Investigating the crustal structure of a strike‐slip “step‐over” zone along the Great Glen fault

Abstract

Strike‐slip step‐overs and bends are regions of anomalous deformation that may yield clues to the mechanical behavior of the Earth's crust. Experimental reprocessing of a marine deep seismic reflection profile cutting across an ∼35‐km‐wide right‐stepping step‐over developed on the Great Glen fault system north of the Inner Moray Firth basin (east of Northwest Highlands, Scotland) reveals a restricted zone of prominent reflections and diffractions beginning at 7–8 km depth, continuing as deep as the interpreted Moho discontinuity at 26–27 km. Geological interpretation of this zone suggests a concentration of possible diffractors marking sharp structural disruption underlain by a ∼12‐km thick layer of subhorizontal and moderately dipping reflectors that dominates the lower crust in the step‐over region. The known kinematic history of the Great Glen fault system, together with the observed direction of the step‐over, implies that a zone of contraction would have formed within the step‐over during early Caledonian sinistral strike slip. The seismic reflection structure can be interpreted as developing by deformation associated with contraction and possible block rotation between the two fault segments such that the diffractive zone represents steep structure in a near‐vertical zone of strike slip which passes deeper into a layer of low‐angle dipping thrusts or shear zones. The results of this study are consistent with a concept of mechanical detachment in the middle crust as documented for areas of contraction along active strike‐slip faults.