New Articles for Geosphere Posted Online in July and August
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied include St.
Lawrence Island, the Kodiak Islands, and Yakutat Bay, Alaska; Lake City
caldera, Colorado; the Canadian Cordillera; and crushed fossil turtle
shells. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Fluid-driven cyclic reorganization in shallow basaltic fault zones
Bob Bamberg; Richard Walker; Marc Reichow; Audrey Ougier-Simonin
Abstract:
Faults represent a critical heterogeneity in basaltic sequences, yet few
studies have focused on their architectural and hydromechanical evolution.
We present a detailed, multi-scale characterization of passively exhumed
fault zones from the layered basalts of the Faroe Islands, which reveals
cyclic stages of fault evolution. Outcrop-scale structures and fault rock
distribution within the fault zones were mapped in the field and in 3-D
virtual outcrop models, with detailed characterization of fault rock
microstructure obtained from optical and scanning electron microscopy. The
fault zones record deformation localization from decameter-wide Riedel
shear zones into meter-wide fault cores that contain multiple cataclastic
shear bands and low-strain lenses organized around a central slip zone.
Shear bands and the slip zone consist of (ultra-) cataclasites with a
zeolite-smectite assemblage replacing the original plagioclase-pyroxene
host rock composition. Low-strain lenses are breccias of weakly altered
host rock or reworked fault rocks. Slip zone-proximal zones show
significant late-stage dilatation in the form of hydrothermal breccias or
tabular veins with up to decimeter apertures. We interpret these structures
as evolving from alternating shear-compaction and dilation through
hydrofracture. The fault core preserves slip zone reworking, which is
interpreted to indicate repeated shear zone locking and migration. The
alternating deformation styles of shear-compaction and dilatation suggest
episodic changes in deformation mechanisms driven by transient overpressure
and release. The fault zone mechanical properties are thus governed by the
combined effects of permanent chemical weakening and transient
fluid-mediated mechanical weakening, alternating with cementation and
healing. We suggest that the model presented for fault evolution should
apply widely to shallow, basalt-hosted fault zones.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02488.1/616519/Fluid-driven-cyclic-reorganization-in-shallow
Middle Miocene faulting and basin evolution during central Basin and
Range extension: A detailed record from the upper Horse Spring
Formation and red sandstone unit, Lake Mead region, Nevada, USA
Melissa A. Lamb; Thomas A. Hickson; Paul J. Umhoefer; Zachary W. Anderson;
Crystal Pomerleau ...
Abstract:
Miocene basins of the Lake Mead region (southwestern United States) contain
a well-exposed record of rifting and the evolving paleogeography of the
eastern central Basin and Range. The middle Miocene Horse Spring Formation
and red sandstone unit allow for detailed stratigraphic,
chronostratigraphic, and structural analysis for better understanding the
geologic history of extension in this region. We present new data from the
White Basin and Lovell Wash areas (Nevada) to interpret the evolution of
faulting, basin fill, and paleogeography. We conclude that tectonics
strongly influenced sedimentation and hypothesize that climate may have
played a secondary but important role in creating stratigraphic variations.
Deposited from 14.5 to 13.86 Ma, the microbialitic Bitter Ridge Limestone
Member of the Horse Spring Formation, the stratigraphically lowest unit in
this study, records a widespread shallow and uniform lake which had
moderate and steady sedimentation rates, both of which were controlled by a
few faults. The persistent lake was broken up by fault reorganization
followed by deposition of the highly variable fluvial-lacustrine facies of
the Lovell Wash Member from 13.86 to 12.7 Ma. During this time, faulting
shifted from the northeast-trending, oblique normal left-lateral White
Basin fault to the northwest-trending, normal Muddy Peak fault and other
smaller northwest-trending faults. The lower and middle portions of the red
sandstone unit, 12.7–11.4 Ma, record an increase in the sedimentation rate
of basin fill near the Muddy Peak fault as well as the return to widespread
lacustrine conditions. Sedimentation and faulting slowed during deposition
of the uppermost red sandstone unit, but some deformation occurred
post–11.4 Ma. This study records basin-fill evolution including variations
in depositional environments laterally and vertically, documents changes in
the location and magnitude of faulting, supports earlier work that
hypothesized faulting proceeded in discrete westward steps across the Lake
Mead area, and helps constrain the paleogeographic and tectonic evolution
of the region.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02463.1/616489/Middle-Miocene-faulting-and-basin-evolution-during
Kinematics and paleogeology of the western United States and northern
Mexico computed from geologic and paleomagnetic data: 0 to 48 Ma
Peter Bird; Raymond V. Ingersoll
Abstract:
Fault traces and offsets, cross-section length changes, paleomagnetic
inclination and declination anomalies, and stress-direction indicators with
ages back to 90 Ma are collected from the geologic literature on the
western United States and northern Mexico. Finite-element program Restore
simulates paleokinematics by weighted least squares and integrates
displacements, strains, and rotations back in time, producing paleogeologic
maps, as well as maps of velocity, heave rate, strain rate, and stress
direction at 6 m.y. intervals. After calibrating three program parameters
against neotectonic velocities from geodesy, all classes of data except
inclination anomalies are fit reasonably well. The kink in the San Andreas
fault near San Gorgonio Pass has been gradually restored by slip on
adjacent faults and automated smoothing. Piercing-point pairs successfully
restored along the San Andreas–Gulf of California plate boundary include
the Pelona and Orocopia Schists at 6 Ma, the Pinnacles and Neenach
Volcanics at 21 Ma, and the Jolla Vieja and Poway conglomerates adjacent to
their Sonoran source at 48–42 Ma. During 18–6 Ma, rapid extension on the
Oceanside detachment fault system was restored, placing present San Nicolas
Island adjacent to present Rosarito, Baja California, at 18 Ma. Since ca.
18 Ma, the western Transverse Ranges have rotated 70° clockwise,
restoration of which implies that sinistral faults in this province
originated with NNE trends. The first contact between the Pacific and North
America plates at ca. 28 Ma was not associated with any dramatic increase
in dextral faulting on land; instead, the primary result was extension in
the Plush Ranch–Vasquez-Diligencia basins and Colorado River corridor,
probably driven by an unstable triple-junction and accelerated by heating
and uplift of North America above enlarging slab windows.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02474.1/616490/Kinematics-and-paleogeology-of-the-western-United
Thermal history modeling techniques and interpretation strategies:
Applications using HeFTy
Kendra E. Murray; Andrea L. Stevens Goddard; Alyssa L. Abbey; Mark Wildman
Abstract:
Advances in low-temperature thermochronology, and the wide range of
geologic problems that it is used to investigate, have prompted the routine
use of thermal history (time-temperature, tT) models to quantitatively
explore and evaluate rock cooling ages. As a result, studies that
investigate topics ranging from Proterozoic tectonics to Pleistocene
erosion now commonly require a substantial numerical modeling effort that
combines the empirical understanding of chronometer thermochemical behavior
(kinetics) with independent knowledge or hypotheses about a study area’s
geologic history (geologic constraints). Although relatively user-friendly
programs, such as HeFTy and QTQt, are available to facilitate thermal
history modeling, there is a critical need to provide the geoscience
community with more accessible entry points for using these tools. This
contribution addresses this need by offering an explicit discussion of
modeling strategies in the program HeFTy. Using both synthetic data and
real examples, we illustrate the opportunities and limitations of thermal
history modeling. We highlight the importance of testing the sensitivity of
model results to model design choices and describe a strategy for
classifying model results that we call the Path Family Approach. More
broadly, we demonstrate how HeFTy can be used to build an intuitive
understanding of the thermochronologic data types and model design
strategies that are capable of discriminating among geologic hypotheses.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02500.1/616296/Thermal-history-modeling-techniques-and
Crushed turtle shells: Proxies for lithification and burial-depth
histories
Holger Petermann; Tyler R. Lyson; Ian M. Miller; James W. Hagadorn
Abstract:
We propose a new proxy that employs assemblages of fossil turtle shells to
estimate the timing and depth at which fossilization and lithification
occur in shallowly buried terrestrial strata. This proxy, the Turtle
Compaction Index (TCI), leverages the mechanical failure properties of
extant turtle shells and the material properties of sediments that encase
fossil turtle shells to estimate the burial depths over which turtle shells
become compacted. Because turtle shells are one of the most abundant
macroscopic terrestrial fossils in late Mesozoic and younger strata, the
compactional attributes of a suite of turtle shells can be paired with
geochronologic and stratigraphic data to constrain burial histories of
continental settings—a knowledge gap unfilled by traditional burial-depth
proxies, most of which are more sensitive to deeper burial depths. Pilot
TCI studies of suites of shallowly buried turtle shells from the Denver and
Williston basins suggest that such assemblages are sensitive indicators of
the depths (~10–500 m) at which fossils and their encasing sediment become
sufficiently lithified to inhibit further shell compaction, which is when
taphonomic processes correspondingly wane. This work also confirms
previously hypothesized shallow Cenozoic burial histories for each of these
basins. TCI from mudstone-encased turtle shells can be paired with
thicknesses and ages of overlying strata to create geohistorical burial
curves that indicate when such post-burial processes were active.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02513.1/616297/Crushed-turtle-shells-Proxies-for-lithification
Volcano-pluton connections at the Lake City magmatic center (Colorado,
USA)
Ayla S. Pamukçu; Blair Schoene; Chad D. Deering; C. Brenhin Keller; Michael
P. Eddy
Abstract:
Exposed at the Lake City caldera (Colorado, USA) is the ca. 23 Ma reversely
stratified (rhyolite to trachyte) Sunshine Peak Tuff and post-collapse
syenite and monzonite resurgent intrusions. Existing models for this system
suggest that the rhyolites are related to the trachyte and resurgent
syenite through fractional crystallization, separation, and remobilization
(crystal mush model), and that multiple magma batches were involved in the
system (Hon, 1987; Kennedy et al., 2016; Lubbers et al., 2020). We use U-Pb
zircon CA-ID-TIMS-TEA and zircon trace-element modeling to further probe
age and geochemical relationships between the extrusive and intrusive
units. Zircon ages and compositions from the erupted units and the syenite
overlap, suggesting these magmas were related and may have mixed prior to
eruption. Results from the monzonite suggest it was a contemporaneous but
distinct magma batch that mixed with parts of the larger system. Trends in
zircon geochemistry are decoupled from time, reflecting a complex history
of accessory mineral saturation and mixing of magma batches, and a distinct
high-Hf population of zircon grains hints at the existence of an
additional, independent batch of rhyolitic magma in the system. The new
ages we present shorten the lifetime of the Lake City magmatic system from
80 to 300 k.y. (Bove et al., 2001) to 60 to 220 k.y. and suggest the
high-silica rhyolite magma crystallized over a minimum of ~160 k.y. This
latter timescale likely reflects a protracted history that includes
differentiation of a parent melt prior to extraction of eruptible
high-silica rhyolite magma.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02467.1/616165/Volcano-pluton-connections-at-the-Lake-City
Development of the Whitehorse trough as a strike-slip basin during
Early to Middle Jurassic arc-continent collision in the Canadian
Cordillera
Leigh H. van Drecht; Luke P. Beranek; Maurice Colpron; Adam C. Wiest
Abstract:
The Whitehorse trough is a synorogenic basin in the northern Cordillera
that resulted from arc-collision processes along the northwestern margin of
North America, but its filling history and tectonic significance remain
uncertain. New detrital zircon U-Pb-Hf isotope analyses of 12 rock samples,
including six basal sandstones that sit unconformably on Triassic rocks of
Stikinia, were combined with published detrital zircon and fossil data to
establish the depositional ages of synorogenic Laberge Group strata in
Yukon and test proposed links between Intermontane terrane exhumation and
basin-filling events. Laberge Group strata yielded 205–170 Ma and 390–252
Ma detrital zircon populations that indicate derivation from local Late
Triassic to Middle Jurassic arc and syncollisional plutons and
metamorphosed Paleozoic basement rocks of the Stikinia and Yukon-Tanana
terranes. Basal sandstone units have Early Jurassic depositional ages that
show the Whitehorse trough filled during early Sinemurian, late Sinemurian
to Pliensbachian, and Toarcian subsidence events. Late Triassic to Early
Jurassic detrital zircon grains confirm that syn-collisional plutons near
the northern trough were exhumed at 0.5–7.5 mm/yr and replicate their
excursion to subchondritic Hf isotope compositions as a result of
increasing crustal contributions from Rhaetian to Sinemurian time. The new
detrital zircon data, combined with recent constraints for Triassic–
Jurassic metamorphism and magmatism in Yukon, require modification of
published forearc to syncollisional basin models for the Whitehorse trough.
We reinterpret Jurassic subsidence patterns and architecture of the
Whitehorse trough to reflect sinistral transtension within a transform
fault system that resulted from the reorganization of subduction after
end-on arc collision.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02510.1/616164/Development-of-the-Whitehorse-trough-as-a-strike
Detrital zircon ages from upper Paleozoic–Triassic clastic strata on
St. Lawrence Island, Alaska: An enigmatic component of the Arctic
Alaska–Chukotka microplate
Jeffrey M. Amato; Julie A. Dumoulin; Eric S. Gottlieb; Thomas E. Moore
Abstract:
New lithologic and detrital zircon (DZ) U-Pb data from Devonian–Triassic
strata on St. Lawrence Island in the Bering Sea and from the western Brooks
Range of Alaska suggest affinities between these two areas. The Brooks
Range constitutes part of the Arctic Alaska–Chukotka microplate, but the
tectonic and paleogeographic affinities of St. Lawrence Island are unknown
or at best speculative. Strata on St. Lawrence Island form a
Devonian–Triassic carbonate succession and a Mississippian(?)–Triassic
clastic succession that are subdivided according to three distinctive DZ
age distributions. The Devonian–Triassic carbonate succession has
Mississippian-age quartz arenite beds with Silurian, Cambrian,
Neoproterozoic, and Mesoproterozoic DZ age modes, and it exhibits similar
age distributions and lithologic and biostratigraphic characteristics as
Mississippian-age Utukok Formation strata in the Kelly River allochthon of
the western Brooks Range. Consistent late Neoproterozoic, Cambrian, and
Silurian ages in each of the Mississippian-age units suggest efficient
mixing of the DZ prior to deposition, and derivation from strata exposed by
the pre-Mississippian unconformity and/or Endicott Group strata that
postdate the unconformity. The Mississippian(?)–Triassic clastic succession
is subdivided into feldspathic and graywacke subunits. The feldspathic
subunit has a unimodal DZ age mode at 2.06 Ga, identical to Nuka Formation
strata in the Nuka Ridge allochthon of the western Brooks Range, and it
records a distinctive depositional episode related to late Paleozoic
juxtaposition of a Paleoproterozoic terrane along the most distal parts of
the Arctic Alaska–Chukotka microplate. The graywacke subunit has Triassic
maximum depositional ages and abundant late Paleozoic grains, likely
sourced from fringing arcs and/or continent-scale paleorivers draining
Eurasia, and it has similar age distributions to Triassic strata from the
Lisburne Peninsula (northwestern Alaska), Chukotka and Wrangel Island
(eastern Russia), and the northern Sverdrup Basin (Canadian Arctic), but,
unlike the Devonian–Triassic carbonate succession and feldspathic subunit
of the Mississippian(?)–Triassic clastic succession, it has no obvious
analogue in the western Brooks Range allochthon stack. These correlations
establish St. Lawrence Island as conclusively belonging to the Arctic
Alaska–Chukotka microplate, thus enhancing our understanding of the
circum-Arctic region in late Paleozoic–Triassic time.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02490.1/615820/Detrital-zircon-ages-from-upper-Paleozoic-Triassic
Upper-plate structure and tsunamigenic faults near the Kodiak Islands,
Alaska, USA
Marlon D. Ramos; Lee M. Liberty; Peter J. Haeussler; Robert Humphreys
Abstract:
The Kodiak Islands lie near the southern terminus of the 1964 Great Alaska
earthquake rupture area and within the Kodiak subduction zone segment. Both
local and trans-Pacific tsunamis were generated during this devastating
megathrust event, but the local tsunami source region and the causative
faults are poorly understood. We provide an updated view of the tsunami and
earthquake hazard for the Kodiak Islands region through tsunami modeling
and geophysical data analysis. Using seismic and bathymetric data, we
characterize a regionally extensive seafloor lineament related to the
Kodiak shelf fault zone, with focused uplift along a 50-km-long portion of
the newly named Ugak fault as the most likely source of the local Kodiak
Islands tsunami in 1964. We present evidence of Holocene motion along the
Albatross Banks fault zone, but we suggest that this fault did not produce
a tsunami in 1964. We relate major structural boundaries to active forearc
splay faults, where tectonic uplift is collocated with gravity lineations.
Differences in interseismic locking, seismicity rates, and potential field
signatures argue for different stress conditions at depth near presumed
segment boundaries. We find that the Kodiak segment boundaries have a clear
geophysical expression and are linked to upper-plate structure and splay
faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf
fault zone when compared to the nearby Albatross Banks fault zone,
suggesting short wave travel paths and little tsunami warning time for
nearby communities.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02486.1/615137/Upper-plate-structure-and-tsunamigenic-faults-near
Revisiting the 1899 earthquake series using integrative geophysical
analysis in Yakutat Bay, Alaska
Maureen A.L. Walton; Sean P.S. Gulick; Peter J. Haeussler
Abstract:
A series of large earthquakes in 1899 affected southeastern Alaska near
Yakutat and Disenchantment Bays. The largest of the series, a M W 8.2 event on 10 September 1899, generated an ~12-m-high
tsunami and as much as 14.4 m of coseismic uplift in Yakutat Bay, the
largest coseismic uplift ever measured. Several complex fault systems in
the area are associated with the Yakutat terrane collision with North
America and the termination of the Fairweather strike-slip system, but
because faults local to Yakutat Bay have been incompletely or poorly
mapped, it is unclear which fault system(s) ruptured during the 10
September 1899 event. Using marine geophysical data collected in August
2012, we provide an improved tectonic framework for the Yakutat area, which
advances our understanding of earthquake hazards. We combined 153 line km
of 2012 high-resolution multichannel seismic (MCS) reflection data with
compressed high-intensity radar pulse (Chirp) profiles, basin-scale MCS
data, 2018 seafloor bathymetry, published geodetic models and
thermochronology data, and previous measurements of coseismic uplift to
better constrain fault geometry and subsurface structure in the Yakutat Bay
area. We did not observe any active or concealed faults crossing Yakutat
Bay in our high-resolution data, requiring faults to be located entirely
onshore or nearshore. We interpreted onshore faults east of Yakutat Bay to
be associated with the transpressional termination of the Fairweather fault
system, forming a series of splay faults that exhibit a horsetail geometry.
Thrust and reverse faults on the west side of the bay are related to
Yakutat terrane underthrusting and collision with North America. Our
results include an updated fault map, structural model of Yakutat Bay, and
quantitative assessment of uncertainties for legacy geologic coseismic
uplift measurements. Additionally, our results indicate the 10 September
1899 rupture was possibly related to stress loading from the earlier
Yakutat terrane underthrusting event of 4 September 1899, with the majority
of 10 September coseismic slip occurring on the Esker Creek system on the
northwest side of Yakutat Bay. Limited (~2 m) coseismic or postseismic slip
associated with the 1899 events occurred on faults located east of Yakutat
Bay.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02423.1/615138/Revisiting-the-1899-earthquake-series-using
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