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At least 1,495 records · Page 83Linked to original sources
USGS2012

Lu-Hf and Sm-Nd evolution in lunar mare basalts

Lu-Hf and Sm-Nd data for mare basalts combined with Rb-Sr and total REE data taken from the literature suggest that the mare basalts were derived by small (≤10%) degrees of partial melting of cumulate sources, but that the magma ocean from which these sources formed was light REE and Hf-enriched. Calculated source compositions range from lherzolite to olivine websterite. Nonmodal melting of small amounts of ilmenite (≤3%) in the sources seems to be required by the Lu/Hf data. A comparison of the Hf and Nd isotopic characteristics between the mare basalts and terrestrial oceanic basalts reveals that the εHf/εNd ratios in low-Ti mare basalts are much higher than in terrestrial oceanic basalts. The results are qualitatively consistent with the hypothesis that terrestrial basalt sources are partial melt residues whereas mare basalt sources are cumulates. Alternatively, the results may imply that the terrestrial mantle has evolved in two (or more) stages of evolution, and that the net effect was depletion of the mantle during the first ∼1–3 b.y. followed by enrichment during the last 1–2 b.y.; or simply that there is a difference in Lu-Hf crystal-liquid partitioning (relative to Sm-Nd between the lunar and terrestrial mantles.

Journal of Geophysical Research Solid Earth↗
USGS2012

Partition coefficients of Hf, Zr, and ree between phenocrysts and groundmasses

Partition coefficients of Hf, Zr, and REE between olivine, orthopyroxene, clinopyroxene, plagioclase, garnet, amphibole, ilmenite, phlogopite, and liquid are presented. Samples consist of megacrysts in kimberlite, phenocrysts in alkaline basalts, tholeiitic basalts and andesitic to dacitic rocks, and synthetic garnet and clinopyroxene in Hawaiian tholeiites. The Hf-Lu and Zr-Lu elemental fractionations are as large as the Lu-Sm or Lu-Nd fractionation. The Hf and Zr partition coefficients between mafic phenocrysts and liquids are smaller than the Lu partition coefficients, but are similar to the Nd or Sm partition coefficients. The Hf and Zr partition coefficients between ilmenite, phlogopite, and liquid are larger than the Lu partition coefficients for these minerals and their corresponding liquids. The Hf-Zr elemental fractionation does not occur except for extreme fractionation involving Zr-minerals and extremely low fo 2 . These data have an important bearing on chronological and petrogenetic tracer studies involving the Lu-Hf isotopic system.

Journal of Geophysical Research Solid Earth↗
USGS2012

Forecast model for moderate earthquakes near Parkfield, California

Earthquake instability models have possible application to earthquake forecasting because the models simulate both preseismic and coseismic changes of fault slip and ground deformation. In the forecast procedure proposed here, repeated measurements of preseismic fault slip and ground deformation constrain the values of model parameters. The early part of the model simulation corresponds to the available field data, and the subsequent part constitutes an estimate of future faulting and ground deformation. In particular, the time, location, and size of unstable faulting are estimates of the pending earthquake parameters. The forecast accuracy depends on the model realism and parameter resolution. The forecast procedure is applied to fault creep and trilateration data measured near Parkfield, California, where at least five magnitude 5.5 to 6 earthquakes have occurred regularly since 1881, the last in 1966. The quasi-static model consists of a flat vertical plane embedded in an elastic half space. Spacially variable fault slip of strike-slip sense is driven by an increasing regional shear stress but is impeded by a relatively strong patch of brittle, strain-softening fault. The field data are consistent with these approximate values of patch parameters: radius of 3 km, patch center 5 km deep and 8 km southeast of the 1966 epicenter, and maximum brittle strength of 26 bars. Fluctuations in the available field data prevent estimating the earthquake time with any more precision than use of the 21±8 year recurrence interval. However, the model may later give a more precise estimate of the earthquake time if the fault slip rate near the inferred patch increases before the earthquake, as predicted by the model.

Journal of Geophysical Research Solid Earth↗
USGS2012

Excitation of a buried magmatic pipe: A seismic source model for volcanic tremor

Recent observations of seismic events at various volcanoes suggest that harmonic tremor results from the sustained occurrence of so-called long-period or low-frequency events. Accordingly, we can view the long-period volcanic event as the elementary process of tremor and interpret it as the impulse response of the tremor-generating system. We present a seismic model in which the source of tremor is the acoustic resonance of a fluid-filled volcanic pipe triggered by excess gas pressure. The model consists of three elements, namely, a triggering mechanism, a resonator, and a radiator. For simplicity, we assume a hemispherical trigger, cylindrial resonator, and circular radiator set in a vertical configuration with the trigger capping the top of the pipe and the disk-shaped radiator shutting off its bottom. Considering the simple case of a source buried in a homogeneous half space, we then apply the discrete wave number method to obtain a complete representation of the ground motion response at near and intermediate distances. The results demonstrate that the displacement attributed to the pipe dominates the near-field motion, while that due to the disk is representative of the intermediate and far fields. The trigger itself has a smaller contribution, mainly limited to the field in the proximity of the source. The characteristics displayed by the free surface response evolve from a strong impulsive signature in the immediate vicinity of the epicenter to a well-developed harmonic wave train dominated by Rayleigh waves at larger distances. No clear shear arrival can be detected in the synthetic seismograms. The displacement spectrum reflects the organ-pipe modes of the conduit, and the bandwidth associated with the dominant spectral peak of motion is controlled by the combined losses due to viscous attenuation in the fluid and elastic radiation into the solid. In the case of the cylindrical magma column considered, the radiation loss is proportional to the square of the pipe radius, while the loss related to viscous damping is inversely proportional to the same factor, indicating that the relative importance of the two loss mechanisms is critically dependent on the geometry of the magma reservoir. The relative importance of the pipe and disk elements, likewise, is a function of the conduit cross section. This suggests the possibility of determining the geometry of the source as well as the radiation loss and in situ magma viscosity from a comparison of near- and far-field observations.

Journal of Geophysical Research Solid Earth↗
USGS2012

Recent movement on the Garlock Fault as suggested by water level fluctuations in a well in Fremont Valley, California

Water levels have been continuously recorded since March 1978 in a well in Fremont Valley, where several strands of the adjacent Garlock fault zone have exhibited both left-lateral displacement and components of normal displacement. Differences in water levels indicate that a fault segment lies between the observation well and a nearby irrigation well. During the 4-year recording period, six sharp fluctuations, or “spikes,” were noted. These fluctuations, occurring over 2- to 4-day periods, have amplitudes of 15–30 cm. They appear to be the result of creep events on a nearby fault. Two types of creep events are plausible: (1) normal slip on an en echelon trace of the Garlock fault less than 300 m south of the well, with the north side up relative to Fremont Valley or (2) left-lateral slip on the same fault. Because of the nature of the fluctuations we favor the latter interpretation. Dislocation models utilizing exponential, arc tangent, and skewed cosine functions were used to analyze the water level fluctuations, associated pressure distribution, and fault displacements. The results suggest that creep on the fault ranges from several millimeters to a centimeter for individual events. Estimates of cumulative creep for the period 1978–1982 range from 20 to 50 mm, depending on the particular model employed.

Journal of Geophysical Research Solid Earth↗
USGS2012

Global map of eolian features on Mars

Ten basic categories of eolian features on Mars were identified from a survey of Mariner 9 and Viking orbiter images. The ten features mapped are (1) light streaks (including frost streaks), (2) dark streaks, (3) sand sheets or splotches, (4) barchari dunes, (5) transverse dunes, (6) crescentic dunes, (7) anomalous dunes, (8) yardangs, (9) wind grooves, and (10) deflation pits. The features were mapped in groups, not as individual landforms, and recorded according to their geographic positions and orientations on maps of 1:12.5 million or 1:25 million scale. In the north polar region, light and dark streaks indicate winds from the west and northwest. Frost streaks show northeast and northwest winds. Barchan dunes show southwest and west winds. Transverse dunes show east and west winds. Local erosional features show winds from the northwest or southeast and northeast or southwest. In the middle and low northern latitudes, streaks show northeast winds; yardangs and deflation pits show mostly easterly and northeasterly winds. In the low southern latitudes, light streaks and dunes record northwest and northeast winds; dark streaks record southeast winds. In high southern latitudes, most streaks and dunes record southeast and east winds. In the south polar region, light and frost streaks record southwest and northwest winds, whereas dark streaks and transverse dunes show southeast winds. The patterns recorded for ephemeral features conform to global conditions of strong southern spring and summer (northern fall and winter) wind circulation. Erosional features in bedrock indicate long-term and perhaps ancient wind trends, whereas depositional features may record relatively more recent winds. Wind directions indicated by yardangs, pits, and other erosional features seldom correspond to those shown by streaks. These erosional features indicating winds of different directions than those that formed the streaks may have been carved when the effective regional winds on Mars were different due to relative hemisphere insolation differences, which are related to obliquity variations. Alternatively, many yardangs and pits may be largely controlled by structure and carved along surface structural patterns or features that channeled both ancient and present-day winds. Deflation pits in the mantled terrain may contain the best record of ancient wind trends. These pits seem to have stratigraphically related orientations. Different stratigraphic units composing the mantle contain wind erosion pits that possibly record large-scale wind direction changes because of long-term changes in the obliquity of the Mars.

Journal of Geophysical Research Solid Earth↗
USGS2012

In situ stress, natural fracture distribution, and borehole elongation in the Auburn Geothermal Well, Auburn, New York

Hydraulic fracturing stress measurements and a borehole televiewer survey were conducted in a 1.6‐km‐deep well at Auburn, New York. This well, which was drilled at the outer margin of the Appalachian Fold and Thrust Belt in the Appalachian Plateau, penetrates approximately 1540 m of lower Paleozoic sedimentary rocks and terminates 60 m into the Precambrian marble basement. Analysis of the hydraulic fracturing tests indicates that the minimum horizontal principal stress increases in a nearly linear fashion from 9.9±0.2 MPa at 593 m to 30.6±0.4 MPa at 1482 m. The magnitude of the maximum horizontal principal stress increases in a less regular fashion from 13.8±1.2 MPa to 49.0±2.0 MPa over the same depth range. The magnitudes of the horizontal principal stresses relative to the calculated overburden stress are somewhat lower than is the norm for this region and are indicative of a strike‐slip faulting regime that, at some depths, is transitional to normal faulting. As expected from the relative aseismicity of central New York State, however, analysis of the magnitudes of the horizontal principal stresses indicates, at least to a depth of 1.5 km, that frictional failure on favorably oriented preexisting fault planes is unlikely. Orientations of the hydraulic fractures at 593 and 919 m indicate that the azimuth of the maximum horizontal principal stress at Auburn is N83°E±15°, in agreement with other stress field indicators for this region. The borehole televiewer log revealed a considerable number of planar features in the Auburn well, the great majority of which are subhorizontal (dips < 5°) and are thought to be bedding plane washouts or drill bit scour marks. In addition, a smaller number of distinct natural fractures were observed on the borehole televiewer log. Of these, the distinct steeply dipping natural fractures in the lower half of the sedimentary section at Auburn tend to strike approximately east‐west, while those in the upper part of the well and in the Precambrian basement exhibit no strong preferred orientation. The origin of this east‐west striking fracture set is uncertain, as it is parallel both to the contemporary direction of maximum horizontal compression and to a late Paleozoic fracture set that has been mapped to the south of Auburn. In addition to these planar features the borehole televiewer log indicates paired dark bands on diametrically opposite sides of the borehole throughout the Auburn well. Processing of the borehole televiewer data in the time domain revealed these features to be irregular depressions in the borehole wall. As these depressions were consistently oriented in a direction at right angles to the direction of maximum horizontal compression, we interpret them to be the result of stress‐induced spalling of the borehole wall (breakouts).

New York↗
USGS2012

Well bore breakouts and in situ stress

The detailed cross-sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data. Breakout shapes are shown for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations. Both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression. Refs.

Journal of Geophysical Research↗
USGS2012

Mineralogy and geochemistry of a sediment‐hosted hydrothermal sulfide deposit from the Southern Trough of Guaymas Basin, Gulf of California

Samples dredged from a 15‐m‐high hydrothermal mound atop the flat turbidite pond in the Southern Trough of Guaymas Basin consist of pyrrhotite‐rich massive sulfide, barite, barite + calcite, talc, and opaline silica as well as substrate material composed of fossiliferous, clay‐rich ooze. An 11‐m‐long sediment core taken near the dredge site shows increasing hydrothermal alteration with depth; anhydrite‐filled fractures near the base of the core appear to be channels for hydrothermal discharge. Oxidation of the sulfide‐rich samples to an assemblage of geothite, lepidocrocite, and amorphous Fe oxyhydroxide is ubiquitous. Compared to other massive sulfide deposits on sediment‐starved oceanic ridges, the hydrothermal deposit dredged in Guaymas Basin has a high pyrrhotite/pyrite ratio, a low Zn sulfide and combined ore metal (Cu + Zn + Pb + Ag + Cd) content, and a greater abundance of sulfate, carbonate, and silicate phases. Venting hydrothermal solutions are alkaline with moderately high p H; high Ca, Ba, and SiO 2 content; low ƒS 2 and ƒo 2 ; and very low transition metal content. Disequilibrium assemblages of pyrrhotite and sulfate minerals form during rapid mixing of this evolved vent fluid with ambient bottom waters at the discharge site. Talc is formed at a temperature near 270°C by mixing or entrainment of Mg‐rich bottom water or pore fluid with upwelling hydrothermal fluid that is saturated with silica. Calcite may precipitate from the alkaline, Ca‐rich fluid during degassing of CO 2 . The minimum temperature range for sulfide and nonsulfide deposition is approximately 190°–326°C. The composition of hydrothermal deposits, vent solutions, and altered sediment requires that circulating fluids evolve during deep penetration into the basaltic basement complex, further interaction with the organic‐and carbonate‐rich sediment pile, and near‐surface mixing with ambient seawater. Although the stable assemblage albite‐epidote‐clinochlore present at depth in the sediment pile requires very low dissolved Mg and Fe in the altering fluid, the addition of Mg to deeply buried sediment indicates significant recharge of the system by Guaymas Basin bottom water.

Southern Trough of Guaymas Basin↗
USGS2012

A plate flexure approximation to postseismic and interseismic deformation

The rather large postseismic deformation that is associated with two‐dimensional dip‐slip faulting in the lithosphere is related to the bending of a free plate generated by dip‐slip faulting. In the absence of gravity, asthenosphere relaxation eventually permits the faulted lithosphere to assume the dihedral configuration of a faulted free plate. For thrust faulting, the faulted area is depressed into the asthenosphere, and the flanks of the plate slope uniformly upward. In the presence of gravity, buoyancy forces act upon the plate, and the ultimate ( t →∞) postseismic uplift is approximated ( y /α > 0.2) by w 1 exp (− y /α) (cos y /α ‐ sin y /α), where w 1 is an explicit function of the fault parameters and lithosphere properties, y is the horizontal distance from the downdip end of the fault, and α is the flexural parameter for the lithosphere plate. The relaxed‐asthenosphere response is the sum of the coseismic deformation (unrelaxed‐astheriosphere response) and this ultimate postseismic deformation. The annual, steady deformation associated with strain accumulation at a subduction zone is simply the relaxed‐asthenosphere response to virtual, normal slip on the main thrust zone of an amount equal to the annual plate convergence. From these relations we have estimated the deformation (strain and uplift) expected along the Pacific coast of northern Honshu. The estimated deformation is about twice the observed deformation, but the predicted geographical distribution of the deformation is similar to the observed deformation.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Scarp degraded by linear diffusion: Inverse solution for age

Under the assumption that landforms unaffected by drainage channels are degraded according to the linear diffusion equation, a procedure is developed to invert a scarp profile to find its “diffusion age.” Diffusion age, having dimension [length] 2 , is the product of diffusivity times chronological age. The second moment of scarp slope grows linearly with age. This fact, together with an assumption about initial scarp shape, allows the inverse determination of diffusion age. Age found assuming a vertical initial scarp is termed “apparent age”; any nonvertical initial scarp profile has a nonzero initial apparent age. True diffusion age differs from apparent age by a fraction of scarp offset squared. The inverse procedure applied to synthetic data yields the following rules of thumb. Evidence of initial scarp shape has been lost when apparent age reaches twice its initial value. If a scarp is formed by two events, the inversion gives their offset-weighted-mean age with an error that is a fraction of offset squared. A scarp that appears to have been formed by one event may have been formed by two with an interval between them as large as apparent age. After scarps of two fault traces have diffused to appear as one, the error in inferred age may be as large as half the apparent age. Variation of apparent age along strike would indicate multiple fault traces. The simplicity of scarp profile measurement and this inversion makes profile analysis attractive. If linearity of the flow law, time for a free face to be reduced to the angle of repose, and variation of diffusivity with climate and material could be established, profile analysis would become a reliable dating technique.

Journal of Geophysical Research Solid Earth↗
USGS2012

The relocation of microearthquakes in the northern Mississippi Embayment

Three-component seismograms, recorded by a small array of digital instruments in the northern Mississippi embayment, consistently show a high-amplitude phase on the vertical component that arrives approximately 0.8 s before the shear wave. On the basis of its timing and apparent velocity, this phase is identified as an S-P conversion from the boundary between the unconsolidated Cenozoic sediments and the underlying Paleozoic rocks. Synthetic models of ground motion in the Mississippi embayment indicate that vertical displacement amplitudes are higher for S-P conversions than for transmitted shear waves at all angles of incidence. The models and their agreement with observations of the three-component seismograms suggest that true shear waves cannot be reliably identified from vertical component seismograms recorded in this area. The travel times of converted phases are used, together with crustal velocity models derived from a recent seismic refraction survey, to relocate approximately 500 microearthquakes recorded by the Central Mississippi Valley regional seismic network. Since the network data are recorded by vertical component seismographs, we assume that the S phases, observed at the stations sited on unconsolidated sediments, are S-P conversions. This assumption significantly reduces the average rms residual and provides well-constrained hypocentral locations for 350 earthquakes. The relocated microearthquakes cluster tightly along previously identified epicentral trends. Focal depths range from 0.5 to 22 km, but events deeper than 14 km are rare, and events shallower than 3 km are confined, almost exclusively, to the area between Ridgely, Tennessee, and New Madrid, Missouri. Focal mechanisms are generally consistent with the results of previous studies. The axial seismicity trend, which extends from Caruthersville, Missouri, to Marked Tree, Arkansas, is characterized by right-lateral strike-slip motion on a northeast trending, nearly vertical fault plane. The events between Ridgely, Tennessee, and New Madrid, Missouri, commonly show strike-slip focal mechanisms, with minor components of both normal and reverse dip slip.

Northern Mississippi Embayment↗
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This effort has resulted in the first detailed history of total geomagnetic field behavior during a reversal of polarity. A comparison of the directional variation history of the reversed and normal polarity intervals on either side of the transition with the Holocene record has allowed an estimate of the duration of these periods to be made. These time estimates were then used to calculate accumulation rates for the volcanic sequence and thereby provide a means for estimating time periods within the transition itself. The polarity transition was found to consist of two phases, each with quite different characteristics. At the onset of the first phase, a one‐third decrease in magnetic field intensity may have preceded the first intermediate field directions by about 600 years. Changes in field direction were confined near the local north‐south vertical plane when the actual reversal in direction occurred and normal polarity directions may have been attained within 550±150 years. The end of the first phase of the transition was marked by a brief (possibly 100–300 years) period with normal polarity and a pretransitional intensity which suggests a quasi‐normal dipole field structure existed during this interval. The second phase of the transition was characterized by a return to very low field intensities with the changes in direction describing a long counterclockwise loop in contrast to the earlier narrowly constrained changes. This second phase lasted 2900±300 years, and both normal directions and intensities were recovered at the same time. Both directional and intensity data document very erratic geomagnetic field behavior during the polarity transition. Changes in magnetic field direction were variable and occurred either (1) in a regular, progressive manner, (2) with sudden, extremely rapid angular changes (58°±21°/year), or (3) with little or no movement for periods of the order of 600±200 years. Changes in magnetic intensity occurred in a like manner and were sometimes correlated with changes in direction, but during other periods both directional and intensity changes occurred independently. Directional changes following the polarity transition occurred in a seemingly normal manner, although intensity fluctuations attest to some instability of the newly reestablished dipole.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition, 2. Field intensity variations and discussion of reversal models

We carried out an extensive paleointensity study of the 15.5±0.3 m.y. Miocene reversed‐to‐normal polarity transition recorded in lava flows from Steens Mountain (south central Oregon). One hundred eighty‐five samples from the collection whose paleodirectional study is reported by Mankinen et al. (this issue) were chosen for paleointensity investigations because of their low viscosity index, high Curie point and reversibility, or near reversibility, of the strong field magnetization curve versus temperature. Application of the Thellier stepwise double heating method was very successful, yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows. After grouping successive lava flows that did not differ significantly in direction and intensity, we obtained 51 distinguishable, complete field vectors of which 10 are reversed, 28 are transitional, and 13 are normal. The record is complex, quite unlike that predicted by simple flooding or standing nondipole field models. It begins with an estimated several thousand years of reversed polarity with an average intensity of 31.5±8.5 μT, about one third lower than the expected Miocene intensity. This difference is interpreted as a long‐term reduction of the dipole moment prior to the reversal. When site directions and intensities are considered, truly transitional directions and intensities appear almost at the same time at the beginning of the transition, and they disappear simultaneously at the end of the reversal. Large deviations in declination occur during this approximately 4500±1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site. The transitional intensity is generally low, with an average of 10.9±4.9 μT for directions more than 45° away from the dipole field and a minimum of about 5 μT. The root‐mean‐square of the three field components X , Y , and Z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude. However, a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions, which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field. Changes in the field vector are progressive but jerky, with at least two, and possibly three, large swings at astonishingly high rates. Each of those transitional geomagnetic impulses occurs when the field intensity is low (less than 10 μT) and is followed by an interval of directional stasis during which the magnitude of the field increases greatly. For the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700±2700 nT yr −1 , which is about 15–50 times larger than the maximum rate of change of the nondipole field observed during the last centuries. The occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions. The record closes with an estimated several thousand years of normal polarity with an average intensity of 46.7±20.1 μT, agreeing with the expected Miocene value. However, the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

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

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 km 3 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 km 3 of lava, the record over the past 0.5 m.y. suggests that eruptions of 1 km 3 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 km 3 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.

Journal of Geophysical Research Solid Earth↗
USGS2012

Implications of silicic vent patterns for the presence of large crustal magma chambers

On the basis of the distribution of silicic vents, many volcanic fields can be grouped with (1) igneous systems that may be small and whose vent locations are controlled by regional tectonics, (2) those that include sizable crustal magma bodies which erupt at sites determined by their anomalous local stress fields, or (3) relatively small volume systems that are transitional between categories 1 and 2. Linear vent patterns that are aligned normal to the regional least principal stress (σ 3 ) commonly are associated with absence of evidence for large, shallow magma bodies. The Coso volcanic field and the Inyo‐Mono domes in California and probably the South Sister area in Oregon are examples of such systems. The 1960 dacitic fissure eruption at Cordón Caulle in southern Chile evidently is linked to tectonic stress relaxation associated with the great earthquake that occurred 48 hours before the eruption began. Large, shallow magma chambers are thought to perturb the local stress field so that areal patterns of silicic vents are diffuse, radial, or arcuate. Such systems may erupt great volumes of pyroclastic material catastrophically and produce large calderas. Well‐preserved examples of late precaldera leaks of silicic magma occur at Long Valley, California, and Mount Mazama (Crater Lake), Oregon. Other possible examples are noted. Some of these, those which formed calderas smaller than Crater Lake, apparently were preceded by silicic eruptions from aligned vents. Bearing in mind that there exists a transitional group between tectonically controlled small systems and very large magma chambers, vent distributions can be useful in evaluating potential volcanic hazards for silicic volcanic fields.

California↗
USGS2012

Root zone of the late Proterozoic Salma caldera, northeastern Arabian Shield, Kingdom of Saudi Arabia

The eroded root of the late Proterozoic Salma caldera crops out in a striking, roughly elliptical feature, about 27 km long and 22 km wide, near the northeastern edge of the Arabian Shield. The caldera is genetically part of an elongate alkalic granitic massif (Jabal Salma) that extends 35 km from the caldera to the southwest. Comenditic ash flow tuff and lava(?) of the caldera fill, probably more than 1 km thick, are the oldest recognized rocks of the caldera complex. These rocks were erupted during caldera collapse associated with the rapid evacuation of the upper, mildly peralkalic part of a zoned magma reservoir. Within the caldera fill, a massive, lithic-rich intracaldera rhyolite, probably a lava in excess of 1 km thick, is overlain by a layered ash flow sequence. Numerous megabreccia blocks, probably derived from the caldera wall, occur in the massive rhyolite. Open folds in the layered volcanic rocks may be due to high-temperature slumping of the rocks toward the center of the caldera following collapse. Later peralkalic granite that intruded the caldera ring fracture zone occurs in an arcuate pattern outside the area of exposed caldera fill. After caldera collapse, metaluminous to peraluminous magma rose beneath the caldera at approximately 580 Ma and solidified as biotite alkali granite, rim syenogranite, and late, high-level granophyre. Rare earth element abundances indicate that the layered rhyolite tuff, peralkalic granite, and granophyre are chemically more evolved than the biotite alkali granite and rim syenogranite. The granophyre intruded the caldera fill as a dome-shaped body composed of numerous sheetlike masses. Granophyric texture resulted from rapid pressure release and quenching accompanying the intrusion of each sheet. Maximum penetration of the granophyre into overlying rocks occurred in the central region and along the west side of the caldera, where the caldera fill volcanic rocks have been removed by erosion. No apparent structural doming of the exposed volcanic rocks along the east side of the caldera took place; the layered ash flows commonly dip steeply toward the center of the caldera. Postemplacement deformation and metamorphism of the caldera are minimal. Small-displacement strike-slip faults cut the complex, which is tilted to the northeast by no more than about 2°.

Journal of Geophysical Research Solid Earth↗
USGS2012

Links between magma-tectonic rate balances, plutonism, and volcanism

With Smith's (1979) model of ash flow magmatism as the point of departure, quantitative regimes of crustal magmatic evolution are outlined. The average extrusive regime of ash flow magmatism operates at a rate of about 0.001 km 3 /yr; progressively larger episodes of increasing repose times characterize the evolution of caldera-forming eruptions. The intrusive regime operates at an inferred average rate of about 0.01 km 3 /yr expressed in terms of the volumetric influx of mafic magma from the mantle, based on the record of silicic rates of extrusion and a 10:1 intrusive to extrusive ratio derived from evidence of zoned silicic systems. This rate is almost identical with the mean cumulative rate for the Hawaiian-Emperor system of basaltic volcanism, which is determined to be 0.015 km 3 /yr. Fluctuations about a mean rate of 10 −2 km 3 /yr lead to a threefold classification of magmatic regimes in terms of source power, depending on whether they are persistently greater than (HHER regime), about equal to (MHER regime), or lower than (LHER regime) the mean Hawaiian-Emperor rate. Ash flow magmatism correlates with the MHER regime, averaged over a sufficiently long time. Thus the conditions of Hawaiian-Emperor magmatism are identified with a characteristic power of magma transfer rates that optimize mechanisms for creation of high-level chambers of derivative and fractionated silicic magma (terminology of Smith (1979)) within sufficiently localized regions of high residence times. These conditions represent a balance point, and a bifurcation, in dynamic paths of magmatic evolution leading to widely divergent types of plutonic-volcanic associations. Categorical examples of the principal types of igneous path are discussed relative to continental basalt, silicic ash flow, batholithic granite, and intrusive porphyry systems. The Henry Mountains, Utah, are thought to represent an example of an intrusive porphyry generated in the LHER regime of a mafic source system lacking sufficient power for volcanic expression or significant mineralization. Long Valley, California (and possibly other systems along the eastern front of the Sierra Nevada), is an example operating at the mean rate with transient excursions to higher and lower rates. In regions of rapid extension and/or volcanic propagation (whether linear or areal) these regimes give rise to basaltic eruptions ranging from isolated cinder cones to flood basalts.

Journal of Geophysical Research Solid Earth↗