Trenching and bucket auger borehole studies of the Red Mountain fault were conducted in May of 1997. Seven bucket auger boreholes were dug prior to and after the trench for stratigraphic control. The trench was excavated at the eastern end of the 45 ka Punta Gorda terrace (Figure 1), north of Ventura, where the 2 main strands of the Red Mountain fault merge and where the Punta Gorda terrace is overlain by a small alluvial fan.
The Punta Gorda terrace was dated by Lajoie et al. (1979, 1982) using amino acid racemization of marine shells found at the base of the terrace at 45 ka.
We found that the basal marine beach sand (Qtm1 and Qtm2) exposed within the trench (Figure 2) was vertically separated 34 m across the fault. On the hanging wall, the marine platform was exposed in both the trench and borings, overlaying the upper Miocene Sisquoc Formation. On the footwall, this platform is exposed in a road cut ~120 m east of the trench site overlaying Pliocene Fernando Formation, and it was also encountered in previous geotechnical borings across the terrace. The 34 m separation potentially limits the dip-slip rate of the Red Mountain fault to 1.5 mm/yr using a 30° dip on the fault (intermediate between the 16° dip in the trench and ~60° dip in oil wells). This may be a minimum rate if there is another strand of the fault not exposed in the trench, if the fault has any lateral component of slip, or if the footwall block is uplifting as well. Correlation of the Punta Gorda terrace across the fault and its age of 45 ka are corroborated using oxygen isotopes on Olivella shells.
The trench exposures reveal that a significant portion of the near surface deformation of this reverse fault has occurred as folding. At the surface, there is a recumbent fold composed of terrace deposits which formed as the 60° dip on the fault, (found at 3 km depth in oil wells), shallows near the surface to ~10°. Subsequent movement on the fault occurred on a ~30° dipping fault, cutting the hanging-wall block of the fault and isolating the recumbent fold in the footwall block. There is evidence of past earthquakes in a colluvial wedge and a fissure fill in the hanging-wall block. Attempts are being made to constrain the timing of the most recent event by using 14C dating of detrital charcoal fragments found in the fissure fill.
DISCUSSION
The work done on this trench shows a 34 m uplift across the fault. However, the 45 ka Punta Gorda terrace was formed at about 38 m below present-day sea level. Now near the trench, but on the footwall block of the Red Mountain fault, the terrace is at 169 m above sea level. Thus the footwall block of the fault is rising at over 4 mm/yr. The hanging-wall block of the fault has exceeded that rate, rising an additional 34 m. At seismogenic depth the faulting rate on the Red Mountain fault must combine both values if the causes of the uplifts are unrelated.
Huftile and Yeats (1995) modelled these structures as unrelated in that the Red Mountain fault is a north-dipping reverse fault which can be shown to extend down to seismogenic depths. The Ventura Avenue anticline was modelled as a north vergent lift-off fold uplifting Miocene and younger rocks above a decollement which is related to the south-dipping Oak Ridge fault. This model requires that the uplift rates of the footwall and hanging-wall blocks be combined to yield the slip rate on the Red Mountain fault at seismogenic depths. This would mean a total vertical uplift rate on the Red Mountain fault of 5.1 mm/yr, and a slip rate on a 68°-dipping fault of 5.9 mm/yr.
REFERENCES CITED
Huftile, G. J., and Yeats, R. S., 1995, Convergence rates across a displacement transfer zone in the western Transverse Ranges near Ventura, California: Jour. Geophys. Res., v. 100, p. 2043-2067.
Jackson, P. A., 1980, Structural evolution of the Carpinteria basin, western Transverse Ranges, California: unpub. MS thesis, Oregon State University, Corvallis, Oregon, 107 p.
Jackson, P. A., and Yeats, R. S., 1982, Structural evolution of the Carpinteria basin, western Transverse Ranges, California: Am. Assoc. Petrol. Geol. Bull., v. 66, p. 805-829.
Lajoie, K. R., et al., 1979, Quaternary marine shoreline and crustal deformation San Diego to Santa Barbara, California, in Geological excursions in the southern California area: San Diego State University, p. 3-15.
Lajoie, K. R., et al., 1982, Quaternary chronology and rates of crustal deformation in the Ventura area, California, in Cooper, J. D., compiler, Neotectonics of southern California: Geol. Soc. America Cordilleran Section Field Trip Guidebook, p. 43-51.
PUBLICATIONS
Huftile, G. J., and Yeats, R. S., Tectonics of the Montebello anticline, the East Montebello fault and the Whittier narrows earthquake: in prep.
