Transition from Continental Rifting to Seafloor Spreading:
South San Clemente Basin

Proposal for a University of California Student Cruise Aboard R/V Revelle

P.I.—Prof. Ken Macdonald, UC Santa Barbara; Co-P.I.—Dr. Mark Legg, Visit. Res Geophys, UCSB/ICS

Using five days of ship time aboard R/V Revelle, we propose to examine the transition from continental rifting to seafloor spreading as recorded in the geology of the California Continental Borderland, where South San Clemente Basin appears to evoke nascent seafloor spreading.

Tectonic Background

The tectonic evolution of the California Continental Borderland passed from Mesozoic subduction of the Farallon Plate beneath the western North America continental margin to the modern right-slip along the San Andreas transform fault system bounding the Pacific (PAC) and North America (NOAM) tectonic plates (Atwater, 1970). Miocene transtension, involving partitioned right-slip and extension as well as vertical-axis rotation of large crustal blocks, occurred as the PAC-NOAM transform grew, and the last remnants of the ancient Farallon plate foundered (Kamerling and Luyendyk, 1979; Legg, 1991). The Inner Borderland Rift, a zone of ridge-and-basin morphology floored by basement of the Catalina Schist subduction complex (Legg, 1991; Nicholson and others, 1994) was created in the wake of the large-scale block rotations and translations as overall extension exceeded 100-200 km. Wide spread middle Miocene volcanism tapped upper mantle magma sources (Weigand, 1994), and aligned volcanic ridges may represent leaky transform faults or, as within the Gulf of California, local areas of nascent seafloor spreading connected by major transform fault segments (Lonsdale, 1989).

South San Clemente Basin within the Borderland resembles Gulf of California spreading center basins (Legg and others, 2000) with a symmetrical series of north-trending ridges and troughs interpreted to be horsts and grabens within an evolving spreading center (Figs. 1 & 2). This contrasts sharply with the more typical northwest, transform parallel, trend of major Borderland ridges and basins. Yet, the north trending rift structure implies an east-west principal extension direction, consistent with a northwest-trending dextral PAC-NOAM transform fault system. The northern boundary of this rift consists of the N40oW trending San Clemente fault, inferred to be a Miocene to Recent transform fault. Irregular-shaped Animal Basin bounds the southwest margin of rift, where steep northwest-trending basin margins are inferred to delineate another dextral transform fault zone. The north-trending rift and northwest-trending dextral transform fault pattern resembles oblique continental rift systems such as the Basin-and-Range of the southwestern United States. Northeast-trending, active, seafloor spreading rift centers, within the Guaymas Basin of the Gulf of California, are nested within a broad, older, north-trending "continental" rift basin, and bounded by the northwest-trending transform faults (Lonsdale, 1989).

Compilation of underway SeaBeam data collected by the Scripps Institution of Oceanography on numerous cruises from San Diego allow a more detailed examination of the rift morphology within South San Clemente Basin (Fig. 2).

Like the Guaymas Basin in the Gulf of California, nested within the larger north-trending horst and graben rift structure of South San Clemente Basin lie sets of northeast-trending sub-basins and ridges. These features are orthogonal to the San Clemente transform fault and are proposed to represent nascent seafloor spreading centers within the evolving continental rift. Sinistral separation of ridge crests, such as along San Salvador Knoll and the Salsipuedes Knolls might be inferred to represent left-slip on northwest-trending transcurrent faults, contrary to the known dextral fault character of the San Clemente fault system. If, instead, these ridges are interpreted as volcanic constructs within the rift, the left separation can be explained as short dextral transform fault segments between offset spreading centers--short transform faults in the evolving spreading center have offset growing volcanic ridges. We infer that the transformation from north-trending volcanic ridges and basins to northeast-trending volcanic ridges and basins represents the transition from continental rifting to seafloor spreading preserved within South San Clemente Basin.

The proposed transition from continental rifting to seafloor spreading appears to have occurred during the Mio-Pliocene time, about the same time as seafloor spreading began in the Gulf of California to the southeast. Few seafloor samples have been acquired from the South San Clemente Basin area, however, among those collected include Mesozoic rocks of the Catalina Schist basement retrieved from the flanks of La Victoria Knoll and abundant volcanic and volcaniclastic rocks (Vedder and others, 1974; Vedder, 1990). Although most of the volcanic rocks retrieved from Borderland ridges to the north in U.S. waters are inferred to be of Middle Miocene age, several basalt and andesite samples retrieved from the southern or Baja California Borderland, have been radiometrically dated showing post-Miocene ages (Doyle and Gorsline, 1979). One sample retrieved from the flanks of the Maximinos Knolls, east of the South San Clemente Basin rift, yielded a Pliocene age, about 3.6-4.8 Ma (Doyle and Gorsline, 1979). Basalt cored beneath Animal Basin, overlain by fossiliferous late Miocene and younger sedimentary rocks, is inferred to be about 9.3 Ma (Lyle, and others, 1997). High-resolution seismic reflection profiles and observations from DSV Alvin show Holocene activity along the San Clemente fault (Legg, 1985; Goldfinger and others, 2000). Post-Miocene rifting in the Borderland, contemporaneous with Gulf of California rifting, raises important questions regarding the coupling of Pacific lithosphere with North America, microplate tectonics, and the evolution of oblique-transform plate boundaries.

We propose to conduct a student cruise aboard the R/V Roger Revelle to South San Clemente Basin to test our hypothesis regarding the existence of nascent seafloor spreading within the rift.

* This cruise will be a significant part of Geological Sciences 281, Field Studies
In Marine Geophysics, taught by Professor Ken Macdonald. It will also be open to undergraduates and students from other UC campuses.

* Acquire Simrad EM120 high-resolution swath bathymety to fill in the major gaps in the existing SeaBeam coverage of the rift system and allow more accurate mapping of the rift/transform morphology.

* Acquire dredge and gravity core seafloor samples at carefully selected sites to retrieve volcanic and other bedrock materials to examine the geochemistry of rift-related volcanism and possibly provide control on the age of most recent volcanism.

* Acquire piston core seafloor samples in each of the two sub-basins to examine changes in sedimentation likely due to tectonic effects and provide stratigraphic control on timing.

* Acquire high-resolution seismic reflection profiles to provide sub-bottom imaging necessary to map buried tectonic structures and provide seismic stratigraphy for interpreting timing of tectonic events. (3.5 kHz sub-bottom profiling system and a small airgun (40 in3) with 3-24 channel mini-streamer.)

* Acquire geomagnetic profiles along traverses orthogonal to the rift and seafloor spreading axes in order to identify possible magnetic anomaly patterns associated with seafloor spreading.

* Five (5) days of ship time are requested to fill in the bathymetry gaps, collect two dredges and five gravity/piston cores, and acquire about 240 km of high-resolution subbottom seismic profiles.

* The proposed project will be a joint effort with collaborating scientists and students from University of California, Santa Barbara, Scripps Institution of Oceanography, and CICESE and UABC in Ensenada, Baja California, Mexico.

* It is imperative that the process of obtaining clearance for research within the Mexican EEZ be started immediately to facilitate the ship scheduling and timely cruise departure.

Cruise Benefits

The proposed cruise will provide important sea time and experience for marine geology and geophysics students in the acquisition of seafloor and subbottom data as well as preliminary data for further thesis work. Indeed, if our hypothesis of nascent seafloor spreading is further corroborated by the data acquired during the proposed cruise, a major research expedition and project to examine in detail the processes involved in the transition from continental rifting to seafloor spreading will be forthcoming. That project will provide important funding and research material for scientists and students involved in the initial student cruise as well as future students and scientists at the University of California. As a joint U.S.-Mexico research project, this will further the goals of NAFTA and ongoing efforts to promote cooperation and collaboration between U.S. and Mexican institutions.

Proposed Schedule

Day 1

Steam from San Diego to South San Clemente Basin acquiring Simrad, high-resolution seismic profiles, and magnetometer data. Acquire swath bathymetry, magnetic, and seismic data along traverses to fill in major gaps in existing SeaBeam coverage.

Day 2

Continue reconnaissance swath bathymetry mapping and identify candidate sites for dredge and core sampling. First dredge and one core sample late in day; swath bathymetry, geophysics and high-resolution seismic profiling at night.

Day 3

Focus on seafloor sampling, with two piston cores. Swath bathymetry, geophysics and high-resolution seismic profiling at night.

Day 4

Complete seafloor sampling with two cores and second dredge. Swath bathymetry, geophysics and high-resolution seismic profiling at night.

Day 5

Swath bathymetry, geophysics, and high-resolution seismic profiling on return to port.

Cruise Participants

Prof. Ken Macdonald and 5 (or more) UCSB students (Jeff Blasius, Robert Descesari, Kate Gans, Karen Hurley, Chris Earhardt)
Dr. Mark Legg, Dr. Marc Kamerling, Dr. Craig Nicholson (UCSB, Inst Crustal Studies)
Prof. Jorge Ledesma Vasquez, 2 other students/researchers from UABC, Ensenada
Prof. Lance Forsythe, 2 other student researchers from CICESE, Ensenada
Dr. Chris Goldfinger, (Oregon State Univ, Ocean Sciences)
Marine Tech (Bob Wilson) and Geophysical/Seismic (Seth Mogk) from Scripps

References

Atwater, Tanya, 1970, Implications of plate tectonics for the Cenozoic evolution of western North America: GSA Bulletin, v. 81, p. 3515-3536.

Doyle, L. J., and Gorsline, D. S., 1977, Marine geology of Baja California continental borderland, Mexico: AAPG Bulletin, v. 61, p. 903-917.

Goldfinger, C., Legg, M., and Torres, M., 2000, New mapping and submersible observations of recent activity on the San Clemente fault: [abstract] EOS, Trans. AGU, v. 81, p. 1069.

Kamerling, Marc, and Luyendyk, B. P., 1979, Tectonic rotations of the Santa Monica Mountains region, western Transverse Ranges, California, suggested by paleomagnetic vectors: GSA Bulletin, v. 90, p. 331-337.

Legg, M. R., 1985, Geologic Structure and tectonics of the inner continental borderland offshore northern Baja California, Mexico [unpublished Ph.D. dissertation]: U. California, Santa Barbara, 410 p.

Legg, M.R., 1991, Developments in understanding the tectonic evolution of the California Continental Borderland: in Osborne, R.H., ed., SEPM Spec. Publ. 46, p. 291-312.

Legg, M.R., White, S., and Macdonald, K., 2000, Late Cenozoic seafloor spreading in South San Clemente Basin: A hole through the Inner Borderland lithosphere: [abs] EOS, Trans. AGU, v. 81, p. 1068.

Lonsdale, P., 1989, Geology and tectonic history of the Gulf of California: in Winterer, E.L, Husson, D.M., and Decker, R.W., eds., The Geology of North America: Volume N: The Eastern Pacific Ocean and Hawaii, Geological Society of America, Boulder, CO, p. 499-521.

Lyle, M., Koizumi, I., Richter, C., and others, 1997, Proceedings of the Ocean Drilling Program, Initial Reports, vol. 167, ch. 5, Site 1011, p. 86-127.

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.

Vedder, J.G., 1990, Maps of California Continental Borderland showing compositions and ages of bottom samples acquired between 1968 and 1979: U.S.G.S. Misc. Field Studies Map MF-2122, three sheets, scale 1:250,000.

Vedder, J. G., Beyer, L. A., Junger, Arne, Moore, G. W., Roberts, A. E., Taylor, J. C., and Wagner, H. C., 1974, Preliminary report on the geology of the Continental Borderland of southern California: U.S.G.S., Misc. Field Studies Map MF-624, Scale 1:500,000.

Weigand, P. 1994, Petrology and geochemistry of Miocene volcanic rocks from Santa Catalina and San Clemente Islands, California: in Halvorson, W.L., and Maender, G.J., eds., The Fourth California Islands Symposium: Update on the Status of Resources: Santa Barbara Museum of Natural History, Santa Barbara, CA, p. 267-280.