CT scan of a small-scale fault network: 3D fault geometries and their interpretation

Fault surfaces and networks have been shown to have complex geometries. Outcrop observations are typically two-dimensional and limited in size by the exposure dimensions, while three-dimensional (3D) seismic data lack the resolution to characterize and quantify fault complexities on length scales less than a decameter. Defining the geometries of faults and their networks (high-resolution in 3D) is critical for understanding the interactions between faults and fluids. This presentation will examine the geometries of a network of small-scale normal faults displacing (by <1 cm) well bedded sand and silt layers in the Mount Messenger and Mohakatino formations in Taranaki, New Zealand. A 3D model of faulting was produced from high-resolution multi-band CT scanner (MARS Bioimaging Ltd.) imagery of a 10x8x3 cm rock sample. The digitally sectioned rock contains calcified fault rock that is distinguishable from wall rock and mapped throughout the rock volume at sub-millimeter scale. Fault-rock thicknesses vary by in excess of an order of magnitude, with greatest thicknesses at fault steps and fault bends. Fault zones comprise a series of lenses that have strike lengths greater than dip lengths and lens shapes that are often elongate parallel to bedding. The fault network is highly connected with branch lines, fault steps and fault bends most often sub-parallel to bedding. These observations suggest that mechanical heterogeneity of beds may partly control the geometries of both fault zones and the fault network. At the time of formation, the interconnected fault network likely increased bedding-parallel permeability (at scales from sub-millimeter and above) along fault zones.