Boulder, Colo., USA: Article topics and locations include lagoon infilling by coral reef sand aprons as a proxy for carbonate sediment productivity; mobilization of tin during subduction; and reworked pollen reduces apparent floral change during the Paleocene-Eocene Thermal Maximum. These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent .
Fault surface morphology as an indicator for earthquake nucleation potential
Agathe M. Eijsink; James D. Kirkpatrick; François Renard; Matt J. Ikari
Abstract: Laboratory measurements can determine the potential for geologic materials to generate unstable (seismic) slip, but a direct relation between sliding behavior in the laboratory and physical characteristics observable in the field is lacking, especially for the phyllosilicate-rich gouges that are widely observed in natural faults. We integrated laboratory friction experiments with surface topography microscopy and demonstrated a quantitative correlation between frictional slip behavior and fault surface morphology of centimeter-scale samples. Our results show that striated, smooth fault surfaces were formed in experiments that exhibited stable sliding, whereas potentially unstable sliding was associated with rougher, isotropic fault surfaces. We interpret that frictional stability and fault surface morphology are linked via the evolution of asperity contacts on localized slip surfaces. If fault surface roughness obeys a fractal relationship over a large range of length scales, then we infer that the morphological characteristics observed in the laboratory could indicate the earthquake nucleation potential on natural fault surfaces.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50258.1/618283/Fault-surface-morphology-as-an-indicator-for
Raman thermometry and (U-Th)/He thermochronometry reveal Neogene transpressional exhumation in the Nacimiento block of central California, USA
B. Lacroix; A. Lahfid; C. Ward; N.A. Niemi; A.D. Chapman ...
Abstract: We present a novel approach for mapping vertical uplifts in exhumed metasedimentary rocks by coupling Raman spectroscopy of carbonaceous material with (U-Th)/He thermochronometry on apatite and zircon. We apply this approach to carbonaceous metasedimentary rocks of the Franciscan subduction complex, exposed in the Nacimiento block of central California, USA, an area that records high-pressure–low-temperature metamorphism prior to entrainment within the present-day transform plate boundary. We reveal the extent and magnitude of previously unrecognized exhumation gradients, which, combined with regional structural observations, can be used to quantify vertical crustal motion associated with localized transpression. We propose that the Nacimiento block was affected by a kilometer-scale, post-subduction thermal anomaly linked to a localized transpressive regime since ca. 25 Ma, with an uplift rate of ~0.3 mm/yr.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49882.1/618284/Raman-thermometry-and-U-Th-He-thermochronometry
Early Au-rich sulfide liquid saturation explains the low Au endowment of continental intraplate alkaline magmas
Bertrand Rottier; Cyril Chelle-Michou; Vincent Casanova; Camille Pastore; Peter Tollan
Abstract: Alkaline magmatic systems formed in intraplate settings have a low Au endowment, contrasting with the high Au fertility of alkaline magmatic systems in arc and rift settings, which host numerous Au-rich porphyries and Cu-Au and Au-Ag epithermal deposits. Among other factors, the Au fertility of a magmatic system is determined by the Au concentration of the ore-forming magma, which is strongly controlled by the amount and chemistry of magmatic sulfides that fractionated during magma differentiation. To better explore the factors controlling the low Au endowment of alkaline magmatic systems formed in intraplate settings, we studied magmatic sulfide and silicate melt inclusions hosted in various phenocrysts from the Mont-Dore Massif (France). The magmatic system was saturated with Cu-poor, Au-rich sulfide liquid during its differentiation from basanite to trachyandesite, leading to a strong depletion of both Cu and Au in the residual melt. This presumably reduced the capacity of the magmatic system to form Au-rich magmatic-hydrothermal deposits. Such evolution contrasts with previous studies conducted in sulfide-saturated magmatic systems formed in arc settings, where an Au-poor monosulfide solid solution was the main sulfide phase to crystallize, only marginally affecting the Au budget of those systems. We conclude that the observed saturation of Au-rich sulfide liquid during evolution of the studied volcanic products could explain the low Au endowment of intraplate alkaline magmatic systems.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50515.1/618285/Early-Au-rich-sulfide-liquid-saturation-explains
High-magnitude stresses induced by mineral-hydration reactions
Oliver Plümper; David Wallis; Floris Teuling; Evangelos Moulas; Stefan M. Schmalholz ...
Abstract: Fluid-rock interactions play a critical role in Earth’s lithosphere and environmental subsurface systems. In the absence of chemical mass transport, mineral-hydration reactions would be accompanied by a solid-volume increase that may induce differential stresses and associated reaction-induced deformation processes, such as dilatant fracturing to increase fluid permeability. However, the magnitudes of stresses that manifest in natural systems remain poorly constrained. We used optical and electron microscopy to show that one of the simplest hydration reactions in nature [MgO + H2O = Mg(OH)2] can induce stresses of several hundred megapascals, with local stresses of as much as ~1.5 GPa. We demonstrate that these stresses not only cause fracturing but also induce plastic deformation with dislocation densities (1015 m –2) exceeding those typical of tectonically deformed rocks. If these reaction-induced stresses can be transmitted across larger length scales, they may influence the bulk stress state of reacting regions. Moreover, the structural damage induced may be the first step toward catastrophic rock failure, triggering crustal seismicity.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50493.1/618286/High-magnitude-stresses-induced-by-mineral
Hematite accommodated shallow, transient Pleistocene slow slip in the exhumed southern San Andreas fault system, California, USA
Alexandra A. DiMonte; Alexis K. Ault; Greg Hirth; Kelly K. Bradbury
Abstract: Slow slip is part of the earthquake cycle, but the processes controlling this phenomenon in space and time are poorly constrained. Hematite, common in continental fault zones, exhibits unique textures and (U-Th)/He thermochronometry data patterns reflecting different slip rates. We investigated networks of small hematite-coated slip surfaces in basement fault damage of exhumed strike-slip faults that connect to the southern San Andreas fault in a flower structure in the Mecca Hills, California, USA. Scanning electron microscopy shows these millimeter-thick surfaces exhibit basal hematite injection veins and layered veinlets comprising nanoscale, high-aspect-ratio hematite plates akin to phyllosilicates. Combined microstructural and hematite (U-Th)/He data (n = 64 new, 24 published individual analyses) record hematite mineralization events ca. 0.8 Ma to 0.4 Ma at <1.5 km depth. We suggest these hematite faults formed via fluid overpressure, and then hematite localized repeated subseismic slip, creating zones of shallow off-fault damage as far as 4 km orthogonal to the trace of the southern San Andreas fault. Distributed hematite slip surfaces develop by, and then accommodate, transient slow slip, potentially dampening or distributing earthquake energy in shallow continental faults.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50489.1/618287/Hematite-accommodated-shallow-transient
Tibetan Plateau insights into >1100 °C crustal melting in the Quaternary
Xiu-Zheng Zhang; Qiang Wang; Derek Wyman; Andrew C. Kerr; Wei Dan ...
Abstract: Partial melting during high-temperature to ultrahigh-temperature (UHT) metamorphism facilitates crustal differentiation, element transfer, and the evolution of topography in orogens, however the mechanisms that drive heating of Earth’s crust remain controversial. We provide new evidence from ca. 2.3 Ma dacites in the Tibetan Plateau, representing the youngest known UHT metamorphic event. Our results show that these dacites were mainly generated by fluid-absent melting of metasedimentary rocks and minor mafic rocks at peak temperatures of 1100–1150 °C and pressures of 0.8–0.9 GPa. The dacites represent mixtures of UHT melts and granulite residues and are geochemically similar to A-type granites with extremely high heat-production values (5.33–5.99 μW m–3). Compared with the geological and geophysical observations, numerical modeling indicates that the key factor determining the thermal evolution of Tibet is the thickness of the radioactive layer. Orogens dominated by rocks of felsic composition, like Tibet, could easily reach UHT conditions within a short period of time (20–40 m.y.) after crustal thickening by radioactive heating, without the need for an additional tectonic mechanism.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50387.1/618288/Tibetan-Plateau-insights-into-gt-1100-C-crustal
Molybdenum isotopes in mafic igneous rocks record slab-mantle interactions from subarc to postarc depths
Wei Fang; Li-Qun Dai; Yong-Fei Zheng; Zi-Fu Zhao
Abstract: Arc volcanic rocks erupted above oceanic subduction zones generally have heavy molybdenum (Mo) isotopic compositions, which are attributed to contributions from oceanic slab–derived fluids with isotopically heavy Mo at subarc depths. Given that mafic igneous rocks with light Mo isotopic compositions have been rarely identified, it is unclear where light Mo isotope reservoirs reside, as these must exist due to oceanic subduction through geologic time. We present Mo isotope data from Mesozoic–Cenozoic mafic igneous rocks from the eastern North China craton, which were affected by continuous subduction of the oceanic slab. The Mesozoic mafic igneous rocks have island-arc basalt (IAB)–like features and high δ 98Mo values of –0.15‰ to +0.09‰. In contrast, the Cenozoic mafic igneous rocks have ocean-island basalt (OIB)–like features and low δ 98Mo values of –0.53‰ to –0.19‰. In addition, these two suites of rocks exhibit systematic differences in Sr-Nd isotopic compositions and fluid- and melt-mobile element contents, indicating that their mantle sources contained crustal components derived from different depths. The heavy Mo and weakly enriched Sr-Nd isotopic compositions and IAB-like trace-element signatures were inherited from slab fluids (including oceanic crust–derived aqueous solutions and seafloor sediment–derived hydrous melts) liberated at subarc depths. The light Mo and depleted Sr-Nd isotopic compositions and OIB-like trace-element signatures were inherited from dehydrated slab–derived melts (with rutile breakdown) at postarc depths. Therefore, Mo isotope systematics of mafic igneous rocks are a powerful means with which to trace the geochemical fluxes from subducting oceanic slabs to the mantle wedge at different depths.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50456.1/618289/Molybdenum-isotopes-in-mafic-igneous-rocks-record
Global Ba/Nb systematics in arc magmas reflect the depths of mineral dehydration in subducted slabs
Nicholas D. Barber; Marie Edmonds; Frances Jenner; Helen Williams
Abstract: The transfer of material from subducting slabs to the overlying mantle is one of the most important processes regulating Earth’s geochemical cycles. A major part of this material cycling involves slab devolatilization and the release of sediment- and slab-derived fluids to the mantle wedge, triggering melting and subsequent arc volcanism. Previous geodynamic, geophysical, and geochemical studies have revealed many important controls on fluid fluxing to the mantle and its manifestations in arc magmas. However, it remains difficult to identify the specific mineral breakdown reactions that control element fluxes from the subducting slab into the overriding mantle. To address this challenge, we combine global arc whole-rock compositional data with geophysical information (e.g., depths to slab) and thermodynamic data. We observe three peaks in Ba/Nb in global arc magma whole-rock compositions corresponding to depths to slab of 60, 120, and >290 km. Using published thermodynamic and geodynamic models of slab evolution, we show that these peaks can be linked to the progressive breakdown of hydrous minerals (e.g., epidote, actinolite, lawsonite) in subducting slabs.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50447.1/618208/Global-Ba-Nb-systematics-in-arc-magmas-reflect-the
Mobilization of tin during continental subduction-accretion processes
Rolf L. Romer; Uwe Kroner; C. Schmidt; Claus Legler
Abstract: Major tin (Sn) deposits within the Variscan orogen are closely related to 325–270 Ma postkinematic granites that intruded the metamorphic rocks of the former precollisional accretionary wedge of the Gondwana margin. In the Erzgebirge (Germany), some of these metasedimentary rocks have high Sn contents (locally more than 1000 ppm Sn). We report cassiterite (SnO 2) U-Pb ages of 395–365 Ma and high Sn contents in prograde biotite in these metasedimentary rocks. These data demonstrate that Sn was already introduced into these rocks during accretion and prograde metamorphism. Mobilization of Sn from sedimentary source rocks during prograde fluid loss in a subduction-accretion setting represents an important process of pre-enrichment of sedimentary source rocks that upon partial melting may produce Sn-enriched melts. The large-scale metamorphic mobilization of Sn, documented here for the first time, highlights the possible importance of metamorphic Sn enrichment in accretionary complexes, thereby explaining the spatial distribution of major Sn districts within the Variscan orogen.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50466.1/618209/Mobilization-of-tin-during-continental-subduction
Dominant precessional forcing of the East Asian summer monsoon since 260 ka
Benhong Guo; Junsheng Nie; Thomas Stevens; Jan-Pieter Buylaert; Tingjiang Peng ...
Abstract: One of the most perplexing problems in paleoclimate research is how orbital cyclicities force East Asian summer monsoon (EASM) precipitation variation over the middle to late Quaternary. Chinese loess records suggest that EASM precipitation was dominated by 100 k.y. cycles controlled by Northern Hemisphere ice sheet forcing. In contrast, speleothem records suggest that EASM precipitation was dominated by 23 k.y. cycles caused by Northern Hemisphere summer insolation forcing. In order to resolve this inconsistency, we present high-resolution paleoclimate records from Xijin drill cores on the western Chinese Loess Plateau for the past 260 k.y., the rough upper limit of luminescence dating. Magnetic susceptibility (χ) shows clear 23 k.y. precessional cycles over interglacials but has constant low values over glacials. This is interpreted as indicating a lack of pedogenesis, such that χ cannot record EASM precipitation variations, rather than an absence of EASM variation itself. To circumvent this issue, we use inversed sand content as an alternative proxy for EASM precipitation over glacials and splice this with the interglacial logarithmic χ from Xijin drill cores. This new record reveals dominant 23 k.y. cycles over both interglacials and glacials, consistent with speleothem δ18O data and dominant insolation forcing. These findings allow a consistent understanding of EASM variability and forcing mechanisms from both loess and speleothem archives, resolving one of the largest debates in past monsoon research. These results challenge suggestions of high-latitude ice sheet forcing of the EASM based on slowly accumulated loess records from the central Loess Plateau.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50206.1/618210/Dominant-precessional-forcing-of-the-East-Asian
Oceanographic consequences of the Bransfield Strait (Antarctica) opening
Shan Liu; F. Javier Hernández-Molina; Chupeng Yang; Cuimei Zhang; Xiaoxia Huang ...
Abstract: The Bransfield Strait (Antarctica) is an important region for evaluating changes in Weddell Sea shelf waters on geological time scales because of its restricted connections to the surrounding ocean. However, the detailed oceanographic consequences of the opening of the strait remain unclear. We present bottom-current-related sedimentary features in the Bransfield Strait and examine the impact of the strait’s opening on deep-water circulation. Our findings show that the ocean circulation started to resemble that of the present day after a period of volcanic activity, possibly around the Middle Pleistocene. Coeval changes in Bransfield Strait morphology and an increase in seafloor irregularities due to the formation of volcanic chains finally determined new pathways for the Bransfield deep and bottom waters, enhanced due to the new climatic scenario of 100 k.y. cycles. The fact that “modern-like” oceanic circulation occurred only during previous interglacial periods demonstrates the significant impact of 100 k.y. climate cycles on the thermohaline changes of Antarctic deep waters. Hence, establishing a modern-day circulation model would enable researchers to assess paleoproductivity and local upwelling that have profoundly influenced the marine ecosystem of the Antarctic Peninsula after the Middle Pleistocene.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50389.1/618211/Oceanographic-consequences-of-the-Bransfield
Novel age constraints for the onset of the Steptoean Positive Isotopic Carbon Excursion (SPICE) and the late Cambrian time scale using high-precision U-Pb detrital zircon ages
Hannah R. Cothren; Thomas P. Farrell; Frederick A. Sundberg; Carol M. Dehler; Mark D. Schmitz
Abstract: The Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent +4–5‰ shift in the Cambrian δ13C record used for global chronostratigraphic correlation. The onset of this excursion is traditionally placed at the base of the Pterocephaliid trilobite biomere (base of the Furongian Series). Recent studies have documented local controls on the expression of the SPICE and emphasize the need for chronostratigraphic standards for these complex biogeochemical signals. We build upon prior work in western Laurentia by integrating δ13C and biostratigraphy with high-precision isotope dilution U-Pb detrital zircon maximum depositional ages that are coincident with the onset, peak, and falling limb of the SPICE. Our study provides the first useful numerical age constraint for the onset of the SPICE and the Laurentian trilobite biozones and requires revision of the late Cambrian geologic time scale boundaries by several million years.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50434.1/618212/Novel-age-constraints-for-the-onset-of-the
Reworked pollen reduces apparent floral change during the Paleocene-Eocene Thermal Maximum
Vera A. Korasidis; Scott L. Wing; David M. Nelson; Allison A. Baczynski
Abstract: Plant megafossils from the Paleocene-Eocene Thermal Maximum (PETM) in the Bighorn Basin, north-central Wyoming, USA, document a dramatic shift in floral composition, whereas palynofloral change from the same sections has appeared to be more subtle. We investigated this discrepancy by quantifying pollen preservation and measuring the stable carbon isotope composition of specific pollen taxa. Pollen grains belonging to two common latest Paleocene taxa are poorly preserved in PETM samples, and their δ 13Cpollen is similar during the latest Paleocene and PETM. In contrast, pollen grains of a thermophilic taxon that became more abundant during the PETM are pristine, and the δ13C pollen of PETM specimens is ~4‰ lower than that of latest Paleocene specimens. More broadly, pollen grains belonging to lineages currently centered in temperate climates are poorly preserved when found in PETM samples, whereas in the same samples, pollen belonging to lineages now centered in the tropics are well preserved. These differences in preservation and isotopic composition indicate extensive redeposition of older pollen grains during the PETM. Increased abundance of Cretaceous palynotaxa in PETM samples confirms erosion and redeposition, likely resulting from more episodic and intense precipitation. Exclusion of reworked palynotaxa from analyses reveals that, as in the megaflora, temperate taxa were absent during the PETM at the time when dry tropical taxa briefly appeared. Major climate changes like the PETM may commonly destabilize landscapes, increase reworking, and thus smooth patterns of change in microfloras, leading to underestimates of the rate and magnitude of floral response to past global change.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50441.1/618213/Reworked-pollen-reduces-apparent-floral-change
When knickzones limit upstream transmission of base-level fall: An example from Kauaʻi, Hawaiʻi
L. Wren Raming; Kelin X Whipple
Abstract: A threshold drainage area limits fluvial transmission of base-level fall and may be expressed in the form of a waterfall or a series of waterfalls, defined here as a knickzone. Knickzones on the west coast of Kauaʻi, Hawaiʻi (USA), exhibit evidence of a threshold drainage area. Eighteen (18) of the 25 knickzones in our study area are located at the coast or a tributary junction, have a drainage area <1.5 km2, and have been stationary for at least 1.5 m.y. The other seven knickzones are located >1 km upstream from the coast or nearest tributary junction and range in drainage area from 1 to 5.5 km2. Both sets of knickzones limit incision relative to canyons without knickzones. Field observations show strong ʻaʻā flows and dikes always crop out at the lip of knickzones, suggesting these resistant rocks and coarse sediment generated from them act to inhibit knickzone migration. A model incorporating flood records and channel conditions above knickzones shows thresholds of coarse sediment entrainment are never exceeded below 1 km2. Our results demonstrate knickzones on the west coast of Kaua‘i are enduring features explained by resistant lava flows and physical limits in bedrock incision.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50019.1/618175/When-knickzones-limit-upstream-transmission-of
Lagoon infilling by coral reef sand aprons as a proxy for carbonate sediment productivity
Ana Vila-Concejo; Sarah M. Hamylton; Jody M. Webster; Stephanie J. Duce; Thomas E. Fellowes
Abstract: Sand aprons are distinctive landforms that offer important insights into sedimentary dynamics for reef platform development. Here we link temporal and spatial scales of 21 sand aprons in the southern Great Barrier Reef to understand their Holocene formation and evolution in response to relative sea-level changes, the depth of the Pleistocene base, and contemporary morphodynamics. Our results show that lagoon infilling is a function of reef size and is a self-limiting process controlled by hydrodynamics and relative sea-level changes. Lagoon infilling does not depend on the type of reef or degree of exposure to waves, but it could reflect past wave climates. Our carbonate productivity estimates based on lagoon infilling are remarkably similar to those inferred from habitat classification. Finally, we hypothesize that current carbonate productivity has slowed because of the effects of climate change.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50453.1/618176/Lagoon-infilling-by-coral-reef-sand-aprons-as-a
Crystal plasticity enhances trace element mobility in garnet
Tommaso Tacchetto; Steven M. Reddy; Denis Fougerouse; Chris Clark; David W. Saxey ...
Abstract: Chemical heterogeneities along grain boundaries in garnet occur across a wide range of metamorphic conditions, yet the processes underlying their development remain poorly understood. Here we integrate electron backscattered diffraction (EBSD) and atom probe tomography (APT) to evaluate the mechanisms driving nanoscale trace element mobility to deformation microstructures in a granulite-facies garnet. This approach shows that low-angle boundaries can be enriched in Ca, Ti, P, Cu, K, Na, Cl, and H. Based on the correlation between EBSD and APT data, we propose that solute ions (Ca, Ti, P, and Cu) were segregated to the interface during the migration of dislocation associated with ductile deformation of the grain. In contrast, elements such as K, Na, Cl, and H are interpreted to reflect diffusion along the low-angle boundary from an externally derived fluid source. These results provide the missing link between chemical heterogeneity and deformation-related microstructures in garnet. Our approach shows that a combination of microstructural and nanoscale geochemical analyses can provide unprecedented insights into mechanisms of element transfer within minerals.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50283.1/618177/Crystal-plasticity-enhances-trace-element-mobility
Forced Cenozoic continental subduction of Tarim craton-like lithosphere below the Tianshan revealed by ambient noise tomography
Weijia Sun; Songjian Ao; Qingya Tang; Marco G. Malusà; Liang Zhao ...
Abstract: The possibility that craton-like lithosphere may undergo subduction during convergence of major tectonic plates is still poorly investigated. We addressed this issue using ambient noise tomography of the Tarim Basin and the Tianshan (Central Asia). Our S-wave velocity model reveals a flat-lying high-velocity anomaly beneath the Tarim Basin in the 45–60 km depth range, consistent with intrusion of mafic rocks at the base of the lower crust above a depleted lithospheric mantle, as expected after interaction of the lithosphere with a mantle plume. This high-velocity anomaly can be followed northward, steeply dipping (~45°) beneath the Tianshan, which indicates that the Tarim craton-like lithosphere was subducted to mantle depths. It is connected with a fast P-wavespeed anomaly in the upper mantle, interpreted as a relict of the South Tianshan Ocean. A long period of tectonic quiescence, after the closure of the South Tianshan Ocean and before the Cenozoic tectonic rejuvenation of the Tianshan, suggests a minor role of oceanic slab pull in controlling continental subduction. The major player is instead the northward push of India within the framework of Cenozoic India-Asia convergence. We conclude that forced subduction can be experienced not only by thinned continental crust but also by a strong craton-like lithosphere.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50510.1/618178/Forced-Cenozoic-continental-subduction-of-Tarim
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