Deformation styles ________________________ 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, form at about 15° to the underlying fault with the same shear sense. The R shears are then linked by a second set, the R shear that form 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, often with the development of a further set of shears known as P shears, which are roughly symmetrical to the R shears with respect to the overall shear direction, until a throughgoing fault is formed. 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 and/or braided segments in many cases 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, 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. Strike slip duplexes Strike slip duplexes occur at the step over regions of faults, forming a lens shaped near parallel arrays of horses. These occur between two or more large bounding faults which usually have large displacement. An idealized strike-slip fault runs in a straight line with a vertical dip and has only horizontal motion, thus there is no change in topography due to motion of the fault. In reality, as strike slip faults become large and developed, their behavior changes and becomes more complex. A long strike slip fault follows a staircase-like trajectory consisting of interspaced fault planes that follow the main fault direction.[6] These sub parallel stretches are isolated by offsets at first, but over long periods of time they can become connected by step overs in order to accommodate the strike slip displacement.[5] In long stretches of strike-slip the fault plane can start to curve, giving rise to structures similar to step overs. Right lateral motion of a strike slip fault at a right step over (or overstep) gives rise to extensional bends characterised by zones of subsidence, local normal faults, and pull apart basins. On extensional duplexes, normal faults will accommodate the vertical motion, creating negative relief. Similarly, left stepping at a dextral fault generates contractional bends; shortening the step overs which is displayed by local reverse faults, push-up zones, and folds. On contractional duplex structures, thrust faults will accommodate vertical displacement rather than being folded, as the uplifting process is more energy efficient.[ Strike slip dulexes are passive structures; they form as a response to displacement of the bounding fault rather than by the stresses from plate motion. Each horse has a length that varies from half to twice the spacing between the bounding fault planes. Depending on the properties of the rocks and the fault, the duplexes will have different length ratios and will develop on either major or subtle offsets, although it is possible to observe duplex structures that develop on nearly straight fault segments. Because the motion of the duplexes may be heterogeneous, the individual horses can experience a rotation with a horizontal axis, which results in the formation of scissor faults. Scissor faults exhibit normal motion at one end of the horse and a thrust motion ant the other end.[ Because strike slip duplexes structures have more horizontal motion than vertical motion, they are best observed on a map rather than a vertical projection, and are a good indication that the main fault has a strike slip motion. An example of strike slip duplexes were observed in the Lambertville sill, New Jersey.[ Flemington and the Hopewell faults, the two main faults in the region, experienced 3km of dip slip and over 20km of strike slip motions to accommodate regional extension. It is possible to trace the lensoidal structures which are interpreted as horses that form duplexes. The lens structures observed in the 3M quarry are 180 meters long and 10 meters wide. The main duplex is 30 m in length and other smaller duplexes are also present. _____________________________________ Geo.Bayan Aleker
Posted on: Mon, 15 Sep 2014 17:47:08 +0000
Trending Topics
Recently Viewed Topics
© 2015