Thick sequences of marine Jurassic and Lower Cretaceous deposits are well exposed in the Andes of northern Chile. These rocks constitute relicts of back arc basin in fill, that in most if not all cases, are cross-cut by andesitic dykes and sills. Dykes usually feed the sills that in turn concentrate at a certain stratigraphic level. Field studies of contact relationships and of dyke and sill internal structures lead to the conclusion that at least in the Sierra de Moreno and in El Salvador-Potrerillos areas, the intrusives were emplaced into wet and semi-consolidated deposits, during burial and basin subsidence. The analysis of the intrusive process scaled to the sill and dyke geometry let conclude that the elastic constants of the host rock at the time of intrusion were low, as those one would expect in wet, semi-lithified sediments. An analysis of the magmatic overpressures that caused the intrusion, indicates that the magma chamber was located at a depth varying between 3 and 5 km below the level at which the sills are concentrated. The emplacement of tabular bodies was mainly controlled by the magnitude and orientation of the principal stress field. The dyke to sill transition and the stratigraphical concentration of sills resulted from a local inversion of the stress field probably due to the effect of the interception of rocks with higher elastic constants than the rest of the sequence. The sill to dyke transition is also very sensible to the strain regime active at the time. In those places where the sediments are intruded by sills the deformation of the host rock is much less than in those places where the sills are absent. It can be inferred that the stresses required to initiate buckling of the multilayer with the sill intercalations will not be attained before the fracture strenght limit of the sill-sediment multilayer system is reached. From a different view-point it can be concluded that in those sedimentary basins with low porosity and low permeability rocks, the fractured sills may become effective hydrocarbon reservoirs. Contrary to what would be expected, the reservoirs will be located where the sequence is less deformed. Conclusions derived from this study also allow to predict the size and distribution of veins, flats and veinlets, provided that the hydraulic and stress constants have been adequately calibrated.