III. Geology and Tectonics
A. Structure
The MBNMS is located along the active transform boundary (the San Andreas
fault system) separating the Pacific Plate from the North American Plate.
Here the fault system is over 100 km wide and incorporates faults in the
offshore, including those of the Palo Colorado-San Gregorio and Monterey
Bay fault zones (Figure 2). These fault zones are
seismically active, and in many places offset the seafloor or Quaternary
sedimentary rocks (Greene et al. 1973, 1989; Greene 1977, 1990; McCulloch
and Greene 1990; Cockerham et al. 1990). A paleo-subduction zone occurs
along the MBNMS western boundary (McCulloch and Greene 1990); the fossil
thrust faults in this zone appear to control the structure at the base of
the continental slope.
Most of the northern and central parts of the MBNMS lies within the Salinian
block. It is composed of allochthonous (i.e. transported to local region) Cretaceous granitic
basement material, primarily overlain with Neogene marine sedimentary units;
it has been tectonically slivered into its present position (Page 1970,
Page and Brocher 1993) This block has been carried upon the Pacific Plate
as the plate moves northward, slipping along the San Andreas fault for about
the past 21 million years.
In the Monterey Bay region, the plate boundary between the North American
and Pacific plates is comprised of the San Andreas fault system, consisting
of the Hayward-Calaveras and San Andreas fault zones on land, and the offshore
Palo Colorado-San Gregorio fault zones (Figure 3).
The Palo Colorado-San Gregorio is the major active fault zone within the
MBNMS. It is a right-lateral strike-slip fault zone oriented generally north-south,
comprised of two or more parallel and fairly continuous fault segments that
extend at least 100 km from Point Año Nuevo (Weber 1990; Weber et
al. 1980,1979) in the north to Garrapata Beach (10 km north of Point Sur)
(Greene et al. 1973; McCulloch and Greene 1990; Greene 1990). The amount
of right-lateral offset along this fault zone has been measured by different
methods and at several locations (Clark et al. 1984, Graham and Dickenson
1978, Greene 1977, Howell and Vedder 1978, Silver 1977, Underwood 1995);
offset varies from 80-90 km (Silver 1977) to as much as 150 km (Clark et
al. 1984).
The Monterey Bay fault zone is a wide (~10 km), en echelon (i.e. composed
of short, discontinuous, offset, roughly parallel faults) formation comprised
of many fault segments ranging from 5 km or less up to 15 km in length (Greene
et al. 1973; Greene 1977, 1990; Gardner-Taggart 1991; Gardner-Taggart et
al. 1993).The Monterey Bay fault zone is either truncated or merges with
the San Gregorio fault segment of the Palo Colorado-San Gregorio fault zone
(Greene 1970, 1990; Mullins and Nagel 1981, 1983; Nagel et al. 1986; Mullins
and Nagel 1990).
Monterey Canyon cuts across the generally north-south trending offshore
faults in Monterey Bay. It is a large submarine canyon that bisects the
Bay and has eroded deeply into the Salinian block and the overlying Neogene
sedimentary rocks of the Miocene Monterey Formation, Santa Cruz Mudstone,
Santa Margarita Formation, and the Pliocene Purisima Formation (Shepard
and Dill 1966; Martin 1969; Greene 1970, 1990; Greene et al. 1991). The
canyon is the result of tectonic activity occurring ever since subduction
of the Pacific Plate ceased and transform motion began, about 21 million
years ago (Atwater 1970; Greene 1977, 1990; Greene et al. 1989, 1991). Landslides
and turbidity currents created by mass wasting events (Greene et al. 1991,
and see Mass Wasting) steepen the canyon's walls, expose basement and bedrock,
and erode the canyon.
Offshore, the Ascension fault (questionable Sur-Nacimiento fault of McCulloch
1989a) generally extends northwestward for over 180 km from the Palo Colorado-San
Gregorio fault zone, where it appears to be truncated near the southernmost
head of Cabrillo Canyon (Figure 4). However, continuation
of this fault to the south for 60 km, where it may tie into the Sur thrust
fault zone, has been proposed by Greene (1977) and is questionably mapped
by McCulloch and Greene (1990).
The southern part of the MBNMS includes allochthonous Sur-Obispo terraine,
composed of basement rocks of Jurassic to Cretaceous age (McWilliams and
Howell 1982). The eastern boundary of this block is defined by the Sur-Nacimiento
and Hosgri fault zones along the Big Sur coastline (Page 1970; Graham 1978;
McCulloch 1989a). The western boundary is defined by the Santa Lucia Bank
fault zone at the base of the continental shelf (McCulloch et al. 1980;
McCulloch 1989b).
B. Stratigraphy
Nearly everywhere on the Salinian block the granitic surface is eroded.
In the northern and east-central part of the MBNMS, deep water Miocene marine
sedimentary rocks (Monterey and Santa Cruz Mudstone Formations) unconformably
overlie the basement rocks (Greene 1977, 1990; Clark and Reitman 1969).
Older Tertiary sedimentary rocks are found in basement depressions both
onshore and offshore. The most extensively preserved accumulation of these
older rocks are found in the Outer Santa Cruz Basin of the northern segment,
where Paleocene, Eocene and Oligocene rocks underlie the Miocene sequence
ubiquitous to the whole region (Hoskins and Griffiths 1971, Heck et al.
1990). In addition, a small Paleocene proximal submarine canyon/distal submarine
fan deposit (the Carmelo Formation) is found at Point Lobos. With the exception
of the Miocene volcanic seamounts located outside the MBNMS boundary, few
volcanic rocks have been mapped in the northern or central segments of the
MBNMS seafloor. An exception occurs in Soquel Canyon, where a small outcrop
of volcanic rocks was discovered recently and appear to have been slivered
into place by movement along faults of the Monterey Bay fault zone. Patches
of volcanic rocks do occur on shore (Clark et al. 1974).
Two major unconformities exist within the MBNMS (Greene 1990). One unconformity
is found on the basement rock surface of the Salinian block, and the other
at the top of the Miocene sequence. A regional Miocene unconformity stretches
from the mouth of the San Francisco Bay southward into the Salinas Valley
and the northern part of the Santa Lucias. A physiographic representation
of the truncated surface of the unconformity can be seen in the northern
segment of the MBNMS, where a Pliocene and later prograding shelf is well
defined (Figure 1).
Basement rocks in the southern segment of the MBNMS are mainly Jurassic
Franciscan melange and metamorphic rocks, Cretaceous sandstones, and carbonates
of the Sur Series. Tertiary volcanic and sedimentary rocks locally overlie
the basement rocks, but are not well mapped. Overlying the basement
and Tertiary rocks are Quaternary shelf and slope deposits.
C. Mass wasting
Mass wasting events are common to the MBNMS (Figure 4), often due to earthquakes
(see III.D.). Landslides occur nearly everywhere along the
continental slope and on the walls of submarine canyons. Mass wasting due
to earthquakes in the Monterey Canyon produces turbidity currents, which
steepen and erode the canyon walls (Greene 1991). Major slumps and rockfalls
have also been mapped in the Ascension-Monterey canyon system, and in Sur
and Pioneer Canyons (Greene 1977, 1990; Greene et al. 1990; McHugh et al.
1992). A meander in Sur Canyon is the result of a slump (Greene et al. 1989).
An extensive landslide (Sur Slide) is located on the continental slope offshore
of Point Sur (Hess et al. 1979, Normark and Gutmacher 1988); its steep headwall
scarp is located just inside the MBNMS boundary. Coastal landslide and debris
flows regularly impact the nearshore part of the southern segment of the
MBNMS along the very steep cliff areas of the Santa Lucias, as well as the
Santa Cruz Mountains just south of Point Año Nuevo in the central
segment and the Devil's slide area to the north.
D. Earthquake activity (Seismicity)
The geologic structure and active seismicity of the MBNMS region indicate
that the central California margin has been, and is currently, subjected
to complex tectonic processes (Figure 5 and Figure 5 legend). The region
was subjected to nearly orthogonal (i.e. head-on) collision and subduction
until about 21 million years ago, when a transform margin formed (Atwater
1970; Atwater and Molnar 1973). Transtensional and transpressional structures
resulted from widespread transcurrent movement along faults of the San Andreas
fault system. These structures were either overprinted or altered and deformed
during and after a shift in the stress field (Cox and Engerbretson 1985).
Change in direction of the Pacific plate in relation to the North American
plate some 3-5 million years ago produced a more orthogonal convergence
that initiated formation of compressional structures parallel to the San
Andreas fault (Cox and Engerbretson 1985). Recent seismicity and geophysical
studies offshore indicate thrusting is occurring with the Palo Colorado-San
Gregorio and other fault zones in this region (Cockerham et al. 1990, Greene
1990).
The 1989 Loma Prieta earthquake exhibited the complex deformation characteristic
of this central California boundary (Plafker and Galloway 1989; Cockerham
et al. 1990; Figure 6). Both thrusting and strike-slip
displacement occurred, indicating that the structural pattern of the region
is the result of both transform movement and compression between the two
plates.
The largest recorded earthquakes in the MBNMS region occurred in 1926 and
1989. Steinbrugge (1968) has described the 1926 earthquakes as follows:
Detection of earthquakes and determination of their locations and focal
depths (hypocenters) in the Sanctuary has been difficult because the area
lies largely outside the network of permanently located seismographic stations.
However, ongoing refinement and expansion of the California Seismographic
Network (CALNET) has resulted in more accurate location and analysis of
earthquakes in this region.
On October 17, 1989, the 7.1 magnitude Loma Prieta earthquake occurred along
the San Andreas fault within the Santa Cruz Mountains (37°02' N latitude,
121°53' W longitude), approximately 15 km east-northeast of the city
of Santa Cruz and about 95 km southeast of San Francisco (Figure
5). Although this earthquake was not of great magnitude, the destruction
and damage in the Monterey Bay region was similar to that which occurred
during the great San Francisco earthquake of 1906 (Lawson et al. 1908).
The initial fault rupture (focal point) occurred at a depth of 18.5 km and
propagated along strike for nearly 40 km. In contrast, the 1906 earthquake
rupture length was about 450 km.
During the 1989 event, a subsurface area of over 300 km² ruptured along
a fault plane striking N50°W + 8° and dipping 70° +10°
to the southwest; direction of slip was 130° +15° (Plafker and
Galloway 1989). The earthquake resulted in approximately 4 m lateral displacement
and 3 m vertical displacement (ibid).
In general, both compressional and strike-slip motion is occurring along
the plate boundary of the MBNMS region. Recurrent earthquakes, although
not rupturing the ground surface, cause significant uplift in the coastal
mountains and stimulate erosion through landslides. In the valleys and coastal
areas, liquefaction and associated subsidence accelerates erosion locally
(Greene et al. 1990).
Next - Section IV. Ground Water
Geology Table of Contents