M.Burkhard, S. Caritg, and U. Helg

Institut de Géologie, Neuchâtel University, Switzerland

ABSTRACT: Forced, Disharmonic Multilayer Buckle Folding in the Late Variscan Antiatlas of Morocco

The late Paleozoic Antiatlas of Morocco is a well exposed folded chain which displays some intriguing deviations from the standard model of foreland fold-andthrust belts. Folding style is strikingly similar to the Valley and Rich province of the Appalachians. Basement involvement, however, in the form of widely distributed large scale "plis de fonds" amidst the folded Paleozoic sediments does not have any counterpart in the Appalachian Blue Ridge, the Alpine External Crystalline Massifs or the Wind River style basement uplifts. Alternative tectonic interpretations of the Antiatlas tectonics include : 1) Two phased evolution: thin skinned first, followed by basement inversion, 2) Strike slip, wrench tectonics or, 3) One phase of "thick skinned" basement inversion.

New structural observations include a systematic mapping of fold styles, axial planes and paleo-stresses. No thrust faults of importance can be indentified anywhere in the Antiatlas fold-belt. A major detachment level needs to be postulated in the lower Cambrian shales in order to explain the discrepancy of shortening determined from balancing cross sections on the regional scale. A second important "detachment" or disharmony exists in Silurian shales: shortening is 20 to 25%; folding wavelength and amplitude, however, are markedly different on either side. An extremely high cylindricity of the fold trains, together with paleo-stress axes perpendicular to axial planes permit to rule out any significant component of wrenching. We conclude that the Antiatlas of Morocco represents a large scale example of a disharmonically folded multilayer with only minor thrust faults. Folding was most probably forced by basement inversion "from below", rather than by horizontal push "from behind".

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado