Geophys. J . R . astr. Soc (1987), 89, 345-352

Synoptic interpretation of seismic reflection and refraction data St. Mueller, J. Ansorge, N. Sierro, and P. Finckh, lnstitul fur Geophysik, ETH-Honggerberg, CH-8093 Zurich, Switzerland

D . Emter, Geowissenschajiliches Gemeinschajisobservatorium SchiltachlSchwarzwald, Universitaten Karlsruhe und Stuttgart, Heubach 206,0-7620Wo[fach,F R . Germany.

Summary. The structure of the upper lithosphere beneath southern Germany, northern Switzerland and west-central Utah (U.S.A.) has been investigated in detail by various geophysical methods. A synoptic interpretation of travel time and amplitude data obtained in seismic refraction and wide-angle reflection surveys, combined with near-normal incidence reflection observations, now permits the elucidation of the fine structure in a more quantitative and unified manner. With this scheme it is possible to unambiguously identify low-velocity zones and to deduce velocity gradients if reliable amplitude information is included in the inversion process.

Key words: seismic structure, synoptic interpretation, southern Germany, northern Switzerland, central Utah.

1. Introduction

Over the past few decades, reliable travel time and amplitude data of seismic waves produced by artificial sources have accumulated, and velocity-depth sections for the earth’s crust can now be determined in great detail. With the aid of synthetic seismogram sections it is possible to discriminate between different models which, for instance, on the basis of travel time observations alone, were considered to be equivalent. It is the purpose of this paper to demonstrate that there is a considerable variation of the P- and S-velocities with depth in the lithosphere which can be delineated if travel time and amplitude information obtained by seismic refraction and reflection surveys in the same area is utilized in a synoptic interpretation. The results presented lead to a consistent picture of crustal structure in three regions (southern Germany, northern Switzerland, west-central Utah) where extensive seismic studies have been carried out. 2. Southern Germany A rather extensive programme of normal-incidence reflection measurements has been carried out in West Germany since 1952, where there has been active cooperation between the geophysical exploration industry and academic institutions. The data to be analysed were normally obtained with standard prospecting equipment which was permitted to record for

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Figure 1. Sketch map showing the osition of seismic refraction profiles (a ..... d) and the location of deep position cf crustal cross section reflection surveys (I ..... V) in central fiurope. Stars denote shotpoints. (-.-.-) in Fig. 2. Crystalline massifs: BF = Black Forest; V = Vosges Mountains.

several seconds after the returns from horizons of commercial interest had been received. Deep crustal reflections from the crystalline part of the crust have been observed in a number of locations in southern Germany, four of which in the western Molasse basin north of the Alps are indicated in Fig. 1 Summary plots of the observed reflections in the form of histograms are shown in Fig. 2. (Liebscher 1962), where the vertical downward oriented axis represents two-way travel time from the surface to each of the reflecting horizons, and the horizontal axis (here oriented to the left) shows the relative number of seismic events per tenth of a second determined from numerous recordings of seismic reflections in the four areas (cf. Fig. 1). Consistent reflections were identified at nearly the same times in widely separated regions, and all efforts have failed which tried to show that these signals are mutliple reflections. In all the histograms of Fig. 2 there are three clearly separated peaks at about -4.2, -7.3 and -10.5 s with conspicuously more reflecting elements between the latter two maxima. A minor, less-developed peak appears at around 2.7 s in locations I and 111. Using available seismic refraction and wide-angle observations (Giese, Prodehl & Stein, 1976) in the same area (cf. Fig. l), Emter (1971) combined both data sets in a joint interpretation which led to the velocity-depth section through the Molasse basin as depicted in Fig. 2. Following Mueller & Landisman (1966) the first two reflection peaks (-2.7 s and -4.2 s) were associated with the top and bottom of the pronounced low-velocity "channel" in the upper crust, the third (-7.3 s) with the Conrad discontinuity and the fourth (-10.5 s) with the MohoroviEiC discontinuity. The insertion of two low-velocity zones (with a minimum velocity of 5.5 and 6.2 km/s, respectively) within the crust was necessary to resolve the discrepancies between seismic reflection and refraction observations, in particular to explain the position of the critical points closer to the shotpoints than is to be expected for models without the presence of the two velocity reversals. 3. Northern Switzerland

In 1982 a network of Vibroseis reflection profiles with a total length of 180 km was surveyed in northern Switzerland to investigate the suitability of the crystalline basement as

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host rock for the deposition of highly radioactive waste (Finckh et al. 1986). The recording configuration was designed to resolve in great detail the structures in the uppermost 4 s two-way travel time. By applying a special processing procedure in which the field data were correlated with only the first half (10 s) of the up-sweep covering a frequency range from 11 to 36 Hz the correlated record length could be extended to a maximum of 14 s (Finckh et al. 1984). Portions of a profile running along the strike of the Swiss Jura Mountains from west to east, and of a crossline running from the Black Forest in the north to the folded part of the Jura Mountains in the south (see crossed profiles V in Fig. 1) were subjected to the special processing scheme described above. Line drawings of the reflecting elements for the two crossed profiles based on the actual seismic sections (cf. Finckh et at., 1984, 1986) are reproduced in the Figs. 3a and 3b. They clearly show returns of energy from the entire crust with major reflectors which can be traced across both sections. Pronounced reflected energy is observed at two-way travel times of 2.5 - 3.3 s (G 1) from the upper crust, at 6.0 - 6.5 (C 1) from the middle crust and at 8.5 - 9.2 s (M). The latter reflections must be associated with the crust-mantle transition zone (Mueller et al. 1980), while the sloping reflector (1.5 to 3.0 s) at the northern end of the N-S section (Fig. 3b) has been ascribed to horizontal energy returns from the vertical Vonvald fault in the southern Black Forest crystalline massif (Finckh et al. 1986). In the same area of northern Switzerland detailed seismic refraction and wide-angle reflection measurements were carried out with station spacings of less than one kilometer as part of the national geophysical survey (Sierro et al. 1983). As an example, part of the record section for a S-N refraction profile extending from the Molasse basin to the Black Forest massif (line d in Fig. 1) is shown (Fig. 3d) which practically coincides with the N-S reflection line across the Jura Mountains (Fig. 3b). The clearly delineated signal phases permit an indisputable correlation of first and later arrivals as indicated. Both surveys are detailed enough for a synoptic interpretation of the reflection and refraction data. The inversion leads to a consistent Pvelocity-depth section in that area (Fig. 3c)

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Figure 2. Cross section through the earth's crust in the western Molasse basin of southern Germany. The depth scale (2) is linear in time based on the deep reflection histograms (I ... IV, cf Fig. 1). P-wave velocities are determined from the seismic refraction profiles (a, b, c) of which a and c are reversed.

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St. Mueller, J . Ansorge, N . Sierro and P . Finckh

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Figure 3. (a), (b): Line drawings of reflections for the crossed profiles 70 and 10 (location V in Fig. 1) in northern Switzerland. (d): Part of refraction record section with correlations (reduced time t, = t - d/6,where d = distance in km)for a profile from the Molasse basin to the Black Forest (line d in Fig. 1). (c): P velocity-depth structure based on synoptic interpretation. The +signs below the low-velocity zone C2/C1 signify lamination superimposed on a positive velocity gradient in the lower crust (see text).

which, except for the absolute depth, resembles the velocity-depth section in Fig. 2. The velocity structure derived from the refraction and wide-angle reflection data contains relatively abrupt interfaces which cause normal-incidence reflections from the top and bottom of low-velocity zones in the upper and middle crust. It is worth noting that the wide-angle reflections from the top of the velocity reversals (G2 and C2) are less pronounced if compared to the corresponding reflections from the bottom (G1 and Cl) of those structures. Both low-velocity zones exhibit significantly smaller velocity contrasts than the crustal structures proposed for the Rhine Graben area by Mueller ef al. (1969, 1973) and more recently for the Black Forest by Gajewski & Prodehl (1986), which are characterized by a single well-developed low-velocity "channel" at mid-crustal depths. The conspicuous reverberations between C1 and M in the refraction record section (Fig. 3d) are most likely due to lamination of the lower crust analogous to the model by Deichmann & Ansorge (1983) for a nearby profile close to the eastern margin of the Black Forest. The schematic velocity-depth function for the lower crust presented here is concordant with the more detailed model of Sandmeier & Wenzel(l986) derived from a N-S refraction profile through the Black Forest. A lamination at the top of the mantle is also indicated by the reverberations following the onset of the PMP wide-angle reflections () and by the normal-incidence reflections with two-way travel times of more than -9 s.

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Figure 4. (a): Line drawing abstracted from COCORP line 1 in west-central Utah (cf. location map, bwer left). @): Interpreted depth section of the same line (after Allmendinger el a[. 1983). (C): P Velocity-depth section for DELTA-West refraction profile (model N 7 of Miiller & Mueller 1979). A pronounced low-velocity zone (LVZ) lies around VP 1 between EflectoI'S(F)and (A) (0,where the lower boundary (A) is identified as the Sevier Desert detachment fault.

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St. Mueller, J . Ansorge, N . Sierro and P . Finckh

4. West-Central Utah A few years ago the eastern Basin and Range province in western North America has been the site of a major seismic reflection survey conducted by the Consortium for Continental Reflection Profiling (COCORP). The data collected provide valuable information on Mesozoic thrusting, Cenozoic extensional tectonics and their interrelationships. A series of remarkably continuous, low-angle reflectors which extend for more than 120 km perpendicular to strike can be followed to a depth of 15 to 20 km (Allmendinger et af. 1983). Beneath the Sevier Desert in west-central Utah a clearly defined detachment (labelled (A) and (C) in Fig. 4a) can be traced from a surface zone of normal faulting ((B) in Fig. 4a) down to 12 - 15 km depth with an apparent westward dip of -12". An extensional displacement of 30 to 60 km along this decoupling horizon has been suggested (Allmendinger et al. 1983). Another set of seismic reflection data from the same region (for a NE-SW section along U.S. Highway 50) also shows clearly the same subhorizontal detachment fault (Smith & Bruhn 1984). Detailed travel time studies of normal-incidence reflection observations combined with seismic refraction and wide-angle reflection data from the U.S. Geological Survey's crustal refraction profile DELTA-West in central Utah revealed that there are two velocity reversals within the crust at depths of about 10 and 25 km (Mueller & Landisman 1971). A more comprehensive interpretation of the DELTA-West data set carried out later (Miiller & Mueller, 1979) which was based on a systematic hedgehog search for "permissible" travel time models compatible with the observed amplitude-distance curves, confirmed that a low-velocity zone exists between depths of about 6 and 12 km with a minimum p-velocity of less than 5.4 km/s. The preferred velocity-depth model (N 7, with a minimum velocity of 5.2 km/s) is shown in Fig. 4c. If this model, which is valid for a region about 35 km north of and parallel to the COCORP line 1, is projected into the W-E reflection section (arrow in Fig. 4a, around Vibration Point 1000) and the interpreted crustal section (Fig. 4b) it is apparent that the depth range between 6 and 12 km as measured from the surface contains a pronounced velocity reversal (10%) and that the Sevier Desert detachment must be identical with the lower boundary of this low-velocity zone (LVZ). The layer below this velocity reversal has a P-velocity of 6.4 km/s. In the present discussion the general question of the nature of low-velocity zones in the crust (see e.g. Mueller, 1977) is deliberately not addressed nor is it meant to suggest that such zones are a common feature of the continental crust everywhere.

5. Conclusion The examples presented in this paper emphasize that the seismic reflection and refraction techniques are complementary and that both ought to be used together in order to permit a quantitative determination of crustal structure in terms of wave velocities and depth which will ultimately provide clues to the physical state and chemical composition of the Jithosphere and its tectonic evolution. Contribution No. 521, ETH-Geophysics, Zurich, Switzerland

References Allmendinger, R.W., Sharp, J.W., Von Tish, D., Serpa, L., Brown, L., Kaufman, S., Oliver, J. & Smith, R.B., 1983. Cenozoic and Mesozoic structure of the eastern Basin and Range province, Utah, from COCORP seismic-reflection data, Geology, 11, 532-536. Deichmann, N. & Ansorge, J., 1983. Evidence for lamination in the lower continental crust beneath the Black Forest (Southwestern Germany), J . Geophys., 52, 109-1 18.

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Emter, D., 1971. Ergebnisse seismischer Untersuchungen der Erdkrust und des obersten Erdmantels in Stidwestdentschland,P h D . Thesis, University of Stuttgart (Germany). Finckh, P., Ansorge, J., Mueller, St. & Sprecher, Ch., 1984. Deep crustal reflections from a Vibroseis survey in northern Switzerland, Technophys., 109, 1-14. Finckh, P., Frei, W., Fuller, B., Johnson, R., Mueller, St., Smithson, S . & Sprecher, Ch., 1986. Detailed crustal structure from a seismic reflection survey in northern Switzerland, in Reflection Seismology: A Globar Perspective: Geodynamics Series,vol. 13, pp 43-54, eds Barazangi, M. & Brown, L., American Geophysical Union, Washington, DC. Gajewski, D. & Prodehl, C., 1986. Crustal evolution of the Rhinegraben area, Part 11: Seismic-refraction investigation of the Black Forest, Berichtsband A, Sonderforschungsbereich 108, Spannung und Spunnungsumwundlung in a'er Lithosphiire, University of Karlsruhe (Germany),313-360. Giese, P., Prodehl, C. & Stein, A. (eds), 1976. Explosion seismology in central Europe, Springer, Berlin-Heidelberg-New York. Liebscher, H.-J., 1962. Reflexionshorizonte der tieferen Erdkruste im Bayerischen Alpenvorland, abgeleitet aus Ergebnissen der Reflexionsseismik,Z. Geophys., 28, 162-184. Mueller, St. and Landisman, M., 1971. An example of the Unified Method of Interpretation for crustal seismic data, Geophys. JR. astr. SOC.,23, 365-37 1. Mueller, St., 1977. A new model of the continental crust, in The Earfh's Crmf: Geophys. Monogr. ed. Heacock, J.G., 20,289-317. American Geophysical Union, Washington, DC. Mueller, St. & Landisman, M., 1966. Seismic studies of the earth's crust in continents, Part I: Evidence for a low-velocity zone in the upper part of the lithosphere Geophys. J . R . astr. Soc., 10,525-538. Mueller, St., Peterschmitt, E., Fuchs, K. & Ansorge, J., 1969. Crustal structure beneath the Rhinegraben from seismic refraction and reflection measurements, Tectonophys., 8,529-542. Mueller, St., Peterschmitt, E., Fuchs, K., Emter, D. & Ansorge, J., 1973. Crustal structure of the Rhinegraben area, Tectonophys., 20,381-392. Mueller, St., Ansorge, J., Egloff, R. & Kissling, E., 1980. A crustal section along the Swiss Geotraverse from the Rhinegraben to the Po lain, Eclogae geol. Helv., 73,463-483. Miiller, G. & Mueller, St., 1976. Travel-time and amplitude interpretation of crustal phases on the refraction profile DELTA-W, Utah, Bull. seirm. SOC.Am., 69, 1121-1132. Sandmeier, K.-J. & Wenzel, F., 1986. Synthetic seismograms for a complex crustal model, Geophys. Res. Lett., 13, 22-25. Sierro, N., Bindschaedler, A., Ansorge, J. & Mueller, St., 1983. Geophysikalisches Untersuchungsprogramm Nordschweiz: Regionale refraktionsseismische Messungen 81/82, NAGRA Tech. Report N T B 83-21, Baden (Switzerland). Smith, R.B. & Bruhn, R.L., 1984. Intraplate extensional tectonics of the eastern Basin-Range: Inferences on structural style from seismic reflection data, regional tectonics, and thermal-mechanical models of brittle-ductile deformation, J . geophys. Res., 89,5733-5762.