Fault and bed ‘rotation’ during continental extension: block rotation or vertical shear?

Abstract

For almost a century, the view has existed that the tilting of blocks between closely-spaced planar normal faults is rigid-body rotation. This interpretation requires only simple geometry, and has consequently found widespread application. However, it is not consistent with the deformation expected around normal faults given the present knowledge of stress fields and rheology in basement in the brittle upper crust, which is better regarded instead as distributed vertical simple shear. Rigid-body rotation and vertical shear involve different relations between fault and bed tilting, and thus predict different initial fault dips for particular present-day dips of faults and beds. These two schemes also predict different amounts of extension, and it is consequently important to establish which is correct. With this aim in mind, we examine normal faults associated with Neogene extension in western Turkey and the western United States, and with Mesozoic extension in the North Sea. Except where extension and the associated tilting are minimal, rigid-body rotation predicts unrealistically steep initial fault dips. Some extensional basins also exhibit reversals of normal fault polarity and tilt polarity of beds, which are incompatible with rigid-body rotation. We therefore conclude that the general cause of the tilting is vertical shear, not rigid-body rotation. This has three main observational consequences. First, the heave on any normal fault equals the amount of extension across it. Second, no feature near a normal fault can rotate through the vertical. A normal fault thus cannot rotate through the vertical and appear as a reverse fault. Third, any initially-vertical feature near a normal fault will remain vertical. A vertical dyke in the tilted surroundings of a normal fault is thus not necessarily younger than the extension that caused the tilting.