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New GSA bulletin articles published ahead of print in February


Boulder, Colo., USA: Several articles were published online ahead of print
for GSA Bulletin in February. Topics include earthquake cycles in
southern Cascadia, fault dynamics in the Gulf of Mexico, debris flow after
wildfires, the assembly of Rodinia, and the case for no ring fracture in
Mono basin.

Jurassic evolution of the Qaidam Basin in western China: Constrained by

succession, detrital zircon U-Pb geochronology and Hf isotope analysis

Tao Qian; Zongxiu Wang; Yu Wang; Shaofeng Liu; Wanli Gao …

The formation and evolution of an intracontinental basin triggered via the
subduction or collision of plates at continental margins can record
intracontinental tectonic processes. As a typical intracontinental basin
during the Jurassic, the Qaidam Basin in western China records how this
extensional basin formed and evolved in response to distant subduction or
collisional processes and tectonism caused by stresses transmitted from
distant convergent plate margins. The Jurassic evolution of the Qaidam
Basin, in terms of basin-filling architecture, sediment dispersal pattern
and basin properties, remains speculative; hence, these uncertainties need
to be revisited. An integrated study of the stratigraphic succession,
conglomerates, U-Pb geochronology, and Hf isotopes of detrital zircons was
adopted to elucidate the Jurassic evolutionary process of the Qaidam Basin.
The results show that a discrete Jurassic terrestrial succession
characterized by alluvial fan, braided stream, braided river delta, and
lacustrine deposits developed on the western and northern margins of the
Qaidam Basin. The stratigraphic succession, U-Pb age dating, and Hf isotope
analysis, along with the reconstructed provenance results, suggest
small-scale distribution of Lower Jurassic sediments deposited via
autochthonous sedimentation on the western margin of the basin, with
material mainly originating from the Altyn Tagh Range. Lower Jurassic
sediments in the western segment of the northern basin were shed from the
Qilian Range (especially the South Qilian) and Eastern Kunlun Range. And
coeval sediments in the eastern segment of the northern basin were
originated from the Quanji massif. During the Middle-Late Jurassic, the
primary source areas were the Qilian Range and Eastern Kunlun Range, which
fed material to the whole basin. The Jurassic sedimentary environment in
the Qaidam Basin evolved from a series of small-scale, scattered, and
rift-related depressions distributed on the western and northern margins
during the Early Jurassic to a larger, extensive, and unified depression
occupying the whole basin in the Middle Jurassic. The Altyn Tagh Range rose
to a certain extent during the Early Jurassic but lacked large-scale
strike-slip tectonism throughout the Jurassic. At that time, the North
Qaidam tectonic belt had not yet been uplifted and did not shed material
into the basin during the Jurassic. The Qaidam Basin experienced
intracontinental extensional tectonism with a northeast-southwest trend
throughout the Jurassic in response to far-field effects driven by the
sequential northward or northeastward amalgamation of blocks to the
southern margin of the Qaidam Block and successive accretion of the
Qiangtang Block and Lhasa Block onto the southern Eurasian margin during
the Late Triassic?Early Jurassic and Late Jurassic?Early Cretaceous,

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Late Mesoproterozoic low-P/T?type metamorphism in the North Wulan
terrane: Implications for the assembly of Rodinia

Lu Wang; Stephen T. Johnston; Nengsong Chen; Heng Wang; Bin Xia …

Regional metamorphism provides critical constraints for unravelling
lithosphere evolution and geodynamic settings, especially in an orogenic
system. Recently, there has been a debate on the Rodinia-forming Tarimian
orogeny within the Greater Tarim block in NW China. The North Wulan
terrane, involved in the Paleozoic Qilian orogen, was once part of the
Greater Tarim block. This investigation of petrography, whole-rock and
mineral geochemistry, phase equilibrium modeling, and in situ monazite U-Pb
dating of garnetite, pelitic gneiss, and quartz schist samples from the
Statherian?Calymmian unit of the North Wulan terrane provides new
constraints on the evolutionary history of the Greater Tarim block at the
end of the Mesoproterozoic during the assembly of Rodinia. The studied
samples yielded three monazite U-Pb age groups of ca. 1.32 Ga, 1.1 Ga, and
0.45 Ga that are interpreted to be metamorphic in origin. The tectonic
significance of the early ca. 1.32 Ga metamorphism is uncertain and may
indicate an extensional setting associated with the final breakup of
Columbia. The ca. 1.1 Ga low-pressure, high-temperature (low-P/T
)?type granulite-facies metamorphism is well preserved and characterized by
a clockwise PT path with a minimum estimation of
?840?900 °C and ?7?11 kbar for peak metamorphism, followed by postpeak
decompression and cooling. A tectonothermal disturbance occurred at ca.
0.45 Ga, but with limited influence on the preexisting mineral compositions
of the studied samples. The characteristics of the metamorphism indicate an
arc?back-arc environment with ongoing subduction of oceanic lithosphere at
ca. 1.1 Ga. Combined with previous studies, we suggest that the Greater
Tarim block probably experienced a prolonged subduction-to-collision
process at ca. 1.1?0.9 Ga during the assembly of Rodinia, with a position
between western Laurentia and India?East Antarctica.

View article:

Imaging the Late Triassic lithospheric architecture of the Yidun
Terrane, eastern Tibetan Plateau: Observations and interpretations

Qiong-Yao Zhan; Di-Cheng Zhu; Qing Wang; Peter A. Cawood; Jin-Cheng Xie …

The present-day lithospheric architecture of modern and ancient orogens can
be imaged by geophysical techniques. For ancient orogens, unravelling their
architecture at the time of formation is hindered by later tectono-magmatic
events. In this paper, we use spatial variations in radiogenic isotopic
compositions of Late Triassic magmatism from the Yidun Terrane, eastern
Tibetan Plateau, to establish its lithospheric architecture during the
Triassic. Comprehensive geochemical and isotopic data of Late Triassic
magmatic rocks from four transects across the Yidun Terrane document
eastward enrichment in whole-rock Nd, Sr, and zircon Hf isotopic
compositions. Mafic and felsic rocks of major plutons show coherent and
nonlinear trends in the Zr and P2O5 systematics and
have limited variation of isotopic compositions. This indicates that Late
Triassic magmatic differentiation was dominated by fractionation of
mantle-derived mafic magmas. The spatial isotopic trends result from
changing mantle sources, including variable contributions of isotopically
depleted asthenospheric mantle and isotopically enriched subcontinental
lithospheric mantle (SCLM) to magma sources. The spatial variation of
mantle sources suggests a westward thinning of the SCLM during the
Triassic. We propose that this architecture is most likely associated with
eastward subduction of oceanic lithosphere of the Jinshajiang Ocean located
at the west of the Yidun Terrane, immediately prior to the Late Triassic

View article:

Cambrian magmatic flare-up, central Tibet: Magma mixing in
proto-Tethyan arc along north Gondwanan margin

Pei-yuan Hu; Qing-guo Zhai; Peter A. Cawood; Guo-chun Zhao; Jun Wang …

Accompanying Gondwana assembly, widespread but diachronous Ediacaran?early
Paleozoic magmatism of uncertain origin occurred along the supercontinent’s
proto-Tethyan margin. We report new geochemical, isotopic, and
geochronological data for Cambrian magmatic rocks (ca. 500 Ma) from the
Gondwana-derived North Lhasa terrane, located in the present-day central
Tibetan Plateau. The magmatic rocks are composed of basalts, gabbros,
quartz monzonites, granitoids (with mafic microgranular enclaves), and
rhyolites. Nd-Hf isotopic and whole-rock geochemical data indicate that
these rocks were probably generated by mixing of mantle-derived mafic and
crust-derived felsic melts. The mantle end-member volumes of mafic,
intermediate, and felsic rocks are ?75%?100%, 50%?60%, and 0?30%,
respectively. Integration of our new data with previous studies suggests
that the North Lhasa terrane experienced long-term magmatism through the
Ediacaran to Ordovician (ca. 572?483 Ma), with a magmatic flare-up at ca.
500 Ma. This magmatism, in combination with other Ediacaran?early Paleozoic
magmatism along the proto-Tethyan margin, was related to an Andean-type
arc, with the magmatic flare-up event related to detachment of the oceanic
slab following collisional accretion of Asian microcontinental fragments to
northern Gondwana. Diachroneity of the proto-Tethyan arc system along the
northern Gondwanan margin (ca. 581?531 Ma along the Arabian margin and ca.
512?429 Ma along the Indian-Australian margin) may have been linked to
orogenesis within Gondwana. The North Lhasa terrane was probably involved
in both Arabian and Indian-Australian proto-Tethyan Andean-type orogens,
based on its paleogeographic location at the northern end of the East
African orogen.

View article:

Rift inheritance controls the switch from thin- to thick-skinned
thrusting and basal décollement re-localization at the
subduction-to-collision transition

Stefano Tavani; Pablo Granado; Amerigo Corradetti; Giovanni Camanni;
Gianluca Vignaroli …

In accretionary convergent margins, the subduction interface is formed by a
lower plate décollement above which sediments are scraped off and
incorporated into the accretionary wedge. During subduction, the basal
décollement is typically located within or at the base of the sedimentary
pile. However, the transition to collision implies the accretion of the
lower plate continental crust and deformation of its inherited rifted
margin architecture. During this stage, the basal décollement may remain
confined to shallow structural levels as during subduction or re-localize
into the lower plate middle-lower crust. Modes and timing of such
re-localization are still poorly understood. We present cases from the
Zagros, Apennines, Oman, and Taiwan belts, all of which involve a former
rifted margin and point to a marked influence of inherited rift-related
structures on the décollement re-localization. A deep décollement level
occurs in the outer sectors of all of these belts, i.e., in the zone
involving the proximal domain of pre-orogenic rift systems. Older–and
shallower–décollement levels are preserved in the upper and inner zones of
the tectonic pile, which include the base of the sedimentary cover of the
distal portions of the former rifted margins. We propose that thinning of
the ductile middle crust in the necking domains during rifting, and its
complete removal in the hyperextended domains, hampered the development of
deep-seated décollements during the inception of shortening. Progressive
orogenic involvement of the proximal rift domains, where the ductile middle
crust was preserved upon rifting, favors its reactivation as a décollement
in the frontal portion of the thrust system. Such décollement eventually
links to the main subduction interface, favoring underplating and the
upward motion of internal metamorphic units, leading to their final
emplacement onto the previously developed tectonic stack.

View article:

No ring fracture in Mono Basin, California

Wes Hildreth; Judy Fierstein; Juliet Ryan-Davis

In Mono Basin, California, USA, a near-circular ring fracture 12 km in
diameter was proposed by R.W. Kistler in 1966 to have originated as the
protoclastic margin of the Cretaceous Aeolian Buttes pluton, to have been
reactivated in the middle Pleistocene, and to have influenced the arcuate
trend of the chain of 30 young (62?0.7 ka) rhyolite domes called the Mono
Craters. In view of the frequency and recency of explosive eruptions along
the Mono chain, and because many geophysicists accepted the ring fracture
model, we assembled evidence to test its plausibility. The shear zone
interpreted as the margin of the Aeolian Buttes pluton by Kistler is 50?400
m wide but is exposed only along a 7-km-long set of four southwesterly
outcrops that subtend only a 70° sector of the proposed ring. The southeast
end of the exposed shear zone is largely within the older June Lake pluton,
and at its northwest end, the contact of the Aeolian Buttes pluton with a
much older one crosses the shear zone obliquely. Conflicting attitudes of
shear structures are hard to reconcile with intrusive protoclasis. Also
inconsistent with the margin of the ovoid intrusion proposed by Kistler,
unsheared salients of the pluton extend ?1 km north of its postulated
circular outline at Williams Butte, where there is no fault or other
structure to define the northern half of the hypothetical ring. The shear
zone may represent regional Cretaceous transpression rather than the margin
of a single intrusion. There is no evidence for the Aeolian Buttes pluton
along the aqueduct tunnel beneath the Mono chain, nor is there evidence for
a fault that could have influenced its vent pattern. The apparently arcuate
chain actually consists of three linear segments that reflect Quaternary
tectonic influence and not Cretaceous inheritance. A rhyolitic magma
reservoir under the central segment of the Mono chain has erupted many
times in the late Holocene and as recently as 700 years ago. The ring
fracture idea, however, prompted several geophysical investigations that
sought a much broader magma body, but none identified a low-density or
low-velocity anomaly beneath the purported 12-km-wide ring, which we
conclude does not exist.

View article:

Timing and amount of southern Cascadia earthquake subsidence over the
past 1700 years at northern Humboldt Bay, California, USA

Jason S. Padgett; Simon E. Engelhart; Harvey M. Kelsey; Robert C. Witter;
Niamh Cahill …

Stratigraphic, lithologic, foraminiferal, and radiocarbon analyses indicate
that at least four abrupt mud-over-peat contacts are recorded across three
sites (Jacoby Creek, McDaniel Creek, and Mad River Slough) in northern
Humboldt Bay, California, USA (?44.8°N, ?124.2°W). The stratigraphy records
subsidence during past megathrust earthquakes at the southern Cascadia
subduction zone ?40 km north of the Mendocino Triple Junction. Maximum and
minimum radiocarbon ages on plant macrofossils from above and below
laterally extensive (6 km) contacts suggest regional synchroneity of
subsidence. The shallowest contact has radiocarbon ages that are consistent
with the most recent great earthquake at Cascadia, which occurred at 250
cal yr B.P. (1700 CE). Using Bchron and OxCal software, we model ages for
the three older contacts of ca. 875 cal yr B.P., ca. 1120 cal yr B.P., and
ca. 1620 cal yr B.P. For each of the four earthquakes, we analyze
foraminifera across representative mud-over-peat contacts selected from
McDaniel Creek. Changes in fossil foraminiferal assemblages across all four
contacts reveal sudden relative sea-level (RSL) rise (land subsidence) with
submergence lasting from decades to centuries. To estimate subsidence
during each earthquake, we reconstructed RSL rise across the contacts using
the fossil foraminiferal assemblages in a Bayesian transfer function. The
coseismic subsidence estimates are 0.85 ± 0.46 m for the 1700 CE
earthquake, 0.42 ± 0.37 m for the ca. 875 cal yr B.P. earthquake, 0.79 ±
0.47 m for the ca. 1120 cal yr B.P. earthquake, and ?0.93 m for the ca.
1620 cal yr B.P. earthquake. The subsidence estimate for the ca. 1620 cal
yr B.P. earthquake is a minimum because the pre-subsidence paleoenvironment
likely was above the upper limit of foraminiferal habitation. The
subsidence estimate for the ca. 875 cal yr B.P. earthquake is less than
View article:

Fault kinematics: A record of tectono-climatically controlled
sedimentation along passive margins, an example from the U.S. Gulf of

Abah P. Omale; Juan M. Lorenzo; Ali AlDhamen; Peter D. Clift; A. Alexander
G. Webb

Faults offsetting sedimentary strata can record changes in sedimentation
driven by tectonic and climatic forcing. Fault kinematic analysis is
effective at evaluating changes in sediment volumes at salt/shale-bearing
passive margins where sediment loading drives faulting. We explore these
processes along the northern Gulf of Mexico. Incremental throw along 146
buried faults studied across onshore Louisiana revealed continual Cenozoic
fault reactivation punctuated by inactive periods along a few faults. Fault
scarp heights measured from light detection and ranging (LiDAR) data are
interpreted to show that Cenozoic fault reactivation continued through the
Pleistocene. The areas of highest fault throw and maximum sediment
deposition shifted from southwest Louisiana in the early Miocene to
southeast Louisiana in the middle?late Miocene. These changes in the locus
of maximum fault reactivation and sediment deposition were controlled by
changing tectonics and climate in the source areas. Early Miocene fault
throw estimates indicate a depocenter farther east than previously mapped
and support the idea that early Miocene Appalachian Mountain uplift and
erosion routed sediment to southeast Louisiana. By correlating changes in
fault throw with changes in sediment deposition, we suggest that (1) fault
kinematic analysis can be used to evaluate missing sediment volumes because
fault offsets can be preserved despite partial erosion, (2) fault throw
estimates can be used to infer changes in past tectonic and climate-related
processes driving sedimentation, and (3) these observations are applicable
to other passive margins with mobile substrates and faulted strata within
overfilled sedimentary basins.

View article:

An extreme climate gradient-induced ecological regionalization in the
Upper Cretaceous Western Interior Basin of North America

Landon Burgener; Ethan Hyland; Emily Griffith; Helena Mitášová; Lindsay E.
Zanno …

The Upper Cretaceous Western Interior Basin of North America provides a
unique laboratory for constraining the effects of spatial climate patterns
on the macroevolution and spatiotemporal distribution of biological
communities across geologic timescales. Previous studies suggested that
Western Interior Basin terrestrial ecosystems were divided into distinct
southern and northern communities, and that this provincialism was
maintained by a putative climate barrier at ?50°N paleolatitude; however,
this climate barrier hypothesis has yet to be tested. We present mean
annual temperature (MAT) spatial interpolations for the Western Interior
Basin that confirm the presence of a distinct terrestrial climate barrier
in the form of a MAT transition zone between 48°N and 58°N paleolatitude
during the final 15 m.y. of the Cretaceous. This transition zone was
characterized by steep latitudinal temperature gradients and divided the
Western Interior Basin into warm southern and cool northern biomes.
Similarity analyses of new compilations of fossil pollen and leaf records
from the Western Interior Basin suggest that the biogeographical
distribution of primary producers in the Western Interior Basin was heavily
influenced by the presence of this temperature transition zone, which in
turn may have impacted the distribution of the entire trophic system across
western North America.

View article:

Debris flow initiation from ravel-filled channel bed failure following
wildfire in a bedrock landscape with limited sediment supply

Marisa C. Palucis; Thomas P. Ulizio; Michael P. Lamb

Steep, rocky landscapes often produce large sediment yields and debris
flows following wildfire. Debris flows can initiate from landsliding or
rilling in soil-mantled portions of the landscape, but there have been few
direct observations of debris flow initiation in steep, rocky portions of
the landscape that lack a thick, continuous soil mantle. We monitored a
steep, first-order catchment that burned in the San Gabriel Mountains,
California, USA. Following fire, but prior to rainfall, much of the
hillslope soil mantle was removed by dry ravel, exposing bedrock and
depositing ?0.5 m of sandy sediment in the channel network. During a
one-year recurrence rainstorm, debris flows initiated in the channel
network, evacuating the accumulated dry ravel and underlying cobble bed,
and scouring the channel to bedrock. The channel abuts a plowed terrace,
which allowed a complete sediment budget, confirming that ?95% of sediment
deposited in a debris flow fan matched that evacuated from the channel,
with a minor rainfall-driven hillslope contribution. Subsequent larger
storms produced debris flows in higher-order channels but not in the
first-order channel because of a sediment supply limitation. These
observations are consistent with a model for post-fire ravel routing in
steep, rocky landscapes where sediment was sourced by incineration of
vegetation dams–following ?30 years of hillslope soil production since the
last fire–and transported downslope by dry processes, leading to a
hillslope sediment-supply limitation and infilling of low-order channels
with relatively fine sediment. Our observations of debris flow initiation
are consistent with failure of the channel bed alluvium due to grain size
reduction from dry ravel deposits that allowed high Shields numbers and
mass failure even for moderate intensity rainstorms.

View article:

The assembly of the South China and Indochina blocks: Constraints from
the Triassic felsic volcanics in the Youjiang Basin

Chengshi Gan; Yuejun Wang; Yuzhi Zhang; Xin Qian; Aimei Zhang

The Youjiang Basin is usually regarded as an important foreland basin in
the southern part of the South China Block that is related to the
convergence of the South China and Indochina blocks during the
Permian-Triassic. However, the nature of the basin remains controversial
due to questions about the subduction polarity and suture boundary between
these two blocks. Permian-Triassic felsic volcanics across the Dian-Qiong
and Song Ma suture zones could offer new insights into the convergent
processes of the South China and Indochina blocks. This study presents
detailed petrological, zircon U-Pb dating, and Hf-O isotope and whole-rock
geochemical analyses for the Triassic felsic volcanics of the Youjiang
Basin (northeast of the Dian-Qiong). The dacites and rhyolites from the
Beisi and Baifeng Formations were dated at ca. 240?245 Ma. All of the
felsic volcanics are characterized by high SiO2 (69.40?73.15
wt%), FeOt/MgO, 10000*Ga/Al, and TZr, ? 18O (9.7?11.8‰) and negative ?Nd(t) (from ?9.6 to
?12.3) and zircon ?Hf(t) (from ?6.2 to ?14.5) with A-type
granitoid geochemical affinities, suggesting the reworking of crustal rocks
in an extensional setting. Permian-Triassic felsic igneous rocks display
similar geochemical signatures across the Dian-Qiong suture zone, whereas
they show distinctive Sr-Nd and zircon Hf-O isotopes across the Song Ma
suture zone. The felsic igneous rocks to the northeast of the Song Ma
suture zone have much lower ?Nd(t) and higher ?18O
with negative zircon ?Hf(t) than those to the southwest, which
have positive zircon ?Hf(t). Combined with other geological and
geophysical features, it is inferred that the Song Ma suture zone was
probably the suture boundary between the South China and Indochina blocks,
and the Youjiang Basin was likely a peripheral foreland basin in response
to the southwestward convergence of the South China Block toward the
Indochina Block.

View article:

High-latitude ice and climate control on sediment supply across SW
Gondwana during the late Carboniferous and early Permian

N. Griffis; I. Montañez; R. Mundil; D. Le Heron; P. Dietrich …

The response of sediment routing to climatic changes across
icehouse-to-greenhouse turnovers is not well documented in Earth’s
pre-Cenozoic sedimentary record. Southwest Gondwana hosts one of the
thickest and most laterally extensive records of Earth’s penultimate
icehouse, the late Paleozoic ice age. We present the first high-resolution
U-Pb zircon chemical abrasion?isotope dilution?thermal ionization mass
spectrometry (CA-ID-TIMS) analysis of late Paleozoic ice age deposits in
the Kalahari Basin of southern Africa, which, coupled with existing
CA-ID-TIMS zircon records from the Paraná and Karoo Basins, we used to
refine the late Paleozoic ice age glacial history of SW Gondwana. Key
findings from this work suggest that subglacial evidence in the Kalahari
region is restricted to the Carboniferous (older than 300 Ma), with
glacially influenced deposits culminating in this region by the earliest
Permian (296 Ma). The U-Pb detrital zircon geochronologic records from the
Paraná Basin of South America, which was located downstream of the Kalahari
Basin in the latest Carboniferous and Permian, indicate that large-scale
changes in sediment supplied to the Paraná were contemporaneous with shifts
in the SW Gondwana ice record. Gondwanan deglaciation events were
associated with the delivery of far-field, African-sourced sediments into
the Paraná Basin. In contrast, Gondwanan glacial periods were associated
with the restriction of African-sourced sediments into the basin. We
interpret the influx of far-field sediments into the Paraná Basin as an
expansion of the catchment area for the Paraná Basin during the
deglaciation events, which occurred in the latest Carboniferous (300?299
Ma), early Permian (296 Ma), and late early Permian (
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