Geographically, the east-west trend of the western Transverse Ranges has been an enigma for some time. The range was given its name by physiographers because of its generally oblique orientation to all of the other mountain ranges in California. So the dilemma was recognized by physiographers even before geologists got involved in trying to decipher it. As an historical precedent, Francis Bacon in 1660 (Press and Siever, 2001) noted that the shape of the Atlantic Ocean basin was important long before its geological importance was deciphered.
An early indication that something was tectonically anomalous about the Los Angeles basin and southern California Inner Borderland (LAB/IB; defined by Crouch and Suppe in 1993 as the area between the Newport-Inglewood-Rose Canyon fault on the east, the Malibu Coast-Santa Monica-Raymond fault on the north, and the East Santa Cruz Basin fault on the west) was the difference in the stratigraphic sequence in the LAB/IB when compared with the surrounding regions of the western Transverse Ranges and the Peninsular Ranges (WTR/PR). The WTR/PR contain a relatively complete sedimentary sequence from the Jurassic through the Pleistocene. The sedimentary sequence throughout a majority of the LAB/IB contains no Mesozoic or Paleogene strata; where observed, lower and middle Miocene and younger strata rest directly on Catalina Schist basement rocks (Crouch and Suppe, 1993). In order to create this long hiatus in the geologic record in the LAB/IB requires either that the LAB/IB was a long-standing highland throughout the time interval from the Jurassic through the Paleogene (Reed, 1933; Corey, 1954), or that in the early Miocene the area was uplifted and all earlier deposited sedimentary rocks were eroded. This proposed highland was called "Catalinia" (Reed, 1933; Woodford et al., 1954). If Catalinia had been a long-standing highland, all strata deposited in the surrounding WTR/PR would contain debris eroded from the Catalina Schist, and current structures in those strata would indicate south-to-north transport in the WTR and west-to-east transport in the PR. If Catalinia had been uplifted and eroded during the early Miocene from a previously depositional setting, then lower Miocene strata in the WTR/PR would contain debris from the Catalina Schist as well as debris from any earlier strata that had been deposited, and early Miocene current directions would indicate south-to-north transport in the WTR and west-to-east transport in the PR. Of all the above-stated criteria that would demonstrate the existence of Catalinia, only the south-to-north transport directions in the WTR have been shown to exist. It is impossible on a three-day field trip to show the apparent absence of such pre-medial-Miocene debris in all the strata of the WTR and the PR, but the absence of such debris is what has kept geologists searching for a better explanation for the origin of the LAB/IB. The Catalina Schist and overlying middle Miocene Monterey Formation will be observed at Stop 11.
In 1925, Woodford described a unique lower to middle Miocene sedimentary unit that he named the San Onofre Breccia that is exposed in several places in southern California. This unit is a marine to nonmarine breccia composed mostly of angular, boulder to sand-size fragments of Catalina Schist (Stuart, 1979). The paleogeographic and tectonic importance of this breccia has been recognized ever since Woodford's (1925) paper was published, and the existence of the breccia was one of the best lines of evidence in support of the proposed highland of Catalinia discussed in Evidence Line 2 above. Certainly the provenance of the San Onofre Breccia had to be exposures of almost 100% Catalina Schist. For the PR, therefore, the type area for the San Onofre Breccia, the provenance had to be toward the west because erosion of any rocks in the PR would have produced a deposit that was not nearly 100% schist debris. Current directions in the deposit would be west-to-east directed. Likewise, for exposures in the WTR, the provenance had to be from what is today toward the south and current directions would have been south-to-north directed. If Catalinia had been a long-standing highland, there would be Catalina Schist debris in Paleogene strata in the PR and the WTR, as noted in Evidence Line 2 above, but there is not. This means that Catalinia had to be uplifted rapidly around the end of the early Miocene. But if Catalinia were not an eroding highland before the early Miocene, it would have been an area of pre-Miocene deposition. So when it became exposed in the early and medial Miocene, these pre-Miocene deposits would have to have been eroded before erosion reached the schist, and this pre-Miocene debris would be found in the breccia, but it is not. The presence of nearly 100% Catalina Schist debris in the San Onofre Breccia is important, therefore, because the only way to create a Catalina Schist source area that is devoid of younger rock types is by extensional tectonic denudation of the overlying rocks along low-angle detachment faults. Extension of the crust causes fracturing in the brittle surface rocks, which are then pulled apart in the hanging wall of a detachment fault so that the footwall schist rocks become exposed. The schist is then uplifted isostatically until it reaches the surface of the ocean where it can be eroded and serve as source material for the San Onofre Breccia. Because the San Onofre Breccia is not found everywhere in the LAB/IB (most areas have Monterey Shale on top of the Catalina Schist), it is likely that Catalinia was not a large highland, but rather a series of scattered islands, providing detritus at local sites of deposition. Note that it took six decades of expanding geologic understanding before we, as professional geologists, were able to understand this key piece of information that has perplexed us for so long. The models for the tectonic history of southwestern California by Yeats et al. (1974), Crouch (1979), Kamerling and Luyendyk (1985), and Crouch and Suppe (1993) involve crustal extension and would produce the San Onofre Breccia. The model by Howell et al. (1974), involving strike-slip faulting, is not an extensional model and would not produce the San Onofre Breccia. The San Onofre Breccia will be seen at Stops 6, 14, and 15. A similar unit having the same implications will be studied at Stop 12.
The oldest sedimentary unit in the WTR is the Santa Monica Formation (Hoots, 1931), which is Late Jurassic in age. The oldest sedimentary unit in the PR is the Bedford Canyon Formation (Allison, 1970), which is Medial Jurassic in age. Fossils are rare in the Santa Monica Formation, so the age could be extended; fossils in the Bedford Canyon Formation are more common and from several stratigraphic levels (Imlay, 1963, 1964). Although the ages are different, the units are broadly similar in lithology (Durrell, 1954; Dibblee, 1982), both are mildly metamorphosed (the metamorphism coming before deposition of the overlying Cretaceous units), and both have an affinity with the Mariposa Formation of the Sierran western foothills belt of Jurassic rocks in northern California (Imlay, 1963; Jones et al., 1976). Structural trend (bedding, fold axes, etc.) in the Bedford Canyon Formation of the PR and the Mariposa Formation of the Sierran western foothills belt is generally north-south, whereas the structural trend in the Santa Monica Formation of the WTR is east-west. This difference in structural trend prompted Jones and Irwin (1975) to propose post-Late Jurassic rotation for the Santa Monica Mountains. The Santa Monica Formation will be seen at Stop 1 and the Bedford Canyon Formation at Stop 18.
A suite of unique, exotic, and ultrahard rhyolitic clasts has been known from Paleogene strata in the southern California coastal area for some time (Woodford et al., 1968). These clasts are referred to as "Poway" clasts because of their abundance in the middle Eocene Poway Formation near the town of Poway, California, northeast of San Diego. Their source has been traced to rocks in northwestern Sonora, Mexico (Merriam, 1968; Abbott and Smith, 1989). Discovery of these clasts in conglomerate units on the northern Channel Islands (Merschat, 1971; Parsley, 1972; Yeats et al., 1974; confirmed chemically by Abbott and Smith, 1978) presented a problem because the transport distance from the San Diego area to the present location of the islands is much too great. Yeats et al. (1974) shortened the transport distance by suggesting large, eastward, lateral backtranslation for the Santa Monica Mountains and the northern Channel Islands along the Malibu Coast-Santa Monica-Raymond-Cucamonga fault, bringing the Santa Monica Mountains up against the northern end of the Santa Ana Mountains and thus closer to the San Diego area. Howell et al. (1974) suggested that the problem could be solved with left-lateral backslip along offshore right-lateral faults that would bring the northern Channel Islands southward toward the San Diego area. Kies and Abbott (1982), in their summary paper on Paleogene conglomerates, apparently used the Howell et al. (1974) model in their paleogeographic reconstructions. Crouch (1979), Kamerling and Luyendyk (1985), Crouch and Suppe (1993), and Fritsche (1998) shortened the distance between the northern Channel Islands and the San Diego area by backrotating the WTR in a counterclockwise direction until the two areas became adjacent. Poway clasts will be discussed and searched for at Stops 8, 10, and 17.
Paleogene current directions, summarized by Yeats et al. (1974), are dominantly east-to-west in the PR and south-to-north in the WTR. Yeats et al. (1974) used this paleocurrent data, along with Poway-clast distributions, to postulate large, eastward, lateral backtranslation for the Santa Monica Mountains along the Malibu Coast-Santa Monica-Raymond-Cucamonga fault. These paleocurrent directions do not as easily support the strike-slip models of Howell et al. (1974) and Kies and Abbott (1982). Paleogene paleocurrent data can also be used, however, to support the rotation hypothesis of Crouch (1979), Kamerling and Luyendyk (1985), and Crouch and Suppe (1993) because after ~90Á of counterclockwise backrotation, the paleocurrents of the PR and WTR become parallel. Ripple marks indicating south-to-north paleocurrents will be seen at Stop 10.
The nonmarine parts of the Paleocene successions that occur in the Santa Monica Mountains of the WTR province (Colburn and Novak, 1989) and in the PR province (Colburn, 1995) are so similar in lithologic character and stratigraphic order that they seem to require very close proximity of the two provinces during the time they were deposited. The successions consist of (1) an erosion surface named the Runyon Canyon erosion surface (Colburn et al., 1988) cut on Cretaceous strata, (2) a deeply weathered zone in the underlying Cretaceous strata, (3) a basal conglomerate, (4) a carbonaceous interval, (5) an interval of white, biotite-rich, cross-bedded arkose interbedded with red and green mudstone, and (6) an iron-pisolite-bearing porcellanite that was created by intense weathering of an arkose. These Paleocene sequences will be seen at Stops 7 and 20.
Large volumes of lower to middle Miocene volcanic flows and intrusive dikes and sills occur in the WTR/PR and the LAB/IB regions (Weigand, 1982). These igneous rocks were intruded and extruded during a relatively short period of time soon after the San Onofre Breccia was deposited. Because it is well established that subduction was no longer taking place in this region, subduction cannot be called on as a melting mechanism for the production of the volcanics. Their origin, therefore, is postulated to result from decompression melting (Weigand and Savage, 1999), which would only be produced in an extensional setting. In addition, the abundance of dikes favors an extensional setting. Presence of these extensional volcanics favors the models of Yeats et al. (1974), Crouch (1979), Kamerling and Luyendyk (1985), and Crouch and Suppe (1993) over the model of Howell et al. (1974), which does not require extension. These igneous rocks will be studied at Stops 2, 3, 4, and 13.
The single most important discovery in the search for a paleogeographic model to explain Evidence Lines 1 through 8 was evidence produced from paleomagnetic studies of rocks, mostly volcanic, in the southern California region. In 1979, Kamerling and Luyendyk published the first of several studies that made use of the paleomagnetic properties of rocks to indicate the direction to the north magnetic pole at the time the rock was formed. As each new study appeared over the next decade, the evidence mounted that indeed the entire WTR had rotated clockwise about 90Á since the time the rocks had either cooled or been deposited. This discovery proved to be the key that unlocked the solution to the Tertiary tectonic history of southern California. Backrotation of the WTR about 90Á into a north-south orientation produces the following results: (1) the physiographic trend of the WTR (Evidence Line 1) and the structural trend of the Santa Monica Formation (Evidence Line 4) are brought to north-south, parallel to the PR and the Sierrra Nevada; (2) the future site of the Catalina Schist basement rocks and the Catalinia islands is covered by WTR strata, bringing the separated Mesozoic and Paleogene sequences of the Santa Monica Mountains and the PR into contact with each other (Evidence Lines 2 and 7); (3) the paleocurrents of the WTR and the PR (Evidence Line 6) become parallel and allow Poway clasts (Evidence Line 5) to be transported to the future northern Channel Islands area in just a short distance; and (4) extension and clockwise rotation of the WTR away from the PR during the early and medial Miocene by a detachment process creates both decompression melting and intrusion and extrusion of volcanic rocks (Evidence Line 8) and the tectonic denudation necessary to produce the San Onofre Breccia (Evidence Line 3).
Soon after paleomagnetic evidence for WTR rotation was discovered, rotation models for the Tertiary tectonic history of southwestern California were introduced because the previous models of Yeats et al. (1974) and Howell et al. (1974), which did not include rotation, were no longer valid. The earliest ones (e.g., Luyendyk et al., 1980; Luyendyk and Hornafius, 1987) used right-shearing shutter panels in the LAB/IB to account for rotation of the WTR. These models did not use a full 90Á of rotation and did not fully close up the LAB/IB, thus failing to address the problems associated with Evidence Lines 1 through 4 above. Crouch (1979), Kamerling and Luyendyk (1985), and Crouch and Suppe (1993) combined the concepts of rotation and extension to produce palinspastic models that eliminated the right-shearing shutter panels and restored the western Transverse Ranges to a position parallel with and immediately adjacent to the Peninsular Ranges before rotation and extension. These models satisfy all the constraints of Evidence Lines 1 through 8, but do not suggest a mechanism by which the rotation of the WTR might be accomplished. The rotation-extension model has since received support from Nicholson et al. (1994), who suggested that capture of the partially subducted Monterey microplate by the Pacific plate and rotation of the WTR block on top of the Monterey microplate as it was pulled by the Pacific plate provides a mechanism for the operation of the rotation-extension model. A more recent model by Fritsche (1998) is similar to Crouch and Suppe's (1993), but better addresses the details of the paleomagnetic rotation data.
Abbott, P. L., and Smith, T. E., 1989, Sonora, Mexico, source for the Eocene Poway conglomerate of southern California: Geology, v. 17, p. 329-332.
Allison, E. C., 1970, Basement rocks of the northern Santa Ana Mountains, in Vernon, J. W., and Warren, A. D., leaders, Geologic guide book, southeastern rim of the Los Angeles basin, Orange County, California: Pacific Section, American Association of Petroleum Geologists, p. 33-36.
Colburn, I. P., 1995, The Paleocene successions of the northern Peninsular Range province field trip stops, in Colburn, I. P., and Ramirez, P. C., eds., The Paleocene stratigraphic successions of the northern Peninsular Ranges, Orange and Riverside Counties, California: Pacific Section, Society of Economic Paleontologists and Mineralogists, book 79, p. 25-53.
Colburn, I. P., and Novak, G. A., 1989, Paleocene conglomerates of the Santa Monica Mountains, California: petrology, stratigraphy, and environment of deposition, in Colburn, I. P., Abbott, P. L., and Minch, J., eds., Conglomerates in basin analysis: a symposium dedicated to A. O. Woodford: Pacific Section, Society of Economic Paleontologists and Mineralogists, book 62, p. 227-253.
Corey, W. H., 1954, Tertiary basins of southern California, in Jahns, R. H., ed., Geology of southern California: California Division of Mines Bulletin 170, chap. 3, p. 73-83.
Crouch, J. K., 1979, Neogene tectonic evolution of the California continental borderland and western Transverse Ranges: Geological Society of America Bulletin, part I, v. 90, p. 338-345.
Crouch, J. K., and Suppe, J., 1993, Late Cenozoic tectonic evolution of the Los Angeles basin and inner California borderland: A model for core complex-like crustal extension: Geological Society of America Bulletin, v. 105, p. 1415-1434.
Dibblee, T. W., Jr., 1982, Geology of the Santa Monica Mountains and Simi Hills, southern California, in Fife, D. L., and Minch, J. A., eds., Geology and mineral wealth of the California Transverse Ranges: South Coast Geological Society, Mason Hill volume, p. 94-130.
Durrell, C., 1954, Geology of the Santa Monica Mountains, Los Angeles and Ventura Counties [California], in Jahns, R. H., ed., Geology of southern California: California Division of Mines Bulletin 170, map sheet 8.
Fritsche, A. E., 1998, Miocene paleogeography of southwestern California and its implications regarding basin terminology: International Geology Review, v. 40, p. 452-470.
Hoots, H. W., 1931, Geology of the eastern part of the Santa Monica Mountains, Los Angeles County, California: U.S. Geological Survey Professional Paper 165-C, p. 83-134, pls. 16-34.
Howell, D. G., Stuart, C. J., Platt, J. P., and Hill, D. J., 1974, Possible strike-slip faulting in the southern California continental borderland: Geology, v. 2, p. 93-98.
Imlay, R. W., 1963, Jurassic fossils from southern California: Journal of Paleontology, v. 37, p. 97-107.
Imlay, R. W., 1964, Middle and Upper Jurassic fossils from southern California: Journal of Paleontology, v. 38, p. 505-509.
Jones, D. L., Blake, M. C., Jr., and Rangin, C., 1976, The four Jurassic belts of northern California and their significance to the geology of the southern California Borderland, in Howell, D. G., ed., Aspects of the geologic history of the California Continental Borderland: Pacific Section, American Association of Petroleum Geologists, Miscellaneous Publication 24, p. 343-362.
Kamerling, M. J., and Luyendyk, B. P., 1979, Tectonic rotations of the Santa Monica Mountains region, western Transverse Ranges, California, suggested by paleomagnetic vectors: Geological Society of America Bulletin, part I, v. 90, p. 331-337.
Kamerling, M. J., and Luyendyk, B. P., 1985, Paleomagnetism and Neogene tectonics of the northern Channel Islands, California: Journal of Geophysical Research, v. 90, p. 12,485-12,502.
Kies, R. P., and Abbott, P. L., 1982, Sedimentology and paleogeography of lower Paleogene conglomerates, southern California continental borderland, in Fife, D. L., and Minch, J. A., eds., Geology and mineral wealth of the California Transverse Ranges: Santa Ana, California, South Coast Geological Society, p. 337-349.
Luyendyk, B. P., and Hornafius, J. S., 1987, Neogene crustal rotations, fault slip, and basin development in southern California, in Ingersoll, R. V., and Ernst, W. G., eds., Cenozoic basin development of coastal California, Rubey volume 6: Englewood Cliffs, New Jersey, Prentice-Hall, p. 259-283.
Luyendyk, B. P., Kamerling, M. J., and Terres, R. R., 1980, Geometric model for Neogene crustal rotations in southern California: Geological Society of America Bulletin, v. 91, p. 211-217.
Merriam, R. H., 1968, Geologic reconnaissance of northwest Sonora, in Dickinson, W. R., and Grantz, A., Proceedings of conference on geologic problems of San Andreas fault system: Stanford University Publication, Geological Sciences, v. 11, p. 287.
Merschat, W. R., 1971, Lower Tertiary paleocurrent trends, Santa Cruz Island, California [M.S. thesis]: Athens, Ohio University, 77 p.
Nicholson, C., Sorlien, C. C., Atwater, T., Crowell, J. C., and Luyendyk, B. P., 1994, Microplate capture, rotation of the western Transverse Ranges, and initiation of the San Andreas transform as a low-angle fault system: Geology, v. 22, p. 491-495.
Parsley, R. M., 1972, Late Cretaceous through Eocene paleocurrent directions, paleoenvironment and paleogeography of San Miguel Island, California [M.S. thesis]: Athens, Ohio University, 135 p.
Press, F., and Siever, R., 2001, Understanding Earth, 3rd edition: New York, Freeman and Company, 121 p.
Reed, R. R., 1933, Geology of California: Tulsa, Oklahoma, American Association of Petroleum Geologists, 355 p.
Stuart, C. J., 1979, Lithofacies and origin of the San Onofre Breccia, coastal southern California, in Stuart, C. J., ed., Miocene lithofacies and depositional environments, coastal southern California and northwestern Baja California: Pacific Section, Society of Economic Paleontologists and Mineralogists, p. 25-42.
Weigand, P. W., 1982, Middle Cenozoic volcanism of the western Transverse Ranges, in Fife, D. L., and Minch, J. A., eds., Geology and mineral wealth of the California Transverse Ranges: Santa Ana, California, South Coast Geological Society, p. 170-188.
Weigand, P. W., and Savage, K. L., 1999, Summary of the Miocene igneous rocks of the Channel Islands, southern California, in Browne, D. R., Chaney, H., and Mitchell, K. L., eds., Proceedings of the Fifth California Islands Symposium: a CD publication available from D. R. Browne at Minerals Management Service in Camarillo, California, p. 106-114.
Woodford, A. O., 1925, The San Onofre Breccia; Its nature and origin: California University Publications in the Geological Sciences, v. 15, p. 159-280.
Woodford, A. O., Schoellhamer, J. E., Vedder, J. G., and Yerkes, R. F., 1954, Geology of the Los Angeles basin, in Jahns, R. H., ed., Geology of southern California: California Division of Mines Bulletin 170, chap. 2, p. 65-81.
Woodford, A. O., Welday, E. E., and Merriam, R., 1968, Siliceous tuff clasts in the upper Paleogene of southern California: Geological Society of America Bulletin, v. 79, p. 1461-1486.
Yeats, R. S., Cole, M. R., Merschat, W. R., and Parsley, R. M., 1974, Poway fan and submarine cone and rifting of the inner southern California borderland: Geological Society of America Bulletin, v. 85, p. 293-302.
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