New GSA Bulletin Articles Published Ahead of Print in July
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics studied this
month include the nature and dynamics of China and Tibet; the Rae and Slave
cratons; Vega Island, Antarctica; and continental geological evidence for
Solar System chaotic behavior. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Rates and processes controlling periglacial alluvial fan formation:
Implications for martian fans
Marisa C. Palucis; A.M. Morgan; J.V. Strauss; F. Rivera-Hernandez; J.A.
Marshall ...
Abstract:
Alluvial fans are found across a range of climates and are built from a
combination of fluvial and debris flow processes. Correct identification of
process is critical to reconstructing the climate and water histories of
alluvial fans on Earth and Mars. Theory and data from subaerial Earth fans
are often used to estimate paleoflow discharges and sediment fluxes for
martian fans; however, most terrestrial work has been conducted on fans
that are in hot, dry climates with runoff sourced from rainfall. This
differs from the prevailing interpretation that martian fans were sourced
from snowmelt under warming periglacial conditions. To characterize
processes and rates of periglacial fan formation, we conducted a
field-based study of the Black Mountain alluvial fan in the Aklavik Range,
Canada. We observed active fluvial bedload transport as well as several
small debris flows that had initiated from ice-filled gullies. Following a
runoff event of ∼0.005 mm/hr to ∼0.2 mm/hr across the fan, we estimated
sediment fluxes of ∼0.04 m3/hr. Under bankfull conditions, we
estimated runoff rates between ∼0.01 mm/hr to ∼14 mm/hr and corresponding
sediment fluxes of ∼0.3 m3/hr to ∼550 m3/hr. This
suggests that moderate flow events, well below the maximum runoff
production rates suggested for Mars, are capable of entraining and
transporting appreciable amounts of sediment by fluvial processes. However,
sedimentological and geomorphological observations suggest that ∼67% of the
fan was deposited fluvially; the remainder was deposited by mass flows. Our
results emphasize the need to take care in interpreting martian sedimentary
processes and climate from fan surface morphology alone.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36459.1/615885/Rates-and-processes-controlling-periglacial
A comprehensive genetic model for the world’s largest Sb deposit
(Xikuangshan, China)
Shanling Fu; Ruizhong Hu; Jiantang Peng; Liyan Wu; Dongsheng Ma
Abstract:
The Mesozoic (160−130 Ma), fault-controlled Xikuangshan Sb deposit within
Devonian limestone strata of Hunan Province, Southern China is the world’s
largest Sb deposit containing a proven reserve of ∼2.5 m Sb. Although mined
for over a century, its genesis remains poorly understood. Here we use new
He-Ar isotope data of hydrothermal stibnite and both new and existing
C-O-Sr-Nd isotopes of hydrothermal calcite with known stages to decipher
its genesis and the major constraints on mineralization intensity. The 3He/4He and 40Ar/36Ar ratios of
fluid inclusions trapped in stibnite are from 0.01 to 0.04 Ra (Ra:
atmospheric 3He/4He ratio) and 304−1077,
respectively, indicating the ore-forming fluids at Xikuangshan were
dominated by air-saturated meteoric groundwater after interaction with
crustal rocks. Ore-stage calcite C and O isotopes indicated that most CO 2 in the fluids was acquired from marine carbonate rocks by
dissolution; whereas Sr and Nd isotopes differed from deposited Devonian
country rocks but were similar to the underlying regional Proterozoic
clastic rocks in the region. Calcite from early and late stages showed a
strong positive correlation between δ18O and 87Sr/ 86Sr, consistent with the mixing between the circulating
groundwater and compounds released from the Proterozoic rocks due to
extensive fluid-rock interaction. The 3He/Q ratios of the fluid
inclusions are low, varying from 4.3 to 18.5 × 10−15 cm 3 standard temperature and pressure (STP) J−1,
indicating deep-seated magma could have provided heat by conduction but no
volatiles into the ore-forming fluids. Based on these new results, we
suggest that deep-seated granitic magma heated the down-going meteoric
groundwater along fault zones, after which the groundwater extensively
interacted with and extracted Sb from the Proterozoic Sb-rich rocks to form
Sb-rich fluids. The Sb-rich fluids then ascended through regional faults
and deposited Sb as stibnites at favorable structural traps to form the
Xikuangshan Sb deposit. This study highlights that extensive water-rock
interaction is essential to form the deposit, and more intensive water-rock
interaction at an early stage allowed for early-stage mineralization
yielding higher Sb reserves (>80%) at Xikuangshan.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36424.1/615819/A-comprehensive-genetic-model-for-the-world-s
The transition from continental to lithospheric breakup recorded in
proto-oceanic crust: Insights from the NW South China Sea
Peng Chao; Gianreto Manatschal; Cuimei Zhang; Pauline Chenin; Jianye Ren
...
Abstract:
The formation of a new plate boundary and creation of the first oceanic
crust, two of the most important processes of plate tectonics, still
remains little understood. While older studies used to assumed a sharp
ocean-continent boundary between continent and ocean, recent studies
suggest a progressive ocean-continent transition (OCT) between unequivocal
continental and oceanic crusts. In the latter view, breakup is not
instantaneous but a lasting phase, which raises questions about the nature
of the OCT basement and the processes operating between continental and
lithospheric breakup. Based on detailed observations of high-quality and
yet unpublished reflection seismic data, we describe and interpret the
characteristic structures of the NW-South China Sea OCT and their
relationship with overlying syn-breakup phase sediments. We show that the
OCT displays a transition from fault-dominated rifting to magma-dominated
seafloor spreading. On its continent-ward side, the OCT is made of hybrid
crust where tectonic thinning of continental crust is compensated by
syn-extensional magmatic thickening. Oceanward, the hybrid crust evolves
into a fully magmatic but fault-dominated proto-oceanic crust, and finally
turns into a mature Penrose-type oceanic crust. Relying on the growth
structures observed in the syn-breakup sedimentary sequences and magmatic
additions, we propose a kinematic restoration of the breakup phase. We
suggest out-of-sequence flip-flop faulting to explain the switch from
asymmetrical, fault-dominated-extension, to fully magmatic and largely
symmetrical syn-extension accretion recorded in the syn-breakup sedimentary
sequences overlying the OCT.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36371.1/615563/The-transition-from-continental-to-lithospheric
Middle Jurassic intracontinental evolution of East Asia: Insights from
the Tianshifu-Dongyingfang basin of the Liaodong Peninsula, NE China
Zhiheng Ren; Shuwen Dong; Yueqiao Zhang; Xuanhua Chen; Wei Shi ...
Abstract:
The Middle Jurassic intracontinental shortening event in East Asia, known
as episode A of the Yanshanian Movement in the eastern North China Craton
and the first phase of the Daebo Orogeny in the Korean Peninsula,
respectively, has been thoroughly studied during the past two decades.
However, the timing of its beginning, duration, and deformational processes
are still highly controversial because few studies have probed the
correlations between episode A of the Yanshanian Movement and the first
phase of the Daebo Orogeny. The Tianshifu-Dongyingfang basin, located in
the northern Liaodong Peninsula, is filled with complete Jurassic sequences
that can be compared with those in the eastern North China Craton to the
west of the Tan-Lu fault zone and the Korean Peninsula. Regional
stratigraphic correlations suggest that the Tianshifu-Dongyingfang basin
contains not only the syntectonic conglomerates, but also the lower and
upper unconformities A1 and A2 of episode A of the
Yanshanian Movement, which separated the syntectonic conglomerates from the
underlying Early−Middle Jurassic coal measures and the overlying Late
Jurassic red beds, respectively. Detailed field survey and structural
analysis show that the development of syntectonic conglomerates and
unconformities A1 and A2 were controlled by the
imbricate thrust fault system in the basement, which resulted from the
WNW−ESE compression inversed from the fault-slip data. The youngest
detrital zircon U-Pb ages obtained near unconformities A1 and A 2 show that the WNW−ESE compression of episode A of the
Yanshanian Movement in the Liaodong Peninsula initiated at ca. 171 Ma and
ended at least by 163 Ma. Our new structural and geochronological data,
combined with data from the neighboring areas, suggest that episode A of
the Yanshanian Movement and the first phase of the Daebo Orogeny belong to
a single intracontinental shortening event, the compressional deformation
of which was derived from the westward, low-angle subduction of the
Paleo-Pacific plate and transmitted to the Korean and Liaodong Peninsulas
at ca. 172−170 Ma and to the eastern North China Craton to the west of the
Tan-Lu fault zone at 170−165 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36373.1/615564/Middle-Jurassic-intracontinental-evolution-of-East
Incipient collision of the Rae and Slave cratons at ca. 1.95 Ga
Jamie Cutts; Brendan Dyck
Abstract:
Incipient collision between the Rae and Slave cratons represents a critical
initial stage in the final assembly of the composite Laurentian craton.
However, the timing and kinematics of this event remain poorly constrained.
Here, we integrate phase equilibria modeling and Lu-Hf garnet and U-Pb
monazite and zircon petrochronology to precisely date the timing of peak
metamorphism along the Great Slave Lake shear zone, the central segment of
the western Rae cratonic margin that was the locus of terminal Rae-Slave
collision. Zircon and monazite U-Pb ages broadly constrain prograde-to-peak
metamorphism to 1933−1913 Ma, whereas the timing of upper-amphibolite
facies peak metamorphism (0.8−1.0 GPa and 750−800 °C) is best constrained
by two garnet Lu-Hf ages of 1931 ± 12 and 1917 ± 6 Ma. Our results
highlight the inherent strengths and limitations of garnet, monazite, and
zircon petrochronology in the context of dating Paleoproterozoic-aged
metamorphism. Based on the timing of crustal thickening constrained by
Lu-Hf garnet geochronology, we posit that the Slave craton first collided
with the western Rae margin at ca. 1950 Ma. Taken together with prior
constraints on the timing of metamorphism elsewhere along the western Rae
margin, we propose incipient collision was followed by the northwards
zippering of the margins and the eventual ∼500 km of dextral translation of
the Slave craton along the Great Slave Lake shear zone.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36393.1/615565/Incipient-collision-of-the-Rae-and-Slave-cratons
New age constraints support a K/Pg boundary interval on Vega Island,
Antarctica: Implications for latest Cretaceous vertebrates and
paleoenvironments
Eric M. Roberts; Patrick M. O’Connor; Julia A. Clarke; Sarah P. Slotznick;
Christa J. Placzek ...
Abstract:
A second K/Pg boundary interval in the northern sector of the Antarctic
Peninsula on Vega Island has been proposed, yet current temporal resolution
of these strata prohibits direct testing of this hypothesis. To not only
test for the existence of a K/Pg boundary on Vega Island but also provide
increased age resolution for the associated vertebrate fauna (e.g., marine
reptiles, non-avian dinosaurs, and avian dinosaurs), the Vega Island
succession was intensively re-sampled. Stratigraphic investigation of the
Cape Lamb Member of the Snow Hill Island Formation, and in particular, the
overlying Sandwich Bluff Member of the López de Bertodano Formation, was
conducted using biostratigraphy, strontium isotope stratigraphy,
magnetostratigraphy, and detrital zircon geochronology. These data indicate
a Late Campanian−early Maastrichtian age for the Cape Lamb Member and
present three possible correlations to the global polarity time scale
(GPTS) for the overlying Sandwich Bluff Member. The most plausible
correlation, which is consistent with biostratigraphy, detrital zircon
geochronology, sequence stratigraphy, and all but one of the Sr-isotope
ages, correlates the base of the section to C31N and the top of the section
with C29N, which indicates that the K/Pg boundary passes through the top of
the unit. A second, less plausible option conflicts with the
biostratigraphy and depends on a series of poorly defined magnetic
reversals in the upper part of the stratigraphy that also correlates the
section between C31N and C29R and again indicates an inclusive K/Pg
boundary interval. The least likely correlation, which depends on favoring
only a single Sr-isotope age at the top of the section over
biostratigraphy, correlates the section between C31N and C30N and is
inconsistent with an included K/Pg boundary interval. Although our
preferred correlation is well supported, we failed to identify an
Ir-anomaly, spherules/impact ejecta, or other direct evidence typically
used to define the precise position of a K/Pg boundary on Vega Island. This
study does, however, confirm that Vegavis, from the base of the
Sandwich Bluff Member, is the oldest (69.2−68.4 Ma) phylogenetically placed
representative of the avian crown clade, and that marine vertebrates and
non-avian dinosaurs persisted in Antarctica up to the terminal Cretaceous.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36422.1/615298/New-age-constraints-support-a-K-Pg-boundary
Genetic mechanism of permeability anisotropy in conglomerate reservoir
and its controlling factors
Feng-Qi Tan; Xian-Kun Li; Chun-Miao Ma; Jian-Rong Lv; Wen-Tao Liu
Abstract:
The near-source, multistream, and rapidly changing sedimentary environments
of conglomerate reservoirs lead to reservoir characteristics such as strong
heterogeneity and complex modal pore structure, which create unique seepage
pathways and strong permeability anisotropy, which in turn affect the
displacement mode and oil recovery of crude oil in pores and throats. Thus,
it is necessary to clarify the genetic mechanism and controlling factors of
this permeability anisotropy, providing the basis and premise of
sustainable and efficient development of conglomerate reservoirs. In this
paper, a typical conglomerate reservoir from the Kexia Formation in the 7th
Block of the Karamay oilfield (Xinjiang, China) was selected as the
research object to solve the permeability anisotropy problem. According to
the sealed coring data, eight conglomerate samples were cut preliminarily
in the field and prepared finely in the laboratory to make standard cube
samples with side lengths of 4.5 cm, and then these samples were
investigated using casting thin section, computed tomography scanning, and
constant velocity mercury injection experimental methods to determine the
genetic mechanism of permeability anisotropy and its controlling factors at
macro- and microscales. The results showed that sedimentation and
hydrodynamic conditions controlled the distribution of permeability
anisotropy, the separation, rounding, and arrangement of mineral particles,
the development of sedimentary structural planes, and the different types
of packing textures, which affected the seepage paths and resistance in
different directions, resulting in great differences in permeability
anisotropy. The permeability in the direction parallel to water flow was
the largest, followed by the direction perpendicular to water flow, and the
permeability in the vertical sedimentation direction was the lowest because
of the combined influences of the tortuosity of seepage, the connectivity
of pore throats, and the sedimentary structural planes. In addition,
diagenesis in later stages can reduce the absolute value of permeability in
different directions and the strength of permeability anisotropy by
changing the micropore structure of the conglomerate reservoir, but it will
not change the distribution of permeability anisotropy. Based on these
observations, a generalized linear mixed model was used to analyze the
micropore structure parameters and standard deviation of permeability in
different directions from the eight conglomerate samples, and the results
showed that the relative sorting coefficient had the greatest influence on
the standard deviation of permeability, and the uniformity of pore-throat
size distribution was the main microscale controlling factor on the
strength of permeability anisotropy. When the pore-throat size distribution
was less uniform, the difference in the amplitude of permeability
anisotropy was more obvious. The average capillary radius, seepage
tortuosity, and maximum pore-throat radius showed less importance than
relative sorting coefficient, so they are considered secondary factors. The
other micropore structure parameters showed some influence on the
permeability anisotropy, but the degree of influence was limited.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36403.1/615134/Genetic-mechanism-of-permeability-anisotropy-in
Albian−Cenomanian granitoid magmatism in Eastern and Central Tibet as a
result of diachronous, continental collision induced slab tear
propagation
Xue Gao; Yildirim Dilek
Abstract:
A discrete belt of Albian−Cenomanian granitoid plutons occurs in the Lhasa
and Qiangtang terranes in the Central (CTP) and Eastern Tibetan Plateau
(ETP) and represents a major magmatic pulse in the plateau’s crustal
evolution during the Cretaceous. The geochemistry, petrogenesis, and
magmatic development of these granitoids are different from those of
magmatic arc granitoids along the southern edges of the Lhasa and Qiangtang
terranes, indicating different heat and melt sources and tectonic setting
of their formation. We present here new mineral, whole-rock and isotope
geochemistry, and zircon U-Pb age data from the Xiasai pluton in the ETP
and discuss its geochemical-petrological characteristics and magmatic
development in comparison to the other Cretaceous plutons in the ETP and
CTP, and within the tectonic framework of the Mesotethyan geology of Tibet.
Zircons from the Xiasai and other plutons in the ETP have yielded U-Pb ages
ranging from 106 Ma to 93 Ma in comparison to 115 Ma and 100.3 Ma zircons
from the Southern Qiangtang Terrane (SQT) and 113.4 Ma and 109 Ma zircons
from the Northern Lhasa Terrane (NLT) farther west. The Cretaceous
granitoids in the ETP and CTP range in composition from granite, K-feldspar
granite to monzogranite and biotite monzogranite, representing highly
fractionated I-type granites with relatively high SiO2 and K 2O contents, variable (Na2O + K2O)/CaO and
FeOT/MgO ratios, and (Zr + Nb + Ce + Y) abundances. They display
significant negative Eu anomalies (Eu/*Eu) = 0.04−0.12) and strong
depletions in Sr and Ba, and are strongly enriched in large ion lithophile
elements but depleted in high field strength elements. Their εHf (t) values correspond to wide ranging Hf isotope crustal model
ages (TDMC) of 0.3−1.9 Ga, and their Sr-Nd isotopic
signatures show elevated (87Sr/86Sr)i
ratios (0.7034−0.7105) and negative εNd(t) values of −8.8 to
−4.9. These high whole-rock (87Sr/86Sr)i
ratios and relatively high Th/Nb and Th/Yb ratios indicate incorporation of
melts derived from partial melting of subducted sediments into the melt
evolution of these granitoids that involved partial melting of the
subduction-metasomatized lithospheric mantle and the mafic- to
intermediate-composition continental crust. The extant zircon
crystallization ages of the granitoid intrusions in the CTP and ETP show
eastward younging of their emplacement from 115 Ma to 93 Ma, suggesting an
apparent eastward migration of the heat source through time. A diachronous
collision of the NLT with the SQT during 145−120 Ma and the subsequent slab
breakoff induced, eastward propagated slab tear and asthenospheric
upwelling produced the hybrid melts of the Albian−Cenomanian granitoids and
their emplacement in a discrete, narrow magmatic belt in the CTP and ETP.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36452.1/615027/Albian-Cenomanian-granitoid-magmatism-in-Eastern
A newly defined, long-lived Paleozoic intra-oceanic arc in the South
Tianshan (NW China): Implications for multiple accretionary tectonics
in the southern Altaids
Limin Gao; Wenjiao Xiao; Zhou Tan; Xinshui Wang; Yuhong Guo
Abstract:
The South Tianshan Orogenic Belt marks the final assembly of the southern
Altaids and the Karakum-Tarim Cratons. Integrated petrological,
geochemical, and geochronological studies were carried out on the Akeyazi
high-alumina basaltic lavas (HABs) and their host sandstones from the
southern margin of the Central Tianshan Block. Given their relative high
alumina contents (Al2O3 > 16 wt%) and geochemical
similarity to basalts from modern arcs (e.g., Aleutian, Kamchatka, Izu, and
Honshu arc), the Akeyazi basaltic lavas are classified as HABs. The Akeyazi
HABs are distinguished by strong enrichment in large ion lithophile
elements (LILE) and negative Nb-Ta anomalies. Furthermore, their
mid-oceanic ridge basalt (MORB)-like Sr-Nd-Hf (87Sr/86Sr(i) ∼0.705766, εNd(t) ∼+2.5, εHf (t) ∼+9.5), along with arc-like trace element patterns, indicate
a petrogenesis derived from a mantle wedge metasomatized by pelagic,
sediment-derived melts (2%−5% melts mixed, as confirmed by melt modeling).
Zircon U-Pb ages of Akeyazi HABs and associated arc-related gabbros suggest
that they were mainly erupted during ca. 415 Ma. Moreover, host sandstones
of Akeyazi HABs exhibit a unimodal distribution with a peak at 480−410 Ma
in the detrital zircon spectrum, which is notably distinct from those of
the Central Tianshan Block, South Tianshan Accretionary Complex, and North
Tarim Craton, each of which is multimodal. The maximum depositional age
(MDA) of the meta-sandstones is 351.0 ± 4.3 Ma, with the weighted average
of three oldest Paleozoic grains at 502 ± 26 Ma. Collectively, our data,
for the first time, discloses an intra-oceanic arc setting from ca. 502−351
Ma during the northward subduction of the South Tianshan Ocean, which
further reveals the history of multiple episodes of accretion in the
southern Altaids.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36355.1/615016/A-newly-defined-long-lived-Paleozoic-intra-oceanic
Asymmetric propagation mechanism of hydraulic fracture networks in
continental reservoirs
Jingshou Liu; Lianfu Mei; Wenlong Ding; Ke Xu; Haimeng Yang ...
Abstract:
Hydraulic fracturing technology is relatively mature in North America, but
under complex geological conditions, such as those in China, the
application of this technology still faces great challenges. At present,
techniques for the numerical simulation of hydraulic fracture networks are
mainly based on the prediction of the fracture half-height and half-length,
which cannot capture the heterogeneity of continental low-permeability
sandstone reservoirs in China and the distribution of the asymmetric
hydraulic fracture network present in them. Therefore, determining the
asymmetric propagation mechanism of hydraulic fracture networks is very
important for improving the recovery rates of continental reservoirs. In
this paper, taking the Ordos Basin in China as an example, the spatial
distribution of the stress field of a heterogeneous continental reservoir
is precisely predicted by reservoir mechanical heterogeneity modeling. By
using a microseismic monitoring method, the 3-D morphology of the hydraulic
fracture network is determined. Through the coupling of multisource data,
the frequency distributions of the determined in situ stress magnitudes in
different hydraulic fracturing stages are obtained. The propagation
direction of the hydraulic fracture network changes under the control of
the horizontal stress difference (Δσ) and the presence of natural
fractures. The smaller Δσ is, the greater the deflection of the hydraulic
fracture propagation direction. The asymmetric propagation of these
fractures is related to the frequency distribution of Δσ. As the frequency
of Δσ approaches a normal distribution, the two wings of the hydraulic
fracture network become basically equal in length, and as Δσ deviates more
from a normal distribution, the difference between the two wings of the
hydraulic fracture network increases. These research results will provide
new insight for modeling, exploring, and developing continental reservoirs.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36358.1/615017/Asymmetric-propagation-mechanism-of-hydraulic
Continental geological evidence for Solar System chaotic behavior
in the Late Cretaceous
Huaichun Wu; Linda A. Hinnov; Shihong Zhang; Ganqing Jiang; Tianshui Yang
...
Abstract:
The Earth’s geologic record of Milankovitch cycles closely tracks Solar
System solutions for the past 50 million years. Prior to 50 million years
ago (Ma), however, the solutions lose accuracy rapidly due to chaotic
behavior of the Solar System. Here we reconstruct a 10.173 million
year-long record from 82.358 Ma to 92.531 Ma of Earth’s orbital parameters
from a continental lacustrine sequence in the Songliao Basin, China
constrained by four in situ high-resolution radioisotopic U-Pb ages and
magnetic reversal stratigraphy. Analysis of thorium and ostracode shell
abundance records from the Songliao Basin reveal evidence for two chaotic
secular resonance transitions in the orbital motions of Earth and Mars from
85.2 Ma to 91.55 Ma. The evidence validates similar observations in western
North American marine stratigraphy. A unique phasing between the observed
orbital eccentricity and obliquity modulations may explain the anoxic
events that occurred in both marine and continental environments during
this time. Taken together, the continental and marine stratigraphic
evidence demonstrates a strong global reach of Late Cretaceous Milankovitch
cycles, and provides an important constraint on Solar System chaoticity and
the calculation of accurate orbital solutions prior to 50 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36340.1/615018/Continental-geological-evidence-for-Solar-System
Tectonic evolution of the Beishan orogen in central Asia: Subduction,
accretion, and continent-continent collision during the closure of the
Paleo-Asian Ocean
Jie Li; Chen Wu; Xuanhua Chen; Andrew V. Zuza; Peter J. Haproff ...
Abstract:
The Beishan orogen is part of the Neoproterozoic to early Mesozoic Central
Asian Orogenic System in central Asia that exposes ophiolitic complexes,
passive-margin strata, arc assemblages, and Precambrian basement rocks. To
better constrain the tectonic evolution of the Beishan orogen, we conducted
field mapping, U-Pb zircon dating, whole-rock geochemical analysis, and
Sr-Nd isotopic analysis. The new results, when interpreted in the context
of the known geological setting, show that the Beishan region had
experienced five phases of arc magmatism at ca. 1450−1395 Ma, ca. 1071−867
Ma, ca. 542−395 Ma, ca. 468−212 Ma, and ca. 307−212 Ma. In order to explain
the geological, geochemical, and geochronological data from the Beishan
region, we present a tectonic model that involves the following five phases
of deformation: (1) Proterozoic rifting that separated the North Beishan
block from the Greater North China craton that led to the opening of the
Beishan Ocean, (2) early Paleozoic north-dipping subduction (ca. 530−430
Ma) of the Beishan oceanic plate associated with back-arc extension
followed by collision between the North and South Beishan microcontinental
blocks, (3) northward slab rollback of the south-dipping subducting
Paleo-Asian oceanic plate at ca. 450−440 Ma along the northern margin of
the North Beishan block that led to the formation of a northward-younging
extensional continental arc (ca. 470−280 Ma) associated with bimodal
igneous activity, which indicates that the westward extension of the
Solonker suture is located north of the Hongshishan-Pengboshan tectonic
zone, (4) Late Carboniferous opening and Permian north-dipping subduction
of the Liuyuan Ocean in the southern Beishan orogen, and (5)
Mesozoic-Cenozoic intracontinental deformation induced by the final closure
of the Paleo-Asian Ocean system in the north and the Tethyan Ocean system
in the south.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36451.1/615019/Tectonic-evolution-of-the-Beishan-orogen-in
Mesoproterozoic oxygenation event: From shallow marine to atmosphere
Baozeng Xie; Jian-ming Zhu; Xiangli Wang; Dongtao Xu; Limin Zhou ...
Abstract:
Oxygen availability is crucial for the evolution of eukaryotes in
geological history, but the redox conditions of the mid-Proterozoic
atmospheric-oceanic system remain vigorously debated. Previous geochemical
studies suggested a very low mid-Proterozoic atmospheric oxygen level of
probably below 0.1−1% of the present atmospheric level (PAL) with weakly
oxygenated surface ocean, which may have suppressed the early evolution of
eukaryotes in the ocean. A recent study, however, revealed a pulsed
oxygenation event that was recorded in shallow marine carbonate of the
middle Gaoyuzhuang Formation (at ca. 1.57 Ga, North China) with atmospheric pO2 of up to ≥4% PAL based on iodine speciation. This
event is associated with the earliest known multi-cellular eukaryotic
fossils. To elucidate whether this pulsed oxygenation was limited to
shallow local seawaters or could also reflect an increase in atmospheric
oxygen, chromium (Cr), and carbonate-associated sulfate (CAS), sulfur
isotopes in the middle Gaoyuzhuang Formation were studied in two sections
of the North China Platform. The results show a positive shift in
authigenic Cr isotope from −0.18‰ to +0.66‰, which was followed by a
decline to −0.16‰. This suggests a rise in atmospheric oxygen sufficient to
initiate oxic Cr cycling and isotope fractionation (pO2
> 0.1%−1% PAL). This positive Cr isotope excursion was associated with a
transient positive shift in δ34SCAS from ∼20‰ to ∼50‰
and a subsequent decline to ∼10‰, which provides independent evidence of
atmospheric oxygenation. This oxygenation may have enhanced oxidative
terrestrial weathering and increased sulfate input to the ocean, thus
stimulating bacterial sulfate reduction and pyrite burial in deep anoxic
seawaters. This is broadly consistent with previous results of carbonate
I/(Ca+Mg) and Ce anomaly but also reveals a lag between shallow seawater
and atmospheric oxygenation. Fluctuated redox conditions and decoupled
oxygenation between shallow seawater and atmosphere during the
mid-Proterozoic may help to account for the current debate.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36407.1/615020/Mesoproterozoic-oxygenation-event-From-shallow
The influence of Late Ordovician volcanism on the marine environment
based on high-resolution mercury data from South China
Yong Wang; Jingqiang Tan; Wenhui Wang; Lian Zhou; Peng Tang ...
Abstract:
Volcanic ash interbeds in the Late Ordovician strata in South China record
high-frequency volcanic activity. To assess the impact of these volcanic
events on the climate and marine biological evolutionary crisis during the
Late Ordovician Mass Extinction (LOME), we present high-resolution
measurements of mercury (Hg) concentrations and organic carbon isotope
ratios (δ13Corg) in an Ordovician−Silurian
sedimentary succession (Muchanggou section). The results show that
high-frequency volcanic ash layers and Hg enrichments developed
synchronously in the Katian graptolite P. pacificus (Lower
Subzone) and T. typicus biozones and are coupled with a negative
shift of ∼1‰ in δ13Corg, which reflects the
perturbation of the surface carbon and mercury cycles by intense volcanic
activity. Based on volcanic Hg fluxes, it is estimated that more than 1.1 ×
1013 tons of CO2 were emitted by the Katian volcanic
activities in South China, which interrupted the growth of the polar ice
sheet and the rapid cooling of tropical oceans in the mid-Boda event. As
volcanism weakened, increased weathering led to the growth of the polar ice
sheet and cooling in the Hirnantian. Both regional and global biodiversity
declined rapidly with the end of volcanism, which suggests that the marine
ecological crisis was related to a series of disruptions in biogeochemical
cycles in the post-volcanic period.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36257.1/615021/The-influence-of-Late-Ordovician-volcanism-on-the
Giant deep submarine depressions: A combined dissolution-mechanical
process along carbonate margins
T. Mulder; T. Cavailhes; V. Hanquiez; H. Gillet; A. Recouvreur ...
Abstract:
Submarine depressions are common features on the eastern Bahamian seafloor
but the genesis of the deepest ones (>4000 m in water depth) is not well
understood. We conducted a morphometric analysis and compared them to a
worldwide database of rounded depressions, whatever their genesis is. The
deep Bahamian depressions are large elongated structures, among the largest
on Earth, with a width greater than 1000 m and a depth sometimes greater
than 200 m. They extend at the toe of the Blake Bahama Escarpment (BBE),
one of the tallest escarpments on Earth. Some of them align parallel to the
BBE. Other depressions align along large submarine canyon axes. When
aligned along canyon axes, the depressions closest to the canyon head are
flanked by a topographic high interpreted as a slope-break deposit, i.e.,
sediment deposited after flow expansion following a hydraulic jump.
Turbidity currents in the carbonate canyon system are not permanent
processes, but are rather triggered during sea-level highstands when the
carbonate platform is flooded. In addition, some depressions are not
located in canyon axes. Consequently, the size and location of the
depressions are not likely explained by a simple plunge pool mechanical
erosion. Rather, our data suggest that all depressions could be initiated
by giant karstic dissolution structures (dolines or sinkholes). Under
interpretation, those located in canyon axes are sporadically refreshed by
carbonate-laden turbidity currents. The height of the outsized chutes
marking the crossing of the BBE by the canyon mouth generates a hydraulic
jump allowing sediment deposition toward the bottom of the depression.
Large depressions observed at this location in the Bahamas were the result
of an initial dissolution phase related to retreat of the BBE and the more
recent sediment-laden flow activity. The depressions orientated along
canyon axes facilitated the regressive erosion that formed the canyons. At
present, the depressions located at canyon mouths act as regular plunge
pools.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36142.1/614952/Giant-deep-submarine-depressions-A-combined
Geophysical extent of the Wyoming Province, western USA: Insights into
ancient subduction and craton stability
Paul A. Bedrosian; Carol D. Frost
Abstract:
A new 3-D resistivity model, estimated from inversion of magnetotelluric
data, images crustal and upper-mantle structure of the Wyoming Province and
adjacent areas. The Archean province is imaged as a coherent resistive
domain, in sharp contrast to active tectonic domains of the western U.S.
Prominent high-conductivity belts define the northern, eastern, and
southern margins of the Wyoming Province and are interpreted as sutures
marking the remnants of Paleoproterozoic orogens. The model results suggest
the northern boundary of the Wyoming Province is located 150 km south of
its traditional placement and adjacent to a composite orogen separating the
Wyoming Province and Medicine Hat block. The eastern province boundary is
clearly imaged along the Black Hills, whereas the western margin is
obscured by Cenozoic extension and magmatism. An internal boundary within
the Wyoming Province is interpreted to represent a Neoarchean suture; in
stark contrast to Proterozoic sutures, though, it is not marked by a
high-conductivity belt. This difference in conductivity is speculated to
reflect changes in the subduction process through time. The absence of
high-conductivity along Archean sutures appears to be global in nature and
related to reduced continental freeboard in the Archean which limited
continental weathering and the delivery of carbon-rich sediments to the
seafloor. Although the entire Wyoming Province has been proposed to have
undergone lithospheric modification that lessened its stability, the
resistivity model suggests a thick lithospheric root remains in place
except along its western margin. These results suggest that Archean cratons
may be more resistant to lithospheric modification by influx of heat and
fluids associated with extension and plumes than previously thought, and
that metasomatism does not necessarily weaken the lithosphere and set a
craton on the path to destruction.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36417.1/614953/Geophysical-extent-of-the-Wyoming-Province-western
Relationship between stylolite morphology and the sealing potential of
stylolite-bearing carbonate cap rocks
Jun Wu; Tailiang Fan; Enrique Gomez-Rivas; Qian Cao; Anna Travé ...
Abstract:
We analyzed the sealing effectiveness of cap rocks bearing different types
of stylolites using a combination of petrographic, petrophysical, pore
structure, and sealing capacity characterization techniques. This study was
based on examples of carbonate cap rocks that seal ultradeep hydrocarbon
reservoirs of the Tarim Basin (China). Samples from both drill cores and
their outcrop analogues were investigated to quantify how morphology
influences the sealing capacity of different types of stylolite-bearing
rocks. The study cap rocks consisted of mudstone, wackestone to packstone,
grainstone, and dolomitic limestone. Four types of stylolites were
identified: rectangular layer, seismogram pinning, suture and sharp peak,
and simple wave-like types. The difference in the sealing capacity of
carbonate cap rocks is attributed to their pore structure connections and
the types of stylolites they develop. Samples bearing simple wave-like
stylolites showed the best sealing capacity, followed by those with
rectangular layer and suture and sharp peak types, whereas carbonates
hosting seismogram pinning types had the lowest sealing capacity. The
impact of stylolite segments on the rock sealing properties, however,
differed from one segment to another. Rectangular layer−type stylolites
could be divided into three distinct segments (with good, moderate, and
poor sealing, respectively). Both the seismogram pinning and suture and
sharp peak stylolite types were divided in two parts, with the former one
having moderate and poor sealing and the latter exhibiting good and
moderate sealing. The simple-wave−like type had a good sealing capacity all
along the pressure-solution seam. The most effective sealing barriers for
vertical fluid flow form when (1) calcite and siliceous cements are
pervasively distributed in the vicinity of stylolites, forming highly
cemented zones with lower porosity and permeability than their surrounding
host rocks; (2) stylolites are enriched in insoluble residues; and (3) rare
microfractures and dissolution vugs are found along the stylolites. This
work provides useful examples for the prediction of the sealing potential
of stylolite-bearing carbonate rocks according to stylolite morphology in
other geologic settings.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36297.1/614954/Relationship-between-stylolite-morphology-and-the
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