New Articles for Geosphere Posted Online in November
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations and topics studied this month
include seismicity recorded in hematite fault mirrors in the Rio Grande
rift and evolution of the Nanaimo Basin, British Columbia, Canada.You can
find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Porphyry copper deposit formation in arcs: What are the odds?
Jeremy P. Richards
Abstract:
Arc magmas globally are H2O-Cl-S–rich and moderately oxidized (ΔFMQ = +1 to
+2) relative to most other mantle-derived magmas (ΔFMQ ≤ 0). Their
relatively high oxidation state limits the extent to which sulfide phases
separate from the magma, which would otherwise tend to deplete the melt in
chalcophile elements such as Cu (highly siderophile elements such as Au and
especially platinum-group elements are depleted by even small amounts of
sulfide segregation). As these magmas rise into the crust and begin to
crystallize, they will reach volatile saturation, and a hydrous, saline,
S-rich, moderately oxidized fluid is released, into which chalcophile and
any remaining siderophile metals (as well as many other water-soluble
elements) will strongly partition. This magmatic-hydrothermal fluid phase
has the potential to form ore deposits (most commonly porphyry Cu ± Mo ± Au
deposits) if its metal load is precipitated in economic concentrations, but
there are many steps along the way that must be successfully negotiated
before this can occur. This paper seeks to identify the main steps along
the path from magma genesis to hydrothermal mineral precipitation that
affect the chances of forming an ore deposit (defined as an economically
mineable resource) and attempts to estimate the probability of achieving
each step. The cumulative probability of forming a large porphyry Cu
deposit at any given time in an arc magmatic system (i.e., a single
batholith-linked volcanoplutonic complex) is estimated to be ~0.001%, and
less than 1/10 of these deposits will be uplifted and exposed at shallow
enough depths to mine economically (0.0001%). Continued uplift and erosion
in active convergent tectonic regimes rapidly remove these upper-crustal
deposits from the geological record, such that the probability of finding
them in older arc systems decreases further with age, to the point that
porphyry Cu deposits are almost nonexistent in Precambrian rocks. A key
finding of this paper is that most volcanoplutonic arcs above subduction
zones are prospective for porphyry ore formation, with probabilities only
falling to low values at late stages of magmatic-hydrothermal fluid
exsolution, focusing, and metal deposition. This is in part because of the
high threshold required in terms of grade and tonnage for a deposit to be
considered economic. Thus, the probability of forming a porphyry-type
system in any given arc segment is relatively high, but the probability
that it will be a large economic deposit is low, dictated to a large extent
by mineral economics and metal prices.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02086.1/609629/Porphyry-copper-deposit-formation-in-arcs-What-are
The rise and demise of deep accretionary wedges: A long-term field and
numerical modeling perspective
Samuel Angiboust; Armel Menant; Taras Gerya; Onno Oncken
Abstract:
Several decades of field, geophysical, analogue, and numerical modeling
investigations have enabled documentation of the wide range of tectonic
transport processes in accretionary wedges, which constitute some of the
most dynamic plate boundary environments on Earth. Active convergent
margins can exhibit basal accretion (via underplating) leading to the
formation of variably thick duplex structures or tectonic erosion, the
latter known to lead to the consumption of the previously accreted material
and eventually the forearc continental crust. We herein review natural
examples of actively underplating systems (with a focus on circum-Pacific
settings) as well as field examples highlighting internal wedge dynamics
recorded by fossil accretionary systems. Duplex formation in deep
paleo–accretionary systems is known to leave in the rock record (1)
diagnostic macro- and microscopic deformation patterns as well as (2)
large-scale geochronological characteristics such as the downstepping of
deformation and metamorphic ages. Zircon detrital ages have also proved to
be a powerful approach to deciphering tectonic transport in ancient active
margins. Yet, fundamental questions remain in order to understand the
interplay of forces at the origin of mass transfer and crustal recycling in
deep accretionary systems. We address these questions by presenting a suite
of two-dimensional thermo-mechanical experiments that enable unravelling
the mass-flow pathways and the long-term distribution of stresses along and
above the subduction interface as well as investigating the importance of
parameters such as fluids and slab roughness. These results suggest the
dynamical instability of fluid-bearing accretionary systems causes either
an episodic or a periodic character of subduction erosion and accretion
processes as well as their topographic expression. The instability can be
partly deciphered through metamorphic and strain records, thus explaining
the relative scarcity of paleo–accretionary systems worldwide despite the
tremendous amounts of material buried by the subduction process over time
scales of tens or hundreds of millions of years. We finally stress that the
understanding of the physical processes at the origin of underplating
processes as well as the forearc topographic response paves the way for
refining our vision of long-term plate-interface coupling as well as the
rheological behavior of the seismogenic zone in active subduction settings.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02392.1/609941/The-rise-and-demise-of-deep-accretionary-wedges-A
Slow slip in subduction zones: Reconciling deformation fabrics with
instrumental observations and laboratory results
O. Oncken; S. Angiboust; G. Dresen
Abstract:
Cataclasites are a characteristic rock type found in drill cores from
active faults as well as in exposed fossil subduction faults. Here,
cataclasites are commonly associated with evidence for pervasive pressure
solution and abundant hydro fracturing. They host the principal slip of
regular earthquakes and the family of socalled slow earthquakes (episodic
slip and tremor, low to very low frequency earthquakes, etc.). Slip
velocities associated with the formation of the different types of
cataclasites and conditions controlling slip are poorly constrained both
from direct observations in nature as well as from experimental research.
In this study, we explore exposed sections of subduction faults and their
dominant microstructures. We use recently proposed constitutive laws to
estimate deformation rates, and we compare predicted rates with
instrumental observations from subduction zones. By identifying the maximum
strain rates using fault scaling relations to constrain the fault core
thickness, we find that the instrumental shear strain rates identified for
the family of “slow earthquakes” features range from 10−3s −1 to 10−5s−1. These values agree with
estimated rates for stress corrosion creep or brittle creep possibly
controlling cataclastic deformation rates near the failure threshold.
Typically, porefluid pressures are suggested to be high in subduction
zones triggering brittle deformation and fault slip. However, seismic slip
events causing local dilatancy may reduce fluid pressures promoting
pressuresolution creep (yielding rates of <10−8 to 10 −12s−1) during the interseismic period in agreement
with dominant fabrics in plate interface zones. Our observations suggest
that cataclasis is controlled by stress corrosion creep and driven by fluid
pressure fluctuations at nearlithostatic effective pressure and shear
stresses close to failure. We posit that cataclastic flow is the dominant
physical mechanism governing transient creep episodes such as slow slip
events (SSEs), accelerating preparatory slip before seismic events, and
early afterslip in the seismogenic zone.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02382.1/609942/Slow-slip-in-subduction-zones-Reconciling
Early Pennsylvanian sediment routing to the Ouachita Basin
(southeastern United States) and barriers to transcontinental sediment
transport sourced from the Appalachian orogen based on detrital zircon
U-Pb and Hf analysis
Isaac J. Allred; Michael D. Blum
Abstract:
Carboniferous sediment dispersal from the Appalachian orogenic system
(eastern United States) has become a topic of widespread interest. However,
the actual pathways for continental-scale, east-to-west sediment transfer
have not been documented. This study presents detrital zircon (DZ) U-Pb
ages and Hf isotopic values from the Lower Pennsylvanian (Morrowan)
Jackfork Group and Johns Valley Shale of the synorogenic Ouachita deepwater
basin of Arkansas to document provenance and delineate the likely
sediment-routing systems within the broader context of sediment dispersal
across Laurentia. Twelve (12) DZ U-Pb age distributions are interpreted to
indicate that sediments were derived from the Appalachians to the east and
northeast, as well as the midcontinent region to the north. All samples
display prominent ca. 500– 400 Ma, 1250–950 Ma, 1550–1300 Ma, and 1800–1600
Ma grains, consistent with ultimate derivation from the Appalachian,
Grenville, Midcontinent, and Yavapai-Mazatzal provinces. DZ Hf values
obtained from the Ouachita Basin are similar to published Hf values from
Pennsylvanian samples in the Appalachian and Illinois Basins. Age
distributions are generally consistent for seven samples collected from the
Jackfork Group and Johns Valley Shale in the southern Ouachita Mountains
through ~2400 m of stratigraphic section and are interpreted to indicate
little change in provenance during the Morrowan in this part of the system.
However, samples from the most northern and most source-proximal site in
Little Rock, Arkansas, exhibit modest percentages of Appalachian ages and
elevated contributions of Yavapai-Mazatzal ages when compared with samples
collected farther to the south and west. We interpret differences between
DZ signatures to indicate distinct sediment-routing pathways to the
Ouachita Basin. We infer the strong Appalachian and Grenville signals to
represent an axial system flowing through the Appalachian foredeep, whereas
the more diverse signals represent a confluence of rivers from the
northeast through the backbulge of southern Illinois and western Kentucky
and from the north across the Arkoma shelf. Collectively, the Ouachita
Basin represents a terminal sink for sediments derived from much of the
eastern and central United States.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02408.1/609943/Early-Pennsylvanian-sediment-routing-to-the
Late Cretaceous upper-crustal thermal structure of the Sevier
hinterland: Implications for the geodynamics of the Nevadaplano
Nolan R. Blackford; Sean P. Long; Austin Stout; David W. Rodgers; C.M.
Cooper ...
Abstract:
Crustal temperature conditions can strongly influence the evolution of
deformation during orogenesis. The Sevier hinterland plateau in Nevada and
western Utah (“Nevadaplano”) experienced a Late Cretaceous episode of
shallow-crustal metamorphism and granitic magmatism. Here, we investigate
the thermal history of the Nevadaplano by measuring peak thermal field
gradients attained in the upper 10–20 km of the crust along an east-west
transect through nine ranges in eastern Nevada and western Utah, by
integrating Raman spectroscopy of carbonaceous material thermometry and
published conodont alteration indices with reconstructed cross sections.
Thermal field gradients of 29 ± 3 °C/km were obtained in the House and
Confusion Ranges in westernmost Utah. The Deep Creek, Schell Creek, and
Egan Ranges in easternmost Nevada yielded elevated gradients of 49 ± 7
°C/km, 36 ± 3 °C/km, and 32 ± 6 °C/km, respectively. Moving westward, the
White Pine, Butte, Pancake, and Fish Creek Ranges exhibit gradients
typically between ~20–30 °C/km. The elevated thermal gradients in
easternmost Nevada are interpreted to have been attained during ca. 70–90
Ma granitic magmatism and metamorphism and imply possible partial melting
at ~18 km depths. Our data are compatible with published interpretations of
Late Cretaceous lithospheric mantle delamination under the Sevier
hinterland, which triggered lower-crustal anatexis and the resulting rise
of granitic melts. The lack of evidence for structures that could have
accommodated deep burial of rocks in the nearby Northern Snake Range
metamorphic core complex, combined with thermal gradients from adjacent
ranges that are ~1.5–3 times higher than those implied by thermobarometry
in the Northern Snake Range, further highlights the debate over possible
tectonic overpressure in Cordilleran core complexes. Cross-section
retro-deformation defines 73.4 ± 4.6 km (76 ± 8%) of extension across
eastern Nevada and 15 km of shortening in the Eastern Nevada fold belt.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02386.1/609944/Late-Cretaceous-upper-crustal-thermal-structure-of
Seismicity recorded in hematite fault mirrors in the Rio Grande
rift
M.L. Odlum; A.K. Ault; M.A. Channer; G. Calzolari
Abstract:
Exhumed fault rocks provide a textural and chemical record of how fault
zone composition and architecture control coseismic temperature rise and
earthquake mechanics. We integrated field, microstructural, and hematite
(U-Th)/He (He) thermochronometry analyses of exhumed minor
(square-centimeter-scale surface area) hematite fault mirrors that crosscut
the ca. 1400 Ma Sandia granite in two localities along the eastern flank of
the central Rio Grande rift, New Mexico. We used these data to characterize
fault slip textures; evaluate relationships among fault zone composition,
thickness, and inferred magnitude of friction-generated heat; and document
the timing of fault slip. Hematite fault mirrors are collocated with and
crosscut specular hematite veins and hematite-cemented cataclasite.
Observed fault mirror microstructures reflect fault reactivation and strain
localization within the comparatively weaker hematite relative to the
granite. The fault mirror volume of some slip surfaces exhibits polygonal,
sintered hematite nanoparticles likely created during coseismic temperature
rise. Individual fault mirror hematite He dates range from ca. 97 to 5 Ma,
and ~80% of dates from fault mirror volume aliquots with high-temperature
crystal morphologies are ca. 25–10 Ma. These aliquots have
grain-size–dependent closure temperatures of ~75–108 °C. A new mean apatite
He date of 13.6 ± 2.6 Ma from the Sandia granite is consistent with prior
low-temperature thermochronometry data and reflects rapid, Miocene rift
flank exhumation. Comparisons of thermal history models and hematite He
data patterns, together with field and microstructural observations,
indicate that seismicity along the fault mirrors at ~2–4 km depth was
coeval with rift flank exhumation. The prevalence and distribution of
high-temperature hematite grain morphologies on different slip surfaces
correspond with thinner deforming zones and higher proportions of quartz
and feldspar derived from the granite that impacted the bulk strength of
the deforming zone. Thus, these exhumed fault mirrors illustrate how
evolving fault material properties reflect but also govern coseismic
temperature rise and associated dynamic weakening mechanisms on minor
faults at the upper end of the seismogenic zone.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02426.1/609628/Seismicity-recorded-in-hematite-fault-mirrors-in
Stratigraphy of the Eocene–Oligocene Titus Canyon Formation, Death
Valley, California, and Eocene extensional tectonism in the Basin and
Range
Nikolas Midttun; Nathan A. Niemi; Bianca Gallina
Abstract:
Geologic mapping, measured sections, and geochronologic data elucidate the
tectono-stratigraphic development of the Titus Canyon extensional basin in
Death Valley, California, and provide new constraints on the age of the
Titus Canyon Formation, one of the earliest syn-extensional deposits in the
central Basin and Range. Detrital zircon maximum depositional ages (MDAs)
and compiled 40Ar/39Ar ages indicate that the Titus
Canyon Formation spans 40(?)–30 Ma, consistent with an inferred Duchesnean
age for a unique assemblage of mammalian fossils in the lower part of the
formation. The Titus Canyon Formation preserves a shift in depositional
environment from fluvial to lacustrine at ca. 35 Ma, which along with a
change in detrital zircon provenance may reflect both the onset of local
extensional tectonism and climatic changes at the Eocene–Oligocene
boundary. Our data establish the Titus Canyon basin as the southernmost
basin in a system of late Eocene extensional basins that formed along the
axis of the Sevier orogenic belt. The distribution of lacustrine deposits
in these Eocene basins defines the extent of a low-relief orogenic plateau
(Nevadaplano) that occupied eastern Nevada at least through Eocene time. As
such, the age and character of Titus Canyon Formation implies that the
Nevadaplano extended into the central Basin and Range, ~200 km farther
south than previously recognized. Development of the Titus Canyon
extensional basin precedes local Farallon slab removal by ca. 20 Ma,
implying that other mechanisms, such as plate boundary stress changes due
to decreased convergence rates in Eocene time, are a more likely trigger
for early extension in the central Basin and Range.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02363.1/609630/Stratigraphy-of-the-Eocene-Oligocene-Titus-Canyon
Evolution of the Late Cretaceous Nanaimo Basin, British Columbia,
Canada: Definitive provenance links to northern latitudes
J. Brian Mahoney; James W. Haggart; Marty Grove; David L. Kimbrough;
Virginia Isava ...
Abstract:
Accurate reconstruction of the Late Cretaceous paleogeography and tectonic
evolution of the western North American Cordilleran margin is required to
resolve the long-standing debate over proposed large-scale, orogen-parallel
terrane translation. The Nanaimo Basin (British Columbia, Canada) contains
a high-fidelity record of orogenic exhumation and basin subsidence in the
southwestern Canadian Cordillera that constrains the tectonic evolution of
the region. Integration of detrital zircon U-Pb geochronology, conglomerate
clast U-Pb geochronology, detrital muscovite 40Ar/39
Ar thermochronology, and Lu-Hf isotopic analysis of detrital zircon defines
a multidisciplinary provenance signature that provides a definitive linkage
with sediment source regions north of the Sierra Nevada arc system (western
United States). Analysis of spatial and temporal provenance variations
within Nanaimo Group strata documents a bimodal sediment supply with a
local source derived from the adjacent magmatic arc in the southern Coast
Mountains batholith and an extra-regional source from the Mesoproterozoic
Belt Supergroup and the Late Cretaceous Atlanta lobe of the Idaho
batholith. Particularly robust linkages include: (1) juvenile (εHf >+10)
Late Cretaceous zircon derived from the southern Coast Mountains batholith;
(2) a bimodal Proterozoic detrital zircon signature consistent with
derivation from Belt Supergroup (1700–1720 Ma) and ca. 1380 Ma plutonic
rocks intruding the Lemhi subbasin of central Idaho (northwestern United
States); (3) quartzite clasts that are statistical matches for
Mesoproterozoic and Cambrian strata in Montana and Idaho (northwestern
United States) and southern British Columbia; and (4) syndepositional
evolved (εHf >−10) Late Cretaceous zircon and muscovite derived from the
Atlanta lobe of the Idaho batholith. These provenance constraints support a
tectonic restoration of the Nanaimo Basin, the southern Coast Mountains
batholith, and Wrangellia to a position outboard of the Idaho batholith in
Late Cretaceous time, consistent with proposed minimal-fault-offset models
(<~1000 km).
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02394.1/609512/Evolution-of-the-Late-Cretaceous-Nanaimo-Basin
Late Pleistocene and early Holocene sea-level history and glacial
retreat interpreted from shell-bearing marine deposits of southeastern
Alaska, USA
James F. Baichtal; Alia J. Lesnek; Risa J. Carlson; Nicholas S. Schmuck;
Jane L. Smith ...
Abstract:
We leverage a data set of >720 shell-bearing marine deposits throughout
southeastern Alaska (USA) to develop updated relative sea-level curves that
span the past ~14,000 yr. This data set includes site location, elevation,
description when available, and 436 14C ages, 45 of which are
published here for the first time. Our sea-level curves suggest a
peripheral forebulge developed west of the retreating Cordilleran Ice Sheet
(CIS) margin between ca. 17,000 and 10,800 calibrated yr B.P. By 14,870 ±
630 to 12,820 ± 340 cal. yr B.P., CIS margins had retreated from all of
southeastern Alaska’s fjords, channels, and passages. At this time,
isolated or stranded ice caps existed on the islands, with alpine or
tidewater glaciers in many valleys. Paleoshorelines up to 25 m above sea
level mark the maximum elevation of transgression in the southern portion
of the study region, which was achieved by 11,000 ± 390 to 10,500 ± 420
cal. yr B.P. The presence of Pacific sardine (Sardinops sagax) and
the abundance of charcoal in sediments that date between 11,000 ± 390 and
7630 ± 90 cal. yr B.P. suggest that both ocean and air temperatures in
southeastern Alaska were relatively warm in the early Holocene. The
sea-level and paleoenvironmental reconstruction presented here can inform
future investigations into the glacial, volcanic, and archaeological
history of southeastern Alaska.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02359.1/609513/Late-Pleistocene-and-early-Holocene-sea-level
Testing local and extraregional sediment sources for the Late
Cretaceous northern Nanaimo basin, British Columbia, using 40Ar/39Ar detrital K-feldspar thermochronology
V. Isava; M. Grove; J.B. Mahoney; J.W. Haggart
Abstract:
Detrital K-feldspar 40Ar/39Ar thermochronology was
conducted on clastic sedimentary rock samples collected from northern
exposures of the Upper Cretaceous Nanaimo Group on Vancouver Island and
adjacent Gulf Islands of British Columbia to constrain the denudation
history of the local Coast Mountains batholith source region and determine
the origin of extraregional sediment supplied to the basin. Strata of the
northern Nanaimo Group deposited between 86 and 83 Ma (Comox and Extension
formations) exhibit a 130–85 Ma age distribution of detrital K-feldspar 40Ar/39Ar ages that lack age maxima. These are
interpreted to have been sourced from the southwestern Coast Mountains
batholith. Younger strata deposited between 83 and 72 Ma (Cedar District
and De Courcy formations) yield a broader age range (150–85 Ma) with an age
maximum near the depositional age. These results indicate focused
denudation of deeper-seated rocks east of the Harrison Lake fault. The
youngest units deposited after 72 Ma (Geoffrey, Spray, and Gabriola
formations) primarily yield younger than 75 Ma detrital K-feldspar ages
with pronounced age maxima near the depositional age. This sediment was
sourced extraregionally relative to the Coast Mountains batholith. We
sought to constrain the origin of the extra-regional sediment by measuring
the thermal histories of 74 samples of basement rocks from throughout the
Pacific Northwest, and by compiling a database of over 2400 biotite 40Ar/39Ar and K/Ar cooling ages from predominantly
Cretaceous batholiths along the western North American margin. This
analysis focused upon two previously proposed source regions: the Idaho
batholith and the Mojave-Salina margin of southern California. The Nanaimo
detrital K-feldspar 40Ar/39Ar age distributions favor
the peraluminous Late Cretaceous Idaho batholith and its Proterozoic
Belt-Purcell Supergroup sedimentary wall rock as the more likely source of
the extraregional sediment and disfavor the Baja–British Columbia
hypothesis for 2000–4000-km-scale translation of rocks along the margin
during the Late Cretaceous.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02395.1/609514/Testing-local-and-extraregional-sediment-sources
A graphic method for depicting basin evolution and changes in the
dominant hydrodynamic process from paleocurrent data
Domenico Chiarella; Dario Gioia
Abstract:
Paleocurrent data measured on depositional elements and sedimentary
structures (e.g., channels, cross-strata) are commonly utilized in the
description of sedimentary strata. Paleocurrent data provide information
about the depositional setting and in some cases can be useful for
immediately detecting specific depositional processes (e.g., herringbone
cross-strata for bimodal tidal currents). The typical graphical
representation used to report paleocurrent data is the rose diagram.
However, rose diagrams are not able to disclose all information contained
in paleocurrent data, limiting the potentiality of such a representation
method. In particular, there is presently no method to highlight changes in
the paleogeographic configuration that can ultimately have an impact on the
evolution of depositional processes and paleocurrent direction through
time. Here, we present a graphic method that permits instant visualization
of anomalies in paleocurrent distributions of the stratigraphic record that
can be linked to changes in the paleogeography due to tectonic evolution or
in the dominant hydrodynamic process. It is important to highlight that the
proposed method does not aspire to replace rose diagrams but to provide an
additional tool to be used before and in combination with rose diagrams in
order to extrapolate as much information as possible from paleocurrent
data.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02403.1/609381/A-graphic-method-for-depicting-basin-evolution-and
Latest Permian–Triassic magmatism of the Taimyr Peninsula: New evidence
for a connection to the Siberian Traps large igneous province
Mikhail Kurapov; Victoria Ershova; Andrei Khudoley; Marina Luchitskaya;
Daniel Stockli ...
Abstract:
This study presents new whole rock major and trace element, Sr-Nd isotopic,
petrographic, and geochronologic data for seven latest Permian
(Changhsingian)–Late Triassic (Carnian) granitoid intrusions of the
northwestern and northeastern Taimyr Peninsula in the Russian High Arctic.
U-Pb zircon ages, obtained using secondary ion mass spectrometry (SIMS),
sensitive high-resolution ion microprobe (SHRIMP), and laser
ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), define
the crystallization age of the Taimyr intrusions studied as ranging from
ca. 253 Ma to 228 Ma, which suggests two magmatic pulses of latest
Permian–Early Triassic and Middle–Late Triassic age. Ar-Ar dating of
biotite and amphibole indicate rapid cooling of the intrusions studied, but
Ar-Ar ages of several samples were reset by secondary heating and
hydrothermal activity induced by the Middle–Late Triassic magmatic pulse.
Petrographic data distinguish two groups of granites: syenite–monzonites
and granites–granodiorites. Sr-Nd isotopic data, obtained from the same
intrusions, show a variation of initial (87Sr/86Sr) i ratios between 0.70377 and 0.70607, and εNd(t) values range
between –6.9 and 1.2. We propose that the geochemical and isotopic
compositions of the Late Permian–Triassic Taimyr granites record the
existence of a magma mush zone that was generated by the two pulses of
Siberian Traps large igneous province (LIP) magmatism.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02421.1/609382/Latest-Permian-Triassic-magmatism-of-the-Taimyr
Testing models of Laramide orogenic initiation by investigation of Late
Cretaceous magmatic-tectonic evolution of the central Mojave sector of
the California arc
Rita C. Economos; Andrew P. Barth; Joseph L. Wooden; Scott R. Paterson;
Brody Friesenhahn ...
Abstract:
The Mojave Desert region is in a critical position for assessing models of
Laramide orogenesis, which is hypothesized to have initiated as one or more
seamounts subducted beneath the Cretaceous continental margin.
Geochronological and geochemical characteristics of Late Cretaceous
magmatic products provide the opportunity to test the validity of Laramide
orogenic models. Laramide-aged plutons are exposed along a transect across
the Cordilleran Mesozoic magmatic system from Joshua Tree National Park in
the Eastern Transverse Ranges eastward into the central Mojave Desert. A
transect at latitude ~33.5°N to 34.5°N includes: (1) the large
upper-crustal Late Cretaceous Cadiz Valley batholith, (2) a thick section
of Proterozoic to Jurassic host rocks, (3) Late Cretaceous stock to
pluton-sized bodies at mesozonal depths, and (4) a Jurassic to Late
Cretaceous midcrustal sheeted complex emplaced at ~20 km depth that
transitions into a migmatite complex truncated along the San Andreas fault.
This magmatic section is structurally correlative with the Big Bear Lake
intrusive suite in the San Bernardino Mountains and similar sheeted rocks
recovered in the Cajon Pass Deep Scientific Drillhole. Zircon U-Pb
geochronology of 12 samples via secondary ionization mass spectrometry
(SIMS) (six from the Cadiz Valley batholith and six from the Cajon Pass
Deep Scientific Drillhole) indicates that all Cretaceous igneous units
investigated were intruded between 83 and 74 Ma, and Cajon Pass samples
include a Jurassic age component. A compilation of new and published SIMS
geochronological data demonstrates that voluminous magmatism in the Eastern
Transverse Ranges and central Mojave Desert was continuous throughout the
period suggested for the intersection and flat-slab subduction of the
Shatsky Rise conjugate deep into the interior of western North America.
Whole-rock major-element, trace-element, and isotope geochemistry data from
samples from a suite of 106 igneous rocks represent the breadth of Late
Cretaceous units in the transect. Geochemistry indicates an origin in a
subduction environment and intrusion into a crust thick enough to generate
residual garnet. The lack of significant deflections of compositional
characteristics and isotopic ratios in igneous products through space and
time argues against a delamination event prior to 74 Ma. We argue that Late
Cretaceous plutonism from the Eastern Transverse Ranges to the central
Mojave Desert represents subduction zone arc magmatism that persisted until
ca. 74 Ma. This interpretation is inconsistent with the proposed timing of
the docking of the Shatsky Rise conjugate with the margin of western North
America, particularly models in which the leading edge of the Shatsky Rise
was beneath Wyoming at 74 Ma. Alternatively, the timing of cessation of
plutonism precedes the timing of the passage of the Hess Rise conjugate
beneath western North America at ca. 70–65 Ma. The presence, geochemical
composition, and age of arc products in the Eastern Transverse Ranges and
central Mojave Desert region must be accounted for in any tectonic model of
the transition from Sevier to Laramide orogenesis.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02225.1/609383/Testing-models-of-Laramide-orogenic-initiation-by
Evolution of mafic lavas in Central Anatolia: Mantle source domains
Tanya Furman; Barry B. Hanan; Megan Pickard Sjoblom; Biltan Kürkcüoğlu;
Kaan Sayit ...
Abstract:
We present new Sr-Nd-Pb-Hf isotopic data on mafic lavas from the Sivas,
Develidağ, Erciyes, and Erkilet volcanic complexes in central Turkey and
Tendürek in eastern Turkey to evaluate the mantle sources for volcanism in
the context of the geodynamic evolution of the Anatolian microplate. Early
Miocene through Quaternary volcanism in Western Anatolia and latest Miocene
through Quaternary activity in Central Anatolia were dominated by
contributions from two distinct source regions: heterogeneous metasomatized
or subduction-modified lithosphere, and roughly homogeneous sublithospheric
ambient upper mantle; we model the source contributions through mixing
between three end members. The sublithospheric mantle source plots close to
the Northern Hemisphere reference line (NHRL) with radiogenic 206Pb/204Pb of ~19.15, while the other contributions
plot substantially above the NHRL in Pb isotope space. The lithospheric
source is heterogeneous, resulting from variable pollution by
subduction-related processes likely including direct incorporation of
sediment and/or mélange; its range in radiogenic isotopes is defined by
regional oceanic sediment and ultrapotassic melts of the subcontinental
lithospheric mantle. The geochemical impact of this contribution is
disproportionately large, given that subduction-modified lithosphere and/or
ocean sediment dominates the Pb isotope signatures of mafic Anatolian
lavas. Subduction of the Aegean or Tethyan seafloor, associated with marked
crustal shortening, took place throughout the region until ca. 16–17 Ma,
after which time broad delamination of the thickened lower crust and/or the
Tethyan slab beneath Central Anatolia allowed for sediment and/or mélange
and slab-derived fluids to be released into the overlying evolving modified
mantle. Aggregation of melts derived from both mantle and lithospheric
domains was made possible by upwelling of warm asthenospheric material
moving around and through the complexly torn younger Aegean-Cyprean slab
that dips steeply to the north beneath southern Anatolia.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02329.1/609368/Evolution-of-mafic-lavas-in-Central-Anatolia
Insights from the geological record of deformation along the subduction
interface at depths of seismogenesis
Donald M. Fisher; John N. Hooker; Andrew J. Smye; Tsai-Wei Chen
Abstract:
Subduction interfaces are loci of interdependent seismic slip behavior,
fluid flow, and mineral redistribution. Mineral redistribution leads to
coupling between fluid flow and slip behavior through decreases in
porosity/permeability and increases in cohesion during the interseismic
period. We investigate this system from the perspective of ancient
accretionary complexes with regional zones of mélange that record
noncoaxial strain during underthrusting adjacent to the subduction
interface. Deformation of weak mudstones is accompanied by low-grade
metamorphic reactions, dissolution along scaly microfaults, and the removal
of fluid-mobile chemical components, whereas stronger sandstone blocks
preserve veins that contain chemical components depleted in mudstones.
These observations support local diffusive mass transport from scaly
fabrics to veins during interseismic viscous coupling. Underthrusting
sediments record a crack porosity that fluctuates due to the interplay of
cracking and precipitation. Permanent interseismic deformation involves
pressure solution slip, strain hardening, and the development of new shears
in undeformed material. In contrast, coseismic slip may be accommodated
within observed narrow zones of cataclastic deformation at the top of many
mélange terranes. A kinetic model implies interseismic changes in physical
properties in less than hundreds of years, and a numerical model that
couples an earthquake simulator with a fluid flow system depicts a
subduction zone interface governed by feedbacks between fluid production,
permeability, hydrofracturing, and aging via mineral precipitation. During
an earthquake, interseismic permeability reduction is followed by coseismic
rupture of low permeability seals and fluid pressure drop in the
seismogenic zone. Updip of the seismogenic zone, there is a post-seismic
wave of higher fluid pressure that propagates trenchward.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02389.1/609369/Insights-from-the-geological-record-of-deformation
Interpretation of hydrothermal conditions, production-injection induced
effects, and evidence for enhanced geothermal system- type heat
exchange in response to >30 years of production at Roosevelt Hot
Springs, Utah, USA
Stuart F. Simmons; Rick G. Allis; Stefan M. Kirby; Joseph N. Moore; Tobias
P. Fischer
Abstract:
The Roosevelt Hot Springs hydrothermal system is located at the base of the
Mineral Mountains in southwestern Utah on the eastern side of the Basin and
Range. Hydrothermal activity is related to relatively recent bimodal
magmatism, and the system is hosted in coarsely crystalline rock made of
Oligocene–Miocene granitoids and Precambrian gneiss. The hydrothermal plume
covers ~5 km2, with a maximum temperature of 268 °C at ~750 m
depth, and a vertically extensive fault-fracture mesh east of the Opal
Mound fault controls the upflow of hydrothermal fluids. Power generation
(currently 38 MWe gross) began in 1984, and up through 2016, four wells
were used for fluid production, and three wells were used for edge-field
injection. Chemical analyses of produced fluids show that modern reservoir
fluid compositions are similar to but more concentrated than those at the
start of production, having near-neutral pH, total dissolved solids of
7000–10,000 mg/kg, and ionic ratios of Cl/HCO3 ~50–100, Cl/SO 4 ~50–100, and Na/K ~4–5. Chemical geothermometers indicate
equilibration temperatures that mainly range between 240° and 300 °C. Early
production induced a steep drop in pressure (~3.0–3.5 MPa), which was
accompanied by a 250–300 m lowering of piezometric levels in wells and
development of a shallow steam zone across the system. Hydrothermal fluid
compositions evolved continuously in response to production-related
steam-loss and injection breakthrough, which is reflected by gradual
increases in chloride of up to 35% and stable isotope ratios of up to ~2‰ δ 18O and ~10‰ #x03B4;D. Simple mixing model calculations suggest
that there has been a significant amount, ~10–20 MWth, of
sustained multi-decadal heat mining and enhanced geothermal system
(EGS)–type heat transfer by the injectate as it returns to the production
zone. Overall, the two factors that have sustained long-term power
production (currently 38 MWe gross) are the increased upflow of deep
chloride water and, to a lesser extent, the mining of heat at <1 km
depth.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02348.1/609370/Interpretation-of-hydrothermal-conditions
Detrital zircon geothermochronology reveals pre-Alleghanian exhumation
of regional Mississippian sediment sources in the southern Appalachian
Valley and Ridge Province
Matthew McKay; William T. Jackson, Jr.; Derek Spurgeon; Adelie Ionescu;
Barry Shaulis
Abstract:
The Black Warrior foreland basin records sedimentation associated with the
development of intersecting Ouachita and Alleghanian thrust belts along the
southern margin of Laurentia. Mississippian–Pennsylvanian units in the
Black Warrior basin are interpreted to be sourced from either the northern
Appalachians and mid-continent or more regionally from the southern
Appalachians or nearby Ouachita thrust belt. We present detrital zircon
U-Pb ages and Th/U values from Paleozoic units that indicate zircon from
the Mississippian Hartselle Sandstone are temporally and chemically
compatible with being sourced from the southern Appalachians. Zircon mixing
models suggest sediment was primarily recycled from Cambrian, Ordovician,
and Devonian strata in the Appalachian Valley and Ridge, with minor influx
from Piedmont units. A ca. 415 Ma zircon population requires additional
input from the Maya Block of the Yucatan Peninsula or similar outboard
terranes. We present zircon (U-Th)/He analysis and thermal history modeling
of Paleozoic units, which detail pre-Alleghanian exhumation in the
Appalachian Valley and Ridge. Both the Cambrian Chilhowee Group and
Pennsylvanian Pottsville Formation exhibit (U-Th)/He dates ranging from 507
to 263 Ma with a Mississippian subset (353–329 Ma, n = 4), which indicates
rapid cooling and inferred exhumation during Late Devonian–Early
Mississippian Neoacadian tectonism. We propose a Mississippian drainage
system that transported material along southern Appalachian structural
fabrics to the juncture between Appalachian and Ouachita thrust belts
followed by a sediment-routing rotation toward the Black Warrior foreland.
This interpretation honors chemical-age zircon data, accounts for
metamorphic grains in thin section petrography, and matches
Mississippian–Pennsylvanian Black Warrior foreland lithostratigraphic
relationships.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02427.1/609359/Detrital-zircon-geothermochronology-reveals-pre
Quaternary basaltic volcanic fields of the American Southwest
Greg A. Valentine; Michael H. Ort; Joaquín A. Cortés
Abstract:
The southwestern United States contains numerous monogenetic basaltic
volcanoes distributed in intraplate volcanic fields. We review, on a
regional scale, our current understanding of the Quaternary basalts with a
focus on aspects pertinent to hazard assessment, such as physical
volcanology and geochronology, while also summarizing the several
petrogenetic conceptual models that have been proposed for the range of
local tectonic settings in the region. We count 2229 volcanoes in 37
volcanic fields (including the Pinacate volcanic field, which is mostly in
northern Sonora, Mexico). Volcanic landforms are dominantly scoria cones
and ramparts with attendant lava fields that have a spectrum of ‘a’ā and
blocky to pāhoehoe morphologies, while a small percentage of the volcanoes
are maars and tuff cones. Explosive eruption styles that were driven mainly
by magmatic volatiles, where they have been studied in detail, included
Hawaiian, Strombolian, violent Strombolian, and sub-Plinian activity. The
latter two have resulted in substantial fallout deposits that can be
traced tens of kilometers from source vents. Phreatomagmatic styles have
produced pyroclastic current (mainly pyroclastic surges), ballistic, and
fallout deposits. These eruption styles pose hazards to humans when they
occur in populated areas and to air travel and regional infrastructure even
in sparsely populated areas. All but one of the major volcanic fields
(fields that contain ~100 or more Quaternary volcanoes) together form a
northwest-southeast–trending band, which we suggest may reflect an
influence of plate-boundary-related shearing on melt segregation in the
upper mantle along with other factors; this view is consistent with recent
global positioning system (GPS) and structural geologic data indicating the
influence of dextral motion along the North America-Pacific plate boundary
deep inside the Southwest. Of the 2229 Quaternary volcanoes identified,
~548 (25%) have been dated, and only ~15% have been dated with methods such
as 40Ar/39Ar and cosmogenic surface exposure methods
that are considered optimal for young basalts. Acknowledging the large
uncertainty due to the poor geochronological data coverage, we use a simple
Poisson model to provide a first-order estimate of recurrence rates of
monogenetic volcanoes on the scale of the region as a whole; recurrence
rates using our compiled age data set range from 3.74 × 10−4 yr −1 to 8.63 × 10−4 yr−1. These values are
only based on dated and mapped volcanoes, respectively, and do not account
for undated and buried volcanoes or other uncertainties in the volcano
count. The time between monogenetic eruptions in the Southwest is similar
to the repose times of some polygenetic volcanoes, which suggests that the
regional hazard is potentially commensurate with the hazard from a
reawakening stratovolcano such as those in the Cascade Range. Notable in
our review is that only a few volcanoes have been the subject of physical
volcanological characterization, interpretation, and detailed petrologic
study that may elucidate factors such as magma generation, ascent
(including time scales), and controls on eruption style.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02405.1/609360/Quaternary-basaltic-volcanic-fields-of-the
Giant meandering channel evolution, Campos deep-water salt basin,
Brazil
Jacob A. Covault; Zoltán Sylvester; Can Ceyhan; Dallas B. Dunlap
Abstract:
Submarine channels are conduits for sediment delivery to continental
margins, and channel deposits can be sandy components of the fill in
tectonically active salt basins. Examples of salt-withdrawal basin fill
commonly show successions of sandy channelized or sheet-like systems
alternating with more mud-rich mass-transport complexes and hemipelagites.
This alternation of depositional styles is controlled by subsidence and
sediment-supply histories. Salt-basin fill comprising successions of
largely uninterrupted meandering-channel deposition are less commonly
recognized. This begs the questions: can sediment supply be large enough to
overwhelm basin subsidence and result in a thick succession of channel
deposits, and, if so, how would such a channel system evolve? Here, we use
three-dimensional seismic-reflection data from a >1500 km2
region with salt-influenced topography in the Campos Basin, offshore
Brazil, to evaluate the influence of salt diapirs on an Upper
Cretaceous–Paleogene giant meandering submarine-channel system (channel
elements >1 km wide; meander wavelengths several kilometers to >10
km). The large scale of the channels in the Campos Basin suggests that
sediment discharge was large enough to sustain the meandering channel
system in spite of large variability in subsidence across the region. We
interpreted 22 channel centerlines to reconstruct the detailed kinematic
evolution of this depositional system; this level of detail is akin to that
of recent studies of meandering fluvial channels in time-lapse Landsat
satellite images. The oldest channel elements are farther from salt diapirs
than many of the younger ones; the centerlines of the older channel
elements exhibit a correlation between curvature and migration rate, and a
spatial delay between locations of peak curvature and maximum migration
distance, similar to that observed in rivers. As many of the younger
channel centerlines expanded toward nearby salt diapirs, their migration
pattern switched to downstream translation as a result of partial
confinement. Channel segments that docked against salt diapirs became less
mobile, and, as a result, they do not show a correlation between curvature
and migration rate. The channel migration pattern in the Campos Basin is
different compared to that of a tectonically quiescent continental rise
where meander evolution is unobstructed. This style of channelized basin
filling is different from that of many existing examples of salt-withdrawal
minibasins that are dominated by overall less-channelized deposits. This
difference might be a result of the delivery of voluminous coarse sediment
and high discharge of channel-forming turbidity currents to the Campos
Basin from rivers draining actively uplifting coastal mountains of
southeastern Brazil. Detailed kinematic analysis of such well-preserved
channels can be used to reconstruct the impact of structural deformation on
basin fill.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02420.1/609361/Giant-meandering-channel-evolution-Campos-deep
The effect of along-strike variation in dip on rupture propagation on
strike-slip faults
Julian C. Lozos
Abstract:
Strike-slip faults can be nonplanar in both their strike and dip
dimensions. While a large body of work has investigated the effects of
changes in strike on earthquake rupture and arrest, no previous studies
have investigated the role of along-strike variations in dip on strike-slip
ruptures. Here, I use the three-dimensional finite-element method to
conduct dynamic simulations of ruptures on strike-slip faults with linear
surface traces and changes in dip along strike. I experiment with the
amount of dip change as well as the abruptness of that change under a
variety of initial stress conditions. In all of my initial stress cases, I
find that a change in dip along strike can cause rupture to stop, and that
larger dip changes over shorter distances are more likely to do so. This is
largely due to the change in strike at depth that inherently comes from
changing the dip; the majority of these behaviors are a result of the
rupture front being forced to change direction mid-rupture. While some
dip-slip movement does occur on the nonvertical parts of the model fault,
it does not have a significant effect on rupture extent. However,
linear-surface-trace, nonvertical-dip faults do produce different surface
slip, stress, and ground motion patterns compared to corresponding
nonlinear-strike, vertical-dip faults. Together, my results show that
changes in dip along strike-slip faults do considerably impact the rupture
process, suggesting that this type of geometrical complexity should be
considered in rupture forecasts and hazard assessments.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02391.1/609330/The-effect-of-along-strike-variation-in-dip-on
Bringing sedimentology and stratigraphy into the StraboSpot data
management system
Casey J. Duncan; Marjorie A. Chan; Elizabeth Hajek; Diane Kamola; Nicolas
M. Roberts ...
Abstract:
The StraboSpot data system provides field-based geologists the ability to
digitally collect, archive, query, and share data. Recent efforts have
expanded this data system with the vocabulary, standards, and workflow
utilized by the sedimentary geology community. A standardized vocabulary
that honors typical workflows for collecting sedimentologic and
stratigraphic field and laboratory data was developed through a series of
focused workshops and vetted/refined through subsequent workshops and field
trips. This new vocabulary was designed to fit within the underlying
structure of StraboSpot and resulted in the expansion of the existing data
structure. Although the map-based approach of StraboSpot did not fully
conform to the workflow for sedimentary geologists, new functions were
developed for the sedimentary community to facilitate descriptions,
interpretations, and the plotting of measured sections to document
stratigraphic position and relationships between data types. Consequently,
a new modality was added to StraboSpot— Strat Mode—which now accommodates
sedimentary workflows that enable users to document stratigraphic positions
and relationships and automates construction of measured stratigraphic
sections. Strat Mode facilitates data collection and co-location of
multiple data types (e.g., descriptive observations, images, samples, and
measurements) in geographic and stratigraphic coordinates across multiple
scales, thus preserving spatial and stratigraphic relationships in the data
structure. Incorporating these digital technologies will lead to better
research communication in sedimentology through a common vocabulary, shared
standards, and open data archiving and sharing.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02364.1/609331/Bringing-sedimentology-and-stratigraphy-into-the
Extreme metamorphism and metamorphic facies series at convergent plate
boundaries: Implications for supercontinent dynamics
Yong-Fei Zheng; Ren-Xu Chen
Abstract:
Crustal metamorphism under extreme pressure-temperature conditions produces
characteristic ultrahigh-pressure (UHP) and ultrahigh-temperature (UHT)
mineral assemblages at convergent plate boundaries. The formation and
evolution of these assemblages have important implications, not only for
the generation and differentiation of continental crust through the
operation of plate tectonics, but also for mountain building along both
converging and con- verged plate boundaries. In principle, extreme
metamorphic products can be linked to their lower-grade counterparts in the
same metamorphic facies series. They range from UHP through high-pressure
(HP) eclogite facies to blueschist facies at low thermal gradients and from
UHT through high-temperature (HT) granulite facies to amphibolite facies at
high thermal gradients. The former is produced by low-temperature/pressure
(T/P ) Alpine-type metamorphism during compressional heating in
active subduction zones, whereas the latter is generated by high- T/P Buchan-type metamorphism during extensional heating in rifting
zones. The thermal gradient of crustal metamorphism at convergent plate
boundaries changes in both time and space, with low-T/P ratios in
the compressional regime during subduction but high-T/P ratios in
the extensional regime during rifting. In particular, bimodal metamorphism,
one colder and the other hotter, would develop one after the other at
convergent plate boundaries. The first is caused by lithospheric subduction
at lower thermal gradients and thus proceeds in the compressional stage of
convergent plate boundaries; the second is caused by lithospheric rifting
at higher thermal gradients and thus proceeds in the extensional stage of
convergent plate boundaries. In this regard, bimodal metamorphism is
primarily dictated by changes in both the thermal state and the dynamic
regime along plate boundaries. As a consequence, supercontinent assembly is
associated with compressional metamorphism during continental collision,
whereas supercontinent breakup is associated with extensional metamorphism
during active rifting. Nevertheless, aborted rifts are common at convergent
plate boundaries, indicating thinning of the previously thickened
lithosphere during the attempted breakup of supercontinents in the history
of Earth. Therefore, extreme metamorphism has great bearing not only on
reworking of accretionary and collisional orogens for mountain building in
continental interiors, but also on supercontinent dynamics in the Wilson
cycle.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02334.1/609332/Extreme-metamorphism-and-metamorphic-facies-series
Identification of seasonal varves in the lower Pliocene Bouse
Formation, lower Colorado River Valley, and implications for Colorado
Plateau uplift
Jon E. Spencer; Kurt N. Constenius; David L. Dettman; Kenneth J. Domanik
Abstract:
The cause of Cenozoic uplift of the Colorado Plateau is one of the largest
remaining problems of Cordilleran tectonics. Difficulty in discriminating
between two major classes of uplift mechanisms, one related to lithosphere
modification by low-angle subduction and the other related to active mantle
processes following termination of subduction, is hampered by lack of
evidence for the timing of uplift. The carbonate member of the Pliocene
Bouse Formation in the lower Colorado River Valley southwest of the
Colorado Plateau has been interpreted as estuarine, in which case its
modern elevation of up to 330 m above sea level would be important evidence
for late Cenozoic uplift. The carbonate member includes laminated marl and
claystone interpreted previously in at least one locality as tidal, which
is therefore of marine origin. We analyzed lamination mineralogy, oxygen
and carbon isotopes, and thickness variations to discriminate between a
tidal versus seasonal origin. Oxygen and carbon isotopic analysis of two
laminated carbonate samples shows an alternating pattern of lower δ 18O and δ13C associated with micrite and slightly
higher δ18O and δ13C associated with siltstone, which
is consistent with seasonal variation. Covariation of alternating δ 18O and δ13C also indicates that post-depositional
chemical alteration did not affect these samples. Furthermore, we did not
identify any periodic thickness variations suggestive of tidal influence.
We conclude that lamination characteristics indicate seasonal genesis in a
lake rather than tidal genesis in an estuary and that the laminated Bouse
Formation strata provide no constraints on the timing of Colorado Plateau
uplift.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02419.1/609333/Identification-of-seasonal-varves-in-the-lower
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