New GSA Bulletin Articles Published Ahead of Print in April
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, including the
Central Asian Orogenic Belt, the North China Craton, the Lhasa terrane, and
the Sichuan Basin. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Kinematic evolution of the central Andean retroarc thrust belt in
northwestern Argentina and implications for coupling between shortening
and crustal thickening
Susana Henriquez; Peter G. DeCelles; Barbara Carrapa; Amanda N. Hughes
Abstract:
The Andes are the culmination of shortening and crustal thickening that
commenced during Late Cretaceous time. First-order questions regarding the
tectonic evolution of the central Andes include the magnitude and timing of
shortening, and controls on the along-strike variability in observed styles
of shortening and deformation. Along-strike differences in the time of
surface uplift have spawned two contrasting hypotheses: (1) uplift is
related to dynamic and isostatic processes accompanying lithospheric
removal and is decoupled in space and time from crustal thickening and
shortening, and (2) uplift is directly coupled with shortening and crustal
thickening. Although considerable work has been done in Bolivia to address
these hypotheses, work in northern Argentina has not yet produced a
trans-orogenic balanced structural cross section from which the total
amount and kinematic history of shortening can be evaluated. To help
understand the evolution of the thrust belt in northernmost Argentina, we
present a regional, retrodeformable cross section at 23°−24°S across the
Puna and Eastern Cordillera. New apatite fission-track thermochronological
data integrated with other geochronological, sedimentological, and
structural data constrain incremental retrodeformation of the cross section
between ca. 45 and 6.5 Ma. Regional shortening was facilitated by at least
12 major thrust systems, linked to a regional mid-crustal décollement.
Deformation generally propagated eastward through time and involved two
major episodes of eastward advance of the orogenic front, separated by
periods of internal out-of-sequence shortening and kinematic stagnation of
the orogenic front. A new minimum estimate of ∼271 km of total shortening
from the Cordillera de Domeyko to the eastern orogenic front explains
crustal thickening at northern Puna latitudes. Together with previously
published paleoaltimetry data, our new structural and thermochronologic
data indicate that regional uplift in the northern Argentine Puna and
Eastern Cordillera was synchronous with, and thus directly linked to,
crustal shortening and thickening.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36231.1/613418/Kinematic-evolution-of-the-central-Andean-retroarc
(U-Th)/He chronology: Part 1. Data, uncertainty, and reporting
R.M. Flowers; P.K. Zeitler; M. Danišík; P.W. Reiners; C. Gautheron ...
Abstract:
The field of (U-Th)/He geochronology and thermochronology has grown
enormously over the past ∼25 years. The tool is applicable across much of
geologic time, new (U-Th)/He chronometers are under continuous development,
and the method is used in a diverse array of studies. Consequently, the
technique has a rapidly expanding user base, and new labs are being
established worldwide. This presents both opportunities and challenges.
Currently there are no universally agreed-upon protocols for reporting
measured (U-Th)/He data or data derivatives. Nor are there standardized
practices for reporting He diffusion kinetic, 4He/3
He, or continuous ramped heating data. Approaches for reporting
uncertainties associated with all types of data also vary widely. Here, we
address these issues. We review the fundamentals of the methods, the types
of materials that can be dated, how data are acquired, the process and
choices associated with data reduction, and make recommendations for data
and uncertainty reporting. We advocate that both the primary measured and
derived data be reported, along with statements of assumptions, appropriate
references, and clear descriptions of the methods used to compute derived
data from measured values. The adoption of more comprehensive and uniform
approaches to data and uncertainty reporting will enable data to be
re-reduced in the future with different interpretative contexts and data
reduction methods, and will facilitate inter-comparison of data sets
generated by different laboratories. Together, this will enhance the value,
cross-disciplinary use, reliability, and ongoing development of (U-Th)/He
chronology.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36266.1/613176/U-Th-He-chronology-Part-1-Data-uncertainty-and
(U-Th)/He chronology: Part 2. Considerations for evaluating,
integrating, and interpreting conventional individual aliquot data
R.M. Flowers; R.A. Ketcham; E. Enkelmann; C. Gautheron; P.W. Reiners ...
Abstract:
The (U-Th)/He dating technique is an essential tool in Earth science
research with diverse thermochronologic, geochronologic, and detrital
applications. It is now used in a wide range of tectonic, structural,
petrological, sedimentary, geomorphic, volcanological, and planetary
studies. While in some circumstances the interpretation of (U-Th)/He data
is relatively straightforward, in other cases it is less so. In some
geologic contexts, individual analyses of the same mineral from a single
sample are expected to yield dates that differ well beyond their analytical
uncertainty owing to variable He diffusion kinetics. Although much
potential exists to exploit this phenomenon to decipher more detailed
thermal history information, distinguishing interpretable intra-sample data
variation caused by kinetic differences between crystals from
uninterpretable overdispersion caused by other factors can be challenging.
Nor is it always simple to determine under what circumstances it is
appropriate to integrate multiple individual analyses using a summary
statistic such as a mean sample date or to decide on the best approach for
incorporating data into the interpretive process of thermal history
modeling. Here we offer some suggestions for evaluating data, attempt to
summarize the current state of thinking on the statistical characterization
of data sets, and describe the practical choices (e.g., model structure,
path complexity, data input, weighting of different geologic and
chronologic information) that must be made when setting up thermal history
models. We emphasize that there are no hard and fast rules in any of these
realms, which continue to be an important focus of improvement and
community discussion, and no single interpretational and modeling
philosophy should be forced on data sets. The guiding principle behind all
suggestions made here is for transparency in reporting the steps and
assumptions associated with evaluating, integrating, and interpreting data,
which will promote the continued development of (U-Th)/He chronology.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36268.1/613175/U-Th-He-chronology-Part-2-Considerations-for
Locating Lhasa terrane in the Rodinia and Gondwana supercontinents: A
key piece of the reconstruction puzzle
Xiu-Zheng Zhang; Qiang Wang; Wei Dan; Derek Wyman
Abstract:
The debate over global continental reorganization from Rodinia to Gondwana
likely stems from some key Neoproterozoic units being overlooked in
previous reconstructions. Here we provide a self-consistent set of
evidence, based on magmatism and metamorphism, that the Lhasa terrane is
the “lost” part of NW India and retains crucial archives, including 760−730
Ma ophiolitic-arc magmatic rocks and evidence of a distinctive metamorphic
event at ca. 660 Ma. Hence, the Lhasa terrane together with Madagascar and
Seychelles formed in a late Neoproterozoic juvenile arc system laying
either along the periphery of rifting Rodinia or outboard of the
supercontinent. Subsequently, these terranes made up the northern East
African Orogen (EAO) during the initial assembly of Gondwana. As a result
of lateral-escape tectonics after collision and dextral strike-slip along
the northern margin of Gondwana, the Lhasa terrane escaped from the EAO and
had migrated to the northern margin of Australia by the Middle Cambrian.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36152.1/613137/Locating-Lhasa-terrane-in-the-Rodinia-and-Gondwana
Two-stage exhumation of deeply subducted continental crust: Insight
from zircon, titanite, and apatite petrochronology, Sulu belt of
eastern China
Songjie Wang; Michael Brown; Lu Wang; Tim E. Johnson; Hugo K.H. Olierook
...
Abstract:
The rates and mechanisms by which deeply subducted continental crust was
exhumed back to the surface are not well understood, but can be better
characterized using multimineral petrochronology. Here, we combine zircon,
titanite, and apatite U-Pb ages from leucogranite and phengite gneiss with
a pressure−temperature (P−T) path from eclogite to provide robust
quantitative constraints on cooling and exhumation of the Sulu belt, a
large ultrahigh-pressure metamorphic terrane in eastern China. The
leucogranite, which formed during exhumation, is enriched in light rare
earth elements (REE) relative to heavy REE and in large ion lithophile
elements relative to high field strength elements, similar to hydrous
crustal melts. Whole-rock Sr-Nd isotope compositions indicate that the
leucogranite was not directly derived from the host phengite gneiss, but
was more likely sourced from deeper in the exhuming crust. For the gneiss,
mantles on inherited zircon yield an age of 230 ± 2 Ma and a temperature of
802 ± 36 °C based on a minimum pressure of 2.9 GPa, which records the
minimum timing and P−T of initial decompression. Overgrowths on
inherited zircon from the leucogranite constrain crystallization to 224 ± 1
Ma, coeval with the growth of zircon rims in the gneiss, at a temperature
of 764 ± 42 °C and a pressure within the quartz-eclogite facies. Titanite
and apatite define single populations with lower concordia intercept ages
of 222 ± 3 Ma and 198 ± 7 Ma, at temperatures of 720 ± 30 °C and ∼450 ± 100
°C, respectively, recording the timing of passage through the
quartz-eclogite to the amphibolite facies and then the transition to the
upper greenschist facies. Although the data yield a nearly constant cooling
rate of 10.9+4.5−3.6 °C/m.y., exhumation was
completed in two stages. The first stage from coesite-eclogite facies to
∼1.2 GPa, corresponding to the depth of the Moho, occurred at a rate of 7.5 +5.8−2.6 km/m.y. Thereafter, exhumation into the
mid-crust occurred at a much slower rate of 0.87+0.86 −0.71 km/m.y. The first stage of faster exhumation was
accompanied by migration of leucogranite melt along foliation in the
gneiss, which would have decreased the average density and weakened the
crust, enhancing the rate of return flow.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36309.1/613138/Two-stage-exhumation-of-deeply-subducted
Contrasting mechanisms and timescales of subduction and exhumation as
recorded by Paleoproterozoic and late Paleozoic high-pressure
granulites in the North China Craton
Shan-Shan Li; Richard M. Palin; M. Santosh
Abstract:
The North China Craton records multiple metamorphic events related to
supercontinent assembly during the Paleoproterozoic, forming Columbia, and
again during the late Paleozoic, forming Pangea. Here we show that the
Paleoproterozoic high-pressure granulites (HPGs) formed from enriched
mid-ocean ridge basalt protoliths and record a clockwise
pressure-temperature-time (P−T−t) path with prograde metamorphism
at 7.8−10.0 kbar and 780−820 °C, peak granulite-facies metamorphism at
12−12.3 kbar and ∼860−880 °C, and retrograde metamorphism at 8.7−9.1 kbar
and 850−855 °C. Subduction initiated prior to 1.90 Ga, with final collision
and orogeny at 1.88 Ga, followed by post-collision/exhumation at 1.80 Ga,
defining a prolonged exhumation period (∼90 m.y.) that occurred at a slow
velocity of ∼0.16 ± 0.08 mm/y. Late Paleozoic HPGs are normal mid-ocean
ridge basalt type and record a near clockwise P−T−t path, with
peak/post-peak amphibolite-facies metamorphism at 11.0−12.5 kbar and
860−890 °C, isothermal decompression to 7.2−7.5 kbar and 810−820 °C, and
retrogression to 5.5−7.2 kbar and 805−850 °C. Subduction initiated earlier
than ca. 340 Ma, exhumation and uplift initiated at 335−309 Ma and
continued to 297−287 Ma. The exhumation was short-lived (∼50 m.y.) and
relatively fast (0.38 ± 0.14 mm/y). When compared to granulite-facies
metamorphism documented in many Paleoproterozoic HPGs, late Paleozoic HPGs
appear to commonly form with an initial period of steep subduction leading
to eclogite-facies metamorphism, with subsequent exhumation to middle/lower
levels of the crust. Our results further reveal that the exhumation
velocity for supercontinent collision was facilitated and duration
shortened through time, and that the exhumation mechanism might have been
controlled by subduction angle, compression pressure, and temperature.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36380.1/612999/Contrasting-mechanisms-and-timescales-of
Early Paleozoic subduction fingerprints of the Paleo-Asian ocean in
easternmost Central Asian Orogenic Belt (CAOB): Identification of the
oldest Alaskan-type complex in the CAOB
Meng-Meng Cui; Ben-Xun Su; Jing Wang; Yong Wu; Anton Kutyrev
Abstract:
The early-stage subduction records of the Paleo-Asian ocean are poorly
preserved in the eastern segment of the Central Asian Orogenic Belt (CAOB),
which hinders constraints on the evolution of the whole CAOB. This study
presents new age data and zircon Hf-O isotopes as well as bulk-rock
geochemistry of the Wuxing mafic-ultramafic complex in the Xingkai massif
in northeastern China, which has been identified as Alaskan-type complex
with aspects of field occurrence, petrological and mineral assemblages, and
mineral chemistry in our recently published work. The results indicate that
the complex formed mainly between 517 Ma and 510 Ma with a lithological
sequence of Sanying clinopyroxenite (517 Ma), then Sanying gabbro (514 Ma),
and Erying hornblendite (513 Ma), and finally Erying hornblende
clinopyroxenite (510 Ma). The lithological formation sequence is consistent
with the intrusive relations between lithological phases and their
irrelevant major element compositions and variable trace element patterns
of the bulk rocks. The εHf(t) values of zircon in two samples in
this study vary from from +4.45 to +7.61 and from −11.8 to +4.42,
respectively and tend to be more depleted with age. These features suggest
that the Wuxing complex was a product of long-term arc magmatism and
experienced significant ancient crustal assimilation in early-stage magmas
and negligible contamination in later ones. The presence of 1222 Ma and 706
Ma inherited zircon grains implies existence of Proterozoic basement in the
Xingkai massif and its continental arc setting in Cambrian. The Wuxing
complex is the oldest Alaskan-type complex found so far in the entire CAOB
and is a good witness of the Paleo-Asian oceanic subduction in the
easternmost CAOB. The older age of the Wuxing complex compared to the
regional Hongqiling intrusion is also compatible with its Alaskan-type
nature and platinum-group element mineralization, which are distinct to the
Permian-Triassic Ni-Cu sulfide deposit-hosting mafic-ultramafic intrusions
in the southern CAOB.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36307.1/613000/Early-Paleozoic-subduction-fingerprints-of-the
New Late Cretaceous paleomagnetic results from the Lhasa terrane and
their implications for the suturing of India and Eurasia and the
closure of the Neo-Tethys Ocean
Yabo Tong; Zhenyu Yang; Junling Pei; Jianfeng Li; Shuchen Jin ...
Abstract:
The forms of the margins of the Lhasa terrane and the Tethyan Himalaya
prior to the collision of India and Eurasia as constrained by
paleomagnetism are ambiguous due to the disordered Cretaceous paleomagnetic
data from the central Lhasa terrane and the counterclockwise rotation of
the Indian plate during the Cretaceous. This ambiguity has induced
controversy over the processes of suturing of India and Eurasia and the
closure of the Neo-Tethys Ocean. We obtained a set of high-quality Late
Cretaceous paleomagnetic data from the central Lhasa terrane, which,
integrated with reliable Cretaceous and Paleogene paleomagnetic data sets
from the other parts of the Lhasa terrane and Tethyan Himalaya, confirmed
that the southern margin of the Lhasa terrane and the northern margin of
the Tethyan Himalaya were originally oriented ∼317° and ∼326°,
respectively, prior to the collision of India and Eurasia. The margins of
the Lhasa terrane and Tethyan Himalaya were almost consistent with the
original straight fold axes of Cretaceous strata in the southern part of
the Lhasa terrane, which were oriented 332.5° ± 8.5°, indicating that the
subduction of the Neo-Tethys Ocean beneath Eurasia and the movement of the
Tethyan Himalaya consistently maintained a stabilized direction of 62.5° ±
8.5° during the Late Cretaceous. The different kinematic characteristics of
the Indian plate and Tethyan Himalaya and the overlap of the margins of the
Tethyan Himalaya and Lhasa terrane during 59.0−56.0 Ma indicate that the
Tethyan Himalaya was already rifted from the Indian plate prior to
62.5−59.2 Ma, and then it quasi-parallelly collided with the Lhasa terrane
during 59.0−56.0 Ma, quasi-synchronously closing the Neo-Tethys Ocean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36310.1/613001/New-Late-Cretaceous-paleomagnetic-results-from-the
Mechanisms of strain localization and nucleation of earthquake faulting
by grain-scale processes at the middle crustal level
Chunru Hou; Junlai Liu; Yuanyuan Zheng; Yanqi Sun; Tieying Zhang ...
Abstract:
The mechanism of strain localization is the key to our understanding of the
transition from steady-state to unstable flow, and therefore of earthquake
faulting in the middle crust. In this study, biotite grains in mylonitic
gneisses along the Jinzhou detachment fault zone, Liaodong peninsula,
northeast China, acted as a preexisting weak phase that had important
influences on deformation of mid-crustal rocks. High phase strength
contrasts between biotite and other mineral phases resulted in stress
concentrations at the tips of biotite grains and induced semi-brittle
deformation of neighboring quartz and feldspar grains. As a consequence,
the biotite grains became interconnected to form zones of weakness, while
basal plane slip and grain boundary sliding operated in biotite grains and
fine-grained biotite-feldspar-quartz aggregates, respectively. The zones
filled with biotite grains and fine-grained quartz-feldspar aggregates
continued to propagate and coalesce during the deformation. These processes
led to transition from load-bearing (i.e., coarse plagioclase grains)
framework to interconnected weak phase (i.e., biotite grains and
fine-grained feldspar aggregates) domination, that further led to the
formation of initial strain localization zones (SLZs). With the propagation
and linkage of the SLZs, high stress concentrations at the tips of the SLZs
led to nucleation of rupture along the SLZs. As a consequence, there
occurred an abrupt increase in strain rates that resulted in transition
from stable to unstable slip within the SLZs. The processes were
accompanied by occurrence of mid-crustal earthquake faulting and formation
of pseudotachylites along the SLZs.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36303.1/612977/Mechanisms-of-strain-localization-and-nucleation
High-resolution record of multiple organic carbon-isotope excursions in
lacustrine deposits of Upper Sinemurian through Pliensbachian (Early
Jurassic) from the Sichuan Basin, China
Marco Franceschi; Xin Jin; Zhiqiang Shi; Bin Chen; Nereo Preto ...
Abstract:
The Sinemurian-Pliensbachian boundary event (ca. 193 Ma) is recorded as a
global perturbation of the carbon cycle, as evidenced by a large negative
carbon-isotope excursion recorded in many marine sedimentary successions.
Whereas multiple lines of evidence testify that the
Sinemurian-Pliensbachian boundary event was associated with environmental
and climatic changes, sea-level oscillations, and biotic turnovers in
marine settings, the record and effects of the event on continents are
poorly known. In this paper, we report a high-resolution δ13C org record and palynological data from the Lower Jurassic
lacustrine succession of the Sichuan Basin that allow a prominent 8‰
negative carbon-isotope excursion to be identified at the
Sinemurian-Pliensbachian transition. We therefore interpret this
perturbation as the expression of the Sinemurian-Pliensbachian boundary
event in the Sichuan Basin, and we propose a correlation with the marine
realm. Facies evolution illustrates that the Sinemurian-Pliensbachian
boundary event coincided with a phase of expansion of the lacustrine
systems. Palynological analyses indicate a sharp shift from arid to humid
climate conditions coincident with the carbon-isotope perturbation that
supports a scenario of lake expansion driven by increased rainfall. In
contrast to observations in the Sichuan Basin, where deep lake conditions
persisted across the Sinemurian-Pliensbachian boundary event, a global drop
in the sea level is documented at the onset of the isotope perturbation.
This suggests that eustatic oscillations due to increased continental water
storage in lakes and aquifers in the context of a wetter climate phase may
have been associated with the early stages of the Sinemurian-Pliensbachian
boundary event.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36235.1/612978/High-resolution-record-of-multiple-organic-carbon
New constraints on the timing and character of the Laramide Orogeny and
associated gold mineralization in SE California, USA
Tarryn K. Cawood; Amy Moser; Ariel Borsook; Alan D. Rooney
Abstract:
The timing of deformation and associated gold mineralization in SE
California, USA, is contentious, partly due to the challenges involved with
dating ductile deformation. We therefore combine modern geo- and
thermochronology with field and microscopic observations to show that the
Cargo Muchacho Mountains preserve evidence of northward thrusting in a
kilometer-scale ductile shear zone during the Late Cretaceous Laramide
Orogeny, accompanied by hydrothermal fluid flow, gold mineralization, and
pegmatite emplacement. Penetrative strain was largely accommodated within
the Jurassic metavolcaniclastic Tumco Formation, whereas intrusive Jurassic
granitoids behaved as competent bodies. Quartz microstructures suggest
deformation at ∼500 °C, which is consistent with fabrics defined by
amphibolite facies minerals. The timing of thrusting is constrained by
dynamically recrystallized titanite with a U-Pb age of 68 ± 1 Ma and late
syn-kinematic pegmatites that yield U-Pb zircon ages of 65.0 ± 4.2−63.2 ±
4.8 Ma. Syn-kinematic fluid flow was focused into a lateral thrust ramp
where the shear zone foliation was deflected around a relatively rigid
pluton, creating zones rich in magnetite-quartz veins and epidote, and
precipitating gold associated with pyrite and chalcopyrite. Dating of these
sulfides via Re-Os yields an age of 64.7 ± 0.8 Ma, which confirms a
Laramide age for the gold mineralization. Together, apatite from the
pegmatites and a nearby Jurassic granite yields a U-Pb age of 60.4 ± 3.5
Ma, reflecting cooling to below 530−450 °C. Comparison with published
studies suggests that thick-skinned deformation in the Cargo Muchacho
Mountains was driven by flat-slab subduction of the conjugate Hess Plateau,
which occurred several million years after and to the south of flat-slab
subduction of the conjugate Shatsky Rise. This suggests that the conjugate
Hess Plateau may have been subducted up to several hundred kilometers
farther north than previously thought. Metamorphic devolatilization of
underplated Orocopia Schist likely generated the gold-bearing hydrothermal
fluids, and anatexis of the schist formed the peraluminous pegmatites,
which highlights the importance of schist underplating and devolatilization
along much of the Californian and Mexican cordillera.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36251.1/612979/New-constraints-on-the-timing-and-character-of-the
U-Pb dating of oil charge in superimposed basins: A case study from the
Tarim Basin, NW China
Jiaxu Chen; Xiaowen Guo; Ze Tao; Zicheng Cao; Bin Wang ...
Abstract:
Direct dating of oil charge in superimposed basins is essential to
understanding the evolutionary histories of petroleum systems, especially
in sedimentary basins with complicated tectonic evolution and thermal
histories. Based on analyses of different phases of calcite veins and
primary oil inclusions, episodes of oil charge were determined by laser
ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) in situ
U-Pb dating of calcite veins from an Ordovician reservoir within the Tahe
Oilfield of the Tarim Basin, NW China. This basin has been subjected to
multiple uplifts and erosions and repeated oil charges. The U-Pb dating
results indicate that the first phase of oil charge occurred from 329.7 ±
1.6 Ma to 308.1 ± 4.1 Ma, and the second phase occurred from 249.3 ± 2.6 Ma
to 220.5 ± 7.3 Ma. The timing of oil charge determined by fluid inclusion
analysis alone can lead to great uncertainties due to the existence of
multiple phases of oil charge and complex thermal evolution in superimposed
basins. Our study demonstrates that U-Pb dating of calcite veins
originating from the reservoirs offers a unique solution to determining the
oil charge history, which avoids the multi-solution uncertainties in the
timing of oil charge inferred from fluid inclusion analysis in superimposed
basins.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36324.1/612933/U-Pb-dating-of-oil-charge-in-superimposed-basins-A
Ridge subduction and episodes of crustal growth in accretionary belts:
Evidence from late Paleozoic felsic igneous rocks in the southeastern
Central Asian Orogenic Belt, Inner Mongolia, China
Jialiang Li; Jingao Liu; Di-Cheng Zhu; Bruce K. Nelson; Ruohan Gao
Abstract:
The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary
orogens on Earth and preserves evidence that more than 50% of this orogen
represents juvenile crustal growth over an extended period of some 750
million years from ca. 1000 Ma to ca. 250 Ma. However, the mechanism of
crustal growth is controversial, as implied by a variety of proposed models
ranging from contributions of mantle-derived basaltic underplating in a
post-collisional extensional setting to subduction-related processes in an
island arc setting. To distinguish among these models, we report
petrological, geochemical, whole-rock Sr-Nd isotope and zircon U-Pb and Hf
isotope analyses of late Paleozoic felsic igneous rocks from the northern
Inner Mongolia region, southeastern CAOB. New zircon U-Pb analyses of three
plutonic and extrusive magmatic suites yield Late Carboniferous to Early
Permian ages of 319−279 Ma. The Xi Ujimqin granodiorites have low-K
tholeiitic to calc-alkaline, metaluminous to weakly peraluminous
compositions, and are magnesian I-type granitoids. These granitoids are
also characterized by relatively high MgO and Sr contents, high Mg # values, mid-ocean ridge basalt (MORB)-like Nd-Hf isotope
compositions and young Nd-Hf model ages of 600−298 Ma. These features
indicate that the parental magmas of the Xi Ujimqin I-type granitoids
originated from a depleted lithospheric mantle that had been metasomatized
by fluids released from a subducting slab. By contrast, the Xilinhot
alkali-feldspar granites and Dashizhai rhyolites display geochemical
signatures of ferroan granites. These, together with their moderately
depleted Nd-Hf isotope compositions and young Nd-Hf model ages of 809−277
Ma, suggest that they were produced by re-melting of juvenile lower crust
via underplating of mantle-derived magmas. The coexistence of an
association of boninite-adakite-high-Mg andesite-Nb-enriched basalt, and
MORB to ocean island basalt-type mafic rocks in the northern Inner Mongolia
region implies a dramatic change in composition from fluid-related
calc-alkaline arc magmatism to melt-related and mantle-derived magmatism
initiated by upwelling asthenosphere. We infer that ridge subduction may
have occurred in this region during Late Carboniferous to Early Permian.
Combined with previous studies, there are two ridge subduction events
during the early and late Paleozoic in Inner Mongolia within the
southeastern CAOB, which coincide with two large-volume magmatic flare-ups
(300 ± 20 and 450 ± 20 Ma). Furthermore, our model calculations suggest
that most of the juvenile crust in the southeastern CAOB was generated in a
relatively short interval (∼20−40 m.y.) during each long-lived tectonic
cycle (>140 m.y. spanning oceanic rifting, subduction, and collision) of
magmatic activity. Ridge subduction may have played a significant role in
the rhythmic growth of juvenile crust in the CAOB during the Phanerozoic.
Repeated cycles of punctuated, rapid growth of juvenile crust associated
with long-lived subduction systems represent a pattern that may be
generalized to other Phanerozoic accretionary belts on Earth.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35986.1/612907/Ridge-subduction-and-episodes-of-crustal-growth-in
3-D seismic chronostratigraphy of reefs and drifts in the Browse Basin,
NW Australia
Sebastian Thronberens; Stefan Back; Julien Bourget; Tony Allan; Lars
Reuning
Abstract:
In its subsurface, the Browse Basin on the Australian Northwest Shelf
accommodates one of the largest Miocene−Pliocene carbonate platform
provinces on Earth. This study presents a comprehensive three-dimensional
(3-D) documentation and investigation of the Neogene Browse Basin
carbonates based on the analysis of 15 3-D seismic reflection volumes
covering ca. 33,000 km2 and 22 industry boreholes. Eight
basin-wide seismic marker horizons tied to Sr isotope and biostratigraphic
data provide a robust 3-D chronostratigraphic framework that (1) newly
documents the occurrence of Miocene to recent carbonate systems in vast
inboard basin areas; (2) presents the 3-D distribution and dynamic
development of carbonate platforms through time constrained by absolute
ages; and (3) supports the detailed 3-D interpretation of Miocene to recent
depositional processes and key stratigraphic controls. Around 18.6 Ma, the
extent of all Browse Basin carbonate platforms was ca. 8600 km2,
rising to >10,000 km2 between 15.6 Ma and 12 Ma. By ca. 9.7
Ma, the total platform extent had decreased to ca. 5700 km2,
waning to <2800 km2 in the Pliocene. The observed reef demise
is less abrupt than previously thought and comprises two steps, with the
first in the late Miocene on the outer shelf and the second in the Pliocene
on the middle and inner shelf. The Miocene outer-shelf platform demise
coincided with (1) strong subsidence that outpaced sedimentation, (2) an
increase in NW−SE-oriented tidal current activity, (3) the development of
NNE−SSW-oriented bottom currents, and (4) the onset of drift sedimentation.
The Pliocene demise of middle-shelf reefs coincided with (1) an increase of
clastic sediment input from land and (2) stalling of the Indonesian
Throughflow around 3.7 Ma. The basin-wide, 3-D seismic chronostratigraphic
analysis presented supports a re-evaluation of the key controls for Miocene
and Pliocene reef growth and decay on the NW Shelf and highlights the
complexity of interacting global, regional, and local processes and
peculiarities in carbonate platform development.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36286.1/612885/3-D-seismic-chronostratigraphy-of-reefs-and-drifts
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