New GSA Bulletin Articles Published Ahead of Print in August
Boulder, Colo., USA: The Geological Society of America regularly publishes
articles online ahead of print. GSA Bulletin topics include the
Dead Sea transform system; the Tethyan Himalaya, southern Tibet; the
Surprise Canyon Formation; the Nacimiento Formation; and the Trachyandesite
of Kennedy Table. You can find these articles at
https://bulletin.geoscienceworld.org/content/early/recent
.
Long-term (7 Ma) strain fluctuations within the Dead Sea transform
system from high-resolution U-Pb dating of a calcite vein
John P. Craddock; Perach Nuriel; Andrew R.C. Kylander-Clark; Bradley R.
Hacker; John Luczaj ...
Abstract:
The onset of the Dead Sea transform has recently been reevaluated by U-Pb
age-strain analyses of fault-related calcite taken from several fault
strands along its main 500-km-long sector. The results suggest that the
relative motion between Africa and Arabia north of the Red Sea was
transferred northward to the Dead Sea transform as early as 20 Ma and along
a ∼10-km-wide deformation zone that formed the central rift with
contemporaneous bounding sinistral motion. The Gishron fault is the western
bounding fault with normal and sinistral fault offsets that placed
Proterozoic crystalline rocks and a cover of Cambrian sandstones in fault
contact with Cretaceous-Eocene carbonates. Fault-related calcite veins are
common in the Gishron fault zone, and we report the results of a detailed
study of one sample with nine calcite fillings. Low fluid inclusion
entrapment temperatures <50 °C, stable isotopes values of −3.3−0‰ (δ 13C) and −15 to −13‰ (δ18O), and low rare earth
element (REE) concentrations within the nine calcite fault fillings
indicate that a local, meteoric fluid fed the Gishron fault zone over ca. 7
Ma at depths of <2 km. Laser ablation U-Pb ages within the thin section
range from 20.37 Ma to 12.89 Ma and allow a detailed fault-filling
chronology with the oldest calcite filling in the middle, younging outward
with shearing between the oldest eight zones, all of which are finally
crosscut by a perpendicular (E-W) vein. All nine calcite fillings have
unique mechanical twinning strain results (n = 303 grains). Shortening
strain magnitudes (−0.28% to −2.8%) and differential stresses (−339 bars to
−415 bars) vary across the sample, as do the orientations of the shortening
(ε1) and extension (ε3) axes with no evidence of any
twinning strain overprint (low negative expected values). Overall, the
tectonic compression and shortening is sub-horizontal and sub-parallel to
the Gishron fault (∼N-S) and Dead Sea transform plate boundary. Most
strikingly, the 7 m.y. period of vein growth correlates exactly with the
timing of fault activity as evident within the 10-km-wide deformation zone
in this evolving plate boundary (between 20 Ma and 13 Ma).
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36000.1/606863/Long-term-7-Ma-strain-fluctuations-within-the-Dead
Eocene thickening without extra heat in a collisional orogenic belt: A
record from Eocene metamorphism in mafic dike swarms within the Tethyan
Himalaya, southern Tibet
Yuhua Wang; Lingsen Zeng; Li-E Gao; Zhenyu Chen; Sanzhong Li
Abstract:
Knowledge of the nature of the earliest metamorphism experienced by
collisional orogenic belts is essential for reconstruction of tectonic
processes that build high mountain chains and their environmental
consequences. Understanding the metamorphic nature of Eohimalayan-phase
orogeny of the Himalayan orogen, one of the typical examples of orogenic
belts worldwide, could provide some important constraints to test different
tectonic models (shallow continental subduction vs. slab breakoff) for the
early phases of the development of large-scale orogenic belts. As exhumed
middle- to lower-crustal rocks in the Kangmar gneiss dome, the garnet
amphibolites with a protolith age of 176.4 ± 3.6 Ma experienced a phase of
metamorphism at 47.2 ± 1.8 Ma with an increase in pressure as well as
temperature from 3−5 kbar and 550−600 °C to over ∼11 kbar and 650 °C. This
suggests that the middle- to lower-crustal rocks experienced heating at
least by ∼50 °C while they underwent compression and thickening. Heat-flow
estimation further demonstrates that the self-produced heat was high enough
to achieve the observed pressure-temperature conditions recorded by the
garnet amphibolite. Therefore, an additional heat supply is not required
during early Eocene metamorphism. A breakoff of the leading part of the
subducting Indian continental slab, if it occurred, should be younger than
ca. 47 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35679.1/606653/Eocene-thickening-without-extra-heat-in-a
Geodynamic significance of a buried transient Carboniferous landscape
Fergus McNab; Nicky White
Abstract:
It is increasingly clear that present-day dynamic topography on Earth,
which is generated and maintained by mantle convective processes, varies on
timescales and length scales on the order of 1−10 m.y. and 103
km, respectively. A significant implication of this behavior is that
Phanerozoic stratigraphic records should contain indirect evidence of these
processes. Here, we describe and analyze a well-exposed example of an
ancient landscape from the Grand Canyon region of western North America
that appears to preserve a transient response to mantle processes. The
Surprise Canyon Formation lies close to the Mississippian-Pennsylvanian
boundary and crops out as a series of discontinuous lenses and patches that
are interpreted as remnants of a westward-draining network of paleovalleys
and paleochannels within a coastal embayment. This drainage network is
incised into the marine Redwall Limestone whose irregular and karstified
upper surface contains many caves and collapse structures. The Surprise
Canyon Formation itself consists of coarse imbricated conglomerates,
terrestrial plant impressions including Lepidodendron, and marine
invertebrate fossils. It is overlain by marine, fluvial, and aeolian
deposits of the Supai Group. These stratal relationships are indicative of
a transient base-level fall whose amplitude and regional extent are
recognized as being inconsistent with glacio-eustatic sea-level variation.
We propose that this transient event is caused by emplacement and decay of
a temperature anomaly within an asthenospheric channel located beneath the
lithospheric plate. An analytical model is developed that accounts for the
average regional uplift associated with landscape development and its rapid
tectonic subsidence. This model suggests that emplacement and decay of a
∼50 °C temperature anomaly within a channel that is 150 ± 50 km thick can
account for the observed vertical displacements. Our results are
corroborated by detrital zircon studies that support wholesale drainage
reorganization at this time and by stratigraphic evidence for spatially
variable regional epeirogeny. They are also consistent with an emerging
understanding of the temporal and spatial evolution of the
lithosphere-asthenosphere boundary.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36002.1/606654/Geodynamic-significance-of-a-buried-transient
A model involving amphibolite lower crust melting and subsequent melt
extraction for leucogranite generation
Liqiang Wang; Wenbin Cheng; Teng Gao; Yong Wang
Abstract:
In the southern Tibetan Plateau, leucogranites are dominantly distributed
in the Himalayan orogenic belt with minor occurrences in the southern Lhasa
subterrane. In this paper, we report the first Miocene Anglonggangri
leucogranites in the northern Lhasa subterrane. This finding provides
important constraints on both leucogranite petrogenesis and the
tectono-magmatic evolution of the Lhasa terrane. The Anglonggangri
leucogranites include biotite-muscovite granite and slightly younger
garnet-muscovite granite and pegmatite. Zircon U-Pb and muscovite 40Ar-39Ar dating of these leucogranites yields
Miocene ages of 11.1−10.2 Ma. The biotite-muscovite and garnet-muscovite
granites are characterized by high SiO2 (72.3−74.4 wt%) and Al 2O3 contents (14.4−15.4 wt%) and are peraluminous.
The biotite-muscovite granite displays geochemical signatures with high
Sr/Y (29.2−81.0) and (La/Yb)N (37.5−98.9) ratios, low Y
(4.30−7.22 ppm) and Yb contents (0.26−0.47 ppm), low to moderate initial ( 87Sr/86Sr)i ratios (0.7085−0.7192), and
moderate εNd(t) values (−10.17 to −6.94). Furthermore, they also
exhibit radiogenic Pb isotope and variable zircon εHf(t) values
(−9.6 to +4.4) with Proterozoic Nd (1.1−1.4 Ga) and Hf model ages (0.8−1.7
Ga). By comparison, the garnet-muscovite granite has lower CaO, MgO, TiO 2, and total FeO contents and is enriched in Rb (380−466 ppm)
and depleted in Sr (24.1−38.5 ppm) and Ba (30.7−58.6 ppm) and further
characterized by a significant rare earth element (REE) tetrad effect and
non-charge and radius-controlled (CHARAC) trace element behaviors. The
garnet-muscovite granite shows a negative Eu anomaly and positive
correlations among Sr and Eu, Sr and Ba, and Th and light rare earth
elements (LREEs). Pegmatite comprising Nb-Ta oxides and cassiterite occurs
in the garnet-muscovite granite. Geochronological and geochemical
characteristics of the Anglonggangri leucogranites indicate that the magma
of the biotite-muscovite granite was derived from partial melting of
amphibolite lower crust contaminated with Proterozoic-Archean upper crustal
materials. The garnet-muscovite granite was generated through melt
extraction from the biotite-muscovite granite crystal mush. These results
confirm that partial melting of the amphibolite lower crust not only
occurred in the southern and central Lhasa subterranes but also in the
northern Lhasa subterrane.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36055.1/606597/A-model-involving-amphibolite-lower-crust-melting
Nonclimatic and extrabasinal processes controlled pedogenesis in
paleosols of the Paleocene Nacimiento Formation, New Mexico, USA
Kevin M. Hobbs; Peter J. Fawcett
Abstract:
Paleosols can represent intervals of nondeposition in sedimentary packages
and are used with increasing frequency as proxies for paleoenvironmental
conditions during basin filling. However, the complexities of factors both
internal and external to pedogenesis require consideration of paleosols in
any basin or sedimentary package in a broader context than is often
assumed. With this in mind, we measured and analyzed pedogenic features,
stratigraphic position, geochemical composition, and petrography of
paleosols in the Nacimiento Formation to gain insight into the
paleoenvironmental conditions of the early Paleocene in the San Juan Basin.
During this interval, the San Juan Basin was located in middle northern
latitudes (∼40°N) and saw rapid terrestrial siliciclastic sedimentation
related to Laramide tectonism. Evidence from earlier researchers suggests
that prevailing climate conditions during Nacimiento Formation deposition
were warm and humid. We used morphological properties of paleosols to
categorize paleosols into pedotypes indicative of distinct pedogenic
conditions. The general stratigraphic distribution of these pedotypes shows
an up-section increase in soil drainage conditions through the Nacimiento
Formation that cannot be correlated with known climate changes. We suggest
that fluvial system evolution was the major control on pedogenic
conditions. We investigated Nacimiento Formation paleosols with widely used
paleosol geochemical climate analyses, which provided paleoclimate
estimates that are in disaccord with independently derived estimates. We
show that in alluvial depositional systems with source areas in weathered
sedimentary rocks, these analyses can be difficult to interpret and likely
lead to estimates that do not reflect true pedogenic conditions during the
postdepositional near-surface alteration of sediments. Petrographic
analysis of Nacimiento Formation paleosols showed that some likely formed
under semiarid to subhumid conditions that allowed pedogenic accumulation
and illuviation of smectite clays yet did not substantially chemically
alter primary detrital plagioclase feldspar grains in paleosol B horizons.
The paleosols of the Nacimiento Formation, when analyzed at the basin
scale, show that sedimentary aggradational processes can overpower climate
processes in creation of a sequence of paleosols in a stratigraphic
section. In addition, the incorporation of clays from sedimentary rocks in
a basin’s source area can drastically skew the geochemical signatures and
therefore interpretations of paleosols in that basin.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B35803.1/606537/Nonclimatic-and-extrabasinal-processes-controlled
Trachyandesite of Kennedy Table, its vent complex, and post−9.3 Ma
uplift of the central Sierra Nevada
Wes Hildreth; Judy Fierstein; Fred M. Phillips; Andy Calvert
Abstract:
Tectonic interpretation of the central Sierra Nevada—whether the crest of
the Sierra Nevada (California, USA) was uplifted in the late Cenozoic or
whether the range has undergone continuous down-wearing since the Late
Cretaceous—is controversial, since there is no obvious tectonic explanation
for renewed uplift. The strongest direct evidence for late Cenozoic uplift
of the central Sierra Nevada comes from study of the Trachyandesite of
Kennedy Table, which followed the course of the Miocene San Joaquin River
but has a steeper gradient than the modern river. Early workers attributed
this steeper gradient to tilting of the Sierra Nevada block since the late
Miocene, resulting in 2 km of range-crest uplift. However, this
interpretation has been contested on grounds that the Miocene river
gradient had to be assumed and that the Sierran Batholith could have warped
during tilting, thus failing to uplift the range crest. The objective of
this study was to obtain quantitative data that test these criticisms. The Trachyandesite of Kennedy Table is a chain of 33
remnants of a single lava flow as thick as 65 m, preserved for 21 km from
Squaw Leap to Little Dry Creek, close to the modern San Joaquin River in
the foothills of the Sierra Nevada. Several remnants lie on fluvial gravel
of the late Miocene San Joaquin River. Early workers speculated that the
lava concealed its own (unrecognized) vent, but in 2011, we identified the
vent on the Middle Fork of the San Joaquin River, 13.5 km south of Deadman
Pass and 70 km northeast of Kennedy Table. The vent complex intrudes
Cretaceous granite, has 285 m relief, and is an intricately jointed
intrusion that grades up into a glassy lava flow. Composition (58% SiO 2) and 40Ar/39Ar age (9.3 Ma) are
identical at the vent and downstream. Basal elevations of remnants were
recorded, and the present-day basal gradients of several were adjusted for
apparent dip and projected along a vertical plane at 220° (the estimated
tilt azimuth). The basal gradients are far steeper than that of the modern
river, but they differ slightly from reach to reach and are thus
inconsistent measures of the post-Miocene tilt. Likewise, relief eroded
atop most remnants renders modeling of upper surfaces suspect. At Little
Dry Creek, however, a chain of nine remnants rests on fluvial floodplain
sand and gravel; this chain trends 230°, and its smooth basal contact now
dips 1.36° (adjusted at 220°). Projection of this dip 89 km from the 207 m
base of the most distal remnant at Little Dry Creek to the vent intrusion
falls far below the 2760 m intrusion-to-lava-flow transition near the
Sierran crest, showing that the Sierran block has not undergone pronounced
convex warping. Using elevation data on paleoriver meanders preserved by
the lava flow, we show that the paleogradient has a cosine dependence on
meander-section azimuth, indicating tilting. Subtraction of 1.07° of dip
restores the data to an azimuth-independent configuration, indicating total
tilting since 9.3 Ma of 1.07° and an original large-scale gradient of
0.46°, similar to the published value of 0.33° at Squaw Leap, but larger
than the previously obtained value of 0.057° at Little Dry Creek.
Subtraction of those Miocene estimates from the observable 1.643° tilt
along the section from Little Dry Creek to the vent yields vent uplift of
2464 m (for 0.057°), 1835 m (for 0.46°), and 2040 m (for 0.33°).
Confirmation of earlier assumptions regarding Miocene river gradient and
block rigidity greatly strengthens the case for ∼2 km of late Cenozoic
uplift of the central Sierra Nevada crest.
View article:
https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36125.1/606499/Trachyandesite-of-Kennedy-Table-its-vent-complex
GSA BULLETIN articles published ahead of print are online at
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