NEOGENE TECTONIC ROTATION OF THE PALOS VERDES PENINSULA

Previous studies have shown that the western Transverse Ranges (WTR) have undergone approximately 90˚ of vertical-axis rotation since 18 Ma. The model proposed by Crouch and Suppe (1993) suggests that the WTR initially diverged from the Peninsular Range Province atop a regional-scale, low-angle detachment fault. This formed an extensional terrane that presently underlies the Los Angeles basin and the offshore, inner-borderland. The Palos Verdes Peninsula (PVP) is situated in the eastern part of the extensional basin and is composed of Miocene sedimentary rocks and volcanic rocks of the Monterey Formation that were deposited on exhumed basement rocks in the basin during extension. These rocks provide the opportunity to use paleomagnetic data to test mechanical models of rotation in southern California.

Core samples for paleomagnetic analysis were collected from both sedimentary (16.5 – 7.5 Ma) and volcanic rocks (15.5 – 14.5 Ma) of the Monterey Formation along the Palos Verdes and San Pedro coastlines. The samples were subjected to alternating field and thermal demagnetization to isolate and resolve the characteristic magnetization. Early Miocene sedimentary rocks from the southern coastline of the PVP have stable, primary remnant magnetic directions that have passed reversal tests and indicate an average of 10.2 ± 9˚ of clockwise, vertical-axis rotation. This relatively minor amount of rotation is in contrast to the large amount of rotation recorded in the WTR to the north.

The Miocene sedimentary rocks of the PVP were deposited on basement rocks (the Catalina Schist) that comprise the footwall exhumed during rotation along the regional detachment fault of Crouch and Suppe (1993). The lack of significant rotation recorded in the PVP rocks suggests that Neogene rotation was largely confined to the WTR in the upper plate of the detachment and that rotation occurred on a discrete surface at depth. This supports previous models of rotation and hold implications for the driving mechanisms of vertical-axis rotation in southern California.