Restricted rifting and its coexistence with compressional structures: results from the CROP 3 traverse (Northern Apennines, Italy)

A geological interpretation of the deep seismic reflection line CROP 3 (Italian program CROsta Profonda: deep crust, profile no. 3), which crosscuts the Northern Apennines from the Tyhrrenian to the Adriatic coast, is presented. The profile images the lithosphere up to 15 s TWT and highlights several peculiarities: (a) the lower crust is bedded by discontinuous sub parallel reflective markers which terminate at ≈ 7 s beneath the western side of the profile; (b) in the same area a notable reflection is recognizable at 10 s and is interpreted as the top of the asthenosphere; (c) east-plunging shear zones are recognizable throughout the crust. By contrast, the Adriatic (outer) side of this line shows: (a) reflections deepening westward, from 13 to 15 s TWT, related to the base of the crust (33–38 km depth); (b) the absence of thick bedding of reflective markers within the lower crust; and (c) tectonic structures affecting the basement which are different from those which deform the cover of the Northern Apennines. Geological interpretation is based on the eastward migration of ductile shear zones which have been recognized on the seismic line. The bending of the outer zone crust is considered to be a consequence of the rifting process with the application of pushing forces against the Adriatic lithosphere which cannot escape toward east.     

Quaternary normal faulting in southeastern Sicily (Italy):a seismic source for the 1693 large earthquake

We present geological and morphological data, combined with an analysis of seismic reflection lines across the Ionian offshore zone and information on historical earthquakes, in order to yield new constraints on active faulting in southeastern Sicily. This region, one of the most seismically active of the Mediterranean, is affected by WNW–ESE regional extension producing normal faulting of the southern edge of the Siculo–Calabrian rift zone. Our data describe two systems of Quaternary normal faults, characterized by different ages and related to distinct tectonic processes. The older NW–SE-trending normal fault segments developed up to ≈400  kyr ago and, striking perpendicular to the main front of the Maghrebian thrust belt, bound the small basins occurring along the eastern coast of the Hyblean Plateau. The younger fault system is represented by prominent NNW–SSE-trending normal fault segments and extends along the Ionian offshore zone following the NE–SW-trending Avola and Rosolini–Ispica normal faults. These faults are characterized by vertical slip rates of 0.7–3.3  mm  yr ⁻¹ and might be associated with the large seismic events of January 1693. We suggest that the main shock of the January 1693 earthquakes ( M ~ 7) could be related to a 45  km long normal fault with a right-lateral component of motion. A long-term net slip rate of about 3.7  mm  yr ⁻¹ is calculated, and a recurrence interval of about 550 ± 50  yr is proposed for large events similar to that of January 1693.     

A seismic reflection-refraction experiment across the exposed lower crust in Calabria (southern Italy): first results

Exposed crystalline basement of the Serre Mountains in Calabria presents a tilted block of a nearly complete section of the Hercynian continental lower crust (HCLC). In addition to petrological and structural data from surface mapping, and petrophysical data from the laboratory, a seismic reflection-refraction experiment was conducted in May 1990. This consisted of a 40 km long N-S profile crossing the HCLC and of four short transverse profiles, each recorded using 3-component receivers with an 80 m spacing and explosive sources.
The reflectivity of the outcropping lower crustal units is lower than theoretically predicted from the observed compositional layering. A low-velocity zone, outcropping in the north, and dipping to the south, marks the contact between the HCLC and the underlying Alpine metamorphic units. Below this zone, the deeper crust appears well-structured by strong and continuous, discrete reflections down to 6.5–8 s t.w.t. (presumably the crust-mantle boundary at 19–24 km depth) with a dominant dip toward the south.
Analysis of refracted-wave velocities reveals values systematically lower by up to 30% than laboratory data on rock samples or calculated data from modal analysis. This discrepancy can only partly be explained by the effect of microcracks (10%), the underestimation of the amount of leucosomes (2–5%) and the effect of seismic anisotropy (0–5%). The remaining discrepancy must be attributed to large scale alteration of the rocks due to Apennine tectogenetic events.