Strains related to normal faulting: mapping, analysis and elastic modelling

Fault interactions involve the partitioning of displacement between mechanically coupled fault surfaces and the three-dimensional strain of the intervening volume. We discuss an example from seismic reflection data from the Inner Moray Firth where, in a series of left-stepping en-echelon normal faults, the total vertical offset of a geological interface is partitioned between observed, discrete, fault offset and apparently continuous deformation (at least at the scale of observation). The latter continuous deformation is manifested as rotation of the geological interface and is recorded by measuring apparent dip along a series of parallel transects (Long & Imber 2010). Maps of the apparent dip variation serve as semi-quantitative estimates of strain in the vicinity of the faults. We use this pattern of faulting and discrete offsets to drive an elastic dislocation forward-model and examine the distribution of elastic displacements and strains in the volume. Despite the obvious mechanical contrast between a homogeneous elastic continuum and a syn-rift sand shale sequence, there is a strong correspondence between the observation-based strains and the strains predicted by the elastic model. This correspondence is particularly pronounced in the units above the fault tips where our elastic displacements recreate the seismically-imaged monoclines.