IV. Structural Geology of Transpression

The plate boundary is perhaps the most obvious strike-slip feature in partitioned zones. This fault is the largest strike-slip feature in the deformation zone and directly caused by simple shear. Even in homogeneous zones small scale strike-slip features are created, being Riedel Shears. These formations are not restricted to just homogeneous areas as they also form in partitioned regions. Most commonly forming in brittle zones, Riedel Shears are a pair of strike-slip faults that form so that their acute bisector forms parallel to the direction of the maximum compressive stress, illustrated in the transpression model in (Figure 7). Primarily strike-slip, Riedel Shears do have associated with them a small component of dip-slip. One fault will form at a low angle to the transpression zone with synthetic displacement to the boundaries. The second fault is antithetic and forms at a high angle.  

Figure 7: Shows the types of structures that form under Transpressional and Transtensional Strains.

IV.2 Thrust Faults 

Reverse faults are the product of any compressive and indeed transpressive zone that undergoes faulting. Thrust faulting tends to strike perpendicular to the direction of maximum compression, in the same direction as folds and foliation Figure 8.(b) (c) and (f). 

Figure 8: Shows structures formed in Wrench Tectonics through Simple Shear.

In many transpression settings we observe vertical thickening of the deformation zone, this proving an ideal environment for thrust faulting to arise especially when pure shear is large in magnitude and dominates. Normal faulting does occur in net transpressive settings although only as a minor feature, but when they form they strike at high angles to the zone. 

IV.3 Folds 

Folds tend to form sigmoidal patterns in rocks of a low metamorphic grade within heterogeneous shear zones. Hinges form perpendicular to the Z-axis (vertical axis) in the x-y plane, but can be subject to rotation in this plane by changing shear regimes. The standard type of folding in deformation zones undergoing transpression is the average fold system. In locations such as the San Andreas Transpression System (Figure IV.3), this is well illustrated as we see a number of separate fold axeisaligned sub-parallel to the plate boundary. 

This is a good representation of how deformation evolves due to pure shear compression. In this deformation zone it is estimated that up to 95% of lateral simple shear is taken up by the fault system, leaving the remaining component of pure shear to dominate (H. Fossen et al, 1994).

Folds are created in conjunction with Riedel Shears adopting a helicoidal geometry as the fractures twist to meet the basement of the fault zone. Two separate types of Riedel Shears exist in three dimensions (Figure 10 & 11). Firstly, concave upwards fractures are called Tulip or Negative Flower structures and exist in transtensional environments. In transpression, Palm Tree or Positive Flower structures are convex up. These features have not only been modeled in sandboxes, but have been imaged using seismic data. 

Figure 10: Diagram showing the form of Tulip Structures.

Figure 11: Diagram showing the form of Positive and Negative Flower Structures.