Sedimentary Fracturing and Faulting Around Rapidly Moving Salt Sheets

Abstract

ABSTRACT High speed motion of salt sheets at various rates (50-150 km/Ma), depths, and salt-sheet thicknesses through sediments are illustrated in this study. Mohr's criterion for failure is used to examine the domains of primary and secondary fracturing surrounding the salt body. Sediment structures are also displayed. It is shown that (1) the higher the rate of salt motion, the larger the range of fracturing; (2) the domain of fracturing around the salt body depends on the salt size and sediment properties as well as on the salt speed. Generally the fracturing domain extends as far as 2-4 kilometers in the vertical, and to about 3-5 kilometers in the lateral directions. INTRODUCTION From recent studies of salt sheets in the Gulf of Mexico (Lowrie et al., 1990; Lowrie, 1987), there have arisen questions concerning the effects of rapidly moving salt (-150 km/Ma). Emplacement of salt masses can have a profound effect on the sedimentary formations overlying, underlying, and in front of a salt sill. This influence may be manifested in many ways, such as deformation of formations to accommodate the intrusion, development of faulting in formations overlying and underlying the salt intrusion, modification of the depositional pattern resulting in thinning of beds on approaching the sheet, and development of a thermal anomaly around the "nose" of the sill (O'Brien and Lerche, 1988). Much work has been done in connection with salt diapir and sheet emplacement (see references in Halhouty, 1979; O'Brien and Lerche, 1987). Some numerical simulations of faulting initiation in formations around a salt sheet for cases of varying salt thickness, depth of emplacement, speed of salt motion, were presented by Cao, Lerche and O'Brien (1989) when the salt motion was less than 1 km/Ma. However, studies of salt development in the Gulf of Mexico suggest that salt motion through sediments could be as fast as tens of kilometers per million years (Lowrie et al., 1990) based on ideas presented by West (1989). A problem is to estimate the effects on sediment structures and fracturing/faulting in the vicinity of such rapidly moving salt sheets. In this paper, numerical results are presented for the initiation of faulting in formations overlying and underlying a rapidly moving salt sheet. These results may be extended to all rifting basins characterized by salt deposition and subsequent salt tectonics. A better understanding of the high speed salt tectonic results will help in the evaluation of salt development and its impacts in Gulf of Mexico and other similar basins. The technique used here is based on analysis of the stress induced in the deformed formations surrounding a moving salt sheet. To underscore the physical processes which are operating, a model is invoked of the deformation of the sedimentary formations surrounding a salt sheet; however, this analysis may also be extended to the case where the deformation of the formations is estimated from available observational data. Having specified the deformation of the sedimentary formations, the stresses which generated these deformations are then inferred through application of the theory of elasticity. In this way, the state of stress can be modelled in the formations surrounding a salt sheet during the time in which the sheet is being salt sheet which, in turn, forces a greater degree of strain on the sediments.