How initial basin geometry influences gravity-driven salt tectonics: Insights from laboratory experiments

Frank Zwaan,Matthias Rosenau,Daniele Maestrelli

doi:10.1016/j.marpetgeo.2021.105195

Abstract

As a rifted margin starts to tilt due to thermal subsidence, evaporitic bodies can become unstable, initiating gravity-driven salt tectonics. Our understanding of such processes has greatly benefitted from tectonic modelling efforts, however a topic that has gotten limited attention so far is the influence of large-scale salt basin geometry on subsequent salt tectonics. The aim of this work is therefore to systematically test how salt basin geometry (initial salt basin depocenter location, i.e. where salt is thickest, as well as mean salt thickness) influence salt tectonic systems by means of analogue experiments. These experiments were analyzed qualitatively using top view photography, and quantitatively through Particle Image Velocimetry (PIV), and 3D photogrammetry (Structure-from-Motion, SfM) to obtain their surface displacement and topographic evolution. The model results show that the degree of (instantaneous) margin basin tilt, followed by the mean salt thickness are dominant factors controlling deformation, as enhancing basin tilt and/or mean salt thickness promotes deformation. Focusing on experiments with constant basin tilt and mean salt thickness to filter out these dominant factors, we find that the initial salt depocenter location has various effects on the distribution and expression of tectonic domains. Most importantly, a more upslope depocenter leads to increased downslope displacement of material, and more subsidence (localized accommodation space generation) in the upslope domain when compared to a setting involving a depocenter situated farther downslope. A significant factor in these differences is the basal drag associated with locally thinner salt layers. When comparing our results with natural examples, we find a fair correlation expressed in the links between salt depocenter location and post-salt depositional patterns: the subsidence distribution due to the specific salt depocenter location creates accommodation space for subsequent sedimentation. These correlations are applicable when interpreting the early stages of salt tectonics, when sedimentary loading has not become dominant yet.

Highlights

The deposition of extensive evaporite deposits is a common occurrence during and after continental break-up and the associated marine transgressions
Our analogue modelling efforts to study the effects of evaporite basin geometry on gravity-gliding style salt tectonics leads us to the following conclusions:
An assessment of the whole model population shows that first the degree of basin tilt, followed by the mean salt thickness are dominant factors controlling deformation

Summary

Introduction

The deposition of extensive evaporite (salt) deposits is a common occurrence during and after continental break-up and the associated marine transgressions. As the margin starts tilting due to thermal subsi dence of the adjacent oceanic basin (Fig. 1b), sufficiently large evaporitic bodies can become gravitationally unstable, initiating gravity gliding-type salt tectonics in which post-salt sediments are detached from the pre-salt substratum and transported downslope (e.g. at the Angolan and Brazilian margins of the South Atlantic, Marton et al, 2000; Fort et al, 2004a; Quirk et al, 2012; Jackson et al, 2015). It must be stressed that next to margin tilt, sedimentary loading can have an important influence on the development of salt tectonic systems and the relative significance of both driving forces remains debated (e.g. Schultz-Ela, 2001; Brun and Fort, 2011, 2012; Rowan et al, 2012; Goteti et al, 2013; Peel, 2014; Warren, 2016)

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Conclusion