New Geology Articles Published Online Ahead of Print
Boulder, Colo., USA: Article topics and locations include the Red Lake
greenstone belt, Canada; Anak Krakatau volcano, Indonesia; martian soil;
Glacial Lake Missoula, Montana, USA; and findings from IODP Expedition 385.
These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent
.
Crustal conductivity footprint of the orogenic gold district in the
Red Lake greenstone belt, western Superior craton, Canada
Ademola Q. Adetunji; Gaetan Launay; Ian J. Ferguson; Jack M. Simmons; Chong
Ma ...
A magnetotelluric (MT) study across the Red Lake greenstone belt of the
western Superior craton, Canada, images a 50-km-long north-dipping
conductor (<20 Ω·m) at 20–25 km depth and subvertical conductors
spatially correlated with crustal-scale shear zones and large orogenic gold
deposits. The conductors are interpreted to be the conductivity signature
of the deep crustal source of the auriferous fluids and pathways of the
orogenic gold system. The geophysical results, supported by existing
geochemical and fluid inclusion studies, suggest that the Au- and CO 2-rich fluids responsible for gold mineralization were released
by devolatilization of supracrustal rocks underthrust to mid- to
lower-crustal levels during subduction. This MT study links shallow gold
mineralization to a deep crustal source region, demonstrating the
connection between a crustal suture zone and the formation of orogenic gold
deposits in an Archean greenstone belt.
Interactions of magmatic intrusions with the multiyear flank
instability at Anak Krakatau volcano, Indonesia: Insights from
InSAR and analogue modeling
Edgar U. Zorn; Magdalena Vassileva; Thomas R. Walter; Herlan Darmawan;
Leonie Röhler ...
Volcano flank collapses have been documented at ocean islands worldwide and
are capable of triggering devastating tsunamis, but little is known about
the precursory processes and deformation changes prior to flank failure.
This makes the 22 December 2018 flank collapse at Anak Krakatau in
Indonesia a key event in geosciences. Here, we provide direct insight into
the precursory processes of the final collapse. We analyzed interferometric
synthetic aperture radar (InSAR) data from 2014 to 2018 and studied the
link between the deformation trend and intrusion occurrence through
analogue modeling. We found that the flank was already moving at least 4 yr
prior to collapse, consistent with slow décollement slip. Movement rates
averaged ~27 cm/yr, but they underwent two accelerations coinciding with
distinct intrusion events in January/February 2017 and in June 2018.
Analogue models suggest that these accelerations occurred by (re)activation
of a décollement fault linked to a short episode of magma intrusion. During
intrusion, we observed a change in the internal faults, where the
outward-directed décollement accelerated while inward faults became
partially blocked. These observations suggest that unstable oceanic flanks
do not disintegrate abruptly, but their collapse is preceded by observable
deformations that can be accelerated by new intrusions.
Martian soil as revealed by ground-penetrating radar at the
Tianwen-1 landing site
Ruonan Chen; Ling Zhang; Yi Xu; Renrui Liu; Roberto Bugiolacchi ...
Much of the Martian surface is covered by a weathering layer (regolith or
soil) produced by long-term surface processes such as impact gardening,
eolian erosion, water weathering, and glacial modifications. China’s first
Martian mission, Tianwen-1, employed the Mars Rover Penetrating Radar
(RoPeR) to unveil the detailed structure of the regolith layer and assess
its loss tangent. The RoPeR radargram revealed the local regolith layer to
be highly heterogeneous and geologically complex and characterized by
structures that resemble partial or complete crater walls and near-surface
impact lenses at a very shallow depth. However, comparable radar data from
the Lunar far side are rather uniform, despite the two surfaces being
geologically contemporary. The close-to-surface crater presented in this
study shows no detectable surface expression, which suggests an accelerated
occultation rate for small craters on the surface of Mars as compared to
the rate on the Moon. This is probably due to the relentless eolian
processes on the Martian surface that led to the burial of the crater and
thus shielded it from further erosion. The high loss tangent indicates that
the regolith at the Tianwen-1 landing site is not dominated by water ice.
Rock surface luminescence dating of gravel determines the age of a
glacial outburst megaflood, Glacial Lake Missoula, Montana, USA
Larry N. Smith; Reza Sohbati; Mayank Jain
Giant gravel bars are important archives of megafloods; however,
determining their depositional ages requires reliable geochronometric
methods. Five gravel bars, reaching heights of 150–170 m, formed in the
bedrock-lined Alberton Gorge along the Clark Fork River, Montana (USA),
during draining of Glacial Lake Missoula (GLM). We report the first
numerical ages for megaflood deposits in the GLM basin by successfully
applying the novel optically stimulated luminescence (OSL) rock surface
dating technique to date cobbles collected from three locations along one
bar’s transport direction. Depth-dependent infrared stimulated luminescence
and post-infrared pulsed OSL signals showed that exteriors of only 3 out of
the 38 collected cobble samples were well bleached by exposure to daylight
before burial, and hence suitable for dating. The cobbles provided ages of
16.5 ± 0.9, 18.5 ± 1.4, and 21.7 ± 1.1 ka, all of which are
indistinguishable from the average cosmogenic nuclide age of 18.2 ± 1.5 ka
(n = 4) for a large megaflood in the Channeled Scabland, eastern
Washington State. The average of the two younger ages, 17.5 ± 1.0 ka, is
our best estimate of the deposit age. We interpret the older age to be from
a cobble that was reworked by the flood. Our results show that these
techniques have great potential for providing reliable chronologies for
paleofloods and other high-energy depositional events.
Stranding continental crustal fragments during continent breakup:
Mantle suture reactivation in the Nain Province of Eastern Canada
Philip J. Heron; A.L. Peace; K.J.W. McCaffrey; A. Sharif; A.J. Yu ...
Earth’s continental crust has evolved through a series of supercontinent
cycles, resulting in a patchwork of Archean cores surrounded by terranes,
fragments, and slivers of younger crustal additions. However, the dispersal
(and/or stranding) of continental fragments during breakup is not well
understood. Inherited structures from previous tectonic activity may
explain the generation of continental terranes by controlling first-order
deformation during rifting. Here, we explored the influence of lithospheric
deformation related to ancient orogenesis, focusing on the impact of the
Torngat orogen in the genesis of the Nain Province continental fragment in
Eastern Canada. We present three-dimensional continental extension models
in the presence of an inherited lithospheric structure and show that a
narrow continental terrane could be separated and stranded by deep
lithospheric scarring. The results show that continental terranes formed by
this method would be limited to a width of 100–150 km, imposed by tectonic
conditions during continental suturing. The findings have broad
implications, demonstrating an original theory on the fundamental geologic
problem of terrane generation and continent breakup.
Carbon released by sill intrusion into young sediments measured
through scientific drilling
Daniel Lizarralde; Andreas Teske; Tobias W. Höfig; Antonio
González-Fernández; IODP Expedition 385 Scientists
The intrusion of igneous sills into organic-rich sediments accompanies the
emplacement of igneous provinces, continental rifting, and sedimented
seafloor spreading. Heat from intruding sills in these settings alters
sedimentary organic carbon, releasing methane and other gasses. Recent
studies hypothesize that carbon released by this mechanism impacts global
climate, particularly during large igneous province emplacements. However,
the direct impacts of sill intrusion, including carbon release, remain
insufficiently quantified. Here, we present results from International
Ocean Discovery Program (IODP) Expedition 385 comparing drill-core and
wireline measurements from correlative sedimentary strata at adjacent sites
cored in Guaymas Basin, Gulf of California, one altered by a recently
intruded sill and one unaffected. We estimate 3.30 Mt of carbon were
released due to this sill intrusion, representing an order of magnitude
less carbon than inferences from outcrops and modeling would predict. This
attenuated carbon release can be attributed to shallow intrusion and the
high heat capacity of young, high-porosity sediments. Shallow intrusion
also impacts sub-seafloor carbon cycling by disrupting advective fluxes,
and it compacts underlying sediments, increasing potential carbon release
in response to subsequent intrusions.
Carbon isotope and biostratigraphic evidence for an expanded
Paleocene–Eocene Thermal Maximum sedimentary record in the deep
Gulf of Mexico
Lucas Vimpere; Jorge E. Spangenberg; Marta Roige; Thierry Adatte; Eric De
Kaenel ...
In this study, we present evidence of a Paleocene–Eocene Thermal Maximum
(PETM) record within a 543-m-thick (1780 ft) deep-marine section in the
Gulf of Mexico (GoM) using organic carbon stable isotopes and
biostratigraphic constraints. We suggest that climate and tectonic
perturbations in the upstream North American catchments can induce a
substantial response in the downstream sectors of the Gulf Coastal Plain
and ultimately in the GoM. This relationship is illustrated in the
deep-water basin by (1) a high accommodation and deposition of a shale
interval when coarse-grained terrigenous material was trapped upstream at
the onset of the PETM, and (2) a considerable increase in sediment supply
during the PETM, which is archived as a particularly thick sedimentary
section in the deep-sea fans of the GoM basin. Despite other thick PETM
sections being observed elsewhere in the world, the one described in this
study links with a continental-scale paleo-drainage, which makes it of
particular interest for paleoclimate and source-to-sink reconstructions.
Melting of fault gouge at shallow depth during the 2008 MW 7.9
Wenchuan earthquake, China
H. Wang; H.B. Li; G. Di Toro; L.-W. Kuo; E. Spagnuolo ...
Typical rocks at shallow depths of seismogenic faults are fluid-rich
gouges. During earthquakes, on-fault frictional heating may trigger thermal
pressurization and dynamic fault weakening. We show that frictional
melting, rather than thermal pressurization, occurred at shallow depths
during the 2008 MW 7.9 Wenchuan earthquake, China. One year
after the Wenchuan earthquake, we found an ~2-mm-thick, glass-bearing
pseudotachylyte (solidified frictional melt) in the fault gouges retrieved
at 732.6 m depth from the first borehole of the Wenchuan Earthquake Fault
Scientific Drilling Project. The matrix of pseudotachylyte is enriched in
barium and cut by barite-bearing veins, which provide evidence of co- and
post-seismic fluid percolation. Because pseudotachylyte can be rapidly
altered in the presence of percolating fluids, its preservation suggests
that gouge melting occurred in a recent large earthquake, possibly the
Wenchuan earthquake. Rock friction experiments on fluid-rich fault gouges
deformed at conditions expected for seismic slip at borehole depths showed
the generation of pseudotachylytes. This result, along with the presence of
a second slip zone attributed to the Wenchuan earthquake at 589.2 m depth,
implies that during large earthquakes, frictional melting can occur at
shallow depths and that seismic slip can be accommodated by multiple
faults. This conclusion is consistent with the evidence from surface
faulting that multiple ruptures propagated during the Wenchuan earthquake.
In situ Raman observations reveal that the gas fluxes of diffuse
flow in hydrothermal systems are greatly underestimated
Lianfu Li; Zhendong Luan; Zengfeng Du; Shichuan Xi; Jun Yan ...
Reduced gases released from hydrothermal vents supply energy to local
deep-marine ecosystems and play an important role in global biogeochemical
cycles of sulfur and carbon. The habitable, lower-temperature diffuse flow
sites in a hydrothermal system generally have higher biomass than focused
flow sites. However, a scarcity of observational data of diffuse flows
limits our understanding of the role of volatile gases in these
environments. We deployed in situ Raman spectroscopy in the Iheya
North hydrothermal field of the mid–Okinawa Trough (East China Sea). A
Raman probe inserted directly into hydrothermal vent orifices with
temperatures of 30–302 °C collected Raman spectra of hydrothermal fluids. In situ observation data show that the greater volume of diffuse
flows results in a flux of volatile gases one to two orders of magnitude
higher than that from focused flow environments. This indicates the great
potential of diffuse flow for supplying energy and material to hydrothermal
systems. The role played by diffuse flow should be reassessed.
Sediment waves control origins of submarine canyons
Xingxing Wang; Benjamin Kneller; Qiliang Sun
Submarine canyons commonly occur on virtually all continental slopes. Their
varied origins are widely studied but still debatable. Eastward
(along-slope)–migrating submarine canyons, with nearly regular spacing, are
well developed at the northern South China Sea. High-resolution
three-dimensional seismic data show that these canyons are localized in the
troughs between sediment waves. The waves were present on the slope since
before ca. 10.5 Ma and were especially well developed during the late
Miocene (ca. 10.5–5.5 Ma). This interval can be divided into two units, of
which the upper unit (SU II) has larger sediment waves and much
better-developed canyons compared to the lower unit (SU I). Submarine fans
developed at the canyon mouths within SU II at the downdip termination of
the confinement caused by the sediment waves. Gravity currents were
captured between the waves, resulting in erosion mainly along the troughs
between them. The canyons were forced to migrate eastward by the migration
of the confining sediment waves. In this study, we present a new mechanism
for the origin of such regularly spaced submarine canyons for the first
time, which we attribute to the formation of regularly spaced sediment
waves generated by contour currents.
Inclination and heterogeneity of layered geological sequences
influence dike-induced ground deformation
Matías Clunes; John Browning; Carlos Marquardt; Jorge Cortez; Kyriaki
Drymoni ...
Constraints on the amount and pattern of ground deformation induced by dike
emplacement are important for assessing potential eruptions. The vast
majority of ground deformation inversions made for volcano monitoring
during volcanic unrest assume that dikes are emplaced in either an elastic
half-space (a homogeneous crust) or a crust made of horizontal layers with
different mechanical properties. We extend these models by designing a
novel set of two-dimensional finite-element method numerical simulations
that consider dike-induced surface deformation related to a mechanically
heterogeneous crust with inclined layers, thus modeling a common geometry
in stratovolcanoes and crustal segments that have been folded by tectonic
forces. Our results confirm that layer inclination can produce localized
ground deformation that may be as much as 40× higher in terms of
deformation magnitude than would be expected in a non-layered model,
depending on the angle of inclination and the stiffness of the rock units
that host and are adjacent to the dike. Generated asymmetrical deformation
patterns produce deformation peaks located as much as 1.4 km away from
those expected in non-layered models. These results highlight the necessity
of accurately quantifying both the mechanical properties and attitude of
the geology underlying active volcanoes.
Buoyant doming generates metamorphic core complexes in the North
American Cordillera
Drew A. Levy; Andrew V. Zuza; Zachary D. Michels; Joel W. DesOrmeau
Metamorphic core complexes (MCCs) are considered to be a hallmark of
large-magnitude crustal extension, with their characteristic high-strain
mylonitic fabrics attributed to simple-shear strain downdip of a detachment
fault. However, some MCCs exhibit pure-shear–dominated mylonitic fabrics
temporally decoupled from regional extension, which may be related to
magmatically enhanced buoyant doming of the lower crust. We tested the
viability of buoyant doming for the formation of MCCs in the North American
Cordillera by investigating the kinematics and conditions of mylonitic
shear in the Ruby Mountains–East Humboldt Range (REH) MCC. Field
observations and geochronology demonstrate an ~10 m.y. gap between
midcrustal attenuation and regional extension in the brittle upper crust.
Mylonites in the REH record general shear strain with >80% bulk
attenuation at strain rates of 10–13 to 10–12 s –1 and temperatures of 400–600 °C. The REH mylonites developed
at the culmination of 40–29 Ma magmatism involving mantle-derived mafic
intrusions and leucogranite crustal melts prior to post–17 Ma detachment
faulting. We posit diapirism driven by thermal and melt buoyancy could have
generated shear zones along the diapir flanks at our documented strain
rates. Characteristics of the buoyant doming model are expressed in many
low- to moderate-melt-fraction MCCs globally, and the pre-extensional
high-strain mylonitic fabrics may therefore be an important mechanism for
localizing temporally decoupled brittle detachment faulting.
Extreme isotopic heterogeneity in impact melt rocks: Implications
for Martian meteorites
Steven J. Jaret; E. Troy Rasbury; Peter Reiners; John G. Spray; Lucy M.
Thompson ...
Lead isotope ratios have been determined in multiple feldspar grains from
hand samples of impact melt rock at the Manicouagan and Sudbury impact
structures in Canada. The results reveal an extreme range of isotope
values. This indicates that melt sheets are not homogeneous with respect to
Pb at the millimeter scale. Such heterogeneity is significantly larger than
that seen in non-impact-generated igneous rocks. Individual Pb isotope
ratios of feldspars from Martian shergottites show a large range in 206Pb/204Pb values within one sample, more similar to
the terrestrial impact melt sheets than to nonimpact igneous rocks. We
suggest crystallization from impact melt sheets rather than volcanic
sources as a petrogenetic model for some of the Martian shergottites.
Disparate crustal thicknesses beneath oceanic transform faults and
adjacent fracture zones revealed by gravity anomalies
Zhikui Guo; Sibiao Liu; Lars Rüpke; Ingo Grevemeyer; Jason P. Morgan ...
Plate tectonics describes oceanic transform faults as conservative
strike-slip boundaries, where lithosphere is neither created nor destroyed.
Therefore, seafloor accreted at ridge-transform intersections should follow
a similar subsidence trend with age as lithosphere that forms away from
ridge-transform intersections. Yet, recent compilations of high-resolution
bathymetry show that the seafloor is significantly deeper along transform
faults than at the adjacent fracture zones. We present residual mantle
Bouguer anomalies, a proxy for crustal thickness, for 11 transform fault
systems across the full range of spreading rates. Our results indicate that
the crust is thinner in the transform deformation zone than in either the
adjacent fracture zones or the inside corner regions. Consequently, oceanic
transform faulting appears not only to thin the transform valley crust but
also leads to a secondary phase of magmatic addition at the transition to
the passive fracture zones. These observations challenge the concept of
transform faults being conservative plate boundaries.
Transient tin mineralization from cooling of magmatic fluids in a
long-lived system
Yang Li; Jun-Yi Pan; Li-Guang Wu; Sheng He; Olivier Bachmann ...
Fertility and longevity of hydrothermal systems are key parameters required
to improve our ability in predicting new deposits and directly extracting
metals from metalliferous fluids. Reconstructing evolutional trajectories
of metalliferous fluids with high temporal resolution is critical for
pushing our understanding forward, but this is inevitably challenging
because traditional approaches for obtaining this information either have
poor temporal resolution and/or bear considerable uncertainties. We present
a novel approach (translating texture-controlled information to temporal
patterns) to reconstruct the thermal and isotopic history of the Weilasituo
vein-type tin deposit (Inner Mongolia, China) at the millennial scale. In situ oxygen isotope thermometry of paragenetically constrained
quartz and cassiterite reveals that tin deposition was accompanied by
gradual cooling of pure magmatic fluids from ~500 °C to ~390 °C at
lithostatic conditions, while fluid mixing and/or water-rock interaction
were not required. The system then transitioned to hydrostatic conditions
and permitted penetration of meteoric water and further cooling. Aluminum
diffusion in quartz chronometry yields time scales of ~50 k.y., ~5 k.y.,
and ~200 k.y. for pre-, syn-, and post-ore stages, respectively. Our
results highlight that the magmatic-hydrothermal system did not form ore
minerals for most of its lifetime, with mineralization occurring only
briefly (i.e., <5% of its lifetime). Hence, the rates and efficiency of
ore formation may need significant revision. For magmatic-hydrothermal
systems with felsic magmas being stored at high crystallinity after
extensive volatile exsolution, the efficiency of scavenging metals from
melts to fluids critically controls their fertility. To directly extract
metals from metalliferous fluids, the key is targeting systems with a high
degree of magma crystallization (e.g., higher metal contents in fluids) in
warm crust (e.g., able to sustain long-lived systems).
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