Gordon Gastil

The record of Cenozoic volcanism around the Gulf of California

Potassium/argon dating and chemical analyses of major oxides of volcanic rocks in areas adjacent to the Gulf of California provide a stratigraphic record of tectonic and magmatic evolution that has occurred during the past 30 m.y. The important volcanic provinces are: the Pliocene-Holocene Gulf of California dacite; the Pliocene-Holocene west Baja California alkaline basalt-andesite; the Trans-Mexican Volcanic Belt; the “proto-Gulf” basalt from the coast of Nayarit; the late Miocene alkaline basalt of the Commondu Formation found in the Peninsula; the late Miocene basalt-andesite-rhyolite rocks straddling the northern half of the Gulf; the 18- to 22-m.y.-old hornblende andesite belt in the Peninsula of Baja California and the central coast of Sonora; and the Oligocene–early Miocene basalt-rhyolite belt, largely east of the Gulf. Tectonics interpretation suggests that the subduction plane moved westward between Oligocene and middle Miocene time and that active calc-alkaline volcanism continued over a broad area around the northern Gulf even after the trench west of Baja California had been annihilated.

Is there a Oaxaca-California megashear? Conflict between paleomagnetic data and other elements of geology

After two decades, paleomagnetic study of Cretaceous plutonic, volcanic, and sedimentary rocks consistently shows that both the continental borderland and the batholithic terrane of peninsular California have moved north 14{degree} {plus minus} 5{degree} and rotated 25{degree} to 40{degree} clockwise between the end of the Cretaceous and the end of the Paleocene. A variety of geologic data across the borderland-batholith boundary, with the peninsula, across the Gulf of California, and within the State of Sonora, Mexico, appear to preclude the existence of a megashear along which peninsular California could have traveled northward. Lacking an acceptable scenario for moving the rocks from their proposed position of magnetization to their present position, the data of paleomagnetism and other elements of geology are in direct conflict. The resolution of this enigma will require an important revision of current assumptions.

OLDER PRECAMBRIAN ROCKS OF THE DIAMOND BUTTE QUADRANGLE, GILA COUNTY, ARIZONA

Rocks intruded by Precambrian granite have been previously described in several districts of central Arizona, but it has not been possible to demonstrate their stratigraphic order. In the Diamond Butte quadrangle five older Precambrian formations can be placed in depositional sequence. The oldest is the Alder Formation, consisting of wacke, slate, quartzite, and conglomerate. It is conformably overlain by the Flying W formation consisting of interbedded basic and acidic volcanic rocks and conglomerate. This is overlain with slight unconformity by the conglomerate, quartzite, and slate of the Houden formation. Above the Houden is the Board Cabin formation composed of porphyritic, pillow, and pyroclastic volcanic rocks, and volcanic sediments. The rhyolites and conglomerates of the Haigler formation complete the sequence. The Haigler is believed to be overlain by still younger rhyolites, and the Alder may be underlain by an older sequence of slate and basic volcanic rocks. This volcanic-sedimentary rock sequence portrays a recurrently unstable marine environment in which a large portion, if not all, of the sediment was derived from contemporary volcanic rocks. Through time the parent magma became enriched in potassium and silicon, and finally engulfed its own extrusive and sedimentary deposit to crystallize at relatively shallow depths as quartz porphyry, granophyre, and granite. Evidence of pre-existing metamorphic or plutonic rocks has not been found, and this depositional-orogenic cycle may represent the initial continental formation in this area.

The tectonic history of the southwestern UNited States and Sonora, Mexico, during the past 100 M.Y.

The area between the Colorado Plateau on the north, New Mexico and Chihuahua to the east, and southern and Baja California to the southwest has undergone a complicated thermal and tectonic history climaxing in medial Cretaceous time and manifested in a sequence of events which continue to the present time, independent of plate boundary transform faulting. This sustained sequence of events was initiated by subduction-related magmatic emplacement, largely between 120 and 50 Ma, beginning in the west and sweeping eastward across the region. The emplacement of these rocks generated a large isostatic welt which began to elevate in late medial Cretaceous time in the west central portion of the area and spread outward until in Oligocene time the west coast of southern California was emergent almost to the continental escarpment. Both erosional and tectonic unloading accompanied elevation, with the axis of the welt achieving its maximum elevation in earliest Cenozoic time when north central Sonora may have had elevations of the order of 5000 m. Body forces caused the welt to relax laterally, producing thrust and nappe structures which were westward vergent in the west and eastward vergent in the east. To the east these structures are included in what is called the Larimide Orogeny; to the west they include such features as the Santa Rosa mylonite belt and related structures. As with the magmatic emplacement, the gravitational spreading began in the west. During the Eocene, additional rebound in response to deep erosion and tectonic unroofing began to expose midcrustal rocks within the core area of the welt, and subhorizontal detachment began along the brittle-ductile boundary. During Oligocene and early Miocene time, renewed isostatic uplift resulted from rising magma, and continued erosional and tectonic denudation spawned both brittle and ductile extension, hydrothermal alteration, and mineral deposition. From mid-Miocene to the present time the central portion of the area has subsided, while the marginal mountain ranges have elevated.

Correlation between seismicity and the distribution of thermal and carbonate water in southern and Baja California, United States and Mexico

A comparison of the distribution of thermal and thermal-related springs and wells in southern California, United States, and Baja California, Mexico, with the abundance of earthquakes of magnitude 4 or greater shows as close a relationship between thermal waters and the distribution of seismicity as to the distribution of active faults. It appears that the distribution of thermal water variations in the geothermal gradient in turn influences the stress accumulation capability of the rocks at depth. Thus, areas with abundant thermal waters (and hence steep geothermal gradients) release stress by frequent moderate earthquakes; areas lacking thermal waters, such as the central Transverse Ranges, accumulate stresses that are released by infrequent large earthquakes.

LEAD-ALPHA DATES FOR SOME BASEMENT ROCKS OF SOUTHWESTERN CALIFORNIA

Lead-alpha measurements for zircon and monazite from Woodson Mountain Granodiorite, Ramona and Cuyamaca Peak quadrangles, give ages of about 110 m.y. Bonsall Tonalite, Bonsall and Cuyamaca Peak quadrangles, gives ages of about 120 m.y. Green Valley Tonalite, Escondido and Cuyamaca Peak quadrangles, gives ages of 130 m.y. Prebatholithic volcanic rocks from the Santiago Peak and Escondido quadrangles give ages of about 150 m.y. Granite and granodierite from the Palm Desert quadrangle give ages of 110 and 105 m.y. A quartz diorite from the Superstition Hills, Plaster City quadrangle, indicates 155 m.y. Detrital zircon in the prebatholithic Bedford Canyon Formation, Corona South quadrangle, and Julian Schist, Santa Ysabel quadrangle, give composite ages of 750–1000 m.y. A metasedimentary migmatite gneiss from the Orocopia Mountains, Canyon Spring quadrangle, yields a zircon age of 2400 m.y.

The tectonic implications of asymmetrically zoned plutons

Abstract Asymmetrically zoned plutons have been mapped in many parts of the world. And whereas many of these are asymmetrical for reasons unrelated to regional tectonic environment, there are large areas in which the majority of asymmetric plutons are asymmetric in the same direction. In these “domains” of asymmetry either the mechanisms of emplacement, or post-emplacement deformation have produced a systematic exposure orientation. Recognition of these domains may lead to discovery of the tectonic mechanisms responsible for systematic orientation. Ultimately domains of pluton asymmetry may serve to identify the manner of tectonic emplacement or post-emplacement deformation.

LEAD-ALPHA AGES AND POSSIBLE SOURCES OF METAVOLCANIC ROCK CLASTS IN THE POWAY CONGLOMERATE, SOUTHWEST CALIFORNIA

Zircons in volcanic rock clasts of the Eocene Poway Conglomerate in San Diego County, California, have lead-alpha dates that range from 190 to 260 m.y. Three similar felsites of the Sidewinder and Hodge Volcanic Series in the central Mojave Desert range from 210 to 260 m.y. in age. Because no other similar rocks of this age are known to exist in Southern California or adjacent areas, it is tentatively concluded that many felsite clasts of the Poway Conglomerate were originally derived from Permian-Triassic rocks of the Mojave Desert.