Continental breakup is a highly variable process. Differences occur in the relative timing and extent of volcanism, uplift, and graben formation as well as in the mode and amount of continental extension before breakup. We propose a model which reconciles this variability with the previously recognized tendency for breakup to occur along preexisting weak trends. Continental lithosphere is viewed as a composite material composed of two strong layers, one in the upper mantle and one in the middle crust. Finite element simulation indicates that extensional failure at weaknesses in the mantle causes concentrated extension in the mantle and diffuse extension in the crust. This leads to early melt segregation and volcanism, margin uplift during the late stages of the extension process, and relatively narrow symmetrical extended margins. In contrast, failure at weaknesses in the crustal strong zone causes focused extension in the crust and diffuse extension in the mantle. This produces initial graben formation, cooling in the lower crust and upper mantle, and broad asymmetrical extended margins. Volcanism only occurs late in the process. Failure at laterally offset weaknesses within both strong layers leads to a deformation pattern dominated simple shear. Thus differences in the prerift configuration of the continental lithosphere can control the overall style of continental breakup.

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Decision tables have been used for many years in data processing and business applications to simulate complex rule sets. Several computer languages have been developed based on rule systems and they are easily programmed in several current languages. Land management and river-reservoir models simulate complex land management operations and reservoir management in highly regulated river systems. Decision tables are a precise yet compact way to model the rule sets and corresponding actions found in these models. In this study, we discuss the suitability of decision tables to simulate management in the river basin scale Soil and Water Assessment Tool (SWAT+) model. Decision tables are developed to simulate automated irrigation and reservoir releases. A simple auto irrigation application of decision tables was developed using plant water stress as a condition for irrigating corn in Texas. Sensitivity of the water stress trigger and irrigation application amounts were shown on soil moisture and corn yields. In addition, the Grapevine Reservoir near Dallas, Texas was used to illustrate the use of decision tables to simulate reservoir releases. The releases were conditioned on reservoir volumes and flood season. The release rules as implemented by the decision table realistically simulated flood releases as evidenced by a daily Nash-Sutcliffe Efficiency (NSE) of 0.52 and a percent bias of −1.1%. Using decision tables to simulate management in land, river, and reservoir models was shown to have several advantages over current approaches, including: (1) mature technology with considerable literature and applications; (2) ability to accurately represent complex, real world decision-making; (3) code that is efficient, modular, and easy to maintain; and (4) tables that are easy to maintain, support, and modify.

The anomalously thin crust and subsidence history of passive margins suggest that significant extension occurs within continental lithosphere prior to the onset of seafloor spreading. We estimate the amount of continental extension prior to seafloor spreading along the conjugate margins of the central and North Atlantic Oceans and Labrador Sea from the total tectonic subsidence of the margins. In the central Atlantic basin the integrated continental extension varies from 225 km for the break between the southern Appalachian segment of the North American margin and Africa to over 600 km for the break between the Blake Plateau region of North America and the Cape Verde Terrace region of Africa. The original width of the extended zone varies from 240 to 770 kni. In the North Atlantic and Labrador basins the integrated extension varies from 60 km for the break between northern Labrador and Greenland to over 600 km for the break between Newfoundland and Europe. The original width of the extended zones in these basins varies from 100 km to over 1460 km.In general, continental breakup involves two to three times less extension where it follows the preexisting structural grain than where it crosses the preexisting grain. As a result, seafloor spreading tends to begin first along segments which follow the structural grain. Extension tends to be symmetrically distributed between conjugate margins where the break follows the grain and asymmetrically distributed where the break crosses the grain. Failure parallel to the 1Now at Shell Development Company, Houston, Texas. Paper number 89TC00955.0278-7407/89/89TC-00955510.00 structural grain exhibits greater variability from margin to margin than failure across the grain. This phenomenon appears to result from differences in the nature of preexisting weaknesses within the structural grain. We conclude that the distribution of the continental extension along passive margins is to a large extent controlled by the orientation of the break relative to the structural grain and the nature of the preexisting weaknesses within that structural grain.

The prerift reconstruction of continental plates bounded by rifted margins requires the closure of relative motion accomplished both by seafloor spreading and by the extension of continental crust during the rift phase of continental breakup. Continental extension is not expected to be pervasive throughout the plate, but to be confined to a zone of up to several hundred kilometers in width. The direction of particle motion in this zone is expected to be parallel to flow lines followed by the rigid portions of the plates. The “best fit” between rifted continental margins is then described by the rotation angle about an Euler pole which best closes both the oceanic crust and the extension within the continental crust, along small circles about the Euler pole. As an example application of these concepts, the pattern of Late Mesozoic crust extension within the Gulf of Mexico basin is used to constrain the location of the Euler pole and angle of rotation of the Yucatan block with respect to North America. Both crust type and the degree of extension within the transitional crust surrounding the basin are estimated on a point‐by‐point basis from bathymetry and basement depth. The root‐mean‐square (rms) misfit in the apparent total closure of both oceanic crust and extension within the continental crust is computed for a range of possible poles of opening. A contour map of misfit versus pole location reveals a global minimum rms misfit of ±47 km for a 45° counterclockwise opening of the Gulf of Mexico basin about an Euler pole at 25°N, 79°W. An elongate trend of poles, for which the misfit is less than 100 km, extends from the Bahama Islands eastward across the Atlantic and through North Africa. The reconstruction of the Gulf of Mexico for the best fit pole places the palinspastically restored Yucatan block in contact with North America along the Texas and Louisiana shelf. The resulting restored southern margin of North America fits against the the northern margin of South America in its prerift position to within the uncertainty in the North America‐Africa‐South America plate circuit. We conclude that the prerift placement of the Yucatan block between North and South America along the Texas‐Louisiana shelf is consistent with both the pattern of extension within the Gulf of Mexico basin and seafloor spreading history of the central Atlantic basin.

The airline industry may be an occupational setting with specific health risks. Two environmental agents to which flight crews are known to be exposed are cosmic radiation and magnetic fields generated by the aircraft's electrical system. Other factors to be considered are circadian disruption and conditions specific to air travel, such as noise, vibration, mild hypoxia, reduced atmospheric pressure, low humidity, and air quality. This study investigated mortality among US commercial pilots and navigators, using proportional mortality ratios for cancer and noncancer end points. Proportional cancer mortality ratios and mortality odds ratios were also calculated for comparison to the proportional mortality ratios for cancer causes of death. Results indicated that US pilots and navigators have experienced significantly increased mortality due to cancer of the kidney and renal pelvis, motor neuron disease, and external causes. In addition, increased mortality due to prostate cancer, brain cancer, colon cancer, and cancer of the lip, buccal cavity, and pharynx was suggested. Mortality was significantly decreased for 11 causes. To determine if these health outcomes are related to occupational exposures, it will be necessary to quantify each exposure separately, to study the potential synergy of effects, and to couple this information with disease data on an individual basis.

Vertisols are complex soils with high clay content (>30%), high shrink–swell potential, and microrelief features known as gilgai We applied field and laboratory electrical‐resistivity measurements to characterize seasonal wetting and drying of a Texas Vertisol, and to quantify the effects of gilgai and cracks on seasonal hydrodynamics of the soil. Thirty‐two multielectrode resistivity‐profiling lines were collected along the same profile from 1 May 2005 to 22 Apr. 2006, using combined dipole–dipole and Schlumberger electrode configurations. The profiles were 17.5 m long and intersected two sets of microlows and microhighs of the gilgai. The resistivity data were inverted using the RES2DINV program and the inverted data were corrected for temperature. We measured variations of resistivity with soil moisture in the laboratory and the results were used to calibrate the field data. To evaluate the resistivity results, in situ measurements of soil moisture were made using auger sampling. During the wetting cycle, three distinct soil moisture regimes were recognized in the upper 1.4 m of the Vertisol: an upper zone (0–0.5‐m depth), which is the most dynamic with regard to wetting and drying; a middle zone (0.5–1.1 m), which is relatively saturated and less dynamic; and a lower zone (below 1.1 m), which is relatively less saturated compared with the middle layer. The saturation of the middle layer appears to be enhanced by preferential flow through cracks. Also, the microrelief topography exercises a control on spatiotemporal variations in soil moisture in that the microhighs dry faster than the microlows.