On the Growth of Normal Faults and the Existence of Flats and Ramps along the El Asnam Active Fold and Thrust System

The combination of detailed topographic leveling on the southwest segment of the El Asnam thrust fault with existing seismic and geologic data implies that the geometry of this fault involves shallow dipping flats and steep ramps. The fault appears to be growing along strike toward the southwest end, where the main shock initiated in 1980. From a depth of about 10 km, the main thrust appears to ramp to the basement-Cenozoic cover interface on a plane soiking N40°E and dipping 50°550 to the nonhwest. Along the southwest segment where folding has not yet developed, the thrust continues steeply through the Cenozoic cover to the near surface where it flattens, causing normal faulting. Along the central and nonheast segments, which display a more evolved fold structure, the deep thrust probably flattens at a depth of 5-6 km, into a decollement along the Cenozoic-Jurassic interface before ramping to the surface. The Sara El Marouf and Kef El Mes anticlines have thus formed as fault propagation folds. Normal faults at Beni Rached probably branch with the thrust to maintain kinematic compatibility between the deep ramp and decollement The greater separation ( -1 km) between the normal faults at Beni Rached and the thrust where it crosses Oued Cheliff than along the southwest segment (-1 km) reflects the greater depth of the ramp to flat bend. We infer that the September 9, 1954, eanhquake activated only the central deep segment of the main thrust together with the Beni Rached normal faults, while that of October 10, 1980, activated the whole system of flat decollements, ramp thrusts and compatibility normal faults. Further complexities of the faulting in map view are related to changes of strike of the thrust (in particular nonh of Oued Cheliff).