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Influence of Fault Separation on the Structural and Geomechanical Characteristics of Paired Crestal Grabens Developed Over an Uplifted Fault Block

Abstract

Two-dimensional geomechanical simulations of folding of a sequence of clastics, carbonate and shales over a series of high angle to vertical curved faults to create upthrown and downthrown blocks has shown the geometric and geomechanical consequence of that folding (Couples et al 2007). Typically the stratigraphically centrally placed carbonate package produces a rather simple anticlinal and synclinal form over each high angle or vertical fault with little to no thinning, the basal shales thin significantly in the anticline and thicken is the syncline, and the uppermost clastics show a behaviour intermediate between these two extremes. In all cases simulated, a graben develops at the edge of each upthrown block at or near the fold crest in the carbonates and clastics, developing patterns of shear deformation showing a distribution and offset for crestal grabens. In this study the same sedimentary package is used as for Couples et al (2007). But all faults are vertical and the separation between the two faults is varied systematically in a series of sequential simulations such that the size of the upthown block varies, and the two crestal graben approach each other as the size of the upthrown block decreases. All simulations are two-dimensional, in cross-sectional view, and start from the undeformed shape. The simulations show that when fault spacing is wide and the upthrown block is long, each crestal graben develops independent of the other. But as the two faults approach each other the crestal grabens appear to sense the presence of the other, passing through slight modifications then an intermediate stage where two superposed grabens can be distinguished. At these stages the details of both the graben faults and of the folded carbonates and clastics changes but they are still recognisable. Eventually the individual features merge to create a structure which resembles an anticlinal form termed a pop-up by a number of workers. While these are 2D simulations and miss the full consequences of true a true 3D simulation they do show characteristics that should be of use in situations where two high-angle faults approach each other. Couples, G.D. Lewis, H., Olden, P., Higgs, N, and Workman, G. 2007. New insights into the faulting process using numerical simulations of rock-layer bending. In Lewis, H. & Couples, G.D. (eds) 2007. The Relationship between Damage and Localization. Geological Society, London, Special Publications, 289, 161-186.