A lithospheric-scale seismogenic thrust in central Italy

In this paper, the relatively deep (up to 90 km) seismicity of central Italy is considered in comparison with the upper crust seismicity at the outer front of the Apennine fold-and-thrust belt system and with the major crust-scale tectonic discontinuities outlined by deep crust seismic reflection and refraction profiles. In particular, attention is given to the presence of a Plio-Quaternary SW-dipping thrust zone cutting the entire Adriatic crust and emerging at the outer front of the Apennine thrust system, along the Po Plain-Adriatic belt. This discontinuity, detectable from the CROP 03 near-vertical seismic reflection profile, but only partially highlighted in previous studies, is here named Adriatic Thrust (AT). The geometry of the AT is compared with the distribution and kinematics of the Coastal-Adriatic upper crust seismicity, of the Marche Foothill lower crust seismicity and of the Apennine deep seismicity (base of the crust and upper mantle). A good spatial correspondence between the down-dip fault geometry and the seismicity is observed. The rheological stratification of the lithosphere controls the location of the seismic activity at different depth intervals along the fault plane (0–10, 15–25, 35–45 km). The reconstructed rheological conditions do not allow to justify in terms of brittle behavior the deepest earthquakes, located at depths from 55 to 70 km within the possible down-dip intra-mantle prosecution of the AT. These events might represent either co-seismic plastic instabilities due to high strain concentrations within the AT shear zone or seismic failure of the continental Adriatic lithosphere due to a strong increase in pore fluid pressure (infiltration from the asthenosphere of hydroxil-CO2 rich volatiles). The proposed interpretation of the AT as a lithosphere-scale seismogenic fault zone, has several implications: (1) the Po Plain-Adriatic front of the Apennine thrust system is still active; (2) the Apennine deep seismicity is not associated with a SW-dipping subduction plane; (3) seismogenic deformation in association with lithospheric failure is possible.

[1]  R. Sibson,et al.  Frictional constraints on thrust, wrench and normal faults , 1974, Nature.

[2]  S. Pondrelli,et al.  Active stress map of Italy , 1999 .

[3]  Carlo Meletti,et al.  Construction of a Seismotectonic Model: The Case of Italy , 2000 .

[4]  Giusy Lavecchia,et al.  Architecture and seismotectonics of a regional low‐angle normal fault zone in central Italy , 2000 .

[5]  J. Platt,et al.  Late orogenic extension of the Betic Cordillera and the Alboran Domain: A lithospheric view , 1995 .

[6]  Artyushkov.  Steep bending of continental lithosphere without its stretching or plate collision: an indication for lithospheric failure and phase transitions , 1998 .

[7]  Giovanni Chiodini,et al.  Quantification of deep CO2 fluxes from Central Italy. Examples of carbon balance for regional aquifers and of soil diffuse degassing , 1999 .

[8]  Liotta,et al.  Restricted rifting and its coexistence with compressional structures: results from the CROP 3 traverse (Northern Apennines, Italy) , 1998 .

[9]  S. Kirby.  Rheology of the lithosphere , 1983 .

[10]  S. Parolai,et al.  Hypocenter location accuracy and seismicity distribution in the Central Apennines (Italy) , 2001 .

[11]  A. Frepoli,et al.  Contemporaneous extension and compression in the Northern Apennines from earthquake fault-plane solutions , 1997 .

[12]  G. Panza,et al.  Properties of the lithosphere in collisional belts in the Mediterranean—a review , 1990 .

[13]  Giorgio Ranalli,et al.  Rheological stratification of the lithosphere , 1987 .

[14]  E. Tondi,et al.  Active tectonics in the central Apennines and possible implications for seismic hazard analysis in peninsular Italy , 1997 .

[15]  S. Mazzoli,et al.  Active tectonics of the Northern Apennines and Adria geodynamics: new data and a discussion , 2002 .

[16]  A. Frepoli,et al.  Spatial variation in stresses in peninsular Italy and Sicily from background seismicity , 2000 .

[17]  P. Renda,et al.  From Mesozoic extension to Tertiary collision; deformation patterns in the units of the north-western Sicilian chain , 2002 .

[18]  Mauro De Donatis,et al.  Frontal part of the northern Apennines fold and thrust belt in the Romagna‐Marche area (Italy): Shallow and deep structural styles , 1999 .

[19]  A. Cundari.  Petrogenesis of leucite-bearing lavas in the Roman volcanic Region, Italy , 1979 .

[20]  V. Pasquale,et al.  Rheology and seismotectonic regime in the northern central Mediterranean , 1997 .

[21]  G. Cello,et al.  Role and effects of pore fluid pressure in thrusting: The case history of the Umbria‐Marche Apennines, central Italy , 1995 .

[22]  M. Dragoni,et al.  Evaluation of stresses in two geodynamically different areas: Stable foreland and extensional backarc , 1996 .

[23]  A. Pizzi,et al.  Methods for determining the Pleistocene–Holocene component of displacement on active faults reactivating pre-Quaternary structures: examples from the Central Apennines (Italy) , 2000 .

[24]  A. Argnani,et al.  Foreland deformation in the Central Adriatic and its bearing on the evolution of the Northern Apennines , 1997 .

[25]  G. Selvaggi.  Strain pattern of the Southern Tyrrhenian slab from moment tensors of deep earthquakes: implications on the down-dip velocity , 2001 .

[26]  R. Console,et al.  Seismogenic structures activated during the 1987 seismic sequences along the Adriatic coast , 1992 .

[27]  V. Picotti,et al.  Extension controls Quaternary tectonics, geomorphology and sedimentation of the N-Appennies foothills and adjacent Po Plain (Italy) , 1997 .

[28]  R. Scarpa,et al.  Fault-plane solutions and seismicity of the Italian peninsula , 1985 .

[29]  G. Selvaggi,et al.  Subcrustal earthquakes in the northern Apennines (Italy): Evidence for a still active subduction? , 1992 .

[30]  B. Hobbs,et al.  Earthquakes in the ductile regime? , 1986 .

[31]  C. Petit,et al.  Depth distribution of earthquakes in the Baikal rift system and its implications for the rheology of the lithosphere , 2001 .

[32]  R. Console,et al.  The Perugia (Italy) earthquake of 29, April 1984: A microearthquake survey , 1988 .

[33]  P. Montone,et al.  Recent tectonic evolution and present stress in the Northern Apennines (Italy) , 1999 .

[34]  New criteria for seismotectonic zoning in central Italy: insights from the Umbria-Marche Apennines , 2002 .

[35]  F. Calamita,et al.  EVOLUZIONE STRUTTURALE E MORFOGENESI PLIO-QUATERNARIA DELL'APPENNINO UMBRO-MARCHIGIANO TRA IL PREAPPENNINO UMBRO E LA COSTA ADRIATICA , 1999 .

[36]  C. Federico,et al.  Crustal shortening and duplication of the Moho in the Northern Apennines: a view from seismic refraction data , 1995 .

[37]  K. Priestley,et al.  A re‐assessment of focal depth distributions in southern Iran, the Tien Shan and northern India: do earthquakes really occur in the continental mantle? , 2000 .

[38]  Marco Menichetti,et al.  Seismotectonic zoning in east-central Italy deduced from an analysis of the Neogene to present deformations and related stress fields , 1994 .

[39]  R. Nicolich.  Deep seismic transects , 2001 .

[40]  G. B. Cimini,et al.  Tomographic constraints on the geodynamic evolution of the Italian region , 1999 .

[41]  M. Pipan,et al.  Crustal section based on CROP seismic data across the North Tyrrhenian–Northern Apennines–Adriatic Sea , 2001 .

[42]  G. Selvaggi,et al.  The Mw 5.4 Reggio Emilia 1996 earthquake: active compressional tectonics in the Po Plain, Italy , 2001 .

[43]  A. Tertulliani,et al.  Study of the seismic sequence of Porto San Giorgio (Marche)–3 July 1987 , 1989 .

[44]  E. Pili,et al.  Lithospheric shear zones and mantle-crust connections , 1997 .

[45]  G. Vai,et al.  Anatomy of an orogen : the Apennines and adjacent Mediterranean basins , 2001 .

[46]  G. Ranalli,et al.  The role of rheology in extensional basin formation modelling , 1997 .

[47]  Roberta M. Johnson,et al.  Observations of high‐latitude lower thermospheric winds from Thule Air Base and Søndre Strømfjord, Greenland , 1999 .

[48]  B. Hobbs,et al.  The strength of the continental crust, detachment zones and the development of plastic instabilities , 1989 .

[49]  Alessandro Maria Michetti,et al.  Sintesi delle conoscenze sulle faglie attive in Italia Centrale: parametrizzazione ai fini della caratterizzazione della pericolosità sismica , 2000 .

[50]  P. Molnar,et al.  Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere , 1983 .