Geometry of the Deep Calabrian Subduction (Central Mediterranean Sea) From Wide‐Angle Seismic Data and 3‐D Gravity Modeling

Abstract

The Calabrian subduction zone is one of the narrowest arcs on Earth and a key area to understand the geodynamic evolution of the Mediterranean and other marginal seas. Here in the Ionian Sea, the African plate subducts beneath Eurasia. Imaging the boundary between the downgoing slab and the upper plate along the Calabrian subduction zone is important for assessing the potential of the subduction zone to generate megathrust earthquakes and was the main objective of this study. Here we present and analyze the results from a 380‐km‐long, wide‐angle seismic profile spanning the complete subduction zone, from the deep Ionian Basin and the accretionary wedge to NE Sicily, with additional constraints offered by 3‐D gravity modeling and the analysis of earthquake hypocenters. The velocity model for the wide‐angle seismic profile images thin oceanic crust throughout the basin. The Calabrian backstop extends underneath the accretionary wedge to about 100 km SE of the coast. The seismic model was extended in depth using earthquake hypocenters. The combined results indicate that the slab dip increases abruptly from 2–3° to 60–70° over a distance of ≤50 km underneath the Calabrian backstop. This abrupt steepening is likely related to the rollback geodynamic evolution of the narrow Calabrian slab, which shows great similarity to the shallow and deep geometry of the Gibraltar slab.

Comment on “Geometry of the Deep Calabrian Subduction (Central Mediterranean Sea) From Wide‐Angle Seismic Data and 3‐D Gravity Modeling” by Dellong Et Al.

“…The AE T. Becker and an anonymous reviewer are thanked for the assistance and the useful suggestions, respectively. This paper is a Comment and bears mostly on the data available through Dellong et al, 2020.…”

Reply to Comment by A. Argnani on “Geometry of the Deep Calabrian Subduction From Wide‐Angle Seismic Data and 3‐D Gravity Modeling”

“…As to differences in crustal thickness between the oceanic crust imaged along DY-P4 and DY-P3, the top of the oceanic crust along the DY-P3 velocity model was never shown in Dellong et al (2018) nor in Dellong et al (2020), as it was not directly observed in the DY-P3 OBS data. If the author of the comment is referring to the oceanic crust location presented in the gravity models, it is worth mentioning that this specific "best fit" model was built to fit the free-air gravity anomaly and consequently does not reflect our latest interpretation of the area.…”

“…We agree that the lower portion of the crustal block, initially interpreted as Calabrian continental crust along profile DY-P3, in fact represents oceanic crust of the downgoing Ionian Sea slab (Figure 1; Comment Key Point 2). We note in the text, "While along DY-P3 no slab was modeled, along the profile DY-P4, the slab is clearly imaged by the data from the land stations […] The Moho depth along model distance 80-120 km on profile DY-P3 (31 km) corresponds to the depth of the oceanic Moho along DY-P4; however, the backstop-slab interface was not detected along DY-P3 ( Figure 9 and Figure S6 in Dellong et al, 2020; electronic supplements)". This new interpretation is due to the better data quality of the well-coupled land stations than the sea bottom instruments.…”

“…(1) In both articles (Dellong et al, 2018(Dellong et al, , 2020 this hypothesis was investigated, but a satisfactory answer with solid proof could not be achieved that this layer was an upper-crustal layer. We still prefer our final interpretation for at least two main reasons:…”

“…First of all, we would like to thank Andrea Argnani for the interest he has shown in our work. Andrea Argnani in his comment on Dellong et al (2020) (Geometry of the deep Calabrian subduction (Central Mediterranean Sea) from wide‐angle seismic data and 3‐D gravity modeling) proposes an alternate interpretation of the final velocity models presented by Dellong et al (2018) and Dellong et al (2020). Additionally, he claims that a part of the literature was not reported properly.…”