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Folds with axes almost parallel to the mean tectonic transport direction are frequent in fold-thrust belts, particularly along the borders of arcuate belts. Such structures have been frequently explained in terms of superimposed tectonic events but, in some cases, they are most realistically understood in terms of lateral folds inherited from lateral footwall ramps and initiated subparallel to the tectonic transport direction. In order to distinguish frontal and lateral folds, we have established criteria based on field examples from two regions where folds occur parallel to the regional transport direction: the Betic Cordilleras (Southern Spain) and the Corbieres thrust-front (northeast Pyrenees, France). The main differences between frontal and lateral folds concern the geometry, the evolution of the structures and the modes of strain accommodation in the roof strata enveloping imbricate stacks. Recalling that, above a duplex or even on individual horse, the roof layers are thinned along sections normal to transport, we examine the precise location where the extension is expressed, illustrated by examples from the two regions. This leads us to address some tectonic problems of these erogenic zones, such as the thrust-sequence and development of thrust-related cleavage. |
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