Thrust sequences: have we been asking the wrong question?
Rob Butler
School of Earth Sciences, The University of Leeds, Leeds LS2 9JT
butler@earth.leeds.ac.ukClassic analyses of thrust systems are commonly aimed at establishing sequences of thrust activity, with conclusions being drawn as to when thrust "propagation" was in a piggy-back sequence or perhaps "out-of-sequence". The underlying premise is that there is only one fault strand active at any one time (as averaged over say, several earthquake cycles). Since these ideas were mooted in the early 1980s research in emergent, recently active thrust belts has uncovered a more complex picture where thrusts are active not sequentially across strike but simultaneously. Examples of where growth strata can be used to deduce such activity include the southern ranges of the Pyrenees, the frontal Apennine-Sicilian system and the Pakistan Himalayas. Most accretionary prisms and toe-of-slope deep-water fold belts show synchronous amplification of folds and thrusts across a section. It is tempting to suggest that the mechanics of these emergent thrust systems are different to buried, duplex structures, where the original notions of thrust sequencing were developed. However, as part of a re-appraisal of thrust geometry in NW Scotland, a series of hitherto neglected field relationships have emerged that are better explained by synchronous displacements on imbricate thrusts and duplex roofs. A common feature of many apparent duplex structures is that, although the duplex roof is folded and bulged by underlying imbricate slices (in accordance with simple duplex models), the footwalls to the duplex cut up and down stratigraphic section (in conflict with the simple duplex models). Examples of this behaviour are found in the footwall to the Arnabol Thrust (Eriboll), in the Foinaven "duplex" , in the footwall to the Glencoul Thrust (Loch More and in its type area), in the Kinlochewe window and in the Achnashellach culmination. In the past some of these relationships have been interpreted as representing low-angle extensional faults that post-data thrusting. However, the geometries can be linked directly to the evolution of the thrust stack. Simultaneous slip on an array of imbricate thrusts can bulge duplex roofs and even lead to back-steepening of thrust slices (relationships classically used to infer piggy-back sequences of thrust "propagation"). However, activity on more hinterland-ward imbricate thrusts can cause the roof onto which they branch to truncate more forelandward structures. In this fashion the duplex roof can cut up and down stratigraphic section (mimicking an extensional fault). The model predicts that truncated thrust geometries should decorate the duplex roofs towards the foreland. Understanding these geometries is important in many situations, not least in oil and gas prone thrust belts where the integrity of sub-thrust structures can be prejudiced by roof thrust truncations.