Folding Due to Back Rotation Between Extensional Detachments in Fold-Thrust Belts
Lyal B. Harris
Water, Earth and Environment (INRS-ETE),
Quebec, QC
Folding of shallowly dipping layers
is commonly assumed to necessitate
regional
horizontal
shortening, and most
models
of folding, thermal histories
and fluid flow for fold-thrust belts are based on this assumption. This
presentation shows how inclined to recumbent folds in fold-thrust belts may also
be created by back-rotation between pairs of extensional detachments, either
within localised extensional sites during regional thrusting (e.g. due to
basement wedging) or during orogenic collapse.
Back-rotation is common between pairs of faults and ductile shear zones
in nature and in analogue models
, however back-rotated
layers
have been
previously thought not to exceed orthogonality to their bounding structures.
Greater than normal back-rotation between extensional detachments may, however,
be facilitated by the presence of fluids and/or shale smearing.
Layers
back-rotated into the contractional field of the incremental strain ellipse
undergo additional buckle folding due to a component of layer-parallel
shortening. Back-rotation between extensional detachments therefore produces
folds with an apparent thrust vergence. This can be easily misinterpreted as
implying that folds formed during thrusting, and were later cut by detachments.
Folds formed by back-rotation between extensional faults have shallowly dipping
long limbs and often display thicker steep, short limbs with abundant parasitic
folds. These contrast with folds formed during layer-parallel shortening, where
layers
on the steeply dipping short limb initially thicken, then thin when they
are further deformed and overturned. Examples of this mechanism are provided
from the Proterozoic Ophthalmia fold-thrust belt in the Hamersley Province of
Western Australia and from analogue modelling.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005