Boulder, Colo., USA: Article topics and locations include natural levees in Texas; internal breadcrust surfaces; a submarine volcano near Tokyo Bay, Japan; what controlled the thickness of continental crust in the Archean?; and Silurian wildfire proxies and atmospheric oxygen. These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent .
Halogen ratios in crustal fluids through time—Proxies for the emergence of aerobic life?
Tobias Fusswinkel; Paula Niinikoski-Fusswinkel; Thomas Wagner
Abstract: Halogens (Cl, Br, I) are exceptional provenance tracers in crustal fluids because their ratios are not strongly altered during most fluid-rock interaction processes. The halogen systematics of metamorphic fluids are of particular interest because such fluids are key drivers of crustal-scale element fluxes and ore formation in orogenic belts, but they remain poorly studied due to analytical challenges. We present novel triple-halogen laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) fluid-inclusion data from metamorphic systems ranging in age from Archean to Phanerozoic. Our results show that the halogen signatures in Phanerozoic metamorphic fluids are controlled by variable degrees of organic-matter interaction in their source rocks, leading to increased I/Cl and decreased Br/I ratios relative to seawater. By contrast, Archean metamorphic fluids from organic matter–rich source rocks have low I/Cl and very high Br/I ratios, distinctly different from any known fluid source signature. We propose that these signatures nevertheless are consistent with organic-matter interaction because dominantly prokaryotic Archean lifeforms did not yet produce iodine-bearing metabolites. This prevented biosequestration and accumulation of iodine-rich organic matter in sediments and imposed halogen signatures onto Archean metamorphic fluids entirely unlike those in younger fluids.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50182.1/614875/Halogen-ratios-in-crustal-fluids-through-time
Past eruptions of a newly discovered active, shallow, silicic submarine volcano near Tokyo Bay, Japan
Iona M. McIntosh; Kenichiro Tani; Alexander R.L. Nichols; Qing Chang; Jun-Ichi Kimura
Abstract: Oomurodashi is a newly discovered active, shallow, silicic submarine volcano only 60 km from Tokyo Bay. We reveal its past eruptive activity, and potential future hazards, by examining volatile contents of its subaerial and submarine pumice and lava deposits. These novel data for shallow silicic submarine eruption products were obtained using new Fourier transform infrared spectroscopy (FTIR) analytical techniques for vesicular and hydrated glasses. All matrix glasses have H2O species data consistent with low-temperature hydration following eruption. We therefore used unaltered OH data to investigate past eruptions. Geochemistry confirmed that Oomurodashi was the source of a ca. 13.5 ka subaerial tephra deposit on nearby inhabited islands. We infer from pumice OH contents and tephra characteristics that this deposit was formed by explosive submarine phreatomagmatic activity that produced the shallow crater in the submarine edifice. OH contents of in-place submarine lavas are lower than expected for their current water depth; comparison with past sea level implies that these lavas erupted at ca. 7–10 ka and ca. 14 ka when sea level was lower. Oomurodashi has also erupted submarine pumice with different densities, quench depths, and dispersal histories; however, any pumice sufficiently buoyant to produce floating pumice rafts will have been lost from the local geological record, so pumice rafts remain a potential future hazard.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50148.1/614876/Past-eruptions-of-a-newly-discovered-active
Evidence for a lithospheric step and pervasive lithospheric thinning beneath southern New England, northeastern USA
Gillian B. Goldhagen; Heather A. Ford; Maureen D. Long
Abstract: In this study, we use data from the SEISConn seismic experiment to calculate Sp receiver functions in order to characterize the geometry of upper-mantle structure beneath southern New England (northeastern United States). We image robust negative-velocity-gradient discontinuities beneath southern New England that we interpret as corresponding to the lithosphere-asthenosphere boundary (LAB) and identify a well-defined step of 15 km in LAB depth at a longitude of 73°W, which we interpret to be the boundary between Laurentian and Appalachian lithosphere, although the offset may be larger if the putative LAB phase is reinterpreted to be a mid-lithospheric discontinuity. We infer that the lithosphere throughout the region is substantially thinner than elsewhere in the continental interior, consistent with regional tomographic studies and previously published Sp receiver function results. The presence of thinned lithosphere suggests that the low-velocity Northern Appalachian Anomaly (NAA) in the upper mantle may extend as far south as coastal Connecticut. The presence of regionally thinned lithosphere and a step in lithospheric thickness suggests that inherited structure may be preserved in present-day lithosphere, even in the presence of more recent dynamic processes associated with the NAA.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50133.1/614551/Evidence-for-a-lithospheric-step-and-pervasive
Eruptive tempo of Emeishan large igneous province, southwestern China and northern Vietnam: Relations to biotic crises and paleoclimate changes around the Guadalupian-Lopingian boundary
Hu Huang; Magdalena H. Huyskens; Qing-Zhu Yin; Peter A. Cawood; Mingcai Hou ...
Abstract: The Emeishan large igneous province (LIP), southwestern China and northern Vietnam, is thought to have been a potential driver for the biotic crises and paleoclimate changes around the Guadalupian-Lopingian boundary (GLB; Permian), but the lack of high-precision radiometric dates to constrain the duration and eruption rates of the volcanism has limited the assessment of their relationship. We present new chemical abrasion–isotope dilution–isotope ratio mass spectrometry U-Pb zircon geochronology of three samples from the lowermost and uppermost parts of the volcanic succession in the central portion of the Emeishan LIP. The results demonstrate that Emeishan volcanism began by 260.55 ± 0.07 Ma and persisted until at least 257.22 ± 0.37 Ma. Combined with a previously published age of 259.1 ± 0.5 Ma for silicic ignimbrites, we estimate that ~85% of Emeishan LIP volume erupted within 1.45 ± 0.50 m.y. Our new results confirm that the Emeishan volcanism began slightly prior to the initiation of the associated mass extinction event and was contemporaneous with the associated warming interval. The new data support the hypothesis that the Emeishan LIP likely triggered the biotic crises and paleoclimate changes around the GLB.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50183.1/614552/Eruptive-tempo-of-Emeishan-large-igneous-province
What controlled the thickness of continental crust in the Archean?
Vuong V. Mai; Jun Korenaga
Abstract: Exposed continents are one of Earth’s major characteristics. Recent studies on ancient ocean volume and exposed landmasses suggest, however, that early Earth was possibly a water world, where any significant landmass was unlikely to have risen above sea level. On modern Earth, the thickness of continental crust seems to be controlled by sea level and the buoyancy of continental crust. Simply applying this concept to the Archean would not explain the absence of exposed continents, and we suggest that a third element that is currently insignificant was important during early Earth: the strength of continental upper crust. Based on the pressure imbalance expected at continent-ocean boundaries, we quantified the conditions under which rock strength controls the thickness of continental crust. With the level of radiogenic heat production expected for early Earth, continents may have been too weak to have maintained their thickness against a deep ocean.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50350.1/614553/What-controlled-the-thickness-of-continental-crust
Crustal transpressional fault geometry influenced by viscous lower crustal flow
Haibin Yang; Louis N. Moresi; Mark Quigley; Metin Kahraman; Doğan Kalafat
Abstract: The San Andreas fault (California, USA) is near vertical at shallow (< 10 km) depth. Geophysical surveys along the San Andreas fault reveal that, at depths of 10–20 km, it dips ~50–70° to the southwest near the Western Transverse Ranges and dips northeast in the San Gorgonio region. We investigate the possible origin of along-strike geometric variations of the fault using a three-dimensional thermomechanical model. For two blocks separated by transpressional faults, our model shows that viscous lower crustal material moves from the high-viscosity block into the low-viscosity block. Fault plane-normal flow in the viscous lower crust rotates the fault plane due to the simple shear flow at the brittle-ductile transition depth. This occurs irrespective of initial fault dip direction. Rheological variations used to model the lower crust of Southern California are verified by independent observations. Block extrusion due to lower crustal viscosity variation facilitates the formation of the Garlock Fault and sustains the geometric complexity of the fault.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50061.1/614554/Crustal-transpressional-fault-geometry-influenced
Quantitative relationships between river and channel-belt planform patterns
Tian Y. Dong; Timothy A. Goudge
Abstract: Channel planform patterns arise from internal dynamics of sediment transport and fluid flow in rivers and are affected by external controls such as valley confinement. Understanding whether these channel patterns are preserved in the rock record has critical implications for our ability to constrain past environmental conditions. Rivers are preserved as channel belts, which are one of the most ubiquitous and accessible parts of the sedimentary record, yet the relationship between river and channel-belt planform patterns remains unquantified. We analyzed planform patterns of rivers and channel belts from 30 systems globally. Channel patterns were classified using a graph theory-based metric, the Entropic Braided Index (eBI), which quantifies the number of river channels by considering the partitioning of water and sediment discharge. We find that, after normalizing by river size, channel-belt width and wavelength, amplitude, and curvature of the belt edges decrease with increasing river channel number (eBI). Active flow in single-channel rivers occupies as little as 1% of the channel belt, while in multichannel rivers it can occupy >50% of the channel belt. Moreover, we find that channel patterns lie along a continuum of channel numbers. Our findings have implications for studies on river and floodplain interaction, storage timescales of floodplain sediment, and paleoenvironmental reconstruction.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49935.1/614344/Quantitative-relationships-between-river-and
Natural levees increase in prevalence in the backwater zone: Coastal Trinity River, Texas, USA
Hima J. Hassenruck-Gudipati; Paola Passalacqua; David Mohrig
Abstract: Flood dynamics in low-relief landscapes control the lateral exchange of water and sediment between a river and its floodplain. Locations where these exchanges occur for any given river discharge depend on local bank elevations, which in turn depend on the type of landform present immediately adjacent to the river channel. Our analysis separated landforms bordering a river into six categories: levee, scroll bar, counter point bar, channel-bend cutoff, erosional surface, and inactive surface. Each landform is associated with a different range of elevations. Levees are the highest, and counter point bars and cutoffs are the lowest. Using a combination of lidar-derived measurements of topography and water-surface profiles derived from U.S. Geological Survey gauge data, we show that landforms at the margins of the river change with downstream position on the coastal reach of the Trinity River in the southern United States. The fractions of counter point bars and cutoffs decrease downstream, while the fraction and continuity of levees increase to nearly 100%. This spatial change correlates with downstream reductions in channel-bend migration and deformation, and the measured range in river stage. As a result, the greatest range in bank elevations occurs upstream where variation in river stage is also highest. Meanwhile, the smallest range in bank elevation and river stage exists in the coastal backwater zone. Our analysis indicates that essentially all levees within the backwater zone are overtopped by flow associated with a single river discharge. Moving upriver, the discharge associated with levee-overtopping flow systematically increases. This study highlights the morphodynamic control on coastal river flooding.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50011.1/614345/Natural-levees-increase-in-prevalence-in-the
Mid-loaf crisis: Internal breadcrust surfaces in rhyolitic pyroclasts reveal dehydration quenching
Hugh Tuffen; Jamie I. Farquharson; Fabian B. Wadsworth; Cameron Webb; Jacqueline Owen ...
Abstract: Breadcrust bombs are pyroclasts displaying fractured, dense surfaces enveloping expanded interiors, and are associated with Vulcanian explosions. We document pyroclasts from the 2008–2009 CE eruption of Chaitén (Chile) that are internally as well as externally breadcrusted. The pyroclasts are cut by intersecting micrometer- to millimeter-thick tuffisites with dense glassy walls, which grade into strongly inflated pumiceous material. We find H2O diffusion gradients proximal to the breadcrusted surfaces, such that H2O is depleted from far-field magma (0.68 ± 0.04 wt%) into dense, fractured vein walls (0.2–0.3 wt%), indicating a spatial association between H2O mass transfer within the pyroclast interior and both suppressed vesiculation and breadcrusting. We experimentally confirm that diffusive H2O depletion suppresses bubble growth at shallow conduit conditions. Therefore, we interpret the breadcrust formation to be induced by H 2O diffusion and the associated rise in viscosity rather than by cooling in the classical breadcrust-formation models. We posit that a “dehydration quench” is important as degassing continues to very low H 2O contents in shallow-conduit magma that continues to vesiculate.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49959.1/614346/Mid-loaf-crisis-Internal-breadcrust-surfaces-in
Epidote U-Pb geochronology and H isotope geochemistry trace pre-orogenic hydration of midcrustal granitoids
Veronica Peverelli; Alfons Berger; Andreas Mulch; Thomas Pettke; Francesca Piccoli ...
Abstract: Hydrothermal veins and altered feldspar are evidence for fluid circulation in granitic rocks in the continental crust. The hydrothermal alteration of feldspar affects the deformation behavior of granitoids, especially if it occurs before orogeny. Geochronology can establish the timing of fluid circulation to determine if this fluid-driven alteration plays a role in crustal deformation. Although existing dating techniques cannot be applied to feldspar alteration directly, absolute ages of fluid circulation can be obtained from hydrothermal veins. We combined U-Pb geochronology and hydrogen isotope data (δD) from epidote [Ca2Al2(Al,Fe 3+)Si3O12(OH)] to unravel the hydration of post-Variscan granitoids in the Alpine orogen. The recent protocol for epidote U-Pb dating proves for the first time that fluids of meteoric origin infiltrated the granitoids in Permian times by exploiting synrift faults, consistent with the δD values of the epidote-forming fluids. This hydration event caused at least some degree of feldspar hydration and weakening of the granitic rocks ~260 m.y. before their pervasive structural overprint by the Alpine orogeny. The preservation of Permian U-Pb ages despite Alpine orogenic processes confirms epidote as a powerful tool with which to unveil pre-orogenic hydration events in metagranitoids. Our analytical approach broadens insights into the water cycle in the middle continental crust in orogens.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50028.1/614347/Epidote-U-Pb-geochronology-and-H-isotope
Silurian wildfire proxies and atmospheric oxygen
Ian J. Glasspool; Robert A. Gastaldo
Abstract: The earliest evidence of wildfire is documented from two localities: the early mid-Silurian Pen-y-lan Mudstone, Rumney, Wales (UK), and the late Silurian Winnica Formation, Winnica, Poland. Nematophytes dominate both charcoal assemblages. Reflectance data indicate low-temperature fires with localized intense conditions. Fire temperatures are greater in the older and less evolved assemblage. These charcoal assemblages and others, new and previously documented, from the Silurian and earliest Devonian are compared to box models of atmospheric oxygen concentration (pO2 ). Based on modern charring experiments, these data indicate pO 2 is divergent from the broad trends predicted by the COPSE-revisited and GEOCARBSULFOR models. Sustained burns require a minimum pO2 threshold of 16%, or ~0.75 present atmospheric level. This threshold was first met and, our charcoal data indicate, was exceeded in the mid-Silurian and then, later in the Silurian, attained again repeatedly.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50193.1/614348/Silurian-wildfire-proxies-and-atmospheric-oxygen
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