Temporary Array Data for Studying Seismic Anisotropy of Variscan Massifs – The Armorican Massif,...

TEMPORARY ARRAY DATA FOR STUDYING SEISMIC ANISOTROPY OFVARISCAN MASSIFS - THE ARMORICAN MASSIF, FRENCH MASSIF

CENTRAL AND BOHEMIAN MASSIF

JAROSLAVA PLOMEROVA1, MICHEL GRANET2, SEBASTEN JUDENHERC2, ULRICH ACHAUER2,VLADISLAV BABUSKA1, PETRJEDLICKA1, DANIEL KouBA1, LUDEKVECSEY1

1 Geophysical Institute, Czech Acad. Sci., Prague, Czech Republic

2EOST - Institut de Physique du Globe Strasbourg, France

S u m m a r y : We present the first results of a comparison of deep lithosphere structure of threeVariscan massifs - the Armorican Massif, French Massif Central and Bohemian Massif, as revealedby recent tomographic studies of seismic anisotropy. The data originate from several fieldmeasurements made in temporary arrays of stations equipped with both short-period and broad-band seismometers with digital recording. The study is based on teleseismic body waves and a jointinversion of anisotropic data (P-residual spheres, the fast shear-wave polarizations and split times)and demonstrates that the three Variscan massifs appear to consist of at least two parts withdifferent orientation of large-scale fabric derived from seismic anisotropy. The boundaries ofanisotropic lithospheric domains are related to prominent tectonic features recognised on thesurface as sutures, shear zones or transfer fault zones, as well as grabens, thus indicating that someof them extend deep through the entire lithosphere.

K e y w o r d s : Mantle lithosphere, seismic anisotropy, Variscan massifs

1. INTRODUCTION

During the Paleozoic period between 500 - 250 Ma, the Variscan erogenic belt of central andwestern Europe was created by the accretion of a complex of lithospheric blocks and magmatic arcsand by a closure of oceanic domains between Laurasia (Laurentia-Baltica) and Africa (Gondwana).The general northwest direction of accretion is typical of the northern part of the Variscan belt, butirregular shapes of colliding blocks resulted in a very complicated structure of convergence,lithospheric subductions and crustal shortening, which were followed by extensional processes andrifting (Franke, 1989; Matte, 1991).

Among many questions concerning the Paleozoic tectonics which remain unsolved, is therelation between near-surface structures and the structure of the deep lithosphere. The subcrustallithosphere obviously plays a dominant role during collisions of lithospheric blocks, and newlycreated boundaries or sutures are potential zones of weakness for the occurrence of intraplateearthquakes, as well as for later periods of volcanic activity. Such tectonic environments areobserved in several massifs of the European Variscides.

1 Address: Bocni II, 141 31 Praha4, Czech Republic (e-mail: [email protected])

2 Address: 5, rue R. Descartes, 67084 France (e-mail: [email protected])

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Teleseismic studies of the anisotropic structure of the deep lithosphere beneath Europe revealeda consistent large-scale fabric within large geo-tectonic units, extending several hundreds of km,which changes at major tectonic boundaries (Babuska et al., 1984; Babuska and Plomerova, 1992;Plomerova et al., 2000). In general, the north-western Europe is characterised by structures withhigh velocities dipping predominantly to the NW, whereas in the southern part of central Europe thehigh velocities plunge prevailingly to the S.

This paper at presents new results on seismic anisotropy of the deep structure of three Variscanmassifs (Fig. 1): the Armorican Massif (AM), French Massif Central (MC) and Bohemian Massif(BM). The results were obtained partly by reprocessing the data previously acquired for isotropictomography (MC), as well as by evaluating data from recent (AM) or ongoing (BM) passivetomographic experiments focused on seismic anisotropy. Detection of shear-wave splitting, namelyin the SKS phase, is the most diagnostic sign of seismic anisotropy beneath a station. Variations inorientation of the fast shear wave polarization and splitting time St. evaluated in 3D in dependenceon the propagation direction, allow us to infer anisotropic tensors characterising the generalorientation of anisotropy in 3D (Sileny and Plomerova, 1996). Significant inferences of theanisotropic structure of the subcrustal lithosphere can be retrieved from P-residual spheres, in whichthe anisotropic part, i.e. incidence angle-azimuth dependent terms of the relative residuals is shown.As we subtract the weighted directional mean of the residuals from the individual values at eachstation, the spheres of different stations can be directly compared (e.g., Babuska and Plomerova,1992). A joint analysis and inversion of the P-residual spheres and the shear-wave splittingparameters result in constructing 3D self-consistent anisotropic models of the individual domains ofthe subcrustal lithosphere with generally orientated symmetry axes (Plomerova et al., 1996; 2000).

Fig. 1. Location of several European Variscan Massifs. In this paper we study anisotropicdomains of the deep lithosphere beneath the Armorican Massif (AM), French Massif Central (MC)and Bohemian Massif (BM).

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The study is based on field measurements in temporary arrays consisting of both short-period andbroad-band seismometers with digital recording. In previous regional studies, based on permanentobservatories in central Europe (Babuska and Plomerova, 1992), or those focused on isotropictomography of the MC (Granet et al., 1995a,b), distinct lateral variations of the lithosphere-asthenosphere boundary were mapped, especially lithosphere thinning related to grabens: theLimagne Graben (LG) in the MC and the Ohre Graben (OG) in the BM. No such distinct upwellingof the lithosphere-asthenosphere transition is observed in the AM (Judenherc et al., 2000).However, for all the three Variscan massifs, the results reported in this paper indicate that themassifs consist of at least two parts with different orientations of anisotropic structures within thesubcrustal lithosphere.

2. ARMORICAN MASSIF

Two parallel lines of seismometers perpendicular to the major shear zones of the AMwere deployed for 6 months in 1997 to record teleseismic data for a tomographic image ofthe lithosphere (Judenherc et al., 1999a,b), Although the azimuthal coverage of incomingrays is rather limited, the variation of velocities of longitudinal waves with the direction ofpropagation through the lithosphere differs at stations in the northern part from that in thesouthern part of the array, as can be seen in the P-residual spheres (Fig. 2). The patterns ofthe spheres are reversed, i.e. those residuals, which are negative in the north, are positivein the south, and vice versa. While the maximum amplitude of negative residuals,reflecting relatively high velocities orientated to the south-west, are detected in thenorthern part, the high velocities are orientated to the north-east at the southern stations.Such a pattern of the residual spheres, showing the azimuth-incidence angle dependentterms of relative P residuals, invoke the presence of inclined anisotropic structures in thesubcrustal lithosphere with high velocities orientated approximately convergently to theboundary separating two different lithospheric domains, one in the northern and the otherin the southern part of the AM. The boundary between both orientations of P velocities,i.e. between the two different orientations of the large-scale fabrics, is close to theboundary between the high- and low-velocity heterogeneities in the deep lithospheredetected by isotropic tomography (Judenherc et al., 1998; 2000). However, such a deepboundary, which separates two tectonic units with different orientations of anisotropicstructures, has no clear surface manifestation related to the North Armorican Shear Zone(NASZ) and South Armorican Shear Zone (SASZ). Similarly, lateral variations of theshear-wave splitting parameters (time delay dt and fast S azimuths, Fig. 2) do not seem tobe simply related to the SASZ or NASZ (Judenherc et al., 2000). The boundary betweenthe two patterns of P-residual spheres reflecting two different orientations of anisotropicstructures might be associated with the Quessoy-Nort/Erdre transfer fault zone (QFZ,Fig. 2) which originated in the Cenozoic Pyrenean collision (Nicolas et al., 1990).

Our results of analysing the body-wave anisotropy are not compatible with theanisotropic models of olivine aggregate with horizontal or vertical symmetry axes(Babuska et al., 1993). A joint inversion of the P-residual spheres and shear-wave splittingparameters evaluated in 3D (Sileny and Plomerova, 1996), retrieves the seismicanisotropy within the lithosphere domain in the northern part of the AM and approximatesit by a preliminary anisotropic model with the coefficient of anisotropy kp = 5%. The'slow velocity' symmetry axis b of the hexagonal model is orientated N50°E, dipping at

Studia geoph. et geod. 44 (2000) 197

Temporary Array Data for Studying Seismic Anisotropy of Variscan Massifs...

25° from the horizontal, i.e., the high velocity (a,c) plane of the model strikes 140° anddips 65° from the horizontal to the SW. On the other hand, the inversion of the meanP-residual sphere and the SKS splitting at the southern part of the array retrieved the highvelocity (a,c) plane dipping at about 64° to the NE (Judenherc et al., 2000).

The preliminary results on the deep lithosphere structure of the AM, obtained from thejoint inversion of body-wave data recorded during the first teleseismic field experiment,show that the deep lithosphere of the AM might be formed by two terrains withconvergently dipping anisotropic structures. A boundary of these anisotropic domains issituated between the NASZ and SASZ and seems to coincide with the QFZ (Fig. 2).

Fig. 2. Body-wave anisotropy in the Armorican Massif: azimuths of the fast SKS wavesevaluated in 3D (red arrows) and the smoothed P-residual spheres averaged over the stations of thenorthern (green circles) and southern (orange circles) parts of the array. Directional means at thestations are subtracted from relative residuals. Blue diamonds represent early arrivals (high-velocitydirections within the lithosphere) and red diamonds the late arrivals. Only the residuals in absolutevalue larger than 0.1s are plotted and the size of the signs is proportional to the residual value.Maximum incidence angle is 50° at the M-discontinuity at a reference depth of 33 km,corresponding to an epicentral distance of about 20°, is marked by the perimeter of the residualspheres. The North (NASZ) and South (SASZ) Armorican Shear Zones as well as Quessoy-Nort/Erdre (QFZ) transfer fault zone are marked schematically.

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Fig. 3. Azimuthal anisotropy derived from SKS waves (green arrows. Granet et al., 1998) and thesmoothed P-residual spheres in the Massif Central. Letters A-D mark groups of stations withcorresponding patterns of the spheres shown in the bottom of the figure. Blue triangles representearly arrivals (high-velocity directions within the lithosphere) and red circles the late arrivals. Blackplus signs mark residuals within <-0.1 s, 0.1 s> interval. The perimeter marks the incidence angleof 60°.


3. FRENCH MASSIF CENTRAL

The deep structure of the MC is dominated by a thinning of the crust to about 24 kmbeneath the Limagne Graben (LG). The crustal thinning is accompanied by a shallowingof the lithosphere-asthenosphere transition to depths of about 70 - 80 km (Babuska andPlomerova, 1992), or even less - to about 50 - 60 km, beneath the main volcanic fields(Sobolev et al, 1997). P-velocity tomography, geodynamic modelling and geochemicalcharacteristics of the Tertiary-Quaternary volcanic activity provide evidence of the ascentof a thermally and chemically anomalous mantle plume beneath the Massif Central duringthe Tertiary (Granet et al., 1995a,b).

A dense network of portable stations was installed among the permanent observatoriesof the MC for a period of 8 months in 1991 - 92 to record teleseismic events for the 3DP-velocity tomographic image of an area of 425 x 300 km (Granet et al., 1995a,b). Wereprocessed the available dataset and focused on the seismic mapping of anisotropicdomains within the subcrustal lithosphere beneath the MC. The existence of seismicanisotropy within the lower lithosphere of this region was indicated by the directionaldependence of the P-residual spheres constructed from long-term observations ofpermanent observatories (Babuska et al., 1984) and later proved by detecting shear-wavesplitting (Granet et al., 1998).

Both the shear-wave splitting parameters (fast polarization azimuth and splitting delaytime dt) and the P-residual spheres, constructed from the dense array of portable stations,show lateral changes of anisotropy within the subcrustal lithosphere of the MC (Fig. 3).Fast split shear waves orientated NNE-SSW were detected in its western part, while in itseastern part the fast shear waves were orientated NW-SE. Similarly to shear-waveobservations, a distinct change in the orientation of anisotropy can be traced in theP-residual spheres. Three different types of the residual patterns can be recognised at fourgroups of stations: groups A (300 observations) and B (390 observations), in the westernpart of the MC, with fast P waves arriving from the W-NW, and group D(720 observations), in its eastern part, with high P velocities from the E or NE. Bothpatterns with different orientations of anisotropy are separated by a relatively narrowzone, marked by stations belonging to group C (750 observations), in which no clear-cutpattern as to the directional dependence of the P residuals is observed. Also small valuesof dt were observed in this part of the MC. The narrow zone strikes parallel to the SillonHouiller (SH) transfer fault zone across the array from the LG to the eastern part of theCantal volcano (Fig. 3). This narrow zone may be associated with a suture zone separatingtwo lithospheric domains with anisotropic structures dipping divergently.

4. BOHEMIAN MASSIF

In co-operation between the geophysical institutions in Prague and Strasbourg, anarray of portable digital three-component stations (Table 1 and Fig. 4) was installed in theBM in August 1998. The field measurement ran for 8 months. The array consisted of sixbroad-band (BB) and two short-period (SP) stations. Three of the BB stations (BM1-3)still continue to record data with a better azimuthal coverage. The data base of the recordsis complemented by waveforms provided by permanent seismological observatorieswithin the BM.

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Table 1. List of portable seismological stations deployed in the BM from August 98 to April 99(three-component broad-band - BB and short-period - SP seismometers with continuous digitalrecordings)

Station

BM1 Zamek Aleje (BB)BM2 Gravi obs. Pribram (BB)BM3 Novohr. Hory (BB)BM4 Sipin (BB)BM5 Blanik(SP)BM6 Kutiny (BB)BM7 Ionosph_obs. Panska Ves (BB)BM8 Meteo_obs. Sous (SP)

Coordinates

°E

15.6313.9914.7013.0314.8916.1814.5715.32

°N

49.2649.7048.6049.8649.6449.6750.5450.79

Instrument / Acquisition system

Guralp CMG-3T / VISTEC-JUPITERGuralp CMG-3T / VISTEC-JUPITERGuralp CMG-40T / VISTEC-JUPITERLenartz 20s / MINITITAN 3XTLenartz Is / MINITITAN 3 XTLenartz 20s / MINITITAN 3XTLenartz 20s / TITAN DATLenartz 1 / MINITITAN 3 XT

Fig. 4. Array configuration (stations BM1-8, red triangles) during the field measurement in theBohemian Massif, along with permanent seismological observatories (black triangles). Table 1gives parameters of the sensors and the aquisition systems of the eight portable stations used duringthe field experiment.


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Previous studies based on observations of permanent observatories (Babuska et al.,1984, 1993) and field measurements (Plomerova et al., 1998; Babuska and Plomerova,2000a; Plenefisch et al., 1999), proved the existence of seismic anisotropy in the uppermantle of central Europe (see also Granet et al., 1998; Brechner et al., 1998). Two basicpatterns of P-residual spheres reflecting two different orientations of anisotropic structuresin the subcrustal lithosphere were recognised (e.g., Babuska and Plomerova, 1992). TheSaxothuringian pattern is characterised by early arrivals from the NW, while the earlyarrivals from the S are typical in the Moldanubian. The patterns were interpreted byanisotropic structures dipping divergently relative to the suture between both tectonicunits (Babuska and Plomerova, 2000b, this volume). The Moldanubian pattern of theP-spheres, computed from a recent ISC European dataset for a ten-year period of1986 - 1995, is also observed at several other stations of the Variscan central Europe, e.g.at stations GRF and FUR to the SW of the BM (Fig. 5) and continues southward to theAlps. A comparison of this pattern with those in, e.g., Babuska et al. (1984) or Babuskaand Plomerova (1992), confirms that the residual pattern is independent of different timewindows and normalizations (Plomerova, 1997). In the following paragraphs we shallgive new results of anisotropic studies predominatly within the Moldanubian (s.l.) part ofthe BM.

Figure 6 presents the fast shear-wave polarization vectors at stations PRU and KHC(unpublished data evaluated in 3D by Spacek, 1995). The fast shear-wave polarizationschange in dependence on the direction of propagation, thus indicating dipping anisotropiesin the mantle lithosphere (Sileny and Plomerova, 1996; Plomerova et al., 1998). While themean values of the fast S azimuths at KHC retain their E-W orientation, the mean value atPRU is slightly rotated to the WNW-ESE. By inverting the P-residual spheres for thesame stations, we retrieved a general anisotropic model of the Moldanubian part of theBM, in which the subcrustal lithosphere is characterised by a hexagonal medium withslow symmetry axis b and the high-velocity plane (a,c) steeply dipping dominantly to thesouth (Table 2, Fig. 7). In Table 2,p (azimuth from the N) and T(measured upwards from


Fig. 5. P-residual spheres at the permanent observatories located in the Moldanubian part of theVariscan central Europe. The residuals are computed for a ten-year period: 1986 - 1995. The high-velocity directions prevail for the propagation from the south. For more details see caption of Fig. 3.

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the vertical) are spherical angles, a is the dip from the horizontal. The thickness of theanisotropic medium (130 and 165 km, see Table 2) is larger than the model thickness ofthe subcrustal lithosphere (about 95 and 110km, Babuska and Plomerova, 1992). Thismay be partly explained by the relatively conservative value of the anisotropy (kp = 5%)used in the inversion. In the westernmost part of the BM, Babuska and Plomerova (2000b,this volume) estimate the mantle anisotropy in the N-S profile at about 7%. In comparingthe two independent datasets in Figures 6 and 7 we observe a similar rotation of the strikeof the dipping (a,c) high-velocity plane (Fig. 7) and the mean fast shear-wave azimuths(Fig. 6).

Fig. 6. Fast S polarization vectors and splitting delay times dt at stations PRU and KHC in theprojection of lower hemisphere. The splitting parameters were evaluated in 3D (Sileny andPlomerova, 1996) and the vectors display both the azimuth and inclination of the fast shear waves.


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Table 2. Anisotropic structure of the mantle lithosphere beneath the BM as retrieved by inversionof the P-residual spheres.

PRUKHC

symmetry axis b

p

49°7°

T

10°40°

ad

20°50°

(a,c) plane

strike

139°97°

pd

229°187°

ad

70°40°

thickness

(km)

130165

Fig. 7. 3D anisotropic model of the subcrustal lithosphere evaluated from theP-residual spheres at observatories PRU and KHC. The orientation of anisotropic structures (seeTable 2) is approximated by the hexagonal model of olivine aggregate (coefficient of anisotropykp = 5%) with the slow symmetry axis b, in the projection of lower hemisphere. Centre of the violetregion in the upper part of both diagrams marks the maximum of concentration of the b axes,yellow-red region marks the minimum of the b axes, i.e. the orientation of the (a,c) plane.



Fig. 8a

Fig. 8b


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Fig. 8. Shear waveforms (a) of 1998-09-02 event (Mindanao, epicentral distance = 99°.backazimulh = 70° and MS = 6.7) recorded at the portable station BM1. along with the particlemotions and evaluation of the SKS (b) and SKKS (c) splitting parameters. A minimum (dark blue)of the misfit function in the ( d t , c ) space determines the orientation of the fast shear wave (given byan angle c) in the plane perpendicular to the ray of the shear phase and the delay time dt betweenthe fast and slow shear waves. The orientation (c) of the fast shear wave is recomputed into thegeographical azimuth "phi" and the inclination. which is defined by an angle 'theta' measured fromthe vertical upwards.

In spite of the relative seismic quiescence during the field measurement in the BM,several quality waveforms have been recorded. Figure 8 presents the waveforms of SKSand SKKS at station BM1 and evaluations of the splitting parameters in 3D from therecords after using a 4 - 40 s band-pass Butherworth filter of the 3rd order. The splittingtime dt and orientation of the fast shear wave are practically identical for both core phases.The same feature of the consistency between splitting parameters evaluated for SKS andSKKS is observed also at two other stations, where both core phases of the same eventwere recorded (NKC and BM6, Fig. 9). There are distinct lateral variations of the dt andorientation of the fast shear waves within the BM. While in the central part large dt anda general E-W orientation of the fast split wave prevail, in the eastern part of the BM thedt is smaller and the fast polarization rotates to the WNW-ESE. Smaller dt and similarrotation are indicated also at station NKC situated at the southern rim of theSaxothuringian unit (cf. a change of anisotropy, Babuska and Plomerova, 2000b, thisvolume).

Studia geoph. et geod. 44 (2000)206

Fig. 8c


Fig. 9. Fast S-polarization azimuths and the time delays dt of the SKS and SKKS phases arrivingfrom approximately opposite directions (regions of Mindanao and Peru). The arrows mark azimuthsof the fast S polarization vectors evaluated in 3D: an inclination of the vectors is not shown. Lengthof the symbols is proportional to the St.

In this short paper we have repotted only the preliminary results of anisotropymeasurements in the BM acquired from the array of portable stations which were installedin the region for the first time. At present, we continue in evaluating both teleseismic Parrivals and shear waveforms, with the aim to perform the joint inversion of the P-residualspheres and shear-wave splitting parameters and to model the anisotropic structure ofvarious lithospheric domains of the BM.

5. CONCLUSION

Velocity anomalies in the Armorican and Bohemian Massifs, as well as in the FrenchMassif Central, are strongly affected by at least two differently orientated dippinganisotropic structures within the subcrustal lithosphere. A number of recent studiesconfirm that the seismic anisotropy within the upper mantle cannot be considered as asecond-order effect. Further research will focus on 3D modelling of the individualanisotropic domains of the mantle lithosphere, with the aim to differentiate between thevelocity variations caused by a heterogeneity and by anisotropy.


Acknowledgements: Our sincere thanks go to K. Klinge and to an anonymous referee for theircritical reviews and constructive remarks, which improved the original manuscript substantially.Providing the contours of the AM by O. Lexa is appreciated. The research was partly supported byGrants No. A3012908 of the Czech Academy of Sciences, No. 205/98/K004 of the Grant Agency ofthe Czech Republic and the Barrande Project No.98087-2.

Manuscript received: 21 December 1999 Revisions accepted: 15 March 2000

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