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 (Central Italy)

The central Apennines is among the most seismically active sector of the central Mediterranean region, as testified by the recent sequence of earthquakes with Mw >6 (L’Aquila Mw 6.3, 06/04/2009; Amatrice Mw 6.2, 24/08/2016), which recently struck it. Although many recent studies focus on the recent history of the active and seismogenic faults, due to the poor understanding on the age of the tectonically bounded intermontane basins and on the early stage of their continental deposition, less is known on their long-term behavior.To try to fill this gap of knowledge, in this work an integrated multiple-dating approach, including 40Ar/39Ar dating, magnetostratigraphy, multiproxy paleoclimatic data (palynological analyses, Pollen Temperature Index and clumped isotopes), and astrocyclostratigraphy, was applied on 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-S. Demetrio-Castelnuovo Sub-basin -PSC). Combining the results from the CN-1 pollen record, the 40Ar/39Ar dating of a tephra (1.77 ±0.15 Ma), and the magnetostratigraphy of the CN-1 sediment core, we can refer the longest normal polarity interval (N3) to the Olduvai subchron, which we use for constraining 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 the Marine Isotopic Stages (MIS) 75 to 50. 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 record and clumped isotopes show warm-to-cold climate changes along the CN-1 sediment core, with temperatures 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 to MIS 67 and MIS 64, respectively. The PSC Sub-basin shows a complex subsurface architecture, with thresholds and depocenters showing maximum thickness of the lacustrine deposits up to 450 m. Since 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 Ma. This age for the post-orogenic formation of the tectonically active L’Aquila Basin is well in agreement with the kinematic evolution of the central Apennines, where the last documented compressional tectonic phase occurred at ca. 3.5 Ma. This post-orogenic extensional domain, which is still active, represents an archive of ca. 3 Myr of continued crustal extension and one of the most seismically active sectors of the central Mediterranean region.