Boulder, Colo., USA: Article topics and locations include submarine landslides; Deccan volcanism; North America's Midcontinent Rift magma volume; the Cordillera-craton boundary of western Canada; north Hellas rim, Mars; magmatism in the Canary Islands; and the Chinese Loess Plateau. These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent .
New constraints on the source of loess from U/Th-Pb geochronology of detrital multi-minerals
Guangyan Zhou; Long Li; Guoqiao Xiao; Luo Yan; D. Graham Pearson ...
Abstract: The origin and provenance of eolian sediments such as loess can potentially reveal paleo-air circulation directions that drive temporal changes in climate. The provenance of these deposits has been mostly assessed by detrital zircon U-Pb geochronology and remains debated. Recent studies reveal that constraining sediment provenance using only detrital zircon yields a restricted view of the overall sediment sources, because zircon commonly records high-temperature tectonothermal events and thus only represents a portion of provenance lithologies. We applied a new strategy for constraining the provenance of eolian sediments from the Chinese Loess Plateau—Earth's most extensive loess deposit—by using a multi-proxy (i.e., zircon, monazite and rutile) U/Th-Pb geochronology approach. We show that while zircon U-Pb-Lu-Hf isotopic data cannot conclusively discriminate the potential sources, monazite and rutile grains display dominant U/Th-Pb age peaks in the early Paleozoic, early Mesozoic, and late Cenozoic, mirroring the orogenic episodes of the northeast Tibetan Plateau. Monazite and rutile records also reveal spatial variability in sediment provenance between the western and central parts of the Chinese Loess Plateau, which is potentially affected by dust transport from westerly/northwesterly winds. Our integrated data set provides diagnostic constraints on the dust source, provenance variability, and transportation trajectory of the Chinese Loess Plateau, and highlights the importance of this new approach to provenance study of eolian deposits worldwide.
Ancient structural inheritance explains gold deposit clustering in northern Perú
Daniel Wiemer; Steffen G. Hagemann; Jon Hronsky; Anthony I.S. Kemp; Nicolas Thébaud ...
Abstract: The punctuated distribution of major gold deposits along orogenic belts is poorly understood. In northern Perú, gold deposits cluster along trends that transect the subparallel Phanerozoic belts that accreted onto the western margin of the Archean–Proterozoic Amazonian craton. This suggests the influence of as yet unrecognized pre-Andean basement structures in controlling the localization of Phanerozoic deposits. We report the newly discovered Tomac ophiolite, which represents a missing link in tracing a cryptic basement collisional suture striking obliquely to the Peruvian Andes. The Tomac ophiolite signals the opening of a former Neoproterozoic ocean basin between the Amazonian craton and a continental ribbon of dispersed microterranes. Re-accretion of the continental ribbon resulted in the formation of a collisional belt between ca. 465 and 440 Ma. Rifting and re-accretion occurred parallel to the strike of the ca. 1400–1000 Ma Grenvillian-Sunsás orogen, indicating that structural corridors that transect the Andes at an oblique strike angle reflect inheritance of preexisting lithospheric weak zones. Our study demonstrates that Phanerozoic gold deposits in northern Perú cluster at the intersection between the newly defined basement suture and the superimposed Andean belt and associated well-documented cross faults. Gold mineralization was localized at ancient structural intersections, where enhanced permeability facilitated anomalous energy and mass transfer, promoting major ore accumulation during episodes of accretion.
Differentiation between carbonate and silicate metasomatism based on lithium isotopic compositions of alkali basalts
Dong-Bo Tan; Yilin Xiao; Li-Qun Dai; He Sun; Yangyang Wang ...
Abstract: Carbonate and silicate metasomatism occurring in subduction zones is an important mechanism underlying mantle heterogeneity and compositional diversity of mantle-derived rocks. However, distinguishing between the two kinds of metasomatism is often difficult. Lithium (Li) and its isotopes have great potential in this regard because of the different Li isotopic compositions of recycled marine carbonate and silicate components. We report Li isotopic data from Cenozoic and Mesozoic alkali basalts of the West Qinling orogen in central China. Relative to those for normal basalts, very high δ7Li values (up to +11.2‰) were observed for the Cenozoic alkali basalts, but significantly and systematically lower values (as low as –3.3‰) were estimated for the Mesozoic alkali basalts. Their abnormal Li isotopic compositions, combined with major- and trace-element contents and Sr-Mg isotope ratios, indicate that the Cenozoic and Mesozoic alkali basalts originated from carbonated and silicated mantle sources, respectively, reflecting metasomatism of the mantle by slab-derived carbonate and silicate melts during Paleotethyan oceanic subduction. Interactions of such melts with the mantle peridotite in subduction channels can account for the elemental and isotopic differences of the studied alkali basalts. The present study demonstrates an effective way to distinguish between carbonate and silicate metasomatism in subduction zones by studying Li isotopic compositions of alkali basalts and highlights the prospect of Li isotopes in tracing the deep carbon cycle.
A Rosetta stone linking melt trajectories in the mantle to the stress field and lithological heterogeneities (Trinity ophiolite, California)
Georges Ceuleneer; Mathieu Rospabé; Tom Chatelin; Hadrien Henry; Romain Tilhac ...
Abstract: Infiltration triggered by selective dissolution of pyroxenes is a major mode of melt migration in the mantle. A common view, supported by experiments and numerical models, is that the geometry of the melt plumbing system is governed by the stress field induced by solid-state flow of the host peridotite. Yet, salient melt migration structures frozen at an early stage of development in the mantle section of the Trinity ophiolite reveal that lithological heterogeneities drastically impact melt trajectories. Where melts reach a pyroxenite layer, dissolution-induced permeability abruptly increases, initiating a feedback loop confining melt migration to that layer regardless of its orientation relative to the stress field. This process results in the development of a network of interweaved dunitic channels evolving to thick tabular dunites where the melt reacts with closely spaced pyroxenite layers. This reacting melt was rich in alkali elements and water, as evidenced by the minerals (mostly amphibole and micas) encapsulated in the Cr-spinel grains that crystallized during the reaction. This "pioneer melt" differs from the volumetrically dominant depleted andesite that fed the crustal section. In fact, the migration of andesite benefited from the enhanced permeability provided by the dunites formed by the pioneer melt. As a result, dunites are palimpsests, the compositions of which record successive percolation events. The geometry of the melt pathways is extremely challenging to model because the abundance, spacing, and orientation of lithological heterogeneities cannot be predicted, being inherited from a long geological history.
Autobrecciation and fusing of mafic magma preceding explosive eruptions
Aaron A. Marshall; Michael Manga; Brittany D. Brand; Benjamin J. Andrews
Abstract: Bubble and crystal textures evolve during magma ascent, altering properties that control ascent such as permeability and viscosity. Eruption style results from feedbacks between ascent, bubble nucleation and growth, microlite crystallization, and gas loss, all processes recorded in pyroclasts. We show that pyroclasts of the mafic Curacautín ignimbrite of Llaima volcano, Chile, record a history of repeated autobrecciation, fusing, and crystallization. We identified pyroclasts with domains of heterogeneous vesicle textures in sharp contact with one another that are overprinted by extensive microlite crystallization. Broken crystals with long axes (l) >10 μm record fragmentation events during the eruption. A second population of unbroken microlites with l ≤10 μm overprint sutures between fused domains, suggesting the highly crystalline groundmass formed at shallow depths after autobrecciation and fusing. Nearly all pyroclasts contain plutonic and ancestral Llaima lithics as inclusions, implying that fusing occurs from a few kilometers depth to as shallow as the surface. We propose that Curacautín ignimbrite magma autobrecciated during ascent and proto-pyroclasts remained melt rich enough to fuse together. Lithics from the conduit margins were entrained into the proto-pyroclasts before fusing. Autobrecciation broke existing phenocrysts and microlites; rapid post-fusing crystallization then generated the highly crystalline groundmass. This proposed conduit process has implications for interpreting the products of mafic explosive eruptions.
Hydrodynamic invalidation of synformal traps for dissolved CO 2
S.A. Stewart; L.A. Riddell
Abstract: Reservoirs with the geometry of structural lows trap negatively buoyant fluids, mirroring the way structural highs trap positively buoyant fluids. In both scenarios, lateral flow of reservoir water alters the trapping geometry by causing fluid contacts to tilt. Tilt increases in proportion to hydraulic head gradient increases and the density contrast between the flowing and trapped fluids decreases. Positively buoyant, immiscible supercritical CO2 is at least 20% lighter than typical saline formation waters at CO2 subsurface storage pressures and temperatures, and would experience relatively low tilts, 2-saturated brines are within 1% of the density of CO2-free equivalent brine. This relatively low density contrast creates tilted contacts at much higher angles even at the low hydraulic head gradients that typify deep saline aquifers. For example, a fluid contact with 0.5% density contrast exceeds 2° tilt for hydraulic head gradients of only 15 cm/km. Many large and basin-scale synformal traps are formed with structural dips of just a few degrees and therefore cannot trap CO2 solutions under hydrodynamic conditions. This problem could be overcome by utilization of hydrodynamic traps or appropriately configured structural-stratigraphic traps for CO2 sequestration.
Extreme erosion by submarine slides
Harya D. Nugraha; Christopher A.-L. Jackson; Howard D. Johnson; David M. Hodgson; Michael A. Clare
Abstract: Submarine slides (including slides, slumps, and debris flows) pose major geohazards by triggering tsunami and damaging essential submarine infrastructure. Slide volume, a key parameter in hazard assessments, can increase markedly through substrate and/or water entrainment. However, the erosive potential of slides is uncertain. We quantified slide erosivity by determining the ratio of deposited (Vd) to initially evacuated (Ve) sediment volumes; i.e., slides that gain volume through erosion have a Vd/Ve ratio >1. We applied this method to the Gorgon slide, a large (500 km 3), seismically imaged slide offshore northwestern Australia, and reviewed Vd/Ve ratios for 11 other large slides worldwide. Nine of the 11 slides have V d/Ve >1 (median value = 2), showing emplaced volumes increased after initial failure. The Gorgon slide is the most erosive slide currently documented (Vd/V e = 13), possibly reflecting its passage across a highly erodible carbonate ooze substrate. Our new approach to quantifying erosion is important for hazard assessments given substrate-flow interactions control slide speed and runout distance. The variations in slide volume also have important implications for submarine infrastructure impact assessments, including more robust tsunami modeling.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50164.1/615405/Extreme-erosion-by-submarine-slides
Latitudinal diversity gradient dynamics during Carboniferous to Triassic icehouse and greenhouse climates
Shu-Han Zhang; Shu-Zhong Shen; Douglas H. Erwin
Abstract: Understanding the complex patterns of latitudinal diversity gradients (LDGs) in deep time has been hampered by the absence of long-term records of LDGs through multiple climatic changes. We used records of marine invertebrate fossils to generate LDGs in each age bin from the Carboniferous icehouse to the Triassic greenhouse climates. We evaluated LDGs by subsampling, calculated evolutionary rates for different latitudinal zones, and assessed the modularity of the fossil data within latitudinal zones using the Louvain algorithm. Our results suggest that the LDG peaks may be shaped by multiple factors rather than alternating icehouse and greenhouse climates, although icehouse climates usually restrict diversity at high latitudes. In nearly all clades, peak diversity shifted northward during the late Carboniferous and early Permian, reflecting the northward drift of plates and increased habitat area. Changes in the steepness of the LDG was most pronounced at low latitudes and during biotic crises and recovery, while icehouse to greenhouse transitions created more deviation at high latitudes. Our results show a strong historical influence from previous LDG patterns in LDG dynamics, but one that was interrupted by upheavals in composition after dramatic environmental changes.
Mercury evidence of Deccan volcanism driving the Latest Maastrichtian warming event
Sha Li; Stephen E. Grasby; Xiangdong Zhao; Jiubin Chen; Daran Zheng ...
Abstract: The timing and ecological impacts of the Deccan Traps large igneous province eruption are vigorously debated. Pre–Cretaceous-Paleogene (KPg) boundary impacts of Deccan volcanism have been widely identified in marine sediments, but direct evidence of terrestrial impacts remains rare. We used mercury concentrations and isotopic compositions, a proxy for volcanic activity, to assess impacts on terrestrial environments. We studied two drill cores across the KPg boundary in eastern China that represent two different depositional environments: clastic deposits in the Jiaolai Basin and carbonate deposits in the Pingyi Basin. Both drill cores exhibit strong Hg enrichment prior to the KPg boundary. Near consistent mass-independent fractionation (MIF) of odd-Hg isotopes (odd-MIF) in the Jiaolai Basin likely indicates a volcanogenic source of Hg spikes below the KPg boundary. Odd-MIF isotopes in the Pingyi Basin likewise suggest a volcanogenic Hg source but with a terrestrial Hg signature of lower Δ199Hg values before and after the Hg spike interval. The Hg enrichment level can be stratigraphically correlated to the beginning of the Latest Maastrichtian warming event (LMWE) and is consistent with a strong, negative carbon-isotope excursion (CIE) in both δ13C org (organic matter) and δ13Ccarb (carbonate), suggesting a disturbance of the global carbon cycle induced by a major pulse of Deccan Traps volcanism. Our discovery of a terrestrial record of pre-KPg boundary Deccan volcanism provides robust evidence of global influence of the Deccan Traps large igneous province during the LMWE.
North America's Midcontinent Rift magma volume: A coincidental rendezvous of a plume with a rift
Prasanna Mahesh Gunawardana; Robert Moucha; Tyrone O. Rooney; Seth Stein; Carol A. Stein
Abstract: The Midcontinent Rift of North America is a ca. 1.1 Ga, 3000-km-long failed rift that nearly split the Precambrian continent of Laurentia. Unlike most continental rifts, which are filled with a mixture of volcanic rocks and sediments, the Midcontinent Rift contains a large volume of flood basalts that were emplaced during both syn- and post-rift stages. Consequently, the Midcontinent Rift, which comprises the Keweenaw large igneous province, is the most significant positive anomaly on gravity maps of central North America. We investigated the mantle conditions required to produce this large volume of flood basalt and the observed two main stages of emplacement. To explore whether these magma volumes required a plume or, instead, could have resulted from the increased ambient mantle temperatures expected for the Neoproterozoic, we used a geodynamic model for a range of ambient mantle and plume temperatures under different scenarios of lithospheric extension. The most favorable scenario for the generation of both syn-rift and post-rift lavas combines a plume with excess temperatures between 175 and 225 °C introduced during the syn-rift phase and ambient mantle potential temperatures between 1393 and 1443 °C, with an initial lithospheric thickness not exceeding 150 km for 3 mm/yr extension rates.
Physical transport of magmatic sulfides promotes copper enrichment in hydrothermal ore fluids
Christoph A. Heinrich; James A.D. Connolly
Abstract: Loss of magmatic sulfides to the mantle is posited to explain the copper deficit of evolved arc magmas and the depleted Cu/Ag ratio of the continental crust. We address the question of whether saturating sulfides may instead be mechanically entrained with rising magmas, and how this would affect their geochemical fate in the upper crust. Entrainment is plausible considering sulfide wetting properties and settling velocities relative to magma ascent velocities. Entrained sulfide increases the pressure at which magmas become saturated with respect to H-O-S fluids in the upper crust by 10–100 MPa, with the pressure difference increasing with temperature, water content, and oxidation. Bubbles are likely to nucleate on sulfide particles, allowing transfer of S and Cu from the sulfide to the fluid over a small crystallization interval without limitations by diffusion through the silicate melt. This sequence of processes gives magmatic sulfides an active role in ore metal transport and enrichment to form porphyry copper deposits, and may have global implications for crustal Cu budgets.
Xenoliths reveal a hot Moho and thin lithosphere at the Cordillera-craton boundary of western Canada
Dante Canil; J. Kelly Russell
Abstract: Properties of the lithosphere control the transitions in elevation and plate deformation from hot, mobile orogenic belts to cooler, stronger cratons. The sharp, abrupt boundary of the North American Cordillera with the craton in western Canada has been suggested to be a result of recent (View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50151.1/615119/Xenoliths-reveal-a-hot-Moho-and-thin-lithosphere
Hikurangi megathrust slip behavior influenced by lateral variability in sediment subduction
Andrew C. Gase; Nathan L. Bangs; Harm J.A. Van Avendonk; Dan Bassett; Stuart A. Henrys
Abstract: Subduction megathrusts exhibit a range of slip behaviors spanning from large earthquakes to aseismic creep, yet what controls spatial variations in the dominant slip mechanism remains unresolved. We present multichannel seismic images that reveal a correlation between the lithologic homogeneity of the megathrust and its slip behavior at a subduction zone that is world renowned for its lateral slip behavior transition, the Hikurangi margin. Where the megathrust exhibits shallow slow-slip in the central Hikurangi margin, the protolith of the megathrust changes ~10 km downdip of the deformation front, transitioning from pelagic carbonates to compositionally heterogeneous volcaniclastics. At the locked southern Hikurangi segment, the megathrust forms consistently within pelagic carbonates above thickened nonvolcanic siliciclastic sediments (unit MES), which subduct beyond 75 km horizontally. The presence of the MES layer plays a key role in smoothing over rough volcanic topography and establishing a uniform spatial distribution of lithologies and frictional properties that may enable large earthquake ruptures.
Extensive and ancient feldspathic crust detected across north Hellas rim, Mars: Possible implications for primary crust formation
Michael S. Phillips; Christina E. Viviano; Jeffrey E. Moersch; A. Deanne Rogers; Harry Y. McSween ...
Abstract: Due to its size and observable record of ancient rock, Mars is key to understanding crustal formation on planetary bodies, including Venus and Earth, which may have derived their first stable crust from mantle-overturn melting. Recent evidence that ancient martian crust contains an evolved component supports inferences of a pervasive, buried feldspathic component to the crust. With data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), we searched for feldspathic lithologies in pre-Noachian (older than ca. 4.1 Ga) crustal blocks uplifted by the Hellas basin-forming impact. We present evidence for ancient feldspathic rocks exposed across an ~2200 × 600 km area north of Hellas. Given their pre-Noachian age and stratigraphic position directly above putative mantle material, it is possible that these outcrops represent martian primary crust. Our discovery supports the hypothesis that there exists a pervasive, subsurface feldspathic component to the martian crust—a hypothesis that has, until now, been supported only by inferences from geodynamic data and small-scale observations.
Long-runout pyroclastic density currents: Analysis and implications
Olivier Roche; Christopher D. Henry; Nourddine Azzaoui; Arnaud Guillin
Abstract: Pyroclastic density currents are hazardous, ground-hugging, hot mixtures of gas and solid particles produced by volcanoes. Currents generated by the largest explosive eruptions have traveled distances on the order of 100 km, and their devastating impact has repeatedly marked Earth's geologic history. We show that pyroclastic density currents from super-eruptions during the Oligocene in the Great Basin of the United States had exceptional runout distances that may have exceeded 300 km. We present a quantitative analysis of the data from these currents and consider, in particular, the areal extent of their deposits (ignimbrites) as well as the relationship between their runout and the eruption mass discharge rate. The ignimbrites have elliptical distributions characterized by axis length ratios of ~2–6, in contrast to common subcircular ignimbrites, while the maximum runouts versus the discharge rates are clearly outside of the prediction intervals defined by other eruption data. We argue that the long runouts resulted from the channeling of concentrated currents in regional paleovalleys whose gentle slope worked to lengthen the travel distance. Our study demonstrates that these additional factors should be considered in assessing hazards posed by future super-eruptions.
Unleashing alkali feldspar: Ra/Th ages and chemical and isotopic constraints on Holocene phonolite magmatism, Canary Islands
Bryce S. Brown; Frank C. Ramos; John A. Wolff; Olaya Dorado; Joan Martí
Abstract: Accurately dating phenocrysts in Holocene volcanic rocks poses many challenges but is critical to placing magmatic processes that occur prior to eruption into a temporal framework. We dated alkali feldspar (i.e., orthoclase Or10 to Or46) crystals in four young phonolites from the Teide–Pico Viejo volcanic complex, Tenerife (Spain), using (226Ra)/(230Th) isotopes. Partition coefficients of Ra (DRa) and DRa/ DBa of feldspars were predicted using an approach based on the lattice strain model, which yielded crystallization ages that overlap or predate known eruption ages for the Lavas Negras (ca. 1 ka), Montaña Blanca (ca. 2 ka), Arenas Blancas (ca. 2–4 ka), and Teide H (ca. 6 ka) phonolites. Crystallization of feldspar may occur up to the time of eruption, with >8 ka crystals also present, possibly suggesting extended magma differentiation times. However, feldspars yielding finite ( 226Ra)/(230Th) ages are mostly in equilibrium with the groundmass, unlike >8 ka crystals, which were therefore identified as antecrysts/xenocrysts. The 87Sr/86Sr ratios of feldspars indicate that crystallization predated late-stage assimilation, affecting 87Sr/86Sr ratios of some melts. The ( 226Ra)/(230Th) ages also constrain the tempo of phonolite magma evolution on Tenerife. Integration of (226Ra)/( 230Th) ages with feldspar major elements, trace elements, and isotopes provides a powerful means for investigating crystallization histories using a dominant mineral that controls the overall magmatic evolution of phonolites on thousand-year time scales.
Duplex kinematics reduces both frontal advance and seismic moment deficit in the Himalaya
Wan-Lin Hu; Victoria L. Stevens
Abstract: Duplexing plays important roles in the evolution of fold-and-thrust belts and accretionary wedges, and causes internal shortening of the system, which then impacts both rates of frontal advance and seismic-moment deficit. Nevertheless, the significance of this internal shortening has not yet been highlighted in previous studies in the Himalaya or elsewhere. We invoke geometric solutions to constrain the ratio of transferred slip ( R; i.e., the ratio of updip slip to downdip slip) for the midcrustal ramp—the most active ramp within the midcrustal duplex—in the Himalayan wedge. We find that R is ~0.9, and then used this ratio to calculate the accumulating seismic moment. The reduction in seismic-moment accumulation over the past 1000 yr along the entire Himalayan arc (~2200 km) is equivalent to at least one ~Mw 8.72 earthquake, and potentially reduces the seismic moment deficit by ~23%–54%, which may reconcile the long-term unbalanced seismic moment in the Himalaya.
An absolutely dated record of climate change over the last three glacial–interglacial cycles from Chinese loess deposits
Junjie Zhang; Qingzhen Hao; Sheng-Hua Li
Abstract: Chinese loess-paleosol sequences have long been regarded as the continental counterpart of deep-sea sediments in terms of their record of the Quaternary glacial–interglacial cycles. However, absolute chronologies for loess-paleosol sequences older than ca. 130 ka on the Chinese Loess Plateau are scarce. We conducted the first high-resolution luminescence dating, extending back to 350 ka, of the classical Luochuan loess section of the central Chinese Loess Plateau. Bacon age-depth modeling of the luminescence ages was used to obtain an age framework. The results indicate that on orbital timescales, loess accumulation at Luochuan was continuous over the last three glacial–interglacial cycles. The new age framework resolves discrepancies between orbitally tuned age models at several boundaries and provides new evidence supporting the reassignment of the Marine Isotope Stage 8/9 boundary age from 300 ka to ca. 280 ka. Our results also suggest that published astronomical timescales for the loess deposits of the Chinese Loess Plateau over the last ~2.6 m.y. are probably broadly reliable, as are the related regional and global climatic interpretations.
GEOLOGY articles are online at https://geology.geoscienceworld.org/content/early/recent
