Normal fault terminations in limestones from the SE-Basin (France): implications for fluid flow

Abstract

AbstractThe geometry and evolution of normal fault terminations were studied in Tithonian limestones exposed on a vertical cliff in the French SE-Basin. The rocks consist of mainly limestone layers alternated with thin clayey interlayers. All studied fault zones die out vertically into bed-perpendicular veins striking approximately parallel to the fault. Displacement decreasing to zero towards the fault tip is accommodated horizontally by bed-parallel opening of calcite veins, and vertically by bed-perpendicular localized compaction. The latter mechanism leads the clayey interlayers to be thinned and in places completely pushed out, and enhances pressure solution in bed-parallel seams. The respective thicknesses of the limestone layers and clayey interlayers, and the ratio between local displacement amount and bed thickness influence the geometries of the fault termination and of the steps between slip surfaces. Relatively thick clayey interlayers localize low-angle slip surfaces and may impede the vertical propagation of the slip surface. Vertical fault restriction is also related to thick limestone layers, which are deflected and affected by outer arc extensional fractures, localized pressure solution and dilational jogs connecting adjacent propagating slip surfaces. However, beds keep their continuity if thicker than the local displacement amount. Where the local displacement is larger than the layer thickness, limestone beds are disconnected and clayey interlayers are completely cut but the slip surfaces. Tip-point veins, as well as outer arc veins, do not cross the clayey interlayers and fluid flow is local and confined within one limestone layer. In contrast, dilational jogs in places cut through several layers, and the breaking of clayey interlayers causes an increase in fluid flow. Conduits can be opened along the fault zone and fluids are driven into the jog, so slip surfaces may communicate separate reservoirs, until dilational jogs are sealed by mineral precipitation.