Mapping Crustal and Uppermost Mantle Deformation in the Westernmost Mediterranean by Radial Anisotropy

The dynamics of crustal extension and the crust-mantle interaction in the Central-Western Mediterranean and Italian peninsula (i.e. Liguro-Proven&#231;al and Tyrrhenian Basin), and plate convergence (i.e. Alpine and Apennines chains) are key for the understating of the current geodynamics setting and its evolution in the region. However, open questions such as the style, depth and extent of the deformation still exist despite the wealth of seismological and non-seismological data acquired in the past decades. In this context, it is necessary to provide improved subsurface models in terms of seismic velocities, from which better constraints on the geodynamic models can be derived.We use seismic ambient noise for retrieving phase velocities of Rayleigh and Love waves in the 4-35 s period range, using private (LiSard network in Sardinia island) and publicly available continuous recordings from more than 500 seismic stations. Considering the excellent coverage and the short period of recovered phase velocities, our study aims to provide an unprecedented, high-resolution image of the shallow crust and uppermost mantle.We employ a Bayesian trans-dimensional, Monte Carlo Markov chain inversion approach that requires no a-priori model nor a fixed parametrization. In addition to the (isotropic) shear wave velocity structure, we also recover the values of radial anisotropy (&#958;=(VSH/VSV)2) as a function of depth, thanks to the joint inversion of both Rayleigh and Love phase velocities.Focusing on radial anisotropy, this appears clearly uncoupled with respect to the shear wave velocity structure. The largest negative anisotropy anomalies (VSH<VSV, &#958;<0.9) are found in the Liguro-Proven&#231;al and western Tyrrhenian basins in the top 10-15 km, suggesting a common structural imprint inherited during the extensional phases of such basins. Conversely, the eastern Tyrrhenian basin shows positive radial anisotropy (VSH>VSV, &#958;>1.1) within the same depth range. This evidence, combined with the observed shear wave velocities typical of the uppermost mantle, corroborates the presence of a sub-horizontal asthenospheric flow driving the current extension and oceanization of the eastern Tyrrhenian basins.Moving towards the Italian mainland, a strong anomaly of negative anisotropy appears in the eastern portion of the Apennines chain. We relate such an anisotropic signal with the ongoing compressive regime affecting the area. Here, the high-angle thrust faults and folds, that accommodates the horizontal shortening, obliterate the horizontal layering of the sedimentary deposits, currently constituting the flanks of the fold system.Our results suggest that the combination of radial anisotropy and shear wave velocities can unravel key characteristics of the crust and uppermost mantle, such as inherited or currently active structures resulting from past or ongoing geodynamic processes.

Download Full-text

Radial anisotropy in Europe from surface waves ambient noise tomography and transdimensional hierarchical inversion

10.5194/egusphere-egu2020-20109 ◽

2020 ◽

Author(s):

Chloé Alder ◽

Eric Debayle ◽

Thomas Bodin ◽

Anne Paul ◽

Laurent Stehly ◽

...

Keyword(s):

Group Velocity ◽

Shear Wave ◽

Rayleigh Waves ◽

3D Model ◽

Joint Inversion ◽

Shear Velocity ◽

Dispersion Curves ◽

Uppermost Mantle ◽

Radial Anisotropy ◽

Anisotropy Model

We present a 3D probabilistic model of shear wave velocity and radial anisotropy of the European crust and uppermost mantle mainly focusing on the Alps and the Apennines.The model is built using continuous seismic noise recorded between 2010 and 2018 at 1521 broadband stations, including the AlpArray network (Hete&#769;nyi et al., 2018).We use a large dataset of more than 730&#160;000 couples of stations representing as many virtual source-receiver pairs. For each path, we calculate the cross-correlation of continuous vertical- and transverse-components of the noise records in order to get the Green&#8217;s function. From the Green&#8217;s function, we then obtain the group velocity dispersion curves of Love and Rayleigh waves in the period range 5 to 149 s.Our 3D model is built in two steps. First, the dispersion data are used in a linearized least square inversion providing 2D maps of group velocity in Europe at each period. These maps are obtained using the same coverage for Love and Rayleigh waves. Dispersion curves for both Love and Rayleigh waves are then extracted from the maps, at each geographical point. In a second step, these curves are jointly inverted to depth for shear velocity and radial anisotropy. The inversion in done within a Bayesian Monte-Carlo framework integrating some a priori information coming either from PREM (Dziewonski and Anderson 1961) or the recent 3D shear wave model of Lu et al. 2018 performed for the same region.Therefore, this joint inversion of Rayleigh and Love data allows us to derive a new 3D model of shear velocity and radial anisotropy of the European crust and uppermost mantle. The isotropic part of our model is consistent with the shear velocity model of Lu et al. 2018. The 3D radial anisotropy model of the region adds new constraints on the deformation of the lithosphere in Europe. Here we present and discuss this new radial anisotropy model, with particular emphasis on the Apennines.

Download Full-text

A 3‐D Shear Velocity Model of the Crust and Uppermost Mantle Beneath Alaska Including Apparent Radial Anisotropy

Journal of Geophysical Research Solid Earth ◽

10.1029/2019jb018122 ◽

2019 ◽

Vol 124 (10) ◽

pp. 10468-10497 ◽

Cited By ~ 9

Author(s):

Lili Feng ◽

Michael H. Ritzwoller

Keyword(s):

Shear Velocity ◽

Velocity Model ◽

Uppermost Mantle ◽

Radial Anisotropy

Download Full-text

Uppermost Mantle Structure in Southern Eurasia from PN Tomography and Sn Attenuation.

10.21236/ada310351 ◽

1996 ◽

Author(s):

Thomas M. Hearn ◽

James F. Ni

Keyword(s):

Mantle Structure ◽

Uppermost Mantle

Download Full-text

Velocity structure and radial anisotropy of the lithosphere in southern Madagascar from surface wave dispersion

Geophysical Journal International ◽

10.1093/gji/ggaa550 ◽

2020 ◽

Vol 224 (3) ◽

pp. 1930-1944 ◽

Cited By ~ 1

Author(s):

E J Rindraharisaona ◽

F Tilmann ◽

X Yuan ◽

J Dreiling ◽

J Giese ◽

...

Keyword(s):

Receiver Functions ◽

Dispersion Curves ◽

Eastern Coast ◽

Small Scale ◽

Seismic Structure ◽

Lithospheric Thickness ◽

Velocity Models ◽

Phase Velocities ◽

Maximum Value ◽

Radial Anisotropy

SUMMARY We investigate the upper mantle seismic structure beneath southern Madagascar and infer the imprint of geodynamic events since Madagascar’s break-up from Africa and India and earlier rifting episodes. Rayleigh and Love wave phase velocities along a profile across southern Madagascar were determined by application of the two-station method to teleseismic earthquake data. For shorter periods (<20 s), these data were supplemented by previously published dispersion curves determined from ambient noise correlation. First, tomographic models of the phase velocities were determined. In a second step, 1-D models of SV and SH wave velocities were inverted based on the dispersion curves extracted from the tomographic models. As the lithospheric mantle is represented by high velocities we identify the lithosphere–asthenosphere boundary by the strongest negative velocity gradient. Finally, the radial anisotropy (RA) is derived from the difference between the SV and SH velocity models. An additional constraint on the lithospheric thickness is provided by the presence of a negative conversion seen in S receiver functions, which results in comparable estimates under most of Madagascar. We infer a lithospheric thickness of 110−150 km beneath southern Madagascar, significantly thinner than beneath the mobile belts in East Africa (150−200 km), where the crust is of comparable age and which were located close to Madagascar in Gondwanaland. The lithospheric thickness is correlated with the geological domains. The thinnest lithosphere (∼110 km) is found beneath the Morondava basin. The pre-breakup Karoo failed rifting, the rifting and breakup of Gondwanaland have likely thinned the lithosphere there. The thickness of the lithosphere in the Proterozoic terranes (Androyen and Anosyen domains) ranges from 125 to 140 km, which is still ∼30 km thinner than in the Mozambique belt in Tanzania. The lithosphere is the thickest beneath Ikalamavony domain (Proterozoic) and the west part of the Antananarivo domain (Archean) with a thickness of ∼150 km. Below the eastern part of Archean domain the lithosphere thickness reduces to ∼130 km. The lithosphere below the entire profile is characterized by positive RA. The strongest RA is observed in the uppermost mantle beneath the Morondava basin (maximum value of ∼9 per cent), which is understandable from the strong stretching that the basin was exposed to during the Karoo and subsequent rifting episode. Anisotropy is still significantly positive below the Proterozoic (maximum value of ∼5 per cent) and Archean (maximum value of ∼6 per cent) domains, which may result from lithospheric extension during the Mesozoic and/or thereafter. In the asthenosphere, a positive RA is observed beneath the eastern part Morondava sedimentary basin and the Proterozoic domain, indicating a horizontal asthenospheric flow pattern. Negative RA is found beneath the Archean in the east, suggesting a small-scale asthenospheric upwelling, consistent with previous studies. Alternatively, the relatively high shear wave velocity in the asthenosphere in this region indicate that the negative RA could be associated to the Réunion mantle plume, at least beneath the volcanic formation, along the eastern coast.

Download Full-text