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Salt Tectonics in the Northern Lusitanian Basin: An example of Interplay between Late-Variscan Heritage and Regional and Local Stress Fields

Abstract

Salt structures can play an important role in the tectono-sedimentary evolution of basins. In west Iberia, the Alpine regional stress field had a horizontal maximum compressive stress striking NNW-SSE, related to the Late Miocene inversion event. However, this stress field cannot produce most of the observed and mapped structures in the onshore of the northern Lusitanian Basin.

Therefore, we paid special attention to diapir tectonics, which can impose a local stress field (vertical maximum compressive stress) and be responsible for significant vertical movements of Mesozoic and Cenozoic cover rocks.

Based on fieldwork, tectonic analysis and interpretation of geological maps (Portuguese Geological Survey, 1:50000 scale), our work shows: (1) the presence of high angle faults and anticlines with N-S, NNE-SSW, ENE-WSW or WNW-ESE trends, which cannot be the result of Alpine compression; (2) some structures can be related to late-Variscan fracturing, by reactivation of basement faults with NNE-SSW and ENE-WSW trends; (3) the anticlines show radial faulted Jurassic cores which points to diapir upward pushing; (4) some anticlines are aligned with exposed salt diapirs, showing lateral continuity between these structures; (5) geometry and sedimentary filling (up to the Pliocene) of basins show relationship to salt-related anticlines, with salt withdrawal from the base of the basin (subsidence) and movement into the neighbouring anticlines/diapirs; and (6) unconformities and folded unconformities, which means that there have been several deformation events prior to the Late Miocene Alpine event.

These data suggest that: (1) most of these structures result from local diapir tectonics, initiated before the Cretaceous; (2) some of the structures may be related to reactivation of inherited late-Variscan basement fracturing; and (3) considerable vertical movements can be deduced from salt-related anticlines and neighbouring basins.