Possible causes of arc development in the Apennines, central Italy

In central Italy, the geometry, kinematics,
and tectonic evolution of the late Neogene
Umbrian Arc, which is one of the main
thrusts of the northern Apennines, have
long been studied. Documented evidence
for orogenic curvature includes vertical axis
rotations along both limbs of the arc
and a positive orocline test along the entire
arc. The cause of the curvature is, however,
still unexplained. In this work, we focus
our attention on the southern portion of
the Umbrian Arc, the so-called Olevano-
Antrodoco thrust. We analyze, in particular,
gravity and seismic-reflection data and consider available paleomagnetic, stratigraphic,
structural, and topographic evidence from
the central Apennines to infer spatial extent,
attitude, and surface effects of a midcrustal
anticlinorium imaged in the CROP-11 deep
seismic profile. The anticlinorium has horizontal
dimensions of ~50 by 30 km, and
it is located right beneath the Olevano-
Antrodoco thrust. Stratigraphic, structural,
and topographic evidence suggests that the
anticlinorium produced a surface uplift during
its growth in early Pliocene times. We
propose an evolutionary model in which,
during late Neogene time, the Olevano-
Antrodoco thrust developed in an out-of sequence
fashion and underwent ~16° of
clockwise rotation when the thrust ran into
and was then raised and folded by the growing
anticlinorium (late Messinian–early Pliocene
time). This new model suggests a causal
link between midcrustal folding and surficial
orogenic curvature that is consistent with
several available data sets from the northern
and central Apennines; more evidence is,
however, needed to fully test our hypothesis.
Additionally, due to the occurrence of midcrustal
basement-involved thrusts in other
orogens, this model may be a viable mechanism
for arc formation elsewhere.

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