Intra-salt structure and strain partitioning in layered evaporites: implications for drilling through Messinian salt in the eastern Mediterranean

Abstract

We use 3D seismic reflection data from the Levant margin, offshore Lebanon to investigate the structural evolution of the Messinian evaporite sequence, and how intra-salt structure and strain varies within a thick salt sheet during early-stage salt tectonics. Intra-Messinian reflectivity reveals lithological heterogeneity within the otherwise halite-dominated sequence. This leads to rheological heterogeneity, with the different mechanical properties of the various units controlling strain accommodation within the deforming salt sheet. We assess the distribution and orientation of structures, and show how intra-salt strain varies both laterally and vertically along the margin. We argue that units appearing weakly strained in seismic data, may in fact accommodate considerable sub-seismic or cryptic strain. We also discuss how the intra-salt stress state varies through time and space in response to the gravitational forces driving deformation. We conclude that efficient drilling through thick, heterogeneous salt requires a holistic understanding of the mechanical and kinematic development of the salt and its overburden. This will also enable us to build better velocity models that account for intra-salt lithological and structural complexity in order to accurately image sub-salt geological structures.

Taking the Pulse of Salt‐Detached Gravity Gliding in the Eastern Mediterranean

“…They do, however, show some variation in the precise direction of translation along margin, rotating from a more WNW bearing in the northern part of the data set (295°) to a more NNW bearing in the south (335°). This rotation in transport direction is consistent with the observed change in the strike of faults in the updip extensional domain (Evans & Jackson, 2021).…”

“…The Messinian salt layer overall thickens westward into the deep Levantine Basin and thins updip, pinching out onto the Levant Margin (Figure 5a). The evaporite sequence is lithologically heterogeneous, leading to internal seismic reflectivity within the deforming salt sheet (e.g., Figure 6) (Evans & Jackson, 2021; Feng et al., 2016; Gvirtzman et al., 2013; Meilijson et al., 2019; Netzeband et al., 2006). The intrasalt reflections are folded and faulted, and are truncated landward against the top‐salt due to an earlier, syn‐depositional phase of deformation, erosion, and dissolution (Evans & Jackson, 2021; Feng et al., 2017; Gvirtzman et al., 2013, 2017; Kartveit et al., 2018; Kirkham et al., 2020).…”

“…The supra‐salt structure of the basin can be divided into kinematically linked domains of updip extension and downdip contraction, separated by a relatively undeformed translational domain (Figure 2). The present data set mostly covers the extensional and upper translational domains, with minimal contraction in the overburden, except where the Latakia Ridge locally restricts salt flow in the north, resulting in the formation of NE‐trending buckle folds (Figure 5b) (Allen et al., 2016; Evans & Jackson, 2021). The extensional domain is dominated by salt‐detached normal faults and associated salt rollers that strike sub‐parallel to the margin, perpendicular to the base‐salt dip (Figures 5b and 6).…”

“…Due to its complex geodynamic setting, two discrete phases of salt deformation took place on the Levant margin; an early syn‐depositional phase and a later post‐overburden phase (Bertoni & Cartwright, 2007; Evans & Jackson, 2021; Feng et al., 2017; Gvirtzman et al., 2013; Kartveit et al., 2018; Netzeband et al., 2006). The early syn‐depositional phase of salt flow took place during the Messinian, folding the intra‐salt layers in the absence of a post‐salt overburden.…”

Pre‐salt rift morphology controls salt tectonics in the Campos Basin, offshore SE Brazil

“…Jackson & Hudec, 2005, 2017; Pichel et al., 2018). The RSBs in the extensional domain here are slightly unusual in that they form along‐strike of extensional rollovers (Figure 7a—structure 1, Figure 7b—structure 1; Figure 10), which means that translation of salt and its overburden downdip is partially accommodated by the coeval formation of and slip on, salt‐detached faults (Evans & Jackson, 2021).…”