Growth of Supra-Salt Normal Fault Arrays

Abstract

The structural style and evolution of normal fault systems above mobile salt is less understood compared to those developed in rifts where strong variations in mechanical stratigraphy are absent. In this study we examine how extensional fault arrays evolve during the reactive rise, collapse and subsequent active rise of salt diapirs. We use 3D seismic data from the Egersund Basin, offshore Norway to investigate the three-dimensional growth history of supra-salt fault arrays. We study two fault arrays that are orientated nearly perpendicular to one another; Fault Array 1 (FA1), which trends NW, is located above a c. 600 m thick salt wall separated by tall (2.7 km) salt stocks, whereas Fault Array 2 (FA2), trends N and is developed above a relatively thin (c. 100 m) salt layer and a sub-salt fault array. FA1 is c. 35 km long and comprises segments up to 16 km long with throws up to 2 km. FA2 is 16 km long, comprises segments up to 6 km long with throws up to 230 m. The evolution of these two fault arrays is divided into four main stages; (1) initial syn-sedimentary fault growth during the Late Triassic–to-Middle Jurassic; (2) continued growth of FA1 during the Late Jurassic–to-Early Cretaceous at a time activity on FA2 has ceased; (3) Late Cretaceous reactivation of FA1 and FA2; and (4) death of FA2 by the end of the Late Cretaceous, whereas FA1 continued to grow until the Neogene. Even though the fault arrays were active during broadly the same time period and both initiated by cover stretching related to activity on the large, thick-skinned, basin-bounding faults, their growth mechanisms are different; FA1 was initiated through gliding, with further slip controlled by reactive diapirism, diapir collapse and active diapirism, which occurred simultaneously along strike during stages 2 to 4. FA2 was initiated by re-activation of a pre-existing sub-salt fault array, mainly through kinematic coupling, during stage 1, before being buried during stage 2. Parts of FA2 were then reactivated during stage 3, in response to salt mobilization driven by basin inversion. The reactivated faults are located where the underlying salt is thick, while the non-reactivated faults are found where salt is depleted. This study illustrates how two closely related supra-salt fault arrays may evolve differently in response to different salt-related forcing mechanisms.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015