Imprint of foreland structure on the deformation of a thrust sheet: The plio-pleistocene gela nappe (southern Sicily, Italy)

Abstract

In Sicily, the progressive imbrication of the Apenninic thrust belt above the Pelagian-African Foreland is traced by the southward migration of marine basins that were progressively shortened during the late Miocene-Pleistocene. The outermost and youngest thrust sheet (Gela Nappe) displays a peculiar shortening, with Messinian to early Pliocene E-W folds refolded in the late Pliocene - early Pleistocene by approximately N-S folds (subparallel to the transport direction of the thrust sheets). This structural interference is documented in south Sicily within localized belts of refolding spaced ∼5 - 8 km apart. The significance of this fold interference pattern is highlighted by our analysis of the offshore seismic reflection line M23A (CROP Mare Project) that intersects the Gela Nappe along a trace suborthogonal to the thrust transport direction. Migration and depth conversion of the line reveal multiple imbrications and draping of the allochthonous units above structural highs of the foreland, delimited by inherited N-S faults. The largest faults bound mid-late Miocene extensional basins but were reactivated in compression during the late Pliocene - early Pleistocene, causing (1) superposed folding along discordant N-S structural trends, (2) compressional extrusion of the whole wedge of the Gela Nappe, and (3) offset of its sole thrust. The reactivation of faults subparallel to the transport direction accommodates differential flexure of the rigid foreland beneath the Apenninic wedge, and these late stage deformations in the foreland are responsible for the superposition of E-W finite shortening onto N-S shortening. Copyright 2009 by the American Geophysical Union.