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Effects of Triassic Disharmonic Strata During Successive Distensive and Compressive Mesozoic and Cenozoic Events in Cevennes Foothills, France

F. Roure, M. Briend, G. Jacquart, J. Letouzey

Recent onland seismic reflection lines across a Mesozoic basin southeast of the Massif Central, France, provide new interpretations of the deep structures of Eocene thrust faults and Oligocene normal faults, previously interpreted as superficial extensions of basement reverse and normal faults. The post-Triassic deformations observed are strictly confined to the Mesozoic sedimentary cover and do not affect the Paleozoic basement. A main disharmonic level, locally even providing "decollements," was thus outlined near the bottom of the Triassic sequence. Downfaultings of lower Lias limestone blocks along listric normal faults disappearing inside the Triassic strata and obliterated by upper Lias mudstones or Middle Jurassic limestones are the only tectonic effects of the ear y Jurassic Tethyan opening observed in this part of the southwest Mesozoic European margin.

During the Eocene Pyrenean orogeny, meridian compressive strains reactivated some old discontinuities in the Paleozoic basement. Late Hercynian northeast-southwest high-angle faults were then acting as left-lateral strike-slips. Simultaneously, due to a tectonic "decouplage" between basement and cover (Triassic disharmonic formations), the Mesozoic strata were shortened by oblique folding and thrusting above a decollement zone.

By Oligocene time, another distensive event occurred. It originated a new set of listric normal faults quite similar to the Lias ones, but cutting across all the previous structures. Oligocene lacustrine deposits accumulated along these faults in the downfaulted blocks. Crustal flexures and normal faulting known eastward gave the minimum initial dip to the Paleozoic basement required to explain the Early Jurassic or Oligocene sliding and faulting of the Mesozoic cover.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.