Geochemistry Geophysics Geosystems最新文献

Using bathymetry and ROV dives, we investigate two successive flip-flop detachment faults (D1 active, D2 older) in the near-amagmatic 64°35′E region of the SWIR. Kilometer-sized benches on the upper slopes of D1 footwall form the D1 degraded breakaway. Scarps at the top expose the D2 fault zone with deformed serpentinized peridotite, sigmoidal phacoids, planar fractures, and serpentinite microbreccia/gouge horizons. Two ROV sections of the D1 footwall show contrasting deformation styles, corresponding to distinct morphological domains, which relate to contrasting fault and footwall strength. One section documents corrugations, outcrops dominated by sigmoidal phacoids, and planar fractures with thin, discontinuous serpentinite microbreccia/gouge horizons. ROV dives in this corrugated domain show that NNE-trending km-spaced ridges and WNW-trending narrow benches in the shipboard bathymetry correspond, respectively, to broad undulations (mega-corrugations) of the D1 fault and to several antithetic minor normal faults (cumulated horizontal offset of ∼285 m). The other section, lacking corrugations, broad ridges, and antithetic fault, has thicker and more continuous serpentinite microbreccia/gouge horizons, indicating a weaker fault. The abundance of such weak gouges probably reflects hydrous fluid availability during deformation. We link mega-corrugations in the western domain and km-scale lobes of D1 emergence to a broad detachment damage zone with up to ∼600 m-thick mega-phacoids of less deformed serpentinized peridotite. Small antithetic normal faults in the corrugated domain are interpreted as due to bending forces in the D1 footwall. Our findings highlight the three-dimensional, non-planar structural and morphological variability of the exhumed D1 detachment fault zone along the ridge-axis.

The Chilean margin is one of the Earth's tectonically most active plate boundaries, and yet, some of its segments are still underexplored. Here, we present amphibious data from the Copiapó region at ∼27°S located within the mature Atacama seismic gap. Combined 2D seismic refraction, multibeam bathymetry, and local seismicity data show a typical oceanic crust thickness of 6–7 km and seismic P-wave velocities between 3.0 and 7.3 km/s with slightly lower velocities and increased thicknesses underneath the Copiapó Ridge seamounts. The latter is most likely due to predominantly extrusive formation. Elevated velocities underneath one of the seamounts indicate a local region of magmatic underplating, while bending-related faults visible in the bathymetry and reduced mantle velocities near the trench suggest mantle hydration. The subduction angle of the down-going Nazca plate smoothly increases from 12° below the marine forearc to 22° at greater depths (40–60 km) with no abrupt change in the dip angle as observed at ∼22°S. The local seismicity off- and onshore Copiapó shows three separated bands of earthquakes sub-parallel to the down-going plate, and are most likely related to the plate interface, the oceanic Moho and the Double Benioff Zone. The largest event (M_W 5.9) during our observation period (December 2022–June 2023) and its aftershocks occurred in the deepest band ∼20 km below the subduction interface. Along the interface, seismicity is most pronounced in areas of high locking offshore, whereas areas of low locking are characterized by previously observed slow slip events and sparse seismicity.

Our understanding of the geological evolution of mid-ocean ridges in response to tectonic reconfigurations and associated mantle processes is hampered by a lack of exploration in off-axis areas. A notable exception is the Reykjanes Ridge, where multibeam bathymetry, magnetics, and gravity surveys have been conducted up to ∼150 km from the ridge axis. Previous work shows that the ridge has undergone a major reorganization following changes in spreading direction, resulting in the progressive formation and then elimination of transform faults from north to south under the influence of the regional mantle melting anomaly. Notably, this process is incomplete near the southern termination of the ridge, providing a window into the processes of crustal accretion and segmentation prior to and immediately following this reorganization. Here, we employ remote-predictive geological and structural mapping methods linked to chrono-magnetic data to elucidate changes in segment morphology, magma supply, and structural fabrics along the southern ∼200 km of the ridge over the past ∼11 M.y. We identify two new fracture zones and three new non-transform discontinuities, with elimination of transform motion occurring between ∼9.7 and 4.2 Ma, which is later than previously thought. Transform elimination coincides with rift propagation and the emergence of a new magmatically robust segment at ∼58°N at ∼9.7–8.2 Ma. This transition is also associated with a reorientation of seafloor fabric from dominantly N-trending to NE-trending, associated with the dissection of axial volcanic ridges by the oblique (NE-trending) plate boundary, resulting in more crustal accretion to the North American plate overall.

Gravity data provide constraints on lateral subsurface density variations and thus provide crucial insights into the geological evolution of the region. Previously, gravity data from the Norwegian Arctic archipelago of Svalbard comprised an onshore regional gravity database with coarse station spacing of 2–20 km, offshore gravity profiles acquired in some fjords, airborne gravity, and satellite altimetry. The sparse regional point-based onshore coverage hampered the direct integration of gravity data with seismic profiles acquired onshore Svalbard in the late 1980s and early 1990s. In April 2022, we acquired gravity data at 260 new stations along seven profiles from western to eastern Spitsbergen, with a cumulative length of 329 km. The profiles were acquired directly along selected seismic profiles and provide much closer station spacing (0.5–2 km) compared to the regional inland grid (2–20 km) acquired in the late 1980s (total number of onshore stations: 1,037). Having processed the data, we compared the first-order density trends of our new data with the legacy regional grid. The new gravity data are consistent with the regional data, imaging a gravity low in the western part of the area underlying a foreland basin and a gravity high in the northwestern part of the area likely associated with a basement high or denser basement. We compare the new and vintage gravity using maps and profiles, linked to the known major tectonic features such as major basinal axes and fault zones, as well as other geophysical data sets including seismics and magnetics.

Oceanic island basalts are the products of mantle plume melting and their chemistry provides insights into the Earth's deep interior. We report a statistical and machine learning study of 8 radiogenic isotopes and 19 incompatible trace element ratios in basalts from 27 oceanic island volcanic chains associated with mantle plumes compiled from the GEOROC and EARTHCHEM-PetDB databases. Machine-learning hierarchical analysis and agglomerative clustering results based on t-distributed stochastic neighbor embedding (t-SNE) reveal distinct clusters of isotopic compositions corresponding to canonical ones of HIMU, EM I, EM II, PREMA. and DM as well as a LOND cluster, which however do not reflect the existence of discrete components. The HIMU clan is restricted to only a couple of plumes and is characterized by low K/U, Pb/Ce, Ba/Nb and strong REE fractionation. EM I has higher K/U, Ce/Rb, Ba/Nb and lower U/Pb, Rb/Ba, Rb/Sr than EM II reflecting a difference in prevalent recycled components. Stepwise multiple regression reveals that fractionations of incompatible element ratios can be explained by variations in partial melting controlled by lithospheric thickness and plume buoyancy flux; the latter indicates that buoyancy flux primarily reflects plume temperature. ³He/⁴He also correlates with plume buoyancy flux, suggesting that the hottest plumes carry the least radiogenic He. The hottest plumes may be those rising from the core-mantle boundary. This, and the absence of evidence of a primordial mantle reservoir suggest that unradiogenic He may be derived from the core.

The degree of organic sulfurization is broadly relevant yet underreported. We present a statistically significant correlation between whole rock S/Fe and the measured degree of organic sulfurization in the thermally immature Cenomanian–Turonian Eagle Ford Group. This relationship shows a sink switch for sulfur from pyrite to organic matter. Excess iron and excess sulfur relative to pyrite, which are mathematically related to S/Fe, provide better insights into organic sulfurization than previous approaches that calculate excess iron relative to detrital iron based on aluminum concentrations. Organic sulfurization and S/Fe are tightly coupled in the Eagle Ford partially due to limited sulfur- and iron-bearing components. Similar relationships could exist in other thermally immature, organic-rich, anoxia-prone, calcareous mudstones. The degree of organic sulfurization was estimated from S/Fe, which was used to map stratigraphic and regional variations of organic sulfurization across the Eagle Ford and to investigate how organic sulfurization relates to organic enrichment, organic preservation, and depositional redox chemistry. The extent of organic sulfurization is more tightly linked to organic preservation than enrichment. Together, organic sulfurization and Mo provide concordant evidence for depositional euxinia. The relationship between Mo and degree of organic sulfurization could indicate that sulfurized organic matter provides a pathway for Mo enrichment, but future work needs to disentangle direct mechanisms from indirect covariations between Mo, organic content, and degree of organic sulfurization. Whole rock elemental chemistry and programmed pyrolysis provide insights into organic sulfurization variations that can be upscaled and can guide subsequent detailed organic sulfur analyses.

The relationships between the serpentinized continental mantle in orogens, its geophysical signature at depth and hydrogen seepages are poorly understood. A petro-physical modeling approach accounting for serpentinization shows that a large domain of serpentinized mantle (1,800 km²) is present in the northern Pyrenees. The serpentinization reached a maximum of 40% during the mid-Cretaceous rifting, according to the predicted temperature and pressure. Although high-temperature serpentinization could have generated large quantify of hydrogen during the Mesozoic, the shallow and inactive faulting in Northern Pyrenees make this process unlikely to explain the entire serpentinization inferred by petro-physical modeling. A combination of low-temperature alteration of mafic and ultramafic rocks in the North Pyrenean Zone, active normal faulting in the North Pyrenean Fault, accumulation in local traps and transport of H₂-rich fluids along inactive but permeable fault may explain the hydrogen seepages observed today.

Cosmogenic nuclide dating is an essential component of studying Earth surface processes, but it requires knowledge of how nuclide production rates vary in time and space. Typically, production rates are calibrated at sites with independently well-constrained exposure histories and then scaled to other sites of interest using scaling frameworks that account for spatial and temporal variations in the secondary cosmic-ray flux at Earth's surface. To date, scaling schemes for terrestrial cosmogenic nuclide production rates have been developed for the Quaternary, yet cosmogenic nuclide applications that extend beyond the Quaternary are becoming more prevalent. For these deeper time applications, production rate calculations using scaling models optimized for the latest Quaternary neglect longer term spatiotemporal variations in geomagnetic field intensity, paleogeography, and paleoatmospheric depth. We present a production rate scaling scheme for the past 70 million years, SPRITE (Scaling Production Rates In deep TimE). This framework extends existing scaling schemes into deeper time by (a) accounting for site-specific changes in paleolatitude, (b) integrating a geomagnetic field intensity model rooted in data from a global paleomagnetic database, and (c) incorporating climate-driven, time-varying atmospheric depths. We evaluate the efficacy of our model by applying it to existing data sets from paleoexposure sites, and from sites with apparent continuous million-year exposure histories. This scaling model can be applied with measurements of stable cosmogenic nuclides to research questions such as constraining hiatus durations between ancient lava flows and calculating the formation timescales of stable landforms in arid environments over millions of years.

In volcanic arcs, magma evolves from basaltic to intermediate and felsic composition, resulting in arc crust maturation. It remains unclear whether processes involving mush during magmatic flare-ups would enhance this evolution. This study revealed a temporal-compositional evolution of plutonic rocks from mafic (∼94 Ma) to intermediate (∼92–88 Ma) to felsic (∼88 Ma) during a magmatic flare-up event in the Gangdese arc, Tibet, with increasing radiogenic Sr–Nd isotope enrichment. Apatites in mafic and felsic rocks have ε_Nd(t) values similar to their hosts, while intermediate rocks show higher values. The elemental composition of apatites in mafic and intermediate rocks is similar but differs from those in felsic rocks. Textural and compositional features indicate varying degrees of influence of mafic rock compositions by accumulation. Triangular and linear covariation relationships between apatite-compatible (e.g., La) and -incompatible (e.g., Rb) elements with SiO₂, respectively, for all plutonic rocks as a whole, confirm the incorporation of apatite-rich mushes into the mixing process. These findings suggest that mafic magma crystallized into apatite-rich mush, which was later remobilized and mixed with felsic magma to form intermediate magma. Felsic rocks represent end-member magmas resulting from crustal anatexis and/or mafic magma differentiation. Thus, the Gangdese arc's maturation during the magmatic flare-up progressed sequentially through mafic magma crystallization and mush formation, mush remobilization and mixing with felsic magma, and the eventual accumulation and segregation of felsic magma. This sequence of events during flare-ups illustrates a common crustal maturation process in volcanic arcs, as also seen in the Andean Cordillera.

Lower crustal metasedimentary xenoliths (garnet-sillimanite granulites) from the Bournac breccia pipe in the Massif Central, France, provide a robust example of sediments transported to depth and incorporated into stable lower continental crust during a collisional orogeny. Dates for detrital igneous zircon range from the Archean (up to 3,300 Ma) to the Devonian and record sedimentation prior to the onset of the collisional phase of the Variscan orogeny. Metamorphic zircon and monazite document the presence of the metasediments in the lower crust by ca. 330 Ma during the later phase of Variscan collision. Zircon and monazite crystallization continued within the lower crust until ca. 265 Ma, corresponding to a period of slow cooling following an episode of ultra-high temperature (UHT) metamorphism that peaked at 313 Ma. Zr-in-rutile thermometry and GASP barometry applied to these samples record conditions of 0.63–0.77 GPa and 830–910°C, which correspond to Ti-in-zircon temperatures from the latter part of the Variscan orogeny and geotherms in excess of typical continent-continent collisions. Rutile in these samples remained open to Pb loss until their eruption at ca. 11.6 Ma, providing an indirect date of the Bournac eruption. These rocks record the incorporation of felsic sedimentary material into the stable deep continental crust during a collisional orogen and their residence there for over 300 Ma. More broadly, the addition of sediments to stable lower crust contributes to changes in crustal composition and has significant implications for the heterogeneity of the deep continental crust, as well as overall crustal heat production and mantle heat flow.