Two‐dimensional anisotropic tomography of lithosphere beneath France using regional arrival times

Judenherc, S.; Granet, M.; Boumbar, N.

doi:10.1029/1999jb900079

Cited by 18 publications

(23 citation statements)

References 38 publications

(17 reference statements)

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“…The fast shear wave polarization plane is usually oriented parallel to the belt, and the delay between the fast and slow S-wave arrivals is larger than 1 s, even beyond the Mesozoic Pyrenees belt. Pn anisotropy measurements (Judenherc et al 1999) are in good agreement with S-wave splitting measurements; the fast propagation direction of Pn is also parallel to the Hercynian/Pyrenean tectonic fabric, suggesting that the entire lithosphere beneath the probed area has a coherent 'wrench fault type' fabric. The analysis of the seismic anisotropy data for the active orogen of Taiwan, the Neoproterozoic Ribeira belt and the Hercynian/Alpine Pyrenean belt leads to similar conclusions.…”

Section: T R a N S P R E S S I O N A L O R O G E N I C D O M A I N Ssupporting

confidence: 69%

Wrench faults down to the asthenosphere: geological and geophysical evidence and thermomechanical effects

Vauchez

Tommasi

2003

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We review a set of geological and geophysical observations that strongly support a coherent deformation of the entire lithosphere in major intracontinental wrench faults. Tectonic studies of wrench faults eroded down to the middle to lower crust show that, even in cases in which the lower to middle crust is partially melted, strain remains localized (although less efficiently) in transcurrent shear zones. Seismic profiling as well as seismic tomography and magnetotelluric soundings provide strong argument in favour of major wrench faults crosscutting the Moho and deforming the upper mantle. Pn velocity anisotropy, shear-wave splitting and electric conductivity anisotropy measurements over major wrench faults and in transpressional domains support that a wrench fault fabric exists over most or even the entire lithosphere thickness. These seismic and electrical anisotropies are generated by a crystallographic preferred orientation of olivine and pyroxenes developed in the mantle during the fault activity, which is frozen in the lithospheric mantle when the deformation stops. The preservation of such a 'wrench fault type' fabric within the upper mantle may have major effects on the subsequent tectonothermal behaviour of continents, because olivine is mechanically and thermally anisotropic. Indeed, the association of numerical models and laboratory data on textured mantle rocks strongly suggests that the orogenic continental lithosphere is an anisotropic medium with regards to its stiffness and to heat diffusion. This anisotropy may explain the frequent reactivation, at the continents scale, of ancient lithospheric-scale wrench faults and transpressional belts during subsequent tectonic events.

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Section: T R a N S P R E S S I O N A L O R O G E N I C D O M A I N Ssupporting

confidence: 69%

Wrench faults down to the asthenosphere: geological and geophysical evidence and thermomechanical effects

Vauchez

Tommasi

2003

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“…Another difference between the seismic anisotropy observed by this work and on the Iberian Peninsula is in the degree of anisotropy. On land, in both the Pyrenees (Judenherc et al 1999) and in the Ossa Morena Zone (Díaz et al 1993), a 3–5 per cent anisotropy was observed, whereas in this work, higher values of 5–8 per cent were seen.…”

Section: Discussioncontrasting

confidence: 69%

“…There, the fast direction determined in the mantle from the ILIHA Deep Sounding Seismic experiment, from a limited sampling of azimuths, was 040° in the Ossa Morena Zone, southwest Iberia (Díaz et al 1993). From regional P n tomography, the fast direction is roughly E–W in the Pyrenees, corresponding to the strike and possible ‘shear’ of that mountain chain (Judenherc et al 1999). Smith & Ekström (1999), using earthquake P n arrivals, found that the fast direction at a station in the southeast of the peninsula was northeast–southwest and on the east coast, it was more eastnortheast–westsouthwest.…”

Section: Discussionmentioning

confidence: 99%

“… Compressional‐wave upper‐mantle anisotropy of the Iberia Peninsula compared with our result. In the Pyrenees (NE Spain), the fast velocity direction is roughly parallel to the strike of the mountain chain (Judenherc et al 1999); and in the Ossa–Morena Zone, between the dotted lines, the fast velocity direction is parallel to pre‐existing geological structures (Díaz et al 1993). The pale grey arrows are the directions and degree of anisotropy found by Smith & Ekström (1999) (see text).…”

Section: Discussionmentioning

confidence: 99%

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Azimuthal seismic anisotropy in a zone of exhumed continental mantle, West Iberia margin

Cole

Minshull

Whitmarsh

2002

Geophysical Journal International

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Summary P‐wave azimuthal seismic anisotropy of the uppermost mantle has been shown to indicate directions of extension, both recent and fossil, in oceanic and continental settings. We have determined the P‐wave anisotropy of a zone of exhumed continental mantle beneath the southern Iberia Abyssal Plain, as a possible indication of the initial direction of separation of Iberia from North America in a region where there are no magnetic lineations or fracture zone lineations adjacent to the margin with which to constrain this direction. Over 10 000 airgun shots were recorded by 11 ocean‐bottom seismometers/hydrophones from which 24 900 arrivals that had traversed unaltered/weakly serpentinized upper mantle were picked. Complete 360° azimuthal coverage was attained between the shots and receivers for ranges up to 55 km. P‐wave raypaths through the zone of exhumed continental mantle were ray‐traced through a velocity model that incorporated the 3‐D relief of the seabed and acoustic basement. Time residuals with respect to this model were fitted using an azimuthally dependent function. Small anisotropy (≲7 per cent) with a fast direction of 143° was determined to exist between 3.1 and 6.7 km below the top of the basement. Although the fast direction suggests that the post‐breakup mantle stretching between Iberia and North America was northwest–southeast, this hypothesis is hard to reconcile with other observations. Instead, we conclude that motion along concave‐downward faults during the exhumation of the upper mantle could have been sufficient to change an initially roughly east–west margin‐normal fast direction to the 143° azimuth observed today. The degree of serpentinization was estimated to decrease from ∼20 to 0 per cent between 3 and 7 km depth into the basement in the zone of exhumed continental mantle. The degree of upper‐mantle anisotropy is similar to that observed in other oceanic and continental settings where similar degrees of serpentinization are inferred from mean P‐wave velocities.

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“…The negative delays (nearly 0.5 s) in northern Africa could be explained in the same way. On the other hand, to the north and northwest the negative delays (from 0.2 to 0.5 s) could reflect the moderate crustal thinning toward the Iberian massif and/or would be an effect of the old and stable Hercynian crust (Judenherc et al, 1999). One of the longest negative delays (>0.5 s) was obtained at the station on the south Portuguese coast (near Faro).…”

Section: Station Delaysmentioning

confidence: 86%

Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula

Serrano

Hearn

Morales

et al. 2005

Physics of the Earth and Planetary Interiors

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Travel times of 11,612 Pn arrivals collected from 7675 earthquakes are inverted to image the uppermost mantle velocity and anisotropy structure beneath the southern half of the Iberian Peninsula and surrounding regions. Pn phases are routinely identified and picked for epicentral distances from 200 to 1200 km. The method used in this study allows simultaneous imaging of variations of Pn velocity and anisotropy. The results show an average uppermost mantle velocity beneath the study area of 8.0 km/s. The peninsular area covered by the Iberian massif is characterized by high Pn velocity, as expected in tectonically stable regions, indicating areas of the Hercynian belt that have not recently been reactivated. The margins of the Iberian Peninsula have undergone a great number of recent tectonic events and are characterized by a pronouncedly low Pn velocity, as is common in areas greatly affected by recent tectonic and magmatic activity. Our model indicates that the Betic crustal root might be underlined by a negative anomaly beneath the southeastern Iberian Peninsula. In the Atlantic Ocean, we find a sharp variation in the uppermost mantle velocities that coincides with the structural complexity of the European and African plate boundary in the Gulf of Cadiz. Our results show a very pronounced low-velocity anomaly offshore from Cape San Vicente whereas high velocities are distributed along the coast in the Gulf of Cadiz. In the Alboran Sea and northern Morocco, the direction of the fastest Pn velocity found is almost parallel to the Africa-Eurasia plate convergence vector (northwest-southeast) whereas to the north, this direction is almost parallel to the main trend of the Betic Cordillera, i.e. east-west in its central part and north-south in the curvature of the Arc of Gibraltar. This suggests that a significant portion of the uppermost mantle has been involved in the orogenic deformation that produced the arcuate structure of the Betic Cordillera. However, we assume that the Neogene extension had no major influence on a lithospheric scale in the Alboran Sea. Our results also show a quite complex pattern of anisotropy in the southwest Iberian lithospheric mantle since the relationship between the direction of fastest Pn velocity and major Hercynian tectonic trends cannot be directly established.

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Two‐dimensional anisotropic tomography of lithosphere beneath France using regional arrival times

Cited by 18 publications

References 38 publications

Wrench faults down to the asthenosphere: geological and geophysical evidence and thermomechanical effects

Wrench faults down to the asthenosphere: geological and geophysical evidence and thermomechanical effects

Azimuthal seismic anisotropy in a zone of exhumed continental mantle, West Iberia margin

Seismic anisotropy and velocity structure beneath the southern half of the Iberian Peninsula

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