Paleomagnetism and Tectonics of southern California and Mojave Desert.



Tectonics, 1996


Schermer, E.R., Luyendyk, B.P., and Cisowski, S.



In the Ft. Irwin region of the northern Mojave desert (NE Mojave domain), Late Cenozoic east-striking sinistral faults predominate over northwest-striking dextral faults of the same age. East-striking faults curve to northwest strikes and shallower dips at the east ends, and locally at the west ends. Kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east-striking portions of the faults and sinistral-thrust slip on northwest-striking, moderately dipping segments. Crustal blocks ~7-10 km wide by ~50 km long are bounded by complex fault zones up to 2 km wide. Anastamosing fault strands and subsidiary fault blocks result in a complex pattern of small, local fault blocks at the edges and ends of each larger crustal block. Faulting initiated after ~11 Ma, and Quaternary deposits are faulted and folded. We document a minimum of 13 km cumulative sinistral offset in a north-south transect across Ft. Irwin from south of the Bicycle Lake fault to north of the Drinkwater Lake fault.
Paleomagnetic results from 50 sites drilled in Miocene volcanic rocks disclose two direction groups defining northeast and north-to-northwest declinations in early and middle Miocene rocks. The north-to-northwest declinations are close to the expected direction for the middle Miocene North American paleomagnetic pole of Calderone, et al. [1990] . However, rock magnetic studies suggest that both primary and remagnetized directions are present in this group. The northeast directions are primary and 63.5° ± 7.6°clockwise from the reference pole and suggest net post middle Miocene clockwise rotation of several of the east-trending blocks in the northeast Mojave domain. We observe that the Jurassic Independence Dike Swarm within Ft. Irwin may be rotated 25-80° clockwise relative to the swarm north of the Garlock fault, thus supporting the inference of clockwise rotation.
Using a simple-shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks of appropriate dimensions, we predict ~23° clockwise rotation using the observed fault slip, or less than half that inferred from the paleomagnetic measurements. The discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks, where most of our paleomagnetic sites are located. However, at least 25°, and possibly 40° of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible "rigid-body" rotation of the region evidenced by clockwise bending of northwest-striking domain-bounding faults.
Our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the Eastern California shear zone over the last 10 m.y. occurred within the northeast Mojave Domain. The remainder must be accommodated in adjacent structural domains, e.g., east of the Avawatz Mountains and west of the Goldstone Lake fault.

AAPG 1998


Bruce P. Luyendyk, Phil B. Gans, and Marc J. Kamerling



Mafic rock samples from the northern Channel Islands (western Transverse Ranges) and the Peninsular Ranges and offshore islands were dated by 40Ar/39Ar incremental heating experiments on groundmass and plagioclase concentrates (24 experiments total). Many of the ages are substantially different from published K-Ar dates. Early Miocene 40Ar/39Ar dates on two lavas from San Miguel Island agree with the paleontology control there (Saucesian) and conflict with prior late Oligocene K-Ar dates from the same units. Hypabyssal intrusions from Santa Rosa Island are around 18 Ma. The top of the Santa Cruz Island volcanic section is 16.33 ± 0.26 Ma. Anacapa ages near 16 Ma are the youngest found for the northern Channel Islands. Published 40Ar/39Ar dates from the Tranquillon Volcanics are 17.8 Ma ,suggesting a widespread volcanic event between 18 and 16 Ma. We interpret this as timing the beginning of rifting of the northern borderland. The apparent age progression of volcanism from west (18 to 17 Ma) to east (16 Ma) is suggestive of a propagating rift prior to major extension and rotation of the western Transverse Ranges. The El Modeno volcanics in the Peninsular Ranges give ages of about 11 Ma, several million years younger than published K-Ar dates. The Rosarito Beach Formation comprises Miocene basalts, tuffs, and sediments that accumulated in two basins between Tijuana and Ensenada. The section spans from 15.51 ± 0.14 to 16.19 ± 0.28 Ma. This formation was derived from a western (offshore) source that has since been translated away, submerged, or eroded. It is possible that part of the northern Channel Islands platform was adjacent there before rifting and rotation. Ages from San Clemente Island are younger than the Rosarito section and span 14.5 to 16.0 Ma. The combined sections on San Clemente and at Rosarito span several reversed magnetic polarity intervals. More ages from these sections could aid in calibrating the magnetic reversal time scale.

Geol. Soc. Amer., 2000

Map restoration of folded and faulted late Cenozoic strata across the Oak Ridge fault, onshore and offshore Ventura basin, California

Sorlien, C. C., Gratier, J. P., Luyendyk, B.P.,Hornafius, J. S., and Hopps, T. E.



The Ventura basin lies within the east-west-trending active fold-and-thrust belt of the western Transverse Ranges, California. This basin has been the site of significant earth-quakes on structures within it and bordering it. The purpose of our study is to identify the main structures in the basin and its borders and to quantify their rate of deformation. Our study includes the onshore and offshore Ventura basin, the arcuate basin-bounding Oak Ridge reverse fault, and the Oxnard shelf to the south. Shortening, fault-slip, and crustal-block motions were studied using a three-dimensional map-restoration technique. Structure- contour maps on the 6 Ma surface and other horizons were digitized and restored to the initial horizontal state by unfolding them using the computer program UNFOLD and then fitting the unfolded surfaces across faults. Comparing the restored and present configuration allows us to estimate total net finite displacements relative to a fixed horizontal reference line. Average post-5 Ma shortening rates estimated from our restoration are slower than both post-1 Ma rates and present rates deter-mined by global positioning systems. Most shortening due to folding in the onshore basin is post-1 Ma, although slip on the Oak Ridge fault has occurred both before and after 1 Ma. Displacement due to faulting and folding includes left-lateral strike-slip motion on the northeast-southwest coastal segment of the Oak Ridge fault and associated clockwise rotation of the adjacent Ventura basin. The Oxnard shelf is bordered to the south by mountains and islands that have been previously interpreted as folds above thrust-fault ramps. This onshore-offshore block moves as one continuous thrust sheet. Similarly, beyond the well-studied onshore fault, a kinematically continuous offshore Oak Ridge-Mid-Channel left-oblique fault system is interpreted to continue at least an additional 100 km westward beneath the Santa Barbara Channel.


Geol. Soc. Amer., 2001

A model for Neogene crustal rotations, transtension, and transpression in southern California

Luyendyk, B. P



Paleomagnetic data from southern California south of 35.5 N latitude suggest that eight crustal domains have rotated clockwise at various times during the Neogene. Earthquake data suggest that some domains may be continuing to rotate today. Rotated domains include the western and eastern Transverse Ranges blocks, a crustal block near Morro Bay, Catalina Island block, San Gabriel Mountains, Tehachapi Mountains, the central Mojave Desert and the northeast Mojave Desert. These rotated domains comprise about 25% of the surface area of this region. Paleomagnetic data constrain the amounts of net clockwise rotation to be as much as 100 degrees in the California Borderland and western Transverse Ranges and from 60 to 40 degrees for the other domains. The average rotation rate can be estimated for the western Transverse Ranges at 5-6 degrees/m.y. since 17 Ma. These rotations have occurred during three distinct tectonic regimes. Early Miocene rotations and extension in the central Mojave may have been related to a slab window that formed beneath the region after the Pacific and North American plates came into contact; the Tehachapi and Morro blocks began rotating in this regime also. Capture of the Monterey Plate by the Pacific Plate caused the initiation of rotation under transtension of the western Transverse Ranges, San Gabriel and Catalina blocks at 17 Ma. Shift of the Pacific-North American plate boundary eastward to open the Gulf of California at about 5 Ma created a restraining geometry to the plate boundary. This caused clockwise rotation of the eastern Transverse Ranges and the northeast Mojave Desert and smaller counterclockwise rotation of portions of the western Mojave Desert and the San Gabriel block.