STRESS FIELD IN A SHEAR ZONE, AND FORMATION OF THE MAIN FAULT

Using the analytical approximation method, we calculated stress field parameters for cases with different relative positions of Riedel shears and loads required for shearing. Considering an internal friction angle of 30°, and the distance between adjacent shears exceeding 0.7 of the characteristic shear length, we estimated the Coulomb stress that can lead to fracturing. In the areas between the shears, it is below the shear strength value. This means that if an increase in the external load is lacking, there are no prerequisites for the formation of new fractures that may connect adjacent shears. If the shears are spaced closer to each other (i.e. at distances less than 0.7 of the shear length), the shear strength is exceeded in the areas between them, and new shears can occur there and connect the Riedel shears to each other. Therefore, in observations of a natural system of Riedel shears, it becomes possible to assess whether this system is sufficiently stable in its current status, or, in case of a critical increase in the Coulomb stress in the areas between adjacent shears, the equilibrium can be easily disturbed, and there is a possibility that the main fault forms in the strike-slip zone under study.

New structural model of strike-slip tectonics of the Gulf of Aqaba, southern Dead Sea Transform

<p>The Dead Sea Transform is an active left lateral, strike-slip plate boundary. The Gulf of Aqaba corresponds to its southern segment, where the largest amount of opening is observed. The gulf itself is deformed by a set of en echelon faults which are bounded by normal faults. These en echelon faults show structural styles of Riedel shears which are typically observed in strike-slip tectonics. However, their orientation is the opposite to the one observed in well described models or natural cases. In this study, we compare a compiled dataset to analogue models which simulate the displacement in various strike-slip systems. This comparison to a sandbox model highlights the importance of the tectonic load in a strike-slip fault system. The model is composed of two base plates with only one straight velocity discontinuity. X-Ray Computed Tomography is used as a technique to carry out a 4D analysis of internal fault structures of the model. The 10°-transtensional model generates a set of Riedel shear faults, which merge during the later stages of deformation. The 30°-transtensional tectonic load shows two major steep bounding faults with a dip-slip component and a set of en echelon faults - opposite Riedel shears in between them. A higher amount of transtension rotates the classic Riedel shear faults to the opposite position. This fault pattern is very similar to the one observed in the Gulf of Aqaba, where the internal fault system is composed of opposite Riedel shears bounded by normal faults. These observations can increase the understanding of the structural styles seen in the Gulf of Aqaba. Moreover, our study describes a new strike-slip fault system.</p>

Mesofabric, microfabric, and submicrofabric of ice-thrust bedrock, Highvale mine, Wabamun Lake area, Alberta

The fabric of the ice-thrust argillaceous bedrock from a shear zone was studied in hand specimens, under a polarizing microscope and a scanning electron microscope. The fabric included principal displacement shears, Riedel shears, conjugate sets of particle alignments, cutans, lithorelics, and aggregations which dense cores of randomly oriented groups of clay platelets wrapped by an external layer of oriented clay particles in a turbostatic arrangement. In addition, the bedrock has been disturbed by permafrost, cycles of loading and unloading, and weathering, causing the magnitude of deformation to vary within the ice-thrust shear zone.The fabric of the ice-thrust shear zone is similar to that of shear zones formed by tectonic activity and by laboratory shear tests, suggesting that all these shear zones were formed under conditions of similar kinematic restraint.

The analysis of en-échelon veins

SummaryMeasurements of en-échelon veins cutting small chevron folds in Carboniferous greywackes, near Hartland in Devon, allow the following new conclusions about naturally occurring second-order fractures to be stated. 1. Veins may be initiated between 10° and 46° to the shear zone in which they occur. Previous studies of en-échelon fractures have generally classified them as either tension gashes formed at 45° to an array, or Riedel shears at 15° to an array. 2. Veins at about 20° to 40° to an array probably occupy surfaces which are transitional between shear and extension fractures. 3. Proof of a component of shear, in addition to dilation, on many of the veins at less than 40° to an array is provided by crystal fibres at acute angles to vein margins, and third-order en-échelon veins in arrays which are parallel to and part of a set of second-order veins.