Oak Ridge fault, Ventura fold belt, and the Sisar decollement, Ventura basin, California Robert S. Yeats, Gary J. Huftile, F. Bryan Grigsby* Department of Geology, Oregon State University Corvallis, Oregon 97331-5506 ABSTRACT (Yeats, 1977). The Fernando Formation of Pliocene and early Pleistocene The rootless Ventura Avenue, San Miguelito, and Rincon anti- age consists of abyssal-plain turbidite sandstone alternating with and over- clines (Ventura fold belt) in Pliocene-Pleistocene turbidites are fault- lain by hemipelagic mudstone; this is overlain by shallow-marine to non- propagation folds related to south-dipping reverse faults rising from a marine conglomerate and sandstone of the Saugus Formation. On the decollement in Miocene shale. To the east, the Sulphur Mountain anti- basis of amino-acid racemization age estimates near Ventura (Yerkes et al., clinorium overlies and is cut by the Sisar, Big Canyon, and Lion 1987; K. R. Lajoie, 1987, personal commun.), the youngest Saugus is south-dipping thrusts that merge downward into the Sisar decolle- 0.2-0.4 Ma. The Fernando is underlain by the Sisquoc, Monterey, and ment in lower Miocene shale. Shortening of the Miocene and younger Rincon Formations, all dominated by mudstone and shale of much higher sequence is ~3 km greater than that of underlying competent Paleo- ductility than overlying strata or the underlying Paleogene sequence. The gene strata in the Ventura fold belt and ~7 km greater farther east at Paleogene is dominated by sandstone; i.e., the marine Vaqueros Forma- Sulphur Mountain. Cross-section balancing requires that this differ- tion, the underlying nonmarine Sespe Formation, and various marine ence be taken up by the Paleogene sequence at the Oak Ridge fault to formations of Eocene and Cretaceous age. These formations are competent the south. Convergence is northeast to north-northeast on the basis of enough that they are not folded within the central part of the basin, unlike earthquake focal mechanisms, borehole breakouts, and piercing-point the overlying Miocene and younger formations. offset of the South Mountain seaknoll by the Oak Ridge fault. A The Ventura basin (Fig. 1) is flanked on the north by the Red northeast-trending line connecting the west end of Oak Ridge and the Mountain and San Cayetano reverse faults on which microearthquakes east end of the Sisar fault separates an eastern domain where late have been recorded (Lee et al., 1979; Yeats et al., 1987) and on the south Quaternary displacement is taken up entirely on the Oak Ridge fault by the Oak Ridge reverse fault, which has no instrumental or historical and a western domain where displacement is transferred to the Sisar seismicity onshore, but is probably seismogenic (Yeats, in prep.). All three decollement and its overlying rootless folds. This implies that (1) the faults cut through the brittle crust and may flatten downward at the Oak Ridge fault near the coast presents as much seismic risk as it does brittle-ductile transition (Yeats, 1983; Webb and Kanamori, 1985). To farther east, despite negligible near-surface late Quaternary move- evaluate displacement on the decollement in Miocene shale, we pin the ment; (2) ground-rupture hazard is high for the Sisar fault set in the north end of balanced cross sections against the Red Mountain and San upper Ojai Valley; and (3) the decollement itself could produce an Cayetano faults, and we consider separately the displacement on the Oak earthquake analogous to the 1987 Whittier Narrows event in Los Ridge fault. Angeles. The data and structural interpretation are presented in four cross sections extending from the Oak Ridge fault across the fold and thrust belt to the northern end of the basin (Fig. 2); displacements are given in INTRODUCTION Table 1. Much of the late Quaternary convergence across the western Trans- Cross section A-A' (modified from Grigsby, 1986) is constrained by verse Ranges is taken up in the Ventura basin and is characterized by well data north of the Pitas Point fault, but is speculative in the Pitas Point thin-skinned thrusting and decollements at two levels. The deeper level is anticline, where well data are proprietary (except for the report of Way- the brittle-ductile transition in the middle crust, as described by Yeats land et al., 1978). The San Miguelito and Rincon anticlines are related to (1983) and Webb and Kanamori (1985), and the shallower level is near the Padre Juan and C-3 faults, respectively, and the Padre Juan fault loses the base of a ductile sequence of Miocene shale and mudstone separating a separation upward toward its fault tip as it crosses the anticlinal axis. A 1.2 folded, competent sequence of Pliocene-Pleistocene turbidites (Ventura Ma ash bed near horizon 5 is folded in the Rincon anticline, but a sheared fold belt) from unfolded competent Paleogene strata (Yeats, 1983). We unconformity with some of the post-horizon 5 strata missing on the north analyzed these thin-skinned structures by using retrodeformable (bal- flank of the anticline (Grigsby, 1986) suggests that some deformation took anced) cross sections, whereby rock layers are restored to their unde- place during upper Fernando deposition. Well data show that the Padre formed state without loss of bed length or bed thickness (Dahlstrom, 1969; Juan fault and the south flank of the Rincon anticline flatten southward Price, 1981; Namson, 1987). Restoration of displacement on thrusts ramp- (Grigsby, 1986). On the basis of relations in the Venetura Avenue anticline ing upward from the brittle-ductile transition and on structures above the to the east, we suggest that the Vaqueros is not involved in the Rincon fold decollement in ductile Miocene beds permits an estimation of total crustal and is not cut by the Padre Juan and C-3 faults. Comparison of thicknesses shortening across the Ventura basin. For the upper decollement and fold north and south of the Oak Ridge fault shows that all displacement on that belt to be fully retrodeformed, the shortening of the Miocene and younger fault occurred prior to the end of Saugus deposition. Similarly, much of the sequence must be taken up by a ramp thrust cutting up from the brittle- movement on the Red Mountain fault occurred during Fernando deposi- ductile transition through the Paleogene and older, sequence. tion (Yeats et al., 1987), bringing indurated Paleogene strata against duc- tile Miocene rocks so that the Red Mountain fault acted as a backstop and GEOLOGIC SETTING caused the Padre Juan and C-3 faults to ramp up toward the surface. The The central Ventura basin contains an enormously thick succession of Padre Juan fault arched across the crest of the Rincon anticline, and the post-Miocene strata, including the world's thickest Pleistocene sequence younger Javon Canyon fault (JCF in Fig. 2) broke through to the surface (Grigsby, 1986) and displaced Holocene sediments (Sarna-Wojcicki et al., 1987). •Present address: 706-M Eagle Heights, Madison, Wisconsin 53705. Cross section B-B' extends from the Oak Ridge fault across the 1112 GEOLOGY, v. 16, p. 1112-1116, December 1988 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/16/12/1112/3510492/i0091-7613-16-12-1112.pdf?casa_token=_kBljaooCGMAAAAA:0hZt65X7FXyCj17oDC8XqJ86uhFT-CzV1e8XwWxHNxHzi9byZTKJnh5pdwmJ2RUp91jXNows by California Geological Survey, 19774 on 09 November 2019 Ventura fold belt (Ventura Avenue anticline and Canada Larga syncline) the folding of Saugus at the east end of the Cañada Larga syncline indicate to the Ojai Valley (Yeats, 1981,1983). The stratigraphy in the Santa Clara that folding of the anticline occurred after the Saugus was deposited and is Valley is controlled by wells as deep as the Repetto Member of the still underway (Rockwell et al., 1984). Detritus from the anticline first Fernando Formation, and a 6553-m-deep test well in the Ventura Avenue appears in sediment unconformably overlying the Saugus in the Santa anticline, Shell Taylor 653, bottoms in lower Mohnian (early-late Mio- Clara syncline (Oxnard and Mugu aquifers and overlying "clay cap" of cene) siltsone of the Monterey Formation. This well would have reached ground-water geologists) that contains abundant Fernando foraminifers the Vaqueros Formation if the Vaqueros had been folded along with reworked from the rising anticlinal crest (Yeats, in prep.). overlying strata. Assuming normal thicknesses of middle and lower Mio- On the north flank of the Cañada Larga syncline, the competent cene shale beyond well depth, the Vaqueros is projected to occur at about strata rise to the surface in a homocline and are folded in a south-verging the same depth as it would occur in the Santa Clara syncline, showing that syncline in the Ojai Valley. In a retrodeformable cross section, Namson the Ventura Avenue anticline is rootless (Nagle and Parker, 1971; Yeats, (1987) attributed these folds to a north-dipping thrust in the Paleogene and 1983). The south-dipping Barnard thrust set loses separation as it arches older competent sequence; we suggest that this may be a blind thrust over the anticline, and the anticline is interpreted as a fault-propagation related to the San Cayetano fault. The Monterey-Rincon contact is cut by fold (terminology of Suppe, 1985; for more detailed section, see Yeats, several reverse faults that Rockwell et al. (1984) showed cutting the Pa- 1983). The cross section is drawn near the Saugus exposures that are dated leogene sequence. We suggest that they do not, but are the surface expres- as young as 0.4-0.2 Ma (Yerkes et al., 1987; K. R. Lajoie, 1987, personal sion of the deep decollement underlying the Ventura Avenue anticline commun.). The absence of thinning of the Saugus toward the outcrop and from which the Barnard fault set rises. South of the Santa Clara syncline, Figure 1.