MOHO DEPTH AND CRUSTAL STRUCTURE IN PENINSULAR ITALY

Transcription

1 MOHO DEPTH AND CRUSTAL STRUCTURE IN PENINSULAR ITALY Neutrino Geoscience 2010 Alessandro Amato Istituto Nazionale di Geofisica e Vulcanologia

2 Talk outline Italy: tectonic setting and deep structures Moho depth through Receiver Functions Crustal structures Complexities and uncertainties Future improvements: What can be done? Is it worth doing?

3 Evolution of the Mediterranean subduction and collision (courtesy of G. Rosenbaum) Apennines Ligurian Algerian Tyrrhenian Alboran 300 km

4 Evolution of the Mediterranean subduction and collision. Complex structures expected at LNGS

5 About 56,000 earthquakes from 2005 to 2010 by the INGV seismic network The LNGS is located in a very complex region: - Boundary between NAP and central Apennines - Extensional tectonics

6 GPS velocities in Italy Reference: Eurasia Strong improvement after 2003 (ongoing) Coherent patterns Clear trends: Extension across the Apennines Convergence in NE Italy and Sicily (N-S) LNGS located in an area of active extension across the Apennines D Agostino et al., 2010

7 Middle and Lower Crust -- Seismic evidence 0 Rifted Margin Contractional Rift Shield & Platform Paleozoic Orogen Extensional Arc Forearc V p Km Central Italyhas very different crustal characteristics at short distance (see next slides) From Rudnick & Fountain, 1995

8 Receiver Functions: why? Deep well data: few and shallow (< 7km) Active seismology: shallow commercial lines and very few deep seismic lines (CROP, DSS), 2D and with low resolution at depth. New lines unlikely, expensive Receiver functions depend on seismic networks and teleseisms (M>6 at >30 distance, ~1/day) Great network improvement in Italy in last decade Possible ad hoc experiments (1-2 years) A lot of detailed studies in the last years in the world unraveled important processes (subduction fine structure, fluids transportation, etc.)

9 Seismic network in 1997 and De Luca et al. (2009)

10 Receiver functions: principles Receiver Functions are time series made of Ps converted waves generated at interfaces at depth RFs reveal velocity jumps in the structure RFs are obtained from seismic waveforms of teleseismic events rotated in the radial and transverse planes, through deconvolution of the vertical component. Central Apennines seismic structure, SGI 2010, Pisa

11 Receiver functions: data One RF computed via deconvolution of the Vertical seismogram from the horizontal one A RF data-set is made of hundreds of RFs binned as a function of the back-azimuth direction of the incoming P-wave (i.e. all RF computed from events occurred in the same place are stacked to increase the S/N ratio.)

12 Data sets: radial and transverse RFs a simple crustal structure (left) and complex (right) P diretta Ps-Moho Ps-Moho

13 Simple vs. doubled Moho The ZK2000 technique allows us to retrieve Moho depth, crust is assumed homogeneous Simple, well identified, shallow Moho Doubled (?) Moho in the belt

14 Receiver functions: application to Italy Moho depth from teleseismic receiver functions Zhu and Kanamori, JGR, 2000

15 Receiver functions: application to Italy Northern Apennines Central Apennines Southern Apennines Piana Agostinetti and Amato (2009)

16 Seismicity: transition from northern to central Apennines

17 Integration of CSS and RF data for MOHO mapping R. Di Stefano, I. Bianchi, M.G. Ciaccio (in prep.) La mappa e ottenuta dall interpolazione su tre superfici (poligoni in figura) della profondità della Moho ricavata da dati CSS e RF, seguendo il metodo sviluppato da Waldhauser et al. 1998, ed implementato per l utilizzo delle RF. Le profondita della Moho ricavate dalle RF provengono da tre lavori: Geisller et al., 2008, Lombardi et al., 2008 e Piana Agostinetti & Amato I risultati evidenziano le differenze di profondità ai limiti di placca, sia in pianta che nei profili. Il vantaggio di interpolare su tre superfici permette la definizione delle zone di sovrapposizione delle moho appartenenti alle diverse placche.

18 Vp anomalies in the crust and upper mantle in central Italy from seismic tomography (Di Stefano et al., 2009) (z=22km) Low Vp suggests fluid filled volumes in the lower-crust; (z=38km) Low Vp suggests an hydrated mantle nose (?); (z=52km) High and Low Vp associated to the Adria lid and the Tyrrhenian hydrated upper mantle, respectively. P-wave velocity perturbations of the crust and uppermost mantle in the central northern Apennines (tomography by Di Stefano et al., 2009). SKS simplified from Salimbeni et al. (2007) are mapped on the upper mantle layer Di Stefano et al. (2009) Central Apennines seismic structure, SGI 2010, Pisa

19 RF: what else do they tell us? Details of the crustal and upper mantle structure Fluid migration (?) in the continental subduction of northern Apennines Examples from Apulia and Abruzzi

20 Importance of retrieving reliable estimates of Moho depth and crustal structure is demonstrated by these numbers: Dye (2010)

21 Fluid migration from the slab in Northern Apennines is hypothesized from RF studies Piana Agostinetti et al., in review on EPSL

22 Apulia foreland (Puglia 1 well): results from a RJMCMC inversion (Piana Agostinetti and Malinverno, 2010) ~16 strati Two-layered Meso-Cenozoic Apulian Platform (limestones/evaporites?) LVs below (Permo-Trias), thickness ~3-4 km LVs in the lower crust (~20-24km) Layered lower crust (~24-30km) MRVN Amato et al., in prep.

23 Abruzzi (Teramo preliminary) TERO (Teramo, not far from here) Preliminary from Amato et al. In prep. Left: from Di Luzio et al. (2008)

24 Struttura della crosta al M. Conero AOI M. Conero Preliminary from Amato et al., in prep. From Finetti, 2005 In this interpretation of CROP line, the UC is crystalline-metamorphic. The RF UC has low Vs (Vp 6km/s: granitic gneiss? Higher U,Th!)

25 Abruzzi: Local Earthquakes Tomography and RF High velocity shallow bodies from seismic tomography and RF: mafic basement rocks (Chiarabba et al., 2010) Local earthquake data (ML<3.7) recorded by temporary ( ) and permanent seismic stations belonging to the INGV and the Abruzzi regional network, for a total of 139 stations, to compute three-dimensional Vp and Vp/Vs models Map showing the instrumental seismicity of the past 28 years in central Apennines and the map location of the HV bodies in the upper crust.

26 Conclusions The lithosphere structure around LNGS is very complex, strongly 3D A detailed Moho(s) depth map is available Current models are good but can be improved, particularly for middle and lower crust Teleseismic Receiver Functions are a powerful tool to unravel details of crust structure (and composition?)

27 Possible improvements Detailed study of Central Apennines seismic stations Analyze extensively seismic station GIGS ( ) Ad hoc experiments(?) Better joint interpretation of Vp, Vs, etc. at various depths (incl. the HV bodies in the upper crust, and anisotropic patterns)

28 Thank you for your attention

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30 Types of crust and typical P velocities for rock types (after Christensen and Mooney, 2005)