New Articles for Geosphere Posted Online in September
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied include the
Altiplano-Puna volcanic complex of the Central Andes; the Tornillo Group of
Big Bend National Park, West Texas, USA; the San Gorgonio Pass, California,
USA, region; and the central Cascadia arc, Oregon, USA. You can find these
articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Rhyolitic melt production in the midst of a continental arc
flare-up—The heterogeneous Caspana ignimbrite of the Altiplano-Puna
volcanic complex of the Central Andes
Charles T. Lewis; Shanaka L. de Silva; Dale H. Burns
Abstract:
The ~5 km3, 4.54–4.09 Ma Caspana ignimbrite of the
Altiplano-Puna volcanic complex (APVC) of the Central Andes records the
eruption of an andesite and two distinct rhyolitic magmas. It provides a
unique opportunity to investigate the production of silicic magmas in a
continental arc flare-up, where small volumes of magma rarely survive
homogenization into the regional magmatic system that is dominated by
supereruptions of monotonous dacitic ignimbrites. The fall deposit and thin
flow unit that record the first stage of the eruption (Phase 1) tapped a
crystal-poor peraluminous rhyolite. The petrological and geochemical
characteristics of Phase 1 are best explained by partial melting of or
reheating and melt extraction from a granodioritic intrusion. Phase 2 of
the eruption records the emplacement of a more extensive flow unit with a
crystal-poor, fayalite-bearing rhyolite and a porphyritic to
glomeroporphyritic andesite containing abundant
plagioclase-orthopyroxene-Fe-Ti oxide (norite) glomerocrysts. The isotopic
composition of Phase 2 is significantly more “crustal” than Phase 1,
indicating a separate petrogenetic path. The mineral assemblage of the
noritic glomerocrysts and the observed trend between andesite and Phase 2
rhyolite are reproduced by rhyolite-MELTS–based models.
Pressure-temperature-water (P-T-H2O) estimates indicate that the
main (Phase 2) reservoir resided between 400 and 200 MPa, with the andesite
recording the deeper pressures and a temperature range of 920–1060 °C.
Rhyolite phase equilibria predict an estimated temperature of ~775 °C and
~5 wt% H2O. Pressures derived from phase equilibria indicate
that the rhyolite was extracted directly from the noritic cumulate at ~340
MPa and stored at slightly shallower pressures (200–300 MPa) prior to
eruption. The rhyolite-MELTS models reveal that latent-heat buffering
during the extraction and storage process results in a shallow liquidus
during the extensive crystallization that produced a noritic cumulate in
equilibrium with a rhyodacitic residual liquid. Spikes in latent heat
facilitated the segregation of the residual liquid, creating the
pre-eruptive compositional gap of ~16 wt% SiO2 between the
andesite and the Phase 2 rhyolite. Unlike typical Altiplano-Puna volcanic
complex (APVC) magmas, low ƒO2 conditions in the andesite
promoted co-crystallization of orthopyroxene and ilmenite in lieu of
clinopyroxene and magnetite. This resulted in relatively high Fe
concentrations in the rhyodacite and Phase 2 rhyolite. Combined with the
co-crystallization of plagioclase, this low oxidation state forced high Fe 2+/Mg and Fe/Ca in the Phase 2 rhyolite, which promoted fayalite
stability. The dominance of low Fe3+/FeTot and Fe-Ti
oxide equilibria indicates low ƒO2 (ΔFMQ 0 − ΔFMQ − 1)
conditions in the rhyolite were inherited from the andesite. We propose
that the serendipitous location on the periphery of the regional thermal
anomaly of the Altiplano-Puna magma body (APMB) permitted the small-volume
magma reservoir that fed the Caspana ignimbrite eruption to retain its
heterogeneous character. This resulted in the record of rhyolitic liquids
with disparate origins that evaded assimilation into the large dacite
supereruption-feeding APMB. As such, the Caspana ignimbrite provides a
unique window into the multiscale processes that build long-lived
continental silicic magma systems.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02462.1/617788/Rhyolitic-melt-production-in-the-midst-of-a
Chronostratigraphic correlation of the Upper Silurian Salina Group for
the Michigan and Appalachian Basins through coupled (δ13
Ccarb) chemostratigraphy and subsurface geophysical analyses
Stephan C. Oborny; Bradley D. Cramer; Carlton E. Brett; Alyssa M. Bancroft
Abstract:
The Upper Silurian Salina Group of eastern North America is well known for
its thick evaporite successions and hydrocarbon resources. These strata
have been assigned to numerous chronostratigraphic schemes within Ohio and
Michigan and are currently identified by varying subsurface and outcrop
nomenclatural schemes. These chronostratigraphic challenges have persisted
for over 50 yr and dramatically inhibit the correlation of events recorded
in the Silurian section of eastern North America with the global record of
Silurian biogeochemical events. To help resolve the chronostratigraphic
correlation of these units, we provide new high-resolution δ13C carb chemostratigraphic analyses of a core located in central
Ohio for strata assigned to the Greenfield and Tymochtee Formations and
integrate existing biostratigraphic, chemostratigraphic, and subsurface
geophysical data in western, southern, and eastern Ohio. The new data
presented here, integrated for the first time with basinwide subsurface
geophysical data, demonstrate a mid-late Homerian Stage global sea-level
lowstand, suggest a short interval of tectonic stability within the study
area at the beginning of “Salina B–G” deposition, during which
accommodation was occupied by the Greenfield Formation and laterally
equivalent strata, and provide chronostratigraphic constraints for basin
flexure and potential forebulge migration associated with renewed tectonic
activity. The new chronostratigraphic correlation of these strata provides
a broader picture of Silurian environmental change across the eastern half
of the Laurentian paleocontinent.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02515.1/617787/Chronostratigraphic-correlation-of-the-Upper
The Cretaceous-Paleogene contact in the Tornillo Group of Big Bend
National Park, West Texas, USA
Thomas M. Lehman; Jacob Cobb; Paul Sylvester; A. Kate Souders
Abstract:
The Cretaceous-Paleogene (K-Pg) contact interval is constrained by
vertebrate fossil sites at seven sites in the Tornillo Group and lies
within an 80–100-m stratigraphic section between the top of the Javelina
Formation and the base of the “log jam sandstone” marker bed in the Black
Peaks Formation. In western exposures of this interval, the highest
occurrence of in situ dinosaur specimens and the lowest occurrence of
Paleocene mammal specimens are separated by an unusual conglomerate bed.
This thin conglomerate bed coincides with the contact between Cretaceous
and Paleogene strata and contains reworked Cretaceous fossils. It is
superficially similar to conglomerate beds elsewhere attributed to the
effects of tsunamis generated by the Chicxulub impact; however, the maximum
depositional age of ca. 63 Ma based on detrital zircons indicates that the
conglomerate was deposited about three million years after the K-Pg
boundary event. Paleocene mammalian fossils from immediately above the
conglomerate bed represent a fauna that can be no older than the middle
Torrejonian (To2 interval zone). The contact between Cretaceous and
Paleocene strata is therefore disconformal and represents a hiatus of at
least three million years. A condensed section occurs at the westernmost
exposure of the K-Pg contact, where at least 80 m of strata are absent
below the conglomerate bed; these strata are present in exposures farther
east. This condensed section likely records an erosional event resulting
from uplift and deformation of the nearby Terlingua monocline. Although the
80 m of strata below the conglomerate bed are poorly fossiliferous, several
clearly in situ dinosaur specimens indicate that this entire interval is
Late Cretaceous in age. There is no compelling evidence for preservation of
the K-Pg boundary event horizon at any of the seven sites in the Tornillo
Group, and so the hiatus represented at the Cretaceous/ Paleocene contact
here likely also includes some part of latest Cretaceous time. Mammalian
specimens from sites in the “log jam sandstone,” ~40 m above the middle
Torrejonian sites, represent an early Tiffanian fauna (Ti1 interval zone).
Latest Torrejonian (To3) sites have not been recognized, and therefore a
second disconformity likely coincides with the base of the “log jam
sandstone” marker horizon in the Black Peaks Formation.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02519.1/617786/The-Cretaceous-Paleogene-contact-in-the-Tornillo
Spatiotemporally heterogeneous deformation, indirect tectonomagmatic
links, and lithospheric evolution during orogenic activity coeval with
an arc flare-up
Snir Attia; Scott R. Paterson; Dazhi Jiang; Robert B. Miller
Abstract:
Broad overlap between deformation and magmatism in active margins has
spurred the development of a conceptual framework of direct tectonomagmatic
links in both active and ancient arcs. Although widespread and highly
influential, such models have only rarely been critically evaluated.
Rigorously linking tectonism, geodynamics, lithospheric evolution, and arc
activity requires detailed reconstructions of the spatiotemporal patterns
of magmatism and deformation across both a sufficiently wide area and a
range of observational scales. Herein, new constraints on the timing,
extent, and characteristics of deformation during mid-Cretaceous tectonism
in the central Sierra Nevada (eastern California, USA) are synthesized with
published geologic mapping, structural studies, and geochronology to create
an updated reconstruction of one of the type examples of a hot, magma-rich
orogen. Tilted strata, tectonic fabrics, and shear zones with variable
geometries, kinematics, intensity, and timing reveal a significantly
revised record of ~25 m.y. of heterogeneous deformation ca. 105–80 Ma.
Deformation and magmatism show distinct and unrelated spatiotemporal
patterns throughout this orogenic episode. Contrary to previous models of
direct tectonomagmatic links, many of which were developed in the central
Sierra Nevada, arc activity did not control the location, intensity, or
kinematics of intra-arc deformation, nor did shear zones control the
location of magmatism. Furthermore, arc lithosphere appears to have
strengthened, rather than weakened, as the arc-orogenic flare-up proceeded.
In addition to changing plate-scale boundary conditions, lithospheric-scale
rheological evolution likely played a key role in the patterns of Late
Cretaceous deformation observed across strike of the entire Cordilleran
margin.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02478.1/617785/Spatiotemporally-heterogeneous-deformation
The effects of pre-stress assumptions on dynamic rupture with complex
fault geometry in the San Gorgonio Pass, California, region
Jennifer M. Tarnowski; Christodoulos Kyriakopoulos; David D. Oglesby;
Michele L. Cooke; Aviel Stern
Abstract:
We use three-dimensional (3-D) dynamic finite-element models to investigate
potential rupture paths of earthquakes propagating along faults through the
western San Gorgonio Pass, a structurally complex region along the San
Andreas fault system in southern California (USA). We focus on the
right-lateral San Bernardino strand of the San Andreas fault system, the
oblique thrust–right-lateral San Gorgonio Pass fault zone, and a portion of
the right-lateral Garnet Hill strand of the San Andreas fault system. We
use the 3-D finite-element method to model rupture propagation along a
fault geometry that reflects current understanding of the local geometrical
complexity and is consistent with long-term loading and observed surface
deformation. We test three different types of pre-stress assumptions: (1)
constant tractions (assuming pure right-lateral strike-slip motion on the
San Bernardino and Garnet Hill strands and oblique thrust–right-lateral
strike-slip motion on the San Gorgonio Pass fault zone), (2) a uniform
regional stress regime, and (3) long-term (evolved) stress from
quasi-static crustal deformation modeling. Our results imply that under the
more realistic regional stress and evolved stress assumptions, throughgoing
rupture propagation from the southeast to northwest (i.e., from the Garnet
Hill to the San Bernardino strand) may be more likely than throughgoing
rupture in the reverse direction (from the San Bernardino to the Garnet
Hill strand). The results may have implications for the earthquake
potential in the region as well as for ground motion in the Los Angeles
Basin. The results also emphasize how fault geometry and stress patterns
combine to influence rupture propagation on complex fault systems.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02511.1/617784/The-effects-of-pre-stress-assumptions-on-dynamic
Structure, chronology, kinematics, and geodynamics of tectonic
extension in the greater Catalina metamorphic core complex,
southeastern Arizona, USA
Jon E. Spencer; Stephen M. Richard; Ann Bykerk-Kauffman; Kurt N.
Constenius; Victor A. Valencia
Abstract:
Oligocene and early Miocene displacement on the Catalina–San Pedro
detachment fault and its northern correlatives uncovered mylonitic fabrics
that form the greater Catalina metamorphic core complex in southeastern
Arizona, USA. Gently to moderately dipping mylonitic foliations in the
complex are strongly lineated, with a lineation-azimuth average of 064–244°
and dominantly top-southwest shear sense over the entire 115-km-long
mylonite belt. Reconstruction of detachment fault displacement based on a
variety of features indicates 40–60 km of displacement, with greater
displacement in more southern areas. Widespread 26–28 Ma volcanism during
early extensional basin genesis was followed by 24–26 Ma granitoid
magmatism. Cooling of footwall mylonites continued until 22–24 Ma, as
indicated by 40Ar/39Ar mica dates. Lower temperature
thermochronometers suggest that footwall exhumation was still underway at
ca. 20 Ma. Tectonic reconstruction places a variety of unmetamorphosed
supracrustal units in the Tucson and Silver Bell Mountains above equivalent
units that were metamorphosed and penetratively deformed in the Tortolita
and Santa Catalina Mountains. This restored juxtaposition is interpreted as
a consequence of older Laramide thrust burial of the deformed units, with
northeast-directed thrusting occurring along the Wildhorse Mountain thrust
in the Rincon Mountains and related but largely concealed thrusts to the
northwest. Effective extensional exhumation of lower plate rocks resulted
from a general lack of internal extension of the upper plate wedge. This is
attributed to a stable sliding regime during the entire period of
extension, with metamorphic core complex inflation by deep crustal flow
leading to maintenance of wedge surface slope and detachment fault dip that
favored stable sliding rather than internal wedge extension.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02485.1/617715/Structure-chronology-kinematics-and-geodynamics-of
Implications of glacial deposit ages for the timing and rate of active
crustal faulting in the central Cascade arc, Oregon, USA
Katherine A. Alexander; Colin B. Amos; Greg Balco; William H. Amidon;
Douglas H. Clark ...
Abstract:
New cosmogenic 3He chronologies and geologic mapping of faulted
glacial drift provide new constraints for the slip rates of active faulting
in the central Cascade arc, Oregon, USA. The White Branch and Dilman
Meadows fault zones cut deposits created by three distinct glacial
advances, which provide timing, kinematics, and rate constraints for fault
motion. New cosmogenic 3He data from landforms comprising the
youngest and most widespread deposits have ages between 19.4 +10.1/–6.2 ka
and 21.3 ± 4.9 ka; therefore, they were deposited during the last glacial
maximum (LGM). A second, older outwash surface reveals an age of 74.2 ± 3.8
ka, which suggests glaciation possibly associated with marine isotope stage
(MIS) 5b. Dip-slip displacement across fault scarps expressed by lidar data
reveal similar magnitudes of extensional deformation for LGM and older
glacial deposits on the White Branch fault zone, which implies a lack of
earthquake ruptures between the oldest and LGM advances. In contrast, scarp
profiles along the Dilman Meadows fault zone reveal progressive cumulative
slip for surfaces of increasing age. Taken together, our measurements
provide the first constraints on the rate of extensional faulting derived
from Quaternary geochronology along the White Branch and Dilman Meadows
faults, which total 0.1–0.4 mm/yr since ca. 75 ka and 0.6 ± 0.04 mm/yr
since the LGM, respectively. The White Branch fault zone accommodates
predominately fault-normal extension, whereas right-oblique slip
characterizes the Dilman Meadows fault zone. Active deformation across the
central Cascade Range thus reflects the combined effects of ongoing crustal
block rotation and arc magmatism.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02476.1/617716/Implications-of-glacial-deposit-ages-for-the
GEOSPHERE articles are available at
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