Deformation mechanisms of deeply buried and surface-piercing Late Pre-Cambrian to Early Cambrian Ara Salt from interior Oman

Abstract

We compared microstructures of Late PreCambrian to Early Cambrian Ara Salt diapirs from the deep subsurface (3.5-5 km) of the South Oman Salt Basin and from surface-piercing salt domes of the Ghaba Salt Basin. Laterally, these basins are approximately 500 km apart but belong to the same tectono-sedimentary system. The excellent data situation from both wells and outcrops allows a unique quantification of formation and deformation mechanisms, spanning from sedimentation to deep burial, and via re-activated diapir rise to surface piercement. Microstructures of gamma-irradiated and etched thin sections indicate dislocation creep and fluid-assisted grain boundary migration as the main deformation mechanisms operating in the deep subsurface. Microstructures from the surface are characterised by large 'old' subgrain-rich crystals. These 'old' grains are partly replaced by 'new' subgrain-free and subgrain-poor crystals, which show gamma irradiation-decorated growth bands and fibrous microstructures, indicative of pressure solution creep and static recrystallisation, most likely due to surface piercement and exposure. Using subgrain size piezometry, the maximum differential stresses for the subsurface salt is 1.7 MPa and those for the surface-piercing salt is 3.4 MPa, the latter value displaying the high stress conditions in the diapir 'stem' as the salt rises on its way to the surface.

Experimental deformation of coarse‐grained rock salt to high strain

“…During dislocation creep two dynamic recrystallization processes can occur: subgrain rotation and grain boundary migration. Although subgrains in naturally deformed rock salt are ubiquitous, the inferred main dynamic recrystallization process in naturally deformed rock salt is fluid assisted grain boundary migration recrystallization [ Urai et al ., ; Schléder and Urai , ; Schléder et al ., ; Desbois et al ., ; Schoenherr et al ., ; Kneuker et al ., ]. The size of the subgrains that form in the rock salt is related to the differential stress as derived from experimentally deformed natural rock salt [ Guillope and Poirier , ; Carter et al ., ; Schléder and Urai , ].…”

“…Deformation experiments on rock salt have led to the formulation of flow laws for dislocation creep [Carter et al, 1993;Franssen, 1994] and pressure solution [Spiers et al, 1990]. In naturally deformed rock salt evidence for both deformation mechanisms have been observed [Urai et al, 1987;Desbois et al, 2010;Schoenherr et al, 2010;Kneuker et al, 2014].Deformation experiments on single crystals of rock salt, accommodated by dislocation creep, show three different stages of strain hardening and the stress-strain curves are sigmoidal [Davidge and Pratt, 1964;Carter and Heard, 1970]. Davidge and Pratt [1964] related the first stage, at low strain, to easy glide on two orthogonal slip planes where one slip plane dominates.…”

40 Ar/ 39 Ar ages of crystallization and recrystallization of rock-forming polyhalite in Alpine rocksalt deposits

“…Prucha 1968;Schauberger 1986;Schoenherr et al 2009;Stern et al 2011). In contrast to the rheologically weak salt (halite) rocks, the rheologically stiff enclaves of, for example, polyhalite rocks are assumed to preserve information from the times of diagenesis and intense deformation events.…”

Long-term cavity closure in non-linear rocks

“…Renard et al (2001); Zubtsov et al (2004) also reported that the addition of hard particles into homogeneous salts can greatly enhance deformation by pressure solution. The law established by bodies and associated with rainwater (Desbois et al, 2010;Schléder and Urai, 2007;Schoenherr et al, 2010). In buried salts, only microstructural evidences of dislocation creep and grain boundary migration have been found.…”