New GSA Bulletin Articles Published Ahead of Print in March
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics studied this
month include the nature and dynamics of China and Tibet; the setting and
process evolution of the central Newfoundland gold district; and observations
from Kīlauea Iki lava lake, Hawaii. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Impacts of late Miocene normal faulting on Yarlung Tsangpo River
evolution, southeastern Tibet
Tianyi Shen; Guocan Wang; Peter van der Beek; Matthias Bernet; Yue Chen ...
Abstract:
The Jiacha Gorge in southeastern Tibet is the second-largest deeply incised
gorge of the Yarlung-Tsangpo River, after the Tsangpo Gorge. A late
Cenozoic N-S−trending normal fault, the Woka Rift fault, coincides with the
western limit of the gorge. However, the relationship between the formation
of the gorge, drainage evolution, and rift activity remains unclear.
Analysis of the river long profile suggests that the Jiacha Gorge developed
as a ∼45-km-wide knickzone, rather than a local knickpoint. Projection of
tributary stream profiles indicate significantly deeper incision in the
gorge than in the downstream area, suggesting different controls on
incision. Thermochronological data collected along two age-elevation
profiles in the Woka Rift footwall and the Jiacha Gorge record rapid
cooling at ca. 12−10 Ma followed by moderate cooling between ca. 10 and 7
Ma, with additional accelerated cooling after ca. 5 Ma in the gorge. We
interpret late Miocene (ca. 12−10 Ma) rapid cooling to reflect the onset of
east-west extension and normal faulting along the Woka Rift, whereas the
Pliocene (post−ca. 5 Ma) accelerated cooling is inferred to be driven by
river incision in the Jiacha Gorge. Geomorphic and sedimentologic
observations suggest diversion of the Yarlung-Tsangpo River through the
Jiacha Gorge, from an earlier more southerly course, after the onset of
rifting. Therefore, we suggest that normal faulting of the Woka Rift
anchored the Jiacha Gorge knickzone on its shoulder, while footwall uplift
and drainage diversion led to enhanced incision, forming the steepest
channel upstream of the Tangpo Gorge along the Yarlung-Tsangpo River.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36210.1/612828/Impacts-of-late-Miocene-normal-faulting-on-Yarlung
Subduction initiation of the western Paleo-Asian Ocean linked to global
tectonic reorganization: Insights from Cambrian island-arc magmatism
within the West Junggar, NW China
Yunying Zhang; Min Sun; Jiyuan Yin; Chao Yuan; Zhen Sun ...
Abstract:
The subduction initiation associated with the beginning of accretionary
orogens has been thought to be related to global plate reorganization. To
characterize the initial subduction within the western Central Asian
Orogenic Belt, this integrated study focuses on Cambrian tholeiitic to
calc-alkaline plutons in the Barleik-Mayile-Saleinuohai area of West
Junggar, NW China. Zircon U-Pb results of felsic plutons reveal a wide
range (511−488 Ma) of ages with older ages up to 514−511 Ma. The felsic
rocks exhibit variable SiO2 (53.0−77.4 wt%) and K2O
(0.05−2.24 wt%) contents and can be classified as diorite, granodiorite,
trondhjemite, and tonalite. On the basis of their low TiO2
(0.12−0.71 wt%) contents and characteristic trace element trends as well as
high zircon εHf(t) (+10.5 to +14.5) and mantle-like zircon δ18O
(5.0 ± 0.48‰ to 5.4 ± 0.43‰, two standard deviations) values, we interpret
that the Cambrian felsic rocks have diverse origins, involving
differentiation of arc basalts and partial melting of subducted oceanic
crust, arc mafic crust, and metasomatized mantle wedge. The Saleinuohai
gabbroic pluton shows zircon δ18O ratios from 4.2 to 4.7‰, which
are lower than those of igneous zircons in equilibrium with mantle and thus
reflect modification of their mantle source by hydrothermal fluids with
seawater-like oxygen isotopes at high temperature. Combined with regional
data, we propose that the West Junggar arc represents the extending of the
Boshchekul-Chingiz arc in the Early Cambrian, defining a long (>1000 km)
E-W−trending subduction zone. The earliest island-arc tholeiitic felsic
plutons in the West Junggar took place at ca. 514−511 Ma, which, coupled
with other early subduction records (e.g., 530 Ma SSZ-type Kopu-relisay
ophiolites) in the western Paleo-Asian Ocean, indicates that initial stages
of subduction of the western Paleo-Asian Ocean probably occurred in the
Early Cambrian. The simultaneity between the initial subduction of the
western Paleo-Asian Ocean, Gondwana assembly, and Laurasia breakup suggests
a causal link between the three, collectively correlated to a global plate
adjustment event.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36304.1/612799/Subduction-initiation-of-the-western-Paleo-Asian
A composite Llandovery δ13Ccarb record from the Michigan
Basin, USA
Mohammed Al-Musawi; Stephen E. Kaczmarek; William B. Harrison, III; Peter
J. Voice; Jeffrey J. Kuglitsch ...
Abstract:
A new high-resolution, composite δ13Ccarb curve for
the Michigan Basin, USA, was constructed using two stratigraphically
overlapping subsurface cores that span the complete Llandovery interval.
The Llandovery succession of the Michigan Basin measures between 140 m and
265 m thick and is represented by three lithostratigraphic groups: the
Cataract (Manitoulin formation and Cabot Head Shale), the Burnt Bluff (Lime
Island, Byron, and Hendricks formations), and the Manistique (Schoolcraft
and Cordell formations). The composite δ13Ccarb curve
exhibits five positive carbon isotope excursions (CIEs). Conodont and 87Sr/86Sr data suggest that the CIEs correlate with
the Hirnantian, Early Aeronian, Late Aeronian, Valgu, and Ireviken global
excursions. The new composite δ13Ccarb curve from the
Michigan Basin represents the most stratigraphically complete, single-basin
Llandovery record worldwide. The composite δ13Ccarb
curve permits the ages of three Llandovery lithostratigraphic groups to be
chronostratigraphically determined within ca. 1 Ma resolution. The
composite δ13Ccarb curve also implies that the
Ordovician−Silurian boundary in the Michigan Basin is located at the top of
the Manitoulin formation and not at the base of the Manitoulin formation,
which is consistent with recent findings from Manitoulin Island, Ontario.
From the regional perspective, the high-resolution composite δ13
Ccarb curve provides a valuable tool for delineating the
depositional history of the Michigan Basin during the Llandovery. From a
global perspective, the new composite δ13Ccarb curve
sheds light on the dynamics of the global carbon cycle, particularly during
the Rhuddanian, which is rarely preserved in other carbonate basins
worldwide.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article/doi/10.1130/B36317.1/612800/A-composite-Llandovery-13Ccarb-record-from-the
Arc-continent collision during culmination of Proto-Tethyan Ocean
closure in the Central Qilian belt, NE Tibetan Plateau
Zhen Yan; Changlei Fu; Jonathan C. Aitchison; Manlan Niu; Solomon Buckman
...
Abstract:
Accurate lithostratigraphy framework of the Central Qilian belt is key to
understand subduction-collision of the Proto-Tethyan Ocean in the NE
Tibetan Plateau. The Dongchagou and the Moshigou formations share similar
detrital zircon age populations with youngest age peak at ca. 1.15−1.21 Ga,
which is consistent with the granitoid gneiss of the Liujiatai Formation.
In combination with rock assemblages, metamorphic facies, and associated
oldest granitoid plutons, we revised Precambrian lithostratigraphy of the
Central Qilian belt as the >1.2 Ga Huangyuan Group and the ca. 1200−930
Ma Huangzhong Group in this study. The former is the same as the Liujiatai
Formation, whereas the later includes the Dongchagou and Moshigou
formations. Siliciclastic turbidites of the Qingshipo Formation gradually
pass upward into carbonates of the Huashishan Group, we therefore ascribe
them to the Huashishan Group. This group unconformably overlies the
Precambrian basement of the Central Qilian belt and consists of continental
slope to coastal plain deposits with a bi-direction of the S- and N-ward
paleocurrent. These sediments contain abundant metamorphic, magmatic, and
minor ophiolitic detritus with main age populations of ca. 910 Ma, ca. 487
Ma, and ca. 447 Ma detrital zircons, probably derived from the Central and
South Qilian belts. The basement of the Huangzhong Group experienced
440−430 Ma continent-continent collision-related metamorphism and
deformation. These results and regional geology demonstrate that the
Huashishan Group was formed by arc-continent collision deposition during
culmination of the Proto-Tethyan Ocean closure in the Qilian orogenic belt,
NE Tibetan Plateau.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36328.1/612653/Arc-continent-collision-during-culmination-of
Melt surges, flow differentiation, and remobilization of crystal-rich
mushes in response to unloading: Observations from Kīlauea Iki lava
lake, Hawaii
Rosalind T. Helz
Abstract:
Field and laboratory studies of the 1959 Kīlauea Iki lava lake have
provided insight into differentiation processes in mafic magma chambers.
This paper explores how partially molten basaltic mushes responded to
unloading as a consequence of drilling. Most holes drilled from 1967 to
1979 terminated in a melt-rich internal differentiate with a sharp
crust-melt interface. These interfaces were not stable, so the boreholes
were backfilled by melt-rich (<5% crystal) ooze. This process, with melt
ascent rates of 1.3−4.2 m/s, occurred within minutes of intersecting the
bodies, mimicking volcanic eruptions, albeit on a small scale. One borehole
(KI79-1), which did not encounter such a discontinuity, was backfilled over
a period of 16 days by upward flow of crystal-rich mushes rather than
melt-rich ooze. The first interval of ooze recovered had undergone
extensive internal differentiation. Its most conspicuous feature was
production of melt-rich layers by lateral migration of interstitial melt
from the wallrock into the rising crystal-rich mush. In addition, two
smaller-scale processes occurred within the rising mush: segregation of
melt into discrete blebs within the rising mush column and aggregation of
groundmass crystals into crystal-rich clumps formed adjacent to coarser
olivine crystals. The upper parts of the ooze are enriched in melt relative
to deeper samples, which suggests that the melt blebs rose relative to
their olivine-rich matrix. Similar melt blebs and crystal-rich clumps are
observed in naturally occurring diapiric bodies within the lava lake. These
processes appear to be intrinsic to the upwelling of narrow cylindrical
mush bodies whether constrained within a borehole (like the oozes) or
unconstrained (as were the diapirs in the lava lake). The most striking
behavior observed during repeated reentry of KI79-1 was a sharp change in
rheology during the second and third re-entries of the borehole. The shift
in behavior observed was that the oozes rose up the borehole, with ascent
rates of 1.0−1.7 m/s, which are comparable to the rates of the crystal-poor
oozes from melt-rich internal differentiates. These oozes contain more melt
than the original core at equivalent depths, presumably because melt moved
relative to crystals down the pressure gradient created by the open
borehole. Groundmass textures in these inflated mushes show erosion of
crystal outlines, especially of grain-to-grain contacts between different
phases, so that the tenuous crystalline network observed in the original
core samples was replaced by rounded crystals in continuous melt at
crystallinities of 55−65 vol%. The transition from stable coherent mush to
inflatable mush occurred at 25−28 vol% melt. This behavior appears similar
to certain types of reactive transport observed in other studies.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36248.1/612593/Melt-surges-flow-differentiation-and
Tectonic burial of sedimentary rocks drives the building of juvenile
crust of magmatic arc
Sheng-Kai Qin; Ze-Ming Zhang; Richard M. Palin; Hui-Xia Ding; Xin Dong ...
Abstract:
Continental arcs grow primarily by addition of mantle-derived magmas, thus
forming juvenile crust, although geophysical evidence, alongside field
investigation of exhumed terranes, show that supracrustal rocks are common
components of the lower portions of continental arcs. The mechanisms by
which metasedimentary rocks are transported to the deep arc crusts and
their contributions to the juvenile arc crusts are ambiguous. Here, we
conduct a systematic petrological, geochronological, and geochemical study
of pelitic migmatites within Late Cretaceous meta-gabbros from the lower
crust of the eastern Gangdese arc, southern Tibet. Our results show that
the pelitic migmatites were derived from the Late Carboniferous sedimentary
rocks of the upper arc crust, have significantly enriched Sr-Nd-Hf-O
isotopic compositions, and underwent Late Cretaceous (95−80 Ma)
high-pressure granulite-facies metamorphism and partial melting at ∼850 °C
and 15 kbar to generate voluminous granitic melts. This indicates that the
eastern Gangdese arc underwent significant crustal thickening and chemical
differentiation during the final stages of subduction of the Neo-Tethys. We
suggest that the metasedimentary rocks were transported into the lower
crust of the Gangdese arc by underthrusting of arc crust and accretion of
mantle-derived magma, and the deeply buried supracrustal rocks altered the
petrological constitution and chemical compositions of juvenile lower arc
crust, and assimilated the mantle- and juvenile crust-derived melts. We
propose that tectonic burial of sedimentary rocks is a key mechanism
driving the building and reworking of juvenile crust of magmatic arcs
throughout most of geological time.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36271.1/612594/Tectonic-burial-of-sedimentary-rocks-drives-the
Large-scale, flat-lying mafic intrusions in the Baltican crust and
their influence on basement deformation during the Caledonian orogeny
Rodolphe Lescoutre; Bjarne Almqvist; Hemin Koyi; Théo Berthet; Peter Hedin
...
Abstract:
The Fennoscandian Shield in central Sweden displays a complex structural
and compositional architecture that is mainly related to the Proterozoic
history of the Baltica paleocontinent. In its western parts, the
Precambrian basement is covered by the allochthonous rocks of the
Caledonide orogen, and direct information about the underlying crust is
restricted to a few unevenly distributed basement windows in western Sweden
and Norway. In this study, we use preliminary results from the second
borehole of the Collisional Orogeny in the Scandinavian Caledonides project
(COSC-2), new gravity data, forward gravity, and magnetic modeling and
interpretation of seismic reflection profiles to assess the 3-D
architecture of the basement. Our results reveal a wide (∼100 km) and dense
network of mainly flat-lying and saucer-shaped dolerites intruding the
volcanic and granitic upper crustal rocks of the Transscandinavian Igneous
Belt. Similar intrusion geometries related to 1.2 Ga dolerites can be
recognized in the Fennoscandian Shield. We discuss that the formation of
these sill complexes occurred in a lithologically and structurally
heterogeneous crust during transtension, which is in disagreement with the
current understanding of sill emplacement that involves crustal shortening,
layering, or anisotropy of the host rock. Our seismic interpretation and
the structural observations from the COSC-2 drilling show that part of the
Caledonian-related basement deformation was localized along the margins of
the dolerite sheets. We propose that the dolerite intrusion geometry, akin
to a flat-ramp geometry, guided the basement deformation during the
Caledonian orogeny.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36202.1/612534/Large-scale-flat-lying-mafic-intrusions-in-the
Late Mesozoic intracontinental deformation and magmatism in the Chinese
Tianshan and adjacent areas, Central Asia
Fujun Wang; Meng Luo; Zhiyuan He; Rongfeng Ge; Yuanyuan Cao ...
Abstract:
The Tianshan Range−Junggar Basin−Kalamaili Range system represents the
southwestern Central Asian Orogenic Belt and is a natural laboratory for
studying intracontinental deformation processes. Its current topography is
a product of the far-field effects of the Cenozoic India-Asia collision.
However, the Mesozoic topographic and tectonic evolution of the Tianshan
and Kalamaili Ranges and their impacts on the Junggar Basin remain
enigmatic due to the scarcity of data. Here, we present a comprehensive
synthesis of sedimentological and geochronological data on these ranges and
adjacent basins to reconstruct the intracontinental evolution from the
Early Jurassic to the Early Cretaceous. Based on field observations and
seismic profile analysis, we identified several unconformities within the
late Mesozoic strata in the Tianshan Range and the Junggar Basin. Detrital
zircon U-Pb dating results for Lower Jurassic to Lower Cretaceous
sandstones of the eastern and southern Junggar Basin, with published
paleocurrent data, reveal a complex intracontinental topographic evolution.
Moreover, tuffaceous gravels and tuff samples yielded weighted mean zircon 206Pb/238U ages of 156.5 ± 3.2 Ma and 156.3 ± 2.2 Ma,
respectively, which indicates the presence of contemporary magmatic
activity. The deformation and magmatism mentioned above were possibly
related to multi-plate convergence in East Asia during the late Mesozoic.
This study provides new insights into the late Mesozoic tectonic-magmatic
evolution of the Tianshan Range and its adjacent areas.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36318.1/612535/Late-Mesozoic-intracontinental-deformation-and
Provenance and metamorphic records of forearc sediments from the
Heilongjiang Complex, NE China: Implications for subduction erosion
associated with the Mudanjiang Ocean
Jiahao Jing; Hao Yang; Wenchun Ge; Zheng Ji; Yanlong Zhang ...
Abstract:
Subduction erosion is widely thought to play a significant role in crustal
recycling at modern convergent plate margins. However, identifying
subduction erosion in fossil accretionary margins is difficult due to
multiple episodes of tectonic superimposition and an absence of direct
geophysical evidence. By assessing provenance and metamorphic records in
this study, we are the first to document the record of subduction erosion
in Late Triassic to Middle Jurassic meta-sedimentary rocks from the Luobei
Heilongjiang Complex, NE China. Most detrital zircons from four
meta-sedimentary samples have typical core-rim textures. Approximately 90%
of the detrital zircon cores yielded Phanerozoic ages with εHf
(t) values of +12.8 to −17.6. Geochronological and isotopic comparisons
indicate that the meta-sedimentary rocks have a provenance in the eastern
Songnen Block and were deposited in a forearc basin. The youngest ages of
detrital zircon cores span from 229 to 176 Ma, while the metamorphic ages
obtained from detrital zircon rims range from 209 to 185 Ma. Combined with
the 186−165 Ma phengite 40Ar-39Ar ages of the
meta-sedimentary rocks, this suggests that their protoliths were deposited
during the Late Triassic−Middle Jurassic. Mineral inclusions define peak
blueschist-facies metamorphism at pressure-temperature conditions of
0.9−1.0 GPa and 359−365 °C; this indicates that forearc sediment from the
overriding plate was abraded into the subduction channel and underwent
high-pressure metamorphism. Metamorphic ages (209−165 Ma) obtained from
phengite and detrital zircon rims suggest that a period of subduction
erosion occurred during slab subduction. These results reveal rapid
progression from early forearc sedimentation (229−165 Ma) to subduction
erosion (209−165 Ma) during westward subduction of the Mudanjiang Ocean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36218.1/612536/Provenance-and-metamorphic-records-of-forearc
Two magma fractionation paths for continental crust growth: Insights
from the adakite-like and normal-arc granites in the Ailaoshan fold
belt (SW Yunnan, China)
Jian Xu; Xiao-Ping Xia; Qiang Wang; Christopher J. Spencer; Chun-Kit Lai
...
Abstract:
Geochemical similarities between the continental crust and arc magmas have
led to the inference that subduction zones may be the primary sites of
crustal growth. Thus, it is necessary to unravel the petrogenetic
mechanism(s) of granitoid generation in subduction-related settings to
understand crustal growth through magmatic differentiation processes. In
this study, we focused on granitoid generation in oceanic-continental
subduction zones. We analyzed the whole-rock geochemistry and Sr-Nd
isotopes, together with zircon U-Pb-Hf-O isotopes, of the newly identified
Middle Triassic granitoids in the Ailaoshan high-grade metamorphic complex
(Yunnan, SW China). All the studied granite samples were characterized by
large ion lithophile element (e.g., Rb, Sr, and Ba) enrichments and high
field strength element (e.g., Nb, Ta, and Ti) depletions, similar to
arc-type rocks. They also showed a range of whole-rock Sr-Nd, (87Sr/86Sr)i = 0.7020−0.7048, εNd (t) = +0.6 to +4.2, and zircon
Hf-O, εHf(t) = +10.3 to +18.1, δ 18Ozircon = 5.09‰−6.65‰, isotope compositions, which
overlap with those of previously reported coeval (ca. 237−235 Ma)
hornblende diorite and granodiorite, the formation of which was interpreted
to have originated from a mantle wedge metasomatized by a sediment-derived
melt. Furthermore, the fractionation trends of some of the granitic samples
and diorite-granodiorite suite overlap. They can be divided into two
geochemical groups: Group 1 has intermediate to high SiO2
(66.9−73.8 wt%) and K2O (3.40−5.42 wt%) and low MgO (0.19−1.09
wt%) contents and shows depletion in heavy rare earth elements (HREEs;
e.g., Yb and Y), resulting in adakite-like high Sr/Y (61−183) and La/Yb
(47−90) ratios. Group 1 shows positive SiO2 versus Sr/Y and
La/Yb correlations and negative SiO2 versus HREE and Y
correlations, implying fractionation of a garnet-bearing assemblage. The
negative correlations between SiO2 and εNd( t) and Nb/La reveal a crustal assimilation
trend. Group 2 has relatively high SiO2 (72.6−76.5 wt%) and low
K2O (1.93−3.82 wt%) and MgO (0.05−0.83 wt%) contents and shows
depletion in middle REEs (MREEs; e.g., Gd and Dy) with low Sr/Y (1−10) and
La/Yb (4−11) ratios. Group 2 granites show negative Gd/Yb versus SiO 2 correlation, which indicates significant fractionation of an
amphibole-bearing assemblage. Our results suggest that both group 1 and 2
granites were formed in a subduction setting from a common mantle-derived
parental dioritic magma, but they experienced two distinct fractionation
processes. While group 1 granites were likely formed by crustal
assimilation and high-pressure (lower-crustal) garnet-dominated
fractionation, group 2 granites were generated through low-pressure
(middle-/upper-crustal) amphibole-/plagioclase-dominated fractionation. We
suggest that these two fractionation trends are critical to crustal growth
and the development of a more fractionated (felsic) upper crust.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36230.1/612537/Two-magma-fractionation-paths-for-continental
A shallow-water depositional interpretation for the upper Miocene
Chagres Formation (Caribbean coast of Panama)
Elena Stiles; Camilo Montes; Carlos Jaramillo; Murray K. Gingras
Abstract:
The upper Miocene Chagres Formation, cropping out along the Caribbean coast
of Panama, has been previously interpreted as the record of the last
deep-water connection between the Caribbean and Pacific oceans based on
bodyfossil paleodepth estimates. We test this interpretation by presenting
an integrated ichnological and sedimentological analysis on eight coastal
localities of the Chagres Formation (Toro Limestone and Chagres Sandstone
members). The Toro Limestone Member accumulated under fully marine
conditions in a mixed carbonate and siliciclastic marine shelf above storm
wave base. Fluctuating high-moderate energy conditions modulated by a
combination of tides and high energy currents likely led to the sparse
bioturbation of Toro Limestone sediments, corresponding to an archetypalCruziana ichnofacies assemblage. Bioturbation, dominated by Thalassinoides and Ophiomorpha isp., increases in the
overlying Chagres Sandstone Member and corresponds to a proximal expression
of the Cruziana ichnofacies. The Chagres Sandstone Member was deposited
within the lower shoreface-upper offshore transition zone in a shelf with
low sedimentation rates under combined storm, wave, and tidal influence,
and includes fossils and sediments of continental origin suggesting
proximity to a river mouth. Our results suggest that the Chagres Formation
accumulated in shallow waters, contrasting with a previously proposed
deep-water accumulation, and are consistent with deposition on a
tectonically stable platform of the northern Canal Basin at the apex of the
Panama orocline.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36291.1/612538/A-shallow-water-depositional-interpretation-for
Episodic massive release of methane during the mid-Cretaceous
greenhouse
Biao Chang; Junhua Huang; Thomas J. Algeo; Richard D. Pancost; Xiaoqiao Wan
...
Abstract:
Methane-derived carbonates (MDCs) are common along modern and ancient
continental margins, and the majority of such formations are associated
with seafloor cold seeps. Here, we document petrographic, rare earth
element + yttrium (REE+Y), carbonate clumped isotope temperature (T Δ47), and carbon-isotopic evidence from a shale succession in
southern Tibet spanning a ∼28 m.y. interval (ca. 113−85 Ma) that coincided
with the mid-Cretaceous greenhouse event. At least nine limestone
nodule-bearing horizons exhibit seep-associated sedimentary structures,
including carbonate fabrics (e.g., micritic crusts, crystal fans, and
botryoidal textures) and 13C-depleted isotopic compositions (δ 13Ccarb < ‒32.3‰), which are indicative of
methane-derived carbon sources. Along with sedimentary evidence, the
patterns of TΔ47−δ13Ccarb−δ18O carb support precipitation of these MDCs over a large
temperature range. The REE+Y compositions and europium (Eu) anomalies
indicate that the release of methane was associated with hydrothermal
fluids. Methane may have been derived from both thermogenic and biogenic
sources based on the inorganic carbon isotopic signatures of the carbonate.
These nodular carbonate horizons document multiple episodes of seafloor
methane release during the mid-Cretaceous and represent exceptionally
long-lived, active methane seepage. Massive methane release events may have
played a role in generating the greenhouse climate of the mid-Cretaceous.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36169.1/612491/Episodic-massive-release-of-methane-during-the-mid
Middle-lower crustal flow in response to the India-Eurasia collision:
Structural evidence from the southern Chong Shan belt within the
Sundaland block, southeastern Tibetan Plateau
Xiaoyu Chen; Junlai Liu; Yinchuan Qi; Xiaoxi Bao; Chengyang Ling
Abstract:
In this contribution, we present new structural, microstructural, fabrics,
and geochronological data from the southern Chong Shan complex, one of the
metamorphic complexes in the southeastern Tibetan Plateau that were sheared
and exhumed during the India-Eurasia convergence. The NW-SE−striking
complex is comprised of a central high-grade metamorphic zone (Unit I)
flanked by two low-grade metamorphic zones (units II and III) on the
northeastern and southwestern sides, respectively. High-grade metamorphic
rocks (e.g., amphibolites, sillimanite-mica schists) of up to amphibolite
facies, of the Proterozoic Chong Shan group and granitic intrusions of
Permo-Triassic to Cenozoic in age in Unit I are characterized by
high-temperature deformation. Units II (i.e., the Wuliangshan group) and
III (i.e., the Lancang group) on both sides of the high-grade Unit I
consist of metamorphic rocks of low greenschist facies (e.g., phyllites)
with low-temperature deformation. The high- and low-grade units possess
consistent kinematics, i.e., northwestward motion of the core rocks
relative to the two limbs, and they are separated by large scale shear
discontinuities. Thereby, the high- and low-grade units are kinematically
linked but mechanically decoupled. Zircon laser ablation−inductively
coupled plasma−mass spectrometry U-Pb dating of syn-shearing granitic dikes
reveals that ductile shearing occurred from 29 to 19 Ma. Structural
analysis reveals that these units constitute an A-type dome that has long
axis parallel to the stretching lineations and fold axes of outcrop-scale
A-type folds. It is shown that three stages of deformation contributed to
the formation of the southern Chong Shan dome, during which subhorizontal
shearing were in connection with regional doming. The events occurred as
the consequence of middle to lower flow that led to lateral crustal flow
and vertical exhumation of crustal masses. Therefore, the lateral crustal
flow was not only limited along the boundary high strain zones of the
Sundaland block, but distributed within the southeastern Tibetan Plateau.
We would argue that the tectonic extrusion of the Sundaland block occurred
through ductile crustal flow of a viscous middle and lower crust in the
plate interior combined with concurring channel flow along the block
margins.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36244.1/612434/Middle-lower-crustal-flow-in-response-to-the-India
Latest Silurian syntectonic sedimentation and magmatism and Early
Devonian orogenic gold mineralization, central Newfoundland
Appalachians, Canada: Setting, structure, lithogeochemistry, and
high-precision U-Pb geochronology
I.W. Honsberger; W. Bleeker; S.L. Kamo; H.A.I. Sandeman; D.T.W. Evans ...
Abstract:
The eastern Dunnage Zone of the central Newfoundland Appalachians hosts
Paleozoic orogenic gold mineralization along a northeast-trending,
crustal-scale fault corridor that extends for more than 200 km. This
orogenic gold system is characterized by polyphase, structurally
controlled, quartz vein systems that cut Neoproterozoic granitoid rocks and
unconformably overlying syntectonic, polymict conglomerate and associated
transitional to calc-alkaline bimodal igneous rocks. High-precision
chemical abrasion−isotope dilution−thermal ionization mass spectrometry
(CA-ID-TIMS) U-Pb geochronology confirms a widespread, latest Silurian
magmatic pulse (422−420 Ma) that is attributed to a transient phase of
lithospheric extension resulting from asthenospheric and crustal melting
related to slab break-off. Syntectonic conglomerate was deposited as a
basal unit during extension-related uplift and erosion that lasted until
ca. 418 Ma in north-central Newfoundland. Orogenic gold mineralization
associated with syntectonic sedimentation and magmatism is hosted within
third-order shear vein systems that form offshoots within a triangle
zone−like structural corridor between southeast- and northwest-dipping,
second-order fault splays and shear zones. The southeast-dipping fault
system formed during northwest migration of the Acadian thrust front,
whereas the northwest-dipping faults and shear zones are Salinic structures
that were reactivated in the Early Devonian. Primary hydrothermal rutile in
the orogenic gold-mineralized quartz veins produced ages of ca. 410 Ma.
These ages are consistent with quartz vein emplacement and orogenic gold
mineralization as a result of hydrothermal fluid-pressure cycling related
to far-field compression and thermal perturbations during the Early
Devonian Acadian orogenic cycle. The setting and process evolution of the
central Newfoundland gold district are remarkably similar to that of
world-class orogenic gold systems of the Canadian Shield.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36083.1/612305/Latest-Silurian-syntectonic-sedimentation-and
Orbital forcing of late Early Devonian storm events on a mixed
carbonate-siliciclastic shelf, Longmenshan area, Sichuan Province,
China
Fengjie Li; Zheng’an Chen; James G Ogg; Zuoqiang Li; Xiaokang Ma ...
Abstract:
The Yangmaba Formation of latest Early Devonian age in the Longmenshan area
of Sichuan Province, China, is a shelfal facies that consists of four types
of carbonate-siliciclastic deposits: clay-rich, siliciclastic sand-rich,
carbonate-dominated, and hybrid mixed. Storm deposits vary in their
thickness, composition, and abundance-per-meter within these mixed
carbonate-siliciclastic deposits. Meter-scale statistics of the relative
storm frequency (events-per-meter) and magnitude (bed thickness) were
compiled in two coeval sections through each of the hybrid facies in the
lower Yangmaba Formation, which spans approximately the entire conodont Polygnathus patulus Zone of the uppermost Emsian Stage. Low-pass
filtering, spectral analysis, and Acycle software interpretation of these
tempestite statistics yield 3.5 main oscillations in each section, with an
average wavelength of ∼23 m. These long-wavelength trends are
semi-coincident with interpreted long-term variations in sea level, where
shallower depths allowed a greater influence by storms. Superimposed on the
long-wavelength cycles are medium-wavelength cycles of 5.5−6.3 m. The
estimated ∼1.5 m.y. time-span of this conodont zone of the lower Yangmaba
Formation and the approximate 1:4 ratios of these wavelengths indicate that
frequency and intensity of major storms and the recording of tempestites in
the sedimentary record were modulated by ∼100- and
95-k.y.-short-eccentricity orbital-climate oscillations superimposed on a
main 405-k.y.-long-eccentricity cycle. These eccentricity climate cycles
governed storm intensity and regional sea level on this margin of the
tropical Yangtze Platform of South China.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36105.1/612261/Orbital-forcing-of-late-Early-Devonian-storm
Mercury enrichments during the Carnian Pluvial Event (Late Triassic) in
South China
He Zhao; Stephen E. Grasby; Xiangdong Wang; Lei Zhang; Yongsheng Liu ...
Abstract:
The Carnian Pluvial Event (CPE) was an interval marked by global climatic
and environmental change, along with biotic turnover, which occurred during
the early Late Triassic. Although the causes and consequences of this event
remain unclear, one possible scenario is enhanced volcanism injecting
greenhouse gases into the atmosphere, perturbing the global carbon cycle,
and negatively impacting the global environment. However, there is an
underlying challenge in showing a true cause-and-effect relationship
between volcanism and the CPE, as both the sedimentary and volcanic records
are difficult to date accurately enough to demonstrate temporal
correspondence. However, mercury (Hg) can be used to fingerprint
catastrophic volcanism in the sedimentary record. We examined two sections
that record the CPE at Laishike and Wayao in Guizhou Province, southwest
China, which display high Hg contents along with spikes of Hg/total organic
content (TOC), Hg/Al, Hg/total sulfur (TS), and Hg/(Mo/Al) during the CPE
that indicate a shift to excess Hg loading. These Hg anomalies are
correlative with the global negative excursion in δ13C carb values at the CPE, which suggests that increased volcanism
injected both massive amounts of Hg and isotopically light carbon into the
atmosphere, and these were ultimately recorded in marine sediments. This
interpretation is supported by slightly negative or near-zero Δ 199Hg values that are consistent with a volcanic Hg source. Our
study supports the hypothesis that enhanced volcanism played a major role
in the evolution of biota and the environment during the CPE.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36205.1/612262/Mercury-enrichments-during-the-Carnian-Pluvial
Timing of mass redeposition of sponge spicules from the peri-Tethyan
shelf into the deep Carpathian Basin and their relation to
mid-Cretaceous global sea level changes
Marta Bąk; Krzysztof Bąk; Zbigniew Górny
Abstract:
Extensive Albian−Cenomanian sponge growth on the European peri-Tethyan
shelves depended on sea level fluctuations that caused significant
shoreline shifts and forced facies migration across shelves and partial
redeposition of sponge spicules by gravitational currents into deep basins.
Such phenomena played important roles in accumulation in the Carpathian
Basins, a part of the western Tethys Ocean along the southern slopes of the
European platform. Spiculitic deposits formed thick bodies in the
mid-Cretaceous flysch of the Silesian nappe. This study reports new,
detailed biostratigraphic and chemostratigraphic (δ13C org) data from a reference section in the Outer Carpathians that
were used to date this paleoceanographic phenomenon. A high-resolution
analysis of a continuous, 46.5-m-long sedimentary deep-water succession
exposed in the Silesian nappe reveals that biogenic input of numerous
sponge spicules occurred from latest Albian through middle Cenomanian time,
controlled by third-order sea level oscillations. Chemostratigraphic data
with biostratigraphic control mark carbon isotope excursions that are
characteristic of the mid-Cretaceous (Albian−Cenomanian boundary interval
containing oceanic anoxic event (OAE)1d, mid-Cenomanian event (MCE) Ia, MCE
Ib, and MCE II) and allow precise determination of the beginning and
termination of mass sponge spicule redeposition. The onset of redeposition
corresponds to the top of OAE1d, which records a global regressive event
(KAl8). The mass redeposition of biogenic material ended between MCE Ib and
MCE II during the KCe3 eustatic event. The stratigraphic data indicate that
mass redeposition of spiculitic deposits lasted ca. 4.5 Ma, with an average
sedimentation rate of ∼5 mm kyr−1. Precisely determining the
duration of mass redeposition of spicule-bearing material into the Silesian
Basin enables their correlation with deposits in epicontinental basins.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36178.1/612257/Timing-of-mass-redeposition-of-sponge-spicules
A Mississippi Valley−type Zn-Pb mineralizing system in South China
constrained by in situ U-Pb dating of carbonates and barite and in situ
S-Sr-Pb isotopes
Suo-Fei Xiong; Shao-Yong Jiang; Zuo-Hao Chen; Jian-Xin Zhao; Ying Ma ...
Abstract:
The ages of Zn-Pb deposits are exceptionally challenging to determine owing
to the lack of suitable mineral chronometers and techniques. Here we
present the first result for in situ LA-ICP-MS U-Pb dating of
carbonates and barite from a Mississippi Valley−type (MVT) Zn-Pb deposit in
South China. Hydrothermal dolomite in close textural and paragenetic
association with Zn-Pb sulfides, and calcite and barite cement from the
breccia ores, yield ages of 473.4 ± 2.7 Ma and 368.7 ± 3.1 Ma,
respectively. Together with new in situ S-Pb-Sr isotope values,
these data reveal an epigenetic Zn-Pb mineralization history, agreeing well
with a model involving basinal brine accumulation and MVT Zn-Pb sulfide
precipitation. Because carbonate is a common mineral in Zn-Pb deposits
worldwide, and other minerals in such deposits suitable for isotope dating
are generally absent, in situ U-Pb dating of gangue carbonates
opens a new window for better defining the ore genesis of this globally
important Zn-Pb deposit type and for tracking hydrothermal fluid flow in
sedimentary basins.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36289.1/612258/A-Mississippi-Valley-type-Zn-Pb-mineralizing
Kinematics and 40Ar/39Ar geochronology of the
Lincang-Inthanon tectonic belt: Implication for Cenozoic tectonic
extrusion of SE Asia
Yang Wang; Yuejun Wang; Peizhen Zhang; Jinjiang Zhang; Bo Zhang ...
Abstract:
The Lincang-Inthanon tectonic belt is a major tectonic boundary within the
southeastern Tibetan Plateau and Indo-China Peninsula, which are typical
examples of tectonic extrusion in SE Asia. The Lincang strain zone,
Lancang-Gengma fault, and Inthanon metamorphic complex make up this nearly
N−S-striking tectonic belt, which separates the Baoshan−Shan Thai and
Simao-Indochina terranes. New petrographic, structural, and mica 40Ar/39Ar geochronological studies were conducted to
reveal their deformation styles and constrain the timing of their tectonic
evolution. W−E-directed compression related to the subduction of the
Paleotethyan Ocean with subsequent continental collision and sinistral
ductile shearing in the early Oligocene are recorded along the Lincang
strain zone. The Lancang-Gengma fault zone switched from sinistral shearing
to dextral motion in the late Cenozoic and shows a deformation history
similar to that of the parallel Red River fault. The Inthanon metamorphic
complex may have experienced crustal shortening in the early Cenozoic,
followed by sinistral transtension in the early Miocene. The
Lincang-Inthanon tectonic belt shows many lithological, tectonic
evolutionary, and metamorphic similarities with the Gaoligong, Chongshan,
and Ailaoshan−Red River shear zones. Therefore, the sinistral shearing
along the Lincang-Inthanon tectonic belt and the Chongshan shear zone in
the north, which may have initiated since the early Oligocene, played an
important role in adjusting differential extrusion and rotation of the
Baoshan−Shan Thai and Simao-Indochina terranes. Our results delineate the
regional tectonic framework and provide insights into the characteristics
and geodynamics of intracontinental deformation in the eastern
India-Eurasia oblique convergence zone.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36187.1/612259/Kinematics-and-40Ar-39Ar-geochronology-of-the
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