Constraining the early stage of the post-orogenic extensional tectonics in central Italy: New evidence from a long sediment core from the tectonically active L’Aquila Basin

The central Apennines are among the most
seismically active sectors of the central Mediterranean
region, as testified by the recent sequence
of Mw >6 earthquakes (L’Aquila Mw
6.3, 6 April 2009; Amatrice Mw 6.2, 24 August
2016), that struck the region. Although
many recent studies have focused on the
recent history of the active and seismogenic
faults, due to the poor understanding on the
age of the fault-bounded intermontane basins
and on the early stage of their continental
deposition, less is known about the faults’
long-term behavior.
To try to fill this knowledge gap, we analyzed
a long sediment core (230 m depth,
Castelnuovo 1 borehole; CN-1) recovered
from the infill of the Plio-Pleistocene
tectonically active L’Aquila Basin (Paganica–
San Demetrio–Castelnuovo [PSC]
Sub-basin) by employing 40Ar/39Ar dating,
magnetostratigraphy, multiproxy paleoclimatic
data (palynological analyses, pollen
temperature index and clumped isotopes),
and astrocyclostratigraphy.
Combining the results from the CN-1
pollen record, 40Ar/39Ar dating of a tephra
(1.77 ± 0.15 Ma), and magnetostratigraphy
of the CN-1 sediment core, we can refer the
longest normal polarity interval (N3) to the
Olduvai subchron, which we use to constrain
the CN-1 age model. Moreover, spectral
analysis of the CN-1 calcimetry data series
shows the presence of 13 obliquity-modulated
cycles, resulting in an age of ca. 1490
ka for the top of the core and an age of ca.
2027 ka for its base. This time span encompasses
Marine Isotopic Stages (MIS) 50–75.
The occurrence of lacustrine ostracod fauna
since the lowermost portion of the CN-1 core
points to the presence in the PSC Sub-basin,
already at 2 Ma, of a well-developed intermontane
lake. Both pollen assemblages and
clumped isotopes show warm-to-cold climate
changes along the CN-1 sediment core, with
clumped-isotope–derived temperatures of
the lake waters of 15.4 ± 1.6 °C (MIS 53) and
11.5 ± 1.3 °C (MIS 52), whereas temperatures
of 21 ± 1.7 °C and 15.6 ± 1.7 °C correspond
respectively to MIS 67 and MIS 64.
The PSC Sub-basin shows a complex subsurface
architecture, with highs and depocenters
showing maximum thickness of the
lacustrine deposits up to 510–450 m. Considering
that the CN-1 age model points to a
sedimentation rate of 0.3 mm/yr for the deep
lacustrine deposits, the onset of the continental
sedimentation in the L’Aquila intermontane
basin started at ca. 3.2–3.0 Ma.
This post-orogenic extensional domain
responsible for the onset and subsequent development
of the L’Aquila intermontane basin
is still active, representing an archive of
∼3 m.y. of continued crustal extension from
one of the most seismically active sectors of
the central Mediterranean region.