Investigating the Effect of Sediment Loading on the Growth of a Shale-Cored Anticline Using Finite Element Modelling

Abstract

ABSTRACT Large-scale folding of sedimentary rock is generally considered to be a response to horizontal tectonic shortening. We test an alternative hypothesis where we propose that in basins with high sedimentation rates where folds are cored by mechanically weak mobile shale, fold growth can be amplified by the gravitational loading of the weak underlying shale. We use two-dimensional plane-strain, finite element code to investigate the mechanics of growth of a shale-cored fold in the South Caspian Sea Basin, where c.10 km of sediment was deposited in the last 6 Myr. The overburden and syn-kinematic sediments are modelled as poro-elastoplastic materials using a modified Cam-Clay critical state model and the mobile shale is modelled as visco-plastic Herschell-Bulkley material, at critical state. The results show that the atypical geometries of the fold strata can be explained by the application of horizontal shortening and simultaneous sediment loading of the visco-plastic layer. The viscosity of the shale determines whether differential loading will cause fold growth and its density controls the magnitude of fold amplification, with a lower density causing greater fold amplification. Results demonstrate that the magnitude of shale inflation is controlled by complex interaction of the relative amounts of shortening and sedimentation rate. INTRODUCTION Mobile shale tectonics has been documented in 65 areas across the world, including the Niger Delta, the Gulf of Mexico, the Caribbean Basin and the South Caspian Basin (Soto et al., 2021a). However, unlike salt tectonics, very few studies which investigate the role of sediment loading on the deformation history within these regions have been published. Although both salt and mobile shale can produce thin-skinned deformation through extension, shortening and loading (Morley and Guerin, 1996), the intrinsic (material) properties of both mediums mean they act and respond differently depending on the local conditions (Jackson and Vendeville, 1994). Whist properties of salt remain relatively constant, both the composition of shale and its properties can vary through time, depending on compaction and depth (Ewy et al., 2020), with shale movement dependent primarily on the degree of overpressure within the pore fluids (Morley and Guerin, 1996). The effect of sediment load in salt tectonics has been demonstrated frequently (Waltham, 1997, Gemmer et al., 2004, Hudec and Jackson, 2007) and is well understood, however, deformation of mobile shale is relatively poorly understood. It is generally agreed that shale can behave as a visco-plastic solid (Dean et al., 2015, Albertz et al., 2010, Ings and Beaumont, 2010, Soto et al., 2021a) but can also behave as a suspension fluid as demonstrated by the wide-spread occurrence of mud volcanoes in shale provinces such as Trinidad, Nigeria and both on and offshore Azerbaijan (Barboza and Boettcher, 2000, Boettcher et al., 2000, Fowler et al., 2000, Graue, 2000, Hudec and Soto, 2021). Several large-scale numerical models have attempted to model mobile shale tectonics, for example the effect of delta propagation on mobile shales in the Niger Delta (Albertz et al., 2010, Ings and Beaumont, 2010, Dean et al., 2015). Here we examine the growth of a single, large km-scale, fold and conduct numerical models to test whether sediment loading above a thick mobile shale layer can contribute to the growth of such structures.