Strike-slip tectonics or wrench tectonics is a type of tectonics that is dominated by lateral (horizontal) movements within the Earth's crust (and lithosphere). Where a zone of strike-slip tectonics forms the boundary between two tectonic plates, this is known as a transform or conservative plate boundary. Areas of strike-slip tectonics are characterised by particular deformation styles including: stepovers, Riedel shears, flower structures and strike-slip duplexes. Where the displacement along a zone of strike-slip deviates from parallelism with the zone itself, the style becomes either transpressional or transtensional depending on the sense of deviation. Strike-slip tectonics is characteristic of several geological environments, including oceanic and continental transform faults, zones of oblique collision and the deforming foreland of zones of continental collision.
Deformation styles
Stepovers When strike-slip fault zones develop, they typically form as several separate fault segments that are offset from each other. The areas between the ends of adjacent segments are known as stepovers. In the case of a dextral fault zone, a right-stepping offset is known as an extensional stepover as movement on the two segments leads to extensional deformation in the zone of offset, while a left-stepping offset is known as a compressional stepover. For active strike-slip systems, earthquake ruptures may jump from one segment to another across the intervening stepover, if the offset is not too great. Numerical modelling has suggested that jumps of at least 8 km, or possibly more are feasible. This is backed up by evidence that the rupture of the 2001 Kunlun earthquake jumped more than 10 km across an extensional stepover. The presence of stepovers during the rupture of strike-slip fault zones has been associated with the initiation of supershear propagation (propagation in excess of the S wave velocity) during earthquake rupture.
Riedel shear structures In the early stages of strike-slip fault formation, displacement within basement rocks produces characteristic fault structures within the overlying cover. This will also be the case where an active strike-slip zone lies within an area of continuing sedimentation. At low levels of strain, the overall simple shear causes a set of small faults to form. The dominant set, known as R shears, forms at about 15° to the underlying fault with the same shear sense. The R shears are then linked by a second set, the R' shears, that forms at about 75° to the main fault trace. These two fault orientations can be understood as conjugate fault sets at 30° to the short axis of the instantaneous strain ellipse associated with the simple shear strain field caused by the displacements applied at the base of the cover sequence. With further displacement, the Riedel fault segments will tend to become fully linked until a throughgoing fault is formed. The linkage often occurs with the development of a further set of shears known as 'P shears', which are roughly symmetrical to the R shears relative to the overall shear direction. The somewhat oblique segments will link downwards into the fault at the base of the cover sequence with a helicoidal geometry.
Flower structures In detail, many strike-slip faults at surface consist of en echelon or braided segments, which in many cases were probably inherited from previously formed Riedel shears. In cross-section, the displacements are dominantly reverse or normal in type depending on whether the overall fault geometry is transpressional (i.e. with a small component of shortening) or transtensional (with a small component of extension). As the faults tend to join downwards onto a single strand in basement, the geometry has led to these being termed flower structure. Fault zones with dominantly reverse faulting are known as positive flowers, while those with dominantly normal offsets are known as negative flowers. The identification of such structures, particularly where positive and negative flowers are developed on different segments of the same fault, are regarded as reliable indicators of strike-slip.
… excerpt ends here. Continue reading the full article.
