Restoration of multiphase salt tectonic deformation using passive strain markers

Chris Kirkham,Joe Cartwright

doi:10.1111/bre.12564

Abstract

AbstractSeveral salt basins globally have been subject to multiphase deformation. The geometry of structures formed within a salt sheet during an early phase of deformation can be concealed by overprinting during later deformation phases, impeding an informed investigation into the early drivers for salt flow. The layered Messinian Evaporites in the deep basins of the Eastern Mediterranean have undergone a Late Messinian phase of deformation, punctuated by truncation of the top of the deformation structures, followed by a Late Pliocene‐Recent phase of deformation. Seismic reflection data show that the Messinian Evaporites are internally dominated by contraction structures that verge basinward in the Late Pliocene‐Recent flow direction, with several apparent detachments. Planform linear trails of fluid escape pipes, documented in the North Levant Basin, that cross‐cut the Messinian Evaporites present natural passive markers for the internal flow kinematics of the salt sheet throughout the Late Pliocene‐Recent phase of deformation. Using the Couette (simple shear) strain profile implied by the fluid escape pipes, we remove the effects of the Late Pliocene‐Recent deformation through cross section and map restorations. The subsequent geometries are far simpler, with upright folds that are now vertically aligned. Once apparently detached structures are demonstrably connected through the Messinian Evaporites. The vergence of the present day intrasalt folds can be taken as an indicator for the Late Pliocene‐Recent flow direction and distinct deformation geometries that connect at various levels through the salt sequence once retro‐deformed can assist when interpreting the salt flow profile. The retro‐deformation is essential for how intra‐Messinian deformation and its drivers are interpreted going forward. Furthermore, simple shear deformation results in erroneous apparent percentage shortening calculations as line length is not conserved, obscuring the strain from the pre‐Pliocene deformational phase. The methodology employed here has important implications for all salt basins that have undergone multiphase deformation.

Highlights

One of the major unresolved questions in salt tectonics is how an originally thick, basin scale salt sheet flows and deforms internally in response to different drivers for the deformation, be it differential loading, differential uplift/subsidence and tilting, or regional plate kinematics (Albertz & Ings, 2012; Brun & Fort, 2011; Gemmer et al, 2004; Rowan et al, 2004)
Using the strain implied by the deformed fluid escape pipes, we restore the observed present day deformation of an area within the translational domain and remove the effects of the Phase 2 deformation, via 2D cross section and map restorations. We show that this restoration takes account of the strain in the salt sheet related to larger scale kinematics of the linked, updip gravity-driven extensional domain
Followed by an Early-Mid Pliocene quiescent period (Elfassi et al, 2019), and a second phase in the Later Pliocene to Recent (Ben Zeev & Gvirtzman, 2020; Cartwright & Jackson, 2008; Elfassi et al, 2019). This two phase model is fully supported by the results of this study, in which we show that retro-deformation of the salt to account for the Late Pliocene to Recent phase does not remove all the internal strain: far from it, the restored maps and profiles instead reveal a simpler style of internal folding than would be apparent if it was tacitly assumed that all the folding accrued in a single phase

Summary

| INTRODUCTION

One of the major unresolved questions in salt tectonics is how an originally thick, basin scale salt sheet flows and deforms internally in response to different drivers for the deformation, be it differential loading, differential uplift/subsidence and tilting, or regional plate kinematics (Albertz & Ings, 2012; Brun & Fort, 2011; Gemmer et al, 2004; Rowan et al, 2004). To examine the effect of the Phase 2 (Late Pliocene to Recent) deformation on the internal deformation of the salt sheet, we undertook cross-sectional and map restorations from two representative subareas within the southern sector of the study area (Figure 4; see Sections 4.4 and 4.5) These two subareas were selected for the proximity to the fluid escape pipe trails related to the Oceanus and Saida-Tyr presalt structures, with a position basinward from them and the salt flow direction interpreted (Cartwright et al, 2018; Kirkham et al, 2019), the completeness of the salt sequence without significant truncation at the Top Salt and clear and distinctive intrasalt deformations that can be mapped (Figure 4; subareas A and B, respectively). | 2459 restore the structures within the maps to their original position post-Phase 1 deformation but prior to Phase 2 deformation

| RESULTS

Findings

| CONCLUSIONS