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