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New Geology articles published online ahead of print in April

Boulder, Colo., USA: Thirty-one new articles were published online ahead of
print for Geology in April. Topics include shocked zircon from the
Chicxulub impact crater; the Holocene Sonoran Desert; the architecture of
the Congo Basin; the southern Death Valley fault; missing water from the
Qiangtang Basin; sulfide inclusions in diamonds; how Himalayan collision
stems from subduction; ghost dune hollows; and the history of the Larsen C Ice Shelf. These Geology articles are online at


Nonlinear fault damage zone scaling revealed through analog modeling

Sylvain Mayolle; Roger Soliva; Stéphane Dominguez; Christopher Wibberley;
Yannick Caniven

Fault damage zones strongly influence fluid flow and seismogenic behavior
of faults and are thought to scale linearly with fault displacement until
reaching a threshold thickness. Using analog modeling with different
frictional layer thicknesses, we investigate damage zone dynamic evolution
during normal fault growth. We show that experimental damage zone growth
with displacement is not linear but progressively tends toward a threshold
thickness, being larger in the thicker models. This threshold thickness
increases significantly at fault segment relay zones. As the thickness
threshold is approached, the failure mode progressively transitions from
dilational shear to isochoric shear. This process affects the whole layer
thickness and develops as a consequence of fault segment linkage as
inferred in nature when the fault matures. These findings suggest that
fault damage zone widths are limited both by different scales of mechanical
unit thickness and the evolution of failure modes, ultimately controlled in
nature by lithology and deformation conditions.

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History of the Larsen C Ice Shelf

reconstructed from sub-ice shelf and offshore sediments

J.A. Smith; C.-D. Hillenbrand; C. Subt; B.E. Rosenheim; T. Frederichs …

Because ice shelves respond to climatic forcing over a range of time
scales, from years to millennia, an understanding of their long-term
history is critically needed for predicting their future evolution. We
present the first detailed reconstruction of the Larsen C Ice Shelf (LCIS),
eastern Antarctic Peninsula (AP), based on data from sediment cores
recovered from below and in front of the ice shelf. Sedimentologic and
chronologic information reveals that the grounding line (GL) of an expanded
AP ice sheet had started its retreat from the midshelf prior to 17.7 ± 0.53
calibrated (cal.) kyr B.P., with the calving line following ~6 k.y. later.
The GL had reached the inner shelf as early as 9.83 ± 0.85 cal. kyr B.P.
Since ca. 7.3 ka, the ice shelf has undergone two phases of retreat but
without collapse, indicating that the climatic limit of LCIS stability was
not breached during the Holocene. Future collapse of the LCIS would
therefore confirm that the magnitudes of both ice loss along the eastern AP
and underlying climatic forcing are unprecedented during the past 11.5 k.y.

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Confirmation of slow Ti diffusion in quartz by diffusion couple
experiments and evidence from natural samples

Andreas Audétat; Nobuyoshi Miyajima; Dorothea Wiesner; Jean-Nicolas Audinot

Titanium diffusion profiles in natural quartz crystals have become an
increasingly popular tool to reconstruct the time scales of various
magmatic, metamorphic, and hydrothermal processes. However, the original
calibration of Ti diffusion rates in quartz has recently been challenged,
and diffusivities were found to be about three orders of magnitude lower.
We performed annealing experiments on crystal-crystal diffusion couples
consisting of Ti-free synthetic quartz seeds over which Ti-rich quartz
(100-3000 μg/g Ti) was grown hydrothermally. The annealing experiments were
performed at 1000-1600 °C and 0.1 MPa to 2.0 GPa, and they lasted for 3-84
days. The resulting diffusion profiles were mapped by cathodoluminescence
(CL), transmission electron microscope-energy-dispersive X-ray spectroscopy
(TEM-EDXS), and, for the first time, by helium ion microscope-secondary ion
mass spectrometry (HIM-SIMS). Obtained diffusion coefficients range from
values similar to the lower range in previous research to values up to two
orders of magnitude lower. In addition, inversely zoned quartz and sanidine
phenocrysts in a natural rhyolite were studied. Comparison of the diffusion
profiles suggests that at ~735 °C, the Ti diffusivity in quartz is ~1.5 and
3.0 orders of magnitude lower than that of Ba and Sr, respectively, in
sanidine. The combined evidence confirms that Ti diffusion in quartz is
very slow, potentially even slower than proposed earlier. Consequently,
previous time scales derived from Ti diffusion profiles in quartz are
likely orders of magnitude too short, and further experiments are necessary
to fully clarify the issue.

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Did accretion of the Caribbean oceanic plateau drive rapid crustal
thickening in the northern Andes?

Sarah W.M. George; Brian K. Horton; Cristian Vallejo; Lily J. Jackson; E.
Gabriela Gutierrez

The onset of orogenic shortening in the northern Andes Mountains coincided
with latest Cretaceous accretion of the Caribbean oceanic plateau. We
present isotopic data (εHf in zircon and εNd) coupled with arc position to
test whether accretion led to abrupt crustal thickening in the northern
Andes of Ecuador and Colombia. A rapid isotopic excursion toward more
evolved crustal compositions was synchronous with ca. 75-70 Ma collision in
Ecuador and preceded a similar deviation in Colombia at ca. 70-55 Ma. The
rapid but diachronous shift to more evolved isotopic signatures is
attributed to progressive northward accretion of the oceanic plateau and
associated thickening of continental crust. We emphasize the effects of
accretion on the magmatic evolution of Cordilleran-type margins, initially
provoking shortening and crustal thickening, and ultimately providing a
substrate for subsequent arcs.

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How Himalayan collision stems from subduction

M. Soret; K.P. Larson; J. Cottle; A. Ali

The mechanisms and processes active during the transition from continental
subduction to continental collision at the plate interface are largely
unknown. Rock records of this transition are scarce, either not exposed or
obliterated during subsequent events. We examine the tectono-metamorphic
history of Barrovian metamorphic rocks from the western Himalayan orogenic
wedge. We demonstrate that these rocks were buried to amphibolite-facies
conditions from ≤47 Ma to 39 ± 1 Ma, synchronously with the formation (46
Ma) and partial exhumation (45-40 Ma) of the ultrahigh-pressure eclogites.
This association indicates that convergence during continental subduction
was accommodated via development of a deep orogenic wedge built through
successive underplating of continental material, including the partially
exhumed eclogites, likely in response to an increase in interplate
coupling. This process resulted in the heating of the subduction interface
(from ~7 to ~20 °C/km) through advective and/or conductive heat transfer.
Rapid cooling of the wedge from 38 Ma, coeval with the formation of a
foreland basin, are interpreted to result from indentation of a promontory
of thick Indian crust.

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Mud in sandy riverbed deposits as a proxy for ancient fine-sediment

N. Wysocki; E. Hajek

The amount of silt and clay available to rivers reflects source-terrain
composition and weathering and can be a primary control on the form and
dynamics of channel networks. Fine sediment also affects the permeability
of buried fluvial reservoirs. Despite this significance, there is currently
a lack of methods for reconstructing how much fine sediment was transported
by ancient rivers. Mud accumulations in sandy river deposits are often
interpreted as indicators of variable flow conditions; however, these
deposits may present an opportunity to constrain how much fine sediment was
transported through ancient rivers. We report results from a series of
experiments designed to evaluate how much clay and silt are preserved in
sandy riverbed deposits under constant and variable discharge conditions.
Our results demonstrate that (1) mud deposits, including drapes and lenses,
form readily under constant, high-discharge conditions, (2) the amount of
fine sediment recovered from bed-material deposits increases as
fine-sediment supply increases, and (3) fine-sediment retention is higher
during bed aggradation than during bypass conditions. These results
indicate that the net retention of clay and silt in sandy riverbed deposits
may be a simple but powerful proxy for comparing the overall amount of fine
sediment supplied to ancient rivers.

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Subduction of the Izanagi-Pacific Ridge-transform intersection at the
northeastern end of the Eurasian plate

Toru Yamasaki; Gen Shimoda; Kenichiro Tani; Jinichiro Maeda; Futoshi

Recent reconstructions of global plate motions suggest that the
Izanagi-Pacific Ridge was subducted along the eastern margin of Eurasia at
ca. 50 Ma. In the Hidaka magmatic zone (HMZ), which was located at the
northeastern end of the Eurasian plate, three magmatic pulses occurred
(46-45, 40-36, and 19-18 Ma). We report whole-rock geochemical and Sr-Nd-Pb
isotopic data for 36 Ma high-Sr/Y (adakitic) rocks from the HMZ and show
that these rocks formed by partial melting of oceanic crust and were
emplaced as near-trench intrusions during ridge subduction. We reevaluate
the nature of plutonic rocks in the HMZ and show that both the 46-45 and
40-36 Ma granitoids have essentially identical geochemical features. The
distribution of plutons and magmatic cessation between 45 and 40 Ma are
best explained by subduction of a ridge-transform intersection with a large
offset of the ridge axis. The boundary between the Eocene granitoids
corresponds to the position of a paleo-transform fault, and adakitic
magmatism was caused by partial melting triggered by slab tearing at an
overlapping spreading center. The paleoridge-transform configuration
coincides with the locations of later large faults and a peridotite body.

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Climatically driven instability of marine methane hydrate along a
canyon-incised continental margin

Richard J. Davies; Miguel Ángel Morales Maqueda; Ang Li; Mark Ireland

Establishing how past climate change affected the stability of marine
methane hydrate is important for our understanding of the impact of a
future warmer world. As oceans shallow toward continental margins, the base
of the hydrate stability zone also shallows, and this delineates the
feather edge of marine methane hydrate. It is in these rarely documented
settings that the base of the hydrate stability zone intersects the seabed
and hydrate can crop out where it is close to being unstable and most
susceptible to dissociation due to ocean warming. We show evidence for a
seismically defined outcrop zone intersecting canyons on a canyon-incised
margin offshore of Mauritania. We propose that climatic, and hence ocean,
warming since the Last Glacial Maximum as well as lateral canyon migration,
cutting, and filling caused multiple shifts of the hydrate stability field,
and therefore hydrate instability and likely methane release into the
ocean. This is particularly significant because the outcrop zone is longer
on canyon-incised margins than on less bathymetrically complex submarine
slopes. We propose considerably more hydrate will dissociate in these
settings during future ocean warming, releasing methane into the world’s

View article:

Enhanced Quaternary exhumation in the Namche Barwa syntaxis, eastern

Rong Yang; Frédéric Herman; Ting Liu; Rabiul Haque Biswas; Maria Giuditta
Fellin …

The Namche Barwa syntaxis in the eastern Himalaya is rapidly evolving in
terms of its tectonics and topography. Here we constrain the exhumation
history of the Yigong River to the immediate north of the syntaxis across
different time scales using a multidisciplinary approach. Our new
thermochronometric data reveal an acceleration of exhumation rates since 2
Ma in the downstream of the Yigong. Cosmogenic nuclides and
thermoluminescence thermochronometry analyses confirm persistent rapid
exhumation in the lower Yigong over the Quaternary with further increased
exhumation in the last 100 ka. Together with the analysis of the morphology
of the Yigong River profile, we interpret that northward expansion of the
syntaxis together with capture of the Yigong by the Yarlung Tsangpo River
during this expansion is responsible for the exhumation history of the
Yigong River in the Quaternary.

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Protogenetic sulfide inclusions in diamonds date the diamond formation
event using Re-Os isotopes

M.G. Pamato; D. Novella; D.E. Jacob; B. Oliveira; D.G. Pearson …

Sulfides are the most abundant inclusions in diamonds and a key tool for
dating diamond formation via Re-Os isotopic analyses. The manner in which
fluids invade the continental lithospheric mantle and the time scale at
which they equilibrate with preexisting (protogenetic) sulfides are poorly
understood yet essential factors to understanding diamond formation and the
validity of isotopic ages. We investigated a suite of sulfide-bearing
diamonds from two Canadian cratons to test the robustness of Re-Os in
sulfide for dating diamond formation. Single-crystal X-ray diffraction
(XRD) allowed determination of the original monosulfide solid-solution
(Mss) composition stable in the mantle, indicating subsolidus conditions of
encapsulation, and providing crystallographic evidence supporting a
protogenetic origin of the inclusions. The results, coupled with a
diffusion model, indicate Re-Os isotope equilibration is sufficiently fast
in sulfide inclusions with typical grain size, at mantle temperatures, for
the system to be reset by the diamond-forming event. This confirms that
even if protogenetic, the Re-Os isochrons defined by these minerals likely
reflect the ages of diamond formation, and this result highlights the power
of this system to date the timing of fluid migration in mantle lithosphere.

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New statistical quantification of the impact of active deformation on
the distribution of submarine channels

Marco Pizzi; Alexander C. Whittaker; Lidia Lonergan; Mike Mayall; W. Hamish

Submarine channel systems play a crucial role in governing the delivery of
sediments and pollutants such as plastics from the shelf edge to deep
water. Understanding their distribution in space and time is important for
constraining the locus, magnitude, and characteristics of deep-water
sedimentation and for predicting stratigraphic architectures and
depositional facies. Using three-dimensional seismic reflection data
covering the outer fold-and-thrust belt of the Niger Delta, we determined
the pathways of Miocene to Pliocene channels that crossed, at 173
locations, 11 fold-thrust structures for which the temporal and spatial
evolution of strain rates has been constrained over a period of 11 m.y. We
use a statistical approach to quantify strain and shortening rate
distributions recorded where channels have crossed structures compared to
the fault array as a whole. Our results prove unambiguously that these
distributions are different. The median strain rate where channels cross
faults is 0.6%/m.y. (~40 m/m.y.), 2.5× lower than the median strain
rate of active fault segments (1.5%/m.y.) with a marked reduction in the
number of channel-fault crossings where fault strain rates are >1%/m.y.
Our results quantify the sensitivity of submarine channels to active
deformation at a population level for the first time and enable us to
predict the temporal and spatial routing of submarine channels affected by
structurally driven topography.

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Paleozoic evolution of crustal thickness and elevation in the northern
Appalachian orogen, USA

Ian W. Hillenbrand; Michael L. Williams

The Acadian and Neoacadian orogenies are widely recognized, yet poorly
understood, tectono-thermal events in the New England Appalachian Mountains
(USA). We quantified two phases of Paleozoic crustal thickening using
geochemical proxies. Acadian (425-400 Ma) crustal thickening to 40 km
progressed from southeast to northwest. Neoacadian (400-380 Ma) crustal
thickening was widely distributed and varied by 30 km (40-70 km) from north
to south. Doubly thickened crust and paleoelevations of 5 km or more
support the presence of an orogenic plateau at ca. 380-330 Ma in southern
New England. Neoacadian crustal thicknesses show a strong correlation with
metamorphic isograds, where higher metamorphic grade corresponds to greater
paleo-crustal thickness. We suggest that the present metamorphic field
gradient was exposed through erosion and orogenic collapse influenced by
thermal, isostatic, and gravitational properties related to Neoacadian
crustal thickness. Geobarometry in southern New England underestimates
crustal thickness and exhumation, suggesting the crust was thinned by
tectonic as well as erosional processes.

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Diapiric relamination of the Orocopia Schist (southwestern U.S.) during
low-angle subduction

James B. Chapman

The Orocopia Schist and related schists are sediments subducted during the
Laramide orogeny and are thought to have been underplated as a laterally
extensive layer at the base of the crust in the southwestern United States
Cordillera. This concept is hard to reconcile with the existence of
continental mantle lithosphere in southeastern California and western
Arizona. Analytical solutions and numerical modeling suggest that the
Orocopia Schist may have ascended through the mantle lithosphere as
sediment diapirs or subsolidus crustal plumes to become emplaced in the
middle to lower crust. Modeled time-temperature cooling paths are
consistent with the exhumation history of the Orocopia Schist and explain
an initial period of rapid cooling shortly after peak metamorphism. The
Orocopia Schist represents a potential example of relaminated sediment
observable at the surface.

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Annually resolved sediments in the classic Clarkia lacustrine deposits
(Idaho, USA) during the middle Miocene Climate Optimum

Daianne Höfig; Yi Ge Zhang; Liviu Giosan; Qin Leng; Jiaqi Liang …

The world-renowned Miocene Clarkia paleolake in northern Idaho (USA) is
closely associated with Columbia River Basalt Group volcanism. The flood
basalt dammed a local drainage system to form the paleolake, which
preserved a plant fossil Lagerstätte in its deposits. However, the
precise age and temporal duration of the lake remain unsettled. We present
the first unequivocal U-Pb zircon ages from interbedded volcanic ashes at
the P-33 type location, constraining the deposition to 15.78 ± 0.039 Ma.
Using micro-X-ray fluorescence and petrographic and spectral analyses, we
establish the annual characteristics of laminations throughout the
stratigraphic profile using the distribution of elemental ratios, mineral
assemblages, and grain-size structures, as well as organic and fossil
contents. Consequently, the ~7.5-m-thick varved deposit at the type
location P-33 represents ~840 yr of deposition, coincident with the end of
the main phase of Columbia River Basalt Group eruptions during the Miocene
Climate Optimum. The timing and temporal resolution of the deposit offer a
unique opportunity to study climate change in unprecedented detail during
global warming associated with carbon-cycle perturbations.

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The carbonate compensation depth in the South Atlantic Ocean since the
Late Cretaceous

Adriana Dutkiewicz; R. Dietmar Müller

Deep-sea carbonate deposition is a complex process that is encapsulated in
the carbonate compensation depth (CCD)–a facies boundary separating
calcareous sediments from non-carbonates. Knowing how the CCD has varied
over time is important for understanding and predicting the distribution of
seafloor sediments and assessing their role in the global carbon cycle. We
focus on the South Atlantic Ocean where the most recent CCD curve is based
on Deep Sea Drilling Project (DSDP) Leg 73 sites drilled in 1980 in the
South Atlantic Ocean. We compute the South and central South Atlantic CCD
from the Late Cretaceous to the present day using updated age models from
45 DSDP and Ocean Drilling Program sites and backtracking with
lithology-specific decompaction, eustasy, and dynamic topography. Our
models extend further back in time and show more fluctuations than previous
reconstructions, with the CCD varying by hundreds of meters during a span
of 2-3 m.y. The addition of eustasy and dynamic topography deepens the CCD
by as much as 500 m between 74 Ma and 45 Ma, and by ~200 m during the
Cenozoic. The central South Atlantic CCD diverges from the average South
Atlantic CCD during the Eocene and Miocene, when it was ~1 km shallower.
These regional deviations may be due to changes in primary productivity
and/or carbonate dissolution leading to reduced carbonate accumulation
rates. Our CCD curves highlight the importance of regional processes in
carbonate deposition across the South Atlantic and provide improved
constraints for the modeling of geochemical cycles.

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Transient Permian-Triassic euxinia in the southern Panthalassa deep

S.E. Grasby; D.P.G. Bond; P.B. Wignall; R. Yin; L.J. Strachan …

Both the duration and severity of deep-water anoxic conditions across the
Permian-Triassic mass extinction (PTME) are controversial. Panthalassa
Ocean circulation models yield varying results, ranging from a
well-ventilated deep ocean to rapidly developing northern-latitude, but not
southern-latitude, anoxia in response to Siberian Traps-driven global
warming. To address this uncertainty, we examined a southern-paleolatitude
pelagic record. Trace metal and pyrite framboid data suggest bottom-water
euxinic conditions developed in the southern Panthalassa Ocean at the PTME,
coincident with enhanced volcanic activity indicated by Hg geochemistry.
While a global ocean euxinic event at the PTME placed extraordinary stress
on marine life, southern surface waters appear to have recovered more
quickly as radiolarian populations returned several million years before
they did in northern Panthalassa.

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Water transfer to the deep mantle through hydrous, Al-rich silicates in
subduction zones

Jörg Hermann; Shayne Lakey

Constraining deep-water recycling along subduction zones is a first-order
problem to understand how Earth has maintained a hydrosphere over billions
of years that created conditions for a habitable planet. The
pressure-temperature stability of hydrous phases in conjunction with slab
geotherms determines how much H2O leaves the slab or is
transported to the deep mantle. Chlorite-rich, metasomatic rocks that form
at the slab-mantle interface at 50-100 km depth represent an unaccounted, H 2O-rich reservoir in subduction processes. Through a series of
high-pressure experiments, we investigated the fate of such chlorite-rich
rocks at the most critical conditions for subduction water recycling (5-6.2
GPa, 620-800 °C) using two different natural ultramafic compositions. Up to
5.7 GPa, 740 °C, chlorite breaks down to an anhydrous peridotite
assemblage, and H2O is released. However, at higher pressures
and lower temperatures, a hydrous Al-rich silicate (11.5 Å phase) is an
important carrier to enable water transfer to the deep mantle for cold
subduction zones. Based on the new phase diagrams, it is suggested that the
deep-water cycle might not be in secular equilibrium.

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Eruption risks from covert silicic magma bodies

Shane M. Rooyakkers; John Stix; Kim Berlo; Maurizio Petrelli; Freysteinn

Unintentional encounters with silicic magma at ~2-2.5 km depth have
recently occurred during drilling at three volcanoes: Kilauea (Hawaii),
Menengai (Kenya), and Krafla (Iceland). Geophysical surveys had failed to
warn about shallow magma before each encounter, and subsequent surveys at
Krafla have been unable to resolve the size or architecture of its silicic
magma body. This presents a conundrum for volcano monitoring: Do such
shallow “covert” magma bodies pose an eruption risk? Here, we show that
Krafla’s most recent explosive eruption, a mixed hydrothermal-magmatic
event in 1724 C.E. that formed the Víti maar, involved rhyolite essentially
indistinguishable in composition from magma encountered during drilling in
2009. Streaks of quenched basalt in some Víti pumices provide direct
evidence for interaction between co-erupted rhyolitic and basaltic magmas,
but crystals in these pumices show no evidence for late-stage heating or
re-equilibration with more mafic melt, implying mixing time scales of at
most several hours. Covert silicic magma thus presents an eruption risk at
Krafla and may be mobilized with little warning. Difficulties in resolving
magma bodies smaller than ~1 km3 with geophysical surveys mean
that covert silicic magma may exist at many other volcanoes and should be
considered in hazard and risk assessments.

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Ghost-dune hollows of the eastern Snake River Plain, Idaho: Their
genesis, evolution, and relevance to Martian ghost-dune pits

David R. Gaylord; Tammy M. Rittenour; Paul K. Link; Brent D. Turrin; Mel A.

Ghost-dune hollows on the eastern Snake River Plain (ESRP), Idaho, USA, are
topographically inverted, crescent-shaped depressions that record the
partial encasement of sand dunes by ca. 61 ka basalt lava flows. Deflation
of these “ghost” sand dunes produced approximately two dozen, 5-10-m-deep
ghost-dune hollows now incompletely filled with pedogenically altered
eolian and colluvial sediment. Optically stimulated luminescence (OSL) and 40Ar/39Ar ages constrain a ghost-dune hollow model
that illuminates the late Pleistocene to Holocene environmental and climate
history of the ESRP. Detrital zircon analyses indicate sand-dune supply
routes changed following the burial of Pleistocene Henrys Fork (tributary
of the Snake River) alluvium by ca. 70 ka basalt flows. Removal of Henrys
Fork alluvium from the eolian supply system made Lake Terreton sediment the
primary source for later ESRP sand dunes. Such sediment supply changes
highlight the potential impacts of effusive volcanism on sand-dune
histories and landscapes. Our results support stratigraphic and sedimentary
modeling of comparable ghost-dune “pit” deposits older than ca. 2 Ga on
Mars that may have served as refugia for early life on that planet.
Analogous ancient ghost-dune hollow deposits on Earth may also have served
as early life refugia.

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Controls on the origin and evolution of deep-ocean trench-axial

Adam D. McArthur; Daniel E. Tek

The type and volume of sediment entering subduction zones affects the style
of plate-boundary deformation and thus sedimentary and tectonic cycles.
Because submarine channels significantly increase the transport efficiency
of turbidity currents, their presence or absence in subduction trenches is
a primary control on trench fill. To date, comprehensive architectural
characterization of trench-axial channels has not been possible,
undermining efforts to identify the factors controlling their initiation
and evolution. Here, we describe the evolution of the Hikurangi Channel,
which traverses the Hikurangi Trench, offshore New Zealand. Analysis of
two- and three-dimensional seismic data reveals that the channel was
present only during the last ~3.5 m.y. of the ~27 m.y. of the trench’s
existence; its inception and propagation resulted from increased sediment
supply to the trench following amplified hinterland exhumation. To test if
the controls on the evolution of the Hikurangi Channel are universal,
multivariate statistical analysis of the geomorphology of subduction
trenches globally is used to investigate the formative conditions of axial
channels in modern trenches. Terrigenous sediment supply and thickness of
sediment cover in a trench are the dominant controls; subsidiary factors
such as trench length and rugosity also contribute to the conditions
necessary for trench-axial channel development. Axial channels regulate
sediment distribution in trenches, and this varies temporally and spatially
as a channel propagates along a trench. The presence of a trench-axial
channel affects plate-boundary mechanics and has implications for the style
of subduction-margin deformation.

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Joint inversion of surface wave and gravity data reveals subbasin
architecture of the Congo Basin

A. Raveloson; A. Nyblade; R. Durrheim

We investigated the architecture of the greater Congo Basin, one of the
largest and least-well-studied sedimentary basins on any continent.
Seismograms from a large number of M > 4.5 earthquakes within and
surrounding the African plate were used to make event-to-station Rayleigh
wave group velocity measurements between periods of 5 and 100 s. Group
velocities for discrete periods across the basin, obtained by inverting the
event-station measurements, were jointly modeled with gravity data to
obtain a three-dimensional S-wave and density model of the basin. The model
corroborates the existence of two previously suggested subbasins, one to
the north and one to the south, each ~8 km deep and separated by an
east-west structural high. Our results favor a salt tectonics origin for
the structural high but cannot rule out uplifted basement rock. The
northern subbasin is offset to the west from the southern subbasin,
consistent with previous studies suggesting sinistral motion along basement
faults during periods of transpressional tectonics in late
Neoproterozoic-early Paleozoic times.

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Hydrothermal recycling of sedimentary ammonium into oceanic crust and
the Archean ocean at 3.24 Ga

E.E. Stüeken; T.J. Boocock; A. Robinson; S. Mikhail; B.W. Johnson

The Archean ocean supported a diverse microbial ecosystem, yet studies
suggest that seawater was largely depleted in many essential nutrients,
including fixed nitrogen. This depletion was in part a consequence of
inefficient nutrient recycling under anoxic conditions. Here, we show how
hydrothermal fluids acted as a recycling mechanism for ammonium (NH 4+) in the Archean ocean. We present elemental and
stable isotope data for carbon, nitrogen, and sulfur from shales and
hydrothermally altered volcanic rocks from the 3.24 Ga Panorama district in
Western Australia. This suite documents the transfer of NH4 + from organic-rich sedimentary rocks into underlying
sericitized dacite, similar to what is seen in hydrothermal systems today.
On modern Earth, hydrothermal fluids that circulate through sediment
packages are enriched in NH4+ to millimolar
concentrations because they efficiently recycle organic-bound N. Our data
show that a similar hydrothermal recycling process dates back to at least
3.24 Ga, and it may have resulted in localized centers of enhanced
biological productivity around hydrothermal vents. Last, our data provide
evidence that altered oceanic crust at 3.24 Ga was enriched in nitrogen,
and, when subducted, it satisfies the elemental and isotopic source
requirements for a low-N, but 15N-enriched, deep mantle nitrogen
reservoir as sampled by mantle plumes.

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Missing water from the Qiangtang Basin on the Tibetan Plateau

Bin Yong; Chi-Yuen Wang; Jiansheng Chen; Jiaqi Chen; D.A. Barry …

The Qiangtang Basin is a large endorheic basin in the inner part of the
Tibetan Plateau, and has been thought to be a dry region in contrast with
the surrounding wet outer region that feeds all the major Asian rivers.
Combining surface hydrological data with modeling and satellite data from
2002 to 2016 CE, our study reveals that an enormous amount of water, ~54 ±
4 km3, is unaccounted for annually in the Qiangtang Basin. The
amount of missing water is comparable to the total annual discharge of the
Yellow River. Data from the Gravity Recovery and Climate Experiment (GRACE)
satellite mission show little increase of local terrestrial water storage.
Thus, the missing water must have flowed out of the basin through
underground passages. Interpreting this result in the context of recent
seismic and geological studies of Tibet, we suggest that a significant
amount of meteoric water in the Qiangtang Basin leaks out by way of
groundwater flow through deep normal faults and tensional fractures along
the nearly north-south rift valleys that are oriented subnormal to and
cross the surficial hydrological divide on the southern margin of the
basin. Cross-basin groundwater outflow of such a magnitude defies the
traditional view of a basin-scale water cycle and leads to a very different
picture from the previous hydrological view of the Qiangtang Basin. This
finding calls for major rethinking of the regional water balance.

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Alternating wet and dry depositional environments recorded in the
stratigraphy of Mount Sharp at Gale crater, Mars

W. Rapin; G. Dromart; D. Rubin; L. Le Deit; N. Mangold …

The Curiosity rover is exploring Hesperian-aged stratigraphy in Gale
crater, Mars, where a transition from clay-bearing units to a layered
sulfate-bearing unit has been interpreted to represent a major
environmental transition of unknown character. We present the first
description of key facies in the sulfate-bearing unit, recently observed in
the distance by the rover, and propose a model for changes in depositional
environments. Our results indicate a transition from lacustrine mudstones
into thick aeolian deposits, topped by a major deflation surface, above
which strata show architectures likely diagnostic of a subaqueous
environment. This model offers a reference example of a depositional
sequence for layered sulfate-bearing strata, which have been identified
from orbit in other locations globally. It differs from the idea of a
monotonic Hesperian climate change into long-term aridity on Mars and
instead implies a period characterized by multiple transitions between
sustained drier and wetter climates.

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Evidence for 40-41 km of dextral slip on the southern Death Valley
fault: Implications for the Eastern California shear zone and
extensional tectonics

Terry L. Pavlis; Ghislain Trullenque

Recognition of a pair of pre-Neogene markers together with analysis of
published data indicate ~40 km of dextral slip across the southern Death
Valley fault zone, California, USA. Stratigraphic overlaps on fault rocks
indicate much of the dextral slip predates the late Miocene, placing a
significant fraction of the dextral slip in the same time window as
regional extension and challenging interpretations that the modern
strike-slip system became active post-6-3 Ma. However, these results are
consistent with regional evidence that dextral transtension began by ca. 12

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A metasedimentary source of gold in Archean orogenic gold deposits

Iain K. Pitcairn; Nikolaos Leventis; Georges Beaudoin; Stephane Faure; Carl
Guilmette …

The sources of metals enriched in Archean orogenic gold deposits have long
been debated. Metasedimentary rocks, which are generally accepted as the
main metal source in Phanerozoic deposits, are less abundant in Archean
greenstone belts and commonly discounted as a viable metal source for
Archean deposits. We report ultralow-detection-limit gold and trace-element
concentrations from a suite of metamorphosed sedimentary rocks from the
Abitibi belt and Pontiac subprovince, Superior Province, Canada. Systematic
decreases in the Au content with increasing metamorphic grade indicate that
Au was mobilized during prograde metamorphism. Mass balance calculations
show that over 10 t of Au, 30,000 t of As, and 600 t of Sb were mobilized
from 1 km3 of Pontiac subprovince sedimentary rock metamorphosed
to the sillimanite metamorphic zone. The total gold resource in orogenic
gold deposits in the southern Abitibi belt (7500 t Au) is only 3% of the Au
mobilized from the estimated total volume of high-metamorphic-grade Pontiac
sedimentary rock in the region (25,000 km3), indicating that
sedimentary rocks are a major contributor of metals to the orogenic gold
deposits in the southern Abitibi belt.

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Similar curvature-to-width ratios for channels and channel belts:
Implications for paleo-hydraulics of fluvial ridges on Mars

Alistair T. Hayden; Michael P. Lamb; Alexander J. Carney

The surface of Mars contains abundant sinuous ridges that appear similar to
river channels in planform, but they stand as topographic highs. Ridges
have similar curvature-to-width ratios as terrestrial meandering rivers,
which has been used to support the hypothesis that ridges are inverted
channels that directly reflect channel geometry. Anomalously wide ridges,
in turn, have been interpreted as evidence for larger rivers on Mars
compared to Earth. However, an alternate hypothesis is that ridges are
exhumed channel-belt deposits–a larger zone of relatively coarse-grained
deposits formed from channel lateral migration and aggradation. Here, we
measured landform wavelength, radius of curvature, and width to compare
terrestrial channels, terrestrial channel belts, and martian ridges. We
found that all three landforms follow similar scaling relations, in which
ratios of radius of curvature to width range from 1.7 to 7.3, and
wavelength-to-width ratios range from 5.8 to 13. We interpret this
similarity to be a geometric consequence of a sinuous curved line of finite
width. Combined with observations of ridge-stacking patterns, our results
suggest that wide ridges on Mars could indicate fluvial channel belts that
formed over significant time rather than anomalously large rivers.

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Outer forearc uplift and exhumation during high-flux magmatism:
Evidence from detrital zircon geochemistry of the Nacimiento forearc
basin, California, USA

Scott M. Johnston; Andrew R.C. Kylander-Clark

We present new coupled detrital zircon trace-element and U-Pb age data from
Valanginian-Santonian strata of the Nacimiento forearc basin (California,
USA) to enhance provenance discrimination and investigate the evolution of
the late Mesozoic California margin. Our data document at least five
different Jurassic-earliest Cretaceous zircon populations with variable
U/Yb ratios, and zircon that displays systematically increasing U/Yb from
130 to 80 Ma. Based on the presence of a distinctive population of
geochemically primitive, 168-157 Ma low-U/Yb zircon that is found in
Albian-Lower Cenomanian strata but not in older Valanginian strata, we
infer a period of uplift and Albian-early Cenomanian erosion of forearc
basement (the Coast Range ophiolite) that was coincident with increasing
Cordilleran arc magmatic flux.

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Shutting down dust emission during the middle Holocene drought in the
Sonoran Desert, Arizona, USA

Guy Tau; Onn Crouvi; Yehouda Enzel; Nadya Teutsch; Paul Ginoux …

Long-term relationships between climate and dust emission remain unclear,
with two prevailing but opposite hypotheses for effects of climate shifts:
(1) increased dust emission due to increasing aridity imposing a vegetation
change, or (2) decreased dust emission due to increasing aridity which
imposes less stormy climate and reduced sediment supply. Here we test these
hypotheses by analyzing an ~11-m-long core archiving Holocene dust trapped
in Montezuma Well, a natural sinkhole in Arizona (southwestern United
States), alongside current dust sources and transport pathways. Major
elements indicate that Montezuma Well sediments originate from two end
members: local carbonate bedrock and external siliceous dust. Core
sediments are similar to the adjacent siliceous soils accumulated atop the
bedrock, pointing to their eolian origin. Particle-size distributions
reveal fine dust transported during winter from the northwestern Sonoran
Desert and the Mojave Desert and coarse dust transported during summer from
the southwestern Sonoran Desert, similar to current climate systems and
dust pathways. A survey of potential dust sources indicates that current
summer and winter dust sources in the Sonoran Desert are under a
supply-limited state. Dust fluxes were higher during wetter phases of the
Holocene when winter sources dominated. During the middle Holocene drought,
dust fluxes were minimal and dominated by summer sources until dust input
ceased as drought conditions did not produce enough floods to refill
sources with sediments. We propose that in the Sonoran Desert, dust
emission is strongly connected with climate, increasing during humid
intervals and enhanced by fluvial sediment replenishment at dust sources.

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Extensive decentralized hydrogen export from the Atlantis Massif

Susan Q. Lang; Marvin D. Lilley; Tamara Baumberger; Gretchen L. Früh-Green;
Sharon L. Walker …

Hydrogen is an important energy source for subsurface microbial
communities, but its availability beyond the flow focused through
hydrothermal chimneys is largely unknown. We report the widespread export
of H2 across the Atlantis Massif oceanic core complex (30°N,
Mid-Atlantic Ridge; up to 44 nM), which is distinct from the
circulation system feeding the Lost City Hydrothermal Field (LCHF) on the
massif’s southern wall. Methane (CH4) abundances are generally
low to undetectable (3 nM) in fluids that are not derived from
the LCHF. Reducing fluids exit the seafloor over a wide geographical area
and depth range, including the summit of the massif and along steep areas
of mass wasting east of the field. The depth of the fluids in the water
column and their H2/CH4 ratios indicate that some are
sourced separately from the LCHF. We argue that extensive H2
export is the natural consequence of fluid flow pathways strongly
influenced by tectonic features and the volume and density changes that
occur when ultramafic rocks react to form serpentinites, producing H 2 as a by-product. Furthermore, the circulation of H2
-rich fluids through uplifted mantle rocks at moderate temperatures
provides geographically expansive and stable environmental conditions for
the early evolution of biochemical pathways. These results provide insight
into the spatial extent of H2– and CH4-bearing fluids
associated with serpentinization, independent of the focused flow emanating
from the LCHF.

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Shock-deformed zircon from the Chicxulub impact crater and implications
for cratering process

Jiawei Zhao; Long Xiao; Zhiyong Xiao; Joanna V. Morgan; Gordon R. Osinski

Large impact structures with peak rings are common landforms across the
solar system, and their formation has implications for both the interior
structure and thermal evolution of planetary bodies. Numerical modeling and
structural studies have been used to simulate and ground truth peak-ring
formative mechanisms, but the shock metamorphic record of minerals within
these structures remains to be ascertained. We investigated impact-related
microstructures and high-pressure phases in zircon from melt-bearing
breccias, impact melt rock, and granitoid basement from the Chicxulub peak
ring (Yucatán Peninsula, Mexico), sampled by the International Ocean
Discovery Program (IODP)/International Continental Drilling Project
(IODP-ICDP) Expedition 364 Hole M0077A. Zircon grains exhibit shock
features such as reidite, zircon twins, and granular zircon including “former reidite in granular neoblastic” (FRIGN) zircon. These features
record an initial high-pressure shock wave (>30 GPa), subsequent
relaxation during the passage of the rarefaction wave, and a final heating
and annealing stage. Our observed grain-scale deformation history agrees
well with the stress fields predicted by the dynamic collapse model, as the
central uplift collapsed downward-then-outward to form the peak ring. The
occurrence of reidite in a large impact basin on Earth represents the first
such discovery, preserved due to its separation from impact melt and rapid
cooling by the resurging ocean. The coexistence of reidite and FRIGN zircon
within the impact melt-bearing breccias indicates that cooling by seawater
was heterogeneous. Our results provide valuable information on when
different shock microstructures form and how they are modified according to
their position in the impact structure, and this study further improves on
the use of shock barometry as a diagnostic tool in understanding the
cratering process.

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