ABSTRACT Mafic rock samples from the northern Channel Islands (western Transverse Ranges) and the Peninsular Ranges and offshore islands were dated by 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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.

We report structural, paleomagnetic, and magnetic fabric data for mid-Cretaceous plutons of the Peninsular Ranges batholith along a transect at ~30°N latitude. Four plutons in the western sector are characterized by characteristic magnetizations residing in magnetite. In this sector, El Milagro, Aguaje del Burro, La Zarza, and San Telmo plutons yield a combined paleopole at 82.1°N, 169.7°E (K = 137.6, A 95 = 7.9°; n = 4–38 sites), which, rotated for closure of the Gulf of California, falls at 79.3°N, 179.5°E, and it is concordant with the North America reference pole. Plutons in the transition zone, between the eastern and western sectors of the Peninsular Ranges, have magnetizations residing in hematite. El Potrero and San José plutons yield highly discordant paleopoles, indicating apparent clockwise rotation (R) and flattening (F) of 33.0° ± 5.1° and −27.6° ± 6.1°, respectively (San José), and 46.1° ± 5.9° and −31.0° ± 7.0° (El Potrero). The discordance is best explained by west-down tilt of the crustal block between the Main Mártir thrust and the Rosarito fault, which are major compressional structures parallel to the trend of the Peninsular Ranges. The San Pedro Mártir pluton, a large La Posta–type pluton on the eastern sector of the transect, has magnetizations that reside primarily in hematite. The mean paleomagnetic pole (71.3°N, 335.5°E; K = 40.7 and A 95 = 7.2°) is slightly discordant, indicating westward tilt of ~15°. The different paleopoles obtained for individual plutons convincingly show that the Peninsular Ranges batholith has suffered internal deformation, which is more intense along the transition zone. The magnetic fabric for plutons representative of the western, eastern, and transitional sectors of the range show marked contrasts in the deformation recorded by anisotropy of magnetic susceptibility (AMS). Anisotropy is weakly developed in the western sector (El Milagro), very strongly developed in the transition zone (San José), and moderately developed in the eastern sector (Sierra San Pedro Mártir). Within the plutons, El Milagro fabrics record emplacement-related stress. In contrast, San José and San Pedro Mártir appear to record regional stress linked to evolution of the Main Mártir thrust. Overall, our data are consistent with rotation of the crustal block where Potrero and San José plutons are located; rotation was accommodated by major crustal faults in a compressional stress field, as the crustal block moved to occupy the space abandoned by the ascending (and westward expanding) San Pedro Mártir diapir batholith. The rotation could be related to interaction between the large Sierra San Pedro Mártir pluton and the Main Mártir thrust, or to mechanical controls such as wedging against a rigid salient.

ABSTRACT This chapter expands upon a model, first proposed in 1998 by Busby and others, in which Mesozoic oceanic-arc rocks of Baja California formed along the Mexican continental margin above a single east-dipping subduction zone, and were extensional in nature, due to rollback of an old, cold subducting slab (Panthalassa). It expands on that model by roughly tripling the area of the region representing this fringing extensional oceanic-arc system to include the western third of mainland Mexico. This chapter summarizes the geologic, paleomagnetic, and detrital zircon data that tie all of these oceanic-arc rocks to each other and to the Mexican margin, herein termed the Guerrero-Alisitos-Vizcaino superterrane. These data contradict a model that proposes the oceanic-arc rocks formed in unrelated archipelagos some 2000–4000 km west of Pangean North America. Following the termination of Permian–Triassic (280–240 Ma) subduction under continental Mexico, the paleo-Pacific Mexico margin was a passive margin dominated by a huge siliciclastic wedge (Potosí fan) composed of sediments eroded from Gondwanan basement and Permian continental-arc rocks. I propose that a second fan formed further north, termed herein the Antimonio-Barranca fan, composed of sediment eroded from southwest Laurentian sources. Zircons from these two fans were dispersed onto the ocean floor as turbidites, forming a unifying signature in the Guerrero-Alisitos-Vizcaino superterrane. The oldest rocks in the Guerrero-Alisitos-Vizcaino superterrane record subduction initiation in the oceanic realm, producing the 221 Ma Vizcaino ophiolite, which predated the onset of arc magmatism. This ophiolite contains Potosí fan zircons as xenocrysts in its chromitites, which I suggest were deposited on the seafloor before the trench formed and then were subducted eastward. This is consistent with the geophysical interpretation that the Cocos plate (the longest subducted plate on Earth) began subducting eastward under Mexico at 220 Ma. The Early Jurassic to mid-Cretaceous oceanic arc of western Mexico formed above this east-dipping slab, shifting positions with time, and was largely extensional, forming intra-arc basins and spreading centers, including a backarc basin along the continental margin (Arperos basin). Turbidites with ancient Mexican detrital zircons were deposited in many of these basins and recycled along normal fault scarps. By mid-Cretaceous time, the extensional oceanic arc began to evolve into a contractional continental arc, probably due to an increase in convergence rate that was triggered by a global plate reorganization. Contraction expanded eastward (inboard) throughout the Late Cretaceous, along with inboard migration of arc magmatism, suggesting slab shallowing with time.

Abstract New field observations, petrology, geochemistry, and 40 Ar/ 39 Ar geochronology are reported for the Scripps Dike, which crops out at the coast north of La Jolla, California. The northeast–southwest-trending and laterally discontinuous dike has a basaltic–trachyandesite bulk composition, with an emplacement age of 13.89 ± 0.13 Ma. Modeling of the dike composition indicates that it formed from 0.5 to 1.5% partial melting of a primitive mantle-type source, metasomatized by slab fluids, predominantly in the garnet stability field. The composition of the dike, including relatively high MgO (6.6 wt.%) and Sr/Y (~105), makes it akin to magnesian andesites in Baja California, Mexico, termed “bajaites.” Field evidence indicates that the current exposure of the dike is close to the original stalling depth, it was probably associated with explosive volcanism, and the dike flowed laterally. After accounting for alteration, the dike has an initial 87 Sr/ 86 Sr composition of 0.70390, with limited evidence for crustal contamination, consistent with derivation from a slab-fluid-metasomatized mantle source. The composition of the dike places it broadly in the range of Miocene California Continental Borderland (hereafter referred to as Borderland) volcanic rocks studied previously. A comparison of ages of volcanic rocks occurring along the Borderland margin reveals an approximately age-progressive trend to the southeast. This represents an opposite sense to the apparent age-progressive trend for Miocene to Recent volcanic rocks north of the Western Transverse Ranges. Possible models to explain the compositions and age relationships of Miocene to Recent volcanic rocks of the Borderland region include southeasterly migration of volcanism in response to Rivera Triple Junction movement and slab window formation, or the presence of a weak “hotspot” that has been active since at least the Miocene. Identification of the process(es) responsible for Borderland volcanism is currently limited by dissection and northwestward movement of Borderland rocks in response to northwest–southeast shearing of the Pacific–North American plate boundary, and by the quality and quantity of reported age-dates and paleomagnetic information. The formation processes of volcanism in the Borderland have ramifications for palinspastic reconstruction of the margin, as well as for the thermal and magmatic evolution of western California in response to a change in plate motion in a subduction to transform setting. The Scripps Dike provides evidence that regions of the mantle beneath the California Continental Borderland were metasomatized by slab fluids in a manner similar to portions of mantle beneath central Baja California, Mexico.

Abstract Study of shear zones and associated basins within an oblique rift can shed light on the development of a young transform continental margin. San Pedro Basin lies within the Inner Borderland Rift offshore southern California, where it is bisected by the San Pedro Basin Fault (SPBF). Based on seismic reflection and multibeam bathymetry data, we show that the SPBF attained continuity with the San Diego Trough Fault (SDTF) between 1 Ma and 800 ka, to form a 350-km-long shear zone. Prior to that time, the SDTF was linked to the Catalina Fault, forming a restraining bend that contributed to the uplift of Catalina Ridge. Seismically defined depositional sequences in San Pedro Basin record a multistage history of uplift and subsidence for the basin. Young, flat-lying sequences filling a sigmoidal depocenter indicate that subsidence has been occurring since about 1 Ma. This date is corroborated by a series of submarine lowstand depositional terraces surrounding Santa Catalina Island. A 5 Ma to 1 Ma progressively tilted sequence, onlapped by the flat-lying strata, is confined to the present basin. Folded sequences older than ca. 5 Ma extend beyond the present basin onto Catalina Ridge and are correlated to Mohnian and Luisian strata on Santa Catalina Island and Palos Verdes Peninsula. From these data, we interpret the growth history of San Pedro Basin to involve at least three successive, nested basins. The first, which we call the “San Pedro (SP) protobasin,” formed before 5 Ma and was of indeterminate size, including within its boundaries areas flanking the current basin that were subsequently uplifted, Catalina Ridge and Palos Verdes Anticlinorium. Between 5 Ma and 1 Ma, approximately, a second basin, nested within the first, formed as the two flanking structural highs initiated. Finally, a third basin, nested within the first two, began to form when the SPBF–SDTF link was established and rapid local subsidence began; this is the depocenter of the current San Pedro Basin, and its southwestern boundary is occupied by the trace of the SPBF. Our model of basin formation begins with the initial oblique Inner Borderland (IB) Rift, which formed during rotation and translation of crustal blocks away from the continental margin (about 20 Ma). The IB Rift was segmented due to preexisting structural configurations. Published reconstructions show that the SP protobasin was originally in a narrow zone flanked by active volcanoes. Continued extension widened and deepened the rift, while volcanism continued along the flanks of the rift until about 10 Ma. As the rift widened, nested basins formed within the original protobasins along the axis of the rift. These basins were later fragmented (after 5 Ma for the SP protobasin) by transpressive processes associated with the shift of the transform plate boundary to the southern San Andreas Fault. New nested basins also formed during this time as shear zones reorganized to shortcut restraining geometries.