Geochemistry Geophysics Geosystems最新文献

Zircon grains from the metasedimentary lower crust of the Rio Grande Rift, New Mexico, preserve a metamorphic record of the transition from Laramide compression to Eocene extension. Zircon U-Pb isotopes and trace-element concentrations from five two-pyroxene metaigneous granulite xenoliths define discrete populations: older zircon cores (∼15–50 Ma) that are depleted in heavy rare-earth elements (HREE) but Ti-rich, and younger zircon rims (∼3–15 Ma) with elevated HREE and lower Ti concentrations. Coupled phase equilibria and garnet-melt-zircon trace-element partitioning calculations show that the older zircon cores equilibrated in thick (>40 km), hot (800–900°C), garnet-bearing lower crust during the cessation of compression at the end of the Laramide orogeny. Zircon rim domains equilibrated at lower pressures, consistent with >9 km of thinning of the lower crust. Thermal-kinematic calculations show that these pressure-temperature-time constraints require thinning of the lithospheric mantle prior to and during regional Cenozoic extension. Convective erosion of the mantle lithosphere over tens of millions of years, possibly facilitated by dynamics of the Farallon slab, provides a mechanism to facilitate lower crustal heating and extension.

Ratios of glycerol dialkyl glycerol tetraethers (GDGT), which are membrane lipids of bacteria and archaea, are at the base of several paleoenvironmental proxies. They are frequently applied to soils as well as lake- and marine sediments to generate records of past temperature and soil pH. To derive meaningful environmental information from these reconstructions, high analytical reproducibility is required. Based on submitted results by 39 laboratories from across the world, which employ a diverse range of analytical and quantification methods, we explored the reproducibility of brGDGT-based proxies (MBT′_5ME, IR, and #rings_tetra) measured on four soil samples and four soil lipid extracts. Correct identification and integration of 5- and 6-methyl brGDGTs is a prerequisite for the robust calculation of proxy values, but this can be challenging as indicated by the large inter-interlaboratory variation. The exclusion of statistical outliers improves the reproducibility, where the remaining uncertainty translates into a temperature offset from median proxy values of 0.3–0.9°C and a pH offset of 0.05–0.3. There is no apparent systematic impact of the extraction method and sample preparation steps on the brGDGT ratios. Although reported GDGT concentrations are generally consistent within laboratories, they vary greatly between laboratories. This large variability in brGDGT quantification may relate to variations in ionization efficiency or specific mass spectrometer settings possibly impacting the response of brGDGTs masses relative to that of the internal standard used. While ratio values of GDGT are generally comparable, quantities can currently not be compared between laboratories.

We document an apparent downward displacement of the Matuyama-Brunhes magnetic reversal by ∼20 m at Scotia Sea International Ocean Discovery Program Site U1538 (Pirie Basin) by comparison with the well-defined paleomagnetic record at nearby Site U1537 (Dove Basin). Detailed stratigraphic correlation between the two sites is possible due to similar lithologic variations. However, the two sites have distinctly different porewater geochemistry. Notably, Site U1538 indicates a greater demand for electron acceptors to oxidize organic carbon and Fe²⁺ enrichment below the depth of SO₄²⁻ depletion. Magnetic parameters indicate enrichment of an authigenic magnetic mineral with strong remanence properties around the depth of SO₄²⁻ depletion (∼46 m at Site U1538) relative to magnetic parameters at correlative depths at Site U1537. Fe²⁺ enrichment below the depth of SO₄²⁻ depletion is not predicted based on the energetically favorable order of electron acceptors for microbial respiration but is documented here and in other depositional settings. This indicates Fe²⁺ production exceeds the production of H₂S by SO₄²⁻ reduction, providing a geochemical environment that favors the production and preservation of ferrimagnetic remanence-bearing iron sulfides over paramagnetic pyrite and, thus, a mechanism for deep chemical remanent magnetization acquisition at depths of tens of meters. The influence of authigenic ferrimagnetic iron sulfides on paleomagnetic signals can be difficult to demonstrate with magnetic properties alone; therefore, this finding has implications for evaluating the fidelity of magnetostratigraphic records with complementary geochemical data. Such situations should be considered in other depositional environments with similar porewater Fe²⁺ accumulation below the SO₄²⁻ reduction depth.

The amalgamation of Laurentia was initiated along the western margin of the Rae craton. However, the tectonic setting that generated magmatic rocks along this margin has long been debated, with the Thelon tectonic zone in the north having formed in an arc setting, and the Taltson magmatic zone in the south variably attributed to either continental arc or intracratonic magmatism. The magmatic rocks of the Great Slave Lake shear zone (GSLsz) lie between these two tectonic belts and, thus, may be critical to the interpretation of the evolution of the western Rae margin. To understand the origin of the rocks in the GSLsz, we have applied U-Pb geochronology, trace-element geochemistry, and O and Hf isotope analyses to zircons from a suite of samples that transect the La Loche River fault (LRf)—a major structure that bisects the GSLsz. Samples collected to the north of the LRf are Neoarchean in age, have mantle-like δ¹⁸O (4.7–5.8‰) and chondritic to juvenile εHf values (0–4.5), whereas those to the south are exclusively Paleoproterozoic in age and have more elevated δ¹⁸O (6.3–7‰) and much more evolved εHf values (−12 to −6); these results indicate that the LRf marks a crustal-scale suture between the Slave craton and the Taltson magmatic zone. Our isotopic data, together with other regional constraints from the area, are most consistent with the Taltson magmatic zone having formed in a continental arc setting emplaced into ca. 2.3 Ga juvenile basement crust.

Investigating rock-uplift variations in time and space provides insights into the processes driving mountain-belt evolution. The Apennine Mountains of Italy underwent substantial Quaternary rock uplift that shaped the present-day topography. Here, we present linear river-profile inversions for 28 catchments draining the eastern flank of the Northern-Central Apennines to reconstruct rock-uplift histories. We calibrated these results by estimating an erodibility coefficient (K) from incision rates and catchment-averaged erosion rates obtained from cosmogenic-nuclide data, and we tested whether a uniform or variable K produces a rock-uplift model that satisfactorily fits independent geochronological constraints. We employ a landscape-evolution model to demonstrate that our inversion results are reliable despite substantial seaward lengthening of the catchments during uplift. Our findings suggest that a rock-uplift pulse started around 3.0–2.5 Ma, coinciding with the onset of extension in the Apennines, and migrated southward at a rate of ∼90 km/Myr. The highest reconstructed rock-uplift rates (>1 km/Myr) occur in the region encompassing the highest Apennine massifs. These results are consistent with numerical models and field evidence from other regions exhibiting rapid rock-uplift pulses and uplift migration related to slab break-off. Our results support the hypothesis of break-off of the Adria slab under the central Apennines and its southward propagation during the Quaternary. Moreover, the results suggest a renewed increase in rock-uplift rates after the Middle Pleistocene along the Adriatic coast, coeval with recent uplift acceleration along the eastern coast of southern Italy in the Apulian foreland.

Recently, integrated geophysical-geological surveys in the Nankai subduction zone in Japan have revealed that slow earthquakes repeatedly occur beneath the outer wedge of the forearc. During December 2020 to February 2021, clustered slow earthquakes propagated around the frontal thrust of the accretionary wedge. The frontal thrust ramps up from the basal décollement and slips over trench-filling sediment along the landward edge of the Nankai trough floor. Here, the Paleo-Zenisu ridge has been subducted beneath the inner-outer slope border. In addition, ocean floor topography and geologic structure revealed by seismic reflection surveys completed before 2022 document that the basement of the Philippine Sea Plate beneath the frontal thrust has a seamount and a horst-like basement high. The northern edge of the basement high is located at the ramp-up position of the frontal thrust. The 2020–2021 clustered slow earthquakes started at the Paleo-Zenisu ridge and propagated to the topographic highs beneath the deformation front. Considering that the relative plate convergence between the upper Amurian Plate of the Nankai forearc and the subducting Philippine Sea Plate is ∼6.0 cm/year, the basement high at the deformation front has uplifted the frontal crest of the wedge at an average rate of 2.7–5.7 mm/year for several tens to hundred thousand years. These rates are among some of the highest rock uplift rates measured in the world. The slow earthquakes in the off-Kumano Nankai Trough in 2020–2021 are a snapshot of a “living” Nankai frontal thrust during the megathrust interseismic period.

Cold and diffuse hydrothermal circulation on mid-ocean ridge flanks impacts heat and fluid fluxes between the seafloor and the ocean. One mode of this circulation is given by outcrop-to-outcrop flow, where seawater circulates through a crustal aquifer that connects two or more recharging and discharging seamounts or basement highs that outcrop through the less permeable sediment cover. The physical mechanism driving this flow is a lateral pressure gradient that is sustained by contrasting the hydrological properties of the recharging and discharging outcrops. To investigate the physical controls of this pressure gradient, we performed two-dimensional numerical simulations of coupled heat transfer and fluid flow. We have modified aquifer permeability, outcrop permeability and width, outcrop distance, and sediment thickness to assess their mutual effects on the lateral pressure differences. We have also investigated how different flow patterns, resulting from changes in these parameters, manifest themselves in seafloor observables such as flow rates, aquifer temperatures, and heat flow. Our models show that outcrop-to-outcrop flow generally occurs for aquifer permeabilities ≥10⁻¹⁴ m², depending on the basal heat input. High aquifer permeabilities correspond to fast flow rates and low fluid temperatures, whereas the maximum lateral pressure differences arise for lower permeabilities. The permeability and the geometric shape of the outcrops determine the flow direction, while the aquifer temperature is also affected by the distance between the outcrops. Thicker sediments increase the lateral pressure difference and the flow rate. Our models thus provide constraints for predicting subseafloor hydrothermal ridge flank flow behavior from regional field data.

A summary of the Jurassic-Cretaceous rift to breakup tectonostratigraphy of the onshore Sverdrup Basin is correlated to the offshore Amerasia Basin in order to reconstruct a tectonic setting for the High Arctic Large Igneous Province (HALIP). The rift climax from the Canadian rifted margin is correlated with hyper-extension of the continent-ocean-transition zone. Hyper-extension of the continental lithosphere can accommodate plate motions of Arctic Alaska-Chukotka away from the Canadian Arctic Islands and Lomonosov Ridge between ∼155 Ma and 135–133 Ma. After lithospheric breakup at ∼135–133 Ma, correlation of the post-rift stage to the seafloor spreading anomalies M10n to M4n that are associated with oceanic crustal domains can accommodate plate motions from 135–133 Ma to 128 Ma. The uncertainties associated with the earliest magmas of HALIP overlap with the uncertainties on the timing of the latest seafloor spreading. The first main pulse of HALIP in the Aptian at 124–120 Ma post-dates seafloor spreading and so HALIP was emplaced in a tectonic setting that closely resembles the present state of the south and eastern Amerasia Basin. At the paleogeographic center of the HALIP, the Alpha Ridge complex is consistent with the magmatic character and history of similar Cretaceous oceanic plateau in terms of volume and duration.

The 14 August 2021 Haiti earthquake mainly portrayed reverse motion to the east near L’Asile town and left-lateral strike-slip motion to the west near Camp-Perrin town. To map the rupture and infer its segmentation, we conducted the first post-seismic field reconnaissance along the left-lateral strike-slip Enriquillo fault from L’Asile to Macaya mountain. We identified 98 linear, minor cracks that are not representative of primary fault surface rupture. Analyzing the topographic slope distribution, we detected that the cracks were often located in areas that are prone to topographic instability. About 60% of the cracks are located in Quaternary alluvium and Middle-Miocene continental marls, indicating a preference for soft sediments. The rivers also have an impact, as crack lengths and openings negatively correlate with their distance to neighboring rivers. In addition, the earthquake occurred in a rainy region with up to 2,479.34 mm of rainfall in 2021, increasing soil instability. Above all, we found a contrast and asymmetry between the eastern and the western parts of the rupture. By dividing the 60-km long rupture into two equal parts, we observed 57 cracks to the west against 41 to the east. The longest and the widest cracks are to the west. Analyzing their orientation, the cracks mainly oriented as left-lateral strike-slip faults to the west and mainly thrusts to the east. This configuration appears to be influenced by the slip pattern of the 2021 Haiti earthquake and consistent with the regional stress field.

Recorded earthquake-induced changes in hydrogeological systems date back over 2,000 years. As a part of our ongoing hydrogeochemical monitoring effort to study such changes, we collected 406 groundwater samples twice a week between February 2021 and July 2023 from two bore wells in the Kopili fault zone of Northeast India. We analyzed stable isotope ratios (δ²H, δ¹⁸O) and dissolved element concentrations to obtain a 2.5-year hydrogeochemical time series and responses to multiple regional earthquakes (Mw ≥ 3) within the monitored period. We find significant but transient anomalies in both the chemical and isotopic composition of groundwater at one of the observation wells (OW1) after the 2021 Assam Mw 6.4 earthquake, followed by prolonged alterations in the hydrochemistry at both wells. We do not identify any precursory changes. Using multivariate statistical techniques and analyzing compositional changes before and after the mainshock, we infer that the hydrochemical anomalies at OW1, representing an immediate response to the mainshock, can be attributed to the potential breach of a hydrological barrier. This, in turn, allowed the infiltration of new water into the OW1 aquifer, potentially sourced from the nearby Brahmaputra River. Subsequently, during the post-anomaly period, the earthquake-induced fracturing and the associated changes in permeability sustained a prolonged period of mixing between surface water and groundwater, resulting in newly formed hydrochemistry at both wells. Our findings highlight the dynamic nature of aquifer properties during earthquakes. Long-term continuous evaluation of such changes may provide new insights into feedback between tectonics and fluid flow in the crust.