Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. For March, GSA Bulletin topics
include multiple articles about the dynamics of China and Tibet; the ups
and downs of the Missouri River; the Los Rastros Formation, Argentina; the
Olympic Mountains of Washington State; methane seep deposits; meandering
rivers; and the northwest Hawaiian Ridge. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Transition from a passive to active continental margin setting for the
NE Asian continental margin during the Mesozoic: Insights from the
sedimentary formations and paleogeography of the eastern Jiamusi
Massif, NE China
Yini Wang; Wenliang Xu; Feng Wang
Abstract:
The Mesozoic tectonic evolution of the NE Asian continental margin has
received much attention in recent years. However, previous studies focused
mainly on the petrogenesis of igneous rocks and their relationship with
Mesozoic tectonics, and there have been few studies of the Mesozoic
sedimentary formations of the NE Asian continental margin. We combined
zircon U-Pb ages with Hf isotopic and biostratigraphic data to reconstruct
the Mesozoic paleogeography of the NE Asian continental margin. The results
indicate that Mesozoic strata of the eastern Jiamusi Massif, NE China,
include the Upper Triassic Nanshuangyashan Formation (Norian), Lower
Jurassic volcanic rocks, and Lower Cretaceous Longzhaogou Group. The Upper
Triassic Nanshuangyashan Formation consists of a suite of alternating
marine and terrestrial sedimentary rocks with abundant fossils that formed
in a passive continental margin setting. The Lower Jurassic strata comprise
a suite of calc-alkaline volcanic rocks that include basaltic andesites,
andesites, and rhyolites that formed in an active continental margin
setting related to initial subduction of the Paleo-Pacific Plate beneath
Eurasia. The Lower Cretaceous Longzhaogou Group belong to alternating
marine and terrestrial sedimentary formations that formed in an active
continental margin setting related to subduction of the Paleo-Pacific
Plate. Here, we integrate these data to reconstruct the Mesozoic tectonic
history of the NE Asian continental margin, which comprises a Late Triassic
passive continental margin, the initiation of subduction of the
Paleo-Pacific Plate in the Early Jurassic, and westward subduction and
rollback of the Paleo-Pacific Plate in the Early Cretaceous.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35850.1/595823/Transition-from-a-passive-to-active-continental
Low-δ18O A-type granites in SW China: Evidence for the interaction
between the subducted Paleotethyan slab and the Emeishan mantle plume
Jian Xu; Xiao-Ping Xia; Qiang Wang; Christopher J. Spencer; Bin He ...
Abstract:
The mechanisms and processes by which subducted slab interacted with mantle
plume remain controversial, as direct observation of such interaction is
difficult to impossible. Compositional heterogeneity of large igneous
provinces (LIPs) additionally makes plume-slab interaction hard to detect.
Oxygen isotopes are sensitive enough to trace the source of magmas. Here we
provide evidence for plume-slab interaction mainly based on in situ zircon
Hf-O isotope analyses, as well as whole-rock elemental and Sr-Nd-Hf isotope
analyses, on the Late Permian and Early Triassic A-type granites on the
margin of the Emeishan LIP in SW China. These granites show typical A-type
geochemical characters, such as high total alkali (7.93−9.68 wt%) and field
strength element (HFSE, e.g., Zr and Nb) contents, and high FeOT
/(FeOT+MgO) (0.87−0.98) and Ga/Al (3.67−5.06) values. The Late
Permian (ca. 259 Ma) and Early Triassic (ca. 248 Ma) granites show high
Nb/Th (>3.0) and low Y/Nb (<1.2) and Yb/Ta (<2.0) ratios similar
to the oceanic island basalts and have near-zero εNd(t) (−0.83
to −0.13 and −0.15 to +0.16, respectively) and depleted εHf(t)
(+2.71 to +3.39 and +2.62 to +3.55, respectively). In situ zircon O-Hf
analyses yielded anomalously low δ18O (0.2−2.0‰ and 3.2−4.8‰,
respectively) and positive εHf(t) (1.6−7.0 and 3.9−8.8,
respectively), suggesting varying proportions of hydrothermally altered
oceanic crust in their source region. Our results imply that significant
amounts of altered Paleotethyan oceanic crust have been subducted in the
upper mantle beneath the western South China Block. The nearby rising
Emeishan mantle plume may have rapidly entrained and incorporated these
oceanic crustal materials to the shallow mantle so that their low-δ 18O isotope feature was preserved. Subsequent
decompression-related partial melting of this hybrid source formed parental
rocks of the low-δ18O A-type granites. Our findings also suggest
that LIPs could obtain their compositional (especially oxygen isotope)
diversity through the interaction between the subducting slab and rising
mantle plume.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35929.1/595807/Low-18O-A-type-granites-in-SW-China-Evidence-for
The ups and downs of the Missouri River from Pleistocene to present:
Impact of climatic change and forebulge migration on river profiles,
river course, and valley fill complexity
Justin Anderson; John Holbrook; Ronald J. Goble
Abstract:
The Missouri River is a continent-scale river that has thus far escaped a
rigorous reporting of valley fill trends within its trunk system. This
study summarizes evolution of the lower Missouri River profile from the
time of outwash in the Last Glacial Maximum (LGM) until establishment of
the modern dominantly precipitation-fed river. This work relies on
optically stimulated luminescence (OSL) dating, water-well data, and a
collection of surficial geological maps of the valley compiled from U.S.
Geological Survey EDMAP and National Science Foundation Research Experience
for Undergrads projects. Mapping reveals five traceable surfaces within
valley fill between Yankton, South Dakota, USA, and Columbia, Missouri,
USA, that record two cycles of incision and aggradation between ca. 23 ka
and ca. 8 ka. The river aggraded during the LGM to form the Malta Bend
surface by ca. 26 ka. The Malta Bend surface is buried and fragmented but
presumed to record a braided outwash plain. The Malta Bend surface was
incised up to 18 m between ca. 23 ka and ca. 16 ka to form the Carrolton
surface (ca. 16 ka to ca. 14 ka). The Carrollton surface ghosts a braided
outwash morphology locally through overlying mud. Aggradation followed (ca.
14 ka to ca. 13.5 ka) to within 4 m of the modern floodplain surface and
generated the Salix surface (ca. 13.5 to ca. 12 ka). By Salix time, the
Missouri River was no longer an outwash river and formed a single-thread
meandering pattern. Reincision at ca. 12 ka followed Salix deposition to
form the short-lived Vermillion surface at approximately the grade of the
earlier Carrolton surface. Rapid aggradation from ca. 10 ka to ca. 8 ka
followed and formed the modern Omaha surface (ca. 8 ka to Present). The
higher Malta Bend and Omaha profiles are at roughly the same grade, as are
the lower Carrolton and Vermillion surfaces. The Salix surface is in
between. All surfaces converge downstream as they enter the narrow and
shallow bedrock valley just before reaching Columbia, Missouri. The maximum
departure of the profiles is 18 m near Sioux City, Iowa, USA, at ∼100 km
downstream from the James Lobe glacial input near Yankton, South Dakota.
Incision and aggradation appear to be driven by relative changes in input
of sediment and water related to glacial advance and retreat and then later
by climatic changes near the Holocene transition. The incision from the
Malta Bend to the Carrolton surface records the initial breakdown of the
cryosphere at the end of the LGM, and this same incisional event is found
in both the Ohio and Mississippi valleys. This incisional event records a
“big wash” that resulted in the evacuation of sediment from each of the
major outwash rivers of North America. The direction and magnitude of
incision from the LGM to the modern does not fit with modeled
glacioisostatic adjustment trends for the Missouri Valley. Glaciotectonics
likely influenced the magnitude of incision and aggradation secondarily but
does not appear to have controlled the overall timing or magnitude of
either. Glaciotectonic valley tilting during the Holocene, however, did
likely cause the Holocene channel to consistently migrate away from the
glacial front, which argues for a forebulge axis south of the Missouri
Valley during the Holocene and, by inference, earlier. This is at least 200
km south of where models predict the Holocene forebulge axis. The Missouri
Valley thus appears to reside in the tectonic low between the ice front and
the forebulge crest. The buffer valley component of incision caused by
profile variation could explain as much as 25 m of the total ∼40 m of
valley incision at Sioux City, Iowa. The Missouri Valley also hosted a
glacial lobe as far south as Sioux City, Iowa, in pre-Wisconsinan time,
which is also a factor in valley excavation.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35909.1/595763/The-ups-and-downs-of-the-Missouri-River-from
Segmentation of the Wassuk Range normal fault system, Nevada (USA):
Implications for earthquake rupture and Walker Lane dynamics
Ben Surpless; Sarah Thorne
Abstract:
Normal faults are commonly segmented along strike, with segments that
localize strain and influence propagation of slip during earthquakes.
Although the geometry of segments can be constrained by fault mapping, it
is challenging to determine seismically relevant segments along a fault
zone. Because slip histories, geometries, and strengths of linkages between
normal fault segments fundamentally control the propagation of rupture
during earthquakes, and differences in segment slip rates result in
differential uplift of adjacent footwalls, we used along-strike changes in
footwall morphology to detect fault segments and the relative strength of
the mechanical links between them. We applied a new geomorphic analysis
protocol to the Wassuk Range fault, Nevada, within the actively deforming
Walker Lane. The protocol examines characteristics of footwall morphology,
including range-crest continuity, bedrock-channel long profiles, catchment
area variability, and footwall relief, to detect changes in strike-parallel
footwall characteristics. Results revealed six domains with significant
differences in morphology that we used to identify seismically relevant
fault segments and segment boundaries. We integrated our results with
previous studies to determine relative strength of links between the six
segments, informing seismic hazard assessment. When combined with recent
geodetic studies, our results have implications for the future evolution of
the Walker Lane, suggesting changes in the accommodation of strain across
the region. Our analysis demonstrates the power of this method to
efficiently detect along-strike changes in footwall morphology related to
fault behavior, permitting future researchers to perform reconnaissance
assessment of normal fault segmentation worldwide.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35756.1/595764/Segmentation-of-the-Wassuk-Range-normal-fault
Widespread hydrothermal vents and associated volcanism record prolonged
Cenozoic magmatism in the South China Sea
Fang Zhao; Christian Berndt; Tiago M. Alves; Shaohong Xia; Lin Li ...
Abstract:
The continental margin of the northern South China Sea is considered to be
a magma-poor rifted margin. This work uses new seismic, bathymetric,
gravity, and magnetic data to reveal how extensively magmatic processes
have reshaped the latter continental margin. Widespread hydrothermal vent
complexes and magmatic edifices such as volcanoes, igneous sills, lava
flows, and associated domes are confirmed in the broader area of the
northern South China Sea. Newly identified hydrothermal vents have crater-
and mound-shaped surface expressions, and occur chiefly above igneous sills
and volcanic edifices. Detailed stratigraphic analyses of volcanoes and
hydrothermal vents suggest that magmatic activity took place in discrete
phases between the early Miocene and the Quaternary. Importantly, the
occurrence of hydrothermal vents close to the present seafloor, when
accompanied by shallow igneous sills, suggest that fluid seepage is still
active, well after main phases of volcanism previously documented in the
literature. After combining geophysical and geochemical data, this study
postulates that the extensive post-rift magmatism in the northern South
China Sea is linked to the effect of a mantle plume over a long time
interval. We propose that prolonged magmatism resulted in contact
metamorphism in carbon-rich sediments, producing large amounts of
hydrothermal fluid along the northern South China Sea. Similar processes
are expected in parts of magma-poor margins in association with CO 2/CH4 and heat flow release into sea water and
underlying strata.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35897.1/595765/Widespread-hydrothermal-vents-and-associated
Tectonically conditioned record of continental interior paleoclimate
during the Carnian Pluvial Episode: The Upper Triassic Los Rastros
Formation, Argentina
C.A. Benavente; A.C. Mancuso; R.B. Irmis; K.M. Bohacs; S. Matheos
Abstract:
Discerning paleoclimate parameters in depositional systems of the
continental interior is challenging because the system response and
stratigraphic record of climate are controlled by tectonic processes and
are mediated through landscape and hydrological evolution of fluvial
lacustrine systems. Climate and tectonic signals cannot be deconvolved from
stratigraphic patterns alone but require additional information or data
sets that directly record climate or tectonic influence. The Carnian Los
Rastros Formation in northwest Argentina provides an excellent case study
that integrates an appropriate range of information in a system with strong
climate and tectonic signals, being deposited in part during the Carnian
Pluvial Episode and spanning the active rift phase of the
Ischigualasto−Villa Unión Basin. We examined the stratigraphic and spatial
patterns of carbon (C) and oxygen (O) stable isotopes in lacustrine
carbonates from the Los Rastros Formation in multiple parts of the basin to
constrain paleohydrological conditions and paleotemperatures. Practically
all C and O isotope values are characterized by negative values: δ 18Ocarb −11.6‰ and −15.7‰ (χ average −13.1‰; 1σ =
1.6) and δ13Ccarb −2.6‰ to −8.0‰ (χ average −5.1‰; 1σ
= 2.1), reflecting the latitude, altitude, and continentality of the lake
system and its vegetated and humid catchment area. Stratigraphic patterns
of stable isotope data from two different localities (Cerro Bola North and
Cerro Bola South) show a change from short water-residence time to long
residence time and back to short residence time. This contrasts with
sedimentologic, organic geochemical, and small-scale stratigraphic patterns
that indicate an overfilled lake basin, which is expected to contain a
completely open-hydrology isotopic signature. Paleotemperatures calculated
from marginal lacustrine carbonates show a warm and quite variable
paleothermal range consonant with their continental interior position and
with Global Climate Model estimates for high paleolatitudes. Warmer
paleotemperatures (linked to aridity, probably smaller lake size, and less
thermal mass) precede the Carnian Pluvial Episode, whereas relatively
cooler paleotemperatures coincide with the Carnian Pluvial Episode (linked
to humidity, probably larger lake size, and more thermal mass). Carbon and
oxygen stable isotope signatures integrated with sedimentologic and
physiographic information allow us to propose that tectonics, specifically,
half-graben tilting during the active synrift phase, dominated over climate
effects as the cause of hydrological fluctuations of this system, even
during the Carnian Pluvial Episode. Without appropriate
stratigraphic-tectonic context, single-proxy reconstructions of
continental-interior paleoclimate can be misleading. A robust
interpretation of climate effects requires characterization of tectonic
effects, geomorphology, paleohydrology, and sedimentary system responses.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35847.1/595766/Tectonically-conditioned-record-of-continental
An intracontinental orogen exhumed by basement-slice imbrication in the
Longmenshan Thrust Belt of the Eastern Tibetan Plateau
Zhenhua Xue; Wei Lin; Yang Chu; Michel Faure; Yan Chen ...
Abstract:
The Longmenshan Thrust Belt in Eastern Tibet resulted from a Mesozoic
orogeny and Cenozoic reworking. It is generally believed that the Cenozoic
tectonics along the Longmenshan Thrust Belt are mostly inherited from the
Mesozoic. Reconstructing the Mesozoic tectonics of the Longmenshan Thrust
Belt is therefore important for understanding its evolutionary history. On
the basis of detailed structural analysis, we recognized a Main Central
Boundary that divides the Longmenshan Thrust Belt into a Southeastern Zone
and a Northwestern Zone. Both zones underwent a main D1 event
characterized by D1E top-to-the-SE thrusting in the Southeastern
Zone and D1W top-to-the-NW/N thrusting in the Northwestern Zone.
In the Southeastern Zone, a D2 top-to-the-NW/N normal faulting
that cuts the D1E structures is developed along the NW boundary
of the basement complexes. Newly obtained and previous geochronological
data indicate that the D1E and D1W events occurred
synchronously at ca. 224−219 Ma, and the D2 top-to-the-NW/N
normal faulting was episodically activated at ca. 166−160 Ma, 141−120 Ma,
81−47 Ma, and 27−25 Ma. Episodic and synchronously activated top-to-the-NW
normal faulting and top-to-the-SE thrusting along the northwestern and
southeastern boundaries of the basement complexes, respectively, leads us
to propose that the basement slices were episodically imbricated to the SE
during the Late Jurassic−Early Cretaceous and Late Cretaceous−earliest
Paleocene. The D1 amphibolite facies metamorphic rocks above the
basement complexes recorded fast exhumation during the Late Jurassic−Early
Cretaceous. We propose that the early Mesozoic northwestward basement
underthrusting along a crustal “weak zone” was responsible for the D 1 double-vergent thrusting and amphibolite facies metamorphism.
Subsequent basement-slice imbrications reworked the Longmenshan Thrust Belt
and exhumed the amphibolite facies rocks. Our results highlight the
importance of basement underthrusting and imbrication in the formation and
reworking of the intracontinental Longmenshan Thrust Belt in Eastern Tibet.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35826.1/595732/An-intracontinental-orogen-exhumed-by-basement
Mo isotope records from Lower Cambrian black shales, northwestern Tarim
Basin (China): Implications for the early Cambrian ocean
Guangyou Zhu; Tingting Li; Kun Zhao; Chao Li; Meng Cheng ...
Abstract:
The widely developed black shales deposited during the early Cambrian
recorded paleoenvironmental information about coeval seawater. Numerous
studies have been conducted on these shales to reconstruct the paleomarine
environment during this time period. However, most research has been
conducted on stratigraphic sections in South China, and equivalent studies
of sections from other cratons are relatively rare. Here, we report Mo
isotopic compositions as well as redox-sensitive trace-element and iron
(Fe) speciation data for black shales of the Lower Cambrian Yuertusi
Formation from the Tarim block (i.e., a small craton). The Fe speciation
data show high FeHR/FeT and Fepy/Fe HR ratios, indicating roughly sustained euxinic bottom-water
conditions during their deposition. Based on Mo isotopic compositions (δ 98/95Mo), we further classified the euxinic black shales into
two intervals: a lower interval (0−21.3 m) and an upper interval (21.3−32.3
m). The lower interval is characterized by variable Mo isotopic
compositions (−2.12‰ to +0.57‰, mean = −0.52‰ ± 0.72‰), with an obvious
negative excursion in its middle portion. The overlying upper interval has
relatively heavy δ98/95Mo values up to +1.42‰ (mean = +0.62‰ ±
0.37‰). We ascribe δ98/95Mo differences in the lower and upper
intervals to inadequate aqueous H2S concentrations for
quantitative thiomolybdate formation under euxinic conditions. The most
negative Mo isotope excursion may have been caused by upwelling
hydrothermal inputs during a transgression, consistent with significantly
elevated total organic carbon (TOC) contents, Mo and U enrichments, and Fe
supply. Relatively positive δ98/95Mo values in the upper
interval have roughly similar variations with other coeval sections,
indicating such variations were common for early Cambrian euxinic deposits,
and they were most likely caused by local differences in [H2S] aq. Compilation of Mo isotope data from the early Cambrian and
earlier times further indicates relatively oxygenated seawater, especially
the deep-marine areas during the early Cambrian before reaching a state
like modern seawater.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35726.1/595658/Mo-isotope-records-from-Lower-Cambrian-black
Quantitative analysis of the sedimentary architecture of eolian
successions developed under icehouse and greenhouse climatic conditions
Grace I.E. Cosgrove; Luca Colombera; Nigel P. Mountney
Abstract:
The continental terrestrial record preserves an archive of how ancient
sedimentary systems respond to and record changes in global climate. A
database-driven quantitative assessment reveals differences in the
preserved sedimentary architectures of siliciclastic eolian systems with
broad geographic and stratigraphic distribution that developed under
icehouse versus greenhouse climatic conditions. Over 5600 geological
entities, including architectural elements, facies, sediment textures, and
bounding surfaces, have been analyzed from 34 eolian systems of
Paleoproterozoic to Cenozoic ages. Statistical analyses have been performed
on the abundance, composition, preserved thickness, and arrangement of
different eolian lithofacies, architectural elements, and bounding
surfaces. Results demonstrate that preserved sedimentary architectures of
icehouse and greenhouse systems differ markedly. Eolian dune, sand sheet,
and interdune architectural elements that accumulated under icehouse
conditions are significantly thinner relative to their greenhouse
counterparts; this is observed across all basin settings, supercontinents,
geological ages, and dune field physiographic settings. However, this
difference between icehouse and greenhouse eolian systems is exclusively
observed for paleolatitudes <30°, which suggests that climate-induced
changes in the strength and circulation patterns of trade winds may have
partly controlled eolian sand accumulation. These changes acted in
combination with variations in water table levels, sand supply, and sand
transport, ultimately influencing the nature of long-term sediment
preservation. During icehouse episodes, Milankovitch cyclicity resulted in
deposits typified by glacial accumulation and interglacial deflation.
Greenhouse conditions promoted the accumulation of eolian elements into the
geological record due to elevated water tables and biogenic- and
chemical-stabilizing agents, which could protect deposits from wind-driven
deflation. In the context of a rapidly changing climate, the results
presented here can help predict the impact of climate change on Earth
surface processes.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35918.1/595649/Quantitative-analysis-of-the-sedimentary
Late Eocene post-collisional magmatic rocks from the southern Qiangtang
terrane record the melting of pre-collisional enriched lithospheric
mantle
Yue Qi; Qiang Wang; Gang-jian Wei; Xiu-Zheng Zhang; Wei Dan ...
Abstract:
Diverse rock types and contrasting geochemical compositions of
post-collisional mafic rocks across the Tibetan Plateau indicate that the
underlying enriched lithospheric mantle is heterogeneous; however, how
these enriched mantle sources were formed is still debated. The accreted
terranes within the Tibetan Plateau experienced multiple stages of
evolution. To track the geochemical characteristics of their associated
lithospheric mantle through time, we can use mantle-derived magmas to
constrain the mechanism of mantle enrichment. We report zircon U-Pb ages,
major and trace element contents, and Sr-Nd isotopic compositions for Early
Cretaceous and late Eocene mafic rocks in the southern Qiangtang terrane.
The Early Cretaceous Baishagang basalts (107.3 Ma) are characterized by low
K2O/Na2O (<1.0) ratios, arc-like trace element
patterns, and uniform Sr-Nd isotopic compositions [(87Sr/ 86Sr)i = 0.7067−0.7073, εNd(t) = −0.4 to
−0.2]. We suggest that the Baishagang basalts were derived from partial
melting of enriched lithospheric mantle that was metasomatized by subducted
Bangong−Nujiang oceanic material. We establish the geochemistry of the
pre-collisional enriched lithospheric mantle under the southern Qiangtang
terrane by combining our data with those from other Early Cretaceous mafic
rocks in the region. The late Eocene (ca. 35 Ma) post-collisional rocks in
the southern Qiangtang terrane have low K2O/Na2O
(<1.0) ratios, and their major element, trace element, and Sr-Nd
isotopic compositions [(87Sr/86Sr)i =
0.7042−0.7072, εNd(t) = −4.5 to +1.5] are similar to those of
the Early Cretaceous mafic rocks. Based on the distribution, melting
depths, and whole-rock geochemical compositions of the Early Cretaceous and
late Eocene mafic rocks, we argue that the primitive late Eocene
post-collisional rocks were derived from pre-collisional enriched
lithospheric mantle, and the evolved samples were produced by assimilation
and fractional crystallization of primary basaltic magma. Asthenosphere
upwelling in response to the removal of lithospheric mantle induced the
partial melting of enriched lithospheric mantle at ca. 35 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35864.1/595453/Late-Eocene-post-collisional-magmatic-rocks-from
Reappraisal of the Mesozoic tectonic transition from the Paleo-Tethyan
to Paleo-Pacific domains in South China
Chengshi Gan; Yuzhi Zhang; Yuejun Wang; Xin Qian; Yang Wang
Abstract:
The southeastern (SE) South China Block was mainly influenced by the
Paleo-Tethyan and Paleo-Pacific dynamic domains during the Mesozoic. The
initial timing of the tectonic transition between these two domains in the
SE South China Block still remains debated. The transition would affect the
nature of the lithosphere and material provenance of sediments, and,
therefore, igneous and sedimentary rocks in the area could record such
dynamic processes. In this study, published geochronological and
geochemical data of the Triassic and Jurassic igneous rocks and detrital
zircon data of contemporaneous sedimentary rocks in the SE South China
Block were compiled, aiming to provide constraints on the tectonic
transition via tracing the spatial-temporal variations in the nature of the
lithosphere and sedimentary provenance signals. The compiled results
suggest that the magmatic intensity and volume decreased significantly from
the Late Triassic to Early−Middle Jurassic, with an obvious magmatic
quiescence between them, and increased from the Early−Middle Jurassic to
Late Jurassic. The εNd(t) and
zircon εHf(t) values of mafic
rocks, granitoids, and shoshonitic rocks remarkably increased from the Late
Triassic to Early−Middle Jurassic, indicative of variations in the
lithospheric mantle and continental crust. Such variations suggest that the
initial tectonic transition occurred at the earliest Early Jurassic. Based
on the southward paleocurrents from Early Jurassic sandstone, E-W−trending
extension of Early−Middle Jurassic mafic and shoshonitic rocks, and similar
sedimentary provenances of Late Triassic and Early−Middle Jurassic
sedimentary rocks, these features imply that the SE South China Block was
not immediately influenced by the Paleo-Pacific domain during the
Early−Middle Jurassic. However, from the Early−Middle Jurassic to Late
Jurassic and Early Cretaceous, the spatial distribution, geochemical
signatures, magmatic intensity, and magmatic volume of igneous rocks and
provenance of sedimentary rocks exhibit obvious variations, and the
regional fold hinge direction changed from E-W−trending to NE-trending,
suggesting significant effects from Paleo-Pacific subduction on the SE
South China Block. Thus, the Mesozoic tectonic transition from the
Paleo-Tethyan to the Paleo-Pacific dynamic domain in the SE South China
Block likely occurred during the Early−Middle Jurassic.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35755.1/595454/Reappraisal-of-the-Mesozoic-tectonic-transition
Two key switches in regional stress field during multi-stage
deformation in the Carboniferous−Triassic southernmost Altaids
(Beishan, NW China): Response to orocline-related roll-back processes
Zhonghua Tian; Wenjiao Xiao; Brian F. Windley; Peng Huang; Ji’en Zhang ...
Abstract:
The orogenic architecture of the Altaids of Central Asia was created by
multiple large-scale slab roll-back and oroclinal bending. However, no
regional structural deformation related to roll-back processes has been
described. In this paper, we report a structural study of the Beishan
orogenic collage in the southernmost Altaids, which is located in the
southern wing of the Tuva-Mongol Orocline. Our new field mapping and
structural analysis integrated with an electron backscatter diffraction
study, paleontology, U-Pb dating, 39Ar-40Ar dating,
together with published isotopic ages enables us to construct a detailed
deformation-time sequence: During D1 times many thrusts were
propagated northwards. In D2 there was ductile sinistral
shearing at 336−326 Ma. In D3 times there was top-to-W/WNW
ductile thrusting at 303−289 Ma. Two phases of folding were defined as D4 and D5. Three stages of extensional events (E1−E3) separately occurred during D1−D 5. Two switches of the regional stress field were identified in
the Carboniferous to Early Permian (D1-E1-D 2-D3-E2) and Late Permian to Early
Triassic (D4-E3-D5). These two switches in
the stress field were associated with formation of bimodal volcanic rocks,
and an extensional interarc basin with deposition of Permian-Triassic
sediments, which can be related to two stages of roll-back of the
subduction zone on the Paleo-Asian oceanic margin. We demonstrate for the
first time that two key stress field switches were responses to the
formation of the Tuva-Mongol Orocline.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35898.1/595455/Two-key-switches-in-regional-stress-field-during
Structural styles, deformation, and uplift of the Olympic Mountains,
Washington: Implications for accretionary wedge deformation
M. James Aldrich
Abstract:
The Olympic subduction complex is the exposed subaerial Cascadia
accretionary wedge in the Olympic Mountains of Washington State. Uplift of
the mountains has been attributed to two competing models: margin-normal
deformation from frontal accretion and underplating, and margin-parallel
deformation from the clockwise rotation and northward movement of the
Oregon Coast Range block compressing the Olympic Mountains block against
the Canadian Coast Range. East-northeast−oriented folds and Quaternary
thrust faults and paleostress analysis of faults in the Coastal Olympic
subduction complex, west of the subduction complex massif, provide new
evidence for north-south shortening in the Coastal Olympic subduction
complex that fills a large spatial gap in the north-south shortening
documented in prior studies, substantially strengthening the block rotation
model. These new data, together with previous studies that document
north-south shortening in the subduction complex and at numerous locations
in the Coast Range terrane peripheral to the complex, indicate that
margin-parallel deformation of the Cascadia forearc has contributed
significantly to uplift of the Olympic Mountains. Coastal Olympic
subduction complex shallow-level fold structural style and deformation
mechanisms provide a template for analyzing folding processes in other
accretionary wedges. Similar-shaped folds in shallow-level Miocene
turbidite sediments of the Coastal Olympic subduction complex formed in two
shortening phases not previously recognized in accretionary wedges. Folds
began forming by bed-parallel flow of sediment into developing hinges. When
the strata could no longer accommodate shortening by flexural flow, further
shortening was taken up by flexural slip. Similar-shaped folds in the
deeper accretionary wedge rocks of the subduction complex massif have a
well-developed axial-surface cleavage that facilitated shear folding with
sediment moving parallel to the axial surface into the hinges, a structural
style that is common to accretionary wedges. The pressure-temperature
conditions and depth at which the formation of similar folds transitions
from bed-parallel to axial-surface−parallel deformation are bracketed.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35729.1/595422/Structural-styles-deformation-and-uplift-of-the
Late Paleogene paleotopographic evolution of the northern Cordilleran
orogenic front: Implications for demise of the orogen
Majie Fan; Kurt N. Constenius; Rachel F. Phillips; David L. Dettman
Abstract:
The paleotopographic history of the North American Cordilleran orogen holds
the key to understanding mechanisms of orogenesis and subsequent orogenic
collapse. It has been suggested that the orogenic front in western Montana
(USA) and Alberta (Canada) was more than 4 km high during Late
Cretaceous−early Eocene contractional deformation and during the initial
phase of extension in the middle Eocene; however, the late Eocene−Oligocene
topographic evolution during continued extensional collapse remains poorly
constrained. Here we extend the paleotopographic record in the Kishenehn
Basin in northwestern Montana and southeastern British Columbia (Canada) to
the late Oligocene by studying δ18O values of fossil mollusks
and cement and paleosol carbonates. The molluscan taxa changed from three
sympatric groups with preferred habitats ranging from tropical wet,
semi-arid subtropical, and temperate during the middle and late Eocene, to
mainly a single group associated with temperate environment during the
Oligocene, reflecting a decline in molluscan biodiversity induced by
climate cooling across the Eocene−Oligocene transition. Reconstructed δ 18O values of alpine snowmelt and basinal precipitation
decreased by 1.4‰ and 3.8‰, respectively, from the middle to late Eocene,
reflecting climate cooling and ∼1 km surface uplift of the basin floor. The
reconstructed alpine snowmelt δ18O values then increased by 2.9‰
in the Oligocene suggesting a ∼0.5 km drop in elevation of the orogenic
front. Collectively, the results of our new and previously published δ 18O data chronicle the paleotopographic response to the change
from flat-slab subduction to slab rollback over a 45 m.y. period. These
data suggest that the orogenic front was characterized by high elevation
(>4 km) in the ancestral Lewis-Clark-Livingston ranges during latest
Cretaceous−early Eocene (ca. 75−52 Ma) contraction. The initial phase of
extension related to the Kishenehn Basin created a lowland basin with a
surface elevation of only ∼1.5 km during the early middle Eocene (ca. 46−44
Ma) whereas the ranges remained >4 km high. The high range elevations
were sustained for at least 12 m.y. in the middle to late Eocene concurrent
with extension, while the basin floor elevation was uplifted to ∼2.5 km by
the latest Eocene (ca. 36−34 Ma). Basin aggradation can explain at most
half of the 1 km basin floor uplift. The remaining amount (at least 0.5 km)
and sustained high range elevation suggest that range denudation and
crustal extension was compensated by the isostatic and thermal effects of
slab rollback and/or passage of a slab window and infusion of hot
asthenosphere beneath the continent. The range elevation in the orogenic
front decreased ∼0.5 km by the late Oligocene (ca. 28 Ma), associated with
a decrease in rock uplift rate associated with extension. A post-Oligocene
elevation drop of ∼1 km resulted in both the ranges and basin floor
reaching modern topography in the Kishenehn Basin drainage, likely due to
the regional effect of Neogene Basin and Range extension. This study, along
with the previous investigation of the Kishenehn Basin by Fan et al.
(2017), are the first studies that systematically investigate paleorelief
of the orogenic belt by reconstructing paleoelevations of the mountains and
the basin at the same time. The results highlight that the Cordilleran
orogenic front of northern Montana and southern British Columbia sustained
its high elevation edifice for at least 12 m.y. after the start of
extension. We suggest that initial crustal extension did not result in
orogenic demise because of concurrent thermal and isostatic uplift.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35919.1/595423/Late-Paleogene-paleotopographic-evolution-of-the
The role of slab geometry in the exhumation of cordilleran-type orogens
and their forelands: Insights from northern Patagonia
Marie C. Genge; Massimiliano Zattin; Elisa Savignano; Marta Franchini;
Cécile Gautheron ...
Abstract:
In cordilleran-type orogens, subduction geometry exerts a fundamental
control on the tectonic behavior of the overriding plate. An integrated
low-temperature, large thermochronological data set is used in this study
to investigate the burial and exhumation history of the overriding plate in
northern Patagonia (40°−45°S). Thermal inverse modeling allowed us to
establish that a ∼2.5−4-km-thick section originally overlaid the
Jurassic−Lower Cretaceous successions deposited in half-graben systems that
are presently exposed in the foreland. Removal of the sedimentary cover
started in the late Early Cretaceous. This was coeval with an increase of
the convergence rate and a switch to a westward absolute motion of the
South American Plate that was accompanied by shallowing of the subducting
slab. Unroofing was probably further enhanced by Late Cretaceous to early
Paleogene opening of a slab window beneath the overriding plate. Following
a tectonically quiescent period, renewed exhumation occurred in the orogen
during relatively fast Neogene plate convergence. However, even the highly
sensitive apatite (U-Th)/He thermochronometer does not record any coeval
cooling in the foreland. The comparison between Late Cretaceous and Neogene
exhumation patterns provides clear evidence of the fundamental role played
by inter-plate coupling associated with shallow slab configurations in
controlling plate-scale deformation. Our results, besides highlighting for
the first time how the whole northern Patagonia foreland was affected by an
exhumation of several kilometers since the Late Cretaceous, provide
unrivalled evidence of the link between deep geodynamic processes affecting
the slab and the modes and timing of unroofing of different sectors of the
overriding plate.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35767.1/595424/The-role-of-slab-geometry-in-the-exhumation-of
Flow of Devonian anatectic crust in the accretionary Altai Orogenic
Belt, central Asia: Insights into horizontal and vertical magma
transfer
Sheng Wang; Yingde Jiang; Roberto Weinberg; Karel Schulmann; Jian Zhang ...
Abstract:
Flow of partially molten crust is a key contributor to mass and heat
redistribution within orogenic systems, however, this process has not yet
been fully understood in accretionary orogens. This issue is addressed in a
Devonian migmatite-granite complex from the Chinese Altai through
structural, petrological, and geochronological investigations presented in
this study. The migmatite-granite complex records a gradual evolution from
metatexite, diatexite to granite and preserves a record of two main
Devonian phases of deformation designated D1 and D2. The D1 phase was
subdivided into an early crustal thickening episode (D1B) and a
later extensional episode (D1M) followed by D2 upright folding.
The D1M episode is associated with anatexis in the deep crust.
Vertical shortening, associated with D1M, gave rise to the
segregation of melt and formation of a sub-horizontal layering of stromatic
metatexite. This fabric was reworked by the D2 deformation associated with
the migration of anatectic magma in the cores of F2 antiforms.
Geochronological investigations combined with petro-structural analysis
reveal that: (1) D1M partial melting started probably at 420−410
Ma and formed sub-horizontal stromatic metatexites at ∼30 km depth; (2) The
anatectic magma accumulated and migrated when a drainage network developed,
as attested by the pervasive formation of massive diatexite migmatites, at
410−400 Ma; (3) Soon after, massive flow of the partially molten crust from
orogenic lower to orogenic upper crustal levels, assisted by the interplay
between D2 upright folding and magma diapirism, led to migmatite-granite
emplacement in the cores of regional F2 antiforms that lasted until at
least 390 Ma; (4) a terminal stage was manifested by the emplacement of
370−360 Ma granite dykes into the surrounding metamorphic envelope. We
propose that Devonian anatexis assisted by deformation governed first the
horizontal and then the vertical flow of partially molten orogenic lower
crust, which drove crustal flow, mass redistribution, and crustal
differentiation in the accretionary system of the Chinese Altai.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35645.1/595395/Flow-of-Devonian-anatectic-crust-in-the
Three stages of arc migration in the Carboniferous-Triassic in northern
Qiangtang, central Tibet, China: Ridge subduction and asynchronous slab
rollback of the Jinsha Paleotethys
Yin Liu; Wenjiao Xiao; Brian F. Windley; Kefa Zhou; Rongshe Li ...
Abstract:
Carboniferous-Triassic magmatism in northern Qiangtang, central Tibet,
China, played a key role in the evolution of the Tibetan Plateau yet
remains a subject of intense debate. New geochronological and geochemical
data from adakitic, Nb-enriched, and normal arc magmatic rocks, integrated
with results from previous studies, enable us to determine the
Carboniferous-Triassic (312−205 Ma), arc-related, plutonic-volcanic rocks
in northern Qiangtang. Spatial-temporal relationships reveal three periods
of younging including southward (312−252 Ma), rapid northward (249−237 Ma),
and normal northward (234−205 Ma) migrations that correspond to distinct
slab geodynamic processes including continentward slab shallowing, rapid
trenchward slab rollback, and normal trenchward rollback of the Jinsha
Paleotethys rather than the Longmuco-Shuanghu Paleotethys, respectively.
Moreover, varying degrees of coexistence of adakites/High-Mg andesites
(HMAs)/Nb-enriched basalt-andesites (NEBs) and intraplate basalts in the
above-mentioned stages is consistent with the magmatic effects of slab
window triggered by ridge subduction, which probably started since the Late
Carboniferous and continued into the Late Triassic. The
Carboniferous-Triassic multiple magmatic migrations and ridge-subduction
scenarios provide new insight into the geodynamic processes of the Jinsha
Paleotethys and the growth mechanism of the Tibetan Plateau.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35906.1/595412/Three-stages-of-arc-migration-in-the-Carboniferous
Magnetostratigraphic study of a Late Cretaceous−Paleogene succession in
the eastern Xining basin, NE Tibet: Constraint on the timing of major
tectonic events in response to the India-Eurasia collision
Chi-Cheng He; Yue-Qiao Zhang; Shao-Kai Li; Kai Wang; Jian-Qing Ji
Abstract:
Cretaceous-Cenozoic basins developed in the NE Tibetan Plateau contain key
archives to unravel the growth history of the plateau in response to the
India-Eurasia collision. Here we present magnetostratigraphic results of a
Late Cretaceous to Paleogene succession of the Zhongba section outcropping
at the southern margin of the eastern Xining basin. This succession
consists of three lithological units punctuated by two stratigraphic
unconformities, which best recorded the deformation history of this
foreland basin. Detailed magnetostratigraphic investigation show that the
lower terrestrial sedimentary rock unit, the Minhe Group, was deposited in
latest Cretaceous in the time span of ca. 74.5−69.2 Ma; the middle unit was
deposited in Paleogene in the time span of ca. 49.3−22 Ma; and the upper
conglomeratic unit, not dated, possibly was deposited in early Miocene.
Accordingly, the Cretaceous−Paleogene unconformity, widely observed in the
foreland basins of NE Tibet, represents a sedimentary hiatus duration of
∼19.9 m.y., from ca. 69.2 Ma to ca. 49.3 Ma, which possibly recorded the
far-field response to the tectonic transition from Neo-Tethys oceanic plate
subduction to the India-Eurasia collision in southern Tibet. Changes in
provenance, sedimentary accumulation rate, and mean susceptibility value at
ca. 33−30 Ma, and the total prolate anisotropy of magnetic susceptibility
(AMS) ellipsoids and provenance shifting since ca. 23−19 Ma, point to the
pulsed growth of West Qinling, and rapid uplift of Laji Shan, respectively,
indicating an enhanced effect of the India-Eurasia collision in Oligocene
and early Miocene. AMS results show a clockwise rotation of the shortening
direction from NEN-SWS in latest Cretaceous to NE-SW in Paleogene.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35874.1/595413/Magnetostratigraphic-study-of-a-Late-Cretaceous
Temperatures of Late Cretaceous (Campanian) methane-derived authigenic
carbonates from the Western Interior Seaway, South Dakota, USA, using
clumped isotopes
Yang Gao; Gregory A. Henkes; J. Kirk Cochran; Neil H. Landman
Abstract:
Methane seep deposits, comprising large, carbonate-rich mounds formed from
hydrocarbon seepage, were widely distributed in the Late Cretaceous Western
Interior Seaway (WIS) of North America. Well-preserved, methane-derived
authigenic carbonates (MDACs) from these deposits have been shown to retain
petrological, paleontological, and geochemical imprints of their ancient
depositional setting, all of which are important for understanding the
dynamics and evolution of the shallow, epeiric WIS. To better characterize
the environmental conditions of WIS seeps, we applied clumped isotope
paleothermometry to magnesium calcite MDAC samples from five seep
localities in the upper Campanian Pierre Shale, South Dakota, USA. We
measured 21 subsamples, including 18 micritic carbonates and demonstrated
apparent clumped isotope equilibrium between MDACs and WIS bottom waters.
Extreme 13C depletion in most samples (δ13C ranging
to −45.44‰) indicates they were precipitated with oxidized methane as a
major source of dissolved inorganic carbon, which itself implies a close
association with ancient methanotrophic metabolism. The average clumped
isotope paleotemperature from the micritic carbonates is 23 ± 7 °C (1σ
standard deviation), which agrees with bottom water paleotemperatures
inferred from δ18O measurements of MDACs and well-preserved
mollusk shells at similar localities in the WIS. The calculated average δ 18Ow value for these samples is −0.5 ± 1.7‰ (1σ SD),
which is indistinguishable from previously reported calculation on
Campanian seawater δ18Ow from fossil mollusk shells,
but elevated above younger fossils collected from other locations in the
WIS. Our conclusions are inconsistent with previously hypothesized
disequilibrium for WIS MDAC clumped isotope and therefore we propose that
fossil MDAC deposits may be used as paleotemperature archives.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35846.1/595396/Temperatures-of-Late-Cretaceous-Campanian-methane
Autogenic translation and counter point bar deposition in meandering
rivers
Z. Sylvester; P.R. Durkin; S.M. Hubbard; D. Mohrig
Abstract:
Although it has long been recognized that deposition along meandering
rivers is not restricted to convex banks (i.e., point bars), the consensus
is that sediment deposition on concave banks of channel bends mostly occurs
when meander bends translate downstream because erosion-resistant barriers
inhibit their lateral migration. Using a kinematic model of channel
meandering and time lapse satellite imagery from the Mamoré River in
Bolivia, we show that downstream translation and associated concave bank
deposition are essential, autogenic parts of the meandering process, and
resulting counter point bars are expected to be present whenever
perturbations such as bend cutoffs and channel reoccupations create short
bends with high curvatures. The implication is that zones of concave bank
deposition with lower topography, finer-grained sediment, slack water, and
riparian vegetation that differs from point bars are more common than
previously considered.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35829.1/595343/Autogenic-translation-and-counter-point-bar
Stratigraphy, age, and provenance of the Eocene Chumstick basin,
Washington Cascades; implications for paleogeography, regional
tectonics, and development of strike-slip basins
Erin E. Donaghy; Paul J. Umhoefer; Michael P. Eddy; Robert B. Miller;
Taylor LaCasse
Abstract:
Strike-slip faults form in a wide variety of tectonic settings and are a
first-order control on the geometry and sediment accumulation patterns in
adjacent sedimentary basins. Although the structural and depositional
architecture of strike-slip basins is well documented, few studies of
strike-slip basins have integrated depositional age, lithofacies, and
provenance control within this context. The Chumstick basin formed in
central Washington during a regional phase of dextral, strike-slip faulting
and episodic magmatism associated with Paleogene ridge-trench interaction
along the North America margin. The basin is bounded and subdivided by
major strike-slip faults that were active during deposition of the
intra-basinal, non-marine Chumstick Formation. We build on the existing
stratigraphy and present new, detailed lithofacies mapping, conglomerate
clast counts (N = 16; n = 1429), and sandstone detrital zircon analyses (N
= 16; n = 1360) from the Chumstick Formation to document changes in
sediment provenance, routing, and deposition. These data allow us to
reconstruct regional Eocene paleo-drainage systems of Washington and Oregon
and suggest that drainage within the Chumstick basin fed a regional river
system that flowed to a forearc or marginal basin on the newly accreted
Siletzia terrane. More generally, excellent age control from five
interbedded tuffs and high sediment accumulation rates allow us to track
the evolving sedimentary system over the Formation’s ca. 4−5 m.y.
depositional history. This is the first time lithofacies and provenance
variations can be constrained at high temporal resolution (0.5−1.5 m.y.
scale) for an ancient strike-slip basin and permits a detailed
reconstruction of sediment routing pathways and depositional environments.
As a result, we can assess how varying sediment supply and accommodation
space affects the depositional architecture during strike-slip basin
evolution.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35738.1/595282/Stratigraphy-age-and-provenance-of-the-Eocene
Progressive spatial and temporal evolution of tectonic triggers and
metasomatized mantle lithosphere sources for orogenic gold
mineralization in a Triassic convergent margin: Kunlun-Qinling Orogen,
central China
Hesen Zhao; Qingfei Wang; David I. Groves; Jun Deng
Abstract:
Whether orogenic gold deposits formed from crustal or subcrustal sources is
debated, and their link to orogenic processes is ambiguous. Gold
mineralization in the Triassic East Kunlun−West Qinling Orogen, China,
displays a spatial zonation in terms of its ages and stable isotope
compositions. In the West Qinling segment, most gold deposits formed in a
back-arc setting at 220∼210 Ma during a collisional episode within late
slab rollback. These deposits have dominant δ34S of 5∼15‰ and δ 18Ofluid of 10∼14‰, whereas those formed in the
suture zone at 210∼170 Ma, during a post-collisional episode after slab
break-off, have lower δ34S of −5∼+5‰ and δ18O fluid of 6∼10‰. In the East Kunlun segment, those deposits that
formed in a continental-arc setting and its related suture zone at 240∼200
Ma, in collisional to post-collisional episodes associated with slab
break-off, have δ34S and δ18Ofluid values
that are essentially similar to those in the West Qinling suture. δ 34S values of ore sulfide separates and rims of zoned pyrites
that have mantle-like signatures, in contrast with crustal signatures of
host rocks, are indicative of subcrustal ore-fluid sources. The combined
chronological and stable isotope shifts are consistent with a model in
which ore fluids for gold mineralization in a back-arc setting were sourced
from mantle lithosphere that was metasomatized by subducted oceanic
sediment; whereas those in a continental-arc setting—including its suture
zone—were sourced from fluid derived from altered oceanic crust. This study
thus provides new insights into the complexity of orogenic gold systems in
evolving orogens.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35754.1/595219/Progressive-spatial-and-temporal-evolution-of
Late Mesozoic−Cenozoic cooling history of the northeastern Tibetan
Plateau and its foreland derived from low-temperature thermochronology
Chen Wu; Andrew V. Zuza; Jie Li; Peter J. Haproff; An Yin ...
Abstract:
The growth history and formation mechanisms of the Cenozoic Tibetan Plateau
are the subject of an intense debate with important implications for
understanding the kinematics and dynamics of large-scale intracontinental
deformation. Better constraints on the uplift and deformation history
across the northern plateau are necessary to address how the Tibetan
Plateau was constructed. To this end, we present updated field observations
coupled with low-temperature thermochronology from the Qaidam basin in the
south to the Qilian Shan foreland in the north. Our results show that the
region experienced a late Mesozoic cooling event that is interpreted as a
result of tectonic deformation prior to the India-Asia collision. Our
results also reveal the onset of renewed cooling in the Eocene in the
Qilian Shan region along the northern margin of the Tibetan Plateau, which
we interpret to indicate the timing of initial thrusting and plateau
formation along the plateau margin. The interpreted Eocene thrusting in the
Qilian Shan predates Cenozoic thrust belts to the south (e.g., the Eastern
Kunlun Range), which supports out-of-sequence rather than
northward-migrating thrust belt development. The early Cenozoic deformation
exploited the south-dipping early Paleozoic Qilian suture zone as indicated
by our field mapping and the existing geophysical data. In the Miocene,
strike-slip faulting was initiated along segments of the older Paleozoic
suture zones in northern Tibet, which led to the development of the Kunlun
and Haiyuan left-slip transpressional systems. Late Miocene deformation and
uplift of the Hexi corridor and Longshou Shan directly north of the Qilian
Shan thrust belt represent the most recent phase of outward plateau growth.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35879.1/595220/Late-Mesozoic-Cenozoic-cooling-history-of-the
Distinct responses of late Miocene eolian and lacustrine systems to
astronomical forcing in NE Tibet
Zhixiang Wang; Chunju Huang; David B. Kemp; Ze Zhang; Yu Sui
Abstract:
East Asian summer monsoon (EASM) and winter monsoon (EAWM) variability on
Journal
ACM SIGSAM Bulletin
