New Geology Articles Published Online Ahead of Print in
January
Boulder, Colo., USA: Eleven new articles were published ahead of print for Geology in
January 2021. The include new modeling, geochemical evidence of tropical
cyclone impacts, transport of plastic in submarine canyons, and a porphyry
copper belt along the southeast China coast. These Geology
articles are online at
http://geology.geoscienceworld.org/content/early/recent.
Episodic exhumation of the Appalachian orogen in the Catskill
Mountains (New York State, USA)
Chilisa M. Shorten; Paul G. Fitzgerald
Abstract:
Increasing evidence indicates the eastern North American passive margin has
not remained tectonically quiescent since Jurassic continental breakup. The
identification, timing, resolution, and significance of post-orogenic
exhumation, notably an enigmatic Miocene event, are debated. We add insight
by constraining the episodic cooling and exhumation history of the Catskill
Mountains (New York, USA) utilizing apatite fission-track thermochronology
and apatite (U-Th)/He data from a ~1 km vertical profile. Multi-kinetic
inverse thermal modeling constrains three phases of cooling: Early Jurassic
to Early Cretaceous (1–3 °C/m.y.), Early Cretaceous to early Miocene (~0.5
°C/m.y.), and since Miocene times (1–2 °C/m.y.). Previous thermochronologic
studies were unable to verify late-stage cooling and/or exhumation
(typically post-Miocene and younger) because late-stage cooling was
commonly a spurious artifact of earlier mono-kinetic annealing algorithms.
Episodic cooling phases are correlative with rifting, passive-margin
development, and drainage reorganization causing landscape rejuvenation.
Geomorphologic documentation of increased offshore mid-Atlantic
sedimentation rates and onshore erosion support the documented accelerated
Miocene cooling and exhumation.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48011.1/594234/Episodic-exhumation-of-the-Appalachian-orogen-in
A new model for the optimal structural context for giant porphyry
copper deposit formation
José Piquer; Pablo Sanchez-Alfaro; Pamela Pérez-Flores
Abstract: Porphyry-type deposits are the main global source of copper and
molybdenum. An improved understanding of the most favorable structural
settings for the emplacement of these deposits is necessary for successful
exploration, particularly considering that most future discoveries will be
made under cover based on conceptual target generation. A common view is
that porphyry deposits are preferentially emplaced in pull-apart basins
within strike-slip fault systems that favor local extension within a
regional compressive to transpressive tectonic regime. However, the role of
such a structural context in magma storage and evolution in the upper crust
remains unclear. In this work, we propose a new model based on the
integration of structural data and the geometry of magmatic-hydrothermal
systems from the main Andean porphyry Cu-Mo metallogenic belts and from the
active volcanic arc of southern Chile. We suggest that the magma
differentiation and volatile accumulation required for the formation of a
porphyry deposit is best achieved when the fault system controlling magma
ascent is strongly misoriented for reactivation with respect to the
prevailing stress field. When magmas and fluids are channeled by faults
favorably oriented for extension (approximately normal to σ3), they form
sets of parallel, subvertical dikes and veins, which are common both during
the late stages of the evolution of porphyry systems and in the epithermal
environment. This new model has direct implications for conceptual mineral
exploration.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G48287.1/594235/A-new-model-for-the-optimal-structural-context-for
A new model for the growth of normal faults developed above
pre-existing structures
Emma K. Bramham; Tim J. Wright; Douglas A. Paton; David M. Hodgson
Abstract:
Constraining the mechanisms of normal fault growth is essential for
understanding extensional tectonics. Fault growth kinematics remain
debated, mainly because the very earliest phase of deformation through
recent syn-kinematic deposits is rarely documented. To understand how
underlying structures influence surface faulting, we examined fault growth
in a 10 ka magmatically resurfaced region of the Krafla fissure swarm,
Iceland. We used a high-resolution (0.5 m) digital elevation model derived
from airborne lidar to measure 775 fault profiles with lengths ranging from
0.015 to 2 km. For each fault, we measured the ratio of maximum vertical
displacement to length (Dmax/L) and any nondisplaced portions of the fault.
We observe that many shorter faults (<200 m) retain fissure-like
features, with no vertical displacement for substantial parts of their
displacement profiles. Typically, longer faults (>200 m) are vertically
displaced along most of their surface length and have Dmax/L at the upper
end of the global population for comparable lengths. We hypothesize that
faults initiate at the surface as fissure-like fractures in resurfaced
material as a result of flexural stresses caused by displacements on
underlying faults. Faults then accrue vertical displacement following a
constant-length model, and grow by dip and strike linkage or lengthening
when they reach a bell-shaped displacement-length profile. This hybrid
growth mechanism is repeated with deposition of each subsequent
syn-kinematic layer, resulting in a remarkably wide distribution of Dmax/L.
Our results capture a specific early period in the fault slip-deposition
cycle in a volcanic setting that may be applicable to fault growth in
sedimentary basins.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48290.1/594236/A-new-model-for-the-growth-of-normal-faults
Geochemical evidence of tropical cyclone controls on shallow-marine
sedimentation (Pliocene, Taiwan)
Shahin E. Dashtgard; Ludvig Löwemark; Pei-Ling Wang; Romy A. Setiaji;
Romain Vaucher
Abstract:
Shallow-marine sediment typically contains a mix of marine and terrestrial
organic material (OM). Most terrestrial OM enters the ocean through rivers,
and marine OM is incorporated into the sediment through both suspension
settling of marine plankton and sediment reworking by tides and waves under
fair-weather conditions. River-derived terrestrial OM is delivered
year-round, although sediment and OM delivery from rivers is typically
highest during extreme weather events that impact river catchments. In
Taiwan, tropical cyclones (TCs) are the dominant extreme weather event, and
75% of all sediment delivered to the surrounding ocean occurs during TCs.
Distinguishing between sediment deposited during TCs and that redistributed
by tides and waves during fair-weather conditions can be approximated using
δ13Corg values and C:N ratios of OM. Lower Pliocene shallow-marine
sedimentary strata in the Western Foreland Basin of Taiwan rarely exhibit
physical evidence of storm-dominated deposition. Instead they comprise
completely bioturbated intervals that transition upward into strata
dominated by tidally generated sedimentary structures, indicating extensive
sediment reworking under fair-weather conditions. However, these strata
contain OM that is effectively 100% terrestrial OM in sediment that
accumulated in estimated water depths <35 m. The overwhelming
contribution of terrestrially sourced OM is attributed to the dominance of
TCs on sedimentation, whereby ~600,000 TCs are estimated to have impacted
Taiwan during accumulation of the succession. In contrast, the virtual
absence of marine OM indicates that organic contributions from suspension
settling of marine OM is negligible regardless of the preserved evidence of
extensive reworking under fair-weather conditions. These data suggest that
(1) even in the absence of physical expressions of storm deposition, TCs
still completely dominate sedimentation in shallow-marine environments, and
(2) the organic geochemical signal of preserved shallow-marine strata is
not reflective of day-to-day depositional conditions in the environment.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48586.1/594237/Geochemical-evidence-of-tropical-cyclone-controls
Transport and accumulation of plastic litter in submarine
canyons—The role of gravity flows
Guangfa Zhong; Xiaotong Peng
Abstract: Manned submersible dives discovered plastic litter accumulations
in a submarine canyon located in the northwestern South China Sea, ~150 km
from the nearest coast. These plastic-dominated litter accumulations were
mostly concentrated in two large scours in the steeper middle reach of the
canyon. Plastic particles and fragments generally occurred on the
upstreamfacing sides of large boulders and other topographic obstacles,
indicating obstruction during down-valley transportation. Most of the
litter accumulations were distributed in the up-valley dipping slopes
downstream of the scour centers. This pattern is tentatively linked to
turbidity currents, which accelerated down the steep upstream slopes of the
scours and underwent a hydraulic jump toward the scour centers before
decelerating on the upstream-facing flank. Associated seabed sediment
consisted of clayey and sandy silts, with unimodal or bimodal grain-size
distributions, which are typical for turbidites. The focused distribution
of the litter accumulations is therefore linked to turbidity currents that
episodically flush the canyon. Our findings provide evidence that litter
dispersion in the deep sea may initially be governed by gravity flows, and
that turbidity currents efficiently transfer plastic litter to the deeper
ocean floor.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48536.1/594238/Transport-and-accumulation-of-plastic-litter-in
Revisiting Ediacaran sulfur isotope chemostratigraphy with in situ
nanoSIMS analysis of sedimentary pyrite
Wei Wang; Yongliang Hu; A. Drew Muscente; Huan Cui; Chengguo Guan ...
Abstract:
Reconstructions of ancient sulfur cycling and redox conditions commonly
rely on sulfur isotope measurements of sedimentary rocks and minerals.
Ediacaran strata (635–541 Ma) record a large range of values in bulk sulfur
isotope difference (Δ34S) between carbonate-associated sulfate
(δ34SCAS) and sedimentary pyrite (δ34S py), which has been interpreted as evidence of marine sulfate
reservoir size change in space and time. However, bulk δ34S py measurements could be misleading because pyrite forms under
syngenetic, diagenetic, and metamorphic conditions, which differentially
affect its isotope signature. Fortunately, these processes also impart
recognizable changes in pyrite morphology. To tease apart the complexity of
Ediacaran bulk δ34Spy measurements, we used scanning
electron microscopy and nanoscale secondary ion mass spectrometry to probe
the morphology and geochemistry of sedimentary pyrite in an Ediacaran drill
core of the South China block. Pyrite occurs as both framboidal and
euhedral to subhedral crystals, which show largely distinct negative and
positive δ34Spy values, respectively. Bulk δ 34Spy measurements, therefore, reflect mixed signals
derived from a combination of syndepositional and diagenetic processes.
Whereas euhedral to subhedral crystals originated during diagenesis, the
framboids likely formed in a euxinic seawater column or in shallow marine
sediment. Although none of the forms of pyrite precisely record seawater
chemistry, in situ framboid measurements may provide a more
faithful record of the maximum isotope fractionation from seawater sulfate.
Based on data from in situ measurements, the early Ediacaran ocean
likely contained a larger seawater sulfate reservoir than suggested by bulk
analyses.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48262.1/594239/Revisiting-Ediacaran-sulfur-isotope
Recognition of a Middle–Late Jurassic arc-related porphyry copper
belt along the southeast China coast: Geological characteristics
and metallogenic implications
Jingwen Mao; Wei Zheng; Guiqing Xie; Bernd Lehmann; Richard Goldfarb
Abstract:
Recent exploration has led to definition of a Middle–Late Jurassic copper
belt with an extent of ~2000 km along the southeast China coast. The
171–153 Ma magmatic-hydrothermal copper systems consist of porphyry, skarn,
and vein-style deposits. These systems developed along several
northeast-trending transpressive fault zones formed at the margins of
Jurassic volcanic basins, although the world-class 171 Ma Dexing porphyry
copper system was controlled by a major reactivated Neoproterozoic suture
zone in the South China block. The southeast China coastal porphyry belt is
parallel to the northeast-trending, temporally overlapping, 165–150 Ma
tin-tungsten province, which developed in the Nanling region in a back-arc
transtensional setting several hundred kilometers inboard. A new
geodynamic-metallogenic model linking the two parallel belts is proposed,
which is similar to that characterizing the Cenozoic metallogenic evolution
of the Central Andes.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48615.1/594241/Recognition-of-a-Middle-Late-Jurassic-arc-related
Anisovolumetric weathering in granitic saprolite controlled by
climate and erosion rate
Clifford S. Riebe; Russell P. Callahan; Sarah B.-M. Granke; Bradley J.
Carr; Jorden L. Hayes ...
Abstract:
Erosion at Earth’s surface exposes underlying bedrock to climate-driven
chemical and physical weathering, transforming it into a porous,
ecosystem-sustaining substrate consisting of weathered bedrock, saprolite,
and soil. Weathering in saprolite is typically quantified from bulk
geochemistry assuming physical strain is negligible. However, modeling and
measurements suggest that strain in saprolite may be common, and therefore
anisovolumetric weathering may be widespread. To explore this possibility,
we quantified the fraction of porosity produced by physical weathering, FPP, at three sites with differing climates in granitic
bedrock of the Sierra Nevada, California, USA. We found that strain
produces more porosity than chemical mass loss at each site, indicative of
strongly anisovolumetric weathering. To expand the scope of our study, we
quantified FPP using available volumetric strain and
mass loss data from granitic sites spanning a broader range of climates and
erosion rates. FPP in each case is ≥0.12, indicative of
widespread anisovolumetric weathering. Multiple regression shows that
differences in precipitation and erosion rate explain 94% of the variance
in FPP and that >98% of Earth’s land surface has
conditions that promote anisovolumetric weathering in granitic saprolite.
Our work indicates that anisovolumetric weathering is the norm, rather than
the exception, and highlights the importance of climate and erosion as
drivers of subsurface physical weathering.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48191.1/593942/Anisovolumetric-weathering-in-granitic-saprolite
“Missing links” for the long-lived Macdonald and Arago hotspots,
South Pacific Ocean
L. Buff; M.G. Jackson; K. Konrad; J.G. Konter; M. Bizimis ...
Abstract:
The Cook-Austral volcanic lineament extends from Macdonald Seamount (east)
to Aitutaki Island (west) in the South Pacific Ocean and consists of
hotspot-related volcanic islands, seamounts, and atolls. The Cook-Austral
volcanic lineament has been characterized as multiple overlapping,
age-progressive hotspot tracks generated by at least two mantle plumes,
including the Arago and Macdonald plumes, which have fed volcano
construction for ~20 m.y. The Arago and Macdonald hotspot tracks are argued
to have been active for at least 70 m.y. and to extend northwest of the
Cook-Austral volcanic lineament into the Cretaceous-aged Tuvalu-Gilbert and
Tokelau Island chains, respectively. Large gaps in sampling exist along the
predicted hotspot tracks, complicating efforts seeking to show that the
Arago and Macdonald hotspots have been continuous, long-lived sources of
hotspot volcanism back into the Cretaceous. We present new major- and
trace-element concentrations and radiogenic isotopes for three seamounts
(Moki, Malulu, Dino) and one atoll (Rose), and new clinopyroxene 40Ar/39Ar ages for Rose (24.81 ± 1.02 Ma) and Moki
(44.53 ± 10.05 Ma). All volcanoes are located in the poorly sampled region
between the younger Cook-Austral and the older, Cretaceous portions of the
Arago and Macdonald hotspot tracks. Absolute plate motion modeling
indicates that the Rose and Moki volcanoes lie on or near the reconstructed
traces of the Arago and Macdonald hotspots, respectively, and the 40Ar/39Ar ages for Rose and Moki align with the
predicted age progression for the Arago (Rose) and Macdonald (Moki)
hotspots, thereby linking the younger Cook-Austral and older Cretaceous
portions of the long-lived (>70 m.y.) Arago and Macdonald hotspot
tracks.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48276.1/593943/Missing-links-for-the-long-lived-Macdonald-and
A detrital zircon test of large-scale terrane displacement along
the Arctic margin of North America
Timothy M. Gibson; Karol Faehnrich; James F. Busch; William C. McClelland;
Mark D. Schmitz ...
Abstract:
Detrital zircon U-Pb geochronology is one of the most common methods used
to constrain the provenance of ancient sedimentary systems. Yet, its
efficacy for precisely constraining paleogeographic reconstructions is
often complicated by geological, analytical, and statistical uncertainties.
To test the utility of this technique for reconstructing complex,
margin-parallel terrane displacements, we compiled new and previously
published U-Pb detrital zircon data (n = 7924; 70 samples) from
Neoproterozoic–Cambrian marine sandstone-bearing units across the Porcupine
shear zone of northern Yukon and Alaska, which separates the North Slope
subterrane of Arctic Alaska from northwestern Laurentia (Yukon block).
Contrasting tectonic models for the North Slope subterrane indicate it
originated either near its current position as an autochthonous
continuation of the Yukon block or from a position adjacent to the
northeastern Laurentian margin prior to >1000 km of Paleozoic–Mesozoic
translation. Our statistical results demonstrate that zircon U-Pb age
distributions from the North Slope subterrane are consistently distinct
from the Yukon block, thereby supporting a model of continent-scale
strike-slip displacement along the Arctic margin of North America. Further
examination of this dataset highlights important pitfalls associated with
common methodological approaches using small sample sizes and reveals
challenges in relying solely on detrital zircon age spectra for testing
models of terranes displaced along the same continental margin from which
they originated. Nevertheless, large-n detrital zircon datasets
interpreted within a robust geologic framework can be effective for
evaluating translation across complex tectonic boundaries.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48336.1/593944/A-detrital-zircon-test-of-large-scale-terrane
Quantitative reconstruction of pore-pressure history in sedimentary
basins using fluid escape pipes
Joe Cartwright; Chris Kirkham; Martino Foschi; Neil Hodgson; Karyna
Rodriguez ...
Abstract:
We present a novel method to reconstruct the pressure conditions
responsible for the formation of fluid escape pipes in sedimentary basins.
We analyzed the episodic venting of high-pressure fluids from the crests of
a large anticlinal structure that formed off the coast of Lebanon in the
past 1.7 m.y. In total, 21 fluid escape pipes formed at intervals of 50–100
k.y. and transected over 3 km of claystone and evaporite sealing units to
reach the seabed. From fracture criteria obtained from nearby drilling, we
calculated that overpressures in excess of 30 MPa were required for their
formation, with pressure recharge of up to 2 MPa occurring after each
pipe-forming event, resulting in a sawtooth pressure-time evolution. This
pressure-time evolution is most easily explained by tectonic overpressuring
due to active folding of the main source aquifer while in a confined
geometry.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48406.1/593946/Quantitative-reconstruction-of-pore-pressure
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