Dragged and Detached Primary Welds, and Horizontal Axis Rotation of Minibasins During Contractional Reactivation of Salt Ridges: Insights From Analog Modeling

Abstract

The evolution of salt-bearing fold-and-thrust belts is highly controlled by the distribution of previous salt structures and syntectonic erosion-sedimentation processes. In this scenario, inherited salt walls or diapirs act as weak discontinuities concentrating contractional deformation while syntectonic erosion-sedimentation may localize or hinder it. The combination of both factors results in a characteristic structural style that differs from fold-and-thrust belts without evaporites. In this contribution, we present an experimental program based on physical (sandbox) models designed to test the influence of pre-existing salt structures and different syncontractional sedimentation rates in the development of fold-and-thrust belts. The experimental setup includes a polygonal pattern of minibasins developed by downbuilding and inter-minibasins salt walls and diapirs that have been subsequently shortened. The parameters tested in the experimental program are i) the thickness of the source layer, from flat-lying and constant thickness to along-strike wedge-shaped geometry and ii) the syncontractional sedimentation rate, from absent to high rate. The thickness differences in the source layer control the distribution and geometry of resulting structures. Whereas symmetrical minibasins and salt walls developed by downbuilding when salt thickness was constant, the minibasins geometry was asymmetric showing different thicknesses when the source layer had a wedge-shaped geometry. During contraction, salt walls were reactivated, progressively squeezed, welded and finally faulted mimicking the structural pattern inherited from the downbuilding stage, either symmetric or asymmetric. Once salt walls welded, intersections between them remained open preventing further squeezing of diapirs. Welding of salt walls promoted the transference of deformation to more distal salt walls by means of minibasins displacement and, therefore, primary welds moved forwards (dragged welds). Furthermore, a relatively low syncontractional sedimentary rate prevented a prompt forward propagation of deformation and increased horizontal axis rotation of minibasins. As the sedimentary rate increased the detachment of minibasins countered the minibasins rotation. The orientation of syncontractional beds enhanced the swing of minibasins during the superimposed contractional deformation.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019