In chapter III the wavenumber filtering method helped in the analysis of gravity data of southern California. Many contour maps were created which highlighted various aspects of the gravity field. These maps gave new insight on the size, shape and magnitude of structural features in the region. From this analysis boundaries of similar gravity anomalies were determined. To understand better the tectonics of the region, these boundaries are now compared to other types of geological data.
A great deal of research is currently underway in the field of paleomagnetism. By measuring the orientation of the magnetic particals in igneous and sedimentary rocks, the location of the rocks with respect to the earth's magnetic poles at the time of origin can be determined. This information can be used to track the motion of individual terranes. Preliminary results indicates that much of southern California and Baja California have undergone large amounts of lateral motion since Miocene time. Paleomagnetic data from the western Transverse Ranges also show portions of the region have undergone large amounts of rotation. It is still unclear whether the region traveled as a single unit or many individual terranes.
The filtered gravity maps may show possible boundaries for individually rotated terranes. This chapter compares the gravity in the western Transverse Ranges to geologic and paleomagnetic data of the region. To understand better possible cross-sectional structures, two-dimensional forward modeling of the gravity data is performed along profiles across the Santa Monica Mountains and the California Continental Borderland. From this study, a new model of transport, collision and rotaion is proposed to explain the formation of the western Tranverse Ranges and their relationship to the Borderland.
The major gravitational feature of Western Transverse Ranges is an east-west trending relative maximum anomaly corresponding to the Santa Monica Mountains and the northern Channel Islands. Paleomagnetic data indicate that both the Santa Monica Mountains and the northern Channel Islands of Anacapa, Santa Cruz and perhaps San Miguel, have undergone a northward lateral motion of 15 degrees with respect to the North American continent and clockwise rotation of 90 to 120 degrees (Kamerling and Luyendyk, 1979, Kaplan and others, 1984).
The surface expression of the Santa Monica Mountains consists of a broad asymmetrical westward plunging anticline of middle Miocene sediments (Greene, 1976). Ninety-nine million year old granite intrusions are found in the eastern Santa Monica Mountains. These plutonic rocks intrude Santa Monica slate of the same age (Yerkes, and others, 1965, Campbell, 1976, Elhig, 1981). Volcanics of middle Miocene age are found throughout the mountains; the western end of the range they are called the Conejo Volcanics. These volcanics rest on lower Topanga Formation and have an apparent thickness of 3.6 to 4.5 kilometers. In the east, the volcanics are called Glendora Volcanics. These rest on Cretaceous sediments and have a maximum thickness of 1.3 kilometers. Dikes and sills of diabase, basalt and andesite are found throughout the region intruding rocks older than middle Miocene (Yerkes, and others 1965). Sorensen (1984) has reported pieces of mafic rock in drill cores in the eastern Santa Monica Mountains. Sorensen suggests that these rocks have been brought up from depth along the Santa Monica fault. As will be shown, a small mafic to ultramafic body in the core of the Santa Monica Mountains would explain the high gravity anomaly of the region.
In the northern Channel Islands, Miocene metavolcanic and related intrusive rocks are found on Anacapa and Santa Cruz Islands. These volcanics are similar to the Conejo Volcanics and the Glendora Volcanics in the Santa Monica Mountains and show both tholeiitic and calcalkaline affinities (Higgins, 1976). This similarity suggests that at least Anacapa and Santa Cruz Islands are part of the Santa Monica terrane (Higgins, 1976, Crowe and others, 1976, Fisher and Charlton, 1976). However, some authors believe that the geology of these islands suggest they are more a part of the Borderland than the Transverse Ranges (Howell and others, 1976, Howell and Vedder, 1981, and others). The filtered gravity maps discussed in chapter III supports the view that the northern Channel Islands are part of a larger Santa Monica terrane and that these islands only happen to line up with the ridges of the Borderland.
To the north and parallel to the Santa Monica Mountians - northern Channel Islands gravity anomaly is a smaller relative maximum anomaly corresponding to the Santa Ynez Mountains. In this range are the Santa Ynez, San Rafael and Topa Topa mountains. In the eastern Santa Ynez mountains, Hornafius (1984) shows 5 degrees of northern lateral motion and up to 90 degrees of clockwise rotation. In the western Santa Ynez Mountians, Kaplan (personal communication) shows similar lateral motions, but no rotations.
Between the Santa Monica Mountains-northern Channel Islands anomaly and the Santa Ynez Mountains anomaly is a parallel relative minimum associated with the Ventura Basin. This basin can be divided into the Santa Clara Trough in the northeast, the Oxnard Shelf in the southeast, and the Santa Barbara Basin in the west. The Santa Clara Trough, bounded by the Red Mountain thrust and San Cayetano thrust on the north and the Oakridge fault on the south, contains Neogene sediments of up to 6.5 kilometers thick. South of the Oakridge fault, Neogene sediments only reach 1.2 kilometers in thickness (Crowell, 1976). Wells in the Ventura basin and Santa Barbara Channell have gone as deep as 12 kilometers without reachng basement (Mike Reblin, personal communication).
Crowell (1976) proposed a model for shallow structure of the Santa Clara Trough. By removing deformation found in present day structure, Crowell was able to develop a pre-shortening model that shows a thick wedge of sediments in the Santa Clara Trough separated from a thin wedge on the Oxnard Shelf by the Oakridge fault. His explanation was that the margins of the trough swung away from each other during late Miocene time to form the Santa Clara Trough. This deep basin filled with sediments and closed again during the Pleistocene to cause the deformation. This model does not fit well with the paleomagnetic data currently available.
To understand further the geology of the region, it is useful to perform forward modeling along selected profiles across the region. Forward modeling consists of entering the shape and density of one or more geologic bodies into a computer program to calculate their gravity effect at stations at or above the surface. In two-dimensional modeling, the bodies are represented by polygonal prisms and the stations occur along a profile. After performing the calculations, the user compares the results to the actual gravity measurments. Adjustments are made to the shape or density of the bodies and the program rerun until the calculated anomaly curve matchs the actual curve satisfactorly.
Most forward modeling programs are based on Talwani et al. (1959), and Talwani and Heirtzler (1964). These routines assume that all bodies are infinite along strike. That is they are of infinite length in and out of the profile. This assumtion does not cause too much error if the profile is taken near the center of the bodies. Cady (1980) added end corrections to Talwani's equations that allow the user to specify the length of the bodies to the right and left of the profile. The computer program used in this study is based on Cady (1980) and is listed in Appendix A. It allows the user to model bodies of finite or infinite length.
Modeling along two profiles has been performed. Figure 13 shows the location of these profiles. The first profile is along column 40 of the gravity matrix used for filtering. The profile begins in the south in the Santa Monica basin. It includes the Santa Monica Mountains, the Oxnard shelf and the Santa Clara trough of the Ventura basin, and ends in the Santa Ynez Mountains. The second profile is along row 12 of the matrix. It crosses the California Continental Borderland in a west to east direction. Begining at the Santa Rosa Cortez rigde, the profile includes the Santa Cruz basin, Santa Cruz rigde, Santa Monica basin, the extension of the Palos Verdes peninsula, and ends in the Los Angeles basin. The results of this modeling are used to develop a tectonic explanation for the formation of the western Transverse Ranges.
Two-dimensional modeling across the Santa Monica Mountains shows that this anomaly can be explained by a high density body 40 kilometers wide and less than 10 kilometers thick flanked by two sedimentary basins (figure 14a). The northern and southern limits of the high density body correspond well to the Malibu - Santa Monica fault on the south, and the Oakridge fault on the north. Under these bodies is a moderately dense layer. Assuming that the sediments in the basins have a density of 2.4 g/cm3, the body under the Santa Monica Mountains would have a density of 2.9 g/cm3, and the underlying body would have a density of 2.5 g/cm3.
An alternate model (figure 14b) can be developed where the body causing the Santa Monica Mountain anomaly continues under the basins. Again, the sedimentary basins have a density of 2.4 g/cm3, and the underlying body has a density of 2.9 g/cm3.
The density of the modeled body causing the Santa Monica Mountain anomaly corresponds well with mafic to ultra-mafic rock reported in deep wells near the Santa Monica fault by Sorencen (1984). Ultramafic rocks have been found on Santa Cruz Island by Mattinson and Hill (1976). These authors suggest that the Islands are underlain by oceanic crust similar to the Coast Range ophiolite. The gravity data suggest that the entire length of the Santa Monica Mountain-northern Channel Island terrane has a mafic to ultramafic core. A geological interpretation of the two-dimensional gravity model presented in figure 14c suggests a mafic to ultra-mafic body sitting on Franciscan-Catalina basement material flanked by sediments in the Los Angeles Basin in the south and the Ventura Basin in the north. A similar type of model was proposed by Elhig (1981).
The gravity interpretation conducted in this study, as well as the paleomagnetic data that is now available, leads to a tectonic model of the western Transverse Ranges and the California Continental Borderland. Gravity modeling has shown that the Santa Monica Mountains and the northern Channel Islands have cores of high density material that are floating on the Pacific plate. Similar models may be developed for the ridges of the Borderlands. Paleomagnetic data indicates that these terranes sitting on top of Pacific plate have been transported from low latitudes to their present location during the Miocene. Paleomagnetic data also indicates that the Santa Monica terrane, including the Santa Monica Mountains, the northern Channel Islands, and the southern half of the Ventura Basin, has rotated more than 135 degrees clockwise during the Miocene. A model of rafting, rotation and collision can be used to explain the origin of the Santa Monica Mountains, Santa Clara Trough, Oxnard Shelf, Santa Ynez Mountains, and the Los Angeles Basin.
Gravity modeling indicates that the terrane consists of a high density mafic body extending the length of the Santa Monica Mountains and northern Channel Islands with a sedimentary cover on top and a thick sedimentary wedge on the northern flank. No sedimentary wedge is found on the southern flank. The surface expression of the Santa Monica Mountains is not a result of the high density body, but is a simple plunging anticline formed in the sedimentary cover during compression of the region (Ehlig, 1981).
Paleomagnetic data indicate that prior to the Miocene the Santa Monica terrane was 10 to 15 degrees farther south with respect to North America. Depending on the relative motion of the Peninsular Ranges, this would place the terrane somewhere in between the Santa Ana Mountains and the Vizcaino Peninsula in Baja California (Kamerling and Luyendyk, 1979) with its long axis parallel to the continent. Such a configuration would have the southern side of the terrane facing east against the continent. The present northern side of the terrane, the sedimentary wedge, would have faced west and oceanward, as one would expect. During the Miocene, the terrane detatched from the North American continent, perhaps along a former suture zone, and became attatched to the underlying Pacific plate.
As the terrane was carried on the Pacific plate, its northern end came in contact with the North American continent. With the terrane being long and narrow, this contact caused the terrane to be rotated clockwise. Its long axis collided against the continent. The outboard sedimentary wedge of the terrane collided with the south facing sedimentary wedge of the continent, forming the Oxnard Shelf on the south and the Santa Clara Trough on the north. The collision of the terrane with the continent caused the deformation that formed the east-west structure of the western Transverse Ranges. At a later time, the northern Channel Islands block was faulted away from the Santa Monica Mountains to their present position.
The Santa Monica Mountains, the Santa Ynez Mountains and the Ventura Basin are compressional features formed when the Santa Monica terrane collided with the North American continent as the terrane was being rotated. The Ventura Basin is not a basin formed by extension, but a result of two sedimentary wedges that collided and deformed into a synclinal feature. The Los Angeles Basin just to the south of Santa Monica terrane, could be a sedimentary basin formed in the wake of the rotating terrane. As the Santa Monica terrane rotated clockwise, sediments filled in the region behind.
The ridges of the California Continental Borderland, including the Palos Verdes Peninsula, Santa Catalina Island, the San Clemente Ridge, and the Santa Rosa Cortes Ridge, may be similar terranes that have detatched from the North American continent and have been carried northwestward on the Pacific plate. The Pacific plate acted as a sort of treadmill. The difference in composition of various ridges may be due to the original location of the terranes on the continent with respect to the four Jurassic belts proposed by Jones, et al. (1976). The basins between the ridges in the Borderland may not be a result of extension, but gaps between blocks that have filled with sediments.
A model to explain the tectonics of the Western Transverse Ranges and the
California Continental Borderland has been developed. These provinces were
formed by terranes breaking away from the continent, attaching themselves to
the Pacific plate, and being carried northwestward to their present positions.
One very large piece, the Santa Monica terrane, came in contact with the
continent and was rotated into it. The collision of the terrane with the North
American continent caused the formation of the western Transverse Ranges.
