Analyses and modeling of gravity data in the Dead Sea pull-apart basin reveal the geometry of the basin and constrain models for its evolution. The basin is located within a valley which defines the Dead Sea transform plate boundary between Africa and Arabia. Three hundred kilometers of continuous marine gravity data, collected in a lake occupying the northern part of the basin, were integrated with land gravity data from Israel and Jordan to provide coverage to 30 km either side of the basin. Free-air and variable-density Bouguer anomaly maps, a horizontal first derivative map of the Bouguer anomaly, and gravity models of profiles across and along the basin were used with existing geological and geophysical information to infer the structure of the basin. The basin is a long (132 km), narrow (7-10 km), and deep (-<10 km) full graben which is bounded by subvertical faults along its long sides. The Bouguer anomaly along the axis of the basin decreases gradually from both the northern and southern ends, suggesting that the basin sags toward the center and is not bounded by faults at its narrow ends. The surface expression of the basin is wider at its center (<16 km) and covers the entire width of the transform valley due to the presence of shallower blocks that dip toward the basin. These blocks are interpreted to represent the widening of the basin by a passive collapse of the valley floor as the full graben deepened. The collapse was probably facilitated by movement along the normal faults that bound the transform valley. We present a model in which the geometry of the Dead Sea basin (i.e., full graben with relative along-axis symmetry) may be controlled by stretching of the entire (brittle and ductile) crust along its long axis. There is no evidence for the participation of the upper mantle in the deformation of the basin, and the Moho is not significantly elevated. The basin is probably close to being isostatically uncompensated, and thermal effects related to stretching are expected to be minimal. The amount of crustal stretching calculated from this model is 21 km and the stretching factor is 1.19. If the rate of crustal stretching is similar to the rate of relative plate motion (6 mm/yr), the basin should be --•3.5 m.y. old, in accord with geological evidence. ment discontinuities across en echelon faults in a brittleelastic medium [Rodgers, 1980; $egall and Pollard, 1980; Bilham and King, 1989]. The evolution of deep basins (deeper than 2-3 km) is expected to be more complicated as they result from either larger displacements along the fault system or from rotation of the axis of extension relative to the fault system. Furthermore, the deformation of deep 1U.S. Geological Survey, Woods Hole, Massachusetts. 2Department Paper number 93JB02025. 0148-0227/93/93JB-02025505.00 strike-slip basins can either be thin-skinned (brittle upper crust above a detachment surface [Royden, 1985]) or thickskinned involving the ductile lower crust [Christie-Blick and Biddle, 1985]. The Dead Sea basin is one of the better examp...
Proton magnetometer measurements were made from the USSR satellite Cosmos 49 during the period October 24 to November 3, 1964. A selected number of the 18,000 observations were fitted to a special field model; residual anomalies were less than 100 gammas and appear to be consistent with belts of broad anomalies that are known from surface and near‐surface magnetic measurements. Correlation of the residual anomalies with broad geologic provinces is good. A series of magnetic lows across the continental United States agrees in position with a proposed rift extending from the Cape Mendocino fracture off the west coast to the wrench fault system proposed by C. L. Drake and H. P. Woodward in the east coast area. In the western United States, large‐amplitude magnetic lows are associated with the Basin and Range province and the northern Rocky Mountains; both regions are characterized by high heat flow and similar crustal structure and Cenozoic geologic history. Most of central Wyoming, central Montana, and the area of the Colorado plateau is characterized by normal heat flow and generally positive magnetic anomalies.
Magnetic fields have been computed for 4X6 (depth units) prismatic models with thicknesses ranging from 0.1 depth unit to infinity. For each model, the azimuth and inclination of polarization have been systematically varied from 0° to 90° and from 0° to 150°, respectively. In addition, the fields were computed for inclinations of the earth's field ranging from 0° to 90°. The computation and contouring of the fields were accomplished by high speed digital computers and automatic data plotters. Both formulation and program listing are included.The 825 magnetic fields that make up this 4X6 model include the induced case and the remanent magnetization case. This suite of fields is primarily designed to aid the investigator in the geologic interpretation of observed magnetic anomalies. THE MODEL P (*. y, z)MAGNETIC NORTH MAGNETIC NORTH
A prominent aeromagnetic anomaly in Randolph County, Indiana, suggests the existence of a dikelike structure within the Precambrian basement rocks. Because of the ambiguity inherent in magnetic interpretation, a unique solution for the parameters involved (depth of burial, geometric configuration, and magnetic susceptibility of the mass producing the anomaly) is impossible. However, if one of the parameters is known it is sometimes possible to indicate a range of plausible values for each of the remaining parameters. The depth to the surface of the Precambrian rocks in Randolph County is known from drill‐hole data to be about 3,000 ft. As the depth of burial is known, limits need be set only on the thickness, angle of dip, and susceptibility contrast of the assumed dike. These limits are determined by a graphical method. Theoretical anomalies over the postulated dike are computed for different dike thicknesses, angles of dip, and susceptibility contrasts. The computed profiles are then fitted to an observed magnetic profile flown at right angles to the trend of the feature. Numerical values for the “goodness of fit” are calculated by using the statistical method of sums of squares. Two plots, one of the index of goodness of fit and the other of susceptibility as functions of dip angle and dike thickness, are made to show graphically the interdependence of the variables and the plausible range of each. The contoured plots of the goodness of fit and the susceptibility contrast show that the range of best fit is between dip angles of 30 and 60 degrees and dike thickness of 1,000 to over 4,000 ft. The contoured plot of the susceptibility contrast, translated into percentage of magnetite, shows this parameter to be independent of the dip angle, varying only with thickness.
This r e p o r t and accompanying i l l u s t r a t i o n s a r e preliminary and have not been e d i t e d o r reviewed f o r conformity t o Geological Survey standards and nomenclature. ALASKAN GEOLOGY BRATJCH
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