New GSA Bulletin Articles Published Online Ahead of Print Now
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; a possible new
record of global organic carbon; and a model for natural avalanches. You can
find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Long-lived (>100 m.y.) postcollisional exhumation and cooling in
the Paleoproterozoic Trans−North China orogen: Evidence from phase
equilibria modeling and monazite petrochronology of
granulite-facies metapelites in the Fuping Complex
Li Tang; M. Santosh; Richard M. Palin; Li-Hui Jia; Hua-Wen Cao ...
Long-lived collisional orogens that formed over tens to hundreds of
millions of years are common in the geologic record. The Trans−North China
orogen marks the collision between the Eastern and Western blocks of the
North China craton, and it preserves metamorphic rocks with ages between
1.98 Ga and 1.80 Ga. These units allow detailed assessment of the time
scale and duration of crustal thickening, exhumation, and cooling
associated with a major Proterozoic orogeny. In this study, we present
integrated petrography, mineral chemistry, phase equilibria modeling, and
texturally controlled in situ mass spectrometry of monazite U-Th-Pb and
trace-element analyses performed on a suite of orthopyroxene-bearing
pelitic granulites and garnet-biotite gneisses from the Fuping Complex
within the Trans−North China orogen. These rocks record clockwise
pressure-temperature (P-T) paths involving
granulite-facies peak conditions of 9.9−11.0 kbar and 850−880 °C for
pelitic granulites, and 10.9−11.6 kbar and 860−880 °C for garnet-biotite
gneisses, followed by postpeak decompression to ∼8−9 kbar and later
cooling, with final solidification of melt at <840 °C. Four monazite
populations were identified in these samples. Group I grains are irregular
and elongated and occur in contact with or embay garnet. They have high
rare earth element (REE) and Y contents and metamorphic ages of 1.90−1.86
Ga, which correspond to the breakdown of garnet during postpeak
decompression. Most monazite grains crystallized from melt are represented
by groups II + III + IV and are associated with orthopyroxene, biotite,
plagioclase, and quartz in the matrix. They have crystallization ages
between 1.86 Ga and 1.76 Ga and relatively low REE and Y concentrations.
These data imply a long-lived (>100 m.y.) postcollisional exhumation and
cooling involving decompression from 10−12 kbar to ∼9 kbar during 1.90−1.86
Ga, followed by retrograde cooling from 1.86 to 1.76 Ga under prolonged
residence in the middle to lower crust. Initial collision and peak
metamorphism occurred before 1.90 Ga, ultimately leading to the final
cratonization of the North China craton and its incorporation into the
Columbia supercontinent.
Reconstructing backwater hydrodynamics from fluvial-deltaic
deposits using stratigraphic inversion: An example from the Tullig
Sandstone, Western Irish Namurian Basin, County Clare, Ireland
Chenliang Wu; Jeffrey A. Nittrouer; Eric A. Barefoot; Kurtis C. Burmeister
Fluvial-deltaic systems are subject to non-uniform backwater flow where
rivers approach receiving basins. This hydrodynamic condition results in
sediment aggradation on the channel bed and enhanced downstream fining. In
turn, this impacts river channel dynamics, including lateral migration
rates and the propensity for avulsion. The imprint of non-uniform flow on
stratigraphy has been reported from field, numerical modeling, and
experimental studies. This work provides key observations for evaluating
the impact of non-uniform flow spanning length scales from those of
sediment grains to delta lobes. However, reconstructing paleohydraulic
conditions of non-uniform flow from fluvial-deltaic settings remains a
challenge. Non-uniform flow is a defining characteristic of fluvial-deltaic
environments, but most existing relations linking hydrology and the
depositional record rely on the assumption of steady and uniform flow.
Herein, we present a novel stratigraphic inversion technique, combining it
with morphodynamic modeling and statistical analyses, to evaluate how
backwater conditions manifest in the stratigraphy of the Tullig Sandstone,
an ancient fluvial-deltaic deposit of the Western Irish Namurian Basin. Our
analyses refine estimates of channel properties, including flow depth and
bed slope, and are validated by field measurements of sandstone-facies
properties, including grain size and stratal architecture. For example, bed
sediment fines down-dip, commensurate with increasing cross set and bed
thicknesses. These patterns indicate bed aggradation and are consistent
with reconstructed morphodynamic conditions. This study pinpoints the
extent of non-uniform flow and its influence on fluvial-deltaic
stratigraphy and provides a framework for improving reconstruction
techniques used to interpret the paleohydrology of ancient fluvial-deltaic
systems.
Frontal expansion of an accretionary wedge under highly oblique
plate convergence: Southern Indo-Burman Ranges, Myanmar
Peng Zhang; Shao-Yong Jiang; Raymond A. Donelick; Renyuan Li; Cleber J.
Soares ...
The formation of accretionary wedges with oversteepened slopes and uplifted
axial zones has been demonstrated to be potentially associated with highly
oblique plate convergence by numerical and analog studies. The direct role
of this mechanism, or other factor(s) in producing the described structural
and morphological features in nature, however, has yet to be confirmed. We
used seismic reflection sections, detrital zircon U-Pb ages, and detrital
apatite fission-track thermochronological data to examine the effects of
highly oblique convergence and sediment reworking on accretionary wedge
growth in the Indo-Burma Subduction Zone. A detailed subsurface structural
analysis of a two-dimensional seismic survey from the outer wedge of the
southern Indo-Burman Ranges, Myanmar, yielded three primary
characteristics. These are (1) a narrow, steep deformation front (average
width 15.6 km) and a vast, low-relief shelf terrace (average width 49 km);
(2) a comparatively long-lived growth thrust fault (FT1) with a convex-up
geometry at the rear of the deformation front that controlled the vertical
stack of the progradational sequences in the shelf terrace; and (3) a group
of NE-striking transtensional faults that cut through entire outer-wedge
successions and displays as a series of negative flower-like structures.
These characteristic features are roughly consistent with the results of
laboratory analog modeling of highly oblique plate convergence but
significantly differ from those of natural accretionary wedges that formed
under highly oblique convergence conditions, such as those in the Sumatra,
Hikurangi, Chile, and Cascadia. We further analyzed the sediment provenance
of the southern Indo-Burman Ranges and discovered that the outer-wedge
rocks are a product of sediment reworking of the hinterland wedge that
began to be uplifted and exhumed in the early Miocene (22−12 Ma) due to
transpressional motion between the Indian plate and West Burma Terrane. Our
analyses indicate that active sedimentation behind the major growth thrust
fault (FT1) provided additional basal shear stress that strengthened the
coupling of the interface between the wedge base and décollement and
promoted the vertical expansion of the outer wedge of the southern
Indo-Burman Ranges from the Neogene to the present day. In contrast, the
outer wedge of the central Indo-Burman Ranges has experienced stronger
forward accretion since the late Miocene, which could be explained by a
smaller degree of obliquity and weaker sediment reworking. Our findings
demonstrate that both highly oblique plate convergence and sediment
reworking were the primary driving forces that triggered vertical
development of accretionary wedges. The results of this research have
significant implications for understanding the structures and kinematic
evolution of wedge systems at other convergent plate margins, in which
seamount passage or subduction erosion is often interpreted as the cause of
the steeply tapered wedges.
Tracing high-pressure metamorphism in the eastern Himalayan
syntaxis using detrital zircon and monazite from modern stream
sediments
Yunxuan Zhang; Liang Guo; Hongfei Zhang; Nigel Harris; Wangchun Xu ...
The timing of high-pressure (HP) metamorphism in the eastern Himalayan
syntaxis is important for understanding the India-Asia collisional
processes, but it remains elusive. To reveal the metamorphic history of the
eastern Himalayan syntaxis, we performed a study of geochronology, trace
elements, and mineral inclusions of detrital zircon and monazite from
modern stream sediments in the eastern Himalayan syntaxis. Detrital zircon
comprise magmatic and metamorphic domains with different zoning. Inherited
magmatic zircon domains have high Th/U, low (Dy/Yb)N, and retain
ages of 1798−360 Ma. Metamorphic zircon domains with low Th/U, high (Dy/Yb) N, and inclusions of garnet, kyanite, and/or clinopyroxene
probably formed under HP conditions. They yield age groups of 49−35 Ma,
33−17 Ma, and 12−7 Ma. The low Th/U and low (Dy/Yb)N metamorphic
zircon domains probably formed during retrogression and yield age groups of
27−16 Ma and 10−6 Ma. Detrital monazite yield age distributions similar to
those of the low (Dy/Yb)N metamorphic zircon except for the
821−402 Ma inherited cores. The (Dy/Yb)N of 31.6−5.7 Ma monazite
decreases with increasing Y content, which indicates that it likely formed
under the retrograde stage during garnet breakdown. Based on the oldest
metamorphic ages, the initial India-Asia collision occurred no later than
50−44 Ma in the eastern Himalayan syntaxis. The multimodal age patterns of
the metamorphic zircon and monazite indicate that the Indian continent
underwent multistage HP and retrograde metamorphism in the eastern
Himalayan syntaxis. The nearly contemporaneous HP and retrograde
metamorphism indicate that the Indian continent continued subducting while
the earlier HP metamorphic slices detached and exhumed.
Immiscible two-phase model for air blasts created during natural
avalanches
Jianbo Fei; Zhankui Liu; Yuxin Jie
An immiscible two-phase model based on the incompressible Navier-Stokes
(N-S) equations is used to simulate the air blast generated by an
avalanche. For simplicity, the avalanche is treated as an assembly of
monodisperse spherical grains and described as a continuous media. The
constitutive law of local µ(I) rheology is introduced to
model the moving granular material. The motion of the avalanche and the
induced air blast fits into a unified framework that combines the N-S−type
governing equations with a µ(I)-rheology−based kinematic
viscosity and a constant viscosity. The avalanche-air interface is treated
using the volume-of-fluid method. A numerical program was developed on the
open-source platform OpenFOAM specifically for this model to simulate the
entire evolutionary process of the avalanche as well as the air blast
generated. The model was validated by comparing the results of numerical
simulations with those from inclined-plane laboratory experiments. With
terrain input from the Shuttle Radar Topography Mission data, the model was
further applied to simulate the air blast generated in two natural
avalanches, namely, the Baige and Wenjia valley avalanches fo China, which
occurred in 2008 and 2018, respectively. The simulation results were found
to be consistent with field observations following a statistical analysis
of the properties of the air blast including flow speed and area of impact
of the above-mentioned natural events.
Morphometric constraints on the formation of new terrestrial
analogs for planetary pits
R. Naor; A. Mushkin; I. Halevy
The origin of geological depressions abounding on Mars and other planetary
bodies remains poorly understood, partially due to the limited variability
in the geological settings of existing terrestrial analogs. Here, we
present a new terrestrial analog that is located at the northwestern margin
of the Levantine volcanic field of Harrat Ash−Shaam along the Dead Sea
Transform. The analog site consists of tens of geological depressions
(locally named “juba”) that morphologically resemble Martian bowl-shaped
pits and occur within a Pleistocene basaltic plateau that overlies
Meso-Cenozoic carbonates. To constrain plausible formation mechanisms for
the juba depressions, we carried out detailed field mapping and
morphometric analyses using a 0.25 m/pixel digital terrain model (DTM)
derived from airborne light detection and ranging (LiDAR) survey covering
34 km2 of the study area, and centimeter-scale, ground-based
LiDAR scans of selected juba depressions. We show that variable magnitudes
of slope asymmetry between north- and south-facing walls within the juba
depressions, along with different degrees of sediment infilling, provide
effective proxies for the relative geomorphic maturity of these landforms,
and in turn indicate asynchronous formation of the juba depressions after
the Pleistocene emplacement of the Harrat Ash−Shaam basalts in the study
area. Our findings preclude formation of the juba depressions by
phreatomagmatic explosions and instead point toward collapse into missing
subsurface volume. In a broader context, we propose that the morphometric
analyses developed herein to distinguish between plausible juba formation
mechanisms in the Harrat Ash−Shaam volcanic field can be extended to better
constrain the formation mechanisms of similar pit features on Mars and
other planetary bodies.
Juxtaposition of different-grade metamorphic rocks in an ancient
orogen: Evidence from the Chengde Complex of the Trans-North China
Orogen, North China Craton
Hui C.G. Zhang; Guochun Zhao; Chao Wang; Nanqing Xu; Jinlong Yao
Juxtaposition of different-grade metamorphic slices is a typical feature of
Phanerozoic orogens but is relatively scarce in Precambrian orogens. Here,
we focus on the Chengde Complex at the northern segment of the late
Paleoproterozoic Trans-North China Orogen, the North China Craton as a
case, to explore how different-grade orogeny-related metamorphism was
juxtaposed in Precambrian orogens. The Trans-North China Orogen is a
typical Precambrian collisional orogen and records abundant information
about the Paleoproterozoic orogeny. High-pressure (HP) mafic granulite and
two types of amphibolite samples were collected from the Chengde Complex,
and we conducted a combined study involving litho-structural assemblage
investigation, metamorphic petrology, and geochronology to decipher their
metamorphic P-T−t history. HP mafic granulite and
amphibolite occur as enclaves or dikes within felsic gneisses. HP mafic
granulite records clockwise P-T paths with isothermal
decompression (ITD) segments, and the peak metamorphic P-T
conditions are 13.0−14.9 kbar/790−830 °C. Peak metamorphic P-T
conditions retrieved from garnet-bearing amphibolite are 8.3 kbar/675 °C,
whereas peak metamorphic P-T conditions retrieved from garnet-free
amphibolite are 4.0−5.5 kbar/500−510 °C. Zircon U-Pb dating of HP granulite
and amphibolite yield different prograde (1914−1871 Ma), peak (1869−1816
Ma), and retrograde (ca. 1830 Ma) metamorphic ages, and all of these data
indicate that these metamorphic rocks were diachronously transferred to
different depths in the subduction channel and subsequently exhumed at
shallower crustal levels. Therefore, we conclude that the Chengde Complex
is composed of imbricate rocks with different metamorphic grades and ages,
and such an unordered juxtaposition of diverse metamorphism could also be a
typical feature of Precambrian orogens.
Intense metamorphism-generated radiogenic Sr regulated Cenozoic
water Sr isotope evolution on the NE Tibetan Plateau: A perspective
on Qilian orogen denudation and Asian eolian transport
Yibo Yang; Albert Galy; Rongsheng Yang; Yudong Liu; Weilin Zhang ...
The India-Asia collision reactivated the early Paleozoic Qilian orogen with
an intense metamorphic belt that promoted the release of
metamorphism-generated radiogenic Sr into the drainage systems on the NE
Tibetan Plateau. This metamorphic impact on the regional dissolved Sr cycle
is well observed at the recent−modern scale, but its onset and evolutionary
histories are unclear. We present the first basin-scale 52−5 Ma regional
dissolved Sr isotopic record in water on the NE Tibetan Plateau by
analyzing well-dated basin fluvial-lacustrine sediments in the Xining,
Linxia, and Tianshui Basins. The Xining Basin displays an increase in basin
water 87Sr/86Sr ratio and a decrease in the sediment
εNd values at ca. 25 Ma. This Sr-Nd isotope-deduced provenance
change coincides with the reorganization of drainage and erosion regimes
that is suggested by an evident rise in the youngest peak and the lag time
of detrital apatite fission-track ages from Cenozoic sedimentary basins
surrounding the Qilian orogen. The Qilian-sourced eolian dust during ca.
9−8 Ma significantly elevated the river and lake water 87Sr/ 86Sr ratios in the downwind Linxia and Tianshui Basins, which is
consistent with the expansion of the Asian dust system. Our results suggest
that large-scale denudation of the Qilian orogen in response to the
India-Asia collision initiated in the late Oligocene. Given the remarkable
hydrological impact of the Cenozoic reactivation of the Qilian orogen, our
study highlights the potentially important role of continental
collision−formed metamorphic belts in regulating past regional and even
seawater Sr isotope evolution.
Validation of earthquake ground-motion models in southern
California, USA, using precariously balanced rocks
Anna H. Rood; Dylan H. Rood; Greg Balco; Peter J. Stafford; Lisa Grant
Ludwig ...
Accurate estimates of earthquake ground shaking rely on uncertain
ground-motion models derived from limited instrumental recordings of
historical earthquakes. A critical issue is that there is currently no
method to empirically validate the resultant ground-motion estimates of
these models at the timescale of rare, large earthquakes; this lack of
validation causes great uncertainty in ground-motion estimates. Here, we
address this issue and validate ground-motion estimates for southern
California utilizing the unexceeded ground motions recorded by 20
precariously balanced rocks. We used cosmogenic 10Be exposure
dating to model the age of the precariously balanced rocks, which ranged
from ca. 1 ka to ca. 50 ka, and calculated their probability of toppling at
different ground-motion levels. With this rock data, we then validated the
earthquake ground motions estimated by the Uniform California Earthquake
Rupture Forecast, Version 3 (UCERF3) seismic-source characterization and
the Next Generation Attenuation (NGA)-West2 ground-motion models. We found
that no ground-motion model estimated levels of earthquake ground shaking
consistent with the observed continued existence of all 20 precariously
balanced rocks. The ground-motion model I14 estimated ground-motion levels
that were inconsistent with the most rocks; therefore, I14 was invalidated
and removed. At a 2475 year mean return period, the removal of this invalid
ground-motion model resulted in a 2−7% reduction in the mean and a 10−36%
reduction in the 5th−95th fractile uncertainty of the ground-motion
estimates. Our findings demonstrate the value of empirical data from
precariously balanced rocks as a validation tool for removing invalid
ground-motion models and, in turn, reducing the uncertainty in earthquake
ground-motion estimates.
Tectonically controlled drainage fragmentation in the southwestern
Great Basin, USA
B.M. Lutz; J.R. Knott; F.M. Phillips; M.T. Heizler; K.A. Heitkamp, Jr. ...
The area now occupied by the Great Basin, western USA, contained
paleo-fluvial systems that predated the modern-day endorheic (closed)
basins. The areal extent of these paleo-fluvial systems within the
southwestern Great Basin is known mainly from isolated remnants preserved
in the modern mountain ranges. We document the age, extent, and tectonic
disruption of Mio-Pliocene fluvial systems of the southwestern Great Basin.
Synthesis of new field observations, geochemistry, and geochronology with
existing studies defines two latest Miocene to Pliocene east-southeast
flowing drainages that predated the modern endorheic basins. The drainage
network was ultimately fragmented in Pliocene time (ca. 3.5-4 Ma).
Fragmentation of the drainage network led to lake formation, drying of
lakes, and the formation of isolated springs. The rapid environmental
changes initiated by faulting and volcanism isolated previously
interbreeding populations of spring-dwelling taxa and have caused divergent
evolution since Pliocene time. Modern endemism within the region’s springs
is thus a direct consequence of intraplate tectonism.
Location and shape of the Lhasa terrane prior to India-Asia
collision
Weiwei Bian; Suo Wang; Yong Yao; Xianwei Jiao; Wenxiao Peng ...
The precollisional location and shape of the Lhasa terrane are crucial for
constraining the closure of the Neo-Tethys Ocean and the ensuing India-Asia
collision; however, estimation of these features of the Lhasa terrane
remains highly controversial. Here, we carried out a new paleomagnetic
investigation on the Lower Cretaceous Duoni Formation red beds in the
central-eastern Lhasa terrane. The tilt-corrected site-mean direction is
declination (Ds) = 339.0°, inclination (I s) = 26.8°, ks = 78.4, and α95 =
2.3° (k—precision parameter; α95—the radius
that the mean direction lies within 95% confidence; s
—stratigraphic coordinates) (N = 50), corresponding to a paleopole
at 64.2°N, 324.2°E, with A95 = 1.9° ( A95—the radius that the mean pole lies within 95%
confidence). These new paleomagnetic data pass a positive fold test and
indicate that the studied area was located at 14.3 ± 1.9°N during the Early
Cretaceous. No significant inclination shallowing is present in the Lower
Cretaceous Duoni Formation red beds. Our new results, combined with
previously published reliable Cretaceous paleomagnetic results, show that
the Lhasa terrane was located at a paleolatitude of ∼22.9°N to 10.1°N from
west to east and was oriented at ∼298°−296° prior to India-Asia collision.
Geodynamic controls in the southernmost Northern Andes magmatic
arc: Trace elements and Hf-O isotopic systematics in forearc
detrital zircon
César Witt; Marc Poujol; Massimo Chiaradia; Diego Villagomez; Monique
Seyler ...
U-Pb dating of single detrital zircon grains by laser ablation−inductively
coupled plasma−mass spectrometry (LA-ICP-MS) paired with Hf and O isotopic
and trace-element analyses provide first-order indicators of the Late
Cretaceous−Cenozoic evolution of the southern Ecuadorian magmatic arc.
Detrital zircon U-Pb ages define significant clusters that are tentatively
interpreted as intense arc magmatism at ca. 72 Ma, ca. 60 Ma, and ca. 43
Ma. A major accretionary event in the Late Cretaceous (75−65 Ma) is marked
by a broad range of zircon isotopic values (εHf[ t] > 20 and δ18O > 8‰) that
suggest melting of both the lower and upper crust (most likely of
continental affinity) as well as enriched mantle components. Highly
fractionated signatures in trace-element patterns and Eu/Eu* combined with
mantle-like δ18O and juvenile εHf values characterize zircons
from 60 to 45 Ma, suggesting that the Late Cretaceous−middle Eocene arc
originated from an enriched mantle and likely reflects the persistence of
overthickened crust previously attributed to the main Late Cretaceous
accretionary period. Subsequently, negative shifts in εHf( t) isotopic composition from 45 to 30 Ma are
paired with mantle-like δ18O values as well as decreases in U/Yb
and Eu/Eu*. These signatures could be attributed to magma emplacement in a
thinner crust and the existence of a broad extensional magmatic arc
extending from the current forearc toward areas near the craton; however,
other scenarios cannot be excluded. This event was characterized by
enriched mantle melt sources with residence times pointing to known crustal
events (Sunsás) in the Amazonian craton. From 30 to 10 Ma, the isotopic
record slightly evolved toward a depleted mantle signature with a
substantial increase in fractionation. Our results combined with previously
published isotopic records from detrital zircon grains found in modern
rivers suggest that, for at least the last 30 m.y., the southernmost
Northern Andes magmatic arc has been segmented, with the emplacement of
juvenile magmas to the north and more enriched magmas related to the
recycling of ancient continental crust and/or subducted sediments to the
south—aspects found in other Northern Andes settings in which the
continental arc was constructed in both oceanic and continental crust.
Rapid southeastern Laurentide Ice Sheet thinning during the last
deglaciation revealed by elevation profiles of in situ cosmogenic 10Be
Christopher T. Halsted; Paul R. Bierman; Jeremy D. Shakun; P. Thompson
Davis; Lee B. Corbett ...
Accurate reconstruction of Laurentide Ice Sheet volume changes following
the Last Glacial Maximum is critical for understanding ice sheet
contribution to sea-level rise, the resulting influence of meltwater on
oceanic circulation, and the spatial and temporal patterns of deglaciation.
Here, we provide empirical constraints on Laurentide Ice Sheet thinning
during the last deglaciation by measuring in situ cosmogenic 10
Be in 81 samples collected along vertical transects of nine mountains in
the northeastern United States. In conjunction with 107 exposure age
samples over five vertical transects from previous studies, we reconstruct
ice sheet thinning history. At peripheral sites (within 200 km of the
terminal moraine), we find evidence for ∼600 m of thinning between 19.5 ka
and 17.5 ka, which is coincident with the slow initial margin retreat
indicated by varve records. At locations >400 km north of the terminal
moraine, exposure ages above and below 1200 m a.s.l. exhibit different
patterns. Ages above this elevation are variable and older, while lower
elevation ages are indistinguishable over 800−1000 m elevation ranges, a
pattern that suggests a subglacial thermal boundary at ∼1200 m a.s.l.
separating erosive, warm-based ice below and polythermal, minimally erosive
ice above. Low-elevation ages from up-ice mountains are between 15 ka and
13 ka, which suggests rapid thinning of ∼1000 m coincident with
Bølling-Allerød warming. These rates of rapid paleo-ice thinning are
comparable to those of other vertical exposure age transects around the
world and may have been faster than modern basin-wide thinning rates in
Antarctica and Greenland, which suggests that the southeastern Laurentide
Ice Sheet was highly sensitive to a warming climate.
Bhumichula plateau: A remnant high-elevation low-relief surface in
the Himalayan thrust belt of western Nepal
Tshering Z.L. Sherpa; Peter G. DeCelles; Barbara Carrapa; Lindsay M.
Schoenbohm; Joshua Wolpert
The Himalaya is known for dramatically rugged landscapes including the
highest mountains in the world. However, there is a limited understanding
of the timing of attainment of high elevation and relief formation,
especially in the Nepalese Himalaya. Anomalous high-elevation low-relief
(HELR) surfaces, which exhibit geomorphic antiquity and are possibly
remnants of formerly widespread high-elevation paleosurfaces, provide a
unique opportunity to assess the attainment of regional high elevation in
the Himalaya. The Bhumichula plateau is one such HELR surface (4300−4800 m)
in the western Nepalese Himalayan fold-thrust belt. The Bhumichula plateau
is situated in the Dadeldhura klippe (also called the Karnali klippe), an
outlier of Greater Himalayan Sequence high-grade metasedimentary/igneous
rocks surrounded by structurally underlying Lesser Himalayan Sequence
low-grade metasedimentary rocks. We assess the origin of the Bhumichula
plateau by combining regional geological relationships and zircon and
apatite (U-Th-Sm)/He and apatite fission track thermochronologic ages. The
HELR surface truncates pervasive west-southwestward dipping foliations,
indicating that it post-dates tilting of rocks in the hanging wall of the
Main Central thrust above the Lesser Himalayan duplex. This suggests that
the surface originated at high elevation by erosional beveling of
thickened, uplifted crust. Exhumation through the ∼180−60 °C thermal window
occurred during middle Miocene for samples on the plateau and between
middle and late Miocene for rocks along the Tila River, which bounds the
north flank of the Bhumichula plateau. Cooling ages along the Tila River
are consistent with erosional exhumation generated by early Miocene
emplacement of the Main Central (Dadeldhura) thrust sheet, middle Miocene
Ramgarh thrust emplacement, and late Miocene growth of the Lesser Himalayan
duplex. The most recent middle-late Miocene exhumation took place as the
Tila River and its northward flowing tributaries incised upstream, such
that the Bhumichula plateau is a remnant of a more extensive HELR
paleolandscape. Alpine glaciation lowered relief on the Bhumichula surface,
and surface preservation may owe to its relatively durable lithology,
gentle structural relief, and elevation range that is above the rainier
Lesser Himalaya.
The Late Triassic Longmenshan lateral foreland thrusting: New
insights from geological evidence and 3-D particle discrete-element
simulation
Haonan Zhao; Yuanyuan Zhang; Wei Du; Yang Zhang; Renjie Zhou ...
This work proposes a new lateral foreland thrusting model based on
geological evidence and 3-D particle discrete-element simulation to explain
the Longmenshan southeastward thrusting during the closure of the
Songpan-Ganze basin. The Late Triassic NE−SW compression caused by the
northward movement of the Qiangtang Block and the resulting differential
shortening within the wedge-shaped Songpan-Ganze terrane produced
southeastward topographic gradient. The thick sedimentary pile, driven by
the horizontal tectonic force and the deviatoric stress generated from
gravitational effect, and decoupled from the subducting basement by the
low-strength décollement, was laterally extruded and resulted in the
southeastward Longmenshan thrusting. Therefore, the Longmenshan thrust belt
is a lateral foreland thrust belt of the Songpan-Ganze terrane. For the
first time, 3-D particle discrete-element simulation was used for the
geological study of the Longmenshan area, and it clearly reproduces the
dynamical process of the Longmenshan southeastward thrusting and well
predicts the Xiaojin Arcuate Zone. The particle discrete-element simulation
results verify the new model and reveal that two key factors facilitate the
lateral foreland thrusting: the wedge-shaped geometry that produces
differential shortening and lateral topographic gradient, and the
low-strength décollement that decouples the extruded sedimentary pile from
the basement. The lateral foreland thrust belt, which is unique in its
tectonic location and dynamic behavior, is a new kind of foreland thrust
belt that is different from the pro-foreland and retro-foreland thrust
belts, and it provides new insight into the tectonic evolution of
collisional orogens.
Permian−Middle Triassic floral succession in North China and
implications for the great transition of continental ecosystems
Wenchao Shu; Jinnan Tong; Jianxin Yu; Jason Hilton; Michael J. Benton ...
The global pattern of plant evolution through the Permian−Triassic mass
extinction is uncertain, and the extent to which land plants were affected
is debated. Detailed studies undertaken at a regional scale can help
evaluate this floral transition, and thus we provide a detailed account of
floral evolution from the Permian to Middle Triassic of North China based
on new paleobotanical data and a refined biostratigraphy. Five floral
transition events are identified from before, during, and after the
Permian−Triassic crisis, including the disappearance of the gigantopterid
flora (associated with loss of coal deposits), the end-Permian mass
extinction of Paleophytic taxa, and gradual recovery in the Triassic with
the stepwise appearance of the Mesophytic vegetation. The record begins
with a Cisuralian gigantopterid-dominated rainforest community, and then a
Lopingian walchian Voltziales conifer-ginkgophyte community that evolved
into a voltzialean conifer-pteridosperm forest community. The last is
associated with a change amongst terrestrial vertebrates from the Jiyuan
fauna to a pareiasaur-dominated fauna, found in red beds that lack coal
deposits due to arid conditions. The disappearance of the voltzialean
conifer forest community may represents the end-Permian mass extinction of
plants although it could also be a consequence of the non-preservation of
plants in sedimentary red-beds. The first post-crisis plants are an Induan
herbaceous lycopsid community, succeeded by the Pleuromeia- Neocalamites shrub marsh community. A pteridosperm shrub woodland
community dominated for a short time in the late Early Triassic along with
the reappearance of insect herbivory. Finally, in the Middle Triassic,
gymnosperm forest communities gradually rose to dominance in both uplands
and lowlands along with other diverse plant communities, indicating the
establishment of the Mesophytic Flora.
Buoyancy-driven exhumation deformation: Evidence from the Sulu
orogen, eastern China
Hao Yin; Guang Zhu; Xiaodong Wu; Nan Su; Yuanchao Lu ...
Whether deformation of exhumed crust in a collisional orogen is driven by
buoyancy or tectonic stress remains uncertain. The Sulu orogen in eastern
China contains slowly subducted and exhumed high-pressure
(HP)−ultrahigh-pressure (UHP) terranes, which provide a good opportunity to
understand whether the exhumation deformation was driven by buoyancy. We
used field and microscopic observations as well as quartz c-axis
fabrics to determine the deformation kinematics, temperatures, and
evolution of the exhumed crustal slices. Deformed and undeformed dikes were
dated by using the zircon U-Pb laser ablation−inductively coupled
plasma−mass spectrometry method to constrain the timing of deformation. Our
data demonstrate that each crustal slice was involved in pervasive
top-to-the-NW (hinterland) or top-to-the-SE (foreland) ductile deformation
during Late Triassic exhumation, and that the base of each slice records
deformation that was superimposed during the subsequent exhumation of the
underlying slice. The crustal exhumation of the southern Sulu orogen is
consistent with the multistage ductile extrusion model. The kinematics of
exhumation deformation within each crustal slice conform to an upward
asymmetric flow. The flow velocity and corresponding shear sense were
affected by temperature variations in each HP slice, whereas they were
sensitive to migmatization within the UHP slices. The southern Sulu orogen
examples show that crustal flow deformation during exhumation was driven by
buoyancy and controlled by viscosity. Unlike the consistent kinematics of
tectonics-driven deformation, the kinematics of buoyancy-driven deformation
are characterized by variations in the senses of shear within a single
crustal slice, and this can therefore be used to distinguish the two types
of deformation.
From ka to Ma: A multi-timescale record of accelerating Cenozoic
tectonic uplift between the Qilian Shan and Qaidam Basin, northern
Tibetan Plateau
Zhimin Li; Runchao Liu; John He; Wenjun Zhu; Wanhe Wang ...
Constraining tectonic uplift history within the Tibetan Plateau is critical
to understanding its deformational response to continental collision.
However, it is difficult to extrapolate orogen-scale uplift history from
any single method alone. Here, we combined high-resolution deep and shallow
seismic imaging (on the order of 103 to 102 meters in
depth, respectively) with geologic paleoseismic trenching (on the order of
several meters in depth) in the Qilian Shan−Qaidam Basin (QSQB) transition
zone within the northern Tibetan Plateau, which provide a fault-to-basin,
ka-to-Ma-scale record of mountain building. Tectonic uplift began in the
early Cenozoic (>40 Ma), with slow uplift rates persisting until ca.
15.3 Ma. Tectonic tilting with limited thrust faulting along the QSQB
transition zone was the predominant form of deformation during this period.
Accelerated uplift since the middle Miocene is attributed to the activation
of more thrust faults, and an increase in fault vertical slip rates by an
order of magnitude, reaching ∼0.2−0.25 mm/a.
Early Cretaceous continental-scale sediment routing, the McMurray
Formation, Western Canada Sedimentary Basin, Alberta, Canada
Abdullah M. Wahbi; Michael D. Blum; Caroline Nazworth Doerger
Alternative depositional models for the Early Cretaceous McMurray Formation
between a dominantly continental and marginal marine settings remain a
controversial topic. The source-to-sink model can be inverted to
reconstruct ancient sediment-routing systems by utilizing methods to
estimate ancient drainage basins, which can contribute to understanding of
sediment routing and testing of alternative depositional models. New
detrital zircon U-Pb ages from 31 samples were analyzed to identify source
terranes of the McMurray Formation and the overlying Wabiskaw Member of the
Clearwater Formation to test first-generation alternative sediment-routing
models and estimate maximum depositional ages. In total, 9729 new
concordant U-Pb ages identified multiple source terranes for the McMurray
Formation within an interpreted continental-scale paleodrainage basin that
extended from the SW to SE United States and eastern Canada. The
paleodrainage basin then expanded to include the Western Cordillera arc
system within the overlying Upper Mannville Group. Multidimensional scaling
and mixing models independently support a paleo-upstream mixing of primary
and recycled sources in the McMurray Formation axial system in the United
States and a paleo-downstream confluence between this axial system and
east-derived sediment-routing systems in Canada, which display an evolution
from bedrock-confined to alluvial morphology from the lower to middle-upper
McMurray Formation. The current dataset constrains the maximum depositional
age of the McMurray Formation to the latest Barremian to Aptian, with an
age range from ca. 122 to 115 Ma, which is significantly younger than
previously reported. Age estimates in the overlying Upper Mannville units
from ca. 115 to 110 Ma statistically overlap with the McMurray Formation
age, suggesting continuous Mannville deposition that lasted 10−12 m.y. or
less.
Emplacement dynamics of a crystal-rich, highly viscous trachytic
flow of the Sancy stratovolcano, France
Jean-Marie Prival; Andrew J.L. Harris; Elena Zanella; Claudio Robustelli
Test; Lucia Gurioli ...
Emplacement dynamics of highly viscous, silicic lava flows remain poorly
constrained due to a lack of consideration of crystal-rich cases.
Emplacement models mostly apply to glassy or microlitic, vesiculated
rhyolitic flows. However, crystalline, vesicle-free silicic lava can flow
differently. We studied the Grande Cascade unit, which is a vesicle-free,
phenocryst-rich, trachytic flow in the Monts Dore massif, France. Field
work was carried out to define internal structures, and oriented samples
were collected for chemical, petrological, and anisotropy of magnetic
susceptibility analyses, allowing us to estimate emplacement temperature
and viscosity. These data allow us to define a new silicic lava flow
subtype that is low in temperature (800−900 °C), high in silica content (up
to 66.8 wt%), high in viscosity (109−1011 Pa s), rich
in phenocrysts (∼35%), and lacks vesicles. Brittle deformation of the lava
occurs upon extrusion, generating a cataclasite basal layer and thin
(3-m-thick) shear zone that accommodates all of the stress, allowing most
of the flow’s volume to slide over its base as a 40-m-thick plug in which
there is no deformation. Blocks are rare, of a single size (10 ± 1 cm), and
result from localized break-up of the basal shear zone. Emplacement
dynamics are different from those of glassy, pumiceous lava flows. They are
closer to glacier dynamics, where most of the volume slides over a thin
basal shear zone and till is generated there by abrasion and milling of the
underlying layer. For the Grande Cascade lava flow, abrasion means that the
flow lacks its classical blocky crust and instead the flow base is marked
by a layer rich in fine-grained material. The structures and emplacement
dynamics of this crystal-rich flow are consistent with ideal,
gravity-driven shear flow. We thus argue for a global reassessment of
silicic-rich lava emplacement based on crystal content and using a
multidisciplinary approach focused on well-exposed examples in the rock
record.
Constraining the crustal composition of the continental U.S. using
seismic observables
Siyuan Sui; Weisen Shen; Kevin Mahan; Vera Schulte-Pelkum
The composition of the crust is one of the most uncertain and controversial
components of continental estimates due to (1) limited direct access and
(2) inconsistent indirect assessments. Here we show that by combining
high-resolution shear velocity (Vs) models with newly measured with newly
measured ratio of compressional wave velocity (Vp) and Vs, or Vp/Vs ratio,
for the crystalline crust, a 3-D composition (SiO2 wt%) model of
the continental crust can be derived with uncertainty estimates. Comparing
the model with local xenolith data shows consistency at mid and lower
crustal depths. The spatial patterns in bulk SiO2 content
correlate with major geological provinces, including the footprints of
Cenozoic and Mesozoic mafic volcanism in the western U.S., and offer new
insight into the composition and evolution of the continental U.S.
Finding a VOICE in the Southern Hemisphere: A new record of global
organic carbon?
Ralf J. Weger; Gregor P. Eberli; Leticia Rodriguez Blanco; Maximillian
Tenaglia; Peter K. Swart
Variations in the carbon isotopic composition of carbonate and organic
carbon (δ13Ccarb and δ13Corg)
are generally used to record perturbations in the global carbon cycle,
which are in turn closely linked to changes in climate. However, because of
climate gradients on Earth, assignment of the “global” signal in ancient
records is not straightforward. Here, we report the δ13C values
of organic material in the Upper Jurassic to Lower Cretaceous sedimentary
record of the Vaca Muerta Formation, situated in the Neuquén Basin,
Argentina, which show similar patterns to those observed in several
northern latitude basins. This record of δ13C values in the
organic material differs from those measured in the early Atlantic Ocean, a
record previously considered to be representative of the global values of
organic carbon. As a result of the global synchronicity observed in the δ 13C values of organic material from both northern and southern
latitudes, we suggest that these patterns may represent the global record
of δ13C values in organic material rather than those measured in
the proto−Atlantic Ocean. The δ13C values of the organic
components show a slight initial decrease of ∼2‰ in the early Tithonian
(149−145 Ma) and then another decrease of ∼2‰ before reaching a minimum of
−30.29‰ in the late Tithonian (145−143 Ma), followed by a gradual
increasing trend throughout the Berriasian (143.1−137.7 Ma). The early
Valanginian (137.7−135.5 Ma) was marked by a more substantial increase in δ 13C values up to −23.46‰. These changes mirror those seen in
Northern Hemisphere locations during the Late Jurassic and Early
Cretaceous, where this perturbation has been termed the Volgian isotopic
carbon excursion (VOICE). This difference in the Late Jurassic and Early
Cretaceous δ13C values between the early Atlantic Ocean and the
Neuquén Basin is interpreted to be the result of the climate gradient at
the time, which was characterized by more humid conditions in high
latitudes compared to dry conditions in the Atlantic Ocean basin.
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