Formation, Rotation and Translation of Thrust Systems Formed at Basement Ramps During Early-Stage Salt Flow: Application to the Eastern Gulf of Mexico

Abstract

In the eastern GoM the pattern of early-stage salt flow is complicated by basement topography. This topography consists of a series of plunging arches that trend obliquely to the early flow direction. Seismic lines reveal a puzzling array of structures. Those that trend roughly N-S document predominantly extensional structures whereas E-W lines reveal shortening structures down dip of the Florida-Middle Ground Arch. A physical modeling study was designed to investigate the effect of subsalt topography on salt flow and deformation style. Our models illustrate that when salt flows from a low up onto a high, a ‘standing wave’ and associated shortening zone is formed due to a flux mismatch. Eventually, gravitational collapse of this ‘standing wave’ occurs. On the down dip edge of a subsalt high the greater flux in the thicker salt as it pulls away from the high creates a topographic hinge. Extensional graben that formed above the high due to the gravitational collapse of the ‘standing wave’ are squeezed as they pass through this hinge zone to form high-block-parallel folds. The relatively simple salt isopach in these models resulted in flow that was primarily in the dip direction. Models with dip-parallel plunging arches revealed complex divergent and convergent flow down and around these arches but the strains produced by the flow deviations were too low to produce any shortening structures. Additional models were run with plunging arches oblique to the dip direction and much more complex salt isopachs. These reveal the formation of shortening belts as the salt and its thin prekinematic roof cross the arches. Shortening amplifies as these zones move off the arch across the topographic hinge into thicker salt. The complex salt isopach deflects salt flow to produce convergent and divergent flow, and this, along with flow velocity gradients, resulted in the rotation of early-formed thrust belts. Rotations of up to 70° were recorded in the most complex model, resulting in transported fold belts with trends that were close to dip-parallel, similar to that observed on seismic data from the eastern GoM. Additional zones of shortening are found in and around complex salt pinchouts in the proximal zones of the gravity-gliding system.

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