Every large eruption of nonbasaltic magma taps a magma reservoir that is thermally and compositionally zoned. Most small eruptions also tap parts of heterogeneous and evolving magmatic systems. Several kinds of compositionally zoned ash flow tuffs provide examples of preemptive gradients in T and ƒO2, in chemical and isotopic composition, and in the variety, abundance, and composition of phenocrysts. Such gradients help to constrain the mechanisms of magmatic differentiation operating in each system. Roofward decreases in both T and phenocryst content suggest water concentration gradients in magma chambers. Wide compositional gaps are common features of large eruptions, proving the existence of such gaps in a variety of magmatic systems. Nearly all magmatic systems are ‘fundamentally basaltic’ in the sense that mantle-derived magmas supply heat and mass to crustal systems that evolve a variety of compositional ranges. Feedback between crustal melting and interception of basaltic intrusions focuses and amplifies magmatic anomalies, suppresses basaltic volcanism, produces and sustains crustal magma chambers, and sometimes culminates in large-scale diapirism. Degassing of basalt crystallizing in the roots of these systems provides a flux of He, CO2, S, halogens, and other components, some of which may influence chemical transport in the overlying, more silicic zones. Basaltic magmas become andesitic by concurrent fractionation and assimilation of partial melts over a large depth range during protracted upward percolation in a plexus of crustal conduits. Zonation in the andesitic-dacitic compositional range develops subsequently within magma chambers, primarily by crystal fractionation. Some dacitic and rhyolitic liquids may separate from less-silicic parents by means of ascending boundary layers along the walls of convecting magma chambers. Many rhyolites, however, are direct partial melts of crustal rocks, and still others fractionate from crystal-rich intermediate parents. The zoning of rhyolitic magma is accomplished predominantly by liquid state thermodiffusion and volatile complexing; liquid structural gradients may be important, and thermal gradients across magma chamber boundary layers are critical. Intracontinental silicic batholiths form where extensional tectonism favors coalescence of crustal partial melts instead of hybridization with the intrusive basaltic magma. Cordilleran batholiths, however, result from prolonged diffuse injection of the crust by basalt that hybridizes, fractionates, and preheats the crust with pervasive mafic to intermediate forerunners, culminating in large-scale diapiric mobilization of partially molten zones from which granodioritic magmas separate. Much of the variability among magmatic systems probably reflects the depth variation of relative rates of transport of magma, heat, and volatile components, as controlled in turn by the orientation and relative magnitudes of principal stresses in the lithosphere, the thickness and composition of the affected crust, and variations in the rate and longevity of basaltic magma supply. Extension of the lithosphere may reduce the susceptibility of basaltic magmas to hybridization in the crust, but it can also enhance the role of mantle-derived volatiles in chemical transport.

Gradients in silicic magma chambers: Implications for lithospheric magmatism

Responding to the latest developments in rock physics research, this popular reference book has been thoroughly updated while retaining its comprehensive coverage of the fundamental theory, concepts, and laboratory results. It brings together the vast literature from the field to address the relationships between geophysical observations and the underlying physical properties of Earth materials - including water, hydrocarbons, gases, minerals, rocks, ice, magma and methane hydrates. This third edition includes expanded coverage of topics such as effective medium models, viscoelasticity, attenuation, anisotropy, electrical-elastic cross relations, and highlights applications in unconventional reservoirs. Appendices have been enhanced with new materials and properties, while worked examples (supplemented by online datasets and MATLAB® codes) enable readers to implement the workflows and models in practice. This significantly revised edition will continue to be the go-to reference for students and researchers interested in rock physics, near-surface geophysics, seismology, and professionals in the oil and gas industries.

The Rock Physics Handbook

238U234U230Th232Th systematics and the precise measurement of time over the past 500,000 years

The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

https://science.sciencemag.org/content/sci/260/5114/1617.full.pdf

Seismicity Remotely Triggered by the Magnitude 7.3 Landers, California, Earthquake

stress provinces indicate that these transitions can be abrupt, occurring over <75 km in places. In the western United States, a region of active tectonism characterized by high levels of seismicity and generally high heat flow, the stress pattern is complex, but numerous stress provinces can be well delineated. Despite relative tectonic quiescence in the eastern and central United States, a major variation in principal stress orientation is apparent between the Atlantic Coast and midcontinent areas. Most of the eastern United States is marked by predominantly compressional tectonism (combined thrust and strike slip faulting), whereas much of the region west of the southern Great Plains is characterized by predominantly extensional tectonism (combined normal and strike slip faulting). Deformation along the San Andreas fault and in parts of the Sierra Nevada is nearly pure strike slip. Exceptions to this general pattern include areas of compressional tectonics in the western United States (the Pacific Northwest, the Colorado Plateau interior, and the Big Bend segment of the San Andreas fault) and the normal growth faulting along the Gulf Coastal Plain. Sources of stress are constrained not only by the orientation and relative magnitude of the stresses within a given province but also by the manner of transition of the stress field from one province to another. Much of the modem pattern of stress in the western United States can be attributed to present transform motion and residual thermal and dynamic effects of Tertiary subduction along the western edge of the North American plate. Abrupt stress transitions around actively extending regions in the western United States probably reflect shallow sources of stress and anomalously thin lithosphere. Large areas characterized by a uniform stress field in the central and eastern United States suggest broad scale plate tectonic forces. In the midcontinent region, both ridge push and asthenospheric viscous drag resistance to lithospheric motion can explain the NE-SW compression in the cold, thick lithosphere of the craton, although drag-induced stress directions (resistance to absolute plate motion) correlate better with the data than do the ridge push directions. Asthenospheric counterflow models do not apply in this region, as predicted stress orientations are about 90 o off. A region of compression oriented approximately perpendicular to the continental margin and Appalachian fold belt is defined by the stress data along the Atlantic Coast. This NW-SE compression is in direct contrast with previous models predicting extension perpendicular to passive continental margins due to lateral density contrasts at the continental-oceanic crust interface. Ridge push forces, while capable of producing a component of compression across the coastal area, do not explain the observed orientation of the stress. Two speculative mechanisms are suggested to explain the observed orientations: (1) rotation of stress (or strain) due to anisotropic Appalachian basement structure and (2) flexural effects associated with erosion and isostatic rebound of the Appalachians.

/pdf/state-of-stress-in-the-conterminous-united-states-12cs1w0fvw.pdf

State of stress in the conterminous United States

Long Valley caldera, a 17- by 32-km elliptical depression on the east front of the Sierra Nevada, was formed 0.7 m.y. ago during eruption of the Bishop tuff. Subsequent intracaldera volcanism included eruption of (1) aphyric rhyolite 0.68-0.64 m.y. ago during resurgent doming of the caldera floor, (2) porphyritic hornblende-biotite rhyolite from centers peripheral to the resurgent dome at 0.5, 0.3, and 0.1 m.y. ago, and (3) porphyritic hornblende-biotite rhyodacite from outer ring fractures 0.2 m.y. ago to 50,000 yr ago, a sequence that apparently records progressive crystallization of a subjacent chemically zoned magma chamber. Holocene rhyolitic and phreatic eruptions suggest that residual magma was present in the chamber as recently as 450 yr ago. Intracaldera hydrothermal activity began at least 0.3 m.y. ago and was widespread in the caldera moat; it has since declined due to self-sealing of near-surface caldera sediments by zeolitization, argillization, and silicification and has become localized on recently reactivated northwest-trending Sierra Nevada frontal faults that tap hot water at depth.

Volcanism, structure, and geochronology of Long Valley Caldera, Mono County, California

Geologic map of the Jemez Mountains, New Mexico

Geological, chronological, and structural studies of the Long Valley-Mono/Inyo Craters area document a long history of related volcanic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km3 prior to its climatic caldera-forming eruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8–10 km deep with smaller cupolas as shallow as 4–5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km3 of lava, the record over the past 0.5 m.y. suggests that eruptions of 1 km3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1–0.2 km3 by additional magma since mid-1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8–6.2 earthquakes that occurred in the region beginning in 1978 had non-double-couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low-viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500-year intervals over the past 2000–3000 years, whereas the Long Valley magma chamber has erupted at about 200,000-year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity.

Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview

Paleomagnetism, Potassium-Argon Ages, and Geology of Rhyolites and Associated Rocks of the Valles Caldera, New Mexico

/pdf/stratigraphic-nomenclature-of-volcanic-rocks-in-the-jemez-18c1fy4hh1.pdf

Roy A. Bailey

Papers

Volcanism, structure, and geochronology of Long Valley Caldera, Mono County, California

Geologic map of the Jemez Mountains, New Mexico

Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview

Paleomagnetism, Potassium-Argon Ages, and Geology of Rhyolites and Associated Rocks of the Valles Caldera, New Mexico

Stratigraphic nomenclature of volcanic rocks in the Jemez Mountains, New Mexico