New Articles for Geosphere Posted Early Online
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Topics this month include the Colorado
River extensional corridor; the central Azores volcanic islands; the
Belvidere Mountain Complex, northern Appalachians; and Cretaceous seamount
chains near the northwestern Hawaiian Ridge. You can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Late Triassic tectonic stress field of the southwestern Ordos Basin
and its tectonic implications: Insights from finite-element
numerical simulations
Li-Jun Song; Zeng-Zhen Wang
The tectonic stress field of the southwestern Ordos Basin during the Late
Triassic is controversial. The major controversy is whether the southwest-
ern Ordos Basin was a compressional basin throughout the Late Triassic or
whether it transformed from an extensional into a compressional basin
during this period. We divided the Late Triassic into the early to middle
and late to terminal periods. Two paleotectonic stress field simulation
models of the southwestern Ordos Basin were constructed using
finite-element software (ANSYS 10). Our results showed high consistency
with regional geologic correlations, suggesting the credibility of the
models. We found that the southwestern Ordos Basin was dominated by NE-SW
extensional stress and strain during the early to middle Late Triassic,
associated with strike-slip faulting along the western margin of the Ordos
block. This is consistent with the development of syndepositional normal
faults and was probably induced by the scissor collision from east to west
between the North China craton and Yangtze block. The tectonic stress field
of the southwestern Ordos Basin during the late to terminal Late Triassic
mainly manifested as NE-SW compressive stress and strain. The dominant
tectonic dynamics for the Ordos block during this period may have changed
to northward compression of the Songpan-Ganzi and Qiangtang terranes. The
southwestern Ordos Basin was characterized by compressional deformation and
northeastward migration of the depocenter. The southwestern Ordos Basin
transformed from an extensional basin associated with strike-slip faulting
during the early to middle Late Triassic into a compressional depression
basin during the late to terminal Late Triassic.
Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto
Group, Grand Canyon and Basin and Range, USA
Stephen M. Rowland; Slava Korolev; James W. Hagadorn; Kaushik Ghosh
We describe, interpret, and establish a stratotype for the Frenchman
Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records
key global geochemical and climate signals and is well exposed throughout
the Grand Canyon and central Basin and Range, USA. This flat-topped
carbonate platform deposit is the uppermost unit of the Tonto Group,
replacing the informally named “undifferentiated dolomites.” The unit
records two global chemostratigraphic events—the Drumian Carbon Isotope
Excursion (DICE), when δ13Ccarb (refers to “marine
carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean
Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰.
The formation consists of eight lithofacies deposited in shallow subtidal
to peritidal paleoenvironments. At its stratotype at Frenchman Mountain,
Nevada, the FMD is 371 m thick. Integration of regional trilobite
biostratigraphy and geochronology with new stratigraphy and sedimentology
of the FMD, together with new δ13Ccarb
chemostratigraphy for the entire Cambrian succession at Frenchman Mountain,
illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge
Zone) time. To the west, the unit correlates with most of the Banded
Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east,
at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to
pre–Middle Devonian erosion that cut progressively deeper cratonward.
Portions of the FMD display visually striking, meter-scale couplets of
alternating dark- and light-colored peritidal facies, while other portions
consist of thick intervals of a single peritidal or shallow subtidal
facies. Statistical analysis of the succession of strata in the stratotype
section, involving Markov order and runs order analyses, yields no evidence
of cyclicity or other forms of order. Autocyclic processes provide the
simplest mechanism to have generated the succession of facies observed in
the FMD.
Progressive Miocene unroofing of the Big Maria and Riverside
Mountains (southeastern California, USA) along the southwestern
margin of the Colorado River extensional corridor
Megan E. Flansburg; Daniel F. Stockli
The Colorado River extensional corridor (CREC) consists of Miocene
meta-morphic core complexes exhumed along top-to-the-NE low-angle
detachment faults. The Big Maria and Riverside Mountains of southeastern
California (USA) are located on the southwestern margin of the CREC, where
little is known about the nature and timing of large-magnitude extension.
We present the first detailed (U-Th)/He thermochronometric data from these
ranges, elucidating the geometry and timing of upper-crustal extensional
unroofing and exhumation. The Riverside Mountains yielded ca. 72–50 Ma
zircon (U-Th)/He (ZHe) ages in the hanging wall of the Riverside detachment
fault, and the corrugated footwall yielded ca. 50–18 Ma ZHe ages,
indicating the preservation of an exhumed ZHe partial retention zone.
Apatite (U-Th)/He data further indicate a potential secondary Miocene
breakaway in the northeastern end of the range. Although the Big Maria
Mountains have been thought to lie outside of the CREC, our new zircon and
apatite (U-Th)/He data show that the entirety of the Big Maria Mountains
was tectonically exhumed in the footwall of a detachment fault and cooled
from >6 km depth between 22 and 15 Ma. ZHe data from both ranges suggest
the Big Maria Mountains are part of the CREC and were exhumed from
underneath the Riverside Mountains by a contemporaneous but structurally
lower detachment—the Big Maria detachment—that is regionally correlative
with the breakaway zone that delimits the western CREC margin. This
detachment is temporally coeval with the structurally higher detachment
system that forms the Whipple-Buckskin-Rawhide-Harcuvar-Harquahala
metamorphic core complex belt to the northeast.
Exhumed fluvial landforms reveal evolution of late Eocene–Pliocene
rivers on the Central and Northern Great Plains, US
A
Jesse T. Korus; R.M. Joeckel
Cenozoic strata on the Great Plains are the products of a long-lived,
continental sediment routing system, and yet strikingly little is known
about these ancient rivers. This article details the discovery of ~3100
fluvial ridges—erosionally inverted alluvial-fan, channel-fill,
channel-belt, and valley-fill deposits—extending from the Rocky Mountain
front to the eastern margin of the Great Plains. The direct detection of
these channel bodies reveals new insights into late Eocene–Pliocene
drainage evolution. Late Eocene–Oligocene streams were morphologically
diverse. Alluvial fans adjacent to the Rocky Mountain front changed
eastward to parallel or downstream-divergent, fixed, single-thread,
straight to slightly sinuous (S = 1.0–1.5) streams <50 m in
width. At ~100 km from the Rocky Mountain front, streams became sinuous and
laterally mobile, forming amalgamated channel bodies as much as 3 km in
width. Streamflow in all these systems was highly dispersed (southeast to
northeast) and temporally variable. These characteristics reveal a nascent
Great Plains alluvial apron hosting small, poorly integrated drainages
undergoing abrupt changes. By the Miocene, more uniform streamflow
generally trended east-northeast. Channel deposits are identifiable 500 km
from the Rocky Mountain front. Middle Miocene valley fills gave way to
fixed, multithread channels a few kilometers in width by the late Miocene.
These patterns evince a mature alluvial apron hosting bigger rivers in
well-integrated drainages. We interpret the systematic changes between
fixed and mobile channel styles to record spatially and temporally variable
aggradation rates. The widening of channels in the late Miocene likely
reflects increased discharge relating to wetter climates upstream or the
integration of once-isolated Rocky Mountain drainage basins into a
continental-scale drainage system.
Emplacement history of volcaniclastic turbidites around the central
Azores volcanic islands: Frequencies of slope landslides and
eruptions
Yu-Chun Chang; Neil C. Mitchell; Julie C. Schindlbeck-Belo; Thor H.
Hansteen; Armin Freundt ...
Volcanic islands export clastic material to their surrounding oceans by
explosive eruptions, lava emissions, biogenic production on their shelves,
and failure of their slopes, amongst other processes. This raises the
question of whether geological events (in particular, eruptions and
landslides) can be detected offshore and dated, and whether any
relationships (for example, with climate changes) can be revealed using
sediment cores. The volcanically active central Azorean islands (Faial,
Pico, São Jorge, and Terceira), with their neighboring submarine basins,
are potentially good candidates for such an analysis. Here,
chronostratigraphies of four gravity cores collected amongst the islands
are constructed based on twelve radiocarbon dates and two dates derived by
geochemically correlating primary volcaniclastic turbidites with
ignimbrites on Faial and Terceira Islands. Age-depth models are built from
the hemipelagic intervals to estimate individual turbidite dates. Volumes
of turbidites are modeled by multiplying basin areas with bed thickness,
allowing for various turbidite thinning rates and directions. The volumes
of landslide-generated turbidites are only comparable with the largest
volumes of their adjacent upper-slope submarine landslide valleys;
therefore, such turbidites in the cores likely derive from these largest
landslides. Emplacement intervals between turbidites originating from both
landslides and pyroclastic density currents are found to be mostly a few
thousand years. Frequencies of landslide-generated turbidites and
hemipelagic sedimentation rates were both highest in the past 8 k.y.
compared to preceding periods up to 50 k.y. High hemipelagic sedimentation
rates are interpreted to be related to sea-level rise, allowing more shelf
bioproduction and release of particles by coastal erosion. The coincident
increased frequencies of submarine landslides may also be associated with
the increased sediment supply from the islands, resulting in a more rapid
build-up of unstable sediments on submarine slopes. Notably, the
emplacement frequencies of turbidites of pyroclastic density current
origins do not suggest the decreased eruption frequency toward the Holocene
that has been found elsewhere.
Evolution of slip partitioning in a major continental margin
strike-slip fault system during a transition to oblique
plate-margin tectonics: Insight into the evolution of the Garlock
fault zone, California (USA)
Joseph E. Andrew; J. Douglas Walker; William M. Rittase
The Walker Lane belt and Eastern California shear zone of California, USA,
are active, plate boundary–related dextral systems with transtensional and
transpressional deformation, respectively. They are separated by the
sinistral Garlock fault, creating a complex system without an overall
integrated formation and evolution model. We examine the deformation within
the eastern segment of the Garlock fault zone over geologic timescales by
determining the slip history of faults. We assess the progression of
faulting and associated deformation along the WSW-striking Garlock fault
zone and how it applies to the overall NNW-directed dextral system.
Previous studies found that large synthetic fault strands take up 30% of
the slip of the Garlock fault zone and have proposed multiple mechanisms to
explore how to accommodate regional NNW-directed shear across the Garlock
fault without cutting its trace. We analyze an unstudied section of
faulting in one of the more complex areas of regional deformation via
compiled and reinterpreted published geologic data for an analysis of total
and incremental slip on the main faults of the eastern Garlock fault zone.
We identify geologic offset features to interpret total slip, timing, and
deformation evolution. We find that 30% of the total slip of the Garlock
zone occurs on strands other than the Garlock fault sensu stricto, with the
locus of main slip sidestepping during the evolution of accommodation of
through-going, regional dextral shear. Our results support ideas of the
creation and evolution of the regional dextral system via stress
concentration on a sub-Garlock lithospheric anisotropy with a resulting
lowering of the plastic yield stress. Our results also show an eastward
increase in fault system complexity, which may imply an underappreciated
seismic hazard of the eastern Garlock fault zone.
Newly recognized blueschist-facies metamorphism
(glaucophane-omphacite-garnet), Belvidere Mountain Complex,
northern Appalachians
Ian W. Honsberger
An occurrence of blueschist-facies metamorphism in the Appalachian orogen
is newly recognized in northwestern New England, United States. Inclusions
of glaucophane and omphacite occur in a relict garnet core from a
retrogressed garnet-barroisite amphibolite of the Belvidere Mountain
Complex in Vermont. Pressure-temperature pseudosection and mineral
composition isopleth calculations demonstrate that the Belvidere Mountain
Complex blueschist-facies mineral assemblage of
glaucophane–magnesio-hornblende–omphacite–chlorite–rutile–quartz–clinozoisite–garnet
was stable at ~1.65–2.0 GPa and ~450–480 °C. Garnet-absent amphibolite with
barroisite and chlorite inclusions in clinozoisite records high-pressure
epidote-amphibolite–facies metamorphism at ~1.0–1.4 GPa and ~515–550 °C.
These new findings quantify deep subduction of the Belvidere Mountain
Complex during the Cambrian to Ordovician Taconic orogenic cycle and
suggest that more blueschist-facies mineral assemblages could be revealed
in the Appalachians with detailed analysis of retrogressed rocks.
Syntectonic sediment loading and fold-thrust belt structural
architecture: An example from the central Appalachians (USA)
Mark A. Evans
Fluid inclusion microthermometry of synkinematic veins is used to estimate
the maximum syntectonic load that was deposited on the wedge top in the
central Appalachians (northeastern United States) during the Alleghanian
orogeny. The restored loads indicate two major depocenters during the
Alleghanian orogeny: one above Broadtop synclinorium, with as much as 7 km
of Pennsylvanian–Permian load probably sourced by the erosion of rocks
uplifted by the growing Blue Ridge massif and emplacement of the North
Mountain thrust sheet; the other above the Anthracite belt, with as much as
16 km of syntectonic load likely sourced by the erosion of rocks uplifted
by the growing Reading Prong massif. The loads were generally <3 km in
the intervening Juniata culmination. In areas of high load, the structural
architecture of the basin is that of widely spaced thrusts (~17–22 km) with
large leading-edge anticlines in the Cambrian–Ordovician lithotectonic
unit, while in areas of low load, thrusts are more closely spaced (~15 km)
and deformed into an imbricate stack. The relationship between observed
syntectonic loads, thrust spacing, and structural style reflect modeled
relationships.
New insights into the age and origin of two small Cretaceous
seamount chains proximal to the Northwestern Hawaiian Ridge
Arturo Sotomayor; Andrea Balbas; Kevin Konrad; Anthony A.P. Koppers; Jasper
G. Konter ...
The Northwestern Hawaiian Ridge is an age-progressive volcanic chain
sourced from the Hawaiian mantle plume. Proximal to the Northwestern
Hawaiian Ridge are several clusters of smaller seamounts and ridges with
limited age constraints and unknown geodynamic origins. This study presents
new bathymetric data and 40Ar/39Ar age determinations
from lava flow samples recovered by remotely operated vehicle (ROV) from
two east–west-trending chains of seamounts that lie north of the Pūhāhonu
and Mokumanamana volcanoes. The previously unexplored Naifeh Chain
(28°48′N,167°48′W) and Plumeria Chain (25°36′N, 164°35′W) contain five
volcanic structures each, including three guyots in the Naifeh Chain. New 40Ar/39Ar age determinations indicate that the Naifeh
Chain formed ca. 88 Ma and the Plumeria Chain ca. 85 Ma. The Cretaceous
ages, coupled with a perpendicular orientation of the seamounts relative to
absolute Pacific plate motion at that time, eliminate either a Miocene
Hawaiian volcanic arch or Cretaceous mantle-plume origin. The seamounts lie
on oceanic crust that is modeled to be 10–15 Ma older than the
corresponding seamounts. Here, two models are put forth to explain the
origin of these enigmatic seamount chains as well as the similar nearby
Mendelssohn Seamounts. (1) Diffuse lithospheric extension results in the
formation of these seamounts until the initiation of the Kula-Pacific
spreading center in the north at 84–79 Ma, which alleviates the tension.
(2) Shear-driven upwelling of enriched mantle material beneath young
oceanic lithosphere results in an age-progressive seamount track that is
approximately perpendicular to the spreading ridge. Here we show that all
sampled seamounts proximal to the Northwestern Hawaiian Ridge are
intraplate in nature, but their formations can be attributed to both plume
and plate processes.
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 Justin Samuel at the address above. Please discuss
articles of interest with the authors before publishing stories on their
work, and please refer to GEOSPHERE in articles published. Non-media
requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
https://www.geosociety.org
# # #