The Slab Puzzle of the Alpine‐Mediterranean Region: Insights From a New, High‐Resolution, Shear Wave Velocity Model of the Upper Mantle

Abstract

Mediterranean tectonics since the Lower Cretaceous has been characterized by a multiphase subduction and collision history with temporally and spatially variable, small‐scale plate configurations. A new shear wave velocity model of the Mediterranean upper mantle (MeRE2020), constrained by a very large set of over 200,000 broadband (8–350 s), interstation, Rayleigh wave, phase velocity curves, illuminates the complex structure and fragmentation of the subducting slabs. Phase velocity maps computed using these measurements were inverted for depth‐dependent, shear wave velocities using a stochastic particle‐swarm‐optimization (PSO) algorithm. The resulting three‐dimensional (3‐D) model makes possible an inventory of slab segments across the Mediterranean. Fourteen slab segments of 200–800 km length along‐strike are identified. We distinguish three categories of subducted slabs: attached slabs reaching down to the bottom of the model; shallow slabs of shorter length in downdip direction, terminating shallower than 300 km depth; and detached slab segments. The location of slab segments are consistent with and validated by the intermediate‐depth seismicity, where it is present. The new high‐resolution tomography demonstrates the intricate relationships between slab fragmentation and the evolution of the relatively small and highly curved subduction zones and collisional orogens characteristic of the Mediterranean realm.

The Slab Puzzle of the Alpine-Mediterranean Region: Insights from a new, High-Resolution, Shear-Wave Velocity Model of the Upper Mantle

“…The Aegean Slab is generally the most prominent high-velocity anomaly in tomographic models of the Mediterranean that is interpreted as African oceanic lithosphere subducting beneath the Hellenic Arc (Bijwaard et al, 1998;Biryol et al, 2011;Chang et al, 2010;De Boorder et al, 1998;De Jonge et al, 1994;Hansen et al, 2019;Piromallo & Morelli, 1997Portner et al, 2018;Spakman et al, 1988;van der Meer et al, 2018;Zhu et al, 2012). MeRE2020 shows a continuous high-velocity anomaly down to approximately 300 km depth (Figures 6 and 7, and profile G-G' in Figure 9).…”

“…Random perturbations of a background model within specified depth-dependent velocity ranges are tested with respect to the resulting misfit between the measured and synthetic dispersion curves. Nodewise 1-D background models for the inversion are constructed using CRUST1.0 (Laske et al, 2013) for the crust and the isotropic average of PREM (Dziewonski & Anderson, 1981) for the upper mantle. Topography and bathymetry are taken into account.…”

“…In the eastern Mediterranean, slabs in the Aegean and Antalya subduction zones can be outlined by intermediate-depth seismicity down to a maximum depth of about 200 km (Bocchini et al, 2018;Caputo et al, 1970;Knapmeyer, 1999;Le Pichon & Angelier, 1979;Papazachos & Comninakis, 1971;Papazachos et al, 2005). A patch of intermediate-depth seismicity in the Vrancea area in Romania (e.g., Hurukawa et al, 2008) indicates a slab segment beneath the southeastern Carpathians.…”

“…Furthermore, they showed that the slab in the Hellenic Subduction Zone is subducting into the lower mantle, whereas beneath the western Mediterranean and the Pannonian Basin, the slabs are resting on the 660 km discontinuity. Faccenna et al (2014) reviewed the tectonics and deformation in the Mediterranean and compared the P wave velocity models by Piromallo and Morelli (2003) and by Li et al (2008) with the S wave velocity models by Zhu et al (2012) and Auer et al (2014). They concluded that the large-scale features of these models with wavelength greater than about 300 km agree well but smaller scale features including the about 100 km thick slabs vary considerably due to limited and spatially variable resolution.…”

Gravity effect of Alpine slab segments based on geophysical and petrological modelling

“…A continuous subduction of Eurasia beneath the Central Alps down to at least 200 km depth is imaged by different tomographic models (e.g. Lippitsch et al, 2003;Piromallo and Morelli, 2003;Koulakov et al, 2009;Mitterbauer et al, 2011;Hua et al, 2017;Fichtner et al, 2018;. A potential slab gap with an approximate size of 2 • is separating the subducting slab segments in the Central Alps to the Eastern Alps as imaged by, e.g.…”