Cenozoic evolution of Neotethys and implications for the causes of plate motions

Africa-North America-Eurasia plate circuit rotations, combined with Red Sea rotations and new estimates of crustal shortening in Iran define the Cenozoic history of the Neotethyan ocean between Arabia and Eurasia. The new constraints indicate that Arabia-Eurasia convergence has been fairly constant at 2 to 3 cm/yr since 56 Ma with slowing of Africa-Eurasia motion to <1 cm/yr near 25 Ma, coeval with the opening of the Red Sea. Ocean closure occurred no later than 10 Ma, and could have occurred prior to this time only if a large amount of continental lithosphere was subducted, suggesting that slowing of Africa significantly predated the Arabia-Eurasia collision. These kinematics imply that Africa's disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern Eurasia slowed its motion. The slow, steady rate of northward subduction since 56 Ma contrasts with strongly variable rates of magma production in the Urumieh-Dokhtar arc, implying magma production rate in continental arcs is not linked to subduction rate

Restricted single isocenter for multiple targets dynamic conformal arc (RSIMT DCA) technique for brain stereotactic radiosurgery (SRS) planning.

In stereotactic radiosurgery (SRS), the multiple isocenters for multiple targets dynamic conformal arc (MIMT DCA) technique is traditionally used to treat multiple brain metastases, with one isocenter for each target. The single isocenter for multiple targets (SIMT) technique has recently been adopted to reduce the treatment time at the cost of plan quality. The objective of this study was to develop a restricted single isocenter for multiple targets DCA (RSIMT DCA) technique that can significantly reduce the treatment time but still maintain similar plan quality as the MIMT DCA technique.Treating multiple brain metastases with a single isocenter poses a challenge to SRS planning using DCA beams that are intrinsically 3D and do not modulate the beam intensity to spare the normal tissue between targets. To address this obstacle, we have developed a RSIMT DCA technique and used it to treat SRS patients with multiple brain metastases since February 2015. This planning approach is similar to the SIMT technique except that the number of targets for each isocenter is restricted and the distance between the isocenter and target is limited. In this technique, the targets are first split into batches so that all targets in a batch are within a chosen distance (e.g., 7 cm) of each other. All targets in a batch are combined into one target and the geometric center of the combined target is the isocenter for the group of DCA beams associated with that batch. Each DCA group typically consists of 3-4 DCA beams to irradiate 1-3 targets. For each DCA beam, the collimator angle is adjusted to minimize the exposure of normal tissue between targets. The dose of each treatment group is normalized so that the maximal point dose to the combined target is 125% of the prescription dose, which is equivalent to normalize the prescription dose to 80% isodose line. If the maximal point dose of a target is 95% and V19Gy=100%) was achieved for all plans using either technique. Most PTVs have a maximal point dose between 24.9 and 25.1 Gy, with 2 PTVs between 24.5 and 24.9 Gy. Overall, the plan quality was slightly better for the MIMT DCA technique and the normalized difference was statistically significantly larger than 0 for all investigated dose quality indexes. The normalized difference of body mean dose and conformity index (CI) between the RSIMT and MIMT techniques was respectively 4.2% (p=0.002) and 9.4% (p=0.001), indicating similar plan quality globally and in the high dose area. The difference was more pronounced for the mid-to-low dose spillage with the ratios of V12Gy and V10Gy/VPTV being 13.9% (p=3.8×10-6) and 14.9% (p=1.3×10-5), respectively. The treatment time was reduced by 30%-50% with the RSIMT DCA technique.The RSIMT DCA technique can produce satisfactory SRS plans for treating multiple targets and can significantly reduce the treatment time

Structural discordance between neogene detachments and frontal sevier thrusts, central Mormon Mountains, southern Nevada

This is the published version. Copyright 1985 American Geophysical Union. All Rights Reserved.Detailed geologic mapping in the Mormon Mountains of southern Nevada provides significant insight into processes of extensional tectonics developed within older compressional orogens. A newly discovered, WSW-directed low-angle normal fault, the Mormon Peak detachment, juxtaposes the highest levels of the frontal most part of the east-vergent, Mesozoic Sevier thrust belt with autochthonous crystalline basement. Palinspastic analysis suggests that the detachment initially dipped 20–25° to the west and cut discordantly across thrust faults. Nearly complete lateral removal of the hanging wall from the area has exposed a 5 km thick longitudinal cross-section through the thrust belt in the footwall, while highly attenuated remnants of the hanging wall (nowhere more than a few hundred meters thick) structurally veneer the range. The present arched configuration of the detachment resulted in part from progressive “domino-style” rotation of a few degrees while it was active, but is largely due to rotation on younger, structurally lower, basement-penetrating normal faults that initiated at high-angle. The geometry and kinematics of normal faulting in the Mormon Mountains suggest that pre-existing thrust planes are not required for the initiation of low-angle normal faults, and even where closely overlapped by extensional tectonism, need not function as a primary control of detachment geometry. Caution must thus be exercised in interpreting low-angle normal faults of uncertain tectonic heritage such as those seen in the COCORP west-central Utah and BIRP's MOIST deep-reflection profiles. Although thrust fault reactivation has reasonably been shown to be the origin of a very few low-angle normal faults, our results indicate that it may not be as fundamental a component of orogenic architecture as it is now widely perceived to be. We conclude that while in many instances thrust fault reactivation may be both a plausible and attractive hypothesis, it may never be assumed

Assessment of GPS velocity accuracy for the Basin and Range Geodetic Network (BARGEN)

We assess the accuracy of horizontal velocity estimates from the Basin and Range Geodetic Network (BARGEN), a continuous GPS network that has been in operation since 1996. To make this quantitative assessment, we use a procedure that we term the “whole-error” method. In this method, the measure of the velocity errors is the root-mean-square (RMS) residual velocity relative to a simple geophysical model. This method produces a conservative estimate of the uncertainties, since errors in the geophysical models also contribute to the RMS residual. Using estimates from two different BARGEN subnetworks, the Northern Basin and Range and the Yucca Mountain Cluster, we determine velocity uncertainties of 0.1–0.2 mm yr^(−1). Since BARGEN covers a significant fraction of area of the proposed Plate Boundary Observatory component of EarthScope, our results indicate a good ability of this project to determine highly accurate long-term horizontal crustal velocities and deformation rates in this region

Fluid Flow, Brecciation, and Shear Heating on Faults: Insights from Carbonate Clumped-Isotope Thermometry

Slip on gently dipping detachments in the brittle crust has been enigmatic for decades, because fracture mechanics laws predict frictional resistance is too great for sliding to occur, except under rather unusual circumstances. The Miocene Mormon Peak detachment in Nevada and the Eocene Heart Mountain detachment in Wyoming are two well‐studied examples of upper crustal, carbonate‐hosted low‐angle detachments, with highly debated slip processes. Both low‐angle faults were active during regional magmatism, and a number of proposed slip mechanisms involve magmatic fluids, frictional heating, or both. To address the role that magmatic fluids and frictional heating may have played in reducing friction, we measured clumped‐isotope ratios on 137 carbonate samples from these faults. The majority of fault breccias and gouges on the detachment slip surface record temperatures that are colder than the host rock. Surprisingly, samples from within 5 m of the Heart Mountain detachment average just 65 °C, and not a single sample (out of 37 measurements, excluding metamorphosed host rock at White Mountain) records a temperature greater than 90 °C. Along both faults, most samples are depleted in δ^(18)O relative to the host rock, indicating that meteoric, not magmatic, fluids were present and interacting with the fault rock. However, a few samples preserve temperatures of over 160 °C, which, based on textural and geochemical criteria, are difficult to explain other than by frictional heating during slip. These temperatures are recorded in one sample directly on the Mormon Peak detachment slip surface and in two hanging wall localities above the Heart Mountain detachment

Th–U–total Pb geochronology of authigenic monazite in the Adelaide rift complex, South Australia, and implications for the age of the type Sturtian and Marinoan glacial deposits

The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th–U–total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re–Os isochron age of 643 ± 2.4 Ma (2σ) on shales near the bottom of the Sturtian–Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were folded into their present structural pattern

Structural discordance between neogene detachments and frontal Sevier thrusts, central Mormon Mountains, southern Nevada

Detailed geologic mapping in the Mormon Mountains of southern Nevada provides significant insight into processes of extensional tectonics developed within older compressional orogens. A newly discovered, WSW-directed low-angle normal fault, the Mormon Peak detachment, juxtaposes the highest levels of the frontal most part of the east-vergent, Mesozoic Sevier thrust belt with autochthonous crystalline basement. Palinspastic analysis suggests that the detachment initially dipped 20–25° to the west and cut discordantly across thrust faults. Nearly complete lateral removal of the hanging wall from the area has exposed a 5 km thick longitudinal cross-section through the thrust belt in the footwall, while highly attenuated remnants of the hanging wall (nowhere more than a few hundred meters thick) structurally veneer the range. The present arched configuration of the detachment resulted in part from progressive “domino-style” rotation of a few degrees while it was active, but is largely due to rotation on younger, structurally lower, basement-penetrating normal faults that initiated at high-angle. The geometry and kinematics of normal faulting in the Mormon Mountains suggest that pre-existing thrust planes are not required for the initiation of low-angle normal faults, and even where closely overlapped by extensional tectonism, need not function as a primary control of detachment geometry. Caution must thus be exercised in interpreting low-angle normal faults of uncertain tectonic heritage such as those seen in the COCORP west-central Utah and BIRP's MOIST deep-reflection profiles. Although thrust fault reactivation has reasonably been shown to be the origin of a very few low-angle normal faults, our results indicate that it may not be as fundamental a component of orogenic architecture as it is now widely perceived to be. We conclude that while in many instances thrust fault reactivation may be both a plausible and attractive hypothesis, it may never be assumed

Crustal loading near Great Salt Lake, Utah

Two sites of the BARGEN GPS network are located ∼30 km south of Great Salt Lake (GSL). Lake-level records since mid-1996 indicate seasonal water elevation variations of ∼0.3 m amplitude superimposed on a roughly “decadal” feature of amplitude ∼0.6 m. Using an elastic Green's function and a simplified load geometry for GSL, we calculate that these variations translate into radial crustal loading signals of ±0.5 mm (seasonal) and ±1 mm (decadal). The horizontal loading signals are a factor of ∼2 smaller. Despite the small size of the expected loading signals, we conclude that we can observe them using GPS time series for the coordinates of these two sites. The observed amplitudes of the variations agree with the predicted decadal variations to <0.5 mm. The observed annual variations, however, disagree; this difference may be caused by some combination of local precipitation-induced site motion, unmodeled loading from other nearby sources, errors in the GSL model, and atmospheric errors

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.