DEM: Thrusts 4. The effect of the height of pre-existing basement structures on faulting

 Application of a Discrete Element Model to investigate the effects of the pre-kinematic strength on thrusting when the relative height of basement structures varies. 

The media in these experiments contain 20664 elements in a box that is 300 x 30 units. Elements are distributed randomly with radii of 0.5, 0.4. 0.3 and 0.2 units. The left-hand boundary is incremented to the right at 0.00005 unit/timestep to a total compression of 150 units (50%). The experiments run for 3 million timesteps with outputs every 50,000 which are presented in the movies. The media is divided into 14 layers. Colours indicate the ‘strength’ of the media where numbers in the file name represent the separation between elements as a function of their initial separation that is required for bonds to break. This is distinct for each bond pair where each element is assigned a breaking separation determined from the average of the strength assigned to each element. Experiments are performed with two basement structures representing normal faults dipping towards the compression direction at 50^o. The initial position of the faults is fixed at 150 and 225 units.

These models use an alternating layer strength between weak (1x, grey/black layers) and more competent layers (6x stronger, white and green). The movies show the results from experiments where the height of the faults:

• are the same and reducing in height from 20 units to 10 units in 2.5 unit increments (A_0.06_onf_hf_*),
• the height of the front fault is varying (A_0.06_onf_hf1_*) with respect to the rear fault (H=15.0),
• the height of the rear fault is varying (A_0.06_onf_hf2_*) with respect to the front fault (H=15.0).

The filename of the standard model where both faults are 15.0 unit high (50% of the height) is  A_0.06_onf_hf_15.0.gif. A comparison between it and an experiment with no faults is given in A_0.06_nof_onf_compare.gif. The *_combined movies show three representations of fault evolution. 

• The upper image is the layers in the experiment.
• The central image shows the bonds between elements as they break where the darker the colour, the more bonds are broken with initial neighbours.
• The lower image shows a coherence plot where an element is coloured relative to the difference between its layer number and that of the element immediately below it. Elements within layers are white and  fault locations are highlighted as the colour darkens.

A presentation of final results of the experiments and their associated filenames are in DEM_Thrusts 4.pdf.The methodology can be found in the first reference below. The second links to other thrust experiments with underlying basement faults.