New Articles for Geosphere Posted Online in November
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied include
Anadarko Basin, USA; UAV use using cognitive science principles; and
Ubehebe Crater, Death Valley. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Oceanic intraplate faulting as a pathway for deep hydration of the
lithosphere: Perspectives from the Caribbean
Brandon T. Bishop; Sungwon Cho; Linda Warren; Lillian Soto-Cordero;
Patricia Pedraza ...
Abstract:
The recycling of water into the Earth’s mantle via hydrated oceanic
lithosphere is believed to have an important role in subduction zone
seismicity at intermediate depths. Hydration of oceanic lithosphere has
been shown to drive double planes of intermediate-depth, Wadati-Benioff
zone seismicity at subduction zones. However, observations from trenches
show that pervasive normal faulting causes hydration ~25 km into the
lithosphere and can explain neither locations where separations of 25–40 km
between Wadati-Benioff zone planes are observed nor the spatial variability
of the lower plane in these locations, which suggests that an additional
mechanism of hydration exists. We suggest that intraplate deformation of
>50-m.y.-old lithosphere, an uncommon and localized process, drives
deeper hydration. To test this, we relocated the 25 November 2018 6.0 M W Providencia, Colombia, earthquake mainshock and 575 associated
fore- and aftershocks within the interior of the Caribbean oceanic plate
and compared these with receiver functions (RF) that sampled the fault at
its intersection with the Mohorovičić discontinuity. We examined possible
effects of velocity model, initial locations of the earthquakes, and
seismic-phase arrival uncertainty to identify robust features for
comparison with the RF results. We found that the lithosphere ruptured from
its surface to a depth of ~40 km along a vertical fault and an
intersecting, reactivated normal fault. We also found RF evidence for
hydration of the mantle affected by this fault. Deeply penetrating
deformation of lithosphere like that we observe in the Providencia region
provides fluid pathways necessary to hydrate oceanic lithosphere to depths
consistent with the lower plane of Wadati-Benioff zones.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02534.1/619073/Oceanic-intraplate-faulting-as-a-pathway-for-deep
Illuminating geology in areas of limited exposure using texture shading
of lidar digital terrain models
Richard W. Allmendinger; Paul Karabinos
Abstract:
Regions of sparse exposure challenge geologic mappers because of limited
information available on the underlying structure and continuity of the map
units. We introduce here a little-known technique for post-processing bare
earth digital terrain models (DTMs) that can dramatically improve knowledge
of the underlying structure in covered areas. Texture shading enhances
changes in slope and does not suffer from limitations introduced by
artificial illumination required in hillshade or shaded relief images. When
this technique is applied to lidar DTMs, layers of rock units with variable
resistance to erosion can be clearly imaged, even in areas with limited
outcrop. This technique enables one to collect comprehensive orientation
data in areas of deformed sedimentary strata, assess the continuity of
metamorphic and igneous rock units, and depict basement fracture sets. We
demonstrate the use of texture shading in the Valley and Ridge of northern
Pennsylvania, metamorphic rocks in the Berkshire Hills of western
Massachusetts and Green Mountains of Vermont, and glacial deposits in the
Finger Lakes region of upstate New York (northeastern United States).
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02531.1/618907/Illuminating-geology-in-areas-of-limited-exposure
Strain partitioning in the Moine Nappe, northernmost Scotland
Sarah Collier Southern; Sharon Mosher; Omero Felipe Orlandini
Abstract:
Extreme strain in the form of flattening or constriction during noncoaxial
shear in ductile shear zones provides a record of ductile thrust system
dynamics and the overall tectonic evolution. Within the Moine Nappe in
northern Scotland, between the Ben Hope and Moine thrusts, the Strathan
Conglomerate displays apparent strain partitioning with extreme flattening
(e.g., laterally extensive sheets of deformed pebbles with aspect ratios of
134:113:1 and 88–92% estimated thinning) adjacent to the overlying Ben Hope
Thrust and extreme constriction (e.g., rods with aspect ratios of 21:4:1
and estimated extension of 1000%) lower in the nappe package. We
demonstrate that partitioning of strain is between its intensity and how
deformation is manifested. Field, microstructural, and crystallographic
orientation data from this study indicate that both areas were deformed by
WNW-directed noncoaxial shear and coaxial flattening under
amphibolite-facies conditions. Adjacent to the Ben Hope thrust, flattening
was pervasive during noncoaxial shear, whereas beneath and within the Moine
Nappe package, polyphase folding dominated. There, early, large-scale folds
(F2) rotated into the transport direction. Subsequent
transport-parallel (F3) folds and tubular sheath folds formed on
the F2 limbs and were dismembered to form rods. No evidence of
constriction is observed; instead, pervasive noncoaxial shear was
accompanied by minor flattening under decreasing temperature conditions.
Thus, these S-tectonites in the Moine Nappe are the result of concentrated
flattening of pebbles into sheets during WNW-directed shear, whereas the
L-tectonites result from heterogeneously distributed shear and folding,
coupled with minor flattening, which produced rods without constriction.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02522.1/618908/Strain-partitioning-in-the-Moine-Nappe
Late Paleozoic cratonal sink: Distally sourced sediment filled the
Anadarko Basin (USA) from multiple source regions
Baylee E. Kushner; Gerilyn S. Soreghan; Michael J. Soreghan
Abstract:
The Anadarko Basin (south-central USA) is the deepest basin on the North
American craton and occupies a region largely surrounded by major, late
Paleozoic plate-margin (Marathon-Ouachita-Appalachian) and intraplate
(Ancestral Rocky Mountains) orogenic systems, albeit a distal arm of the
latter. The Anadarko Basin hosts an exceptionally voluminous record of
Pennsylvanian strata, and much of this fill has been attributed to erosion
of the adjacent Wichita uplift—composed of granitic and rhyolitic rocks of
Cambrian age—separated from the basin by a fault zone exhibiting 12 km of
vertical separation. This work incorporates thin-section petrography (102
samples) and U-Pb detrital zircon geochronology of sandstone samples (12
samples) from core and outcrop of the Middle Pennsylvanian Red Fork
Sandstone (and equivalents) as well as slightly younger Upper Pennsylvanian
units (Tonkawa, Chelsea, and Gypsy sandstones) in order to interpret
drainage pathways and evolution of those pathways toward and into the
Anadarko Basin (Oklahoma) and evaluate the relative importance of the major
provenance regions. Our petrographic analysis indicates sandstones with
arkosic compositions are limited to the region immediately adjacent to
(north of) the Wichita uplift. All remaining samples, which reflect the
vast bulk of sediment in the depocenter, including sediment on the northern
and eastern Anadarko shelf, are litharenites. Analysis of kernel density
plots of the U-Pb ages of detrital zircons together with multidimensional
scaling analysis of the Middle Pennsylvanian samples indicate three groups
of similar provenance: (1) samples dominated by Cambrian ages from
locations directly adjacent to the Wichita uplift; (2) samples dominated by
Neoproterozoic ages from locations along the northern shelf of the Anadarko
Basin; and (3) samples dominated by Mesoproterozoic ages from locations
along the eastern Anadarko shelf and the basin center. These samples are
spatially discrete, indicating partitioning of drainage networks during the
Middle Pennsylvanian, with two continental-scale fluvial systems entering
the Anadarko Basin from the north (transversely) and east (axially). The
lack of Cambrian ages in the depocenter and (northern) shelf samples
indicate that the Wichita uplift supplied only limited sediment to the
basin; sediment derived from the uplift was trapped in fringing fans
directly adjacent to the uplift. In contrast to the patterns exhibited by
the Middle Pennsylvanian samples, Upper Pennsylvanian samples exhibit more
uniform U-Pb ages across the basin. This indicates the relatively rapid
evolution of the Appalachian-derived northerly and easterly drainages into
an integrated system that flowed axially across the (overfilled)
mid-continent basins to the ultimate continental sink in the Anadarko
Basin.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02489.1/618751/Late-Paleozoic-cratonal-sink-Distally-sourced
Strategies for effective unmanned aerial vehicle use in geological
field studies based on cognitive science principles
Kathryn M. Bateman; Randolph T. Williams; Thomas F. Shipley; Basil Tikoff;
Terry Pavlis ...
Abstract:
Field geologists are increasingly using unmanned aerial vehicles (UAVs or
drones), although their use involves significant cognitive challenges for
which geologists are not well trained. On the basis of surveying the user
community and documenting experts’ use in the field, we identified five
major problems, most of which are aligned with well-documented limits on
cognitive performance. First, the images being sent from the UAV portray
the landscape from multiple different view directions. Second, even with a
constant view direction, the ability to move the UAV or zoom the camera
lens results in rapid changes in visual scale. Third, the images from the
UAVs are displayed too quickly for users, even experts, to assimilate
efficiently. Fourth, it is relatively easy to get lost when flying,
particularly if the user is unfamiliar with the area or with UAV use.
Fifth, physical limitations on flight time are a source of stress, which
renders the operator less effective. Many of the strategies currently
employed by field geologists, such as postprocessing and photogrammetry,
can reduce these problems. We summarize the cognitive science basis for
these issues and provide some new strategies that are designed to overcome
these limitations and promote more effective UAV use in the field. The goal
is to make UAV-based geological interpretations in the field possible by
recognizing and reducing cognitive load.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02440.1/618752/Strategies-for-effective-unmanned-aerial-vehicle
Foreland basin response to middle Cretaceous thrust belt evolution,
southwestern Montana, USA
Cole T. Gardner; Emily S. Finzel; Justin A. Rosenblume; David M. Pearson
Abstract:
The middle Cretaceous Blackleaf Formation records the first major
transgression-regression of the Western Interior Seaway into the
southwestern Montana retroforeland basin. Although Blackleaf sedimentology
is well documented, sediment provenance and potential linkages with
regional tectonics are not. Recent characterization of hinterland
tectonics, fold-thrust belt detrital zircon signatures, and advances in
high-n detrital zircon analysis allow for significant provenance
refinement. We present new detrital zircon ages (n = 5468) from ten samples
from the upper Blackleaf Formation (Intervals C and D) in southwestern
Montana. Based on maximum depositional ages, sedimentation spanned from 106
to 92 Ma. Jurassic and Cretaceous grains were primarily derived from the
older portion of the Cordilleran magmatic arc in western Idaho. Triassic
and older grains were recycled from older central Idaho sedimentary strata
inboard of the arc. Three depositional stages are identified based on
statistical modeling of detrital age distributions. Stage 1 (106–104 Ma)
records sourcing from lower Paleozoic strata in central Idaho. Stage 2
(105–101 Ma) records initial unroofing of upper Paleozoic–Triassic strata
via propagation of the fold-thrust belt into eastern Idaho, accommodating
shortening of Mississippian and younger rocks above the Lemhi Arch. Stage 3
(102–100 Ma) records continued unroofing in central Idaho down to Cambrian
stratigraphic levels and distal mixing of sources in the eastern part of
the basin. Exhumation in the fold-thrust belt beginning at ca. 105 Ma is
coincident with margin-wide fault slip-rate increases. We infer that
increased sedimentation rates and low-magnitude flexural loading from
shallow thrusting in eastern Idaho drove clastic wedge progradation across
the basin.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02521.1/618753/Foreland-basin-response-to-middle-Cretaceous
Pyroclastic deposits of Ubehebe Crater, Death Valley, California, USA:
Ballistics, pyroclastic surges, and dry granular flows
Greg A. Valentine; Judy Fierstein; James D.L. White
Abstract:
We describe and interpret deposits associated with the final Ubehebe
Crater-forming, phreatomagmatic explosive phase of the multivent,
monogenetic Ubehebe volcanic center. Ubehebe volcano is located in Death
Valley, California, USA. Pyroclastic deposits occur in four main facies:
(1) lapilli- and block-dominated beds, (2) thinly bedded lapilli tuff, (3)
laminated and cross-laminated ash, and (4) massive lapilli ash/tuff.
Lapilli- and block-dominated beds are found mostly within several hundred
meters of the crater and transition outward into discontinuous lenses of
lapilli and blocks; they are interpreted to have been deposited by
ballistic processes associated with crater-forming explosions. Thinly
bedded lapilli tuff is found mainly within several hundred meters, and
laminated and cross-laminated ash extends at least 9 km from the crater
center. Dune forms are common within ~2 km of the crater center, while
finer-grained, distal deposits tend to exhibit planar lamination. These two
facies (thinly bedded lapilli tuff and laminated and cross-laminated ash)
are interpreted to record multiple pyroclastic surges (dilute pyroclastic
currents). Repeated couplets of coarse layers overlain by finer-grained,
laminated horizons suggest that many or most of the surges were transient,
likely recording individual explosions, and they traveled over complex
topography in some areas. These two factors complicate the application of
classical sediment-transport theory to quantify surge properties. However,
dune- form data provide possible constraints on the relationships between
suspended load sedimentation and bed-load transport that are consistent
using two independent approaches. Massive lapilli ash/tuff beds occur in
drainages below steep slopes and can extend up to ~1 km onto adjacent
valley floors beneath large catchments. Although they are massive in
texture, their grain-size characteristics are shared with laminated and
cross-laminated ash facies, with which they are locally interbedded. These
are interpreted to record concentrated granular flows sourced by
remobilized pyroclastic surge deposits, either during surge transport or
shortly after, while the surge deposits retained their elevated initial
pore-gas pressures. Although similar surge-derived concentrated flows have
been described elsewhere (e.g., Mount St. Helens, Washington, USA, and
Soufriére Hills, Montserrat, West Indies), to our knowledge Ubehebe is the
first case where such processes have been identified at a maar volcano.
These concentrated flows followed paths that were independent of the
pyroclastic surges and represent a potential hazard at similar maar
volcanoes in areas with complex terrain.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02526.1/618754/Pyroclastic-deposits-of-Ubehebe-Crater-Death
Cretaceous sequence stratigraphy of the northern Baltimore Canyon
Trough: Implications for basin evolution and carbon storage
Kimberly E. Baldwin; Kenneth G. Miller; William J. Schmelz; Gregory S.
Mountain; Leslie M. Jordan ...
Abstract:
We evaluate the Cretaceous stratigraphy and carbon sequestration potential
of the northern Baltimore Canyon Trough (NBCT) using >10,000 km of
multi-channel seismic profiles integrated with geophysical logs,
biostratigraphy, and lithology from 29 offshore wells. We identify and map
six sequences resolved primarily at the stage level. Accommodation was
dominated by thermal and non-thermal subsidence, though sequence boundaries
correlate with regional and global sea-level changes, and the record is
modified by igneous intrusion, active faulting, and changes in sediment
supply and sources. Our stratigraphic maps illustrate a primary southern
(central Appalachian) Early Cretaceous source that migrated northward
during the Aptian and Albian. During the Cenomanian, sedimentation rates in
the NBCT increased and depocenters shifted northward and landward. We show
that deposition occurred in three phases: (1) earlier Cretaceous
paleoenvironments were primarily terrestrial indicated by variable
amplitude, chaotic seismic facies, serrated gamma logs, and heterolithic
sandstones and mudstones with terrestrial microfossils; (2) the Albian to
Cenomanian was dominated by deltaic paleoenvironments indicated by blocky,
funnel-shaped, gamma-ray logs and clinoforms characterized by continuous
high-amplitude seismic reflections with well-defined terminations; and (3)
the Cenomanian and younger was marine shelf, inferred from mudstone-prone
lithologies, peak gamma-ray values in well logs, and foraminiferal
evidence. Long-term transgression and maximum water depths at the
Cenomanian/Turonian boundary correlative with Ocean Anoxic Event 2 were
followed by a regression and relative sea-level fall. We show that porous
and permeable sandstones of three Aptian to Cenomanian highstand systems
tracts are high-volume reservoirs for supercritical CO2 storage
that are confined by overlying deep water mudstones.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02497.1/618755/Cretaceous-sequence-stratigraphy-of-the-northern
Topographic development of a compressional mountain range, the western
Transverse Ranges of California, USA, resulted from localized uplift
along individual structures and regional uplift from deeper shortening
N. Onderdonk; A.F. Garcia; C. Kelty; A. Farris; E. Tyler
Abstract:
The western Transverse Ranges are a tectonically active mountain belt in
southern California (USA) characterized by fast rates of shortening and
rock uplift. Large drainages at the western end of this mountain belt,
including the Santa Ynez River and its tributaries, transect regional
west–northwest-striking reverse faults and folds. We used fluvial strath
terraces within the Santa Ynez River watershed as geomorphic markers for
measuring Quaternary rock uplift and deformation across these structures.
Mapping, surveying, and numerical dating of these strath terraces in both
hanging-wall and footwall blocks of the major reverse faults allow us to
separate regional uplift from localized uplift along individual structures.
Luminescence dates from 18 sites within the Santa Ynez River watershed show
that the three prominent terrace levels present throughout the area formed
between ca. 85 ka and 95 ka, 55 ka and 75 ka, and 30 ka and 45 ka. All
three fluvial terrace straths grade into marine paleo-shore platforms along
the coast that formed during sea-level highstands. The fluvial straths were
formed as a result of lateral erosion during warm, dry climate intervals
when vertical incision was temporarily arrested. Incision of the terraces
followed during intervening periods of wet climate. Mapping and valley-long
profiles of the terraces document deformation by faults and folds, and we
infer minimum rock-uplift rates from the amount of incision below the
terrace strath surfaces. Rock-uplift rates range from 0.3 mm/yr to 4.9
mm/yr, with faster rates in the hanging-wall blocks of the major reverse
faults and slower rates in the footwall blocks. Rock-uplift rates
calculated from strath terraces in the footwall blocks range from 0.3 mm/yr
to 1.6 mm/yr, which indicates a regional component of uplift that results
from deeper deformation. Higher rates of rock uplift in the hanging-wall
blocks (0.5–4.9 mm/yr) are superposed on this regional component.
Incremental rock-uplift rates calculated over three time intervals and
differences in terrace deformation with age suggest that deformation rates
across some structures have decreased over the past 85 k.y. We conclude
that topographic growth of the western Transverse Ranges results from a
combination of localized uplift along individual structures that varies
both spatially and temporally and a more constant regional uplift that
likely results from deeper ductile deformation or slip along detachment
faults that have been inferred to underlie the area.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02505.1/618756/Topographic-development-of-a-compressional
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