Reinterpretation of a wide angle reflection “fan” across the Central Alps

Journal of Applied Geophysics, 30 ( 1993 ) 43-53

43

Elsevier Science Publishers B.V., Amsterdam

Reinterpretation of a wide angle reflection "fan" across the Central Alps G. Musacchio a, R. D e F r a n c o b, R. Cassinis a a n d G. G o s s o c a University of Milano, Dept. of Earth Sciences, Geophysical Division, Via L. Cicognara 7, Milan, Italy bCNR of Milan, Via Bassini 15, Milan, Italy cUniversity of Milan, Dept. of Earth Sciences, Geologicaland Paleontological Division, Via Mangiagalli 34, Milan, Italy (Accepted after revision September 30, 1992)

ABSTRACT Musacchio, G., De Franco, R., Cassinis, R. and Gosso, G., 1993. Reinterpretation of a wide angle reflection "fan" across the Central Alps. In: R. Cassinis, K. Helbig and G.F. Panza (Editors), Geophysical Exploration in Areas of Complex Geology, II. J. Appl. Geophys., 30: 43-53. Within the framework of a new phase of crustal exploration in the Alpine range (by vertical reflection seismics) all existing seismic data (refraction-wide-angle reflection profiles and fans ) have been reprocessed and reinterpreted. This study discusses the results of the reinterpretation of the fan ZE (recorded in 1986 ), which crosses the Central Alps east of the European Geotraverse (EGT). The geophones (3 components ) were positioned 130 km east of the shot-point C (on the EGT ); therefore, the "mirror" of the supercritical reflections from the Moho is located approximately to the north of the Iseo lake. Reprocessing included a spectral analysis (on noise, PMP and SMS signals), a polarization analysis (on 3-component reoriented recordings ), and a reflectivity analysis (on the vertical component ). A line-drawing was prepared and finally the depth of the correlated reflection segments was calculated using average velocities constrained mainly by the SUDALP 77 profile, which intersects the fan ZE. The analysis shows that the Penninic crust is more reflective than the South-alpine crust and that the transparency of the lower crust increases where the total thickness is greater; here, the frequency of the signal reflected from the Moho also seems to increase. The geometry shown in the depth cross-section is discussed also considering the resolution power of the fan. Furthermore, like in the Western Alps as well as along the EGT, the asymmetry of the lower crust clearly indicates that a trace of lithospheric subduction still exists in the present configuration.

Introduction The recent seismic experiments (refraction-wide-angle reflection and near-vertical reflection), help to draw a new picture (Fig. l ) of the crust beneath the Western and Central Alps (ECORS-CROP Deep Seismic Sounding Group, 1989; Buness and Giese, 1990; Damotte et al., 1990; Rey et al., 1990; Ye and Ansorge, 1990; Holliger and Kissling, 1991 ). Involvement of the Moho discontinuity in the Alpine deformations is shown in all models: from the inner to the outer part of the chain, 0926-9851/93/$06.00

the Moho changes in depth, its shape is strongly asymmetrical and discontinuous and the trace of lithospheric subduction can still be recognized. The reinterpretation of the fan ZE profile provides information about the deep crustal structures and seismic features of the Moho discontinuity in a zone about 30 km east of the EGT-86 Central-Southern segment, along a mirror located to the north of the Iseo Lake. The results are used to compare the structure beneath this transect and those beneath the Central-Western Alps. Fan ZE crosscuts the Insubric line, a tec-

© 1993 Elsevier Science Publishers B.V. All rights reserved.

44

G. MUSACCHIO ET AL.

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insufficient data to discard the hypothesis of a continuous Moho beneath the Central Alps.

Expected geological setting The expected geological characters of the different crustal segments along the transect of the fan ZE should be justified by the geological history of their specific geological domain. At the surface, to the north of the Insubric line,

45

REINTERPRETATION OF A WIDE ANGLE REFLECTION "FAN" ACROSS THE CENTRAL ALPS

the Penninic zone represents the wide suture zone of the Alpine belt due to the presence of ophiolitic nappes. It is rather peculiar because of the multiple repetition of thin nappes which were refolded and metamorphosed together in a subduction environement during the Cretaceous and the Tertiary periods. They are formed by extremely thin layers of rocks which are derived from several crustal domains: oceanic and continental crusts, pelagic, trench, margin, forearc and continental sediment piles as well as subeontinental mantle. By contrast, the South-alpine domain is exposed as an unmetamorphosed sequence of cover and basement thrusts in which even the uppermost sheets were not delaminated, during the Upper Cretaceous to the Tertiary periods, from crus-

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tal depths at which Alpine metamorphism was active. Therefore, the underlying middle crust does not reproduce a Penninic-style nappe system, but rather a sequence of intra-basement thrust horizons, superimposed on a homogeneous pre-Alpine crust.

Data analysis Three-component analogic data were digitized using a sampling frequency of about 100 Hz (Maistrello et al., 1991 ). Prior to processing, an analysis was performed to determine the influence of both the elevation and the geometrical distribution of the recording points. This analysis shows that the elevation effect plays a major role and that the correction time

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Fig. 3. An example of polarization filter (station No. 07). a, b, and c are the three reoriented components before processing; d, e, and fare the filtered three components; g is the rectilinearity function. It could be observerd that after the polarization filter the polarized S-waves became clearer. is an average of + 0.15 s with the exception of stations 6 and 28, which have delays of about 0.17 and 0.2 s, respectively. Data were processed using the PITSA programme (compiled by F. Scherbaum, 1992). Data were acquired using a reference recording system oriented in the N-S, W-E, and z-directions. Then they were reoriented in the

r - t - z system: the first horizontal component was oriented in the source-receiver direction, the second one was perpendicular to the first one, and the z-component was oriented in the vertical direction. The data analysis procedure, shown in Fig. 2, was applied in order to study both S- and P-waves. In general, the Swave analysis is not a trivial task. In our case,

47

REINTERPRETATION OF A WIDE ANGLE REFLECTION " F A N " ACROSS THE CENTRAL ALPS

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the small number of 3-D component traces and the quality of the data made this study very difficult. We had some difficulties in correlating the S-wave energy, mainly because it is always masked by a lot of noise with almost the same spectral characteristics as the signal. To overcome these difficulties, we used the polarization filter technique (Kanasewich, 1981 ). Actually, the availability of the 3-component seismograms allowed us to develop filter functions which utilise the polarization properties of both compressional and shear-waves. These kinds of filters preserve motion if it satisfies specified polarization conditions in a particular direction and they attenuate motion that does not satisfy the imposed criteria. In this case, the three specified directions were (Fig. 3 ): vertical, transversal and radial with respect to the acquisition geometry. To identify the S-wave reflection from the Moho (SMS phases), as a first approximation we assumed that Ts/Tp= 1.73 (elastic, isotropic and homogeneous media) to obtain an initial estimate of the arrival time of SMS reflections. In general, Moho reflections correspond to the high polarization energy showed by the rectilinearity function (Fig. 3g) and SMS

arrivals can be detected with an error on timed readings of about _+0.2 s. SMS phases can be recognized in many traces, but the correlations are doubtful due to the small number of data and to the non-homogeneous distribution of recordings along the fan (Fig. 4a). However, it was possible to correlate three S crustal reflectors, in the northern part of the section, that do not give any detectable P energy. This is particularly clear in the vertical component section which is reduced to a velocity of 3.46 k m . s - l , where the mainly SVconverted energy indicates correlation phases between 1 and 3.5 s of reduced travel-time. We performed a detailed spectral analysis of the Moho signals which are clearer and more stable over a relatively large number of recordings. The spectral analysis shows that the signal is confined to a narrow band (2-8 Hz ) and that the frequency of the P-waves is always higher than that for the S-waves (Fig. 4c ). Only a few recordings became clearer after frequency filtering because noise often has a spectral window with a lower limit coinciding with the one of the signal. Two different pass band filters were used: 3-8 Hz for the vertical component to analyze the P-waves and 2-7 Hz for

49

R E I N T E R P R E T A T I O N O F A W I D E A N G L E R E F L E C T I O N " F A N " A C R O S S T H E C E N T R A L ALPS

the horizontal component to analyze the Swaves. Results

The data analysis led to several considerations about ( 1 ) reflectivity pattern, (2) spectral content of signals coming from the Alpine crust, and (3) Ts/Tp ratio. Finally, a line drawing was the starting point for the construction of an interpretative depth section. --The Mean Squared Amplitude diagram in the northern, central and southern part of the section clearly shows different patterns of reflectivity (Fig. 4d). The southern part is characterized by a strong peak of reflectivity confined to the Moho reflections; instead, reflectivity is low in the upper part of the crust. The central part of the fan profile shows two zones with strong reflectivity; the first peak is related to the upper crust reflections and the second one to the Moho reflections. These peaks are separated by a clear transparent zone between about 2.5 and 5 s. In the northern part, reflectivity is spread throughout the whole crust (Fig. 4d). Crust reflectivity patterns in the northern and southern ends of the profile provide a good image of the different geological structures of the Penninic-Austroalpine and South Alpine domains. This also agrees with the NVR recordings in the Western (ECORS-CROP profiles) and Central-Western Alps (NFP20 EST profiles), where the Penninic pile shows high energy reflectors and the South Alpine thrusts do not give any seismic markers. ---The spectral analysis shows a general increase of P- and S-wave frequencies in the Central part of the fan profile just beneath the Insubric Line (Fig. 4c). A lateral variation in frequency is also seen from north to south beneath the Penninic domain of the Western Alps (ECORS--CROP Deep Seismic Sounding Group, 1989 ). Three interpretations can be proposed: ( 1 ) a variation of quality factor; (2) a disappearance of lower crust lamellation; (3) a

change of the Moho to a complex transitional boundary reflecting only high wave-lengths. We believe that here the presence of a transparent zone between about 2.5 and 5 s (Fig. 4b, d) could be strictly related to the lateral variation of Moho frequencies in the sense that it seems to allow higher-frequency body waves to penetrate down to the Moho. --Another phenomenon that characterizes the transition between the Alpine and African crust seems to be the Ts/Tp ratio which tends to increase (from about 1.71 to about 1.77) going from the south to north of the Insubric lineament. The same feature is observed in the Western Alps (ECORS-CROP Deep Seismic Sounding Group, 1989) and is interpreted as being related to a low S reflector layer (perhaps a fluid saturated rock) on top of a crustal reflector. This agrees with the presence of the previously mentioned S reflectors located in the northern part of our crustal section. Unfortunately, the limited amount of data makes it impossible to conduct a good statistical study of the phenomenon. In effect, we have to consider that the complexity of the geological structures might have a great influence on our interpretation: the absence of longitudinal SUOALP77 E

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Fig. 6. ( A ) Unmigrated depth section. T h r e e N M O v e l o c i t i e s are chosen: a = 6 k m . s - ' ; b = 6 . 2 5 k m - s - ' ; c = 6 . 3 5 k m . s - ' . ( B ) Geological interpretation. Two different dips are hypothized ( 1 , 2 ) f o r t h e I n s u b r i c l i n e . T h e one signed b y " 2 " is hypothesized on the NFP20 profile (Holliger and K i s s l i n g , 1 9 9 1 ).

reflected waves coming from these discontinuities can be explained by a simple geometrical effect, such as reflections out of the plane of the section or the different reflection angles between P- and S-waves (Helbig, pers. commun. during the Int. Sch. Appl. Geophys., ERICE, 1992). - - T h e line drawing was traced using both the longitudinal and transversal waves and, in par-

ticular, for P-waves, a zoom on first arrivals (between 0 and 2 s) (Fig. 4b) was also useful as well as the intersecting profile South-Alp 77. This profile is a good constraint on the interpretation because its shot-point B (Fig. 1 ) is located on the acquisition fan profile (at station No. 17 ). If one constructs a composite film section (Fig. 5), using the recordings of the South-Alp 77 and station number 10 of the fan

REINTERPRETATION OF A WIDE ANGLE REFLECTION "'FAN" ACROSS THE CENTRAL ALPS

ZE, there is very good correlation between the phases coming from the two different experiments. It could be seen that between 2 and 6 s of reduced travel-time ( V ~ = 6 kin- s- 1), there is a high amplitude, low frequency and discontinuous reflector that, according to Deichmann (1984), can be interpreted as the Moho. Some other seismic markers are interpreted as crustal reflectors and the two shallower ones could not be distinguished in the fan section perhaps owing to destructive interference. In the southern part of the profile, Moho reflections (between 3 and 3.5 s, according to Deichmann, 1984) look very peculiar and a high amplitude first arrival is followed by three distinct reverberations. These reverberations are very clear especially on the fan film section (Fig. 4a). - - T h e depth section was constructed adopting a laterally homogeneous crust and assuming (Fig. 6a) three different NMO velocities, (6, 6.25, 6.35 km-s -1) constrained by the 2-D interpretation on the South-Alp 77 (Deichmann, 1984), by the EGT-86, NFP20 (Ye and Ansorge, 1990; Holliger and Kissling, 1991, and references therein) and by some considerations about outcropping and hypothesized shallow geology. By laboratory mesurement on samples average velocity of outcropping rocks (carbonatic and metamorphic ) was computed from published data (Gebrande, 1982, and references therein; Kern, 1982, and references therein) on similar rocks. Because the velocity variation between the outcropping South-Alpine (carbonatic rocks) and outcropping Austroalpine (metamorphic rocks) is small and thus cannot be detected, it can be assumed that there is no horizontal velocity variation. Many other crustal reflectors are drawn with respect to the earlier geophysical interpretation (Scarascia and Maistrello, 1989 ) and the Moho looks very fragmentary and asymmetrical and its remarkable jumps (about 5 km) correspond to the area where the crust is thicker. As previously explained, the Moho drops of about 0.5 s (about 5 km) are not re-

51

lated to the fact that receivers are not coplanar to each other and therefore can be attributed to structural features.

Interpretation Two interaction mechanisms between the European and Adria plates are accepted in the evolutionary models of the Cretaceous-Tertiary convergence: a symmetric and simultaneous subduction of the two lithospheres (e.g. Laubscher, 1990; Suhadolc et al., 1990, and references therein) and an asymmetric, southeast-dipping subduction of the European plate (e.g. Platt, 1986, 1987; Pfiffner et al., 1990; Ring, 1992). The Penninic nappe belt is explained as a zone of imbrication of supracrustal slices delaminated from the top of continental or oceanic crusts, while the excessive crustal volumes are eliminated in the mantle (Frei et al., 1989 ), it is alternatively explained as an uplifted subduction complex (Platt, 1986, 1987; Polino et al., 1990; Ring, 1992). Lithospheric scale thrusting does not occur in both types of models until the collisional stage between the main continental margins is reached. In the Iseo lake fan section (Fig. 6b) a prevalent gentle southward dip of intracrustal reflectors coexists with a northward deepening step-like image of the Moho. This configuration coincides at the surface with a lateral contact of the Penninic subduction-related nappe belt with the South-alpine sequence of thin-skin thrusts. South of the Insubric line no continental crustal slices carting a metamorphic imprint of Alpine age have been delaminated, emplaced and exposed at the surface. The observed crustal duplication to around 50 km below the Insubric Line is not feasible even if thin-skin thrusting in the South-alpine was continuous since the Upper Cretaceous. The abnormal thickness of the South-alpine crust can be explained only by a major indentation of the

52

Penninic and South-alpine belts below the emergence of the Insubric Line whose upward displacement component forces the deep Alpine metamorphosed and unmetamorphosed levels against each other. The seismic characteristic of Pennine pile is the high reflectivity of its internal tectonic boundaries, shown by both wide-angle and NVR seismic experiments. In the fan ZE section this feature seems to extend southward with respect to the Insubric line. This suggests an emplacement beneath the Southern Alps of a crustal element, which evolved under subductive (Penninic) conditions and therefore is characterized internally by multiple repetitions of sheets of oceanic and continental derivation. Consequently, the isolated Moho segments can be interpreted by different indentation mechanisms between the Penninic and South-alpine blocks: an underthrusting of the Penninic below the South-alpine favours the derivation of A - B - C Moho segments from the European plate; a Penninic crustal flake with a subsequent delamination of the Adria crust suggests that B - C - D Moho reflectors belong to the Southern African plate. In both cases, according to current Alpine geological models, the splitting of the Moho into isolated segments can be interpreted as a Tertiary, collisional feature. Conclusion

The main result of the reprocessing and reinterpretation of the fan ZE (Iseo lake) is a crustal model which presents a configuration beneath the Central Alps that confirms the cylindricity of the deep crustal structures. Moho deepening is asymmetric, discontinuous and its main jumps are around 20 km in the Western Alps (ECORS-CROP), 10 km in the Central-Western Alps (EGT-86, NFP20 EST) and 5 km in the Central Alps (fan ZE). With respect to the previous interpretation, many other crustal reflectors are correlated and the internal tectonic boundaries of the Penninic pile are related to high energy crustal reflec-

G. MUSACCHIO ET AL.

tors. An emplacement of crustal elements characterized by multiple indentation of oceanic and continental crusts even beneath the southern Alps is supposed to explain the occurrence of the high energy reflectors in that area. The specific indentation mechanism selected will determine whether the Moho segments come from the European or African plate. References Buness, H. and Giese, P., 1990. A crustal section through the northwestern Adriatic plate. In: R. Freeman, P. Giese and St. Mueller (Editors), The European Geotraverse: Integrative Studies. European Science Foundation, Strasbourg, pp. 297-305. Damotte, B., Nicolich, R., Cazes, M. and Guellec, S., 1990. Mise en ouvre, traitment et prrsentation du profll plaine du Po-Massif Central. In: R. Roure, P. Heitzman and R. Polino (Editors), Deep Structure of the Alps. Mrm. Soc. Grol. Fr., 156: 65-77. Deichmann, N., 1984. Combined travel-time and amplitude interpretation of two seismic refraction studies in Europe. Doctorate Thesis. Swiss Fed. Inst. Technology, Zurich, 142 pp. Dohor, G.P., 1985. Seismic shear waves, part B: applications. In: K. Helbig and S. Treitel (Editors), Handbook of Geophysical Exploration, Vol. 15B. Geophysical Press, London. ECORS-CRoP Deep Seismic Sounding Group, 1989. Mapping the Moho of the Western Alias by wide-angle reflection seismics. Tectonophysics, 162:193-202. Frei, W., Heitzman, P., Lehner, P., Mueller, St., Oliver, R., Pfiffner, A., Steck, A. and Valasek, P., 1989. Geotraverses across the Swiss Alps. Nature, 340: 544-548. Gebrande, H., 1982. Elastic wave velocities and constants of elasticity of rocks at room temperature and pressure up to 1 GPa. In: G. Angenheister (Editor), Physical Properties of Rocks, subvol, b. Springer, Berlin, pp. 3596. Holliger, K. and Kissling, E., 1991. Ray theoretical depth migration: methodology and application to deep seismic reflection data across the eastern and southern Swiss Alps. Eclogae Geol. Helv., 84(2):369-402. Kanasewich, E.R., 1981. Time sequence analysis in geophysics. Univ. Alberta Press, Edmonton, Alta., 480 pp. Kern, H., 1982. Elastic waves velocities and constants of elasticity of rocks at elevated pressures and temperatures. In: G. Angenheister (Editor), Physical Properties of Rocks, subvol, b. Springer, Berlin, pp. 99-139. Laubscher, H., 1990. Deep seismic data from the central Aips: mass distribution and their kinematics. In: R. Roure, P. Heitzman and R. Polino (Editors), Deep

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