Erosion of the continental lithosphere at the cusps of the Calabrian arc: Evidence from S receiver functions analysis

Mediterranean tectonics has been characterized by an irregular, complex temporal evolution with episodic rollback and retreat of the subducted plate followed by period of slow trench-migration. To provide insight into the geodynamics of the Calabrian arc, we image the characteristics and lithospheric structure of the convergent, Apulian and Hyblean forelands at the cusps of the arc. Specifically we investigate the crustal and lithospheric thicknesses using teleseismic S-to-p converted phases, applied to the Adria-Africa plate margin for the first time. We find that the Moho in the Apulian foreland is nearly flat at ∼30 km depth, consistent with previousPreceiver functions results, and that the Hyblean crustal thickness is more complex, which can be understood in terms of the nature of the individual pieces of carbonate platform and pelagic sediments that make up the Hyblean platform. The lithospheric thicknesses range between 70–120 km beneath Apulia and 70–90 km beneath Sicily. The lithosphere of the forelands at each end of the Calabrian arc are continental in nature, buoyant compared to the subducting oceanic lithosphere and have previously been interpreted as mostly undeformed carbonate platforms. Our receiver function images also show evidence of lithospheric erosion and thinning close to Mt. Etna and Mt. Vulture, two volcanoes which have been associated with asthenospheric upwelling and mantle flow around of the sides the slab. We suggest that as the continental lithosphere resists being subducted it is being thermo-mechanically modified by toroidal flow around the edges of the subducting oceanic lithosphere of the Calabrian ar

Analisi strutturale di crosta e mantello in prossimità dell’alta Val di Chiana (Toscana orientale)

L’Appennino Settentrionale è una catena montuosa NE-vergente ed è il risultato dell’affioramento del prisma di accrezione originato in seguito alla subduzione della litosfera adriatica sotto il mar Tirreno ed ancora in atto (Faccenna et al., 2001). Dall’Oligocene ad oggi, l’Appennino Settentrionale è stato interessato da due fasi deformative: inizialmente compressiva con la formazione di thrusts e più recentemente distensiva (Elter et al., 1975). Attualmente è caratterizzato da un regime crostale distensivo con una velocità stimata circa 2.5 mm/anno (Hunstad et al., 2003). Gli effetti e le conseguenze di questi episodi deformativi sono ben visibili attraverso un’analisi geologica e geofisica. L’area in studio è posta in corrispondenza della transizione tra la successione Toscana ed il settore Tirrenico del dominio Umbro-Marchigiano, quindi, una zona particolarmente dibattuta da un punto di vista geodinamico, a causa della presenza di diverse tipologie crostali, flusso di calore e anomalie gravimetriche

Crustal and Upper Mantle Three-Dimensional Stratification and Anisotropy from Receiver Functions (Northern Apennines-Italy)

The Northern Apennines (NA) were predominantly formed by a Meso-Cenozoic sedimentary sequence thrust northeast and stacked over the Adriatic foreland during Late Miocene-Pleistocene. Extension on the Tyrrhenian margin is synchronous with thrust emplacements along the external Apenninic chain and is associated by crustal thinning, normal faulting, ductile deformation, volcanic activity and high heat flow. Both, the extensional and the compressional fronts migrated towards the Adriatic foreland during the Plio-Pleistocene. Crustal extension everywere disrupted structural architectures formed during the preceding compressional phase leading to the development of thinned, uplifted and extended crust in the Tuscany mainland. Several models have been proposed to explain the evolution of the NA that are acknowledged to be tectonically complex. We present results from a Receiver Functions (RFs) analysis of teleseismic events recorded at Arezzo seismic station (Tuscany). A broad-band station (ARZ) is installed on the north-east margin of the “Val di Chiana” extensional syntectonic basin. We selected and grouped in “bins” high signal/noise teleseismic events of four years of recording to compute a data-set of RFs. We applied a classical inversion scheme (a Neighbourhood Algorithm) and we carried out a three-dimensional modelling. As a criterion to identify and to distinguish the effects of azimuthal anisotropy from those of lateral heterogeneity, we included the harmonic angular analysis performed by stacking Radial (R) and Transverse (T) components with weights depending on the backazimuth. The results of these analysis provide a detailed three-dimensional image of the S-velocity lithosphere structure

Slip distribution inversion by trans-dimensional Monte Carlo sampling: application to the 2009 L’Aquila Earthquake (Central Italy)

Non-uniform slip distribution on a fault plane from geodetic data is usually estimated in two steps. First, the geometric fault parameters are inferred by non -linear inversion assuming a uniform slip on a rectangular fault. A second analysis, based on linear inversion techniques, infers the slip distribution on an arbitrary subdivision of the fault plane into patches. Two main concerns arise. First, the fault geometry determined under the assumption of a uniform slip i s not guaranteed to properly represent the fault geometry for a spatially variable slip distribution. Moreover, an arbitrary fault subdivision into patches u nrelated to the observed data could bias the model resolution, introducing spurious features. In recent years, the availability of large coverage data, such as DInSAR images, improved mapping the coseismic displacements. The large amount of geodetic da ta from the area surrounding earthquake faults allows for improving the slip models and refining the knowledge of earthquake dynamics. Less attention has been given to the development of new inversion algorithms that can resolve the main concerns above. In particular, the question is whether the data themselves ca n constrain the slip model complexity, i.e., the unknown number and distribution of the fault patches needed to fit the observations. The reversible jump Mar kov chain Monte Carlo (RJMCMC) algorithm has been recently introduced in the geosciences to solve a variety of non linear inverse problems. RJMCMC combines a classical Markov chain Monte Carlo method with the ability to shift between models with a different number of unknowns. A posterior probability distribution of the num ber of unknowns is obtained at the end of the Markov chain, so that the model resolution is determined by the observed data. In this study, we apply a RJMCMC method to the Mw 6.3 L’Aquila earthquake that occurred on April 6th 2009 in Central Italy. Three DInSAR images, mapping the c oseismic displacement, are inverted to constrain not only the slip distribution but also the number of unknowns (i.e., the number of fault patches) and the ge ometry of non-rectangular patches

Crustal and upper mantle response to lithospheric segmentation in the northern Apennines

Lithospheric tear faults are expected to develop in response to along-strike variations in the rates of slab rollback. However, the exact geometry of such structures and their crustal and upper mantle expressions are still debated. We present an analysis of seismic, structural and morphological features that possibly represent the expression of lithospheric segmentation in the northern Apennines. Geophysical observations show evidence for the existence of a discontinuity in the lithospheric structure beneath the northern Apennines, characterized by a change in the spatial distribution of intermediate-depth seismicity, along-strike variations in the pattern of crustal seismicity, and a bend in the Moho topography. The near-surface expression of this discontinuity is associated with an abrupt change in the morphology and exhumation history of the northern Apennines in the proximity of the Livorno-Sillaro Lineament. We interpret these features as evidence for incipient tearing of the lithospheric slab beneath the northern Apennines, marking the boundary between domains that underwent contrasting styles of lithospheric deformation, which are either associated with different rates of slab rollback or a transition from underplating to retreat. We suggest that similar types of structures may play a crucial role in the evolution of convergent plate boundaries, allowing segmentation of orogenic belts and facilitating the development of orogenic curvatures, Ultimately, further tearing along such structures could potentially lead to the occurrence of tear–related magmatism and the formation of slab windows

Evidenze della rapida variazione di profondità della Moho, in corrispondenza dell'area di Città di Castello (Appennino Settentrionale), dall'analisi di funzioni ricevitore

In questo studio è stata sfruttata l’opportunità di poter analizzare dati provenienti da una densa rete sismica locale temporanea costituita da 30 stazioni a tre componenti, installata nell’ambito di un progetto del Gruppo Nazionale per la Difesa dei Terremoti (GNDT) nel periodo compreso fra l’Ottobre 2000-Maggio 2001, in un’area che si estende per circa 2800 km2 a circa 43° N in Appennino Settentrionale (Piccinini et al., 2003), al fine di ottenere un dettagliato andamento della topografia della Moho, in una zona così complessa, attraverso un’analisi delle Funzioni Ricevitore (Langston, 1979), definendo la struttura di velocità delle onde di taglio (S) al di sotto di ciascuna delle 28 stazioni sismiche. Sono stati analizzati circa 400 eventi telesismici registrati da 28 stazioni con valori di magnitudo M>5 e distanza epicentrale Δ compresi fra 25°-100°. Per il calcolo delle RFs è stato utilizzato il metodo sviluppato da Di Bona (1998), tale metodo consente di ottenere una stima della varianza, permettendo l’utilizzo di forme d’onda generate da eventi di bassa magnitudo (aventi valori di varianza accettabili), con un conseguente ampliamento del data-set. Modellando ampiezze e tempi di arrivo delle fasi Ps in funzione dell’azimuth di provenienza (BAZ) e della relativa distanza epicentrale (Δ), si possono ricostruire le geometrie delle superfici di discontinuità al di sotto delle stazioni sismiche. La fase di modellazione è stata condotta attraverso l’applicazione dell’algoritmo di inversione “neighbourhood” di Sambridge (1999) mediante un approccio monodimensionale. Questo metodo consente di campionare in maniera estensiva lo spazio dei parametri (profondità delle varie interfacce e valori di velocità negli strati compresi fra le interfacce), concentrando la ricerca in quelle regioni dello spazio multiparametrico dove i modelli di velocità trovati hanno un miglior misfit rispetto al dato (la RF) reale. Tale fase di modellazione ha consentito di ricostruire i modelli di velocità delle onde S (Vs) al di sotto di ciascuna stazione. L’analisi comparata dei modelli di velocità delle onde S (Vs) così ottenuti, per ogni singola stazione, mette in luce la natura fortemente eterogenea della porzione più superficiale della crosta dell’area in studio. Nonostante la complessità delle RFs calcolate che si riflette sulla eterogeneità della porzione più superficiale dei profili di Vs ottenuti, è stata individuata con buona continuità l’andamento di una superficie di discontinuità sismica da noi interpretata come transizione crosta-mantello superiore o Moho

Moho-depth and subglacial sedimentary layer thickness in the Wilkes Basin from Receiver Function Analysis

Wilkes Basin lies to the east of the Transantarctic Mountains. The origin of this sub-glacial basin is still controversial. Flexural uplift of the Transantarctic Mountains has been suggested as the geophysical process which generated the basin (Stern & ien Brink, 1989). Other studies proposed a continental rift structure for this region (Ferraccioli et al., 2001). The two models differ mainly in the crustal structure predicted beneath the basin. In the former, crustal thickning is expected to be originated from the high rigidity of the East Antarctic Craton lithosphere. Otherwise, the rift structure hypothesis is consistent with a broad crustal thinning. During the WIBEM 2003 campaign, we deployed five broadband seismic stations across the basin. We selected high signal/noise teleseismic recording to compute a data-set of receiver functions. We applied a classical inversion scheme, the Neighbourhood Algorithm, to our data-set. Here, two different and complementary studies are presented. We constrain the Moho geometry beneath the Wilkes Basin from the analysis of low-frequency P-to-S conversion at the base of the crust. Also, we investigate the nature of the basin mapping the presence of subglacial sediments using the P-to-S conversion at the ice-bedrock interface

Crustal structure and Moho depth profile crossing the central Apennines (Italy) along the N42 degree parallel.

We present results from a teleseismic receiver-function study of the crustal structure in the central Apennines (Italy). Data from fifteen stations deployed in a linear transect running along the N42 degree parallel were used for the analysis. A total number of 364 receiver functions were analyzed. The crustal structure has been investigated using the neighborhood algorithm inversion scheme proposed by Sambridge [1999a], obtaining crustal thicknesses, bulk crustal VP/VS ratio and velocity-depth models. In each inversion, the degree of constraint of the different parameters has been appraised by the Bayesian inference algorithm by Sambridge [1999b]. The study region is characterized by crustal complexities and intense tectonic activity (recent volcanism, orogenesis, active extensional processes), and these complexities are reflected in the receiver functions. However, the relatively close spacing among the seismometers (about 20 km) helped us in the reconstruction of the crustal structure and Moho geometry along the transect. Crossing the Apennines from west to east, the Moho depth varies by more than 20 km, going from a relatively shallow depth (around 20 km) on the Tyrrhenian side, deepening down to about 45 km depth beneath the external front of the Apenninic orogen, and rising up again to about 30 km depth in correspondence of the Adriatic foreland. Despite the strong variability of the crustal thickness, the average crustal VS values show little variation along the transect, fluctuating around 3 km/s. The average VP values obtained from the VS and VP /VS are generally lower than 6 km/s

Crustal Structure Across Northern Victoria Land, Antarctica, From Receiver Function Analysis

Global crustal model, from gravity studies, imaged a thick crust (>40 km) under Eastern Antartic craton (EAC). This global trend ends abruptly west of the Transantarctic Mountains (TAM), which border EAC along its western margins. There, the crust raises up to about 20 km. While this model points out the difference between EAC and the Ross sea crustal structures, its intrinsic spatial resolution gives little help to solve some regional geophysical issues, like the TAM orogenesis and the formation and nature of the Wilkes Basin. In this study, teleseismic Receiver Functions (RFs) are used to image the S-velocity crustal structure in finer details. We computed RFs from teleseismic events recorded during three different austral summer compaigns: BackTAM, WIBEM and WISE. Broadband seismic stations were deployed along a transect which spans from the coast of Northern Victoria land (NVL) to the far interior of the EAC plateau. The transect, almost perpendicular to the regional TAM axis, came across four different geological/geophysical settings: the alloctonhous terranes of the NVL, the TAM sector, the Wilkes Basin and the EAC plateau. Each area shows peculiar crustal structures and we propose both finer local S-velocity models and a regional crustal model