Wolfgang E. Elston

Vesicle zonation and vertical structure of basalt flows

Abstract Observation and measurement of vertical sections of thin ( Numerical simulations, performed for this study, suggest that these characteristic patterns of vesicle zonation are the result of the growth and rise of gas bubbles in cooling lavas rather than the result of dynamic conditions such as flow movement or convection. As a bubble grows, it begins to ascend, and continues to ascend until it is overtaken by solidification progressing inward from either the upper or lower cooling surfaces of the flow. Bubbles that start out high in the flow will ascend ahead of the lower solidification front and cease rising only after encountering the downward-advancing upper solidification front, and bubbles near the base of a flow will be entrapped by the upward-advancing lower solidification front. Bubbles that start and rise just above the lower solidification front form the lower part of the upper vesicular zone. Such bubbles will also have longer times in which to grow than bubbles that are either higher or lower and are therefore among the largest in the flow. A zone free of vesicles will remain between the last bubbles to ascend to the upper solidification front and the last bubbles to be overtaken by the lower solidification front.

Determination of Flow Direction of Rhyolitic Ash-Flow Tuffs from Fluidal Textures

Although mid-Tertiary calc-alkalic volcanics equal in volume and composition to major batholiths played a crucial role in the evolution of western North America, interpretations are hampered by Basin and Range faulting and subsequent erosion and sedimentation which obscure primary volcano-tectonic features. To reconstruct them, criteria for tracing volcanic formations to their source are needed. Seventy-one oriented samples were collected from the Pleistocene Bandelier Rhyolite ash-flow tuff and Battleship Rock welded tuff, which erupted from known centers in the Jemez Mountains, New Mexico. In each sample, orientation of elongated shards, pumice fragments and crystals was measured in thin sections cut parallel to primary layering. Lineation was pronounced and indicates a preferred orientation of microscopic or megascopic components, resulting from primary flowage. The statistical significance of results is documented by the Tukey Chi-square test and the vector method. Statistical parameters for these two techniques were calculated by Fortran IV program. Only seven samples (9 percent) indicated Chi-square values below the 90 percent confidence limit. Flow azimuth, which indicates the absolute direction of movement at any point on a flow, is determined by observing objective textural criteria in thin sections cut parallel to the dip, or in vertical sections cut parallel to the predetermined flow-lineation direction. The orientations of both equidimensional and non-equidimensional fork-shaped shards and penetration effects were found to be reliable objective criteria for determination of flow azimuth in dip-parallel sections. Imbrication, blocking effects, and orientation of spindle-shaped objects were found to be reliable objective criteria for flow azimuth determination in vertical sections. The plot of flow lineations and flow azimuths of the Bandelier Rhyolite tuff indicate flowage radially away from the Valles Caldera. Deviations from this pattern probably can be explained by influence of pre-flow topography. When applying these techniques to a region where the source of ash-flow tuffs is unknown, sampling should be carried out over a large area. More than one sample should be collected at each station to assure a flow pattern relatively free from small-scale variation.

Tertiary Volcanic Rocks, Mogollon-Datil Province, New Mexico, and Surrounding Region: K-Ar Dates, Patterns of Eruption, and Periods of Mineralization

K-Ar dates and field relations show that mid-Tertiary volcanic rocks usually assigned to the Datil Formation or Group in southwestern New Mexico can be interpreted as consisting of three major, slightly overlapping, volcanic cycles, which erupted about 28 to 38 + m.y., 23 to 28 m.y., and 20 to 23 m.y. B.P., respectively. Each cycle consists of lava flows, ranging in composition from andesite to rhyolite, and of voluminous silicic ash-flow tuff (ignimbrite) deposits, and is terminated by basaltic andesite and associated calc-alkalic rocks. Mid-Tertiary volcanism ceased at about 20 m.y. B.P., coincident with the beginning of Basin and Range faulting. Tholeiite and alkali basalt became the most common types of volcanic rocks after 20 m.y. B.P., although some felsic volcanism persisted. The timing of mid-Tertiary events imposes constraints on plate-tectonic or other interpretations of volcanism and tectonism in western North America. Models designed to explain volcanism in the Great Basin could be expected to fit conditions in southwestern New Mexico also; the time of inception, type, variation trends, and volume of volcanism are essentially the same. The ∼20 m.y. date for the beginning of Basin and Range faulting is consistent with the constant-rate model of Atwater (1970). There is no evidence in New Mexico for progressive outward spreading of volcanism as described from the Great Basin by Armstrong and others (1969). As in the Great Basin, the mid-Tertiary rocks of southwestern New Mexico become progressively enriched in alkalis and depleted in calcium with time. This can be most clearly demonstrated by comparing rocks of pre-28 m.y. and post-28 m.y. age. In documenting a plate-tectonic model, Christiansen and Lipman (1972) and Lipman and others (1972) recognized a petrologic transition at about 28 m.y. They correlated it with the start of Basin and Range faulting, in conflict with the ∼20 m.y. date given here. Porphyry-type mineralization is shown to be partly older than mid-Tertiary volcanism and partly contemporaneous with it.

Rhyolite ash-flow plateaus, ring-dike complexes, calderas, lopoliths, and moon craters.

Data supporting idea that terrestrial volcano- tectonic depressions resembling Moon craters exist, noting Mogollon plateau in New Mexico

RIFTING AND VOLCANISM IN THE NEW MEXICO SEGMENT OF THE BASIN AND RANGE PROVINCE, SOUTHWESTERN USA*

The Basin and Range province has been the site of extensional orogeny for the past 40 m.y. Its complexities may be caused by the superposition of three tectonic regimes: (a) an Andean-type volcanic arc, which gradually gave way to (b) a spreading ensialic backarc basin and was in turn succeeded by (c) intraplate block faulting and rifting. Total extension may have exceeded 100 percent.

POSSIBLE SHATTER CONES IN A VOLCANIC VENT NEAR ALBUQUERQUE, NEW MEXICO

Striated cone structure resembling shatter cones in volcanic vent near Albuquerque, New Mexico

Mars: Evidence for dynamic processes from mariners 6 and 7 ☆

Abstract Mariners 6 and 7 photographs of the equatorial region of Mars document a three-stage evolution of that part of the Martian surface: (1) High- and intermediate-albedo cratered terrains in Meridiani Sinus, Margaritifer Sinus-Thymiamata, Deucalionis Regio-Sabaeus Sinus, and Hellespontus; (2) low-albedo moderately cratered terrain and dark crater fill in Meridiani Sinus, Thymiamata, and Deucalionis Regio-Sabaeus Sinus and possible volcanism in the Hellas-Hellespontus border; and (3) high-albedo surficial deposits, banked-up crater fill, a possible bright-ray crater in Meridiani Sinus, chaotic terrain on the edge of the Margaritifer Sinus mesa, featureless terrain in Hellas and Edom, sinuous channel-like reentrants on scarps at the Hellas-Hellespontus boundary. Regional faulting seems to have occurred following formation of the old cratered plains and prior to formation of low-albedo plains in Meridiani Sinus and also prior to formation of canyon-like reentrants and featureless terrain along the Hellas-Hellespontus boundary. Mars has had a complex history of dynamic evolution, possibly analogous to the more stable regions of Earth. Its geochemical differentiation and thermal regime should account for long-term postaccretional tectonic and volcano-tectonic processes as well as for fluid media on its surface sufficient to cause erosion, including the cutting of large canyons.

Distribution of Uranium in Middle Tertiary Volcanic Rocks, Mogollon-Datil Volcanic Field, New Mexico: ABSTRACT

The uranium abundances in middle Tertiary volcanic rocks of the Mogollon-Datil volcanic field, southwestern New Mexico, have been determined as part of a major petrogenetic study. Over 350 samples of middle Tertiary to Quaternary volcanic rocks have been analyzed for their uranium content by delayed neutron activation analysis. Of the volcanic associations previously proposed for southwestern New Mexico, calc-alkalic andesite, ±43 to ±35 m.y., has a mean of 2.3 ppm U (range 0.9 to 5.4 ppm); calc-alkalic quartz latite to rhyolite, ±35 to ±29 m.y., has a mean of 3.9 ppm U (range 1.7 to 6.2 ppm); basaltic andesite and associated rocks, ±32 to ±18 m.y., has a mean of 2.3 ppm U (range 0.8 to 6.9 ppm); and high-silica rhyolite, ±32 to ±18 m.y., has a mean of 5.2 ppm U (range 1.6 to 9.4 ppm). Anomalous values in the range of 14 to 35 ppm U were found for a riebeckite-bearing lava from the central San Mateo Mountains, a sample of intrusive andesite from the Alum Mountain area, and a lithophysal rhyolite lava and associated ash-flow tuff from the Sierra Cuchillo. Post-13-m.y. b omodal basalt-rhyolite is sparse within the Mogollon-Datil volcanic field. A few determinations from this study, and published and unpublished data for other localities in New Mexico, indicate U abundance from 0.3 to 1.5 ppm U in post-13-m.y. basalt and about 7 to 8 ppm U in rhyolite. End_of_Article - Last_Page 755------------