the transition. Processing sequence
Processing steps important to the interpretation of crustal structure on the PG&E-2 record include dip filtering, amplitude correction, migration and depth conversion. On a preliminary stack, steeply dipping linear noise dominates the seis-
of the dipping reflections
with the
mic signal for travel-times greater than 4 s. To bring out reflections from the deep crust a prestack dip filter (Hale and Claerbout, 1983; Larner et al., 1983) was applied to the data in both the shot and receiver domains. In the resulting profiIe (Fig. 21, sub-horizontal reflections underlying the linear noise were enhanced. A time-dependent spherical divergence approach to amplitude correction was chosen over automatic gain control (AGC) since spherical divergence tends to preserve relative amplitude. ~plitude versus twtt curves (Fig. 5) corrected with spherical divergence show substantial amplitude fluctuations while curves corrected with a l-s AGC are flatter. The spherical divergence curves show a highly reflective zone in the upper 2 s of the data, followed by a non-receptive zone from 2 to 5 s, beyond which reflectivity increases again. This pattern of amplitude variability is evident throughout the reprocessed record (Fig. 2) and can be tied to the geology as discussed below. Migration and depth conversion were important to the interpretation of reflector geometry on PG &E-2. A good image of the sedimentary basin (upper 4 s of data) was obtained with a 45-degree finite-difference migration (Fig. 6). Imaging in the middle to lower crust was affected by a large semi-circle created by the migration of the out-of-plane diffraction at * 8.5 s. Therefore,
62
K.C. MILLER
a migrated and depth-converted line drawing was generated for display (Fig. 7). The migrated position of deep crustal reflections was interpreted from a migration of the record in Figure 2 and was converted from time to depth using a velocity model derived from Howie et al. (1992). Data interpretation
The principal seismic features on the PG&E-2 profile (Fig. 2) are reflections from the offshore Santa Maria basin (O-2 s), a seismically transparent mid-crustal region (2-5.5 s) and a more reflective lower crust (5.5-8 s). In the shallow crust, gently folded basin sediments overlie highly irregular basement topography. The basin is relatively shallow to the northwest, but deepens rapidly by more than 0.6 s (1 km) in the central portion of the record. The transition from shallow to deep basement rocks takes place over a distance of only 5-10 km. Below basement, the mid-crust is for the most part seismically transparent, al-
though a few reflections are evident beneath the southern part of the basin. The top of the lower crust is characterized by an increase in reflectivity at 5.5-6.5 s (12-15 km). This boundary coincides with a velocity interface inferred to be the top of oceanic crust from modeling of wide-angle reflection data (Putzig, 1988; Howie et al., 1992). Like the basement reflections above it, this boundary deepens from 5.5 s (12 km) to 6.5 s (15 km) over a distance of 5 km in the central portion of the line. A set of dipping reflectors, inferred to be from out of the plane, crosses the transition from relatively shallow to relatively deep oceanic crust. Reflections from Moho depths are intermittent. A possible Moho reflection occurs at 8 s on the northwest end of the line. Additional Moho reflections occur at kilometer 40 of the transect. A large diffraction occurs below the Moho at around 8.5 s and has been demonstrated to be from out of the plane. The important features on the record are examined in detail in the following sections. Basin structure and stratigraphy
2
4
TWTT'(sec)8
4
TWTf(sec)
10
12
IO
12
0
I.,,,
-50
2
Fig. 5. Amplitude correction spherical
lute rected
geology
curves for the two amplitude
solid
line has been
by multiplying
to a power of 1.5. The dashed
traces
after near
for spherical compared
amplitude
corrected
each sample
The curves were computed
amplitudes
stacked
The
divergence
time raised 1 s AGC.
vs. travel-time
schemes.
8
line has received
by averaging
correction
to the AGC
on 5 adjacent cor-
more clearly reflect changes curves.
The large
near 8.5 s in B is from one of the out-of-the-plane
a
the abso-
(a) 12 km; and (b) 27 km. Curves divergence
for
by its travel
amplitude reflections.
in
The principal elements of the geologic evolution of the offshore Santa Maria basin are apparent from a study of structure and depositional geometry along the PG &E-2 transect. To identify and date the geologic events that affected the basin, seismic sequence analysis (Vail et al., 1977) was employed. Five depositional packages were identified and dated by comparing them with seismic data from the basin that were previously tied to well logs (Meltzer and Levander, 1991; Pacific Gas and Electric, 1988) and with other published well data and stratigraphic columns (Hoskins and Griffith, 1971; Crain et al., 1985; McCulloch, 1987; Clark et al., 1991). Among the published well information is that for the Ocean0 P-060-1 well, located a few hundred meters from this transect (fig. 6, Dept. of Interior, 1983). Each identified sequence is labeled with a letter (A through E in Fig. 6). In terms of regional stratigraphic correlations, unit E includes the Lower Miocene Obispo and Point Sal formations. The Obispo is composed of rhyolitic tuffs and flows while the overlying Point Sal is composed of limey mudstones and dolostones. Unit D corre-
CRUSTAL
STRUCTURE
ALONG
THE
OFFSHORE
SANTA
MARIA
NW
BASIN,
63
CALIFORNIA
SE
Distance (km) PG.&E-l Tie
10.0 I
0.0
20.0 ,
0 40.0
30.0 I
PG4:B
50.0
1.0
p
t!i
p
2.0
m” z $ 3.0
4.0
Fig. 6. Finite-difference migration of upper 4 s of PG&E-2 showing a detailed image of the offshore Santa Maria basin. Vertical exa~eration is _ 6~1. Dots define sequence boundaries. Letters A through E mark different sequences. Sequences D and E are Miocene in age; B and C are Pliocene in age and A is Pliocene to Recent. Franciscan rocks make up the basement. Basin subsidence and sediment deformation are markedly greater south of kilometer 25 than to the north. A solid line near 3.0 s marks a possible fault-plane reflection from within the basement.
sponds to the Miocene Monterey formation, a group of marine shales, cherts and siltstones. Sequences B and C probably correspond to the marine siltstones and shales of the Pliocene Sisquoc formation. Finally, unit A can be correlated to sandstones and siltstones of the PlioceneNW
PGhE-I Tte
S-_ -_-
RU-3 Tie
--
c TOP of Oceanic
10 2
to-Recent Foxen formation. Rocks of the Mesozoic Franciscan Complex are inferred to constitute basement for the basin on the basis of dredging and drilling results (McCulloch, 1987). The PG&E-2 transect images an unusually thick section of these Miocene and Pliocene sedi-
Crust
/
\
SE
-= -I-
-...._
5%
-
p%-3
Top of Ocewiic Crust
Fig. 7. Line drawing of the PGSZE-2 migrated profiie. Out-of-plane artifacts have been omitted from the drawing. The lateral changes in oceanic-crustal structure are clearly seen here. Sub-horizontal reflections at 12 km depth step down to 15 km depth near kilometer 25 of the transect.
Kc‘.
ments as it is located in the deepest part of the basin, 5 km west of the Hosgri fault (Fig. 1). Near kilometer 25, the basin deepens rapidly from 1.5 to 2.5 km (Fig. 6). The pattern of deposition suggests that basin subsidence was consistently greater south of kilometer 25 than to the north. To the south, Lower Miocene sediments of unit E are up to 1 km thick. These strata thin to the north and much of the package truncates against a basement high at kilometer 25. The pattern of strata thinning across this basement high continues through the mid-Miocene (unit D time) into the Pliocene (unit B time). Immediately below the top of basement are occasional events that might be interpreted as fault-plane reflections. In particular, a reflection at kilometer 30 (Fig. 6) takes off from the sediment-basement interface and penetrates the underlying rock at a dip of 7” to 20“. The reflection may be a normal fault associated with basin formation as it occurs at the point where the basin floor deepens by 1 km. McCulloch (1987) describes basement in the offshore Santa Maria basin as a series of en-echelon highs and lows having a northerly strike. Since PG&E-2 is oblique to this trend, the fault dip is probably an apparent one. The true dip of the fault would be 35-65”, assuming that the event is a normal fault initiated during basin subsidence and that it has the same strike as the basement trends. Recent workers (e.g., McCulloch, 1987; Atwater, 1989) have interpreted the offshore Santa Maria basin as a pull-apart basin formed during the initial stages of transform motion along the Early Miocene California margin. Basement structure in a pull-apart basin is typified by enechelon highs and lows that lie at an oblique angle to basin-bounding strike-slip faults (e.g., Wilcox et al., 1973; Harding, 1976). Crain et al. (1985) and McCulloch (1987) have documented this type of structural trend in the offshore Santa Maria basin. The 2-dimensional basin structure observed on PG&E-2 is consistent with this picture. Basement topography is irregular, with presumably en-echelon highs separated by as a little as 5 km. Sediments lie in lows and onlap local basement highs. Basin strata are deformed by shortening near
MILLER
the time of the C-D sequence boundary. Above this boundary, sequences A and B are nearly undeformed. Compression waned during deposition of unit C as fold amplitude decreases upward within the sequence. Beneath sequence C, strata of units D and E are folded. The folds occur above basement highs suggesting that basement faults were reactivated during shortening. Basinwide, folds and reverse faults have N35W trends (McCulloch, 1987). Thus PG&E-2, with its N20W trend provides a highly oblique view of these features. The seismic sequence analysis suggests that shortening in the offshore began before the end of the Miocene (5.3 Ma) and ended some time in the Pliocene. Elsewhere in the offshore Santa Maria basin, some present-day faulting and folding occurs (e.g., McCulloch et al., 1980; Crouch et al., 1984; McIntosh et al., 1991; Meltzer and Levander, 1991; Miller, 1991). Offshore reverse faults and folds as well as the uplift of the California Coast Ranges can be attributed to a small change in Pacific-North America plate motion approximately 3-5 Ma ago (Page and Engebretson, 1984; McCulloch, 1987; Harbert and Cox, 1989). Basement geometry and folds within basin strata suggest that an important intra-basin boundary may occur at kilometer 25-30 of the profile. South of this point, basin subsidence is greater than to the north by up to a kilometer. Larger fold amplitudes indicate more intense deformation to the south. In addition, the age of most recent shortening may be significantly less to the south (Meltzer and Levander, 1991).
A relatively non-reflective layer that correlates to a 12-km-thick layer with a velocity of 5.3-5.8 km/s on wide-angle models (Putzig, 1988; Howie et al., 1992) characterizes the mid-crust on PG& E-2. Several lines of evidence lead to the inference that the entire layer is composed of rocks of the Franciscan accretionary wedge. First, exploration wells have bottomed in Cretaceous basement rocks in the Santa Maria basin (Dept. of Interior, 1983; Pacific Gas and Electric, 1988). Cretaceous metasediments and altered igneous
CRUSTAL
STRUCTURE
ALONG
THE OFFSHORE
SANTA
MARIA
BASIN,
rocks have been dredged from the Santa Lucia Bank, a basement high that bounds the offshore Santa Maria Basin to the southwest (McCulloch, 1987). In addition, Franciscan rocks are also found immediately onshore along the coast (e.g., Page, 1981). Below exploration-well depths, the best evidence for a Franciscan mid-crust comes from the 5.3-5.8 km/s velocity assigned to this layer in the wide-angle velocity models. This velocity range is typical of Franciscan rocks at these depths (Stewart and Peselnick, 1978; Walter and Mooney, 1982). Finally, the presence of a thick accretionary wedge is consistent with long-lived subduction at the California margin. Given what is known of the geology of the Franciscan Complex, it is not surprising that it is transparent to seismic waves. Onshore, these rocks are folded, faulted, and stratally disrupted (e.g., Be&land et al., 1972; Page, 1981). Such a structurally complex assemblage is difficult to image with reflection data because steep dips are pervasive and because geologic structures are varying at wavelengths shorter than the Fresnel zone. The coherent reflections that do occur within the layer are particularly prevalent on the south end of the PG&E-2 profile. These may arrive from a variety of interfaces. As discussed above, fault surfaces formed during basin formation may extend into the Franciscan basement. Onshore, coherent, well-bedded sandstone units are known to occur within Franciscan melange (Page, 1981). Similar blocks may occur offshore. Finally, intrusion of sills into the Franciscan may have accompanied Miocene volcanism within the offshore Santa Maria basin. Deep-crustal reflections The lower crust is marked by an onset of reflectivity that begins at 5-6 s and persists for l-2 s (Fig. 2). The reflections are sub-horizontal and fairly continuous along the entire profile. A transition occurs near kilometer 25. Northwest of this point, reflections arrive at approximately 5.5 s, while to the southeast the reflectivity begins at approximately 6.5 s. The transition is marked only by two sets of reflections (A and B, Fig. 2) that have been interpreted as coming from out of
CALIFORNIA
65
the plane. Continuous reflections appear to die away by 7-8 s to the northwest and by 8.5-9 s to the southeast. This sudden onset of reflectivity is interpreted as a lithologic change from Franciscan metasediments to the gabbros of a tectonically underplated oceanic-crustal layer. The base of reflectivity, at least on the southeast end of the profile, marks the crust-mantle boundary. At 2 s (6-7 km), the reflective package on the southeast half of the profile (Fig. 2) is approximately the correct thickness for a layer of typical oceanic crust. Two velocity models derived from wide-angle reflection data that cross the PG&E-2 transect (Putzig, 1988; Howie et al., 1992) provide further evidence for the change in lithology. In both models, a 12-15-km-thick package of Franciscan rocks with a velocity of 5.3-5.8 km/s is floored by a 6-lokm-thick layer with a velocity of 6.8 km/s, a value typical of gabbro (e.g., Christensen and Salisbury, 1975). The wide-angle models place top of oceanic crust at 12-15&m depth. This corresponds to twtt of 5.5-6.5 s, which is where the inferred lithologic change occurs on PG & E-2. The PG&E-2 record images a large step in the oceanic crust. At the northwest end of the line (Fig. 71, the inferred top of oceanic crust occurs at a depth of 12 km, but near kilometer 25 the top of oceanic crust reflections drops 3 km to 15-km depth. The transition occurs over a distance of 5 km without intervening reflections to suggest the nature of the transition. Wide-angle reflection data confirm the change in depth to oceanic crust across the line. The velocity model of Putzig (1988), which crosses PG&E-2 at its northwest end (PG&E-1, Fig. 7) puts oceanic crust at 12 km depth, whereas the model of Howie et al. (1992) (PG&E-3, Fig. 7), puts top of oceanic crust at 15 km to the south. The Putzig model also constrains the thickness of the oceanic layer to 10 km at the northwest end of PG&E-2. Since PG&E-2 does not image well the base of the crust northwest of km 25, the PG&E-1 velocity model supplies evidence for oceanic crust that is 3-4 km thicker than to the southeast. The step observed on PG&E-2 is interpreted as the crest and limb of an antiform that lies beneath the offshore Santa Maria basin. The
66
K.C.
relatively shallow oceanic crust on the northwest endof the profile (Fig. 7) is the crest of the feature. One limb of the antiform is located near kilometer 25, where the oceanic-crustal layer steps down 3 km over a 5-7-km distance. Based on reflection terminations on either side of the break, the limb has a minimum apparent dip of 30”. The other limb of the antiform is not imaged on PG&E-2 but would be located beyond the northwest end of the profile. PG &E-2 provides the primary seismic evidence for a WNW-trending antiform in the oceanic crust. The presence of the antiform is inferred from the wide-angle velocity models (Putzig, 1988; Howie et al., 19921, but none of the other reflection profiles provide a continuous image of structure at the Franciscan/oceanic crust interface beneath the offshore Santa Maria basin (Meltzer and Levander, 1991; McIntosh et al., 1991; Miller et al., 1992). Reasons that might account for poor imaging on these records include poor signal penetration and scattering of energy in the mid-crustal Franciscan layer. Integration of the PG&E-2 results with crossing wide-angle velocity models constrains the WNW trend of the antiform and shows that it has a minimum length of 50 km. A fence diagram
Fig. 8. A fence diagram basins.
Regions
illustrating
the relationship
filled with + are interpreted
include
the California
oceanic
crust crosses
coast (thick PG&E-2
oceanic
stippled
obliquely.
of PG&E-2
The antiform beneath
(Fig. 8) illustrates PG&E-2’s relationship to the structure and to the cross lines PG&E-1 and PG&E-3. The PG&E-1 and PG&E-3 depth models presented in the diagram follow the wide-angle velocity model of Putzig (1988) along PG&E-1, the gravity model of Miller et al. (1992) along PG&E-1, and the velocity and gravity models of Howie et al. (1992) along PG&E-3. PG&E-2 images the antiform at approximately 40” to the strike of the feature. At its south end, PG&E-2 crosses line PG&E-3 about lo-15 km west of the antiform. Moving northward, line PG& E-2 crosses up onto the structure. The PG & E-l / PG & E-2 intersection lies above the crest of the structure. The 50-km distance between PG&E-1 and PG&E-3 leads to the minimum length estimate for the antiform. Gravity modeling along the RU-13 profile (Miller et al., 1992) extends this minimum length to 80 km. Possible origins for the antiform include primary topography in the oceanic crust and secondary deformation of the crust. Possible primary features include fracture zones and seamounts formed on oceanic crust prior to the time the crust enters the subduction zones. Secondary features might include structures formed in the crust after it entered the subduction zone. Seamounts
to the other
crust. Intervening
line), Hosgri
Fault
MIILEK
(HF)
lies northeast
the PG&E-l-PG&E-2
PG&E
layer represents and Santa
Lucia
transects. Bank
of the PG&E-2-PG&E-3 intersection.
Stippled
Franciscan
regions
are sedimentary
rocks. Geographical
fault (SLBF).
references
The antiform
line intersection,
in the
but lies directly
CRUSTAL
STRUCTURE
ALONG
THE
OFFSHORE
SANTA
MARIA
BASIN,
lack the minimum 3 : 1 aspect ratio required by the 80 by 25 km map dimensions of the antiform (e.g., Batiza, 1989). Large vertical tectonism is not normally associated with a small offset fracture zone (e.g., Bonatti, 1978). Although the antiform is sub-parallel to the Morro fracture zone, magnetic anomalies on the Monterey and Pacific plates (Lonsdale, 1992) lack offsets that might indicate the existence of another fracture zone beneath the margin. A secondary origin, such as imbrication of the oceanic-crustal layer (Putzig, 1988) provides the best fit to the antiform geometry and to Miocene plate tectonic models for central California. An imbrication would result in thickening of the crust and considerable vertical relief. In addition, a thrust could also extend large distances along strike. Miocene plate motion studies by Lonsdale (1992), Severinghaus and Atwater (1991) and Fernandez and Hey (1991) provide a source of the compression. Relative plate motion between the Monterey plate (a fragment of which now lies beneath the central California margin) and the Argue110 plate to the south contained a component of convergence across the intervening Morro fracture zone that may have resulted in as much as 80 km of intraplate shortening (Miller et al., 1992). The antiform may be one manifestation of this shortening. Conclusions
The PG&E-2 seismic profile provides an intriguing image of margin-parallel crustal structure in the central California offshore. Here, the crust is composed of three fundamental layers: Tertiary sediments in the offshore Santa Maria basin, a 12-15 km-thick basement layer of Franciscan rocks and 7-10 km of oce.anic crust. The basement of the offshore Santa Maria basin appears as a series of undulating highs and lows on PG&E-2. Near the middle of the transect, the average depth to basement increases by nearly a kilometer. Patterns of deposition and deformation also change at this location. Seismic sequence analysis along with well log data show that basement blocks began subsiding in Miocene time. A Late Miocene to Early Pliocene shorten-
CALIFORNIA
67
ing episode pushed up basement blocks and folded basin strata. A series of sub-horizontal reflections originating from 12 to 15 km depth are interpreted as an oceanic-crustal layer that has been mechanically underplated to the base of the Franciscan rocks. A 3-km change in depth to the top of oceanic crust that occurs near the middle of the PG&E-2 transect is the seismic evidence for a WNWtrending antiform that lies beneath the central California margin. Although nearby wide-angle velocity models help constrain the trend of the feature, PG&E-2 is the only profile that directly images it. Other profiles in the area lack reflections from the oceanic-crustal depths due to either a poor signal-to-noise ratio or scattering of seismic energy in the upper and middle crust. The seismic image on PG&E-2 provides insight into the tectonic evolution of the central California offshore. In the Early Miocene, previously subducting oceanic crust probably became coupled to the base of the Franciscan Complex as the new transform regime evolved at the California margin. At the same time, subsidence of the offshore Santa Maria basin began. Thus, it was in Miocene times that the basic components of offshore crustal structure observed today were amalgamated. Later, the upper crust, at least, underwent compression in response to a change in Pacific-North America relative plate motion at 3-5 Ma. It is not clear how this event affected the lower crustal structure as the only candidate for a shortening structure, the antiform, strikes at a 25” angle to upper-crustal faults and folds. The antiform may be a secondary feature formed in a heterogeneous stress field that probably existed during the last stages of subduction in the Early Miocene. An alternative model is that the antiform represents primary oceanic crust topography, such as a seamount or a fracture zone.
Acknowledgments
The seismic data were provided by Pacific Gas and Electric Company. Data-processing was done at the facilities of the Stanford Exploration Project. Discussions with Woody Savage, Walter
K.C. MILLER
68
Mooney and John Howie greatly improved the initial manuscript.
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Stewart, R. and Peselnick, L., 1978. Systematic behavior of compressional velocity in Franciscan rocks at high pressure and temperature. J. Geophys. Res., 83: 831-839. Stock, J. and Molnar, P., 1988. Uncertainties and implications of the Late Cretaceous and Tertiary position of North America plate relative to the Farallon, Kula, and Pacific plates. Tectonics, 7: 1339-1384. Vail, P.R., Mitchum, R.M. and Thompson, S., 1977. Seismic stratigraphy and global changes of sea level, part 2: the depositional sequence as a basic unit for stratigraphic analysis. In: C.E. Payton (Editor), Seismic Stratigraphy Applications to Hydrocarbon Exploration. Am. Assoc. Pet. Geol. Mem., 26: 53-62. Walter, A.W. and Mooney, W.D., 1982. Crustal structure of the Diablo and Gabilan Ranges, central California: a reinterpretation of existing data. Bull. Seismol. Sot. Am., 72: 1567-1590. Wilcox, R.E., Harding, T.P. and Seely, D.R., 1973. Basic wrench tectonics. Am. Assoc. Pet. Geol. Bull., 57: 74-96.
57
Elsevier Science Publishers B.V., Amsterdam
Crustal structure along the strike of the offshore Santa Maria basin, California Kate C. Miller Department of Geophysics, Stanford Uniuersity, Stanford, CA, USA
(Received November 18, 1991; revised version accepted April 30, 1992)
ABSTRACT Miller, K.C., 1993. Crustal structure along the strike of the offshore Santa Maria Basin, California. In: A.G. Green, A. Kriiner, H.-J. G&e and N. Pavlenkova (Editors), Plate Tectonic Signatures in the Continental Lithosphere. Tectonophysics, 219: 57-69.
The geology of the central California offshore region contains the record of the Early Miocene transition of the western North American continental margin from a site of subduction to one of transform motion. The fundamental elements of crustal structure and composition within the margin are defined by the seismic reflection image on the marine profile, PG&E-2. The profile shows that the crust in the offshore region is composed of three layers: (a) l-2 km of sediments in the offshore Santa Maria basin; (b) lo-12 km of Franciscan basement rocks; (c) a 7-lo-km thickness of oceanic crust. Seismic sequence analysis provides a foundation for interpreting the evolution of the offshore Santa Maria basin. Results indicate that the basin fill is predominantly Miocene to Pliocene in age and that subsidence began in Early Miocene times. A Late Miocene to Early Pliocene shortening event led to faulting and folding of basin strata and to uplift of basement blocks. The Franciscan Complex, an accretionary wedge assemblage which floors the basin, reveals little internal seismic structure. However, reflections from the Franciscan-oceanic crust boundary at 12-15 km depth supply evidence for the existence of an antiform with 2-3 km relief within the oceanic crustal layer. This antiform probably formed during Early Miocene shortening of the oceanic crust. Alternatively, the antiform could represent primary oceanic-crustal topography such as seamounts or a fracture zone.
Introduction The geology of the central California margin records the transition of the North American continental margin from a site of Mesozoic to early Tertiary subduction of the Farallon plate to one of transform motion between the Pacific and North American plates during Late Oligocene and Early Miocene times. The geologic evolution of this boundary is well studied onshore, but the offshore portion has remained relatively inaccessible. With the advent of offshore oil exploration, a database of seismic reflection profiles and well
Correspondence to: K.C. Miller, Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 799680555, USA.
0040-1951/93/$06.00
logs has accumulated in recent years. Early work with some of these data by Hoskins and Griffith (1971) documented the occurrence of shallow basins floored by Mesozoic rocks and filled with Miocene to Recent sediments. Later studies (Page et al., 1979; Crain et al., 1985; McCulloch, 1987; Clark et al., 1991) established a more detailed stratigraphic and structural framework particularly for the offshore Santa Maria basin (Fig. 1). Subsidence during a period of Miocene deepmarine sedimentation followed by terrigenous sediment input in the Plio-Pleistocene created the basin. Folds and faults representing shortening of Late Miocene to Recent age have reactivated basement structures and deformed the basin fill. The structure and composition of crustal rocks beneath the offshore basins has remained largely unknown, since exploration wells have rarely pen-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
58
ICC. MILLER
to-noise ratios for offsets of up to 150 km (Howie et al., 1992). This paper focuses on crustal structure as it is imaged in the 50-km long profile, PG&E-2 (Fig. 1). The transect lies along the axis of the offshore Santa Maria basin, 5 km west of the basin-bounding Hosgri fault. The PG&E-2 record is unique among the near-vertical-incidence seismic profiles collected in the experiment in that it provides a continuous image of a horizon at 12-15 km (5.545 s) beneath the offshore Santa Maria basin. The reflections probably correspond to a layer of oceanic crust that floors the crust beneath the margin (Putzig, 1988; Howie et al., 1992). In addition, PG&E-2 furnishes an excellent image of the shallow crust. Study of shallow structure and depositional geometry along the profile reveals the principal factors affecting the
etrated basement and industry reflection profiles have only imaged the shallow basins. To explore the deep crust of the west coast of central California, an onshore/offshore nearvertical-incidence and wide-angle reflection experiment was conducted in 1986 by Pacific Gas and Electric Company (PG&E) and the EDGE consortium. During the experiment, eight marine multi-channel and two wide-angle seismic reflection profiles were shot across the central California margin (Fig. 1). To ensure the best possible image of the lower crust, the seismic data were acquired with a large airgun array and were recorded to a two-way travel-time (twtt) of 16 s. Data-processing parameters were carefully chosen to enhance events from the deep crust. Likewise, the wide-angle data were also acquired with a large airgun array that resulted in good signal-
36
Ii&l
Fig. 1. Location
map for PG&E/EDGE
Filled circles represent for Pacific Wide-angle corridors.
ILL
the location
Gas and Electric reflection
profile.
RU profiles
(solid lines) were
These data are treated
Fault is the Santa
survey.
of the PG&E-2
Company.
transects
seismic
in other papers
I&I
Open
circles designate
to Morro
were collected.
for Rice University
in onshore/offshore
(see text). Thin lines are mapped
Lucia Bank fault. The thick line is the California
the single line parallel
near-vertical-incidence
A total of eight profiles
were collected acquired
.L”
in conjunction
experiments
along
line that crosses
that bound
profile
profiles
the PC& E-l
consortium. and PG&E-3
faults are labeled.
the Davidson
the Monterey
locations.
were collected
with the EDGE
faults. Major strike-slip
coast. The double
Ridge mark the dead ridge and transform
seismic S&E
seamount
Plate fragment.
SLB and
CRUSTAL
STRUCTURE
geologic evolution basin. Regional
ALONG
THE OFFSHORE
of the offshore
SANTA
MARIA
BASIN,
Pacific Rise at the North American trench (Atwater, 1970; Stock and Molnar, 1988; Atwater, 1989), the Farallon plate broke into smaller fragments. One of these fragments, called the Monterey plate, was trapped by the Pacific plate and now lies in the central California offshore (Fig. 1) (Lonsdale, 1992). As the transform boundary lengthened in the Early Miocene, a zone of diffuse deformation evolved along the margin. Within this zone, crustal blocks rotated and basins, such as the offshore Santa Maria basin, are thought to have formed (Atwater, 1989). The Santa Maria basin is the principal shallow-crustal feature of offshore central California. It is an elongate coast-parallel basin bounded on the northeast by the Hosgri fault and on the southwest by the Santa Lucia Bank fault (Fig. 11. The Hosgri fault is often cited as a possible Early Miocene locus for PacificNorth America transform motion, before the transform stepped inland to the San Andreas fault (Graham and Dickinson, 1978; Hall, 1975). In the last 5 to 6 Ma, a widespread shortening event has resulted in basin inversion and uplift of
Santa Maria
geology
Mesozoic to early Tertiary subduction of the Farallon plate followed by Late Oligocene to Recent transform motion are the primary plate tectonic processes that helped build the presentday California margin. During subduction, the large accretionary wedge known as the Franciscan Complex (Berkland et al., 1972) accumulated on top of downgoing oceanic crust. The Franciscan Complex is the primary basement rock for coastal and offshore central California. Evidence from this study and other work (e.g., Putzig, 1988; Ewing and Talwani, 1991; Meltzer and Levander, 1991; Howie et al., 1992) resulting from the PG & E/EDGE experiment suggest that oceanic crust has been tectonically underplated to the base of the Franciscan Complex. When the tectonic setting at the California margin changed from subduction to translation in the Late Oligocene (30-28 Ma) with the arrival of the East NW
59
CALIFORNIA
SE
Dishnce (km)
ffig-1
!‘G$Z-3
12.0
12.c
Fig. 2. Seismic divergence. offshore
reflection
Hyperbolic
profile
events
PC&E-2.
inside
Data
the boxes,
Santa Maria basin (O-2 s), a non-reflective reflective
have
A and
been
mid-crust
lower crust (6-8.5
dip filtered
B, are out-of-plane (2-5.5
s), interpreted
and
amplitudes
events.
s), interpreted
have
The principal
been features
corrected
to be made up of Franciscan
to be underplated
oceanic
crust.
for spherical
on the profile
are the
rocks, and a more
60
K.C. MILLER
8
mountains at the margin. The shortening has been associated with a clockwise shift in relative plate motion between the Pacific and North America plates (Cox and Engebretson, 1985; Harbert and Cox, 1989). Data acquisition and processing The PG&E-2 profile lies parallel to the offshore Santa Maria basin margin, 15 km offshore of Point Buchon. The data are 180-channel, 45fold airgun data acquired with a 6000-cubic inch (0.098 m3) array fired at source intervals of 50 m. Receivers were located every 25 m along a 4.5-km cable with a maximum source to receiver offset of approximately 4.7 km. The data were recorded with a 4-ms sample rate to 16 s twtt (two-way travel-time). Preliminary processing of the data by the acquisition contractor, Digicon, included gain recovery, signature deconvolution, velocity analysis, predictive deconvolution, common-depth-point stacking, filtering and migration. The data were reprocessed at the facilities of the Stanford Exploration Project (SEP) using Cogniseis’ DISCO processing software, GECO’s SKS processing software, as well as SEP software. Reprocessing efforts were directed toward evaluating and improving the signal returned from the deep crust (twtt of 5-8 s). A variety of data analysis techniques was used to assess the quality and reliability of the deep-crustal reflections. Subsequent application of time-dependent gain, dip filtering, migration and depth conversion improved the image of the deep crust. Out-of-plane reflections
The complex, laterally varying geologic structure of offshore central California makes it a potential site for numerous out-of-plane reflections. Before a geologic interpretation was assigned to the profile, these reflections had to be isolated from in-plane events. A reflection or diffraction with normal moveout velocity significantly lower than reasonable stacking velocities originates from out of the plane. Using this criterion, two sets of out-of-plane reflections have been identified on the PG&E-2 transect. The
7 t...,.,,l.......l..“.,.I.,...,,I,..,..~,.....,l,.,....l,,.,..~ -8 -6 -4 -2 Distan?e (km 5
Fig. 3. Theoretical hyperbolas Figure event
B near
kilometer velocities.
equation:
curves>
along
kilometer
6
and
8
observed
reflections
seen in
25 and 8.5 s twtt; (b)
15 and 4.5 s twtt. The solid curves moveout
along hyperbolas
They were calculated
t2 = t,’ + x’/
t, is the vertical
and 1’ is velocity. moveout
A near
the theoretical
ent stacking reflector,
(solid
(stars) for the two out-of-plane
2: (a) event
represent time
hyperbolas
4
L.’ where incidence
The theoretical
observed
hyperbola
has a moveout
In Figure
3b, the observed
at differ-
using the travel
t is the twtt to the
travel
curves
time,
x is offset
are used to bracket
events.
In Figure
3a, the observed
velocity
between
1.5 and 2.0 km/s.
hyperbola
has a moveout
velocity
between 3.0 and 4.0 km/s.
first occurs at kilometer 25 and 8.5 s (label A, Fig. 2) of the profile. It is of unusually high amplitude and has easily distinguishable diffraction tails. Comparison of the moveout along the diffraction with theoretical curves for hyperbolas originating at 8.5 s (Fig. 3a) shows that the event best matches a moveout curve with a velocity in the range of 1.5 to 2.0 km/s. These velocities are more typical of the water column and uppermost crust than of the deep crust. The second set of out-of-plane reflections lies between kilometers 15 and 25 at 4-6 s (label B, Fig. 2). This energy cannot be unequivocally distinguished from in-plane reflections based on analysis of the normal moveout of diffractions. The energy has a moveout velocity between 3.5 and 4.0 km/s (Fig. 3b), which is only slightly less than the stacking velocity for this travel-time.
CRUSTAL
STRUC TURE
ALONG
THE
OFFSHORE
SANTA
MARIA
BASIN,
61
CALIFORNIA
Distance (km) 12.0
16.0
20.0
24.0
6.0
Fig. 4. Dipping reflection
B (Fig. 3) migrated at 5.5 km/s. Note crossing relationship sub-horizontal reflections at 5.3 s.
However, examination of a record migrated at 5.5 km/s, the expected interval velocity of rocks at this travel-time (Fig. 4), does suggest that these reflections are from out of the plane. After migration, the dipping reflections from event B still cross the sub-horizontal reflections that lie at 5.2 s. ~though geologically unreasonable, this crossing geometry typically occurs in 2-D migrated sections in which sideswipe is present (French, 1974). Since both sets of out-of-plane reflections occur at the transition in depth to the top of oceanic crust, the possiblity arises that these events are somehow related to this structural transition. It seems likely that event A originates somewhere in the upper crust, since a very slow moveout velocity is associated with it. However, event B is associated with moveout velocities much closer to stacking velocity appropriate for this depth. Thus it may indeed represent out-of-plane structural complications associated with the transition. Processing sequence
Processing steps important to the interpretation of crustal structure on the PG&E-2 record include dip filtering, amplitude correction, migration and depth conversion. On a preliminary stack, steeply dipping linear noise dominates the seis-
of the dipping reflections
with the
mic signal for travel-times greater than 4 s. To bring out reflections from the deep crust a prestack dip filter (Hale and Claerbout, 1983; Larner et al., 1983) was applied to the data in both the shot and receiver domains. In the resulting profiIe (Fig. 21, sub-horizontal reflections underlying the linear noise were enhanced. A time-dependent spherical divergence approach to amplitude correction was chosen over automatic gain control (AGC) since spherical divergence tends to preserve relative amplitude. ~plitude versus twtt curves (Fig. 5) corrected with spherical divergence show substantial amplitude fluctuations while curves corrected with a l-s AGC are flatter. The spherical divergence curves show a highly reflective zone in the upper 2 s of the data, followed by a non-receptive zone from 2 to 5 s, beyond which reflectivity increases again. This pattern of amplitude variability is evident throughout the reprocessed record (Fig. 2) and can be tied to the geology as discussed below. Migration and depth conversion were important to the interpretation of reflector geometry on PG &E-2. A good image of the sedimentary basin (upper 4 s of data) was obtained with a 45-degree finite-difference migration (Fig. 6). Imaging in the middle to lower crust was affected by a large semi-circle created by the migration of the out-of-plane diffraction at * 8.5 s. Therefore,
62
K.C. MILLER
a migrated and depth-converted line drawing was generated for display (Fig. 7). The migrated position of deep crustal reflections was interpreted from a migration of the record in Figure 2 and was converted from time to depth using a velocity model derived from Howie et al. (1992). Data interpretation
The principal seismic features on the PG&E-2 profile (Fig. 2) are reflections from the offshore Santa Maria basin (O-2 s), a seismically transparent mid-crustal region (2-5.5 s) and a more reflective lower crust (5.5-8 s). In the shallow crust, gently folded basin sediments overlie highly irregular basement topography. The basin is relatively shallow to the northwest, but deepens rapidly by more than 0.6 s (1 km) in the central portion of the record. The transition from shallow to deep basement rocks takes place over a distance of only 5-10 km. Below basement, the mid-crust is for the most part seismically transparent, al-
though a few reflections are evident beneath the southern part of the basin. The top of the lower crust is characterized by an increase in reflectivity at 5.5-6.5 s (12-15 km). This boundary coincides with a velocity interface inferred to be the top of oceanic crust from modeling of wide-angle reflection data (Putzig, 1988; Howie et al., 1992). Like the basement reflections above it, this boundary deepens from 5.5 s (12 km) to 6.5 s (15 km) over a distance of 5 km in the central portion of the line. A set of dipping reflectors, inferred to be from out of the plane, crosses the transition from relatively shallow to relatively deep oceanic crust. Reflections from Moho depths are intermittent. A possible Moho reflection occurs at 8 s on the northwest end of the line. Additional Moho reflections occur at kilometer 40 of the transect. A large diffraction occurs below the Moho at around 8.5 s and has been demonstrated to be from out of the plane. The important features on the record are examined in detail in the following sections. Basin structure and stratigraphy
2
4
TWTT'(sec)8
4
TWTf(sec)
10
12
IO
12
0
I.,,,
-50
2
Fig. 5. Amplitude correction spherical
lute rected
geology
curves for the two amplitude
solid
line has been
by multiplying
to a power of 1.5. The dashed
traces
after near
for spherical compared
amplitude
corrected
each sample
The curves were computed
amplitudes
stacked
The
divergence
time raised 1 s AGC.
vs. travel-time
schemes.
8
line has received
by averaging
correction
to the AGC
on 5 adjacent cor-
more clearly reflect changes curves.
The large
near 8.5 s in B is from one of the out-of-the-plane
a
the abso-
(a) 12 km; and (b) 27 km. Curves divergence
for
by its travel
amplitude reflections.
in
The principal elements of the geologic evolution of the offshore Santa Maria basin are apparent from a study of structure and depositional geometry along the PG &E-2 transect. To identify and date the geologic events that affected the basin, seismic sequence analysis (Vail et al., 1977) was employed. Five depositional packages were identified and dated by comparing them with seismic data from the basin that were previously tied to well logs (Meltzer and Levander, 1991; Pacific Gas and Electric, 1988) and with other published well data and stratigraphic columns (Hoskins and Griffith, 1971; Crain et al., 1985; McCulloch, 1987; Clark et al., 1991). Among the published well information is that for the Ocean0 P-060-1 well, located a few hundred meters from this transect (fig. 6, Dept. of Interior, 1983). Each identified sequence is labeled with a letter (A through E in Fig. 6). In terms of regional stratigraphic correlations, unit E includes the Lower Miocene Obispo and Point Sal formations. The Obispo is composed of rhyolitic tuffs and flows while the overlying Point Sal is composed of limey mudstones and dolostones. Unit D corre-
CRUSTAL
STRUCTURE
ALONG
THE
OFFSHORE
SANTA
MARIA
NW
BASIN,
63
CALIFORNIA
SE
Distance (km) PG.&E-l Tie
10.0 I
0.0
20.0 ,
0 40.0
30.0 I
PG4:B
50.0
1.0
p
t!i
p
2.0
m” z $ 3.0
4.0
Fig. 6. Finite-difference migration of upper 4 s of PG&E-2 showing a detailed image of the offshore Santa Maria basin. Vertical exa~eration is _ 6~1. Dots define sequence boundaries. Letters A through E mark different sequences. Sequences D and E are Miocene in age; B and C are Pliocene in age and A is Pliocene to Recent. Franciscan rocks make up the basement. Basin subsidence and sediment deformation are markedly greater south of kilometer 25 than to the north. A solid line near 3.0 s marks a possible fault-plane reflection from within the basement.
sponds to the Miocene Monterey formation, a group of marine shales, cherts and siltstones. Sequences B and C probably correspond to the marine siltstones and shales of the Pliocene Sisquoc formation. Finally, unit A can be correlated to sandstones and siltstones of the PlioceneNW
PGhE-I Tte
S-_ -_-
RU-3 Tie
--
c TOP of Oceanic
10 2
to-Recent Foxen formation. Rocks of the Mesozoic Franciscan Complex are inferred to constitute basement for the basin on the basis of dredging and drilling results (McCulloch, 1987). The PG&E-2 transect images an unusually thick section of these Miocene and Pliocene sedi-
Crust
/
\
SE
-= -I-
-...._
5%
-
p%-3
Top of Ocewiic Crust
Fig. 7. Line drawing of the PGSZE-2 migrated profiie. Out-of-plane artifacts have been omitted from the drawing. The lateral changes in oceanic-crustal structure are clearly seen here. Sub-horizontal reflections at 12 km depth step down to 15 km depth near kilometer 25 of the transect.
Kc‘.
ments as it is located in the deepest part of the basin, 5 km west of the Hosgri fault (Fig. 1). Near kilometer 25, the basin deepens rapidly from 1.5 to 2.5 km (Fig. 6). The pattern of deposition suggests that basin subsidence was consistently greater south of kilometer 25 than to the north. To the south, Lower Miocene sediments of unit E are up to 1 km thick. These strata thin to the north and much of the package truncates against a basement high at kilometer 25. The pattern of strata thinning across this basement high continues through the mid-Miocene (unit D time) into the Pliocene (unit B time). Immediately below the top of basement are occasional events that might be interpreted as fault-plane reflections. In particular, a reflection at kilometer 30 (Fig. 6) takes off from the sediment-basement interface and penetrates the underlying rock at a dip of 7” to 20“. The reflection may be a normal fault associated with basin formation as it occurs at the point where the basin floor deepens by 1 km. McCulloch (1987) describes basement in the offshore Santa Maria basin as a series of en-echelon highs and lows having a northerly strike. Since PG&E-2 is oblique to this trend, the fault dip is probably an apparent one. The true dip of the fault would be 35-65”, assuming that the event is a normal fault initiated during basin subsidence and that it has the same strike as the basement trends. Recent workers (e.g., McCulloch, 1987; Atwater, 1989) have interpreted the offshore Santa Maria basin as a pull-apart basin formed during the initial stages of transform motion along the Early Miocene California margin. Basement structure in a pull-apart basin is typified by enechelon highs and lows that lie at an oblique angle to basin-bounding strike-slip faults (e.g., Wilcox et al., 1973; Harding, 1976). Crain et al. (1985) and McCulloch (1987) have documented this type of structural trend in the offshore Santa Maria basin. The 2-dimensional basin structure observed on PG&E-2 is consistent with this picture. Basement topography is irregular, with presumably en-echelon highs separated by as a little as 5 km. Sediments lie in lows and onlap local basement highs. Basin strata are deformed by shortening near
MILLER
the time of the C-D sequence boundary. Above this boundary, sequences A and B are nearly undeformed. Compression waned during deposition of unit C as fold amplitude decreases upward within the sequence. Beneath sequence C, strata of units D and E are folded. The folds occur above basement highs suggesting that basement faults were reactivated during shortening. Basinwide, folds and reverse faults have N35W trends (McCulloch, 1987). Thus PG&E-2, with its N20W trend provides a highly oblique view of these features. The seismic sequence analysis suggests that shortening in the offshore began before the end of the Miocene (5.3 Ma) and ended some time in the Pliocene. Elsewhere in the offshore Santa Maria basin, some present-day faulting and folding occurs (e.g., McCulloch et al., 1980; Crouch et al., 1984; McIntosh et al., 1991; Meltzer and Levander, 1991; Miller, 1991). Offshore reverse faults and folds as well as the uplift of the California Coast Ranges can be attributed to a small change in Pacific-North America plate motion approximately 3-5 Ma ago (Page and Engebretson, 1984; McCulloch, 1987; Harbert and Cox, 1989). Basement geometry and folds within basin strata suggest that an important intra-basin boundary may occur at kilometer 25-30 of the profile. South of this point, basin subsidence is greater than to the north by up to a kilometer. Larger fold amplitudes indicate more intense deformation to the south. In addition, the age of most recent shortening may be significantly less to the south (Meltzer and Levander, 1991).
A relatively non-reflective layer that correlates to a 12-km-thick layer with a velocity of 5.3-5.8 km/s on wide-angle models (Putzig, 1988; Howie et al., 1992) characterizes the mid-crust on PG& E-2. Several lines of evidence lead to the inference that the entire layer is composed of rocks of the Franciscan accretionary wedge. First, exploration wells have bottomed in Cretaceous basement rocks in the Santa Maria basin (Dept. of Interior, 1983; Pacific Gas and Electric, 1988). Cretaceous metasediments and altered igneous
CRUSTAL
STRUCTURE
ALONG
THE OFFSHORE
SANTA
MARIA
BASIN,
rocks have been dredged from the Santa Lucia Bank, a basement high that bounds the offshore Santa Maria Basin to the southwest (McCulloch, 1987). In addition, Franciscan rocks are also found immediately onshore along the coast (e.g., Page, 1981). Below exploration-well depths, the best evidence for a Franciscan mid-crust comes from the 5.3-5.8 km/s velocity assigned to this layer in the wide-angle velocity models. This velocity range is typical of Franciscan rocks at these depths (Stewart and Peselnick, 1978; Walter and Mooney, 1982). Finally, the presence of a thick accretionary wedge is consistent with long-lived subduction at the California margin. Given what is known of the geology of the Franciscan Complex, it is not surprising that it is transparent to seismic waves. Onshore, these rocks are folded, faulted, and stratally disrupted (e.g., Be&land et al., 1972; Page, 1981). Such a structurally complex assemblage is difficult to image with reflection data because steep dips are pervasive and because geologic structures are varying at wavelengths shorter than the Fresnel zone. The coherent reflections that do occur within the layer are particularly prevalent on the south end of the PG&E-2 profile. These may arrive from a variety of interfaces. As discussed above, fault surfaces formed during basin formation may extend into the Franciscan basement. Onshore, coherent, well-bedded sandstone units are known to occur within Franciscan melange (Page, 1981). Similar blocks may occur offshore. Finally, intrusion of sills into the Franciscan may have accompanied Miocene volcanism within the offshore Santa Maria basin. Deep-crustal reflections The lower crust is marked by an onset of reflectivity that begins at 5-6 s and persists for l-2 s (Fig. 2). The reflections are sub-horizontal and fairly continuous along the entire profile. A transition occurs near kilometer 25. Northwest of this point, reflections arrive at approximately 5.5 s, while to the southeast the reflectivity begins at approximately 6.5 s. The transition is marked only by two sets of reflections (A and B, Fig. 2) that have been interpreted as coming from out of
CALIFORNIA
65
the plane. Continuous reflections appear to die away by 7-8 s to the northwest and by 8.5-9 s to the southeast. This sudden onset of reflectivity is interpreted as a lithologic change from Franciscan metasediments to the gabbros of a tectonically underplated oceanic-crustal layer. The base of reflectivity, at least on the southeast end of the profile, marks the crust-mantle boundary. At 2 s (6-7 km), the reflective package on the southeast half of the profile (Fig. 2) is approximately the correct thickness for a layer of typical oceanic crust. Two velocity models derived from wide-angle reflection data that cross the PG&E-2 transect (Putzig, 1988; Howie et al., 1992) provide further evidence for the change in lithology. In both models, a 12-15-km-thick package of Franciscan rocks with a velocity of 5.3-5.8 km/s is floored by a 6-lokm-thick layer with a velocity of 6.8 km/s, a value typical of gabbro (e.g., Christensen and Salisbury, 1975). The wide-angle models place top of oceanic crust at 12-15&m depth. This corresponds to twtt of 5.5-6.5 s, which is where the inferred lithologic change occurs on PG & E-2. The PG&E-2 record images a large step in the oceanic crust. At the northwest end of the line (Fig. 71, the inferred top of oceanic crust occurs at a depth of 12 km, but near kilometer 25 the top of oceanic crust reflections drops 3 km to 15-km depth. The transition occurs over a distance of 5 km without intervening reflections to suggest the nature of the transition. Wide-angle reflection data confirm the change in depth to oceanic crust across the line. The velocity model of Putzig (1988), which crosses PG&E-2 at its northwest end (PG&E-1, Fig. 7) puts oceanic crust at 12 km depth, whereas the model of Howie et al. (1992) (PG&E-3, Fig. 7), puts top of oceanic crust at 15 km to the south. The Putzig model also constrains the thickness of the oceanic layer to 10 km at the northwest end of PG&E-2. Since PG&E-2 does not image well the base of the crust northwest of km 25, the PG&E-1 velocity model supplies evidence for oceanic crust that is 3-4 km thicker than to the southeast. The step observed on PG&E-2 is interpreted as the crest and limb of an antiform that lies beneath the offshore Santa Maria basin. The
66
K.C.
relatively shallow oceanic crust on the northwest endof the profile (Fig. 7) is the crest of the feature. One limb of the antiform is located near kilometer 25, where the oceanic-crustal layer steps down 3 km over a 5-7-km distance. Based on reflection terminations on either side of the break, the limb has a minimum apparent dip of 30”. The other limb of the antiform is not imaged on PG&E-2 but would be located beyond the northwest end of the profile. PG &E-2 provides the primary seismic evidence for a WNW-trending antiform in the oceanic crust. The presence of the antiform is inferred from the wide-angle velocity models (Putzig, 1988; Howie et al., 19921, but none of the other reflection profiles provide a continuous image of structure at the Franciscan/oceanic crust interface beneath the offshore Santa Maria basin (Meltzer and Levander, 1991; McIntosh et al., 1991; Miller et al., 1992). Reasons that might account for poor imaging on these records include poor signal penetration and scattering of energy in the mid-crustal Franciscan layer. Integration of the PG&E-2 results with crossing wide-angle velocity models constrains the WNW trend of the antiform and shows that it has a minimum length of 50 km. A fence diagram
Fig. 8. A fence diagram basins.
Regions
illustrating
the relationship
filled with + are interpreted
include
the California
oceanic
crust crosses
coast (thick PG&E-2
oceanic
stippled
obliquely.
of PG&E-2
The antiform beneath
(Fig. 8) illustrates PG&E-2’s relationship to the structure and to the cross lines PG&E-1 and PG&E-3. The PG&E-1 and PG&E-3 depth models presented in the diagram follow the wide-angle velocity model of Putzig (1988) along PG&E-1, the gravity model of Miller et al. (1992) along PG&E-1, and the velocity and gravity models of Howie et al. (1992) along PG&E-3. PG&E-2 images the antiform at approximately 40” to the strike of the feature. At its south end, PG&E-2 crosses line PG&E-3 about lo-15 km west of the antiform. Moving northward, line PG& E-2 crosses up onto the structure. The PG & E-l / PG & E-2 intersection lies above the crest of the structure. The 50-km distance between PG&E-1 and PG&E-3 leads to the minimum length estimate for the antiform. Gravity modeling along the RU-13 profile (Miller et al., 1992) extends this minimum length to 80 km. Possible origins for the antiform include primary topography in the oceanic crust and secondary deformation of the crust. Possible primary features include fracture zones and seamounts formed on oceanic crust prior to the time the crust enters the subduction zones. Secondary features might include structures formed in the crust after it entered the subduction zone. Seamounts
to the other
crust. Intervening
line), Hosgri
Fault
MIILEK
(HF)
lies northeast
the PG&E-l-PG&E-2
PG&E
layer represents and Santa
Lucia
transects. Bank
of the PG&E-2-PG&E-3 intersection.
Stippled
Franciscan
regions
are sedimentary
rocks. Geographical
fault (SLBF).
references
The antiform
line intersection,
in the
but lies directly
CRUSTAL
STRUCTURE
ALONG
THE
OFFSHORE
SANTA
MARIA
BASIN,
lack the minimum 3 : 1 aspect ratio required by the 80 by 25 km map dimensions of the antiform (e.g., Batiza, 1989). Large vertical tectonism is not normally associated with a small offset fracture zone (e.g., Bonatti, 1978). Although the antiform is sub-parallel to the Morro fracture zone, magnetic anomalies on the Monterey and Pacific plates (Lonsdale, 1992) lack offsets that might indicate the existence of another fracture zone beneath the margin. A secondary origin, such as imbrication of the oceanic-crustal layer (Putzig, 1988) provides the best fit to the antiform geometry and to Miocene plate tectonic models for central California. An imbrication would result in thickening of the crust and considerable vertical relief. In addition, a thrust could also extend large distances along strike. Miocene plate motion studies by Lonsdale (1992), Severinghaus and Atwater (1991) and Fernandez and Hey (1991) provide a source of the compression. Relative plate motion between the Monterey plate (a fragment of which now lies beneath the central California margin) and the Argue110 plate to the south contained a component of convergence across the intervening Morro fracture zone that may have resulted in as much as 80 km of intraplate shortening (Miller et al., 1992). The antiform may be one manifestation of this shortening. Conclusions
The PG&E-2 seismic profile provides an intriguing image of margin-parallel crustal structure in the central California offshore. Here, the crust is composed of three fundamental layers: Tertiary sediments in the offshore Santa Maria basin, a 12-15 km-thick basement layer of Franciscan rocks and 7-10 km of oce.anic crust. The basement of the offshore Santa Maria basin appears as a series of undulating highs and lows on PG&E-2. Near the middle of the transect, the average depth to basement increases by nearly a kilometer. Patterns of deposition and deformation also change at this location. Seismic sequence analysis along with well log data show that basement blocks began subsiding in Miocene time. A Late Miocene to Early Pliocene shorten-
CALIFORNIA
67
ing episode pushed up basement blocks and folded basin strata. A series of sub-horizontal reflections originating from 12 to 15 km depth are interpreted as an oceanic-crustal layer that has been mechanically underplated to the base of the Franciscan rocks. A 3-km change in depth to the top of oceanic crust that occurs near the middle of the PG&E-2 transect is the seismic evidence for a WNWtrending antiform that lies beneath the central California margin. Although nearby wide-angle velocity models help constrain the trend of the feature, PG&E-2 is the only profile that directly images it. Other profiles in the area lack reflections from the oceanic-crustal depths due to either a poor signal-to-noise ratio or scattering of seismic energy in the upper and middle crust. The seismic image on PG&E-2 provides insight into the tectonic evolution of the central California offshore. In the Early Miocene, previously subducting oceanic crust probably became coupled to the base of the Franciscan Complex as the new transform regime evolved at the California margin. At the same time, subsidence of the offshore Santa Maria basin began. Thus, it was in Miocene times that the basic components of offshore crustal structure observed today were amalgamated. Later, the upper crust, at least, underwent compression in response to a change in Pacific-North America relative plate motion at 3-5 Ma. It is not clear how this event affected the lower crustal structure as the only candidate for a shortening structure, the antiform, strikes at a 25” angle to upper-crustal faults and folds. The antiform may be a secondary feature formed in a heterogeneous stress field that probably existed during the last stages of subduction in the Early Miocene. An alternative model is that the antiform represents primary oceanic crust topography, such as a seamount or a fracture zone.
Acknowledgments
The seismic data were provided by Pacific Gas and Electric Company. Data-processing was done at the facilities of the Stanford Exploration Project. Discussions with Woody Savage, Walter
K.C. MILLER
68
Mooney and John Howie greatly improved the initial manuscript.
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