Lanza, FedericaRoman, Diana C.Power, John A.Thurber, Clifford H....
15页
查看更多>>摘要：On June 15, 2020, at 21:16 UTC, a locally-felt earthquake of magnitude 4.2 struck Unalaska Island, Alaska,similar to 15 km west of the town of Unalaska and the large fishing port of Dutch Harbor. The event was followed by a M4.1 earthquake at 00:34 UTC and several M3+ aftershocks, initiating a prolific sequence with hundreds of earthquakes recorded into late December. The earthquakes all locate about 12 km southeast of the summit of Makushin Volcano at 7 to 10 km depth. To date, no eruptive activity or other surface changes have been observed at the volcano in webcam images, GPS or InSAR. Seismic bursts close to volcanoes are often associated with the onset of unrest that can lead to eruption. However, determining whether seismicity reflects magmatic rather than tectonic stresses is often challenging, although critical for hazard assessments and risk management strategies. To investigate the triggering mechanisms of the recent Makushin seismicity, we integrate information from space-time patterns of the earthquake hypocenters with their fault-plane solutions. We relocate the swarm events using double-difference relocation techniques and a 3D velocity model and find that the earthquakes, although they seem to follow two predominant orientations (NW-SE and SW-NE), do not show clear clustering into preferred alignments. Similarly, we do not observe pronounced migration in time and space. Fault-plane solutions (FPS) for all but one M2.5+ earthquakes have P-axis orientations consistent with subhorizontal NW-SE oriented regional maximum compression, whereas many of the lower-magnitude earthquakes have P-axes perpendicular to regional maximum compression. This provides evidence for the presence of a local stress field likely induced by magma intrusion. Results from Coulomb stress modeling are also consistent with dike inflation modulated by stresses induced by the M4+ earthquakes. The seismic swarm is thus likely linked to a superposition of driving stresses from both magmatic and tectonic processes on pre-existing faults. The case of the 2020 Makushin swarm, with its unusual characteristics, challenges traditional swarm classification schemes and suggests that a reconsideration of the definition of seismic swarms as having the maximum magnitude event in the middle of the swarm is warranted. (c) 2022 The Authors. Published by Elsevier B.V.
查看更多>>摘要：Continental arcs influence the global carbon cycle via degassing and chemical weathering. Several studies also suggested that the isotopic composition of continental arcs plays an important role in controlling the strontium (Sr) isotopic composition of seawater through chemical weathering. Yet, geological or tectonic drivers of isotopic variations of continental arcs on a global scale are missing in previous studies. Here, we compiled two independent global databases for mid-Cretaceous (125-85 Ma) continental arcs. The first database maps the location, geometry, and tectonic nature of the basements upon which the arcs were built. The second compiles Sr-Nd-Hf isotopes data for these continental arcs. We find that isotopically-juvenile arcs form when their basements are generated by tectonic processes such as oceanic terrane accretion, upper plate extension, slab breakoff, and ridge subduction. During the midCretaceous, most continental arcs were fully or partially built on juvenile basements and yielded juvenile isotopes on a global scale. Our compilation reinforces the importance of isotopically-juvenile continental arcs observed in the global Hf isotopes in detrital zircon for the mid-Cretaceous, but also provides the geological context for these trends. Since many of these isotopically-juvenile arcs were located in tropical or warm-temperate regions during the mid-Cretaceous, these uplifted rocks were preferentially eroded and weathered providing juvenile Sr isotopes to the oceans. This study highlights the importance of tectonic processes in controlling isotopic compositions of continental arc, which has implications for ocean chemistry, global chemical weathering feedback, and the long-term carbon cycle. (C)& nbsp;2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：We present a model for terrestrial planet formation by pebble accretion, focusing on core segregation in the early Earth. Our results indicate that if the proto-Earth and the Moon-forming impactor Theia grew by pebble accretion, core-forming metals in each body segregated from mantle-forming silicates within the first few million years of solar system history, while both were enveloped in atmospheres composed of nebular gas. Thermal blanketing by their energy-absorbing atmospheres, heat produced by radioactive decay of aluminum-26, and gravitational energy released by metal segregation resulted in very high internal temperatures, such that the mantle and core of both bodies experienced partial or total melting during accretion. We calculate pressure-temperature conditions where the core-forming metals are predicted to have segregated from magma ocean silicates under pebble accretion. Twobody combinations of these conditions, representing the merger of proto-Earth and Theia, yield average segregation pressures and temperatures that are similar to core segregation conditions previously inferred for impact-driven Earth accretion constrained by metal-silicate partitioning of siderophile elements. (c) 2022 Elsevier B.V. All rights reserved.
查看更多>>摘要：The Ediacaran-Cambrian (E-C) transition witnessed remarkable environmental changes, the extinction of the Ediacaran biota, and subsequent rapid diversification of Cambrian animals. However, the linkages between environmental and biological evolution are still under debate at this critical time. Here, we present new Hg abundance and Hg isotopes in sediments from South China, which are then combined with those published from the Indian craton to explore the co-evolution of environment and complex life during the E-C transition. In both areas, high Hg/TOC ratios and near-zero delta Hg-199 of the Late Ediacaran sediments suggest enhanced volcanism, whereas relatively high Hg/TOC ratios and positive delta Hg-199 shift upsection indicate volcanic-sourced atmospheric Hg(II) deposition in the earliest Cambrian. The dramatically decreasing Hg/TOC ratios and positive delta Hg-199 of early Cambrian Age 2 to 3 sediments indicate scavenging of dissolved seawater Hg(II) by organic matter particulates. Our Hg results suggest volcanism may have likely played a significant role in the extinction of Ediacaran biota and global negative carbon excursions near the E-C boundary. Furthermore, our Hg data provides new evidence of extensive OM burial in the early Cambrian Age 2 to 3 oceans, leading to a rapid increase of Earth-surface O-2 levels that coincided with appearance of more complex large-body animals. Our study provides new insights of the global Hg cycle into the co-evolution of the environment and complex life at this critical time. (C) 2022 Elsevier B.V. All rights reserved.
Elhebiry, Mohamed S.Sultan, MohamedAbotalib, Abotalib Z.Kehew, Alan E....
10页
查看更多>>摘要：Mega-streamlined landforms on Earth and Mars have been attributed to aeolian, glaciogenic, fluvial, and tectonic processes. Identifying the forces that shaped these landforms is paramount for understanding landscape evolution and constraining paleo-climate models and ice sheet reconstructions. In Arabia, eastnortheast trending, kilometer-scale streamlined landforms were interpreted to have been formed by Quaternary aeolian erosion. We provide field and satellite-based evidence for a Late Ordovician glacial origin for these streamlined landforms, which were exhumed during Red Sea-related uplift. Then we use Late Ordovician paleo-topographic data to reconstruct the Late Ordovician ice sheet using identified and previously reported glacial deposits and landforms. Our reconstruction suggests these glacial features are part of a major, topographically controlled, marine-terminating ice stream, with a minimum length of 1000 km extending from the southeast Egypt to northern and central Arabia and possibly more than twice this length if the glaciomarine and iceberg deposits in the present-day western Iran are part of this system. Our results support models that advocate for a single, major, and highly dynamic ice sheet and provide new morphological-based constraints for Late Ordovician climate models. (c) 2022 The Author(s). Published by Elsevier B.V.
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