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A. Structure


logoThe 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.

logo logo 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.

logo 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)

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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).

logo 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:

"1926, October 22, 4:35 A.M. Center on the continental shelf off Monterey Bay. Intensity VIII at Santa Cruz, where many chimneys were thrown down: VII at Capitola, Monterey, Salinas and Soquel. Felt from Healdsburg to Lompoc (a distance of 250 miles [450 km]) and east to the Sierra, an area of nearly 100,000 square miles [180,000 km²]. Another shock one hour later was similar to the first in almost every respect."

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).


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Section II. Physiography
 
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