Soft-linkage in thrust systems – geometry and kinematics
Soft-linked faults can be defined as structures with discontinuous fault surfaces which operate together with folds and other forms of distributed strain as a single kinematic and mechanical entity. Many thrust systems have been traditionally interpreted as hard-linked arrays of fully connected faults, although geometrically soft-linked arrays with both slip-parallel and slip-perpendicular transfer zones are possible. Although soft-linked thrust zones have been inferred in some published cross-sections, there has been little consideration of the processes and geometries expected within these implicit transfer zones.
Using high-resolution mapping data from the French Sub-Alpine Chains, the spatial relationships between individual thrust fault components of a soft-linked thrust system have been revealed in three dimensions, revealing structures including soft-linked imbricate stacks, duplexes and splays. Groups of individual thrust structures successfully define coherent kinematic entities. The critical controls on the development of soft-linked thrust systems are shown to be pre-contraction phase normal faults and the rheological stratigraphy.
It is suggested that, under favourable conditions, a phase of soft-linkage between thrust faults may be the norm, before further shortening causes breaching of the transfer zone to produce a fully-connected, hard-linked thrust system. Relict transfer zones adjacent to one-time soft-linked thrusts may be identified.