THE IMPORTANCE OF SMALL-SCALE FAULTING IN REGIONAL EXTENSION

A RECURRING observation in many studies of extensional basins has been that the amount of extension visible on normal faults (for example, on seismic reflection profiles) is significantly less than the amount of extension indicated by crustal thickness and thermal subsidence 1-5. One mechanism suggested to account for this discrepancy is small-scale faulting, with offsets too small to be resolved seismically 6,7. But earthquake studies 8-10 indicate that small faults are responsible for only a small fraction of the total seismic moment in an active area. Scholz and Cowie 11 have recently attempted to extend this approach to the total strain at the end of a finite deformation interval by combining scaling laws describing the distributions of fault lengths and displacements. Here we present fault displacement data that directly conflict with Scholz and Cowie's conclusions, and imply that up to 40% of the extension may be missed by summing fault offsets on basin profiles. The fault population at the end of a long deformation interval may differ substantially from that responsible for the earthquake population at any one time.