Neogene tectonic rotation of the Santa Ynez Range, Western Transverse Ranges, California, Suggested by paleomagnetic investigation of the Monterey Formation

Abstract

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.

Paleogeography of the western Transverse Range province, California; new evidence from the late Oligocene and early Miocene Vaqueros Formation; Stratigraphy of the fine-grained facies of the Sisquoc Formation, Santa Maria Basin, California; paleoceanographic and tectonic implications; The Sisquoc Formation-Foxen Mudstone boundary in the Santa Maria Basin, California; sedimentary response to the new tectonic regime

“…These stron-tium and foraminiferal data do not contradict these landwardyounging hypotheses; instead when viewed in the context of the early Tertiary paleogeography they suggest that landward younging took place over a very complex late Oligocene and early Miocene topography. This complexity is outlined in detail in the discussions of Vaqueros lithofacies that follow and has been alluded to by previous workers (Hornafius, 1985;Rigsby, 1989aRigsby, , 1994a.…”

“…Hypothetical chronostratigraphy of the Vaqueros Formation. The hypothetical time lines (T1 is older, T2 is younger) are based on the strontium isotope ages reported in this study, on the Vaqueros age ranges reported by Dickinson and others (1987), and on geological evidence (such as the positions of paleoshorelines) presented in this paper and in work by Howard (1988), Hornafius (1985), and others. As discussed in the text, this hypothesis suggests that the Vaqueros Formation decreases in age in all landward directions.…”

“…As discussed in the text, this hypothesis suggests that the Vaqueros Formation decreases in age in all landward directions. Base map is the early Tertiary palinspastic reconstruction of southern California (Hornafius, 1985). See figure 2 for abbreviations.…”

“…Palinspastic reconstruction of southern California during early Tertiary time (modified after Hornafius, 1985). Abbreviations: SLO, San Luis Obispo; SM, Santa Maria; SB, Santa Barbara; V, Ventura; LA, Los Angeles; SD, San Diego; SCI, Santa Cruz Island; SRI, Santa Rosa Island; SMI, San Miguel Island; SNI, San Nicolas Island; F, fault.…”

Mid-Tertiary isopach and lithofacies maps for the Los Angeles region, California: Templates for palinspastic reconstruction to 17.4 Ma

“…Paleomagnetic declination data indicate that the western Transverse Ranges Province block underwent Neogene clockwise transrotation of as much as 100˚ in some western parts beginning no later than 16 Ma (Hornafius, 1985;Morris and others, 1986;Luyendyk and Hornafius, 1987;Liddicoat, 1990;Luyendyk, 1990) and continues to rotate today (Molnar and Gipson, 1994). The more-or-less fixed rotational hub is at the east end of the block, about 25 km east of this study's northwest sector.…”