PROGRESSIVE DEVELOPMENT OF FRACTURES AND HINGE MIGRATION IN A GROWING ANTICLINE

This investigation focuses on determining the temporal and spatial distribution of upper crustal deformation within Eocene - Oligocene synorogenic deposits associated with foreland propagation of the South Pyrenean fold-thrust belt. In particular, synorogenic strata that surround the Oliana anticline appear to record successive generations of fractures related to the early stages folding through fold tightening and cessation of fold growth. Four stratigraphic layers (Units 1-4) were used to establish several episodes of fracture development. At the whole-fold scale, fractures perpendicular to and confined within the bedding interfaces are found all around the anticline, regardless of bedding dip. However, in some cases these fractures are overprinted by fractures whose orientations are similar to bed-perpendicular fractures in younger strata and also cut through the inclined bedding of older strata. Such an observation suggests that the early fractures were re-oriented with fold growth and that subsequent generations of fractures, forming perpendicular to upper units, superposed the earlier stages of fracturing. By sequentially removing the effects of the later generations of deformation, it is possible to constrain the progressive development of fractures associated with development of the Oliana anticline. In this way, we are able to show that the primary set of systematic fractures in Unit 1 strike perpendicular to the fold axis and are interpreted to be the result of layer parallel shortening, with a secondary set oriented parallel to the fold axis. On the other hand, Units 2 and 3 show a primary set of fold axis parallel fractures and a secondary set of fold axis perpendicular fractures. The change in orientation of the primary fracture set may reflect a forward migration of the fold hinge due to motion along a blind thrust duplex under the Oliana anticline. Since the youngest layer (Unit 4) post-dates much of the deformation, the disparate orientations of fractures in Unit 4 may be associated with a phase of regional fracturing.