Thermal Pressurization and Onset of Melting in Fault Zones

We examine how frictional heating drives the evolution of temperature, strength, and fracture energy during earthquake slip. For small slip distances, heat and pore fluid are unable to escape the shearing fault core, and the behavior is well approximated by simple analytical models that neglect any transport. Following large slip distances, the finite width of the shear zone is small compared to the thicknesses of the thermal and hydrological boundary layers, and the fault behavior approaches that predicted for the idealized case of slip on a plane. To evaluate the range in which the predictions of these two sets of approximations are valid, we develop a model that describes how frictional dissipation within a finite shear zone drives heat and mass transport through the surrounding static gouge. With realistic parameter values and slips greater than a few centimeters, the subsequent evolution of strength and fracture energy are approximated well by the planar slip model. However, the temperature evolution is much more sensitive to the finite shear zone thickness, and the ultimate temperature rise tends to be intermediate between that predicted for the two simplified cases. We explore the range of conditions necessary for melting to begin and focus in particular on the potential role of fault zone damage in facilitating fluid transport and promoting larger temperature increases. We discuss how the apparent scarcity of exhumed pseudotachylytes places constraints on some of the more uncertain fault zone parameters.Earth and Planetary SciencesEngineering and Applied Science

The generalized Clapeyron equation and its application to confined ice growth

Most theoretical descriptions of stresses induced by freezing are rooted in the (generalized) Clapeyron equation, which predicts the pressure that a solid can exert as it cools below its melting temperature. This equation is central for topics ranging beyond glaciology to geomorphology, civil engineering, food storage, and cryopreservation. However, it has inherent limitations, requiring isotropic solid stresses and conditions near bulk equilibrium. Here, we examine when the Clapeyron equation is applicable by providing a rigorous derivation that details all assumptions. We demonstrate the natural extension for anisotropic stress states, and we show how the temperature and pressure ranges for validity depend on well-defined material properties. Finally, we demonstrate how the range of applicability of the (linear) Clapeyron equation can be extended by adding higher-order terms, yielding results that are in good agreement with experimental data for the pressure melting of ice.Comment: 2 Figure

Topographic signatures and a general transport law for deep-seated landslides in a landscape evolution model

A fundamental goal of studying earth surface processes is to disentangle the complex web of interactions among baselevel, tectonics, climate, and rock properties that generate characteristic landforms. Mechanistic geomorphic transport laws can quantitatively address this goal, but no widely accepted law for landslides exists. Here we propose a transport law for deep-seated landslides in weathered bedrock and demonstrate its utility using a two-dimensional numerical landscape evolution model informed by study areas in the Waipaoa catchment, New Zealand, and the Eel River catchment, California. We define a non-dimensional landslide number, which is the ratio of the horizontal landslide flux to the vertical tectonic flux, that characterizes three distinct landscape types. One is dominated by stochastic landsliding, whereby discrete landslide events episodically erode material at rates exceeding the long-term uplift rate. Another is characterized by steady landsliding, in which the landslide flux at any location remains constant through time and is greatest at the steepest locations in the catchment. The third is not significantly affected by landsliding. In both the “stochastic landsliding” and “steady landsliding” regimes, increases in the non-dimensional landslide number systematically reduce catchment relief and widen valley spacing, producing long, low angle hillslopes despite high uplift rates. The stochastic landsliding regime captures the frequent observation that deep-seated landslides produce large sediment fluxes from small areal extents while being active only a fraction of the time. We suggest that this model is adaptable to a wide range of geologic settings and is useful for interpreting climate-driven changes in landslide behavior

Thermal controls on ice stream shear margins

Ice stream discharge responds to a balance between gravity, basal friction and lateral drag. Appreciable viscous heating occurs in shear margins between ice streams and adjacent slow-moving ice ridges, altering the temperature-dependent viscosity distribution that connects lateral drag to marginal strain rates and ice stream velocity. Warmer ice deforms more easily and accommodates faster flow, whereas cold ice supplied from ice ridges drives advective cooling that counteracts viscous heating. Here, we present a two-dimensional (three velocity component), steady-state model designed to explore the thermal controls on ice stream shear margins. We validate our treatment through comparison with observed velocities for Bindschadler Ice Stream and verify that calculated temperatures are consistent with results from previous studies. Sweeping through a parameter range that encompasses conditions representative of ice streams in Antarctica, we show that modeled steady-state velocity has a modest response to different choices in forcing up until temperate zones develop in the shear margins. When temperate zones are present, velocity is much more sensitive to changes in forcing. We identify key scalings for the emergence of temperate conditions in our idealized treatment that can be used to identify where thermo-mechanical feedbacks influence the evolution of the ice sheet

Shoulder kinematics during cyclic overhead work are affected by a passive arm support exoskeleton

Purpose: We investigated the influence of passive arm-support exoskeleton (ASE) with different levels of torque (50, 75, and 100%) on upper arm osteokinematics. Methods: Twenty participants completed a cyclic overhead drilling task with and without ASE. Task duration, joint angles, and angular acceleration peaks were analyzed during ascent and descent phases of the dominant upper arm. Results: Maximum ASE torque was associated with decreased peak acceleration during ascent (32.2%; SD 17.8; p < 0.001) and descent phases (38.8%; SD 17.8; p < 0.001). Task duration remained consistent. Increased torque led to a more flexed (7.2°; SD 5.5; p > 0.001) and internally rotated arm posture (17.6°; SD 12.1; p < 0.001), with minimal changes in arm abduction. Conclusion: The small arm accelerations and changes in osteokinematics we observed, support the use of this ASE, even while performing overhead cyclic tasks with the highest level of support

Frost for the trees: Did climate increase erosion in unglaciated landscapes during the late Pleistocene?

Understanding climatic influences on the rates and mechanisms of landscape erosion is an unresolved problem in Earth science that is important for quantifying soil formation rates, sediment and solute fluxes to oceans, and atmospheric CO2 regulation by silicate weathering. Glaciated landscapes record the erosional legacy of glacial intervals through moraine deposits and U-shaped valleys, whereas more widespread unglaciated hillslopes and rivers lack obvious climate signatures, hampering mechanistic theory for how climate sets fluxes and form. Today, periglacial processes in high-elevation settings promote vigorous bedrock-to-regolith conversion and regolith transport, but the extent to which frost processes shaped vast swaths of low- to moderate-elevation terrain during past climate regimes is not well established. By combining a mechanistic frost weathering model with a regional Last Glacial Maximum (LGM) climate reconstruction derived from a paleo-Earth System Model, paleovegetation data, and a paleoerosion archive, we propose that frost-driven sediment production was pervasive during the LGM in our unglaciated Pacific Northwest study site, coincident with a 2.5 times increase in erosion relative to modern rates. Our findings provide a novel framework to quantify how climate modulates sediment production over glacial-interglacial cycles in mid-latitude unglaciated terrain

Ore-forming processes of the daqiao epizonal orogenic gold deposit, west qinling orogen, China: Constraints from textures, trace elements, and sulfur isotopes of pyrite and marcasite, and raman spectroscopy of carbonaceous material

The Daqiao gold deposit is hosted in organic-rich Triassic pumpellyite-actinolite facies metamorphosed turbidites in the West Qinling orogen, central China. Gold mineralization is characterized by high-grade hydraulic breccias (B and C ores) that overprint an earlier tectonic breccia (A ore). A complex paragenesis is defined by four sulfide stages: S1 diagenetic preore pyrite (py), S2 hydrothermal early ore disseminated pyrite and marcasite (mc), S3 main ore pyrite and marcasite aggregates, and S4 late ore coarse-grained marcasite with minor pyrite and stibnite. However, multiple generations of pyrite and marcasite may develop within one individual stage. Ore-related hydrothermal alteration is dominated by intensive silicification, sulfidation, sericitization, and generally distal minor carbonatization. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analyses show that the stage S1 py1 from the shale interlayers within turbidites contains low gold contents (mean of 0.05 ppm) and other trace elements (Mn, Co, Ni, Cu, Mo, Bi, and Pb), indicating an anoxic to euxinic sedimentary environment. Stage S2 contributed only minimally to the gold endowment with relatively low gold in various sulfides including py2 (mean of 0.09 ppm), py3 (0.84 ppm) to py4 (0.70 ppm), along with mc1 (0.02 ppm) and mc2 (0.14 ppm). Most of the gold was deposited in stage S3, which formed rapidly crystallized, irregular (e.g., framboids, colloform and cyclic zonation) cement-hosted py5a (mean of 27.35 ppm), py5b (9.71 ppm), and mc3 (5.94 ppm) during repeated hydraulic fracturing. Other trace elements (e.g., Ag, As, Sb, Hg, Tl, and W) are also significantly enriched in the main ore-stage pyrite and marcasite. Little or no gold is detected in the S4 py6 and mc4. Sulfur isotopes determined from in situ LA-multicollector (MC)-ICP-MS analyses of hydrothermal pyrite and marcasite from the Daqiao deposit vary significantly from –31.3 to 22.0 (d34S values) but fall mostly between –10 to 10 and provide important information on the source and evolution of sulfur and of the ore-forming fluids. The results show that S2 ore fluids (mean d34Ssulfide = –0.8 to 5.2) were most likely derived from deep-seated Paleozoic carbonaceous sediments during regional metamorphism associated with orogenesis of the West Qinling orogen. Main ore S3 fluids (mean d34Ssulfide = –9.7 to –6.0) are relatively depleted in34S relative to those of S2, presumably due to fluid oxidation associated with hydraulic fracturing caused by the overpressurized fluids. The textural, chemical, and isotopic data indicate two distinct gold-introducing episodes at Daqiao, forming sulfide disseminations during early ore S2 and cement-hosted sulfide aggregates during main ore S3. The S2 mineralization took place in a tectonic breccia beneath low-permeability shale seals that capped the flow of deep-seated metamorphic fluids, facilitating reaction with preexisting carbonaceous material and the host turbidites to form sulfide disseminations and pervasive silicification. Raman spectroscopy analysis suggests that carbonaceous material in the ores is poorly crystallized, with low maturity, giving estimated temperatures of 283° to 355°C that are much higher than those of the ore fluids (100°–240°C). This temperature difference indicates an in situ sedimentary origin modified by the regional pumpellyite-actinolite facies metamorphism for the carbonaceous material in the host rocks, rather than a hydrothermal origin. In S3, continuous flux of hydrothermal fluids caused fluid overpressure and consequent hydraulic fracturing of the competent silicified rocks. Subsequent rapid fluid pressure fluctuations led to phase separation and thus massive oxidation of ore fluids, which triggered fast precipitation of gold and other trace elements within the fine-grained irregular sulfides. Results presented here, in combination with geologic evidences, suggest that the Daqiao gold deposit can be best classified as the shallow-crustal epizonal orogenic type, genetically associated with orogenic deformation and regional metamorphism of the West Qinling orogen

Principles Of Heliophysics: a textbook on the universal processes behind planetary habitability

This textbook gives a perspective of heliophysics in a way that emphasizes universal processes from a perspective that draws attention to what provides Earth (and similar (exo-)planets) with a relatively stable setting in which life as we know it can thrive. The book is intended for students in physical sciences in later years of their university training and for beginning graduate students in fields of solar, stellar, (exo-)planetary, and planetary-system sciences.Comment: 419 pages, 119 figures, and 200 "activities" in the form of problems, exercises, explorations, literature readings, and "what if" challenge

Theoretical and experimental investigations into the formation and accumulation of gas hydrates

The substantial volumes of gas hydrates found in the Arctic and in marine sediments are both a possible source of global climate change, and a potential future energy resource. The rate at which a hydrate layer forms, and the spatial distribution of hydrate in the layer are controlled by the physical conditions of the formation environment. To better understand the physical conditions that affect hydrate layer characteristics, I present a quantitative model for the formation of hydrates in a porous medium. The theory is tested using the the results of laboratory simulations of the modelled conditions. Conservation principles are used to derive the full set of governing equations using the minimum number of assumptions and simplifications. Scaling arguments, based on estimates of physical parameters in marine sediments, show that both heat and mass transport are dominated by diffusive processes, so advection may be neglected in most formation environments. Analytical solutions to the leading-order set of equations are obtained for the case of a porous half-space cooled on its boundary. These solutions provide estimates of the growth-rate of a hydrate layer and the volume fraction of hydrate present. The model predicts that the layer grows on the thermal diffusion timescale with the phase-change interface moving at a rate which is proportional to the square root of time. The predicted hydrate volume fraction is determined by the rate at which compositional diffusion can supply gas to the moving interface. For the formation of a methane hydrate layer, the model generally predicts a hydrate volume fraction that is less than 1%. The modelled conditions are simulated in a reaction chamber constructed from a cast acrylic tube, 0.7 m in length, with an inner diameter of 0.14 m. To test the apparatus, experiments were conducted in which the growth-rate of an ice layer in the sand-filled reaction chamber was monitored using RTD temperature probes. These experiments demonstrate that a simplified version of the hydrate formation model describes the formation of an ice layer in a porous medium. CO₂ was used as the hydrate former to test the predictions for the growth-rate of a hydrate layer and the layer's hydrate content. CO₂ has a high solubility in water, and the model predicts a much greater hydrate volume fraction for a CO₂ hydrate layer than that for a low solubility gas such as methane. During hydrate formation experiments, the temperature probes were unsuccessful at detecting the position of the hydrate phase-change interface. At the end of some experiments, the pressure was reduced to dissociate the hydrate so that the presence of hydrate might be detected. However, the temperature probes failed to definitively identify the associated consumption of latent heat. This is despite the recovery of an ice Thydrate mixture following one such experiment. Additional instrumentation employing acoustic or electrical techniques may be necessary to quantitatively assess the model predictions.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat