Journal of Geophysical Research: Solid Earth最新文献

High-resolution seismic model is crucial for advancing our understandings on geological processes and enhancing seismic hazard mitigation programs. We construct a high-resolution China Seismological Reference Model (CSRM-1.0) in the top 100 km of the crust and uppermost mantle in continental China following a top-down construction process. The employed seismic constraints include P-wave polarization angle from tele-seismic event, short-period Rayleigh wave ellipticity from ambient noise, long-period Rayleigh wave ellipticity from earthquake data, receiver function, empirical Green's function from ambient noise, Rayleigh wave phase/group velocity dispersion curves from regional earthquakes, and Pn-wave travel time extracted from seismic data of 4,435 stations. CSRM-1.0 has a spatial crustal resolution of ∼60 km beneath the north-south seismic belt and trans-North China orogen regions and ∼120 km beneath the rest of continental China, and a spatial mantle resolution of ∼300 km. CSRM-1.0 exhibits prominent velocity heterogeneities in the crust and uppermost mantle and an eastward thinning of the crust, geographically correlating with geological settings. CSRM-1.0 improvements include accurate estimation of shallow seismic structure, increased spatial resolution and improved model accuracy. Crustal composition inferred from CSRM-1.0 exhibits a general transition from a felsic upper crust to a mafic lower crust. Mafic rocks in the lower crust are found predominantly along inter-block boundaries and sporadically within the interiors of blocks, likely resulted from preferential inter-block intrusions of magmas related to various oceanic plate subductions and the Emeishan mantle plume. This study contributes seismic constraints and CSRM-1.0 to the CSRM product center (http://chinageorefmodel.org) as a backbone open-access geophysical cyberinfrastructure.

Melting experiments of Fe₃C were conducted to 85 GPa in laser-heated diamond anvil cells with in situ X-ray diffraction and post-experiment textural observation. From the determined pressure-temperature conditions of the melting curve for Fe₃C, together with literature data on the melting point of diamond and eutectic point of the system Fe-Fe₃C/Fe₇C₃ under high pressures, we established a self-consistent thermodynamic model for high-pressure melting of the system Fe-C including the mixing parameters for liquids. The results show that mixing of Fe and C liquids is negatively nonideal from 1 bar to the pressure at the center of the Earth. The departure from ideal mixing becomes progressively larger with increasing pressure, which leads to greatly stabilized liquids under core pressures. The modeled carbon content in eutectic melts under core pressures is 3.3–4.4 wt%. From the Gibbs free energy, we derived an internally consistent parameters for Fe-C outer cores which included the crystallizing points at their bottoms, isentropic thermal profiles, and densities and longitudinal seismic wave speeds (Vp). While the addition of carbon in excess of the eutectic melt composition effectively reduces the density of iron liquid, the Vp of iron liquid is not greatly changed. Therefore, the low density and high Vp of PREM relative to pure iron cannot be reconciled by an Fe-C liquid. Therefore, the Earth's core cannot be approximated by the system Fe-C and should include another light element.

Anelastic attenuation provides key insight in our understanding of thermal and rheological structures and the associated deformation and dynamic mechanisms of the Earth's deep interior. Unfortunately, attenuation tomography is advanced far behind wave-speed tomography due to the challenge in properly excluding the complex effects of elastic heterogeneities on seismic wave amplitude. By taking advantage of phase tracking in seismic array analysis, here we derive a new theory of Helmholtz tomography that well accounts for attenuation, source radiation, and scattering, etc., and present a technique called Helmholtz Multi-Event Tomography (HelMET) to retrieve the attenuation properly. The effectiveness of this method is then validated by synthetic inversions. Our synthetic seismograms are calculated using a newly developed three-dimensional finite-difference algorithm that accounts for physical dispersion and dissipation in anelastic media and remains accurate and stable even if strong attenuation exists. Compared to the traditional method poorly performed in the synthetic inversion, the HelMET well recovers the input attenuation anomalies, suggesting that this method can be used to successfully isolate attenuation from the complicated effects of elastic heterogeneities. Our results underline the implication of the new theory and method in accurately imaging high-resolution attenuation structures and unambiguously interpreting the anelastic heterogeneities of the Earth by array-based earthquake and ambient noise data with inexpensive computation.

Earthquakes are frictional instabilities caused by the shear stress decrease, that is, dynamic weakening, of faults with slip and slip rate. During dynamic weakening, shear stress depends on slip, slip rate, and temperature, according to constitutive laws governing the earthquake rupture process. In the laboratory, technical limitations in measuring temperature during frictional instabilities inhibit the investigation and interpretation of shear stress evolution. Here we conduct high velocity friction experiments on calcite-bearing simulated faults, both on bare-rock and on gouge samples, at 20–30 MPa normal stress, 1–6 m/s slip rate and 1–20 m total slip. Seismic slip pulses are reproduced by imposing boxcar and regularized Yoffe slip rate functions. We measured, together with shear stress, slip, and slip rate, the temperature evolution on the fault by employing an innovative two-color fiber optic pyrometer. The comparison between modeled and measured temperature reveals that for calcite-bearing faults the heat sink caused by decarbonation reaction controls the temperature evolution. In bare-rocks, energy is dissipated as frictional heat, and temperature increase is buffered by the heat sink of the calcite decarbonation reaction. In gouges, energy is dissipated as frictional heat and for plastic deformation processes, balanced by the heat sink caused by the decarbonation reaction enhanced by the mechanochemical effect. Our results suggest that in calcite-bearing rocks, a common fault zone material for earthquake sources in the continental crust at shallow depth, the type of fault materials (bare-rocks vs. gouges) controls the energy dissipation during seismic slip.

Zebra rocks, characterized by their striking reddish-brown stripes, rods, and spots of hematite (Fe-oxide), showcase complex self-organized patterns formed under far-from-equilibrium conditions. Despite their ease of recognition, the underlying mechanisms of pattern-forming processes remain elusive. We introduce a novel advection-dominated phase-field model that effectively replicates the Liesegang-like patterns observed in Zebra rocks. This numerical model leverages the concept of phase separation, a well-established principle governing Liesegang phenomena in a two-dimensional setting. Our findings reveal that initial solute concentration and fluid flow velocity are critical determinants in pattern morphologies. We quantitatively explain the spacing and width of a specific Liesegang-like pattern category. Furthermore, the model demonstrates that vanishingly low initial concentrations promote the formation of oblique patterns, with inclination angles influenced by rock heterogeneity. Additionally, we establish a quantitative relationship between band thickness and geological parameters for orthogonal bands. This enables the characterization of critical geological parameters based solely on static patterns observed in Zebra rocks, providing valuable insights into their formation environments. The diverse patterns in Zebra rocks share similarities with morphologies observed on early Earth and Mars, such as banded iron formations and hematite spherules. Our model, therefore, offers a plausible explanation for the formation mechanisms of these patterns and presents a powerful tool for deciphering the geochemical environments of their origin.

Fluids in subduction zones significantly influence element mobility, isotope fractionation, and mass transfer. However, unraveling the source, composition, and redox state of fluids in continental subduction zones poses a significant challenge. This study focuses on a granitic melt-eclogite contact interface, along with adjacent granite and eclogite from the Sulu ultrahigh-pressure metamorphic belt in East China. The interface exhibits complex mineral assemblages, enriched rare earth elements (REEs), and high field strength elements (HFSEs). Zircon grains from the interface show an age of ∼217 ± 9 Ma, slightly later than peak metamorphism, along with the presence of coesite inclusions. These findings suggest that the interfacial fluid likely formed from the mixing of granitic anatectic melt and aqueous fluid from the eclogite during the initial exhumation of the Sulu terrane. The interaction resulted in the eclogite acquiring substantial REEs and HFSEs, suggesting the interfacial fluid's effective element-transporting capability and potential supercritical fluid properties. Zircon Ce anomaly and Fe³⁺/Fe²⁺ oxybarometer data indicate a highly oxidizing interfacial fluid, analogous to arc magmas in oxygen fugacity. This led to the preferential loss of isotopically heavier Cr from the eclogite during fluid-eclogite interaction, evidenced by heavier Cr isotopic compositions in the interface (δ⁵³Cr = −0.04 to −0.05‰) compared to adjacent eclogite (δ⁵³Cr as low as −0.11‰). In summary, our results highlight the presence of strong oxidizing and element-mobilizing fluids in continental subduction zones, offering insights into supercritical fluid recognition and the genesis of oxidizing arc magmas in subduction zones.

Hydraulic fracturing in shale gas production can induce felt earthquakes, making it crucial to understand and mitigate induced earthquakes. The Cen'gong shale gas block in South China offers extensive data—3D seismic, geological structure, microseismic data, and detailed stimulation operations—allowing a comprehensive investigation into induced earthquakes by hydraulic fracturing. Using a dense temporary seismic array and deep-learning workflows, we build a high precision earthquake catalog and determine their focal mechanisms. Pre-existing fractures are identified through the Ant Tracking attribute derived from the 3D seismic data. We analyze the distribution, frequency, magnitude, and focal mechanisms of induced earthquakes, compare them spatially with the distribution of the pre-existing fractures, and track their temporal changes during and after hydraulic fracturing. Most induced earthquakes occurred along pre-existing fractures, exhibiting relatively larger magnitudes and persistent trailing seismicity. The number of trailing seismicity is proportional to the response time of stimulation earthquakes. The focal mechanism solutions suggest that the rupture mechanism of the trailing seismicity remained unchanged. By analyzing four clusters of earthquakes, we found that in two of these clusters, the induced earthquakes initiated from the far side of the fractures, then linearly migrated along the pre-existing fractures. This directional migration pattern is explained by stress rotation along the fractures. Our analysis suggests that both pre-existing fractures and stimulation operations significantly influence induced earthquake occurrences. Therefore, this work may enhance our understanding of pre-existing fractures, and optimizing stimulation operations can mitigate earthquake hazards in shale gas production.

Curved mountain belts are spectacular natural features that contain crucial 3D information about the tectonic evolution of orogenic systems in the absence of other kinematic markers. The Mesozoic units exposed in the Mexican Fold and Thrust Belt in northeastern Mexico show a striking curvature, whose kinematic history has not been studied. The existing tectonic models of the region simply assumed the shape of the tectonic units as an inherent feature to the orogen. We investigated the kinematic history of this curvature through paleomagnetism and rock magnetism analyses, coupled with an exhaustive review of available published literature. The studied data sets indicate a protracted history of (re)magnetizations that occurred during the Late Jurassic-Paleocene times at least during the Late Jurassic, Cretaceous and early Eocene. More significantly, they show significant counterclockwise rotations in the northern flank of the curvature and moderate clockwise vertical axis rotations along its southern flank. This data set suggests that the Sierra Madre Oriental was a linear belt that experienced oroclinal bending or buckling during the Cretaceous to early Eocene period (120–50 Ma).

Aqueous fluids are extensively present in the middle to lower crust, as revealed by seismic and magnetotelluric soundings. The α−β quartz phase transition significantly affects many physical properties and leads to substantial microcracks that can provide pathways for the migration of crustal fluids. A systematic investigation of macroscopic physical properties and microstructure of quartz is crucial to elucidate their correlation. In the present study, the effects of water content, trace elements, orientations, and phase transition on the electrical conductivity of quartz were thoroughly evaluated at 400−900°C and 1 GPa. Individual annealing experiments were simultaneously conducted on quartz single crystals at different peak temperatures and 1 GPa to investigate the evolution and spatial distribution of microcracks using X-ray microtomography (CT) and backscattered electron imaging. We found that trace element content and orientations, rather than H₂O, are the dominant factors controlling the conductivity of quartz. The distinct changes in conductivity of single crystals at around α−β phase transition temperature are attributed to the transformation of microcracks from isolated to interconnected networks, as confirmed by two-dimensional (2-D) and three-dimensional (3-D) microstructure images. Based on the variation in electrical conductivity and microstructure across the transition, it thus is proposed that the intragranular microcracks caused by quartz phase transition can serve as fluid or melt pathways within highly conductive zones present in the middle to lower crust, while α-quartz acts as an impermeable cap.