New Geology Articles Published Online Ahead of Print in July
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50075.1/615873/New-constraints-on-the-source-of-loess-from-U-Th
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50208.1/615874/Ancient-structural-inheritance-explains-gold
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50090.1/615875/Differentiation-between-carbonate-and-silicate
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50083.1/615876/A-Rosetta-stone-linking-melt-trajectories-in-the
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50180.1/615877/Autobrecciation-and-fusing-of-mafic-magma
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, <1°, similar to
hydrocarbon fields. On the other hand, CO2-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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50071.1/615404/Hydrodynamic-invalidation-of-synformal-traps-for
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50110.1/615406/Latitudinal-diversity-gradient-dynamics-during
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50016.1/615407/Mercury-evidence-of-Deccan-volcanism-driving-the
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49913.1/615408/North-America-s-Midcontinent-Rift-magma-volume-A
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50138.1/615409/Physical-transport-of-magmatic-sulfides-promotes
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
(<50 Ma) heating and delamination of the lithosphere. To test this, we
queried the fine structure in the thermal history of the mantle lithosphere
approaching this transition using mantle xenolith thermobarometry. The
xenoliths sampled by their host lava within ~20 km of the Cordillera-craton
boundary require thinner lithosphere (by 20 km) and a hotter Moho (by 200
°C) compared to those sampled 180 km away. A hot, weak lithosphere at the
boundary explains a topographic low that parallels this transition for a
large length of the North American Cordillera, possibly from focused heat
during edge convection. The cooling pattern of geothermometers applied to
the xenoliths also suggests that any delamination event producing thinner
lithosphere in the orogen is unlikely to have occurred in the past 50 m.y.
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50261.1/615120/Hikurangi-megathrust-slip-behavior-influenced-by
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50341.1/615121/Extensive-and-ancient-feldspathic-crust-detected
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50215.1/615122/Long-runout-pyroclastic-density-currents-Analysis
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50112.1/615123/Unleashing-alkali-feldspar-Ra-Th-ages-and-chemical
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50229.1/615124/Duplex-kinematics-reduces-both-frontal-advance-and
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.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50125.1/615125/An-absolutely-dated-record-of-climate-change-over
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