Hydrocarbon Generation, a Cause of Fluid Overpressure and Thin-Skinned Detachment in Fold-and-Thrust Belts

Peter R. Cobbold
Rennes University, 35042 Rennes, France

Thin-skinned detachments are common features in the frontal parts of fold-and-thrust belts. If a thrust wedge has a small angle of taper, the theory of a critical Coulomb wedge predicts that shear stresses are small over the basal detachment. One possible reason for this is slow shear within a ductile layer of low viscosity, such as salt. Another possibility is frictional slip, assisted by fluid overpressure. However, this begs the question as to the origin of the overpressure. Where thrust belts have developed beneath rapidly accumulating young sediment, as in the frontal part of a delta,compaction is a likely cause of overpressure. However, many detachments have developed in old rocks, which compacted and lithified long before deformation started. In many such examples, the detachment is in potential source rock, raising the possibility that overpressure is due to hydrocarbon generation. The foothills of the southernmost Andes, the Central Andes, and the Brooks Range of Alaska, all are good examples for testing the above ideas, because detachment has occurred in recognized source rocks of Paleozoic or Mesozoic age. More significant, thrusting was coeval with the onset of hydrocarbon generation,as a result of regional subsidence. Finally, in each example, the thrust front has an irregular shape, which coincides on a map with the equally irregular shape of the source rock pod. A similar coincidence is visible in physical models, where fluid overpressure contributes to the balance of forces. On this basis, the oroclinal shape of the modern Central Andes may be due to the subsidence and maturation history of Silurian source rock in a pre-existing Paleozoic basin.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005