The inner Apennines (Italy) are characterized by scattered outcrops of continent-derived orogenic metamorphic units exposed along the Tyrrhenian coast from northern to southern Apennines. At least since the 1970s, some peculiar rocks exposed on Zannone Island (central Italy) have been described as the only Paleozoic-Triassic metamorphic complex linking those exposed in the northern-with those in the southern Apennines. Assessing the protolith nature, thermobaric conditions, and structural features of what is accepted to be the metamorphic unit of Zannone is, therefore, crucial to elucidate the early paleotectonic evolution of the Apennines-Tyrrhenian orogenic system. To that end, we interpreted seismic reflection profiles offshore Zannone, we carried out mesoscale and microscale structural investigations of representative outcrops on the island, performed X-ray diffraction analysis, and K-Ar and U-Pb geochronology of representative clay gouge and syntectonic carbonate veins. Results show that the metamorphic rocks of Zannone can actually be reinterpreted as belonging to nonmetamorphic siliciclastic turbidites, likely deposited in foredeep settings, and coeval to the Oligocene-Miocene Macigno Fm. of the northern Apennines. The turbiditic sequence was overthrust by Triassic dolostone in the early Miocene (~22 Ma), weakly deformed at <200°C, and downfaulted by postorogenic extensional faults starting~7 Ma ago. Hence, Zannone represents a unique geological location in central Italy where to study the innermost (i.e. oldest) thrust sheet of the central-northern Apennines, thereby shedding new light onto the early tectonics of the Apennines. Based on this evidence, a new evolutionary scheme for the early stage of the Apennines tectonic evolution is proposed.
Abstract. We studied the Zuccale Fault (ZF) on Elba,
part of the Northern Apennines, to unravel the complex deformation history that is
responsible for the remarkable architectural complexity of the fault. The ZF
is characterized by a patchwork of at least six distinct, now tightly
juxtaposed brittle structural facies (BSF), i.e. volumes of deformed
rock characterized by a given fault rock type, texture, colour, composition,
and age of formation. ZF fault rocks vary from massive cataclasite to
foliated ultracataclasite, from clay-rich gouge to highly sheared talc
phyllonite. Understanding the current spatial juxtaposition of these BSFs
requires tight constraints on their age of formation during the ZF lifespan
to integrate current fault geometries and characteristics over the time
dimension of faulting. We present new K–Ar gouge dates obtained from three
samples from two different BSFs. Two top-to-the-east foliated gouge and talc
phyllonite samples document faulting in the Aquitanian (ca. 22 Ma),
constraining east-vergent shearing along the ZF already in the earliest
Miocene. A third sample constrains later faulting along the exclusively
brittle, flat-lying principal slip surface to < ca. 5 Ma. The new
structural and geochronological results reveal an unexpectedly long faulting
history spanning a ca. 20 Myr time interval in the framework of the
evolution of the Northern Apennines. The current fault architecture is
highly heterogeneous as it formed at very different times under different
conditions during this prolonged history. We propose that the ZF started as
an Aquitanian thrust that then became selectively reactivated by early
Pliocene out-of-sequence thrusting during the progressive structuring of the
Northern Apennine wedge. These results require the critical analysis of
existing geodynamic models and call for alternative scenarios of continuous
convergence between the late Oligocene and the early Pliocene with a major
intervening phase of extension in the middle Miocene allowing for the
isostatic re-equilibration of the Northern Apennine wedge. Extension
started again in the Pliocene and is still active in the innermost portion
of the Northern Apennines. In general terms, long-lived, mature faults can
be very architecturally complex. Their unravelling, including understanding
the dynamic evolution of their mechanical properties, requires a
multidisciplinary approach combining detailed structural analyses with
dating the deformation events recorded by the complex internal architecture,
which is a phenomenal archive of faulting and faulting conditions through
time and space.
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