Structural style of the offscraped Ligurian oceanic sequences (Early and Late Cretaceous) of the Northern Apennines: new hypothesis concerning the development of mélange block-in-matrix fabric.

Shaly dismembered formations crop out extensively on the eastern side of the Northern Apennines, as overthrusted Cretaceous oceanic units. The lack of metamorphism and the unconformably overlain epi-Ligurian sequence, which represents the trench/slope-basin sedimentary sequence, suggest that these units are accreted sequences accomplished by offscraping and imbrication. These units are characterized by stratal disruption, lack of internal coherence and complete destruction of the original stratigraphic sequence. Block-in-matrix fabric results in symmetric boudins and systematic presence of isolated or intrafolial hinges of isoclinal folds. Two orthogonal sets of tight-to-isoclinal folds are always recognizable, involving the break-up and the boudinage of the competent layers, now transposed parallel to the axial planes of the disrupted isoclinal folds. The superposition of two generations of orthogonal tight-to-isoclinal folds may be locally observed either in single bed fragments or in some preserved coherent units. Disruption and block-in-matrix fabric is a consequence of polyphase folding and intense shortening. This geometry and kinematic interpretation well constrain the tectonic environment where these rocks were deformed to the proximity of the prism toe and within the prism itself. Thus, the Apennine broken formations result from the disruption of a sedimentary pile lying on the subducting plate of the Cretaceous to Eocene trench fore-arc system, which generated the Ligurian accretionary wedge. (C) 2002 Elsevier Science Ltd. All rights reserved.