302 research outputs found
Accreted island arcs and cross-cutting batholithic belts of the North American Cordillera
The basement framework of the western Cordillera consists
in large part of tectonically accreted island arc
terranes and cross-cutting batholithic belts. The arc
terranes are diverse in terms of magmatic history,
tectonic disruption and basement relations, and represent
several distinct systems. Terranes of the two
oldest systems occur in inner and outer belt positions. The inner belt runs from central Alaska to the
northern Sierra Nevada. It was in its major developmental
phases by the Devonian, and was constructed on
imbricated North American continental rise strata outboard
of a passive margin. The outer belt includes the
Alexander Terrane (AT) of SE Alaska and younger amalgamated
arc terranes of the Alaska Peninsula and Queen
Charlotte-Vancouver Islands
Early Mesozoic Paleotectonic-Paleogeographic Reconstruction of Southern Sierra Nevada Region
A paleotectonic-paleogeographic reconstruction was
based on structural, petrologic, and geochronologic
studies of pre-Sierra Nevada batholith framework rocks
exposed between the San Joaquin River and the Garlock
fault. Most available fossil data from roof pendants
of this region indicate Late Triassic to Early Jurassic
ages. An additional fossil locality from the
western wall rocks yields a Late Permian Tethyan fauna.
This is a maximum age for the enclosing rocks, for
the fossils are in a limestone olistolith. As yet there is no
sign of Paleozoic strata in the region except perhaps
along the eastern Sierran crest in small metamorphic
septa, and in the western foothills where ophiolitic rocks
are present
Seismic anisotropy as a constraint on composition in the lower crust
Our current interpretation of the composition of the middle
and lower crust comes mainly from seismic observations, yet
it remains a challenge to link seismic observations directly to
composition. This is because isotropic seismic properties are
similar across a range of compositions. Taking anisotropy into
account allows for further refinement of our interpretation of
composition provided that anisotropy is characterized for
candidate rock types. This study uses electron backscatter
diffraction (EBSD) measurements of crystallographic
preferred orientation of minerals to calculate seismic
anisotropy in samples of the Pelona-Orocopia-Rand (POR)
schist from the Mojave region of southern California. The
goals of this work are to characterize the seismic anisotropy of
the POR schist and its relationship to observed lower crustal
anisotropy in the region, and to refine predictions of lower
crustal composition based on seismic anisotropy
Evaluating Arrow Dynamics via Stochastic Perturbation Methods
Traditional methods of measuring arrow spine, involving static weight tests, fail to account for the dynamic behavior of arrows in flight. Addressing this gap, our study developed a novel apparatus to capture the dynamic properties of arrows, providing a more accurate reflection of their performance. By applying stochastic perturbations through a voice coil actuator and measuring displacements, we were able to determine the natural frequency, damping characteristics, and mechanical stiffness of arrows made from carbon, wood, and aluminum with varying spines. Our findings, based on a second-order parameterized model, correlated well with spine values provided by manufacturers. Additionally, extensive high cycle fatigue tests were conducted on each type of arrow material, revealing minimal impact on the dynamic parameters of the arrows
Magnitude and Timing of Extreme Continental Extension, Central Death Valley Region, California
New geochronologic, stratigraphic, and sedimentologic
data indicate extreme late Cenozoic extension across the
central Death Valley region (fig. 9). ^(40)Ar/^(39)Ar geochronology
of sanidine from tuffs intercalated with steeply tilted
sediments along the eastern margin of the central Death Valley
region, including sections near Chicago Pass and at
Eagle Mountain, indicates deposition from approximately
15 to 11.7 Ma (fig. 10). Clasts of marble, orthoquartzite,
fusilinid limestone, and leucogabbro are prominent at both
locations. The only known source in the Death Valley
region for this clast assemblage is in the southern
Cotton wood Mountains, more than 100 km away on the
western flank of the Death Valley region. U/Pb geochronology
of baddeleyite confirms that leucogabbro clasts from
both sections have the same igneous crystallization age
(~180 Ma) as the leucogabbroic phase of the Hunter Mountain
batholith, in the southern Cottonwood Mountains. The
sediments include debris flows, flood deposits, and monolithic
boulder beds of large leucogabbro clasts (>1 m), suggesting
deposition in an alluvial fan setting. Sedimentary
transport of these deposits is unlikely to have exceeded 20
km. Restoration of the Eagle Mountain and Chicago Valley deposits to a position just east of the southern Cotton wood
Mountains results in approximate net translations of 80 km
and 104 km, respectively, at an azimuth of N. 67° W. (fig.
11). This suggests overall extension magnitudes of at least
500 percent across the Death Valley region since 12 Ma,
with strain rates that approached 10^(-14)/s during maximum
extension. These results support previous reconstructions
based on isopachs and Mesozoic structural features. (See,
for example, Wernicke and others, 1988.
Petrology of the Early Cretaceous Sierra Nevada Batholith; the Stokes Mountain region, CA
Previous studies have shown that the early Cretaceous
batholith (130-110 Ma) contains the least chemically and
isotopically evolved lithologies of the composite Sierra
Nevada batholith.
Mapping at 1:24,000 of a 360 km^2 area
in the foothills ESE of Fresno (the Stokes Mountain region;
latitude 36°30') reveals a smoothly continuous range (SiO_2 = 44-78%) of calcic lithologies dominated by norites,
hornblende gabbros, quartz diorites, tonalites and
granodiorites
Active foundering of a continental arc root beneath the southern Sierra Nevada in California
Seismic data provide images of crust–mantle interactions during ongoing removal of the dense batholithic root beneath the southern Sierra Nevada mountains in California. The removal appears to have initiated between 10 and 3 Myr ago with a Rayleigh–Taylor-type instability, but with a pronounced asymmetric flow into a mantle downwelling (drip) beneath the adjacent Great Valley. A nearly horizontal shear zone accommodated the detachment of the ultramafic root from its granitoid batholith. With continuing flow into the mantle drip, viscous drag at the base of the remaining ~35-km-thick crust has thickened the crust by ~7 km in a narrow welt beneath the western flank of the range. Adjacent to the welt and at the top of the drip, a V-shaped cone of crust is being dragged down tens of kilometres into the core of the mantle drip, causing the disappearance of the Moho in the seismic images. Viscous coupling between the crust and mantle is therefore apparently driving present-day surface subsidence
Oceanic plateau subduction beneath North America and its geological and geophysical implications
We use two independent approaches, inverse models of mantle convection and plate reconstructions, to predict the temporal and
spatial association of the Laramide events to subduction of oceanic plateaus. Inverse convection models, consistent with vertical
motions in western US, recover two prominent anomalies on the Farallon plate during the Late Cretaceous that coincide with
paleogeographically restored Shatsky and Hess conjugate plateaus when they collided with North America. The distributed
deformation of the Laramide orogeny closely tracked the passage of the Shatsky conjugate massif, suggesting that subduction of this
plateau dominated the distinctive geology of the western United States. Subduction of the Hess conjugate corresponds to termination
of a Latest Cretaceous arc magmatism and intense crustal shortening in Early Paleogene in northwest Mexico. At present, conjugates
of the Shatsky and Hess plateaus are located beneath the east coast of North America, and we predict that +4% seismic anomalies in P
and S velocities are associated with the remnant plateaus with sharp lateral boundaries detectable by the USArray seismic experiment.
Flat subduction of the Shatsky conjugate caused drastic subsidence/uplift and tilt of the Colorado Plateau (CP). From the inverse
convection calculations, we find that with the arrival of the flat slab, dynamic subsidence starts at the southwestern CP and reaches a
maximum at ~86 Ma. Two stages of uplift follow the removal of the Farallon slab: one in Latest Cretaceous and the other in Eocene
with a cumulative uplift of ~1.2 km. The southwestern plateau reaches a high dynamic topography in the Eocene which is sustained to
the present. Both the descent of the slab and buoyant upwelling may have contributed to late Cenozoic plateau uplift. The CP tilts
downward to the NE before the Oligocene, caused by NE trending subduction of the Farallon slab. The NE tilt diminishes and
switches to a SW tilt during the Miocene when buoyant mantle upwellings occur
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