Fault Locking and Alternate Fault Activity in Outcrop and Subsurface, a Transfer Mechanism
Tecto Sedi Integrated Inc, Calgary, AB, Canada
A series of observations from outstanding outcrops in Sarawak and in Ecuador led to a concept of fault lock and alternate fault activity that can explain numerous geometric anomalies encountered in the subsurface.
The fault locking mechanism takes place when the interface of two contrasting lithologies on the hanging wall of a fault become aligned with another such interface on the footwall. Compressional, transpressional or oblique stress then create a microdetachment along the lithological interfaces on both sides of the fault. The displacement along the major fault is thus interrupted/locked and the strain is transferred to a neighboring fault.
In the Western Canadian Basin, numerous formations with dramatic vertical lithological changes have confirmed the validity of the proposed mechanism. The lithological contrasts relate to their rheology and invoke the interface between shales and massive sandstones, massive siltstones or competent grain-supported carbonates. The Mississippian Pekisko Formation in West Central Alberta will be used to demonstrate the mechanisms invoked and the resulting geometries.
Fault locking is best recognized with either a perfect repeat (superposition) of a very competent unit or when all of the tops and bases of the same formation occur at exactly the same TVDss depth.
The same mechanism of fault locking and alternating fault activity can be evoked at basin scale, 1) in map view when dealing with conjugate fault sets, 2) in cross section when dealing with triangle zones or when there is interference between horizontal detachments and subduction zones (tsunami of December 26th 2004).
AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas