New GSA Bulletin Articles Published Ahead of Print in December
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; major
reorganization of the Snake River modulated by passage of the Yellowstone
Hotspot; Weddell Sea Embayment ice streams, Antarctica; and geochemical
analysis of black shale. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
New insights into organic matter accumulation from high-resolution
geochemical analysis of a black shale: Middle and Upper Devonian Horn
River Group, Canada
Haolin Zhou; Nicholas B. Harris; Tian Dong; Korhan Ayranci; Jilu Feng ...
Abstract:
Organic matter (OM) accumulation in organic matter-rich mudstones, or black
shales, is generally recognized to be controlled by combinations of
bioproductivity, preservation, and dilution. However, specific triggers of
OM deposition in these formations are commonly difficult to identify with
geochemical proxies, in part because of feedbacks that cause geochemical
proxies for these controls to vary synchronously. This apparent
synchronicity is partly a function of sample spacing, commonly at decimeter
to meter intervals, which may represent longer periods of time than is
required for the development of feedbacks. Higher resolution data sets may
be required to fully interpret OM accumulation.This study applies a novel
combination of technologies to develop a high-resolution geochemical data
set, integrating energy-dispersive X-ray fluorescence (EDXRF) and infrared
imagery analyses, to record proxies for redox conditions, bioproductivity,
and clastic and carbonate dilution in millimeter-resolution profiles of 133
core slabs from the Middle and Upper Devonian Horn River shale in the
Western Canada Sedimentary Basin, which provides decadal-scale temporal
resolution. A comparison to a more coarsely sampled data set from the same
core results in substantially different interpretations of variations in
bioproductivity, redox, and dilution proxies. Stratigraphic distributions
of organic matter accumulation patterns (bioproductivity-control,
siliciclastic/carbonate-dilution, and redox conditions-control) show that
organic enrichment events were highly varied during deposition of the shale
and were closely related to second- and third-order sea-level changes.
High-resolution profiles indicate that bioproductivity was the predominant
trigger for organic matter accumulation in a second-order highstand,
particularly during deposition of third-order transgressive systems tracts.
Organic matter accumulation was largely controlled by dilution from either
carbonate or clastic sediments in a second-order lowstand.
Bioproductivity-redox feedbacks developed on timescales of decades to
centuries.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36091.1/610466/New-insights-into-organic-matter-accumulation-from
Tracing fluid evolution in sedimentary basins with calcite geochemical,
isotopic and U-Pb geochronological data: Implications for petroleum and
mineral resource accumulation in the Nanpanjiang Basin, South China
Xiang Ge; Chuanbo Shen; Renjie Zhou; Peng He; Jianxin Zhao ...
Abstract:
Fluid migration in sedimentary basins enable mass and energy transport and
play critical roles in geochemical and geodynamical evolution of
sedimentary basins. Moreover, reconstructing sedimentary basin fluid
evolution from the geological record aids in constraining the evolution of
associated petroleum and mineralization systems. As a relic of fluid flow
activity, calcite is often a record of fluid flow and therefore can be used
to characterize the fluids responsible for its precipitation. Here we study
the Nanpanjiang Basin in South China where petroleum reservoirs and
Carlin-type gold deposits spatially coincide. Through in situ U-Pb dating
and geochemical analysis (87Sr/86Sr, δ18OVienna standard mean ocean water, δ13C Vienna Peedee belemnite, rare earth elements) of calcite, this
work constrains the key times related to petroleum migration/accumulation
and Carlin-type gold mineralization, defines the basin fluid evolution, and
proposes a genetic model for petroleum accumulation and gold mineralization
within the Nanpanjiang Basin. The U-Pb age (ca. 241.4 Ma) for the
gray/black calcite related to bitumen indicates the petroleum
migration/accumulation occurred during the Triassic. The U-Pb date (ca.
106−121 Ma) of the white calcite associated with the gold-bearing pyrite,
realgar, and fluorite record the lower timing limit of the Carlin-type gold
systems. The geochemical data suggest both calcite types are cogenetic but
suffered complex evolution with the gray/black calcite precipitating under
low temperatures related to the continuous basin burial and the white
calcite affected by post formation alteration related to both hydrothermal
and meteoric fluids. Combined with the regional tectonic history, the Early
Triassic petroleum migration/accumulation and the Early Cretaceous
secondary Carlin-type gold mineralization events are considered to be
related to the collision between the Indo-China and South China blocks, and
the subduction between the Paleo-Pacific and Eurasian plates, respectively.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36168.1/610467/Tracing-fluid-evolution-in-sedimentary-basins-with
Proterozoic−Phanerozoic tectonic evolution of the Qilian Shan and
Eastern Kunlun Range, northern Tibet
Chen Wu; Jie Li; Andrew V. Zuza; Peter J. Haproff; Xuanhua Chen ...
Abstract:
The Proterozoic−Phanerozoic tectonic evolution of the Qilian Shan, Qaidam
Basin, and Eastern Kunlun Range was key to the construction of the Asian
continent, and understanding the paleogeography of these regions is
critical to reconstructing the ancient oceanic domains of central Asia.
This issue is particularly important regarding the paleogeography of the
North China-Tarim continent and South China craton, which have experienced
significant late Neoproterozoic rifting and Phanerozoic deformation. In
this study, we integrated new and existing geologic field observations and
geochronology across northern Tibet to examine the tectonic evolution of
the Qilian-Qaidam-Kunlun continent and its relationships with the North
China-Tarim continent to the north and South China craton to the south. Our
results show that subduction and subsequent collision between the
Tarim-North China, Qilian-Qaidam-Kunlun, and South China continents
occurred in the early Neoproterozoic. Late Neoproterozoic rifting opened
the North Qilian, South Qilian, and Paleo-Kunlun oceans. Opening of the
South Qilian and Paleo-Kunlun oceans followed the trace of an early
Neoproterozoic suture. The opening of the Paleo-Kunlun Ocean (ca. 600 Ma)
occurred later than the opening of the North and South Qilian oceans (ca.
740−730 Ma). Closure of the North Qilian and South Qilian oceans occurred
in the Early Silurian (ca. 440 Ma), whereas the final consumption of the
Paleo-Kunlun Ocean occurred in the Devonian (ca. 360 Ma). Northward
subduction of the Neo-Kunlun oceanic lithosphere initiated at ca. 270 Ma,
followed by slab rollback beginning at ca. 225 Ma evidenced in the South
Qilian Shan and at ca. 194 Ma evidenced in the Eastern Kunlun Range. This
tectonic evolution is supported by spatial trends in the timing of
magmatism and paleo-crustal thickness across the Qilian-Qaidam-Kunlun
continent. Lastly, we suggest that two Greater North China and South China
continents, located along the southern margin of Laurasia, were separated
in the early Neoproterozoic along the future Kunlun-Qinling-Dabie suture.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36306.1/610468/Proterozoic-Phanerozoic-tectonic-evolution-of-the
Kinematics and structural evolution of the Anziling dome-and-keel
architecture in east China: Evidence of Neoarchean vertical tectonism
in the North China Craton
Chen Zhao; Jian Zhang; Guochun Zhao; Changqing Yin; Guokai Chen ...
Abstract:
The debate on the role of vertical versus horizontal tectonism in Archean
cratons is intimately linked to the initiation of plate tectonics. The
dome-and-keel architecture has been considered as a consequence of vertical
tectonism. Although such a structural pattern is documented in some
Mesoarchean and older cratons, such as the Kaapvaal and Pilbara cratons,
whether it also occurs in Neoarchean cratons is poorly constrained.
Determining the kinematics, structural evolution, and the timing of these
structures is crucial in understanding how the tectonic behavior operated
during the evolution of the early Earth. The North China Craton, especially
its eastern part, is a Neoarchean continental block and preserves typical
greenstone-granite rock assemblages. Detailed structural mapping reveals
that the Anziling area (east China) is characterized by a typical dome
structure without significant reworking by later deformation. The dome is
in tectonic contact with a supracrustal rock assemblage that is now the
dip-slip Shuangshanzi ductile shear zone. In the supracrustal rocks,
compositional layers are folded into upright isoclinal folds. Meanwhile,
along the shear zone, foliation varies from NNW to SW with sub-vertical
dip. Mineral stretching lineations indicate a sinistral shear sense with a
slightly oblique-slip component in the north, but show NWW-directed and
SW-directed steep dip-slip shear in the west and south, respectively.
Kinematic indicators imply that the granitic dome formed through a
vertically upward movement accompanied by an uneven clockwise rotation. The
supracrustal rocks sank downwards to form the regional keel structure.
Structural data suggests that the Anziling area is a typical dome-and-keel
structure. U-Pb zircon dating on pre-, syn-, and post-tectonic dykes
indicate that the dome-and-keel structure formed at 2530−2500 Ma, and was
intimately related to the emplacement of tonalite-trondhjemite-granodiorite
granitoids. New data from this study suggest that until the late
Neoarchean, the vertical tectonism was still a dominant tectonic regime
that was operating in the eastern North China Craton.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36225.1/610469/Kinematics-and-structural-evolution-of-the
Late Triassic successive amalgamation between the South China and North
China blocks: Insights from structural analysis and magnetic fabrics
study of the Bikou Terrane and its adjacent area, northwestern Yangtze
block, central China
Zhenhua Xue; Wei Lin; Yang Chu; Wei Wei; Zhentian Feng ...
Abstract:
The Bikou Terrane, located at the conjunction of the Longmenshan
fold-thrust belt and the west Qinling orogenic belt in centeral China, was
involved in the Late Triassic collision between the South China and North
China blocks. The Bikou Terrane has preserved crucial information on
structural geometry and kinematics of Triassic tectonics, and is therefore
of great importance for reconstructing the Paleo-Tethyan evolutionary
history. However, multi-phase tectonic events of the Bikou Terrane are
unsettled. This work presents detailed structural analysis based on both
the field and laboratory works, which reveals three phases of deformation
events in Bikou and its adjacent areas, including top-to-the-SW shearing
related to SW-ward thrusting (DI) mainly to the north of the
Bikou Terrane, top-to-the-NNW shearing related to NNW-ward thrusting (DII) in the Bikou Terrane, and strike-slip faulting (D III) locally developed in the northern Bikou Terrane. Anisotropy
of magnetic susceptibility (AMS) study and related structural analysis not
only support the multiphase deformation but also reveal a gradual
transition from the DII-related magnetic fabrics to the D III-related magnetic fabrics in the Bikou Terrane. Integrating
published geochronological data, it is constrained that DI
occurred at ca. 237−225 Ma, DII occurred at ca. 224−219 Ma, and
DIII possibly occurred during the Early Cretaceous. Based on
regional tectonics, the DI event corresponds to the collision
between the South Qinling block and the Bikou Terrane, and the D II event reflects the intracontinental amalgamation between the
Bikou Terrane and the Yangtze block, which indicates a Late Triassic
successive amalgamation from the North China block to the South China
block. Intracontinental adjustment represented by the strike-slip (D III event) occurred after the final amalgamation between the
North China and South China blocks. By applying AMS on deciphering
structural geometry and multi-phase deformation, our study suggests that
AMS is a useful tool for structural analysis.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36228.1/610455/Late-Triassic-successive-amalgamation-between-the
Quantifying the growth of continental crust through crustal thickness
and zircon Hf-O isotopic signatures: A case study from the southern
Central Asian Orogenic Belt
Yujian Wang; Dicheng Zhu; Chengfa Lin; Fangyang Hu; Jingao Liu
Abstract:
Accretionary orogens function as major sites for the generation of
continental crust, but the growth model of continental crust remains poorly
constrained. The Central Asian Orogenic Belt, as one of the most important
Phanerozoic accretionary orogens on Earth, has been the focus of debates
regarding the proportion of juvenile crust present. Using published
geochemical and zircon Hf-O isotopic data sets for three belts in the
Eastern Tianshan terrane of the southern Central Asian Orogenic Belt, we
first explore the variations in crustal thickness and isotopic composition
in response to tectono‐magmatic activity over time. Steady progression to
radiogenic zircon Hf isotopic signatures associated with syn‐collisional
crustal thickening indicates enhanced input of mantle‐derived material,
which greatly contributes to the growth of the continental crust. Using the
surface areas and relative increases in crustal thickness as the proxies
for magma volumes, in conjunction with the calculated mantle fraction of
the mixing flux, we then are able to determine that a volume of ∼14−22% of
juvenile crust formed in the southern Central Asian Orogenic Belt during
the Phanerozoic. This study highlights the validity of using crustal
thickness and zircon isotopic signatures of magmatic rocks to quantify the
volume of juvenile crust in complex accretionary orogens. With reference to
the crustal growth pattern in other accretionary orogens and the Nd-Hf
isotopic record at the global scale, our work reconciles the rapid crustal
growth in the accretionary orogens with its episodic generation pattern in
the formation of global continental crust.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36046.1/610456/Quantifying-the-growth-of-continental-crust
Late Quaternary active faulting on the inherited Baoertu basement fault
within the eastern Tian Shan orogenic belt: Implications for regional
tectonic deformation and slip partitioning, NW China
Chuanyong Wu; Guangxue Ren; Siyu Wang; Xue Yang; Gan Chen ...
Abstract:
The deformation pattern and slip partitioning related to oblique
underthrusting of the Tarim Basin in the eastern Tian Shan orogenic belt
are not well understood because interior deformation images are lacking.
The Baoertu fault is an E-W−striking, ∼350-km-long reactivated basement
structure within the eastern Tian Shan. In this study, we quantify its late
Quaternary activity based on interpretations of detailed high-resolution
remote sensing images and field investigations. Three field observation
sites along an ∼80-km-long fault segment indicate that the Baoertu fault is
characterized by sinistral thrust faulting. Based on surveying of the
displaced geomorphic surfaces with an unmanned drone and dating of the late
Quaternary sediments using radiocarbon and optically stimulated
luminescence (OSL) methods, we estimate a late Quaternary left-lateral,
strike-slip rate of 1.87 ± 0.29 mm/yr and a N−S shortening rate of 0.26 ±
0.04 mm/yr for this fault. The lithospheric Baoertu fault acts as a
decoupling zone and accommodates the left-lateral shearing caused by the
oblique underthrusting of the Tarim Basin. In the eastern Tian Shan
orogenic belt, the oblique convergence is partitioned into thrust faulting
across the entire range and sinistral slip faulting on the high-dip
basement structure within the orogen. This active faulting pattern in the
eastern Tian Shan of sinistral shearing in the center and thrust faulting
on both sides can be viewed as giant, crustal-scale positive flower
structures.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35887.1/610457/Late-Quaternary-active-faulting-on-the-inherited
Sedimentary response of a structural estuary to Holocene coseismic
subsidence
Laura C. Reynolds; Alexander R. Simms; Thomas K. Rockwell; Yusuke Yokoyama;
Yosuke Miyairi ...
Abstract:
Stratigraphic evidence for coseismic subsidence has been documented in
active-margin estuaries throughout the world. Most of these studies have
been conducted in subduction zone or strike-slip settings; however, the
stratigraphic response to coseismic subsidence in other tectonic settings
would benefit from further study. Here we show evidence of late Holocene
coseismic subsidence in a structural estuary in southern California. Below
the modern marsh surface, an organic-rich mud containing marsh gastropods,
foraminifera, and geochemical signatures indicative of terrestrial
influence (mud facies) is sharply overlain by a blue-gray sand containing
intertidal and subtidal bivalves and geochemical signatures of marine
influence (gray sand facies). We use well-established criteria to interpret
this contact as representing an abrupt 1.3 ± 1.1 m rise in relative sea
level (RSL) generated by coseismic subsidence with some contribution from
sediment compaction and/or erosion. The contact dates to 1.0 ± 0.3 ka and
is the only event indicative of rapid RSL rise in the 7 k.y. sedimentary
record studied. Consistent with observations made in previous coseismic
subsidence studies, an acceleration in tidal-flat sedimentation followed
this abrupt increase in accommodation; however, the recovery of the estuary
to its pre-subsidence elevations was spatially variable and required
500−900 years, which is longer than the recovery time estimated for
estuaries with larger tidal ranges and wetter climates.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35827.1/610458/Sedimentary-response-of-a-structural-estuary-to
LA-ICP-MS U-Pb dating and geochemical characterization of oil
inclusion-bearing calcite cements: Constraints on primary oil migration
in lacustrine mudstone source rocks
Ao Su; Honghan Chen; Yue-xing Feng; Jian-xin Zhao
Abstract:
To date, few isotope age constraints on primary oil migration have been
reported. Here we present U-Pb dating and characterization of two
fracture-filling, oil inclusion-bearing calcite veins hosted in the
Paleocene siliciclastic mudstone source rocks in Subei Basin, China.
Deposition age of the mudstone formation was estimated to be ca. 60.2−58.0
Ma. The first vein consists of two major phases: a
microcrystalline-granular (MG) calcite phase, and a blocky calcite phase,
each showing distinctive petrographic features, rare earth element
patterns, and carbon and oxygen isotope compositions. The early MG phase
resulted from local mobilization of host carbonates, likely associated with
disequilibrium compaction over-pressuring or tectonic extension, whereas
the late-filling blocky calcite phase was derived from overpressured
oil-bearing fluids with enhanced fluid-rock interactions. Vein texture and
fluorescence characteristics reveal at least two oil expulsion events, the
former represented by multiple bitumen veinlets postdating the MG calcite
generation, and the latter marked by blue-fluorescing primary oil
inclusions synchronous with the blocky calcite cementation. The MG calcite
yields a laser ablation−inductively coupled plasma−mass spectrometry U-Pb
age of 55.6 ± 1.4 Ma, constraining the earliest timing of the early oil
migration event. The blocky calcite gives a younger U-Pb age of 47.8 ± 2.3
Ma, analytically indistinguishable from the U-Pb age of 46.5 ± 1.7 Ma
yielded by the second calcite vein. These two ages define the time of the
late oil migration event, agreeing well with the age estimate of 49.7−45.2
Ma inferred from fluid-inclusion homogenization temperature and published
burial models. Thermodynamic modeling shows that the oil inclusions were
trapped at ∼27.0−40.9 MPa, exceeding corresponding hydrostatic pressures
(23.1−26.7 MPa), confirming mild-moderate overpressure created by oil
generation-expulsion. This integrated study combining carbonate U-Pb dating
and fluid-inclusion characterization provides a new approach for
reconstructing pressure-temperature-composition-time points in petroleum
systems.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35804.1/610367/LA-ICP-MS-U-Pb-dating-and-geochemical
Quantitative analysis of hillshed geomorphology and critical zone
function: Raising the hillshed to watershed status
Zachary S. Brecheisen; Daniel D. Richter; Seulgi Moon; Patrick N. Halpin
Abstract:
Landscapes are frequently delineated by nested watersheds and river
networks ranked via stream orders. Landscapes have only recently been
delineated by their interfluves and ridge networks, and ordered based on
their ridge connectivity. There are, however, few studies that have
quantitatively investigated the connections between interfluve networks and
landscape morphology and environmental processes. Here, we ordered
hillsheds using methods complementary to traditional watersheds, via a
hierarchical ordering of interfluves, and we defined hillsheds to be
landscape surfaces from which soil is shed by soil creep or any type of
hillslope transport. With this approach, we demonstrated that hillsheds are
most useful for analyses of landscape structure and processes. We ordered
interfluve networks at the Calhoun Critical Zone Observatory (CZO), a North
American Piedmont landscape, and demonstrated how interfluve networks and
associated hillsheds are related to landscape geomorphology and processes
of land management and land-use history, accelerated agricultural gully
erosion, and bedrock weathering depth (i.e., regolith depth). Interfluve
networks were ordered with an approach directly analogous to that first
proposed for ordering streams and rivers by Robert Horton in the GSA Bulletin in 1945. At the Calhoun CZO, low-order hillsheds are
numerous and dominate most of the observatory’s ∼190 km2 area.
Low-order hillsheds are relatively narrow with small individual areas, they
have relatively steep slopes with high curvature, and they are relatively
low in elevation. In contrast, high-order hillsheds are few, large in
individual area, and relatively level at high elevation. Cultivation was
historically abandoned by farmers on severely eroding low-order hillsheds,
and in fact agriculture continues today only on high-order hillsheds.
Low-order hillsheds have an order of magnitude greater intensity of
gullying across the Calhoun CZO landscape than high-order hillsheds. In
addition, although modeled regolith depth appears to be similar across
hillshed orders on average, both maximum modeled regolith depth and spatial
depth variability decrease as hillshed order increases. Land management,
geomorphology, pedology, and studies of land-use change can benefit from
this new approach pairing landscape structure and analyses.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35724.1/610368/Quantitative-analysis-of-hillshed-geomorphology
First structural observation around the hinge of the Mongolian Orocline
(Central Asia): Implications for the geodynamics of oroclinal bending
and the evolution of the Mongol-Okhotsk Ocean
Pengfei Li; Min Sun; Tserendash Narantsetseg; Fred Jourdan; Wanwan Hu ...
Abstract:
To understand the origin of curved subduction zones has been one of the
major challenges in plate tectonics. The Mongol-Okhotsk Orogen in Central
Asia is characterized by the development of a U-shaped oroclinal structure
that was accompanied by the continuous subduction of the Mongol-Okhotsk
oceanic plate. Therefore, it provides a natural laboratory to understand
why and how a subduction system became tightly curved. In this study, we
provide the first structural observation around the hinge of the Mongolian
Orocline (the Zag zone in Central Mongolia), with an aim to constrain the
oroclinal geometry and to link hinge zone structures with the origin of the
orocline. Our results show that rocks in the Zag zone are characterized by
the occurrence of a penetrative foliation that is commonly subparallel to
bedding. Both bedding and dominant fabric in the Zag zone are steeply
dipping, and their strike orientations in a map view follow a simple curve
around the hinge of the Mongolian Orocline, thus providing the first
structural constraint for 3D geometry of the orocline. A secondary
penetrative fabric parallel to the axial plane of the orocline was not
observed, indicating a low degree of orogen-parallel shortening during
oroclinal bending. Combining with available geological and geophysical
data, we conclude that the Mongolian Orocline was developed in a period of
Permian to Jurassic, and its origin was linked to the subduction of the
Mongol-Okhotsk oceanic slab. We consider that the low-strain oroclinal
bending likely resulted from the along-strike variation in trench retreat,
which was either triggered by the negative buoyancy of the Mongol-Okhotsk
oceanic slab, or driven by the relative rotation of the Siberian and North
China cratons. Our results shed a light on 3D geometry and geodynamic
mechanisms of large-scale oroclinal bending in an accretionary orogen.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36200.1/610062/First-structural-observation-around-the-hinge-of
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