A wide‐angle seismic profile across the western peninsulas of SW Ireland was performed. This region corresponds to the northernmost Variscan thrust and fold deformation. The dense set of 13 shots and 109 stations along the 120 km long profile provides a detailed velocity model of the crust. The seismic velocity model, obtained by forward and inverse modelling, defines a five‐layer crust. A sedimentary layer, 5–8 km thick, is underlain by an upper‐crustal layer of variable thickness, with a base generally at a depth of 10–12 km. Two mid‐crustal layers are defined, and a lower‐crustal layer below 22 km. The Moho lies at a depth of 30–32 km. A low‐velocity zone, which coincides with a well‐defined gravity low, is observed in the central part of the region and is modelled as a Caledonian granite which intruded upper‐crustal basement. The granite may have acted as a buffer to northward‐directed Variscan thrusting. The Dingle–Dungarvan Line (DDL) marks a major change in sedimentary and crustal velocity and structure. It lies immediately to the north of the velocity and gravity low, and shows thickness and velocity differences in many of the underlying crustal layers and even in the Moho. This suggests a deep, pre‐Variscan control of the structural development of this area. The model is compatible with thin‐skinned tectonics, which terminated at the DDL and which incorporated thrusts involving the sedimentary and upper‐crustal layers.
S U M M A R YThis paper presents an updated interpretation of seismic anisotropy within the uppermost mantle of southern Germany. The dense network of reversed and crossing refraction profiles in this area made it possible to observe almost 900 traveltimes of the P, phase that could be effectively used in a time-term analysis to determine horizontal velocity distribution immediately below the Moho. For 12 crossing profiles, amplitude ratios of the P, phase compared to the dominant crustal phase were utilized to resolve azimuthally dependent velocity gradients with depth. A P-wave anisotropy of 3-4 per cent in a horizontal plane immediately below the Moho at a depth of 30 km, increasing to 11 per cent at a depth of 40 km, was determined. For the axis of the highest velocity of about 8.03 kms-' at a depth of 30 km a direction of N31"E was obtained. The azimuthal dependence of the observed P, amplitude is explained by an azimuth-dependent sub-Moho velocity gradient decreasing from 0.06 s in the fast direction to 0 s-' in the slow direction of horizontal P-wave velocity. From the seismic results in this study a petrological model suggesting a change of modal composition and percentage of oriented olivine with depth was derived.Regional P-wave seismic models for the uppermost mantle layer beneath south-west Germany based on time-term analysis were published by Bamford (1973Bamford ( , 1976aBamford ( , 1976bBamford ( , 1977. These models all feature around 7 per cent anisotropy involving azimuthally dependent velocities, with the direction of fast velocity at about N22"E. Fuchs (1983) used these results, together with qualitative observations of the distribution of P , amplitudes with azimuth and the velocity4epth function from the seismic profiles SB-060 and 09-240 beneath south-west Germany, to continue the anisotropic model to greater depth. This resulted in the so-called anvil model. Fuchs (1983) further interpreted this seismic model in terms of petrological composition.Since the time-term analysis of Bamford, recent explosion seismic experiments have provided additional P, observations. In this paper, first these observations are included in a new time-term analysis of the P, data from south-west Germany using the Bamford (1977) formulation. Further, the data are analysed for lateral inhomogeneity based on the extended time-term method developed by Hearn (1984). In order to continue the time-term model to greater depth, the azimuthally dependent velocity gradient in the uppermost mantle layer is determined. As amplitudes are more sensitive to velocity gradients than traveltimes, this is accomplished by quantitatively analysing P, amplitudes from 12 azimuthally distributed profiles in a 250 x 300 km area in the south-west German triangle. The band method, which facilitates the comparison between observed amplitude-ratio curves and a range of theoretical curves, is used for quantitatively analysing P, amplitudes. The derived sub-Moho velocity gradients are then taken into account by extending the time-term method f...
In this paper we present newly acquired high‐quality wide‐angle seismic data of the GRANU95 project and first models of the crustal structure along two profiles (95‐A and 95‐B) beneath the Saxonian Granulites, a major ellipse‐shaped exposure of lower‐crustal material within the mid‐European Variscan belt in southeastern Germany (Saxony). The crust is subdivided into four layers. The crystalline basement with velocities higher than 6.0 km s−1 is generally reached at shallow depths, with three major sedimentary structures as prominent exceptions where velocities considerably lower than 6.0 km s−1 (as low as 5.1 km s −1 ) reach as deep as 4 km. The highest upper‐crustal velocities (up to 6.5 km s−1 ) are not seen below the exposed granulites themselves, but at shallow depths (4 km) SW of the exposure. These shallow high velocities correlate well in depth with highly reflective zones observed on three seismic‐reflection lines of DEKORP (85‐4N, 95‐01, 95‐02) at their intersection with the seismic‐refraction line 95‐B, where they appear as a set of NW–SE‐trending dome‐shaped reflections. On profile 95‐A this high‐velocity upper‐crustal layer (6.3 km s−1 ) dips from 5 to 9 km beneath the SE margin of the exposed granulites. These results suggest that the granulite dome and its western continuation are widely underlain by a NE‐trending antiformal structure (probably a sheet of metabasic rocks) where the exposed felsic granulites form just a local cap on top. Below the upper‐crustal high velocities, a layer with decreased velocity (6.2–6.25 km s−1 ) extends down to an average depth of 15 km along the Variscan strike (95‐B) and to 11–16 km depth (slightly dipping towards the SE) perpendicular to the terrane boundaries (95‐A). At mid‐crustal levels a weak reflection from a layer with a velocity of 6.4–6.6 km s−1 may indicate the classical Conrad discontinuity. At the depth range 22–24 km the velocity jumps to an average value of 7.0 km s−1, thus defining a prominent high‐velocity layer in the lower crust, which may be viewed as the well‐known laminated lower crust typical of Variscan structures, but with higher average velocity than usually detected. The crust–mantle boundary at about 30–31 km is typical for western Europe and confirms the extensional signature of the West European crust. Below the Moho, poorly constrained upper‐mantle velocities of about 7.9–8.0 km s−1 are derived. The high velocities observed in the lower‐crustal layer would not exclude the possibility of mantle‐derived intrusions, but the lack of any sign of an updoming Moho favours the interpretation of a more passively driven extension.
The Saxonian Granulites represent a major exposure of high‐pressure rocks within the mid‐European Variscan belt. The granulites emerge in an extensional dome structure beneath a low‐grade Paleozoic cover. The boundary between the granulites and their cover is a crustal‐scale shear zone with transport top to the SE, juxtaposing high‐pressure (HP) granulites against greenschist‐grade rocks. Seismic reflection and refraction profiling reveal that the granulite dome and its western continuation up to the SW margin of the Bohemian Massif are underlain by a reflective layer up to l s two‐way time (TWT) thickness (∼3.5 km), with P wave velocities Vp generally above 6.0 and up to 7.0 km/s (probably a sheet of metabasic rocks). This layer exhibits a NE trending antiformal structure, in line with the granulite antiform, with an apex at ∼1.2 s TWT. The outcrop of felsic granulite forms a local cap on the NE part of this high‐velocity layer. A magnetotelluric survey has revealed high resistivity in the upper crust and a zone of high conductivity under the high‐velocity layer, in the middle and lower crust, terminating ∼10 km to the south of the granulite outcrop. Similar high‐grade rocks occur in the Erzgebirge antiform SE of the Saxonian Granulites, but their exhumation was later followed by grossly westdirected stacking with medium‐pressure and low‐pressure rocks, followed by backthrusting toward the SE and late open folds. Isotopic data both from the Saxonian Granulites and the Erzgebirge indicate HP metamorphism ∼360–370 Ma, followed by a granulite stage at 350–340 Ma. This is entirely incompatible with the record of low‐grade sediments overlying the crystalline rocks, which document subsidence and marine sedimentation lasting until ∼330 Ma. This paradox is explained by tectonic underplating, differential thinning of the hanging wall lithosphere, and extensional unroofing of the high‐grade rocks derived from one of the subduction zones adjacent towards the NW and SE. Tectonic underplating and exhumation of the granulites must have occurred under the floor of a marine basin.
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