Miocene Basalts from Joshua Tree Show Fault Block Rotation

Abstract

Paleomagnetic study of Miocene basalts collected from a locality 7km north of Desert Center, CA (33.7N, 244.6E) shows that rocks are rotated 46.6? in a clockwise direction, results inconsistent with previous kinematic models of the development of the San Andreas Fault. While the history of the southern segment of the fault from the Holocene to present shows good agreement and consistency between the observed and predicted offset along the fault, the history from the Miocene to the Pleistocene is poorly known, which inhibits further understanding of the early development of the Northern section of the fault system. This paleomagnetic study, which was proposed to be used with existing paleomagnetic evidence to reexamine extend and magnitude of the vertical axis block rotation in the fault system, conclusively showed through AF demagnetization that the blocks had indeed been rotated. Samples collected in 6 lava flows show a general direction of north and down after demagnetization, while lava flows towards the top of the section possesses very strong, randomly directed magnetisms easily cleaned by low levels of alternate field demagnetization. I interpret these components as isothermal remanence magnetizations imparted by lighting strikes. Laboratory magnetization imparted by brief application of a direct field nearly saturate at a field strength of 200mT, strongly suggesting that titanomagnetite is the primary magnetic mineral found in samples.