Set Of Gravity Anomalies Research Articles

3D spatial domain gravity inversion with growing multiple polygonal cross-sections and exponential mass density contrast

An automatic 3D spatial domain inversion technique is developed to estimate basement depths of sedimentary basins from observed gravity anomalies using a prescribed exponential mass density contrast. A collage of vertical polygonal cross-sections, each one with unit thickness, in which the density contrast differs exponentially with depth describes the model space. The proposed technique estimates the optimum depth ordinates of the vertices of polygonal cross-sections from a given set of gravity anomalies following predefined convergence criteria. Initial depths to basement interface at plurality of observations are calculated presuming that the density contrast within the Bouguer slab at each observation is also varying exponentially with depth. A previously reported algorithm that make use of both analytic and numeric approaches to compute the gravity response of such 3D model space with exponential mass density contrast is adopted for forward modelling. The proposed inversion is efficient even when the gravity anomalies are available at non-uniform spatial grid intervals. Recovery of basement depths with modest error from a set of gravity anomalies attributable to a synthetic model in the presence of pseudorandom noise and also the fact that the estimated depth structure of the Almazan Basin in NE Spain correlates reasonably well with the information derived from seismic data demonstrates the applicability of the proposed inversion method. The snags associated with other existing density models in the analysis of gravity anomalies are demonstrated on both synthetic and real field anomalies.

Global tectonics and metallogeny: Deep roots of some ore-controlling fracture zones. A possible relation to small-scale convective cells at the base of the lithosphere?

Criteria suggest that some of the ore-controlling fracture zones have deep roots, some extending into the upper mantle. The question is analyzed of whether a pattern of deep-seated fracture zones, extending to the base of a continental lithosphere, may be related to, or interact with, the small-scale convective cells (rolls) which originate [1,2] in the mantle beneath a moving lithospheric plate. The analysis follows a previous application of the same concept by others [3,4] who explained the origin of a chain of volcanoes by the movement of an oceanic lithospheric plate over a pattern of small-scale convective cells of the underlying mantle. The Canadian Shield is used to discuss the above question. A correlation has been found between: (1) the direction of two sets of long wavelength gravity anomalies detected by Stephenson and Beaumont, [8] in a study of the isostatic response of the Canadian Shield, and (2) the pattern of N-S and E-W trending trajectories of ore-controlling fracture zones, postulated by the author [5,6] in the southern part of the Canadian Shield. The Hudson Bay Paleolineament [7] correlates very well with the N-S set of gravity anomalies. The above correlation suggests that the ore-controlling lineaments and the long wavelength anomalies of the Canadian Shield are related. If the gravity anomalies reflect, as Stephenson and Beaumont [8] tentatively suggest, a pattern of convective cells beneath the base of the lithosphere, then the volcanic activity and metallogenesis may be related to the boundaries and corners of the cells. The author suggests that the convective cells could, in this case, originate by melting of the mantle material proceeding preferentially along the intersecting deep-seated fracture zones. The pattern of deep-seated fracture zones, compiled for the western United States [9] also shows a relationship of major ore deposits and ore clusters to the corners of rectangular blocks, defined by mutual intersection of the E-W and N-S fracture zones. In this case, the size of the blocks, measured in an E-W direction, is about 530 km, and in a N-S direction about 600 km. These figures are significantly close to the distance from the seismic discontinuity at a depth of 650 km, which is considered by others [2,4] as the lower boundary of the small-scale convection.