Publisher: Geological Society of America   (Total: 4 journals)   [Sort by number of followers]

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Bulletin of the Geological Society of America     Hybrid Journal   (Followers: 34, SJR: 2.329, CiteScore: 4)
Geology     Full-text available via subscription   (Followers: 56, SJR: 3.114, CiteScore: 4)
Geosphere     Open Access   (Followers: 2, SJR: 1.752, CiteScore: 3)
Lithosphere     Open Access   (Followers: 2, SJR: 1.892, CiteScore: 3)
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Geology
Journal Prestige (SJR): 3.114
Citation Impact (citeScore): 4
Number of Followers: 56  
 
  Full-text available via subscription Subscription journal
ISSN (Print) 0091-7613 - ISSN (Online) 1943-2682
Published by Geological Society of America Homepage  [4 journals]
  • Could the Réunion plume have thinned the Indian craton' —
           REPLY

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      Authors: Paul J; Ghosh A.
      PubDate: Sun, 01 May 2022 00:00:00 GMT
       
  • Could the Réunion plume have thinned the Indian craton' —
           COMMENT

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      Authors: Yang G.
      PubDate: Sun, 01 May 2022 00:00:00 GMT
       
  • Widespread glasses generated by cometary fireballs during the late
           Pleistocene in the Atacama Desert, Chile: REPLY

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      Authors: Schultz PH; Harris R, Perroud S, et al.
      PubDate: Sun, 01 May 2022 00:00:00 GMT
       
  • Widespread glasses generated by cometary fireballs during the late
           Pleistocene in the Atacama Desert, Chile: COMMENT

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      Authors: Roperch P; Gattacceca J, Valenzuela M, et al.
      PubDate: Sun, 01 May 2022 00:00:00 GMT
       
  • Geology 's Most-Cited Papers

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      Abstract: The most-cited Geology paper published in the journal's 50 years, with nearly 2,500 citations, is one by P. Tapponnier and others, published in 1982. Below are the top 20 articles11. Details for the top 50 articles and a list of the top-cited papers by decade are in the Supplemental Material22. Thanks go to Lisa G. Dunn, Arthur Lakes Library, Colorado School of Mines, for providing the report.
      PubDate: Sun, 01 May 2022 00:00:00 GMT
       
  • Early Earth zircons formed in residual granitic melts produced by tonalite
           differentiation: COMMENT

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      Authors: Ge R.
      PubDate: Sun, 01 May 2022 00:00:00 GMT
       
  • Early Earth zircons formed in residual granitic melts produced by tonalite
           differentiation: REPLY

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      Authors: Laurent O; Moyen J, Wotzlaw J, et al.
      PubDate: Sun, 01 May 2022 00:00:00 GMT
       
  • Quantifying large-scale continental shelf margin growth and dynamics
           across middle-Cretaceous Arctic Alaska with detrital zircon U-Pb dating

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      Authors: Lease RO; Houseknecht DW, Kylander-Clark AC.
      Abstract: AbstractSequence stratigraphy provides a unifying framework for integrating diverse observations to interpret sedimentary basin evolution; however, key time assumptions about stratigraphic elements spanning hundreds of kilometers are rarely quantified. We integrate new detrital zircon U-Pb (DZ) dates from 28 samples with seismic mapping to establish a chronostratigraphic framework across 800 km and ~20 m.y. for the middle-Cretaceous Torok-Nanushuk clinothem of Arctic Alaska (USA). Shelf-margin DZ dates indicate continent-scale sediment routing with Russian Chukotka provenance and provide reliable maximum depositional ages derived from arc volcanism. Shelf-margin advance rates display a clear relationship to toplap trajectories and provide empirical support for long-held inferences linking sediment supply to margin architecture. Two distinct shelf-margin growth regimes are evident: (1) a ca. 115–107 Ma phase of rapid ~50 km/m.y. shelf advance rates with mainly progradational trajectories; and (2) a ca. 107–98 Ma phase of moderate ~13 km/m.y. shelf advance rates with progradational-retrogradational-aggradational trajectories. We established a subsequent shelf–to–deep water correlation by independently dating ca. 98–95 Ma low shelf accommodation and basin-floor deposition as far as 240 km east that indicate lowstand shedding and a change to localized routing with Brooks Range provenance. Finally, we dated a ca. 95 Ma basin-wide transgression at deep-water to shelfal settings across 350 km that exhibits apparent synchroneity consistent with an event-significant surface. In one of the world's largest foreland-basin clinothems, our work constrains the timing and duration of key depositional elements to test large-scale sequence stratigraphic assumptions, enables reliable correlation and quantification of sediment dynamics across 800 km, and captures the chronology of a giant regressive-transgressive cycle.
      PubDate: Tue, 08 Mar 2022 00:00:00 GMT
       
  • Shallow distributed faulting in the Imperial Valley, California, USA

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      Authors: Sahakian VJ; Derosier BJ, Rockwell TK, et al.
      Abstract: AbstractIn the tectonically complex Imperial Valley, California (USA), the Imperial fault (IF) is often considered to be the primary fault at the U.S.-Mexico border; however, its strain partitioning and interactions with other faults are not well understood. Despite inferred evidence of other major faults (e.g., seismicity), it is difficult to obtain a holistic view of this system due to anthropogenic surface modifications. To better define the structural configuration of the plate-boundary strain in this region, we collected high-resolution shallow seismic imaging data in the All American Canal, crossing the Imperial, Dixieland, and Michoacán faults. These data image shallow (<25 m) structures on and near the mapped trace of the Imperial fault, as well as the Michoacán fault and adjacent stepover. Integration of our data with nearby terrestrial cores provides age constraints on Imperial fault deformation. These data suggest that the Michoacán fault, unmapped in the United States, is active and likely produces dynamic or off-fault deformation within its stepover to the Dixieland fault. Together, these data support more strain partitioning than previously documented in this region.
      PubDate: Tue, 08 Mar 2022 00:00:00 GMT
       
  • Hot atmospheric formation of carbonate accretionary lapilli at the
           Cretaceous-Paleogene boundary, Brazos River, Texas, from clumped isotope
           thermometry

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      Authors: Burtt DG; Henkes GA, Yancey TE, et al.
      Abstract: AbstractThe Chicxulub impact (in the northern Yucatan Penninsula, Mexico) marks the Cretaceous-Paleogene (K-Pg) boundary and is implicated in one of the five major extinctions. Researchers have examined ejecta from the Chicxulub impact, and most recently a drill core from the crater itself, yet the processes and chemical reactions occurring in the impact vapor plume are poorly constrained. Rounded carbonate particles, identified as accretionary lapilli, have been found thousands of kilometers from the impact crater and may be a unique record of plume conditions. We present carbon (δ13C), oxygen (δ18O), and clumped (Δ47) isotope ratios of lapilli from the Brazos River, Texas (USA), as well as from foraminifera and a mudstone. Unaltered lapilli δ13C and δ18O values covary, ranging from −9.38‰ to −2.10‰ and from −7.72‰ to −5.36‰, respectively, and they are distinct from mudstones, foraminifera, and secondarily altered lapilli in the same section. Clumped isotope temperatures [T(Δ47)] from the lapilli range from 66 °C to 539 °C and average 155 ± 46 °C (1 standard deviation), with sedimentary and fossil carbonates recording clement, shallow ocean–like T(Δ47). These data are consistent with petrography and hypothesized vapor plume formation, and we argue that the δ13C and δ18O values result from target rock decarbonation. Atmospheric temperatures >100 °C extending >1800 km from the Chicxulub crater imply an uninhabitable zone within seconds to minutes of the impact that was 10× larger in diameter than the crater itself.
      PubDate: Tue, 08 Mar 2022 00:00:00 GMT
       
  • Linking metamorphism and plate boundaries over the past 2 billion years

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      Authors: Liu Y; Mitchell RN, Brown M, et al.
      Abstract: AbstractSince the Jurassic, there has been a clear spatiotemporal correlation between different types of metamorphism and active convergent plate margins. However, the extent to which this relationship extends into the past is poorly understood. We compared paleogeographic reconstructions and inferred plate kinematics with the age and thermobaric ratio (temperature/pressure [T/P]) of metamorphism over the past 2 b.y. The null hypothesis—that there is no spatiotemporal link between inferred plate margins and metamorphism—can be rejected. Low-T/P metamorphism is almost exclusively located near plate margins, whereas intermediate- and high-T/P metamorphism skews toward increasingly greater distances from these margins, consistent with three different tectonic settings: the subduction zone, the mountain belt, and the orogenic hinterland, respectively. However, paleogeographic reconstructions suggest that so-called “paired metamorphic belts” are rare and that high- and low-T/P localities more commonly occur along strike from each other. The observation that bimodal metamorphism is largely a function of distance from the trench and that end-member T/P types rarely occur in the same place can be explained if the style of orogenesis has evolved from hotter to colder, consistent with the abrupt emergence of low-T/P metamorphism in the Cryogenian. The widespread development of high-T/P rocks in orogenic hinterlands in the Proterozoic was followed by the production and efficient exhumation of low-T/P rocks in subduction channels in the Phanerozoic.
      PubDate: Tue, 08 Mar 2022 00:00:00 GMT
       
  • No effect of thermal maturity on the Mo, U, Cd, and Zn isotope
           compositions of Lower Jurassic organic-rich sediments

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      Authors: Dickson AJ; Idiz E, Porcelli D, et al.
      Abstract: AbstractThe isotope ratios of redox-sensitive metals in organic-rich rocks are critical tools for quantifying the timing and severity of deoxygenation and nutrient cycling in Earth's past. The resilience of isotopic data to thermal alteration of the host sediments over millions of years of burial is, however, largely unknown. We present molybdenum, uranium, cadmium, and zinc stable-isotope data from two stratigraphic successions of the same Lower Jurassic Posidonienschiefer unit in the Lower Saxony Basin of northern Germany that were affected by different burial histories. We show that thermal maturity had no effect on the isotopic compositions of these elements but does appear to have increased their concentrations in the rock. The data corroborate the results of laboratory-based maturation studies and provide constraints on the Mo, U, Cd, and Zn isotopic compositions of ca. 182 Ma seawater in the Lower Saxony Basin.
      PubDate: Fri, 25 Feb 2022 00:00:00 GMT
       
  • Zn-, Mg- and O-isotope evidence for the origin of mantle eclogites from
           Roberts Victor kimberlite (Kaapvaal Craton, South Africa)

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      Authors: Huang J; Huang J, Griffin WL, et al.
      Abstract: AbstractWe report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite (Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O isotopes. Type II eclogites are the protoliths of the highly metasomatized Type I. Garnet and clinopyroxene in Type II eclogites have δ66Zn from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed whole-rock δ66Zn is from 0.24‰ to 0.54‰, which is higher than typical mantle values (0.16–0.20‰). Their heavy Zn- and light Mg- and O-isotope compositions (δ26Mg = −1.1‰ to −0.14‰, δ18O = 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The low δ18O may be inherited from parental melts derived from low-δ18O mantle sources like those that produced the Weltevreden komatiites of the Kaapvaal Craton. The high δ66Zn and low δ26Mg reflect diffusion-driven Zn-Mg–isotope exchange between peridotites and the parental melts during their emplacement in the deep lithosphere. Type I eclogites have reconstructed whole-rock δ66Zn from 0.03‰ to 0.43‰ and garnet δ18O from 6‰ to 9.1‰ but show more scatter in inter-mineral Zn-isotope fractionation than Type II, reflecting incomplete equilibration during later metasomatism by carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus suggests that the Roberts Victor eclogites might have crystallized from deep-seated melts at mantle depths.
      PubDate: Fri, 25 Feb 2022 00:00:00 GMT
       
  • Long-lasting viscous drainage of eclogites from the cratonic lithospheric
           mantle after Archean subduction stacking

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      Authors: Wang Z; Kusky TM, Wang L.
      Abstract: AbstractThe origin of early continental lithosphere is enigmatic. Characteristics of eclogitic components in the cratonic lithospheric mantle (CLM) indicate that some CLM was likely constructed by stacking of subducted oceanic lithosphere in the Archean. However, the dynamic process of converting high-density, eclogite-bearing subducted oceanic lithosphere to buoyant CLM remains unclear. We investigate this process through numerical modeling and show that some subducted and stacked eclogites can be segregated into the asthenosphere through an episodic viscous drainage process lasting billions of years. This process increases the chemical buoyancy of the CLM, stabilizes the CLM, and promotes the preservation and redistribution of the eclogites in the CLM, explaining the current status of early subduction relicts in the CLM revealed by geophysical and petrological studies. Our results also demonstrate that the subduction stacking hypothesis does not conflict with the longevity of CLM.
      PubDate: Fri, 25 Feb 2022 00:00:00 GMT
       
  • Decrypting the polymetamorphic record of the Himalaya

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      Authors: Lihter I; Larson KP, Smit MA, et al.
      Abstract: AbstractReconstructions of the tectonometamorphic architectures of orogenic systems rely on petrogenetic indicator minerals, such as garnet, to trace the transport of rocks through space and time. We show the results of new garnet Lu–Hf geochronology and inclusion U-(Th-)Pb geochronology from exhumed midcrustal metamorphic rocks exposed in the Kanchenjunga region of the eastern Nepalese Himalaya. Garnet in three of the five specimens analyzed yielded pre-Himalayan Lu–Hf dates of ca. 292–230 Ma, which contrasts with Himalayan-aged inclusions therein. Garnet in these specimens either grew entirely during the Early Permian opening of the Neo-Tethys Ocean or grew partly during Cambrian–Ordovician orogenesis and partly during the Cenozoic. The remaining two specimens yield Lu–Hf ages of ca. 50 Ma, which are some of the oldest recorded dates for Himalayan metamorphism. The apparent discordance between the geochronological techniques highlights a potential issue with interpretations that rely on single-geochron-method inclusion records. These results further show that some pressure-temperature determinations used in Himalayan models may not be Himalayan in age.
      PubDate: Fri, 25 Feb 2022 00:00:00 GMT
       
  • Tibetan Plateau growth linked to crustal thermal transitions since the
           Miocene

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      Authors: Zhang X; Wang Q, Wyman D, et al.
      Abstract: AbstractThe topographic transition of central–northern Tibet since the early Miocene has created a consistently high and flat plateau similar to that of today. However, to date, the associated deep crust and/or /mantle events are poorly understood, mainly due to an early Miocene metamorphic–magmatic lull within the Qiangtang Block. To address this issue, we undertook a study of crustal xenoliths and zircon xenocrysts in 6.0–2.3 Ma lavas in the Qiangtang Block. The occurrence of 22.6–12.9 Ma high-temperature–low-pressure granulite xenoliths implies that the middle crust of the block has been very hot since that time. Zircon xenocrysts and granitic xenoliths from 6.0–2.3 Ma lavas were studied and shown to have high δ18O values, which supports Miocene crustal melting and the formation of unexposed, coeval felsic plutons. Combined with paleoelevation data from the Tibetan Plateau, our results suggest that the early Miocene cold–hot thermal transition of the middle–lower crust was near-synchronous with topographic evolution from high-relief mountains to a flat plateau, which supports crustal flow as the main topographic smoothing mechanism for central–northern Tibet.
      PubDate: Fri, 25 Feb 2022 00:00:00 GMT
       
  • The continent-to-ocean transition in the Iberia Abyssal Plain

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      Authors: Grevemeyer I; Ranero CR, Papenberg C, et al.
      Abstract: AbstractConceptual models of magma-poor rifting are strongly based on studies of the nature of the basement in the continent-to-ocean transition of the Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended continental crust. Yet, basement has only been sampled at a few sites, and its regional nature and the transition to seafloor spreading inferred from relatively low-resolution geophysical data are inadequately constrained. This uncertainty has led to a debate about the subcontinental or seafloor-spreading origin of exhumed mantle and the rift-related or oceanic nature of magmatic crust causing the magnetic J anomaly. Different interpretations change the locus of break-up by >100 km and lead to debate of the causative processes. We present the tomographic velocity structure along a 360-km-long seismic profile centered at the J anomaly in the Iberia Abyssal Plain. Rather than delineating an excessive outpouring of magma, the J anomaly occurs over subdued basement. Furthermore, its thin crust shows the characteristic layering of oceanic crust and is juxtaposed to exhumed mantle, marking the onset of magma-starved seafloor spreading, which yields the westward limit of an ~160-km-wide continent–ocean transition zone where continental mantle has been unroofed. This zone is profoundly asymmetric with respect to its conjugate margin, suggesting that the majority of mantle exhumation occurs off Iberia. Because the J anomaly is related to the final break-up and emplacement of oceanic crust, it neither represents synrift magmatism nor defines an isochron, and hence it poorly constrains plate tectonic reconstructions.
      PubDate: Fri, 25 Feb 2022 00:00:00 GMT
       
  • Oligocene onset of uplift and inversion of the Cascadia forearc basin,
           southern Oregon Coast Range, USA

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      Authors: Darin MH; Armentrout JM, Dorsey RJ.
      Abstract: AbstractAn extensive detrital zircon U-Pb data set (n = 6324 dates) from Eocene to Miocene sandstones and modern river sands establishes the onset of arc magmatism and forearc uplift along the Cascadia convergent margin in southwestern Oregon (United States). Middle to late Eocene marine strata in the Coos Bay area were primarily sourced from the Klamath Mountains and coeval Clarno-Challis volcanoes in central Oregon and/or Idaho. Ancestral Cascades arc magmatism initiated at 40 Ma and supplied sediment to a broad forearc basin in western Oregon during late Eocene time. Major reduction of Ancestral Cascades arc (40–12 Ma) and Clarno-Challis (52–40 Ma) zircon in the Tunnel Point Sandstone (ca. 33–30 Ma) records the isolation of the Coos Bay area from the Ancestral Cascades arc due to Oligocene onset of forearc uplift, basin inversion, and emergence of the southern Oregon Coast Range. The Tarheel formation (ca. 18–15 Ma) is characterized by disappearance of Ancestral Cascades arc zircon and a substantial increase in Clarno-Challis zircon recycled from underlying forearc strata. The ~15–20 m.y. delay between subduction initiation (ca. 49–46 Ma) and the onset of forearc uplift (ca. 33–30 Ma) supports insights from thermomechanical models that identify tectonic underplating and thermally activated lower-crustal flow as major drivers of deformation and uplift in active forearc regions.
      PubDate: Fri, 25 Feb 2022 00:00:00 GMT
       
  • Giant sheath-folded nappe stack demonstrates extreme subhorizontal shear
           strain in an Archean orogen

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      Authors: Zhong Y; Kusky TM, Wang L.
      Abstract: AbstractGiant sheath-folded nappes are associated with suture zones and emplacement of far-traveled allochthons in Phanerozoic orogens, demonstrating a rare but significant geologic phenomenon indicative of modern-style plate tectonics. We document the world's oldest-known subhorizontal mega-scale sheath fold from Archean Alpine-style nappes of the Central orogenic belt, North China craton. The Zanhuang nappes are recumbent Alpine-style forearc-affinity metabasaltic and metasedimentary nappes emplaced over a passive continental margin in the Archean, marking an ancient suture zone. Field evidence shows multiscale sheath folds from decimeters to tens of meters in size, and our three-dimensional fence profile, fold hinges, kinematic lineations, and lithological traces define an ~1-km-long (parallel to the x-axis) sheath fold in the core of the nappe stack. Structural analysis statistically demonstrates the macro-scale recumbent sheath-folded nappe preserves a complete 180° hinge-line curvature. The giant sheath fold plunges northwest, reflecting its formation during non-coaxial, top-to-the-southeast shearing with extremely high shear strain (γ ≥10), equated to >10 km of ductile slip on the bounding surfaces. Slip vectors derived from S-C fabrics on overturned limbs are consistent with rotation into the southeast-directed transport direction, parallel to the similarly rotated fold hinges. Comparison of the giant sheath-folded nappes from the Archean Zanhuang example with mega-scale sheath folds in Phanerozoic and Proterozoic orogens shows that Neoarchean lithosphere was stiff enough to allow tectonics to operate in a manner analogous to modern-style plate tectonics.
      PubDate: Fri, 25 Feb 2022 00:00:00 GMT
       
  • Implications for Ediacaran biological evolution from the ca. 602 Ma
           Lantian biota in China

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      Authors: Yang C; Li Y, Selby D, et al.
      Abstract: AbstractThe morphologically differentiated benthic macrofossils of algae and putative animal affinities of the Lantian biota in China represents the oldest known Ediacaran macroscopic eukaryotic assemblage. Although the biota provides remarkable insights into the early evolution of complex macroeukaryotes in the Ediacaran, the uncertainty in its age has hampered any robust biological evaluation. We resolve this issue by applying a petrographic-guided rhenium-osmium (Re-Os) organic-bearing sedimentary unit study on the Lantian biota. This work confines a minimum age for the first appearance of the Lantian biota to 602 ± 7 Ma (2σ, including decay constant uncertainty). This new Re-Os date confirms that the Lantian biota is of early–mid Ediacaran age and temporally distinct from the typical Ediacaran macrobiotas. Our results indicate that the differentiation and radiation of macroscopic eukaryotes, and the evolution of the primitive, erect epibenthic ecosystem, occurred in the early–mid Ediacaran and were associated with highly fluctuating oceanic redox conditions. The radiogenic initial 187Os/188Os ratios derived from the Lantian (1.14 ± 0.02) and other Ediacaran shales invoke oxidative weathering of upper continental crust in the early–middle Ediacaran, which may have stimulated the evolution of life and oceanic-atmospheric oxygenation. Integrated with published Ediacaran chronological and geochemical data, our new Re-Os geochemical study of the Lantian black shale provides a refined, time-calibrated record of environment and eukaryote evolution during the Ediacaran.
      PubDate: Fri, 11 Feb 2022 00:00:00 GMT
       
  • Boron proxies record paleosalinity variation in the North American
           Midcontinent Sea in response to Carboniferous glacio-eustasy

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      Authors: Wei W; Yu W, Algeo TJ, et al.
      Abstract: AbstractSalinity variation in ancient epicontinental seas has long been challenging to reconstruct. We determined salinity variation in the Late Pennsylvanian North American Midcontinent Sea based on normalized boron concentrations (B/Ga) and isotopic compositions (δ11B) of bulk shale. We analyzed the Stark Shale Member of the Missourian Stage Dennis Limestone cyclothem in the Charleston core (CC) from the Illinois Basin (eastward, more proximal) and the Iowa Riverton core (IRC) from the Midcontinent Shelf (westward, more distal). Both cores exhibited the same pattern of secular variation, with lower B/Ga and δ11B values at their base and top, recording less saline conditions during lower sea-level stands, and higher values in their middle part, recording more saline conditions at highstands—a pattern that conforms to conventional sequence stratigraphic and glacio-eustatic interpretations of Midcontinent cyclothems. At equivalent stratigraphic levels, B/Ga and δ11B values are systematically lower at CC relative to IRC, reflecting the generally lower salinity conditions of the Illinois Basin relative to the Midcontinent shelf, which is consistent with greater freshwater influence from the east due to heavy river runoff into the Appalachian foreland basin. Our findings serve to demonstrate the utility of paired B concentration and isotope data for paleosalinity analysis of deep-time shale formations.
      PubDate: Fri, 11 Feb 2022 00:00:00 GMT
       
  • Ultrahigh-temperature granites and a curious thermal eye in the
           post-collisional South Bohemian batholith of the Variscan orogenic belt
           (Europe)

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      Authors: Finger F; Schiller D, Lindner M, et al.
      Abstract: AbstractComprehensive zircon thermometry that takes into account zircon saturation temperatures, Ti-in-zircon measurements, and zircon morphologies and microstructures can provide key information on the thermal evolution of a granite batholith. The Variscan South Bohemian batholith (Germany, Austria, and Czech Republic) comprises a series of granitoid units that intruded between ca. 330 and ca. 300 Ma. We categorize the granitic rocks according to their emplacement temperature into very low temperature (T) (VLT; <750 °C), low T (LT; 750–800 °C), medium T (MT; 800–850 °C), high T (HT; 850–900 °C), and ultrahigh T (UHT; >900 °C). The first stage of batholith formation (ca. 330–325 Ma) is characterized by LT to MT melting of mainly metasedimentary sources driven by their isothermal exhumation. In turn, ca. 322 Ma HT and UHT granites in the southern half of the batholith reveal an ephemeral thermal anomaly in the subbatholithic crust, which is presumably linked to a hidden mafic intrusion. The HT and UHT granites are weakly peraluminous, high-K, I-type rocks. Although sharing some features with A-type granites such as high Zr and rare earth element contents, they differ from classical A-type granites in being magnesian, not enriched in Ga over Al, and having high Ba and Sr contents. A ring structure of ca. 317 Ma MT and/or LT plutons is observed around the HT and/or UHT granite complex and interpreted as an aftermath of the hotspot event. This study is an example of how deep-crustal hotspots, presumably caused by mantle magmatism, can significantly enhance the effects of decompressional crustal melting in a post-collisional setting.
      PubDate: Fri, 11 Feb 2022 00:00:00 GMT
       
  • Reconciling persistent sub-zero temperatures in the McMurdo Dry Valleys,
           Antarctica, with Neogene dynamic marine ice-sheet fluctuations

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      Authors: Halberstadt AW; Kowalewski DE, DeConto RM.
      Abstract: AbstractIn the Ross Sea sector of Antarctica, periodic large-scale marine ice-sheet fluctuations since the mid-Miocene are recorded by drill core and seismic data, revealing a dynamic ice-sheet response to past increases in temperature and atmospheric CO2. In the adjacent, predominantly ice-free McMurdo Dry Valleys (MDVs), preserved terrestrial landscapes reflect persistent cold conditions and have been interpreted as indicators of a stable polar ice sheet, implying that the Antarctic Ice Sheet was largely insensitive during past warm periods. These disparate data-based perspectives highlight a long-standing debate around the past stability of the Antarctic Ice Sheet, with direct implications for the future ice-sheet response to ongoing climate warming. We reconcile marine records of dynamic ice-sheet behavior and episodic open-marine conditions with nearby ancient terrestrial landscapes recording consistent cold-polar conditions. Coupled ice-sheet and regional climate models nested at a high resolution are used to investigate surface temperatures in the MDVs during past warm periods. We find that high-elevation regions of the MDVs remain below freezing even when ice-free conditions prevail in the nearby Ross Sea. We compare observed landscapes with the spatial extent of modeled persistent cold conditions required for preservation of these ancient features, demonstrating that frozen MDVs landscapes could have coexisted with receded or collapsed ice sheets during past warm periods.
      PubDate: Fri, 11 Feb 2022 00:00:00 GMT
       
  • Final inversion of the Midcontinent Rift during the Rigolet Phase of the
           Grenvillian Orogeny

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      Authors: Hodgin EB; Swanson-Hysell NL, DeGraff JM, et al.
      Abstract: AbstractDespite being a prominent continental-scale feature, the late Mesoproterozoic North American Midcontinent Rift did not result in the break-up of Laurentia, and subsequently underwent structural inversion. The timing of inversion is critical for constraining far-field effects of orogenesis and processes associated with the rift's failure. The Keweenaw fault in northern Michigan (USA) is a major thrust structure associated with rift inversion; it places ca. 1093 Ma rift volcanic rocks atop the post-rift Jacobsville Formation, which is folded in its footwall. Previous detrital zircon (DZ) U-Pb geochronology conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) assigned a ca. 950 Ma maximum age to the Jacobsville Formation and led researchers to interpret its deposition and deformation as postdating the ca. 1090–980 Ma Grenvillian Orogeny. In this study, we reproduced similar DZ dates using LA-ICP-MS and then dated 19 of the youngest DZ grains using high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS). The youngest DZ dated by CA-ID-TIMS at 992.51 ± 0.64 Ma (2σ) redefines the maximum depositional age of the Jacobsville Formation and overlaps with a U-Pb LA-ICP-MS date of 985.5 ± 35.8 Ma (2σ) for late-kinematic calcite veins within the brecciated Keweenaw fault zone. Collectively, these data are interpreted to constrain deposition of the Jacobsville Formation and final rift inversion to have occurred during the 1010–980 Ma Rigolet Phase of the Grenvillian Orogeny, following an earlier phase of Ottawan inversion. Far-field deformation propagated >500 km into the continental interior during the Ottawan and Rigolet phases of the Grenvillian Orogeny.
      PubDate: Fri, 11 Feb 2022 00:00:00 GMT
       
  • Reassessment of ocean paleotemperatures during the Late Ordovician

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      Authors: Barney BB; Grossman EL.
      Abstract: AbstractThe Late Ordovician is a paradox, with a greenhouse climate shifting to an icehouse climate during a time of presumably high CO2 levels. We used clumped isotope (Δ47) microanalysis of fossil brachiopod shells to determine paleotemperatures of North America (Cincinnati Arch) during the Katian (453–443 Ma). Microanalyses of fossil brachiopods yielded a right-skewed distribution of clumped isotope temperatures [T(Δ47)] ranging from 25 °C to 55 °C with a mode of 32 °C and a mean of 36 °C. To test the impact of Δ47 reordering (reequilibration of 13C-18O bonding) at burial temperatures on T(Δ47), we applied a Monte Carlo simulation to a Δ47 reordering model based on the burial history. Because the reordering simulation indicated partial reordering, we used the simulation results to back-calculate T(Δ47) and correct the reordering effect. Correcting for reordering decreased the mean T(Δ47) by only ~1 °C, to 35 °C, and did not change the mode (32 °C). These temperatures are lower than those of previous studies, but still suggest that the Late Ordovician climate of the Cincinnati Arch was warmer than the modern subtropical seas. However, carbonate facies in these strata argue for upwelling of cool water; thus, typical subtropical surface waters may have been as warm as 35 °C approaching the Hirnantian glaciation. Seawater δ18O values derived from back-calculated T(Δ47) and brachiopod δ18O values average −0.3‰ ± 0.6‰ relative to Vienna standard mean ocean water (VSMOW), consistent with the value expected for subtropical surface waters in a greenhouse Earth, assuming a constant seawater-δ18O model.
      PubDate: Fri, 11 Feb 2022 00:00:00 GMT
       
  • What drives Fe depletion in calc-alkaline magma differentiation: Insights
           from Fe isotopes

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      Authors: Du D; Tang M, Li W, et al.
      Abstract: AbstractThe continental crust is strongly depleted in iron relative to mid-oceanic ridge basalt, broadly identical to the calc-alkaline magmas, suggesting that calc-alkaline differentiation is key to continent formation. However, it remains contentious as to what drives Fe depletion during magmatic differentiation in the crust. The two competing hypotheses for calc-alkaline differentiation—magnetite versus garnet (± amphibole) fractionation—predict contrasting Fe isotopic fractionation pathways in evolved melts because magnetite preferentially depletes ferric, isotopically heavy Fe whereas garnet (± amphibole) does the opposite. We report whole-rock Fe isotope data for two suites of igneous rocks from the central Andes, which represent magmas traversing normal and thickened arc crust, respectively. The magmas traversing thickened crust show a strong Fe depletion trend and consistently high δ56Fe values (0.14‰ ± 0.02‰, 1 standard deviation [SD]), while those traversing normal crust are less depleted in Fe and show variable δ56Fe values (0.10‰ ± 0.05‰, 1SD). The two Andean suites are both isotopically heavier than Mariana arc (Pacific Ocean) magmas that differentiate along tholeiitic (Fe-enriching) paths. These results confirm that garnet (± amphibole) fractionation/retention is the primary driver of Fe depletion in calc-alkaline magmas, and highlight a role for crustal thickening in generating calc-alkaline magmas.
      PubDate: Fri, 11 Feb 2022 00:00:00 GMT
       
  • Coriolis effect recorded in Late Pleistocene Marine Isotope Stage 5e
           Bahamian aeolianites

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      Authors: Rendall B; Wilson K, Kerans C, et al.
      Abstract: AbstractThe windward islands of the Lucayan Archipelago (Bahamas) form an Atlantic Ocean–facing transect spanning >950 km in length and 6° of latitude. The islands' topography is largely constructed from carbonate wind-blown dunes (i.e., aeolianites) deposited during the interglacial phases of the Late Pleistocene and Holocene. New digital elevation data from satellite radar interferometry (TanDEM-X German Earth observation satellite) enables a step change in the ability to map and quantify Bahamian aeolian landforms across the archipelago. A semi-automated mapping approach that leverages object-based image analysis yields a total aeolianite area of ~1674 km2 across Great Abaco, Eleuthera, Cat, San Salvador, Long, Crooked, Acklins, and Mayaguana islands (Bahamas) and the Turks and Caicos Islands. Longitudinal axis measurements from 747 Pleistocene parabolic dunes record increasing consistency of east-west orientation with decreasing latitude. Three U.S. National Data Buoy Center data buoys provided modern wind direction and velocity measurements (n = 730,933 of each) along this transect. Analysis of wind vectors (>P90 [90th percentile], n = 70,095) demonstrates increasing organization of easterlies at southern latitudes and an offset in directionality compared to formational winds of Pleistocene Marine Isotope Stage (MIS) 5e deposits. Southward trends of increasing wind strength and consistency reflect geostrophic flow driven by atmospheric circulation within the Hadley cell and right-hand deflection of the Coriolis effect in the Northern Hemisphere. We propose that the offset in directionality between dune axes and modern wind vectors is related to changes in latitudinal width of the Hadley cell from the Late Pleistocene (MIS 5e) to today. This data set is robust enough to serve as a benchmark against which future atmospheric circulation models can be compared.
      PubDate: Fri, 11 Feb 2022 00:00:00 GMT
       
  • Non-steady-state slip rates emerge along evolving restraining bends under
           constant loading

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      Authors: Elston H; Cooke M, Hatem A.
      Abstract: AbstractRecent field studies provide evidence of fault slip-rate variability over time periods of 10–100 k.y., yet researchers do not know how processes internal to the fault system (e.g., fault reorganization) impact records of fault slip rates. In this study, we directly observed fault-system evolution and measured slip-rate histories within a scaled physical experiment of a dextral strike-slip 15° restraining bend representative of a gentle crustal restraining bend. To assess the degree of slip-rate variability at particular sites along the experimental faults, such as would be revealed in a field study, we tracked fault slip rates at specific locations that advected throughout the experiment with accrued fault slip. Slip rates increased or decreased (5%–25% of the applied velocity) both during fault reorganization (e.g., fault growth and abandonment) and as sites migrated to new structural positions. Sites that advected into the restraining bend showed decreased slip rate. While we expect new fault growth to reduce slip rates along nearby fault segments, we document that the growth of new oblique-slip faults can increase strike-slip rates on nearby fault segments. New oblique-slip thrust faults within the experiment accommodated off-fault convergence and unclamped nearby strike-slip segments. The experimental results show that even under a constant loading rate, slip rates at sites located on stable fault segments can vary due to either reorganization elsewhere in the fault system or site advection.
      PubDate: Thu, 03 Feb 2022 00:00:00 GMT
       
 
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