Relay Ramp Deformation and Throw Patterns Applied to Mechanics-Based Interpretations of Normal Faults: The Hat Creek Fault as a Case Example
Simon A. Kattenhorn and Matthew W. Blakeslee
University of Idaho, Moscow, ID, USA
Normal fault evolution is commonly analyzed in terms of the patterns of throw measured across the fault, whether along the fault trace in the form of throw profiles or across the fault surface as interpreted from offsets of prominent reflectors in 3D seismic data. This approach to fault analysis is possible because faults behave in mechanically predictable ways so that field data can inform perceptions of fault evolution and 3D structural architecture. Faults are initiated in brittle rocks when the frictional strength is overcome, with subsequent fault propagation as slip accrues over time. The surrounding rock volume is approximated as behaving in an elastic manner during interseismic periods, but with irrecoverable deformation in response to fault slip events. The elastic rheology results in general relationships between fault size and fault throw, as well as controlling the manner in which throw is distributed across the fault surface. Theoretical relationships deduced from the field of linear elastic fracture mechanics as well as empirical data obtained from field observations, analog models, and 3D seismic interpretation, all provide strong support for an approximately elliptical distribution of fault throw. This pattern is maintained through time (i.e., fault growth is self-similar) such that the point of maximum throw is commonly representative of the oldest portion of the fault. Accordingly, fault throw profiles along the surface trace (i.e., along fault strike) can be used to deduce fault growth history.
Segmented normal faults are comprised of a number of initially isolated segments that follow these growth characteristics; however, the eventual interaction and linkage of adjacent segments produces predictable changes in throw profiles that relate to the 3D geometry and spatial arrangement of such segments. For example, throw maxima may become skewed toward non-elliptical shapes as a result of mechanical interactions between segments. As these segments become mutually closer, the throw profile evolves toward that of a single segment of equivalent combined length in a phenomenon referred to as kinematic coherence.
AAPG Search and Discovery Article #120140© 2014 AAPG Hedberg Conference 3D Structural Geologic Interpretation: Earth, Mind and Machine, June 23-27, 2013, Reno, Nevada