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?

Abstract

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.