Interacting conjugate faults and detachments: A keel and keystone abrasion model

Conjugate normal faults defining horsts and grabens are generally interpreted as forming by horsts remaining fixed at ‘regional’ levels, while intervening grabens drop-down during overall extension. The question naturally then arises as to how the base of the downthrown graben block is accommodated, with some text books suggesting that displacement dies out downwards, or alternatively, that underlying weak layers such as salt or shale act as a mobile ‘buffer’. However, not all conjugate fault systems are underlain by weak layers, and we here explore relationships where gravity-driven deformation creates conjugates that are instead bound by bed-parallel detachments (BPDs). Using the Late-Pleistocene Lisan Formation exposed around the Dead Sea as a case study, we show that the base (‘keels’) of grabens and the tops (‘keystones’) of horsts form asperities that are continually destroyed by coeval movement on BPDs in a new ‘keel and keystone abrasion model’. This process causes conjugate faults and their associated displacement-distance profiles to be abruptly truncated by BPDs, resulting in destroyed keels and keystones whose projected thickness is equivalent to the adjacent stratigraphic sequence. Bounding BPDs may be defined by multiple sub-parallel surfaces that are locally preserved within adjacent grabens and horsts, reflecting cyclical phases of incremental slip on both conjugate faults and BPDs. This repeated process may ultimately lead to a narrowing of the conjugate fault zone, rather than the progressive widening envisaged in some previous asperity abrasion models. Displaced horst and graben blocks may ‘bulldoze’ the adjacent downslope sediment, resulting in local thickening and folding of the sequence. Conjugate faults and BPDs represent bimodal faulting in a multilayered sequence, with conjugate faults displaying a mean dihedral angle of 97° about the vertical principal stress (σ₁). While the conjugate faults are commonly considered to represent a component of pure shear, the bounding BPDs are generated by non-coaxial shear that operates coevally in a partitioned deformation model. This detailed outcrop-based study helps elucidate the relationships between conjugate faults and detachments, and may also aid the interpretation of seismic sections in other gravity-driven settings where structures may be less clear or ambiguous.