A Geometric Model of Fault Zone and Fault Rock Thickness Variations: Reconciling the Fault Zone Structure of Small to Large Displacement Faults

Conrad Childs¹, Tom Manzocchi¹, John J. Walsh¹, Chris G. Bonson², Andrew Nicol³, and Martin P. Schöpfer^1
1Fault Analysis Group, University College Dublin, Dublin, Ireland
²SRK Consulting (Canada), Vancouver, BC, Canada
³GNS Science, Wellington, New Zealand

The thicknesses of fault rock and fault zones and the fault normal separations for breached and intact relay zones each show a positive correlation with fault displacement. The displacement to thickness ratio, or shear strain, ranges for the different structures increases from intact relay zones (median value = 0.28) to fault rocks (median value = 0.02). The correlation for fault rocks is widely interpreted as a growth trend controlled by fault rock rheology, but recognition of similar correlations for the other fault components suggests an alternative model. In this model a fault initiates as a segmented array of irregular fault surfaces. As displacement increases, relay zones separating fault segments are breached and fault surface irregularities are sheared off, to form fault zones containing lenses of fault bounded rock. With further displacement these lenses are progressively comminuted, and ultimately converted to zones of thickened fault rock. The final fault rock thickness is therefore influenced strongly by fault structure inherited from the geometry of the initial fault array. The large scale range on which fault segmentation and irregularities occur provides the basis for application of this model over a scale range of at least 8 orders of magnitude.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas