The sequential evolution of arrays of thrusts can occur by a number of geometric models: these models form the subject of this contribution. The most common of these is a simple foreland-directed (piggy-back) series which develops into either an emergent imbricate fan or, if the faults converge upwards, a duplex structure. Depending on the geometry and distribution of thrust ramps and the relative magnitude of displacements on the faults, a range of duplex geometries can result where the imbricate slices have either hinterland-dipping, antiformal stack or foreland-dipping attitudes. A frequently recognized complexity occurs when a later fault climbs across from the footwall into the hanging wall of an earlier one leading to a breached geometry. A less common generation of imbricate fans occurs by the hinterland-directed progression of thrusts in a break-back sequence. This can lead to an overstep geometry where earlier structures are truncated in the footwall of a later fault. The evolution of hinterland-directed, back-thrusts is also considered. Both internally piggy-back (regionally hinterland-directed) and break-back (regionally foreland-directed) sequences can develop, depending on the ability of the active displacement front to propagate forwards. Simple imbricate geometries can develop with an internally break-back sequence but piggy-back propagation will generally require substantial strains within the back-thrust slices or the generation of multiple detachment levels. The range of structures which can result from these fore-thrust and back-thrust sequences is illustrated using hypothetical cross-sections and natural examples from the Scottish Moine thrust belt and the western Alps. The implications of each geometry for estimates of orogenic contraction and the construction of balanced cross-sections are also emphasized.

Abstract. The geological structure of the Central Apennines along a section line across the Lazio-Abruzzi carbonate platform has traditionally been interpreted using a thin-skinned thrust tectonic model, in which the sedimentary cover has been detached from an undeformed basement below. Such models have been used to predict that very large amounts of crustal shortening (e.g. 172 km over a section 173 km long) have occurred. Alternatively, in this paper we reinterpret the surface geology and well data along the same section line using a thick-skinned thrust tectonic model. Restoration of this section shows that the amount of shortening (37 km over a section 158 km long) is considerably lower than previously predicted; this is accomplished by open buckling of the carbonate platform, tighter folding of the basin scarp stratigraphy, and reactivation of pre-existing extensional faults. Age bracketing on thrust fault movement allows shortening rates for the two different models to be calculated; these are < 6 mm yr −1 for the new interpretation, but over 24 mm yr −1 for the equivalent thin-skinned model. This latter value is significantly greater than shortening rates reported for most other thrust belts, suggesting that thick-skinned tectonics is a more satisfactory explanation for the structure of this area. The two most important implications of this are that subthrust hydrocarbon plays are largely absent in the area, and Neogene contractional deformation in this part of the Apennines resulted in much less crustal shortening than previously predicted.

The structural restoration of collision orogenic belts onto crustal templates provides important insights into the tectonic evolution, deep structure and amounts of plate convergence after the initial contact between two continental masses. Balanced cross-sections have been constructed, parallel to the local displacement directions, across the western Alps and western Himalayas and demonstrate very large amounts of crustal shortening above intra-crustal detachments. To achieve a balance, substantial volumes of lower crust must have been subducted beneath the two tectonic hinterlands. A model of eclogite metamorphism is invoked to facilitate this subduction and to explain the varying isostatic responses of the Alpine and Himalayan hinterlands. Patterns of eclogite metamorphism are controlled by the geometry of thrust profiles on a lithospheric scale: the development of crust-mantle detachments being of crucial importance. Such a profile is proposed for the Himalayas, suggesting that relatively small volumes of the footwall crust succumbed to eclogite metamorphism. In the Alps however, a steeper thrust profile apparently developed, emplacing mantle onto crust, causing wholescale eclogite metamorphism in the underlying Franco–Swiss crust. The resultant density increase would provide a mechanism of isostatic collapse and flexural subsidence in the Po plain region.

India collided with a northern Kohistan-Asian Plate at about 50 Ma ago, the time of ocean closure being fairly accurately defined from syntectonic sediments as well as the effect on magnetic stripes on the Indian Ocean floor. Since collision, Asia has over-ridden India, developing a wide range of thrust scrapings at the top of the Indian Plate. Sections through the imbricated sedimentary cover suggest a minimum displacement of over 500 km during Eocene to recent plate convergence. This requires the Kohistan region to the north to be underlain by underthrusted middle to lower Indian crust, deformed by ductile shears and recumbent folds. These structures are well seen in the gneisses immediately south of the suture, where they are uplifted in the Indus and Nanga Parbat syntaxes. Here there are several phases of thrust-related small-scale folding and the development of a large folded thrust stack involving basement rocks, the imbrication of metamorphic zones and the local development of large backfolds. Some of the important local structures: the large late backfolds, the Salt Ranges and the Peshawar Basin, can all be related to the necessary changes in thrust wedge shape as it climbs through the crust and the three dimensional nature of the thrust movements associated with interference between the Kohistan and western Himalayan trends.

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