Mantle xenoliths from the Southern Alps, New Zealand, provide insight into the origin of mantle seismic anisotropy related to the Australian‐Pacific plate boundary. Most xenoliths from within 100 km ...lateral distance of the Alpine Fault are coarse grained, but a small number are finer grained protomylonites. The protomylonites contain connected networks of fine grains with a different crystallographic preferred orientation (CPO) to coarse porphyroclasts in the same xenolith, suggesting that protomylonites and coarse‐grained samples record different deformation kinematics. The CPOs of fine grains in protomylonites have monoclinic symmetry, with the 2‐fold rotation axis normal to a plane that contains olivine 010 and orthopyroxene 100 maxima, suggesting that the protomylonite deformation involved significant simple shear. Some coarse‐grained samples contain unconnected lenses and layers of fine grains with the same CPO as the coarse grains. Microstructures suggest that these fine grains formed by subgrain rotation recrystallization and that protomylonites may represent an up‐strain progression of this microstructure, where the connectivity of fine grains has allowed them to localize shear and develop a new Alpine Fault CPO. The samples tell us about the state of the mantle at 25 Ma, in the early history of the plate boundary. If this suite of samples is representative of the mantle beneath the Alpine Fault in the present day, then we can interpret the complex seismic anisotropy patterns in the lithospheric mantle as representative of blocks containing variably rotated older CPOs juxtaposed by narrow shear zones associated with Alpine Fault deformation.
Plain Language Summary
The Earth's mantle lies beneath the crust. Samples of the mantle are ripped‐up by volcanic processes and brought to the Earth's surface: these are known as xenoliths. Slow shearing in the mantle aligns the crystals that make up the xenoliths. Electron Backscatter Diffraction (EBSD) allows us to measure crystal alignments in the xenoliths, whilst seismic data can map the large‐scale patterns of crystal alignment in the current mantle. Seismic data show quite complex patterns in the mantle below the Southern Alps of New Zealand. We use EBSD measurements to help understand these seismic data patterns and their potential relationship to the Alpine Fault; the major structure between the Australian and Pacific plates. The xenoliths include fine crystal sizes that we infer, from the patterns of crystal alignment, to be sheared rocks related to the Alpine Fault. Most xenoliths have much coarser crystals and we suggest that the crystal alignments pre‐date the Alpine Fault. The xenoliths were extracted from the mantle about 25 million years ago, in the earliest stages of the fault. We apply the inferences from the xenoliths to the seismic data patterns in the current mantle and suggest that Alpine Fault deformation in the mantle is limited to very narrow zones. Areas between these zones relate to deformation events that pre‐date the Alpine Fault.
Key Points
Protomylonites cross‐cut coarse‐grained peridotite with different deformation kinematics in mantle xenoliths near the Alpine Fault
Protomylonites represent localized shear in the lithospheric mantle associated with the early stages of the Alpine Fault (∼25 Ma) motion
Seismic anisotropy in the lithosphere probably relates to mantle deformation that predates the Alpine Fault, but has been re‐oriented
A significant issue in the study of orogenic systems concerns the roles played by frontal and basal accretion in the construction of orogenic wedges. These different accretion mechanisms result in ...different thermal histories, with underplated materials experiencing significant heating and deformation during tectonic burial. This work provides new thermal data from Raman spectroscopy of carbonaceous material in combination with structural and stratigraphic observations of the northern Taiwan slate belt to address these questions of wedge development. Sedimentary rocks of the Northern slate belt were deposited on the Chinese continental margin immediately before the onset of the Neogene Taiwan arc‐continent collision. In the slates of the northern Hsüehshan Range, a large‐scale pop‐up structure on the prowedge of the Taiwan Orogen, synorogenic metamorphism has been investigated through analyses of peak temperatures and metamorphic field gradients. Results indicate underthrusting of the margin sediments to ~8‐km depth with significant folding in two major duplexes occurring before underplating. Such basal accretion is considered responsible for the distinct culmination of the Hsüehshan Range in central Taiwan and its relative uplift with respect to the Backbone Range to the east along the Lishan Fault. A similar underthrusting scenario is also suggested for the Backbone Range Slate Belt. We propose that basal accretion is the predominant mechanism in the growth and evolution of the Taiwan orogenic wedge and may have been achieved through inversion of a graben system on the ancient passive margin during continental subduction.
Key Points
Dynamic synorogenic thermal overprint over static basin diagenesis quantified through RSCM geothermometry
Tectonic underplating through duplex formation beneath frontal part of orogenic wedge responsible for dynamic heating
Basal accretion is key in the growth and evolution of the Taiwan orogenic wedge through margin basin inversion during continental subduction
We constrain the thermal state of the central Alpine Fault using approximately 750 Ti-in-quartz secondary ion mass spectrometer (SIMS) analyses from a suite of variably deformed mylonites. ...Ti-in-quartz concentrations span more than 1 order of magnitude from 0.24 to ∼ 5 ppm, suggesting recrystallization of quartz over a 300 °C range in temperature. Most Ti-in-quartz concentrations in mylonites, protomylonites, and the Alpine Schist protolith are between 2 and 4 ppm and do not vary as a function of grain size or bulk rock composition. Analyses of 30 large, inferred-remnant quartz grains ( > 250 µm) as well as late, crosscutting, chlorite-bearing quartz veins also reveal restricted Ti concentrations of 2–4 ppm. These results indicate that the vast majority of Alpine Fault mylonitization occurred within a restricted zone of pressure–temperature conditions where 2–4 ppm Ti-in-quartz concentrations are stable. This constrains the deep geothermal gradient from the Moho to about 8 km to a slope of 5 °C km−1. In contrast, the small grains (10–40 µm) in ultramylonites have lower Ti concentrations of 1–2 ppm, indicating a deviation from the deeper pressure–temperature trajectory during the latest phase of ductile deformation. These constraints suggest an abrupt, order of magnitude change in the geothermal gradient to an average of about 60 °C km−1 at depths shallower than about 8 km, i.e., within the seismogenic zone. Anomalously, the lowest-Ti quartz (0.24–0.7 ppm) occurs away from the fault in protomylonites, suggesting that the outer fault zone experienced minor plastic deformation late in the exhumation history when more fault-proximal parts of the fault were deforming exclusively by brittle processes.
Granitic plutons commonly preserve evidence for jointing, faulting, and ductile fabric development during cooling. Constraining the spatial variation and temporal evolution of temperature during this ...deformation could facilitate an integrated analysis of heterogeneous deformation over multiple length‐scales through time. Here, we constrain the evolving temperature of the Lake Edison granodiorite within the Mount Abbot Quadrangle (central Sierra Nevada, CA) during late Cretaceous deformation by combining microstructural analysis, titanium‐in‐quartz thermobarometry (TitaniQ), and thermal modeling. Microstructural and TitaniQ analyses were applied to 12 samples collected throughout the pluton, representative of either the penetrative “regional” fabric or the locally strong “fault‐related” fabric. Overprinting textures and mineral assemblages indicate the temperature decreased from 400–500°C to <350°C during faulting. TitaniQ reveals consistently lower Ti concentrations for partially reset fault‐related fabrics (average: 12 ± 4 ppm) than for regional fabrics (average: 31 ± 12 ppm), suggesting fault‐related fabrics developed later, following a period of pluton cooling. Uncertainties, particularly in TiO2 activity, significantly limit further quantitative thermal estimates using TitaniQ. In addition, we present a 1‐D heat conduction model that suggests average pluton temperature decreased from 585°C at 85 Ma to 332°C at 79 Ma, consistent with radiometric age data for the field. Integrated with the model results, microstructural temperature constraints suggest faulting initiated by ∼83 Ma, when the temperature was nearly uniform across the pluton. Thus, spatially heterogeneous deformation cannot be attributed to a persistent temperature gradient, but may be related to regional structures that develop in cooling plutons.
Key Points
Joints, ductile shear zones, and faults formed in a cooling granodiorite pluton and preserve overprinting microstructures
Synthesis of thermal model with microstructural data reveals timing of and potential relationships between structural events
Uncertainty in TiO2 activity and partial reequilibration during recrystallization limit utility of titanium‐in‐quartz thermobarometer
Geochemical analyses and geobarometric determinations have been combined to create a depth vs. radiogenic heat production database for the Sierra Nevada batholith, California. This database shows ...that mean heat production values first increase, then decrease, with increasing depth. Heat production is ∼2 μW/m
3 within the ∼3-km-thick volcanic pile at the top of the batholith, below which it increases to an average value of ∼3.5 μW/m
3 at ∼5.5 km depth, then decreases to ∼0.5–1 μW/m
3 at ∼15 km depth and remains at these values through the entire crust below 15 km. Below the crust, from depths of ∼40–125 km, the batholith's root and mantle wedge that coevolved beneath the batholith appears to have an average radiogenic heat production rate of ∼0.14 μW/m
3. This is higher than the rates from most published xenolith studies, but reasonable given the presence of crustal components in the arc root assemblages. The pattern of radiogenic heat production interpreted from the depth vs. heat production database is not consistent with the downward-decreasing exponential distribution predicted from modeling of surface heat flow data. The interpreted distribution predicts a reasonable range of geothermal gradients and shows that essentially all of the present day surface heat flow from the Sierra Nevada could be generated within the ∼35 km thick crust. This requires a very low heat flux from the mantle, which is consistent with a model of cessation of Sierran magmatism during Laramide flat-slab subduction, followed by conductive cooling of the upper mantle for ∼70 m.y. The heat production variation with depth is principally due to large variations in uranium and thorium concentration; potassium is less variable in concentration within the Sierran crust, and produces relatively little of the heat in high heat production rocks. Because silica content is relatively constant through the upper ∼30 km of the Sierran batholith, while U, Th, and K concentrations are highly variable, radiogenic heat production does not vary directly with silica content.
Because shear stress drives plate tectonics and causes earthquakes, important objectives in the Earth Sciences include quantifying stress magnitudes and variability in space and time, and developing ...and improving tools to do so. This thesis addresses both objectives. In the first chapter I demonstrate that the Titanium-in-quartz thermobarometer ("TitaniQ") can be used to accurately record deformation temperatures under greenschist facies conditions. In the second chapter, an experimental study, I show that the relationship between recrystallized grain size and flow stress (the "recrystallized grain size paleopiezometer") can be used to determine the stress history of dynamically recrystallized quartz under non steady state conditions. In the third chapter I apply the paleopiezometer in Taiwan's Hsüehshan range and compare results to independent constraints (e.g. critical taper theory and potential energy considerations). This analysis demonstrates: 1) the piezometer is accurate to within a factor of two or better under conditions at the brittle-ductile transition; 2) piezometric results are consistent with recent flow laws for quartz; 3) the activation energy of naturally deformed quartzite is >133 kJ/mol, consistent with experimental determinations; and 4) Peak differential stress in the Hsüehshan range was ∼210 MPa at temperature ∼300°C. Our results indicate hydrostatic fluid pressure and a low friction coefficient of ∼0.38 within the Taiwan wedge. Integrated crustal strength in Taiwan is 1.5-2.1*1012 N/m, consistent with the force needed to support the topographyof the range. The final chapter investigates stress levels on the Vincent thrust in the San Gabriel Mountains, California by constructing a numerical model of the initiation of flat slab subduction. A model inversion demonstrates that previously hypothesized high stresses are not required to explain inverted metamorphism along the fault.
“The PACE Evaluation”: Two Responses Coulam, Robert F.; Zimmerman, Yvonne A.; Clauser, Steven B. ...
The Gerontologist,
02/1996, Letnik:
36, Številka:
1
Journal Article
Recenzirano
Odprti dostop
In a letter to the editor, Coulam and Zimmerman offer additional information about the evaluation of the Program of All-inclusive Care for the Elderly (PACE) demonstration. Clauser, Kidder and Mauser ...suggest that the PACE sites may be engaging in skimming of clients in another letter to the editor. Branch responds to these suggestions in yet another letter to the editor.
We summarize the post Late Cretaceous regional tectonic evolution of the central California Coast Ranges, west of the San Andreas fault system. The Monterey terrane of North American origin was ...laterally transferred to the Pacific plate via the San Andreas fault. The Monterey terrane is an assembly of three tectonic units, Salinia, Nacimiento and Sierra de Salinas blocks, two of which have been previously identified as separate terranes. These blocks are separated by two regionally important thrust faults: the Sur fault as well as the Salinas shear zone. Based on thermobarometric and thermochronologic constraints and the existence of a common younger cover sequence, these blocks were juxtaposed together after the latest Cretaceous. The Salinian assemblage represents a crustal section through the continental interior side of the Mesozoic California arc and formed during the Late Cretaceous, primarily during a regionally significant magmatic flare-up between 95 and 80 Ma. In the Santa Lucia Range, parts of the arc are exposed to palaeo-depths in excess of 30 km. The Nacimiento and Sierra de Salinas assemblages comprise basement rocks representing Late Cretaceous variants of the Franciscan Complex and are interpreted to be correlative. They represent the lower plate of a regionally important thrust system; the upper plate is the Salinian assemblage, whereas the Sur and Salinas faults are local exposures of the structure. We concur with previous estimates of 150 to 180 km of shortening during a brief time span (<6 my), at a rate of >3 cm/yr. This fault system corresponds to the megathrust of the Farallon subduction beneath North America during the early stages of the regionally extensive episode of shallow subduction (Laramide orogeny). As a result, trench sediment was thrust under North America and tectonically underplated to the lower crust of North America. The Salinas shear zone, in particular, is a ductile expression of shallow subduction; thermobarometry in the upper plate, lower plate and the shear zone itself indicate that this is the fossil subduction megathrust originating at depths of ∼35 km. The entire system collapsed extensionally soon after the trench sediment was underthrust, possibly because of the lack of strength of the lower plate. Arc magmatism in the upper plate ceased at the onset of underplating. This regional example illustrates the significance of tectonic underplating in shallow subduction systems. Accretion-related trench sediment was shuffled from the trench to the sub-arc region of the upper plate, but not recycled into the mantle. This process requires that the subduction megathrust be located solely within the North American crust. This geometry requires a sudden migration of the subduction interface toward the arc and may apply to other regional examples, including the modern shallow subduction of the Cocos plate beneath southern Mexico. The tectonically underplated trench sediment undergoes regional, Barrovian metamorphism, after initially following a high-pressure/low-temperature path. Moreover, the shear zone marking the fossil intracrustal megathrust was subject to granulite-facies metamorphism and limited partial melting.
Objectives. The aim was to determine the safety and efficacy of gemcitabine plus cisplatin for patients with relapsed ovarian carcinoma and to compare
ex vivo drug sensitivity profiles with clinical ...outcomes.
Patients and methods. Previously treated patients with ovarian carcinoma received cisplatin (30 mg/m
2) plus gemcitabine (600–750 mg/m
2) on Days 1 and 8 of each 21-day cycle. Seventeen of the 27 patients underwent
ex vivo analyses for correlation with clinical response.
Results. Of 27 patients, there were 7 (26%) complete and 12 (44%) partial responses, for an overall response rate of 70% (95% CI: 53–87%). Toxicities included neutropenia Grade III in 51.9%, Grade IV in 29.6%; anemia Grade III in 18.5 %; thrombocytopenia Grade III in 66.7 %, Grade IV in 29.6%; nausea and vomiting Grade III in 14.8 %; peripheral neuropathy Grade III in 3.7%; and alopecia Grade IV in 11.1% of patients. The median time to progression for objective responders was 7.9 months with a range of 2.1 to 13.2 months. There were no treatment-related deaths.
Ex vivo results correlated with response, time to progression, and survival, remaining significant when adjusted for platin-resistance and number of prior therapies. Adjustment for platin-free interval decreased the significance but did not, in and of itself, predict significantly for progression-free survival.
Conclusions. Cisplatin plus gemcitabine is active for patients with relapsed ovarian cancer. Toxicities, primarily hematologic, are manageable with dose modifications. Responses observed in heavily pretreated and platin-resistant patients indicate activity in drug-refractory patients. The results of the
ex vivo analyses correlate with clinical outcomes.
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