Thermal Weakening Research Articles

Multiple episodes of late Neoproterozoic rifting in the Tarim Craton during its separation from the supercontinent Rodinia

In this paper, we review and evaluate the magmatic history, regional stratigraphy, structural patterns and sedimentary provenance features of the Late Neoproterozoic Tarim Craton to elucidate the nature and different modes of extensional tectonics during the Latest Neoproterozoic, a critical time coinciding with the disintegration of the supercontinent Rodinia. New detrital zircon U–Pb ages and geochemical data acquired from quartz schists and spatially associated granites in the Tugerming anticline along the northern margin of the Tarim Craton have revealed Cryogenian ages, as opposed to Early Neoproterozoic and older ages as reported in previous studies. Granitic intrusions are dated at 656–637 Ma, which involved remelting of the ancient crust following the decline of the initial (Cryogenian) rift stage. They are unconformably overlain by terrestrial clastic deposits. Based on the available detrital zircons from the Late Neoproterozoic sedimentary rock assemblages, we identify a major change in the provenance of the basinal strata at the end of the Cryogenian period. This inferred provenance change, associated with basement exhumation, overlaps in time with resumed magmatic activities, basin inversion and uplift along the northern margin of the craton. The initial, Cryogenian rifting stage was facilitated by pure shear deformation that produced rift basins in and across the Tarim Craton. The subsequent rifting stage was facilitated by simple shear deformation, confined to the northern edge of the craton as the break-up of the supercontinent Rodinia further progressed. Episodic rifting of the Tarim Craton and its separation from Rodinia were aided by thermal weakening of its lithosphere owing to the periodic impingement of mantle upwelling beneath it throughout the Cryogenian and Ediacaran periods.

Unstable Sliding of Plagioclase Gouge and Deformation Mechanisms Under Hydrothermal Conditions With Effective Normal Stresses of 100–300 MPa

AbstractPlagioclase feldspar is a major mineral in mafic crustal rocks. To better understand the deformation mechanism of plagioclase feldspar during frictional faulting, we conducted shearing experiments on simulated plagioclase gouge in a wide range of effective normal stress of 100–300 MPa, pore‐water pressure of 30–100 MPa, and temperatures ranging from 100°C to 600°C. The coefficient of friction is found to range from 0.65 to 0.74 across the entire temperature range, showing no significant thermal weakening process. Except for a case at 200°C with an effective normal stress of 300 MPa, the frictional sliding is velocity weakening over the whole temperature range, showing a steady‐state rate dependence (a−b) ranging from −0.5 × 10−3 to −8.6 × 10−3. This property facilitates nucleation of unstable slips in frictional faulting. Above 200°C, the direct rate effect parameter (a) and the evolution effect parameter (b) of friction increase with temperature up to a threshold of 400°C or 500°C, depending on the effective normal stress. This thermal enhancement suggests thermally activated creep at contact junctions governed by intergranular pressure solution, as evidenced by microstructural signatures indicating the prevalence of very fine precipitates formed at the surfaces of gouge particles as a result of pressure solution. In frictional sliding of plagioclase, a low effective normal stress of 100 MPa corresponds to a higher degree of velocity weakening and tends to facilitate seismic slip rather than slow slips, whereas the high effective normal stress of 300 MPa corresponds to a minor velocity weakening which may cause slow‐slip events in faults of limited size.

Santos Basin as an Example of a Shear-Driven Core Complex in a Transform Marginal Plateau

It is proposed a new model on the kinematics of strain partitioning for the breakup of the Santos and Namibe conjugate basins. The model is based on observed strike-slip corridors and low-angle detachments with metamorphic core complexes, which accommodated the deformation in a mega relay zone. Through detailed reconstructions of this segment of the South Atlantic, collecting multidisciplinary data from previous researchers, and considering the relative relationship between Africa and South America during the breakup process, it is herein reexamined key evidence of how the Brazilian side became so wide, and acted as an accommodation zone during the Aptian. The 600 km Long transpressional-dextral Proterozoic Ribeira Belt is the cradle of the transtensional-sinistral Santos-Namibe strike-slip rift propagator. These conjugate basins evolved as a large-scale relay zone, developing an oblique-left-lateral extensional corridor during the Aptian, balancing mechanically coeval deformations from two sub-parallel spreading branches, traveling from North and South, but hundreds of kilometers apart from each other. Proterozoic inheritance, and the dynamic clockwise rotation of South America (far-field stresses) controlled strain partitioning between the Campos/Benguela basins and the Pelotas/Walvis basins. The onset of seafloor spreading around the Falkland (Malvinas) Island, triggered the relative clockwise rotation of the southernmost tip of the South America plate, and the intrusion of transversal dike swarms of the Ponta Grossa Arch, which is interpreted as fissural magmatism, comparable to a regional opening mode(type I) fracture system. Plate kinematic transport direction inferred from plate reconstructions is mainly EW. The step-over oblique-slip of this relay zone widened the Santos Basin in the NW-SE direction, often mistakenly misinterpreted as the extension direction in the Santos Basin. This NW path is the direction of the maximum elongation within an E-W shear corridor. Our work details a new and unprecedented strike-slip structural framework of the Santos Basin, with large-scale basement-involved folding around the Outer High, which evolved to an obliquely sheared active low-angle detachment system, simultaneously influenced by the thermal anomaly of the Tristan-Gough plume, responsible for magmatic thermal weakening and thermal-induced uplift. Magmatic underplating may have facilitated the doming process. The Santos Basin is located at the heart of a Transform Marginal Plateau, as previously proposed. It is composed of a magmatic crust, with fragmented slices of continental crust, obliquely sheared during successive transform movements, with possible magmatic underplating. This kinematically linked system of normal faults, strike-slip faults, flexural folds, and detachment faults has direct impact on understanding the properties of world-class oil and gas reservoirs in the Brazilian Pre-Salt Supergiant Province. With the ongoing regional transgression, the active structural highs were the site of deposition of the carbonate reservoirs that host an in-place volume around a hundred billion barrels of hydrocarbons, considering the whole province.

Temperature-dependent acoustic emission characteristics and statistical constitutive model of granite under uniaxial compression

Understanding the temperature-dependent mechanical behavior and fracture characteristics of granite is crucial for many engineering projects. In this study, the real-time temperature curves of granite specimens were obtained during the heating and cooling process, and the thermal treatment tests were conducted. The physical properties of the specimen before and after thermal treatment, including mass, volume, and P-wave velocity, were measured. The acoustic emission (AE) signal in the uniaxial compression is monitored. The results indicate that the physical properties of granite deteriorate with temperature, while the mechanical properties show two effects of thermal strengthening and thermal weakening. This phenomenon is comprehensively analyzed by literature statistical data and optical microscopic observation. Furthermore, the AE characteristic is strongly dependent on temperature. High temperature induces more AE ring count to appear in the early stage of loading. As the temperature increases, the crack initiation stress decreases and the table crack propagation stage becomes longer. The attenuation of high-frequency signals and the enhancement of low-frequency signals are related to the development and interaction mechanism of thermally-induced crack and stress-induced crack. At 600 °C, the global b-value increases significantly. Meanwhile, the evolution of dynamic b-value helps explain the failure process of granite under axial load after thermal treatment. In addition, a new thermo-mechanical damage statistical constitutive model of granite considering temperature effects is proposed by introducing AE parameters. The main advantages of this model can well fit the nonlinear behavior of granite in the early loading stage after thermal treatment, and reflect the failure process of granite before the peak value.

Apatite Triple Dating (Lu–Hf, U–Pb, FT) Constrains Deformation and Cooling in the Coompana and Madura Provinces, Western Australia

Abstract Combined apatite Lu–Hf, U–Pb, and fission track (AFT) triple dating affords the opportunity to investigate the ~60 and 730°C thermal history of a study area. Here, we apply apatite triple dating to resolve the tempo of multiple thermo-tectonic events within the Precambrian basement rocks of the Coompana (COP) and Madura (MAP) Provinces, Western Australia. Apatite Lu–Hf dates for the western COP (~1.52 Ga) and MAP (~1.36 Ga) agree with published Mesoproterozoic magmatic crystallization ages. Younger apatite U–Pb dates (~1.16–1.12 Ga) for the western COP and MAP suggest isotopic decoupling and radiogenic-Pb loss by volume diffusion in response to metamorphism at that time. Further East in the COP, the apatite Lu–Hf, and U–Pb dates are within uncertainty of each other and are interpreted to reflect recrystallization at ~1.20–1.14 Ga, coinciding with the late Mesoproterozoic Maralinga thermomagmatic event. The imprints of such an event were more pervasive towards the eastern COP, resulting in a thermally weakened crust in this area. AFT results constrain the subsequent Phanerozoic low-temperature history which has contrasting thermal trajectories on either side of the Mundrabilla Shear Zone (MSZ). Thermal history modeling suggests an early Carboniferous rapid cooling pulse (~360–330 Ma) for the COP, east of the MSZ, that is contemporaneous with the intraplate Devonian–Carboniferous Alice Springs Orogeny. In contrast, the MAP, west of the MSZ, records a protracted monotonic cooling history since the Middle Devonian, implying long-term crustal stability. The differences in low-temperature thermal histories may be preconditioned by the extent of thermal weakening during the late Mesoproterozoic, as indicated by the Lu–Hf and U–Pb results. Here, we show the value of apatite triple dating applied to grains recovered from drill core samples, demonstrating opportunities for understanding other poorly accessible terranes.

Stratigraphic and geochronologic investigation of the Muddy Creek Basin: Implications for the Eocene tectonic evolution of southwest Montana, USA

Sedimentary basins record crustal-scale tectonic processes related to the construction and demise of orogenic belts, making them an invaluable archive for the reconstruction of the evolution of the North American Cordillera. In southwest Montana, USA, the Renova Formation, considered to locally represent the earliest accumulation following Mesozoic−Cenozoic compressional deformation, is widespread but remains poorly dated, and its origin is debated. Herein, we employed detrital zircon U-Pb and (U-Th)/He double dating and sanidine 40Ar/39Ar geochronology in the context of decimeter-scale measured stratigraphic sections in the Renova Formation of the Muddy Creek Basin to determine basin evolution and sediment provenance and place the basin-scale record within a regional context to illuminate the lithospheric processes driving extension and subsidence. The Muddy Creek Basin is an extensional half graben in southwest Montana that is ∼22 km long and ∼7 km wide, with a >800-m-thick sedimentary package. Basin deposition began ca. 49 Ma, as marked by multiple ignimbrites sourced from the Challis volcanic field, which are overlain by a tuffaceous fluvial section. Fluvial strata are capped by a 46.8 Ma Challis ignimbrite constrained by sanidine 40Ar/39Ar dating. An overlying fossiliferous limestone records the first instance of basinal ponding, which was coeval with the cessation of delivery of Challis volcanics−derived sediment into the Green River Basin. We attribute initial ponding to regional drainage reorganization and damning of the paleo−Idaho River due to uplift and doming of the southern Absaroka volcanic province, resulting in its diversion away from the Green River Basin and backfilling of the Lemhi Pass paleovalley. Detrital zircon maximum depositional ages and sanidine 40Ar/39Ar ages show alternating fluvial sandstone and lacustrine mudstone deposition from 46 Ma to 40 Ma in the Muddy Creek Basin. Sediment provenance was dominated by regionally sourced, Challis volcanics−aged and Idaho Batholith−aged grains, while detrital zircon (U-Th)/He (ZHe) data are dominated by Eocene cooling ages. Basin deposition became fully lacustrine by ca. 40 Ma, based on an increasing frequency of organic-rich mudstone with rare interbedded sandstone. Coarse-grained lithofacies became prominent again starting ca. 37 Ma, coeval with a major shift in sediment provenance due to extension and local footwall unroofing. Detrital zircon U-Pb and corresponding ZHe ages from the upper part of the section are predominantly Paleozoic in age, sourced from the Paleozoic sedimentary strata exposed in the eastern footwall of the Muddy Creek detachment fault. Paleocurrents shift from south- to west-directed trends, supporting the shift to local sources, consistent with initiation of the Muddy Creek detachment fault. Detrital zircon maximum depositional ages from the youngest strata in the basin suggest deposition continuing until at least 36 Ma. These data show that extension in the Muddy Creek Basin, which we attribute to continued lithospheric thermal weakening, initiated ∼10 m.y. later than in the Anaconda and Bitterroot metamorphic core complexes. This points to potentially different drivers of extension in western Montana and fits previously proposed models of a regional southward sweep of extension related to Farallon slab removal.

Blind magmatism abets nonvolcanic continental rifting

Tectonic forces alone cannot drive rifting in old and thick continental lithosphere. Geodynamic models suggest that thermal weakening is critical for lithospheric extension, yet many active rifts lack volcanism, seeming to preclude this process. We focus on one such rift, the Tanganyika-Rukwa segment of the East African Rift System, where we analyze local seismicity for shear wave anisotropy and couple the results with numerical modeling. The strongest splitting measurements are from earthquakes with paths sampling lower crustal regions of high compressional-to-shear wave velocity ratios and have fast polarization directions parallel to the local mantle flow, implying the existence of oriented melt lenses. This lower crustal magmatism and observed high surface heat flow are consistent with substantial lithospheric weakening and explain the enigmatic relief and increasing strain accommodation along the rift axis. We conclude that progressive nonvolcanic rifting is assisted by deep crustal melts yet to breach the surface.

Frictional weakening leads to unconventional singularities during dynamic rupture propagation

Earthquakes i.e. frictional ruptures, are commonly described by singular solutions of shear crack motions. These solutions assume a square root singularity order around the rupture tip and a constant shear stress value behind it, implying scale-independent edge-localized energy. However, recent observations of large-scale thermal weakening accompanied by decreasing shear stress potentially affecting the singularity order can challenge this assumption. In this study, we replicate earthquakes in a laboratory setting by conducting stick-slip experiments on PMMA samples under normal stress ranging from 1 to 4 MPa. Strain gauges rosettes, located near the frictional interface, are used to analyze each rupture event, enabling the investigation of shear stress evolution, slip velocity, and material displacement as a function of distance from the rupture tip. Our analysis of the rupture dynamics provides compelling experimental evidence of frictional rupture driven by enhanced thermal weakening. The observed rupture fronts exhibit unconventional singularity orders and display slip-dependent breakdown work (on-fault dissipated energy). Moreover, these findings elucidate the challenges associated with a priori estimating the energy budget controlling the velocity and final extent of a seismic rupture, when thermal weakening is activated during seismic slip.

Lithospheric weakening by a small-scale plume and its geodynamic implications

The process of small-scale mantle plume emplacement within the continental lithosphere influences the dynamic topography and architecture of the lithosphere-asthenosphere boundary (LAB). Furthermore, plume emplacement alters the rheology of the crust and lithospheric mantle through melt-driven, mechanical and thermal weakening. Previous studies of plume emplacement within the lithosphere have described these outcomes, albeit without considering plume-driven weakening. In this study, we quantitatively evaluated the effects of small-scale plume-driven predefined weakening of the continental lithospheric mantle and crust on dynamic topography and LAB depth changes using a series of two-dimensional numerical models. Model calculations showed that small plume-driven predefined weakening led to short-wavelength dynamic topography and LAB depth reduction through both mechanical and thermal erosion of the lithospheric mantle by the plume. An increase in the degree of weakening decreased the lithospheric strength and reduced the LAB depth, thereby transforming the short-wavelength topography from depressions to elevations as the extensional kinematics shifted to compression. Our model findings explain the topography, lithospheric strength, and LAB depth constrained from natural small-scale plume emplacement zones.

Middle Miocene Onset of the Litang Fault System Records Kinematic Change in Eastern Tibet

AbstractThe ∼400‐km‐long Litang fault system (LTFS) is a major intracontinental strike‐slip fault inside the Chuandian block, eastern Tibet, but its evolution and role in accommodating the India‐Asia convergence remain poorly known. Structural analysis shows that the LTFS splits into 5 strands as a left‐lateral, right‐stepping en‐echelon pattern formed under NW‐directed compression, subsequently reactivated by transtensive faults under NNE‐directed extension. Displaced geological and morphological markers yield a cumulative left‐lateral offset of 28.9–42.8 km. Inverse thermal‐history modeling of thermochronological data of the faulted rocks reveal accelerated cooling at 38–35 Ma, 16–13 Ma, and 7–5 Ma. The late Eocene rapid cooling is ascribed to the reactivation of the Garze‐Litang suture. Rapid cooling events at 16–13 Ma and 7–5 Ma record the onset of transpression and transtension of the LTFS, respectively, yielding a geologic slip rate of 2.6 ± 0.7 mm/yr. Both bifurcated geometry and slow slip rate of the LTFS since 16–13 Ma indicate diffuse deformation inside the Chuandian block, contrasting with strain localized on fast‐slip strike‐slip faults on the block margins. This implies a significant kinematic transition in the middle Miocene, such that the extrusion of the segmented mega‐blocks has been accommodated by both localized and distributed deformation in eastern Tibet. This tectonic transition could be explained by a change in lithospheric rheology from an earlier rigid state to a viscous state underneath the Chuandian block due to thermal weakening of the lower crust. We thus reconcile the end‐member geodynamic models of block extrusion and lower crustal flow in late Cenozoic times.

Petrological insights into connections between the S- and I-type magmatic associations in metamorphic core complexes: a case study of the Çataldağ metamorphic core complex (NW Turkey)

As one of the Cordilleran-type metamorphic core complexes in the western Anatolian extensional province and the broader Aegean region, the Çataldağ metamorphic core complex is composed of a granite-gneiss-migmatite complex and a synkinematic pluton, bounded by a detachment fault zone. The plutonic rocks within the footwall of the Çataldağ metamorphic core complex consist of Eo-Oligocene anatectic S-type leucogranites associated with gneiss-migmatite complex forming the core rocks and an I-type synkinematic pluton that emplaced into the core rocks in the Early Miocene. This paper presents the petrography, mineral chemistry, major-trace element geochemistry, and Sr-Nd-Pb isotope compositions of the Çataldağ I-type synkinematic pluton. It discusses the magma source evolution from the Eo-Oligocene to the Early Miocene and their relations to the Çataldağ metamorphic core complex formation by comparing two contrasting granitic associations from a petrological perspective. The Çataldağ I-type synkinematic pluton primarily comprises porphyritic granodiorite, granite, and peripheral rocks including evolved granite and pegmatite dikes. Thermobarometry calculations estimate crystallisation conditions of approximately 795 °C at 1–2 kbar (averaging 1.5 kbar), corresponding to a shallow crustal magma chamber (⁓5 km deep). The pluton is metaluminous to slightly peraluminous and of a high-K calc-alkaline character. The 87Sr/86Sr(i) and εNd values of the studied samples range from 0.70684 to 0.70772 and from −6.1 to −2.3, respectively. Pb isotopic compositions are 18.74–19.29 for 206Pb/204Pb, 15.69–15.78 for 207Pb/204Pb, and 38.89–39.39 for 208Pb/204Pb. Geochemical modelling shows that a large amount of metasomatised lithospheric mantle-derived melt component (up to 70%) and a minor crustal component (<50%) contributed to the source region of the magma. The combination of isotopic data and thermodynamic modelling suggests that the compositional variations within the pluton were primarily the result of open system processes, predominantly the assimilation of crustal rocks with a dominance of fractional crystallisation. The Sr-Nd-Pb isotopic signatures and trace element characteristics of I-type pluton, as well as their comparisons with those of S-type leucogranites within the Çataldağ core complex, indicate heterogeneous magma sources, evolving from crust-dominated to mantle-dominated magma from Eo-Oligocene to the early Miocene, reflecting mantle upwelling beneath NW Anatolia. We infer that the long-lived, mantle-driven Eocene plutonic activity that preceded the development of core complexes may have led to thermal weakening, melting, and crustal flow in the lower-middle crust beneath NW Anatolia, facilitating the formation of the granite-gneiss-migmatite complex during the incipient phase of extension in the Eo-Oligocene. Coinciding with the rapid exhumation of the S-type granite and migmatite complex, the formation and emplacement of I-type synkinematic pluton along the detachment fault zone of the Çataldağ core complex were developed as a result of asthenospheric upwelling driven by lithospheric removal processes. This inferred lithospheric removal was triggered by Hellenic slab rollback affecting the entire western Anatolia since the Oligocene. The transition from S- to I-type granites observed in the Çataldağ core complex and other Aegean massifs may indeed be a manifestation of the intensity or accumulation of this crustal extension.

Geodynamic regime of the southeastern North China Craton before the Tectono-Magmatic Lull: evidence from ca. 2510 Ma and 2430 Ma igneous rocks in the Xuhuai micro-block

ABSTRACT The Tectono-Magmatic Lull (TML) refers to the period between 2.37 and 2.24 Ga when magmatic activity declined globally, the cause of which remains debated. The North China Craton (NCC), which was formed by assembling several micro-blocks in Neoarchean to Paleoproterozoic, preserves the most records of magmatism across the TML. In this contribution, we report petrological, geochemical, and zircon U-Pb-Hf isotopic data of Paleoproterozoic basement rocks from the Xuhuai (XH) micro-block in the southeastern part of the NCC, and use them to constrain the geodynamic regime at the onset of the TML. Our data reveal that the central part of the XH micro-block is mainly composed of biotite granodioritic gneiss and plagioclase amphibolite, which is divided into a ca. 2510 Ma suite of low- and medium-pressure TTGs and a ca. 2430 Ma bimodal igneous suite including high-pressure TTG and plagioclase amphibolite. Variable magmatic zircon εHf(t) values (1.9 to 7.2), Nb, Ta and Ti anomalies for the 2510 Ma TTGs indicate that the parental magma, which was involved with mantle materials, was generated in an active subduction environment in this pre-TML era. The felsic component of bimodal igneous rocks with high Na/Ta ratios (27 to 30) and stable Mg# (41 to 44) suggests their magma source region was as deep as the rutile stability field, and the delamination-derived asthenosphere mantle ascending. The influx of hot mantle induces thermal weakening and mechanical stretching of the overlying crust, leading to back-arc extension. The results of this study support the hypotheses that the NCC formed from micro-block amalgamation and the trigger of the TML could be the large-scale delamination of the lower crust into the mantle.

Nearshore ice complex breakup is controlled by a balance between thermal and mechanical processes

AbstractShore ice is an important facet of cold‐climate coastal geomorphology yet is generally understudied in comparison to other aspects such as nearshore hydrodynamics. Climate change is resulting in more dynamic shore ice regimes (i.e., shortened ice season and multiple freeze–thaw cycles); thus, a clear understanding of the role of shore ice in coastal geomorphic evolution is needed. The presence of shore ice is generally thought to provide the coast a protective buffer from storm waves though some studies have indicated enhanced nearshore erosion and sediment transport associated with ice development. This is particularly apparent during the breakup phase of shore ice as sediment can be scoured from the bed, deposited in place, or transported offshore. Given this, understanding the mechanics of shore ice breakup is critical. This study documents the first combined field and laboratory evaluation of the physical conditions leading to shore ice breakup, detailing the complex interplay between thermal and mechanical processes in ice deterioration. Through a wave tank experiment as well as field observations, wave impacts alone are shown to be unlikely to cause breakup of shore ice and thermal weakening is required. This has important implications both for predicting when ice will break up as well as for identifying potential nearshore sediment transport pathways. If ice breaks up entirely from thermal degradation, then sediment is likely to be deposited in place, whereas if ice breaks up from a combination of thermal degradation and wave impact, then sediment can be redistributed across the shoreface. Monitoring of meteorological conditions during ice breakup can likely be used as a first‐order predictor of geomorphic changes resulting from shore ice deterioration.

Experimental study of real-time temperature-dependent nonlinear deformation of sandstone

In contrast to thermal heating, the real-time heated rock exhibits significant nonlinearity during the elastic deformation stage. Hence, an experiment assembled by a high and low-temperature test chamber and a microcomputer-controlled rock direct shear instrument is built to evaluate the effect of real-time temperature on the nonlinear deformation behavior of rock. A set of nonlinear stress–strain relationships of forty sandstone samples under moderate real-time temperature conditions are obtained from experiments and agree well with the theoretical predictions from the two-part Hooke’s model (TPHM). The soft and hard parts respond to the real-time temperature are quite different. The ratio of the soft part (γt) increases and the elastic modulus of the hard part (Ee) decreases with the temperature below 125℃. On the contrary, γt decreases and Ee increases with the temperature above 125℃ up to 200℃. Thermally-induced and expandable microcracks (thermal weakening) incur structural damage of the hard part below 125℃, while thermal expansion (thermal strengthening) can lead to tighter compaction between grains above 125℃. Therefore, there is a threshold temperature 125℃ under real-time heating condition due to the thermal cracks and expansion. The elastic modulus of the soft part (Et) decreases continuously with increasing temperature, indicating inconsistent thermal expansion effects on the soft part with mixture of impurity particles and the hard part whose mineral particles are closely contacted at higher temperature. Based on the experimental data and TPHM, empirical relationships between the real-time temperature and nonlinear deformation are proposed. This study will provide support for the safe production and disaster prevention of geological engineering under thermo-mechanical coupling working condition.

Numerical modelling of thermal weakening of granite under dynamic loading

A numerical method to predict thermal weakening of granite rock under low- and moderate rate dynamic loading conditions is developed. Thermal weakening of granite under uniform heating, leading to degradation of the material stiffness and strength, is modelled with the continuum approach, using a damage-viscoplasticity model. The viscoplastic part is based on a special power law criterion for granite under compression and the rounded tensile cut-off surface, while the damage part is equipped with separate damage variables in tension and compression. The strain rate sensitivity of rock is accounted for by viscoplasticity. The global thermo-mechanical initial/boundary value problem is solved with an explicit (in time) staggered method with mass scaling applied to increase the critical time step. Rock heterogeneity is described as random clusters of linear tetrahedral finite elements assigned with the constituent mineral, here quartz, feldspar, and biotite, material properties. The temperature dependence of the thermal and elasticity properties of the minerals is included in the model up to 850 °C, i.e. well beyond the α-β transition of quartz. However, as the model aims to predict the thermal weakening, the temperature dependence of the rock strength is not used as an input. In the numerical examples, low-rate uniaxial tension and compression tests are first simulated on heated and intact rock samples to demonstrate the performance of the model. Finally, crushing of heat-treated and intact granite balls by diametral dynamic compression is numerically replicated with a fair accuracy.

Post-sprouting thermal treatment of green barley malt to produce functional clean-label ingredients: Impact on fermentation, bread-making properties and bread quality

Malt flour represents a potential clean label bread improver, but a high enzymatic activity can lead to some bread defects. Thus, this study was focused on applying different thermal treatments (10 and 40 min; 70–90 °C) to green barley malt in order to promote a partial enzyme inactivation. The addition of 1.5 g of thermally treated malt (TTM) per 100 g of flour in wheat bread formulation was evaluated regarding the resulting bread-making properties, dough fermentation and overall bread quality. Activity of starch-degrading enzymes was not detectable above 80 °C/10 min. TTM incorporation improved the gas production by up to 60% during fermentation, mainly in formulations to which malts thermally treated under mild conditions have been added. Compared to untreated malt, thermal treatment reduced dough thermal weakening, improved gel strength during gelatinization and maintained low setback values. Bread collapse observed by baking follow-up was related to gas inflation and low mechanical resistance. Formulations with the addition of malts thermally treated at 70 °C for 40 min resulted in breads with higher specific volume, improved coloration and a crumb with slightly smaller pores than control and untreated malts. Thus, thermal treatment can be used as a technique to produce standardized malted flour to be used as clean label bread improvers.

Along-strike magma-poor/magma-rich spreading transitions

Abstract 1D/2D data-based studies of active spreading centres brought the knowledge of extension rate-dependent stretching-dominated v. buoyancy-dominated spreading. 3D reflection seismic data from the extinct centre of an initial oceanic corridor in the Caribbean allow us to see an along-strike transition between stretching- and buoyancy-dominated spreading where the spreading through detachment faulting is a precursor to the magma-assisted spreading. Studying progressively more evolved portions of the spreading centre, going from its end towards its centre, we see a progressively higher ascent of the asthenosphere, which heats the developing core complex in the exhuming footwall of the initial stretching-dominated system. The asthenospheric ascent is associated with thermal weakening of the core complex, which eventually results in ductile deformation reaching the upper portion of the complex. Subsequently, the core complex is penetrated by the dyke located at the top of the asthenospheric body. The dyke, which subsequently evolves to a diapir-shaped body, reaches the sea floor and establishes a magma-assisted steady-state seafloor spreading. These observations lead to a model explaining the initiation of the magma-assisted spreading in the initial oceanic corridor. Furthermore, they also improve our knowledge of multiple interacting mechanisms involved in the breakup of the last continental lithospheric layer, subsequent disorganized spreading and younger organized spreading.

High‐Pressure Mechanical Properties of Talc: Implications for Fault Strength and Slip Processes

AbstractThe hydrous mineral talc is stable over a relatively large P‐T field and can form due to fluid migration and metamorphic reactions in mafic and ultramafic rocks and in faults along plate boundary interfaces. Talc is known to be one of the weakest minerals, making it potentially important for the deformation dynamics and seismic characteristics of faults. However, little is known about talc's mechanical properties at high temperatures under confining pressures greater than 0.5 GPa. We present results of deformation experiments on natural talc cylinders exploring talc rheology under 0.5–1.5 GPa and 400–700°C, P‐T conditions simulating conditions at deep faults and subducted slab interface. At these pressures, the strength of talc is highly temperature‐dependent where the thermal weakening is associated with an increased tendency for localization. The strength of talc and friction coefficient inferred from Mohr circle analysis is between 0.13 at 400°C to ∼0.01 at 700°C. Strength comparison with other phyllosilicates highlights talc as the weakest mineral, a factor of ∼3–4 weaker than antigorite and a factor of ∼2 weaker than chlorite. The observed friction coefficients for talc are consistent with those inferred for subducted slabs and the San Andreas fault. We conclude that the presence of talc may explain the low strength of faults and of subducted slab interface at depths where transient slow slip events occur.

Effects of Tectonic Quiescence Between Orogeny and Rifting

AbstractTectonic quiescence is a period of nearly null relative plate motion. During the Atlantic Ocean opening, rifting cut across continental segments of supercontinent Pangea that had been through different periods of quiescence, which varied from tens to hundreds of Myr. Using numerical models, we explored the effects of quiescence between orogeny and rifting on the pre‐rift lithosphere and subsequent conjugate rifted margin configuration. Our model results showed that quiescence of ∼30–60 Myr caused wide orogenic wedges (&gt;300 km) to be heated beneath their centers, which led to the breakup of the lithospheric mantle before the breakup of the continental crust during rifting, resulting in hyperextended conjugate rifted margins. In these scenarios, continental lithosphere breakup away from the suture contributed to the preservation of previously subducted lower crust along the subduction zone. Long periods of tectonic quiescence (∼100 Myr) experienced by a wide orogen permitted efficient orogenic collapse and lateral spreading, with pronounced lithospheric thermal weakening. Consequently, post‐quiescence rifting reactivated the suture and effectively drove plate eduction, exhuming most of the subducted continental lower crust and mantle lithosphere. Ultimately, ultra‐wide domains developed into asymmetric conjugate margins. Contrastingly, in narrow orogenic wedges (&lt;200 km), different quiescence durations had minor effects on the margin architecture. In these cases, a longer quiescence time contributed to a larger preservation of subducted crust in the fossil subduction zone after rifting. Our models agree with conjugate rifted margins in the North and South Atlantic, which probably developed after the collapse of wide orogens and subsequent periods of tectonic quiescence.

Laser-Induced Diaphragm Rupture for Improved Sequencing and Repeatability in a Hypersonic Facility

For hypersonic facilities where the flow conditions are established through the rupture of a diaphragm, such as in the University of Southern Queensland’s hypersonic wind-tunnel facility, the variability in the flow conditions is related to the uncertainty of the pressure at which the diaphragm ruptures. Variability in the diaphragm rupture pressure also results in uncertainty of the time at which the diaphragm will rupture. For experiments that require knowledge of when the test flow will be initiated, the sequencing of events relative to the flow onset is difficult when the flow is initiated using the natural rupture of a diaphragm. The challenge of experiment sequencing that arises due to rupture pressure variability is addressed by introducing a laser for rapid thermal weakening of the diaphragm. Event sequencing challenges are discussed in the context of free-flight testing, including model release strategies for such testing. The work proceeds through a review of Ludwieg tube flow initiation strategies and a discussion of the present context, which requires a reliable method for sequencing the retraction of the free-flight model holder. The natural variability of strength of the Mylar diaphragms in the present work is found to result in around uncertainty in rupture pressure. This rupture pressure variability is demonstrated to have a significant temperature dependence through empirical results and engineering models. Implementation of the laser-induced diaphragm rupture method is demonstrated to enhance repeatability in generating the flow conditions; the variability in rupture pressure was reduced to when the laser method was used. Based on the remaining sequencing uncertainties with the laser-induced rupture method and practical speeds for model platform retraction, uncertainty in the positioning of the free-flight models at the time of flow onset is shown to be .