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Slab Imaging in Continental Subduction under the Northern Apennines Bianchi I. 1,2, J.J. Park 3, T. Mayeau 3, N. Piana Agostinetti 2, V. Levin 4 1 Universita’ “La Sapienza” di Roma 2 INGV Roma 3 Yale University, CT 4 Rutgers University, NJ Thanks to members of the RETREAT project seismology team

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(Piana Agostinetti et al, 2008) Italian peninsula is almost parallel to the convergence direction between Africa and Eurasia. There is considerable earthquake activity along the Apennines BUT Earthquakes are concentrated beneath the mountains and descend to the depth of ~ 100 km. Lucente et al., 1999 APENNINES MARK A SUBDUCTION ZONE Much more slab is imaged and caused by 30 My long subduction. Apennines Slab Vertical transect

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Adapted from Faccenna et al., 2004 An eastward retreating slab has been invoked as principal driving process in this area, able to describe the present position of the orogen and to justify the juxtaposing of extension over compression. (Malinverno & Ryan 1986) Surface extensionSurface convergence Adriatic Sea (Adria microplate) Tyrrhenian Sea (back-arc basin)

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GRAZIE Conceptual Tectonic Model for RETREAT (Brandon, Willett, Pazzaglia, et al) Important Aspects: Crust thickens and thins while passing through orogen Rigid mantle block beneath orogen to match surface tectonics in geodynamic model ADRIA Plate (Africa) TYRRHENIAN Plate (Europe) SLAB Retreat Things to look for with Seismology: Crustal Thickness Top of the Downgoing Lithosphere

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Crustal Models Finetti et al., 2001 Crustal Wavespeeds (Vp & Vs)

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Vertical motion Correlation measure (white is good) Wavelet transform Propagation azimuth Phase velocity (km/sec) RETREAT Seismic Array Test for Crustal Thickness I: surface-wave phase velocity 3-station wavelet-based cross- correlation, obtains both apparent phase velocity and propagation azimuth

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GRAZIE Phase velocity (km/sec) Match the locations of station triangles to a coarse grid of RETREAT Field Area Compute median phase-velocity, discarding values >6 km/sec or <1.5 km/sec Surface Wave period (seconds) ADRIA TYRRHENIAN

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GRAZIE Faster phase- velocity at ~20s period in Tuscany, suggestive of thinner crust there

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Retreat seismic array Epicentral distribution of teleseisms M > 5.5 12659 teleseismic seismic records used Closely-spaced station transect Test of Crustal Thickness II: Receiver Function Transect

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Three components: DV(t) = I(t)*S(t)*EV(t) DR(t) = I(t)*S(t)*ER(t) DT(t) = I(t)*S(t)*ET(t) Receiver Functions overview ER(ω)=DR(ω)/DV(ω) ET(ω)=DT(ω)/DV(ω) (Ammon, 1997)

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Piana Agostinetti et al, EPSL in press 2008 Tuscan Moho Adriatic Moho? Wrong Polarity! Earthquakes From NE Earthquakes From SW

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Back-azimuth RF Dependence implies both Radial & Transverse RF amplitude and implies interface dip or anisotropy Energy only on R component Energy on both R and T components

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How to evaluate both radial & transverse RFs? π/2 Shift 0 Enhance Ps pulses that can be explained in terms of dipping interface or anisotropy By Katsuhiko SHIOMI

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Analysis Procedure: 1- Definee each profile from spots 10 km apart 2- Finding the piercing point at 40 km depth of every RF collected. 3- Every spot grabs RFs for which 40-km piercing points are in Immediately surrounding area. 4- Applying the Harmonic expansion and migration focusing at 40-km depth. Analysis Procedure: 1- Definee each profile from spots 10 km apart 2- Finding the piercing point at 40 km depth of every RF collected. 3- Every spot grabs RFs for which 40-km piercing points are in Immediately surrounding area. 4- Applying the Harmonic expansion and migration focusing at 40-km depth. AA’: spots grab piercing points into a 20x120 km area. BB’: spots grab piercing points into a 20x80 km area. CC’: spots grab piercing points into a 20x120 km area. DD’: spots grab piercing points into a 20x40 km area. EE’: spots grab piercing points into a 20x120 km area. AA’: spots grab piercing points into a 20x120 km area. BB’: spots grab piercing points into a 20x80 km area. CC’: spots grab piercing points into a 20x120 km area. DD’: spots grab piercing points into a 20x40 km area. EE’: spots grab piercing points into a 20x120 km area.

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RESULTS 0 130 km (at least 300 RFs to build one wiggle)

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0 240 km

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0 210 km

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SYNTHETICS DATA Attempt to fit observations with simple dipping isotropic interface: The continuity of dipping plane is not clear under the Tyrrhenian side Or the interface continues but seismic properties change to give a weaker pulse Attempt to fit observations with simple dipping isotropic interface: The continuity of dipping plane is not clear under the Tyrrhenian side Or the interface continues but seismic properties change to give a weaker pulse

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Modeling: SyntheticsObserved 3 Model segments to represent BB’ profile: -Tyrrhenian Model: Moho at 25 km depth for 120 km -Overlapping Model: Tyrrhenian Moho, anisotropic layer and Adriatic dipping Moho for 70km -Adriatic Model: dipping Moho for 50 km 3 Model segments to represent BB’ profile: -Tyrrhenian Model: Moho at 25 km depth for 120 km -Overlapping Model: Tyrrhenian Moho, anisotropic layer and Adriatic dipping Moho for 70km -Adriatic Model: dipping Moho for 50 km

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Discussion Tyrrhenian & Adriatic crusts have different thicknesses, but no confirmation of central thickening at Apennines crest A south-dipping subcrustal anisotropic layer appears to lie beneath under the Northern Apennines orogen, but with limited extent along strike anisotropic texture aligns with interface plunge No evidence for semi-rigid mantle body at base of Apennines orogen POSSIBLE INTERPRETATIONS: -Adriatic lower crust subducting with lithosphere? -Subducted lower crust coming back to shallower depths as for UHP rocks (Beaumont et al. 2001) -Suction of subcrustal material from Tyrrhenian toward Adria as Apennines Slab retreats

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= CUTR SyntheticsCUTR Observed N10ºE Test Analysis for a single station: Cutigliano (CUTR) verifies the direction-dependent subcrustal interface Anisotropy needed

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