New Geology Articles Published Online Ahead of Print in February
Boulder, Colo., USA: Article topics and locations include the Coriolis
effect; insights from iron isotopes; ocean paleotemperatures; the
Midcontinent Rift; the McMurdo Dry Valleys, Antarctica; the South Bohemian
batholith; the creeping section of the San Andreas fault; the growth of the
Tibetan Plateau; and the dwarf planet Ceres. These Geology
articles are online at
https://geology.geoscienceworld.org/content/early/recent
.
Giant sheath-folded nappe stack demonstrates extreme subhorizontal
shear strain in an Archean orogen
Yating Zhong; Timothy M. Kusky; Lu Wang
Abstract:
Giant sheath-folded nappes are associated with suture zones and emplacement
of far-traveled allochthons in Phanerozoic orogens, demonstrating a rare
but significant geologic phenomenon indicative of modern-style plate
tectonics. We document the world’s oldest-known subhorizontal mega-scale
sheath fold from Archean Alpine-style nappes of the Central orogenic belt,
North China craton. The Zanhuang nappes are recumbent Alpine-style
forearc-affinity metabasaltic and metasedimentary nappes emplaced over a
passive continental margin in the Archean, marking an ancient suture zone.
Field evidence shows multiscale sheath folds from decimeters to tens of
meters in size, and our three-dimensional fence profile, fold hinges,
kinematic lineations, and lithological traces define an ~1-km-long
(parallel to the x-axis) sheath fold in the core of the nappe
stack. Structural analysis statistically demonstrates the macro-scale
recumbent sheath-folded nappe preserves a complete 180° hinge-line
curvature. The giant sheath fold plunges northwest, reflecting its
formation during non-coaxial, top-to-the-southeast shearing with extremely
high shear strain (γ ≥10), equated to >10 km of ductile slip on the
bounding surfaces. Slip vectors derived from S-C fabrics on overturned
limbs are consistent with rotation into the southeast-directed transport
direction, parallel to the similarly rotated fold hinges. Comparison of the
giant sheath-folded nappes from the Archean Zanhuang example with
mega-scale sheath folds in Phanerozoic and Proterozoic orogens shows that
Neoarchean lithosphere was stiff enough to allow tectonics to operate in a
manner analogous to modern-style plate tectonics.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49599.1/612161/Giant-sheath-folded-nappe-stack-demonstrates
Oligocene onset of uplift and inversion of the Cascadia forearc basin,
southern Oregon Coast Range, USA
Michael H. Darin; John M. Armentrout; Rebecca J. Dorsey
Abstract:
An extensive detrital zircon U-Pb data set (n = 6324 dates) from
Eocene to Miocene sandstones and modern river sands establishes the onset
of arc magmatism and forearc uplift along the Cascadia convergent margin in
southwestern Oregon (United States). Middle to late Eocene marine strata in
the Coos Bay area were primarily sourced from the Klamath Mountains and
coeval Clarno-Challis volcanoes in central Oregon and/or Idaho. Ancestral
Cascades arc magmatism initiated at 40 Ma and supplied sediment to a broad
forearc basin in western Oregon during late Eocene time. Major reduction of
Ancestral Cascades arc (40–12 Ma) and Clarno-Challis (52–40 Ma) zircon in
the Tunnel Point Sandstone (ca. 33–30 Ma) records the isolation of the Coos
Bay area from the Ancestral Cascades arc due to Oligocene onset of forearc
uplift, basin inversion, and emergence of the southern Oregon Coast Range.
The Tarheel formation (ca. 18–15 Ma) is characterized by disappearance of
Ancestral Cascades arc zircon and a substantial increase in Clarno-Challis
zircon recycled from underlying forearc strata. The ~15–20 m.y. delay
between subduction initiation (ca. 49–46 Ma) and the onset of forearc
uplift (ca. 33–30 Ma) supports insights from thermomechanical models that
identify tectonic underplating and thermally activated lower-crustal flow
as major drivers of deformation and uplift in active forearc regions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49925.1/612162/Oligocene-onset-of-uplift-and-inversion-of-the
The continent-to-ocean transition in the Iberia Abyssal Plain
Ingo Grevemeyer; Cesar R. Ranero; Cord Papenberg; Valenti Sallares; Rafael
Bartolomé ...
Abstract:
Conceptual models of magma-poor rifting are strongly based on studies of
the nature of the basement in the continent-to-ocean transition of the
Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended
continental crust. Yet, basement has only been sampled at a few sites, and
its regional nature and the transition to seafloor spreading inferred from
relatively low-resolution geophysical data are inadequately constrained.
This uncertainty has led to a debate about the subcontinental or
seafloor-spreading origin of exhumed mantle and the rift-related or oceanic
nature of magmatic crust causing the magnetic J anomaly. Different
interpretations change the locus of break-up by >100 km and lead to
debate of the causative processes. We present the tomographic velocity
structure along a 360-km-long seismic profile centered at the J anomaly in
the Iberia Abyssal Plain. Rather than delineating an excessive outpouring
of magma, the J anomaly occurs over subdued basement. Furthermore, its thin
crust shows the characteristic layering of oceanic crust and is juxtaposed
to exhumed mantle, marking the onset of magma-starved seafloor spreading,
which yields the westward limit of an ~160-km-wide continent–ocean
transition zone where continental mantle has been unroofed. This zone is
profoundly asymmetric with respect to its conjugate margin, suggesting that
the majority of mantle exhumation occurs off Iberia. Because the J anomaly
is related to the final break-up and emplacement of oceanic crust, it
neither represents synrift magmatism nor defines an isochron, and hence it
poorly constrains plate tectonic reconstructions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49753.1/612163/The-continent-to-ocean-transition-in-the-Iberia
Tibetan Plateau growth linked to crustal thermal transitions since the
Miocene
Xiu-Zheng Zhang; Qiang Wang; Derek Wyman; Quan Ou; Yue Qi ...
Abstract:
The topographic transition of central–northern Tibet since the early
Miocene has created a consistently high and flat plateau similar to that of
today. However, to date, the associated deep crust and/or mantle events are
poorly understood, mainly due to an early Miocene metamorphic–magmatic lull
within the Qiangtang Block. To address this issue, we undertook a study of
crustal xenoliths and zircon xenocrysts in 6.0–2.3 Ma lavas in the
Qiangtang Block. The occurrence of 22.6–12.9 Ma
high-temperature–low-pressure granulite xenoliths implies that the middle
crust of the block has been very hot since that time. Zircon xenocrysts and
granitic xenoliths from 6.0–2.3 Ma lavas were studied and shown to have
high δ18O values, which supports Miocene crustal melting and the
formation of unexposed, coeval felsic plutons. Combined with paleoelevation
data from the Tibetan Plateau, our results suggest that the early Miocene
cold–hot thermal transition of the middle–lower crust was near-synchronous
with topographic evolution from high-relief mountains to a flat plateau,
which supports crustal flow as the main topographic smoothing mechanism for
central–northern Tibet.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49534.1/612164/Tibetan-Plateau-growth-linked-to-crustal-thermal
Decrypting the polymetamorphic record of the Himalaya
Iva Lihter; Kyle P. Larson; Matthijs A. Smit; John M. Cottle; Kyle T.
Ashley ...
Abstract:
Reconstructions of the tectonometamorphic architectures of orogenic systems
rely on petrogenetic indicator minerals, such as garnet, to trace the
transport of rocks through space and time. We show the results of new
garnet Lu–Hf geochronology and inclusion U-(Th-)Pb geochronology from
exhumed midcrustal metamorphic rocks exposed in the Kanchenjunga region of
the eastern Nepalese Himalaya. Garnet in three of the five specimens
analyzed yielded pre-Himalayan Lu–Hf dates of ca. 292–230 Ma, which
contrasts with Himalayan-aged inclusions therein. Garnet in these specimens
either grew entirely during the Early Permian opening of the Neo-Tethys
Ocean or grew partly during Cambrian–Ordovician orogenesis and partly
during the Cenozoic. The remaining two specimens yield Lu–Hf ages of ca. 50
Ma, which are some of the oldest recorded dates for Himalayan metamorphism.
The apparent discordance between the geochronological techniques highlights
a potential issue with interpretations that rely on single-geochron-method
inclusion records. These results further show that some
pressure-temperature determinations used in Himalayan models may not be
Himalayan in age.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49467.1/612165/Decrypting-the-polymetamorphic-record-of-the
Long-lasting viscous drainage of eclogites from the cratonic
lithospheric mantle after Archean subduction stacking
Zhensheng Wang; Timothy M. Kusky; Lu Wang
Abstract:
The origin of early continental lithosphere is enigmatic. Characteristics
of eclogitic components in the cratonic lithospheric mantle (CLM) indicate
that some CLM was likely constructed by stacking of subducted oceanic
lithosphere in the Archean. However, the dynamic process of converting
high-density, eclogite-bearing subducted oceanic lithosphere to buoyant CLM
remains unclear. We investigate this process through numerical modeling and
show that some subducted and stacked eclogites can be segregated into the
asthenosphere through an episodic viscous drainage process lasting billions
of years. This process increases the chemical buoyancy of the CLM,
stabilizes the CLM, and promotes the preservation and redistribution of the
eclogites in the CLM, explaining the current status of early subduction
relicts in the CLM revealed by geophysical and petrological studies. Our
results also demonstrate that the subduction stacking hypothesis does not
conflict with the longevity of CLM.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49793.1/612166/Long-lasting-viscous-drainage-of-eclogites-from
History of earthquakes along the creeping section of the San Andreas
fault, California, USA
Genevieve L. Coffey; Heather M. Savage; Pratigya J. Polissar; Stephen E.
Cox; Sidney R. Hemming ...
Abstract:
Creeping faults are difficult to assess for seismic hazard because they may
participate in rupture even though they likely cannot nucleate large
earthquakes. The creeping central section of the San Andreas fault in
California (USA) has not participated in a historical large earthquake;
however, earthquake ruptures nucleating in the locked northern and southern
sections may propagate through the creeping section. We used biomarker
thermal maturity and K/Ar dating on samples from the San Andreas Fault
Observatory at Depth to look for evidence of earthquakes. Biomarkers show
evidence of many earthquakes with displacements >1.5 m in and near a
3.5-m-wide patch of the fault. We show that K/Ar ages decrease with thermal
maturity, and partial resetting occurs during coseismic heating. Therefore,
measured ages provide a maximum constraint on earthquake age, and the
youngest earthquakes here are younger than 3 Ma. Our results demonstrate
that creeping faults may host large earthquakes over longer time scales.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49451.1/612167/History-of-earthquakes-along-the-creeping-section
Calibrating the Russøya excursion in Svalbard, Norway, and implications
for Neoproterozoic chronology
Alexie E.G. Millikin; Justin V. Strauss; Galen P. Halverson; Kristin D.
Bergmann; Nicholas J. Tosca ...
Abstract: The Tonian–Ediacaran Hecla Hoek succession of Svalbard, Norway,
represents one of the most complete and well-preserved Neoproterozoic
sedimentary successions worldwide. With diverse fossil assemblages, an
extensive carbonate δ13C record, and sedimentary evidence for
two distinct Cryogenian glaciations, this succession will continue to yield
insights into the Neoproterozoic Earth system; however, at present there
are no direct radiometric age constraints for these strata. We present two
new Re-Os ages and initial Os isotope data that constrain the timing of
Neoproterozoic glaciation in Svalbard, providing further support for two
globally synchronous Cryogenian glaciations and insight into pre- and
post-snowball global weathering conditions. An age from the Russøya Member
(Elbobreen Formation) facilitates correlation of the negative carbon
isotope excursion recorded therein with the pre-glacial “Islay” excursion
of the Callison Lake Formation of northwestern Canada and the Didikama and
Matheos Formations of Ethiopia. We propose that this globally synchronous
ca. 735 Ma carbon isotope excursion be referred to as the Russøya excursion
with northeastern Svalbard as the type locality. This new age provides an
opportunity to construct a time-calibrated geological framework in Svalbard
to assess connections between biogeochemical cycling, evolutionary
innovations within the eukaryotes, and the most extreme climatic changes in
Earth history.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49593.1/612168/Calibrating-the-Russoya-excursion-in-Svalbard
Zn-, Mg- and O-isotope evidence for the origin of mantle eclogites from
Roberts Victor kimberlite (Kaapvaal Craton, South Africa)
Jian Huang; Jin-Xiang Huang; William L. Griffin; Fang Huang
Abstract:
We report Zn-isotope compositions of garnet, clinopyroxene, and whole rocks
for 14 Type I and 10 Type II eclogites from the Roberts Victor kimberlite
(Kaapvaal Craton, South Africa) that were previously analyzed for Mg-O
isotopes. Type II eclogites are the protoliths of the highly metasomatized
Type I. Garnet and clinopyroxene in Type II eclogites have δ66Zn
from 0.14‰ to 0.50‰ and from 0.29‰ to 0.58‰, respectively; reconstructed
whole-rock δ66Zn is from 0.24‰ to 0.54‰, which is higher than
typical mantle values (0.16–0.20‰). Their heavy Zn- and light Mg- and
O-isotope compositions (δ26Mg = –1.1‰ to –0.14‰, δ18O
= 2.3‰ to 4.9‰) cannot originate from subducted, carbonate-rich, altered
oceanic crust, which is enriched in heavy Zn-O and light Mg isotopes. The
low δ18O may be inherited from parental melts derived from low-δ 18O mantle sources like those that produced the Weltevreden
komatiites of the Kaapvaal Craton. The high δ66Zn and low δ 26Mg reflect diffusion-driven Zn-Mg–isotope exchange between
peridotites and the parental melts during their emplacement in the deep
lithosphere. Type I eclogites have reconstructed whole-rock δ66
Zn from 0.03‰ to 0.43‰ and garnet δ18O from 6‰ to 9.1‰ but show
more scatter in inter-mineral Zn-isotope fractionation than Type II,
reflecting incomplete equilibration during later metasomatism by
carbonatitic-to-kimberlitic melts. Our evidence from multiple isotopes thus
suggests that the Roberts Victor eclogites might have crystallized from
deep-seated melts at mantle depths.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49780.1/612169/Zn-Mg-and-O-isotope-evidence-for-the-origin-of
No effect of thermal maturity on the Mo, U, Cd, and Zn isotope
compositions of Lower Jurassic organic-rich sediments
Alexander J. Dickson; Erdem Idiz; Don Porcelli; Melissa J. Murphy; Ricardo
Celestino ...
Abstract:
The isotope ratios of redox-sensitive metals in organic-rich rocks are
critical tools for quantifying the timing and severity of deoxygenation and
nutrient cycling in Earth’s past. The resilience of isotopic data to
thermal alteration of the host sediments over millions of years of burial
is, however, largely unknown. We present molybdenum, uranium, cadmium, and
zinc stable-isotope data from two stratigraphic successions of the same
Lower Jurassic Posidonienschiefer unit in the Lower Saxony Basin of
northern Germany that were affected by different burial histories. We show
that thermal maturity had no effect on the isotopic compositions of these
elements but does appear to have increased their concentrations in the
rock. The data corroborate the results of laboratory-based maturation
studies and provide constraints on the Mo, U, Cd, and Zn isotopic
compositions of ca. 182 Ma seawater in the Lower Saxony Basin.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49724.1/612170/No-effect-of-thermal-maturity-on-the-Mo-U-Cd-and
Coriolis effect recorded in Late Pleistocene Marine Isotope Stage 5e
Bahamian aeolianites
Ben Rendall; Kat Wilson; Charles Kerans; Mark Helper; David Mohrig
Abstract:
The windward islands of the Lucayan Archipelago (Bahamas) form an Atlantic
Ocean– facing transect spanning >950 km in length and 6° of latitude.
The islands’ topography is largely constructed from carbonate wind-blown
dunes (i.e., aeolianites) deposited during the interglacial phases of the
Late Pleistocene and Holocene. New digital elevation data from satellite
radar interferometry (TanDEM-X German Earth observation satellite) enables
a step change in the ability to map and quantify Bahamian aeolian landforms
across the archipelago. A semi-automated mapping approach that leverages
object-based image analysis yields a total aeolianite area of ~1674 km 2 across Great Abaco, Eleuthera, Cat, San Salvador, Long,
Crooked, Acklins, and Mayaguana islands (Bahamas) and the Turks and Caicos
Islands. Longitudinal axis measurements from 747 Pleistocene parabolic
dunes record increasing consistency of east-west orientation with
decreasing latitude. Three U.S. National Data Buoy Center data buoys
provided modern wind direction and velocity measurements (n =
730,933 of each) along this transect. Analysis of wind vectors (>P90
[90th percentile], n = 70,095) demonstrates increasing
organization of easterlies at southern latitudes and an offset in
directionality compared to formational winds of Pleistocene Marine Isotope
Stage (MIS) 5e deposits. Southward trends of increasing wind strength and
consistency reflect geostrophic flow driven by atmospheric circulation
within the Hadley cell and right-hand deflection of the Coriolis effect in
the Northern Hemisphere. We propose that the offset in directionality
between dune axes and modern wind vectors is related to changes in
latitudinal width of the Hadley cell from the Late Pleistocene (MIS 5e) to
today. This data set is robust enough to serve as a benchmark against which
future atmospheric circulation models can be compared.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49454.1/611745/Coriolis-effect-recorded-in-Late-Pleistocene
What drives Fe depletion in calc-alkaline magma differentiation:
Insights from Fe isotopes
De-Hong Du; Ming Tang; Weiqiang Li; Suzanne Mahlburg Kay; Xiao-Lei Wang
Abstract:
The continental crust is strongly depleted in iron relative to mid-oceanic
ridge basalt, broadly identical to the calc-alkaline magmas, suggesting
that calc-alkaline differentiation is key to continent formation. However,
it remains contentious as to what drives Fe depletion during magmatic
differentiation in the crust. The two competing hypotheses for
calc-alkaline differentiation—magnetite versus garnet (± amphibole)
fractionation—predict contrasting Fe isotopic fractionation pathways in
evolved melts because magnetite preferentially depletes ferric,
isotopically heavy Fe whereas garnet (± amphibole) does the opposite. We
report whole-rock Fe isotope data for two suites of igneous rocks from the
central Andes, which represent magmas traversing normal and thickened arc
crust, respectively. The magmas traversing thickened crust show a strong Fe
depletion trend and consistently high δ56Fe values (0.14‰ ±
0.02‰, 1 standard deviation [SD]), while those traversing normal crust are
less depleted in Fe and show variable δ56Fe values (0.10‰ ±
0.05‰, 1SD). The two Andean suites are both isotopically heavier than
Mariana arc (Pacific Ocean) magmas that differentiate along tholeiitic
(Fe-enriching) paths. These results confirm that garnet (± amphibole)
fractionation/retention is the primary driver of Fe depletion in
calc-alkaline magmas, and highlight a role for crustal thickening in
generating calc-alkaline magmas.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49705.1/611746/What-drives-Fe-depletion-in-calc-alkaline-magma
Reassessment of ocean paleotemperatures during the Late Ordovician
Bryce B. Barney; Ethan L. Grossman
Abstract:
The Late Ordovician is a paradox, with a greenhouse climate shifting to an
icehouse climate during a time of presumably high CO2 levels. We
used clumped isotope (Δ47) microanalysis of fossil brachiopod
shells to determine paleotemperatures of North America (Cincinnati Arch)
during the Katian (453–443 Ma). Microanalyses of fossil brachiopods yielded
a right-skewed distribution of clumped isotope temperatures [T(Δ 47)] ranging from 25 °C to 55 °C with a mode of 32 °C and a mean
of 36 °C. To test the impact of Δ47 reordering (reequilibration
of 13C-18O bonding) at burial temperatures onT(Δ47), we applied a Monte Carlo simulation to a Δ 47 reordering model based on the burial history. Because the
reordering simulation indicated partial reordering, we used the simulation
results to back-calculate T(Δ47) and correct the
reordering effect. Correcting for reordering decreased the mean T
(Δ47) by only ~1 °C, to 35 °C, and did not change the mode (32
°C). These temperatures are lower than those of previous studies, but still
suggest that the Late Ordovician climate of the Cincinnati Arch was warmer
than the modern subtropical seas. However, carbonate facies in these strata
argue for upwelling of cool water; thus, typical subtropical surface waters
may have been as warm as 35 °C approaching the Hirnantian glaciation.
Seawater δ18O values derived from back-calculated T(Δ 47) and brachiopod δ18O values average –0.3‰ ± 0.6‰
relative to Vienna standard mean ocean water (VSMOW), consistent with the
value expected for subtropical surface waters in a greenhouse Earth,
assuming a constant seawater-δ18O model.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49422.1/611747/Reassessment-of-ocean-paleotemperatures-during-the
Final inversion of the Midcontinent Rift during the Rigolet Phase of
the Grenvillian Orogeny
Eben B. Hodgin; Nicholas L. Swanson-Hysell; James M. DeGraff; Andrew R.C.
Kylander-Clark; Mark D. Schmitz ...
Abstract:
Despite being a prominent continental-scale feature, the late
Mesoproterozoic North American Midcontinent Rift did not result in the
break-up of Laurentia, and subsequently underwent structural inversion. The
timing of inversion is critical for constraining far-field effects of
orogenesis and processes associated with the rift’s failure. The Keweenaw
fault in northern Michigan (USA) is a major thrust structure associated
with rift inversion; it places ca. 1093 Ma rift volcanic rocks atop the
post-rift Jacobsville Formation, which is folded in its footwall. Previous
detrital zircon (DZ) U-Pb geochronology conducted by laser
ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) assigned
a ca. 950 Ma maximum age to the Jacobsville Formation and led researchers
to interpret its deposition and deformation as postdating the ca. 1090–980
Ma Grenvillian Orogeny. In this study, we reproduced similar DZ dates using
LA-ICP-MS and then dated 19 of the youngest DZ grains using high-precision
chemical abrasion–isotope dilution–thermal ionization mass spectrometry
(CA-ID-TIMS). The youngest DZ dated by CA-ID-TIMS at 992.51 ± 0.64 Ma (2σ)
redefines the maximum depositional age of the Jacobsville Formation and
overlaps with a U-Pb LA-ICP-MS date of 985.5 ± 35.8 Ma (2σ) for
late-kinematic calcite veins within the brecciated Keweenaw fault zone.
Collectively, these data are interpreted to constrain deposition of the
Jacobsville Formation and final rift inversion to have occurred during the
1010–980 Ma Rigolet Phase of the Grenvillian Orogeny, following an earlier
phase of Ottawan inversion. Far-field deformation propagated >500 km
into the continental interior during the Ottawan and Rigolet phases of the
Grenvillian Orogeny.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49439.1/611748/Final-inversion-of-the-Midcontinent-Rift-during
Reconciling persistent sub-zero temperatures in the McMurdo Dry
Valleys, Antarctica, with Neogene dynamic marine ice-sheet fluctuations
Anna Ruth W. Halberstadt; Douglas E. Kowalewski; Robert M. DeConto
Abstract:
In the Ross Sea sector of Antarctica, periodic large-scale marine ice-sheet
fluctuations since the mid-Miocene are recorded by drill core and seismic
data, revealing a dynamic ice-sheet response to past increases in
temperature and atmospheric CO2. In the adjacent, predominantly
ice-free McMurdo Dry Valleys (MDVs), preserved terrestrial landscapes
reflect persistent cold conditions and have been interpreted as indicators
of a stable polar ice sheet, implying that the Antarctic Ice Sheet was
largely insensitive during past warm periods. These disparate data-based
perspectives highlight a long-standing debate around the past stability of
the Antarctic Ice Sheet, with direct implications for the future ice-sheet
response to ongoing climate warming. We reconcile marine records of dynamic
ice-sheet behavior and episodic open-marine conditions with nearby ancient
terrestrial landscapes recording consistent cold-polar conditions. Coupled
ice-sheet and regional climate models nested at a high resolution are used
to investigate surface temperatures in the MDVs during past warm periods.
We find that high-elevation regions of the MDVs remain below freezing even
when ice-free conditions prevail in the nearby Ross Sea. We compare
observed landscapes with the spatial extent of modeled persistent cold
conditions required for preservation of these ancient features,
demonstrating that frozen MDVs landscapes could have coexisted with receded
or collapsed ice sheets during past warm periods.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49664.1/611749/Reconciling-persistent-sub-zero-temperatures-in
Ultrahigh-temperature granites and a curious thermal eye in the
post-collisional South Bohemian batholith of the Variscan orogenic belt
(Europe)
Fritz Finger; David Schiller; Martin Lindner; Christoph Hauzenberger;
Kryštof Verner ...
Abstract:
Comprehensive zircon thermometry that takes into account zircon saturation
temperatures, Ti-in-zircon measurements, and zircon morphologies and
microstructures can provide key information on the thermal evolution of a
granite batholith. The Variscan South Bohemian batholith (Germany, Austria,
and Czech Republic) comprises a series of granitoid units that intruded
between ca. 330 and ca. 300 Ma. We categorize the granitic rocks according
to their emplacement temperature into very low temperature (T)
(VLT; <750 °C), low T (LT; 750–800 °C), medium T (MT;
800–850 °C), high T (HT; 850–900 °C), and ultrahigh T
(UHT; >900 °C). The first stage of batholith formation (ca. 330–325 Ma)
is characterized by LT to MT melting of mainly metasedimentary sources
driven by their isothermal exhumation. In turn, ca. 322 Ma HT and UHT
granites in the southern half of the batholith reveal an ephemeral thermal
anomaly in the subbatholithic crust, which is presumably linked to a hidden
mafic intrusion. The HT and UHT granites are weakly peraluminous, high-K,
I-type rocks. Although sharing some features with A-type granites such as
high Zr and rare earth element contents, they differ from classical A-type
granites in being magnesian, not enriched in Ga over Al, and having high Ba
and Sr contents. A ring structure of ca. 317 Ma MT and/or LT plutons is
observed around the HT and/or UHT granite complex and interpreted as an
aftermath of the hotspot event. This study is an example of how
deep-crustal hotspots, presumably caused by mantle magmatism, can
significantly enhance the effects of decompressional crustal melting in a
post-collisional setting.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49645.1/611750/Ultrahigh-temperature-granites-and-a-curious
Boron proxies record paleosalinity variation in the North American
Midcontinent Sea in response to Carboniferous glacio-eustasy
Wei Wei; Wenchao Yu; Thomas J. Algeo; Achim D. Herrmann; Lian Zhou ...
Abstract:
Salinity variation in ancient epicontinental seas has long been challenging
to reconstruct. We determined salinity variation in the Late Pennsylvanian
North American Midcontinent Sea based on normalized boron concentrations
(B/Ga) and isotopic compositions (δ11B) of bulk shale. We
analyzed the Stark Shale Member of the Missourian Stage Dennis Limestone
cyclothem in the Charleston core (CC) from the Illinois Basin (eastward,
more proximal) and the Iowa Riverton core (IRC) from the Midcontinent Shelf
(westward, more distal). Both cores exhibited the same pattern of secular
variation, with lower B/Ga and δ11B values at their base and
top, recording less saline conditions during lower sea-level stands, and
higher values in their middle part, recording more saline conditions at
highstands—a pattern that conforms to conventional sequence stratigraphic
and glacio-eustatic interpretations of Midcontinent cyclothems. At
equivalent stratigraphic levels, B/Ga and δ11B values are
systematically lower at CC relative to IRC, reflecting the generally lower
salinity conditions of the Illinois Basin relative to the Midcontinent
shelf, which is consistent with greater freshwater influence from the east
due to heavy river runoff into the Appalachian foreland basin. Our findings
serve to demonstrate the utility of paired B concentration and isotope data
for paleosalinity analysis of deep-time shale formations.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49521.1/611751/Boron-proxies-record-paleosalinity-variation-in
Implications for Ediacaran biological evolution from the ca. 602 Ma
Lantian biota in China
Chuan Yang; Yang Li; David Selby; Bin Wan; Chengguo Guan ...
Abstract:
The morphologically differentiated benthic macrofossils of algae and
putative animal affinities of the Lantian biota in China represents the
oldest known Ediacaran macroscopic eukaryotic assemblage. Although the
biota provides remarkable insights into the early evolution of complex
macroeukaryotes in the Ediacaran, the uncertainty in its age has hampered
any robust biological evaluation. We resolve this issue by applying a
petrographic-guided rhenium-osmium (Re-Os) organic-bearing sedimentary unit
study on the Lantian biota. This work confines a minimum age for the first
appearance of the Lantian biota to 602 ± 7 Ma (2σ, including decay constant
uncertainty). This new Re-Os date confirms that the Lantian biota is of
early–mid Ediacaran age and temporally distinct from the typical Ediacaran
macrobiotas. Our results indicate that the differentiation and radiation of
macroscopic eukaryotes, and the evolution of the primitive, erect
epibenthic ecosystem, occurred in the early–mid Ediacaran and were
associated with highly fluctuating oceanic redox conditions. The radiogenic
initial 187Os/188Os ratios derived from the Lantian
(1.14 ± 0.02) and other Ediacaran shales invoke oxidative weathering of
upper continental crust in the early–middle Ediacaran, which may have
stimulated the evolution of life and oceanic-atmospheric oxygenation.
Integrated with published Ediacaran chronological and geochemical data, our
new Re-Os geochemical study of the Lantian black shale provides a refined,
time-calibrated record of environment and eukaryote evolution during the
Ediacaran.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49734.1/611752/Implications-for-Ediacaran-biological-evolution
Comparative morphometric analysis suggests ice-cored pingo-shaped
landforms on the dwarf planet Ceres
Kynan H.G. Hughson; Britney E. Schmidt; Kathrine T. Udell Lopez; Hanna G.
Sizemore; Paul M. Schenk ...
Abstract:
The NASA Dawn mission revealed that the floor of Occator crater on the
dwarf planet Ceres (in the main asteroid belt between Mars and Jupiter) is
populated with small quasi-conical hills. Many of these features exhibit
morphometric properties that are like those of ice-cored periglacial hills
called pingos. Alternatively, some of these Cerean hills have also been
hypothesized to be cryovolcanic in origin. If these hills are analogous to
pingos, they represent ice-rich environments that are attractive targets
for future exploration. We report new constraints on the morphologies of
the Occator hills that aid in determining their origin. We also directly
test how morphologically similar the hills in Occator are to pingos and
volcanic cones on Earth using comparative statistical analyses. Using a
novel application of kernel density estimation and Markov chain Monte Carlo
methods we show that the morphologies of terrestrial pingos and volcanic
cones are quantifiably distinct, and that the Cerean hills share
significant morphometric similarities with pingos on Earth. Our findings
indicate that a statistical treatment of morphometry alone can be a
powerful tool for classifying and comparing planetary surface features, and
that the majority of the resolved Cerean hills are morphometrically more
similar to pingos than to small terrestrial volcanic cones.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49321.1/611753/Comparative-morphometric-analysis-suggests-ice
Non-steady-state slip rates emerge along evolving restraining bends
under constant loading
Hanna Elston; Michele Cooke; Alex Hatem
Abstract:
Recent field studies provide evidence of fault slip-rate variability over
time periods of 10–100 k.y., yet researchers do not know how processes
internal to the fault system (e.g., fault reorganization) impact records of
fault slip rates. In this study, we directly observed fault-system
evolution and measured slip-rate histories within a scaled physical
experiment of a dextral strike-slip 15° restraining bend representative of
a gentle crustal restraining bend. To assess the degree of slip-rate
variability at particular sites along the experimental faults, such as
would be revealed in a field study, we tracked fault slip rates at specific
locations that advected throughout the experiment with accrued fault slip.
Slip rates increased or decreased (5%–25% of the applied velocity) both
during fault reorganization (e.g., fault growth and abandonment) and as
sites migrated to new structural positions. Sites that advected into the
restraining bend showed decreased slip rate. While we expect new fault
growth to reduce slip rates along nearby fault segments, we document that
the growth of new oblique-slip faults can increase strike-slip rates on
nearby fault segments. New oblique-slip thrust faults within the experiment
accommodated off-fault convergence and unclamped nearby strike-slip
segments. The experimental results show that even under a constant loading
rate, slip rates at sites located on stable fault segments can vary due to
either reorganization elsewhere in the fault system or site advection.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49745.1/611420/Non-steady-state-slip-rates-emerge-along-evolving
Quantifying volcanism and organic carbon burial across Oceanic Anoxic
Event 2
Nina M. Papadomanolaki; Niels A.G.M. van Helmond; Heiko Pälike; Appy
Sluijs; Caroline P. Slomp
Abstract:
Oceanic Anoxic Event 2 (ca. 94 Ma; OAE2) was one of the largest Mesozoic
carbon cycle perturbations, but associated carbon emissions, primarily from
the Caribbean large igneous province (LIP) and marine burial fluxes, are
poorly constrained. Here, we use the carbon cycle box model LOSCAR-P to
quantify the role of LIP volcanism and enhanced marine organic carbon (C org) burial as constrained by the magnitude and shape of the
positive stable carbon isotope (δ13C) excursion (CIE) in the
exogenic carbon pool and atmospheric pCO2
reconstructions. In our best fit scenario, two pulses of volcanic carbon
input—0.065 Pg C yr–1 over 170 k.y. and 0.075 Pg C yr –1 over 40 k.y., separated by an 80 k.y. interval with an input
of 0.02 Pg C yr–1—are required to simulate observed changes in δ13C and pCO2. Reduced LIP activity and C org burial lead to pronounced pCO2
reductions at the termination of both volcanic pulses, consistent with
widespread evidence for cooling and a temporal negative trend in the global
exogenic δ13C record. Finally, we show that observed leads and
lags between such features in the records and simulations are explained by
differences in the response time of components of the carbon cycle to
volcanic forcing.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49649.1/611421/Quantifying-volcanism-and-organic-carbon-burial
Biotite as a recorder of an exsolved Li-rich volatile phase in
upper-crustal silicic magma reservoirs
B.S. Ellis; J. Neukampf; O. Bachmann; C. Harris; F. Forni ...
Abstract:
The magmatic-hydrothermal transition is key in controlling the fate of many
economically important elements due to the change in partitioning when melt
and magmatic fluid coexist. Despite its increasing economic importance, the
behavior of lithium (Li) in such environments remains poorly known. We
illustrate how compositionally unusual biotites from the rhyolitic Bishop
Tuff (California, USA) and Kos Plateau Tuff (Greece) may contain a magmatic
volatile phase trapped between layers of biotite crystals. Despite
originating in pristine deposits and showing the expected X-ray diffraction
spectra, these biotites return low (<95 wt%) analytical totals via
electron microprobe (EMP) consistent with the presence of considerable
amounts of light elements (non-measurable by EMP). Lithium contents and
isotope ratios in these biotites are remarkable, with abundances reaching
>2300 ppm, exceptionally light Li isotopic compositions (δ7Li
as low as –27.6‰), and large isotopic fractionation between biotite and
corresponding bulk samples (Δ7Libt–bulk as low as
–36.5‰). Other mineral phases, groundmass glass, and melt inclusions from
the same units do not support an extremely Li-rich melt prior to eruption.
Biotites from phonolitic systems (Tenerife [Canary Islands] and Campi
Flegrei [Italy]) do not show such extreme compositional differences, with
biotite and melt showing roughly equivalent Li contents, underscored by
significantly reduced Δ7Libt–bulk to a maximum of
–10.9‰. We ascribe the difference in behavior to the near-liquidus
appearance of biotite in alkaline magmatic suites, before widespread
exsolution of a magmatic volatile phase in the magma reservoir, while in
rhyolitic suites, biotite crystallizes at low temperature, trapping the
coexisting exsolved fluid phase in the reservoir.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49484.1/611422/Biotite-as-a-recorder-of-an-exsolved-Li-rich
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