New GSA Bulletin Articles Published Online Ahead of Print in
November
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; hydrothermal
systems in the southeastern Canadian Cordillera; and the hyperarid Negev
desert, Israel. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Modern dextral strain controls active hydrothermal systems in the
southeastern Canadian Cordillera
Theron D. Finley; Stephen T. Johnston; Martyn J. Unsworth; Jonathan Banks;
Dinu-Ion Jonathan
Abstract:
Thermal springs in the southeastern Canadian Cordillera occur in
association with several major fault zones, which may permit deep
circulation of fluid through fractured reservoirs to depths greater than 2
km. Both the current stress field and the most recent kinematics of these
faults likely play a significant role in localizing hydrothermal upwellings
but are poorly resolved. In this paper, we present new data from structural
mapping along the Columbia River, Slocan Lake, Purcell Trench, Southern
Rocky Mountain Trench, and Redwall faults. Fault plane and slickenfiber
orientations measured in the field indicate a previously unidentified,
post-Eocene phase of dextral strike-slip kinematics on these faults, which
have historically been mapped primarily as Eocene extensional structures.
The NE−SW maximum principal stress required for these kinematics shares a
similar orientation to the present-day stress field derived from crustal
earthquake focal mechanisms, which suggests dextral slip may be a recent
and ongoing development that continues at the present time. On a regional
scale, there is a positive correlation between the locations of springs and
regions with elevated seismicity. At the local scale, geothermal upwellings
may be controlled by local zones of enhanced permeability including fault
intersections and strain-transfer zones. The identification of favorable
structural settings may enable discovery of hidden (a.k.a. blind)
geothermal energy resources.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36500.1/619110/Modern-dextral-strain-controls-active-hydrothermal
The grainsize of volcanic fall deposits: Spatial trends and physical
controls
Julia Eychenne; Samantha L. Engwell
Abstract:
Volcanic tephra fall deposits, which form during explosive eruptions, are
commonly characterized in terms of their thickness and grainsize. While
significant efforts have been undertaken to relate spatial trends in
thickness to plume dispersion processes, comparably few studies have
focused on understanding variations in grainsize. Yet, grainsize is a key
parameter providing insight into eruption dynamics, from magma
fragmentation to plume transport processes, and modulates the impacts of
tephra. Here, we present a set of grainsize data extracted from the
published record for 56 deposits that represent a range of eruption
intensities and magnitudes. We systematically analyze the deposits in terms
of modality (bimodal or unimodal grainsize distributions) and provide the
median particle diameter with distance from source for component
distribution modes. We found that bimodal fall deposits are formed by
eruptions with large amounts of fine particles (<100 µm) and that all
tephra-fall deposits show characteristic patterns of grainsize decay with
distance from source that can be related to eruption plume height and thus
intensity. The grainsize decay trends are also related to ash dispersion
and deposition processes such as individual particle settling versus
collective settling mechanisms. The maximum distance from source reached by
particles of different sizes is controlled by a combination of source and
transport processes. This data set provides insight into the preservation
potential of deposits of different grainsizes at varying distances from
their sources. Finally, we emphasize the importance of using grainsize
trends in combination with thickness trends to interpret tephra-fall
deposit records.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36275.1/619095/The-grainsize-of-volcanic-fall-deposits-Spatial
Origin of the Songpan-Garze Terrane on the northeastern Tibetan
Plateau, eastern segment of the Tethyan tectonic domain: A Middle
Permian−Early Triassic intracontinental rifting system
Songnan Liu; Yu Wang
Abstract:
The initiation of a continental rift or the birth of an oceanic basin, and
their long-term evolution, are challenges in the understanding of the
evolution of regional and global tectonics. Rifts develop within the
interior or margins of plates, essentially as extensional structures, and
they have been studied widely since the discovery of plate tectonics. The
eastern segment of the Paleo-Tethys traversed central Asia, its structural
features, timing, geological setting, and geodynamic mechanisms are poorly
understood. The Songpan-Garze Terrane (SGT) on the northeastern Tibetan
Plateau has been regarded as either a remnant of the Paleo-Tethys Ocean or
a continental rift basin. Different interpretations of the SGT lead to
various tectonic reconstructions between Gondwana and Laurasia. Here,
sedimentology, geochemistry, and geochronology have been applied to the
southern and northern parts of the SGT to reconstruct the initiation of a
Permian rift system. In this area, for the first time, field investigation
reveals that the SGT is characterized by a continuous succession of
Permian−Lower Triassic terrestrial-neritic deposits and ocean island basalt
(OIB)-type basalts, and that it developed as an intracontinental rift basin
in the Yangtze Block. Zircons from gabbros with OIB-type geochemical
signatures intruded in the southern SGT yield U-Pb ages of 263−254 Ma,
constraining the timing of the initial rifting. Proterozoic felsic rocks
collected from the southern SGT yield U-Pb ages of 761−865 Ma, and they are
petrologically and geochemically comparable to Neoproterozoic basement
rocks along the western margin of the Yangtze Block. The Middle−Late
Permian volcano-sedimentary sequences of the SGT and their geochemistry and
geochronology are comparable to rift sequences in the Panxi region of the
western Yangtze Block. Thus, it has been proposed that the SGT originated
as an intracontinental rift within the Yangtze Block during the Middle
Permian−Early Triassic, and that it continued to deepen as a rift basin
through the Middle−Late Triassic.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36468.1/619064/Origin-of-the-Songpan-Garze-Terrane-on-the
Late Mesozoic elevation history of the north Taihang Mountains, China:
Constraints from clumped isotope geochemistry
Yixiong Wen; Laiming Zhang; Yalin Li; Katharine W. Huntington; Tianjie Jin
...
Abstract:
A series of tectonic events during the late Mesozoic, including the
Yanshanian Orogeny and the Early Cretaceous tectonic extension, reshaped
the tectonic regime and landscape of North China (NC). During this period,
the Taihang Mountains (THM) became a prominent geomorphic and ecological
separation zone, which separates NC into regions with different landforms,
tectonic settings, and climatic regimes. The uplift history of the THM
during the late Mesozoic is of great interest, since it is critical to
constrain the geodynamic and regional climatic evolution of NC. In this
study, we report the first carbonate clumped isotope thermometry (Δ 47) data for paleosol carbonates from this region to constrain
the paleoelevation of the north THM during the late Mesozoic. Our result
indicates that the north THM was almost certainly 2.0 ± 0.8 km elevation at
ca. 160 Ma associated with crust thickening and shortening. After the
collapse of the eastern North China Craton (NCC), the north THM maintained
high elevation at ca. 113 Ma (2.7 ± 0.7 km). The findings suggest that the
present topographic features of NC may have been established in the Early
Cretaceous. Finally, our study suggests elevation decrease at the end of
the Early Cretaceous (ca. 110−100 Ma), which is possibly related to
isostatic adjustment caused by asthenosphere upwelling and partial crustal
thinning in the central NCC.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36364.1/619065/Late-Mesozoic-elevation-history-of-the-north
Cenozoic kinematic histories of the Tidding and Lohit thrusts in the
northern Indo-Burma Ranges: Implications for crustal thickening and
exhumation of Gangdese lower arc crust along the Indus-Yarlung suture
zone
Peter J. Haproff; Drew A. Levy; Andrew V. Zuza; Julian D. Hooker; Matthew
T. Heizler ...
Abstract:
Crustal thickening has been a key process of collision-induced Cenozoic
deformation along the Indus-Yarlung suture zone, yet the timing, geometric
relationships, and along-strike continuities of major thrusts, such as the
Great Counter thrust and Gangdese thrust, remain inadequately understood.
In this study, we present findings of geologic mapping and thermo- and
geochronologic, geochemical, microstructural, and geothermobarometric
analyses from the easternmost Indus-Yarlung suture zone exposed in the
northern Indo-Burma Ranges. Specifically, we investigate the Lohit and
Tidding thrust shear zones and their respective hanging wall rocks of the
Lohit Plutonic Complex and Tidding and Mayodia mélange complexes. Field
observations are consistent with ductile deformation concentrated along the
top-to-the-south Tidding thrust shear zone, which is in contrast to the
top-to-the-north Great Counter thrust at the same structural position to
the west. Upper amphibolite-facies metamorphism of mélange rocks at ∼9−10
kbar (∼34−39 km) occurred prior to ca. 36−30 Ma exhumation during slip
along the Tidding thrust shear zone. To the north, the ∼5-km-wide Lohit
thrust shear zone has a subvertical geometry and north-side-up kinematics.
Cretaceous arc granitoids of the Lohit Plutonic Complex were emplaced at
∼32−40 km depth in crust estimated to be ∼38−52 km thick at that time.
These rocks cooled from ca. 25 Ma to 10 Ma due to slip along the Lohit
thrust shear zone. We demonstrate that the Lohit thrust shear zone,
Gangdese thrust, and Yarlung-Tsangpo Canyon thrust have comparable hanging
wall and footwall rocks, structural geometries, kinematics, and timing.
Based on these similarities, we interpret that these thrusts formed
segments of a laterally continuous thrust system, which served as the
preeminent crustal thickening structure along the Neotethys-southern Lhasa
terrane margin and exhumed Gangdese lower arc crust in Oligocene−Miocene
time.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36323.1/618939/Cenozoic-kinematic-histories-of-the-Tidding-and
Aspect-dependent bedrock weathering, cliff retreat, and cliff
morphology in a hyperarid environment
Yuval Shmilovitz; Yehouda Enzel; Efrat Morin; Moshe Armon; Ari Matmon ...
Abstract:
Deciphering aspect-related hillslope asymmetry can enhance our
understanding of the influence of climate on Earth’s surface morphology and
the linkage between topographic morphology and erosion processes. Although
hillslope asymmetry is documented worldwide, the role of microclimatic
factors in the evolution of dryland cliffs has received little attention.
Here, we address this gap by quantifying aspect-dependent bedrock
weathering, slope-rill morphology, and sub-cliff clast transport rates in
the hyperarid Negev desert, Israel, based on light detection and ranging
(LiDAR)-derived topography, clast-size measurements, and cosmogenic 10Be concentrations. Cliff retreat rates were evaluated using
extrapolated profiles from dated talus flatirons and 10Be
measurements from the cliff face and sub-cliff sediments. We document
systematic, aspect-dependent patterns of south-facing slopes being less
steep and finer-grained relative to east- and north-facing aspects. In
addition, cliff retreat and clast transport rates on slopes of the
south-facing aspect are faster compared to the other aspects. Our data
demonstrate that bedrock weathering of the cliff face and the corresponding
grain size of cliff-derived clasts delivered to the slopes constitute a
first-order control on cliff retreat and sediment transport rates. We
demonstrate that the morphology of the cliff and the pattern of bedrock
weathering co-vary with the solar radiation flux and hence that cliff
evolution in hyperarid regions is modulated by aspect-dependent solar
radiation. These results help to better understand interactions between
climate and dryland surface processes.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36442.1/618876/Aspect-dependent-bedrock-weathering-cliff-retreat
Late Cretaceous−early Paleogene rise of the Gangdese magmatic arc
(south Tibet) from sea level to high mountains
Qiang Xu; Lin Ding; Yu Cao; Fulong Cai; Shun Li ...
Abstract:
Recognition of the existence of an Andean-type continental margin in
southern Tibet prior to its collision with India has provided crucial
constraints on the formation of the Tibetan Plateau and South Asian climate
evolution. Here, we focused on well-dated Late Cretaceous successions in
the Linzhou Basin and determined the elevation changes from sea level to
high mountains in the Gangdese magmatic arc in southern Tibet. Our results
show that the Linzhou Basin was still submerged in the Tethyan Sea at
around 92 Ma when it accumulated Cenomanian (ca. 105−92 Ma) shallow-marine
orbitolinid-bearing limestones (Takena Formation); these limestones are
unconformably overlain by Campanian (83−78 Ma) fluvial-lacustrine deposits
(Shexing Formation) after an ∼9 m.y. depositional hiatus. A prominent
unconformity between the tightly folded Shexing Formation and gently tilted
overlying Paleocene−Eocene Linzizong successions represents the formation
of the regional Lhasaplano ca. 70 Ma, which may have been linked to the
rise of an Andean-type Gangdese mountain range along the southern margin of
the Lhasa terrane. Using carbonate oxygen isotope and clumped isotope
thermometry data from the Shexing Formation paleosols, we quantitatively
documented the rise of the Andean-type Gangdese Mountains with a peneplain
surface at an elevation of 2.7 +0.5/−0.9 km above sea level prior to onset
of the India-Asia collision ca. 65−63 Ma. This scenario of surface gain may
have been an isostatic compensation response to the crust thickening to
over 50 km during the Late Cretaceous. Subsequently, the surface
isostatically rose to its near-present elevation of ∼4.6 km due to the
removal of overthickened mantle lithosphere and revival of intense Gangdese
magmatism by 56 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36438.1/618865/Late-Cretaceous-early-Paleogene-rise-of-the
Growth of early Paleozoic continental crust linked to the Proto-Tethys
subduction and continental collision in the East Kunlun Orogen,
northern Tibetan Plateau
Lebing Fu; Leon Bagas; Junhao Wei; Yao Chen; Jiajie Chen ...
Abstract:
The East Kunlun Orogen (EKO) in the northern Tibetan Plateau records two
continental collisional orogenic events and magmatism in early Paleozoic
and early Mesozoic. However, possible magmatic additions to the continental
crust growth of the EKO in different tectonic stages of early Paleozoic
collisional orogeny have been overlooked. Three phases of early Paleozoic
plutons from the Xiangride-Kuhai area in the east of the EKO have been
chosen for detailed investigation and the results are reported here. The
oldest magmatic suite (Stage 1) includes the ca. 471 Ma Qurelong
Monzodiorite and ca. 454 Ma granodiorite in the Zhiyu Intrusive Complex.
The monzodiorite has a sanukitoid-like composition with high TiO 2 and Y contents and is interpreted as being derived from
partial melting of metasomatized mantle wedge lherzolite. The granodiorite
is typified by its high SiO2 content, high Sr/Y ratio, and
depleted Hf isotope, and is interpreted as an adakite-like melt derived
from the melting of a subducted Proto-Tethys oceanic crust. The magmatism
can be linked to northward subduction of the Proto-Tethys Ocean between 520
and 450 Ma. Stage 2 magmatism is represented by a plutonic suite emplaced
during ca. 450−431 Ma with an I-type granitic composition. Of these, the
ca. 447 Ma Kengdenongshe Intrusion composed of peraluminous granite with
enriched Nd-Hf isotopes is indicative of a Mesoproterozoic igneous source
in the orogen. The ca. 450−434 Ma monzogranite and granodiorite in the
Walega and Zhiyu intrusive complexes exhibit variable element and isotope
compositions. They would have been generated by magma mixing of felsic
melts from the old crust and mafic magmas derived from the metasomatized
lithospheric mantle, with a mafic melt proportion of >30%. The ca. 431
Ma quartz diorite in the Walega Intrusive Complex is formed through crustal
assimilation and fractional crystallization of mafic magmas derived from
the metasomatized lithospheric mantle, with a mafic melt proportion
>60%. Stage 2 suite was emplaced during the closure of Proto-Tethys
oceanic branches and subsequent continental collision during 450−426 Ma.
Magmatism diminished between ca. 426 and 410 Ma during exhumation of the
continental lithosphere as indicated by the presence of retrograde
eclogites in the EKO. Stage 3 magmatic suite includes the ca. 408 Ma
Langmuri Intrusion and ca. 403 Ma Niantang Syenogranite. These plutons are
adakite-like or have an A-type granitic composition and are enriched in
Nd-Hf isotopes. They might have been derived from the remelting of old and
juvenile continental crust in a post-collisional extensional setting during
410−390 Ma. Identification of partial melts, derived from the subducted
Proto-Tethys oceanic crust and metasomatized lithospheric mantle in stage 1
and 2 plutons, show that the subcrustal materials have been significantly
transferred to the overlying continental crust. Hence the magmatism in
oceanic subduction (Stage 1) and continental collision (Stage 2) settings
contributes to the early Paleozoic juvenile continental crust growth of the
EKO. The post-collisional extensional setting (Stage 3) is dominated by the
reworking of a pre-existing continental crust. The early Paleozoic
continental crust growth processes in the EKO are different from the
previous view in which the continental collision orogens have no crust
growth, and inconsistent with the proposal that crust growth is significant
only in a continental collision setting.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36292.1/618866/Growth-of-early-Paleozoic-continental-crust-linked
Termination of the Paleo-Asian Ocean in the Beishan orogen, NW China:
Constraints from detrital zircon U-Pb age and Hf isotope analysis of
turbidites
Qian-Qian Guo; Sun-Lin Chung; Hao-Yang Lee; Wen-Jiao Xiao; Quan-Lin Hou
Abstract:
The termination of the Paleo-Asian Ocean, including duration and
architecture, is controversial and has led to different reconstructions of
the Central Asian orogenic belt. Deep-marine turbidites are of great
significance in constraining the evolution of the oceanic basin. The
youngest marine turbidites in the Beishan orogen in the central part of the
southern Central Asian orogenic belt are separated by the Liuyuan mélange
into two sections, the South turbidites and North turbidites. Detrital
zircon grains from South turbidites yielded a maximum depositional age of
255 Ma. Those from North turbidites yielded a maximum depositional age of
254 Ma. Most of the Precambrian zircons of South turbidites have
concentrated εHf(t) values (−6 to −2.6), whereas those
of North turbidites range from −12.6 to +10.4, indicating different
provenances. The North turbidites also have more positive whole-rock ε Nd(t) values (−0.5 to +1.1) than the South turbidites
(−4.2 to −2.4), suggesting more juvenile or less evolved crustal components
in the source. Detailed comparison of the detrital zircon ages and Hf-Nd
isotopic characteristics with igneous and metamorphic rocks in the south
Beishan area distinguishes two sources. These findings constrain the
Liuyuan suture as the latest suture in the Beishan region, occurring no
earlier than 254 Ma. The youngest peak age of detrital zircons of the
youngest marine sedimentary rocks in the southern Central Asian orogenic
belt becomes younger eastward from 288 Ma to 247 Ma, characterizing the
closure duration of the Paleo-Asian Ocean from west to east in ∼40 m.y.
This study finds that the youngest turbidites in the Beishan orogen not
only record sedimentary processes in response to the final episode of
orogenic evolution in the southern Central Asian orogenic belt, but they
also constrain how and when an archipelago-type accretionary orogen
terminated.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36455.1/618853/Termination-of-the-Paleo-Asian-Ocean-in-the
Tracing decarbonated eclogite in the mantle sources of Tarim
continental flood basalts using Zn isotopes
Zhenchao Wang; Zhaochong Zhang; Marc K. Reichow; Wei Tian; Weiliang Kong
...
Abstract:
Recycled oceanic crust is believed to have played an important role in the
formation of continental flood basalts, whereas the involvement of large
amounts of CO2 derived from recycled marine carbonate in their
mantle sources is highly debated. Zn isotopes have great potential to trace
recycled carbonate due to the distinctly different δ66Zn values
between marine carbonates and the mantle. Representative continental flood
basalt samples from Keping (Group1) and Yingmai and Shengli (Group 2) in
Tarim, NW China, were collected to investigate their mantle sources, and
their Zn isotopes were studied systematically for the first time. The Zn
isotope values of Keping basalts (between 0.29‰ ± 0.03‰ and 0.32‰ ± 0.05‰)
are higher than values in the primitive mantle (δ66Zn = 0.16‰ ±
0.06‰) but similar to those of mid-ocean-ridge basalt (MORB; δ66
Zn = 0.24‰−0.31‰). Considering their high fractionation (e.g., Mg# =
0.28−0.37) and potential involvement of crustal contamination (87Sr/86Sri between 0.70720 and 0.70779; ε Nd between −3.2 and −1.9), their Zn isotopes may not
conclusively point to a carbonated mantle source. In contrast, Yingmai and
Shengli basalts show heavy Zn isotope values (between 0.32‰ ± 0.03‰ and
0.39‰ ± 0.03‰), nonradiogenic 87Sr/86Sri
(0.70459‒0.70518) and εNd(t)
(between −1.3 and 0.1) signatures, and less fractionation (Mg# =
0.46−0.71), implying the involvement of recycled carbonate components in
their mantle source. Nonetheless, the lack of negative Zr, Hf, and Ti
anomalies, low CaO/Al2O3 ratios, and high SiO 2 contents preclude direct melting of carbonate-bearing mantle.
Alternatively, these features may suggest the melting of decarbonated
subducted eclogite and variable interaction with subsolidus peridotite.
This assumption is consistent with the positive correlations of δ 66Zn with Gd/Yb and Sm/Yb ratios and Zn content as a result of
mixing between eclogite-derived high-δ66Zn melt and
peridotite-derived mantle-like δ66Zn melt. Our study provides a
new model to reconcile the geochemical features of the Tarim continental
flood basalts and highlights the potential role of decarbonated eclogite in
the formation of continental flood basalts.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36502.1/618854/Tracing-decarbonated-eclogite-in-the-mantle
Geochemistry of the Cretaceous Mowry Shale in the Wind River Basin,
Wyoming
Katherine L. French; Justin E. Birdwell; Paul G. Lillis
Abstract:
The siliceous nature of the Mowry Shale distinguishes it from many of the
well-studied organic-rich mudstones of the Cretaceous Western Interior
Seaway. Available models of organic enrichment in mudstones rarely
incorporate detailed biomarker, bulk organic, inorganic, and mineralogy
data. Here, we used these data to evaluate how variations in organic matter
source, productivity, dilution, and preservation modulated organic matter
accumulation during the deposition of the Mowry Shale, while also
demonstrating the benefits of this integrated approach. An organic stable
carbon isotope vertical profile for the Mowry Shale is presented to test
whether the Mowry Shale was deposited during oceanic anoxic event 1d (OAE
1d), thereby contributing to organic enrichment in the Mowry Shale. Samples
were analyzed from three thermally immature drill cores collected from the
Wind River Basin, Wyoming to advance our understanding of the conditions
that led to the formation of the Mowry Shale. Results from bulk organic and
inorganic geochemistry and biomarkers show that redox-driven preservation
is closely coupled with stratigraphic changes in organic matter content in
intermediate to distal settings of the Mowry Shale. Biogenic silica
dilution decouples productivity and organic enrichment, such that the
interval that was deposited during the highest productivity is offset from
the interval containing maximum organic content. These findings contrast
with previous studies that identified productivity as the primary driver of
organic enrichment in distal settings of the Mowry Shale. The middle and
lower Mowry Shale contain primarily marine organic matter, whereas the
relative contribution of terrestrial organic matter increases in the upper
Mowry Shale and overlying Frontier Formation as relative sea level declined
and the western shoreline prograded eastward. Finally, the organic stable
carbon isotope profile of the Mowry Shale together with recent
radioisotopic dating suggest that the Mowry Shale postdates OAE 1d by ∼1−2
million years (m.y.), providing another example of temporal offset between
organic matter accumulation in the Cretaceous Western Interior Seaway
compared to other regions that record global OAEs.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36382.1/618835/Geochemistry-of-the-Cretaceous-Mowry-Shale-in-the
Cenozoic intraplate volcanism in central Asia: Mantle upwelling induced
by India-Eurasia collision
Jian Wang; Yuping Su; Jianping Zheng; Hongkun Dai; Gongjian Tang ...
Abstract:
Most of Earth’s volcanism occurs at tectonic plate boundaries associated
with subduction or rifting processes. The mantle plume hypothesis is an
important supplement to plate tectonics for explaining some high-volume
intraplate volcanic fields. However, many intraplate magmatic provinces
occur as low-volume, monogenetic basaltic-suite fields that are neither
associated with plate-boundary processes nor attributable to mantle plumes,
and the origin of such magmatism has long been debated. Identification of
their source characteristics and possible mechanisms that trigger mantle
melting will provide essential insights into Earth’s mantle heterogeneity
and also develop our knowledge of tectonic plate movement through time.
Here, we report new geochronology, mineral chemistry (especially olivine),
and whole-rock chemical and Sr-Nd-Pb-Hf isotopic compositions on Cenozoic
intracontinental alkaline basalts from the northwestern Tarim craton
(central Asia), aiming to better assess the origin of Earth’s low-volume
effusive intraplate volcanic fields. The basalts (ca. 42 Ma) have olivine
(e.g., mean Ni abundances of ∼2250 ppm, mean Mn/Zn ratios of 13.7) and
whole-rock chemistry consistent with their derivation from a mixed
peridotite-pyroxenite source. Moderately depleted Sr-Nd-Pb-Hf isotopes ( 87Sr/86Sr = 0.7039−0.7053; εNd = +4.0 to
+5.5; 206Pb/204Pb = 18.247−18.535; εHf =
+8.1 to +8.7) require a young (ca. 500 Ma) oceanic crust recycled into the
source, possibly related to subduction events during the assembly of
Pangea. Estimated thermal-chemical conditions indicate that the original
melting occurred in a relatively dry (H2O = 1.4 ± 0.9 wt%) and
reduced (logfO2 ΔFMQ = −0.97 ± 0.21, where FMQ is
fayalite-magnetite-quartz) asthenosphere under a mantle potential
temperature of ∼1420 °C and a pressure of ∼3.7 GPa (corresponding to a
depth of ∼120 km). Combining these data with regional tectonic history and
geophysical data (high-resolution P-wave tomography), we propose that the
long-lasting India-Eurasia collision triggered asthenospheric upwelling,
focusing melts along translithospheric zones of weakness; this model
provides a robust explanation for the observed Cenozoic intracontinental
volcanism in central Asia. The integrated geochemical and geophysical
evidence reveals that plate subduction−induced mantle upwelling represents
a likely mechanism for the generation of many regions of plume-absent
intraplate magmatism within continents.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36545.1/618836/Cenozoic-intraplate-volcanism-in-central-Asia
Lithospheric extension of the accretionary wedge: An example from the
Lanling high-pressure metamorphic terrane in Central Qiangtang, Tibet
Xiao Liang; Genhou Wang; Wentao Cao; Marnie A. Forster; Gordon S. Lister
...
Abstract:
Deciphering the exhumation mechanism of high-pressure−low-temperature
(HP-LT) metamorphic rocks can provide important insights into the tectonic
evolution of oceanic subduction zones at active continental margins. Here,
we present a multidisciplinary study examining the exhumation tectonics of
the Permo−Triassic eclogite-bearing HP-LT terranes of the Central Qiangtang
metamorphic belt (CQMB) in the central Tibetan Plateau. Field geological
relations and microscopic observations show that the HP-LT rocks in the
Lanling area are separated from the epimetamorphic late Paleozoic ophiolite
mélange of the hanging wall by low-angle detachment faults and primarily
exhibit five stages of deformation. The HP-LT terranes were exhumed as a
metamorphic core complex and show pervasive synexhumation top-to-the-SW and
-S shear structures. The results of the petrological and mineralogical
analysis and pseudosection modeling of retrograde eclogites indicate that
these rocks are characterized by synexhumation mineral growth pulses with
hairpin-type decompressional pressure-temperature trajectories. A
compilation of previous geochronological data and our 40Ar/ 39Ar dating results of synexhumation minerals and shear bands in
HP-LT rocks indicate continuous exhumation at ca. 244−215 Ma prior to the
continental collision between the North Qiangtang Block (NQB) and South
Qiangtang Block. Moreover, the CQMB is likely an autochthonous accretionary
wedge resulting from northward subduction of the Paleo-Tethys Ocean beneath
the NQB. Combined with ca. 243−230 Ma mantle upwelling to the north of
Lanling, we infer that the CQMB, namely, the accretionary wedge of the
Paleo-Tethys Ocean, experienced Middle−Late Triassic lithospheric extension
in the NW-SE direction.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36476.1/618837/Lithospheric-extension-of-the-accretionary-wedge
Enhanced terrestrial nutrient release during the Devonian emergence and
expansion of forests: Evidence from lacustrine phosphorus and
geochemical records
Matthew S. Smart; Gabriel Filippelli; William P. Gilhooly III; John E.A.
Marshall; Jessica H. Whiteside
Abstract:
The evolution of land plant root systems occurred stepwise throughout the
Devonian, with the first evidence of complex root systems appearing in the
mid-Givetian. This biological innovation provided an enhanced pathway for
the transfer of terrestrial phosphorus (P) to the marine system via
weathering and erosion. This enhancement is consistent with paleosol
records and has led to hypotheses about the causes of marine eutrophication
and mass extinctions during the Devonian. To gain insight into the
transport of P between terrestrial and marine domains, we report
geochemical records from a survey of Middle and Late Devonian lacustrine
and near-lacustrine sequences that span some of these key marine extinction
intervals. Root innovation is hypothesized to have enhanced P delivery, and
results from multiple Devonian sequences from Euramerica show evidence of a
net loss of P from terrestrial sources coincident with the appearance of
early progymnosperms. Evidence from multiple Middle to Late Devonian sites
in Greenland and northern Scotland/Orkney reveal a near-identical net loss
of P. Additionally, all sites are temporally proximal to one or more
Devonian extinction events, including precise correlation with the Kačák
extinction event and the two pulses associated with the Frasnian/Famennian
mass extinction. For all sites, weathering, climate, and redox proxy data,
coupled with nutrient input variability, reveal similar geochemical
responses as seen in extant lacustrine systems. Orbitally forced climatic
cyclicity appears to be the catalyst for all significant terrestrial
nutrient pulses, which suggests that expansion of terrestrial plants may be
tied to variations in regional and global climate.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36384.1/618814/Enhanced-terrestrial-nutrient-release-during-the
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