V.Structural Geology of Transtension

Transtension causes the long axis of the strain ellipsoid to develop at higher angles to the zone boundaries. In brittle zones incurring transtension, Riedel Shears are almost always present. When undergoing transtension, the synthetic set of Riedel Shears develops sub-parallel to the zone boundaries (Figure IV.1). In partitioned transtensional zones we should see the plate strike-slip boundary as well, similar as for the transpressional case.  

Normal faults exist in transtensional zones as they would in any other regions of extension. Normal faults in transtensional areas strike at low angles to the zone of deformation boundaries, a function of the stretching undertaken produced by pure shear. Reverse faulting occurs at high angles to the boundary consistent with fold and foliation orientations. In transtensional situations, the deformation section thins vertically thus meaning that normal faulting will be dominant. A summary of structures and characteristics of wrench tectonics can be found in the table below (Figure V.2). 

Figure 12: Table showing A summary of structures and characteristics of wrench tectonics.

Many strike-slip fault traces are present in the real world due to variations in rock metamorphic grades, existing deformation (folds and faults) and brittle / ductile rheologies Figure 13. These factors produce the variety of structures that we see in Figure 14. 

Figure 14: Shows the types of "real world" boundaries seen in Wrench Tectonic Scenarios.

When dealing with basin formation, the most important structures are bends, straights, stepovers, horses, duplexes and fans. All of these features can be seen in Figure 14 which also indicates possible crustal movement that would produce not only basins, but elevated features such as push-ups.

Straights refer to strike-slip faults that are aligned with the regional slip vector. Irregularities in the composition of faulted lithosphere cause kinks in these straights. A variation in the continuous trace of a fault is a bend. Stepovers result from the separation of the trace of a fault into two sections separated by a slab of lithosphere (Aydin and Nur, 1982). Releasing stepovers (extensional) or bends often undergo subsidence, forming a rhomb graben or a pull-apart basin if the section is sealed by faults. At restraining stepovers (convergent) uplifting occurs forming a rhomb horst or push-up.

Bends are ideal for the production of horses, which may be either subsided or uplifted. Even in pull-apart systems subsidence is most likely but not guaranteed due to the occasional presence of near-surface uplifting (Figure 15). 

In the majority of instances they are subject to subsidence, thus forming basins. Extensional duplexes are a combination of two or more horses that form at the same releasing bend that themselves contain successive normal oblique faults (N.H. Woodcock, 1994). Pull-apart basins can also form at fans the end of a fault trace that is influenced by neighbouring displacement as well as its own fault dynamics. Two types of basins exist, internal and external basins (Figure 16).

Figure 16: Shows the typical structure of a Pull Apart Basin.

This reflects their location in reference to the active strike-slip zone responsible for transtensional features. External to this are basins that have formed due to folding of adjacent upper levels of rock or bending created by upthrust blocks. Strike-slip basins are usually smaller than those created by thermal subsidence or flexure, but are elongated and deeper compared to their width. As a general rule, for every three units in length, pull-apart basins are a single unit in width.

Basin inversion can happen when there is a reversal or rotation in regional tectonics, which can turn transtension into transpression. Given the complexity of real world kinematics, it is possible for units of lithosphere to be undergoing the opposite kinematics to that of the majority of kinematics in that area. Over time if the appropriate changes eventuate, an area may switch from undergoing extension to compression and therefore inverting.