New Articles for Geosphere Posted Online in March
Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere,
posts articles online regularly. Topics include three geologic maps;
insights into the geometry and evolution of the southern San Andreas fault;
and bolide impact effects on the West Florida Platform, Gulf of Mexico. You
can find these articles at
https://geosphere.geoscienceworld.org/content/early/recent
.
Constraints on the timescales and processes that led to high-SiO2
rhyolite production in the Searchlight pluton, Nevada, USA
Michael P. Eddy; Ayla Pamukçu; Blair Schoene; Travis Steiner-Leach;
Elizabeth A. Bell
Abstract:
Plutons offer an opportunity to study the extended history of magmas at
depth. Fully exploiting this record requires the ability to track changes
in magmatic plumbing systems as magma intrudes, crystallizes, and/or mixes
through time. This task has been difficult in granitoid plutons because of
low sampling density, poorly preserved or cryptic intrusive relationships,
and the difficulty of identifying plutonic volumes that record the
contemporaneous presence of melt. In particular, the difficulty in
delineating fossil magma reservoirs has limited our ability to directly
test whether or not high-SiO2 rhyolite is the result of
crystal-melt segregation. We present new high-precision U-Pb zircon
geochronologic and geochemical data that characterize the Miocene
Searchlight pluton in southern Nevada, USA. The data indicate that the
pluton was built incrementally over ~1.5 m.y. with some volumes of magma
completely crystallizing before subsequent volumes arrived. The largest
increment is an ~2.7-km-thick granitic sill that records contemporaneous
zircon crystallization, which we interpret to represent a fossil silicic
magma reservoir within the greater Searchlight pluton. Whole-rock
geochemical data demonstrate that this unit is stratified relative to
paleo-vertical, consistent with gravitationally driven separation of
high-SiO2 melt from early-formed crystals at moderate
crystallinity. Zircon trace-element compositions suggest that our
geochronologic data from this unit record most of the relevant
crystallization interval for differentiation and that this process occurred
in <150 k.y.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02439.1/612795/Constraints-on-the-timescales-and-processes-that
E/I-corrected inclination shallowing in Cenozoic redbeds from the
northern Tarim Basin, NW China: Possible causes and paleogeographic
implications
Zhiliang Zhang; Bai Shen; Jimin Sun; Zhikun Ren
Abstract:
Because of their widespread occurrence and ability to carry stable
remanence, continental redbeds in central Asia are frequently used in
paleomagnetic studies. However, the paleomagnetic inclinations recorded by
redbeds are much shallower than the expected values, as redbeds are usually
subjected to inclination shallowing. To recognize and correct the
inclinations recorded by the Cenozoic redbeds, the paleomagnetic data that
were used for magnetostratigraphic studies in the Kuqa Depression, northern
Tarim Basin, are reanalyzed in this study. The mean inclinations of the
four groups of samples (Eocene, Oligocene, Miocene, and Pliocene) are
systematically ~20° shallower than the expected values calculated from the
apparent polar wander paths (APWPs) of Eurasia, indicating the presence of
inclination shallowing. We apply the elongation/inclination (E/I)
method to correct the inclination shallowing. The mean inclinations of the
Eocene, Oligocene, Miocene, and Pliocene sediments are corrected from 40.5°
to 63.1°, 41.0° to 63.8°, 42.0° to 63.8°, and 44.7° to 63.2°, within 95%
confidence limits between 55.1° and 71.6°, 53.7° and 70.4°, 51.5° and
72.7°, and 52.2° and 71.3°, respectively, which are indistinguishable from
the expected inclination values. Our results suggest that inclination
shallowing in the redbeds of central Asia can be reasonably corrected using
the E/I method, and sedimentary processes such as compaction
and/or imbrication in the very early stage of burial are important causes
for inclination shallowing. Paleolatitudes calculated from the E/I
-corrected inclinations show that the Tarim Basin should have reached or
been at least close to its current latitude since the Cretaceous. The
Cenozoic crustal shortening estimate of the northern Tarim Basin is not
detectible for paleomagnetic study.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02480.1/612575/E-I-corrected-inclination-shallowing-in-Cenozoic
Bolide impact effects on the West Florida Platform, Gulf of Mexico
: End Cretaceous and late Eocene
C. Wylie Poag
Abstract:
This study documents seismic reflection evidence that two different bolide
impacts significantly disrupted stratigraphic and depositional processes on
the West Florida Platform (Gulf of Mexico). The first impact terminated the
Late Cretaceous Epoch (Chicxulub impact, Mexico; ca. 66 Ma;
end-Maastrichtian age). The second took place in the late Eocene
(Chesapeake Bay impact, Virginia, USA, portion of the Chesapeake Bay; ca.
35 Ma; Priabonian age). Both impacts produced far-reaching seismic shaking
and ground roll followed by an impact-generated tsunami, the effects of
which are evident in the seismostratigraphic record. The Chicxulub seismic
shaking caused collapse and shoreward retreat of the Florida Escarpment and
widely disrupted (faulting, folding, slumping) normal flat-lying shelf
beds. The associated tsunami currents redistributed these shelf deposits
and mixed them together with collapse debris from the escarpment to form a
thick wedge of sediments along the base of the escarpment. The Chesapeake
Bay impact created a mounded sedimentary deposit near the outer edge of the
late Eocene ramp slope. This deposit also has a bipartite origin. A lower
layer is marked by en echelon faulting created in situ by seismic shaking,
whereas an upper layer represents sediments redistributed from the late
Eocene shelf and upper ramp slope by tsunami-driven bottom currents (debris
flows, contour currents, slumps). This is the first report of seismic
effects from the Chesapeake Bay impact in the Gulf of Mexico. These results
further demonstrate that large-scale marine bolide impacts have widespread
effects on the stratigraphic and depositional record of Earth.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02472.1/612576/Bolide-impact-effects-on-the-West-Florida-Platform
Assessing the effect of melt extraction from mushy reservoirs on
compositions of granitoids: From a global database to a single
batholith
J. Cornet; O. Bachmann; J. Ganne; A. Fiedrich; C. Huber ...
Abstract:
Mafic and ultramafic plutonic rocks are often considered to be crystal
cumulates (i.e., they are melt-depleted), but such a classification is much
more contentious for intermediate to silicic granitoids (e.g., tonalite,
granodiorite, granite, and syenite). Whether or not a given plutonic rock
has lost melt to feed shallower subvolcanic intrusive bodies or volcanic
edifices has key implications for understanding igneous processes occurring
within the crust throughout the evolution of the Earth. We use statistical
analyses of a global volcanic and plutonic rock database to show that most
mafic to felsic plutonic rock compositions can be interpreted as
melt-depleted (i.e., most of the minerals analyzed are more evolved than
their bulk-rock compositions would allow). To illustrate the application of
the method to natural samples (from the Tertiary Adamello Batholith in the
southern Alps), we estimate the degree of melt depletion using a
combination of magmatic textures, bulk-rock chemistry, modal mineralogy,
distributions of plagioclase composition (using scanning electron
microscope phase mapping/electron microprobe analyses), and thermodynamic
modeling. We find that melt depletion correlates with the magmatic
foliation and is accompanied by bulk depletion in incompatible elements,
low amounts of near-solidus minerals, and mineral compositions that are too
evolved (i.e., depleted in Ca or Mg, depending on the mineral) to be in
equilibrium with their bulk-rock chemistry. The analytical and modeling
workflow proposed in this study provides a path to quantifying melt
depletion in any plutonic samples.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02333.1/612577/Assessing-the-effect-of-melt-extraction-from-mushy
Geologic map of Slate Range Crossing area, California, USA
Joseph E. Andrew
Abstract:
This detailed geologic map and supplemental digital data set examine and
demonstrate the complex Neogene–Quaternary deformation in the Slate Range
Crossing area (California, USA) of the active dextral transtension of the
Death Valley region and Walker Lane belt. This map integrates the late
Cenozoic structures and geologic units with the Mesozoic geologic units and
deformation as a data set to examine the controls on reactivation of older
structures. These geologic data were collected to study pre-, syn-, and
post-kinematic rocks to examine the deformation history of the area and to
find palinspastic markers to examine the late Cenozoic fault displacement
and displacement history across Panamint Valley to the east, as reported in
Andrew and Walker (2009). The study focused on defining the Miocene and
Pliocene rocks and deposits and examining lateral changes and depositional
sources of clasts. There are two different volcanic-sedimentary sequences
in this area. A Miocene section contains mafic to felsic volcanic units,
numerous debris-flow to laharic deposits, and several associated
conglomerates and breccias containing exotic clasts. The exotic clasts are
matched to rocks in the Panamint Range on the east side of Panamint Valley
as reported in Andrew and Walker (2009) as displacement vectors for
palinspastic reconstructions. These Miocene strata ubiquitously dip
eastward 20–40º. A younger volcanic-sedimentary sequence contains
relatively thin mafic lava flows and associated locally derived,
coarse-grained mass wasting deposits. These younger basaltic lavas
generally have gentle dipping lava flow features and foliation. Numerous
faults cut the different age deposits allowing a chronology of Neogene to
Quaternary faulting; additionally, there are numerous fabrics associated
with Jurassic contraction and Cretaceous(?) dextral shear. The area near
Slate Range Crossing has a conspicuous zone of earthquake foci; this study
found that some of this seismic activity coincides with a zone of
southwest-striking, moderately dipping to the north, sinistral-oblique
normal faults, which cut across the northernmost Slate Range. These faults
form a structural boundary between the Argus and Slate Ranges and link the
fault networks in Panamint Valley with those in Searles Valley. This
mapping and structural data demonstrate the two-stage Neogene fault history
of the Walker Lane belt deformation in this area and show that regional
tilting of rocks occurred after ca. 13 Ma and before ca. 4 Ma; this
eastward down-tilting appears to be a discrete event and may mark the
change from extension to transtension. This detailed geologic mapping and
collection of structural data for the rocks in the eastern Argus and
northern Slate Ranges and Panamint Valley were created using digital
in-the-field geographic information systems software running on a
field-hardened laptop computer. This map is a simplification of detailed
geologic mapping data collected at 1:6000 scale and reduced to 1:20000
scale. Structural data includes kinematic and relative timing of
deformation information.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02341.1/612440/Geologic-map-of-Slate-Range-Crossing-area
Geologic map of southern Panamint Valley, southern Panamint Range, and
central Slate Range, California, USA
Joseph E. Andrew
Abstract: This detailed geologic map and supplemental digital data set
examine and demonstrate the complex deformational history and reactivation
relationships of the southern Panamint Valley area (California, USA), from
active transtension of the Walker Lane belt, Miocene extension of the Basin
and Range, multiple Mesozoic events related to subduction, and
Neoproterozoic extension. This collection of map data focuses on the
geometry, kinematics, and relative timing of deformation to understand the
deformation history and effects of structural reactivation. Andrew and
Walker (2009) used these geologic mapping data to palinspastically restore
the Fish Canyon area of the Slate Range to overlapping the western Panamint
Range at Goler Wash. Neogene extension and subsequent dextral transtension
has created a complex network of faults via partial reactivation of
Mesozoic and Neoproterozoic structures and has separated the Slate Range
from the Panamint Range. The Neogene fault system changes from south to
north from dextral strike-slip along the southern Panamint Valley fault to
oblique normal slip along the Emigrant fault at a triple junction with the
sinistral-oblique normal Manly Pass fault. The Mesozoic deformation history
is different in the two ranges across Panamint Valley. The Slate Range was
the hanging wall to Jurassic and Cretaceous contractional deformation; this
same deformation in the Panamint Range to the east was localized along the
western range flank with the majority of the Panamint Range thus being in
the footwall to Mesozoic contraction. The western Panamint Range preserves
migmatitic fabrics and deformation due to Jurassic contraction and
plutonism. The Goldbug fault, along the western Panamint Range, places
Paleoproterozoic to Mesoproterozoic rocks over Neoproterozoic to Cretaceous
rocks. Jurassic contraction has top-to-the-northeast relative transport and
the more discrete Cretaceous thrust faulting in the Panamint Range has
top-to-the-east transport. The Butte Valley fault, previously recognized
farther north of the map area in the Panamint Range, cuts Late Jurassic
rocks and structures. Neoproterozoic to Cambrian sedimentary rocks with
top-to-the-northeast contractional deformation occur as relative
down-dropped block exposed east of the Butte Valley fault. The Butte Valley
fault continues southward and is then deflected by Late Cretaceous thrust
faulting on the Goldbug fault. Neoproterozoic deformation is more difficult
to discern but is hypothesized to relate to abundant olistostromes mapped
within the Kingston Peak Formation in the Panamint Range (i.e., Prave,
1999). This detailed geologic mapping and collection of structural data for
the rocks in the southern Panamint Valley area were created using digital
in-the-field geographic information systems software running on a
field-hardened laptop computer. This map is a simplification of detailed
geologic mapping data collected at 1:6000 scales and reduced to 1:20000
scale. Structural data includes kinematic and relative timing of
deformation information.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02342.1/612441/Geologic-map-of-southern-Panamint-Valley-southern
Geologic map of central Panamint Range, California, USA
Joseph E. Andrew
Abstract:
This detailed geologic map and supplemental digital data set examine and
demonstrate the complex deformational history and reactivation
relationships of the Panamint Range (California, USA), from active
transtension of the Walker Lane belt, Miocene extension of the Basin and
Range, to multiple Mesozoic events related to subduction, and
Neoproterozoic extension. This collection of map data focuses on the
geometry, kinematics, and relative timing of deformation to understand the
deformation history and effects of structural reactivation. A minor portion
of this geologic mapping data was presented in the analysis and figures of
Andrew and Walker (2009). The Neogene extension and subsequent dextral
transtension deformation has created a complex network of faults via
partial reactivation of Mesozoic and Neoproterozoic structures. Structural
data show oblique normal slip overprinting earlier normal slip along the
western range flank fault of the western Panamint Range. Jurassic and
Cretaceous deformation is localized along the western range on the Goldbug
fault. The hanging wall of this fault preserves migmatitic fabrics and
intense deformation due to Jurassic contraction. The Goldbug fault places
Paleoproterozoic to Mesoproterozoic rocks over Neoproterozoic rocks. The
Jurassic contraction has top-to-the-northeast relative transport and the
more discrete Cretaceous thrust faulting has top-to-the-east transport. A
set of Late Cretaceous plutonic rocks and mylonitic gneisses derived from
them, occur along the Goldbug fault and demonstrate the reactivated nature
of this fault in the Late Cretaceous. New data for the Butte Valley fault
show that this fault cuts Late Jurassic plutonic rocks and has normal slip.
The Butte Valley fault ends northward at the linked sinistral slip Warm
Spring Canyon fault, which was previously interpreted to be an intrusive
contact. A previously unrecognized rim syncline structure occurs along the
boundary of the Late Jurassic Manly Peak quartz monzonite. Neoproterozoic
deformation is difficult to discern due to the overprinting deformations.
Numerous Neoproterozoic deformation-related mass wasting deposits can be
seen within this formation, including a set of conspicuous allochthonous
deposits and clasts of older Beck Spring Dolomite that appear to be frozen
in the process of breaking away from intact, normal thickness beds in the
Surprise–Happy Canyons divide. This detailed geologic mapping and
collection of structural data for the rocks in the central Panamint Range
were created using digital in-the-field geographic information systems
software running on a field-hardened laptop computer combined with an
earlier set of field data that were digitized into the digital
georeferenced database. This map is a simplification of detailed geologic
mapping data collected at 1:2000–1:6000 scales and reduced to 1:20000
scale. Structural data include kinematic and relative timing of deformation
information.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02344.1/612442/Geologic-map-of-central-Panamint-Range-California
Insights into the geometry and evolution of the southern San Andreas
fault from geophysical data, southern California
V.E. Langenheim; G.S. Fuis
Abstract:
Two new joint gravity-magnetic models in northern Coachella Valley provide
additional evidence for a steep northeast dip of the Mission Creek strand
of the southern San Andreas fault (southern California, USA). Gravity
modeling indicates a steep northeast dip of the Banning fault in the upper
1–2 km in northern Coachella Valley. The Mission Creek strand and its
continuation to the southeast (Coachella segment) coincide with the
northeastern margin of a Cenozoic basin and are marked by prominent gravity
and magnetic gradients that are consistent with these strands of the San
Andreas fault having accommodated >160 km of right-lateral and 1–5 km of
vertical displacement. These anomalies are best fit by a moderate to steep
northeast dip. Such a geometry is further supported by seismicity,
reflectivity, geodesy, and boundary-element modeling. We explore the
possibility that these fault strands forming the margin of Coachella Valley
were originally near vertical and have rotated into their present
orientation by underplating of a localized high-velocity, lower-crustal
prong within the Peninsular Ranges batholith. Reconstructions of San
Andreas fault offset suggest that this crystalline body was translated into
the San Gorgonio Pass area at the time of major fault reorganization at
1.1–1.3 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02378.1/612449/Insights-into-the-geometry-and-evolution-of-the
Evidence for pre-Cenozoic extension in the eastern Main Ranges of the
southern Canadian Rockies
Robert L. Taerum
Abstract:
The eastern Main Ranges of the southern Canadian Rocky Mountain
thrust-and-fold belt include a network of normal faults (the result of
apparent extensional episodes) that occur within a contractional orogen.
The origin, timing, and nature of these normal faults remain unresolved. A
widely accepted explanation proposes that the normal faults developed as a
consequence of postcontractional transtension that occurred west of the
Rocky Mountain Trench during the Paleogene Period. Detailed field mapping
of deformation in the vicinity of several normal faults has provided
evidence that the normal fault surfaces and adjacent strata underwent
deformation during a contractional episode after the normal faults had
formed. Within the study area, located in the upper Kicking Horse region of
Yoho National Park, British Columbia, Canada, and within the larger region
of the Rocky Mountain belt, the network of normal faults is proposed to
have developed as a consequence of rifting that separated pericratonic
terranes from North America and produced the Slide Mountain Ocean during
the Carboniferous and Permian Periods. Overprinting from more recent
tectonic episodes has obscured most of these inferred extensional faults
throughout the North American Cordillera. Within the study area, however,
the Cretaceous to Paleogene contraction carried the normal faults to their
present location over unattenuated continental crust, without significant
overprinting. This preservation of the network of normal faults allows for
investigation of the relationships among the fault surfaces and the strata
adjacent to each fault.
View article:
https://pubs.geoscienceworld.org/gsa/geosphere/article-abstract/doi/10.1130/GES02347.1/612428/Evidence-for-pre-Cenozoic-extension-in-the-eastern
GEOSPHERE articles are available at
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