Inclined Transpression

Inclined transpression can be defined in terms of simultaneous contraction and strike-slip and dip-slip shearing, and is illustrated here using a strain triangle in which the apices represent the three strain components. Strain matrix modelling shows that all three axes of the finite strain ellipsoid are non-parallel to the Cartesian reference frame, and that they experience complex non-planar rotations during ongoing deformation. As there is a component of vorticity about each axis of the strain ellipsoid, strains have triclinic symmetry. Providing structural fabric at least partially tracks the orientation of the finite strain ellipsoid, then both strike and dip of foliation will be oblique to zone boundaries. Pitches of stretching lineations within the fabric plane are very variable, and can change during progressive deformation.

An example of an upper-crustal inclined transpression zone is exposed at Eyemouth, SE Scotland. As with most natural transpression zones, deformation at Eyemouth is highly heterogeneous, and widespread kinematic strain partitioning has given rise to an irregular distribution of strain components and resulting structures. A wide range of coeval structures are visible in outcrop and allow four different types of deformational domain to be defined. Domains are characterised by the presence or absence of dip-slip, strike-slip, oblique-slip, contractional, and hybrid structures at outcrop and map scales. Localisation of one or more types of structure occurs within deformational domains, and delineation of these can help interpretation of complex transpression zones.

Keywords
Deformational domains
Strain partitioning
Transpression
Triclinic strain symmetry