What controls salt‐detached contraction in the translational domain of the outer Kwanza Basin, offshore Angola?

Aurio Erdi,Christopher A.‐L Jackson

doi:10.1111/bre.12539

Aurio Erdi, Christopher A.‐L Jackson

Open Access

https://doi.org/10.1111/bre.12539

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Abstract

AbstractIt is now well‐established that base‐salt relief drives complex deformation patterns in the mid‐slope domain of salt‐bearing passive margins, in a location classically thought to be dominated by simple horizontal translation. However, due to a lack of detailed studies drawing on high‐quality, 3D seismic reflection data, our understanding of how base‐salt relief controls four‐dimensional patterns of salt‐related deformation in natural systems remains poor. We here use 3D seismic reflection data from, and structural restorations of the Outer Kwanza Basin, offshore Angola to examine the controls on the evolution of variably oriented salt anticlines, rollers, and walls, and related normal and reverse faults. We show that the complex geometries and kinematics of predominantly contractional salt structures reflect up to 22 km of seaward flow of salt and its overburden across prominent base‐salt relief. More specifically, this contractional deformation occurs where the seaward flow of salt is inhibited due to: (a) it flowing being forced to flow up, landward‐dipping ramps; (b) it encountering thicker, slower‐moving salt near the base of seaward‐dipping ramps; or (c) the formation of primary salt welds at the upper hinge of seaward‐dipping ramps. The rate at which salt and its overburden translates seaward varies along strike due to corresponding variations in the magnitude of base‐salt relief and, at a larger, more regional scale, primary salt thickness. As a result of these along‐strike changes in translation rate, overburden rotation accompanies bulk contraction. Our study improves our understanding of salt‐related deformation on passive margins, highlighting the key role of base‐salt relief, and showing contraction, extension and rotation are fundamental processes controlling the structural style of the mid‐slope translational domains of salt basins.

Highlights

Salt-b earing passive margins are typically characterized by thin-s kinned, gravity-d riven deformation above a salt layer
The results of our study are consistent with the latter hypothesis, but we suggest that a key control on salt structure and overburden rotation is the presence of local base-salt relief
We argue that using the geometry and evolution of Ramp-syncline basins (RSBs) alone to understand the kinematics of salt-detached deformation yields an incomplete picture (e.g. Evans & Jackson, 2019; Pichel et al, 2018); such analyses should be supported by the detailed mapping of base-salt relief and overlying salt-tectonic structures

Summary

| INTRODUCTION

Salt-b earing passive margins are typically characterized by thin-s kinned, gravity-d riven deformation above a salt layer. SS1 is flanked by inward-dipping faults overlying a triangular salt pedestal, an incomplete secondary weld (sensu Wagner & Jackson, 2011), and an arched roof (Figure 6c), features characteristic of a reactive diapiric wall that was subsequently squeezed (see Dooley et al, 2009; Jackson et al, 2008; Rowan et al, 2004; Vendeville & Nilsen, 1995) We observe another two walls of this complex type in the northeast of the study area; these structures, which are separated by a primary weld and which overlie a NW-trending, seaward-dipping ramp, are both geometrically. Given the spatial relationship between base-salt structures, salt walls, and supra-s alt faults during the Late Miocene, inferred from our structural restorations, we suggest that shortening and thrusting were driven by a combination of the salt slowing, overburden shortening and primary welding above the seaward-dipping ramp (vii; Figure 9b). For full details of the model design and set-up, please see Dooley et al (2018)

| DISCUSSION

| CONCLUSION