Scientific Quarterly Journal of Geosciences

Scientific Quarterly Journal of Geosciences

Modelling of Strike-Slip Faults

Document Type : Original Research Paper

Author
Hasan Shahrivar
Research Institute for Earth Sciences, Geological Survey of Iran, Tehran, Iran
Abstract
According to Sylvester (1988) a strike slip fault is a fault on which most of the movement is parallel to the fault's strike. They could be 100 km in length and amount of displacement is between few millimeters to tens of kilometers. Geologically we have en echelon arrays of fractures, faults and folds in narrow zones. In addition, strike slip faults displace structures like faults, folds, dikes, foliations, sills; they put rocks with different age, lithology, facies and structure near each other.
     In strike slip faults there are three major structures: 1- Shortening; 2-Extension; 3- Horizontal shear. Two mechanisms explain the geometric and dynamic relation among these faults and associated structures:
1- Pure shear; 2- Simple shear
    The major strike slip faults of the world fall in the domain of simple shear (Sylvester, 1988).
      These faults have been studied by making models in experimental tectonic laboratories. Five sets of fractures form in simple shear in model experiments, experimental deformation of homogeneous rocks under confining pressure, and in alluvium deformed by surface rupturing during earthquakes (Sylvester, 1988):
1- Riedel shears (R) at an angle of 12 with principal displacement zone (PDZ) and the sense of strike slipping is the same as PDZ (the first order structure).
2- Conjugate Riedel shears (R) at angle of 90 - 12 with (PDZ) and the sense of strike slipping is opposite to the PDZ.
3- P shears at an angle of 12 to the (PDZ) and the sense of strike slipping is the same as PDZ (the second order structure).
4- Extension fractures (T) which develop at about 45 to the PDZ.
5- Faults parallel (M) to the PDZ.
Furthermore, in the third dimension the axial surface of en echelon folds and riedel shears, flower structures can be observed, which depending on the direction of movement, they subdivide to positive and negative flower structures. As Harding (1985) shows in seismic profiles, a positive flower structure is defined as a linear antiform with upward and outward diverging movement of blocks in the strike slip zone. The antiform is sub parallel to and thereby differs from the oblique orientation of en echelon that can form externally to the zone, these initial antiforms in positive flower structures can be potential for hydrocarbon traps.
      Experiments of strike slip faults started in 1928 by Cloos. Tchalenko (1968 and 1970) used shear box and clay to study the surface effects of strike slip fault and compared them with an example in natural example of "Dasht-e-Bayaz" fault (Iran). Naylor (1986) studied strike slip fault deformation in dry sand.
      In order to apply model results to nature, model should be scaled kinematically, geometrically and dynamically to nature. Stress ratio for model and nature is;
For a model to simulate its natural prototype, a set of dimensionless ratios which relate the physical properties of model material and natural rocks should be similar (Koyi and Peterson 1993). Here 4 sand model are prepared and defined in order to study the geometry and kinematics of flower structures.
Keywords
Strike-Slip Faults, Modelling

Subjects

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Scientific Quarterly Journal of Geosciences
Volume 5, 17-18
Autumn & Winter 1996, Vol. 5, No. 17-18
Winter 1996
Pages 80-91