FAN-LIKE GROUPS OF STRATA AS AN INDICATION OF CRUSTAL EXTENSION

Abstract

In certain sedimentary basin seismic exploration has revealed geological bodies consisting of layers that taper in one direction, sometimes to the point of pinching out. These layered structures occur widely along the periphery of the Atlantic Ocean and are called 'fan-like'. Analysis of time seismic sections separates fairly objectively the sedimentary troughs created by primary vertical movements from fan-like groups of layers formed by secondary vertical movements produced by crustal extension. In the troughs, the layers taper out to all sides away from the zone of maximum warping, but fan-like groups taper in one direction only, and each layer occupies more area than the underlying one. Whereas one observes strong migration of the axial zone of the trough primary vertical warping, the axial zones remain stationary in secondary vertical warping. The time needed for the formation of the sedimentary trough can be quite long (hundreds of millions of years). On the other hand, fan-like groups form in relatively short times, usually 1 to 2 geological ages (or tens of millions of years).