Abstract

J. P. Turner & P. L. Hancock write: We congratulate Martínez-Peña, Casas-Sainz and Millán-Garrido (1995) on their palaeostress analysis of a complex suite of mesostructures contained in several south Pyrenean thrust sheets. We agree that, in spite of the difficulty in interpreting the multi-phase deformation of thrust belts, analysis of structures from which palaeostress may be inferred yields useful information not apparent from a macro-structural study.

Like Martínez-Peña et al. (1995), we also found that the folded Oligo-Miocene molassic sedimentary rocks of the Jaca thrust-top basin contain remarkably few faults. Indeed, explaining how a near-surface structural unit has travelled 30 km or more over such an irregular tectonic topography yet without being faulted is a challenging structural question. However we are suprised that Martínez-Peña et al. (1995) found in their palaeostress results from the Cretaceous and Palaeocene carbonates to the north of the molasse basin, that: There are no great differences between stress tensors found in the hanging wall and the footwall of thrusts’. Our experience (Turner & Hancock 1989) in that part of the Jaca basin contiguous with and immediately to the west of their study area is that the assemblages of small-scale brittle structures in the immediate (within 1-3 km) hanging walls and footwalls of thrusts are distinctively different, recording locally contrasting stress regimes. In particular, our study showed that the immediate footwalls of thrusts experienced extension parallel to the strike of thrusts before they underwent extension normal to the strike of thrusts.

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