The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
Active Tectonics: Studies in Geophysics
gence between Eurasia and India is being accommodated at the southern margin of the thrust belt rather than being more uniformly distributed between the thrusts at the southern margin and more interior and more poorly dated thrust zones, such as the Main Central Thrust, or southward propagation of thrusts takes place faster than the rate of plate convergence. The rate of underthrusting of India beneath the Himalaya is 18 mm/yr based on data from large earthquakes since the year 1900 (Molnar and Deng, 1984), suggesting that only part of the Indian-Eurasian convergence is being taken up within the Himalaya, with the remainder being taken up by escape-block tectonics farther north.
The Transverse Ranges of California are controlled by north-south contractile tectonics related to the big bend in the San Andreas Fault. In the Ventura Basin, continuous sedimentation throughout most of Quaternary time was strongly influenced by contractile tectonics. These sediments have been age-calibrated by tephrochronology, magnetostratigraphy, and radiometric dating (Izett et al., 1974; Blackie and Yeats, 1976; Lajoie et al., 1979, 1982; Liddicoat and Opdyke, 1981). Based on this calibration and on the construction of balanced cross sections across the Ventura Basin, the rate of convergence of the northern edge of the Ventura Basin against its southern edge is 23 mm/yr over the past 0.2 m.y. at Ventura (Yeats, 1983), over half the northsouth component of Pacific-American plate motion in this area, which is 42 mm/yr (Bird and Rosenstock, 1984). Most of this shortening occurred across the Ventura Avenue anticline and the adjacent syncline to the north (Yeats, 1982).
The Ventura Avenue anticline contains a giant oil field with more than 1460 wells, resulting in an extensive, detailed data set on its internal structure (Figure 4.13). Prior to formation of the anticline, the Taylor low-angle thrust fault set began to form 1.3 Ma along a weak layer in the Pliocene turbidite sequence, moved up a 45° ramp, and stopped motion about 0.65 Ma. Maximum net slip rate was 2.8 mm/yr to the southeast (Yeats, 1983). Following the end of deposition of nonmarine coarse-grained strata of the San Pedro Formation about 0.2 Ma, the anticline began to buckle. The Ventura River was antecedent to this buckle, and uplift of the crest of the fold is calibrated by deformed river terraces that have been dated by 14C, with age extrapolations beyond the limits of 14C dating based on a soils chronosequence (Keller et al., 1982; Rockwell, 1983). Uplift rates on the anticlinal crest were 4.3 to 5.2 mm/yr for the last 29,600 yr, 10.5 to 11.5 mm/yr from 80,000 to 29,600 yr ago, and 15 to 16 mm/yr from 200,000 to 80,000 yr ago (Keller et al., 1982). Rockwell (1983) demonstrated that a rootless buckle fold formed by end loading would undergo rapid displacement normal to its loading direction early in its formation, and this displacement would slow down as the fold became more fully developed [Rockwell, 1983; Yeats, 1983; cf. theoretical considerations by Currie et al. (1962) and Adams (1984)]. The fold is still under high horizontal stress because it is overpressured, with the ratio of fluid pressure to overburden pressure increasing from 0.55 to 0.8 as radius of curvature decreases from 300 to 30 m in the core of the fold (Yeats, 1983).
If the fold is still undergoing contraction as uplift of its crest continues at a decreasing rate, how is this contraction being accommodated? The limbs may still be steepening, but an alternative way to accommodate contraction may be the Ventura Fault (Figure 4.7). The south-facing fault scarp occurs at the sharp boundary between flat-lying strata of the Santa Clara syncline and south-dipping strata of the south flank of the Ventura Avenue anticline. This sharp boundary dips 45° north to depths of several kilometers and is planar, and it is assumed that this boundary is the Ventura Fault at depth (Figures 4.7 and 4.13). It is younger than most of the folding because it is planar, in contrast to other reverse faults that curve over the crest of the anticline and, therefore, formed as low-angle thrusts during the early stages of folding (Figure 4.13). Although the fault scarp is linear and well documented, subsurface well correlations indicate little or no stratigraphic separation across the fault. As shown in Figures 8, 10, and 11 of Yeats (1982), horizons can be correlated across the fault with a sharp change in dip, but no displacement. This led me to propose earlier that the Ventura Fault formed by bending moment (Yeats, 1982), a conclusion that is probably wrong. My present view is that the fault formed so recently that it has not had time to accumulate enough displacement to be documented without ambiguity in the subsurface. The evidence for this is the planar nature of the boundary between flat-lying and steeply dipping strata. The Ventura Avenue fold may have acquired a configuration that is stable to continued horizontal stress if there is a zone south of the fold along which further displacement may take place. If displacement occurs by southward sliding of the Pliocene-Pleistocene turbidite sequence over a subjacent Miocene ductile sequence, this displacement may break through to the surface as the Ventura Fault, as illustrated in the inset to Figure 4.13.
SOCIETAL IMPLICATIONS OF ACTIVE FAULTS AND FOLDS
Yeats et al. (1981) suggested that flexural-slip faults of the Ventura Basin would cause ground-rupture prob-