We develop a large strain, transient evolutionary geomechanical model of a salt extensional basin to study how stress and pressure evolve in such systems. We find that when the diapir widens faster than the imposed extension, salt overburden stress loads the wall rocks, resulting in an elevated horizontal stress despite the regional extension. Overpressures also develop near the diapir, with a pattern similar to overpressure in stable settings. However, they are generally lower than a uniaxial prediction. When the salt-supply rate decreases, owing to thinning of the source layer and/or welding, the diapir may start falling. In this case, the salt cannot absorb all the extension, and extensional strains develop in the basin. As a result, the minimum principal stress decreases. Overall, we discuss how the growth of salt can lead to high values of least principal stress and overpressure, despite the regional extension. We also discuss that diapir fall can lead to extensional turtles characterized by high shear stresses and a decreased minimum principal stress.

1. INTRODUCTION

A large number of salt basins are located in extensional systems (rift basins, passive margins; Jackson & Hudec (2017)). Examples include extensive presence of salt in Gulf of Mexico, North Sea, Campos Basin, Kwanza Basin, Lower Congo Basin (Hudec & Jackson, 2011).

Extensional salt systems have been studied extensively using analogue or physical models (e.g., Allen & Beaumont (2012), Dooley & Hudec (2016), Vendeville & Jackson (1992a), Vendeville & Jackson (1992b)), field studies (e.g., Rowan et al (2003)), subsurface interpretation (e.g., Stewart (2007)), as well as numerical models (e.g., Albertz & Beaumont (2010), Gemmer et al (2004)).

Despite the growing number of evolutionary geomechanical models of salt systems (see Nikolinakou et al (2018a) for a review), very limited studies have directly addressed the geomechanics of salt-sediment interaction under extension (a very recent study is discussed by Hamilton-Wright et al (2019)).

This content is only available via PDF.
You can access this article if you purchase or spend a download.