New Articles for Geosphere Posted Online in June
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Locations studied include the
Turkish-Iranian Plateau and the Zagros Mountain Belt; West Kunlun,
Northwestern China; and the Little Cottonwood stock, Utah, USA. You can find
these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Seismic attenuation tomography of the Sn phase beneath the
Turkish-Iranian Plateau and the Zagros mountain belt
Ayoub Kaviani; Eric Sandvol; Wenfei Ku; Susan L. Beck; Niyazi Türkelli ...
Abstract:
The Turkish-Iranian Plateau and the Zagros highlands are among the most
prominent physiographic features in the Middle East and were formed as a
result of continental collision between the Arabian and Eurasian plates. To
better understand the nature of the lithospheric mantle and the origin of
the observed seismic anomalies in this region, we investigated seismic
attenuation of the uppermost mantle by detailed measurements of the quality
factor of the Sn seismic phase (Sn Q). To that end, we collected a
large data set consisting of 30 years (1990–2020) of waveforms recorded by
1266 permanent and temporary seismic stations, applying both the
two-station method (TSM) and reverse two-station method (RTM) to measure
path-averaged Sn Q. Finally, we performed a tomographic inversion
on the path-averaged Sn Q to map the lateral variations of the
upper-mantle attenuation across the northern Middle East. Our Sn
attenuation maps show moderately low Q (<250) values beneath
the Turkish-Iranian Plateau and high Q values (>350) beneath
the Zagros and northern edge of the Arabian plate. Furthermore, our Sn Q model is broadly consistent with seismic velocity models in the
region suggesting that most of the seismic anomalies are the result of
thermal rather than compositional effects.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article/doi/10.1130/GES02503.1/614763/Seismic-attenuation-tomography-of-the-Sn-phase
Mafic magma-driven magmatic processes and compositional variation in
granitic pluton construction: The Buya intrusion of West Kunlun,
Northwestern China
Chao Wang; Liang Liu; Wen-qiang Yang; Yu-ting Cao; R. Hugh Smithies
Abstract:
To investigate the direct evidence for a number of physico-chemical
processes related to pluton construction and growth, we examine the Buya
pluton of West Kunlun in Northwestern China, which emplaced within the
455–460 Ma time frame. Field observations, geochemical data, and
thermodynamic modeling show that mafic dikes of the Buya pluton were
conduits for magma chamber replenishment during pluton construction. These
mafic inputs, and the enclaves that resulted from them, induced compaction
of the semi-consolidated, crystal-rich, felsic mushes below them. The
accumulation of highly silicic, fine-grained granite at the top of the Buya
pluton is the result of episodic melt segregation events from these mushes.
This sequence of events may reflect a common process that promotes
compositional variation in granite suites. Combined geochemical and Hf- and
Nd-isotopic data suggest that parental magmas of the mafic sheet and
enclave are similar to sanukitoid, which is potentially consistent with a
mantle peridotitic source metasomatized by slab melts. These mafic magmas
intruded the lower crust where the original magma was modified by mafic
lower-crust melt. Following emplacement at shallow crustal levels of the
mafic inputs (~3.7 kbar, ~5.3 km, constrained by amphibole geobarometry),
the felsic mush evolved through the extraction of interstitial melts driven
by hybridization with episodic inputs of mafic magmas as well as crystal
consequent accumulation and fractional crystallization of plagioclase,
hornblende, and accessory phases such as allanite, apatite, and zircon.
This fractional crystallization process may also provide an explanation for
the apparently high Sr/Y features in some silicic high-K, calc-alkaline
magmas.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02451.1/614732/Mafic-magma-driven-magmatic-processes-and
Multi-stage construction of the Little Cottonwood stock, Utah, USA:
Origin, intrusion, venting, mineralization, and mass movement
Collin G. Jensen; Eric H Christiansen; Jeffrey D. Keith
Abstract:
Many porphyry molybdenum deposits are hosted in multi-phase plutons, but it
is unclear in some deposits how these magmas originated and whether the
pluton intruded as it fractionated or was intruded by new batches of magma.
New mapping has clarified field relationships between units in the White
Pine porphyry Mo system hosted in the Little Cottonwood stock, Utah
(western United States), including the White Pine intrusion, the Red Pine
porphyry, rhyolite dikes, and phreatomagmatic pebble dikes. Geologic
relations and geochemistry show the system formed in a continental arc
setting during rollback of the subducting Farallon slab rather than during
extension related to orogenic collapse. Whole-rock geochemistry shows
distinct fractionation trends for each of the major intrusive units in the
composite pluton, suggesting they formed separately, which is supported by
new U-Pb zircon laser ablation–inductively coupled plasma–mass spectrometry
(LA-ICP-MS) ages of ca. 30 Ma for the Little Cottonwood stock, 27 Ma for
the White Pine intrusion, and 26 Ma for the previously undated Red Pine
porphyry. Mineral textures, cross-cutting relationships, and alteration
mineralogy indicate that intrusion of the youngest phase led to a
fluid-saturated magmatic system and triggered venting, including
emplacement of pebble dikes. In the adjacent east Traverse Mountains,
pebble dikes contain clasts that have similar mineral assemblages,
textures, and ages as the major igneous units in the White Pine deposit.
This indicates that the pebble dikes in east Traverse Mountains and in the
pluton are the upper and lower parts of the same magmatic-hydrothermal
system, which was decapitated by a mega-landslide that was likely
facilitated by alteration in the Oligocene hydrothermal system and by later
Basin and Range faulting.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02369.1/614461/Multi-stage-construction-of-the-Little-Cottonwood
Seismostratigraphic analysis of Lake Cahuilla sedimentation cycles and
fault displacement history beneath the Salton Sea, California, USA
Daniel S. Brothers; Neal W. Driscoll; Graham M. Kent; Robert L. Baskin;
Alistair J. Harding ...
Abstract:
The Salton Trough (southeastern California, USA) is the northernmost
transtensional stepover of the Gulf of California oblique-divergent plate
boundary and is also where the southern terminus of the San Andreas fault
occurs. Until recently, the distribution of active faults in and around the
Salton Sea and their displacement histories were largely unknown. Subbottom
CHIRP (compressed high-intensity radar pulse) surveys in the Salton Sea are
used to develop a seismic facies model for ancient Lake Cahuilla deposits,
a detailed map of submerged active faults, and reconstructed fault
displacement histories during the late Holocene. We observe as many as
fourteen Lake Cahuilla sequences in the Salton Sea (last ~3 k.y.) and
develop a chronostratigraphic framework for the last six sequences (last
~1200 yr) by integrating CHIRP data and cone penetrometer logs with
radiocarbon-dated stratigraphy at an onshore paleoseismic site. The Salton
Sea contains northern and southern subbasins that appear to be separated by
a tectonic hinge zone, and a subsidence signal across hinge-zone faults of
6–9 mm/yr (since ca. A.D. 940) increases toward the south to >15 mm/yr.
The faults mapped to the south of the hinge zone appear to accommodate
transtension within the San Andreas–Imperial fault stepover. We identify
8–15 distinct growth events across hinge-zone faults, meaning growth
occurred at least once every 100 yr since Lake Cahuilla sedimentation
began. Several faults offset the top of the most recent Lake Cahuilla
highstand deposits, and at least two faults have offset the Salton Sea
flood deposits. Active faults and folds were also mapped to a limited
extent within the northern subbasin and display growth, but their
kinematics and rupture histories require further study. The broad
distribution of active faulting suggests that strain between the San
Andreas, San Jacinto, and Imperial faults is highly distributed, thus
discrepancies between geologic and geodetic slip-rate estimates from these
major fault systems are to be expected.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02468.1/614460/Seismostratigraphic-analysis-of-Lake-Cahuilla
U-Pb and fission-track data from zircon and apatite resolve latest- and
post-Alleghanian thermal histories along the Fall Line of the Atlantic
margin of the southeastern United States
William H. Craddock; Paul B. O’Sullivan; Ryan J. McAleer
Abstract:
Although the Atlantic continental margin of the eastern United States is an
archetypal passive margin, episodes of rejuvenation following continental
breakup are increasingly well documented. To better constrain this history
of rejuvenation along the southern portion of this continental margin, we
present zircon U-Pb (ZUPb) age, zircon fission-track (ZFT) age, apatite
U-Pb (AUPb) age, and apatite fission-track (AFT) age and length data from
six bedrock samples. The samples were collected along the boundary between
the exposed Appalachian hinterland (Piedmont province) and the updip limit
of passive margin strata (Coastal Plain province). The samples were
collected from central Virginia southward to the South Carolina–Georgia
border. ZUPb age distributions are generally consistent with geologic
mapping in each of the sample areas. The AUPb data are highly discordant
owing to high common-Pb abundances, but for two plutons at the northern and
southern ends of the sample area, they define a discordia regression line
that indicates substantial Permo-Triassic exhumation-driven cooling. ZFT
age distributions are highly dispersed but define central values ranging
from Permian to Jurassic. AFT data mostly appear to define a singular
underlying cooling age, generally approximately Jurassic or Early
Cretaceous. Apatite fission tracks are moderately long (mean lengths in the
range of ~13.5 µm), however track lengths for one sample in central North
Carolina are shorter (~12.5 µm). To interpret the post-breakup thermal
history, we present inverse models of time-temperature history for the five
plutonic samples. The models show a history of (1) rapid cooling (>10
°C/m.y.) from deep-crustal to near-surface temperatures by the Triassic,
(2) hundreds of degrees of Triassic reheating, (3) Jurassic–Early
Cretaceous cooling (at rates of 1–10 °C/m.y.), and (4) slow Late
Cretaceous–Cenozoic cooling (~1 °C/m.y.). An additional suite of forward
models is presented to further evaluate the magnitude of maximum Triassic
reheating at one sample site that is particularly well constrained by
thermal maturity data. The model results and geologic reasoning suggest
that the inverse models may overestimate Triassic paleotemperatures but
that other aspects of the inverse modeling are robust. Overall, this
thermal history can be reconciled with several aspects of the
lithostratigraphy of distal parts of the continental margin, including the
lack of Jurassic–earliest Cretaceous strata beneath the southern Atlantic
coastal plain and Cretaceous–Cenozoic grain-size trends.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02447.1/614326/U-Pb-and-fission-track-data-from-zircon-and
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 Kea Giles 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
# # #