Shear-wave evidence for an anisotropic lower crust beneath the Black Forest, southwest Germany

Tectonophysics,

173 (1990) 483-493

Eisevier Science Publishers

Shear-wave

4x3

B.V.. Amsterdam

- Printed

in The Netherlands

evidence for an anisotropic the Black Forest, southwest

lower crust beneath Germany

E. LfJSCHEN, B. NOLTE and K. FUCHS Geophysical Institute,

lJniversi@

(Received

of Karlsruhe,

October

16, D- 7500 Kurlsruhe 21 (F. R. G. )

Hertzstrasse

31.1988;

accepted

February

28,1989)

Abstract Liischen.

E., No&e, B. and Fuchs,

Forest,

southwest

Seismic Probing Controlled explore

and their Margins.

sources

(a horizontal

of the reflective

differing

A laminated

near- and far-offset

may be resolved.

of preferentially

same depth

crustal S-response.

This seismic

evidence

model

orientated

model

By introducing

regions

ahernating

(and changing anisotropic

with a petrological

Observed

amplitude

with a changing

Introduction

differences

Poisson’s

the eastern shoulder of the in southwest Germany, has

been the site of extensive

geoscientific

exploration

crustal KORP

The data by previous

Poisson’s

and isotropic

model

of deformed

wide-angle

ratio)

cannot

lamellae,

(KTB).

The

wide-angle profiles (Ltischen et al., 1987; Gajewski and Prodehl, 1987). The relatively young and thin (25-27 km) crust is subdivided into a mostly transparent upper crust with few discrete reflections (its lower part coincides with a P-wave lowvelocity zone) and a strongly reflective lower crust starting at about 5 s two-way traveltime (corresponding to 14 km depth) and extending to the crust-mantle boundary at 8.5 s (26 km). The ~40-1951/~/$03.50

0 1990 Eisevier Science Publishers

B.V.

explain

in the

this discrqancy containing

in P- and S-wave reflections

lower

crust,

known

fra,m the

from

programs such as COCORP, and ECORS is not readily its nature

,zontinental BIRPS, DEaccessible by

drilling,

and

troversy.

More light may be shed on this issue by

is still a matter

tive for innovative

Program

S-wave

surieys

amphibolites

Continental

Drilling

studies to

strong

additional geophysical constraints mentary experiments. This provides

Deep

(Editors).

reflection reveal

of the lower crust since 1984. These investigations started as reconnaissance studies for the German uplifted and exposed crystalline basement of Hercynian age is covered with a network of deep reflection profiles and coincident refraction and

the Black

ratio.

laminated

The Black Forest, Tertiary Rift system

Germany.

were not observed

layers

beneath

and B.L.N. Kennett

were used in near-vertical

in southwest

S-reflections

with isotropic

hornblende.

in the lower crust suggest

crust

lower crust

J.C. Dooley

173: 483-493.

and dynamite)

lower

is consistent

for an anisotropic

C. Wright.

Tectonqdzysics.

vibrator

laminated

from the lower crust. These lower crustai

the same region.

lo-30%

K., 1990. Shear-wave

In: J.H. Leven, D.M. Finlayson,

of Continents

shear-wave

the nature

reflections

Germany.

of con-

from complea great incen-

investigations.

From laboratory measurements under high P and T conditions, it is known that shear-wave (S) and compressional-wave (P) velocities react differently to variations in composition. Hence Poisson’s ratio provides a sensitive parameter for compositional interpretations (Kern, 1982). S-wave arrivals from the crust-mantle boundary and from the top of the lower crust, seen in the wide-angle refraction profiles in the Black Forest, encouraged Holbrook et al. (1988) to map the crustal Poisson’s ratio and to interpret this parameter petrologitally. Sandmeier and Wenzel (1990) showed by

484

modelling

the wide-angle

P- and S-wave data that

reflection

network

the laminar

lower crust is also the site of a strongly

lower crustal

alternating

Poisson’s

strong

deduced

from

reflections strong

ratio.

the absence

from

are dominant

crust

by a high content

(Kern,

1982).

has been wide-angle

in contrast

If compositional

then a low Poisson’s

caused

effects

ratio must

of the mineral

to

(Liischen during

November

1987.

A horizontal

be

study aims to extend P- and S-waves

range.

For example,

a 3-week

vibrator

time as an energy

32-fold and CDP-geometry

to the near-vertical

coverage.

not ex-

Recording

for the first

for deep crustal stacking

in

Hz, length

the experiment

vertical

the was

campaign

lo-40

was tested

source

enhancement,

signed to include

where

was particularly

field

(sweep

20 s, with force control)

quartz

the compari-

one would

7) at points signature

et al., 1987). The experiment

conducted

For signal

The present sons between

model

of S-wave

the lower

P-wave reflections.

incidence

This

(Fig.

laminated

work. was de-

rates of 16- and

with maximum

40-fold

was done with a fixed spread

pect significant S-wave arrivals if current velocity models based on wide-angle data are valid.

consisting of 40 three-component stations (horizontal geophone strings for radial component = X.

The experiment

strings

for component

phones

and 80 m length;

transverse

The shear-wave 3-5

km long

experiment

sectors

sited

the existing

PI-

= Y,

vertical

V = Z; strings natural

geophone of 12 geo-

frequency

10 Hz)

with a spacing of 80 m. A separate cable was depIoyed for each component. The instrumenta-

was located on three along

component

tL90

KAALSRUHE.

/

km

Fig. 1. Location map with the reflection (dashed lines) and refraction (solid lines with shotpoints) profiling network after Liischen et al. (1987) and Gajewski and Prodehl(l987)

in the Black Forest (right panel). The lower lefthand panel shows location of three l&es

of the shear-wave experiment relative to the former P-wave reflection lines (blow-up of the central part of the righthand map). Figures 2-4 are from line 3 with the shotpoint marked by a single dot.

SHEAR-WAVE

EVIDENCE

tion consisted with

stacker.

and recorded

On line

ANISOTROPIC

of a 120-channel

a built-in

stacked

FOR

3 (Fig.

LOWER

TI-DFS

The

data

offset

of three

Originally,

this was to test the three-hole

parallel

lines

charged

with 5

recorded create

x 8

Edelmann,

shots

holes

technique

(spacing

The central

inhomogeneity.

by a dot). 1985). Three

kg of explosive,

independently.

a lateral

the two polarized

(marked

of 5 m deep

5 m),

were fired and shot should

It was hoped

outer shots would produce shear-wave events (SH-waves)

higher S-wave signal/noise

dard

that

inversely so that a

ratio (in this case S/P

amplitude ratio) could be produced by subtraction. The inverse polarity method failed, probably due to improperly chosen parameters such as spacing and depth of holes, and size of charges, which require experimental optimization. Never-

485

FOREST

P-reflection

cording ously:

15 km from the nearest

to the southwest method;

BLACK

time

features observations

geophone

(or Camouflet

S-fold

uncorrelated. by recordings

fired in a quarry,

BENEATH

V system

were

1) the Vibroseis

were complemented

CRUST

surveys

which

were

P-wave

already

first arrivals

P-reflective

lower crust

Additionally,

strong

wave are visible to their

(vertical

e.g. 12 s). This

S/P

component, frame

well known (direct

previ-

bottom

from

M,,).

the direct

(S,). They can be identified traveltime

1.73 (43), which

ratio

corresponds

the same region

Sdue

of approximately to a Poisson’s

of 0.25. This ratio has been previously crust (Liischen

the

wave, P,), the

(top TLC,

arrivals

re-

shows

ratio

observed

in

as an average value for the whole et al., 1987; Holbrook

et al., 1988).

When all three components of the seismic motion are recorded for 20 s, strong energy to 17 s TWT is recognized

on the horizontal

(Fig. 2). P- and S-wave arrivals

components

can be separated

by an

appropriate bandpass filter. The ratio of predominant frequencies of P- and S-waves is cf the order of 2. Figure 3 is a high-frequency version of Fig. 2. The shear-wave

energy

on the vertical

#component

theless, the three single shots produced remarkable-quality S-waves. The following considerations

has mostly disappeared. The two horizontal components still show considerable energy to more

are based

than

on these single

may be generated

shots. The shear waves

by conversion

of P-waves

free surface (Fertig, 1984) or at shallow tinuities below the source.

at the discon-

The Vibroseis observations were of poorer quality. Only one of the three lines (line 3) recorded significant S-wave energy returned from the lower crust. On this line, the source-receiver offset had been changed. On lines 1 and 2 the vibrator

16 s TWT.

Most

of this energy

spond to converted energy (P to S) and. to a lesser extent, to the high frequencies of the S-waves. Figure 4 is the low-frequency version of Fig. 2. The vertical component is plotted on the right with a stretched time scale (ratio of time scales is 1.73) to enable direct visual correlation between Pand S-wave reflections from the lower crust. At a little over 16 s TWT (about 1.73 times the

walked through the fixed geophone spread using the same stations. On line 3 the vibrator points were offset to the northeast by about 4 to 7 km

P-traveltime

from

and Mss there is another M,, about 13 s which might correspond

the nearest

geophone

of the stationary

re-

cording spread. The processing of this data set is not yet complete and future studies will include analysis of polarization. The energy source, only one vibrator compared to five vibrators used in standard deep P-wave reflection profiling, was too weak for deep crustal targets. Figure 2 displays the original data set (no automatic gain control, compensated for geometrical spreading) obtained from one of the dynamite shots observed within the offset range from 15 to 18 km. The dashed frame marks that part of the data field which was available in previous stan-

may corre-

from the crust-mantle

energy stops abruptly tions from the same

boundary),

the

suggesting S-wave reflecboundary (MS,). Between ener,:y step at to 2. converted

phase from the crust-mantle boundary (MpS). Reflected S-waves can be expected between 10.5 and 17 s TWT, assuming that they have similar ray paths to the P-waves. P to S converted reflections from the lower crust can be found between 8 and 13 s. The record field between 13 and 17 s therefore contains predomin~tly (low-frequency) S-wave reflections. We were surprised by the following observations: (1) An explosive point source located within

4X6

SINGLE

SHOT

COMPONENTS

X 15 t

2

[km]

BRNDPRSS

X Y Z

l-5-45-49

z

Y 18

________

r---

1

_ ____

-_.--

Pg _-.

4

sg

TLC -.

; I

I

8

--

I

-.-

I

-

I

---4

16

18

Fig. 2. Original The marked factor

increasing

further

record

of a dynamite

events are explained

processing

linearly

from 1 to 2 in order

was applied.

P-wave reflection

shot (the centre

studies.

bole), line 3, with horizontal

in the text. The amptitudes to compensate

The frame on the vertical Note that strong

between

for geometrical

component

components

3 and 17 s of the horizontal marks

seismic energy is present

divergence the data

(X, Y) and vertical components

(Z component:

component

were multipfied between

field which was available

3 and 10 s). No

in previous

down to more than 16 s on the horizontal

(2). with a

standard

components.

SHEAR-WAVE

EVIDENCE

FOR ANISOTROPIC

SINGLE SHOT X 15 2

[km]

LOWER

CRUST

BENEATH

BLACK

FORES-l

COMPONENTS X Y Z

BRNDPFtSS 17-21-45-49

Y

Z

18

_ F

6

8 _

_ M

-

16

-

-

18

Fig. 3. High-frequency

version

of Fig. 2, showing

mainly

P-waves

and converted

waves on all components

4x2(

SINGLE

SHOT

COMPONENTS X Y ;!

X 15

P-----A8

BFlNDPRSS 1-5-1

5-19

BRNDPRSS

17-2 l-45--49 Z

Z

Y

[km1

6

8

16 E

18

:s1 Fig. 4. Low-frequency component

version

of Fig. 2, showing

(Z) in the right panel is stretched

mainly

S-waves

on the X and

by a factor of 1.73 according

compact gneisses was not expected to generate such strong S-energy, especially in the near-vertical range. Obviously, conversion of P-waves to S-waves at the surface or at shallow discontinui-

Y components.

to the mean crustal

The time scale for the vertical value of ~3 for the ratio VP/v,.

ties is very efficient in this case. This idea is supported by strong amplitudes on the horizontal components following the weak direct P-wave (Ps), delayed by about 100 ms. They may correspond to

SHEAR-WAVE

EVlDENCE

conversions

FOR

ANISOTROPIC

of the P-wave, coming

the base of the weathering cally

mapped

at depths

region (Ltischen

wide-angle

BENEATH

from below, at

layer, which is seismiof 50 to 300 m in this

data

from the lower crust were the structural and comdetermined

(Sandmeier

and

from the

Wenzel,

in the next section

1990).

is focused

the

4x9

in terms

of purely

by an alternation of

mafic

between

constituents

compositional

within

et al., 1988;

the

Sandmeier

lower and

crust

Wenzel,

1990). However, present

this model

observations

near-vertical

is inconsistert

with

and fails to predic:

the

the strong

S-wave response.

How do we modify

the previous

seismic

ani-

present

approach

sotropy

into

two

following

dif-

investigations of lower crustal rocks provide evidence for seismic anisotropy caused by a preferred

wavefields,

the

P- and S-waves should

be noted.

introduce

Our

be-

the

is to

model?

agreement

between

numerical

models.

(7) The strong PP-reflector TLC does not correspond to an S-reflector of similar intensity. This is

orientation

indicative

al., 1989). As a starting model anisotropy to the high-velocity

of a change in physical

properties

which

might correspond to changing fluid content in a porous medium (analogous to the ~~/~-indicator for pore fluid and lithology prospecting: Differences

effects

high and low amounts

there is a remarkable

Although ferences

FOREST

plained

on

this key observation. tween

BLACK

(Holbrook

models previously

The modelling

(‘RIJST

et al., 1987).

(2) The S-reflections unexpected, considering positional

LOWER

used in oil- and gas-

e.g. Danbom and Domenico, 1986). in resolution can be ruled out, since P-

and S-wavelengths

(according

sis) show considerable

to frequency

analy-

overlap.

(2) The SS-reflection (at low frequencies) from the crust-mantle boundary (Mss) appears as strong (or even stronger relative to earlier reflections) as the corresponding

PP-reflection.

(3) The contrast between upper and lower crust seen in P-sections is not pronounced in the Swavefield. Superposition of converted reflections from the lower crust must be partly responsible for this effect. On the other hand, it seems that the upper crust is more reflective for S-waves seen on the horizontal components Among many questions evolve from Figs. 2-4, the why S-reflections from the

than for P-waves. and observations which most important one is lower crust are stronger

than expected. Isotropic versus anisotropic modeiling Two important discrepancies between observations have been stated. Previous wide-angle surveys show significant P-wave but no S-wave energy reflected from the lower crust. A laminated structure can explain these observations using a model with vertical alternations in Poisson’s ratio. This may be ex-

and

of minerals

metamorphism

P-velocity

1982;

model of Sandmeier

fig. 3a) (average high-

due to crustal

(Kern,

thickness

and low-velocity

Laboratory

deformation

Siegesmund

and Wenzef (1990,

120 m, total number

lamellae

et

we ascribed the lamellae of the

80, depth

of

15-24

km), and keep the rest of the model isotropic. The model is based on the work of Siegesnund et al. (1989) using crystallographic tory seismic measurements bolite rock samples from

(texture) and laboraon deformed amphithe Ivrea Zone. The

seismic anisotropy is produced by the preferred orientation of the mineral hornblende which is expected to be common at lower crustal levels. In all (P-) high-velocity lamellae the fast quasi-P-axis of the monoclinic hornblende is orientated horizontally (in the profile direction). The X-_; plane is the symmetry plane, so that the S-waves separate

into a pure

(e.g. Crampin,

SH-wave

and a qua:G-SV-wave

1981).

The amount of preferentially orientated hornblende varies in the model between 111 and 30% from top sponding the solid x--z plane S-waves.

to bottom of the lower crust, correto an anisotropy from 4 to 8%. In Fig. 5. lines show the velocity surfaces in the of such an anisotropic layer for P- and The dashed lines represent t7e velocity

surfaces of the adjoining isotropic layer which has a lower P-velocity, but a higher S-veiocity. The S-velocity contrast is small for oblique rays but becomes effective for the near-vertical incidence (for both SH- and quasi-SV-waves). With this model all the synthetic calculations were repeated for P- and S-waves and for the

PHASE VELOCITYIN KM/SEC

PHASE VELOCITYIN KM/SEC 4.0 , . r

-

3.8 --

.. -. .- _

/’

--I 7.0

7.4

-.

‘-.3&fI_ 4.0

‘\ ..

__/‘. 1

Fig. 5. Phase velocity surfaces of an anisotropic layer presented in the x-z plane for the quasi-P-waves (QP, left panel) and for the two Q&waves (right panel). The dashed lines show the corresponding velocities of the adjoining isotropic layer.

wide-angle range as well as for the near-vertical incidence range using a modified version of the reflectivity program (Fuchs and Mtiller, 1971) which includes anisotropic modelling (Nolte, 1988). In the wide-angle range there is no difference in the amplitude response between the isotropic and the anisotropic case for both P-waves and S-waves, the latter showing in both cases no lower crustal response (Fig. 6, A and B). Anisotropy is therefore compatible with the compositional model (Sandmeier and Wenzel, 1990). The same is true for P-waves in the near-vertical range. The effects of anisotropy become significant in the near-vertical range for S-waves (Fig. 6, C and D). The anisotropic model fits the observed data except in the upper crustal region where the true reflectivity appears stronger than that of our model. A and B of Fig. 6 display the radial components of the wide-angle field for the isotropic and the anisotropic case. The amplitude characteristics of the transverse component (not shown here) are identical to the radial one. However, the S-wave reflected from the crust-mantle boundary shows a maximum traveltime difference of 0.3 s at large offsets on the two horizontal components. This is due to S-wave splitting. It cannot be identified in

the observed wide-angle seismic data, because the slower S-wave phase may be masked by the faster one and by other interfering conversions and multiples produced, e.g. in the surface low-velocity (weathering) layer (K.-J. Sandmeier, pers. commun., 1988). . Dm4msion and c&usions

S-wave experiments in the central part of the Hercynian (Variscan) mountain range were performed in the near-vertical incidence range, complementing previously existing wide-angle data. Anisotropic reflectivity modeIling suggests that the observed P- and S-reflection patterns and Pand S-wave velocities of the lower crust can be explained by a combination of composition effects (Holbrook et al., 1988; Sandmeier and Wenzel, 1990) and seismic anisotropy. The seismic anisotropy can be explained in a very natural way by the texture of the mineral hornblende in a deformed amphibolite, which is a common constituent of the lower crust (e.g. Siegesmund et al., 1989). There is no need to introduce other causes for seismic anisotropy, such as periodic thin layers, aligned cracks and microcracks and concentrations of stress (Crampin, 2987), although these

SHEAR-WAVE

EVIDENCE

FOR

ANISOTROPIC

LOWER

CRUST

BENEATH

BLACK

FOREST

,

. i-

=GL--.-.---u, ---------__--

_---_c_--

-.--_.

.-_-__

-_, _ --..

I

---_-_-L--_-A

_._____-

I I

1

7

I-

~~_._. _~~___ -~---_ ___..

-_-_I_

-1

___ _-_c____. t----___&-_. --__t

-

---

W---p

::I: :M ---

-__.-q

----~___*L ---

--

- --+

---

-

-

---I

---

1

-_..__

-y---_-

------77----L -------f

.

. .-_-_

.._--__r

-..__

--

-.-

--

t

__.__~___

.-“I

-..--

3 \--

(33s) ZQ'P/U

- ,I,

__ _-

causes cannot

be explicitly

Experimental Siegesmund the influence that

dominated

even

(Kern,

velocity

by preferred

conditions

anisotropy

anisotropy

1982;

that (under

at low-pressure

of crack-induced

The anisotropic

vanishes

in amphibolites

orientation

direction

(approximately

20°,

see

experimental

Fig.

is

survey

1). According

to

data of Siegesmund

seismic

directions.

data

Modelling

the same anisotropic the profile significant

presented azimuthal

are not yet available of 90°-rotated

in other

profiles

with

model shows differences

from

in this study, suggesting a dependence in the ampli-

tudes of the two horizontal components. Therefore similar experiments with varying azimuths are required in the future. The following conclusions were reached. (1) Wide-angle observations of P- and S-wave energy reveal distinct differences, especially in the lower crust. These differences changes in the Poisson’s ratio.

are

caused

by

vertical

from the lower crust at nearmodelling

of the lower

with a preferred orientation of the mineral blende can resolve the discrepancy between angle and near-vertical S-wave observations.

technical

Edelmann, K.-J.

contribu-

P. Hubrai.

Sandmeier

L.

and

F.

Wenzel for helpful discussions and corrections of the manuscript. Thanks for constructive remarks and

suggested

improvements

are due to the reviewers Geophysical

to the

manuscript

and the editor, Jim Leven.

Institute

Contribution

No. 392.

References

Crampin, S., 1981. A review of wave motion in anisotropic

crust hornwide-

This “case history” emphasises the great potential of S-waves for crustal studies. Their innovative potential can be exploited by combinations of compressional and shear wave evaluation and wide-angle and near-vertical configurations of field experiments.

Crampin,

S., 1987. Geological

extensive-dilatancy Danbom,

S.H. and Domemco,

Exploration.

H.A.K.,

implications

Development

Geophysicists,

Series,

Vol.

1.

Tulsa, Gkla., 275 pp.

1985. Shear-wave

energy sources.

In: G.

Dohr (Editor), Seismic Shear Waves, Part B. Applications. Geophysical

Press, London,

pp. 134-177.

Fertig, J., 1984. Shear waves by an explosive

point-source:

earth surface as a generator of converted

the

P-S waves. Geo-

phys. Prospect., 32: 1-17. Fuchs,

K. and Mtiller, G., 1971. Computation with the reflectivity

with observations. Gajewski,

of synthetic

method and comparison

Geophys. J. R. Astron. Sot., 23: 417-433.

D. and Prodehl, C., 1987. Seismic refraction investi-

gation of the Black Forest. In: R. Freeman and St. Mueller (Editors), The European Geotraverse,

Part 3. Tectonophys-

its, 142: 27-48. W.S., Gajewski,

D., Krammer, A. and Prodehl, C.,

1988. An interpretation

of wide-angle

compressional

shear wave data in southwest Germany: petrological

implications.

J.

and

Poisson’s ratio and

Geophys.

Res.,

93(BlO):

12081-12106. Kern, H., 1982. P- and S-wave velocities in crustal and mantle rocks

under

the simultaneous

pressure and high temperature microstructure. Researches

action

In: W. Schreyer

in Geoscience.

of high confining

and the effect of the rock (Editor),

High-Pressure

Schweizerbartsche

Verlagsbuch-

handlung, Stuttgart, pp. 15-45. Lttschen, E., Wenzel,

F., Sandmeier,

K.-J., Menges, D., Riihl,

Th., Stiller, M., Janoth, W., Keller, F., Siillner, W., Thomas, Eisbacher,

R., Fuchs,

G., 1987. Near-vertical

K., Wilhelm,

H. and

and wide-angle

seismic

surveys in the Black Forest, SW Germany. J. Geophys.,

We thank the Deutsche Forschungsgemeins&aft (DFG) for financial support within the priority programme: Composition, Structure and

of

Nature, 328: 491-496.

S.N. (Editors), 1986. Shear-Wave

Geophysical

Society of Exploration Edelmann,

and industrial

anisotropy.

R., Krohe, A., Stenger,

Acknowledgements

and

cracked elastic media. Wave Motion, 3: 343-391.

Holbrook,

incidence.

(3) Anisotropic

and

H.A.K.

C. Macdonald,

seismograms

(2) Isotropic modelling of lower crustal reflections present in the P- but absent in the S-field for the wide-angle range, does not explain the observed S-reflections

for financial

We thank

et al.

(1989) this direction corresponds to the lineation of hornblende. No tectonic implications are proposed at the present stage because relevant wideangle

Hannover, Knabe,

of minerals.

of the wide-angle

Evolution of the Continental Lower Crust. We are also very grateful to PRAKLA-SEISMOS AG. tions.

model has the fastest P-velocity

in the profile laboratory

data

et al., 1989) demonstrate

dry conditions) and

ruled out in this study.

laboratory

62:

l-30. Nolte,

B., 1988. Erweiterung

tltsprogrammes University

und Anwendung

fur anisotrope

Medien.

des ReflektiviDiploma

of Karlsruhe, Karlsruhe, 120 pp.

Thesis,

SHEAR-WAVE

Sandmeier,

EVIDENCE

K.-J.

and

FOR

Wenzel,

rology from tide-angle Black Forest.

ANISOTROPIC

F., 1990.

In: J.H. Leven, and

CRUST

Lower

crustal

P- and S-wave measurements

J.C. Dooley and B.L.N. Kennett of Continents

LOWER

their

D.M. Finlayson, (Editors),

Margins.

BENEATH

petin the

C. Wright,

Seismic Probing

Tectonophysics,

173:

493

FOREST

of VP and Vs in an amphibolite relationship ural

characteristics

Gebauer Crust:

S., Takeshita,

T. and Kern,

H., 1989. Anisotropy

of the deeper crust and its

to the mineralogical, (Editors),

of the rock. Evolution

Deep Drilling,

istry. Tectonophysics,

495-505. Siegesmund,

BLACK

microstructural

of the European

Geophysics, 157: 25-38.

and text-

In: R. Merssner Geology

and

D.

Continental

and Geochem-