JGR–Solid Earth

Syndicate content Wiley: Journal of Geophysical Research: Solid Earth: Table of Contents
Table of Contents for Journal of Geophysical Research: Solid Earth. List of articles from both the latest and EarlyView issues.
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Hematite Frictional Behavior and He Loss From Comminution During Deformation Experiments at Slow Slip Rates

Tue, 03/12/2024 - 17:39
Abstract

Deformation experiments on hematite characterize its slip-rate dependent frictional properties and deformation mechanisms. These data inform interpretations of slip behavior from exhumed hematite-coated faults and present-day deformation at depth. We used a rotary-shear apparatus to conduct single-velocity and velocity-step experiments on polycrystalline specular hematite rock (∼17 μm average plate thickness) at slip rates of 0.85 μm/s to 320 mm/s, displacements of primarily 1–3 cm and up to 45 cm, and normal stresses of 5 and 8.5 MPa. The average coefficient of friction is 0.70; velocity-step experiments indicate velocity-strengthening to velocity-neutral behavior at rates <1 mm/s. Scanning electron microscopy showed experimentally generated faults develop in a semi-continuous, thin layer of red hematite gouge. Angular gouge particles have an average diameter of ∼0.7 μm, and grain size reduction during slip yields a factor of 10–100 increase in surface area. Hematite is amenable to (U-Th)/He thermochronometry, which can quantify fault-related thermal and mechanical processes. Comparison of hematite (U-Th)/He dates from the undeformed material and experimentally produced gouge indicates He loss occurs during comminution at slow deformation rates without an associated temperature rise required for diffusive loss. Our results imply that, in natural fault rocks, deformation localizes within coarse-grained hematite by stable sliding, and that hematite (U-Th)/He dates acquired from ultracataclasite or highly comminuted gouge reflect minor He loss unrelated to thermal processes. Consequently, the magnitude of temperature rise and associated thermal resetting in hematite-bearing fault rocks based on (U-Th)/He thermochronometry may be overestimated if only diffusive loss of He is considered.

Structure and Tectonic Evolution of the NW Sulu Sea Basin (SE Asia)

Mon, 03/11/2024 - 17:20
Abstract

We discuss the tectonic structure, seismic stratigraphy and evolution of the NW Sulu Sea using reprocessed 2D reflection profiles. The NW Sulu Sea is located between the Palawan continental shelf and the Cagayan Ridge and represents the northern part of the Sulu Sea, a marginal sea resulting from Paleogene extension and subsequent Neogene contraction due to convergence between the Palawan and the Philippine blocks. The basin consists of six seismo-stratigraphic units overlying crystalline basement. Syn-orogenic depocenters contain calibrated Middle Miocene to, possibly, Lower Miocene units, while rift-related depocenters consist of uncalibrated but tentatively dated Paleogene to Lower Miocene units. Thickness and depth maps of the main units and bounding horizons differentiate the Piedra-Blanca and the Rasa domains, separated by the NW-Sulu-Break major tectonic structure. Fault-bounded rift-related depocenters are strongly segmented. We interpret that NW-SE and NE-SW trending zones accommodate shape and trend variations of these depocenters. We suggest that these zones may link rift segments, recording different extensional deformation. Miocene thrusting and folding in the Piedra-Blanca Domain and mudflow with associated gravitational structures in the Rasa Domain influenced the deposition of syn-orogenic units. Rift structures inherited from rift segmentation may have conditioned the style and distribution of contractional deformation during the subsequent incipient reactivation during contraction. In the context of SE Asia, our results support that the timing of rifting of the NW Sulu Sea overlaps with the opening of the South China Sea and the North Palawan margin, which may indicate a common geodynamic driving force triggering extension.

High‐Temperature Deformation of Enstatite‐Olivine Aggregates

Mon, 03/11/2024 - 16:54
Abstract

Synthesized polycrystalline samples composed of enstatite and olivine with different volumetric ratios were deformed in compression under anhydrous conditions in a Paterson gas-medium apparatus at 1150–1300°C, an oxygen fugacity buffered at Ni/NiO, and confining pressures of 300 or 450 MPa (protoenstatite or orthoenstatite fields). Mechanical data suggest a transition from diffusion to dislocation creep with increasing differential stress for all compositions. Microstructural analyses by optical and scanning electron microscopy reveal well-mixed aggregates and homogeneous deformation. Crystallographic preferred orientations measured by electron backscatter diffraction are consistent with activation of the slip systems (010)[100] and (010)[001] for olivine and (100)[001] and (010)[001] for enstatite, as expected at these conditions. Nonlinear least-squares fitting to the full data set from each experiment allowed the determination of dislocation creep flow laws for the different mixtures. The stress exponent is 3.5 for all compositions, and the apparent activation energies increase slightly as a function of enstatite volume fraction. Within the limits of experimental uncertainties, all two-phase aggregates have strengths that lie between the uniform strain rate (Taylor) and the uniform stress (Sachs) bounds calculated using the dislocation creep flow laws for olivine and enstatite. Calculation of the Taylor and Sachs bounds at strain rate and temperature conditions expected in nature (but not extrapolating in pressure) indicates that using the dislocation creep flow law for monomineralic olivine aggregates provides a good estimate of the viscosity of olivine-orthopyroxene rocks deforming by dislocation creep in the deeper lithosphere and asthenosphere.

Low δ18O and δ30Si TTG at ca. 2.3 Ga Hints at an Intraplate Rifting Onset of the Paleoproterozoic Supercontinent Cycle

Mon, 03/11/2024 - 08:34
Abstract

The start of the Paleoproterozoic supercontinent cycle is typically taken as the initiation of orogenesis at ca. 2.1 Ga leading to the assembly of Earth's first supercontinent, Columbia. However, the dearth of ca. 2.5–2.2 Ga geological records makes it difficult to deduce tectonic factors during the onset of the Paleoproterozoic supercontinent cycle. The petrogenesis of tonalite–trondhjemite–granodiorite (TTG) provides useful proxies for tracing prevailing geodynamic regimes of early continental evolution. However, marked decreases of TTG and other magmatism occurred across the Archean–Paleoproterozoic transition and have previously precluded forming testable hypotheses. Early Paleoproterozoic TTGs have been identified in the North China Craton (NCC) and other cratons, which may represent the last major pulse of TTGs globally. Here we present low δ18O and δ30Si ca. 2.3 Ga TTGs from the NCC, together with thermodynamic modeling and compilation of stable O and Si isotopes for TTGs globally through time. The ca. 2.3 Ga TTGs were derived from the partial melting of Archean basaltic crust and give lighter average zircon δ18O (3.15 ± 0.35‰) and whole-rock δ30Si values (−0.17 ± 0.08‰) than most Archean TTGs. Considering coeval mafic-felsic igneous rocks, and lithospheric thinning since ca. 2.5 Ga based on estimated crustal thickness through the Neoarchean–Paleoproterozoic, we posit the onset of an intraplate rifting consistent with the anomalous low-δ18O magmatism. Continental rifting of Archean cratons/supercratons plausibly hints at the formation of rifts driving subduction initiation as the veritable onset of the Paleoproterozoic supercontinent cycle.

Sensitivity of GNSS‐Derived Estimates of Terrestrial Water Storage to Assumed Earth Structure

Mon, 03/11/2024 - 08:19
Abstract

Geodetic methods can monitor changes in terrestrial water storage (TWS) across large regions in near real-time. Here, we investigate the effect of assumed Earth structure on TWS estimates derived from Global Navigation Satellite System (GNSS) displacement time series. Through a series of synthetic tests, we systematically explore how the spatial wavelength of water load affects the error of TWS estimates. Large loads (e.g., >1,000 km) are well recovered regardless of the assumed Earth model. For small loads (e.g., <10 km), however, errors can exceed 75% when an incorrect model for the Earth is chosen. As a case study, we consider the sensitivity of seasonal TWS estimates within mountainous watersheds of the western U.S., finding estimates that differ by over 13% for a collection of common global and regional structural models. Errors in the recovered water load generally scale with the total weight of the load; thus, long-term changes in storage can produce significant uplift (subsidence), enhancing errors. We demonstrate that regions experiencing systematic and large-scale variations in water storage, such as the Greenland ice sheet, exhibit significant differences in predicted displacement (over 20 mm) depending on the choice of Earth model. Since the discrepancies exceed GNSS observational precision, an appropriate Earth model must be adopted when inverting GNSS observations for mass changes in these regions. Furthermore, regions with large-scale mass changes that can be quantified using independent data (e.g., altimetry, gravity) present opportunities to use geodetic observations to refine structural properties of seismologically derived models for the Earth's interior structure.

Type‐B Crystallographic Preferred Orientation in Olivine Induced by Dynamic Dehydration of Antigorite in Forearc Regions

Mon, 03/11/2024 - 08:09
Abstract

The crystallographic preferred orientation (CPO) of olivine, specifically the type-B characterized by c-axes aligned parallel to lineation and b-axes concentrated perpendicular to foliation, is essential for explaining the trench-parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type-B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700–750°C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type-C fabric to a type-B fabric, with increasing grain size and dehydration level. The type-B CPO observed in coarse-grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low-temperature dynamic dehydration mechanism, in contrast to the mechanism of high-temperature plastic flow, for explaining the development of type-B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs.

Thermal Stability of F‐Rich Phlogopite and K‐Richterite During Partial Melting of Metasomatized Mantle Peridotite With Implications for Deep Earth Volatile Cycles

Mon, 03/11/2024 - 07:55
Abstract

Phlogopite and K-richterite constitute important carrier phases for H and F in Earth's lithosphere and mantle. The relative importance depends on their stabilities at high pressure and temperature, which in turn depends on bulk composition. Most previous experimental studies focused on the thermal stability of phlogopite and K-richterite were conducted using simplified chemical compositions. Here, partial melting experiments on metasomatized and carbonated, OH ± F-bearing near-natural peridotite were performed at high pressures (2 and 5 GPa) and temperatures (1,100–1,350°C) to assess the thermal stability of F-free versus F-bearing phlogopite and K-richterite. Experimental results demonstrate that the thermal stability of F-bearing phlogopite is increased by >55°C/wt.% F, relative to F-free phlogopite, whereas K-richterite is absent in all experiments with significant degrees of melting (>2%). The thermal stability of phlogopite containing several wt.% F exceeds continental and oceanic geotherms within the upper 150 km. Fluorine-rich phlogopite would therefore be stable in virtually all of the continental lithosphere, only to be decomposed during large, regional melting events such as continental break-up, thereby acting as a major long-term sink for F and/or H. This could even be the case for the oceanic asthenosphere, depending on the oceanic geotherm of the area of interest.

A Continental Model of Curie Point Depth for China and Surroundings Based on Equivalent Source Method

Fri, 03/08/2024 - 20:24
Abstract

The Curie Point Depth (CPD) marks a significant temperature boundary (∼580°C) within the Earth's lithosphere. However, there has been ongoing debate regarding its spatial distribution. In this research, we utilized the Equivalent Source Method (ESM) based on Gauss-Legendre integration and data obtained from the EMM2017 model, along with a five-layer susceptibility model, to generate a 0.5° × 0.5° grid of continental CPD distribution for China and surroundings. The average CPD in the study area is 30.4 km, which is slightly shallower than the average depth of global continental Moho (∼33 km). Notably, stable and cold cratonic basins, such as the Tarim Basin and the Sichuan Basin, exhibit deep CPD of ∼45 km. In contrast, the North China Craton, which has experienced significant tectono-thermal activity since the Late Mesozoic, shows moderate CPD of ∼30 km and a gradual uplift from west to east. The Tuva-Mongol orocline within the Central Asian Orogenic Belt, the Deccan Volcanic Province in the Indian subcontinent and the Eastern Yangtze Craton have shallow CPD of ∼20 km. We estimate the surface heat flow by CPD, and the result is consistent with measurements within a RMSE of 18.1 mW/m2. When comparing the CPD with Moho, we find that the CPD may lie below Moho in stable and cold cratonic areas. In comparison to two recent global CPD models, our regional model demonstrates better alignment with tectonic features.

Numerical Simulation of the Self‐Organizational Origin of Concentrically Zoned Aggregates of Siderite and Pyrite in Sediment‐Hosted Massive Sulfide Deposits

Fri, 03/08/2024 - 20:16
Abstract

Concentrically zoned pyrite aggregates (CZPA) are common in sediment-hosted massive sulfide (SHMS) deposits and have been widely used to interpret the ore-forming processes. There is considerable uncertainty, however, over the formation of aggregates that are oscillatorily zoned and contain randomly-orientated pyrite microcrystals. Guided by the results of examination of the micro-textures of CZPA and in-situ chemical analyses, we conducted a quantitative diffusion-reaction simulation to assess the mechanism of CZPA formation. Our study shows that oscillatory zoning results from the feedback between the diffusion of reactants and the nucleation-growth of Fe-sulfides. Externally derived Fe2+ diffuses into the early diagenetic sediments containing decomposing organic matter (2CH2O + SO4 2− = 2HCO3 − + H2S) and reacts with H2S to form a pyrite layer via an intermediate pathway (Fe2+ + H2S → FeS + 2H+, FeS + H2S → FeS2 + H2). This growth of pyrite layers depletes the local concentration of reactants and suppresses nucleation until the diffusive reaction front advances and another layer is formed. Intermediate phases, for example, mackinawite, nucleate instead of pyrite, because of their greater ease of nucleation due to the low surface tension, and lead to the domination of nucleation over growth. The nucleation of mackinawite and occurrence of siderite in the CZPA are consistent with a low temperature, high pH, anoxic early diagenetic environment. Our study demonstrates that CZPA in SHMS deposits are formed by intrinsic self-organizational processes rather than by extrinsic changes of ore-forming fluids. The CZPA in SHMS deposits are thus indicative of their diagenetic origin with Fe2+ infiltrated and diffused from hydrothermal fluids into the sediments.

A New Negative Carbon Isotope Interval Caused by Manganese Redox Cycling After the Shuram Excursion

Fri, 03/08/2024 - 08:35
Abstract

Several negative C isotope excursions (CIEs) occurred at the end of the Neoproterozoic era which have been generally attributed to the oxidation of organic carbon using sulfate as the terminal electron acceptor and the subsequent release of 13C-depleted dissolved inorganic C (DIC). Based on new analyses from the Doushantuo Formation in South China, we observe a negative C isotope excursion right after the well-known Shuram excursion. This excursion is equivalent to the ∼550 Ma negative CIE which is globally expressed within several continental margins. However, the origins of this CIE in the termination of Ediacaran remain unresolved. Here, we hypothesize that this post-Shuram negative CIE was caused by a localized manganese cycling that began with the oxidation of hydrothermal Mn(II) in a water column to insoluble Mn(IV)-oxide, followed by accumulation of Mn(IV)-oxide to the seafloor and its subsequent dissolution via Mn(IV) reduction leading to the release of dissolved Mn(II) and 13C-depleted DIC into ambient seawaters. This ultimately led to the precipitation of particulate Mn(II)-carbonate characterized by low δ13Ccarb values ranging from −11.1‰ to −2.8‰. The presence of microbial fabrics in association with the Mn(II)-carbonate further suggests that Mn(II)-carbonate precipitation took place at the seafloor in shallow sun-lit waters rather than in the deeper sediment pile, which archived ambient seawater C isotopic signal. Although most Ediacaran negative CIEs were generally attributed to sulfate reduction, our findings suggest that at a local level, Mn cycling can also lead to negative CIE in the Neoproterozoic, and potentially at other times in Earth's history.

Inverting Geodetic Strain Rates for Slip Deficit Rate in Complex Deforming Zones: An Application to the New Zealand Plate Boundary

Fri, 03/08/2024 - 08:20
Abstract

The potential for future earthquakes on faults is often inferred from inversions of geodetically derived surface velocities for locking on faults using kinematic models such as block models. This can be challenging in complex deforming zones with many closely spaced faults or where deformation is not readily described with block motions. Furthermore, surface strain rates are more directly related to coupling on faults than surface velocities. We present a methodology for estimating slip deficit rate directly from strain rate and apply it to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. The strain rate inversions imply slightly higher slip deficit rates than the preferred geologic slip rates on sections of the major strike-slip systems including the Alpine Fault, the Marlborough Fault System and the northern part of the North Island Fault System. Slip deficit rates are significantly lower than even the lowest geologic estimates on some strike-slip faults in the southern North Island Fault System near Wellington. Over the entire plate boundary, geodetic slip deficit rates are systematically higher than geologic slip rates for faults slipping less than one mm/yr but lower on average for faults with slip rates between about 5 and 25 mm/yr. We show that 70%–80% of the total strain rate field can be attributed to elastic strain due to fault coupling. The remaining 20%–30% shows systematic spatial patterns of strain rate style that is often consistent with local geologic style of faulting.

Rift Propagation Interacting With Pre‐Existing Microcontinental Blocks

Fri, 03/08/2024 - 08:08
Abstract

Rift propagation is a 3D thermo-mechanical process that often precedes continental breakup. Pre-existing microcontinental blocks and the associated lithospheric strength heterogeneities influence the style of rift propagation. Interestingly, some rifts propagate into pre-existing blocks and eventually cut through them (e.g., the Zhongsha Block and the Reed Bank), while others bypass these microcontinental blocks forming distinct overlapping rift branches (e.g., the East African Rift System). In this study, we use 3D numerical models to investigate the interaction between microcontinental blocks and rift propagation under different far-field extension rates. In doing so, we assess the impact of mantle lithospheric thicknesses and lower crustal rheology on the style of rift propagation. Our models reproduce the two types of rift propagation, characterized by propagating rifts that either split or bypass the pre-existing microcontinental blocks. We find that lithospheric thickness exerts dominant control, while lower crustal rheology of microcontinental blocks and the extension rate have less effect on rift propagation. Our model results can explain rift propagation patterns, block rotation and strong lithospheric thinning in the South China Sea, the East African Rift System, and the Woodlark Basin.

Robust Imaging of Fault Slip Rates in the Walker Lane and Western Great Basin From GPS Data Using a Multi‐Block Model Approach

Thu, 03/07/2024 - 14:24
Abstract

The Walker Lane (WL) in the western Great Basin (GB) is an active plate boundary system accommodating 10%–20% of the relative tectonic motion between the Pacific and North American plates. Its neotectonic framework is structurally complex, having hundreds of faults with various strikes, rakes, and crustal blocks with vertical axis rotation. Faults slip rates are key parameters needed to quantify seismic hazard in such tectonically active plate boundaries but modeling them in complex areas like the WL and GB is challenging. We present a new modeling strategy for estimating fault slip rates in complex zones of active crustal deformation using data from GPS networks. The technique does not rely on prior estimates of slip rates from geologic studies, and only uses data on the surface trace location, dip, and rake. The iterative framework generates large numbers of block models algorithmically from the fault database to obtain many estimates of slip rates for each fault. This reduces bias from subjective choices about how discontinuous faults connect and interact to accommodate strain. Each model iteration differs slightly in block boundary configuration, but all models honor geodetic and fault data, regularization, and are kinematically self-consistent. The approach provides several advantages over bespoke models, including insensitivity to outlier data, realistic uncertainties, explicit mapping of off-fault deformation, and slip rates that are more objective and independent of geologic slip rates. Comparisons to the U.S. National Seismic Hazard Model indicate that ∼80% of our geodetic slip rates agree with their geologic slip rates to within uncertainties.

Ocean Bottom Distributed Acoustic Sensing for Oceanic Seismicity Detection and Seismic Ocean Thermometry

Thu, 03/07/2024 - 14:16
Abstract

A T-wave is a seismo-acoustic wave that can travel a long distance in the ocean with little attenuation, making it valuable for monitoring remote tectonic activity and changes in ocean temperature using seismic ocean thermometry (SOT). However, current high-quality T-wave stations are sparsely distributed, limiting the detectability of oceanic seismicity and the spatial resolution of global SOT. The use of ocean bottom distributed acoustic sensing (OBDAS), through the conversion of telecommunication cables into dense seismic arrays, is a cost-effective and scalable means to complement existing seismic stations. Here, we systematically investigate the performance of OBDAS for oceanic seismicity detection and SOT using a 4-day Ocean Observatories Initiative community experiment offshore Oregon. We first present T-wave observations from distant and regional earthquakes and develop a curvelet denoising scheme to enhance T-wave signals on OBDAS. After denoising, we show that OBDAS can detect and locate more and smaller T-wave events than regional OBS network. During the 4-day experiment, we detect 92 oceanic earthquakes, most of which are missing from existing catalogs. Leveraging the sensor density and cable directionality, we demonstrate the feasibility of source azimuth estimation for regional Blanco earthquakes. We also evaluate the SOT performance of OBDAS using pseudo-repeating earthquake T-waves. Our results show that OBDAS can utilize repeating earthquakes as small as M3.5 for SOT, outperforming ocean bottom seismometers. However, ocean ambient natural and instrumental noise strongly affects the performance of OBDAS for oceanic seismicity detection and SOT, requiring further investigation.

Intermittent Criticality Multi‐Scale Processes Leading to Large Slip Events on Rough Laboratory Faults

Thu, 03/07/2024 - 14:00
Abstract

We discuss data of three laboratory stick-slip experiments on Westerly Granite samples performed at elevated confining pressure and constant displacement rate on rough fracture surfaces. The experiments produced complex slip patterns including fast and slow ruptures with large and small fault slips, as well as failure events on the fault surface producing acoustic emission bursts without externally-detectable stress drop. Preparatory processes leading to large slips were tracked with an ensemble of ten seismo-mechanical and statistical parameters characterizing local and global damage and stress evolution, localization and clustering processes, as well as event interactions. We decompose complex spatio-temporal trends in the lab-quake characteristics and identify persistent effects of evolving fault roughness and damage at different length scales, and local stress evolution approaching large events. The observed trends highlight labquake localization processes on different spatial and temporal scales. The preparatory process of large slip events includes smaller events marked by confined bursts of acoustic emission activity that collectively prepare the fault surface for a system-wide failure by conditioning the large-scale stress field. Our results are consistent overall with an evolving process of intermittent criticality leading to large failure events, and may contribute to improved forecasting of large natural earthquakes.

Pseudo‐Prospective Forecasting of Induced and Natural Seismicity in the Hengill Geothermal Field

Wed, 03/06/2024 - 06:36
Abstract

The Hengill geothermal field, located in southwest Iceland, is host to the Hellisheiði power plant, with its 40+ production wells and 17 reinjection wells. Located in a tectonically active area, the field experiences both natural and induced seismicity linked to the power plant operations. To better manage the risk posed by this seismicity, the development of robust and informative forecasting models is paramount. In this study, we compare the forecasting performance of a model developed for fluid-induced seismicity (the Seismogenic Index model) and a class of well-established statistical models (Epidemic-Type Aftershock Sequence). The pseudo-prospective experiment is set up with 14 months of initial calibration and daily forecasts for a year. In the timeframe of this experiment, a dense broadband network was in place in Hengill, allowing us to rely on a high quality relocated seismic catalog. The seismicity in the geothermal field is characterized by four main clusters, associated with the two reinjection areas, one production area, and an area with surface geothermal manifestations but where no operations are taking place. We show that the models are generally well suited to forecast induced seismicity, despite some limitations, and that a hybrid ETAS model accounting for fluid forcing has some potential in complex regions with natural and fluid-induced seismicity.

Impact of Ancient Tectonics on Intracontinental Deformation Partitioning: Insights From Crustal Structures of the East Junggar‐Altai Area

Wed, 03/06/2024 - 06:36
Abstract

Compressive stress generated at collision fronts can propagate over long distances, inducing deformation within the continent's interior. Nevertheless, the factors governing the partitioning of intracontinental deformation remain enigmatic. The Altai Mountains serve as a type-example of ongoing intracontinental deformation. Here, we investigate the crustal architecture of the Chinese Altai Mountains, using receiver functions obtained from newly deployed dense seismic nodal arrays. The new seismic results reveal distinct crustal features, including (a) a negative polarity discontinuity beneath Chinese Altai Mountains, suggesting a low-velocity layer; (b) a north-dipping mid-crustal structure beneath the suture zone between East Junggar and Chinese Altai, indicating underthrusting of East Junggar's lower crust beneath the Chinese Altai Mountains; (c) a double Moho structure beneath East Junggar, revealing a high-velocity lower crustal layer. In conjunction with constraints from previous multi-disciplinary regional studies, the double Moho structures are interpreted as mafic restite from Late Paleozoic magma underplating. The addition of mafic materials can significantly enhance the rheological strength of East Junggar's crust, causing it to function as an indenter that thrust beneath the Chinese Altai Mountains during the subsequent convergence process. As a consequence, significant deformation occurs in the Chinese Altai region, resulting in the emergence of decollements, as evident by the negative polarity discontinuity. The presence of pre-existing decollements makes the Altai Mountains region more susceptible to deformation, thereby facilitating the concentration of intracontinental deformation. These findings illuminate the evolution history of the Chinese Altai Mountains and highlight the great impacts of ancient tectonics on intracontinental deformation partitioning.

Thermal Stressing of Volcanic Rock: Microcracking and Crack Closure Monitored Through Acoustic Emission, Ultrasonic Velocity, and Thermal Expansion

Wed, 03/06/2024 - 06:35
Abstract

Microcracking due to thermal stresses affects the mechanical and flow properties of rocks, which is significant for thermally dynamic environments such as volcanoes and geothermal reservoirs. Compared with other crustal rocks like granite, volcanic rocks have a complex and variable response to temperature; it remains unclear how thermal microcracks form and how they are affected by temperature. We heated and cooled samples of low-porosity basalts containing different amounts of microcracks and a porous andesite over three cycles, whilst monitoring microstructural changes by acoustic emission (AE) monitoring and measurement of P-wave velocity (v P ; up to 450°C) and thermal expansion coefficient (TEC; up to 700°C). During the second and third cycles, the TEC was positive throughout and the rate of detected AE was low. In contrast to studies on granite, we measured a strong and reversible increase in v P with increasing temperature (by 15%–40% at 450°C), which we interpret as due to microcrack closure. During the first cycle, AE and v P measurements indicated thermal microcracking within the andesite and the basalt with a low initial microcrack density. For these samples, strong inflexions in the TEC indicated stress relaxation during heating, preceding significant thermal microcracking during cooling. The basalt with a high initial microcrack density underwent little microcracking throughout all cycles. Our results and a review of the literature relate the initial microstructure to the occurrence of thermal microcracking and explore the potentially significant influence of temperature on volcanic rock properties.

Fracturing and Dome‐Shaped Surface Displacements Above Laccolith Intrusions: Insights From Discrete Element Method Modeling

Tue, 03/05/2024 - 13:55
Abstract

Inflation of viscous magma intrusions in Earth's shallow crust often induces strain and fracturing within heterogeneous host rocks and dome-shaped ground deformation. Most geodetic models nevertheless consider homogeneous, isotropic, and linear-elastic media wherein stress patterns indicate the potential for failure, but without simulating actual fracturing. We present a two-dimensional Discrete Element Method (DEM) application to simulate magma recharge in a pre-existing laccolith intrusion. In DEM models, fractures can propagate during simulations. We systematically investigate the effect of the host rock toughness (resistance to fracturing), stiffness (resistance to deformation) and the intrusion depth, on intrusion-induced stress, strain, displacement and spatial fracture distribution. Our results show that the spatial fracture distribution varies between two end-members: (a) for high stiffness or low toughness host rock or a shallow intrusion: extensive cracking, multiple vertical surface fractures propagating downward and two inward-dipping highly cracked shear zones that connect the intrusion tip with the surface; and (b) for low stiffness or high toughness host rock or a deeper intrusion: limited cracking, one central vertical fracture initiated at the surface, and two inward-dipping fractures at the intrusion tips. Abrupt increases in surface displacement magnitude occur in response to fracturing, even at constant magma injection rates. Our modeling application provides a novel approach to considering host rock mechanical strength and fracturing during viscous magma intrusion and associated dome-shaped ground deformation, with important implications for interpreting geodetic signals at active volcanoes and the exploitation of geothermal reservoirs and mineral deposits.

Deciphering Contribution of Recycled Altered Oceanic Crust to Arc Magmas Using Ba‐Sr‐Nd Isotopes

Tue, 03/05/2024 - 07:23
Abstract

Altered oceanic crust (AOC) plays a critical role in geochemical recycling in subduction zones. However, identifying contributions of subducted AOC to arc magmas remains a conundrum due to the lack of effective tracers. Here, we investigate the Ba-Sr-Nd isotopic compositions of lavas from the Mariana arc and back-arc. Based on a statistical analysis of the Sr-Nd isotopes for global arc volcanoes, we confirm that AOC-derived fluid (or hydrous melt), rather than sediment-derived melt or fluid, is responsible for the Sr-Nd isotope decoupling (i.e., 87Sr/86Sr is “excessively” enriched relative to 143Nd/144Nd when compared to the “normal” mantle derivates) observed in island arc lavas. We show that the arc lavas with increasingly decoupled Sr-Nd isotopes generally have heavier Ba isotope ratios, which is also a characteristic feature of AOC-derived fluids. Thus, these results establish an intimate link between subducted AOC, heavy Ba isotope compositions, and Sr-Nd isotope decoupling signature in island arcs, which provides a powerful tool to trace the AOC recycling in subduction zones. Furthermore, a similar correlation is observed between Sr-Nd isotope decoupling and heavy B isotope ratios for global arc lavas, implying that the recycling of AOC component is generally linked to serpentinite dehydration in subduction zones.

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