New GSA Bulletin Articles Published Ahead of Print in January and
February
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 Hiawatha
impact crater in Northwest Greenland; the Wyoming impact crater field; the
Olds Ferry terrane; the stability of dinosaur communities; the dispersal
processes of supercontinents; and the 1986 Borah Peak earthquake. You can
find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Disequilibrium river networks dissecting the western slope of the
Sierra Nevada, California, USA, record significant late Cenozoic
tilting and associated surface uplift
Helen W. Beeson; Scott W. McCoy
Abstract:
The timing, rates, and spatial patterns of elevation change in the Sierra
Nevada, California, USA, have long been the subject of vigorous debate
owing to their importance in constraining the tectonic history of western
North America and models of orogenesis. Here we present a systematic
analysis of multiple measures of fluvial geomorphology along the entire
length of the range and interpret these observations using 1-D and 2-D
landscape evolution modeling based on the stream power fluvial erosion rule
with the rate parameter calibrated from a large data set of
millennial-scale erosion rates. We demonstrate that westward-draining
rivers in the Sierra Nevada are in a disequilibrium state and that this
state is consistent with the transient fluvial response expected from
significant down-to-the-west tilting in the last 11 m.y. Assuming
rigid-block tilting and using multiple independent measures of tilt
magnitude, we find that the magnitude of surface uplift from late Cenozoic
tilting appears to have been similar along strike and likely resulted in
∼500−1300 m of surface uplift at the crest (0.3−0.8° tilt) from the Yuba
through the Stanislaus rivers (∼39.2−38.2°N) and 1000−1400 m of surface
uplift at the crest (0.6−0.9° tilt) from the Tuolumne River south through
the Kings River (∼38.2−36.4°N). We show that the transient fluvial response
to tilting in the northern Sierra is heavily modulated by heterogeneous
lithology and drainage area exchange in a manner that reconciles the high
spatial variability in basement incision observed in numerous prior
studies. However, we find that heterogeneous lithology alone cannot explain
both the transient state and observed patterns in channel steepness, which
seem to require late Cenozoic changes in tectonic forcing. Beyond the
regional implications of a short-lived rapid pulse of late Cenozoic surface
uplift along the entire length of the range, our results demonstrate that a
range-wide approach in which river networks are analyzed both in planform
and profile can elucidate tectonic histories despite heterogeneous
lithology and ongoing changes in network topology.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35463.1/612214/Disequilibrium-river-networks-dissecting-the
Influence of hydrostratigraphy on the distribution of groundwater
arsenic in the transboundary Ganges River aquifer system, India and
Bangladesh
Madhumita Chakraborty; Abhijit Mukherjee; Kazi Matin Ahmed; Alan E. Fryar;
Animesh Bhattacharya ...
Abstract:
The Ganges River delta complex contains a transboundary aquifer system
shared between India and Bangladesh. Although it serves as the main
freshwater source for the population inhabiting the delta, the aquifer
system is severely contaminated with arsenic (As). This study aimed to
determine the control of the delta hydrostratigraphy on the regional-scale
depth distribution of As within the aquifer system. We developed the first
high-resolution, regional-scale, transboundary hydrostratigraphic model of
the Ganges River delta and analyzed the patterns of As distribution as a
function of the hydrostratigraphy. Model results indicate that, despite the
presence of a single aquifer system across the delta, the hydrostratigraphy
is spatially variable and can be architecturally divided into three
distinct aquifer subsystems from northwest to southeast: a single, thick
continuous aquifer (type I); a vertically segregated, semiconfined aquifer
subsystem (type II); and a multilayered, nearly confined aquifer subsystem
(type III). Results indicate that the spatial distribution of As is
characteristically different in each subsystem. In the type I aquifer, As
concentrations tend to be homogeneous at all depths, while in type II and
type III aquifers, As concentrations sharply decrease with depth. The
intervening aquitards in the type II and type III aquifer subsystems appear
to act as natural barriers to infiltration of surficial As or organic
matter−rich water to the deeper aquifer zones. This delineation of the
regional-scale hydrostratigraphic architecture and resulting understanding
of its plausible controls on the depth-distribution of As within the delta
should significantly aid in the systematic framing of sustainable
management plans for the As-safe aquifers within the Ganges River delta
aquifer system.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36068.1/612215/Influence-of-hydrostratigraphy-on-the-distribution
A trans-Iapetus transform fault control for the evolution of the Rheic
Ocean: Implications for an early Paleozoic transition of accretionary
tectonics
Lei Wu; J. Brendan Murphy; William J. Collins; John W.F. Waldron;
Zheng-Xiang Li ...
Abstract:
The Paleozoic era begins with the final assembly of Gondwana and ends with
the amalgamation of the supercontinent Pangea. Although this tectonic
progression is generally well documented, one fundamental but under-studied
phenomenon during this era is the transition from two-way to one-way
(northward) migration of peripheral terranes between Gondwana and
Laurentia-Baltica from Cambrian to Ordovician time. The two-way terrane
accretion was likely initiated during the opening of the Rheic Ocean at ca.
510 Ma when several Gondwana-derived terranes comprising Carolinia,
Ganderia, Avalonia, and Meguma sequentially drifted from the northern
margin of Gondwana and eventually collided with Laurentia or Baltica. Over
the same time interval, the Laurentia-derived Cuyania terrane (a.k.a., the
“Precordillera terrane” is commonly believed to have accreted to the
proto-Andean margin of Gondwana, whereas the peri-Laurentian/Dashwoods
ribbon continent separated from, then re-accreted to, the Laurentian margin
after being trapped during the collision of the Taconic arc. Alternatively,
the Cuyania terrane is suggested to have remained attached to the Ouachita
Embayment throughout the Cambrian−Ordovician, and then passed onto Gondwana
during the collision between the proto-Andean margin and the hypothesized
Texas Plateau at the leading edge of Cuyania. Here we explain the
enigmatic, pene-contemporaneous migration of these peripheral terranes by a
trans-Iapetus transform fault that was likely active between 510 and 450 Ma
and extended from the proto-Appalachian margin of Laurentia to the
proto-Andean margin of Gondwana. The trans-Iapetus transform fault
terminated after the respective accretion of Carolinia and Ganderia to the
proto-Appalachian margin and of Cuyania to the proto-Andean margin. We
interpret the development of the trans-Iapetus transform during the
Cambrian−Ordovician to be a consequence of the global geodynamic transition
from the break-up of Rodinia (continents/terranes drifting away from
Laurentia) to Pangea amalgamation (continents/terranes drifting toward
assembling Laurussia).
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36158.1/612216/A-trans-Iapetus-transform-fault-control-for-the
Late Mesozoic volcanism in the southern Xiong’er Mountains, eastern
Qinling: Partial melting of the subducted Qinling crust underneath
North China
Jun He; He Zhang; Zeqiu Qi; Zhiyi Wang; Fukun Chen
Abstract:
Volcanism along the continental margins is a powerful tool with which to
probe the orogenic processes and crustal components underlying orogenic
belts. In this study, we report the zircon ages and geochemical
compositions of volcanic rocks exposed in the Luanchuan area, along the
southern margin of the North China block. In previous studies, these
volcanic rocks were reported to be products of Paleoproterozoic (ca. 1.75
Ga) volcanism related to rifting in the Xiong’er Mountains. Our zircon age
data reveal that most volcanic rocks contain late Mesozoic zircon grains of
magmatic origin, suggesting that Late Jurassic to Early Cretaceous (ca.
150−120 Ma) volcanism must have occurred in the southern Xiong’er
Mountains, coeval with emplacement of large granitoid plutons along the
southern margin of the North China block. These late Mesozoic volcanic
rocks mainly consist of trachyandesite, andesite, dacite, and rhyolite.
They vary in their major-element compositions but are relatively uniform in
their normalized patterns of trace and rare earth elements and have
consistently enriched Nd isotopic compositions. Numerous inherited and/or
xenocrystic zircon grains of mostly Paleoproterozoic and occasionally
Neoproterozoic to early Mesozoic ages were found in all of the volcanic
rocks, indicating complex magma sources and/or diverse crustal rocks
overlying the magma chambers. The Paleoproterozoic zircon grains originated
from the Paleoproterozoic volcanic and basement rocks in the southern North
China block. Notably, some volcanic rocks contained early Mesozoic, early
Paleozoic, and Neoproterozoic zircon grains of both metamorphic and
magmatic origin that indicate derivation from the North and South Qinling
blocks, which recorded thermo-tectonic events during the closure of the
Shangdan and Mianlue oceans in the early Paleozoic and early Mesozoic.
Therefore, we propose that these late Mesozoic volcanic rocks originated
from partial melting of crustal rocks and are composed partly of the
subducted basement of the Qinling orogenic belt underneath the southern
Xiong’er Mountains due to asthenospheric upwelling in an extensional
setting.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36237.1/612217/Late-Mesozoic-volcanism-in-the-southern-Xiong-er
The late Holocene Nealtican lava-flow field, Popocatépetl volcano,
central Mexico: Emplacement dynamics and future hazards
Israel Ramírez-Uribe; Claus Siebe; Magdalena Oryaëlle Chevrel; Dolors
Ferres; Sergio Salinas
Abstract:
Popocatépetl, one of the most hazardous volcanoes worldwide, poses
significant threats for nearby populations in central Mexico. Therefore, it
is important to reconstruct its eruptive history, including estimates of
lava-flow emplacement times and their rheological properties. These studies
define possible future eruptive scenarios and are necessary to mitigate the
risk. Stratigraphic studies of the cal 350−50 B.C. Lorenzo Plinian pumice
sequence indicate that effusive activity (Nealtican lava-flow field)
occurred shortly after explosive activity, reflecting drastic changes in
the eruptive dynamics. It was likely due to the efficient degassing of the
magma during the Plinian phase and a decrease of magma ascent and
decompression rates. Magma mixing, fractional crystallization, and a minor
crust assimilation are the processes controlling the differentiation of the
Nealtican lavas. We used lava chemical and mineralogical composition to
estimate lava-flow viscosities, and used high-resolution elevation data to
estimate emplacement times. Results indicate that lava viscosities of
andesites and dacites ranged from 109 to 1012 Pa·s
and emplacement durations were between ∼1 and ∼29 years, depending on the
flow unit and morphological method employed. Considering the entire volume
of emitted lava (4.2 km3) and a mean output rate of ∼1 m 3/s to ∼15 m3/s, we estimated that the effusive phase
that produced the Nealtican lava-flow field may have lasted ∼35 years. This
eruption had a considerable impact on pre-Hispanic settlements around the
volcano, whose population exodus and relocation probably contributed to the
rise of important cities in central Mexico, such as Teotihuacán and
Cholula.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36173.1/612175/The-late-Holocene-Nealtican-lava-flow-field
Mesozoic intracontinental ductile shearing along the Paleozoic Shangdan
suture in the Qinling Orogen: Constraints from deformation fabrics and
geochronology
Shengsi Sun; Yunpeng Dong; Chao Cheng; Dengfeng He; Bo Zhou ...
Abstract:
The Qinling Orogen, located between the North and South China Blocks,
records subduction-collisional orogeny along the Paleozoic Shangdan and
Triassic Mianlue sutures, and Mesozoic intracontinental orogenesis, which
all played an important role in building the present tectonic framework and
topography. The strike-slip Shagou ductile shear zone that overprints the
Paleozoic Shangdan suture between the North and South Qinling Belts is
crucial for understanding the Mesozoic intracontinental deformation in the
Qinling Orogen. The microstructures, asymmetrical fabrics, and kinematic
vorticities (0.81−0.95) suggest sinistral simple shear. The quartz c-axis
patterns from felsic mylonites exhibit Y-maximum fabrics, indicating the
activity of prism <a> slip, while those from amphibole-rich mylonites
display both Y-maximum and Z-maximum fabrics showing the combined activity
of prism <a> and basal <a> slip systems. In the mafic
mylonites, the plagioclase fabrics were induced by combined (010)[100] and
(001)[100] slip systems, while the amphibole fabrics were probably related
to anisotropic growth or passive rotation of rigid clasts. Equilibrium P−T
conditions of 4.28−6.12 kbar and 646−727 °C estimated from
geothermobarometry suggest that the main deformation occurred under
amphibolite facies conditions at middle−lower crustal depths. Zircon U-Pb
ages constrain their protolith rocks to have crystallized at 816 ± 25 Ma
and 726−718 Ma, while the intense mylonitization and sinistral shearing
occurred at ca. 200−187 Ma. A U−Pb zircon age of 132 ± 3 Ma from a granitic
dike cutting the foliation and an amphibole 40Ar/39Ar
plateau age of 119.0 ± 0.9 Ma from mylonites together suggest that the
Shagou shear zone evolved through decompression and exhumation stages in
the time period of 132−119 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36293.1/612176/Mesozoic-intracontinental-ductile-shearing-along
Exhumation of plutons controlled by boundary faults: Insights from the
kinematics, microfabric, and geochronology of the Taibai shear zone,
Qinling Orogen, China
Chao Cheng; Shengsi Sun; Yunpeng Dong; Bin Zhang; Zhao Guo
Abstract:
The Taibai granitic plutons lie between the Taibai ductile shear zone to
the north and the Shangdan suture to the south. The deformation mechanism
of the ductile shearing is crucial to understanding the exhumation
processes of the multiple plutons that formed after the Late Mesozoic
period. Geological investigations, microstructures, and kinematic vorticity
calculations indicate that the Taibai shear zone deformed in response to
pure shear-dominated (54%−65%) transpression and top-to-NW shear sense as a
result of NE−SW oblique contractional tectonics. The quartz
crystallographic preferred orientations of the prism <a> slip system,
the grain boundary migration, and sub-grain rotation dynamic
recrystallization of quartz—combined with the plagioclase−hornblende
thermometer—constrain the main deformation temperatures to a range of
400−650 °C, which suggests amphibolite to greenschist facies conditions. In
addition, it is extremely likely that the mylonites experienced late-stage,
lower temperature deformation as demonstrated by the sporadic bulging
recrystallization, the quartz basal <a> slip system, and the
two-feldspar geothermometer calculation. The samples collected from the
weakly deformed mylonitic granite pluton and the undeformed
quartz-feldspathic dike that intruded into the mylonites yield zircon U−Pb
ages of 129 ± 1 Ma and 115 ± 1 Ma, respectively. This information, with the
lower intercept ages of ca. 120 Ma obtained from the mylonite samples,
suggests that the ductile shearing probably occurred from ca. 129 Ma to 115
Ma. Combined with the regional geological data, these findings suggest that
the Taibai shear zone and the Shangdan suture accommodated the oblique
upward extrusion of the Taibai plutons during Early Cretaceous time.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36073.1/612150/Exhumation-of-plutons-controlled-by-boundary
How similar was the 1983 Mw 6.9 Borah Peak earthquake rupture to its
surface-faulting predecessors along the northern Lost River fault zone
(Idaho, USA)?
Christopher B. DuRoss; Richard W. Briggs; Ryan D. Gold; Alexandra E. Hatem;
Austin J. Elliott ...
Abstract:
We excavated trenches at two paleoseismic sites bounding a trans-basin
bedrock ridge (the Willow Creek Hills) along the northern Lost River fault
zone to explore the uniqueness of the 1983 Mw 6.9 Borah
Peak earthquake compared to its prehistoric predecessors. At the Sheep
Creek site on the southernmost Warm Springs section, two earthquakes
occurred at 9.8−14.0 ka (95% confidence) and 6.5−7.1 ka; each had ∼1.9 m of
vertical displacement. About 4 km to the southeast, across the Willow Creek
Hills, two ruptures at the Arentson Gulch site on the northernmost Thousand
Springs section occurred at 9.0−14.7 ka and 6.1−7.5 ka with ∼1.9 m of
vertical displacement each. We synthesize these and previous paleoseismic
results into a model of five postglacial (<15 ka) ruptures along a ∼65
km reach of the northern Lost River fault zone. Our results show that the
Borah Peak earthquake (34 km; 0.9 m mean displacement) was unique compared
to previous ruptures that had both longer and shorter rupture lengths
(∼25−38 km), more displacement (mean of ∼1.3−1.4 m), and equal or greater
magnitude (Mw 6.9−7.1) than that in the 1983
earthquake. These ruptures support a hypothesis of variable rupture length
and displacement on the northern Lost River fault zone and show that
predecessors to the 1983 rupture have passed unimpeded through the Willow
Creek Hills. Our work demonstrates that normal faults are capable of
producing variable spatial-temporal patterns of rupture that, together with
comparisons of fault geometry and historical rupture length, improve our
understanding of fault segmentation and help inform models of earthquake
rupture probability.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36144.1/611909/How-similar-was-the-1983-Mw-6-9-Borah-Peak
Late Miocene−Quaternary seismic stratigraphic responses to tectonic and
climatic changes at the northeastern margin of the South China Sea
Xingxing Wang; Feng Cai; Zhilei Sun; Qing Li; Ang Li ...
Abstract:
Tectonic and climate evolution could be well archived in deep-water
stratigraphy. Based on newly acquired high-resolution two-dimensional (2-D)
multichannel seismic profiles and multibeam bathymetry, together with Ocean
Drilling Program/International Ocean Discovery Program (ODP/IODP) data,
this study investigated the late Miocene−Quaternary deep-water seismic
stratigraphy, sedimentary evolution, and responses to regional tectonic and
climatic variations at the northeastern margin of the South China Sea. The
late Miocene−Quaternary stratigraphy consists of three units (i.e., SU-1,
SU-2, and SU-3 from bottom to top) that are dated to 10.5−6.5 Ma (stage 1),
6.5−0.9 Ma (stage 2), and 0.9 Ma−present (stage 3), respectively. SU-1 is
dominated by sheeted drifts with slight thickness variation, but SU-2 and
SU-3 mainly consist of mounded, lenticular contourite drifts and
channel-like moats. This suggests that the bottom currents intensified from
stage 1 to stage 2, which was probably caused by the sill uplifting at the
Luzon Strait under the influence of tectonic collision between the Luzon
arc and Eurasia since ca. 6.5 Ma. SU-2 and SU-3 are separated by a
basinwide high-amplitude seismic reflection, across which the average
sedimentation rate shows a dramatic increase from ∼28 m/m.y. in SU-2 to
∼144 m/m.y. in SU-3. The high sediment supply since 0.9 Ma could be linked
to the mid-Pleistocene climate transition, which resulted in abundant
rainfall that promoted the Taiwan orogen to contribute more sediments to
the South China Sea. This study indicates that the deep-water stratigraphy
recorded the tectonic collision around the sole deep-water gateway (i.e.,
Luzon Strait) and mid-Pleistocene climate transition event at the
northeastern South China Sea margin, which has important implications for
understanding the relationships among the tectonic, paleoclimatic,
paleoceanographic, and deep-water sedimentary processes in the largest
marginal basin of the western Pacific Ocean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36224.1/611910/Late-Miocene-Quaternary-seismic-stratigraphic
Rifting of the Indian passive continental margin: Insights from the
Langjiexue basalts in the central Tethyan Himalaya, southern Tibet
Chao Wang; Lin Ding; Fulong Cai; Liyun Zhang; Zhenyu Li ...
Abstract:
The Triassic tectonic setting of the Tethyan Himalaya in southern Tibet
remains controversial and is key to revealing the mechanism of Neo-Tethys
Ocean opening and the breakup history of Gondwana. This paper reports
227−225 Ma mafic volcanic rocks interbedded within the Tethyan Himalaya
sequence in southern Tibet, which were formed in a typical passive
continental margin setting. The basalts are tholeiitic with high TiO 2 (3.20−3.38 wt%) and moderate MgO (4.05−5.40 wt%) contents and
exhibit enrichment in light rare earth elements and weak negative Nb, Ta,
Eu, and Ti anomalies. These geochemical compositions, combined with uniform
whole-rock εNd(t) (+1.50 to +2.70) values, indicate that the
magmas were derived by low-degree melting of a deep-seated garnet source
that was heterogeneously modified by an oceanic island basalt-type of
component. Given the other geological evidence, we propose that the
magmatic evolution of the Tethyan Himalaya during the late
Paleozoic−Mesozoic was generally a lithospheric thinning process, that is,
it evolved from a fully developed continental rift system during the Late
Permian to wane during the Triassic. This interpretation further supports
the hypothesis that the Neo-Tethys Ocean opened in a passive pattern and
that the breakup of Gondwana in the late Paleozoic was initiated by
lithospheric thinning. Therefore, the subsequent magmatism was related to
the passive upwelling of normal asthenospheric materials.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36204.1/611911/Rifting-of-the-Indian-passive-continental-margin
The newly discovered ca. 1.35 Ga metamafic rocks in the Oulongbuluke
Block, NW China, and its record for transition from the Columbia to
Rodinia supercontinent
Pei Lv; Shengyao Yu; Yinbiao Peng; Sanzhong Li; Yongjiang Liu ...
Abstract:
The Oulongbuluke Block is an integral part of the Columbia and Rodinia
supercontinents, but the lack of rock records from the transitional period
between the Columbia and Rodinia supercontinents during the mid−late
Mesoproterozoic has impeded our understanding of the tectonic relationship
of the Oulongbuluke Block, which lies between the Columbia and Rodinia
supercontinents. In this contribution, we present a systematic
petrographic, geochemical, and zircon U-Pb-Hf investigation of newly
discovered metamafic rocks in the Oulongbuluke Block. The results show that
the metamafic rocks have a protolith age of ca. 1.35 Ga and an arc-related
metamorphic age of ca. 1.11−1.09 Ga. The metamafic rock samples are
geochemically characterized by relatively high FeOT/MgO and FeOT and low SiO2, MgO, and K2O + Na 2O, which shows tholeiitic affinity. These metamafic rocks
exhibit slight light rare earth element (LREE) depletion and flat heavy
rare earth element (HREE) content with no obvious Eu anomalies and slightly
negative Nb, Sr, and Zr anomalies. These conditions are similar to those of
enriched mid-oceanic-ridge basalt (E-MORB) and normal mid-oceanic-ridge
basalt (N-MORB). The metamorphic rocks studied also have positive zircon ε Hf(t) values (2.96−7.04). Hence, the protoliths of the metamafic
rocks may have been produced by variable degrees of melting of spinel-phase
lherzolite mantle in a mid-oceanic ridge setting that was probably induced
by a mantle plume. The presence of metamafic rocks indicates that the
Oulongbuluke Block experienced the final breakup of the Columbia
supercontinent at ca. 1.35 Ga, and the ca. 1.11−1.09 Ga arc-related
metamorphism coincided with the convergence of the Rodinia supercontinent.
The tectonic setting of the Oulongbuluke Block changed from a mid-oceanic
ridge setting to an arc setting during the mid−late Mesoproterozoic, which
was likely a response to the transition from the Columbia supercontinent to
Rodinia supercontinent.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36269.1/611912/The-newly-discovered-ca-1-35-Ga-metamafic-rocks-in
Late Paleozoic to Mesozoic tectonic transition in northeastern Eurasia:
Constraints from two island arc magmatic belts in eastern NE China
Gong-Yu Li; Jian-Bo Zhou; Long Li; Zhuo Chen; Hong-Yan Wang
Abstract:
Since the Paleozoic, the tectonic evolutionary process of eastern Eurasia
has been affected by at least three paleo-ocean regimes: the Paleo-Asian
Ocean, the Mongol-Okhotsk Ocean, and the Paleo-Pacific Ocean. However, the
tectonic transition among these ocean regimes is not well understood.
Recently, the Heilongjiang Ocean was proposed to play an important role
during the transition from the Paleo-Asian Ocean regime to the
Mongol-Okhotsk Ocean (and later the Paleo-Pacific Ocean) regime. Here we
report on a new combined study of petrological, geochemical, and
geochronological data of biotite−plagioclase gneisses from the Qinglongcun
Complex in the west Jiamusi-Khanka Block to better understand the tectonic
evolution during this transition period. The results show that the
protoliths of these biotite−plagioclase gneisses are medium-K calc-alkaline
rhyolites that were derived from a volcanic island arc environment related
to paleo-oceanic subduction that occurred at ca. 260 Ma. Integrated with
previous studies of the Permian arc-affinity rocks within the
Jiamusi-Khanka Block, we identified two island arc magmatic belts with ages
of ca. 290 Ma and ca. 260 Ma at the east and west margins of Jiamusi-Khanka
Block, respectively. The east island arc magmatic belt was related to
subduction of the Mongol-Okhotsk Ocean during the Early−middle Permian,
which provided a driving force for the opening of the Heilongjiang Ocean.
Subduction of the Heilongjiang Ocean during the middle−Late Permian
resulted in the west arc island magmatic belt. These two belts provide key
evidence for understanding the tectonic transition from the Paleo-Asian
Ocean to the Mongol-Okhotsk Ocean to the Paleo-Pacific Ocean during the
late Paleozoic to early Mesozoic.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36314.1/611896/Late-Paleozoic-to-Mesozoic-tectonic-transition-in
Hydrogeochemical evolution of formation waters responsible for
sandstone bleaching and ore mineralization in the Paradox Basin,
Colorado Plateau, USA
Ji-Hyun Kim; Lydia Bailey; Chandler Noyes; Rebecca L. Tyne; Chris J.
Ballentine ...
Abstract:
The Paradox Basin in the Colorado Plateau (USA) has some of the most iconic
records of paleofluid flow, including sandstone bleaching and ore
mineralization, and hydrocarbon, CO2, and He reservoirs, yet the
sources of fluids responsible for these extensive fluid-rock reactions are
highly debated. This study, for the first time, characterizes fluids within
the basin to constrain the sources and emergent behavior of paleofluid flow
resulting in the iconic rock records. Major ion and isotopic (δ18Owater; δDwater; δ18OSO4; δ34SSO4; δ34S H2S; 87Sr/86Sr) signatures of formation
waters were used to evaluate the distribution and sources of fluids and
water-rock interactions by comparison with the rock record. There are two
sources of salinity in basinal fluids: (1) diagenetically altered highly
evaporated paleo-seawater-derived brines associated with the Pennsylvanian
Paradox Formation evaporites; and (2) dissolution of evaporites by
topographically driven meteoric circulation. Fresh to brackish groundwater
in the shallow Cretaceous Burro Canyon Formation contains low Cu and high
SO4 concentrations and shows oxidation of sulfides by meteoric
water, while U concentrations are higher than within other formation
waters. Deeper brines in the Pennsylvanian Honaker Trail Formation were
derived from evaporated paleo-seawater mixed with meteoric water that
oxidized sulfides and dissolved gypsum and have high 87Sr/ 86Sr indicating interaction with radiogenic siliciclastic
minerals. Upward migration of reduced (hydrocarbon- and H2
S-bearing) saline fluids from the Pennsylvanian Paradox Formation along
faults likely bleached sandstones in shallower sediments and provided a
reduced trap for later Cu and U deposition. The distribution of existing
fluids in the Paradox Basin provides important constraints to understand
the rock record over geological time.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36078.1/611897/Hydrogeochemical-evolution-of-formation-waters
Modern-like elevation and climate in Tibet since the mid-Miocene (ca.
15 Ma)
Qiang Xu; Shun Li; Yan Bai
Abstract:
When the modern-like geomorphology and climate pattern of the
Himalaya-Tibetan Plateau were established still remains unclear. In this
study, we apply paired stable isotope compositions of carbonate (δ18Oc) and leaf wax-derived n-alkanes (δ 2Halk) from the upper Gazhacun Formation in the
Namling Basin to reconstruct the middle Miocene elevation and climate of
southern Tibet. Depositional age of the upper Gazhacun Formation has been
precisely constrained to between 15.5 Ma and 15.1 Ma by zircon U-Pb ages of
dacite interlayers. Paired carbonate derived δ18Ow
values (−17.9 ± 1.3‰ to −18.3 ± 1.3‰) and leaf-wax derived δ2H w values (−131.5 ± 20‰ to −145.7 ± 20‰) plot on or very close to
the global meteoric water line suggesting that these samples experienced
little evaporation enrichment and isotopic alternation. Based on these two
independent proxies, paleoelevation estimates for the Namling Basin are
consistently between 4.6+0.7/−0.8 km and 5.2 +0.7/−0.8 km, supporting a high elevation for
southern Tibet in the mid-Miocene. Integrated with published paleoelevation
estimates for the Himalaya, central and northern Tibet in literature, a
near-present elevation across the whole Himalaya-Tibetan Plateau has
already been established since the middle Miocene (ca. 15 Ma). Besides,
stable isotopic values across the Himalaya-Tibetan Plateau show that the δ 2Hw values gradually increase northward from the
Himalaya to northeastern Tibet, quite similar to that of the present day.
This pattern suggests that during the middle Miocene, the Himalaya-Gangdese
system may have blocked southerly monsoonal moisture from reaching northern
Tibet. Westerlies or local recycling of moisture might be the dominant
moisture sources across northern Tibet, with enriched δ18O w and δ2Hw precipitation values that could
lead to erroneous paleoelevation estimates over central and northern Tibet.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36198.1/611790/Modern-like-elevation-and-climate-in-Tibet-since
Early Paleozoic Cascadia-type active-margin evolution of the Dunhuang
block (NW China): Geochemical and geochronological constraints
Jérémie Soldner; Chao Yuan; Karel Schulmann; Yingde Jiang; Pavla Štípská
...
Abstract:
Origins of early Paleozoic metabasites (granulites and amphibolites) and
their host metasedimentary rocks in the Dunhuang block, NW China, are
addressed by new geochronological and geochemical data. The metabasites
show back-arc basalt−like geochemical features, marked by high Zr/Nb ratios
and Zr-Hf troughs, but they can be classified into two groups based on
their dissimilar protolith ages and distinct Nd signatures. Most group I
metabasaltic rocks were emplaced before 455 Ma and possess high Ba/Nb
ratios (11.46−224), low (Nb/La)PM (0.10−0.71), and negative
whole-rock εNd(t) values (−12.7 to −2.7), whereas group
II rocks have protolith ages around 445 Ma, low Ba/Nb ratios (0.70−22.93),
low (Nb/La)PM (0.78−1.51), and less evolved whole-rock Nd
isotopic features (εNd[t]: −2.0 to +2.7). It is
proposed that group I metabasites originated from an enriched lithospheric
mantle, while group II metabasites were derived from the depleted
asthenospheric mantle. The metasedimentary rocks received detritus mainly
from the neighboring Cambrian magmatic arc, and they are compositionally
similar to active-margin sediments. Metamorphic zircon U-Pb ages ranging
462−422 Ma from the investigated rocks together with prominent magmatism
further suggest high-grade metamorphism prevailing during the Late
Ordovician−early Silurian. Based on these data, a Cascadia-type evolution
is proposed involving an Ordovician−early Silurian suprasubduction
stretching of the Cambrian active continental margin, which culminated with
mantle upwelling. Recent paleogeographic reconstructions support the
evolution and assembly of interior Proto−Tethys-Ran oceanic and continental
plates, including the Dunhuang block, between 510 and 440 Ma, followed by
Panthalassan subduction beneath the Tarim−North China continental
assemblage at 440−430 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article/doi/10.1130/B36220.1/611791/Early-Paleozoic-Cascadia-type-active-margin
Initial uplift of the Qilian Shan, northern Tibet since ca. 25 Ma:
Implications for regional tectonics and origin of eolian deposition in
Asia
Haijian Lu; Shengping Sang; Ping Wang; Zhiyong Zhang; Jiawei Pan ...
Abstract:
Sedimentary rocks in northern Tibet record uplift of the Tibetan Plateau
and its potential connection with the evolution of the central Asian
aridity, therefore offering a typical example of tectonic-climate linkage.
The coarse-grained conglomeratic and sandy red beds of the Lulehe Formation
(Fm.) in the northern Qaidam Basin (QB), northern Tibet, have long been
held as synorogenic sediment accumulation. There is, however, a heated
debate on its source area (the Qilian Shan, the east Kunlun Shan, or Qimen
Tagh) and initiation age (ca. 52, 25.5, or ca. 21 Ma, respectively). These
proposals lead to distinctly different mountain building processes of the
giant Qilian Shan during the Cenozoic. One view argues that the Qilian Shan
began to uplift substantially as a simultaneous far-field response to the
India-Asia collision at 55−50 Ma. In contrast, others claim that
significant rise of the Qilian Shan and thus northeastward expansion of the
Tibetan Plateau did not occur until 19 or 12 Ma. Based on an updated
magnetostratigraphic framework for the Cenozoic sediments in the northern
QB, here we conducted structural, paleocurrent, pebble composition, zircon
grain shape and surface texture, and detrital geochronological analyses of
the Lulehe Fm., in the northern QB. The results indicate that the Lulehe
Fm. was produced essentially by an initial rush of lithic clasts derived
from the deformed Mesozoic sedimentary cover, which can be attributed to
initial rise of the Qilian Shan since ca. 25 Ma. This finding leads
additional credence to the argument that the onset of significant uplift of
mountain ranges along the periphery of the plateau occurred nearly
synchronously from the latest Oligocene through early Miocene. These
prolonged elevated Mesozoic sediments covering the fold-thrust belts of the
northern QB, on the other hand, may have acted as a sustained source of
material for the Miocene eolian deposits in the western Chinese Loess
Plateau.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36242.1/611792/Initial-uplift-of-the-Qilian-Shan-northern-Tibet
Secondary cratering on Earth: The Wyoming impact crater field
Thomas Kenkmann; Louis Müller; Allan Fraser; Doug Cook; Kent Sundell ...
Abstract:
A large number of small impact structures have been discovered in Wyoming,
USA, and we raise the question of how this accumulation occurred. We
document 31 crater structures of 10−70 m diameter with corresponding shock
features but missing meteorite relics. All craters occur along the outcrops
of the uppermost Permo-Pennsylvanian Casper Sandstone Formation and are
∼280 m.y. old. Their spatial arrangement shows clusters and ray-like
alignments. Several craters have elliptical crater morphologies that allow
the reconstruction of impact trajectories. The radial arrangement of the
trajectories indicates that the craters are secondary craters formed by
ejecta from a primary crater whose likely position and size are
reconstructed. Modeling ballistic trajectories and secondary crater
formation indicates that impacts occurred at around 700−1000 m/s and caused
small shock volumes with respect to crater volumes. This is the first field
of secondary craters found on Earth, and we disentangle its formation
conditions.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36196.1/611743/Secondary-cratering-on-Earth-The-Wyoming-impact
Anthropogenic impact on sediment transfer in the upper Missouri River
catchment detected by detrital zircon analysis
Kelly D. Thomson; Daniel F. Stockli; Andrea Fildani
Abstract:
Downstream sediment transport in river systems is impacted by anthropogenic
modifications, such as dams or levees. This study used detrital zircon U-Pb
geochronology and sediment mixture modeling to investigate the effects of
dams on provenance signatures and sediment transport along the upper
Missouri and Yellowstone Rivers. The Yellowstone River is the longest
dam-free river in North America, while the upper Missouri River has several
major reservoir-forming dams. We present 4777 new individual detrital
zircon U-Pb ages from 32 sand samples collected from sand bars of the
Missouri and Yellowstone Rivers and their major tributaries. These new data
along the Missouri River track downstream modification of detrital zircon
U-Pb age signatures due to sediment sequestration in reservoirs, bank
erosion, and mixing at stream confluences. In contrast, detrital zircon
U-Pb age data evolve more progressively downstream along the Yellowstone
River, which displays less anthropogenic influence and muted tributary
mixing and dilution. U-Pb age component proportions along the extensively
dammed Missouri River vary up to 27% downstream, with amplified changes
occurring at river confluences downstream from dams. These dramatic changes
are attributed to storage of sediment and preferential capture of heavy
minerals (including zircon) in reservoirs, resulting in a reduced sediment
load in the river downstream from dams. Consequently, detrital zircon
mixture models show a disproportionate provenance contribution from
tributaries compared to the trunk river downstream from the dams. Dams not
only reduce the sediment flux from a river but also change the locations
where sediment is generated by initiating erosion in a river downstream
from a dam.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36217.1/611744/Anthropogenic-impact-on-sediment-transfer-in-the
The stability of dinosaur communities
before the K−Pg boundary: A perspective from southern Alberta using
calcium isotopes as a dietary proxy
Jeremy E. Martin; Auguste Hassler; Gilles Montagnac; François Therrien;
Vincent Balter
Abstract:
Reconstructing dinosaur trophic structure prior to the Cretaceous−Paleogene
(K−Pg) boundary may provide information about ecosystem organization and
evolution. Using calcium isotopes, we investigate preserved biogenic
isotope compositions in a set of dinosaur teeth from three continental
formations from Alberta, Canada, to assess latest Cretaceous food web
structure. Tooth enamel δ44/42Ca values are presented for
tyrannosaurids (n = 34) and potential large herbivorous prey (n = 42) in
the upper Campanian Dinosaur Provincial Park Formation, uppermost
Campanian−Maastrichtian Horseshoe Canyon Formation, and upper
Maastrichtian−lower Paleocene Scollard Formation, spanning the last ∼10
m.y. of the Cretaceous. The influence of diagenesis is assessed in a subset
sample through major and trace elemental concentrations and ultraviolet
(UV) Raman spectra, which provides a framework for interpreting calcium
isotope values. In the Dinosaur Park Formation, hadrosaurid δ 44/42Ca values are systematically heavier than ceratopsid
values, a difference that is interpreted to reflect niche partitioning
among megaherbivores. Tyrannosaurid δ44/42Ca values are
scattered but on average, they are 44Ca-depleted relative to
herbivorous dinosaurs in all three formations. As interpreted from the
Dinosaur Park data set, tyrannosaurids may have preferentially fed on
hadrosaurids. These analyses offer possibilities for testing whether
trophic structure among non-avian dinosaur ecosystems changed several
millions of years prior to the K−Pg boundary.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36222.1/611716/The-stability-of-dinosaur-communities-before-the-K
The Castle Rock and Ironside Mountain calderas, eastern Oregon, USA:
Adjacent venting sites of two Dinner Creek Tuff units—the most
widespread tuffs associated with Columbia River flood basalt volcanism
Matthew Cruz; Martin J. Streck
Abstract:
The Dinner Creek Tuff is an important unit of mid-Miocene rhyolite
volcanism contemporaneous to flood basalts of the Columbia River magmatic
province. Field mapping along with analytical data of tuff samples identify
two calderas, the Castle Rock and Ironside Mountain calderas, as the
venting sites of two widespread ignimbrites of the Dinner Creek Tuff. Both
calderas lie within the area of the proposed general storage sites of
main-phase Columbia River Basalt magmas. The Castle Rock caldera formed
during the eruption of the 16.16 Ma Dinner Creek Tuff unit 1. The
northwestern boundary of the caldera is roughly defined by the
juxtaposition of over 300 m of densely welded rheomorphic intra-caldera
tuff and tuffaceous mega-breccia deposits against Mesozoic Weathersby
Formation shale and pre-Miocene Ring Butte trachybasalt lavas. Following
caldera collapse, fluvial and lacustrine volcaniclastic sediments were
deposited on the caldera floor, and outflow tuffs of the Dinner Creek Tuff
units 2 and 4 were deposited into the caldera. Aphyric basaltic andesite
and icelandite (Fe-rich andesite), which correlate stratigraphically to
upper Grande Ronde Basalt lavas, intrude the caldera floor deposits, and
lavas are interbedded with sediments and Dinner Creek Tuff unit 4.The
Ironside Mountain caldera formed during eruption of the 15.6 Ma Dinner
Creek Tuff unit 2, which lies ∼15 km north of the Castle Rock caldera. The
caldera is an 11 × 6 km depression wherein over 900 m of intra-caldera,
rheomorphic, and partially welded tuff are bound by Weathersby Formation
shale and Tureman Ranch granodiorite. Post-caldera collapse, basaltic
andesite and icelandite dikes and sills that are also stratigraphically
correlative to upper Grande Ronde Basalt lavas intruded into the tuff,
mostly along the margins of the caldera, which altered much of the
tuff.Mafic lavas within the study area that closely pre- and post-date
Dinner Creek Tuff units were correlated with regional units of the Columbia
River Basalt Group. Porphyritic and aphyric mafic lava flows underlying
Dinner Creek Tuff unit 1 at Castle Rock are correlated with Picture Gorge
Basalt and Grande Ronde Basalt. Aphyric basaltic andesite and icelandite
that intrude into and overlie the Dinner Creek Tuff units 1 and 2 are
westward extensions of fractionated tholeiitic magmas as seen in late-stage
Grande Ronde Basalt units such as the Hunter Creek Basalt. Finally,
porphyritic basalt lava flows that overlie the Hunter Creek Basalt and
volcaniclastic sediments at the Castle Rock caldera are correlative with
the 13.5 Ma Tim’s Peak Basalt. At Castle Rock, pre-caldera Columbia River
Basalt Group lavas appear to lap onto a mid-Miocene topographic high that
stretches northward and westward for tens of kilometers based on
stratigraphic data, and it may be related to regional uplift at initial
impingement of the mantle upwelling to produce the Columbia River Basalt
Group. The Castle Rock and Ironside Mountain calderas exemplify bimodal
volcanism of the Columbia River magmatic province. Eruption of rhyolites is
closely pre- and post-dated by the eruption of local and regional
tholeiitic lavas belonging to the Columbia River Basalt Group. The local
eruption of evolved tholeiitic lavas likely concealed calderas, but these
lavas also illustrate the close proximity of mafic and rhyolitic magmas at
depth at these rhyolite centers. Consequently, the stratigraphy of both the
Castle Rock and Ironside Mountain calderas somewhat differs from that of
rhyolite calderas dominated by silicic and calc-alkaline intermediate pre-
and post-caldera volcanism.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36070.1/611706/The-Castle-Rock-and-Ironside-Mountain-calderas
Late Miocene−Pliocene onset of fluvial incision of the Cauca River
Canyon in the Northern Andes
N. Pérez-Consuegra; G.D. Hoke; P. Fitzgerald; A. Mora; E.R. Sobel ...
Abstract:
The incision of kilometer-scale canyons into high-standing topography is
often used to constrain the surface uplift history of mountain ranges,
controlled by tectonic and geodynamic processes. However, changes in
climate may also be responsible for canyon incision. This study deciphers
the timing of incision of the ∼2.5-km-deep Cauca River Canyon in the
Central Cordillera of the Northern Andes using the cooling (exhumation)
history of rocks from the canyon walls and a regional analysis of channel
steepness in rivers. Ten bedrock samples and one detrital sample were
collected on the eastern border of the canyon between 300 m and 2300 m of
elevation. Bedrock and detrital AFT data yield ages from 50 to 38 Ma, while
two bedrock AHe ages from the valley bottom yield ages of 7−6 Ma. The AHe
ages and inverse thermal history models reveal a previously unidentified
late Miocene (ca. 7−6 Ma) pulse of exhumation that we interpret as the age
of a single incision event that formed the Cauca River Canyon. We conclude
that the Cauca River Canyon was carved as a response to rock uplift in the
northern Central Cordillera and propagation of an erosion wave into the
mountain range starting in the latest Miocene.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36047.1/611707/Late-Miocene-Pliocene-onset-of-fluvial-incision-of
Genetic mechanism of transfer zones in rift basins: Insights from
geomechanical models
Jingshou Liu; Haimeng Yang; Ke Xu; Zhimin Wang; Xinyu Liu ...
Abstract:
A transfer zone is a kind of structure that is produced to conserve
deformation of a fault structure on both sides. Increasing numbers of
transfer zones are being identified in rift basins, which are areas of
petroleum accumulation and potential exploration targets. This paper
provides a numerical simulation method for the genesis and development of
transfer zones based on geomechanical modeling. On the basis of
three-dimensional (3-D) seismic interpretation, using the Tongcheng fault
as an example, the fault activity parameter and fault activity intensity
index were established to quantitatively characterize the difference in
fault activity on the two sides of a transfer zone. A geomechanical model
was developed for a transfer zone in a rift basin, and the structural
characteristics and genetic mechanism of a convergent fault were studied
using paleostress and strain numerical simulations. Affected by different
movements of boundary faults and basement faults, the evolution of the
Tongcheng fault can be divided into three stages: (1) during the Funing
period, which was the main development period of compound transfer faults,
the activity, stress, and strain of the fault blocks on either side of the
Tongcheng fault were obviously different; (2) during the Dainan period,
which was the development stage of inherited compound transfer faults, the
northern part of the Tongcheng area underwent local compression, and the T3
anticline began to form; and (3) during the Sanduo period, the Tongcheng
fault experienced right-lateral strike-slip activity, where the activity
showed two stages of change, first increasing and then decreasing, and the
Tongcheng fault anticline developed. The superposition of multiple complex
tectonic movements produced a transfer zone that has both strike-slip and
extensional fault properties. The geomechanical model in this paper
provides important insights for analyzing the evolution of transfer zones
in rift basins.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36151.1/611687/Genetic-mechanism-of-transfer-zones-in-rift-basins
A waning Saxothuringian Ocean evidenced in the Famennian tephra-bearing
siliceous succession of the Bardo Unit (Central Sudetes, SW Poland)
Grzegorz Racki; Stanisław Mazur; Katarzyna Narkiewicz; Agnieszka
Pisarzowska; Waldemar Bardziński ...
Abstract:
A tephra-rich cherty-clayey Famennian succession within the major Brzeźnica
olistostrome in the Bardo Mountains, Central Sudetes, SW Poland, preserves
a record of the lost ocean later incorporated into the Variscan orogenic
belt. Fluctuating but mostly oligotrophic regimes and low primary
production levels were influenced by weak upwelling below the perennial
oxygen minimum zone, which controlled the interplay between biosiliceous
and siliciclastic deposition in the oceanic basin, with episodic oxygen
deficiency. The Hangenberg Black Shale has been identified in this oceanic
setting based on its characteristics described worldwide (including mercury
enrichments). A tectonic uplift of the sediment source area near the
Devonian-Carboniferous boundary, recorded in the distinguishing provenance
signal of old continental crust, was paired with a global transgression,
anoxia, and volcanic episode in an interglacial interval. Assuming
paleogeographic affinity with the Bavarian facies of the Saxothuringian
terrane, we interpret the allochthonous sediments as part of an
accretionary prism that was gravitationally redeposited into the late
orogenic basin in front of advancing Variscan nappes. The oceanic basin
parental to the Bardo pelagic succession is therefore thought to represent
a tract of the waning Saxothuringian Ocean in the Peri-Gondwanan
paleogeographic domain that was eventually subducted beneath the
Brunovistulian margin of Laurussia. The sediments of the Bardo Ocean basin
also include a distal record of Famennian explosive volcanic activity that
was likely related to a continental magmatic arc whose remnants are
preserved as the Vrbno Group of the East Sudetes.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35971.1/611657/A-waning-Saxothuringian-Ocean-evidenced-in-the
Mass extinction or extirpation: Permian biotic turnovers in the
northwestern margin of Pangea
Sangmin Lee; G.R. Shi; Hans A. Nakrem; Jusun Woo; Jun-Ichi Tazawa
Abstract:
The Capitanian (Middle Permian) mass extinction event, prior to and
separate from the end-Permian mass extinction, has been suggested as a
severe biotic crisis comparable to the big five mass extinctions of the
Phanerozoic. However, there is still controversy about its global
significance. In particular, this purportedly disastrous event in the
Capitanian was mostly documented in the eastern Tethys, especially South
China and Japan, whereas its extent in higher latitudinal regions remains
unclear. A few recent studies have reported biostratigraphic and
chemostratigraphic evidence for the Capitanian extinction at the
northwestern marginal shelf of Pangea, including in the Kapp Starostin
Formation in Spitsbergen. However, we here report a different result from
these previous studies based on a study of abundant brachiopod fossils
collected from eight geological sections that represent the same formation
in western and central Spitsbergen, Arctic Norway. Our biostratigraphic
investigation recognizes a total of five brachiopod assemblages from the
type section of the Kapp Starostin Formation at Festningen in Spitsbergen.
The most striking biotic change in species composition is observed at the
interval between the lowermost Vøringen Member (late Artinskian) and its
overlying member (Kungurian) of the Kapp Starostin Formation in
Spitsbergen, which makes it much earlier than the Capitanian. A similar
faunal shift at the same stratigraphic interval is also observed from
bryozoan-based biostratigraphic data. This faunal turnover could be linked
to a significant climatic shift (cooling) along the northwestern margin of
Pangea during the Artinskian−Kungurian. Specifically, it is inferred that a
climatic perturbation (cooling) likely drove the extirpation (emigration)
of marine faunas out of Spitsbergen and dispersal eastward into some lower
latitudinal and climatically more habitable areas. Our result indicates
that the Capitanian interval in Spitsbergen does not record a catastrophic
event that corresponds to the Capitanian mass extinction in Tethyan regions
but rather marks gradual faunal transitions throughout the Middle to Late
Permian. This faunal transition, driven by continuous cooling, was
accompanied by major changes in regional lithology, which suggest a degree
of local environmental control, especially in the changes of substrate and
water depth, on the composition of the benthic faunas. The Wegener Halvø
and Schuchert Dal Formations (Lopingian) in central East Greenland contain
a diverse brachiopod fauna that is comparable to that of the post-Vøringen
Member in Spitsbergen. This implies that the brachiopods in the
northwestern marginal shelf of Pangea did not suffer a severe mass
extinction in the Capitanian; instead, many of them migrated southward with
the development of the Zechstein seaway.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36227.1/611658/Mass-extinction-or-extirpation-Permian-biotic
Paleozoic−Mesozoic dispersal of Gondwana: Insights from detrital zircon
geochronology of Lesser Himalaya strata, eastern Nepal
Upendra Baral; Lin Ding; Megh Raj Dhital; Kumar KC; Shun Li
Abstract:
Detrital zircon geochronology has rapidly evolved into a powerful tool for
reconstructing the assembly and dispersal processes of supercontinents.
Currently, the dispersal history of Gondwanaland remains highly
controversial. Here we focus on detrital zircon geochronology of the
Gondwana (Carboniferous−Permian Kokaha Diamictite and Jurassic−Cretaceous
Sapt Koshi Formation) and post-Gondwana (Miocene Tamrang Formation)
sequences of the Lesser Himalaya in eastern Nepal. Detrital zircon U-Pb
dating results show that the Carboniferous−Permian sequence peaks at 544
Ma, 890 Ma, 1178 Ma, and 1752 Ma. Likewise, the Jurassic−Cretaceous
sequence peaks at 531 Ma, 947 Ma, 1176 Ma and 1806 Ma along with a much
younger peak at 123 Ma. Similarly, the Miocene sequence peaks at 526 Ma,
987 Ma, and 1740 Ma. Comparing these newly obtained ages with those of
surrounding regions, we confirm that during the Carboniferous−Permian, the
Indian continent was still connected to Gondwana. The U-Pb age distribution
of the Kokaha Diamictite coincides with that of the Tethys Himalaya, which
further suggests the possibility that this unit either shared the same
provenance with or was recycled from the Tethys Himalaya. During the early
Mesozoic, the Indian plate rifted from Gondwana and drifted northward, as
evidenced by Triassic−Jurassic, rift-related magmatism along the Indian
continental margin. Remarkably, there were significant inputs from the
Rajmahal Basalt during deposition of the Jurassic−Cretaceous sequence.
Besides, the Miocene sequence records a large number of zircons that
closely resemble those of the Greater Himalaya, which implies that the
Greater Himalaya may have already been uplifted and eroded by then.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36216.1/611599/Paleozoic-Mesozoic-dispersal-of-Gondwana-Insights
Ordovician−Silurian back-arc silicic magmatism in the southernmost
Appalachians
Clinton I. Barineau; Douglas A. Sagul; Paul A. Mueller
Abstract:
Ordovician and Silurian siliciclastic, volcanic, and plutonic rocks in the
southern Appalachian eastern Blue Ridge and western Inner Piedmont,
formerly interpreted as components of an obducted Taconic arc terrane, are
recognized as parts of an Ordovician paired arc/back-arc system that formed
on the seaward edge of the Laurentian plate. New crystallization ages from
metamorphosed plutonic-volcanic components intruding and intercalated with
metasedimentary back-arc basin lithologies of the
Wedowee-Emuckfaw-Dahlonega back-arc basin confirm rapid deposition
(hundreds of meters per million years) in an Ordovician−Silurian
extensional basin that received input from continentally derived
Mesoproterozoic crust and bimodal volcanic components. U-Pb (206
Pb/238U) zircon ages from 12 samples of Zana Granite yield
minimum crystallization ages between 459 Ma and 430 Ma, while zircon grains
from six samples of Kowaliga Gneiss have minimum crystallization ages
between 452 Ma and 435 Ma. Zircons from a probable metavolcanic unit within
the Wedowee Group yield a 206Pb/238U age of 454 ± 3
Ma. Mesoproterozoic depleted mantle model ages and negative initial Ɛ Hf values of zircons from the Ordovician−Silurian metaigneous
rocks suggest a significant Mesoproterozoic component in their genesis.
Formation of these silicic plutons was likely associated with the intrusion
of mantle-derived mafic melts and decompression melting of Mesoproterozoic
lower crust as part of a tectonically thickened crustal section with a
contribution from sub-continental mantle lithosphere during back-arc
extension. Silicic magma intruding thick successions of bimodal volcanic
rocks and intercalated sedimentary rocks shortly after deposition is
typical of back-arc extension and contraction phases above B-type
subduction zones (i.e., Benioff oceanic subduction) in Lachlan-style
orogens, which is consistent with the accretionary orogenic nature of the
southern Appalachian Taconic orogen.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35948.1/611600/Ordovician-Silurian-back-arc-silicic-magmatism-in
Linkages between nitrogen cycling, nitrogen isotopes, and environmental
properties in paleo-lake basins
Liuwen Xia; Jian Cao; Eva E. Stüeken; Wenxuan Hu; Dongming Zhi
Abstract:
The linkages between nitrogen cycling, nitrogen isotopes, and environmental
properties are fundamental for reconstructing nitrogen biogeochemistry.
While the impact of ocean redox changes on nitrogen isotopes is relatively
well understood, it is poorly known how nitrogen responds to changes in pH
and salinity. To fill the knowledge gap, we explore the effects of these
environmental parameters using a well-controlled set of samples from
Carboniferous−Paleogene lake sediments in China. Our results show that the
threshold of 10−12‰ in δ15N works to distinguish alkaline (pH
> 9) from circum-neutral conditions. Elevated Mo levels in the alkaline
samples support the idea of NH3 volatilization from a reducing
water column in an alkaline setting. For non-alkaline lakes, δ15
N values tend to be higher (up to +10‰) in more saline, anoxic settings,
which is attributed to either the expansion of stagnant anoxic waters
spurring water-column denitrification or a shift from plant-based toward
more microbially dominated ecosystems or both. Our results imply that
salinity-induced redox stratification and basicity can alter nitrogen
biogeochemical cycling beyond what is shown by the marine nitrogen isotope
record alone. This finding will result in an improved understanding of the
dynamic controls of δ15N in sediments and lead to better
biogeochemical interpretations of paleo-environmental conditions from
unknown environmental settings on Earth and beyond Earth.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36290.1/611601/Linkages-between-nitrogen-cycling-nitrogen
Enhanced precipitation in the Gulf of Mexico during the
Eocene−Oligocene transition driven by interhemispherical temperature
asymmetry
Mingqiu Hou; Guangsheng Zhuang; Brooks B. Ellwood; Xiao-lei Liu; Minghao Wu
Abstract:
Studies reveal that the sea-surface temperature (SST) of the Northern
Hemisphere decreased at a smaller amplitude than that of the Southern
Hemisphere during the Eocene−Oligocene transition (EOT). This
interhemispheric temperature asymmetry has been associated with intensified
Atlantic Meridional Overturning Circulation (AMOC) that may have driven
enhanced precipitation and weathering in low latitudes and the subsequent
drawdown of atmospheric carbon dioxide. However, no quantitative
constraints on paleo-precipitation have been reported in low latitudes to
characterize the AMOC effect across the EOT. Here, we present the results
of high-resolution (ca. 6 k.y. per sample) isotopic and biomarker records
from the Gulf of Mexico. Reconstructed precipitation using leaf wax carbon
isotopes shows an increase of 44% across the EOT (34.1−33.6 Ma), which is
accompanied by a secular increase in SST of ∼2 °C during the latest Eocene.
We attribute the enhanced precipitation in the Gulf of Mexico to the
northward shift of the Intertropical Convergence Zone that was driven by an
enlarged polar-tropic temperature gradient in the Southern Hemisphere and
an invigorated AMOC. Our findings link changes in meridional temperature
gradient and large-scale oceanic circulation to the low-latitude
terrestrial hydroclimate and provide paleohydrological evidence that
supports CO2-weathering feedback during the EOT “greenhouse” to
“icehouse” transition.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36103.1/611602/Enhanced-precipitation-in-the-Gulf-of-Mexico
Emergence of wet conditions in the Mono Basin of the Western USA
coincident with inception of the Last Glaciation
Guleed A.H. Ali; Ke Lin; Sidney R. Hemming; Stephen E. Cox; Philipp
Ruprecht ...
Abstract:
At present, the Basin and Range of the western USA is arid, but geologic
studies show evidence of past wetness. The timing of these wetter
conditions reveals a close association with glacial conditions. This
association has led to the hypothesis of a causal link between glacial
climate and regional wetness, but poor age control on the onset of regional
wetness thwarts a test of this hypothesis. Here we determine the start of
the most recent interval of persistent wetness in the Mono Basin, which is
a hydrologically closed depression that sits at the west-central edge of
the Basin and Range. The most recent emergence of persistent wetness in the
Mono Basin is stratigraphically correlated with the depositional age of Ash
19—a rhyolitic ash bed that represents the oldest tephra of the Wilson
Creek Formation and one of the earliest-known products of explosive
volcanic activity from the Mono Craters. We constrain the depositional age
of Ash 19 by using the U/Th disequilibrium dating method to date carbonates
that are younger and older than Ash 19. Our U/Th dating results show that
Ash 19 was deposited before the formation of a cross-cutting carbonate bed
dated to 69.2 ± 0.3 ka but after an underlying carbonate tufa dated to 67.4
± 3.5 ka, which suggests that the start of wetness in the Mono Basin was
contemporary with the inception of the Last Glaciation—the beginning of
Marine Isotope Stage 4—at ca. 70 ka. This finding corroborates the
hypothesis of a link between glacial climate and regional wetness.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36084.1/611220/Emergence-of-wet-conditions-in-the-Mono-Basin-of
Paleo−Pacific plate subduction on the eastern Asian margin: Insights
from the Jurassic foreland system of the overriding plate
Liang Qiu; Ruoyan Kong; Dan-Ping Yan; Hong-Xu Mu; Weihua Sun ...
Abstract:
The subduction of the paleo−Pacific plate beneath the North China block has
been well documented in terms of magmatic activity, geophysical
investigations, and numerical modeling, but the timing of its onset along
the eastern Asian margin and the tectonic processes involved remain poorly
understood. We have now reconstructed the structural evolution and
sedimentation of the Shihuiyao−Gangzidian−Yuantai region of the southern
Liaodong Peninsula, on the overriding plate at the eastern Asian margin,
using geologic mapping, borehole data sets, field structural studies, and
zircon geochronology. In the Shihuiyao area, based on geologic mapping and
drilling, top-to-the-NW thrust faults truncated earlier top-to-the-NE
reverse faults and then were subsequently overprinted by normal faults. In
the Gangzidian area, geologic mapping and 40 boreholes allowed us to
construct eight cross sections revealing the top-to-the-WNW/W thrust faults
from the surface to a depth of at least 0.5 km; the sections show that the
thrust faults extend to the basement and that subsequent normal faults dip
subvertically. In the Yuantai thrust system, the top-to-the-NW thrusts,
expressed as four tectonic windows and a duplex on the map scale, were
intruded by late porphyry and diabase sills. Integration of the data from
the three study areas allowed us to identify one angular unconformity and
at least two phases of later deformation (D1 and D2).
The pre-D1 unconformity is marked by a foreland basin that was
filled with Middle Jurassic clastic rocks that unconformably overlie the
Neoproterozoic and Cambrian basement. The D1 deformation is
represented by NE-SW−striking thrust faults that displaced Neoproterozoic
or Cambrian strata onto the Middle Jurassic strata. The D2
deformation is defined by kilometer-scale, high-angle normal faults with
variable dips and strikes. Although three samples from the Middle Jurassic
clastic rocks did not yield ideal maximum deposition ages (MDAs; ca. 246
Ma), a porphyry dike and a diabase sill that intruded the Yuantai thrust
system and remain undeformed yielded zircon U-Pb ages of ca. 124 and 117
Ma, respectively. Detrital zircon geochronology yielded a MDA of ca. 129 Ma
for Cretaceous deposits in graben structures with hanging walls formed by D 2 normal faults. Thus, the D1 and D2
deformation events can be constrained to the Late Jurassic and Early
Cretaceous, respectively (D1 between ca. 174 and 124 Ma, and D 2 after ca. 129 Ma). The provenance of the detritus in the
Middle Jurassic Wafangdian Formation suggests that the deposits in the
retroarc foreland basin had a source in a thrust sheet of the
Paleoproterozoic basement, but the source of deposits in a minigraben (D 2) was possibly the nearby Neoproterozoic rocks. Therefore, we
reconstructed the retroarc foreland basin and fold-and-thrust belt of the
southern Liaodong Peninsula in terms of a subduction margin and constrained
a Toarcian−Aptian (ca. 174−124 Ma) age for the onset of paleo−Pacific plate
subduction. We interpret the foreland basin system and the subsequent
synconvergent extension to have been the result of slab flattening and
rollback during episodic subduction of the paleo−Pacific plate.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36118.1/611221/Paleo-Pacific-plate-subduction-on-the-eastern
The Numidian sand event in the Burdigalian foreland basin system of the
Rif, Morocco, in a source-to-sink perspective
Anas Abbassi; Paola Cipollari; M.G. Fellin; M.N. Zaghloul; Marcel Guillong
...
Abstract:
During the Tertiary evolution of the Western Mediterranean subduction
system, a migrating foreland basin system developed between the Maghrebian
orogenic belt and the adjacent African Craton. However, a comprehensive
reconstruction of the foreland basin systems of the Rif Chain is still
missing. By integrating field observations with quantitative
biostratigraphic data from calcareous nannofossil assemblages, sandstone
composition, and detrital zircon U-Pb geochronology from selected
stratigraphic successions, we reconstruct the foreland basin system that
developed in the early Miocene in front of the growing Rif orogen. The
successions analyzed are representative of (1) the classical “Numidian
Facies” from the Intrarifian Tanger Unit and (2) the Numidian-like deposits
(mixed successions) of the “Mérinides Facies” from the “Maghrebian Flysch
Basin” and the “Beliounis Facies” from the Predorsalian Unit. Our
petrographic analyses and detrital zircon U-Pb ages show that the
quartzarenites of the “Numidian Facies” originated from the African Craton,
whereas the sublitharenites and feldspathic litharenites from the Mérinides
and Beliounis Facies originated from a cratonic area and the exhuming Rif
Chain. Our biostratigraphic analyses suggest a simultaneous arrival of the
quartz grains in the Numidian, Mérinides, and Beliounis deposits, which
indicates that their deposition occurred at ∼1 m.y. (ca. 20−19 Ma, early
Burdigalian) and allows us to delineate the early Burdigalian foreland
basin system of the Rif Chain. The foreland depozone received the “Numidian
Facies,” the foredeep-hosted ∼2000 m of the “Mérinides Facies” and the Beni
Ider Flysch, whereas the wedge-top depozone was characterized by deposition
of the “Beliounis Facies.” The Numidian Sandstones and the Numidian-like
deposits analyzed in Morocco show the same age as similar deposits from
Algeria, Tunisia, and Sicily, which suggests a comparable early Burdigalian
tectono-sedimentary evolution along the southern branch of the Western
Mediterranean subduction-related orogen.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36136.1/610772/The-Numidian-sand-event-in-the-Burdigalian
Microporphyritic and microspherulitic melt grains, Hiawatha crater,
Northwest Greenland: Implications for post-impact cooling rates,
hydration, and the cratering environment
Adam A. Garde; Nynke Keulen; Tod Waight
Abstract:
Sand-sized impactite melt grains hand-picked from a glaciofluvial sample
proximal to the Hiawatha impact crater in Northwest Greenland contain new
information about the crystallization and cooling history of this impact
structure, which is concealed by the Greenland Ice Sheet. Of course, the
original locations of the individual sand grains are unknown, but this is
offset by the substantial number and wide variety of impactite grains
available for study. A detailed investigation of 16 melt grains shows that
post-cratering crystallization took place under very variable conditions of
strong undercooling with temperatures that dropped rapidly from high above
their solidus to far below. A distinct event of near-isochemical hydration
at above or ∼250 °C is recorded by intense perlitic fracturing and the
growth of closely packed mordenite spherulites only 1−3 μm across in felsic
melt grains, which was followed by lower temperature hydrothermal
alteration along the pre-existing perlitic fractures. The formation of
abundant mordenite microspherulites appears to be very rare or not
previously recorded in impactite melts and suggests the rapid infilling of
the Hiawatha crater by a hydrous source. The infilling did not occur
immediately after the impact as in submarine impacts, but soon thereafter,
and before the establishment of a low-temperature hydrothermal alteration
system common to the waning stage of cooling in many impact structures.
These observations and previous documentation of terrestrial organic matter
in the impactites are consistent with an impact into a water-rich
terrestrial environment, such as through the Greenland Ice Sheet or into a
forested, lacustrine−fluvial region.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36058.1/610670/Microporphyritic-and-microspherulitic-melt-grains
Middle−Late Triassic southward-younging granitoids: Tectonic transition
from subduction to collision in the Eastern Tianshan−Beishan Orogen, NW
China
Qigui Mao; Songjian Ao; Brian F. Windley; Zhiyong Zhang; Miao Sang ...
Abstract:
To constrain the closure mechanism and time of the Paleo-Asian Ocean, we
report new geochronological and geochemical data for Triassic granites
along a NW−SE corridor from Eastern Tianshan to Beishan, NW China. Seven
granites have U-Pb ages that young southwards from 245 Ma to 234 Ma in the
Kanguer accretionary complex, to 237 Ma to 234 Ma in the eastern Central
Tianshan block, to 229 Ma to 223 Ma in the Liuyuan accretionary complex.
Granites in the Kanguer accretionary complex formed by fractional
crystallization and are peraluminous, high-K, calc-alkaline, and
crust-derived. They have very low MgO (Mg# = 6−9), Cr, and Ni contents, and
their high εNd(t) (+3.40) and εHf(t) (+4.49
to +11.91) isotopes indicate that the Dananhu arc crust was juvenile. The
Huaniushan pluton in the Liuyuan accretionary complex displays the
geochemical signatures of both A1- and A2-type
granites (Y/Nb = 0.32−3.39). All other granites in the Central Tianshan
block and Liuyuan accretionary complex are aluminous A2-types
with high K2O+Na2O, Al, rare earth elements (REE),
Zr+Nb+Y, Ga, Fe/Mg, and Y/Nb and remarkable depletions of Eu, Ba, Nb, Ta,
Sr, P, and Ti. They have a broad range of MgO (Mg# = 9−59), Cr, and Ni
contents, Isr (0.70741−0.70945) values, negative εNd (t) (−2.98 to −1.14), and low to moderate ε Hf(t) (−1.22 to +7.78), which suggests a mixture of
mantle and crustal components. These 245−223 Ma granitoids show marked
Nb-Ta depletions that point to a subduction origin. Notable enrichments in
Nd-Hf isotopes of Late Triassic granites are likely an indication of
collision. Integration with previous data enables us to conclude that the
delamination of an oceanic slab and mantle upwelling induced partial
melting of thickened arc crust during a tectonic transition from a multiple
supra-subduction margin to a collisional setting in the Late Triassic.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36172.1/610671/Middle-Late-Triassic-southward-younging-granitoids
Geochronology and geochemistry of the Huntington Formation, Olds Ferry
terrane, Blue Mountains province, northern U.S. Cordillera:
Implications for accreted terrane correlation and assembly
Bryant Ware; Kyle P. Tumpane; Gene A. Kurz; Mark D. Schmitz; Clyde J.
Northrup
Abstract:
The Olds Ferry terrane is the more inboard of two accreted volcanic arc
terranes in the late Paleozoic−early Mesozoic Blue Mountains province of
the northern U.S. Cordillera. We present geologic, geochronologic, and
geochemical data from the volcano-sedimentary Huntington Formation of the
Olds Ferry arc that place the terrane within a firm temporal and
tectonomagmatic context, and establish its identity as a fringing arc
terrane along the Triassic to Early Jurassic Cordilleran margin. The
Huntington Formation is divided into two unconformity-bounded informal
members: a Norian (ca. 220 Ma) lower member comprising a sequence of
mafic-intermediate volcanics, massive volcaniclastic breccias, and minor
carbonates deposited unconformably onto the 237.7 Ma Brownlee pluton and
intruded by the 210.0 Ma Iron Mountain pluton; and a Rhaetian through
Pleinsbachian (<210−187.0 Ma) upper member composed of massive
conglomerates, abundant rhyodacite to rhyolite effusive and pyroclastic
flows, and interlayered sandstone turbidites, deposited with angular
unconformity onto the lower member. An erosional hiatus and regional
tilting produced an angular unconformity separating the Huntington
Formation from the overlying basal conglomerates of the late Early to
Middle Jurassic Weatherby Formation of the Izee forearc basin transgressive
onlap sequence. Huntington Formation volcanic rocks are isotopically
enriched relative to depleted mantle and coeval igneous rocks in the
outboard Wallowa terrane. A temporal evolution to more radiogenic 87Sr/86Sr ratios (0.7036−0.7057) and εNd values (+5.4
to +3.1) in the upper member volcanics suggests increasing involvement of
continental-derived material in their petrogenesis. Precambrian xenocrystic
zircons in both lower and upper member volcaniclastic rocks strongly
support a proximal location of the Olds Ferry terrane to cratonal North
America during much of its history. The chronology and tectonostratigraphic
architecture of the Olds Ferry terrane allows its robust correlation to
other fringing-arc terranes along the U.S. and Canadian Cordillera.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36036.1/610672/Geochronology-and-geochemistry-of-the-Huntington
Tectonic controls on Quaternary landscape evolution in the Ventura
basin, southern California, USA, quantified using cosmogenic isotopes
and topographic analyses
A. Hughes; D.H. Rood; D.E. DeVecchio; A.C. Whittaker; R.E. Bell ...
Abstract:
The quantification of rates for the competing forces of tectonic uplift and
erosion has important implications for understanding topographic evolution.
Here, we quantify the complex interplay between tectonic uplift,
topographic development, and erosion recorded in the hanging walls of
several active reverse faults in the Ventura basin, southern California,
USA. We use cosmogenic 26Al/10Be isochron burial
dating and 10Be surface exposure dating to construct a
basin-wide geochronology, which includes burial dating of the Saugus
Formation: an important, but poorly dated, regional Quaternary strain
marker. Our ages for the top of the exposed Saugus Formation range from
0.36 +0.18/−0.22 Ma to 1.06 +0.23/ −0.26 Ma, and our burial ages near the base of shallow marine
deposits, which underlie the Saugus Formation, increase eastward from 0.60+0.05/−0.06 Ma to 3.30 +0.30/ −0.41 Ma. Our geochronology is used to calculate rapid long-term
reverse fault slip rates of 8.6−12.6 mm yr−1 since ca. 1.0 Ma
for the San Cayetano fault and 1.3−3.0 mm yr−1 since ca. 1.0 Ma
for the Oak Ridge fault, which are both broadly consistent with
contemporary reverse slip rates derived from mechanical models driven by
global positioning system (GPS) data. We also calculate terrestrial
cosmogenic nuclide (TCN)-derived, catchment-averaged erosion rates that
range from 0.05−1.14 mm yr−1 and discuss the applicability of
TCN-derived, catchment-averaged erosion rates in rapidly uplifting,
landslide-prone landscapes. We compare patterns in erosion rates and
tectonic rates to fluvial response times and geomorphic landscape
parameters to show that in young, rapidly uplifting mountain belts,
catchments may attain a quasi-steady-state on timescales of <10 5 years even if catchment-averaged erosion rates are still
adjusting to tectonic forcing.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36076.1/610673/Tectonic-controls-on-Quaternary-landscape
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