New GSA Bulletin Articles Published Ahead of Print in June
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics studied this
month include the nature and dynamics of China and Tibet; hawaiites in the
Cima volcanic field, California; and the dynamic floor of Yellowstone Lake.
You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Crustal growth/reworking and stabilization of the western Superior
Province: Insights from a Neoarchean gneiss complex of the Winnipeg
River terrane
Chong Ma; Jeffrey Marsh; Robert W.D. Lodge; Ross Sherlock
Abstract:
Long-term stability of the continental lithosphere is attained through a
cumulative increase in net buoyancy and rigidity due to progressive
compositional differentiation (i.e., cratonization). As stable cratons
provided the nucleus for the subsequent accretionary growth and
tectono-magmatic reworking that produced modern continental crust, the
geodynamic processes that facilitated the stabilization of cratons are
critical for understanding the evolution of Earth’s lithosphere. This study
uses a portion of the Winnipeg River terrane, one of the oldest terranes of
the western Superior Province, as a natural laboratory to investigate
Archean crustal growth (partial melting of mantle) and reworking (partial
melting of crust) and provides insights into the geodynamic processes
driving mantle depletion and crustal remelting. Zircon U-Pb data obtained
by laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS)
from an extensive Winnipeg River terrane gneiss complex reveal six major
magmatic events at ca. 3060 Ma, 2930‒2920 Ma, ca. 2910 Ma, 2830‒2800 Ma,
2735‒2730 Ma, and ca. 2700 Ma and regional metamorphism at ca. 2900 Ma.
Whole-rock geochemistry and zircon Lu-Hf and trace element data indicate
that (1) the magmatism at ca. 3060 Ma and ca. 2930‒2920 Ma represents
reworking of the isotopically evolved components of the incipient Winnipeg
River terrane at shallow depths, (2) the ca. 2910 Ma magmatism features a
step-change of Hf isotopic compositions from subchondritic to
suprachondritic and records the formation of new juvenile magmas and the
first reworking of existing juvenile crust, and (3) the magmatism after ca.
2830 Ma largely reflects reworking of the juvenile components of the
incipient Winnipeg River terrane at medium to shallow depths prior to the
ca. 2700 Ma trans-crustal magmatism associated with the convergence of the
Winnipeg River terrane and western Wabigoon terrane. Juvenile magmatism and
crustal growth in the Winnipeg River terrane at ca. 2910 Ma are inferred to
correspond with significant mantle depletion below the Winnipeg River
terrane, which led to a more stable lithospheric mantle in this part of the
western Superior Province. Zircon trace element data support a mantle
upwelling model rather than lithosphere recycling models for the depletion
of mantle at ca. 2910 Ma. This study suggests that crustal growth and
mantle depletion bracketed by prolonged, episodic crustal reworking may be
a fundamental characteristic of the cratonization process.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36441.1/614898/Crustal-growth-reworking-and-stabilization-of-the
The 180-km-long Meers-Willow Fault System in the Southern Oklahoma
Aulacogen: A potential U.S. mid-continent seismic hazard
Brandon F. Chase; Folarin Kolawole; Estella A. Atekwana; Brett M.
Carpenter; Molly Turko ...
Abstract: We integrate new high-resolution aeromagnetic data with seismic
reflection data, well logs, satellite remote sensing, and field
observations to provide a regional view of buried and exposed structures in
the Southern Oklahoma Aulacogen and to assess their potential for future
seismicity. Trends ranging from NW−SE to ∼E−W, peaking at 330° ± 4.5° and
280° ± 3°, dominate the magnetic lineaments of the Southern Oklahoma
Aulacogen, reflecting basement contacts, dikes, and faults, including a
previously unmapped ∼100-km-long basement fault, which is herein referred
to as the Willow fault. The fault disrupts, truncates, and vertically
offsets basement-related seismic reflectors and overlying Paleozoic strata
up through the Permian reflectors. Surface deformation along the trend
includes fault-parallel monoclinal folds, pervasive fractures, and
fracture-hosted mud dikes in Permian evaporite units. These structures
indicate a Permian or post-Permian reactivation of the fault. Along-strike,
the Willow fault connects to the NW-trending, seismically active Meers
Fault to comprise the ∼180-km-long Meers-Willow fault system, which
potentially represents a major seismic hazard along the Southern Oklahoma
Aulacogen. Fault slip potential analyses of the mapped potential fault
traces show that seismic hazards are elevated where faults have steeper
dips. Given some uncertainty in the regional stress state, we also show
that hazards along the NW−SE to E−W trending faults vary considerably
within the uncertainty range. We propose that the Meers-Willow fault system
originated as a Cambrian aulacogen-scale, basement-rooted fault that was
later reactivated as a left-lateral strike-slip fault (with ∼40 km
displacement) during the late Paleozoic Ancestral Rocky Mountain orogeny,
highlighting that lateral offset accommodated a major component of
deformation during the orogen.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36363.1/614762/The-180-km-long-Meers-Willow-Fault-System-in-the
Time-constrained multiphase brittle tectonic evolution of the onshore
mid-Norwegian passive margin
Giulia Tartaglia; Alberto Ceccato; Thomas Scheiber; Roelant van der Lelij;
Jasmin Schönenberger ...
Abstract:
The mid-Norwegian passive margin is a multiphase rifted margin that
developed since the Devonian. Its geometry is affected by the long-lived
activity of the Møre-Trøndelag fault complex, an ENE-WSW−oriented regional
tectonic structure. We propose a time-constrained evolutionary scheme for
the brittle history of the mid-Norwegian passive margin. By means of
remote-sensing lineament detection, field work, microstructural analysis,
paleostress inversion, mineralogical characterization, and K-Ar dating of
fault rocks, six tectonic events have been identified: (1) Paleozoic NE-SW
compression forming WNW-ESE−striking thrust faults; (2) Paleozoic NW-SE
transpression forming conjugate strike-slip faults; (3) Carboniferous
protorifting forming NW-SE− and NE-SW−striking faults; (4) Late
Triassic−Jurassic (ca. 202 and 177 Ma) E-W extension forming approximately
N-S−striking epidote- and quartz-coated normal faults and widespread
alteration; (5) renewed rifting in the Early Cretaceous (ca. 122 Ma) with a
NW-SE extension direction; and (6) Late Cretaceous extensional pulses (ca.
71, 80, 86, 91 Ma ago) reactivating preexisting faults and crystallizing
prehnite and zeolite. Our multidisciplinary and multiscalar study sheds
light onto the structural evolution of the mid-Norwegian passive margin and
confirms the active role of the Møre-Trøndelag fault complex during the
rifting stages. Our 62 new radiometric K-Ar ages define discrete episodes
of faulting along the margin. The proposed workflow may assist in the
interpretation of the structural framework of the mid-Norwegian passive
margin offshore domain and also help to better understand fault patterns of
fractured passive margins elsewhere.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36312.1/614321/Time-constrained-multiphase-brittle-tectonic
Origin of low Mg# hawaiites carrying peridotite xenoliths from the Cima
volcanic field, California, USA: Evidence of rapid magma mixing during
ascent along intersecting fractures
Sarah K. Brehm; Rebecca A. Lange
Abstract:
The Cima volcanic field, in the southern Basin and Range province
(California, USA), includes >70 eruptive units over the last 8 m.y. The
youngest (≤1 Ma) are low Mg# (≥56) hawaiites derived from an asthenospheric
mantle source. The Cima hawaiites, and adjacent Dish Hill basanites, are
known for carrying large mantle xenoliths, which precludes stalling in a
crustal reservoir. This raises the question of how low Mg# hawaiites, which
cannot be in equilibrium with peridotite mantle, formed and differentiated
while carrying dense, mantle xenoliths. Several hypotheses are evaluated
and the only one shown to be viable is mixing between high-MgO basanite
(with entrained mantle xenoliths and sparse olivine phenocrysts) and
low-MgO mugearite liquids, which formed by partial melting of mafic lower
crust under relatively dry and reducing conditions. Multiple lines of
evidence, including the presence of mantle xenoliths in hawaiites,
diffusion-limited growth textures in olivine and clinopyroxene, and notably
thin Fe-rich rims on high-MgO olivine crystals (inherited), indicate magma
mixing must have occurred rapidly (days or less) during ascent to the
surface along intersecting fractures, and not in a stalled crustal
reservoir. Abundant evidence points to clinopyroxene growth immediately
after mixing, and application of clinopyroxene-melt barometry constrains
the depth of mixing to the lower and middle crust (0.8−0.4 GPa). Results
from olivine-melt thermometry/hygrometry (∼1196 °C and ∼1.4 wt% H 2O) applied to a basanite from Dish Hill carrying 5−20 cm mantle
xenoliths leads to calculated ascent velocities ≥0.3−4.9 km/h, enabling
ascent through the 36 km thick crust in ≤7−119 h.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36390.1/614297/Origin-of-low-Mg-hawaiites-carrying-peridotite
New age and lake chemistry constraints on the Aptian pre-salt
carbonates of the central South Atlantic
M. Lawson; J. Sitgreaves; T. Rasbury; K. Wooton; W. Esch ...
Abstract:
The Cretaceous lacustrine carbonates of the offshore Brazilian and West
African pre-salt basins represent some of the most extensive non-marine
carbonates discovered in the geologic record. Despite being intensively
studied over the past decade, the age of these carbonates and the overlying
regional salt sequences is highly controversial. Similarly, the conditions
under which these carbonates were deposited remains poorly understood.
Here, we provide the first integrated geochronology-thermometry study of
these carbonates to develop an improved understanding of when and under
what conditions they formed. We utilize carbonate clumped isotope and 87Sr/86Sr geochemistry alongside traditional
petrographic techniques to identify samples minimally altered from burial
diagenesis that may yield reliable age and lake chemistry constraints.
Carbonate clumped isotope apparent temperatures for the studied carbonates
range from 36 ºC to 91 °C, which we infer to represent a range in sample
preservation from minimally altered depositional temperatures through to
those that have been overprinted by burial diagenesis. 87Sr /86Sr values of our samples are consistent with those of
previous studies for Cretaceous pre-salt carbonates that have not
experienced significant alteration from hydrothermal fluids. Through this
approach, we measured the first high resolution isotope dilution U-Pb age
constraint of 115.83 ± 1.56 Ma (2σ) on a well preserved carbonate. Combined
with overlapping lower resolution laser ablation U-Pb ages for
time-equivalent stratigraphy on two separate carbonate platforms of 114.46
± 4.72 Ma and 109.73 ± 9.26 Ma, these ages provide the first robust direct
age calibration for pre-salt carbonates deposited on either side of the
South Atlantic during the final stages of the break-up of Gondwana in the
Early Cretaceous. These ages also provide the first calibration for a
combined 87Sr/86Sr-facies-log based relative age
framework within the Santos Basin, offshore Brazil. We further utilize δ 18O constraints on samples that yield depositional clumped
isotope apparent temperatures to constrain the δ18O of the water
in these ancient lakes to between 1.9 and 4.9‰ Vienna standard mean ocean water. Such heavy values reveal a
picture of a hot and arid environment. This is consistent with prior
biostratigraphic studies of the carbonates that show a decrease in faunal
diversity in these lakes prior to marine ingress and the development of
open marine conditions in the South Atlantic Ocean.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36378.1/614298/New-age-and-lake-chemistry-constraints-on-the
A one-million-year isotope record from siderites formed in modern
ferruginous sediments
Aurèle Vuillemin; Christoph Mayr; Jan A. Schuessler; André Friese; Kohen W.
Bauer ...
Abstract:
Ancient iron formations hold important records of environmental conditions
during the Precambrian eons. Reconstructions of past oceanic systems
require investigation of modern ferruginous analogs to disentangle water
column and diagenetic signals recorded in iron-bearing minerals. We
analyzed oxygen, iron, and carbon isotopes in siderite, a ferrous carbonate
phase commonly used as an environmental proxy, from a 100-m-long record
spanning a 1 Ma depositional history in ferruginous Lake Towuti, Indonesia.
Combining bulk sediment and pore water geochemistry, we traced processes
controlling siderite isotope signatures. We show that siderite oxygen
isotope compositions (δ18O) reflect in-lake hydrological and
depositional conditions. Low iron isotope values (δ56Fe) record
water column oxygenation events over geological timescales, with minor
diagenetic partitioning of Fe isotopes by microbial iron reduction after
deposition. The carbon isotope compositions (δ13C) reflect the
incorporation of biogenic HCO3−, which is consistent
with sediment organic matter remineralization lasting over ca. 200 ka after
burial. Positive δ13C excursions indicate an increased
production of biogenic methane that escaped the sediment during low lake
levels. Diffusion across the sediment−water interface during initial
formation of siderites tends to align the isotope signatures of bottom
waters to those of pore waters. As microbial reduction of ferric iron and
oxidation of organic matter proceed and saturate pore water conditions with
respect to siderite, overgrowth on nuclei partially mutes the environmental
signal inherited from past bottom waters over ca. 1 Ma. Because high
depositional fluxes of ferric iron and organic matter in early oceans would
have promoted similar microbial processes in ferruginous deposits prior to
lithification, the environmental record contained in siderite grains can
successively integrate depositional and early diagenetic signals over short
geological timescales.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36211.1/614288/A-one-million-year-isotope-record-from-siderites
The dynamic floor of Yellowstone Lake, Wyoming, USA: The last 14 k.y.
of hydrothermal explosions, venting, doming, and faulting
L.A. Morgan; W.C.P. Shanks; K.L. Pierce; N. Iverson; C.M. Schiller ...
Abstract:
This doubtless mere fragment of an ancient inland sea, or great lake, of
perhaps hot or tepid water, surrounded and dotted by active volcanoes, has
been so long, and yet so imperfectly known, and in trapper legends has been
presented in so many different localities, shapes, dimensions, elevations,
etc., that it appropriately merits its designation of “Mystic Lake.” It
has, however, been found to be one of the largest, most elevated, and
peculiarly formed of all the mountain lakes of North America, and yet is
comparatively so little known as to offer a most inviting field for
romantic and interesting exploration. Superintendent Philetus W. Norris,
Annual Report of the Superintendent of the Yellowstone National Park, 1881,
p. 11, (Norris, 1881).
Hydrothermal explosions are significant potential hazards in Yellowstone
National Park, Wyoming, USA. The northern Yellowstone Lake area hosts the
three largest hydrothermal explosion craters known on Earth empowered by
the highest heat flow values in Yellowstone and active seismicity and
deformation. Geological and geochemical studies of eighteen sublacustrine
cores provide the first detailed synthesis of the age, sedimentary facies,
and origin of multiple hydrothermal explosion deposits. New
tephrochronology and radiocarbon results provide a four-dimensional view of
recent geologic activity since recession at ca. 15−14.5 ka of the
>1-km-thick Pinedale ice sheet. The sedimentary record in Yellowstone
Lake contains multiple hydrothermal explosion deposits ranging in age from
ca. 13 ka to ∼1860 CE. Hydrothermal explosions require a sudden drop in
pressure resulting in rapid expansion of high-temperature fluids causing
fragmentation, ejection, and crater formation; explosions may be initiated
by seismicity, faulting, deformation, or rapid lake-level changes. Fallout
and transport of ejecta produces distinct facies of subaqueous hydrothermal
explosion deposits. Yellowstone hydrothermal systems are characterized by
alkaline-Cl and/or vapor-dominated fluids that, respectively, produce
alteration dominated by silica-smectite-chlorite or by kaolinite.
Alkaline-Cl liquids flash to steam during hydrothermal explosions,
producing much more energetic events than simple vapor expansion in
vapor-dominated systems. Two enormous explosion events in Yellowstone Lake
were triggered quite differently: Elliott’s Crater explosion resulted from
a major seismic event (8 ka) that ruptured an impervious hydrothermal dome,
whereas the Mary Bay explosion (13 ka) was triggered by a sudden drop in
lake level stimulated by a seismic event, tsunami, and outlet channel
erosion.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36190.1/614289/The-dynamic-floor-of-Yellowstone-Lake-Wyoming-USA
Mechanism of crustal thickening and exhumation of southern Lhasa
terrane during the Late Cretaceous: Evidence from high-pressure
metamorphic rocks of the Eastern Himalayan Syntaxis
Yanling Zhang; Changqing Yin; Donald W. Davis; Shun Li; Jiahui Qian ...
Abstract:
The mechanism of Late Cretaceous crustal thickening and exhumation of the
southern Lhasa terrane is critical for understanding the tectonic evolution
of the Tibetan Plateau. High-pressure metamorphic rocks from the lower
crust are good candidates for addressing this issue. In this study, we
focus on Late Cretaceous, high-pressure, garnet-bearing amphibolites from
the Nyingchi Complex of the Eastern Himalayan Syntaxis and present an
integrated study of geochronology, petrography, mineral chemistry, and
thermodynamic modeling. Petrographic data determine three metamorphic
stages (M1−M3). The M1 stage is
characterized by a peak mineral assemblage of garnet + hornblende + albite
+ rutile + muscovite + quartz, which is followed by a post-peak (M 2) assemblage of garnet + hornblende + plagioclase + epidote +
biotite + rutile + quartz. The late retrograde stage (M3) is
defined by hornblende + plagioclase symplectites surrounding garnet
porphyroblasts. Mineral chemistry, with thermodynamic modeling, constrains
the P-T conditions of the M1−M3 stages to
14−19 kbar/660−720 °C, 8−10 kbar/650−660 °C, and <7 kbar/<600 °C,
respectively. Metamorphic zircons yield a concordant age at 90 Ma, which
indicates the formation of garnet-bearing amphibolites. These results
indicate a P-T-t path involving near-isothermal decompression for
garnet-bearing amphibolites, which suggests that the Nyingchi Complex
underwent peak-pressure metamorphism (M1) at 90 Ma, followed by
rapid exhumation to the depth of 32−26 km along the subduction channel.
Moreover, the garnet-bearing amphibolites are considered to be the product
of high-pressure metamorphism of mafic crust at the base of the Gangdese
belt. Hence, the crust of the Gangdese belt experienced significant crustal
thickening of up to 60 km at 90 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36366.1/614290/Mechanism-of-crustal-thickening-and-exhumation-of
Unusual δ26Mg values in oceanic crust basalts from the South
China Sea
Renqiang Liao; Hongli Zhu; Lipeng Zhang; He Li; Congying Li ...
Abstract:
Whether or not oceanic crust basalts are affected by plate subduction is a
hot topic of debate. The South China Sea is one of the largest marginal
basins in the western Pacific Ocean and has been surrounded by subduction
of the Pacific plate and Indian plate, yet, to date, no study has clearly
shown evidence of subduction in the geochemistry of volcanism in the basin
due to a lack of sampling of igneous crust basalts on the seafloor. The
International Ocean Discovery Program Expedition 349 cored seafloor basalts
near the fossil spreading ridges of the eastern (Site U1431) and
southwestern (Site U1433 and U1434) subbasins in the South China Sea. The
recovered basalt samples indicated a pyroxenite-bearing peridotite mantle
source. Here, we report Mg isotopic data from 14 of these oceanic crust
basalt samples. The δ26Mg values of most basalts from the three
drill holes were higher (up to −0.10‰) than that of the average mantle
(−0.25‰). The lack of correlations of δ26Mg with geochemical
indices of magmatic processes (e.g., MgO, CaO/Al2O3,
La/Sm, Nb/Zr) suggests that crystal fractionation and partial melting had
insignificant effects on the Mg isotopic compositions of the South China
Sea basalts. Thus, the variations in Mg isotopes were inherited from their
mantle sources. Considering the highly varied Ce/Pb ratios and elevated87Sr/86Sr values but mantle-like 143Nd/ 144Nd values, we propose that the varied δ26Mg values
were likely caused by metasomatism of subduction-released fluids. The
coupling of Mg and Sr-Fe isotopes provides robust evidence that the high-δ 26Mg values of the South China Sea basalts resulted from mixing
among pyroxenite-bearing peridotite mantle, the nearby Hainan plume
materials, and subducting serpentinite-released fluids. Therefore, these Mg
isotopes suggest that the mantle source of the South China Sea basalts was
influenced by subducted materials, providing further evidence of the
initial expansion, formation, and evolution of the South China Sea during
plate subduction.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36320.1/614291/Unusual-26Mg-values-in-oceanic-crust-basalts-from
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