Fault and fracture patterns around a strike-slip influenced salt wall

Abstract

The trends of faults and fractures in overburden next to a salt diapir are generally considered to be either parallel to the salt margin to form concentric patterns, or at right angles to the salt contact to create an overall radial distribution around the diapir. However, these simple diapir-related patterns may become more complex if regional tectonics influences the siting and growth of a diapir. Using the Sedom salt wall in the Dead Sea Fault system as our case study, we examine the influence of regional strike-slip faulting on fracture patterns around a salt diapir. This type of influence is important in general as the distribution and orientation of fractures on all scales may influence permeability and hence control fluid and hydrocarbon flow. Fractures adjacent to the N-S trending salt wall contain fibrous gypsum veins and injected clastic dykes, attesting to high fluid pressures adjacent to the diapir. Next to the western flank of the salt wall, broad (∼1000 m) zones of upturn or ‘drape folds’ are associated with NW-SE striking conjugate extensional fractures within the overburden. Within 300 m of the salt contact, fracture patterns in map view display a progressive ∼30°–35° clockwise rotation with more NNW-SSE strikes immediately adjacent to the salt wall. While some extensional faults display growth geometries, indicating that they were syn-depositional and initiated prior to tilting of beds associated with drape folding, other fractures display increasing dips towards the salt, suggesting that they have formed during upturn of bedding near the diapir. These observations collectively suggest that many fractures developed to accommodate rotation of beds during drape folding. Extensional fractures in the overburden define a mean strike that is ∼45° anticlockwise (counter-clockwise) of the N-S trending salt wall, and are therefore consistent with sinistral transtension along the N-S trending Sedom Fault that underlies the salt wall. Our outcrop analysis reveals fracture geometries that are related to both tilting of beds during drape folding, and regional strike-slip tectonics. The presence of faults and fractures that interact with drape folds suggests that deformation in overburden next to salt cannot be simply pigeon-holed into ‘end-member’ scenarios of purely brittle faulting or viscous flow.