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.

No abstract

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.

No abstract

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.