Superficial granitic layering in shield areas

Tecronophysics, 118 (1985) 75-83 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SUPERFICIAL

75

GRANITIC LAYERING IN SHIELD AREAS

MARKUS BATH Seisrno~og~~a~Seerian, Box I201 9* S - 750 I2

U~ps~~a f.$w&tI)

(Received November 30, 1984; accepted January 24, 1985)

ABSTRACT Bath, M., 1985. Superficial granitic layering in shield areas. Tecronophysics, 118: 75-83. A survey of structural studies suggests that superficial granitic layering is a feature of general occurrence in shield areas. The top granitic layer exhibits both lower seismic wave velocities and above all a considerably lower quality factor than the underlying proper granite, from which it is separated by a first-order discontinuity at a depth of 1-2 km. Because of its fractured and brittle nature. the superficial layer is both of geophysical and engineering significance. not the feast in connection with plans for nuclear waste disposal.

A number of independent structural studies provide convincing evidence that superficial granitic layering is a general and significant property of shield areas. By the present survey we intend to discuss several questions and problems raised from time to time by geologists and seismologists regarding such a layering. In seismic refraction and reflection studies, superficial layers of a thickness from several hundred meters to a few kilometers are frequently reported. In most cases, such superficial layers consist of sediments, as evidenced both by the geology of the investigated region and by the relatively low seismic wave velocities found. It should be made clear that the present study is not concerned with this type of superficial layering. On the contrary, the layering studied here concerns one or more granitic layers on top of the proper granite underneath, to be found in shield areas, where sediments are insignificant or missing. The following notation will be used for the longitudinal (P) waves: P wave propagating in any granitic layer Pg Pgl P wave propagating in the lower, proper granitic layer (above the Conrad discontinuity) Pg2 P wave propagating in the superficial granitic layer (Pg2) P wave propagating in the topmost superficial granitic layer, used only as

~~-1951/85/$03.30

0 1985 Elsevier Science Publishers B.V.

76

distinct P* Pn

from Pg2 in case more than one superficial

P wave propagating discontinuities)

in the basaltic

P wave propagating

in the upper

ity) For the transverse

layer exists

layer (between

the Conrad

mantle

the Mohorovifit

(S) waves. equivalent

(below

notation

and MohorovlG disconttnu-

is used (Sg. Sgl, Sg2). defined

a:,

for the P waves.

LAPLAND

PROFILE

A reversed seismic refraction profile in Swedish Lapland has recently been evaluated in detail (BBth, 1984). It is 315 km long and runs in the direction NNE-SSW parallel to the mountain range in the west (Fig. 1). There are two shot points, N and S at the northern and southern end, respectively, of the profile, where underwater explosions were made in lakes. Records were taken with a seismic refraction equipment at 15 stations or subprofiles, in general with six geophones operating in each of them. Located in the Svecofennian geological

12

setting

16

consists

20

Complex

of crystalline

of the Precambrian

rocks with older granite

Baltic

Shield.

of Svionian

the

age in

E

68

Fig. 1. Part of Fennoscandia. Grilngesberg

showing the Laplsnd

profile (NS), the Blue Road profib

(BR) and the

profile (G). The dotted curve marks the western boundary of the -Baltic Shield.

71

TABLE

1

Crust-upper

mantle

structure

along the profile NS in Swedish

Thickness

Layer or discontinuity

N

(km) S

Lapland

(Fig. 1)

Velocity

Dip

(km/set)

(deg)

(1) Superficial

0.6

0.6

(Pg2)

5.32

0

(2) Superficial

1.8

1.8

Pg2

5.86

0

11.5

16.8

Pgl

6.08

32.9

20.3

P’

6.68

46.8

39.3

N

6.48 A

S

6.37 a

Pn

8.27

(3) Granitic

1.0’ downdip

Conrad (4) Basaltic

1.3’ updip N to S

Mohorovicic Total crust (5) Upper mantle

a Average

crustal

N to S

P wave velocity.

the northern half of the profile, turning into younger granite of Late Svionian age in its southern half. The granite is overlain by a cover of moraine and peat, interrupted by so-called remnant mountains or hills and lakes. Four crustal layers are identified, and it is of particular major granitic

and basaltic

layers, also relatively

interest

thin superficial

that besides

the

layers, (1) and (2)

are clearly established (Table 1). For the waves denoted (Pg2), Pg2 and Pgl, the respective travel time equations for N and S show no significant differences. On the other hand, the equations

for P* as well as those for Pn differ significantly

S, which is due to certain slopes of the Conrad Of the two superficial layers, the greatest

for N and

and MohorovZic discontinuities. interest is attached to layer (2). But

layer (1) belongs also to this layering, probably with a true velocity increasing with depth (of which Table 1 reports an average), possibly also slightly influenced by the moraine

and peat cover. The velocities

be considered substantiated

GRjiNGESBERG

of both (Pg2) and Pg2 are sufficiently

as typical of granite (cf. Heiland, by the geology along the profile.

1951, p. 472), a conclusion

high to that is

PROFILE

Even though the superficial layers in Table 1 constitute an unambiguous result, the Lapland profile was primarily designed to investigate major structural features. For a more detailed exploration of the superficial layering, a dense sequence of recording points at short distances is required. This aspect was realized at Grangesberg in central Sweden (Bath et al., 1976b). See Fig. 1. A linear unreversed profile, extending 17.5 km in a northwesterly direction from the Grangesberg mining area (60.1°N, lS.O’E), was studied by a dense coverage of

78

geophones, the total number of measuring points Gmngesberg were used as seismic wave sources. Located granites,

within

formed

viewpoint,

cal

the Rahic Shield, during

there

Everywhere

nowhere

is the thickness

the rock formations

the Svecofennian-Karelinn

is hardly

granite.

being 73. Regular

along

any difference

the profile,

the

of the Quaternary

are known

evolution.

sedimentary

~b close

biasta at

as leptit~~ ;ttrd

From

to be expected bedrock

mintng

the s&srncdq+

between

icpt~tc ,tnd

to the surfacer: ;rnci

cover estimated

to ex~.~‘t’J20

m. A sharp break in the travel time graphs around 12 km distance demonsrrates significant differences between the velocities of Pg2 and Pgl, as well as hctnccn those of Sg2 and Sgt (Table 2). This result proves the existence of a relativcl! sharp lower boundary of the superficja1 layer at a depth of 1.4 km from Pg and from Sg, the former value being most reliable. The profile being unreversed, the break in the travel time graphs around

T.6 km I-! km

distance could be thought to express some lateral heterogeneity. Howevrr. this suspicion is contradicted partly by the geology along the profile, partly by thl: got~l agreement with independent results elsewhere. Especially the similarity between the results of the present profile and the superficial layer (2) in Tabie both with regard to P wave velocity and layer thickness. The dense coverage

of stations

permits

7 is remarkahte,

an even finer interpretation,

suggesting

that the Pg2 and Sg2 velocities are not exactly constant, but increase slightly with depth, for Pg2 according to the folIowing equation (c = wave velocity, km,/sec: h = depth,

km):

t, (Pg2) = 5.79 -I- 0.043h

(1)

Olhll.4 For h = 1.4 km, eqn. (1) gives v(Pg2) = 5.85 km/set,

marking

a significant,

first-order

discontinuity against o(Pgl) = 6.22 kmjsec. Amplitude studies both in the time and the frequency domain reveal a +-p%dny factor {e) about twice as large in the underlying granite as in the superficial layer (TabIe

2).

TABLE

2

Upper crustai Layer

(I ) Superficial

structure

along the Gr&ngesberg

profile (Fig. I)

Thickness

Velocity

(km)

(km/set)

Pe;

I.4

%

1.6

(2) Granitic

.-

(Q)

p&9 Ss2 Pd

5.82

fw

3.38 6.22

f%l

Sgl

3.62

__~.. .ll.“-l. SO-80 il 120 ” II_~______.__-I.

a Wave period = 0.02 sec.

79 ADDITIONAL

EVIDENCE

Superficial

granitic

FROM SHIELD AREAS

layering

in shield areas could perhaps

be dismissed

local feature, if based on one or two profiles only. Therefore, collect further In a record

information analysis

from the seismological of the Blue Road

it becomes

as a purely desirable

to

literature.

profile

(Fig.

l), Lund

(1979)

finds

an

increase of the P wave velocity from about 5.4 to 6.0 km/set in the uppermost zone of the crust, about 2 km thick. Characteristically, this finding refers only to the shield part of the profile, i.e., east of the mountain range, where the bedrock consists of granite and gneiss. In an interesting, though somewhat debated review of the crustal structure in Fennoscandia, Seguin (1972) reports a superficial granitic layer with thicknesses mostly similar to what has been found above. In such a typical Baltic Shield area as Finland, numerous refraction profiles have been investigated and a superficial granitic layer with a thickness of 2.2 km is found (Penttila et al., 1960). Its P wave velocity is 5.73 km/set, and it overlies granite where the velocity is 5.95 km/set. For comparison, the average

the proper velocity in

our layer (l)+(2) (Table 1) of 2.4 km thickness is 5.73 km/set. In another paper, Penttila (1969) reports P wave velocities of 5.85 and 6.08 km/set in the Finnish upper

and lower granite,

respectively,

i.e., remarkably

velocities for the Lapland profile (Table 1). Besides the quoted papers, referring to conditions

similar

to the Pg2 and Pgl

in the Baltic Shield,

there is

scattered information on similar top layers from other shield areas of the world, e.g., for the Canadian Shield by Weichert and Whitham (1969) Barr (1971) Clee et al. (1974)

Berry

and

Mair

(1977).

For the Yellowknife

area

of the Canadian

Pre-

cambrian Shield, Barr (1971) finds a top granitic layer 3.5 km thick with a P wave velocity of 5.5 km/set overlying granite of 6.1 km/set velocity, while Clee et al. (1974) report a top layer l-2 km thick in which the velocity increases gradually with depth from 5.5 to 6.0 km/set. This kind of layering is a common feature in Canada (Berry and Mair, 1977). In their comprehensive

book,

Steinhart

present similar examples of near-surface layering, especially U.S.A., and emphasize its importance in shield areas. Short-period

Rayleigh

surface waves Rg are generated

and Meyer (1961) clear

by superficial

in Wisconsin, events, such

as near-surface earthquakes, explosions, rockbursts. Due to their short period, their velocity dispersion is governed by the superficial layering. This fact provides a simple method for investigating the top layers, based on ordinary station seismograms,

while

dispersion superficial

the accuracy

is lower

than

for refraction

over paths in Sweden demonstrates beyond granitic layer about 1 km thick (Bath, 1975).

profiles. doubt

Thus,

the

the existence

Rg of a

Amplitude studies yield a quality factor (Q) of 180-300 for Rg of 0.75 set period, typical of the top layer. This value should be compared with quality factors for the underlying granite of 1060 for Sgl of 0.4-0.7 set period and of 1830 for Sgl of 0.7-2.0 set period (Bath et al., 1976a). The quality ratio between the upper and

80

lower granite is then on average about f : 6. i.e., a much more pronounced contrast than 1 : 2 found for Pg of 0.02 set period {Table 2). PurposefulIy, we have collected ~nformatio?~ in favour of a superficral granitrc layering. At first sight, this may seem to be a biased procedure. Perhaps it could be objected that there are other profile studies from shield areas which do not men\ion such a layering and which could be taken as an indication of its absence. However, this behef is no doubt incorrect. The reason for the apparent absence could be that the recording stations are not sufficiently dense to detect minor features and,.‘(yr that the investigators have their attention directed much more to the deeper. major structural features. Moreover, hardly any paper seems to exist that has produc~_I convincing evidence against the superficial shield layering. Also in non-shield areas, the granite nray exhibit a similar hiyering. s[)~~~~tr~les even under a sedimentary cover. in their detailed evaluation of the Has!a& explosions, Rothe and Peterschmitt (1950) find an upper gneissgranite layer. 2.4 km thick and with a P wave velocity of 5.63 km/set. overlying the deep granite, where a velocity of 5.97 km/set prevails. A useful discussion of similar superficial layering in central Europe with additionai references is provided by Mueller (1977). GEOPHYSKAL

SiFNIFlCANC’E

Although our literature survey does not aim at ~~mp~~teness, the cohected seismoiogical references provide sufficient evidence that superficial granitic layering is a normal and general feature of shield areas. The top layer differs markedIdlyfrom the underlying proper granite and it is of interest and si~ifi~an~ both in geophysics and engineering. While layer thicknesses, wave velocities, quality factors and the discontinuity between the upper and lower granite are fairly well established by the seismological data, the physical explanation of these findings is more problematic and calls for information from other branches of geoscience. The lower wave velocity and especially the lower quality of the superficial layer, compared to the proper granite underneath, suggest a fractured structure. tt is known from direct measurements in the up~rmost kilometer of the shield crust that the horizontat compressive stresses increase linearly with depth to several times the weight of the overburden (Hast, 1969). As a cansequence. cracks and fractures are expected to close graduaiIy with increasing depth. which will lead to a gradual increase of wave veiocities with depth. This result agrees with eqn. t’f 1. even quantitatively. Similar ideas have also been advocated by several other authors. PenttiB et al. (1960) interpret the uppermost layer as a part of the granitic layer but conta.i~~ng water-filled cracks and pores, which cause a decrease of the observed impulse velocity. Lund (1979) identifies the top layer with the so-called clastosphere {NoeNygaard, 1962, p. ZOl), where open fissures can develop and exist. However, for the first-order d~~on~~uity between the upper and lower granite,

81

gradual pressure increase and gradual compaction and closing of cracks do not seem to be enough. It would be necessary to include some relatively sudden effect, e.g., some physical change, which could occur when a critical pressure and/or temperature has been reached. Or a change in rock composition could be suspected. In a study of the Polish Precambrian platform, Ryka (1984) says that the proportion between rock varieties changes with depth and that at about 1 km depth, the average composition of the basement changes and deviates from the granitoid composition of rocks at the surface. As a consequence of the fragility of the top layer in combination with its relatively high stress, breaks occur in this layer, so-called secondary earthquakes (B&h, 1983, p. 232). Released by various local external or internal influences, they are purely local in origin and, except for some fragmentary alignment along the east coast of Sweden, they do not form any well-defined belts. In these respects, they differ from the so-called primary earthquakes, which are of tectonic origin, occur at greater depth (generally 5-30 km) and are located in well-defined belts. While these primary events allow a statistical analysis, this is not the case with the secondary events, which therefore imply a less controllable risk factor. Summarizing, we can state the geophysical aspects of the superficial layering as follows. While the prerequisites for the statical, i.e., structural, properties exist generally in shield areas, the prerequisites for the dynamical properties, i.e., natural earthquake release, exist only sporadically at unpredictable locations. ENGINEERING

SIGNIFICANCE

All kinds of engineering work involving the shield crust, are infringements on its superficial granitic layer and generally they are confined to this layer. Therefore, an accurate and detailed knowledge of the properties of this layer is of vital importance for any such work. Especially the relations between the horizontal stresses and the relatively low strength may be critical. Under undisturbed conditions, a certain equilibrium may be established between stress and strength, but this will be disturbed by external influences. Excavations, e.g., for mines, tunnels, storage rooms, imply a disturbance of the natural stress system. Ruptures may then arise wherever the modified stress distribution exceeds the rock strength. In mining operations, excavations cannot be located at will but have naturally to be made where ores exist. But then it is very important for the mining safety to have the ensuing artificial breaks, so-called rockbursts, under control. Rockbursts are a necessary consequence of any mining operation. They occur frequently around the iron ore mines in central and north Sweden, i.e., within the Baltic Shield. Several of these events are discussed in a series of papers from our institute (see, for example, Bath, 1980). On the other hand, storage rooms of various kinds are generally not bound to

x2

certain places to the same extent as mining operations. and then it is tmpormnt to select competent rock. This aspect has recently become one of great signifioanz~* II: cognition with the disposal of radioactive nuclear waste. Relevant plans hake hcrn developed with an extremely great care. a good example being those by S\i-~d14~ authorities (Anonymous, 1983). However. they suggest a deposit at about .Wl) 111 depth in the Swedish granite. i.e., right in the fragile superficial granitic I;fv~i-. It I\ maintained that the layer has been stable over geological epochs of severat hundred million years. and therefore it would probably remain stable for another rttiflion years as well. Although this argument appears ~(~nvin~i~~~.it does not he~.t~nc’$1) when we also take the needed excavation into account. This may disturb the m~tural equilibrium to such an extent that breaks occur. even if a seemingly ~ornpetet3t block is selected and even if remaining cavities are finally refilled. From such viewpoints, the continued search for suitable storage places ih recommended to penetrate to greater depth. and especially to investigate the L‘ondttionh under

the top layer. Stresses certainly

strength of the material layer. Therefore.

continue

to increase with

in the lower granite is considerably

the ratio of strength to

depth.

hut the

higher than in the top

stress is expected to he far more favnurnhlc

in the lower granite than above.

Our literature survey hopefully provides convincing evidence that superficial granitic layering is a typical feature of shield areas and not a purely local phenomenon. Moreover, our discussion of its geophysical and engineering aspects is expected to demonstrate the importance of this layer. Nevertheless, our knowtedge of this layering is not complete and the following projects are suggested for its further exploration: (1) A detailed mapping of the layer within a larger area is recommended. for example by numerous, short refra~tjon profiles, densely covered by geophcrnes. Although the layer probably exists everywhere in shield areas, there are variations from place to place in its thickness as well as in the discontinuity in properties between the upper and the lower granite. (2) Deep drilling and detailed direct examination need to be undertaken at several places for more accurate knowledge about the layering, its physical and chemicaf properties, and especially the nature of the first-order discontinuity between the upper and lower granite. Besides their geophysical si~ifican~, both developments are of importance for the planning of the nuclear waste disposal, (1) by selecting a suitable place where the lower granite is within acceptable reach, (2) by as~erta~~ng the properties of the lower granite.

x3

ACKNOWLEDGEME:NT

The Lapland with regard

profile,

to travel

The evaluation

discussed

above.

times, amplitudes,

is published

has just recently crust-upper

in full as Report

1984), from where it is available

been evaluated

mantle

structure

in detail,

and quality.

No. 2-84 from our institute

to any interested

reader

(B%th.

upon request.

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