New Geology Articles Published Online Ahead of Print in
February
Boulder, Colo., USA: Several new articles were published online ahead of
print for Geology in February. Topics include stress in survivor
plants following the collapse of land ecosystems, the Gulf of Aden, whether
the Denali fault is still active, the first reported Burgess Shale–type
fauna rediscovered, and redefining the age of the lower Colorado River.
These Geology articles are online at
http://geology.geoscienceworld.org/content/early/recent
.
Discovery of a large subsoil nitrate reservoir in an arroyo floodplain
and associated aquifer contamination
Benjamin S. Linhoff; John J. Lunzer
Abstract:
In an area of elevated nitrate (NO3) groundwater concentrations
in the northern Chihuahuan Desert in central New Mexico (United States), a
large reservoir of nitrate was found in the subsoil of an arroyo
floodplain. Nitrate inventories in the floodplain subsoils ranged from
10,000 to 38,000 kg NO3-N/ha—over twice as high as any
previously measured arid region. The floodplain subsoil NO3
reservoir was over 100 times higher than the adjacent desert (59–95 kg NO 3-N/ha). Chloride mass balance calculations of subsoils indicate
arroyo floodplain subsoils have undergone negative recharge since 2600–8600
yr ago, while the surrounding desert has had negative recharge since
13,000–17,000 yr ago. Compared to the adjacent desert, plant communities
are larger and more abundant in the floodplain, though subsoil NO 3 is apparently not utilized. We demonstrate that NO3
accumulates in the subsoil of the floodplain through evaporation of monsoon
season precipitation funneled into the arroyo. Through a one-dimensional
vadose zone model, we show that the NO3 inventories in the
arroyo floodplain could be acquired 8 to 75 times faster than through
atmospheric deposition through the lateral movement.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G47916.1/594886/Discovery-of-a-large-subsoil-nitrate-reservoir-in
Homeostatic response of Aptian gymnosperms to changes in atmospheric CO 2 concentrations
Germán Mora; Ana M. Carmo; William Elliott
Abstract:
The sensitivity of plant carbon isotope fractionation (13Δleaf) to changes in atmospheric CO2 concentrations ( Ca) is the subject of heavy debate, with some studies
finding no sensitivity, while others show a strong dependency. We tested
the hypothesis of photosynthetic homeostasis by using δ13C of
n-alkanes, cuticles, and bulk organic matter of gymnosperm-rich rocks
(Arundel Clay) from two sites deposited during the Aptian, a time that
experienced significant Ca variations. Our results show
no effect of Ca on 13Δleaf, and a
relatively constant Ci/Ca (0.64 ±
0.04, 1σ; i—intercellular space), a value that is similar to that of modern
gymnosperms. These results suggest that Aptian gymnosperms used homeostatic
adjustments with rising Ca, probably involving
increased carbon assimilation and/or stomatal closure, a response also
found in modern gymnosperms. The similarity between Aptian and modern
gymnosperms suggests that the processes responsible for regulating CO 2 and water vapor exchange during photosynthesis have remained
unaltered in gymnosperms for the past 128 m.y.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48037.1/594887/Homeostatic-response-of-Aptian-gymnosperms-to
The Parker Quarry Lagerstätte of Vermont—The first reported Burgess Shale–type fauna rediscovered
Giovanni Pari; Derek E.G. Briggs; Robert R. Gaines
Abstract:
Soft-bodied fossils of Cambrian age, now known as Burgess Shale–type
biotas, were first described from the Parker Slate of the northwest Vermont
(USA) slate belt in the late 19th century, 25 years before the
discovery of the Burgess Shale in British Columbia, Canada. Here, we report
the rediscovery of fossiliferous horizons at Parker’s Cobble, the site of
the original quarry, which was thought to have been exhausted by
excavation. New discoveries include a radiodont, multiple specimens of a
new bivalved arthropod, a priapulid, and other undescribed forms. Pervasive
soft-sediment deformation suggests accumulation near the toe of a steep
unstable slope, similar to the setting of the Burgess Shale. Although
fossils are rare, the exceptional preservation of some soft-bodied taxa
suggests that recovered diversity was limited by transport into an
inhospitable benthic setting rather than by decay, and this implies a
potential for future discoveries of new taxa.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48422.1/594888/The-Parker-Quarry-Lagerstatte-of-Vermont-The-first
Redefining the age of the lower Colorado River, southwestern United
States
R.S. Crow; J. Schwing; K.E. Karlstrom; M. Heizler; P.A. Pearthree ...
Abstract:
Sanidine dating and magnetostratigraphy constrain the timing of integration
of the lower Colorado River (southwestern United States and northern
Mexico) with the evolving Gulf of California. The Colorado River arrived at
Cottonwood Valley (Nevada and Arizona) after 5.24 Ma (during or after the
Thvera subchron). The river reached the proto–Gulf of California once
between 4.80 and 4.63 Ma (during the C3n.2r subchron), not at 5.3 Ma and 5.0 Ma as previously proposed. Duplication of section across
newly identified strands of the Earthquake Valley fault zone (California)
probably explains the discrepancy. The data also imply the start of focused
plate motion and basin development in the Salton Trough (California) at
6–6.5 Ma and relative tectonic stability of the southernmost part of the
lower Colorado River corridor after its integration. After integration, the
Colorado River quickly incised through sediment-filled basins and divides
between them as it also likely excavated Grand Canyon (Arizona). The
liberated sediment from throughout the system led to deposition of hundreds
of meters of Bullhead Alluvium downstream of Grand Canyon after 4.6 Ma as
the river adjusted to its lower base level.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48080.1/594889/Redefining-the-age-of-the-lower-Colorado-River
Subduction erosion and crustal material recycling indicated by adakites
in central Tibet
Zong-Yong Yang; Qiang Wang; Lu-Lu Hao; Derek A. Wyman; Lin Ma ...
Abstract:
Subduction erosion is important for crustal material recycling and is
widespread in modern active convergent margins. However, such a process is
rarely identified in fossil convergent systems, which casts doubt on the
importance of subduction erosion through the geological record. We report
on ca. 155 Ma Kangqiong (pluton) intrusive rocks of a Mesozoic magmatic arc
in the southern Qiangtang terrane, central Tibet. These rocks mainly
consist of trondhjemites and tonalites and are similar to slab-derived
adakites with mantle-like zircon oxygen isotope compositions (δ 18O = 5.2‰–5.6‰), they display more evolved Sr-Nd isotopes and
higher Th/La relative to mid-oceanic ridge basalts from the Bangong-Nujiang
suture, and they contain abundant amphibole and biotite. These
characteristics indicate magma generation via H2O-fluxed melting
of eroded forearc crust debris with subducted oceanic crust at 1.5–2.5 GPa
and 700–800 °C. In addition, the intrusions are exposed <20 km north of
the Bangong-Nujiang suture. Given the formation of adakites, narrow
arc-suture distance, migration of the Jurassic frontal arc toward the
continent interior, and other independent geological archives, we suggest
that the hydrated forearc crust materials were removed from the overlying
plate and carried into the mantle by subduction erosion. Our study provides
the first direct magmatic evidence for a subduction erosion process in
pre-Cenozoic convergent systems, which confirms an important role for such
processes in subduction-zone material recycling.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48486.1/594890/Subduction-erosion-and-crustal-material-recycling
Abrasion regimes in fluvial bedrock incision
Alexander R. Beer; Michael P. Lamb
Abstract:
River incision into bedrock drives landscape evolution and couples surface
changes to climate and tectonics in uplands. Mechanistic bedrock erosion
modeling has focused on plucking—the hydraulic removal of large loosened
rock fragments—and on abrasion—the slower fracturing-driven removal of rock
due to impacts of transported sediment—which produces sand- or silt-sized
fragments at the mineral grain scale (i.e., wear). An abrasion subregime
(macro-abrasion) has been hypothesized to exist under high impact energies
typical of cobble or boulder transport in mountain rivers, in which larger
bedrock fragments can be generated. We conducted dry impact abrasion
experiments across a wide range of impact energies and found that
gravel-sized fragments were generated when the impact energy divided by
squared impactor diameter exceeded 1 kJ/m2. However, the total
abraded volume followed the same kinetic-energy scaling regardless of
fragment size, holding over 13 orders of magnitude in impact energy and
supporting a general abrasion law. Application to natural bedrock rivers
shows that many of them likely can generate large fragments, especially in
steep mountain streams and during large floods, transporting boulders in
excess of 0.6 m diameter. In this regime, even single impacts can cause
changes in riverbed topography that may drive morphodynamic feedbacks.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48466.1/594891/Abrasion-regimes-in-fluvial-bedrock-incision
External signal preservation in halokinetic stratigraphy: A discrete
element modeling approach
Zoë A. Cumberpatch; Emma Finch; Ian A. Kane
Abstract:
Subsurface salt movement in the absence of external tectonic forces can
affect contemporaneous sediment deposition, mask allocyclic signals, and
deform older strata. We used a discrete element model (DEM) to better
understand salt-related modification of a sedimentary sequence with an
increasing sedimentation rate. This permitted quantification of thinning
rates and analysis of the lateral extent of synkinematic layers. Results
show realistic evolution of salt-related faults, defining two
salt-withdrawal basins, beyond which strata are undeformed. Thinning of
stratigraphy is four times greater between the salt flank and crest than
between the undeformed zone and flank, confirming an intense zone of
halokinetic modulation adjacent to the diapir. Early, slowly aggrading
layers are isolated within the salt-withdrawal basin and strongly
influenced by salt growth, whereas later, quickly aggrading layers are more
laterally extensive, matching inferences made from subsurface and outcrop
data. Halokinetic modulation reduces up the stratigraphic section,
mirroring observations around the Pierce diapirs, in the North Sea,
offshore UK. Our DEM provides quantitative insights into the dynamic
interplay between halokinetic and allocyclic controls on salt-stratigraphic
relationships.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48448.1/594892/External-signal-preservation-in-halokinetic
Extreme Quaternary plate boundary exhumation and strike slip localized
along the southern Fairweather fault, Alaska, USA
Richard O. Lease; Peter J. Haeussler; Robert C. Witter; Daniel F. Stockli;
Adrian M. Bender ...
Abstract:
The Fairweather fault (southeastern Alaska, USA) is Earth’s
fastest-slipping intracontinental strike-slip fault, but its long-term role
in localizing Yakutat–(Pacific–)North America plate motion is poorly
constrained. This plate boundary fault transitions northward from pure
strike slip to transpression where it comes onshore and undergoes a
<25°, 30-km-long restraining double bend. To the east, apatite (U-Th)/He
(AHe) ages indicate that North America exhumation rates increase stepwise
from ~0.7 to 1.7 km/m.y. across the bend. In contrast, to the west, AHe
age-depth data indicate that extremely rapid 5–10 km/m.y. Yakutat
exhumation rates are localized within the bend. Further northwest, Yakutat
AHe and zircon (U-Th)/He (ZHe) ages gradually increase from 0.3 to 2.6 Ma
over 150 km and depict an interval of extremely rapid >6–8 km/m.y.
exhumation rates that increases in age away from the bend. We interpret
this migration of rapid, transient exhumation to reflect prolonged
advection of the Cenozoic–Cretaceous sedimentary cover of the eastern
Yakutat microplate through a stationary restraining bend along the edge of
the North America plate. Yakutat cooling ages imply a long-term strike-slip
rate (54 ± 6 km/m.y.) that mimics the millennial (53 ± 5 m/k.y.) and
decadal (46 mm/yr) rates. Fairweather fault slip can account for all
Pacific–North America relative plate motion throughout Quaternary time and
indicates stability of highly localized plate boundary strike slip on a
single fault where extreme rock uplift rates are persistently localized
within a restraining bend.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48464.1/594893/Extreme-Quaternary-plate-boundary-exhumation-and
Phanerozoic variation in dolomite abundance linked to oceanic anoxia
Mingtao Li; Paul B. Wignall; Xu Dai; Mingyi Hu; Haijun Song
Abstract:
The abundance of dolomitic strata in the geological record contrasts with
the general rarity of locations where dolomite forms today, a discrepancy
that has long posed a problem for their interpretation. Recent culture
experiments show that dolomite can precipitate at room temperature, raising
the possibility that many ancient dolomites may be of syngenetic origin. We
compiled a large geodata set of secular variations in dolomite abundance in
the Phanerozoic, coupled with compilations of genus richness of marine
benthic invertebrates and sulfur-isotope variations in marine carbonates.
These data show that dolomite abundance is negatively correlated to genus
diversity, with four dolomite peaks occurring during mass extinctions.
Dolomite peaks also correspond to the rapid increase in sulfur-isotope
composition (δ34S), an indicator of enhanced sulfate reduction,
in anoxic oceans. These results confirm that variations in dolomite
abundance during the Phanerozoic were closely linked with changes in marine
benthic diversity, with both in turn related to oceanic redox conditions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48502.1/594894/Phanerozoic-variation-in-dolomite-abundance-linked
Metal-induced stress in survivor plants following the end-Permian
collapse of land ecosystems
Daoliang Chu; Jacopo Dal Corso; Wenchao Shu; Song Haijun; Paul B., Wignall
...
Abstract:
Teratological spores and pollen are widespread in sediments that record the
Permian- Triassic mass extinction. The malformations are thought to be the
result of extreme environmental conditions at that time, but the mutagenic
agents and the precise timing of the events remain unclear. We examined the
abundance of teratological sporomorphs and metal concentrations in a
Permian-Triassic tropical peatland succession of southwestern China. We
find a significant peak of spore tetrads of lycopsid plants (as much as 19%
of all sporomorphs) coeval with increases in Cu and Hg concentrations above
the main terrestrial extinction interval, which marks the loss of Permian Gigantopteris forests, increased wildfire activity, and the
disappearance of coal beds. Thus, in tropical peatlands, mutagenesis
affected only surviving plants. Mutagenesis was likely caused by metal
toxicity, linked to increased Hg and Cu loading, but was not itself a
direct cause of the terrestrial crisis.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48428.1/594557/Biomineralization-of-the-Cambrian-chancelloriids
Inherited lithospheric structures control arc-continent collisional
heterogeneity
M.S. Miller; P. Zhang; M.P. Dahlquist; A.J. West; T.W. Becker ...
Abstract:
From west to east along the Sunda-Banda arc, convergence of the
Indo-Australian plate transitions from subduction of oceanic lithosphere to
arc-continent collision. This region of eastern Indonesia and Timor-Leste
provides an opportunity for unraveling the processes that occur during
collision between a continent and a volcanic arc, and it can be viewed as
the temporal transition of this process along strike. We collected a range
of complementary geological and geophysical data to place constraints on
the geometry and history of arc-continent collision. Utilizing ~4 yr of new
broadband seismic data, we imaged the structure of the crust through the
uppermost mantle. Ambient noise tomography shows velocity anomalies along
strike and across the arc that are attributed to the inherited structure of
the incoming and colliding Australian plate. The pattern of anomalies at
depth resembles the system of salients and embayments that is present
offshore western Australia, which formed during rifting of east Gondwana.
Previously identified changes in geochemistry of volcanics from Pb isotope
anomalies from the inner arc islands correlate with newly identified
velocity structures representing the underthrusted and subducted
Indo-Australian plate. Reconstruction of uplift from river profiles from
the outer arc islands suggests rapid uplift at the ends of the islands of
Timor and western Sumba, which coincide with the edges of the
volcanic-margin protrusions as inferred from the tomography. These findings
suggest that the tectonic evolution of this region is defined by inherited
structure of the Gondwana rifted continental margin of the incoming plate.
Therefore, the initial template of plate structure controls orogenesis.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48246.1/594556/Inherited-lithospheric-structures-control-arc
Biomineralization of the Cambrian chancelloriids
Hao Yun; Xingliang Zhang; Glenn A. Brock; Luoyang Li; Guoxiang Li
Abstract:
As extinct animals that flourished during the Cambrian explosion,
chancelloriids have a unique body plan lacking guts but with a flexible
integument and a suite of star-shaped, hollow sclerites. Due to this body
plan, along with the paucity of knowledge on sclerite biomineralization,
the phylogenetic position of chancelloriids within the Metazoa is still
controversial. Integration of analyses of diverse fossils from Cambrian
stage 2 to the Wuliuan Stage of China and Australia indicates that
chancelloriid sclerites possess an encasement-like organic layer and a
fibrous aragonitic layer. The organic layer is inferred to be a specialized
trait derived from the epidermal integument of the animal body. The
sclerites were likely biomineralized by using the outer organic layer as a
template to absorb cations and precipitate crystal nuclei, reflecting a
strategy adopted by a range of eumetazoans with a developed epidermis.
Therefore, the hypothesis that chancelloriids represent an
epitheliozoan-grade animal and an early explorer of template-based
biomineralization is supported.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48428.1/594557/Biomineralization-of-the-Cambrian-chancelloriids
High-precision U-Pb age constraints on the Permian floral turnovers,
paleoclimate change, and tectonics of the North China block
Qiong Wu; Jahandar Ramezani; Hua Zhang; Jun Wang; Fangui Zeng ...
Abstract:
The Permian marine-terrestrial system of the North China block provides an
exceptional window into the evolution of northern temperate ecosystems
during the critical transition from icehouse to greenhouse following the
late Paleozoic ice age (LPIA). Despite many studies on its rich hydrocarbon
reserves and climate-sensitive fossil flora, uncertain temporal constraints
and correlations have hampered a thorough understanding of the records of
geologic, biologic, and climatic change from the North China block. We
present a new chronostratigraphy based on high-precision U-Pb chemical
abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS)
geochronology of tuffs from a near-complete latest Carboniferous–Permian
succession in North China. The results indicate that the predominance of
continental red beds, climate aridification, and the disappearance of coals
and characteristic tropical flora were well under way during the Cisuralian
(Early Permian) in the North China block, significantly earlier than
previously thought. A nearly 20 m.y. hiatus spanning the early Kungurian to
the mid-Guadalupian (or later) is revealed in the northern North China
block to have close temporal and spatial associations with the closure
and/or subduction of the Paleo-Asian Ocean and its related tectonic
convergence. This long hiatus was concomitant with the prominent loss of
the highly diverse and abundant Cathaysian floras and the widespread
invasion of the monotonous Angaran floras under arid climate conditions in
the North China block. Similarities in the floral and climate shift
histories between Euramerica and North China suggest that aside from the
regional tectonic controls and continental movement, extensive volcanism
during the Cisuralian may have played a major role in the global warming
and aridification in the aftermath of the LPIA.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48051.1/594558/High-precision-U-Pb-age-constraints-on-the-Permian
Apatite nanoparticles in 3.46–2.46 Ga iron formations: Evidence for
phosphorus-rich hydrothermal plumes on early Earth
Birger Rasmussen; Janet R. Muhling; Alexandra Suvorova; Woodward W. Fischer
Abstract:
Phosphorus is an essential nutrient that is thought to have regulated
primary productivity in global oceans after the advent of oxygenic
photosynthesis. The prime source of seawater phosphorus is regarded to be
continental weathering of phosphate minerals. Ancient seawater phosphorus
concentrations have been constrained using the phosphorus content of
iron-rich chemical sediments—banded iron formations (BIFs); however, the
removal processes and depositional phases remain unclear. Here we report
that nanometer-sized apatite crystals (<500 nm) are ubiquitous in
3.46–2.46 Ga BIFs and cherts from the Kaapvaal (South Africa) and Yilgarn
and Pilbara (Western Australia) cratons. The apatite is uniformly dispersed
in a chemical sediment comprising greenalite nanoparticles, which were
encased in very early diagenetic silica cement that limited compaction and
chemical reactions. The lack of organic carbon (below detection; <0.3
wt%) and absence of primary iron oxides implies that the phosphorus was not
derived from the degradation of organic matter or seawater scavenging by
oxide particles. Instead, the occurrence of apatite in sediments derived
from hydrothermally sourced Fe2+ and SiO2(aq)
suggests that phosphorus too was derived from vent plumes. Today, seawater
P is rapidly removed from vent fluids due to scavenging by oxidized Fe 2+. However, prior to the Great Oxidation Event (2.45–2.32 Ga),
dissolved phosphorus released during anoxic alteration of seafloor basalts
escaped the iron-oxidation trap. Our results point to the existence of a
submarine hydrothermal flux of dissolved phosphorus that supplied nutrients
to the early anoxic oceans. High amounts of seawater P may help to explain
why phosphorus is ubiquitous in cell biology—it was not limiting during the
origin and early evolution of life.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48374.1/594559/Apatite-nanoparticles-in-3-46-2-46-Ga-iron
Recalibrating Rodinian rifting in the northwestern United States
Daniel T. Brennan; Zheng-Xiang Li; Kai Rankenburg; Noreen Evans; Paul K.
Link ...
Abstract:
A lack of precise age constraints for Neoproterozoic strata in the
northwestern United States (Washington State), including the Buffalo Hump
Formation (BHF), has resulted in conflicting interpretations of Rodinia
amalgamation and breakup processes. Previous detrital zircon (DZ) studies
identified a youngest ca. 1.1 Ga DZ age population in the BHF, interpreted
to reflect mostly first-cycle sourcing of unidentified but proximal
magmatic rocks intruded during the amalgamation of Rodinia at ca. 1.0 Ga.
Alternatively, the ca. 1.1 Ga DZ population has been suggested to represent
a distal source with deposition occurring during the early phases of
Rodinia rifting, more than 250 m.y. after zircon crystallization. We
combined conventional laser-ablation split-stream analyses of U-Pb/Lu-Hf
isotopes in zircon with a method of rapid (8 s per spot) U-Pb analysis to
evaluate these opposing models. Our study of ~2000 DZ grains from the BHF
identified for the first time a minor (~1%) yet significant ca. 760 Ma
population, which constrains the maximum depositional age. This new
geochronology implies that the BHF records early rift deposition during the
breakup of Rodinia and correlates with sedimentary rocks found in other
late Tonian basins of southwestern Laurentia.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48435.1/594560/Recalibrating-Rodinian-rifting-in-the-northwestern
Gulf of Aden spreading does not conform to triple-junction formation
Khalid A. Almalki; Peter G. Betts
Abstract:
The Gulf of Aden represents an evolving example of a juvenile ocean system
and is considered the most evolved rift arm of the Afar triple junction. We
have undertaken analysis of recent coupled satellite and marine
potential-field data to understand the first-order crustal architecture
along the entire length of the gulf. Our interpretation suggests the Gulf
of Aden has three domains with distinct free-air gravity and magnetic
characteristics. These domains record a progression from active seafloor
spreading in the eastern domain, through isolated and discontinuous
spreading segments in the central domain, to active continental rifting in
the western domain immediately adjacent to the Afar triple junction.
Forward models suggest the presence of transitional crust, which displays
linear magnetic stripe–like anomalies that bound oceanic stripes in the
central domain and covering the majority of the western domain. Magnetic
anomalies differ from magnetic stripes sensu stricto because they
are discontinuous and cannot be correlated along the length of the gulf.
Detection of northwest-southeast extension in the central domain based on
magnetic stripe orientation is inconsistent with the regional
northeast-southwest extension. Our observations reflect heterogeneous
opening of the Gulf of Aden basins, in which spreading is migrating toward
Afar as a series of isolated spreading segments, rather than initiating at
the junction as proposed by classical platetectonic theory. This mechanism
of ocean initiation is inconsistent with transtensional models that involve
wholesale tearing of continental crust and contradicts conceptual models
that rely on the Afar plume in initiating or driving the extension.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48529.1/594561/Gulf-of-Aden-spreading-does-not-conform-to-triple
Is the Eastern Denali fault still active?
Minhee Choi; David W. Eaton; Eva Enkelmann
Abstract:
The Denali fault, a transcurrent fault system that extends from
northwestern Canada across Alaska toward the Bering Sea, is partitioned
into segments that exhibit variable levels of historical seismicity. A pair
of earthquakes (M 6.2 and 6.3) on 1 May 2017, in proximity to the Eastern
Denali fault (EDF), exhibited source mechanisms and stress conditions
inconsistent with expectations for strike-slip fault activation. Precise
relocation of ~1500 aftershocks revealed distinct fault strands that are
oblique to the EDF. Calculated patterns of Coulomb stress show that the
first earthquake likely triggered the second one. The EDF parallels the
Fairweather transform, which separates the obliquely colliding Yakutat
microplate from North America. In our model, inboard transfer of stress is
deforming and shortening the mountainous region between the EDF and the
Fairweather transform. This is supported by historical seismicity
concentrated southwest of the EDF, suggesting that it now represents a
structural boundary that controls regional deformation but is no longer an
active fault.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48461.1/594562/Is-the-Eastern-Denali-fault-still-active
Wide-blocky veins explained by dependency of crystal growth rate on
fracture surface type: Insights from phase-field modeling
Liene Spruženiece; Michael Späth; Janos L. Urai; Estibalitz Ukar; Michael
Selzer ...
Abstract:
Vein microstructures contain a wealth of information on coupled chemical
and mechanical processes of fracturing, fluid transport, and crystal
growth. Numerical simulations have been used for exploring the factors
controlling the development of vein microstructures; however, they have not
been quantitatively validated against natural veins. Here we combined
phase-field modeling with microtextural analysis of previously unexplained
wide-blocky calcite veins in natural limestone and of the fresh fracture
surface in this limestone. Results show that the wide-blocky vein textures
can only be reproduced if ~10%–20% of crystals grow faster than the rest.
This fraction corresponds to the amount of transgranularly broken grains
that were observed on the experimental fracture surfaces, which are
dominantly intergranular. We hypothesize that transgranular fractures allow
faster growth of vein minerals due to the lack of clay coatings and other
nucleation discontinuities that are common along intergranular cracks. Our
simulation results show remarkable similarity to the natural veins and
reproduce the nonlinear relationship between vein crystal width and vein
aperture. This allows accurate simulations of crystal growth processes and
related permeability evolution in fractured rocks.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48472.1/594563/Wide-blocky-veins-explained-by-dependency-of
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