88 research outputs found
Type III and IV deformation twins in minerals and metals
Type IV twins are defined and shown to exist in triclinic crystal systems, as well as in some monoclinic and trigonal systems. Here, we focus on Pericline twins in triclinic plagioclase as an example. Type IV twins are associated with the irrationality of one of the twinning elements that is rational for a type II twin. The formation of type IV twins is accomplished through the shear on a K2 plane produced by the motion of twinning disconnections on a K1 plane, followed by rotational partitioning. The same systems where type IV twins are present also have type III twins instead of type I. Without using the correct type IV analysis, one would deduce the wrong magnitude and direction of shear associated with the twinning process, the magnitude of which would increase with greater triclinicity. Types I and II twins form if and only if there are rational lattice translation vectors lying in the plane of distortion/shear. Otherwise, the twins are types III and IV.
Historically, two types of twins (I and II) have been categorized for twinning in minerals and metals. When analyzed by the topological model, a crystallographic construction used to define the defect structure of interfaces, triclinic and some other lowsymmetry crystals do not fall into either category and instead form two new twinning types, namely, III and IV. Aside from accurately describing twin structures, these concepts are important for understanding the deformation of minerals such as plagioclase and for deriving constitutive models for the deformation
Rheologic controls on slab dynamics
Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 8 (2007): Q08012, doi:10.1029/2007GC001597.Several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age. However, studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones. Numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics. Employing 2-D numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine, we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity, subducting plate age, and subduction duration can be reproduced without trench motion (i.e., slab roll-back) for locations away from slab edges. Successful models include a stiff slab that is 100â1000 times more viscous than previous estimates from models of plate bending, the geoid, and global plate motions. We find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling, with a small dependence on subducting plate age. Once the slab sinks into the lower mantle the primary processes controlling slab evolution are (1) the ability of the stiff slab to transmit stresses up dip, (2) resistance to slab descent into the higher-viscosity lower mantle, and (3) subduction-induced flow in the mantle-wedge corner.This research was partially supported by NSF award
EAR0125919
The Relationship between Musical Ability and the Perception and Production of L2 Prosodic Features
Studies in L2 acquisition have indicated that musically trained individuals are apt to demonstrate better L2 pronunciation skills. As for music, it was recently clarified that some amusiacs demonstrate selective impairment in L1 prosody discrimination. The purpose of this study was to investigate a relationship between amusical L2 learners and their perception and production of L2 prosody. To investigate this, 24 native-Japanese learners of English either in EFL (n=22) or ESL context (n=2) were examined in terms of their musical ability and L2 intonation perception and production. The musical test indicated that there was one amusiac and 10 low-level musical sufferers in the EFL group. Based on a contrastive analysis between amusical and non-amusical participants, as well as between participant groups with and without musical difficulty, it was found that any level of musical difficulty was correlated with lower auditory processing ability in L2 intonation for these English-language learners. However, the contrastive analysis pertaining to the productive skill indicated that musical difficulty was not associated with their production of accurate L2 intonation patterns. According to these findings, the present study concluded that musical difficulty is only related to these learners\u27 L2 intonation processing. Conversely, the present research found that the ESL learners\u27 learning context appeared to be less associated with their aural performance than with their intonation production. In addition, it was found that the level of previous musical training was related to both better L2 intonation perception and production
Implications of grain size evolution on the seismic structure of the oceanic upper mantle
Author Posting. © Elsevier B.V., 2009. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 282 (2009): 178-189, doi:10.1016/j.epsl.2009.03.014.We construct a 1-D steady-state channel flow model for grain size evolution in the
oceanic upper mantle using a composite diffusion-dislocation creep rheology. Grain size
evolution is calculated assuming that grain size is controlled by a competition between dynamic
recrystallization and grain growth. Applying this grain size evolution model to the oceanic upper
mantle we calculate grain size as a function of depth, seafloor age, and mantle water content.
The resulting grain size structure is used to predict shear wave velocity (VS) and seismic quality
factor (Q). For a plate age of 60 Myr and an olivine water content of 1000 H/106Si, we find that
grain size reaches a minimum of ~15 mm at ~150 km depth and then increases to ~20â30 mm at
a depth of 400 km. This grain size structure produces a good fit to the low seismic shear wave
velocity zone (LVZ) in oceanic upper mantle observed by surface wave studies assuming that the
influence of hydrogen on anelastic behavior is similar to that observed for steady state creep.
Further it predicts a viscosity of ~1019 Pa s at 150 km depth and dislocation creep to be the
dominant deformation mechanism throughout the oceanic upper mantle, consistent with
geophysical observations. We predict larger grain sizes than proposed in recent studies, in which
the LVZ was explained by a dry mantle and a minimum grain size of 1 mm. However, we show
that for a 1 mm grain size, diffusion creep is the dominant deformation mechanism above 100â
200 km depth, inconsistent with abundant observations of seismic anisotropy from surface wave
studies. We therefore conclude that a combination of grain size evolution and a hydrated upper
mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of
the oceanic upper mantle. Our results also suggest that melt extraction from the mantle will be
significantly more efficient than predicted in previous modeling studies that assumed grain sizes
of ~1 mm.Funding for this research was
provided by NSF Grants EAR-06-52707 and EAR-07-38880
Thermal-mechanical behavior of oceanic transform faults : implications for the spatial distribution of seismicity
Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 11 (2010): Q07001, doi:10.1029/2010GC003034.To investigate the spatial distribution of earthquakes along oceanic transform faults, we utilize a 3-D finite element model to calculate the mantle flow field and temperature structure associated with a ridge-transform-ridge system. The model incorporates a viscoplastic rheology to simulate brittle failure in the lithosphere and a non-Newtonian temperature-dependent viscous flow law in the underlying mantle. We consider the effects of three key thermal and rheological feedbacks: (1) frictional weakening due to mantle alteration, (2) shear heating, and (3) hydrothermal circulation in the shallow lithosphere. Of these effects, the thermal structure is most strongly influenced by hydrothermal cooling. We quantify the thermally controlled seismogenic area for a range of fault parameters, including slip rate and fault length, and find that the area between the 350°C and 600°C isotherms (analogous to the zone of seismic slip) is nearly identical to that predicted from a half-space cooling model. However, in contrast to the half-space cooling model, we find that the depth to the 600°C isotherm and the width of the seismogenic zone are nearly constant along the fault, consistent with seismic observations. The calculated temperature structure and zone of permeable fluid flow are also used to approximate the stability field of hydrous phases in the upper mantle. We find that for slow slipping faults, the potential zone of hydrous alteration extends greater than 10 km in depth, suggesting that transform faults serve as a significant pathway for water to enter the oceanic upper mantle.The material presented here
is based on work supported by the National Science Foundation
Division of Ocean Sciences (OCE) grants 0623188 (M.B. and
G.H.) and 0649103 (M.B.) and Division of Earth Sciences
(EAR) grant 0814513 (G.H.)
Using short-term postseismic displacements to infer the ambient deformation conditions of the upper mantle
Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophyscial Research 117 (2012): B01409, doi:10.1029/2011JB008562.To interpret short-term postseismic surface displacements in the context of key ambient conditions (e.g., temperature, pressure, background strain rate, water content, creep mechanism), we combined steady state and transient flow into a single constitutive relation that can explain the response of a viscoelastic material to a change in stress. The flow law is then used to investigate mantle deformation beneath the Eastern California Shear Zone following the 1999 M7.1 Hector Mine earthquake. The flow law parameters are determined using finite element models of relaxation processes, constrained by surface displacement time series recorded by 55 continuous GPS stations for 7 years following the earthquake. Results suggest that postseismic flow following the Hector Mine earthquake occurs below a depth of ~50 km and is controlled by dislocation creep of wet olivine. Diffusion creep models can also explain the data, but require a grain size (3.5 mm) that is smaller than the inferred grain size (10â20 mm) based on the mantle conditions at these depths. In addition, laboratory flow laws predict dislocation creep would dominate at the stress/grain size conditions that provide the best fit to diffusion creep models. Model results suggest a transient creep phase that lasts ~1 year and has a viscosity ~10 times lower than subsequent steady state flow, in general agreement with laboratory observations. The postseismic response is best explained as occurring within a relatively hot upper mantle (e.g., 1200â1300°C at 50 km depth) with a long-term background mantle strain rate of 0.1â0.2 ÎŒstrain/yr, consistent with the observed surface strain rate. Long-term background shear stresses at the top of the mantle are ~4 MPa, then decrease with depth to a minimum of 0.1â0.2 MPa at 70 km depth before increasing slowly with depth due to the pressure dependence of viscosity. These conditions correspond to a background viscosity of 1021 Pa s within a thin mantle lid that decreases to ~5 Ă 1019 Pa s within the underlying asthenosphere. This study shows the utility of using short-term postseismic observations to infer long-term mantle conditions that are not readily observable by other means.This work was supported by the National
Science Foundation grants EAR-0952234 (A.M.F.), EAR-0810188 (G.H.),
and EAR-0854673 (M.D.B.).2012-07-3
Compositional dependence of lower crustal viscosity
Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 8333â8340, doi:10.1002/2015GL065459.We calculate the viscosity structure of the lower continental crust as a function of its bulk composition using multiphase mixing theory. We use the Gibbs free-energy minimization routine Perple_X to calculate mineral assemblages for different crustal compositions under pressure and temperature conditions appropriate for the lower continental crust. The effective aggregate viscosities are then calculated using a rheologic mixing model and flow laws for the major crust-forming minerals. We investigate the viscosity of two lower crustal compositions: (i) basaltic (53âwtâ% SiO2) and (ii) andesitic (64âwtâ% SiO2). The andesitic model predicts aggregate viscosities similar to feldspar and approximately 1 order of magnitude greater than that of wet quartz. The viscosity range calculated for the andesitic crustal composition (particularly when hydrous phases are stable) is most similar to independent estimates of lower crust viscosity in actively deforming regions based on postglacial isostatic rebound, postseismic relaxation, and paleolake shoreline deflection.Woods Hole Oceanographic Institution Summer Student Fellowship Program; NSF. Grant Numbers EAR-13-16333, EAR-12200752016-04-2
Hematite (U-Th)/He Thermochronometry Detects Asperity Flash Heating During Laboratory Earthquakes
Evidence for coseismic temperature rise that induces dynamic weakening is challenging to directly observe and quantify in natural and experimental fault rocks. Hematite (U-Th)/He (hematite He) thermochronometry may serve as a fault-slip thermometer, sensitive to transient high temperatures associated with earthquakes. We test this hypothesis with hematite deformation experiments at seismic slip rates, using a rotary-shear geometry with an annular ring of silicon carbide (SiC) sliding against a specular hematite slab. Hematite is characterized before and after sliding via textural and hematite He analyses to quantify He loss over variable experimental conditions. Experiments yield slip surfaces localized in an âŒ5â30-”m-thick layer of hematite gouge with71% ± 1% (1Ï) and 18% ± 3% He loss, respectively. Documented He loss requires short-duration, high temperatures during slip. The spatial heterogeneity and enhanced He loss from FM zones are consistent with asperity flash heating (AFH). Asperities \u3e200â300 ”m in diameter, producing temperatures \u3e900 °C for âŒ1 ms, can explain observed He loss. Results provide new empirical evidence describing AFH and the role of coseismic temperature rise in FM formation. Hematite He thermochronometry can detect AFH and thus seismicity on natural FMs and other thin slip surfaces in the upper seismogenic zone of Earthâs crust
Diapirs as the source of the sediment signature in arc lavas
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 4 (2011): 641-646, doi:10.1038/ngeo1214Many arc lavas show evidence for the involvement of subducted sediment in the melting process. There is debate whether this âsediment meltâ signature forms at relatively low temperature near the fluid-saturated solidus or at higher temperature beyond the breakdown of trace-element-rich accessory minerals. We present new geochemical data from high- to ultrahigh-pressure rocks that underwent subduction and show no significant depletion of key trace elements in the sediment melt component until peak metamorphic temperatures exceeded ~1050ÂșC from 2.7 to 5 GPa. These temperatures are higher than for the top of the subducting plate at similar pressures based on thermal models. To address this discrepancy, we use instability calculations for a non-Newtonian buoyant layer in a viscous half-space to show that, in typical subduction zones, solid-state sediment diapirs initiate at temperatures between 500â850ÂșC. Based on these calculations, we propose that the sediment melt component in arc magmas is produced by high degrees of dehydration melting in buoyant diapirs of metasediment that detach from the slab and rise into the hot mantle wedge. Efficient recycling of sediments into the wedge by this mechanism will alter volatile fluxes into the deep mantle compared to estimates based solely on devolatilization of the slab.Funding for this work was provided by NSF and WHOIâs Deep Ocean Exploration Institute
Microstructural and geochemical constraints on the evolution of deep arc lithosphere
Mantle xenoliths from the Sierra Nevada, California, USA, sampled a vertical column (60â120 km) of lithosphere that formed during Mesozoic continental arc magmatism. This lithosphere experienced an anticlockwise P-T-t path resulting in rapid cooling that effectively âquenched inâ features inherited from earlier high-temperature conditions. Here we combine new mineral chemistry data (water, trace element, and major element concentrations) with mineral crystallographic preferred orientations (CPOs) to investigate the relationship between melt infiltration and deformation. The peridotites record a refertilization trend with increasing depth, starting from shallow, coarse-protogranular, less-melt-infiltrated spinel peridotite with strong, orthorhombic olivine CPO to deep, fine-porphyroclastic, highly melt-infiltrated garnet peridotite with weak, axial-[010] olivine CPO. In contrast to the observed axial-[010] CPOs, subgrain boundary orientations and misorientation axes suggest the dominant activation of the (001)[100] slip system, suggesting deformation under moderately hydrous conditions. After accounting for effects of subsolidus cooling, we see coherent trends between mineral trace element abundance and water content, indicating that melt infiltration led to an increase in water content of the peridotites. However, measured olivine and pyroxene water contents in all peridotites (5â10 and 30â500 wt ppm, respectively) are lower than that required to promote dominant (001)[100] slip system observed in both natural and experimental samples. These results suggest that deformation occurred earlier along the P-T path, probably during or shortly after hydrous melt infiltration. Subsequent rapid cooling at 90 Ma led to water loss from olivine (owing to decreased solubility at low temperature), leaving behind a deep arc lithosphere that remained viscously coupled to the Farallon slab until the opening of the slab window in the late Cenozoic
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