Clastic dike swarms are intruded into a Cenozoic foreland fold and thrust belt in southern Chile. The dike material consists of sandstone and conglomerate with clasts as much as 25 cm in diameter. Brecciated wall rocks are also found in the clastic matrix. The dikes often exceed 50 cm in width and hundred? of metres in length in outcrop and are oriented overwhelmingly transverse to fold axes and parallel to cross joints. The dike swarms are concentrated near the “toe” of major thrusts, where they are found injected into the hanging wall.
A regional axial planar spaced cleavage is developed in the clastic dikes and sills, clearly postdating injection of clastic material. Dike injection postdates at least the initiation of extension fractures (the cross-joint set). Within the swarms, the dike intrusions represent as much as 20% extension parallel to fold axes, in addition to several percent extension represented regionally by the cross joints. The structural features of the dike swarms and their geometric relationship to other structures suggest rapid, single-phase injection. The arrival of the front edge of a thrust on an already highly overpressured sedimentary sequence would cause an “instantaneous” increase in the overburden pressure and therefore could result in injection of a dense “slurry” of wet, coarse sediments along preexisting planes of weakness. Field relationships of mesoscopic structures suggest the following evolution: cross-joint development, incipient cleavage development, initiation of thrusting and buckling, clastic dike injection, regional spaced-cleavage development, and associated late-stage tightening of folds.