Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes

Slow earthquakes represent an important conundrum in earthquake physics. While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse laboratory observations have provided insights, but the physics of slow fault rupture remain enigmatic. Here we report on laboratory observations that illuminate the mechanics of slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the threshold between stable and unstable failure, and is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the surrounding material. This generalizable frictional mechanism may act in concert with other hypothesized processes that damp dynamic ruptures, and is consistent with the broad range of geologic environments where slow earthquakes are observed

Petrogenesis of Mount St. Helens Dacitic Magmas

The most frequent and voluminous eruptive products at Mount St. Helens are dacitic in composition, although a wide variety of magma types (basalt to rhyodacite) is represented. To address the petrogenesis of the dacites, we present major and trace element analyses of samples of pumice clasts and dome or flow lavas erupted during the past ∼40,000 years. The dacites have similar (in some cases even lower) contents of many incompatible elements (e.g., Zr, Hf, REE, U, Be, Ta, Nb) compared with those in associated basalts and andesites, whereas Ba, Rb, K, Cs, and Sr are relatively enriched. The unusual depleted nature of the dacites and generally low bulk distribution coefficients (estimated from glass/whole‐rock pairs) for numerous trace elements preclude an origin of these magmas principally by crystal fractionation of associated mafic magmas. A more plausible model for their origin involves melting of metabasaltic crustal rocks that have been enriched in Ba, Rb, Cs, and Sr by either intercalation of sediments with depleted basalt or selective metasomatic enrichment of the source region. Melting at crustal levels presumably is related to intrusion of mantle‐derived basaltic magmas. Compositional diversity among the erupted dacites can be attributed to spatial or temporal heterogeneity of the magma sources or, in some specific cases, to such processes as crystal fractionation, assimilation, and magma mixing

Chromian Spinel–Olivine Phase Chemistry and the Origin of Primitive Basalts of the Southern Washington Cascades

Geochemically diverse basalts occur in an ~160-km-wide transect that stretches across the Quaternary southern Washington Cascades (SWC) volcanic arc. Two fundamental groups of SWC basalts can be identified on the basis of major and trace element chemistry. On primitive mantle-normalized “spidergrams”, Group I lavas resemble within-plate basalts, whereas Group II lavas exhibit chemical characteristics (e.g., Nb, Ta, and Ti depletions) typical of subduction-related magmas. The primitive nature of many SWC basalts is indicated by their high MgO (\u3e6.5 wt.%, up to 9.5 wt.%), Ni (\u3e85 ppm) and Cr (\u3e180 ppm) contents. Electron microprobe analyses of olivine–spinel pairs in a diverse suite of SWC basalts are used to further evaluate the primitive nature of these magmas. Some of the observed variations in olivine–spinel compositions can be attributed to oxidation, fractionation, mixing, and/or pressure variations during magma evolution. To minimize these effects, we focus on olivine-hosted spinels in samples for which olivine-host rock equilibrium can be demonstrated. Spinels in such rocks have Fe2O33+/(Fe3++Cr+Al)2+)\u3e44] and low Cr# [=100 Cr/(Cr+Al) 45) for Group II lavas; (3) spinel Mg# ranges from 45 to 71, and overlaps for Groups I and II. However, each group defines distinct trends on Mg# vs. Cr# and other variation diagrams, with Group II having more refractory characteristics. This observation is consistent with higher whole-rock Mg# and Fo contents of olivines for Group II lavas, and suggests that sources for those magmas are more refractory (i.e., more melt-depleted) than sources for Group I lavas. The “within-plate”-like chemistry of Group I lavas precludes significant slab-derived contributions and is consistent with their derivation by decompression melting of little modified and relatively fertile mantle wedge material upwelling beneath the arc. The fluid-mobile element-enriched nature of Group II lavas is consistent with melting of a mantle source(s) that has been modified by slab-derived fluids. However, the standard “flux melting” process invoked to explain formation of many arc magmas is problematic for the Cascades because (a) high temperatures inferred for the Cascadia subduction zone imply that the slab may be extensively dehydrated, in which case the inventory of volatiles and fluid-mobile elements is likely to be strongly depleted; and (b) Group I lavas occur at the volcanic front and appear to originate at depths proximal to the subducting slab. These geochemical and geometric constraints seem inconsistent with formation of Group II magmas by flux melting of the mantle wedge as commonly proposed for arcs. An alternative source for Group II lavas is chemically refractory lithospheric mantle that was previously metasomatized by earlier (40 Ma to present) Cascadia subduction and associated magmatism. Reheating and melting of such fluid-enriched domains could result from influx of hot decompression melts (Group I magmas). In either case, chemical and mineralogical evidence implicates two (or more) distinct processes of melt production and at least two distinct types of mantle source

Mineralogy and Phase Chemistry of Mount St. Helens Tephra Sets W and Y as Keys to Their Identification

Voluminous and widespread tephras were produced frequently during the last 36,000 yr of volcanic activity at Mount St. Helens. Numerous tephra sets have been defined by D. R. Mullineaux, J. H. Hyde, and M. Rubin (1975, U.S. Geological Survey Journal of Research, 3, 329–335) on the basis of field relations, Fe-Mg phenocryst assemblage, and 14C chronology and are valuable marker beds for regional stratigraphic studies. In this study modal abundances and mineral compositions were determined (via petrographic and electron microprobe techniques) for numerous samples of individual layers within tephra sets W and Y to evaluate the degree of compositional variability within and between tephra layers and criteria by which to distinguish among Mount St. Helens and other Pacific Northwest tephras. Although individual layers within a set (e.g., We, Wn) cannot be distinguished from each other on the basis of mineralogic characteristics examined, mineral compositions allow distinction among layers W and Y and other Pacific Northwest tephras (e.g., Mazama, Glacier Peak). Fe-Ti oxide compositions and T-ƒO2 estimates derived using coexisting magnetite-ilmenite are especially useful due to the compositional homogeneity of these minerals both within and between samples of a given unit over a wide geographic area. The silicates show more compositional variability than the oxides, but SiO2/Al2O3 contents in hornblende and Fe/Mg ratios in hypersthene aid in distinguishing among Pacific Northwest tephras

The Origin of Mount St. Helens Andesites

Mount St. Helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites. Petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system. Mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants. The mafic lavas include: (a) transitional, olivine + plagioclase basalts with low K2O and incompatible trace-element abundances: (b) calc-alkaline, olivine + plagioclase ± clinopyroxene basalts enriched in K 2O, TiO2, and incompatible trace elements: and (c) calc-alkaline, olivine + plagioclase basaltic andesites with incompatible trace-element contents transitional between the two basalt types. Intermediate lavas include (a) tholeiitic, two-pyroxene andesites, (b) calc-alkaline, plagioclase + two-pyroxenes ± olivine ± amphibole mafic andesites (56-59% wt.% SiO2), and (c) calc-alkaline, plagioclase + two-pyroxenes + amphibole high-silica andesites (61-62 wt.% SiO2). Eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages, and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches. It is unlikely that any of the andesites (or dacites) are derived by fractional crystallization of the recognized basaltic variants. Formation of the andesites simply by contamination (or assimilation-fractional crystallization) of basaltic magma is also improbable. More plausibly, the andesites represent mixing between basaltic and dacitic end-member magmas, each of which may be somewhat heterogeneous or vary in composition with time. In this model, efficient mixing must occur in some parts of the magma plumbing system, while some conduits or storage reservoirs must be effectively isolated

Compositional Diversity of Late Cenozoic Basalts in a Transect Across the Southern Washington Cascades: Implications for Subduction Zone Magmatism

Major volcanoes of the Southern Washington Cascades (SWC) include the large Quaternary stratovolcanoes of Mount St. Helens (MSH) and Mount Adams (MA) and the Indian Heaven (IH) and Simcoe Mountain (SIM) volcanic fields. There are significant differences among these volcanic centers in terms of their composition and evolutionary history. The stratovolcanoes consist largely of andesitic to dacitic lavas and pyroclastics with minor basalt flows. IH consists dominantly of basaltic with minor andesite lavas, all erupted from monogenetic rift and cinder cone vents. SIM has a poorly exposed andesite to rhyolite core but mainly consists of basaltic lavas erupted from numerous widely dispersed vents; it has the morphology of a shield volcano. Distribution of mafic lavas across the SWC is related to north‐northwest trending faults and fissure zones that indicate a significant component of east–west extension within the area. There is overlap in eruptive history for the areas studied, but it appears that peak activity was progressively older (MSH (Ka), IH (mostl

Origin of Hawaiian Tholeiites: Trace Element Constraints

We report here geochemical studies of Hawaiian tholeiites and ultramafic xenoliths from Salt Lake Crater, Oahu. We focus attention on tholeiitic basalts that comprise the bulk of Hawaiian volcanoes. When the samples are screened to include only those lying neat the log-MgO (about 7 percent) end of olivine-control lines (Wright, 1971), tholeiites from individual volcanoes are remarkably uniform. On this basis, we show that, for tholeiites from six volcanoes, systematic geochemical differences exist that cannot be attributed to differentiation of these magmas from a common parental magma. Apparently there have been important differences in the processes of magma generation, source composition, or source mineral constitution. Partial melting calculations based on REE contents emphasize these distinctions, but unique melting models are not presented. In these models, relative REE abundances in the source material is a major uncertainty. Nd isotopic studies of Hawaiian basalts require systematic differences in Sm/Nd for the source material of each volcano. Furthermore, the time-integrated Sm/Nd of the sources must be less than that in chondrites. REE analyses of Hawaiian garnet lherzolite xenoliths show that they have chondritic to light REE-enriched relative abundances with absolute contents (for light REE) about 3 to 8 times chondrites. These data obviously conflict with interpretations of the Nd isotopic data. Several possibilities follow: (1) the available xenoliths are not parental to tholeiite, (2) our simple interpretation of the Nd isotopic data is wrong, and (3) the source regions may have been invaded at geologically recent times by a light REE-enriched phase, in which case the xenoliths may represent the course material. If the xenoliths are characteristic of the source, partial melting calculations indicate that the tholeiites may be generated by 15 to 20 percent melting of garnet lherzolite and at the sane tune conform to constraints imposed by the REE and Ni contents and the partitioning of Fe and Mg between melts and residues. We propose that the primary tholeiitic magmas contain no more than about 12 percent MgO, and that erupted magmas probably fractionated less than 10 to 15 percent of olivine during ascent and storage in high-level chambers

Physical Activity Facilitators and Barriers Among Retired Women in North Carolina: A qualitative study

BACKGROUND Women are less likely than men to be physically active and more likely to reduce their physical activity as they age. The objective of this research was to understand barriers that might prevent North Carolina women from being physically active after retirement as well as aspects of retirement that might facilitate a more physically active lifestyle to inform intervention strategies applicable to retired women. METHOD Semi-structured interviews were conducted with 15 recently retired women living in North Carolina. Interviews were recorded and transcribed. Content analysis was used to identify themes related to barriers and facilitators of physical activity after retirement. RESULTS Six themes were identified. One theme was the development of leisure-time physical activity habits over the lifespan. Five other themes described how physical activity after retirement was influenced by prior occupational physical activity, concurrent life transitions (e.g., becoming a caregiver), health, social support, and the community environment. LIMITATIONS Women in this study were active participants in community organizations, which might make their experiences unique from those of women who are not engaged with their communities. However, similarities in themes in this and other qualitative studies corroborate the broader transferability of findings. CONCLUSION Interventions to promote physical activity among retired North Carolina women should consider emphasizing health benefits of physical activity and improving walking environments and access to physical activity facilities. Local residents should be involved in intervention design to address unique barriers among women who retire from physically demanding jobs or become caregivers

Dynamic Impedance of Two-Dimensional Superconducting Films Near the Superconducting Transition

The sheet impedances, Z(w,T), of several superconducting a-Mo77Ge23 films and one In/InOx film have been measured in zero field using a two-coil mutual inductance technique at frequencies from 100 Hz to 100 kHz. Z(w,T) is found to have three contributions: the inductive superfluid, renormalized by nonvortex phase fluctuations; conventional vortex-antivortex pairs, whose contribution turns on very rapidly just below the usual Kosterlitz-Thouless-Berezinskii unbinding temperature; and an anomalous contribution. The latter is predominantly resistive, persists well below the KTB temperature, and is weakly dependent on frequency down to remarkably low frequencies, at least 100 Hz. It increases with T as e-U'(T)/kT, where the activation energy, U'(T), is about half the energy to create a vortex-antivortex pair, indicating that the frequency dependence is that of individual excitations, rather than critical behavior.Comment: 10 pages, 10 figs; subm PR