GSA Bulletin Articles Published Ahead of Print in January
Boulder, Colo., USA: Sixteen articles were published online ahead of print
for GSA Bulletin in January. Topics include insights from the
Sawtooth metamorphic complex in Idaho, fingerprinting sand from ancient
rivers, eroding Cascadia, and the Troodos ophiolite.
Three-dimensional geometry and growth of a basement-involved fault
network developed during multiphase extension, Enderby Terrace,
North West Shelf of Australia
Hongdan Deng; Ken McClay
Abstract: Basement fault reactivation, and the growth, interaction, and
linkage with new fault segments are fundamentally three-dimensional and
critical for understanding the evolution of fault network development in
sedimentary basins. This paper analyzes the evolution of a complex,
basement-involved extensional fault network on the Enderby Terrace on the
eastern margin of the Dampier sub-basin, North West Shelf of Australia. A
high-resolution, depth-converted, 3-D seismic reflection data volume is
used to show that multiphase, oblique extensional reactivation of
basement-involved faults controlled the development of the fault network in
the overlying strata. Reactivation of the pre-existing faults initially led
to the formation of overlying, en échelon Late Triassic−Middle Jurassic
fault segments that, as WNW-directed rifting progressed on the margin,
linked by breaching of relay zones to form two intersecting fault systems
(F1 and F2−F4). Further reactivation in the latest Jurassic−Early
Cretaceous (NNW-SSE extension) produced an additional set of en échelon
fault arrays in the cover strata. The final fault network consists of main
or principal faults and subordinate or splay faults, together with branch
lines that link the various components. Our study shows that breaching of
relay ramps and/or vertical linkages produces vertical and horizontal
branch lines giving complex final fault geometries. We find that repeated
activity of the basement-involved faults tends to form continuous and
planar fault architectures that favor displacement transfer between the
main constituent segments along strike and with depth.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35779.1/594311/Three-dimensional-geometry-and-growth-of-a
Age and deposition of the Fort Crittenden Formation: A window into
Late Cretaceous Laramide and Cenozoic tectonics in southeastern
Arizona
Emilia A. Caylor; Barbara Carrapa; Kurt Sundell; Peter G. DeCelles; Joshua
M. Smith
Abstract: The Upper Cretaceous Fort Crittenden Formation exposed in the
Santa Rita and Huachuca Mountains of southeastern Arizona is a syntectonic
deposit that has been associated with Laramide tectonic activity. However,
the spatio-temporal relationships among Cretaceous sedimentation,
magmatism, basement exhumation, and possible flat slab-related processes in
the southern Laramide region remain poorly understood. Age controls for
uplift and erosion of local topography and syntectonic deposition in
response to deformation remain particularly poor. The Fort Crittenden
Formation comprises 800−2500 m of locally derived fluvial to alluvial fan
sedimentary rocks and records paleodrainage reorganization in response to
active tectonics. Changes in sedimentary facies, provenance, and paleoflow
suggest deposition in a tectonically partitioned intraforeland basin. New
detrital zircon data constrain the timing of deposition of the Fort
Crittenden Formation between ca. 86 Ma and ca. 76 Ma. The lack of
depositional age zircons throughout the majority of the Fort Crittenden
Formation is consistent with a magmatic lull in the Cordilleran arc between
ca. 90 Ma and ca. 76 Ma. The overlying Salero Formation and Late Cretaceous
intrusions are expressions of renewed magmatism in southeastern Arizona at
ca. 75 Ma. New Lu-Hf data indicate that magmas evolved from contamination
of old juvenile crust. When interpreted in conjunction with other regional
data sets, our study indicates that the Laramide deformation front migrated
eastward into southwestern New Mexico by 75 Ma. Thermal modeling of apatite
fission track and (U-Th)/He data from granitic clasts are consistent with
Late Cretaceous−Paleocene (ca. 76−55 Ma) heating related to magmatism and
cooling and exhumation during the Eocene and Oligocene.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35808.1/594257/Age-and-deposition-of-the-Fort-Crittenden
Mesozoic crustal melting and metamorphism in the U.S. Cordilleran
hinterland: Insights from the Sawtooth metamorphic complex, central
Idaho
Chong Ma; David A. Foster; Paul A. Mueller; Barbara L. Dutrow; Jeffery
Marsh
Abstract: In this study, we present whole-rock geochemistry and Sm-Nd data;
zircon trace element, U-Pb, and Lu-Hf data; titanite U-Pb dating; and
structural analysis of igneous and metasedimentary rocks of the Sawtooth
metamorphic complex that provide insight into regional metamorphism,
partial melting, and crustal thickening in the Idaho batholith segment of
the Cordilleran orogen. Four magmatic events are revealed: (1) pre-tectonic
felsic magmatism at ca. 156 Ma, (2) syn-tectonic mafic and felsic magmatism
between ca. 100 Ma and ca. 92 Ma, (3) felsic magmatism concurrent with
late-stage deformation at ca. 89−84 Ma, and (4) post-tectonic felsic
magmatism at ca. 77 Ma. The multiple generations of felsic magmatism
include a variety of sedimentary- and igneous-derived granitoids
distinguished by zircon trace element compositions (e.g., U/Ce versus Th
and Ce/Sm versus Yb/Gd) and were sourced from progressively more evolved
crustal components as shown by Lu-Hf and Sm-Nd isotopic data. U-Pb data of
metamorphic zircons and titanites from high-grade metasedimentary rocks
suggest that regional metamorphism occurred from ca. 100−93 Ma, which was
characterized by granulite-facies partial melting and concurrent growth of
metamorphic zircons and garnets. The episodic magmatism in the Sawtooth
metamorphic complex records pervasive melt migration in a hot, mid-crustal
setting at ca. 100‒92 Ma and additional magma ascent in a cool,
upper-crustal setting at ca. 77 Ma. The uplift of the Sawtooth metamorphic
complex from mid- to upper-crust was likely caused by underthrusting at
lower crustal levels coupled with erosion and thinning of the upper crust.
This work suggests that the crust of the Cordilleran hinterland in the
Idaho batholith region underwent significant thickening from ca. 100‒84 Ma,
and a crust of Andean-like thickness was probably achieved by ca. 84 Ma. By
ca. 77 Ma, the central Idaho crust started to thin likely due to
mid-crustal flow and surface erosion. The new data from the Sawtooth
metamorphic complex are consistent with the two major magmatic flare-ups in
the Late Jurassic and Late Cretaceous in the U.S. Cordilleran orogen.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35837.1/594189/Mesozoic-crustal-melting-and-metamorphism-in-the-U
Contribution of an Eastern Indochina-derived fragment to the
formation of island arc systems in the Philippine Mobile Belt
Lin Gong; Pete Hollings; Yu Zhang; Jing Tian; Dengfeng Li ...
Abstract: The Philippine Mobile Belt is a complex plate boundary with
multiple terranes in Southeast Asia, yet its early tectonic evolution is
still not fully understood due to a scarcity of solid evidence. Here we
report new whole rock geochemical, Sr-Nd isotopic, and zircon U-Pb-Hf
isotopic data for Cretaceous-Miocene arc magmatic rocks from the Cebu and
Bohol Islands, Philippine Mobile Belt. Bulk geochemical data display arc
affinities with enriched large ion lithophile elements (e.g., Sr and Ba)
and depleted high field strength elements (e.g., Nb, Ta, and Ti). The high
positive εNd(t) (+4.6 to +9.1) values and low initial 87Sr/86Sr ratios (0.7032−0.7048) suggest that these
igneous rocks were generated by partial melting of mantle wedge in an arc
setting. U-Pb dating of zircons revealed Cretaceous (ca. 120−90 Ma), middle
Eocene to early Oligocene (ca. 43−30 Ma), and middle Miocene (ca. 14 Ma)
crystallization ages for the arc magmatism with abundant Permian-Triassic
zircon xenocrysts clustering at ca. 250 Ma. The Permian-Triassic grains
show dominantly negative εHf(t) values ranging from −16.2 to
−6.6, which are similar to those of coeval rocks in Eastern Indochina.
Combined with previous paleomagnetic studies, we propose that an Eastern
Indochina-derived continental fragment was involved during the formation of
arcs in the Cebu and Bohol Islands, which highlights the potential
contribution of ancient continental materials in the formation of
intra-oceanic arcs. This scenario does not support the previously proposed
model that the Cretaceous arc in the Philippine Mobile Belt formed in the
northern margin of the proto-Philippine Sea Plate and Australian margin.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35793.1/594151/Contribution-of-an-Eastern-Indochina-derived
Fingerprinting sand from Asian rivers to the deep central South
China Sea since the Late Miocene
Licheng Cao; Tao Jiang; Jingke He
Abstract: The complex sedimentary processes from source to sink lead to a
substantial fractionation of sediment size and composition. Relatively
coarse-grained, continent-derived detritus is rarely transported and
deposited in the deep ocean, and the terminus of this sediment routing
system is poorly understood. Sandy turbidite deposits within the Upper
Miocene−Pleistocene strata drilled in the deep central South China Sea
during the International Ocean Discovery Program (IODP) Expedition 349
provide valuable samples for evaluating the evolution of sediment
contributions from different Asian landmasses. This study reconstructs this
ancient source-to-sink system based on an integration of heavy mineral and
detrital zircon analyses (including U-Pb age, trace element, grain size and
shape), obtained from IODP sites U1431 and U1432, as well as a zircon
age-based mixture modeling of well-defined provenance end-members. The
results show several provenance shifts that correspond to more complex and
dynamic source-to-sink scenarios than previously envisaged. Certain source
areas, like East Vietnam, present a different provenance signature than
that of today. Multiple provenances have been mixed and diluted during
sediment transport, exhibiting a large regional variability. We interpret
that siliciclastic turbidite deposits in the central South China Sea were
mainly derived from East Vietnam during the early Late Miocene and
Pliocene, and the Pearl River Basin during the late Late Miocene and
Pleistocene. Additional, but less significant, contributions from the Red
and Mekong river basins and coastal Southeast China are also observed.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35845.1/594152/Fingerprinting-sand-from-Asian-rivers-to-the-deep
Metamorphic P-T-t evolution of amphibolite in the north
Hengshan terrane, North China Craton: Insights into the late
Paleoproterozoic tectonic processes from initial collision to final
exhumation
Jiahui Qian; Changqing Yin; Shun Li; Jian Zhang
Abstract: Amphibolite retrograded from high-pressure (HP) mafic granulite
can provide valuable insight into exhumation of deeply buried crust in
orogenic belts. In the north Hengshan terrane of the North China Craton,
amphibolite occurs as rims of HP mafic granulite block or as smaller
homogeneous boudins representing retrograde products of the granulite.
Three amphibolite samples were selected. The rocks are mainly composed of
hornblende + plagioclase + quartz + biotite + ilmenite with or without
garnet pseudomorph consisting of plagioclase + hornblende + ilmenite and
symplectite of hornblende + plagioclase ± clinopyroxene. The pseudomorph-,
symplectite-bearing sample experienced a post-peak isothermal decompression
at >800 °C that was accompanied by breakdown of garnet and
clinopyroxene. Isopleths of the maximum An in plagioclase and Ti in
hornblende were used to constrain the Tmax stage of ca. 6
kbar/825−850 °C, which was followed by cooling and post-cooling
decompression. For the sample showing an “equilibrated” mineral assemblage,
a medium-temperature decompression from >6.8 kbar/685 °C to 3.6−4.8
kbar/640−660 °C was inferred. P-T evolution of the north Hengshan terrane
is characterized by two discrete (post-peak and post-cooling) decompression
processes. Zircon U-Pb dating of amphibolite yields a metamorphic age of
1868 ± 15 Ma, which is interpreted to record the timing of late
amphibolization. Synthesized metamorphic P-T-t data in the Hengshan-Wutai
area indicate a complicated tectonic evolution that includes an older
collisional orogeny at ca. 1.95 Ga and a younger metamorphism at ca. 1.85
Ga. The post-cooling decompression path of the amphibolite may reflect
final exhumation of the north Hengshan terrane through the late
deformation/metamorphism of the Zhujiafang shear zone.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35810.1/594134/Metamorphic-P-T-t-evolution-of-amphibolite-in-the
Protracted northward drifting of South China during the assembly of
Gondwana: Constraints from the spatial-temporal provenance
comparison of Neoproterozoic−Cambrian strata
Qiong Chen; Guochun Zhao; Min Sun
Abstract: Neoproterozoic to Paleozoic sedimentation shows systematic
temporal-spatial variations within South China, which must be considered in
reconstructing geological evolution of South China in response to global
plate reorganization from the breakup of Rodinia to the assembly of
Gondwana. We use >1000 new U-Pb and Hf isotopic data for detrital
zircons from Neoproterozoic−Cambrian strata across the western (i.e.,
Longmenshan) and eastern (i.e., Wuyishan) margins of South China, coupled
with compiled stratigraphic and magmatic information, to constrain change
in provenance through time. First-order conclusions are as follows: (1)
detrital zircons from the Neoproterozoic strata of the two margins were
mainly sourced from the Panxi-Hannan arc and the Jiangnan orogen, signaling
a rough self-sufficient sedimentary system; (2) newly identified Cambrian
molasse-like sediments in the western margin, in which abundant detrital
zircons are 550−500 Ma old with positive εHf(t) values, were
mainly derived from the 580−500 Ma Cadomian arc belt along the Iran-Turkey
margin; and (3) the Cambrian sediments in the eastern margin document more
increased contributions from the Grenvillian-age provinces most possibly in
Australia. Such spatial-temporal provenance variations signal the northward
drifting of South China, from a position connecting with Iran-Turkey and
northern India to that approaching Australia during the late
Neoproterozoic−Cambrian period. We highlight that the activity of oblique
oceanic-continental convergence accreted Asian terranes onto the northern
margin of Gondwana, hence contributing to the ultimate Gondwana
architecture under global plate reorganization.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35791.1/594114/Protracted-northward-drifting-of-South-China
The convexity of carbonate hilltops: 36Cl constraints on denudation
and chemical weathering rates and implications for hillslope
curvature
Matan Ben-Asher; Itai Haviv; Onn Crouvi; Joshua J. Roering; Ari Matmon
Abstract: Carbonate hillslopes are often soil mantled and display a classic
convex morphology. In this study we examine controls on carbonate hillslope
denudation and morphology using a modified regolith mass balance equation
to account for chemical weathering and dust input—two fluxes that are
commonly neglected in settings with silicate-dominated bedrock. We utilize
seven study sites in the Eastern Mediterranean across a significant
gradient in the mean annual rainfall and dust deposition flux. Combining
cosmogenic 36Cl-derived hilltop denudation rates with an estimate of the
regolith chemical depletion and the quantified fraction of dust in the
regolith we predict hilltop curvature and compare our predictions with
observations based on high-resolution airborne LiDAR (light detection and
ranging). Denudation rates vary from 5 to 210 mm/k.y. and increase with
mean annual rainfall. Less resistant carbonates (chalk) experience faster
denudation rates relative to more resistant dolo-limestone and are less
prone to chemical weathering. Soil production exhibits a humped dependency
on soil thickness. The observed hilltop curvature varies as a function of
rainfall and dust flux with a minimum at sub-humid sites. While trends in
hilltop convexity are often solely attributed to variations in erosion
rate, our results illustrate the additional effects of dust production and
chemical depletion. Our mass balance model implies that drier sites in the
south probably experienced a more intricate history of regolith production
due to dust flux fluctuations. Thus, by incorporating dust flux and
chemical weathering to the classic hillslope evolution model we are able to
identify a complex relation between hilltop curvature, soil production, and
climate.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35658.1/594041/The-convexity-of-carbonate-hilltops-36Cl
Surface slip distributions and geometric complexity of intraplate
reverse-faulting earthquakes
Haibin Yang; Mark Quigley; Tamarah King
Abstract: Earthquake ground surface ruptures provide insights into faulting
mechanics and inform seismic hazard analyses. We analyze surface ruptures
for 11 historical (1968−2018) moment magnitude (Mw)
4.7−6.6 reverse earthquakes in Australia using statistical techniques and
compare their characteristics with magnetic, gravity, and stress trajectory
data sets. Of the total combined (summative) length of all surface ruptures
(∼148 km), 133 km (90%) to 145 km (98%) align with the geophysical
structure in the host basement rocks. Surface rupture length (SRL
), maximum displacement (MD), and probability of surface rupture
at a specified Mw are high compared with equivalent Mw earthquakes globally. This is attributed to (1) a
steep cratonic crustal strength gradient at shallow depths, promoting
shallow hypocenters (∼1−6 km) and limiting downdip rupture widths (∼1−8.5
km), and (2) favorably aligned crustal anisotropies (e.g., bedrock
foliations, faults, fault intersections) that enhanced lateral rupture
propagation and/or surface displacements. Combined (modeled and observed) MDs are in the middle third of the SRL with 68%
probability and either the ≤33rd or ≥66th percentiles of SRL with
16% probability. MD occurrs proximate to or directly within zones
of enhanced fault geometric complexity (as evidenced from surface ruptures)
in 8 of 11 earthquakes (73%). MD is approximated by 3.3 ± 1.6 (1σ)
× AD (average displacement). S-transform analyses indicates that
high-frequency slip maxima also coincide with fault geometric complexities,
consistent with stress amplifications and enhanced slip variability due to
geometric and kinematic interactions with neighboring faults. Rupture slip
taper angles exhibite large variations (−90% to +380% with respect to the
mean value) toward rupture termini and are steepest where ruptures
terminate at obliquely oriented magnetic lineaments and/or lithology
changes. Incremental slip approximates AD between the 10th and
90th percentiles of the SRL. The average static stress drop of the
studied earthquakes is 4.8 ± 2.8 MPa. A surface rupture classification
scheme for cratonic stable regions is presented to describe the prevailing
characteristics of intraplate earthquakes across diverse crustal
structural-geophysical settings. New scaling relationships and suggestions
for logic tree weights are provided to enhance probabilistic fault
displacement hazard analyses for bedrock-dominated intraplate continental
regions.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35809.1/594001/Surface-slip-distributions-and-geometric
Eroding Cascadia—Sediment and solute transport and landscape
denudation in western Oregon and northwestern California
Jim E. O’Connor; Joseph F. Mangano; Daniel R. Wise; Joshua R. Roering
Abstract: Riverine measurements of sediment and solute transport give
empirical basin-scale estimates of bed-load, suspended-sediment, and
silicate-solute fluxes for 100,000 km2 of northwestern
California and western Oregon. This spatially explicit sediment budget
shows the multifaceted control of geology and physiography on the rates and
processes of fluvial denudation. Bed-load transport is greatest for steep
basins, particularly in areas underlain by the accreted Klamath terrane.
Bed-load flux commonly decreases downstream as clasts convert to suspended
load by breakage and attrition, particularly for softer rock types.
Suspended load correlates strongly with lithology, basin slope,
precipitation, and wildfire disturbance. It is highest in steep regions of
soft rocks, and our estimates suggest that much of the suspended load is
derived from bed-load comminution. Dissolution, measured by basin-scale
silicate-solute yield, constitutes a third of regional landscape
denudation. Solute yield correlates with precipitation and is
proportionally greatest in low-gradient and wet basins and for high parts
of the Cascade Range, where undissected Quaternary volcanic rocks soak in
2−3 m of annual precipitation. Combined, these estimates provide
basin-scale erosion rates ranging from ∼50 t ∙ km−2 ∙ yr −1 (approximately equivalent to 0.02 mm ∙ yr−1) for
low-gradient basins such as the Willamette River to ∼500 t ∙ km −2 ∙ yr−1 (∼0.2 mm ∙ yr−1) for steep
coastal drainages. The denudation rates determined here from modern
measurements are less than those estimated by longer-term geologic
assessments, suggesting episodic disturbances such as fire, flood, seismic
shaking, and climate change significantly add to long-term landscape
denudation.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35710.1/593893/Eroding-Cascadia-Sediment-and-solute-transport-and
Hydrothermal formation of iron-oxyhydroxide chimney mounds in a
shallow semi-enclosed bay at Satsuma Iwo-Jima Island, Kagoshima,
Japan
Shoichi Kiyokawa; Takashi Kuratomi; Tatsuhiko Hoshino; Shusaku Goto; Minoru
Ikehara
Abstract: Hydrothermal iron-oxyhydroxide chimney mounds (iron mounds) have
been discovered in a fishing port in Nagahama Bay, located on the southwest
coast of Satsuma Iwo-Jima Island, south of Kyushu Island, Japan. In the
fishing port, uncovered ∼1.0-m-high iron mounds in shallow waters formed
under relatively calm conditions. Typically, the fishing port has
orange-colored turbid waters that mix with outer ocean waters during high
tide. Colloidal iron-oxyhydroxides form due to the oxidation of ferrous
iron in hydrothermal waters (pH = 5.5; temperature = 55 °C) as they mix
with seawater. The mounds are made of two types of material: hard, dark
brown−orange, high-density material; and soft, brownish orange−yellow,
low-density material. Computed tomography scans of the harder iron mound
material revealed a cabbage-like structure consisting of micropipe
structures with diameters of 2−5 mm. These micropipes have relatively hard
walls made of iron oxyhydroxides (FeOH) and are identified as discharge
pipes. Nucleic acid staining genetic sequencing and scanning electron
microscope observations suggest that the mounds formed mainly from
bacterial stalks with high concentrations of FeOH colloidal matter. In the
harder parts of the mounds, these “fat stalks,” which contain oxyhydroxide
colloidal aggregates, are entwined and concentrated. The softer material
contains twisted stalk-like structures, which are coated with FeOH
colloidal particles. Deoxyribonucleic acid (DNA) examination of the iron
mounds revealed the presence of iron-oxidizing bacteria, especially at the
mound surface. We estimate that the iron mounds accumulated at a rate of
∼1700 tons/1000 m2/yr. This is an order of magnitude higher than the rate
of FeOH sedimentation via chemical precipitation of FeOH colloids within
the fishing port. This suggests that biogenic activity, resulting in the
production of entwined FeOH stalks, leads to the rapid accumulation of FeOH
beds and that biogenic activity within the water mass rich in FeOH colloids
is an efficient means of generating thick iron-rich sedimentary sequences.
As such, we propose that some ancient iron formations may have also formed
through the biogenic production of FeOH stalks rather than solely through
chemical sedimentation in a water mass rich in FeOH colloids. It appears
that these rapidly forming biogenic FeOH iron mounds, distributed over a
wide area of ocean floor, are also relatively protected from erosion and
diagenetic alteration (reduction). Previous studies have reported that
ancient iron formations were commonly deposited in deeper environments via
direct iron oxidation from the water column in a ferruginous ocean.
However, there are several hydrothermal vent inflows preserved with FeOH
that would have formed appropriate redox boundary conditions in an
otherwise anoxic ocean. Under these conditions, iron mound mat-type
sedimentary deposits might have formed and been well preserved and affected
by early diagenesis where higher heat flow occurred in the Archean ocean.
The FeOH mounds in Nagahama Bay provide an example of the iron formation
sedimentary environment and important information for estimating the past
depositional state of iron formations.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35782.1/593894/Hydrothermal-formation-of-iron-oxyhydroxide
Textural and geochemical window into the IDDP-1 rhyolitic melt,
Krafla, Iceland, and its reaction to drilling
E. Saubin; B. Kennedy; H. Tuffen; A.R.L. Nichols; M. Villeneuve ...
Abstract: The unexpected intersection of rhyolitic magma and retrieval of
quenched glass particles at the Iceland Deep Drilling Project-1 geothermal
well in 2009 at Krafla, Iceland, provide unprecedented opportunities to
characterize the genesis, storage, and behavior of subsurface silicic
magma. In this study, we analyzed the complete time series of glass
particles retrieved after magma was intersected, in terms of distribution,
chemistry, and vesicle textures. Detailed analysis of the particles
revealed them to represent bimodal rhyolitic magma compositions and
textures. Early-retrieved clear vesicular glass has higher SiO2, crystal,
and vesicle contents than later-retrieved dense brown glass. The vesicle
size and distribution of the brown glass also reveal several vesicle
populations. The glass particles vary in δD from −120‰ to −80‰ and have
dissolved water contents spanning 1.3−2 wt%, although the majority of glass
particles exhibit a narrower range. Vesicular textures indicate that
volatile overpressure release predominantly occurred prior to late-stage
magma ascent, and we infer that vesiculation occurred in response to
drilling-induced decompression. The textures and chemistry of the rhyolitic
glasses are consistent with variable partial melting of host felsite. The
drilling recovery sequence indicates that the clear magma (lower degree
partial melt) overlays the brown magma (higher degree partial melt). The
isotopes and water species support high temperature hydration of these
partial melts by a mixed meteoric and magmatic composition fluid. The
textural evidence for partial melting and lack of crystallization imply
that magma production is ongoing, and the growing magma body thus has a
high potential for geothermal energy extraction. In summary, transfer of
heat and fluids into felsite triggered variable degrees of felsite partial
melting and produced a hydrated rhyolite magma with chemical and textural
heterogeneities that were then enhanced by drilling perturbations. Such
partial melting could occur extensively in the crust above magma chambers,
where complex intrusive systems can form and supply the heat and fluids
required to re-melt the host rock. Our findings emphasize the need for
higher resolution geophysical monitoring of restless calderas both for
hazard assessment and geothermal prospecting. We also provide insight into
how shallow silicic magma reacts to drilling, which could be key to future
exploration of the use of magma bodies in geothermal energy.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35598.1/593788/Textural-and-geochemical-window-into-the-IDDP-1
Intracontinental extension and geodynamic evolution of the
Paleoproterozoic Jiao-Liao-Ji belt, North China craton: Insights
from coeval A-type granitic and mafic magmatism in eastern Liaoning
Province
Jian-Hui Liu; Xiang-Jian Wang; Hui Chen
Abstract: To better understand the origin and tectonic evolution of the
Paleoproterozoic Jiao-Liao-Ji belt, North China craton, which have long
been debated, we have conducted whole-rock geochemical and zircon U-Pb and
Hf isotope analyses on Paleoproterozoic monzo(syeno)granitic gneisses and
metamafic rocks from eastern Liaoning Province. The results indicate that
these monzo(syeno)granitic gneisses and metamafic rocks formed concurrently
at ca. 2.2−2.1 Ga. The monzo(syeno)granitic gneisses show geochemical
features of A-type granites and were derived from dehydration melting of
Archean tonalite-trondhjemite-granodiorite (TTG) with minor involvement of
coeval mantle-derived magma in an extensional setting. The metamafic rocks
are geochemically similar to mid-ocean-ridge basalt; they were generated by
partial melting of isotope-depleted asthenospheric mantle in an
intracontinental extensional setting and experienced crustal and
continental lithospheric mantle contamination and fractional
crystallization. Based on this study and published geological evidence, we
propose the following tectonic model for the origin and geodynamic
evolution of the Jiao-Liao-Ji belt: (1) Generation of high-density
eclogitized lower crust accompanied by formation of voluminous Archean TTGs
in the Eastern block; (2) intracontinental extension driven by upwelling of
an asthenospheric hotspot along the Jiao-Liao-Ji belt in period of 2.20−2.0
Ga; (3) convergent orogenic processes between the Longgang block and
Nangrim block governed by gravity-driven subduction of high-density
eclogitized lower crust at ca. 1.95−1.85 Ga; and (4) postorogenic extension
caused by delamination of the orogenic root and subsequent hot
asthenospheric upwelling. This tectonic model can well explain the origin
and geodynamic evolution of the Paleoproterozoic Jiao-Liao-Ji belt.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35819.1/593789/Intracontinental-extension-and-geodynamic
High-Ca boninitic melt inclusions in lavas of the Troodos ophiolite
and a reappraisal of genetic relationships between different lava
types
Wen-Jun Hu; Mei-Fu Zhou; John Malpas; Zhong-Yuan Ren
Abstract: Boninites in many supra-subduction zone ophiolites are thought to
mark the early stage of subduction. The Troodos ophiolite in Cyprus
contains a variety of volcanic rocks including the lower pillow lavas and
the boninite-bearing upper pillow lavas (UPL). Here we present major, trace
element and Pb isotope data for melt inclusions in olivine from the UPL on
the northern flank of the Troodos ophiolite. Melt inclusions hosted by
high-Mg (Fo = 89.4−93.5) olivine grains have typical high-Ca boninitic
compositions with high MgO (11.2−18.4 wt%) and SiO2 (51.2−55.0
wt%) and low TiO2 (0.2−0.6 wt%) contents, in line with the
existence of boninites in the northern part of the ophiolite. The melt
inclusions have trace element compositions similar to the Troodos boninitic
lavas and can be regarded as representing the parental magma of the
boninites. The boninitic magmas were derived from high-degrees of partial
melting of a refractory source at 1400 °C and 1.5 GPa based on
thermobarometers and phase equilibria. The melt inclusions have
significantly variable Pb isotopic compositions (208Pb/ 206Pb = 2.059−2.122; 207Pb/206Pb =
0.828−0.862) and are enriched in fluid-mobile trace elements (e.g., U, Sr,
and Pb), indicating that the refractory source was metasomatized by slab
fluids. In comparison, recalculation of literature data suggests that the
lower pillow lavas were derived from a less refractory and shallower
source. We propose a subduction initiation model in which melting of two
unique sources was needed to form the chemostratigraphy of lavas in the
Troodos ophiolite.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35717.1/593793/High-Ca-boninitic-melt-inclusions-in-lavas-of-the
Structural geometry and kinematics of thrust belts between the
Dabashan and eastern Sichuan Basin, South China block: Constraints
from (U-Th)/He dating and seismic data
Guo Lu; Chuanxin Li; Wenzheng Li; Shang Deng; Jianyong Zhang
Abstract: The relationship between the North and South China blocks,
particularly their spatial-temporal framework and evolutionary history, has
been widely debated. We conducted a comprehensive study of the structural
geometry and kinematics of the thrust belts in the intersection zone
between the Dabashan and eastern Sichuan Basin based on seismic data,
drilling data, field investigation data, and zircon and apatite (U-Th)/He
thermochronology data. These data sets provide important insights into the
relationship between the North and South China blocks during the
post-orogenic process. Due to the detachments, the strata of the eastern
Sichuan Basin can be divided into four structural intervals characterized
by duplex structures and fault-related fold structures. Balanced
restoration indicates the vertical differences are represented by the
shortening ratio of the upper structural interval of ∼6%, that of the
middle ∼10%, and that of the lower ∼7%. Besides, the shortening ratios
indicate an increasing tendency of the deformation intensity from the west
to the east. The (U-Th)/He dating results suggest two age ranges, i.e.,
from 180 Ma to 130 Ma and from 50 Ma to 20 Ma, respectively. These age data
sets also imply a younger tendency westward. The intersection zone may have
experienced the following stages since the late Mesozoic era: (1) the
stable continental sedimentation stage from the Late Triassic to the Late
Jurassic epochs; (2) the continuous thrust stage from the Late Jurassic to
the early Paleogene epochs, which is associated with the subduction of the
Paleo-Pacific Ocean; and (3) the uplift and denudation stage as a whole
caused by the uplift of the Tibetan Plateau since the Paleogene period.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35781.1/593776/Structural-geometry-and-kinematics-of-thrust-belts
Timing of closure of the Meso-Tethys Ocean: Constraints from
remnants of a 141−135 Ma ocean island within the Bangong−Nujiang
Suture Zone, Tibetan Plateau
Jian-Jun Fan; Yaoling Niu; Yi-Ming Liu; Yu-Jie Hao
Abstract: The relationship between the North and South China blocks,
particularly their spatial-temporal framework and evolutionary history, has
been widely debated. We conducted a comprehensive study of the structural
geometry and kinematics of the thrust belts in the intersection zone
between the Dabashan and eastern Sichuan Basin based on seismic data,
drilling data, field investigation data, and zircon and apatite (U-Th)/He
thermochronology data. These data sets provide important insights into the
relationship between the North and South China blocks during the
post-orogenic process. Due to the detachments, the strata of the eastern
Sichuan Basin can be divided into four structural intervals characterized
by duplex structures and fault-related fold structures. Balanced
restoration indicates the vertical differences are represented by the
shortening ratio of the upper structural interval of ∼6%, that of the
middle ∼10%, and that of the lower ∼7%. Besides, the shortening ratios
indicate an increasing tendency of the deformation intensity from the west
to the east. The (U-Th)/He dating results suggest two age ranges, i.e.,
from 180 Ma to 130 Ma and from 50 Ma to 20 Ma, respectively. These age data
sets also imply a younger tendency westward. The intersection zone may have
experienced the following stages since the late Mesozoic era: (1) the
stable continental sedimentation stage from the Late Triassic to the Late
Jurassic epochs; (2) the continuous thrust stage from the Late Jurassic to
the early Paleogene epochs, which is associated with the subduction of the
Paleo-Pacific Ocean; and (3) the uplift and denudation stage as a whole
caused by the uplift of the Tibetan Plateau since the Paleogene period.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35781.1/593776/Structural-geometry-and-kinematics-of-thrust-belts
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