Heat flow, heat production and crustal dynamics in the Central Alps, Switzerland

Tertot~ophysics, 0 Elsevier

41 Scientific

113

(1975)113-126 Publishing

Company,

Amsterdam

- Printed

in The

Netherlands

HEAT FLOW, HEAT PRODUCTION AND CRUSTAL DYNAMICS CENTRAL ALPS, SWITZERLAND *

LADISLAUS

RYBACH

GERARD

BERSET

‘, DIETRICH

WERNER

I, STEPHAN

MUELLER

IN THE

’ and

’

’ Inslitutc of Geophysics, ETH Zurich (Switzerland) 2 %etlfralschweizcrisclzes Technikum, Lucerne (Switzerland) (Received

August

11, 1976)

ABSTRACT Rybach, L., Werner, crustal dynamics and Geodynamics.

D., Mueller, S. and Berset, G., 1977. Heat flow, in the Central Alps, Switzerland. In: A.M. Jessop Tectonophysics, 41: 113-l 26.

heat production (editor), Heat

and Flow

Interrelations between temperature field, crustal structure and crustal dynamics (vertical crustal movements) have been investigated along a selected profile: the Swiss Geotraverse which cross-sects in a NW-SE direction the following tectonic units: Rhine-Graben, Jura Mountains, Molasse Basin, Helvetic Nappes, Central Massifs with autochthonous cover, Penninic units, basement and sedimentary units of the Southern Alps (total length: 220 km). The corrected heat flow is slightly elevated along or close to the traverse (z 75 mW/m’). Thermal effects of Alpine overthrusting and metamorphism on the surface gradient are negligible today. For steady-state calculations of the temperature field heat production has been determined experimentally for surface samples; for deep crustal rocks it was inferred from an empirical relationship between heat production and seismic compressional wave velocity or density. The temperature field shows downwarped isotherms where a pronounced inversion of seismic velocity and density occurs in the upper crust. The same area of the Central Alps (Lepontine gneiss region) exhibits the strongest recent crustal movements (vertical uplift 5 1 mm/yr). The MohoroviEiE discontinuity is clearly not an isothermal surface; its asymmetric shape found by seismic and gravimetric measurements is likely to be a result of the early Alpine subduction tectonics.

INTRODUCTION

The Swiss part of the Central Alps, a classical area of geologic investigations, has been subjected to intensive geophysical studies in the past few years. Numerous seismic profiles have revealed its crustal structure (Knopoff et al., 1966; Choudhury et al., 1971; Mueller and Talwani, 1971; Giese et al.,

* Contribution land.

No.

132,

Institute

of Geophysics,

ETH

Zurich,

CH-8093

Zurich,

Switzer-

113

1973), gravity measurements have given further proof of crustal thickening and its lateral variation (Kahle et al., 1976); geochronologic data (Jager, 1973) and geodetic results (Jeanrichard, 1973; Schaer and Jeanrichard, 1974) have indicated ancient and recent vertical movements. The abundance and complexity of all this information calls for a synoptic, summary type of representation. For this purpose a characteristic profile, the Swiss Geotraverse * was selected along which most geologic and geophysical features display their maximum variability. The aim of this paper is to link geothermic data to other geophysical evidence about the structure and dynamics of the Central Alps. Special attention is paid to thermal effects of overthrusting, the predominant tectonic mechanism in forming the Alpine st.ructures. In order to evaluate the thermal conditions within the Alpine crust in the section of the Swiss Geotraverse one has to start with surface data: heat flow and heat production. The temperature field at depth can be calculated by taking into account the heat production and thermal conductivity of successively deeper strata and thus leaving open the boundary conditions - which are by no means well known - at the crust-mantle boundary: mantle heat gradient (dT/dz), and Moho-temperature TM. flow c!rn, temperature GEOLOGIC

AND

GEOPHYSICAL

CHARACTERISTICS

OF

THE

SWISS

GEOTRA-

VERSE

The trace of the Swiss Geotraverse runs roughly in a NW-SE direction straight through Switzerland from Base1 at its northern border to Chiasso in the south (total length: 220 km). It crosses, more or less perpendicularly to the general strike directions, the main tectonic units of the country: the Rhine-Graben, the Jura Mountains, the Molasse Basin, the Helvetic Nappes, the Central Massifs with their autochthonous cover, the Penninic units and the basement and sedimentary units of the Southern Alps (Fig. 1). The geologic profile at an original scale of 1 : 100,000 down to a depth of about 5 kilometers has been drawn by R. Triimpy in 1975 using all available information. For the section through the Molasse Basin information has been kindly provided by LEAG Oil prospecting group, through Dr. U.P. Biichi. This profile is reproduced in the upper part of Fig. 4. Though many fascinating details of structural geology are lost in this reduced scale, overthrust phenomena are evident: the Helvetic units of the Alps are overriding the northern Foreland Molasse by more than 10 kilometers. The seismic section, obtained by refraction profiling is shown in the lower part of Fig. 4. The predominant features are the deepening of the crust-mantle boundary

;“- For tional 1974.

the Swiss Geotraverse Geodynamics Project

a working group within the Swiss Committee of the Internawas initiated by Prof. E. Niggli (Berne) and established in

Fig. 1. The Swiss Geotraverse (thick bar) cross-cuts from Base1 to Chiasso the tectonic units Rhine-Graben (I), Folded and Plateau Jura (Z), Molasse Basin (3), Helvetic Nappes (4), Central Massil’s with autochthonous cover (5), Penninic units (6) and the basement (7) and sedimentary (8) units of the Southern Alps. Sites of new heat-flow determinations: BU = Buggingen mine, LU = Lake of Lucerne, GU = Guspisbach shaft of the new Gotthard Highway Tunnel, BI = Biaschina drill hole, LC = Lake of Como.

and a zone of lowered seismic velocities in the upper crust (shaded area): the compressional wave velocity VP first increases from the surface down to about 10 km depth where it reaches a value of 6.25 km/s in the central part of the profile, then it decreases down to about 20 km depth where VP may be as low as 5.5 km/s. The recently measured gravity profile (Kahle et al., 1976) shows the strongest Bouguer anomaly of -160 mGa1 displaced about 20 km to the south of the crest of the Alps. Detailed geodetic measurements (precise levelling, performed along or close to the profile) in 1918 and repeated in 1969/70 revealed that the Alps are still uplifted at a rate of approximately 1 mm/year. The region of maximum uplift is situated about 40 km south of the crest of the Alps (Jeanrichard, 1972, 1973). The rate of uplift in the central part of the profile seems to have been constant over the last several million years as demonstrated by fission track dating (Schaer and Jeanrichard, 1974; Schaer et al., 1975). The coincidence of the region of maximum uplift with the zone of markedly lowered seismic velocities (and densities) in the upper crust (Fig. 4) is striking. Geothermic information concerning the Swiss Geotraverse is rather scarce. The classical work of Clark and Niblett (1956) indicates normal heat flow in

116

the central part of the profile (Gotthard area). It has been further found by Clark and Jager (1969) that a substantial portion of the heat flow in the Swiss Alps results from simultaneous uplifting and denudation. The Lepontine gneiss region in the southern part of the traverse underwent intensive thermal metamorphism with its peak event about 30 m.y. ago (Frey et al., 1974). A substantial heat flow from the mantle (“mantle diapir”) must have been acting at that time in order to explain the mineral reactions that can clearly be identified (Oxburg and Turcotte, 1974; Den Tex, 1975). Today the steady-state heat flow in this area (Haenel, 1974) indicates normal conditions again. NEW

HEAT-FLOW

Heat-flow

DATA

measurements

in lakes and boreholes

Since the pioneer work of Clark and Niblett (1956) quite a number of heat-flow determinations have been carried out along or close to the Swiss Geotraverse. Table I lists the relevant data, obtained by measurements in lakes (Von Herzen et al., 1974; Haenel, 1974), in a borehole (Haenel, 1974) and in a mine (Creutzburg, 1964). The sites of these heat-flow determinations are shown in Fig. 1. Heat-flow

determination

in the new Gotthard

Highway

Tunnel

During the construction of the new Gotthard Highway Tunnel two vertical shafts were accessible for measurements. In the deeper one (Guspisbach shaft, 525 m total depth) rock-temperature determinations were performed every 10 meters. There were no signs of water circulation. The coordinates of the Guspisbach site are: S”34’ East, 46”36’ North, elevation 1685 m above sea level. The temperature gradient was calculated for the depth inter-

TABLE New

I

heat-flow

Locality

(cf.

determinations Fig.

1)

Buggingen mine (W. Germany) Lake of Lucerne (Beckenried) New Gotthard Tunnel (Guspisbach shaft) Biaschina drill hole Lake of Como (Italy)

along

to the Swiss mW/m2

HFU 1.7

or close

_t 0.2

70+

Reference 8

1.6 ? 0.6

67 I 25

1.7

71

1.54 1.71

i 0.4

64 71

Geotraverse

’ 17

Creutzburg Van

Herzen

this

work

Haenel Haenel

(1964)

(1974) (1974)

et al. (1974)

val 280-520 m. Corrections have been applied to the gradient, based on the procedure given in Kappelmeyer and Haenel, 1974, pp. 89-107, to account uplift/erosion and past climatic changes. for the effect of topography, The Guspisbach shaft penetrates over its entire length the nearly vertically dipping Gamsboden granite--gneiss of the Gotthard Massif, a coarse-grained, homogeneous muscovite-biotite gneiss with potassic feldspar and quartz porphyroblasts. The average grain size of this rock is about 2 cm. Thermalconductivity determinations have been performed on correspondingly large, cylindrical samples (10 cm diameter, 3 cm thickness) using the single disc absolute determination technique (see e.g. Kappelmeyer and Haenel, 1974, 11. 127). The equipment has been specially designed for this study and is characterized by electronic compensation of the temperature difference between the sample heater and guard heater (Berset, in preparation). The mean thermal conductivity of twelve samples from the depth interval 310-510 m is 2.73 W/m.K” (6.5 mcal/cm.sec,‘C). Anisotropy is, as already demonstrated by Clark and Niblett (1956), negligible. The heat flow at the Guspisbach site is (71 I 17)mW/m’ or (1.7 c 0.4)HFU (Table I). THERMAL

EFFECTS

OF OVERTHRUSTING

Besides correcting for the above-mentioned effects another time-dependent disturbance of the temperature gradient must be considered: the influence of mass movement due to tectonic processes. Overthrusting is especially evident in the central portion of the Swiss Geotraverse. Oxburgh and Turcotte (1974) investigated the thermal effects of overthrusting by means of a rather simple model, namely by following the temperature equilibration after an instantaneous emplacement of the thrust sheet. Since overthrusting happened in the Swiss Alps at speeds of a few cm/year at best (Triimpy, 1972) a more realistic model is presented here: continuous overthrusting with simultaneous erosion at a constant rate (see the model sketch in the lower left corner of Fig. 2). The surface is kept at a constant temperature of 0°C for all times. The calculation procedure (a finite-difference method) is described in a paper by Kahle and Werner (1975). The results of numerical calculations for a thrust model, suited to the geologic constraints characteristic of the central portion of the Swiss Geotraverse (start of overthrusting: 50 m.y. ago, duration: 10 m.y., thrust sheet thickness: 15 km) are displayed in Fig. 2. They indicate that after 20 m.y. steady-state conditions are attained in the basement. Thus today, in measuring the temperature gradient in the basement rocks of the Swiss Central Massifs, the effects of overthrusting can be neglected. VARIATION

OF HEAT PRODUCTION

The distribution in the continental

WITH DEPTH

of radioactive heat sources governs the temperature field crust. Direct measurements of heat production can be per-

TEMPERATURE -

THRUST --

I-

MODEL

km

km

Fig. 2. Thermal effects of overthrusting in the Central Alps, Switzerland. Thrust model (lower left corner) accounts for continuous overthrusting with simultaneous, constantrate erosion. Curves indicate temperature equilibration with time (for input parameters see text). After 20 m.y. steady-state conditions are attained in the basement.

TABLE

II

Heat production

in characteristic

Rock type

rock types of the Swiss Alps

Width of variation (HCU)

Pegmatites Granites Gneisses Metamorphic Quartzites Carbonates Amphibolites Ultrabasites

schists

11.0-14.0 4.5-14.5 2.0-12.0 0.65.8 0.81.0 0.07-2.2 0.22.5 O.C’l-9.05

--

_

Mean value (HCU)

(fiW/m3 )

12.7 7.6 5.8 3.6 0.9 0.8 0.8 0.03

5.3 3.2 2.4 1.5 0.38 0.33 0.33 0.0012

119

formed on surface samples only. Gamma-ray spectrometry is the most frequently used technique for such determinations; it enables the simultaneous determination of U, Th and K, the relevant heat-producing radioelements. Table II lists average heat-production figures for characteristic surface-rock types from the Swiss Alps. Nondestructive gamma-ray spectrometric measurements have been performed for this study on the same cylindric rock samples that were used for thermal-conductivity determinations. A mean heat-production value of 4.2 pW/m” (= 10.0 HGU) was found for the Gamsboden granite-gneiss at the Guspisbach heat-flow site. It is easy to show that the heat production in the continental crust decreases with depth. Direct information about the exact manner of this decrease is, however, very scanty. The existing data (Lachenbruch and Bunker, 1971; Swanberg, 1972; Rybach, 1973; Hawkesworth, 1974) point towards a decrease of the surface heat production A(0) with depth z according to the exponential law A(z) = A, exp(-z/11). The scale factor H of the decrease is of the order of a few kilometers; its value varies from region to region. The geochemical processes that lead to the exponential distribution of the heat-producing radioisotopes in the continental crust are by no means clear though there are different hypotheses (Turcotte and Oxburgh, 1972; Rybach and Labhart, 1973; Albarede, 1975). Even for a given area with wellknown petrologic structure and geologic history it is hardly possible to postulate a reasonable figure for H and thus for the vertical distribution of heat production. By means of an empirical relationship between seismic compressionalwave velocity VP and heat production A (Rybach, 1973) the vertical distribution A(z) can be estimated in a given region if the velocity structure V,(z) is known. Figure 3 shows the measured velocity distribution V,(z) at the SEISMIC 4.

VELOCITY 5.

I V(i)

6

VP 7

HEAT 6

kdsec

PRODUCTION

A

0

--.. \

L=J,k,,,__DEPTH A

.

1

5&,--

..J

DEPTH B

Fig. 3. Vertical distribution of heat production (B), deduced from the measured seismicvelocity distribution (A) for the Biaschina heat-flow site; data from Giese et al. (1973). The surface value V(0) has been obtained from a short-range refraction profile shot in 1973. The constraints given by surface heat flow and mantle heat flow (see text) point towards an intermediate heat production (dotted line in Fig. 3B) for the rocks occupying the inversion zone.

120

Biaschina heat-flow site (data from Giese et al., 1973) along with the heat production distribution A(z) as determined by using the empirical curve A(V,) given in Rybach (1973). There is evidence that the A( V,,) relationship is not valid in the zone of lowered seismic velocities and densities (= inversion zone), Firstly there are no geochemic-petrologic arguments in favour of such pronounced gradients regarding the radioelement concentrations as e.g. indicated by the peak of the heat production curve in Fig. 3B: a radioelement distribution pattern of this kind is not likely to prevail in the p/T regime of the crust. Another line of evidence is given by the boundary conditions represented by the surface heat flow q(0) and the mantle flow qm. For the latter, something like 25 mW/m* (= 0.6 HFU) is a reasonable figure (Clark and Ringwood, 1964). Calculating q, for a given q(0) value with the local A(z) distribution according to: 4,

= q(0) -

j” A(z)

dz

0 where zM is the Moho depth, unreasonably low qm values result for crustal sections with pronounced inversion zones: e.g. 7.5 mW/m* (= 0.18 HFU) at the Guspisbach site and 6.7 mW/m2 (= 0.16 HFU) at the Biaschina site. Thus an average value for the heat production, as indicated by the dotted line in Fig. 3B, was taken for the depth interval of the inversion zone. The corresponding q,-values for the different sites, calculated for the modified A(z) distributions, are in the order of 29 mW/m* ( = 0.7 HFU). For individual figures see Fig. 4 (bottom). These modified patterns of heat-production distribution were used as input parameters for the calculation of the temperature field. TEMPERATURE DYNAMICS

FIELD

AND

ITS

INTERRELATIONS

WITH

STRUCTURE

AND

OF THE CRUST

For all heat-flow sites the vertical distribution of heat production has been determined from the measured seismic velocities (data from Edel et al., 1975; Giese et al., 1973 and Choudhury et al., 1971) and then modified as described above to match the constraints represented by the surface heat distriflow q(0) and the mantle heat flow qm. With these heat-production butions temperature-depth curves have been calculated by assuming steadystate conditions and by accounting for the temperature dependence of thermal conductivity which may be particularly significant for rocks in the upper crust (see Cermak, 1975). The heat-production curves (e.g. Fig. 3B) were approximated by exponential or step functions. For the uppermost mantle a constant value of thermal conductivity K, = 1.9 W/m.“K (4.5 mcal/cm.sec.“C), which is more or less a characteristic figure, was inserted in the calculations.

121

The isotherms determined from the individual Tfzf curves are displayed in Fig, 4 (bottom). They are downwarped in that segment of the Geotraverse where a pronounced inversion zone in the upper crust is present in the scismic profile. This same segment again coincides with the zone of the most pronounced recent crustal movements (vertical uplift z 1 mm/y) as indicated by the pattern of uplift and/or subsidence along the Grotraverse (upper part of Fig. 4). The coincidence of the thermal low with the zone of velocity (and density) inversion on the one hand, and of the inversion zone with the zone of strongest crustal movements on the other is remarkable. The MohoroviciC disc~~nt~nu~ty along the Swiss Geotraverse is also plotted in the bottom part of Fig. 4. Its shape was determined by seismic measurcment.s (thick bars = wide an&e reflections) and its retief has been confirmed by model calculations to fit the Bouguer anomalies observed along the profile (Kahle et at., 1976). Temperatures at the Moho (TNI) range from 600 to 900°C. The Moho is clearly not an isothermal surface, its asymmetry is very likely to arise from the early Alpine subduction tectonics. There is now increasing evidence from the Eastern Alps for a southward dipping suhduct,ion plane (Hawkesworth et al., 1975; Bicklc et al., 1975). The shape of the Moho as shown in Fig. 4 (bottom) is a further argument for this.

It has been demonstrated that a striking correlation exists between the temperature field, the crustal dynamics (with respect to recent vertical movements) and the crustal structure (with respect to petrophysical parameters like density or seismic velocity) in the Central Alps, Switzerland. The thermal effects of the Alpine orogeny are negligible today, the asymmetric shape of the Moho along the Swiss Geotraverse is most probably attributed to the southward subducting lithosphere during the early Alpine tectonic phase. The zone of lowered seismic velocities and densities in the upper crust seems to play a central role in the interplay of the temperature field, the crustal structure and dynamics. ft has been suggested by Giese (1968, 1970) that partially m&en material of granitie composition is present in these zones. We present now two arguments against this hypothesis: (I) The material cannot be partially moften at temperatures which are clearly between 400 and 500°C in the corresponding depth range (see Fig. ‘I). (2) The material cannot be of granitic composition: this would imply heat production figures around 3 ,uW/m” (= 7 HGIJ) whereas the constraints represented by the measured surface heat flow q(0) and the mantle heat flow qn, call, as discussed above, for a value of about 1 yW/m” (= 2.4 HGU) and thus indicating intermediate chemical composition. The temperature range of 400~500°C in the depth intervaI of 15-25 km and the corresponding Lithostatic pressure of 4-7 kbar define a field in the facies diagram of

Recent

3

- 1.0 1

5

__~

Subsidence

-

_7_

V

50 1 (after

Lurern

Geology

SWISS

Movements

y

Vertical uplift

-- -- ~-.-.-

Crustal

,,,~~.

4

~___

o-L +,\--

-;

+1.0--i mm/y

2

:I::;:jrpqzg--q

Okm

V

Base1

NW 19751

100 1

_L

6

._

7

9

._ (-)

---m-I-+I.0

8

/TZj to

0 km

SE

150 km ~1 I*lttJ m] 11

V

Chiasso

JEANRICHARD, 1972 ; SCHAER & JEANRICHARD, 1974)

V

Gott hard

(TRUNPY,

GEOTRAVERSE

Field

----___----

~a~~,~~-,:r-:-_~~,,l~~~~~~~-

---cc

---0;-

/

‘Y

---

Biaschina

-----------

__s

25OT--

,.-

~6.0 kmlsec

---

9

---

mtn.

L. di Corn0

_~ ----_ Vp ---~,_6.2 __- --

-I_“=“I

___5&)‘C_-.35_--_--.

,o

-

30

20

10

0

10 .20

0

-‘c----..0.6

Moho (after KAHLE et a1J976)

Fig. 4. Characteristics of the Swiss Geotraverse. Top: Geology. - Molasse (1) and flysch (2) sediments, Mesozoic sediments of the HP]vetic units (3), of the Southern Alps (4) and of the Penninic units (5); Jurassic and other autochthonous mesozoic sediments (6); crystalline basement, including the Central Massifs (7); crystalline rocks intensively deformed by the Alpine tectonics, including the basement rocks of the Southern Alps (8); ultramafitites, Ivrea zone (9); Upper Palaeozoic vofcanics (10). The pattern of recent crustal movements indicates the maximum uplift in a region clearly displaced to the south of the crest of the Alps (Gotthard). In the seismic section, represented by lines of equal compressional-wave velocities, lhe pronounced zone of lowered seismic velocities (shaded area} is evident. In the same section of the traverse the isotherms (bottom) are downwarped. It may be noted that the temperature field in the north passes over t.o the Rhinegraben area which is marked by a deep-seated thermal anomaly (Werner, 1975). The shape c)f the Moho, determined by seismic and gravimctric methods (thick bars = wide angle reflections) is highfy asymmetric and could be attributed to subduction tectonics during the early Alpine phase.

Temperature

5Okm

!

750&X---_____

_$qOT----------------______

250~~----.-----------___

4

Guspisbach Beckenried Surface heat flow:160 HFU/67.mW/m2

---..____--

_

40

50

------------

__~_J----_------

-----------

__-------~-_____

_----zr____v_?--

50. km-----.--lOoO~-----_-___

---

km

----__.._

30

40

--_-_--___

---

-i-iT-_--_ 40 0!9

30

20

10

0

Buggingen 1.70/71.

---___

-._-___-I-

20

10

0

121

metamorphism (see e.g. Winkler, 1967, p. 180) which is located clearly in the greenschist facies. This agrees well with the intermediate chemical composition as indicated by the heat production. Thus melanocratic gneisses, subjected to recent metamorphism, are more probable constituents of the inversion zone. Dehydration reactions (e.g. break-down of hydrous minerals) are most likely to occur which may explain the lowered seismic velocities. ACKNOWLEDGEMENT

The authors appreciate the continued encouragement of Professor Ernst Niggli (Bern) to carry out these geophysical studies along the Swiss Geotraverse. They are indebted to Professor R. Triimpy (Ziirich) for providing the detailed geologic profile, and to Dr. R. Haenel (Hannover) and Dr. P.C. England (Oxford) for stimulating discussions. Dipl. Ing. W. Rutschmann (Adliswil) generously permitted the firing of borehole shots in the deep Biaschina drillhole for seismic-refraction measurements, which were subsequently evaluated by Dr. H. Scriba (Ziirich). Thanks also go to Miss B. Winkler and Messrs. W. Finger and H.P. Weber for field and laboratory assistance. The work reported in this paper has been supported by the Swiss National Science Foundation, Project No. 2.230.-0.74. REFERENCES Albarede, F., 1975. The heat flow/heat generation relationship: An interaction model ol fluids with cooling intrusions. Earth Planet. Sci. Lctt., 27: 73-78. Bickle, M.J., Hawkesworth, C.J. and England, P.C., 1975. A preliminary thermal model for regional metamorphism in the Eastern Alps. Earth Planet. Sci. Lctt., 26: 13-2X. Cermak, V., 1975. Temperature-depth profiles in Czechoslovakia and some adjacent areas derived from heat-flow measurements, deep seismic sounding and otller grophysical data. Tectonophysics, 26: 103-l 19. Choudhury, M., Giese, P. and de Visintini, G., 1971. Crustal structure of the Alps: Some general features from explosion seismology. Boll. Geofis. Teor. Appl., 13: 211-210. Clark, S.P. and JHger, E., 1969. Denudation rate in the Alps from grochronologic and heat flow data. Am. J. Sci., 267: 111?-1160. Clark. S.P. and Niblett, E.R., 1956. Terrestrial heat flow in the Swiss Alps. Mon. Not. R. Astron. Sot. Geophys. Suppl., 7: 176-195. A.E., 196.1. Density distribution and constitution of the Clark, S.P. and Ringwood, mantle. Rev. Geophys., 2: 35-88. Creut.zburg, H., 1964. Untersuchungen iibrr den Warmestrom der Erde in Wcstdcutschland. Kali Steinsalz, 4: 73-108. Den Tcx, E., 1975. Thermally mantled gneiss domes: The case for convective heat flow in more or less solid erogenic basement. In: G.J. Borradailc, A.R. Ritscma, H.E. Rondrel. D.J. Simon (editors), Progress in Geodynamics. North-Holland, Amsterdam, pp. 62-79. Edel, J.B., Fuchs, K., Gclbkc, C. and Prodehl, C., 1975. Deep strurt.urr of the Southern Rhinegraben area from seismic refraction investigations. J. Gtxophys., 11 : 333-356. Frey, M., Hunziker, J.C., Frank, W., Bocquct, J., Dal Piaz, G.V., JAgt’r. E. and Niggli, E., 1974. Alpine metamorhism OI the Alps. A review. Schweiz. Mineral. Prtrogr. Mitt., 54: 217-290.

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