The structural and kinematic evolution of an upper crustal transpression zone: The Lusitanian Basin, Portugal.

Abstract

The Lusitanian Basin forms a long (250km), narrow (50-100km), NE-SW trending outcrop of inverted Mesozoic carbonates and elastics. The outcrop of the Mesozoic sequence is flanked by two Neogene sub-basins, the contact between which is commonly faulted indicating that the Lusitanian basin underwent inversion during the Miocene. This inversion was achieved by reactivation of Jurassic extensional faults, which were originally formed above reactivated Hercynian basement faults during the Atlantic rifting event. The Serra de Montejunto/Candeeiros fault system (SMCF) forms the eastern boundary to this inverted zone, and is dominated by the NNE trending Serra dos Candeeiros fault (SCF). Towards the northern end of this fault system is a complex, uplifted, 15x20km fault bounded block, the Candeeiros block. At the southern termination of the SMCF, the fault orientation changes abruptly from NNE to ENE. This change in fault orientation coincides with the Serra de Montejunto mountain range, a narrow zone of tectonic uplift. Detailed studies of the main fault zones and adjacent wall rock deformation, allows the kinematics and strain to be characterised. These studies have revealed a complex history of Miocene to Recent reactivation along the boundaries of the Candeeiros block. Initial reactivation of the cross-faults of the block was synthetic to the sinistral SCF. Initial sinistral motion along the cross-faults was superimposed by a late period of dextral transpression, as evidenced by the inverted Alvados pull-apart basin preserved along the Alvados/Minde cross-faults. Dextral transpressive strains along the cross faults of the Candeeiros block, appear to be intimately related to the southerly directed extrusion and anti-clockwise rotation of the block. Structural relationships suggest up to 30 of rotation has occurred. Rotation of the Candeeiros Block is believed to be associated with the change from sinistral simple shear to sinistral transpression along the SCF, during the Late Miocene. The Serra de Montejunto region also displays a complex kinematic history, initiated by N-S shortening across the range, which was superseded by a late stage of sinistral transpression. This transpressive phase manifests itself as both simple and complex transpressive strains. The overall structural geometry of the region is that of a flower structure, formed in a major restraining bend at the termination of the SMCF. The kinematic and deformation style of the early Miocene reactivation in both the Candeeiros and Montejunto regions are consistent with the application of a N-S regional shortening direction, or σ(_1), on the pre-existing Jurassic age structure of the areas. This initial phase of deformation is overprinted by a distinct late phase of deformation related to sinistral transpression along the SCF, which was probably responsible for the inversion of the Lusitanian Basin. The orientation, and chronology of the regional shortening directions responsible for the evolution of the SMCF are in agreement with the relative plate motion history for Africa-Europe (Dewey et al., 1989).A comparison of instrumental seismicity and neotectonic surface faults associated with the SMCF, suggests that the basin is undergoing basement driven sinistral deformation, which is decoupled from the cover sequence by a thick evaporate sequence. The sinistral displacement along the proposed concealed, steep, basement faults appears to be transferred laterally along the sub-horizontal evaporite horizon, for up to 20km, to the steep cover faults of the SMCF.