New Articles for Geosphere Posted Online in February
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Topics include using the differences
between soil and air temperatures for geological reconstructions of past
climate; the western Snake River Plain, Idaho; the state of the art in
numerical modeling of subduction; and the Pacific margin of Zealandia. You
can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Petrogenesis of voluminous silicic magmas in the Sierra Madre
Occidental large igneous province, Mexican Cordillera: Insights from
zircon and Hf-O isotopes
Graham D.M. Andrews; Cathy J. Busby; Sarah R. Brown; Christopher M. Fisher;
Pablo Davila-Harris ...
Abstract:
Combined Hf-O isotopic analyses of zircons from tuffs and lavas within the
Sierra Madre Occidental (SMO) silicic large igneous province are probes of
petrogenetic processes in the lower and upper crust. Existing petrogenetic
and tectonomagmatic models diverge, having either emphasized significant
crustal reworking of hydrated continental lithosphere in an arc above the
retreating Farallon slab or significant input of juvenile mantle melts
through a slab window into an actively stretching continental lithosphere.
New isotopic data are remarkably uniform within and between erupted units
across the spatial and temporal extent of the SMO, consistent with
homogeneous melt production and evolution. Isotopic values are consistent
with enriched mantle magmas (80%) that assimilated Proterozoic paragneisses
(~20%) from the lower crust. 18Ozircon values are
consistent with fractionation of mafic magma and not with assimilation of
hydrothermally altered upper crust, suggesting that the silicic magmas
evolved at depth. Isotopic data agree with previous interpretations where
voluminous juvenile melts entered the lithosphere during the transition
from a continental arc experiencing slab rollback (Late Eocene) to the
arrival of a subducting slab window (Oligocene and Early Miocene) and
failure of the upper plate leading to the opening of the Gulf of California
(Late Miocene). An anomalously large heat flux and extension of the upper
plate allow for the sustained fractionation of the voluminous SMO magmas
and assimilation of the lower crust.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02430.1/612177/Petrogenesis-of-voluminous-silicic-magmas-in-the
Spatial skill predicts success on sequence stratigraphic interpretation
B.Z. Kreager; N.D. LaDue; T.F. Shipley; R.D. Powell; B.A. Hampton
Abstract:
Sequence stratigraphic interpretation and three-dimensional spatial and
spatiotemporal skills are considered important for the petroleum industry.
However, little is known about the relationship between the two. This study
begins to fill this gap by testing whether spatial skills predict success
on a sequence stratigraphic interpretation task. Students in this study ( N = 78) were enrolled in undergraduate or graduate
stratigraphy-focused courses at three U.S. state universities. Students
completed (1) a sequence stratigraphic interpretation task with a sequence
stratigraphic diagram and Wheeler diagram and (2) two spatial skills tests.
Findings of simple linear regressions show that both disembedding
(extracting or finding a pattern among other features, which is typically
assessed by the hidden-figures test) and mental folding and unfolding (as
assessed by the surface development test) are predictive of student success
on the full sequence stratigraphic interpretation task. A nested
regression, entering mental folding as the initial variable and
disembedding as the secondary variable, showed that mental folding and
unfolding accounted for almost all of the variance accounted for by
disembedding in the simple regression. This may reflect the need to employ
disembedding for the test of mental folding. Because the test of
disembedding and the test of mental folding and unfolding were correlated,
the distinct role of disembedding in stratigraphy remains unclear. However,
the results clearly show that mental folding and unfolding is related to
student success in sequence stratigraphic interpretation. Future studies
should characterize how students utilize these skills, try to determine the
causal direction of this effect, and identify good practices for supporting
students in the classroom.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02428.1/612178/Spatial-skill-predicts-success-on-sequence
Geologic and geomorphic evidence for multi-phase history of strands of
the San Andreas fault through the San Gorgonio Pass structural knot,
southern California
Katherine J. Kendrick; Jonathan C. Matti; Nicolas C. Barth
Abstract:
The San Gorgonio Pass region of southern California is a locus of extensive
Quaternary deformation within a multi-strand section of the San Andreas
fault zone. The geomorphology of the San Gorgonio Pass region reflects the
complicated history of geologic events in the formation of this
structurally complex region. We define fault-bounded blocks in San Gorgonio
Pass and focus on two that are characterized by extensive crystalline
bedrock outcrops with similar bedrock lithologies. These two blocks are
separated by the San Bernardino strand of the San Andreas fault.
Morphometric variables, including local relief, slope, slope distribution,
and surface roughness, consistently demonstrate distinctions between the
bedrock upland regions of the two blocks. Geologic observations of the
region highlight differences in Quaternary units within the two blocks,
reflective of the differing surficial processes active in each block.
Within the Kitching Peak block, the morphology highlights a lineament that
we informally name the Lion Canyon lineament. This boundary more clearly
differentiates the two regions, as compared to the mapped San Bernardino
strand, and may represent the previously active strand or bounding
structure in this section. The distinction in morphology and surficial
processes leads to our interpretation that the Kitching Peak and Pisgah
Peak blocks have experienced different uplift histories. This further leads
to the conclusion that the San Bernardino strand, broadly defined, has been
integrated, at some point in the past, with the Banning strand, allowing
for through-going rupture along the fault system. This connectivity may
have occurred along the Burro Flats section of the San Bernardino strand or
the Lion Canyon lineament. The fault connection along the mapped trace of
the San Bernardino strand is not currently evident at the surface, however,
suggesting that the integration has been disrupted. We propose this is due
to intervals of N-S compression in the region, manifest as slip along the
San Gorgonio Pass fault zone and other regional faults. We present evidence
for lateral displacement along the San Bernardino and Banning strands of
the San Andreas fault, discuss the implications of these displacements, and
propose a sequence of fault activity, including multiple phases of activity
along the San Bernardino and Banning strand pathway to account for the
structural complexity and lack of surficial fault continuity.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02424.1/611975/Geologic-and-geomorphic-evidence-for-multi-phase
A robust age model for the Cryogenian Pocatello Formation of
southeastern Idaho (northwestern USA) from tandem in situ and isotope
dilution U-Pb dating of volcanic tuffs and epiclastic detrital zircons
Vincent H. Isakson; Mark D. Schmitz; Carol M. Dehler; Francis A. Macdonald;
W. Adolph Yonkee
Abstract:
Tandem in situ and isotope dilution U-Pb analysis of zircons from
pyroclastic volcanic rocks and both glacial and non-glacial sedimentary
strata of the Pocatello Formation (Idaho, northwestern USA) provides new
age constraints on Cryogenian glaciation in the North American Cordillera.
Two dacitic tuffs sampled within glacigenic strata of the lower diamictite
interval of the Scout Mountain Member yield high-precision chemical
abrasion isotope dilution U-Pb zircon eruption and depositional ages of
696.43 ± 0.21 and 695.17 ± 0.20 Ma. When supplemented by a new
high-precision detrital zircon maximum depositional age of ≤670 Ma for
shoreface and offshore sandstones unconformably overlying the lower
diamictite, these data are consistent with correlation of the lower
diamictite to the early Cryogenian (ca. 717–660 Ma) Sturtian glaciation.
These 670–675 Ma zircons persist in beds above the upper diamictite and cap
dolostone units, up to and including a purported “reworked fallout tuff,”
which we instead conclude provides only a maximum depositional age of ≤673
Ma from epiclastic volcanic detritus. Rare detrital zircons as young as 658
Ma provide a maximum depositional age for the upper diamictite and
overlying cap dolostone units. This new geochronological framework supports
litho- and chemostratigraphic correlations of the lower and upper
diamictite intervals of the Scout Mountain Member of the Pocatello
Formation with the Sturtian (716–660 Ma) and Marinoan (≤650–635 Ma)
low-latitude glaciations, respectively. The Pocatello Formation thus
contains a more complete record of Cryogenian glaciations than previously
postulated.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02437.1/611977/A-robust-age-model-for-the-Cryogenian-Pocatello
Impact of bending-related faulting and oceanic-plate topography on slab
hydration and intermediate-depth seismicity
Jacob Geersen; Christian Sippl; Nicholas Harmon
Abstract:
It is commonly assumed that intermediate-depth seismicity is in some way
linked to dehydration reactions inside subducting oceanic plates. Although
there is growing evidence that the hydration state of an oceanic plate is
controlled by its structure and degree of faulting, we do not have a
quantitative understanding of this relationship. Double seismic zones offer
the possibility of investigating changes in oceanic-plate hydration not
only along strike but also with depth beneath the slab surface. To quantify
the impact of oceanic-plate structure and faulting on slab hydration and
intermediate-depth seismicity, with a focus on the genesis of double
seismic zones, we correlate high-resolution earthquake catalogs and
seafloor maps of ship-based bathymetry for the northern Chilean and Japan
Trench subduction zones. The correlations show only a weak influence of
oceanic-plate structure and faulting on seismicity on the upper plane of
the double seismic zone, which may imply that hydration is limited by slow
reaction kinetics at low temperatures 5–7 km below the seafloor and by the
finite amount of exposed wall rock in the outer-rise region. These factors
seem to limit hydration even if abundant water is available. Seismicity in
the lower plane is, in contrast, substantially enhanced where deformation
of the oceanic plate is high and distributed across intersecting faults.
This likely leads to an increase in the volume of damaged wall rock around
the faults, thereby promoting the circulation of water to mantle depths
where serpentinization is faster due to elevated temperatures. Increased
lower-plane seismicity around subducting oceanic features such as seamounts
or fracture zones may also be caused by enhanced faulting around these
features. Our results provide a possible explanation for the globally
observed presence of rather homogeneous upper-plane seismicity in double
seismic zones as well as for the commonly patchy and inhomogeneous
distribution of lower-plane seismicity.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02367.1/611956/Impact-of-bending-related-faulting-and-oceanic
The “Nazas Arc” is a continental rift province: Implications for
Mesozoic tectonic reconstructions of the southwest Cordillera, U.S. and
Mexico
C.J. Busby; E. Centeno-García
Abstract:
We reject the notion of a Jurassic continental arc in eastern Mexico,
termed the “Nazas arc,” on geologic grounds. Instead, we propose that the
Jurassic continental arc of the SW Cordilleran U.S. and northern Sonora,
Mexico, passed southward into the oceanic realm and is represented by
Jurassic arc volcanic and plutonic rocks that fringed the Mexican
paleo-Pacific margin, which are currently found in the western Peninsular
Ranges of southern California, USA, and Baja California, the Vizcaino
Peninsula of Baja California, and western mainland Mexico. To show this, we
present a summary of the geologic features of a continental arc, using the
geology of the southern end of the Jurassic continental arc, in southern
Arizona and northern Sonora. These features include multi-kilometer–thick
sections of volcanic rock; large volcanic centers, including silicic
calderas; major eruptive units that can be correlated for distances of 100
km or more; abundant, large plutonic suites; and continuity of these
features for distances of hundreds of kilometers along the length of the
continental arc. Then we show that the “Nazas arc” consists of scattered,
small continental rift basins with thin (meters to tens of meters thick)
volcanic sections at the base of clastic sections that are hundreds of
meters thick. Plutonic rocks are entirely absent from the “Nazas arc,”
despite the fact that post-Jurassic tectonic events should have exposed
them if they existed. This paper also presents a tabulation of all
published U-Pb zircon dates in the Jurassic continental arc of southern
Arizona, USA, and northern Sonora (Table 1A), and in the “Nazas arc” of
eastern Mexico (Table 1B), with ages, methods, the rock type dated, and
notes on geologic relations. We use this to detail the abundance of thick,
laterally extensive volcanic sections and large plutonic suites in a
continental arc (the Jurassic arc of southern Arizona–northern Sonora),
which contrasts sharply with the “Nazas arc. The term “Nazas arc” has been
in widespread usage for volcanic rocks in eastern Mexico for decades in
many dozens of papers, and it is portrayed as a 2000-km-long, 250-km-wide
belt that extends from Sonora through eastern Mexico to Chiapas. It has
been misunderstood to form a subduction-related silicic large igneous
province (SLIP), and it has been proposed that the Gulf of Mexico formed as
a backarc basin behind the “Nazas arc.” The “Nazas arc” model also requires
an east-dipping subduction zone under Mexico, and a separate west-dipping
subduction zone under the oceanic arc rocks of western Mexico, which those
models portray as an exotic arc, despite the presence of abundant detrital
zircon from the Mexican margin. We urge workers to abandon the term “Nazas
arc” and replace it with “Nazas rift province,” which represents
continental rift basins formed during the breakup of Pangea.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02443.1/611957/The-Nazas-Arc-is-a-continental-rift-province
Detrital zircon provenance of the Cretaceous–Neogene East Coast Basin
reveals changing tectonic conditions and drainage reorganization along
the Pacific margin of Zealandia
Jared T. Gooley; Nora M. Nieminski
Abstract:
The Upper Cretaceous–Pliocene strata of New Zealand record ~100 m.y. of
Zealandia’s evolution, including development of the Hikurangi convergent
margin and Alpine transform plate boundary. A comprehensive, new detrital
zircon U-Pb data set (8315 analyses from 61 samples) was generated along a
~700 km transect of the East Coast Basin of New Zealand. Age distributions
were analyzed and interpreted in terms of published data available for
Cambrian–Cretaceous igneous and metasedimentary source terranes using a
Monte Carlo mixture modeling approach. Results indicate a widespread Early
Cretaceous transition in sediment source from the Gondwana interior to the
Median Batholith magmatic arc prior to Late Cretaceous rifting from
Antarctica. Submergence of Zealandia during a Late Cretaceous–Paleogene
drift phase led to major drainage reorganization and the influx of Eastern
Province sediment to the East Coast Basin. A long-lived sediment conduit
that transported extraregional Western Province detritus to the
south-central East Coast Basin may have developed along a structural
precursor to the Alpine Fault. Marked Neogene increase of Upper
Jurassic–Lower Cretaceous Torlesse Composite Terrane sediment to the
central East Coast Basin resulted from exhumation of the Axial Ranges,
convergence along the Hikurangi subduction margin, and concurrent
development of the Alpine Fault. Concurrent influx of contemporaneous
Neogene zircon in the northern East Coast Basin indicated the onset of
subduction-related volcanism of the Northland–Coromandel Volcanic Arc.
Middle Miocene–Pliocene exhumation and dextral translation of the Nelson
region along the Alpine Fault resulted in the eastward routing of Western
Province sediment to the central East Coast Basin. Finally, topography
developed across the plate boundary and ultimately partitioned continental
drainage of Zealandia, such that sediment from the Murihiku, Caples, and
Rakaia Terranes in the Otago region was routed to the southern extent of
the East Coast Basin. These results illuminate the evolution of the
Zealandia continental drainage divide in response to the initiation of the
Pacific-Australian plate boundary and demonstrate the power of mixture
modeling and large data sets for deciphering sediment routing in dynamic
tectonic environments.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02404.1/611958/Detrital-zircon-provenance-of-the-Cretaceous
Linking exhumation, paleo-relief, and rift formation to magmatic
processes in the western Snake River Plain, Idaho, using apatite
(U-Th)/He thermochronology
Kelsey F. Wetzel; Jessica R. Stanley
Abstract:
The western Snake River Plain (WSRP) in southwest Idaho has been
characterized as an intracontinental rift basin but differs markedly in
topography and style from other Cordilleran extensional structures and
structurally from the down-warped lava plain of the eastern Snake River
Plain. To investigate mechanisms driving extension and topographic
evolution, we sampled granitoid bedrock from Cretaceous and Eocene-aged
plutons from the mountainous flanks of the WSRP to detail their exhumation
history with apatite (U-Th)/He (AHe) thermochronometry. AHe cooling dates
from seventeen samples range from 7.9 ± 1.4 Ma to 55 ± 10 Ma. Most cooling
dates from Cretaceous plutons adjacent to the WSRP are Eocene, while Eocene
intrusions from within the Middle Fork Boise River canyon ~35 km NE of the
WSRP yield Miocene cooling dates. The AHe dates provide evidence of
exhumation of the Idaho batholith during the Eocene, supporting a high
relief landscape at that time, followed by decreasing relief. The Miocene
AHe dates show rapid cooling along the Middle Fork Boise River that we take
to indicate focused river incision due to base level fall in the WSRP.
Eocene AHe dates limit magnitudes of exhumation and extension on the flanks
of the WSRP during Miocene rift formation. This suggests extension was
accommodated by magmatic intrusions and intrabasin faults rather than
basin-bounding faults. We favor a model where WSRP extension was related to
Columbia River Flood Basalt eruption and enhanced by later eruption of the
Bruneau-Jarbidge and Twin Falls volcanic fields, explaining the apparent
difference with other Cordilleran extensional structures.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02453.1/611754/Linking-exhumation-paleo-relief-and-rift-formation
The formation of high-Sr/Y plutons in cordilleran-arc crust by crystal
accumulation and melt loss
Adam J. Brackman; Joshua J. Schwartz
Abstract:
Bulk-rock data are commonly used in geochemical studies as a proxy for melt
compositions in order to understand the evolution of crustal melts.
However, processes of crystal accumulation and melt migration out of
deep-crustal, crystal-rich mush zones to shallower storage regions raise
questions about how faithfully bulk-rock compositions in plutons
approximate melt compositions. This problem is particularly acute in the
lower crust of arcs, where melt reservoirs are subject to periodic melt
extraction that leaves behind a cumulate residue. Here, we examine
bulk-rock data from the perspective of high-Sr/Y plutonic rocks in the
lower crust of a well-exposed Early Cretaceous cordilleran-arc system in
Fiordland, New Zealand. We test the validity of using high-Sr/Y bulk-rock
compositions as proxies for melts by comparing bulk-rock compositions to
melts modeled from >100 major- and trace-element analyses of 23 magmatic
clinopyroxene grains from the same samples. The sampling locations of the
igneous clinopyroxenes and encompassing bulk rocks are distributed across
~550 km2 of exhumed lower crust and are representative of
Mesozoic lower-crustal arc rocks in the Median batholith. We confirm that
bulk-rock data have characteristics of high-Sr/Y plutons (Sr/Y >50, Na 2O >3.5 wt%, Sr >1000 ppm, and Y <20 ppm), features
that have been previously interpreted to indicate the presence of garnet as
a residual or fractionating phase. In contrast to bulk rocks, igneous
clinopyroxenes have low Sr (<100 ppm), high Y (25–100 ppm), and low
molar Mg# [100 × Mg/(Mg + Fe)] values (60–70), which are consistent with
derivation from fractionated, low-Sr/Y melts. Chondrite-normalized
rare-earth-element patterns and Sm/Yb values in clinopyroxenes also show
little to no evidence for involvement of garnet in the source or in
differentiation processes. Fe-Mg partitioning relationships indicate that
clinopyroxenes are not in equilibrium with their encompassing bulk rocks
but could have been in equilibrium with melt compositions determined from
chemometry of coexisting igneous hornblendes. Moho-depth calculations based
on bulk-rock Sr/Y values also yield Moho depths (average = 69 km) that are
inconsistent with Moho depths based on bulk-rock Ce/Y, contact aureole
studies, Al-in-horn- blende crystallization pressures, and our modeled
clinopyroxene crystallization pressures. These data indicate that most
Mesozoic high-Sr/Y bulk rocks in the lower crust of Fiordland are cumulates
formed by plagioclase + amphibole + clinopyroxene accumulation and
interstitial melt loss from crystal-rich mush zones. Our data do not
support widespread fractionation of igneous garnet nor partial melting of a
garnet-bearing source in the petrogenesis of these melts. We speculate that
melt extraction and the production of voluminous cumulates in the lower
crust were aided by unusually high heat flow and high magma addition rates
associated with an Early Cretaceous arc flareup. We conclude that bulk-rock
compositions are poor proxies for melt compositions in the lower crust of
the Median batholith, and geochemical modeling of these high-Sr/Y bulk
rocks would overemphasize the role of garnet in their petrogenesis.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02400.1/611755/The-formation-of-high-Sr-Y-plutons-in-cordilleran
A new geological map of the Lau Basin (southwestern Pacific Ocean)
reveals crustal growth processes in arc-backarc systems
Margaret S. Stewart; Mark D. Hannington; Justin Emberley; Alan T. Baxter;
Anna Krätschell ...
Abstract:
A 1:1,000,000-scale lithostratigraphic assemblage map of the Lau Basin
(southwestern Pacific Ocean) has been created using remote predictive
mapping (RPM) techniques developed by geological surveys on land.
Formation-level geological units were identified in training sets at scales
of 1:100,000–1:200,000 in different parts of the basin and then
extrapolated to the areas where geological data are sparse. The final
compilation is presented together with a quantitative analysis of
assemblage-level crustal growth based on area-age relationships of the
assigned units. The data sets used to develop mapping criteria and an
internally consistent legend for the compilation included high-resolution
ship-based multibeam, satellite- and ship-based gravity, magnetics,
seafloor imaging, and sampling data. The correlation of units was informed
by published geochronological information and kinematic models of basin
opening. The map covers >1,000,000 km2 of the Lau-Tonga
arc-backarc system, subdivided into nine assemblage types: forearc crust
(9% by area), crust of the active volcanic arc (7%), backarc rifts and
spreading centers (20%), transitional arc-backarc crust (13%), relict arc
crust (38%), relict backarc crust (8%), and undivided arc-backarc
assemblages (<5%), plus oceanic assemblages, intraplate volcanoes, and
carbonate platforms. Major differences in the proportions of assemblage
types compared to other intraoceanic subduction systems (e.g., Mariana
backarc, North Fiji Basin) underscore the complex geological makeup of the
Lau Basin. Backarc crust formed and is forming simultaneously at 12
different locations in the basin in response to widely distributed
extension, and this is considered to be a dominant pattern of crustal
accretion in large arc-backarc systems. Accelerated basin opening and a
microplate breakout north of the Peggy Ridge has been accommodated by seven
different spreading centers. The result is an intricate mosaic of small
intact assemblages in the north of the basin, compared to fewer and larger
assemblages in the south. Although the oldest rocks are Eocene (~40 m.y.
old basement of the Lau and Tonga Ridges), half of the backarc crust in the
map area formed within the last 3 m.y. and therefore represents some of the
fastest growing crust on Earth, associated with prolific magmatic and
hydro-thermal activity. These observations provide important clues to the
geological evolution and makeup of ancient backarc basins and to processes
of crustal growth that ultimately lead to the emergence of continents.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02340.1/611756/A-new-geological-map-of-the-Lau-Basin-southwestern
Numerical modeling of subduction: State of the art and future direction
Taras Gerya
Abstract:
During the past five decades, numerical modeling of subduction, one of the
most challenging and captivating geodynamic processes, remained in the core
of geodynamic research. Remarkable progress has been made in terms of both
in-depth understanding of different aspects of subduction dynamics and
deciphering the diverse and ever-growing array of subduction zone
observations. However, numerous key questions concerning subduction remain
unanswered defining the frontier of modern Earth Science research. This
review of the past decade comprises numerical modeling studies focused on
12 key open topics.Future progress will require conceptual and technical
progress in subduction modeling as well as crucial inputs from other
disciplines (rheology, phase petrology, seismic tomography, geochemistry,
numerical theory, geomorphology, ecology, planetology, astronomy, etc.). As
in the past, the multi-physics character of subduction-related processes
ensures that numerical modeling will remain one of the key quantitative
tools for integration of natural observations, developing and testing new
hypotheses, and developing an in-depth understanding of subduction. The
review concludes with summarizing key results and outlining 12 future
directions in subduction and plate tectonics modeling that will target
unresolved issues discussed in the review.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02416.1/611709/Numerical-modeling-of-subduction-State-of-the-art
The Cenozoic evolution of the Intrarif (Rif, Morocco)
Manuel Martín-Martín; Francesco Guerrera; Alí Maaté; Rachid Hlila;
Francisco Serrano ...
Abstract:
This paper provides an understanding of the sedimentary-tectonic evolution
of the Cenozoic strata of the El Habt and Ouezzane Tectonic Units
(Intrarif, External Rif) in Morocco. New data provide information about the
depositional architecture and enable a correlation of the evolution of the
External Rif in Morocco with that of the Betic Cordillera in Spain and the
Tunisian Tell, which provides new insights for hydrocarbon exploration in
the region regarding possible source, reservoir, and seal rocks. The
reconstructed Cenozoic succession was bio-chronologically defined, and the
major unconformities and stratigraphic gaps were identified. The presence
of these unconformities allowed three main stratigraphic sequences to be
defined by age: Danian p.p., early Ypresian–early Bartonian p.p., and the early Rupelian–early Serravallian p.p.
Three secondary stratigraphic sequences in the former upper main sequence
were also defined by age: early Rupelian–late Chattian p.p.,
Burdigalian p.p., and the Langhian–Serravallian p.p. The
depositional setting evolved from deep basin during the Late
Cretaceous–Paleocene to external platform-slope during the Eocene–Miocene.
The Cenozoic sandstones contain metamorphic and sedimentary rock fragments
derived from a recycled orogen source area. The clay mineralogy in the
Cenozoic strata consists of associations of Ill+(I–S) ± Sme, Ill+(I–S) ±
Sme+Kln and Ill+(I–S) ± Sme+- Kln+Chl. These associations indicate an
initial unroofing in the Paleogene period, then in the Cretaceous period,
and finally in the Late Jurassic period during the Eocene–Oligocene. This
detritus was followed by variable amounts of a sedimentary mix of Paleogene
to Late Jurassic terrains due to several phases of erosion and deposition
partly related to syn-sedimentary tectonics during the Miocene. Equivalent
features (similar types of sediments, tectofacies, gaps, and unroofing)
were also recognized along the Betic Cordillera in Spain and Maghrebian
Chain (Morocco and Tunisia) and interpreted as related to a pre-nappe
tectonic activity of soft basement folding, which occurred during the
Paleogene after the generalized tectonic inversion (from extension to
compression) occurred in the Late Cretaceous. The Upper Cretaceous is
considered to be the hydrocarbon source rock, while the fractured Eocene
and the porous Oligo-Miocene suites are proposed as possible hydrocarbon
reservoirs. The Cenozoic stratigraphic architecture and the nappe structure
of the region could provide the necessary trap structures.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02199.1/611688/The-Cenozoic-evolution-of-the-Intrarif-Rif-Morocco
Open AR-Sandbox: A haptic interface for geoscience education and
outreach
Florian Wellmann; Simon Virgo; Daniel Escallon; Miguel de la Varga;
Alexander Jüstel ...
Abstract:
Virtual reality concepts have been widely adapted to teach geoscientific
content, most notably in virtual field trips—with increased developments
due to recent travel restrictions and challenges of field access. On the
spectrum between real and fully virtual environments are also combinations
of digital and real content in mixed-reality environments. In this
category, augmented-reality (AR) sandboxes have been used as a valuable
tool for science outreach and teaching due to their intuitive and haptic
interaction-enhancing operation. Most of the common AR-sandboxes are
limited to the visualization of topography with contour lines and colors,
as well as water simulations on the digital terrain surface. We show here
how we can get beyond this limitation, through an open-source
implementation of an AR-sandbox system with a versatile interface written
in the free and cross-platform programming language Python. This
implementation allows for creative and novel applications in geosciences
education and outreach in general. With a link to a 3-D geomodelling
system, we show how we can display geologic subsurface information such as
the outcropping lithology, creating an interactive geological map for
structural geology classes. The relations of subsurface structures,
topography, and outcrop can be explored in a playful and comprehensible
way. Additional examples include the visualizations of geophysical fields
and the propagation of seismic waves, as well as simulations of Earth
surface processes. We further extended the functionality with ArUco-marker
detection to enable more precise and flexible interaction with the
projected content. In combination, with these developments, we aim to make
AR-sandbox systems, with the additional dimension of haptic interactions,
accessible to a wider range of geoscientific applications for education and
outreach.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02455.1/611689/Open-AR-Sandbox-A-haptic-interface-for-geoscience
Neogene sedimentary record of the evolution of a translated strike-slip
basin along the Denali fault system: Implications for timing of
displacement, composite basin development, and regional tectonics of
southern Alaska
Wai K. Allen; Kenneth D. Ridgway; J.A. Benowitz; T.S. Waldien; S.M. Roeske
...
Abstract:
Analysis of the late Miocene to Holocene McCallum sedimentary basin,
located along the south side of the eastern Denali fault system, provides a
better understanding of strike-slip basin evolution, timing of displacement
on the Denali fault, and tectonics of the southern Alaska convergent
margin. Analysis of the McCallum basin utilizing measured stratigraphic
sections, lithofacies analyses, and 40Ar/39Ar tephra
ages documented a 564-m-thick, two-member stratigraphy. Fine-grained,
lacustrine-dominated environments characterized deposition of the lower
member, and coarse-grained, stream-dominated alluvial-fan environments
characterized deposition of the upper member. The 40Ar/ 39Ar dating of tephras indicated that the lower member was
deposited from 6.1 to 5.0 Ma, and the upper member was deposited from 5.0
to 3.8 Ma. Our stratigraphic analysis of the McCallum basin illuminates the
development of a composite strike-slip basin, with the deposition of the
lower member occurring along a transtensional fault section, and deposition
of the upper member occurring along a transpressional fault section. This
change in depositional and tectonic settings is interpreted to reflect
~79–90 km of transport of the basin along the Denali fault system based on
Pleistocene–Holocene slip rates. Previous studies of the timing of Cenozoic
displacement on the Denali fault system utilizing sedimentary records
emphasized a Paleogene component; our findings, however, also require a
significant Neogene component. Neogene strike-slip displacement and basin
development along the Denali fault system were broadly coeval with
development of high topography and related clastic wedges across southern
Alaska in response to flat slab subduction of the Yakutat microplate.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02435.1/611690/Neogene-sedimentary-record-of-the-evolution-of-a
Latest Triassic–Early Jurassic Stikine–Yukon-Tanana terrane collision
and the onset of accretion in the Canadian Cordillera: Insights from
Hazelton Group detrital zircon provenance and arc–back-arc
configuration
JoAnne L. Nelson; Bram van Straaten; Richard Friedman
Abstract:
The Hazelton Group is a Rhaetian–Bajocian (uppermost Triassic–Middle
Jurassic) volcano-sedimentary sequence that represents both the last
pre-accretionary arc volcanic cycle of Stikinia and its early
synaccretionary aftermath. Hazelton magmatism of central Stikinia succeeded
the Late Triassic (mainly Carnian–Norian) Stuhini arc, which ceased
activity as a result of end-on collision with the pericratonic Yukon-Tanana
terrane. The Hazelton volcanic belt lies to the south along strike with the
coeval Whitehorse trough, the synorogenic clastic basin that developed on
top of the Stikinia–Yukon-Tanana collision zone. Whereas the sources of
voluminous clastic sediments in the Whitehorse trough were its rapidly
exhuming shoulders, the thin clastic intervals in the Hazelton Group in
northwestern British Columbia were derived from local to subregional block
uplifts that supplied mainly ca. 230–215 Ma zircons eroded from the
plutonic roots of the Stuhini arc. Lesser components include late Paleozoic
(ca. 350–330 Ma) zircons from Stikinia’s basement and penecontemporaneous
(ca. 205–172 Ma) zircons from Hazelton volcanic/subvolcanic sources.
Reexamination of the four main volcanic fields that make up the lower
Hazelton Group suggests that the main Hazelton volcanic belt formed a
southward-convex magmatic arc from eastern Stikinia across the Skeena arch,
including the Toodoggone and Telkwa belts, with the Spatsizi and
Stewart-Iskut regions of northwestern British Columbia in its back-arc. The
Whitehorse trough and Hazelton belt represent a collision zone to active
arc pair. Southward advance of the arc and counterclockwise rotation of the
Stikinia microplate contributed to closure against the Quesnellia arc and
assembly of the inner Canadian Cordilleran terrane collage.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02444.1/611691/Latest-Triassic-Early-Jurassic-Stikine-Yukon
Differences between soil and air temperatures: Implications for
geological reconstructions of past climate
Peter Molnar
Abstract:
Among quantities of interest in paleoclimate, the mean annual air
temperature, Ta, directly over the surface looms
prominently. Most geologic estimates of past temperatures from continental
regions, however, quantify temperatures of the soil or other material below
the surface, Ts, and in general Ta
< Ts. Both theory and data from the FLUXNET2015 data
set of surface energy balance indicate systematic dependences of
temperature differences ΔT = Ts − T a and also of Bowen ratios—ratios of sensible to latent heat
fluxes from surface to the atmosphere—on the nature of the land-surface
cover. In cold regions, with mean annual temperatures ≲5 °C, latent heat
flux tends to be small, and values of ΔT can be large, 3–5 °C or
larger. Over wet surfaces, latent heat fluxes dominate sensible heat
fluxes, and values of both ΔT and Bowen ratios commonly are small.
By contrast, over arid surfaces that provide only limited moisture to the
overlying atmosphere, the opposite holds. Both theory and observation
suggest the following, albeit approximate, mean annual values of Δ T: for wetlands, 1 °C; forests, 1 ± 1 °C; shrublands, 3–4 °C;
savannas, 3.5 °C < ΔT < 5.5 °C; grasslands, 1 °C where wet
to 3 °C where arid; and deserts, 4–6 °C. As geological tools for inferring
past land-surface conditions improve, these approximate values of Δ T will allow geologic estimates of past mean annual surface
temperatures, Ts, to be translated into estimates of
past mean annual air temperatures, Ta.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02448.1/611659/Differences-between-soil-and-air-temperatures
Quantitative mapping of dolomitization using close-range hyperspectral
imaging: Kimmeridgian carbonate ramp, Alacón, NE Spain
Tobias H. Kurz; Galo San Miguel; Dominique Dubucq; Jeroen Kenter; Veronique
Miegebielle ...
Abstract:
Geological models from outcrop analogues are often utilized as a guide, or
soft constraint, for distributing reservoir properties in subsurface
models. In carbonate outcrops, combined sequence stratigraphic,
sedimentological, and petrographic studies constrain the heterogeneity of
geobodies and diagenetic processes, including dolomitization, at multiple
scales. High-resolution digital outcrop modeling further aids geometric
mapping, geobody definition, and statistical analysis, though its
usefulness for detailed mineralogical and lithological mapping is limited.
Hyperspectral imaging offers enhanced spectral resolution for mapping
subtle mineralogical differences. In both outcrops and subsurface,
differences in carbonate composition can provide key information for
distributing porosity and permeability, yet this mapping is highly
challenging in field studies due to access difficulties, visible material
differences, and sampling resolution. Spectral analysis of
limestone–dolomite ratios conducted in laboratory studies indicates
theoretical measures for quantitative identification and mapping of
dolomite degrees within carbonate rocks. In this study, close-range
hyperspectral imaging is applied to outcrops of the Alacón Member, Barranco
del Mortero, northeastern Spain, to identify exposed limestone–dolomite
geobodies and to quantify the degree of dolomitization across outcrop
faces. Hyperspectral imaging is supplemented with photogrammetric outcrop
modeling, field spectroscopy, and laboratory sample analysis for empirical
validation and uncertainty analysis. Hyperspectral mapping shows that
earlier fieldwork utilizing visual inspection of difficult to access
outcrop surfaces had overestimated the amount of dolomite in the outcrop.
Results indicate that hyperspectral imaging identified dolomite bodies more
accurately and reliably than conventional field methods and facilitates the
mapping of dolomite contribution in areas modified by dedolomitization,
where dolomite content changes by more than ~20%.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02312.1/611660/Quantitative-mapping-of-dolomitization-using-close
Eocene dike orientations across the Washington Cascades in response to
a major strike-slip faulting episode and ridge-trench interaction
Robert B. Miller; Kathleen I. Bryant; Brigid Doran; Michael P. Eddy; Franco
P. Raviola ...
Abstract:
The northern Cascade Mountains in Washington (USA) preserve an exceptional
shallow to mid-crustal record of Eocene transtension marked by dextral
strike-slip faulting, intrusion of dike swarms and plutons, rapid
non-marine sedimentation, and ductile flow and rapid cooling in parts of
the North Cas- cades crystalline core. Transtension occurred during
ridge-trench interaction with the formation of a slab window, and slab
rollback and break-off occurred shortly after collision of the Siletzia
oceanic plateau at ca. 50 Ma. Dike swarms intruded a ≥1250 km2
region between ca. 49.3 Ma and 44.9 Ma, and orientations of more than 1500
measured dikes coupled with geochronologic data provide important snapshots
of the regional strain field. The mafic Teanaway dikes are the southernmost
and most voluminous of the swarms. They strike NE (mean = 036°) and average
~15 m in thickness. To the north, rhyolitic to basaltic dikes overlap
spatially with 49.3–46.5 Ma, mainly granodioritic plutons, but they
typically predate the nearby plutons by ca. 500 k.y. The average
orientations of five of the six dike domains range from 010° to 058°; W-NW–
to NW–striking dikes characterize one domain and are found in lesser
amounts in a few other domains. Overall, the mean strike for all Eocene
dikes is 035°, and the average extension direction (305°–125°) is oblique
to the strike (~320°) of the North Cascades orogen. Extension by diking
reached ~45% in one >7-km-long transect through the Teanaway swarm and
ranged from ~5% to locally ~79% in shorter transects across other swarms,
which corresponds to a minimum of ~12 km of extension. The dominant
NE-striking dikes are compatible with the dextral motion on the N- to
NW-striking (~355–320°) regional strike-slip faults. Some of the W–NW- to
NW-striking dikes were arguably influenced by pre-existing faults, shear
fractures, and foliations, and potentially in one swarm where both NE- and
lesser W-NW–striking dikes are present, by a switch in principal stress
axes induced by dike emplacement. Alternatively, the W-NW– to NW-striking
dikes may reflect a younger regional strain field, as ca. 49.3–47.5 Ma U-Pb
zircon ages of the NE-striking dikes are older than those of the few dated
W-NW– to NW-trending dikes. In one scenario, NE-striking dikes intruded
during an interval when strain mainly reflected dextral strike-slip
faulting, and the younger dikes record a switch to more arc-normal
extension. Diking ended as magmatism migrated into a N-S–trending belt west
of the North Cascade core that marks the initiation of the ancestral
Cascade arc.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02387.1/611597/Eocene-dike-orientations-across-the-Washington
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