--> --> Abstract: Folding Due to Back Rotation Between Extensional Detachments in Fold-Thrust Belts, by Lyal B. Harris; #90039 (2005)
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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 Previous HitlayersNext Hit is commonly assumed to necessitate regional Previous HithorizontalNext Hit shortening, and most Previous HitmodelsNext Hit 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 Previous HitmodelsNext Hit, however back-rotated Previous HitlayersNext Hit 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. Previous HitLayersNext Hit 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 Previous HitlayersTop 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