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J.S. Pallister

Publications and source records attributed to J.S. Pallister.

16 recordsLinked to original sources
USGS2012

Pillow basalts of the Angayucham terrane: Oceanic plateau and island crust accreted to the Brooks Range

The Angayucham Mountains (north margin of the Yukon-Koyukuk province) are made up of an imbricate stack of four to eight east-west trending, steeply dipping, fault slabs composed of Paleozoic (Devonian to Mississippean), Middle to Late Triassic, and Early Jurassic oceanic upper crustal rocks (pillow basalt, subordinate diabase, basaltic tuff, and radiolarian chert). Field relations and geochemical characteristics of the basaltic rocks suggest that the fault slabs were derived from an oceanic plateau or island setting and were emplaced onto the Brooks Range continental margin. The basalts are variably metamorphosed to prehnite-pumpellyite and low-greenschist facies. Major element analyses suggest that many are hypersthene-normative olivine tholeiites. Classification based on immobile trace elements confirms the tholeiitic character of most of the basalts but suggests that some had primary compositions transitional to alkali basalt. Although field and petrographic features of the basalts are similar, trace element characteristics allow definition of geographically distinct suites. A central outcrop belt along the crest of the mountains is made up of basalt with relatively flat rare earth element (REE) patterns. This belt is flanked to the north and south by LREE (light rare earth element)-enriched basalts. Radiolarian and conodont ages from interpillow and interlayered chert and limestone indicate that the central belt of basalts is Triassic in age, the southern belt is Jurassic in age, and the northern belt contains a mixture of Paleozoic and Mesozoic ages. Data for most of the basalts cluster in the “within-plate basalt” fields of trace element discriminant diagrams; none have trace-element characteristics of island arc basalt. The Triassic and Jurassic basalts are geochemically most akin to modern oceanic plateau and island basalts. Field evidence also favors an oceanic plateau or island setting. The great composite thickness of pillow basalt probably resulted from obduction faulting, but the lack of fault slabs of gabbro or peridotite suggests that obduction faults did not penetrate below oceanic layer 2, a likely occurrence if layer 2 were anomalously thick, as in the vicinity of an oceanic island. The presence of basaltic tuff interbeds indicates proximity to an explosive basaltic eruptive center. The juxtaposition of submarine basalts of differing chemical affinity and age, adjacent to higher-grade Paleozoic metamorphic rocks of the Brooks Range to the north, may be explained by obduction of internally complex (thickened) oceanic crust formed in an ocean plateau setting. Emplacement and rotation of thrust plates to steep attitudes occurred during accretion of the Brooks Range passive margin, probably beginning in the Late to Middle Jurassic.

Journal of Geophysical Research Solid Earth↗
USGS2005

The 2003 phreatomagmatic eruptions of Anatahan volcano - Textural and petrologic features of deposits at an emergent island volcano

Stratigraphic and field data are used in conjunction with textural and chemical evidence (including data from scanning electron microscope, electron microprobe, X-ray fluorescence, X-ray diffraction, and instrumental neutron activation analysis) to establish that the 2003 eruption of Anatahan volcano was mainly phreatomagmatic, dominated by explosive interaction of homogeneous composition low-viscosity crystal-poor andesite magma with water. The hydromagmatic mode of eruption contributed to the significant height of initial eruptive columns and to the excavation and eruption of altered rock debris from the sub-volcanic hydrothermal system. Volatile contents of glass inclusions in equilibrium phenocrysts less abundances of these constituents in matrix glass times the estimated mass of juvenile magma indicate minimum emissions of 19 kt SO2 and 13 kt Cl. This petrologic estimate of SO2 emission is an order-of-magnitude less than an estimate from TOMS. Similarly, inferred magma volumes from the petrologic data are an order of magnitude greater than those modeled from deformation data. Both discrepancies indicate additional sources of volatiles, likely derived from a separate fluid phase in the magma. The paucity of near-source volcanic-tectonic earthquakes preceding the eruption, and the dominance of sustained long-period tremor are attributed to the ease of ascent of the hot low-viscosity andesite, followed by a shallow phreatomagmatic mode of eruption. Phreatomagmatic eruptions are probably more common at emergent tropical island volcanoes, where shallow fresh-water lenses occur at near-sea-level vents. These relations suggest that phreatomagmatic explosions contributed to the formation of many of the near-sea-level craters and possibly even to the small calderas at the other Mariana islands.

Northern Mariana Islands↗
USGS2005

Volcano surveillance by ACR silver fox

Recent growth in the business of unmanned air vehicles (UAVs) both in the US and abroad has improved their overall capability, resulting in a reduction in cost, greater reliability and adoption into areas where they had previously not been considered. Uses in coastal and border patrol, forestry and agriculture have recently been evaluated in an effort to expand the observed area and reduce surveillance and reconnaissance costs for information gathering. The scientific community has both contributed and benefited greatly in this development. A larger suite of light-weight miniaturized sensors now exists for a range of applications which in turn has led to an increase in the gathering of information from these autonomous vehicles. In October 2004 the first eruption of Mount St Helens since 1986 caused tremendous interest amoUg people worldwide. Volcanologists at the U.S. Geological Survey rapidly ramped up the level of monitoring using a variety of ground-based sensors deployed in the crater and on the flanks of the volcano using manned helicopters. In order to develop additional unmanned sensing methods that can be used in potentially hazardous and low visibility conditions, a UAV experiment was conducted during the ongoing eruption early in November. The Silver Fox UAV was flown over and inside the crater to perform routine observation and data gathering, thereby demonstrating a technology that could reduce physical risk to scientists and other field operatives. It was demonstrated that UAVs can be flown autonomously at an active volcano and can deliver real time data to a remote location. Although still relatively limited in extent, these initial flights provided information on volcanic activity and thermal conditions within the crater and at the new (2004) lava dome. The flights demonstrated that readily available visual and infrared video sensors mounted in a small and relatively low-cost aerial platform can provide useful data on volcanic phenomena. This was made possible by utilizing GPS and computer-controlled flight direction and stabilization to acquire and track target areas within the Mount St. Helens crater. It was also determined that additional light-weight sensor development will be needed to enable autonomous measurements of volcanic gasses and imaging in poor-weather conditions. Copyright ?? 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Conference Paper↗
USGS1999

Recrystallization and anatexis along the plutonic-volcanic contact of the Turkey Creek caldera, Arizona

Unusual geologic and geochemical relations are preserved along the contact between intracaldera tuff and a resurgent intrusion within the 26.9 Ma Turkey Creek caldera of southeast Arizona. Thick intracaldera tuff is weakly argillically altered throughout, except in zones within several hundred meters of its contact with the resurgent intrusion, where the groundmass of the tuff has been variably converted to granophyre and unaltered sanidine phenocrysts are present. Dikes of similarly granophyric material originate at the tuff-resurgent intrusion contact and intrude overlying intracaldera megabreccia and tuff. Field relations indicate that the resurgent intrusion is a laccolith and that it caused local partial melting of adjacent intracaldera tuff. Geochemical and petrographic relations indicate that small volumes of partially melted intracaldera tuff assimilated and mixed with dacite of the resurgent intrusion along their contact, resulting in rocks that have petrographic and compositional characteristics transitional between those of tuff and dacite. Some of this variably contaminated, second-generation magma coalesced, was mobilized, and was intruded into overlying intracaldera rocks.

Arizona↗
USGS1994

Campground talk and slide show on volcanoes for Chiricahua National Monument

The slides and the accompanying script presented here are based on a campground presentation at Chiricahua National Monument in 1994. Examples of eruptions at active volcanoes are used to help the audience visualize events that took place in the National Monument 27 million years ago. This presentation stresses the following themes: 1) The National Monument lies on the flank of an ancient volcano known as the Turkey Creek caldera. 2) This volcano produced giant explosive eruptions, much larger than any in the 20th century. Similar volcanoes that are still capable of producing giant eruptions exist today, and include examples in the United States. 3) We can learn how these giant volcanoes operate and what their typical "life histories" are by examining ancient analogues such as the Turkey Creek caldera that have been dissected by erosion. Studies of ancient volcanoes provide the conceptual framework for evaluating hazards at active volcanoes. 4) Large eruptions of the type that occurred in the Chiricahua Mountains are highly explosive. As a result, the molten rock, or magma, that feeds such eruptions is "blown to bits", producing volcanic ash and pumice instead of lava flows. 5) The volcanic history of Chiricahua National Monument can be reconstructed by geologic mapping and by "reading" the record of past eruptions from the layers of rock that were deposited by the volcano. The slides and script follow the format of the 1994 campground presentation. Slides are keyed to the script by number. Italic print is used to identify the subject and source of each slide, and to provide additional background information. This document is intended to be used by interpreters at the National Monument for presentations on geology. It may be used as a complete "packaged" talk or as background material for customized presentations. Additional information for the non-specialist on the geology of Chiricahua National Monument is available in a separate report (Pallister and others, 1993).

Open-File Report↗
USGS1992

Mount St. Helens a decade after the 1980 eruptions: magmatic models, chemical cycles, and a revised hazards assessment

Available geophysical and geologic data provide a simplified model of the current magmatic plumbing system of Mount St. Helens (MSH). This model and new geochemical data are the basis for the revised hazards assessment presented here. The assessment is weighted by the style of eruptions and the chemistry of magmas erupted during the past 500 years, the interval for which the most detailed stratigraphic and geochemical data are available. This interval includes the Kalama (A. D. 1480-1770s?), Goat Rocks (A.D. 1800-1857), and current eruptive periods. In each of these periods, silica content decreased, then increased. The Kalama is a large amplitude chemical cycle (SiO2: 57%-67%), produced by mixing of arc dacite, which is depleted in high field-strength and incompatible elements, with enriched (OIB-like) basalt. The Goat Rocks and current cycles are of small amplitude (SiO2: 61%-64% and 62%-65%) and are related to the fluid dynamics of magma withdrawal from a zoned reservoir. The cyclic behavior is used to forecast future activity. The 1980-1986 chemical cycle, and consequently the current eruptive period, appears to be virtually complete. This inference is supported by the progressively decreasing volumes and volatile contents of magma erupted since 1980, both changes that suggest a decreasing potential for a major explosive eruption in the near future. However, recent changes in seismicity and a series of small gas-release explosions (beginning in late 1989 and accompanied by eruption of a minor fraction of relatively low-silica tephra on 6 January and 5 November 1990) suggest that the current eruptive period may continue to produce small explosions and that a small amount of magma may still be present within the conduit. The gas-release explosions occur without warning and pose a continuing hazard, especially in the crater area. An eruption as large or larger than that of 18 May 1980 (???0.5 km3 dense-rock equivalent) probably will occur only if magma rises from an inferred deep (???7 km), relative large (5-7 km3) reservoir. A conservative approach to hazard assessment is to assume that this deep magma is rich in volatiles and capable of erupting explosively to produce voluminous fall deposits and pyroclastic flows. Warning of such an eruption is expectable, however, because magma ascent would probably be accompanied by shallow seismicity that could be detected by the existing seismic-monitoring system. A future large-volume eruption (???0.1 km3) is virtually certain; the eruptive history of the past 500 years indicates the probability of a large explosive eruption is at least 1% annually. Intervals between large eruptions at Mount St. Helens have varied widely; consequently, we cannot confidently forecast whether the next large eruption will be years decades, or farther in the future. However, we can forecast the types of hazards, and the areas that will be most affected by future large-volume eruptions, as well as hazards associated with the approaching end of the current eruptive period. ?? 1992 Springer-Verlag.

Bulletin of Volcanology↗
USGS1992

A basalt trigger for the 1991 eruptions of Pinatubo volcano?

The eruptive products of calc-alkaline volcanos often show evidence for the mixing of basaltic and acid magmas before eruption (see, for example, refs 1, 2). These observations have led to the suggestion3 that the injection of basaltic magma into the base of a magma chamber (or the catastrophic overturn of a stably stratified chamber containing basaltic magma at its base) might trigger an eruption. Here we report evidence for the mixing of basaltic and dacitic magmas shortly before the paroxysmal eruptions of Pinatubo volcano on 15 June 1991. Andesitic scoriae erupted on 12 June contain minerals and glass with disequilibrium compositions, and are considerably more mafic than the dacitic pumices erupted on 15 June. Differences in crystal abundance and glass composition among the pumices may arise from pre-heating of the dacite magma by the underlying basaltic liquid before mixing. Degassing of this basaltic magma may also have contributed to the climatologically important sulphur dioxide emissions that accompanied the Pinatubo eruptions.

Pinatubo volcano↗
USGS1988

Precambrian ophiolites of Arabia: Geologic settings, U Pb geochronology, Pb-isotope characteristics, and implications for continental accretion

Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740–780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820–870 Ma and $ ̆ 1250 Ma "> 1250Ma . The 820–870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the northern end of the 900 km-long Nabitah mobile belt was dated by the zircon U Pb method at ∼ 830 Ma. This date is in the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture, but is ∼ 100 Ma older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq suture that was transposed parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data from the ophiolites are of three types: (1) lead from the ophiolitic rocks and arc tonalites of the northwestern Shield and ophiolitic rocks of the Nabitah suture is similar to lead in modern mid-ocean ridge basalt, (2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and (3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous and perhaps primitive oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.

Precambrian Research↗
USGS1987

Magmatic history of Red Sea rifting: Perspective from the central Saudi Arabian coastal plain

A newly recognized Tertiary dike complex and comagmatic volcanic rocks exposed on the central Saudi Arabian coastal plain record early stages of magmatism related to Red Sea rifting. Intrusive and stratigraphic relationships and new potassium-argon dating indicate episodic magmatism from about 30 Ma to the present. Additional stratigraphic and radiometric evidence suggests that limited rift-related magmatism began as early as about 50 Ma. An early phase of crustal extension in the region was accompanied by faulting and graben formation and by dike-swarm intrusion. The style of extension and intrusion changed at ∼20 Ma. Localized volcanism and sheeted dike injection ceased and were replaced by the intrusion of thick gabbro dikes. This change may mark the onset of sea-floor spreading in the central Red Sea. The dikes and volcanic rocks consist of a bimodal mafic-felsic suite with transitional subalkaline to alkaline chemistry. Although no unique petrogenetic model can be developed for the suite, the following observations and conclusions have been drawn from the available reconnaissance geochemistry: (1) strontium isotopic initial ratios overlap in the range 0.7031 to 0.7047 and are not clearly related to alkalinity or silica content. The bimodality and alkalinity of the suite are therefore not products of contamination by the radiogenic Precambrian upper-crustal granitic rocks exposed in the region. (2) Several of the evolved rocks (rhyolites and comendite) show large to extreme negative europium anomalies and heavy rare-earth–element (HREE) enrichment, features that cannot be explained by low-pressure crystal fractionation of potential parent magmas. An intriguing possibility is that HREE enrichments and large negative Eu anomalies of some peraluminous rhyolites (and granites) may result from the complete melting of garnet from middle or lower crustal rocks. (3) Most of the basalts are light rare-earth element (LREE) enriched, consistent with derivation from fertile mantle in a continental-rift setting; however, interaction of the mafic melts with lower continental crust cannot be ruled out. A model of poly-baric mantle-melt derivation, producing several alkaline-subalkaline cycles, best explains magmatism in the Red Sea region. Differences in the depths and dynamics of mantle-melt extraction and transport brought about through changes in crust and mantle structure as the rift and paar developed may account for the transition from mixed alkaline-subalkaline bimodal magmatism of the pre-20 Ma rift basin to exclusively subalkaline (tholeiitic) magmatism at the Red Sea spreading axis and to predominantly alkali basalt volcanism inland (within the Arabian Shield).

Geological Society of America Bulletin↗
USGS1986

Red-Sea rift magmatism near Al Lith, Kingdom of Saudi Arabia

A newly recognized Tertiary dike complex and comagmatic volcanic rocks exposed on the central Saudi Arabian coastal plain record early stages of magmatism related to Red Sea rifting. Intrusive and stratigraphic relationships, and new potassium-argon dating indicate episodic magmatism from about 30 Ma to the present. Additional stratigraphic and radiometric evidence suggests that limited rift-related magmatism may have began as early as about 50 Ma ago. An early phase of crustal extension in the region was accompanied by faulting and graben formation and by dike-swarm intrusion. The style of extension and intrusion changed approximately 20 Ma ago. Localized volcanism and sheeted dike injection ceased and were replaced by the intrusion of thick gabbro dikes. This change may mark the onset of sea-floor spreading in the central Red Sea. The dikes and volcanic rocks consist of a bimodal mafic-felsic suite with transitional subalkaline to alkaline chemistry. Although no unique petrogenetic model can be developed for the suite, the following observations and conclusions are apparent from the available reconnaissance geochemistry: 1) Strontium isotopic initial ratios overlap in the range 0.7031 to 0.7047 and are not clearly related to alkalinity or silica content. Therefore, the bimodality and alkalinity of the suite are not products of contamination by the radiogenic Precambrian upper-crustal granitic rocks exposed in the region. 2) Several of the evolved rocks (rhyolites and comendite) show large to extreme negative europium anomalies and heavy rare-earth element (HREE) enrichment, features that cannot be explained by low-pressure crystal fractionation of potential parent magmas. An intriguing possibility is that HREE enrichments and large negative Eu anomalies of some peraluminous rhyolites (and granites) may result from the complete melting of garnet from middle or lower crustal rocks. 3) Most of the basalts are light rare-earth element (LREE)-enriched, consistent with derivation from fertile mantle in a continental-rift setting, however, interaction of the mafic melts with lower continental crust cannot be ruled out. A model of poly-baric mantle-melt derivation, producing several alkalinesubalkaline cycles, best explains magmatism in the Red Sea region. Differences in the depths and dynamics of mantle-melt extraction and transport brought about through changes in crust and mantle structure as the rift and paar developed may account for the transition from mixed alkaline-subalkaline bimodal magmatism of the pre-20 Ma rift basin to exclusively subalkaline (tholeiitic) magmatism at the Red Sea spreading axis and to predominantly alkali basalt volcanism within the Arabian Shield.

Open-File Report↗
USGS1984

Samarium-neodymium data on two late Proterozoic ophiolites of Saudi Arabia and implications for crustal and mantle evolution

Whole-rock and mineral samples from the Jabal al Wask and Jabal Ess ophiolites, northwestern Saudi Arabia, yield Sm-Nd isochron ages of 743+24 Ma and 782??38 Ma, respectively. These formation ages, which provide maximum limits for possible obduction ages, are in broad but not precise agreement with the previously known geologic history of the Arabian Shield. They indicate that the ophiolitic rocks are roughly coeval with nearby volcanic and plutonic rocks, supporting a back-arc origin for the two ophiolites. We suggest that the Jabal al Wask and Jabal Ess ophiolites were parts of the same northeast-southwest trending ophiolite belt, now offset along the Najd fault system. Initial e{open}Nd values range from +6.6 to+ 7.6, indicating derivation from a mantle source that has been LIL-depleted for at least 2 Ga. Reported e{open}Nd values from the Arabian Shield that are lower than this suggest the presence of older, reworked continental crust. ?? 1984 Springer-Verlag.

Contributions to Mineralogy and Petrology↗