New Articles for Geosphere Posted Online in May
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied this month
include the Moine thrust zone in northwestern Scotland; the Eastern
California shear zone; implementation of “OpenTopography”; the finite
evolution of “mole tracks”; the southern central Andes; the work of
International Ocean Discovery Program (IODP) Expedition 351; and the
Fairweather fault, Alaska, USA. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Detrital-zircon analyses, provenance, and late Paleozoic sediment
dispersal in the context of tectonic evolution of the Ouachita orogen
William A. Thomas; George E. Gehrels; Kurt E. Sundell; Mariah C. Romero
Abstract:
New analyses for U-Pb ages and εHft values, along with
previously published U-Pb ages, from Mississippian–Permian sandstones in
synorogenic clastic wedges of the Ouachita foreland and nearby
intracratonic basins support new interpretations of provenance and sediment
dispersal along the southern Midcontinent of North America. Recently
published U-Pb and Hf data from the Marathon foreland confirm a provenance
in the accreted Coahuila terrane, which has distinctive Amazonia/Gondwana
characteristics. Data from Pennsylvanian–Permian sandstones in the Fort
Worth basin, along the southern arm of the Ouachita thrust belt, are nearly
identical to those from the Marathon foreland, strongly indicating the same
or a similar provenance. The accreted Sabine terrane, which is documented
by geophysical data, is in close proximity to the Coahuila terrane,
suggesting the two are parts of an originally larger Gondwanan terrane. The
available data suggest that the Sabine terrane is a Gondwanan terrane that
was the provenance of the detritus in the Fort Worth basin. Detrital-zircon
data from Permian sandstones in the intracratonic Anadarko basin are very
similar to those from the Fort Worth basin and Marathon foreland,
indicating sediment dispersal from the Coahuila and/or Sabine terranes
within the Ouachita orogen cratonward from the immediate forelands onto the
southern craton. Similar, previously published data from the Permian basin
suggest widespread distribution from the Ouachita orogen. In contrast to
the other basins along the Ouachita-Marathon foreland, the
Mississippian–Pennsylvanian sandstones in the Arkoma basin contain a more
diverse distribution of detrital-zircon ages, indicating mixed dispersal
pathways of sediment from multiple provenances. Some of the Arkoma
sandstones have U-Pb age distributions like those of the Fort Worth and
Marathon forelands. In contrast, other sandstones, especially those with
paleocurrent and paleogeographic indicators of southward progradation of
depositional systems onto the northern distal shelf of the Arkoma basin,
have U-Pb age distributions and εHft values like those of the
“Appalachian signature.” The combined data suggest a mixture of detritus
from the proximal Sabine terrane/Ouachita orogenic belt with detritus
routed through the Appalachian basin via the southern Illinois basin to the
distal Arkoma basin. The Arkoma basin evidently marks the southwestern
extent of Appalachian-derived detritus along the Ouachita-Marathon foreland
and the transition southwestward to overfilled basins that spread detritus
onto the southern craton from the Ouachita-Marathon orogen, including
accreted Gondwanan terranes.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02288.1/598714/Detrital-zircon-analyses-provenance-and-late
Structural, petrological, and tectonic constraints on the Loch Borralan
and Loch Ailsh alkaline intrusions, Moine thrust zone, northwestern
Scotland
Robert Fox; Michael P. Searle
Abstract:
During the Caledonian orogeny, the Moine thrust zone in northwestern
Scotland (UK) emplaced Neoproterozoic Moine Supergroup rocks, metamorphosed
during the Ordovician (Grampian) and Silurian (Scandian) orogenic periods,
westward over the Laurentian passive margin in the northern highlands of
Scotland. The Laurentian margin comprises Archean–Paleoproterozoic
granulite and amphibolite facies basement (Scourian and Laxfordian
complexes, Lewisian gneiss), Proterozoic sedimentary rocks (Stoer and
Torridon Groups), and Cambrian–Ordovician passive-margin sediments. Four
major thrusts, the Moine, Ben More, Glencoul, and Sole thrusts, are well
exposed in the Assynt window. Two highly alkaline syenite intrusions crop
out within the Moine thrust zone in the southern Assynt window. The Loch
Ailsh and Loch Borralan intrusions range from ultramafic melanite-biotite
pyroxenite and pseudoleucite-bearing biotite nepheline syenite (borolanite)
to alkali-feldspar–bearing and quartz-bearing syenites. Within the thrust
zone, syenites intrude up to the Ordovician Durness Group limestones and
dolomites, forming a high-temperature contact metamorphic aureole with
diopside-forsterite-phlogopite-brucite marbles exposed at Ledbeg quarry.
Controversy remains as to whether the Loch Ailsh and Loch Borralan syenites
were intruded prior to thrusting or intruded syn- or post-thrusting.
Borolanites contain large white leucite crystals pseudomorphed by alkali
feldspar, muscovite, and nepheline (pseudoleucite) that have been flattened
and elongated during ductile shearing. The minerals pseudomorphing leucites
show signs of ductile deformation indicating that high-temperature (~500
°C) deformation acted upon pseudomorphed leucite crystals that had
previously undergone subsolidus breakdown. New detailed field mapping and
structural and petrological observations are used to constrain the
geological evolution of both the Loch Ailsh and the Loch Borralan
intrusions and the chronology of the Moine thrust zone. The data supports
the interpretation that both syenite bodies were intruded immediately prior
to thrusting along the Moine, Ben More, and Borralan thrusts.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02330.1/598715/Structural-petrological-and-tectonic-constraints
Tectonostratigraphic record of late Miocene–early Pliocene
transtensional faulting in the Eastern California shear zone,
southwestern USA
Rebecca J. Dorsey; Brennan O’Connell; Kevin K. Gardner; Mindy B. Homan;
Scott E.K. Bennett ...
Abstract:
The Eastern California shear zone (ECSZ; southwestern USA) accommodates
~20%–25% of Pacific–North America relative plate motion east of the San
Andreas fault, yet little is known about its early tectonic evolution. This
paper presents a detailed stratigraphic and structural analysis of the
uppermost Miocene to lower Pliocene Bouse Formation in the southern Blythe
Basin, lower Colorado River valley, where gently dipping and faulted strata
provide a record of deformation in the paleo-ECSZ. In the western Trigo
Mountains, splaying strands of the Lost Trigo fault zone include a
west-dipping normal fault that cuts the Bouse Formation and a steeply
NE-dipping oblique dextral-normal fault where an anomalously thick (~140 m)
section of Bouse Formation siliciclastic deposits filled a local
fault-controlled depocenter. Systematic basinward thickening and stratal
wedge geometries in the western Trigo and southeastern Palo Verde
Mountains, on opposite sides of the Colorado River valley, record basinward
tilting during deposition of the Bouse Formation. We conclude that the
southern Blythe Basin formed as a broad transtensional sag basin in a
diffuse releasing stepover between the dextral Laguna fault system in the
south and the Cibola and Big Maria fault zones in the north. A palinspastic
reconstruction at 5 Ma shows that the southern Blythe Basin was part of a
diffuse regional network of linked right-stepping dextral, normal, and
oblique-slip faults related to Pacific–North America plate boundary dextral
shear. Diffuse transtensional strain linked northward to the Stateline
fault system, eastern Garlock fault, and Walker Lane, and southward to the
Gulf of California shear zone, which initiated ca. 7–9 Ma, implying a
similar age of inception for the paleo-ECSZ.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02337.1/598716/Tectonostratigraphic-record-of-late-Miocene-early
Intra-oceanic submarine arc evolution recorded in an ~1-km-thick
rear-arc succession of distal volcaniclastic lobe deposits
Kyle Johnson; Kathleen M. Marsaglia; Philipp A. Brandl; Andrew P. Barth;
Ryan Waldman ...
Abstract:
International Ocean Discovery Program (IODP) Expedition 351 drilled a
rear-arc sedimentary succession ~50 km west of the Kyushu-Palau Ridge, an
arc remnant formed by rifting during formation of the Shikoku Basin and the
Izu-Bonin-Mariana arc. The ~1-km-thick Eocene to Oligocene deep-marine
volcaniclastic succession recovered at Site U1438 provides a unique
opportunity to study a nearly complete record of intra-oceanic arc
development, from a rear-arc perspective on crust created during subduction
initiation rather than supra-subduction seafloor spreading. Detailed facies
analysis and definition of depositional units allow for broader
stratigraphic analysis and definition of lobe elements. Patterns in
gravity-flow deposit types and subunits appear to define a series of
stacked lobe systems that accumulated in a rear-arc basin. The lobe
subdivisions, in many cases, are a combination of a turbidite-dominated
subunit and an overlying debris-flow subunit. Debris flow–rich lobe-channel
sequences are grouped into four, 1.6–2 m.y. episodes, each roughly the age
range of an arc volcano. Three of the episodes contain overlapping lobe
facies that may have resulted from minor channel switching or input from a
different source. The progressive up-section coarsening of episodes and the
increasing channel-facies thicknesses within each episode suggest
progressively prograding facies from a maturing magmatic arc. Submarine
geomorphology of the modern Mariana arc and West Mariana Ridge provide
present-day examples that can be used to interpret the morphology and
evolution of the channel (or channels) that fed sediment to Site U1438,
forming the sequences interpreted as depositional lobes. The abrupt change
from very thick and massive debris flows to fine-grained turbidites at the
unit III to unit II boundary reflects arc rifting and progressive waning of
turbidity current and ash inputs. This interpretation is consistent with
the geochemical record from melt inclusions and detrital zircons. Thus,
Site U1438 provides a unique record of the life span of an intra-oceanic
arc, from inception through maturation to its demise by intra-arc rifting
and stranding of the remnant arc ridge.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02321.1/598717/Intra-oceanic-submarine-arc-evolution-recorded-in
Measuring change at Earth’s surface: On-demand vertical and
three-dimensional topographic differencing implemented in
OpenTopography
Chelsea Scott; Minh Phan; Viswanath Nandigam; Christopher Crosby; J Ramon
Arrowsmith
Abstract:
Topographic differencing measures landscape change by comparing
multitemporal high-resolution topography data sets. Here, we focused on two
types of topographic differencing: (1) Vertical differencing is the
subtraction of digital elevation models (DEMs) that span an event of
interest. (2) Three-dimensional (3-D) differencing measures surface change
by registering point clouds with a rigid deformation. We recently released
topographic differencing in OpenTopography where users perform on-demand
vertical and 3-D differencing via an online interface. OpenTopography is a
U.S. National Science Foundation–funded facility that provides access to
topographic data and processing tools. While topographic differencing has
been applied in numerous research studies, the lack of standardization,
particularly of 3-D differencing, requires the customization of processing
for individual data sets and hinders the community’s ability to
efficiently perform differencing on the growing archive of topography data.
Our paper focuses on streamlined techniques with which to efficiently
difference data sets with varying spatial resolution and sensor type (i.e.,
optical vs. light detection and ranging [lidar]) and over variable
landscapes. To optimize on-demand differencing, we considered algorithm
choice and displacement resolution. The optimal resolution is controlled by
point density, landscape characteristics (e.g., leaf-on vs. leaf-off), and
data set quality. We provide processing options derived from metadata that
allow users to produce optimal high-quality results, while experienced
users can fine tune the parameters to suit their needs. We anticipate that
the differencing tool will expand access to this state-of-the-art
technology, will be a valuable educational tool, and will serve as a
template for differencing the growing number of multitemporal topography
data sets.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02259.1/598718/Measuring-change-at-Earth-s-surface-On-demand
Coseismic deformation of the ground during large-slip strike-slip
ruptures: Finite evolution of “mole tracks”
T.A. Little; P. Morris; M.P. Hill; J. Kearse; R.J. Van Dissen ...
Abstract:
To evaluate ground deformation resulting from large (~10 m) coseismic
strike-slip displacements, we focus on deformation of the Kekerengu fault
during the November 2016 Mw 7.8 Kaikōura earthquake in New
Zealand. Combining post-earthquake field observations with analysis of
high-resolution aerial photography and topographic models, we describe the
structural geology and geomorphology of the rupture zone. During the
earthquake, fissured pressure bulges (“mole tracks”) initiated at stepovers
between synthetic Riedel (R) faults. As slip accumulated, near-surface
“rafts” of cohesive clay-rich sediment, bounded by R faults and capped by
grassy turf, rotated about a vertical axis and were internally shortened,
thus amplifying the bulges. The bulges are flanked by low-angle
contractional faults that emplace the shortened mass of detached sediment
outward over less-deformed ground. As slip accrued, turf rafts fragmented
into blocks bounded by short secondary fractures striking at a high angle
to the main fault trace that we interpret to have originated as antithetic
Riedel (R¢) faults. Eventually these blocks were dispersed into strongly
sheared earth and variably rotated. Along the fault, clockwise rotation of
these turf rafts within the rupture zone averaged ~20°–30°, accommodating
a finite shear strain of 1.0–1.5 and a distributed strike slip of ~3–4 m.
On strike-slip parts of the fault, internal shortening of the rafts
averaged 1–2 m parallel to the R faults and ~1 m perpendicular to the main
fault trace. Driven by distortional rotation, this contraction of the rafts
exceeds the magnitude of fault heave. Turf rafts on slightly transtensional
segments of the fault were also bulged and shortened—relationships that can
be explained by a kinematic model involving “deformable slats.” In a
paleoseismic trench cut perpendicular the fault, one would observe
fissures, low-angle thrusts, and steeply dipping strike-slip faults—some
cross-cutting one another—yet all may have formed during a single
earthquake featuring a large strike-slip displacement.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02336.1/598719/Coseismic-deformation-of-the-ground-during-large
Detrital zircon record of Phanerozoic magmatism in the southern Central
Andes
T.N. Capaldi; N.R. McKenzie; B.K. Horton; C. Mackaman-Lofland; C.L. Colleps
...
Abstract:
The spatial and temporal distribution of arc magmatism and associated
isotopic variations provide insights into the Phanerozoic history of the
western margin of South America during major shifts in Andean and
pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf
isotopic results across continental magmatic arc systems of Chile and
western Argentina (28°S–33°S) with igneous bedrock geochronologic and
zircon Hf isotope results to define isotopic signatures linked to changes
in continental margin processes. Key tectonic phases included: Paleozoic
terrane accretion and Carboniferous subduction initiation during Gondwanide
orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene
continental arc magmatism, and Neogene flat slab subduction during Andean
shortening. The ~550 m.y. record of magmatic activity records spatial
trends in magma composition associated with terrane boundaries. East of
69°W, radiogenic isotopic signatures indicate reworked continental
lithosphere with enriched (evolved) εHf values and low (<0.65) zircon
Th/U ratios during phases of early Paleozoic and Miocene shortening and
lithospheric thickening. In contrast, the magmatic record west of 69°W
displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U
values consistent with increased asthenospheric contributions during
lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc
width provide a rough approximation of relative subduction angle, such that
an increase in arc width reflects shallower slab dip. Comparisons among
slab dip calculations with time-averaged εHf and Th/U zircon results
exhibit a clear trend of decreasing (enriched) magma compositions with
increasing arc width and decreasing slab dip. Collectively, these data sets
demonstrate the influence of subduction angle on the position of
upper-plate magmatism (including inboard arc advance and outboard arc
retreat), changes in isotopic signatures, and overall composition of
crustal and mantle material along the western edge of South America.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02346.1/596772/Detrital-zircon-record-of-Phanerozoic-magmatism-in
Prehistoric earthquakes on the Banning strand of the San Andreas fault,
North Palm Springs, California
Bryan A. Castillo; Sally F. McGill; Katherine M. Scharer; Doug Yule; Devin
McPhillips ...
Abstract:
We studied a paleoseismic trench excavated in 2017 across the Banning
strand of the San Andreas fault and herein provide the first detailed
record of ground-breaking earthquakes on this important fault in Southern
California. The trench exposed an ~40-m-wide fault zone cutting through
alluvial sand, gravel, silt, and clay deposits. We evaluated the
paleoseismic record using a new metric that combines event indicator
quality and stratigraphic uncertainty. The most recent paleoearthquake
occurred between 950 and 730 calibrated years B.P. (cal yr B.P.),
potentially contemporaneous with the last rupture of the San Gorgonio Pass
fault zone. We interpret five surface-rupturing earthquakes since 3.3–2.5
ka and eight earthquakes since 7.1–5.7 ka. It is possible that additional
events have occurred but were not recognized, especially in the deeper
(older) section of the stratigraphy, which was not fully exposed across the
fault zone. We calculated an average recurrence interval of 380–640 yr
based on four complete earthquake cycles between earthquakes 1 and 5. The
average recurrence interval is thus slightly less than the elapsed time
since the most recent event on the Banning strand. The average recurrence
interval on the Banning strand is thus intermediate between longer
intervals published for the San Gorgonio Pass fault zone (~1600 yr) and
shorter intervals on both the Mission Creek strand of the San Andreas fault
(~215 yr) and the Coachella section (~125 yr) of the San Andreas fault.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02237.1/596773/Prehistoric-earthquakes-on-the-Banning-strand-of
Geomorphic expression and slip rate of the Fairweather fault, southeast
Alaska, and evidence for predecessors of the 1958 rupture
Robert C. Witter; Adrian M. Bender; Katherine M. Scharer; Christopher B.
DuRoss; Peter J. Haeussler ...
Abstract:
Active traces of the southern Fairweather fault were revealed by light
detection and ranging (lidar) and show evidence for transpressional
deformation between North America and the Yakutat block in southeast
Alaska. We map the Holocene geomorphic expression of tectonic deformation
along the southern 30 km of the Fairweather fault, which ruptured in the
1958 moment magnitude 7.8 earthquake. Digital maps of surficial geology,
geomorphology, and active faults illustrate both strike-slip and dip-slip
deformation styles within a 10°–30° double restraining bend where the
southern Fairweather fault steps offshore to the Queen Charlotte fault. We
measure offset landforms along the fault and calibrate legacy 14
C data to reassess the rate of Holocene strike-slip motion (≥49 mm/yr),
which corroborates published estimates that place most of the plate
boundary motion on the Fairweather fault. Our slip-rate estimates allow a
component of oblique-reverse motion to be accommodated by contractional
structures west of the Fairweather fault consistent with geodetic block
models. Stratigraphic and structural relations in hand-dug excavations
across two active fault strands provide an incomplete paleoseismic record
including evidence for up to six surface ruptures in the past 5600 years,
and at least two to four events in the past 810 years. The incomplete
record suggests an earthquake recurrence interval of ≥270 years—much longer
than intervals <100 years implied by published slip rates and expected
earthquake displacements. Our paleoseismic observations and map of active
traces of the southern Fairweather fault illustrate the complexity of
transpressional deformation and seismic potential along one of Earth’s
fastest strike-slip plate boundaries.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02299.1/596774/Geomorphic-expression-and-slip-rate-of-the
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
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