Metamorphism of the deepest exposed arc rocks in the Cretaceous to Paleogene Cascades belt, Washington: evidence for large-scale vertical motion in a continental arc

The Swakane Gneiss and the overlying Napeequa Complex in the North Cascade range, Washington, were metamorphosed and deformed during development of a Cretaceous‐Paleogene continental arc, and are among the structurally deepest exposed rocks within the Cordilleran arcs of North America. Peak metamorphic conditions in both the Swakane Gneiss and Napeequa Complex were c. 640–750 °C, 9–12 kbar. Clockwise paths and widespread evidence for high‐P metamorphism in meta‐supracrustal rocks (burial to >40 km) document major vertical tectonic motion during arc construction and unroofing. These and other moderately high‐pressure rocks in the North Cascades‐Coast Mountains experienced a dramatically different tectonometamorphic history than metamorphic rocks within other Cordilleran arcs. The exhumed arc complexes of the Sierra Nevada and Peninsular Ranges are dominated by relatively low‐P metamorphic and plutonic rocks (typically <6 kbar). There is no evidence that the northern Cordillera was thickened to a greater degree than these other belts, suggesting that the greater magnitude of vertical motion in the Cascades may have been related to exhumation mechanisms: Eocene extension in the northern Cordillera vs. erosional unroofing in the central and southern Cordillera.     

Versatile Two-Level Schwarz Preconditioners for Multiphase Flow

Numerically modeling groundwater flow on finely discretized two- and three-dimensional domains requires solution algorithms appropriate for distributed memory multiprocessor architectures. Multilevel and domain decomposition algorithms are appropriate for preconditioning or solving linear systems in parallel and have, therefore, been applied to linear models for saturated groundwater flow. These algorithms have also been incorporated into more complex nonlinear multiphase flow models in the context of a linearization procedure such as Newton's method. In this work, we study a class of parallel preconditioners based on two-level Schwarz domain decomposition applied in a nonlinear two-phase flow numerical model. The restriction and interpolation operators are based on an aggregation approach that has a straightforward implementation for a variety of applications arising in subsurface modeling: structured and unstructured discretizations, finite elements and finite differences, and multicomponent model equations. We present model formulations, results from numerical experiments, and a comparison of a standard one-level Schwarz method to three two-level aggregation-based methods.     

Morphology and surface brightness evolution of z∼1.1 radio galaxies

We use K '-band (2.1-μm) imaging to investigate the angular size and morphology of 10 6C radio galaxies, at redshifts 1≤ z ≤1.4. Two radio galaxies appear to be undergoing mergers, another contains, within a single envelope, two intensity peaks aligned with the radio jets, while the other seven appear consistent with being normal ellipticals in the K band.
Intrinsic half-light radii are estimated from the areas of each radio galaxy image above a series of thresholds. The 6C galaxy radii are found to be significantly smaller than those of the more radio-luminous 3CR galaxies at similar redshifts. This would indicate that the higher mean K -band luminosity of the 3CR galaxies reflects a difference in the size of the host galaxies, and not solely a difference in the power of the active nuclei.
The size–luminosity relation of the z ∼1.1 6C galaxies indicates a 1.0–1.6 mag enhancement of their rest frame R -band surface brightness relative to either local ellipticals of the same size or FRII radio galaxies at z <0.2. The 3CR galaxies at z ∼1.1 show a comparable enhancement in surface brightness. The mean radius of the 6C galaxies suggests that they evolve into ellipticals of L ∼ L * luminosity, and is consistent with their low-redshift counterparts being relatively small FRII galaxies ∼25 times lower in radio luminosity, or small FRI galaxies ∼1000 times lower in radio luminosity. Hence the 6C radio galaxies appear to undergo as much optical and radio evolution as the 3CR galaxies.     

Inexact Newton methods and the method of lines for solving Richards' equation in two space dimensions

Richards' equation (RE) is often used to model flow in unsaturated porous media. This model captures physical effects, such as sharp fronts in fluid pressures and saturations, which are present in more complex models of multiphase flow. The numerical solution of RE is difficult not only because of these physical effects but also because of the mathematical problems that arise in dealing with the nonlinearities. The method of lines has been shown to be very effective for solving RE in one space dimension. When solving RE in two space dimensions, direct methods for solving the linearized problem for the Newton step are impractical. In this work, we show how the method of lines and Newton-iterative methods, which solve linear equations with iterative methods, can be applied to RE in two space dimensions. We present theoretical results on convergence and use that theory to design an adaptive method for computation of the linear tolerance. Numerical results show the method to be effective and robust compared with an existing approach.     

Population Estimation Error and Its Impact on 1991–1999 Cancer Rates*

The error of closure in the 2000 census, or the difference between estimated and enumerated populations, poses special problems for public agencies that rely on census data. Nationally and at the state level, populations were only slightly underestimated, but underestimations were high in rapidly growing counties in the South and intermontaine West, as well as in New York City. Race‐specific estimates proved far less reliable, with severe overestimates and underestimates of all racial groups in various counties nationwide. We offer explanations for the estimation error and discuss its impact on cancer rates and trends and its implications for cancer surveillance research.     

The chemistry of star formation

Understanding the chemistry of molecular clouds is now realized to be fundamental to our understanding of the whole star-formation process. A two-day meeting was held within NAM2002 to bring together chemists and astronomers to discuss this complex subject. Derek Ward-Thompson, Helen Fraser and Jonathan Rawlings report.  

     

Comparing shear-wave velocity profiles inverted from multichannel surface wave with borehole measurements

Recent field tests illustrate the accuracy and consistency of calculating near-surface shear (S)-wave velocities using multichannel analysis of surface waves (MASW). S-wave velocity profiles (S-wave velocity vs. depth) derived from MASW compared favorably to direct borehole measurements at sites in Kansas, British Columbia, and Wyoming. Effects of changing the total number of recording channels, sampling interval, source offset, and receiver spacing on the inverted S-wave velocity were studied at a test site in Lawrence, Kansas. On the average, the difference between MASW calculated V_s and borehole measured V_s in eight wells along the Fraser River in Vancouver, Canada was less than 15%. One of the eight wells was a blind test well with the calculated overall difference between MASW and borehole measurements less than 9%. No systematic differences were observed in derived V_s values from any of the eight test sites. Surface wave analysis performed on surface data from Wyoming provided S-wave velocities in near-surface materials. Velocity profiles from MASW were confirmed by measurements based on suspension log analysis.