| Transpression and transtension were terms introduces by Harland (1971) to define
deformation that involves both transcurrent (strike-slip) movement along a zone and
compression or extension across it. Sanderson & Marchini (1984) produced a strain model for
transpression, and the concept has subsequently been applied in a variety of tectonic settings
over a wide range of scales.
Transpression is modelled by the simultaneous application of a transcurrent shear and
horizontal shortening orthogonal to a block, with no lateral stretch. Sanderson &
Marchini originally used two parameters α (the vertical elongation) and γ (the
shear strain on the zone boundary) to define the deformation within the block. For
constant volume deformation, the shortening across the zone is simply β = α-1, but
volume change (Δ) is easily incorporated in the models, where α β = (1+Δ).
One may also specify transpression in terms of the strain rates (-Ésgγ) and the
direction (A) and amount (S) of convergence/divergence, where tan A = -γ /
-É.
The transpressional model has a number of important implications, which include:
It generally leads to triaxial deformation, hence is intrinsically 3-dimensional,
e.g. flattening strains characterise transpressional zones, whereas constrictional
strains result from transtension.
It represents a spectrum of strain states, providing a useful way of
classifying deformational styles between generalised compressional, strike-slip
and extensional regimes. The vorticity axis will be normal to the shear direction
(vertical) and does not need to be parallel to the intermediate principle stain axis.
At a convergence angle of A -70.5O the incremental and finite strain axes may
be differently oriented and this may produce situations where structures may
appear to develop in unusual orientations with respect to the finite strain fabrics
Both the compressional and shear components contribute to the stretch Sn
normal to the zone, where Sn = (α2 + γ2)-1 -2
. Thus the shortening (stretch)
measured across a transpression zone (including simple shear zones), by methods
such as balancing sections, should not be confused with the parameter β, which
measures the convergence of the zone boundaries.
It may be applied to zones of deformation on a variety of scales.
It has been generalised to include several other components of deformation,
including lateral extrusion, vertical shear both parallel and normal to the zone
and oblique shear, inclined shear, etc.
The controlling parameters may vary spatially within a zone (spatial partitioning) or with time
during its evolution (temporal partitioning). In many settings involving oblique convergence,
the strike-slip component may be taken up mainly on faults within the zone, whilst the
compression is more broadly distributed. Spatial partitioning can allow continuity across the
zone boundary (e.g. Robin & Cruden 1994). |