Fold uplift versus regional subsidence and sedimentation rate

Fold uplifts compete during their development with regional subsidence in the frontal parts of thrust belts and accretionary wedges. Two cases exist in frontal thrust belts. In the first, more common case, the uplift rate of the external folds is higher than the subsidence rate of the foredeep. In the second case the fold uplift is less than the regional subsidence. The total fold uplift may be considered as the fold uplift rate minus the regional subsidence rate; this value can be either positive or negative. Positive total fold uplift occurs when folds rise faster than regional subsidence and the envelope of the fold crest rises towards the hinterland of the accretionary wedge. In the opposite case, negative total fold uplift, the envelope dips towards the hinterland because folds rise more slowly than regional subsidence. In the first positive case, the folds are deeply eroded and the onlap of the growing strata moves away from the fold crest if the sedimentation rate is lower than the fold uplift rate in marine or alluvial environments. In the second negative case, where folds rise at lower rates than the regional subsidence, the onlap moves towards the fold crest if the sedimentation rate is higher than the fold uplift rate. This is also the more favorable case for hydrocarbon traps where growth strata can seal the fold, Moreover the fast subsidence rates in the foredeep provide a higher thermal maturation. This second tectonic setting is commonly associated with accretionary wedges forming along west-directed subduction zones, which are characterized by high subsidence rates in the foredeep or trench, due to the fast 'eastward' roll-back of the subduction hinge (e.g. Apennines, Carpathians,Banda are).