New Articles for Geosphere Posted Online in January
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Topics include Farallon plate subduction;
3-D digital outcrop scanning and modeling; and the Cerro Blanco volcanic
complex. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Numerical models of Farallon plate subduction: Creating and removing a
flat slab
Claire A. Currie; Peter Copeland
Abstract:
Flat-slab subduction has affected parts of North America, South America,
and Asia over the past 250 m.y. In these areas, reconstructions show that
the subducting plate became subhorizontal below the continent for ~5 to
>30 m.y., followed by foundering of the slab and resumption of
steep-angle subduction. Using two-dimensional numerical models, we examine
the factors that control the development and removal of a flat slab. Models
are based on the Late Cretaceous to Oligocene Farallon flat slab below the
southwestern United States. We find that the primary control on subduction
geometry is the oceanic plate density structure. Subduction of a buoyant
oceanic plateau creates a flat-slab segment that moves inboard at
approximately the rate of continental trenchward motion (4–5 cm/yr).
Steepening is initiated with eclogitization of the oceanic plateau crust.
Once the plateau density exceeds that of the mantle, the slab undergoes
rollback through progressive trenchward-directed detachment from the
continent at a rate of 2–10 cm/yr. Rollback is enhanced by: (1) weakening
of the overlying continental mantle lithosphere, inferred to result from
slab-derived hydrous fluids, and (2) a slowdown in plate velocities; the
rate and amount of oceanic eclogitization are second-order effects.
Conversely, rollback is hindered by a strong oceanic plate and interactions
between the slab and high-viscosity lower mantle. For the ~2000-km-long
Farallon slab, the Conjugate Shatskey Rise plateau must have remained
buoyant for 20–30 m.y. after subduction. This was followed by rapid
rollback caused by both plateau eclogitization and continental weakening,
leaving an area of thinned and hydrated continental lithosphere.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02393.1/611097/Numerical-models-of-Farallon-plate-subduction
Augmenting geological field mapping with real-time, 3-D digital outcrop
scanning and modeling
Callum Walter; Fouad Faraj; Georgia Fotopoulos; Alexander Braun
Abstract:
Hand scanners are compact, lightweight, and capable of generating 3-D
digital models. Although they do not compare to conventional methods
(terrestrial laser scanning and photogrammetry) in terms of coverage,
resolution, and accuracy, they offer increased mobility, speed, and
real-time processing capabilities in the field. This study investigates the
use of hand scanners for real-time, 3-D digital outcrop modeling to augment
geological field mapping campaigns and highlights the advantages and the
limitations. The utility of incorporating hand scanners as an additional
tool for augmenting geological mapping is assessed based on 41 outcrop
scans from the Gould Lake area, which is located 20 km north of Kingston,
Ontario, Canada. The 3-D digital outcrop models gathered included two
distinct metamorphic lithologies (marble and quartzofeldspathic gneiss)
measuring up to 2.5 m high × 7 m long with an average surface area of 18 m 2. This average scan size would take less than 10 min to
capture, result in ~18 million individual points per scan, and provide a
spatial resolution of ~1 cm for outcrop features. Throughout the course of
the investigation, the main benefit of capturing multiple 3-D digital
outcrop models was the ability to integrate this real-time, in situ
geospatial, and geologic information across multiple visualization scales.
This utility and retention of outcrop-scale geospatial information was
shown to enhance the understanding of multi-scale geological relationships.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02452.1/611098/Augmenting-geological-field-mapping-with-real-time
Magmatic evolution and architecture of an arc-related, rhyolitic
caldera complex: The late Pleistocene to Holocene Cerro Blanco volcanic
complex, southern Puna, Argentina
S.L. de Silva; J. Roberge; L. Bardelli; W. Báez; A. Ortiz ...
Abstract:
Through the lens of bulk-rock and matrix glass geochemistry, we
investigated the magmatic evolution and pre-eruptive architecture of the
siliceous magma complex beneath the Cerro Blanco volcanic complex, a Crater
Lake–type caldera complex in the southern Puna Plateau of the Central Andes
of Argentina. The Cerro Blanco volcanic complex has been the site of two
caldera-forming eruptions with volcanic explosivity index (VEI) 6+ that
emplaced the ca. 54 ka Campo Piedra Pomez ignimbrite and the ca. 4.2 ka
Cerro Blanco ignimbrite. As such, it is the most productive recent
explosive volcano in the Central Andes. The most recent eruptions (younger
than 4.2 ka) are dominantly postcaldera effusions of crystal-rich domes and
associated small explosive pulses. Previous work has demonstrated that
andesitic recharge of and mixing with rhyolitic magma occurred at the base
of the magma complex, at ~10 km depth. New isotopic data (Sr, Nd, Pb, and
O) confirm that the Cerro Blanco volcanic complex rhyolite suite is part of
a regional southern Puna, arc-related ignimbrite group. The suite defines a
tight group of consanguineous siliceous magmas that serves as a model for
the evolution of arc-related, caldera-forming silicic magma systems in the
region and elsewhere. These data indicate that the rhyolites originated
through limited assimilation of and mixing with upper-crustal lithologies
by regional basaltic andesite parent materials, followed by extensive
fractional crystallization. Least squares models of major elements in
tandem with Rayleigh fractionation models for trace elements reveal that
the internal variations among the rhyolites through time can be derived by
extensive fractionation of a quartz–two feldspar (granitic minimum)
assemblage with limited assimilation. The rare earth element character of
local volumes of melt in some samples of the Campo Piedra Pomez ignimbrite
basal fallout requires significant fractionation of amphibole. The
distinctive major- and trace-element characteristics of bulk rock and
matrix of the Campo Piedra Pomez and Cerro Blanco tephras provide useful
geochemical fingerprints to facilitate regional tephrochronology. Available
data indicate that rhyolites from other neighborhood centers, such as
Cueros de Purulla, share bulk chemical characteristics with the Campo
Piedra Pomez ignimbrite rhyolites, but they appear to be isotopically
distinct. Pre-eruptive storage and final equilibration of the rhyolitic
melts were estimated from matrix glass compositions projected onto the
haplogranitic system (quartz-albite-orthoclase-H2O) and using
rhyolite-MELTS models. These revealed equilibration pressures between 360
and 60 MPa (~10–2 km depth) with lowest pressures in the Holocene
eruptions. Model temperatures for the suite ranged from 695 to 790 °C.
Integrated together, our results reveal that the Cerro Blanco volcanic
complex is a steady-state (low-magmatic-flux), arc-related complex,
standing in contrast to the flare-up (high-magmatic-flux) supervolcanoes
that dominate the Neogene volcanic stratigraphy. The silicic magmas of the
Cerro Blanco volcanic complex were derived more directly from mafic and
intermediate precursors through extensive fractional crystallization,
albeit with some mixing and assimilation of local basement. Geochemical
models and pressure-temperature estimates indicate that significant volumes
of remnant cumulates of felsic and intermediate composition should dominate
the polybaric magma complex beneath the Cerro Blanco volcanic complex,
which gradually shallowed through time. Evolution to the most silicic
compositions and final equilibration of some of the postcaldera domes
occurred during ascent and decompression at depths less than 2 km. Our work
connotes an incrementally accumulated (over at least 54 k.y.),
upper-crustal pluton beneath the Cerro Blanco volcanic complex between 2
and 10 km depth. The composition of this pluton is predicted to be
dominantly granitic, with deeper parts being granodioritic to tonalitic.
The progressive solidification and eventual contraction of the magma
complex may account for the decades of deflation that has characterized
Cerro Blanco. The presently active geothermal anomaly and hydrothermal
springs indicate the Cerro Blanco volcanic complex remains potentially
active.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02294.1/611099/Magmatic-evolution-and-architecture-of-an-arc
Geochemical indications for the Paleocene-Eocene Thermal Maximum (PETM)
and Eocene Thermal Maximum 2 (ETM-2) hyperthermals in terrestrial
sediments of the Canadian Arctic
Lutz Reinhardt; Werner von Gosen; Andreas Lückge; Martin Blumenberg;
Jennifer M. Galloway ...
Abstract:
During the late Paleocene to early Eocene, clastic fluvial sediments and
coals were deposited in northern high latitudes as part of the Margaret
Formation at Stenkul Fiord (Ellesmere Island, Nunavut, Canada).
Syn-sedimentary tectonic movements of the Eurekan deformation continuously
affected these terrestrial sediments. Different volcanic ash layers occur,
and unconformities subdivide the deposits into four sedimentary units. Rare
vertebrate fossils indicate an early Eocene (Graybullian) age for the upper
part of the Stenkul Fiord outcrop. Here, we present carbon isotope data of
bulk coal, related organic-rich mud and siltstones, a plant leaf
wax-derived alkane, and additional plant remains. These data provide a
complete carbon isotope record of one stratigraphic section with defined
unconformity positions and in relation to other Eurekan deformation
features. A previously dated ash layer MA-1 provided a U-Pb zircon age of
53.7 Ma and is used as a stratigraphic tie point, together with a discrete
negative carbon isotope excursion found above MA-1 in a closely sampled
coal seam. The excursion is identified as the likely expression of the I-1
hyperthermal event. Based on our isotope data that reflect the early Eocene
dynamics of the carbon cycle, this tie point, and previous paleontological
constraints from vertebrate fossils, the locations of the Paleocene-Eocene
Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM-2) hyperthermals
and their extent along the complete section are herein identified. Within
the intervals of the PETM and ETM-2 hyperthermal events, increasing amounts
of clastic sediments reached the site toward the respective end of the
event. This is interpreted as a response of the fluvial depositional system
to an intensified hydrological system during the hyperthermal events. Our
study establishes an enhanced stratigraphic framework allowing for the
calculation of average sedimentation rates of different intervals and
considerations on the completeness of the stratigraphic record. As one of
the few high-latitude outcrops of early Eocene terrestrial sediments, the
Stenkul Fiord location offers further possibilities to study the effects of
extreme warming events in the Paleogene.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article/doi/10.1130/GES02398.1/610709/Geochemical-indications-for-the-Paleocene-Eocene
The building blocks of igneous sheet intrusions: Insights from 3-D
seismic reflection data
Jonas Köpping; Craig Magee; Alexander R. Cruden; Christopher A.-L. Jackson;
James R. Norcliffe
Abstract:
The propagating margins of igneous sills (and other sheet intrusions) may
divide into laterally and/or vertically separated sections, which later
inflate and coalesce. These components elongate parallel to and thus record
the magma flow direction, and they can form either due to fracture
segmentation (i.e., “segments”) or brittle and/or non-brittle deformation
of the host rock (i.e., “magma fingers”). Seismic reflection data can image
entire sills or sill-complexes in 3-D, and their resolution is often
sufficient to allow us to identify these distinct elongate components and
thereby map magma flow patterns over entire intrusion networks. However,
seismic resolution is limited, so we typically cannot discern the
centimeter- to meter-scale host rock deformation structures that would
allow the origin of these components to be interpreted. Here, we introduce
a new term that defines the components (i.e., “elements”) of sheet-like
igneous intrusions without linking their description to emplacement
mechanisms. Using 3-D seismic reflection data from offshore NW Australia,
we quantify the 3-D geometry of these elements and their connectors within
two sills and discuss how their shape may relate to emplacement processes.
Based on seismic attribute analyses and our measurements of their 3-D
geometry, we conclude that the mapped elements likely formed through
non-elastic-brittle and/or non-brittle deformation ahead of the advancing
sill tip, which implies they are magma fingers. We show that thickness
varies across sills, and across distinct elements, which we infer to
represent flow localization and subsequent thickening of restricted areas.
The quantification of element geometries is useful for comparisons between
different subsurface and field-based data sets that span a range of host
rock types and tectonic settings. This, in turn, facilitates the testing of
magma emplacement mechanisms and predictions from numerical and physical
analogue experiments.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02390.1/610701/The-building-blocks-of-igneous-sheet-intrusions
Tectonostratigraphy and major structures of the Georgian Greater
Caucasus: Implications for structural architecture, along-strike
continuity, and orogen evolution
Charles C. Trexler; Eric Cowgill; Nathan A. Niemi; Dylan A. Vasey; Tea
Godoladze
Abstract:
Although the Greater Caucasus Mountains have played a central role in
absorbing late Cenozoic convergence between the Arabian and Eurasian
plates, the orogenic architecture and the ways in which it accommodates
modern shortening remain debated. Here, we addressed this problem using
geologic mapping along two transects across the southern half of the
western Greater Caucasus to reveal a suite of regionally coherent
stratigraphic packages that are juxtaposed across a series of thrust
faults, which we call the North Georgia fault system. From south to north
within this system, stratigraphically repeated ~5–10-km-thick thrust sheets
show systematically increasing bedding dip angles (<30° in the south to
subvertical in the core of the range). Likewise, exhumation depth increases
toward the core of the range, based on low-temperature thermochronologic
data and metamorphic grade of exposed rocks. In contrast, active shortening
in the modern system is accommodated, at least in part, by thrust faults
along the southern margin of the orogen. Facilitated by the North Georgia
fault system, the western Greater Caucasus Mountains broadly behave as an
in-sequence, southward-propagating imbricate thrust fan, with older faults
within the range progressively abandoned and new structures forming to
accommodate shortening as the thrust propagates southward. We suggest that
the single-fault-centric “Main Caucasus thrust” paradigm is no longer
appropriate, as it is a system of faults, the North Georgia fault system,
that dominates the architecture of the western Greater Caucasus Mountains.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02385.1/610702/Tectonostratigraphy-and-major-structures-of-the
GEOSPHERE articles are available at
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