PRODUCTION OF COMPLEX JOINT NETWORKS DUE TO TEMPORALLY CHANGING MECHANICAL STRATIGRAPHY

The strata deposited within basins flanking growing folds experiences deformation, such as tilting and fracturing, during diagenesis. The increased cementation of these sediments with time gives rise to changes in the strata rheology during fold growth. Consequently, the earliest fractures within the growth strata may develop within a different mechanical stratigraphy than later fractures. Within the Oliana Anticline of the Spanish Pyrenees we observed overprinting of younger joints across beds containing older joints. Whereas the older joint sets are perpendicular to and contained within individual beds, the younger joint sets cross obliquely through multiple beds. We interpret this pattern to reflect changing properties of the beds and/or changing conditions along contacts during diagenesis and folding. In one outcrop, Schmidt hammer results show that present-day contrast in material properties cannot account for the observed termination of older joints against the less resistant beds. From this we suggest that, at the time of the older joints, the contrast in strength between the resistant and less resistant beds must have been greater in order for the joints within the resistant beds to terminate against the softer material. Later, after the beds were tilted along the limb of the growing fold, increased cementation of the less-resistant material permits joints to propagate through the entire sequence. Overprinting relationships of joints at another outcrop with uniform bed rheology maybe explained by two alternative hypotheses. Firstly, with greater tilting, greater compression across bed contacts may facilitate propagation of younger joints obliquely across several beds containing older, bedding confined and bed-perpendicular joints. Alternatively, the cohesion of the bed contacts may have increased with time. The different orientation of the older and younger joints sets in both outcrops within the growth strata reflects the difference in timing of the fracture events with respect to folding. Our work shows that the evolving mechanical stratigraphy of growth strata during fold growth can yield complex networks of joints and can give rise to otherwise unexpected fluid flow networks.