Earth and Planetary Science Letters最新文献

Igneous sills and interconnected sill complexes transport magma both vertically through the Earth's crust and laterally over potentially long distances. Although cooling and solidification of magma are acknowledged to play a major role in the propagation and emplacement of sills, their contributions to sill formation remain poorly understood. Here, the effects of solidification on sill propagation dynamics and the resulting intrusion morphologies are investigated using scaled laboratory experiments. Molten coconut oil (magma analogue), which solidifies during emplacement, is injected directly into the horizontal interface between two layers of a colder, layered, solid visco-elasto-plastic gel (Laponite RD®, host rock analogue) to facilitate sill formation. The injection temperature and volumetric flow rate of the coconut oil, and the temperature of the host material, are varied between experiments to control the relative degree of solidification. When solidification effects are relatively weak, corresponding to high injection temperatures, sill propagation is continuous and forms penny-shaped intrusions that later turn into saucer-shaped sills with marginal segmentation. Conversely, when solidification effects are intermediate to relatively strong, corresponding to lower injection temperatures, sills develop complex elongate morphologies that lengthen parallel to the long-axis of the magma flow direction. Such sills also form in a discontinuous manner and propagate in pulses by growth of discrete marginal lobes, representing periods of tip arrest due to freezing, followed by growth of new lobes at the sill margins. A striking morphological feature that occurs in experiments with intermediate to relatively strong solidification effects is the presence of internal flow channels within sills, which can be: (a) thermally controlled, long-lived channels in experiments with intermediate solidification effects; or (b) structurally controlled, randomly oriented short-lived channels in experiments with relatively strong solidification effects. Our experimental findings are consistent with field and seismic observations of sill geometries, and they highlight that the relative degree of solidification during magma emplacement controls both how sills propagate and their internal flow dynamics.

Fluid inclusions (FI) entrapped in phenocrysts carried by Ocean Island Basalts (OIB) contain key information on volatiles’ abundance and origin in their mantle sources. Here, we add new piece of knowledge to our understanding of volatile geochemistry in global OIB magmas, by presenting new noble gas (He-Ne-Ar) and carbon (C) isotope results for olivine- and clinopyroxene-hosted FI from enclaves, lavas, tephra and volcanic gas samples from Fogo, the only frequently active volcano at the Cape Verde archipelago (eastern Atlantic Ocean). FI, together with crater fumaroles, constrain the Fogo ³He/⁴He signature at 7.14–8.44 R_c/R_a (where R_C is the air-corrected ³He/⁴He isotope ratio, and R_a is the same ratio in air), which is within the typical MORB (Mid-Ocean Ridge Basalt) mantle. The carbon isotopic ratio (δ¹³C vs. Pee Dee Belemnite) of CO₂ in FI and fumaroles range from -6.04 to -4.41 ‰. We identify systematic variations of δ¹³C and He/Ar* with FI entrapment pressure (estimated from a combination of host mineral barometry and FI microthermometry), from which we develop a model for volatile degassing in the mantle-to-crustal magma storage system. The model predicts a crustal-like signature for carbon (δ¹³C of -0.4 ± 1.0 ‰) in primary melts formed by mantle melting at ∼2200 MPa (∼77 km) and a source He/Ar* ratio of 0.90–0.24, which are indicative of variably depleted mantle metasomatized by carbon enriched melts/fluids from a crustal component. We also use our results to characterise regional (in the Cape Verde and Canary archipelagos) and global trends in C and He isotope composition from OIB. From a comparison with the few other OIB localities for which δ¹³C are available, we propose that a carbon enriched crustal component could be recurrent at a global scale in OIB magmatism, although often masked by isotope fractionation during magmatic degassing. We additionally find that, at regional scale, He isotopes in OIB scale inversely correlate with the degree of partial melting of the mantle beneath individual islands’ (inferred from the La/Yb ratio of erupted basalts). More widely, our results corroborate previously established global relationships between OIB He isotopic signature, plume buoyancy flux and overlying plate velocity. In this interpretation, the MORB-like ³He/⁴He (8 ± 1 Ra) at Fogo reflects a combination of (i) low to medium magma productivity, (ii) relatively low plume buoyancy flux (∼1.1 Mg/s), and (iii) slow average velocity (∼3 cm/yr) of the overlying plate.

When and how plate tectonics started and evolved to the style as we know it today is a fundamental yet highly controversial question. Numerical geodynamic modelling predicts that the transition into plate tectonics in the Archean was episodic with possible alternation between success and failure of subduction. However, direct geological evidence for failed subduction is scarce. While this possibility has been suggested by numerical modelling, it is still lack of geological evidence. Here we present a combined study of zircon U-Pb ages and Hf-O isotopes, as well as whole-rock major and trace elements, for meta-igneous rocks from the Alxa Block in the westmost North China Craton (NCC). Two periods of Late Archean magmatism ca. 2.75 Ga and 2.5 Ga are identified to occur surrounding a pre-3.0 Ga continental nucleus. Geochemical analyses of the ca. 2.75 Ga meta-mafic and meta-felsic igneous rocks show two different modes of tectonic regime. The felsic rocks resemble typical Archean TTG (tonalite-trondhjemite-granodiorite) in composition, with variable ε_Hf(t) values from -2.8 to 11.0 and δ¹⁸O values from 4.3 ‰ to 7.9 ‰. Petrogenetic modelling suggests that their magmatic source was the ca. 3.1–2.75 Ga oceanic crust that was hydrothermally altered at different temperatures and then mixed with the older continental crust and partially melted in lower crust in the garnet stability field. This requires tectonic accretion of the oceanic crust to the continental nucleus, signifying an attempted or failed subduction. On the contrary, the meta-mafic rocks exhibit arc-like trace element patterns and calc-alkaline evolution trend, indicating a metasomatic mantle source due to successful subduction of the oceanic slab to subarc depths. Taken together, the present results provide robust constraints on the behavior of oceanic slab at the Archean convergent margin, where successful oceanic subduction would be achieved after several failed attempts. Such failure-success processes of oceanic subduction may be applicable to the whole NCC and other cratons elsewhere in the world, reflecting the gradual maturation of plate tectonics in the Archean, consistent with predictions of numerical modelling.

To investigate the three-dimensional spatial distribution of subducted oceanic slab segments and their consequential effect on the thermal and hydrous composition of the mantle transition zone (MTZ) beneath the entire Tibetan Plateau, we conducted a comprehensive analysis of approximately 655,000 P-to-s receiver functions (RFs), obtained from 735 broadband seismic stations. These RFs were utilized to delineate the 410 km (d410) and 660 km (d660) seismic discontinuities, which represent the upper and lower boundaries of the MTZ, respectively. The RFs were grouped within circular bins with a radius of 1 degree and stacked to image the discontinuities. The mean apparent depths derived based on the 1-D IASP91 Earth model for the d410 and d660 across the entire study area are 412.2±8.3 km and 668.9±8.5 km respectively, and the MTZ thickness is 256.5±6.9 km. The observed apparent depths underwent subsequent correction utilizing multiple velocity models. Several areas in central Tibet exhibit a normal d410 and an anomalously deep d660, which can be attributed to the combined effect of the negative thermal anomaly and dehydration associated with subducted slab segments that have penetrated at least to the d660 depth. The anomalous thickening of the MTZ beneath the southeastern Tibetan Plateau surrounding the Tengchong volcanic field can be explained by the dehydration of the subducted Indian Slab. Significant thinning of the MTZ associated with the deepening of the d410 beneath the western Tian Shan may indicate active thermal upwelling originating from the MTZ.

The impact of climate on mountain relief is often questioned, mainly due to the difficulties of measuring surface processes at the timescale of glacial-interglacial cycles. An appropriate setting for studying mountain erosion in response to Quaternary climate change is found in the Tateyama mountains in the Hida mountain range (northern Japanese Alps) due to distinct geomorphological features. The Japanese Alps uplifted within the past ∼1–3 Myr and experienced multiple glaciations during the late Quaternary. We use ultra-low temperature thermochronometers based on the luminescence of feldspar minerals from 19 rock samples and the electron spin resonance (ESR) of quartz minerals from 8 rock samples, in combination with inverse modelling to derive rock cooling rates and exhumation rate histories at 10⁴–10⁶ year timescales from three transects in the Tateyama region. While luminescence signals have already reached their upper dating limit, ESR signals (Al and Ti centres) yielded ESR ages of ∼0.3–1.1 Ma, implying surface processes active in the Pleistocene. Based on a negative age-elevation relationship, local relief reduction at a cirque-basin scale is identified over the past 1 Myr, whereas a positive age distribution with elevation for samples close to the mountain top does not follow this trend. Inverse modelling reveals rock cooling rates on the order of 20–70 °C/Myr, with slightly faster cooling for cirque-floor samples, which equate with erosion rates of 0.5–1 mm/yr that exceed rates from periglacial and slope processes in the same locality. Thus, our data suggest that Quaternary climate change coupled with distinct surface processes modified the slopes of the Tateyama mountains leading to a localised decrease in relief within an individual cirque basin over the second half of the Quaternary.

Measuring surface displacements driven by afterslip and viscoelastic relaxation following large earthquakes enables inferring frictional and rheological properties of Earth's outermost layers where hazardous earthquakes occur. However, the two concurrent mechanisms generate similar and intertwined surface deformations, complicating the extraction of physical information from the measurements. Here, we quantify the spatiotemporal dominance of co-evolving afterslip and viscoelastic relaxation following the 1973 Ms 7.6 Luhuo earthquake in eastern Tibet, using 42 years (1976‒2018) of fault-crossing short-baseline observations. The finite-fault slip of this M7+ earthquake was constructed from triangulation data measured in 1961‒1975. Based on the model incorporating two postseismic relaxation mechanisms and calibrated by the measurements over four decades, we show that, in the temporal domain, afterslip may produce geodetically measurable deformations (>1.5 mm/yr) in local near-field regions even 20 years after the mainshock, with spatially broader impact within ∼10 years. Viscoelastic relaxation may last for nearly six decades, imposing a broad influence for three decades. In the spatial domain, afterslip may produce deformations within ∼2 times the seismogenic depth (SD; 20 km) from the fault, but dominantly in the near-field of ∼1 times the SD. In contrast, viscoelastic relaxation may affect a much wider region reaching 200 km. While afterslip and viscoelastic relaxation collectively affected the region ∼2 times the SD from the fault in the early postseismic period, their resulting deformations gradually separated in space with time, with afterslip-induced deformation shrinking toward the fault, and surface deformation caused by viscoelastic relaxation concentrating in the mid-field, ∼2–3 times the SD from the fault. Their spatiotemporal partitioning helps better learn fault frictional properties and lithospheric rheology based on geodetic data acquired from different domains.

We develop a joint inversion method in the spectral domain that accounts for different signal characteristics in Global Navigation Satellite System (GNSS) and Gravity Recovery and Climate Experiment (GRACE) observations for quantifying terrestrial water storage (TWS) changes in the Yangtze River Basin (YRB). The method seamlessly integrates these geodetic datasets with distinct spatial scales and coverages. We exploit the Slepian basis functions, being spectrally band-limited and spatially concentrated, to put these datasets together in the regional TWS modeling. Our integrated geodetic results reveal variable spatiotemporal patterns of water storage changes within the YRB, hydrological extremes, and their linkage to interannual climate variability. The results indicate that annual precipitation in the middle and lower YRB is twice of that in the Jinsha River Basin (the uppermost basin of YRB), yet the TWS change is less than a half. This large discrepancy in the ratio of water storage and precipitation can be attributed to substantial runoff in the middle and lower YRB. The joint geodetic inversion identifies extreme droughts and flood events in the basin, consistent with the assessments from precipitation anomalies and Global Flood Awareness System. We also find that interannual TWS variations in the Jinsha River Basin are modulated by Indian Ocean Dipole, while those in the upper YRB and middle and lower YRB are modulated by El Niño-Southern Oscillation. Our findings highlight the potential use of geodetic data combination to advance hydrological variabilities in the region and inform water resource management strategies in response to climate change.

A quantitative kinematic model of present-day deformation across the Tibetan Plateau is essential to understand its growth mechanism. Dominant dextral movement along the ≥2500 km-long Karakorum-Jiali fault zone, long considered the southern boundary of plateau-wide eastward extrusion, was later challenged by a locally restrained, symmetrical shortening model co-involving short (200-300 km) conjugate strike-slip faults. Here, based on meter-resolution satellite data and field measurements, we document a previously undocumented, ∼1000 km-long, ∼ENE to E-trending sinistral fault, the Qixiang Co fault (QXCF), that wholly crosses central Tibet to connect the Ganzi-Xianshuihe fault in eastern Tibet with the Gyaring Co fault and Tangra Yum Co rift in central/southern Tibet. High-resolution unmanned aerial vehicle (UAV) topographic surveys, ¹⁰Be cosmogenic and U-series dating of a lake shoreline and several alluvial fans at 3 sites ∼240 km apart, show that up to ∼450 m of sinistral offset accrued in the last ∼120 ka, implying a horizontal slip rate of 3.5 ± 0.2 mm/yr along the fault. Four recent large paleoearthquakes (M_w7.9–8.0) may have ruptured a > 300 km stretch of the QXCF with characteristic slips of ∼9–10 m and return times of ∼2500–2900 a. Such results demonstrate that the hitherto previously undocumented QXCF is the single most important tectonic boundary across the Qiangtang terrane, contributing predominantly to large-scale eastward escape of central Tibet and challenging the ‘V-shaped conjugate faults’ model. Including this major fault is thus now essential in models of Tibetan Plateau deformation.