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.
Updated: 1 day 6 hours ago

High‐Latitude Geomagnetic Secular Variation at the End of the Cretaceous Normal Superchron Recorded by Volcanic Flows From the Okhotsk‐Chukotka Volcanic Belt

Wed, 12/27/2023 - 11:34
Abstract

The Cretaceous Normal Superchron (CNS, 84–121 Ma) is a singular period of the geodynamo's history, identified by a prolonged absence of polarity reversals. To better characterize the paleosecular variation (PSV) of the geomagnetic field at the end of this interval, we sampled seven continuous sequences of lava flows from the Okhotsk-Chukotka Volcanic Belt, emplaced 84–89 Ma in the vicinity of the Kupol ore deposit (NE Russia). From a collection of 1,024 paleomagnetic cores out of 82 investigated lava flows, we successfully determined the paleodirections of 78 lava flows, which led to 57 directional groups after removing the serial correlations. The resulting paleomagnetic pole is located at 170.0°E, 76.8°N (A 95 = 5.2°, N = 57), in good agreement with previous estimates for north-eastern Eurasia. Aiming at quantifying PSV at a reconstructed paleolatitude (λ) of ∼80°N, we obtained a virtual geomagnetic pole (VGP) scatter Sb=21.5°|19.3°24.0°(N=57) ${{S}_{\mathrm{b}}=21.5{}^{\circ}\vert }_{19.3{}^{\circ}}^{24.0{}^{\circ}}\,(N=57)$, the value of which was corrected for within-site dispersion and is little dependent on the choice of the selection criteria. Compared to previous paleodirectional data sets characterizing PSV at various paleolatitudes during the CNS, our S b estimate confirms a relative latitudinal increase S b(λ = 90°)/S b(λ = 0°) on the order of 2–2.5. Focusing on PSV at high paleolatitude within the 70°–90° range, we show that S b was ∼15% lower at the end of the CNS than during the past 10 Myr, confirming that the singular polarity regime of the geodynamo observed during the CNS is likely accompanied with reduced PSV.

Quantifying Magma Overpressure Beneath a Submarine Caldera: A Mechanical Modeling Approach to Tsunamigenic Trapdoor Faulting Near Kita‐Ioto Island, Japan

Wed, 12/27/2023 - 11:24
Abstract

Submarine volcano monitoring is vital for assessing volcanic hazards but challenging in remote and inaccessible environments. In the vicinity of Kita-Ioto Island, south of Japan, unusual M ∼ 5 non-double-couple volcanic earthquakes exhibited quasi-regular recurrence near a submarine caldera. Following the earthquakes in 2008 and 2015, a distant ocean bottom pressure sensor recorded distinct tsunami signals. In this study, we aim to find a source model of the tsunami-generating earthquake and quantify the pre-seismic magma overpressure within the caldera's magma reservoir. Based on the earthquake's characteristic focal mechanism and efficient tsunami generation, we hypothesize that submarine trapdoor faulting occurred due to highly pressurized magma. To investigate this hypothesis, we establish mechanical earthquake models that link pre-seismic magma overpressure to the size of the resulting trapdoor faulting, by considering stress interaction between a ring-fault system and a reservoir of the caldera. The trapdoor faulting with large fault slip due to magma-induced shear stress in the submarine caldera reproduces well the observed tsunami waveform. Due to limited data, uncertainties in the fault geometry persist, leading to variations of magma overpressure estimation: the pre-seismic magma overpressure ranging approximately from 5 to 20 MPa, and the co-seismic pressure drop ratio from 10% to 40%. Although better constraints on the fault geometry are required for robust magma pressure quantification, this study shows that magmatic systems beneath calderas are influenced significantly by intra-caldera fault systems and that tsunamigenic trapdoor faulting provides rare opportunities to obtain quantitative insights into remote submarine volcanism hidden under the ocean.

Issue Information

Wed, 12/27/2023 - 08:00

No abstract is available for this article.

A Granitic Mylonite Is Strongest Parallel to Lineation in a High‐Temperature Plastic Field

Sat, 12/23/2023 - 06:50
Abstract

The effects of preexisting fabrics on the flow laws and anisotropic deformation of rocks require further study. We conducted triaxial compression experiments on a granitic mylonite parallel to lineation (X), perpendicular to lineation and parallel to foliation (Y), and perpendicular to foliation (Z) under a pressure of 300 MPa, temperatures of 800–1,000°C, and strain rates of ∼2.5 × 10−6–10−4 s−1 using a Paterson gas-medium apparatus. The low stress exponent (n = 1.9–5.8), high activation energy (Q = 325–802 kJ/mol), and macrostructures (distributed for most samples) and microstructures (such as kinked, folded and elongated biotite, elongated quartz and feldspar, microcracks within quartz and feldspar, and melt wetting and dissolution of quartz and feldspar) suggest that the deformation is dominated by dislocation creep, along with brittle regime at ≤∼850°C and likely diffusion creep at ≥900°C. Dehydration melting of biotite causes more obvious melt wetting of quartz and feldspar boundaries, lower n values at ≥950°C, and the maximum changes in n and Q along the Z-direction, since the biotite alignment defines the foliation. Under the same conditions, the X-direction samples consistently display the greatest strengths, which would have been for the Z-direction samples as reported previously, and most obvious deformation localization, mainly due to the alignment of the elongated quartz and feldspar along this direction. Microcracks always occur in quartz but are tensile when compressed perpendicular to the foliation plane and compressively sheared when shortened parallel to the foliation plane. These tensile microcracks further weaken the rock samples with axes perpendicular to foliation.

Adjoint‐State Teleseismic Traveltime Tomography: Method and Application to Thailand in Indochina Peninsula

Sat, 12/23/2023 - 06:40
Abstract

We propose a novel framework for teleseismic traveltime tomography that requires no ray tracing. The tomographic inverse problem is formulated as an Eikonal equation-constrained optimization problem, aiming at the determination of a slowness model that minimizes the difference between observational and predicted differential traveltimes. Two improvements have been made over previous ray-based methods. First, the traveltimes from the source outside the study region to any positions within the study region are computed using a hybrid approach. This involves solving a 2D Eikonal equation to obtain the traveltimes from the source to the boundary of the study region and solving a 3D Eikonal equation to compute the traveltimes from the boundary to any positions within the study region. Second, we compute the sensitivity kernel using the adjoint-state method. This method avoids the computation of ray paths and makes the computational cost nearly independent of the number of receivers. We apply our new method in Thailand and adjacent regions. The final velocity model reveals a thick lithosphere beneath the Khorat Plateau and two mantle upwelling branches beneath its southern and western margins. The mantle upwelling may result from the mantle convection triggered by surrounding subduction systems and/or a slab window of the Indian Plate. The presence of the mantle upwelling corresponds to the source zone of the erupted Cenozoic basalts in the Khorat Plateau, indicating lithospheric modification beneath the plateau. The insightful tomographic result verifies our method and provides new perspectives on the structural heterogeneities and dynamics of the Indochina Block.

CANVAS: An Adjoint Waveform Tomography Model of California and Nevada

Sat, 12/23/2023 - 06:14
Abstract

We present the California-Nevada Adjoint Simulations (CANVAS) model, an adjoint waveform tomography model of the crust and uppermost mantle of the states of California and Nevada. We used WUS256 (Rodgers et al., 2022, https://doi.org/10.1029/2022jb024549) as the starting model and iteratively decreased the minimum period of CANVAS from 30 to 12 s. CANVAS was iterated in two distinct stages: the first stage with source mechanisms from the Global Centroid Moment Tensor (GCMT) catalog and the second stage with inverted moment tensors (MT) using the CANV_WUS model (Doody et al., 2023, https://doi.org/10.1029/2023jb026463). We show that updating the MTs with 3D Green's functions improved waveform fits and azimuthal coverage of windowed data used to calculate the gradients. As for the model itself, we improved waveform fits over WUS256, particularly in the dispersed surface waves. CANVAS resolved tectonic features seen in other models and accurately defined the depth to basement of major basins, including the Central Valley and the Ventura Basin. We propose CANVAS as a starting model for crustal tomography models on smaller scales.

Joint Inversion of SPREE Receiver Functions and Surface Wave Dispersion Curves for 3‐D Crustal and Upper Mantle Structure Beneath the U.S. Midcontinent Rift

Wed, 12/20/2023 - 15:11
Abstract

Broadband seismograms from the EarthScope Transportable Array and Superior Province Rifting EarthScope Experiment (SPREE) deployments are used to map the crust and uppermost mantle structures beneath the failed Midcontinent Rift (MCR) of Minnesota/Wisconsin, USA. The results suggest the existence of a variable zone of mafic underplating that is up to 20 km thick (40–60 deep). We jointly invert receiver functions and Rayleigh wave dispersion curves to quantify the region's crustal and mantle shear-wave velocity structure. Basin sediment thicknesses are mildly asymmetric about the rift axis, with thickest regions immediately beneath the rift. 3-D modeling shows anomalous lower crust and crust-mantle transitions beneath the MCR. Sub-MCR crustal thicknesses are generally >50 km with lower crust Vs of 4.0–4.2 km/s. Away from the MCR, the crust is typically ∼40 km thick. Strong variations in apparent crustal thickness are found along the MCR, increasing significantly in places. An additional layer of shear velocities intermediate between typical lower crust and upper mantle velocities (4.1–4.6 km/s) exists beneath most of the MCR which is thickest beneath the rift axis and pinches out away from the rift. This structure corroborates previous proposals of the presence of an underplated layer near the Moho. Results cannot distinguish between different mechanisms of emplacement (e.g., mafic interfingering within a subsequently down-dropped lower crust vs. development of a high-density pyroxenitic residuum at the top of the mantle). Also observed are anomalously high (>4.7 km/s) sub-rift shear-wave velocities at ∼70–90-km depths, suggesting the presence of cold, depleted upper mantle material.

Bayesian Inversion of Lithology and Liquid Phase Parameters From Seismic Velocity and Electrical Conductivity in the Crust and Uppermost Mantle

Wed, 12/20/2023 - 09:45
Abstract

To deeply understand various geodynamic processes, including volcanic activities and earthquakes, it is essential to extract detailed information about Earth materials, such as lithology and geofluid, from geophysical, petrological, and geochemical observations of Earth's interior. We developed a Bayesian probabilistic framework that can estimate the lithology and geofluid type (aqueous fluid or melt), geofluid amount (porosity), and parameters related to the fluid geometry (aspect ratio and critical fluid fraction related to connectivity) from P-wave and S-wave seismic velocities and electrical conductivity data obtained from geophysical tomography. By conducting synthetic inversion tests, we showed that methods based on a joint probability distribution, which simultaneously determines all parameters, sometimes fail to narrow the number of possible answers from 78 lithologies (e.g., basalt, granite, and eclogite) × two geofluid type (melt or aqueous fluid) candidate sets. This failure is derived directly from the difficult nature of inversion problems with relatively large data uncertainties. The proposed method uses a marginalization technique that first estimates lithology and geofluid type and then quantifies geofluid parameter values, which can narrow the probable lithology, geofluid type, and geofluid parameter sets in many cases. In addition, the computational cost of the proposed marginalization method is comparative to a former joint-estimation method, which is theoretically identical to a previous heuristic method based on the least squares method. Therefore, the marginalization method is useful for geophysical data analyses that involve large amounts of observational data with relatively large uncertainties.

Lower‐Crustal Normal Faulting and Lithosphere Rheology in the Atlas Foreland

Wed, 12/20/2023 - 09:40
Abstract

Earthquakes beneath the foreland basins of the Andes and Tibet are rare but follow a simple pattern, with normal faulting from 0 to 20 km depth and reverse-faulting from 30 to 50 km depth. The switch in faulting style with depth suggests that the stresses generated by foreland flexure are large enough to break faults, with opposite senses of horizontal strain either side of a neutral surface in the mid-crust. In this study, we document a 31 km-deep M w 5.2 normal-faulting earthquake in the forelands of the Algerian Atlas Mountains near Biskra. The Biskra earthquake is of interest, as it indicates that the lower crust of the Atlas forelands is in extension at the same depth that the Tibetan and Andean forelands are in compression. In order to match the gravity anomalies and the depth of normal faulting near Biskra, we find that models of lithospheric flexure require the neutral surface to be >35 km deep and at least the top 5–10 km of the lithospheric mantle supports elastic bending stresses. The differences in the pattern of earthquakes between the forelands of Tibet, the Andes and the Algerian Atlas can be explained by differences in the buoyancy forces acting between these mountain ranges and their lowlands that place the forelands into varying amounts of net compression. Our results suggest the upper mantle beneath cratonic foreland lithosphere may therefore support bending stresses of the order of 10s of MPa, likely because it is cool and the strain rates associated with bending are low.

Low‐Velocity Structure of Subducted Oceanic Crust in the Upper Mantle: Insights From High Pressure and Temperature Elasticity Measurements of Aragonite

Wed, 12/20/2023 - 07:29
Abstract

This study examines the effects of water and carbon on the velocity profiles of subducted oceanic crust in the upper mantle. High pressure and temperature Brillouin measurements were conducted to determine the single-crystal elasticity of aragonite (CaCO3) up to 20 GPa and 600 K, respectively. Using the finite-strain method, we determined the elastic parameters: K S0 = 70.7 GPa, G 0 = 36.1(3) GPa with K S′ = 5.0(1), G′ = 1.3(1), ∂K S/∂T = −0.020(2) GPa/K, and ∂G/∂T = −0.015(1) GPa/K. When combined with literature results, our findings reveal that neither hydrous minerals nor carbonate alone can explain the observed 3%–4% and 3%–7% low velocity anomalies in the compressional (V P) and shear-wave (V S) velocities of subducted oceanic crust in the circum-Pacific region at 150–250 km depth. Considering the combined effect of water and carbonate, the addition of 5.9–6.9 vol.% aragonite together with 10 vol.% lawsonite in the oceanic crust can produce a 3%–4% and 6.1%–8.1% low velocity anomalies in the V P and V S at 150–250 km depth, respectively, consistent with the seismic observations in the region. Complete dehydration of lawsonite accompanied by the gradual decarbonization of the subducted oceanic crust explains the absence of low-velocity anomalies below 300-km depth. Our findings help for a better understanding of the possible influence of carbon and water on seismic velocities of the mantle. As a result, the circulation of carbon and water may be better understood while taking the complex velocity structure of subduction zones and mineral physics findings into account.

A Diffuse Interface Method for Earthquake Rupture Dynamics Based on a Phase‐Field Model

Wed, 12/20/2023 - 07:29
Abstract

In traditional modeling approaches, earthquakes are often depicted as displacement discontinuities across zero-thickness surfaces embedded within a linear elastodynamic continuum. This simplification, however, overlooks the intricate nature of natural fault zones and may fail to capture key physical phenomena integral to fault processes. Here, we propose a diffuse interface description for dynamic earthquake rupture modeling to address these limitations and gain deeper insight into fault zones' multifaceted volumetric failure patterns, mechanics, and seismicity. Our model leverages a steady-state phase-field, implying time-independent fault zone geometry, which is defined by the contours of a signed distance function relative to a virtual fault plane. Our approach extends the classical stress glut method, adept at approximating fault-jump conditions through inelastic alterations to stress components. We remove the sharp discontinuities typically introduced by the stress glut approach via our spatially smooth, mesh-independent fault representation while maintaining the method's inherent logical simplicity within the well-established spectral element method framework. We verify our approach using 2D numerical experiments in an open-source spectral element implementation, examining both a kinematically driven Kostrov-like crack and spontaneous dynamic rupture in diffuse fault zones. The capabilities of our methodology are showcased through mesh-independent planar and curved fault zone geometries. Moreover, we highlight that our phase-field-based diffuse rupture dynamics models contain fundamental variations within the fault zone. Dynamic stresses intertwined with a volumetrically applied friction law give rise to oblique plastic shear and fault reactivation, markedly impacting rupture front dynamics and seismic wave radiation. Our results encourage future applications of phase-field-based earthquake modeling.

On the Constitutive Equations for Coupled Flow, Chemical Reaction, and Deformation of Porous Media

Tue, 12/19/2023 - 10:13
Abstract

Deformation, chemical reactions, fluid flow in geological formations, and many engineering materials are coupled processes. Most existing models of chemical reactions coupled with fluid transport assume the dissolution-precipitation process or mineral growth in rocks. However, these models have limitations, such as predicting restricted reaction extent due to pore clogging or disregarding porosity changes resulting from mineral growth. Recent studies indicate mineral replacement involves coupled dissolution-precipitation, maintaining porosity while altering the solid volume. This has multiple practical implications for natural geological processes and within petroleum and environmental engineering. We present a novel model for reaction-driven mineral expansion that preserves porosity and allows solid volume change. First, we look at fluid-rock interaction at the pore scale and derive effective rheology of a reacting porous media. On a larger scale, we adopt a two-phase continuum medium approach to investigate the coupling between reaction, deformation, and fluid flow. Our micromechanical model based on observations assumes that rock or cement consists of an assembly of solid reactive grains, initially composed of a single, pure phase. The reaction occurs at the fluid-solid contact and progresses into the solid grain material. We approximate the pores and surrounding solid material as an idealized cylindrical shell to simplify the problem and obtain tractable results. We derive macroscopic stress-strain constitute laws that account for chemical alteration and viscoelastic deformation of porous rocks. Our model explains the possibility of achieving a complete reaction, preservation of porosity during chemical reactions, and dependence of mechanical rock properties on fluid chemistry.

Improved Observations of Deep Earthquake Ruptures Using Machine Learning

Tue, 12/19/2023 - 06:35
Abstract

Elevated seismic noise for moderate-size earthquakes recorded at teleseismic distances has limited our ability to see their complexity. We develop a machine-learning-based algorithm to separate noise and earthquake signals that overlap in frequency. The multi-task encoder-decoder model is built around a kernel pre-trained on local (e.g., short distances) earthquake data (Yin et al., 2022, https://doi.org/10.1093/gji/ggac290) and is modified by continued learning with high-quality teleseismic data. We denoise teleseismic P waves of deep Mw5.0+ earthquakes and use the clean P waves to estimate source characteristics with reduced uncertainties of these understudied earthquakes. We find a scaling of moment and duration to be M 0 ≃ τ 4, and a resulting strong scaling of stress drop and radiated energy with magnitude (Δσ≃M00.21 ${\Delta }\sigma \simeq {M}_{0}^{0.21}$ and ER≃M01.24 ${E}_{R}\simeq {M}_{0}^{1.24}$). The median radiation efficiency is 5%, a low value compared to crustal earthquakes. Overall, we show that deep earthquakes have weak rupture directivity and few subevents, suggesting a simple model of a circular crack with radial rupture propagation is appropriate. When accounting for their respective scaling with earthquake size, we find no systematic depth variations of duration, stress drop, or radiated energy within the 100–700 km depth range. Our study supports the findings of Poli and Prieto (2016, https://doi.org/10.1002/2016jb013521) with a doubled amount of earthquakes investigated and with earthquakes of lower magnitudes.

Investigating Dynamic Weakening in Laboratory Faults Using Multi‐Scale Flash Heating Coupled With mm‐Scale Contact Evolution

Tue, 12/19/2023 - 06:19
Abstract

Flash-weakening models typically show good agreement with the total magnitude of weakening in high-speed rock friction experiments, however deviations during the acceleration and deceleration phases, and at low and intermediate sliding velocities, remain unresolved. Here, we incorporate inhomogeneous mm-scale normal stress evolution into a model for flash heating and weakening to resolve outstanding transient and hysteretic friction observed in laboratory experiments and to identify unique solutions to constitutive parameters. We conduced 37 rock friction experiments on Westerly granite using a high-speed biaxial apparatus outfitted with a high-speed infrared camera. We initiated velocity steps from quasi-static rates of 1 mm/s to sliding velocities ranging from 300 to 900 mm/s and conducted both constant- and decreasing-velocity tests following the velocity step. Two sliding surfaces geometries were used to control mm-scale life-times and rest-times. Constant-strength sliding is achieved within 2–3 mm of initiating the velocity step in all constant-velocity experiments. Macroscopic surface temperature is inhomogeneous and increases with slip distance, velocity, and decreasing rest-time. Weakening increases with sliding velocity and decreasing rest-time. We combine thermal models with measured surface temperatures to constrain the evolution of local normal stress at the mm-scale and incorporate this evolution into a flash-weakening model that considers weakening at both the µm- and mm-scale. The flash-weakening model improves when the effects of mm-scale wear processes are incorporated and multi-scale weakening is considered, however some transient friction remains undescribed. Models will be advanced by further incorporating wear processes and by considering processes at the mm-scale and above.

Comparison of Continuously Recorded Seismic Wavefields in Tectonic and Volcanic Environments Based on the Network Covariance Matrix

Mon, 12/18/2023 - 05:50
Abstract

Extended and sufficiently dense seismic networks capture spatiotemporal properties of the continuously recorded wavefields and can be used to compute the level of their coherence at different frequencies via the analysis of the network covariance matrix, which has been successfully used to study volcanic seismicity. Here, we present an application of the covariance matrix method in a subduction zone environment. We show that most coherent signals identified through the covariance matrix analysis are related to regional earthquakes with the wavefield properties affected by the scattering, which depends on the source location. Tectonic tremors, on the other hand, are not characterized by a high level of coherence. We compare real data results with a set of synthetic tests aimed at mimicking the properties of seismic sources and the main features of wave propagation. We conclude that highly coherent volcanic tremor wavefields could be produced in two ways: by a spatially localized group of monochromatic seismic sources or by a single source located in a highly heterogeneous medium. In both cases, the stability of the source position is a necessary condition to reproduce the observations in volcanoes. On the other hand, the low coherence of tectonic tremor wavefields can be explained by a spatially extended distribution of sources, in agreement with large portions of the subduction interface being nearly simultaneously involved in the episodes of slow deformation.

Physical Mechanism for a Temporal Decrease of the Gutenberg‐Richter b‐Value Prior to a Large Earthquake

Mon, 12/18/2023 - 05:45
Abstract

Observations of seismicity prior to large earthquakes show that the slope of a Gutenberg-Richter magnitude-frequency relation, referred to as a b-value, sometimes decreases with time to the mainshock. Yet, underlying physical processes associated with the temporal change of a b-value remain unclear. Here we utilize continuum models of fully dynamic earthquake cycles with fault frictional heterogeneities and aim to simulate the temporal variation of a b-value. We first identify a parameter regime in which the model gives rise to an active and accelerating foreshock behavior prior to the mainshock. We then focus on the spatio-temporal pattern of the simulated foreshocks and analyze their statistics. We find that the b-value of simulated foreshocks decreases with time prior to the mainshock. A marked decrease in the resulting b-value occurs over the duration of less than a few percent of the mainshock recurrence interval, broadly consistent with foreshock behaviors and b-value changes as observed in nature and laboratory, rock-friction experiments. In this model, increased shear stresses on creeping (or velocity-strengthening) fault patches resulting from numerous foreshocks make these creeping patches more susceptible to future coseismic slip, increasing the likelihood of large ruptures and leading to a smaller b-value with time. This mechanism differs from a widely invoked idea that the decrease of a b-value is caused by a rapid increase in shear stress that promotes micro-crack growth, and offers a new interpretation of b-value changes prior to a large earthquake.

Revisiting the San Andreas Heat Flow Paradox From the Perspective of Seismic Efficiency and Elastic Power in Southern California

Mon, 12/18/2023 - 04:50
Abstract

We investigate the relation between frictional heating on a fault and the resulting conductive surface heat flow anomaly using the fault's long-term energy budget. Analysis of the surface heat flow surrounding the fault trace leads to a constraint on the frictional power generated on the fault—the mechanism behind the San Andreas fault (SAF) heat flow paradox. We revisit this paradox from a new perspective using an estimate of the long-term accumulating elastic power in the region surrounding the fault, and analyze the paradox using two parameters: the seismic efficiency and the elastic power. The results show that the constraint on frictional power from the classic interpretation is incompatible with the accumulating elastic power and the radiated power from earthquake catalogs. We then explore four mechanisms that can resolve this extended paradox. First, stochastic fluctuations of surface heat flow could mask the fault-generated anomaly (we estimate 21% probability). Second, the elastic power accumulating in the region could be overestimated (≥550 MW required). Third, the seismic efficiency—ratio of radiated energy to elastic work—of the SAF could be higher than that of the remaining faults in the region (≥5.8% required). Fourth, the scaled energy—ratio of radiated energy to seismic moment—on the SAF could be lower than on the remaining faults in the region (a factor 5 difference required). In the last three hypotheses, we analyze the interplay of the energy budget on a single fault with the total energy budget of the region.

The Thermal Structure of Central Te Riu a Māui/Zealandia Continent as Determined From the Depth to Base of Magnetic Sources

Sat, 12/16/2023 - 11:06
Abstract

To investigate the thermal structure of central Te Riu a Māui/Zealandia continent we estimate the depth to base of magnetic sources (DBMS), using an updated compilation of magnetic data. We calculate thousands of power spectral density estimates and solve an analytic solution for the fractal distribution of magnetization using a Bayesian method constrained by a sediment thickness model. Our DBMS results are broadly similar to a global model yet show complex relationships with independent models of crust and elastic thicknesses, and depth to base of seismicity (D90). Across central Zealandia, DBMS occurs above, coincident with, and below the Moho reflecting that factors additional to temperature influence DBMS. Away from subduction zones crustal thickness likely modulates DBMS while plate margins show varying influence dependent on margin characteristics. In the Taupō Volcanic Zone we interpret DBMS to reflect Curie point temperature at an average depth of 9–11 km. Error analysis shows that the choice of prior influences the posterior standard deviation and can result in counter intuitive error versus DBMS relationships. We use the DBMS as a basal temperature isotherm in heat flow models to compare with heat flow derived from borehole temperature measurements. Using the standard Curie point temperature of 580°C we find that DBMS derived heat flow models over estimate heat flow in areas underlain by plutonic rocks. We propose this reflects a higher titanomagnetite content in these rocks, although detailed studies of the magnetic mineralogy of Zealandia's plutonic rocks are required.

Estimating post‐Depositional Detrital Remanent Magnetization (pDRM) Effects: A Flexible Lock‐In Function Approach

Sat, 12/16/2023 - 10:55
Abstract

The primary data sources for reconstructing the geomagnetic field of the past millennia are archeomagnetic and sedimentary paleomagnetic data. Sediment records, in particular, are crucial in extending the temporal and spatial coverage of global geomagnetic field models, especially when archeomagnetic data are sparse. The exact process on how sediment data acquire magnetization including post-depositional detrital remanent magnetization is still poorly understood. However, it is widely accepted that these effects lead to a smoothing of the magnetic signal and offsets with respect to the sediment age. They impede the direct inclusion of sediment records in global geomagnetic field modeling. As a first step, we model these effects for a single sediment core using a new class of flexible parameterized lock-in functions. The parameters of the lock-in function are estimated by the maximum likelihood method using archeomagnetic data as a reference. The effectiveness of the proposed method is evaluated through synthetic tests. Our results demonstrate that the proposed method is capable of estimating the parameters associated with the distortion caused by the lock-in process.

First‐Order Transition in Appalachian Orogenic Processes Revealed by Along‐Strike Variation of the Moho Geometry

Thu, 12/14/2023 - 12:01
Abstract

Along-strike variation of the Laurentian rifted margin and the Appalachian orogen has long been recognized in the geologic record. We investigated the manifestation of this along-strike variation at depth by generating scattered wavefield migration profiles from four dense seismic arrays deployed across the Appalachian orogen at different latitudes. All profiles exhibit a similar crustal thickness decrease of 15–20 km from the Mesoproterozoic Grenville Province to the Paleozoic Appalachian accreted terranes, but the Moho architecture differs dramatically along strike. The profiles beneath the central and southern Appalachians show a smoothly varying Moho geometry; in contrast, there is an abrupt Moho depth offset beneath the New England Appalachians. This contrast in Moho geometry may result from variations in the Laurentian rifted margin architecture, changes in Taconic orogeny subduction polarity, and greater crustal shortening during the Acadian-Neoacadian orogeny in southern New England and the Alleghanian orogeny in the central and southern Appalachians. A first-order along-strike transition in the behavior of Appalachian orogenic processes is located between the central and New England Appalachians.

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