Tectónica da cadeia da Arrábida

Abstract

Geociências, Museu Nac. Hist. Nat. Univ. Lisboa, nº 2, 35-84A tectonic map of the Arrábida fold and thrust belt ( 30 km x 6 km) based on aerial-photo interpretation and field work is presented together with geological cross-sections constrained by seismic and gravimetric data, one restored section and kinematic models for the evolution of the Sesimbra salt-wall and the Palmela gravitational slide. Two rifting episodes of Lias-Dogger, and Malm ages, were recognized in the Arrábida rotated fault-block that were accommodated by two normal fault systems striking approximately N-S and E-W. Two episodes of tectonic inversion occurred during Miocene times (Burdigalian and Late Tortonian) during which the N-S faults were reactivated as lateral ramps and the E-W faults as thrusts that formed on a backward propagation sequence. The structure and style of deformation of the Arrábida belt were controlled by the inherited Mesozoic structure and the non-stretched basement that bounds the Arrábida block to the south and east. Although most of the inversion structure appears to be thin-skinned gravimetric modelling (SILVA, 1992) indicates some involvement of the basement in the Viso duplex. The existence of only one detachment, the Hettangian evaporite complex, whose depth to is estimated between 3.5 km and 2.2 km, as constrained by gravimetric and geometric models, is the reason for the simple and elegant geometry of the Arrábida fold and thrust belt. Shortening across the belt was estimated in e = 35% (RAMSAY, 1967) using a 10 km section drawn between Quinta do Anjo (loose line) and Albarquel (pin point) in the eastern sector of the Arrábida belt, across the Serra de São Luís and Viso structures.Apositive strain gradient from north to south and west to east is observed at cartographic scale and was corroborated by independent strain analysis ( P. RIBEIRO, et al., 1996; A. RIBEIRO, et al., 1996