Abstract: The Frontal Fold Systems of Collisional Belts: Thin-Skinned Versus Thick-Skinned Tectonics

Michael P. Coward, Alison C. Ries

The importance of syn-sedimentary faults is recognized in many orogenic belts and it is known that much intra-continental deformation is accommodated by the reactivation of pre-existing structures. Frontal thrust zones of many collisional orogenic belts, previously interpreted as thin-skinned thrust zones, are now known to involve basin inversion and fault reactivation.

In the post-rift sequence, folds produced by thin-skinned and thick-skinned deformation are superficially similar, but various geometrical models and construction techniques have been derived for determining their fault and fold structure at depth. It is in the syn-rift and pre-rift sequences that the models and interpretations differ.

Regional section balancing techniques which ignore basin inversion can lead to serious errors in the interpretation of the structure of a mountain front and its hydrocarbon potential. Examples include the western thrust zones of Pakistan (Kirthar-Sulaiman-Kohat Ranges), previously interpreted as thin skinned with over 100 km displacement, are now re-interpreted as the inverted margin of the western Indian sub-continent with only 20 km shortening. This re-interpretation considerably modifies models for source rock potential and hydrocarbon maturation.

Thus it is important to be able to differentiate between folds produced by thin-skinned tectonics and folds produced by inversion. If one or more of the following criteria hold then inversion tectonics should be considered viable: i) the compressive structures show simultaneous growth, i.e. in thin-skinned thrust belts the structures generally propagate towards the foreland, hence increasing the size of the thrust wedge with time, while in thick-skinned or inversion tectonic regimes the original normal faults often reactivate simultaneously, ii) there is often a lack of a mountain belt or surface slope which could drive the thin-skinned tectonics, and iii) there is independent evidence for the presence of an earlier basin, e.g. the rapid change in sedimentary thickness or facies.

As well as the thrust zones of western Pakistan, examples of reverse faults and folds, which have previously been interpreted as thin-skinned thrusts but may warrant re-interpretation as thick-skinned inversion structures, include parts of the Apennines of Italy, many parts of the eastern Cordillera of the South American Andes, the Palmyrides of Syria and the Zagros Ranges of Iraq and Iran.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994