70-7 Stratigraphic, geomorphic, and structural record of migrating slab tear during the past ~1 Myr in southern Calabria and NE Sicily, Italy

Abstract:

Southern Italy is well suited for studies of coupled deep Earth and surface processes because it preserves a well-documented record of deformation and uplift related to migrating collision, trench retreat, and tearing of subducted ocean slabs. However, mechanisms of mantle-driven uplift remain incompletely understood due to a lack of reliable ages for Pleistocene marine terraces >200 m above sea level (masl). To address this gap, we investigated regional stratigraphy, geochronology, geomorphology, and structural geology in southern Calabria and NE Sicily to assess rates and drivers of vertical crustal motions. Our results, combined with previous studies, reveal four regionally correlative stratigraphic units: (1) shallow marine Vinco calcarenite (~1.0 Ma at the top; age of base is uncertain); (2) bathyal marine claystone and marl (~1.0–0.48 Ma); (3) Messina Gravels and Sands Formation Gilbert-delta deposits (~480–120 ka); and (4) ~120-ka shallow marine terraces preserved at ~100–1,300 masl. ⁴⁰Ar/³⁹Ar step-heating experiments on fresh glass from volcanic tuff interbedded with claystone near the top of unit 2 yield reproducible age spectra with a mean age of 0.481 ± 0.019 Ma (± 2σ). Luminescence dating of marine terrace sands (unit 4) reveals burial ages that overlap across the full range of sampled elevations (~100–1,000 masl) with a weighted mean of 118.8 ± 8.7 ka (± 1σ). These ages, combined with surface mapping and fault-zone morphology, indicate that the marine terraces are erosional remnants of a single, once-continuous surface that has been offset by active normal faults and uplifted at rates ranging from <1 mm/yr near the coast to >12 mm/yr in the high Aspromonte Mountains, with time-averaged fault throw rates of 0.8–4.2 mm/yr. Rates of surface uplift and fault throw are 5–10 times faster than prior estimates that were based on a widely accepted but unsubstantiated age of ~1.0–1.2 Ma for marine terraces >1,000 masl. We infer that rapid post-120 ka uplift is an ongoing isostatic response to an active migrating tear in the subducted Ionian slab, consistent with recent geophysical observations and predictions of geodynamic models. The brief period of subsidence recorded in unit 2 (~1.0–0.48 Ma) is also predicted by numerical models for slab tearing, though the geometry and direction of tear migration currently are not known.  

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