Overview of Tectonic Structure of Montejunto Area

From a tectonic and structural point of view, the Montejunto area is divided in two parts by a transversal group of faults. The NE zone is limited, in the east side, by fault with an orientation NE-SW and the contact between the Meso-Cenozoic formations is more or less vertical. The zone SW of Montejunto area is characterized by the presence of several faults of NE-SW orientation.

The Montejunto anticline resulted from the Cenozoic alpine compression and uplift. Basement faults and salt motion controlled that uplift but also controlled the Mesozoic differential subsidence as may be seen from the variable thicknesses of Upper Jurassic rift-related sequences. Three sub-basins may be defined around the Montejunto anticline: Bombarral, Arruda and Turcifal.

One of the big geometrical transformations of the LB basin is this fragmentation in sub-basins, especially in the Central sector. For the first time the influence of Torres Vedras-Montejunto fault is visible, which is the main contributor for the segmentation of this area in the LB from a tectonic perspective. Torres Vedras-Montejunto fault (FTVM) corresponds to a sequence of united faults by soft-links, with normal movement. During Upper Jurassic and Lower Cretaceous, it is responsible by the differentiation between Bombarral basin, in the subsident block, and Arruda and Turcifal basins to south. This fault has an important role in the geological development of the region. With an orientation NE-SW it formed in 3rd rifting phase of the Lusitanian basin.

This basin’s separation of tectonic nature works during sedimentation as a paleogeographical barrier, differentiating sedimentation environments. Turcifal and Arruda sub-basin are divided by an average of 20km long north to North-Northeast striking Runa Fault Complex; and Turcifal and Arruda half grabens are separated from Bombarral-Alcobaça sub-basin by 70km long northeast to east trending structural lineament; the Torres Vedras-Montejunto lineament (Figure 4).The development of complex fault- and diapir-bound sub-basins resulted by the presence of halite at depth (latest Triassic-Hettangian age, Dagorda Formation) limited and modified the propagation of basement faults into the post-salt overburden, contributing to the development of salt pillows as it can be seen in seismic data, and extensionally-forced folds during the Jurassic extensional phasesю This development resulted in the separation the southern part into two half-grabens resulting in Arruda and Turcifal sub-basins with the consequent salt moving from the Arruda sub-basin into the Montejunto Anticline separated the Arruda and Bombarral sub-basins.

The Arruda sub-basin corresponds to a half-graben developed during the Middle Oxfordian-Late Oxfordian as a consequence of transtensional rifting episodes that have affected the Estremadura Basin. This sub-basin represents an intra-continental pull-apart basin with a rhomb-like shape.