New Articles for Geosphere Posted Online in May
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations studied this month include
southwestern Greenland; Colorado; eastern Nevada; and the Canadian
Cordillera. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Evidence for a more extensive Greenland Ice Sheet in southwestern
Greenland during the Last Glacial Maximum
Christopher M. Sbarra; Jason P. Briner; Brandon L. Graham; Kristin Poinar;
Elizabeth K. Thomas ...
Abstract
:
The maximum extent and elevation of the Greenland Ice Sheet in southwestern
Greenland during the Last Glacial Maximum (LGM, 26–19.5 ka) is poorly
constrained. Yet, the size of the Greenland Ice Sheet during the LGM helps
to inform estimates of past ice-sheet sensitivity to climate change and
provides benchmarks for ice-sheet modeling. Reconstructions of LGM ice
extents vary between an inner continental shelf minimum, a mid-shelf
position, and a maximum extent at the shelf break. We use three approaches
to resolve LGM ice extent in the Sisimiut sector of southwestern Greenland.
First, we explore the likelihood of minimum versus maximum Greenland Ice
Sheet reconstructions using existing relative sea-level data. We use an
empirical relationship between marine limit elevation and distance to LGM
terminus established from other Northern Hemisphere Pleistocene ice sheets
as context for interpreting marine limit data in southwestern Greenland.
Our analysis supports a maximum regional Greenland Ice Sheet extent to the
shelf break during the LGM. Second, we apply a simple 1-D crustal rebound
model to simulate relative sea-level curves for contrasting ice-sheet sizes
and compare these simulated curves with existing relative sea-level data.
The only realistic ice-sheet configuration resulting in relative sea-level
model-data fit suggests that the Greenland Ice Sheet terminated at the
shelf break during the LGM. Lastly, we constrain the LGM ice-sheet
thickness using cosmogenic 10Be, 26Al, and 14C exposure dating from two summit areas, one at 381 m above
sea level at the coast, and another at 798 m asl 32 km inland. Twenty-four
cosmogenic radionuclide measurements, combined with results of our first
two approaches, reveal that our targeted summits were likely ice-covered
during the LGM and became deglaciated at ca. 11.6 ka. Inventories of in
situ 14C in bedrock at one summit point to a small degree of
inherited 14C and suggest that the Greenland Ice Sheet advanced
to its maximum late Pleistocene extent at 17.1 ± 2.5 ka. Our results point
to a configuration where the southwestern part of the Greenland Ice Sheet
reached its maximum LGM extent at the continental shelf break.
View article
:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02432.1/614013/Evidence-for-a-more-extensive-Greenland-Ice-Sheet
Post-Laramide, Eocene epeirogeny in central Colorado—The result of a
mantle drip?
Lon D. Abbott; Rebecca M. Flowers; James Metcalf; Sarah Falkowski; Fatima
Niazy
Abstract:
The Southern Rocky Mountains first rose during the Laramide Orogeny (ca.
75–45 Ma), but today’s mountains and adjacent Great Plains owe their
current height to later epeirogenic surface uplift. When and why epeirogeny
affected the region are controversial. Sedimentation histories in two
central Colorado basins, the South Park–High Park and Denver basins,
shifted at 56–54 Ma from an orogenic to an epeirogenic pattern, suggesting
central Colorado experienced epeirogeny at that time. To interrogate that
hypothesis, we analyzed thermal histories for seven samples from central
Colorado’s Arkansas Hills and High Park using thermochronometers with
closure temperatures below ~180 °C, enabling us to track sample exhumation
from ~5–7 km depth. Three samples are from the Cretaceous Whitehorn pluton,
and four are Precambrian granitoids. All zircon and titanite (U-Th)/He
dates (ZHe and THe) and one apatite fission-track (AFT) date are similar to
the 67 Ma pluton emplacement age. Whitehorn dates using the
lower-temperature apatite (U-Th)/He (AHe) thermochronometer are 55–41 Ma.
These data require two exhumation episodes, one ca. 67–60 Ma, the second
beginning at 54–46 Ma. The pluton reached the surface by 37 Ma, based on
the age of volcanic tuff filling a pluton-cutting paleovalley. The
Precambrian samples do not further refine this thermal history owing to the
comparatively higher He closure temperature of their more radiation-damaged
apatite. Laramide crustal shortening caused 67–60 Ma exhumation. Arkansas
Hills shortening ended before 67 Ma, so shortening could not have caused
the exhumation event that began 54–46 Ma; thermochronology supports the
Eocene epeirogeny hypothesis. Epeirogeny affected >2.0 × 104
km2, from the Sawatch Range to the Denver Basin. We attribute
epeirogeny to an Eocene mantle drip that likely triggered subsequent drips,
causing younger exhumation events in adjacent areas.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02434.1/613637/Post-Laramide-Eocene-epeirogeny-in-central
A juvenile Paleozoic ocean floor origin for eastern Stikinia, Canadian
Cordillera
Luke Ootes; Dejan Milidragovic; Richard Friedman; Corey Wall; Fabrice
Cordey ...
Abstract:
The Cordillera of Canada and Alaska is a type example of an accretionary
orogen, but the origin of some terranes remains contentious (e.g., Stikinia
of British Columbia and Yukon, Canada). Presented herein are igneous and
detrital zircon U/Pb-Hf and trace-element data, as well as the first
radiolarian ages from the Asitka Group, the basement to eastern Stikinia.
The data are used to evaluate the role of juvenile and ancient crust in the
evolution of Stikinia and the tectonic environment of magmatism. Two
rhyolites are dated by U-Pb zircon at 288.64 ± 0.21 Ma and 293.89 ± 0.31
Ma, with εHf(t) = +10. Red chert contains radiolarians that are correlated
with P. scalprata m. rhombothoracata + Ruzhencevispongus uralicus assemblages (Artinskian–Kungurian).
Detrital zircon U/Pb-Hf from a rare Asitka Group sandstone have a mode at
ca. 320 Ma and εHf(t) +10 to +16; the detrital zircon suite includes five
Paleoproterozoic zircons (~5% of the population). Detrital zircons from a
stratigraphically overlying Hazelton Group (Telkwa Formation) volcanic
sandstone indicate deposition at ca. 196 Ma with zircon εHf(t) that are on
a crustal evolution line anchored from the Asitka Group. Zircon
trace-element data indicate that the Carboniferous detrital zircons formed
in an ocean arc environment. The Proterozoic detrital zircons were derived
from a peripheral landmass, but there is no zircon εHf(t) evidence that
such a landmass played any role in the magmatic evolution of eastern
Stikinia. The data support that eastern Stikinia formed on Paleozoic ocean
floor during the Carboniferous to early Permian. Consistent with previous
fossil modeling, zircon statistical comparisons demonstrate that Stikinia
and Wrangellia were related terranes during the Carboniferous to Permian,
and they evolved separately from Yukon-Tanana terrane and cratonic North
America.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02459.1/613638/A-juvenile-Paleozoic-ocean-floor-origin-for
The low-angle breakaway system for the Northern Snake Range décollement
in the Schell Creek and Duck Creek Ranges, eastern Nevada, USA:
Implications for displacement magnitude
Sean P. Long; Jeffrey Lee; Nolan R. Blackford
Abstract:
Documenting the kinematics of detachment faults can provide fundamental
insights into the ways in which the lithosphere evolves during
high-magnitude extension. Although it has been investigated for 70 yr, the
displacement magnitude on the Northern Snake Range décollement in eastern
Nevada remains vigorously debated, with published estimates ranging between
<10 and 60 km. To provide constraints on displacement on the Northern
Snake Range décollement, we present retrodeformed cross sections across the
west-adjacent Schell Creek and Duck Creek Ranges, which expose a system of
low-angle faults that have previously been mapped as thrust faults. We
reinterpret this fault system as the extensional Schell Creek Range
detachment system, which is a stacked series of top-down-to-the-ESE brittle
normal faults with 5°–10° stratigraphic cutoff angles that carry
0.1–0.5-km-thick sheets that are up to 8–13 km long. The western portion of
the Schell Creek Range detachment system accomplished ~5 km of structural
attenuation and is folded across an antiformal culmination that
progressively grew during extension. Restoration using an Eocene
unconformity as a paleohorizontal marker indicates that faults of the
Schell Creek Range detachment system were active at ~5°–10°E dips. The
Schell Creek Range detachment system accommodated 36 km of displacement via
repeated excision, which is bracketed between ca. 36.5 and 26.1 Ma by
published geochronology. Based on their spatial proximity, compatible
displacement sense, overlapping deformation timing, and the similar
stratigraphic levels to which these faults root, we propose that the Schell
Creek Range detachment system represents the western breakaway system for
the Northern Snake Range décollement. Debates over the pre-extensional
geometry of the Northern Snake Range décollement hinder an accurate
cumulative extension estimate, but our reconstruction shows that the Schell
Creek Range detachment system fed at least 36 km of displacement eastward
into the Northern Snake Range décollement.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02482.1/613639/The-low-angle-breakaway-system-for-the-Northern
GEOSPHERE articles are available at
https://geosphere.geoscienceworld.org/content/early/recent
. Representatives of the media may obtain complimentary copies of GEOSPHERE
articles by contacting Kea Giles at the address above. Please discuss
articles of interest with the authors before publishing stories on their
work, and please refer to GEOSPHERE in articles published. Non-media
requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
https://www.geosociety.org
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