Shear Heating Research Articles

Fault geometry and low frictional control of the near-field postseismic deformation of the 2021 Mw 7.3 Maduo earthquake

The 2021 Maduo earthquake caused ∼160 km-long complex coseismic surface rupture and postseismic deformation. However, the afterslip pattern and primary reason for the overlapped coseismic slip and afterslip in shallow portion in Maduo earthquake are still unclear. Here, we used ∼0.85 yr Sentinel-1 data to obtain the time-dependent postseismic deformation of the 2021 Mw 7.3 Maduo earthquake and inverted afterslip distribution. We calculated the frictional parameter of the seismogenic fault based on the time series afterslip and depicted the rupture surface based on three-dimensional coseismic displacement. Our findings indicated that the more considerable postseismic deformation in the near-field was concentrated on the bending region near the epicenter. The maximum afterslip was ∼0.35 m in the shallow upper crust. The shallow afterslip overlapped with the coseismic rupture, mainly caused by the low frictional parameter (a – b = 0.002) and shear heating in the shallow region. The fault geometry was gentle in the supershear rupture area, while changed substantially in the west of the epicenter. This highlights that the fault bending could have controlled both coseismic and postseismic deformation pattern in near-field. It also indicates that fault bending controls the stress propagation for large strike-slip fault, which provides a new perspective for seismic risk assessment of other strike-slip faults in the Bayan Har block.

Viscous shear flow and heating of impact-extruded composite energetic materials

Investigating the macroscopic deformation and localized heating behaviors of high-ductility composite energetic material (CEM) under impact extrusion is crucial to understand the accidental ignition phenomenon of CEM charged weapon systems with structural defects and design the advanced safe munitions. In this study, the rapid viscous shear flow and ignition responses of impact-extruded CEM are first experimentally studied and then simulated using a physically based thermomechanical model. The experimental results show that CEM sample impacted by a 6 kg and ∼2 m/s falling drop-weight could be extruded into the narrow crack with a high flow speed (∼20 m/s) and ignited. A thermomechanical model is developed to describe the continuum viscoelastic-plastic deformation and localized heating due to viscoplastic deformation in solid material, viscous flow in melted liquid and chemical reaction. By simulation, a semicircular extrusion region with a shear concentration is formed in CEM sample above the crack, and high shear stress and strain rate at this region contributes to the localized viscous shear heating and ignition of CEM. The effects of the diameter ratio between crack and sample (0.1∼0.4) on dynamic responses of CEM are further investigated. The simulated main features of CEM sample, including flow history into the crack, ignition time and location, are consistent with the experimental observations.

Timescale of Emplacement and Rheomorphism of the Green Tuff Ignimbrite (Pantelleria, Italy)

AbstractWe present a multidisciplinary study based on Differential Scanning Calorimetry (DSC), paleomagnetic analysis, and numerical modeling to gain information on the timescales of syn‐ and post‐depositional ductile deformation of the strongly welded and rheomorphic Green Tuff ignimbrite (GT; Pantelleria, Italy). DSC measurements allow the determination of glass fictive temperatures (Tf; i.e., the parameter accounting for the cooling dependence of glass structure and properties). Using a Tf‐based geospeedometry procedure, we infer the cooling rate (qc) experienced by the glassy phases in different lithofacies within the GT formation. Glass shards from the basal pumice fall deposit record a fast qc of ∼10°C/s. In contrast, the ignimbrite body returns slow qc values depending on the stratigraphic position and lithofacies (basal/upper vitrophyres, fiamme‐rich and rheomorphic layers), ranging from ∼10−2 to ∼10−6 °C/s. Moreover, paleomagnetic analyses of the natural remanent magnetization of ignimbrite matrix and embedded lithic clasts indicate an emplacement temperature higher than 550–600°C. By integrating calorimetric and paleomagnetic datasets, we constrain a conductive cooling model, describing the ignimbrite's temperature‐time‐viscosity (T–t–η) evolution from the eruptive temperature to below Tf. Outcomes suggest that the upper and basal vitrophyres deformed and quenched over hours, indicating that the entire GT underwent intense syn‐depositional ductile deformation. Furthermore, the central body remained above Tf for a much longer timespan (>1 month), enabling post‐emplacement rheomorphic flow. Lastly, we discuss the critical role of mechanisms such as shear heating and retrograde solubility of volatiles, in locally controlling the rheological behavior of the GT.

Crustal and Upper Mantle Velocity Structure of SE Tibet From Joint Inversion of Rayleigh Wave Phase Velocity and Teleseismic Body Wave Data

AbstractSince the continental collision between the Indian and Eurasian plates began about 50 Ma ago, southeastern Tibet (SET) has undergone complex tectonic deformation. In this study, we investigate fine scale structural features of the crustal and upper mantle depths (<250 km) beneath SET, which hold important clues to understanding the dynamic processes related to this collision. A 3D shear velocity model is constructed through jointly inverting Rayleigh wave phase velocity and teleseismic body wave data from more than 650 stations. Our 3D model identifies three independent low‐velocity zones (LVZs) in the mid‐lower crust with unprecedented details. More specifically, we observe a prominent LVZ beneath the North Chuan‐Dian Block, which is well separated from another LVZ beneath the Tengchong volcano in the south. This LVZ beneath the volcano represents a focused magma reservoir in the crust whose origin is potentially linked to the mantle upwelling associated with the eastward subduction of the Indian plate. The third LVZ, observed around the Xiaojiang Fault, likely represents a separated and mechanically weak layer in the mid‐lower crust due to the combined effects of regional crustal thickening under the southeastward plateau expansion, mantle upwelling, and shear heating of strike‐slip faults. In the upper mantle, we observe strong velocity reductions both in localized areas beneath the Tibetan Plateau and the broad region south of 26°N. These low velocity anomalies are sitting above high velocity anomalies at deeper depths, suggesting their association with lithospheric thickening and delamination processes.

Comparative Analysis of the Physicochemical Properties and Metabolites of Farinose and Crisp Lotus Roots (Nelumbo nucifera Gaertn.) with Different Geographical Origins.

Lotus roots are widely consumed vegetables because of their great taste and abundant nutrients, but their quality varies with the environments and cultivar. This study systematically compared farinose (Elian No. 5) and crisp (Elian No. 6) lotus root cultivars from three geographical origins. Pasting and texture characteristics verified that Elian No. 5 possessed lower hardness and lower ability to withstand shear stress and heating during cooking compared with Elian No. 6. Untargeted metabolite profiling was first performed using ultrahigh-performance liquid chromatography coupled with electrospray ionization quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) combined with a Zeno trap. In total, 188 metabolites were identified based on the matching chemistry database. Multivariate analysis demonstrated that lotus roots from different cultivars and origins could be adequately distinguished. Sixty-one differential metabolites were identified among three Elian No. 5 samples, and 28 were identified among three Elian No. 6 samples. Isoscopoletin, scopoletin, and paprazine were the most differential metabolites between Elian No. 5 and Elian No. 6. These results can inform future research on the discrimination and utilization of lotus roots.

Crustal structure and the seismogenic environment in Yunnan imaged by double-difference tomography

The large-scale faulting and earthquake activities that developed extensively in the Yunnan area are associated with the collision of India and Eurasia. The fine crustal structure can provide a better understanding of the crustal deformation, seismogenic environment, and rupture processes. We performed a new 3-dimensional (3D) P wave velocity structure and seismic relocation using double-difference tomography based on seismic observations. The tomography images show that large-scale low-velocity anomalies spread around the margin of the south Chuan–Dian Block, Xiaojiang fault (XJF), and the Lijiang–Xiaojinhe fault (LJ-XJHF) in the middle and lower crust. There is an obvious high-speed anomaly in the Emeishan large igneous province (ELIP). We infer that the low-velocity anomaly under the LJ-XJHF zone may be derived from the lower crustal flow extruded from the central Tibetan plateau and obstructed by the ELIP, while the velocity anomalies around the XJF might be caused by shear heating, which is associated with the large-deep strike–slip fault and the transmission of stress in the southeast direction. The inversion results also show that the Yangbi earthquake occurred at the NW–SE boundary of high and low velocity from the upper crust to the lower crust, which coincides well with the location of the Yangbi earthquake sequence and the Weixi–Qiaohou fault. Meanwhile, the earthquake relocations show that the aftershocks are mainly distributed at low velocities. All the aforementioned research results indicate that the Yangbi earthquake might be attributed to the intrusion of the soft material flow along the Weixi–Qiaohou fault in the NW–SE direction. These low-viscosity crustal materials would cause brittle fractures and result in NW–SE sinistral strike–slip faults.

Carbonates and intermediate-depth seismicity: Stable and unstable shear in altered subducting plates and overlying mantle

A model for intermediate-depth earthquakes of subduction zones is evaluated based on shear localization, shear heating, and runaway creep within thin carbonate layers in an altered downgoing oceanic plate and the overlying mantle wedge. Thermal shear instabilities in carbonate lenses add to potential mechanisms for intermediate-depth seismicity, which are based on serpentine dehydration and embrittlement of altered slabs or viscous shear instabilities in narrow fine-grained olivine shear zones. Peridotites in subducting plates and the overlying mantle wedge may be altered by reactions with CO2-bearing fluids sourced from seawater or the deep mantle, to form carbonate minerals, in addition to hydrous silicates. Effective viscosities of magnesian carbonates are higher than those for antigorite serpentine and they are markedly lower than those for H2O-saturated olivine. However, magnesian carbonates may extend to greater mantle depths than hydrous silicates at temperatures and pressures of subduction zones. Strain rates within altered downgoing mantle peridotites may be localized within carbonated layers following slab dehydration. A simple model of shear heating and temperature-sensitive creep of carbonate horizons, based on experimentally determined creep laws, predicts conditions of stable and unstable shear with strain rates up to 10/s, comparable to seismic velocities of frictional fault surfaces. Applied to intermediate-depth earthquakes of the Tonga subduction zone and the double Wadati-Benioff zone of NE Japan, this mechanism provides an alternative to the generation of earthquakes by dehydration embrittlement, beyond the stability of antigorite serpentine in subduction zones.

Thermal Structure of the Paleo‐Continental Subduction Zone: Insights From Quantitatively Constrained Prograde P–T Paths of Exhumed LT/UHP Eclogites in the Dabie Orogen

AbstractThe discrepancy of geothermal gradients predicted by numerical modeling and recorded by peak metamorphic conditions of exhumed high/ultrahigh pressure (HP/UHP) metamorphic rocks triggers many uncertainties on the thermal structure of subduction zones, which is especially significant for continental subduction zones. Well‐constrained prograde P‒temperature (T) paths of HP/UHP rocks reflect the change in P–T conditions during continental subduction and are insusceptible to later processes, making them robust recorders of the thermal structure of subduction zones. To make a reliable estimation of the thermal structure of a continental subduction zone, the prograde P–T paths of three low (L)T‒UHP eclogites in the Dabie orogen were robustly constrained using a multiple thermobarometry method. The results demonstrate that these eclogites underwent three stages of metamorphic evolution during continental subduction from epidote amphibolite‐facies at ∼450°C–470°C/11–13 kbar, through amphibole eclogite‐facies at 585°C–600°C/17–19 kbar, to peak UHP eclogite‐facies at 600°C–630°C/27–29 kbar, indicating a low and continuously changing geothermal gradient from ∼12°C/km at lower crust depth (40 km) to 10°C/km at depth of 60 km and to ∼6°C/km at sub‐arc depth (100 km). The consistency of this result and the average thermal gradient recovered by peak metamorphic conditions of exhumed HP/UHP metamorphic rocks of continental origin suggests that the continental subduction zones may have a common thermal structure characterized by strongly concave upward geothermal gradient. A comparison of the results of this study and analytical models shows that shear heating may play a crucial role in the thermal evolution of continental subduction zones.

Meltwater generation in ice stream shear margins: case study in Antarctic ice streams

Liquid water within glacier ice and at the glacier beds exerts a significant control on ice flow and glacier stability through a number of processes, including altering the rheology of the ice and lubricating the bed. Some of this water is generated as melt from regions of rapid deformation, including shear margins, due to heating by viscous dissipation. However, how much meltwater is generated and drained from shear margins remains unclear. Here, we apply a model that describes the evolution of ice temperature, melting, and water transport within deforming ice to estimate the flux of meltwater from shear margins in glaciers. We estimate the flux of meltwater from temperate ice zones in three Antarctic regions: Bindschadler and MacAyeal Ice Streams, Pine Island Glacier, and Byrd Glacier. We show that the flux of meltwater from shear margins in these regions may be as significant as the meltwater produced by frictional heating at the bed, with average fluxes of ∼ 0.005 – 0.1 m yr − 1 . This contribution of shear heating to meltwater flux at the bed may thus affect both the rheology of the ice as well as sliding at the bed, both key controls on fast ice flow.

Importance of self‐induced vertical wind shear and diabatic heating on the Fujiwhara effect

AbstractThis study hypothesises that the motion of binary tropical cyclones (TCs) can be modified by diabatic heating (DH) asymmetry associated with self‐induced vertical wind shear. To demonstrate this hypothesis in the quiescent environment, a set of the idealised f‐plane numerical experiments such as identical (varied in the initial separation distance) and non‐identical (varied in TC intensity and size) experiments are conducted. Results show that the binary TC tracks are affected by the initial separation distance, and TC intensity and size, that is, TC influence radius. Identical experiments show that the critical distance for separating, merging or repelling motion is 8° (where 1° ≃ 111 km). The binary TC interaction shows that single large‐scale cyclonic and anticyclonic circulations can develop as a result of the superposition of the circulation of the binary TCs in the lower and upper troposphere. These two different large‐scale circulations result in a vertically sheared environment in each TC. Potential vorticity (PV) tendency diagnosis shows that the TC motion is consistent with a positive region of the local tendency of PV. To quantify the contribution of each PV diagnostic term to the TC motion, we calculate each vector of the PV tendency diagnostic equation. In the merger case, the horizontal advection vector (HADV) appears to be greater than the DH vector (DHV) due to attraction. In the repulsion case, the DHV is largely compensated by the HADV. Specifically, the strong shear‐induced DH generated in the lower troposphere and downshear or downshear‐left quadrant can modify the TC motion in collaboration with the transport of PV to the middle and upper troposphere by offsetting the PV imbalance via the HADV. Overall, these results validate our hypothesis, which we refer to as the three‐dimensional Fujiwhara effect.

Influence of Injection Molding Parameters on the Peel Strength between Plasma-Treated Fluoropolymer Films and Polycarbonate

Light guiding is used to direct light from an emitting source to a different location. It is frequently realized through a clad-core structure with a difference in the refractive index of the materials. This paper explores the possibility of combining a fluoropolymer (THV) film of low refractive index, serving as a cladding layer, with a polycarbonate (PC) core, via injection molding. Pristine THV lacks adherence to the PC. However, when treated with O2 plasma prior to overmolding, bonding can be established that was quantified in peel tests. The effect of this surface treatment was further investigated by adjusting the plasma treatment duration and time to overmolding. Furthermore, parameter studies comprising the four molding parameters, namely packing pressure, injection speed, melt temperature, and mold temperature, were performed. Numerical injection molding simulations assessed the prevailing temperatures at the PC-THV boundary. Consequently, the temperature-time integral could be calculated and linked with the measured peel strengths by fitting a proportionality constant. While the plasma treatment duration showed minor influence, the activation diminished with time, halving the measured peel loads within 24 h. The adhesion was experimentally found to increase with a lower packing pressure, faster injection speed, and higher melt and mold temperature. Those same molding relations influencing the peel loads were also found with the temperature-time integral when scaled by the proportionality constant in the simulations (R2=85%). Apparently, adhesion is added by molding settings which promote higher interface temperatures that prevail for longer. Hereby, the faster injection speed increases the melt temperature through shear heating. A higher packing pressure, in contrast, presumably increases the heat transfer at the PC-THV interface, accelerating the cooling. The measured peel loads were 0.3-1.6 N/mm for plasma-treated samples and nearly zero for pristine THV.

Confined ionic liquids films under shear: The importance of the chemical nature of the solid surface.

Ionic liquids have generated interest in applications as lubricants and as additives to conventional lubricants due to their unique physical properties. In these applications, the liquid thin film can be subjected simultaneously to extremely high shear and loads in addition to nanoconfinement effects. Here, we use molecular dynamics simulations with a coarse-grained model to study a nanometric film of an ionic liquid confined between two planar solid surfaces both at equilibrium and at several shear rates. The strength of the interaction between the solid surface and the ions was changed by simulating three different surfaces with enhanced interactions with different ions. The increase in the interaction with either the cation or the anion leads to the formation of a solid-like layer that moves alongside the substrates; however, this layer can exhibit different structures and stability. An increase in the interaction with the high symmetry anion produces a more regular structure that is more resistant to the effects of shear and viscous heating. Two definitions were proposed and used for the calculation of the viscosity: a local definition based on the microscopic characteristics of the liquid and an engineering definition based on the forces measured at the solid surfaces, with the former displaying a correlation with the layered structure induced by the surfaces. Because of the shear thinning behavior of the ionic liquids as well as the temperature rise brought on by viscous heating, both the engineering and the local viscosities decrease as the shear rate increases.

Emplacement ages and petrogenesis of the Sunchang and Namwon granitoids, South Korea

We present new sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages for two adjacent plutons (Sunchang and Namwon) along the Middle Jurassic Honam Shear Zone, South Korea, and report their major- and trace-element and Sr-Nd-Pb isotopic compositions to constrain the emplacement ages and petrogenesis of the plutons. The Sunchang foliated granites yield U-Pb ages of 186–178 Ma, suggesting long-lasting magmatic activity. The emplacement age of the Namwon pluton (192–180 Ma) indicates that it was broadly coeval with or slightly older than the Sunchang pluton. Some zircon grains contain ancient (Neoarchean to Paleo-Proterozoic) inherited cores. The REE patterns of the granitoids are roughly U-shaped with no strong negative Eu anomalies and show arc-like enrichments in LILEs and depletion in Nb or Ta, similar to I-type granites. The Sunchang granites yield initial 87Sr/86Sr ratios of 0.71543–0.71675, εNd(t) values of −29.0 to −26.0, and 206Pb/204Pb ratios of 18.44–18.90, and the Namwon granitoids yield initial 87Sr/86Sr ratios of 0.71229–0.71652, εNd(t) values of −27.5 to −20.9, and 206Pb/204Pb ratios of 17.93–18.23. The granitoids have adakite-like elevated Sr/Y (25–78) and La/Yb (15–66) ratios but are potassic, which is inconsistent with slab-derived melts. Furthermore, they have relatively low MgO at a given SiO2 content compared with high-SiO2 adakites and low Mg# (30–48) and Ni (<5 ppm), Co (<8 ppm) and Cr (<52 ppm) contents. They resemble C-type adakites, which form by the partial melting of thickened lower continental crust with residual garnet and amphibole. Underplating mafic magma from the mantle wedge or shear heating may have been the source of heat for the crustal melting. The close spatiotemporal association of these C-type adakitic rocks suggests a genetic link between them. The theory that the massive to weakly foliated Namwon granitoids were emplaced post-tectonically needs to be revised, as the Namwon and Sunchang granitoids formed during the same magmatic episode in the regional-scale shear zone.

Implications of New Geological Mapping and U‐Pb Zircon Dating for the Barrovian Tectono‐Metamorphic Evolution of the Lepontine Dome (Central European Alps)

AbstractThe Barrovian metamorphism of the Lepontine dome is manifested by isograds that cross‐cut tectonic nappe contacts, which is commonly interpreted as metamorphism that occurred after nappe emplacement. However, the pervasive mineral and stretching lineation in amphibolite facies, associated with top‐to‐foreland shearing, suggests that peak Barrovian conditions are coeval with nappe‐overthrusting. Here, we combine mapping and U‐Pb zircon dating to better constrain the relation between metamorphism and overthrusting. Metamorphic zircon rims show two age populations at 31–33 and 22–24 Ma. The younger population is locally observed in post‐foliation dikes (and associated metasomatism) likely sourced from deep‐migmatites exhuming along the Alpine backstop. The older population occurs regionally and is found in syn‐kinematic migmatites which occur along a crustal‐scale shear zone. Below this shear zone, magmatic and detrital zircon cores suggest that the Cima Lunga unit, previously interpreted as a tectonic mélange with Mesozoic fragments, was a pre‐Variscan metasedimentary sequence intruded by Permian granitic sills, now orthogneisses. This unit was strongly sheared along the top of the Simano nappe during overthrusting of a rock pile here‐termed Maggia‐Adula nappe. This large‐scale nappe emplacement imprinted the regional lineation and peak temperatures until 31–33 Ma. Péclet (1–10) and Brinkman (0.002–1.8) numbers, estimated for the overthrusting, suggest an advection‐dominated heat transfer caused by rock exhumation, with some diffusion (conduction) during nappe emplacement. Diffusion contributed to Barrovian isograds discordant to the thrust. Shear heating was important if stress times shearing rate &gt;∼5·10−6 W·m−3 within the nappe. The thermal evolution after overthrusting was spatially heterogeneous until ca. 22 Ma.

Two Unconnected Low-Velocity Zones in the Eastern Boundary of the Sichuan-Yunnan Block Revealed with High-Resolution Ambient Noise Tomography

The Anninghe-Zemuhe-Xiaojiang fault zone (AZXFZ) is an important boundary fault zone on the southeastern margin of the Tibetan Plateau, with frequent strong earthquakes. Previous studies have imaged widespread low-velocity zones in this area. However, there are still many disputes on the connectivity and genesis of the low-velocity zones. In this study, we obtain the Rayleigh wave phase velocity dispersion curves at 4–25-s periods using observations from 378 broadband stations located near the AZXFZ. The new 3D S-wave velocity model has a lateral resolution of about 30 km in 0–35-km depth and is obtained by direct inversion of surface wave dispersion data. The new results clearly image two low-velocity zones and a high-velocity zone in the middle crust of the study region. The low-velocity zone on the western side of the Lijiang-Xiaojinhe fault is related to the eastward flow of crustal material and the movement of the left lateral strike-slip faults in the Tibetan Plateau, while the low-velocity anomaly distributed along the Daliangshan fault and Xiaojiang fault is the superimposition effect of shear heating of the faults and upwelling of mantle material. The uplift of Gongga Shan is a combination of the continuous accumulation of crustal material in the middle and lower crust of the southeastern margin of the Tibetan Plateau as well as the bending and compression of the Sichuan-Yunnan Block and the Xianshuihe-Anninghe fault zone.

Thixotropic composite hydrogels based on agarose and inorganic hybrid gellants

Water can be used as oxidant in conjunction with metal particles to form metal–water propellant to increase the energy of propellant.For this application, water needs to be stored in form of solid and capable of becoming liquid when use. Stable and thixotropic hydrogel has good potential as water-retaining material and oxidant of metal-based propellant. In this study, we prepared organic/inorganic composite hydrogels by combining inorganic gellants hectorite and fumed silica with organic gellant agarose, respectively. The total content of the gellants can be reduced to less than 2% by adding agarose. The influence of agarose on water content, phase transition temperature, centrifugal stability and other basic physical properties of composite hydrogels were discussed. The results show that the composite hydrogels have better thixotropy and stability thanpure inorganic hydrogels, and the gel–sol transformation can be realized by applying shear force or heating to the phase transition temperature. The composite hydrogels have good shear thinning ability and improved mechanical stability. Fumed silica/agarose hydrogels have better physical stability, while the thixotropy and shear thinning ability of hectorite/agarose hydrogels are better.

Three-dimensional magnetotelluric signatures and rheology of subducting continental crust: Insights from Sikkim Himalaya, India

3D inversion of broad band MT data present variation of electrical signatures across the subducting Indian crust in Sikkim Himalaya. The vertical and horizontal geoelectric cross-sections are dominated by north-east dipping conductive zones. Two high conductivity zones (4–8 Ω m) at a depth of 5–18 km in Lesser Himalayan Domain (LHD) are explained by conductive mineral assemblage associated with abundant low saline and entrapped fluids. Another conductive feature (6–16 Ω m) in Main Himalayan Thrust Zone close to Main Himalayan Thrust ramp could have arisen from entrapment of CO2-H2O fluids and fluids released by metamorphic reactions. The high conductive anomaly (4–10 Ω m) at a depth of 5–16 km in Greater Himalayan Sequence (GHS) is caused by the presence of partial melts/aqueous fluids derived by present day fluid-absent melting of leucogranite source rocks. A combination of leucogranite intrusion, shear heating, and radiogenic heat production (4–17 μW/m3) are the heat sources for inferred partial melting. Though, the constrained melt fractions of 1.4–3.8% in GHS are lower than the estimation in south Tibet that might be due to the less intrusion of leucogranites. The obtained moderate viscosities of (104.19-105.49 Pa.s) from empirical relation with low melt and fluid fractions of 5–6 wt% in high conductive zone suggest viscous/ductile deformation and weakening mid-crust beneath northern Sikkim Himalaya. However, the estimated values of melt fractions and viscosities at mid-crustal depth of GHS are insufficient to develop a melt channel to flow southward between Main Central Thrust-1(MCT-1) and South Tibet Detachment (STD) envisaged by channel flow model.

Pulse-like ruptures, seismic swarms, and tremorgenic slow-slip events with thermally activated friction

The evolution of frictional resistance on a fault affects the characteristics of seismic ruptures. A wide range of rupture styles, from slow-slip events to fast earthquakes, can be explained under the isothermal rate- and state-dependent friction framework. However, laboratory experiments indicate that friction also depends on temperature, with a largely unknown impact on rupture patterns. Here, we explore how thermally activated friction affects rupture behavior in quasi-dynamic models of seismic cycles with a single velocity-weakening, temperature-strengthening asperity, whereby frictional healing occurs behind the rupture front due to shear heating. A transition from crack-like to pulse-like rupture propagation with self-healing fronts occurs as the temperature strengthening effect increases, spontaneously inducing steady, decaying, or growing pulses. With increasing activation energy, the cycle turns into earthquake swarms and tremorgenic slow-slip events, both characterized by strong interactions between slow and fast ruptures. The temperature sensitivity of friction may contribute to the natural complexity of the seismic phenomenon, potentially explaining a much wider spectrum of rupture behaviors and recurrence patterns.

Metamorphism and Deformation on Subduction Interfaces: 2. Petrological and Tectonic Implications

AbstractThe compositional range of ∼2,000 marine sediments and ∼19,000 oceanic igneous rocks is encapsulated by a set of 12 sedimentary and 10 mafic rock compositions, allowing computation of phase relationships on P‐T paths along subduction interfaces. These are described economically by a partitioning analysis, which connects the mineral assemblages to different parts of the subduction P‐T space and facilitates assessment of prograde dehydration, melting, densification, and rheological systematics. Dehydration and densification occur at shallower depths than in studies that neglect shear heating. Lawsonite stability is limited to interfaces where convergence is slower than 20 mm/yr; such rates also favor transport of volatiles beyond the arc. Terrigenous sediments and mafic rocks reach their solidi close to the top of the wedge‐slab interface; melt fractions are enhanced by fluid from the dehydrating slab interior. Rheological calculations show that the most abundant sediment types have interface capacities of hundreds of meters to kilometers, and that the strengths of mafic rocks comfortably exceed their buoyancy stresses. Above ∼650°C sediments are weak enough to rise as diapirs into the mantle wedge. Carbonate‐ and serpentinite‐rich lithologies are weaker than other interface rocks, and ascend most rapidly at the cessation of subduction. Ascent rates drop abruptly as rocks enter the plate interface, probably leading to retrograde equilibrium at P ∼ 1–1.5 GPa. The seismic‐aseismic transition is expected at about 500°C in mafics, and 400°C in metasediments. Seamounts are weaker than most other interface rocks, and unlikely to form asperities. Slow slip and tremor may be associated with the blueschist‐eclogite transition.

Metamorphism and Deformation on Subduction Interfaces: 1. Physical Framework

AbstractA thermal and mechanical framework is presented for analysis of pressure‐temperature (P‐T) data and structural observations from high‐pressure‐low‐temperature (HPLT) terrains. P‐T data from 281 HPLT rocks exhibit two regimes separated at a pressure of ∼1.5 GPa, which corresponds to the modal maximum depth of thrust faulting in subduction zones. At pressures ≲1.5 GPa, interpreted as recording conditions on the plate interface, temperatures increase at about 350°C/GPa and are consistent with conditions calculated for shear stresses of ∼30–100 MPa on the interface. Such shear stresses are high enough to carry several kilometers' thickness of sediment at least to the base of the plate interface. Burial of material on plate interfaces occurs predominantly during large‐to‐great earthquakes; the exhumation phase involves contrasts in ascent rates of adjacent units, because of their differing buoyancies and strengths. In consequence, juxtaposition of unrelated rock types is expected to be ubiquitous, during both descent and ascent. The scarcity of temperatures higher than ∼650°C at pressures ≳1.5 GPa may reflect loss of material from the wedge‐slab interface by buoyant ascent. Exhumation of rocks in the subduction interface requires substantial reduction in shear stress, most plausibly by (near‐)cessation of subduction. During prograde metamorphism temperatures increase smoothly with depth in the plate interface, with almost isothermal compression in the wedge‐slab interface. Following cessation of subduction, rocks rising along the wedge‐slab interface are likely to heat slightly during decompression. Within the plate interface, temperatures drop following the cessation of shear heating, and rocks follow counter‐clockwise hairpin PT paths.