Boulder, Colo., USA: Article topics and locations include the use of high-resolution three-dimensional seismic data to analyze the infill of buried tunnel valleys in the North Sea; volcanic plumbing; rivers in reverse; the 2014–2015 Holuhraun lava flow field, Iceland; Lake Magadi, Kenya; and measuring wind via sand dunes on Mars. These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent.
Interaction bounding surfaces exposed in migrating transverse aeolian ridges on Mars
Mackenzie Day
Abstract: Wind-blown sand self-organizes into bedforms that have now been identified on six different planetary bodies. These bedforms, including ripples and dunes, exhibit patterns that are diagnostic of surface-atmosphere interactions and can be used to interpret winds and sediment supply from satellite images of planetary surfaces. Patterns in dune and ripple fields change when one or more bedforms interact, for example, by linking, colliding, or merging with one another. When two bedforms interact, the cross-strata developed by the bedforms include a bounding surface where the two bedforms combined. These “interaction bounding surfaces” have been interpreted from ancient and modern strata in recent literature, but they have not yet been identified beyond Earth. On Mars, aeolian dunes and ripples form much as they do on Earth, but additional enigmatic bedform types are also present. Transverse aeolian ridges are straight-crested bedforms found abundantly on Mars, but with few analogs on Earth. Formation mechanisms for these enigmatic bedforms range from dune-like migration and construction to growth in place via wedge stacking or kinetic sieving. In this work, I studied exposed stoss-slope stratification on these enigmatic Martian bedforms to (1) identify the first in situ examples of interaction bounding surfaces captured visually, and (2) demonstrate that the transverse aeolian ridges must have been forward migrating.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49373.1/608087/Interaction-bounding-surfaces-exposed-in-migrating
The influence of rock uplift rate on the formation and preservation of individual marine terraces during multiple sea-level stands
Luca C. Malatesta; Noah J. Finnegan; Kimberly L. Huppert; Emily I. Carreño
Abstract: Marine terraces are a cornerstone for the study of paleo sea level and crustal deformation. Commonly, individual erosive marine terraces are attributed to unique sea-level high stands based on the reasoning that marine platforms could only be significantly widened at the beginning of an interglacial. However, this logic implies that wave erosion is insignificant at other times. We postulate that the erosion potential at a given bedrock elevation datum is proportional to the total duration of sea-level occupation at that datum. The total duration of sea-level occupation depends strongly on rock uplift rate. Certain rock uplift rates may promote the generation and preservation of particular terraces while others prevent them. For example, at rock uplift of ~1.2 mm/yr, the Marine Isotope Stage (MIS) 5e (ca. 120 ka) high stand reoccupies the elevation of the MIS 6d–e mid-stand, favoring creation of a wider terrace than at higher or lower rock uplift rates. Thus, misidentification of terraces can occur if each terrace in a sequence is assumed to form uniquely at successive interglacial high stands and to reflect their relative elevations. Developing a graphical proxy for the entire erosion potential of sea-level history allows us to address creation and preservation biases at different rock uplift rates.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49245.1/608088/The-influence-of-rock-uplift-rate-on-the-formation
Thermodynamic limits for assimilation of silicate crust in primitive magmas
Jussi S. Heinonen; Frank J. Spera; Wendy A. Bohrson
Abstract: Some geochemical models for basaltic and more primitive rocks suggest that their parental magmas have assimilated tens of weight percent of crustal silicate wall rock. But what are the thermodynamic limits for assimilation in primitive magmas? We pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems—the Magma Chamber Simulator (https://mcs.geol.ucsb.edu)—and focus on modeling assimilation of wall-rock partial melts, which is thermodynamically more efficient compared to bulk assimilation of stoped wall-rock blocks in primitive igneous systems. In the simulations, diverse komatiitic, picritic, and basaltic parental magmas assimilate progressive partial melts of preheated average lower, middle, and upper crust in amounts allowed by thermodynamics. Our results indicate that it is difficult for any subalkaline primitive magma to assimilate more than 20–30 wt% of upper or middle crust before evolving to compositions with higher SiO2 than a basaltic magma (52 wt%). On the other hand, typical komatiitic magmas have thermodynamic potential to assimilate as much as their own mass (59–102 wt%) of lower crust and retain a basaltic composition. The compositions of the parental melt and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition given typical temperatures. These findings have important implications for the role of assimilation in the generation and evolution of, e.g., ultramafic to mafic trans-Moho magmatic systems, siliceous high-Mg basalts, and massif-type anorthosites.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49139.1/608089/Thermodynamic-limits-for-assimilation-of-silicate
New estimates of the magnitude of the sea-level jump during the 8.2 ka event
Jonathan Obrist-Farner; Mark Brenner; Jeffery R. Stone; Marta Wojewódka-Przybył; Thorsten Bauersachs ...
Abstract: We analyzed sediment cores from coastal Lake Izabal, Guatemala, to infer Holocene biogeochemical changes in the lake. At ca. 8370 calibrated yr B.P. (cal. yr B.P.), marine waters entered the lake, which presently lies ~38 km from the Caribbean coast. Temporal correlation between Early Holocene drainage of high-latitude Lakes Agassiz and Ojibway (in North America) and marine flooding of Lake Izabal suggests a causal link between the two processes. Our data indicate a relative sea-level jump of 2.60 ± 0.88 m, which is larger than previous estimates of sea-level rise during the 8.2 ka event. The inferred sea-level jump, however, cannot be explained solely by the volume of water released during drainage of Lakes Agassiz and Ojibway. Instead, we propose that previous studies underestimated the magnitude of Lakes Agassiz and Ojibway discharge, or that additional meltwater sources contributed to global sea-level rise at that time.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49296.1/608090/New-estimates-of-the-magnitude-of-the-sea-level
Late Quaternary geochronologic record of soil formation and erosion: Effects of climate change on Mojave Desert hillslopes (Nevada, USA)
L.P. Persico; L.D. McFadden; J.R. McAuliffe; T.M. Rittenour; T.E. Stahlecker ...
Abstract: Climate change is an often-cited control on geomorphic processes in the arid southwestern United States, but links to direct climatic factors and vegetation change remain under debate. Hillslopes at a site in the eastern Mojave Desert in southern Nevada are mantled by 0–1.5 m of colluvial deposits. Accumulation of weathered bedrock combined with eolian inputs of fine sand and silt led to the formation of well-developed soil profiles. Surface sediments from both sources were incorporated into colluvium, allowing both processes to be dated with optically stimulated luminescence (OSL). OSL ages indicate a period of increased colluviation in the Late Pleistocene facilitated by enhanced bedrock weathering and dust deposition. Hillslope aspect strongly controls predominant soil environments and associated vegetation. Well-developed soils with dense grass cover extensively mantle the mesic north-aspect hillslopes, while more xeric south-aspect hillslopes are dominated by thin colluvium with minimal soil development, extensive bedrock exposure, and desertscrub vegetation. Remnants of older colluvium with moderately developed soils on south aspects, however, indicate they were once more extensively mantled by thicker colluvial deposits. The transition to drier conditions in the Holocene diminished vegetation cover on more xeric south aspects, triggering widespread erosion, whereas the more mesic north aspects retained denser grass cover that minimized erosion. The transition to drier conditions in the Holocene altered the vegetation; however, persistent perennial grass cover minimized erosion into the middle Holocene. Increasing aridity during the middle Holocene significantly reduced grass cover on more xeric south aspects, triggering erosion and alluvial deposition. OSL dates of dust incorporated into terrace sediments indicate late Middle Holocene aggradation and soil development in the Late Holocene. In contrast, maintenance of substantial perennial grass cover on mesic north aspects minimized erosion from those hillslopes throughout the Holocene.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49270.1/608091/Late-Quaternary-geochronologic-record-of-soil
Guttulatic calcite: A carbonate microtexture that reveals frigid formation conditions
Eva L. Scheller; John Grotzinger; Miquela Ingalls
Abstract: The paragenesis of carbonate pseudomorphic textures in the rock record that are inferred to represent replaced metastable ikaite (CaCO3·6H2O), which forms at frigid temperatures, is uncertain. Petrographic analysis of Mono Lake (California, USA) Pleistocene tufas allowed recognition of a distinctive calcite microtexture, termed guttulatic calcite, that forms during carbonate dehydration and is diagnostic for precursor ikaite. The texture is characterized by pseudo-hexagonal or spherical low-Mg cores, which likely formed initially as vaterite, with an ellipsoidal overgrowth, and a secondary high-Mg sparry or micritic cement. Observations of Mono Lake ikaite pseudomorphs, combined with a review of more ancient examples, indicate that guttulatic texture records carbonate dehydration of precursor ikaite and can be used to infer frigid paleotemperatures.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49312.1/607791/Guttulatic-calcite-A-carbonate-microtexture-that
Mantle heating at ca. 2 Ga by continental insulation: Evidence from granites and eclogites
R. Tamblyn; D. Hasterok; M. Hand; M. Gard
Abstract: Igneous and metamorphic rocks contain the mineralogical and geochemical record of thermally driven processes on Earth. The generally accepted thermal budget of the mantle indicates a steady cooling trend since the Archean. The geological record, however, indicates this simple cooling model may not hold true. Subduction-related eclogites substantially emerge in the rock record from 2.1 Ga to 1.8 Ga, indicating that average mantle thermal conditions cooled below a critical threshold for widespread eclogite preservation. Following this period, eclogite disappeared again until ca. 1.1 Ga. Coincident with the transient emergence of eclogite, global granite chemistry recorded a decrease in Sr and Eu and increases in yttrium and heavy rare earth element (HREE) concentrations. These changes are most simply explained by warming of the thermal regime associated with granite genesis. We suggest that warming was caused by increased continental insulation of the mantle at this time. Ultimately, secular cooling of the mantle overcame insulation, allowing the second emergence and preservation of eclogite from ca. 1.1 Ga until present.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49288.1/607792/Mantle-heating-at-ca-2-Ga-by-continental
Rivers in reverse: Upstream-migrating dechannelization and flooding cause avulsions on fluvial fans
Douglas A. Edmonds; Harrison K. Martin; Jeffery M. Valenza; Riley Henson; Gary S. Weissmann ...
Abstract: The process of river avulsion builds floodplains and fills alluvial basins. We report on a new style of river avulsion identified in the Landsat satellite record. We found 69 examples of retrogradational avulsions on rivers of densely forested fluvial fans in the Andean and New Guinean alluvial basins. Retrogradational avulsions are initiated by a channel blockage, e.g., a logjam, that fills the channel with sediment and forces water overbank (dechannelization), which creates a chevron-shaped flooding pattern. Dechannelization waves travel upstream at a median rate of 387 m/yr and last on average for 13 yr; many rivers show multiple dechannelizing events on the same reach. Dechannelization ends and the avulsion is complete when the river finds a new flow path. We simulate upstreammigrating dechannelization with a one-dimensional morphodynamic model for open channel flow. Observations are consistent with model results and show that channel blockages can cause dechannelization on steep (10–2 to 10–3), low-discharge (~101 m3 s–1) rivers. This illustrates a new style of floodplain sedimentation that is unaccounted for in ecologic and stratigraphic models.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49318.1/607793/Rivers-in-reverse-Upstream-migrating
Propagating uplift controls on high-elevation, low-relief landscape formation in the southeast Tibetan Plateau
X.P. Yuan; K.L. Huppert; J. Braun; X. Shen; J. Liu-Zeng ...
Abstract: High-elevation, low-relief surfaces are widespread in many mountain belts. However, the origin of these surfaces has long been debated. In particular, the southeast Tibetan Plateau has extensive low-relief surfaces perched above deep valleys and in the headwaters of three of the world’s largest rivers (Salween, Mekong, and Yangtze Rivers). Various geologic data and geodynamic models show that many mountain belts grow first to a certain height and then laterally in an outward propagation sequence. By translating this information into a kinematic propagating uplift function in a landscape evolution model, we propose that the high-elevation, low-relief surfaces in the southeast Tibetan Plateau are simply a consequence of mountain growth and do not require a special process to form. The propagating uplift forms an elongated river network geometry with broad high-elevation, low-relief headwaters and interfluves that persist for tens of millions of years, consistent with the observed geochronology. We suggest that the low-relief interfluves can be long-lived because they lack the drainage networks necessary to keep pace with the rapid incision of the large main-stem rivers. The propagating uplift also produces spatial and temporal exhumation patterns and river profile morphologies that match observations. Our modeling therefore reconciles geomorphic observations with geodynamic models of uplift of the southeast Tibetan Plateau, and it provides a simple mechanism to explain the low-relief surfaces observed in several mountain belts on Earth.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49022.1/607794/Propagating-uplift-controls-on-high-elevation-low
Diversification of Archean tonalite-trondhjemite-granodiorite suites in a mushy middle crust
Jillian Kendrick; Manuel Duguet; Chris Yakymchuk
Abstract: Tonalite-trondhjemite-granodiorite (TTG) suites are the dominant component of Earth’s first continents, but their origins are debated. The trace element concentrations of these rocks are conventionally linked to their source depths and inferred geodynamic settings with the implicit assumption that TTG compositions are source-controlled. Alternatively, their variable compositions may be caused by fractional crystallization in TTG plutons after emplacement and less clearly linked to source depth. Most TTGs in Archean mid-crustal exposures are the dominant component of igneous gray gneiss complexes; the processes that influence the evolution of TTG magmas in this setting are poorly understood. We present a petrological–geochemical model that explains the coexistence of TTGs in the middle crust with low-pressure and high-pressure geochemical trends, irrespective of tectonic setting or depth of the TTG source. We propose that mid-crustal TTGs were long-lived crystal mushes with compositions controlled by the separation of early-crystallizing plagioclase and melt. Using phase equilibrium modeling, we demonstrate that a suite of TTGs from the southern Superior Province in Canada represents variably plagioclase-rich and melt-rich fractions from a common parent magma. The behavior of plagioclase may have a strong influence on the geochemical trends of TTGs, including the degree of rare earth element fractionation. Our results suggest that trace element compositions of TTGs may not primarily reflect the depth of the source and cannot be used alone to infer Archean geodynamic settings.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49287.1/607795/Diversification-of-Archean-tonalite-trondhjemite
Linking lava morphologies to effusion rates for the 2014–2015 Holuhraun lava flow field, Iceland
Joana R.C. Voigt; Christopher W. Hamilton; Gregor Steinbrügge; Ármann Höskuldsson; Ingibjörg Jónsdottir ...
Abstract: Determining the parameters that control fissure-fed lava morphologies is critical for reconstructing the complex emplacement histories of eruptions on Earth and other planetary bodies. We used a geomorphological map of the 2014–2015 Holuhraun lava flow field, in combination with new constraints on lava emplacement chronology and two independently derived time-averaged discharge rate (TADR) data sets, to analyze correlations between lava morphology and effusion rate. Results show that lava morphologies are dominantly controlled by effusion rate at the vent during the early phases of the eruption and by lava transport processes as the system evolves. Initially, TADR and its variance, which reflect pulsation in the lava supply rate from the vent, directly affect lava emplacement styles. However, as the eruption progresses, the lava transport system exerts a stronger control with channels and ponds that can either dampen variation in local effusion rate or create surges during sudden drainage events. The Holuhraun eruption predominantly produced rubbly lava in its earlier eruption phases and transitioned into the production of spiny lava toward the end of the eruption. However, a drop of TADR during the first phase of the eruption correlates with a decrease in rubbly lava formation and an increase in spiny lava production. This suggests that a change in effusion rate caused the observed transition in lava type. Our findings show that rubbly lava is formed under relatively high local effusion rates with pulsating supply conditions, whereas spiny lava is formed under lower local effusion rates and steadier supply.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49251.1/607796/Linking-lava-morphologies-to-effusion-rates-for
Orbital control of Pleistocene euxinia in Lake Magadi, Kenya
D.M. Deocampo; R.B. Owen; T.K. Lowenstein; R.W. Renaut; N.M. Rabideaux ...
Abstract: Lake Magadi is an internally drained, saline and alkaline terminal sump in the southern Kenya Rift. Geochemistry of samples from an ~200 m core representing the past ~1 m.y. of the lake’s history shows some of the highest concentrations of transition metals and metalloids ever reported from lacustrine sediment, including redox-sensitive elements molybdenum, arsenic, and vanadium. Elevated concentrations of these elements represent times when the lake’s hypolimnion was euxinic—that is, anoxic, saline, and sulfide-rich. Euxinia was common after ca. 700 ka, and after that tended to occur during intervals of high orbital eccentricity. These were likely times when high-frequency hydrologic changes favored repeated episodes of euxinia and sulfide precipitation. High-amplitude environmental fluctuations at peak eccentricity likely impacted water balance in terrestrial habitats and resource availability for early hominins. These are associated with important events in human evolution, including the first appearance of Middle Stone Age technology between ca. 500 and 320 ka in the southern Kenya Rift.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49140.1/607797/Orbital-control-of-Pleistocene-euxinia-in-Lake
Unraveling the histories of Proterozoic shales through in situ Rb-Sr dating and trace element laser ablation analysis
Darwinaji Subarkah; Morgan L. Blades; Alan S. Collins; Juraj Farkaš; Sarah Gilbert ...
Abstract: Authigenic components in marine sediments are important archives for past environment reconstructions. However, defining reliable age constraints and assessing the effects of post-depositional overprints in Precambrian sequences are challenging. We demonstrate a new laser-based analytical approach that has the potential to rapidly and accurately evaluate the depositional and alteration histories of Proterozoic shales. Our study employs a novel application of in situ Rb-Sr dating coupled with simultaneous trace-element analysis using reaction-cell laser ablation–inductively coupled plasma–tandem mass spectrometry (LA-ICPMS/MS). We present results from shales sourced from two wells in the Proterozoic McArthur Basin, northern Australia. These rocks have been widely used by previous studies as a key section for ancient biogeochemical and paleo-redox reconstructions. Shales from well UR5 yielded initial 87Sr/86Sr ratios, Rb-Sr ages, and rare earth element plus yttrium (REEY) patterns similar to those of a dolerite sampled from the same core. We propose that the UR5 samples chronicle hydrothermal alteration instigated by the dolerite intrusion. In contrast, a correlative shale from well UR6 yielded an age consistent with the expected depositional age (1577 ± 56 Ma) with REEY and initial 87Sr/86Sr ratios similar to ca. 1.5 Ga seawater. We suggest that this sample records the minimum depositional age and early marine diagenetic history for this unit. This new technique can date Proterozoic shales quickly, cheaply, and with minimum sample preparation. Importantly, ages are triaged to differentiate between those recording primary marine versus secondary processes. This novel approach provides a potentially powerful tool for dating and fingerprinting the vast array of ancient marine shales for further studies of Earth systems through deep time.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49187.1/607798/Unraveling-the-histories-of-Proterozoic-shales
Metamorphism obscures primary taphonomic pathways in the early Cambrian Sirius Passet Lagerstätte, North Greenland
Morten Lunde Nielsen; Mirinae Lee; Hong Chin Ng; Jeremy C. Rushton; Katharine R. Hendry ...
Abstract: Correct interpretation of soft-bodied fossils relies on a thorough understanding of their taphonomy. While the focus has often been on the primary roles of decay and early diagenesis, the impacts of deeper burial and metamorphism on fossil preservation are less well understood. We document a sequence of late-stage mineral replacements in panarthropod fossils from the Sirius Passet Lagerstätte (North Greenland), an important early Cambrian Burgess Shale–type (BST) biota. Muscle and gut diverticula were initially stabilized by early diagenetic apatite, prior to being pervasively replaced by quartz and then subordinate chlorite, muscovite, and chloritoid during very low- to low-grade metamorphism. Each new mineral replicates the soft tissues with different precision and occurs in particular anatomical regions, imposing strong biases on the biological information retained. Muscovite and chloritoid largely obliterate the tissues’ original detail, suggesting that aluminum-rich protoliths may have least potential for conserving mineralized soft tissues in metamorphism. Overall, the fossils exhibit a marked shift toward mineralogical equilibration with the matrix, obscuring primary taphonomic modes. Sequential replacement of the phosphatized soft tissues released phosphorus to form new accessory monazite (and apatite and xenotime), whose presence in other BST biotas might signal the prior, more widespread, occurrence of this primary mode of preservation. Our results provide critical context for interpreting the Sirius Passet biota and for identifying late-stage overprints in other biotas.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48906.1/607799/Metamorphism-obscures-primary-taphonomic-pathways
Carbon isotope excursions during the late Miocene recorded by lipids of marine Thaumarchaeota, Piedmont Basin, Mediterranean Sea
Mathia Sabino; Daniel Birgel; Marcello Natalicchio; Francesco Dela Pierre; Jörn Peckmann
Abstract: Group I mesophilic Thaumarchaeota fix dissolved inorganic carbon (DIC), accompanied by a biosynthetic fractionation factor of ~20‰. Accordingly, the δ13C signature of their diagnostic biomarker crenarchaeol was suggested as a potential δ13CDIC proxy in marine basins if input from nonmarine Thaumarchaeota is negligible. Semi-enclosed basins are sensitive to carbon-cycle perturbations, because they tend to develop thermohaline stratification. Water column stratification typified the semi-enclosed basins of the Mediterranean Sea during the late Miocene (Messinian) salinity crisis (5.97–5.33 Ma). To assess how the advent of the crisis affected the carbon cycle, we studied sediments of the Piedmont Basin (northwestern Italy), the northernmost Mediterranean subbasin. A potential bias of our δ13CDIC reconstructions from the input of soil Thaumarchaeota is discarded, since high and increasing branched and isoprenoid tetraether (BIT) index values do not correspond to low and decreasing δ13C values for thaumarchaeal lipids, which would be expected in case of high input from soil Thaumarchaeota. Before the onset of the crisis, the permanently stratified distal part of the basin hosted a water mass below the chemocline with a δ13CDIC value of approximately –3.5‰, while the well-mixed proximal part had a δ13CDIC value of approximately –0.8‰. The advent of the crisis was marked by 13C enrichment of the DIC pool, with positive δ13CDIC excursions up to +5‰ in the upper water column. Export of 12C to the seafloor after phytoplankton blooms and limited replenishment of remineralized carbon due to the stabilization of thermohaline stratification primarily caused such 13C enrichment of the DIC pool.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49360.1/607640/Carbon-isotope-excursions-during-the-late-Miocene
Volcanic plumbing filters on ocean-island basalt geochemistry
Teresa Ubide; Patricia Larrea; Laura Becerril; Carlos Galé
Abstract: Ocean-island basalts (OIBs) are considered to be messengers from the deep mantle, yet the filtering effect of the plumbing systems that bring OIB melts to the surface remains poorly assessed. We investigated volcanic products from El Hierro island (Canary Islands) from textural and chemical perspectives. The majority of geochemical data cluster at relatively fractionated basaltic compositions of 5 wt% MgO. Compositions ≥10 wt% MgO are porphyritic whole rocks that accumulate mafic minerals. Near-primary melts do not erupt. Instead, we show that carrier melts (crystal-free whole rocks, glasses, and melt inclusions) are consistently buffered to low-MgO compositions during passage through the plumbing system. We tested our model of melt fractionation and crystal accumulation on a global compilation of OIBs. Similar to El Hierro, the majority of data cluster at evolved compositions of 5 wt% MgO (alkaline) to 7 wt% MgO (tholeiitic). Modeling the fractionation of OIB parental melts, we show that with 50% crystallization, OIB melts reach 5 wt% MgO with reduced density, increased volatile content, and overall low viscosity, becoming positively buoyant relative to wall rocks and highly eruptible when reaching volatile saturation at depths around the crust-mantle boundary. Under these conditions, 5 wt% MgO OIB “sweet spot” melts are propelled to the surface and erupt carrying an assortment of recycled crystals. This mechanism is consistent with the petrography and chemistry of erupted products and suggests OIB volcanoes are dominated by low-MgO basaltic melts.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49224.1/607641/Volcanic-plumbing-filters-on-ocean-island-basalt
Boninitic blueschists record subduction initiation and subsequent accretion of an arc–forearc in the northeast Proto-Tethys Ocean
Dong Fu; Bo Huang; Tim E. Johnson; Simon A. Wilde; Fred Jourdan ...
Abstract: Subduction of oceanic lithosphere is a diagnostic characteristic of plate tectonics. However, the geodynamic processes from initiation to termination of subduction zones remain enigmatic mainly due to the scarcity of appropriate rock records. We report the first discovery of early Paleozoic boninitic blueschists and associated greenschists from the eastern Proto-Tethyan North Qilian orogenic belt, northeastern Tibet, which have geochemical affinities that are typical of forearc boninites and island arc basalts, respectively. The boninitic protoliths of the blueschists record intra-oceanic subduction initiation at ca. 492–488 Ma in the eastern North Qilian arc/forearc–backarc system, whereas peak blueschist facies metamorphism reflects subsequent subduction of the arc/forearc complex to high pressure at ca. 455 Ma. These relations therefore record the life circle of an intra-oceanic subduction zone within the northeastern Proto-Tethys Ocean. The geodynamic evolution provides an early Paleozoic analogue of the early development of the Izu–Bonin–Mariana arc and its later subduction beneath the extant Japanese arc margin. This finding highlights the important role of subduction of former upper plate island arc/forearcs in reducing the likelihood of preservation of initial subduction-related rock records in ancient orogenic belts.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49457.1/607464/Boninitic-blueschists-record-subduction-initiation
Detrital signals of coastal erosion and fluvial sediment supply during glacio-eustatic sea-level rise, Southern California, USA
G.R. Sharman; J.A. Covault; D.F. Stockli; Z.T. Sickmann; M.A. Malkowski ...
Abstract: Coastal erosion, including sea-cliff retreat, represents both an important component of some sediment budgets and a significant threat to coastal communities in the face of rising sea level. Despite the importance of predicting future rates of coastal erosion, few prehistoric constraints exist on the relative importance of sediment supplied by coastal erosion versus rivers with respect to past sea-level change. We used detrital zircon U-Pb geochronology as a provenance tracer of river and deep-sea fan deposits from the Southern California Borderland (United States) to estimate relative sediment contributions from rivers and coastal erosion from late Pleistocene to present. Mixture modeling of submarine canyon and fan samples indicates that detrital zircon was dominantly (55%–86%) supplied from coastal erosion during latest Pleistocene (ca. 13 ka) sea-level rise, with lesser contributions from rivers, on the basis of unique U-Pb age modes relative to local Peninsular Ranges bedrock sources. However, sediment that was deposited when sea level was stable at its highest and lowest points since the Last Glacial Maximum was dominantly supplied by rivers, suggesting decreased coastal erosion during periods of sea-level stability. We find that relative sediment supply from coastal erosion is strongly dependent on climate state, corroborating predictions of enhanced coastal erosion during future sea-level rise.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49430.1/607465/Detrital-signals-of-coastal-erosion-and-fluvial
Extreme event-driven sediment aggradation and erosional buffering along a tectonic gradient in southern Taiwan
Clarke DeLisle; Brian J. Yanites; Chia-Yu Chen; Bruce H. Shyu; Tammy M. Rittenour
Abstract: In most landscape evolution models, extreme rainfall enhances river incision. In steep landscapes, however, these events trigger landslides that can buffer incision via increased sediment delivery and aggradation. We quantify landslide sediment aggradation and erosional buffering with a natural experiment in southern Taiwan where a northward gradient in tectonic activity drives increasing landscape steepness. We find that landscape response to extreme rainfall during the 2009 typhoon Morakot varied along this gradient, where steep areas experienced widespread channel sediment aggradation of >10 m and less steep areas did not noticeably aggrade. We model sediment export to estimate a sediment removal timeline and find that steep, tectonically active areas with the most aggradation may take centuries to resume bedrock incision. Expected sediment cover duration reflects tectonic uplift. We find that despite high stream power, sediment cover may keep steep channels from eroding bedrock for up to half of a given time period. This work highlights the importance of dynamic sediment cover in landscape evolution and suggests a mechanism by which erosional efficiency in tectonically active landscapes may decrease as landscape steepness increases.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49304.1/607466/Extreme-event-driven-sediment-aggradation-and
Tunnel valley infill and genesis revealed by high-resolution 3-D seismic data
James D. Kirkham; Kelly A. Hogan; Robert D. Larter; Ed Self; Ken Games ...
Abstract: Landforms produced beneath former ice sheets offer insights into inaccessible subglacial processes and present analogues for how current ice masses may evolve in a warming climate. Large subglacial channels cut by meltwater erosion (tunnel valleys [TVs]) have the potential to provide valuable empirical constraints for numerical ice-sheet models concerning realistic melt rates, water routing, and the interplay between basal hydrology and ice dynamics. However, the information gleaned from these features has thus far been limited by an inability to adequately resolve their internal structures. We use high-resolution three-dimensional (HR3-D) seismic data (6.25 m bin size, ~4 m vertical resolution) to analyze the infill of buried TVs in the North Sea. The HR3-D seismic data represent a step-change in our ability to investigate the mechanisms and rates at which TVs are formed and filled. Over 40% of the TVs examined contain buried glacial landforms including eskers, crevasse-squeeze ridges, glacitectonic structures, and kettle holes. As most of these landforms had not previously been detected using conventional 3-D seismic reflection methods, the mechanisms that formed them are currently absent from models of TV genesis. The ability to observe such intricate internal structures opens the possibility of using TVs to reconstruct the hydrological regimes of former mid-latitude ice sheets as analogues for contemporary ones.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49048.1/607467/Tunnel-valley-infill-and-genesis-revealed-by-high
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Journal
Geology