New Geology Articles Published Online Ahead of Print in November
Boulder, Colo., USA: Article topics and locations include stalagmites from
Soreq Cave, Israel: Hadean zircon; the eolian stratigraphic record of
environmental change through geological time; and waterfalls in the San
Gabriel Mountains, California. These Geology articles are online
at
https://geology.geoscienceworld.org/content/early/recent
.
Bedrock gorge incision via anthropogenic meander cutoff
Adrian M. Bender
Abstract:
Bedrock river-gorge incision represents a fundamental landscape-shaping
process, but a dearth of observational data at >10 yr timescales impedes
understanding of gorge formation. I quantify 102 yr rates and
processes of gorge incision using historical records, field observations,
and topographic and image analysis of a human-caused bedrock meander cutoff
along the North Fork Fortymile River in Alaska (USA). Miners cut off the
meander in 1900 CE, abruptly lowering local base level by 6 m and forcing
narrowing and steepening of the channel across a knickpoint that rapidly
incised upstream. Tectonic quiescence, consistent rock erosivity, and low
millennial erosion rates provide ideal boundary conditions for this 10 2 yr gorge-formation experiment. Initial fast knickpoint
propagation (23 m/yr; 1900–1903 CE) slowed (4 m/yr; 1903–1981 CE) to
diffusion (1981–2019 CE) as knickpoint slope decreased, yielding an
~350-m-long, 6-m-deep gorge within the pre–1900 CE channel. Today,
diffusion dominates incision of a 500-m-long knickzone upstream of the
gorge, where sediment transport likely limits ongoing adjustments to the
anthropogenic cutoff. Results elucidate channel width, slope, discharge,
and sediment dynamics consistent with a gradual transition from detachment-
to transport-limited incision in fluvial adjustment to local base-level
lowering.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49479.1/609921/Bedrock-gorge-incision-via-anthropogenic-meander
Water in omphacite fingerprints the thermal history of eclogites
Peilin Jiang; Hanyong Liu; Henrik Skogby; Ren-Xu Chen; Xiaozhi Yang
Abstract:
Omphacite is a diagnostic mineral of high- and ultrahigh-pressure rocks,
and its association with garnet is characteristic of eclogites from
subduction-related massifs and volcano-entrained xenoliths. Omphacite can
accommodate significant amounts of water as structurally bound hydroxyl
(OH) groups, and is able to convey water into Earth’s interior. We show,
for the first time, experimental evidence that the infrared absorption
patterns of water in omphacite are temperature sensitive. This provides a
new framework for discriminating water in natural omphacite equilibrated at
different temperatures. We also demonstrate that in low-temperature
omphacite, the integral absorbance ratio between the infrared OH absorption
bands at 3620 cm–1 and 3450 cm–1 is linearly related
to temperature. Water in omphacite of massif eclogites records the
temperature of OH equilibrium, allowing reconstruction of the
fluid-involved thermal history and tectonics of rock evolution.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49566.1/609922/Water-in-omphacite-fingerprints-the-thermal
Holocene wet episodes recorded by magnetic minerals in stalagmites from
Soreq Cave, Israel
Yuval Burstyn; Ron Shaar; Jonathan Keinan; Yael Ebert; Avner Ayalon ...
Abstract:
This study demonstrates the feasibility of speleothem magnetism as a
paleo-hydrology proxy in speleothems growing in semi-arid conditions.
Soil-derived magnetic particles in speleothems retain valuable information
on the physicochemical conditions of the overlying soil, and changes in
bedrock hydrology. Yet, the link between magnetic and isotopic proxies of
speleothems has been only partly established. We reveal strong coupling
between the inflow of magnetic particles (quantified using the magnetic
flux index, IRMflux) and δ13C in two Holocene
speleothems from Soreq Cave (Israel). The stalagmite record spans from ca.
9.7 to ca. 5.4 ka, capturing the warm-humid conditions associated with the
early Holocene and the transition to mid-Holocene wet-dry cycles. Extremely
low IRMflux during the early Holocene, indicating minimal
contribution from the overlying soil, is accompanied by anomalously high δ 13C (approaching bedrock values) hypothesized to be caused by
high rainfall and soil erosion. By contrast, IRMflux during the
mid-Holocene covaries with the saw-tooth cyclicity of δ13C and δ 18O, interpreted as rapid fluctuations in rainfall amount. The
peaks in IRMflux precede the negative (wet) δ13C
peaks by ~60–120 yr. The apparent lag is explained as a rapid physical
translocation of overlying soil particles via groundwater (high IRM flux) as a response to increasing rainfall, compared with slower
soil organic matter turnover rates (10–102 yr).
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49383.1/609923/Holocene-wet-episodes-recorded-by-magnetic
Hadean zircon formed due to hydrated ultramafic protocrust melting
Anastassia Y. Borisova; Anne Nédélec; Nail R. Zagrtdenov; Michael J.
Toplis; Wendy A. Bohrson ...
Abstract:
Hadean zircons, from the Jack Hills (Western Australia) and other
localities, are currently the only window into the earliest terrestrial
felsic crust, the formation of which remains enigmatic. Based upon new
experimental results, generation of such early crust has been hypothesized
to involve the partial melting of hydrated peridotite interacting with
basaltic melt at low pressure (<10 km), but it has yet to be
demonstrated that such liquids can indeed crystallize zircons comparable to
Jack Hills zircon. We used thermodynamic and geochemical modeling to test
this hypothesis. The predicted zircon saturation temperatures of <750
°C, together with the model zircon Th, U, Nb, Hf, Y, and rare earth element
(REE) contents at 700 °C, δ18OVSMOW (Vienna standard
mean ocean water) signatures, and co-crystallizing mineral assemblage were
compared to those of the Jack Hills zircon. This comparison was favorable
with respect to crystallization temperature, most trace-element contents,
and mineral inclusions in zircon. The discrepancy in δ18O VSMOW signatures may be explained by hotter conditions of Hadean
protocrust hydration. Our work supports the idea that felsic magma
generation at shallow depths involving a primordial weathered ultramafic
protocrust and local basaltic intrusions is indeed a viable mechanism for
the formation of felsic crust on early Earth.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49354.1/609924/Hadean-zircon-formed-due-to-hydrated-ultramafic
Mississippian southern Laurentia tuffs came from a northern Gondwana
arc
Hepeng Tian; Majie Fan; Victor A. Valencia; Kevin Chamberlain; Robert J.
Stern ...
Abstract:
A Paleozoic arc that formed by southward subduction of the Rheic oceanic
plate beneath northern Gondwana has long been inferred, but its history and
geochemical signatures remain poorly understood. New U-Pb ages, juvenile ε Hf signatures, and trace-element composition data of young
zircons from tuffs at two southern Laurentia sites indicate their
derivation from a continental arc that was active from ca. 328 to ca. 317
Ma and permit correlation of sedimentary sequences 800 km apart in southern
Laurentia. These include the Stanley tuffs in the Ouachita Mountains of
southeastern Oklahoma and southwestern Arkansas and the newly discovered
Barnett tuff in the subsurface of the Midland Basin in west Texas (USA).
The Barnett tuff has a zircon chemical abrasion–isotope dilution–thermal
ionization mass spectrometry U-Pb date of 327.8 ± 0.8 Ma, similar to the
oldest Stanley tuff in the Ouachita Mountains. Zircon Hf isotope depleted
mantle model ages further suggest that the source was a continental arc on
basement with both Grenville and Pan-African affinities, pointing to
northern Gondwana or peri-Gondwana terranes. The new data link the tuffs to
granitoids (326 Ma) of the Maya block in southern Mexico, which was part of
northern Gondwana. Correlation of the Stanley-Barnett tuffs across southern
Laurentia suggests the likely presence of Mississippian tuffs over a broad
region in southern Laurentia, and their usefulness for constraining
absolute ages of basin fills and characterizing the Gondwanan arc.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49502.1/609925/Mississippian-southern-Laurentia-tuffs-came-from-a
Eolian stratigraphic record of environmental change through geological
time
G.I.E. Cosgrove; L. Colombera; N.P. Mountney
Abstract:
The terrestrial sedimentary record provides a valuable archive of how
ancient depositional systems responded to and recorded changes in Earth’s
atmosphere, biosphere, and geosphere. However, the record of these
environmental changes in eolian sedimentary successions is poorly
constrained and largely unquantified. Our study presents the first
global-scale, quantitative investigation of the architecture of eolian
systems through geological time via analysis of 55 case studies of eolian
successions. Eolian deposits accumulating (1) under greenhouse conditions,
(2) in the presence of vascular plants and grasses, and (3) in rapidly
subsiding basins associated with the rifting of supercontinents are
represented by significantly thicker eolian dune-set, sand-sheet, and
interdune architectural elements. Pre-vegetation eolian systems are also
associated with more frequent interactions with non-eolian environments.
The interplay of these forcings has resulted in dune-set thicknesses that
tend to be smallest and largest in Proterozoic and Mesozoic successions,
respectively. In the Proterozoic, the absence of sediment-binding plant
roots rendered eolian deposits susceptible to post-depositional wind
deflation and reworking by fluvial systems, whereby highly mobile channels
reworked contiguous eolian deposits. During the Mesozoic, humid greenhouse
conditions (associated with relatively elevated water tables) and high
rates of basin subsidence (associated with the breakup of Pangea) favored
the rapid transfer of eolian sediment beneath the erosional baseline. The
common presence of vegetation promoted accumulation of stabilizing eolian
systems. These factors acted to limit post-depositional reworking. Eolian
sedimentary deposits record a fingerprint of major environmental changes in
Earth history: climate, continental configuration, tectonics, and
land-plant evolution.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49474.1/609926/Eolian-stratigraphic-record-of-environmental
Solid as a rock: Tectonic control of graben extension and dike
propagation
S. Kolzenburg; J. Kubanek; M. Dirscherl; C.W. Hamilton; E. Hauber ...
Abstract:
The 2014–2015 CE rift event associated with the Bárðarbunga eruption at
Holuhraun, Iceland, offers a unique opportunity to study the spatial and
temporal evolution of a rift graben. We present the first four-dimensional
(three-dimensional plus time) monitoring of the formation and evolution of
a graben during active magma transport using a suite of digital elevation
models spanning from shortly before the eruption throughout 6 months of
magma transport and up to 4.5 years after the eruption. This multiscale
data set enables investigations of how magma supply and eruption dynamics
affect tectonic structures that feed eruptions. After formation (time scale
of a few days), the graben is remarkably stable throughout the eruption and
for years beyond. It is unaffected by large changes in eruptive activity
and effusion and seismicity rates within the plumbing system. These data
document that (1) there was no direct feedback between eruptive dynamics
and graben topography, and (2) graben formation is near instantaneous on
tectonic time scales. These results challenge the overarching role ascribed
to magma transport in recent studies of tectonomagmatic relationships in
rift events, favoring regional tectonics as the fundamental driving force.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49406.1/609927/Solid-as-a-rock-Tectonic-control-of-graben
Microstructural evidence for convection in high-silica granite
Brendan Dyck; Marian Holness
Abstract:
High-silica (>70 wt% SiO2) granites (HSGs) are critical
carriers of tin, copper, and other melt-incompatible elements, yet much
remains unknown about the mechanisms responsible for their formation. One
of the key issues is the apparent lack of evidence for crystal-melt
segregation (e.g., modal layering), without which little can be inferred
about the dynamics (or lack thereof) of crystallizing HSGs. We examined the
crystallographic orientation relationships of clustered quartz crystals
from the 300-m-thick Bobbejaankop sill, Bushveld Complex, South Africa. We
report an inward increase in the number density and size of quartz clusters
toward the central horizon of the sill, coinciding with a significant
increase in concentrations of tin, copper, and tungsten. The majority of
crystal pairs within each cluster exhibit coincident-site lattice
orientation relationships, representing low grain-boundary energy
configurations. These clusters must have formed by synneusis in a magmatic
environment where crystals could have moved freely, rotating into
low-energy orientations on contact. We argue that this not only
demonstrates that 100-m-scale crystal-poor and liquid-rich regions can be
present in bodies of HSG, but also that such bodies can undergo long-lived
convection during crystallization, driven by downwards movement of
crystal-rich plumes at the roof, without significant crystal-melt
segregation. This dynamic behavior provides a mechanism to homogenize
major-element distribution across HSGs and to concentrate highly
incompatible and economic elements into central mineralized horizons.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49431.1/609928/Microstructural-evidence-for-convection-in-high
Eocene fault-controlled fluid flow and mineralization in the Paradox
Basin, United States
Lydia R. Bailey; Jason Kirk; Sidney R. Hemming; Robert W. Krantz; Peter W.
Reiners
Abstract:
Sedimentary rocks of the Paradox Basin of the Colorado Plateau
(southwestern USA) record widespread manifestations of paleo–fluid flow and
fluid-rock reactions including Cu, U-V, and Fe-Mn mineral deposits, Si and
Ca metasomatism, hydrocarbon accumulations, and bleached sandstones. Many
of these are spatially associated with faults. Here we show evidence for a
widespread phase of fault-related fluid migration and mineralization at
41–48 Ma in the Paradox Basin. We measured K-Ar dates of multiple size
fractions of clay-rich fault gouge, yielding statistically overlapping
dates of authigenic (1Md) illite for the Salt Valley (47.0 ± 3.0
Ma), Kane Springs (47.7 ± 3.8 Ma), Cliffdweller (43.4 ± 4.6 Ma), Courthouse
(41.9 ± 2.3 Ma), Lisbon Valley (45.3 ± 0.9 Ma), and GTO (48.1 ± 2.6 Ma)
faults. The latter two have an illite Rb-Sr isochron age of 50.9 ± 3.5 Ma,
and fault-adjacent bornite has a Re-Os isochron age of 47.5 ± 1.5 Ma.
Authigenic illite from a paleo–oil reservoir near the Courthouse fault
formed from the interaction of reduced fluids with oxidized red-bed
sandstones at 41.1 ± 2.5 Ma. The Moab and Keystone faults have older
authigenic illite ages of 59.1 ± 5.7 Ma and 65.2 ± 1.0 Ma, respectively.
Our results show a close temporal relationship between fault gouge
formation, red-bed bleaching, and Cu mineralization during an enigmatic
time interval, raising questions about drivers of Eocene fluid flow.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49466.1/609929/Eocene-fault-controlled-fluid-flow-and
Morphologic signatures of autogenic waterfalls: A case study in the San
Gabriel Mountains, California
Erika L. Groh; Joel S. Scheingross
Abstract:
Waterfalls can form due to external perturbation of river base level,
lithologic heterogeneity, and internal feedbacks (i.e., autogenic
dynamics). While waterfalls formed by lithologic heterogeneity and external
perturbation are well documented, there is a lack of criteria with which to
identify autogenic waterfalls, thereby limiting the ability to assess the
influence of autogenic waterfalls on landscape evolution. We propose that
autogenic waterfalls evolve from bedrock bedforms known as cyclic steps and
therefore form as a series of steps with spacing and height set primarily
by channel slope. We identified 360 waterfalls split between a transient
and steady-state portion of the San Gabriel Mountains in California, USA.
Our results show that while waterfalls have different spatial distributions
in the transient and steady-state landscapes, waterfalls in both landscapes
tend to form at slopes >3%, coinciding with the onset of Froude
supercritical flow, and the waterfall height to spacing ratio in both
landscapes increases with slope, consistent with cyclic step theory and
flume experiments. We suggest that in unglaciated mountain ranges with
relatively uniform rock strength, individual waterfalls are predominately
autogenic in origin, while the spatial distribution of waterfalls may be
set by external perturbations.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49320.1/609930/Morphologic-signatures-of-autogenic-waterfalls-A
The influence of off-fault deformation zones on the near-fault
distribution of coseismic landslides
Colin K. Bloom; Andrew Howell; Timothy Stahl; Chris Massey; Corinne
Singeisen
Abstract:
Coseismic landslides are observed in higher concentrations around
surface-rupturing faults. This observation has been attributed to a
combination of stronger ground motions and increased rock mass damage
closer to faults. Past work has shown it is difficult to separate the
influences of rock mass damage from strong ground motions on landslide
occurrence. We measured coseismic off-fault deformation (OFD) zone widths
(treating them as a proxy for areas of more intense rock mass damage) using
high-resolution, three-dimensional surface displacements from the 2016 M w 7.8 Kaikōura earthquake in New Zealand. OFD zones vary in
width from ~50 m to 1500 m over the ~180 km length of ruptures analyzed.
Using landslide densities from a database of 29,557 Kaikōura landslides, we
demonstrate that our OFD zone captures a higher density of coseismic
landslide incidence than generic “distance to fault rupture” within ~650 m
of surface fault ruptures. This result suggests that the effects of rock
mass damage within OFD zones (including ground motions from trapped and
amplified seismic waves) may contribute to near-fault coseismic landslide
occurrence in addition to the influence of regional ground motions, which
attenuate with distance from the fault. The OFD zone represents a new path
toward understanding, and planning for, the distribution of coseismic
landslides around surface fault ruptures. Inclusion of estimates of fault
zone width may improve landslide susceptibility models and decrease
landslide risk.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49429.1/609931/The-influence-of-off-fault-deformation-zones-on
Pyrrhotite–silicate melt partitioning of rhenium and the deep rhenium
cycle in subduction zone
Shuo Xue; Yuan Li
Abstract:
The Re-Os isotopic system serves as an important tracer of recycled crust
in Earth’s deep mantle because of the large Re/Os ratios and
time-integrated enrichment of radiogenic Os in Earth’s crust. However, the
Re distribution in Earth’s known reservoirs is mass imbalanced, and the
behavior of Re during subduction remains little understood. We performed
laboratory experiments to determine the partition coefficients of Re
between pyrrhotite and silicate melt (DRepo/sm) at
950–1080 °C, 1–3 GPa, and oxygen fugacities (in log units relative to the
fayalite-magnetite-quartz [FMQ] buffer) of FMQ –1.3 to FMQ +2. The obtained
DRepo/sm values are 200–25,000, which increase with
decreasing oxygen fugacity and the total iron content (FeOtot)
of silicate melt but decrease with increasing temperature or decreasing
pressure. Applying DRepo/sm to constrain the behavior
of Re during slab melting demonstrates that slab melts contribute minimal
Re to the sub-arc mantle, with most Re dissolved in sulfides subducted into
Earth’s deep mantle. Deep storage of recycled oceanic basalts and sediments
can explain the mass imbalance of Re in Earth’s primitive mantle, depleted
mantle, and crust.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49374.1/609538/Pyrrhotite-silicate-melt-partitioning-of-rhenium
Identifying crystal accumulation and melt extraction during formation
of high-silica granite
Tian-Yu Lu; Zhen-Yu He; Reiner Klemd
Abstract:
High-silica (<70 wt% SiO2) magmas are usually believed to
form via shallow crustal–level fractional crystallization of intermediate
magmas. However, the broad applicability of this model is controversial,
because the required crystal-melt separation processes have rarely been
documented globally up to now. The ca. 50 Ma Nyemo composite pluton of the
Gangdese batholith belt in southern Tibet, which comprises intrusive rocks
with intermediate- to high-silica compositions (65–78 wt%), offers a unique
opportunity for substantiating the coexistence of extracted melts and
complementary silicic cumulates in one of Earth’s most complete
transcrustal silicic magmatic systems. The Nyemo pluton intrusive rocks
exhibit similar zircon Hf isotopic compositional ranges (mean ε Hf(t) = +5.7 to +8.3), suggesting a common, non-radiogenic magma
source with crustal assimilation in the deep crust. Yet, these rocks have
distinct geochemical characteristics. High-silica miarolitic and rapakivi
granites are strongly depleted in Ba, Sr, and Eu, and their zircon trace
elements show extremely low Eu/Eu* and Dy/Yb. In contrast, monzogranite is
relatively enriched in Ba and Sr with minor Eu anomalies, and the zircon
trace elements are characterized by relatively high Eu/Eu* and Dy/Yb.
Therefore, we propose that the high-silica granites represent highly
fractionated melt extracted from a mush reservoir at unusually low storage
pressure (~99–119 MPa), and that the monzogranite constitutes the
complementary residual silicic cumulates.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49434.1/609539/Identifying-crystal-accumulation-and-melt
Evaluating preservation bias in the continental growth record against
the monazite archive
Jacob A. Mulder; Peter A. Cawood
Abstract:
Most recent models of continental growth are based on large global
compilations of detrital zircon ages, which preserve a distinctly episodic
record of crust formation over billion-year timescales. However, it remains
unclear whether this uneven distribution of zircon ages reflects a true
episodicity in the generation of continental crust through time or is an
artifact of the selective preservation of crust isolated in the interior of
collisional orogens. We address this issue by analyzing a new global
compilation of monazite ages (n >100,000), which is comparable in size,
temporal resolution, and spatial distribution to the zircon continental
growth record and unambiguously records collisional orogenesis. We
demonstrate that the global monazite and zircon age distributions are
strongly correlated throughout most of Earth history, implying a link
between collisional orogenesis and the preserved record of continental
growth. Our findings support the interpretation that the continental crust
provides a preservational, rather than generational, archive of crustal
growth.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49416.1/609540/Evaluating-preservation-bias-in-the-continental
Uneven spatial sampling distorts reconstructions of Phanerozoic
seawater temperature
Lewis A. Jones; Kilian Eichenseer
Abstract:
Paleotemperature proxy records are widely used to reconstruct the global
climate throughout the Phanerozoic and to test macroevolutionary
hypotheses. However, the spatial distribution of these records varies
through time. This is problematic because heat is unevenly distributed
across Earth’s surface. Consequently, heterogeneous spatial sampling of
proxy data has the potential to bias reconstructed temperature curves. We
evaluated the spatiotemporal evolution of sampling using a compilation of
Phanerozoic δ18O data. We tested the influence of variable
spatial coverage on global estimates of paleotemperature by sampling a
steep “modern-type” latitudinal temperature gradient and a flattened
“Eocene-type” gradient, based on the spatial distribution of δ18
O samples. We show that global paleotemperature is overestimated in ~70% of
Phanerozoic stages. Perceived climatic trends for some intervals might be
artifactually induced by shifts in paleolatitudinal sampling, with
equatorward shifts in sampling concurring with warming trends, and poleward
shifts concurring with cooling trends. Yet, the magnitude of some climatic
perturbations might also be underestimated. For example, the observed
Ordovician cooling trend may be underestimated due to an equatorward shift
in sampling. Our findings suggest that while proxy records are vital for
reconstructing Earth’s paleotemperature in deep time, consideration of the
spatial nature of these data is crucial to improving these reconstructions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49132.1/609542/Uneven-spatial-sampling-distorts-reconstructions
Global-scale emergence of continental crust during the
Mesoarchean–early Neoarchean
Wei Wang; Peter A. Cawood; Christopher J. Spencer; Manoj K. Pandit;
Jun-Hong Zhao ...
Abstract:
The timing of the emergence of subaerial landmasses is equivocally
constrained as post-Archean and continues to be a much-debated issue. In
this study, we document exceptionally 18O-depleted (δ 18O < 4.7‰) Mesoarchean to early Neoarchean magmatism in
India that shows a similarity with the coeval low-δ18O magmas
reported from Australia, South America, and northern China. Such
global-scale low-δ18O magmatism would require high-temperature
meteoric water–rock interaction in the uppermost crust synchronous with
magma generation, necessitating the emergence of a substantial volume of
the continental crust. The timing of this low-δ18O magmatism
coincides with the development of extensive, subaerial large igneous
provinces, a downward shift in δ18O and Δ17O values
in pelitic rocks, the rise of normalized 87Sr/86Sr in
seawater, and an intermittent upsurge in the proportion of atmospheric
oxygen. We propose that the emergence of substantial volumes of continental
crust initiated at ca. 3.2 Ga and peaked at 2.8–2.6 Ga, facilitating the
generation of globally distributed low-δ18O magmas, and this
event can be linked to the first appearance of atmospheric oxygen.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49418.1/609543/Global-scale-emergence-of-continental-crust-during
Centennial to millennial variability of greenhouse climate across the
mid-Cenomanian event
Chao Ma; Linda A. Hinnov; James S. Eldrett; Stephen R. Meyers; Steven C.
Bergman ...
Abstract:
Centennial- to millennial-scale climate variations are often attributed to
solar forcing or internal climate system variability, but recognition of
such variations in the deep-time paleoclimate record is extremely rare. We
present an exceptionally well-preserved, millimeter-scale laminated
marlstone from a succession of precession-driven limestone-marlstone
couplets deposited in the Western Interior Seaway (North America)
immediately preceding and during the Cretaceous mid-Cenomanian event (ca.
96.5 Ma). Sedimentological, geochemical, and micropaleontological data
indicate that individual pairs of light-dark laminae record alternations in
the extent of water-column mixing and oxygenation. Principal component
analysis of X-ray fluorescence element counts and a grayscale scan of a
continuous thin section through the marlstone reveal variations with 80–100
yr, 200–230 yr, 350–500 yr, ~1650 yr, and 4843 yr periodicities. A
substantial fraction of the data indicates an anoxic bottom water variation
with a pronounced 10,784 yr cycle. The centennial to millennial variations
are reminiscent of those found in Holocene total solar irradiance
variability, and the 10,784 yr anoxia cycle may be a manifestation of
semi-precession-influenced Tethyan oxygen minimum zone waters entering the
seaway.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48734.1/609544/Centennial-to-millennial-variability-of-greenhouse
The ubiquitous creeping segments on oceanic transform faults
Pengcheng Shi; Meng (Matt) Wei; Robert A. Pockalny
Abstract:
Oceanic transform faults are a significant component of the global plate
boundary system and are well known for generating fewer and smaller
earthquakes than expected. Detailed studies at a handful of sites support
the hypothesis that an abundance of creeping segments is responsible for
most of the observed deficiency of earthquakes on those faults. We test
this hypothesis on a global scale. We relocate Mw ⩾5 earthquakes
on 138 oceanic transform faults around the world and identify creeping
segments on these faults. We demonstrate that creeping segments occur on
almost all oceanic transform faults, which could explain their deficiency
of earthquakes. We also find that most of the creeping segments are not
associated with any large-scale geological structure such as a fault
step-over, indicating that along-strike variation of fault zone properties
may be the main reason for their existence.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49562.1/609351/The-ubiquitous-creeping-segments-on-oceanic
Crustal fluids cause strong Lu-Hf fractionation and Hf-Nd-Li isotopic
provinciality in the mantle of continental subduction zones
Dominik Gudelius; Sonja Aulbach; Hans-Michael Seitz; Roberto Braga
Abstract:
Metasomatized mantle wedge peridotites exhumed within high-pressure
terranes of continental collision zones provide unique insights into
crust-mantle interaction and attendant mass transfer, which are critical to
our understanding of terrestrial element cycles. Such peridotites occur in
high-grade gneisses of the Ulten Zone in the European Alps and record
metasomatism by crustal fluids at 330 Ma and high-pressure conditions (2.0
GPa, 850 °C) that caused a transition from coarse-grained, garnet-bearing
to fine-grained, amphibole-rich rocks. We explored the effects of crustal
fluids on canonically robust Lu-Hf peridotite isotope signatures in
comparison with fluid-sensitive trace elements and Nd-Li isotopes. Notably,
we found that a Lu-Hf pseudo-isochron is created by a decrease in bulk-rock 176Lu/177Hf from coarse- to fine-grained peridotite
that is demonstrably caused by heavy rare earth element (HREE) loss during
fluid-assisted, garnet-consuming, amphibole-forming reactions accompanied
by enrichment in fluid-mobile elements and the addition of unradiogenic Nd.
Despite close spatial relationships, some peridotite lenses record more
intense fluid activity that causes complete garnet breakdown and high field
strength element (HFSE) addition along with the addition of crust-derived
unradiogenic Hf, as well as distinct chromatographic light REE (LREE)
fractionation. We suggest that the observed geochemical and isotopic
provinciality between peridotite lenses reflects different positions
relative to the crustal fluid source at depth. This interpretation is
supported by Li isotopes: inferred proximal peridotites show light δ 7Li due to strong kinetic Li isotope fractionation (–4.7–2.0‰)
that accompanies Li enrichment, whereas distal peridotites show Li contents
and δ7Li similar to those of the depleted mantle (1.0–7.2‰).
Thus, Earth’s mantle can acquire significant Hf-Nd-Li-isotopic
heterogeneity during locally variable ingress of crustal fluids in
continental subduction zones.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49317.1/609352/Crustal-fluids-cause-strong-Lu-Hf-fractionation
Lithium isotopic composition of soil pore water: Responses to
evapotranspiration
Zhewen Xu; Tao Li; Gaojun Li; David W. Hedding; Yunqiang Wang ...
Abstract:
Lithium isotopes show great potential to trace Earth surface processes due
to the large mass discrimination between 6Li and 7Li
associated with clay uptake. However, factors controlling the Li isotopic
composition (δ7Li) of river water, especially those with a
water-bedrock δ7Li difference higher than that of the
equilibrium fractionation associated with clay formation (ΔW-C),
have not yet been fully resolved. Traditional interpretation involves the
Rayleigh distillation, but it unrealistically separates the stage of clay
formation from that of silicate dissolution using fractionation factors
that are much lower than laboratories can constrain. We propose an in situ mechanism that simulates high δ7Li by
evapotranspiration. A model with coupled mineral dissolution and clay
precipitation shows that evaporative enrichment of pore-water Li
progressively increases the incorporation of Li into clays with light δ 7Li, resulting in higher δ7Li values in the residual
water than ΔW-C. We also provide evidence from the Chinese Loess
Plateau, where an evaporative effect readily explains the observed δ7Li. The influence of evapotranspiration on riverine δ 7Li implies that changes in aridity may partly explain the
variations of seawater δ7Li. The same principle may also apply
to other stable isotopic systems whereby incorporation into secondary
precipitates controls the isotopic fractionation.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49366.1/609353/Lithium-isotopic-composition-of-soil-pore-water
Widespread glasses generated by cometary fireballs during the late
Pleistocene in the Atacama Desert, Chile
Peter H. Schultz; R. Scott Harris; Sebastián Perroud; Nicolas Blanco;
Andrew J. Tomlinson
Abstract:
Twisted and folded silicate glasses (up to 50 cm across) concentrated in
certain areas across the Atacama Desert near Pica (northern Chile) indicate
nearly simultaneous (seconds to minutes) intense airbursts close to Earth’s
surface near the end of the Pleistocene. The evidence includes mineral
decompositions that require ultrahigh temperatures, dynamic modes of
emplacement for the glasses, and entrained meteoritic dust. Thousands of
identical meteoritic grains trapped in these glasses show compositions and
assemblages that resemble those found exclusively in comets and CI group
primitive chondrites. Combined with the broad distribution of the glasses,
the Pica glasses provide the first clear evidence for a cometary body (or
bodies) exploding at a low altitude. This occurred soon after the arrival
of proto-Archaic hunter-gatherers and around the time of rapid climate
change in the Southern Hemisphere.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49426.1/609354/Widespread-glasses-generated-by-cometary-fireballs
Zircon (U-Th)/He thermochronology of Grand Canyon resolves 1250 Ma
unroofing at the Great Unconformity and <20 Ma canyon carving
Olivia G. Thurston; William R. Guenthner; Karl E. Karlstrom; Jason W.
Ricketts; Matthew T. Heizler ...
Abstract:
Our study used zircon (U-Th)/He (ZHe) thermochronology to resolve cooling
events of Precambrian basement below the Great Unconformity surface in the
eastern Grand Canyon, United States. We combined new ZHe data with previous
thermochronometric results to model the <250 °C thermal history of
Precambrian basement over the past >1 Ga. Inverse models of ZHe
date-effective uranium (eU) concentration, a relative measure of radiation
damage that influences closure temperature, utilize He diffusion and damage
annealing and suggest that the main phase of Precambrian cooling to <200
°C was between 1300 and 1250 Ma. This result agrees with mica and potassium
feldspar 40Ar/39Ar thermochronology showing rapid
post–1400 Ma cooling, and both are consistent with the 1255 Ma depositional
age for the Unkar Group. At the young end of the timescale, our data and
models are also highly sensitive to late-stage reheating due to burial
beneath ~3–4 km of Phanerozoic strata prior to ca. 60 Ma; models that best
match observed date-eU trends show maximum temperatures of 140–160 °C, in
agreement with apatite (U-Th)/He and fission-track data. Inverse models
also support multi-stage Cenozoic cooling, with post–20 Ma cooling from ~80
to 20 °C reflecting partial carving of the eastern Grand Canyon, and late
rapid cooling indicated by 3–7 Ma ZHe dates over a wide range of high eU.
Our ZHe data resolve major basement exhumation below the Great Unconformity
during the Mesoproterozoic (1300–1250 Ma), and “young” (20–0 Ma) carving of
Grand Canyon, but show little sensitivity to Neoproterozoic and Cambrian
basement unroofing components of the composite Great Unconformity.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48699.1/609355/Zircon-U-Th-He-thermochronology-of-Grand-Canyon
Lithosphere–asthenosphere interactions beneath northeast China and the
origin of its intraplate volcanism
Anqi Zhang; Zhen Guo; Juan Carlos Afonso; Heather Handley; Hongkun Dai ...
Abstract:
Northeast China hosts one of the largest Cenozoic intraplate volcanic
regions in the world. However, the mechanisms that generate the volcanism,
its spatial-temporal distribution, and compositional signatures remain
highly debated due to the lack of high-resolution images of the mantle’s
thermochemical structure. We jointly inverted new surface-wave dispersion
data, surface heat flow, geoid height, and elevation data to image the
fine-scale thermal and compositional structures beneath northeast China and
infer regions of partial melting in the mantle. Our model reveals a complex
circulation pattern in the asthenosphere and a highly variable lithospheric
structure. Combining predictions from our model with independent
geochemical data from recent basaltic volcanism, we demonstrate that the
generation, location, and composition of intraplate volcanism in this
region are controlled by the interaction between shallow asthenospheric
circulation and lithospheric thickness. The modeling approach and
correlations between basaltic composition and mantle state identified in
our study are globally applicable to assessing mantle conditions over time
in other continental regions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49375.1/609356/Lithosphere-asthenosphere-interactions-beneath
Oceanic strike-slip faults represent active fluid conduits in the
abyssal sub-seafloor
Christian Hensen; Florian Scholz; Volker Liebetrau; Norbert Kaul; Marianne
Nuzzo ...
Abstract:
We present pore-fluid geochemistry and heat-flow data along the SWIM1 fault
in the Horseshoe Abyssal Plain (northeastern Atlantic Ocean). The SWIM1
fault is part of the transcurrent plate boundary between Africa and Eurasia
and cuts through as much as 5-km-thick sediments overlying >140 Ma
oceanic lithosphere. In a number of places, restraining segments (as long
as 15 km) of the SWIM1 fault generate anticlines (positive flower
structures) that protrude as ~100-m-high hills above the abyssal plain.
Heat flow and gradients of dissolved constituents in pore water are
enhanced at these seafloor highs. Transport-reaction modeling confirms that
slow advection of deep-seated fluids, depleted in Mg and enriched in Sr and
CH4, can explain the observations. The geochemical signature is
similar to the one observed at deep-sea mud volcanoes located eastward on
the SWIM1 fault. The upward-migrating fluids have interacted with carbonate
rocks at maximum 5 km depth, which represent the oldest sedimentary unit on
top of the basement. We argue that deep-rooted fluids can generally be
mobilized and transported upward along flower structures that formed in
restraining-bend segments of long strike-slip faults. Such tectonic
settings represent largely unrecognized corridors for mass exchange between
lithosphere and ocean.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49344.1/609357/Oceanic-strike-slip-faults-represent-active-fluid
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