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PSFault
Locking and Alternate
Fault
Activity in Outcrop and Subsurface, a Transfer
Mechanism*
Jean-Yves Chatellier1
Search and Discovery Article #40368 (2009)
Posted January 26, 2009
*Adapted from poster presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008
1Talisman Energy Inc., Calgary, AB, Canada ([email protected])
Abstract
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).