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Kinematic evolution of thrust-related structures in the Umbro-Romagnan parautochthon (northern Apennines, Italy)

1994; Wiley; Volume: 6; Issue: 6 Linguagem: Inglês

10.1111/j.1365-3121.1994.tb00523.x

1365-3121

Mauro De Donatis, Stefano Mazzoli,

Geological and Geophysical Studies Worldwide

ABSTRACT In the internal part of the Umbro‐Marchean‐Romagnan Apennines, the foredeep clastic wedge constituting the Neogene part of the sedimentary cover is completely detached from the underlying Mesozoic–Palaeogene succession. The resulting (Umbro‐Romagnan) parautochthon consists of tectonostratigraphic units with a general geometry of broad synclinal blocks separated by narrow faulted anticlines. Thrust‐related structures observed in the field require thrust ramp propagation to have occurred within already folded rocks; therefore, they cannot be restored using simple fault‐bend fold or fault‐propagation folding models. Evidence for a passive fold origin in the studied rocks suggests that an early detachment folding episode preceded ramp propagation. The latter was facilitated by the enhanced thickness of incompetent material in the cores of detachment anticlines, which became the preferential sites where thrust ramps cut up‐section. Depending on the trajectory of such thrust ramps, different types of fault‐related structures could develop. Hanging‐wall anticlines which give way to monoclinal structures higher up in the section are associated with listric thrust ramps, whereas hanging wall monoclines approximately parallel to the underlying fault surface are associated with straight‐trajectory ramps. This kinematic evolution, which occurred partly during syn‐depositional compression, also accounts for the observed lithofacies distribution. The latter reflects an early differentiation of the foredeep trough into sub‐basins that are progressively younger towards the foreland. The detachment anticlines that originally bounded such sub‐basins were the site of later thrust propagation, leading to a tectonic juxtaposition of different tectonostratigraphic units consisting of broad NW‐SE elongate synclinal blocks.