Paleomagnetic definition of crustal fragmentation and Quaternary block rotations in the east Ventura Basin and San Fernando valley, southern California

Abstract

Paleomagnetic studies of the Pliocene‐Quaternary Saugus Formation in the eastern part of the western Transverse Ranges of California show that the crust is fragmented into small domains, tens of kilometers in linear dimension, identified by rotation of reverse‐fault blocks. In an area approximately 35 × 25 km in the San Fernando valley and east Ventura Basin we identified four distinct domains. Two domains, southwest of and adjacent to the San Gabriel fault, are rotated clockwise: (1) The Magic Mountain domain, R = 30° ± 5° and (2) the Merrick syncline domain, R = 34° ± 6°. The Magic Mountain domain has rotated since 1 Ma. Both rotated sections occur in hanging walls of active reverse faults, the Santa Susana and San Fernando faults, respectively. Structural data suggest that the fault tip of the Santa Susana fault is the rotation pivot of the Magic Mountain domain. Two additional blocks are unrotated: (1) the Van Norman Lake domain, directly south of the Santa Susana fault, and (2) the Soledad Canyon domain, immediately across the San Gabriel fault from the Magic Mountain domain, suggesting that the San Gabriel fault might be a domain boundary. Our results suggest that part of the clockwise rotation of some Miocene and older rocks in this area might have occurred in the Quaternary. The Plio‐Pleistocene fragmentation and clockwise rotations continue at present, based on geodetic data, and represent crustal response to diffuse, oblique dextral shearing within the San Andreas fault system.

Clockwise rotation and implications for northward drift of the western Transverse Ranges from paleomagnetism of the Piuma Member, Sespe Formation, near Malibu, California

“…New paleomagnetic results reported here from Oligocene and early Miocene sedimentary beds in the Santa Monica Mountains strengthen the argument for large‐scale rotation of the western Transverse Ranges, originally postulated from observations of basement structure trends [ Hamilton and Myers , 1966; Jones et al , 1976] and subsequently supported by paleodeclination data from Eocene and younger rocks [ Kamerling and Luyendyk , 1979, 1985; Hornafius et al , 1986; Levi et al , 1986; Levi and Yeats , 2003; Liddicoat , 1990, 2001; Prothero et al , 1996]. Their paleomagnetic data indicate that post‐Oligocene rotation amounts to 70°–110° clockwise, affecting the northern Channel Islands, Santa Monica Mountains, Santa Ynez Mountains, and areas between (Figure 1).…”

Paleomagnetism-based limits on earthquake magnitudes in northwestern metropolitan Los Angeles, California, USA

“…The Saugus sites at Soledad Canyon and the Merrick syncline have predominantly reverse polarity, and each section contains several normal intervals. By analogy with the Magic Mountain and Van Norman Lake sections, Levi and Yeats (2003) concluded that the Saugus Formation in Soledad Canyon and the Merrick syncline was deposited during the Matuyama chron, sometime between 2.6 and 0.78 Ma. In addition to normal and reverse polarity, most of the sites of the Saugus Formation retained stable magnetic remanence with accurate paleomagnetic direc-on August 5, 2015 geology.gsapubs.org Downloaded from tions (declination and inclination) Yeats, 1993, 2003).…”

“…This is considered the main reason for clockwise rotations of many formations west of the San Andreas fault, from the Gulf of California to San Francisco (see references in Levi and Yeats, 2003). The study area is west of the San Andreas fault in the Big Bend region between latitudes 34Њ and 35ЊN ( Fig.…”

Transtension in the Brittle Field: The Coso Region, Southern California

“…However, we do not subscribe to the view that, in the brittle upper crust of plate boundary zones, relative plate motion is partitioned, generally, into the relative motion of very large rotating TRANSTENSION IN THE BRITTLE FIELD 195 blocks such that relative motion occurs almost wholly on large displacement faults that bound large rigid blocks (Yeats, 1981;McKenzie and Jackson, 1983;Nur et al, 1986;Nur, 1996, Levi andYeats, 2003). The neotectonics of western North America conforms well to the McCaffrey (2005) model of plate boundary zone behavior with rigid blocks at many scales in a deforming sea of "continuum rubble" at small scales with a wide variety of displacement, strain, and rotation rates, perhaps following a fractal pattern (Turcotte, 1986;Wu, 1993;Marrett et al, 1999).…”

“…Only exceptionally will there be a retreating subduction zone, or fluid igneous body into which escape is possible that will allow blocks to remain rigid. Most models of brittle deformation are based on sets of fault-bounded rigid crustal blocks that do not change shape (Yeats, 1981;McKenzie and Jackson, 1983;Nur et al, 1986;Ron and Nur, 1996;Levi and Yeats, 2003), and that have difficulty reconciling strain compatibility issues such as gaps, overlaps, tilting, and zone boundary constraints. In transtension, zones of shortening, elongation, and rotation produce blocks and structures that form in an initial orientation, and then may be simultaneously shortened and/or elongated, rotated out of a favorable orientation to accommodate strain, and then superimposed by newer structures (Dewey, 2002).…”