Abstract. We used 50 kHz sonar data to estimate natural hydrocarbon emission rates from the 18 km 2 marine seep field offshore from Coal Oil Point, Santa Barbara, California. The hydrocarbon gas emission rate is 1.7 + 0.3 x 105 m3d -1 (including gas captured by a subsea seep containment device) and •he associated oil emission rate is 1.6 + 0.2 x 104 Ld -l (100 barrels d-l). The nonmethane hydrocarbon emission rate from the gas seepage is 35 + 7 td -1 and a large source of air pollution in Santa Barbara County. Our estimate is equal to twice the emission rate fi-om all the on-road vehicle traffic in the county.

No abstract

We have studied the paleomagnetism of various Neogene age rocks in southern California as a means of determining the amounts of Oligocene and younger tectonic rotation and translation which has occurred in this region. Our results suggest that fully 25% of this area, in particular the Transverse Ranges, has undergone extreme clockwise rotation. Work in southeastern California implies that 40° of clockwise rotation has occurred here, although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200°. The crustal block bounded by the San Gabriel and San Andreas fault has undergone a net clockwise rotation of 35° although the data here can be interpreted to show that an original early Miocene rotation of about 50° was followed by a late Miocene or Pliocene counterclockwise rotation of 15°. Paleomagnetic results from the offshore islands suggests that San Clemente, Santa Barbara, and San Nicolas islands have not rotated but Santa Catalina has undergone about 90° of clockwise rotation. All of the northern Channel Islands, including Anacapa, Santa Cruz, Santa Rosa, and San Miguel, are implied to have rotated 70° or 80° clockwise. This result is also found for the Santa Monica Mountains east of Anacapa Island. In the Santa Ynez Range north of the Channel Islands, paleomagnetic study of the Monterey Formation also indicates large clockwise rotations of near 90°. These data also suggest that the Santa Maria Basin is not rotated and that the north boundary of the rotated region is the Santa Ynez River fault. Stratigraphic control on the paleomagnetic data from the Monterey Formation implies that the rotation began about 16 m.y. ago and may be continuing today in the western region. Paleomagnetic inclination data from our study show that the northern Channel Islands, in particular, may have translated 15° northward since middle Miocene time. However, equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections, non dipole magnetic field behavior and right offset on the San Andreas fault system. Palinspastic reconstruction of southern California regions for the early Miocene implies that parts of the Transverse Ranges structures were once aligned with north trending extensional structures in the southwestern United States. We propose that Pacific‐American plate interactions both rifted the continental crust to create this pattern and rotated the western‐most structures within a dextral simple shear zone which had a half width of about 400 km.

A paleomagnetic investigation was conducted on dolomite beds in the middle and upper Miocene Monterey Formation in order to determine the timing and amount of post‐early Miocene tectonic rotation experienced by the Santa Ynez Range (part of the western Transverse Ranges province of California). Five stratigraphic sections of the Monterey Formation exposed along the coastline west of Santa Barbara were extensively sampled. The sections were correlated using tuff beds, and ages were assigned to each dolomite bed on the basis of magnetostratigraphy and diatom biostratigraphy. The dolomite beds possess stable primary remanent magnetic directions which pass fold tests and reversal tests, and show a consistent decrease in magnetic declination with decreasing age. Rocks 14.8 to 15.8 m.y. old have a mean magnetic declination of 92±7°, whereas rocks 8 to 11 m.y. old have a mean magnetic declination of 36±7°. The consistent upsection decrease in magnetic declination within these stratigraphic sections is attributed to a rapid 56±10° clockwise rotation of the entire Santa Ynez Range between 15.2±0.1 and 10±1 m.y. B.P. The mean magnetic declination of 36±7° in rocks 8 to 11 m.y. old is probably due to continued clockwise rotation of the Santa Ynez Range during the Plio‐Pleistocene. An early Miocene palinspastic reconstruction is proposed in which the area of the presently east‐west trending Santa Ynez Range is restored to an initially north‐south orientation.

No abstract