Thermal Weakening Research Articles

Understanding the influence of in situ produced dextran on wheat dough baking performance: Maturograph, biaxial extension, and dynamic mechanical thermal analysis

Rheological tests performed under conditions relevant to those experienced during proof and oven rise are necessary for understanding the mechanisms of dextran addition on wheat dough baking performance. This study evaluated the effect of a high molecular weight ( Mw ) dextran, produced in situ by Weissella confusa A16 or externally added, on wheat dough rheological properties including, (i) proofing behavior using a maturograph; (ii) bi-axial extensional profile using a dough inflation system; and (iii) viscoelastic characters (proof) and thermo-mechanical properties (simulated baking) by dynamic mechanical thermal analysis (DMA). The externally-added dextran increased dough elasticity, tenacity, and viscoelastic characters, but reduced dough extensibility at bubble rupture. DMA tests of doughs under dynamic heating conditions showed a sharp increase of elastic and loss moduli until maximum between 75 and 95 °C, accompanied by a drastic decrease of Tan δ (dough stiffening). Dextran addition exhibited a weakening effect on the dough thermal properties i.e., decreased peak moduli during heating. On the other hand, the mild acidic conditions during sourdough fermentation favored the activity of in situ produced dextran, conferring significantly improved thermal-mechanical properties and dough extensibility. This may explain the superior ability of in situ produced dextran to improve bread volume and crumb softness compared to the external-added dextran. By analyzing rheological parameters, we showed that the maximum proofing moduli in DMA, fermentation stability, dough level, and elasticity in maturogram were predictors of good baking quality. Overall, our study provides the mechanistic underpinning and optimum of dextran as a natural improver of bread quality. • Rheological properties of doughs are strongly associated with baking performance. • Dextran increases dough elasticity and tenacity but reduces dough extensibility. • Dextran added externally exhibits a thermal weakening effect on wheat dough. • Dextran produced in situ is preferred in improving dough rheology and bread quality. • Favorable effects of in situ -produced dextran are mediated by mild acidification.

Origin of subhorizontal shear zones: Examples from a syntectonic laccolith within a metamorphic core complex

The origins of subhorizontal shear zones in the tensile middle crust are unclear. Structures in the syntectonic Congjia laccolith (128–126 Ma) in the Linglong metamorphic core complex (MCC; 137–108 Ma) of eastern China provide an opportunity to investigate the formation of such subhorizontal shear zones. Heterogeneous uplift of the MCC triggered SE-directed subhorizontal magmatic flow to solid-state deformation within the syntectonic laccolith, leading to the development of top-to-the-SE subhorizontal high-to low-temperature shear zones at 128–125 Ma. The magmatic flow and solid-state deformation were driven by gravity and facilitated by intrusion-induced thermal weakening. As the laccolith was exhumed, magmatic fluid overpressure and corresponding cyclical changes in the principal stress axis led to the development of alternating vertical and subhorizontal hydrofracturing within the brittle-ductile regime. Some of the subhorizontal hydrofractures were overprinted by top-to-the-SE brittle-ductile shear zones during the period from 125 to 123 Ma, and subsequent top-to-the-SE brittle faults formed between 123 and 108 Ma, indicating persistent subhorizontal shearing in the ductile, brittle-ductile, and brittle regimes. The structural evolution of the Congjia laccolith demonstrates that gravity-driven subhorizontal magmatic flow can trigger the development of subhorizontal shear zones in middle crust under a regional tensile setting.

Space and Time Variability of Detachment‐ Versus Ramp‐Dominated Thrusting: Insights From the Outer Albanides

AbstractDespite their markedly different structural setting, the northern and southern outer Albanides share a common tectonic evolution from detachment‐dominated to ramp‐dominated, basement‐involved thrusting. The former process (mainly Oligocene to Miocene) is essentially related with the occurrence of a thick décollement level represented by Triassic evaporites, while the latter involves basement ramps splaying out from a middle crustal décollement. As this weak crustal layer is inherited from the Mesozoic rifting stage, the original continental margin architecture is interpreted to strongly influence subsequent convergent deformation. The profoundly different nature of the two dominant décollements in the study area controlled the structural style of the fold and thrust belt. The decoupling capacity of the upper décollement is strongly dependent on the thickness of the Triassic evaporites. Where this is significant (≫1 km; southern outer Albanides), the occurrence of such a thick incompetent layer at the base of competent carbonate units favored the development of break‐thrust folds and imbrication of the sedimentary cover. Fold and thrust belt propagation was instead hindered where original stratigraphic variations resulted in a reduced thickness (≤1 km) of Triassic evaporites. On the other hand, the deeper middle crustal décollement is controlled by basement rheology. Its reactivation during plate convergence was assisted by collision‐related thermal weakening of the crust. This process governed late‐stage (<5 Ma) crustal‐scale tectonic inversion and plays a major role in controlling present‐day seismicity.

3D numerical prediction of thermal weakening effects on granite

AbstractThis paper presents a numerical method to predict the temperature weakening effects on tensile and compressive strength and stiffness of granitic rock. Thermally induced cracking, leading to degradation of the material stiffness and strength, is modelled in the continuum sense by using a damage‐viscoplasticity model based on the Drucker‐Prager criterion with a rounded tensile cut‐off surface. The governing thermo‐mechanical initial/boundary value problem is solved with an explicit (in time) staggered method while using extreme mass scaling to increase the critical time step. Rock heterogeneity is described as random clusters of finite elements assigned with the constituent mineral, here Quartz, Feldspar, and Biotite, material properties further randomized by Weibull distribution. In the present approach, only Quartz thermal expansion coefficient is assumed temperature dependent due to its strong and anomalous temperature dependence upon approaching the α‐β transition. In the numerical testing, the sample is first volumetrically heated to a target temperature. Then, the uniaxial tension and compression tests are performed on the cooled down numerical samples. The simulations demonstrate the validity of the proposed approach as the experimental weakening effects on the rock strength and stiffness as well as the macroscopic failure modes, both in tension and compression, are realistically predicted in a non‐circular way, that is, not using the temperature dependence of any material parameter, save Quartz thermal expansion, as an input data.

Wide Versus Narrow Back‐Arc Rifting: Control of Subduction Velocity and Convective Back‐Arc Thinning

AbstractBack‐arc basins such as the ones behind the island‐arcs of the Western Pacific Ocean or the ones in the Mediterranean Sea are ubiquitous structures of the Earth. They are extensional basins forming in the overriding plate behind subduction zones and similarly to continental rifts, they can exhibit different structural styles from narrow, localized rifting to wide‐rift extension. While these different structural styles have been long recognized, the factors controlling the style of extension in these basins have not been explored properly. We use thermo‐mechanical models to investigate how the relative rates of progressive build‐up of slab‐pull force and of convective thinning and thermal weakening of the overriding plate control the style of back‐arc rifting. Following subduction‐initiation, a high subducting plate velocity results in rapid build‐up of the slab‐pull force. The relatively low rate of convectively thinning and associated moderate weakening of the overriding plate require slab‐pull to build up to close to its maximum value to overcome the high back‐arc integrated strength resulting in a narrow back‐arc rift. In turn a low subducting plate velocity in comparison with the timescale of convective thinning of the overriding plate allows for significant back‐arc weakening before the slab‐pull force becomes large enough to drive back‐arc extension. In this case, the back‐arc exhibits a wide rifting style as extension occurs at significantly reduced overriding plate integrated strength. Our model results provide an explanation why some subduction zones exhibit wide, distributed extension in the overriding plate such as for instance observed in the Pannonian basin.

Down under and under Cover—The Tectonic and Thermal History of the Cooper and Central Eromanga Basins (Central Eastern Australia)

The Cooper subregion within the central Eromanga Basin is the Swiss army knife among Australia’s sedimentary basins. In addition to important oil and gas resources, it hosts abundant coal bed methane, important groundwater resources, features suitable conditions for enhanced geothermal systems, and is a potential site for carbon capture and storage. However, after seven decades of exploration, various uncertainties remain concerning its tectonic and thermal evolution. In this study, the public-domain 3D model of the Cooper and Eromanga stacked sedimentary basins was modified by integrating the latest structural and stratigraphic data, then used to perform numerical basin modelling and subsidence history analysis for a better comprehension of their complex geologic history. Calibrated 1D/3D numerical models provide the grounds for heat flow, temperature, thermal maturity, and sediment thickness maps. According to calibrated vitrinite reflectance profiles, a major hydrothermal/magmatic event at about 100 Ma with associated basal heat flow up to 150 mW/m2 caused source rock maturation and petroleum generation and probably overprinted most of the previous hydrothermal events in the study area. This event correlates with sedimentation rates up to 200 m/Ma and was apparently accompanied by extensive crustal shear. Structural style and depocentre migration analysis suggest that the Carboniferous–Triassic Cooper Basin initially has been a lazy-s shaped triplex pull-apart basin controlled by the Cooper Basin Master Fault before being inverted into a piggy-back basin and then blanketed by the Jurassic–Cretaceous Eromanga Basin. The interpreted Central Eromanga Shear Zone governed the tectonic evolution from the Triassic until today. It repeatedly induced NNW-SSE directed deformation along the western edge of the Thomson Orogen and is characterized by present-day seismicity and distinct neotectonic features. We hypothesize that throughout the basin evolution, alternating tectonic stress caused frequent thermal weakening of the crust and facilitated the establishment of the Cooper Hot Spot, which recently increased again its activity below the Nappamerri Trough.

The influence of orogenic collision inheritance on rifted margin architecture: Insights from comparing numerical experiments to the Mid-Norwegian margin

Most rifts and rifted margins around the world developed on former orogens. This implies that the pre-rift lithospheric configuration is heterogeneous in most cases. Here we investigate how collision inheritance in the form of inherited weak thrusts, long-term thermal weakening, compositional changes, and orogenic collapse, could have played into the spatio-temporal evolution and final architecture of rifted margins. We present interpretations of dynamic numerical experiments, including constraints representative of the North Atlantic Mid-Norwegian rift system phases of continental collision, orogenic collapse, and extension, and compare these to interpretations of seismic reflection profiles.The experiments form rifted margins characterized by basement structures and sedimentary geometries very similar to the Møre and Vøring rifted margins - with onshore collapse-related basins, extensively deformed continental crust with detachments, shear zones, core complexes, rotated thrusts and an offshore succession of distinct structural domains (proximal, necking, hyperextension, exhumation, and outer).Although extensional models developed on homogeneous lithosphere are a good approximation of rifted margin architecture, our results suggest that models that consider pre-rift orogenic inheritance tend to reproduce more accurately the geometries observed in our natural example.

3D Numerical Prediction of Thermal Weakening of Granite under Tension

This paper deals with numerical prediction of temperature (weakening) effects on the tensile strength of granitic rock. A 3D numerical approach based on the embedded discontinuity finite elements is developed for this purpose. The governing thermo-mechanical initial/boundary value problem is solved with an explicit (in time) staggered method while using extreme mass scaling to increase the critical time step. Rock fracture is represented by the embedded discontinuity concept implemented here with the linear (4-node) tetrahedral elements. The rock is modelled as a linear elastic (up to fracture by the Rankine criterion) heterogeneous material consisting of Quartz, Feldspar and Biotite minerals. Due to its strong and anomalous temperature dependence upon approaching the α-β transition at the Curie point (~573 °C), only Quartz in the numerical rock depends on temperature in the present approach. In the numerical testing, the sample is first volumetrically heated to a target temperature. Then, the uniaxial tension test is performed on the cooled down sample. The simulations demonstrate the validity of the proposed approach as the experimental deterioration, by thermally induced cracking, of the rock tensile strength is predicted with a good accuracy.

The nature and origin of cratons constrained by their surface geology

AbstractCratons, defined by their resistance to deformation, are guardians of crustal and lithospheric material over billion-year time scales. Archean and Proterozoic rocks can be found in many places on earth, but not all of them represent cratonic areas. Some of these old terrains, inappropriately termed “cratons” by some, have been parts of mobile belts and have experienced widespread deformations in response to mantle-plume-generated thermal weakening, uplift and consequent extension and/or various plate boundary deformations well into the Phanerozoic.It is a common misconception that cratons consist only of metamorphosed crystalline rocks at their surface, as shown by the indiscriminate designation of them by many as “shields.” Our compilation shows that this conviction is not completely true. Some recent models argue that craton formation results from crustal thickening caused by shortening and subsequent removal of the upper crust by erosion. This process would expose a high-grade metamorphic crust at the surface, but greenschist-grade metamorphic rocks and even unmetamorphosed supracrustal sedimentary rocks are widespread on some cratonic surfaces today, showing that craton formation does not require total removal of the upper crust. Instead, the granulitization of the roots of arcs may have been responsible for weighing down the collided and thickened pieces and keeping their top surfaces usually near sea level.In this study, we review the nature and origin of cratons on four well-studied examples. The Superior Province (the Canadian Shield), the Barberton Mountain (Kaapvaal province, South Africa), and the Yilgarn province (Western Australia) show the diversity of rocks with different origin and metamorphic degree at their surface. These fairly extensive examples are chosen because they are typical. It would have been impractical to review the entire extant cratonic surfaces on earth today. We chose the inappropriately named North China “Craton” to discuss the requirements to be classified as a craton.

Magmatism and Extension in the Anaconda Metamorphic Core Complex of Western Montana and Relation to Regional Tectonics

AbstractMetamorphic core complexes (MCCs) are a product of crustal extension, but their dynamics are still debated. Early research suggests that the formation of MCCs in the western United States was due to gravitational collapse of crust that had been thickened during Cordilleran orogenesis. However, the instability of overthickened crust alone cannot explain the diachronous formation of core complexes with a strong spatial dependency, as there was relatively uniform crustal thickness along strike of the Cordillera. For this reason, there is an interest in what role other lithospheric processes (such as subducted slab removal) play in the evolution of MCCs. We investigate the role of such processes by determining the temporal relation between magmatism and extension in the Anaconda MCC (AMCC) of western Montana. Geologic mapping, zircon U‐Pb geochronology, and zircon (U‐Th)/He thermochronology reveal that the initiation of extension in the AMCC in the Eocene (∼53 Ma) began at least 3 Myr after the emplacement of voluminous Paleocene two‐mica plutons. We interpret that the AMCC is an example of a core complex that was primed for extension by magmatic thermal weakening and suggest that foundering of the Farallon flat slab and the onset of the ignimbrite flareup in western Montana was responsible for the initiation of AMCC extension. An updated compilation of MCC cooling ages and Cenozoic volcanic activity across the western United States supports previous interpretations that the removal of Farallon oceanic lithosphere likely initiated MCC exhumation in some regions.

Exploring the Impact of Thermally Controlled Crustal Viscosity on Volcanic Ground Deformation

AbstractVolcanoes undergoing unrest often produce displacements at the ground surface, providing an important window to interpret the dynamics of the underlying magmatic system. The thermomechanical properties of the surrounding host rock are expected to be highly heterogeneous, with key physical parameters having a strong dependence on temperature. Deformation models that incorporate nonelastic rheological behaviors are therefore heavily reliant on the assumed thermal conditions, and so it is critical to understand how the thermomechanical crustal structure affects the observed deformation field. Here, we use a series of thermo‐viscoelastic Finite Element models to explore how variations in thermal constraints (i.e., reservoir temperature and background geothermal gradient) affect surface displacement patterns when using the Maxwell and Standard Linear Solid (SLS) viscoelastic configurations. Our results demonstrate a strong variability in the viscoelastic deformation response when changing the imposed thermal constraints, caused by the partitioning of deformation and the dissipation of induced stresses. When using the SLS rheology, we identify that cumulative long‐term displacements can vary by over 20%, relative to a reference model with a reservoir temperature of 900°C and background geothermal gradient of 30 K km−1. The relative change increases to a maximum of 35% when thermal weakening of the Young's modulus is also considered. Contrastingly, the deformation patterns of the Maxwell rheology are governed by unbounded displacements and complete stress relaxation. Ultimately, we outline that uncertainties in the thermal constraints can have a significant impact on best‐fit source parameters (e.g., size and depth) and overpressure/volume‐change loading histories inferred from thermo‐viscoelastic models.

Migmatite and leucogranite in a continental-scale exhumed strike-slip shear zone: Implications for tectonic evolution and initiation of shearing

AbstractPlutons within continental strike-slip shear zones bear important geological processes on late-stage plate transpression and continent-continent collision and associated lateral block extrusion. Where, when, and how intrusions and shearing along transpressional strike-slip shear zones respond to plate interactions, however, are often debated. In this study, we investigated migmatite associated leucogranite and pegmatite from the exhumed >1000-km-long Ailao Shan-Red River left-lateral strike-slip shear zone in Southeast Asia that was active during India-Eurasia plate convergence. Most zircons from the migmatites and leucogranitic intrusions present inherited core-rim structure. The depletion of rare earth element patterns and positive Eu anomalies suggest that leucosomes and leucogranites are the result of crustal anatexis. Zircon rims from the foliated migmatites and leucogranites record U-Pb ages of 41–28 Ma, revealing the timing of the Cenozoic crustal anatexis event along this strike-slip shear zone. Ages of the magmatic zircons from the unfoliated pegmatites provide the timing of the termination of a high-temperature tectono-thermal event and ductile left-lateral shearing at 26–23 Ma. The Cenozoic crustal anatexis along the Ailao Shan-Red River strike-slip shear zone indicates that thickened crust underneath the shear zone involved previously subducted crust. We propose that the Cenozoic thermal state has an important effect on the crustal anatexis and thus on the rheological behavior of the lithosphere by thermal weakening, which plays an essential role in localizing the initiation of the deep-seated lower-crustal shear zone.

Thermal Weakening Friction During Seismic Slip: Experiments and Models With Heat Sources and Sinks

AbstractExperiments that systematically explore rock friction under crustal earthquake conditions reveal that faults undergo abrupt dynamic weakening. Processes related to heating and weakening of fault surfaces have been invoked to explain pronounced velocity weakening. Both contact asperity temperature Ta and background temperature T of the slip zone evolve significantly during high‐velocity slip due to heat sources (frictional work), heat sinks (e.g., latent heat of decomposition processes), and diffusion. Using carefully calibrated High‐Velocity Rotary Friction experiments, we test the compatibility of thermal weakening models: (1) a model of friction based only on T in an extremely simplified, Arrhenius‐like thermal dependence; (2) a flash heating model which accounts for the evolution of both V and T; (3) same but including heat sinks in the thermal balance; and (4) same but including the thermal dependence of diffusivity and heat capacity. All models reflect the experimental results but model (1) results in unrealistically low temperatures and model (2) reproduces the restrengthening phase only by modifying the parameters for each experimental condition. The presence of dissipative heat sinks in stage (3) significantly affects T and reflects on the friction, allowing a better joint fit of the initial weakening and final strength recovery across a range of experiments. Temperature is significantly altered by thermal dependence of (4). However, similar results can be obtained by (3) and (4) by adjusting the energy sinks. To compute temperature in this type of problem, we compare the efficiency of three different numerical approximations (finite difference, wavenumber summation, and discrete integral).

Miocene uplift and exhumation history of northwestern Anatolia (Turkey): Implications from apatite (U-Th)/He thermochronology of syn-extensional plutons

• Apatite U-Th/He ages indicate a fast uplift and exhumation throughout the Miocene . • Ductile exhumation of the Solarya and Çataldağ plutons occurred in the early-middle Miocene. • Exhumation of the Eybek Pluton was controlled by brittle faults in late Miocene. • Crustal exhumation was related to back-arc extension driven by Hellenic slab rollback. In this study, we report the first robust apatite (U-Th)/He thermochronology data from the syn-extensional Eybek, Solarya and Çataldağ plutons from NW Turkey. The Solarya and Çataldağ plutons display similar (U-Th)/He apatite ages which range from 21.5 ± 1.6 to 12 ± 1.9 and 22.4 ± 1.2 to 13.7 ± 1.3 Ma, respectively. The Eybek Pluton, on the other hand, yielded younger (U-Th)/He apatite ages between 8.4 ± 1.9 Ma and 5.6 ± 0.5 Ma. Exhumation rates of the plutons change between 214 m/Myr and 122 m/Myr. The exhumation rates and thermal modelling results show that the Solarya and Çataldağ plutons experienced fast uplift and exhumation from early to middle Miocene. A low angle detachment fault(s) associated with Çataldağ Metamorphic Core Complex was responsible for the exhumation of the Çataldağ and Solarya plutons. In contrast, late Miocene cooling and exhumation of the Eybek Pluton was controlled by high angle normal faults. The collective evaluation of our data together with published low-temperature thermochronology data obtained from the spatially associated basement rocks and the metamorphic core complexes indicate that the region-wide exhumation of NW Anatolia was accommodated by different fault systems throughout the Miocene. We infer that the crustal exhumation in northwestern and centralwestern Anatolia occurred contemporaneously during the Miocene, and developed as a result of back-arc extension driven by slab roll-back beneath Hellenic arc and the preceding thermal weakening of the western Anatolian orogenic crust.

Thermo‐Mechanical Numerical Modeling of the South American Subduction Zone: A Multi‐Parametric Investigation

AbstractThe South American subduction zone is enigmatic due to its fast subducting plate consumption, strong oscillation in subducting plate velocity, and deep mantle subduction. A key to help understand these subduction zone characteristics is through studying its dynamics with buoyancy‐driven numerical modeling that uses independent variables to best approximate the dynamics of the real subduction system. We conduct a parametric investigation on the effect of upper mantle rheology, subduction interface yield stress and slab thermal weakening. To constrain those model variables, we attempt to find best‐fits by comparing our model outcomes with the present‐day slab geometry and estimates of Cenozoic velocities in the center of the South American subduction zone. Key ingredients that need to be reproduced are low slab dip angles close to the surface, steeper lower mantle dip angles, strong oscillation of the fast Farallon‐Nazca subducting plate velocity and a progressive decrease in trench retreat rate during long‐term subduction. We include these ingredients to define a model fitting score using 10 criteria. Our best fitting models involve a weak subduction zone interface (yield stress of ∼20 MPa) and significant slab thermal weakening to attain the fast Farallon‐Nazca subducting plate velocity and to better reproduce the subduction partitioning since 48 Ma due to reduced shear stresses resisting downdip slab sinking and reduced slab bending resistance. Furthermore, a non‐Newtonian upper mantle promotes slab folding and realistic oscillation of the subducting plate velocity. Whether and how this slab folding process induces temporal variations in Andean deformation remains an open question.

Along‐Margin Variations in Breakup Volcanism at the Eastern North American Margin

AbstractWe model the magnetic signature of rift‐related volcanism to understand the distribution and volume of magmatic activity that occurred during the breakup of Pangaea and early Atlantic opening at the Eastern North American Margin (ENAM). Along‐strike variations in the amplitude and character of the prominent East Coast Magnetic Anomaly (ECMA) suggest that the emplacement of the volcanic layers producing this anomaly similarly varied along the margin. We use three‐dimensional magnetic forward modeling constrained by seismic interpretations to identify along‐margin variations in volcanic thickness and width that can explain the observed amplitude and character of the ECMA. Our model results suggest that the ECMA is produced by a combination of both first‐order (~600–1,000 km) and second‐order (~50–100 km) magmatic segmentation. The first‐order magmatic segmentation could have resulted from preexisting variations in crustal thickness and rheology developed during the tectonic amalgamation of Pangaea. The second‐order magmatic segmentation developed during continental breakup and likely influenced the segmentation and transform fault spacing of the initial, and modern, Mid‐Atlantic Ridge. These variations in magmatism show how extension and thermal weakening was distributed at the ENAM during continental breakup and how this breakup magmatism was related to both previous and subsequent Wilson cycle stages.

Post-rift regional volcanism in southern Santos Basin and the uplift of the adjacent South American coastal range

The Serra do Mar coastal mountain range plays a crucial role in the sedimentary budget of both the South American continent and its adjacent Atlantic margin. This 1500-km range began to rise during the Late Cretaceous. The uplift prevented the direct input of sediments from the continent to Santos Basin, thus diverting the Paraná River drainage inland and southwards. However, the cause for the uplift is still debated: distinct phases of the Andean orogeny, thermal activity of the Trindade hotspot, and a general thermal weakening caused by earlier mantle plume activity. The current study attempts to explain the uplift initiation by introducing a previously unknown 150-km wide magmatic cluster discovered in multichannel seismic reflection data across the southern Santos Basin. The newly discovered “Santos cluster” includes plutons, sills, and volcanic edifices within the Turonian-Campanian stratigraphic interval, indicating an Early Campanian magmatic phase. We suggest that the widespread magmatism uplifted the southern Santos Basin, and included lateral NNE salt migration. The marine magmatism occurred along with previously reported Campanian volcanic activity in three localities along the Serra do Mar, which uplifted the coastal range and diverted the drainage inland (westward). We suggest that both marine and continental magmatism relate to the SW sub-crustal flow of the Trindade hotspot head.

Contributions of Grain Damage, Thermal Weakening, and Necking to Slab Detachment

We investigate the impact of three coupled weakening mechanisms on the viscous detachment of a stalled lithospheric slab: structural weakening due to necking, material weakening due to grain size reduction, using a two-phase grain damage model, and thermal weakening due to shear heating (thermal damage). We consider a combined flow law of dislocation and diffusion creep. To understand and quantify the coupling of these three nonlinear weakening processes, we derive a mathematical model, which consists of three coupled nonlinear ordinary differential equations describing the evolution of slab thickness, grain size and temperature. With dimensional analysis, we determine the dimensionless parameters which control the relative importance of the three weakening processes and the two creep mechanisms. We derive several analytical solutions for end-member scenarios that predict the detachment time, that is the duration of slab detachment until slab thickness becomes zero. These analytical solutions are then tested against numerical solutions for intermediate cases. The analytical solutions are accurate for end-member scenarios where one of the weakening mechanisms and one of the creep mechanisms is dominant. Furthermore, we use numerical solutions of the system of equations to systematically explore the parameter space with a Monte Carlo approach.. The numerical approach shows that the analytical solutions typically never deviate by more than 50% from the numerical ones, even for scenarios where all three weakening and both creep mechanisms are important. When both grain and thermal damage are important, the two damage processes generate a positive feedback loop resulting in the fastest detachment times. For Earth conditions, we find that the onset of slab detachment is controlled by grain damage and that during later stages of slab detachment thermal weakening becomes increasingly important and can become the dominating weakening process. We argue that both grain and thermal damage are important for slab detachment and that both damage processes could also be important for lithosphere necking during continental rifting leading to break-up and ocean formation.

Dynamic Frictional Slip Along Rock Faults

Abstract The dynamic behavior of rock faults reflects the response of the shape and composition of the fault to the applied loading and environmental conditions. The interaction between the fault properties and the loading system is controlled by multiple variables that act simultaneously to generate an inherently complex behavior. Integrating multiple variables can illuminate the controlling mechanisms of fault dynamic, and five examples of such integration are evaluated here, including power-density, which quantifies the energy dissipation rate on the slipping fault, PV-factor, which evaluates the damage potential, impulse-density, which combines effect of slip-velocity and slip-duration, the weakening distance of transition to thermal weakening, and the kinematic-load, which integrates slip-velocity and slip-distance. The examination of relevant experimental observations indicates that these variables provide effective quantification tools for fault dynamic strength, fault wear, fault damage, melting, and mineralogical transformation.

Rupture to the trench? Frictional properties and fracture energy of incoming sediments at the Cascadia subduction zone

The mechanical properties of sediment inputs to subduction zones are important for understanding rupture propagation through the accretionary prism during megathrust earthquakes. Clay minerals strongly influence the frictional behavior of fault gouges, and the clay content of subduction input materials varies through a stratigraphic section as well as for subduction margins globally. To establish the frictional properties of the shallow Cascadia subduction zone and place the results in a global context, we conducted high velocity rotary shear experiments on ODP core samples retrieved from Cascadia input sediments (35-45% clay) and a suite of individual clay species. We compared our results to a compilation of published high velocity experiments conducted on samples of wet gouge, dry gouge, and intact rock. For each sample type, three trends were identified with increasing normal stress: 1) the stress drop (τp−τss) increases linearly, 2) the characteristic thermal weakening distance (Dth) decreases as a power law function except for wet clay-rich gouges, and 3) the fracture energy (Wb) shows no dependence. However, fracture energy does vary with sample type. Clay-rich gouges under wet conditions have the lowest fracture energy, and fracture energy for both dry and wet gouges is at least an order of magnitude lower than estimates from intact rocks. Therefore when clay-rich lithologies are present, they may minimize spatial variations in frictional behavior, allowing earthquakes to propagate to the trench. For Cascadia input sediments, there is little variation in the fracture energy between lithologies, but the fracture energy of Cascadia sediments is around an order of magnitude higher than input sediments from other subduction margins. The high fracture energy of Cascadia sediments relative to other subduction margins may inhibit large amounts of shallow earthquake slip and dynamic overshoot.