Crustal heat production and mantle heat flow in Central and Eastern Europe

Tectonophysics,

195

159 (1989) 195-215

Elsevier Science Publishers

B.V., Amsterdam

- Printed

in The Netherlands

Crustal heat production and mantle heat flow in Central and Eastern Europe VLADIMiR

CERMAK

Geophysical Institute, Czechoslovak Academy of Sciences, BOrni II, e. P. 1401, I41 -31 Prague I-Spoiilou (Revised

version accepted

(Czechoslovakia)

June 25.1987)

Abstract term&k,

V., 1989. Crustal

Chapman

heat production

and H.J. Zwart (Editors),

and mantle

Thermal

heat flow in Central

Aspects

of Tectonics,

and Eastern

Magmatism

Europe.

In: S. Uyeda,

and Metamorphism.

D.

Tectonophysics,

159: 195-215.

The conversion sources

of seismic velocity

to be estimated.

profiles located

along five continental

on up was taken microcracks

into account,

Furthermore,

owing

be determined

to the highly variable

is problematic

within

compared distribution confirm

by very high with crustal

of radioactive

crust,

production the Moho

platforms.

heat

flow, such by other

Younger

Moho

elements

with a generally

variability

in the mantle

lower Moho heat flow typical

the above estimates

can be accounted

of consolidated

enriched

terrains attain Basin

for by a heat generation

Introduction The deep structure and the composition of the crust are still relatively little known and most of our information comes from the results of deep seismic sounding complemented by laboratory petrophysical studies. For the deep temperature calculation knowledge of the distribution of radiogenie heat production is indispensable. Usually, its presumed distribution is based either on the downward extrapolation of the near-surface data 0 1989 Elsevier Science Publishers

B.V.

was therefore

hump

beneath

the use of a of which can

treated

separately.

from 14 to 26 mW m-*

are characterized

based

data

were

exponential

crust. Our data

tectonothermal

crust. The discrepancy

Moho

in regions

The present on a simple

to stable continental

an upper

in the

by elevated

values over 50 mW m-* for example.

by

migration.

crust,

zone, the thickness

to the large-scale

stable continental

heat

is dominated

by deep groundwater

heat flow estimates

heat flow corresponding

of crustal

and temperature

of the crust

of up in the uppermost

relationship,

as the Pannonian

authors.

part

heat flow ranges

and the Moho heat flow may eventually

proposed

The effect of pressure

of heat sources valid for the whole crust might be too high if applied

a considerable

0040-1951/89/$03.50

derivative

range. This radioactivity

for the upper

surface

models

Europe.

the distribution

heat flow for 49 seismic velocity-depth

of A. The uppermost

value of the pressure

this depth

(A) enables

(Moho)

and Eastern

redistribution

areas such as the shields or ancient

heat flows of 20 to 40 mW m-* characterized

in Central

of the heat flow-heat

to the model adopted

stable continental

traverses a certain

from the parameters

According

heat production

was used to assess the mantle

as was the age dependence

that may have allowed

A-up relationship

(up) into radiogenic

This technique

evolution, in relation

zone of exponential

to

decline.

or on the acceptance of defined petrological structural model together with characteristic radioactivities of specific crustal rock types. Rybach (1973) reported the empirical relationship between the seismic velocity of the compressional wave and heat production, which can be used directly to estimate the vertical heat production profile from explosion seismology data. Since then several other authors (Allis, 1979; Kutas 1979; Gordienko, 1980; Glaznev et al., 1985; Stegena and Meissner, 1985) have used a similar approach in an attempt to

acquire approximate information on crustal heat production. More recently, Rybach and Buntebarth

(1984)

(see

also

Rybach

and Buntebarth

specific

rock

single

quantity,

index

of the

can

Buntebarth

be clearly

the k-value several

minerals),

empirical

relate the k-value

to density,

heat

These

production.

numerous

laboratory

rocks greatly senting

packing and

they which

seismic velocity

formulae

are

and

based

on

on a set of

in composition

the entire crustal

by a

formulae

measurements

varying

and

characterized (the cation

rock-forming

have proposed

(1982)

(1982)) have shown that a

and

repre-

profile.

greater

depth,

beneath

the Black

which

heat

Sea-Crimea-Dnieper-Donetz

gen--Moscow

syncline-Pechora

km). Two traverses North (2200

km)

and

(Fig.

across

the

(3200

(4) across shield Bohemian

Basin- East Carpathians

km). In addition, profiles

W-E:

Ukrainian

(5)

Massif-Pannonian

aulacosyncline

are oriented

Germany--Poland-

detailed

seismic

(1500

velocity-depth

were made at 49 sites along these traverses l),

various cambrian

The heat flow measured in stable continental crust can be described by two components: (1) the heat flow from the upper mantle (or from below a relatively

(1100 km), (2) across the Dinarides-Pannonian Basin-Ukrainian shield (1800 km) and (3) across

which tectonic

define units

to Alpine.

the ranging

Seismic

crustal

structure

of

in age from

Pre-

data were converted

into heat production--depth profiles, and together with knowledge of the surface heat flow pattern (Cermak and Hurtig, 1979) they provided a unique opportunity for assessing the geothermal structure

sources are generally uniform), and (2) the heat generated by radiogenic heat production within

on a regional

the crust. geothermal

Tectonic setting and regional heat flow pattern

plained

The local variations in the observed activity on the surface can be ex-

by the variations

in the heat production

of

crustal rocks (in addition to the variations produced by hydrogeology and local geological effects). The mantle heat flow may be more dependent on the tectonic and magmatic activity than on the age of the overlying crust, and knowledge of this heat flow is therefore more important for the interpretation of heat flow data on a global or regional scale. Moreover, the recognition of the outflow of heat from depth is an essential constraint

for two-dimensional

temperature

modelling

of the crust. The objects of this paper are the application of the seismic velocity-heat production relationship (Rybach and Bunterbarth. 1984) to plentiful experimental material and the evaluation of the mantle and crustal components of heat flow in a large territory. For this purpose we use the information obtained during the joint deep seismic sounding programme organized throughout several countries of Central and Eastern Europe, especially information on the proposed crustal structure along five long-distance traverses (Sollogub et al.. 1978, 1980). Three of these traverses are oriented mately S-N: (1) across the Alpine cline-Bohemian Massif-East European

approxigeosynplatform

scale.

The dominant structure in the study area is the ancient East European craton, which is surrounded by younger folded units. This, the oldest part of Europe (= 3100-600 Ma), forms the nucleus of the whole continent. It comprises the Baltic and Ukrainian shields with exposed Precambrian crust and where Precambrian

the East European platform crystalline rocks occur be-

neath younger sediments. This area is acterized by a relatively thick consolidated

charcrust

(40-50 km) and a low heat flow (30-40 mW m ’ in both shields, and 40-50 mW m- ’ in the major part of the ancient platform). In general, the elevated structures of the craton exhibit lower heat flow while the depressed units show a slightly increased heat flow. To the southwest the craton is surrounded by the Variscan (Hercynian) fold belt, an area of late Paleozoic consolidation (= 400-230 Ma). Once a wide mountain belt, it is now composed of a number of massifs of various sizes outcropping in recently uplifted mountain units while in the plains the Paleozoic basement is buried under a MesoCenozoic platform cover. The thickness of the crust varies from 30 to 40 km and the characteristic heat flow values reach 50-60 mW m 2. The heat flow pattern is more variable compared with

197 12

Fig. 1. Simplified

tectonic

setting

of Central

which crustal

and Eastern

Europe

60

48

36

24

together

seismic velocity profiles were converted

with the position

of five traverses

into heat generation

and the locations

at

profiles.

that of the craton, and a number of local anomalies appear, reflecting the heterogeneity of the crust. Additionally, the zones of a locally elevated

European and African plates. It includes Apennines, Alps, Carpathians, Balkans and narides, and extends to the east comprising

part of the Moho usually exhibit an elevated heat flow of up to 70 mW m-2. Still more complicated and varied are the patterns of the crustal thickness and heat flow in the Alpine realm (5 230 Ma-present). This part of Europe comprises young mountainous belts which were formed in the context of the collision of

Crimean Mountains and the Caucasus. Of particular significance is the large intramontane Pannonian Basin wedged between two branches of Alpine

the Dithe

ranges. The Carpathians directly border on the European craton along the Carpathian foredeep, but to the east between the Alpine belt and the ancient craton there is a zone of younger plat-

19x

forms with a Variscan and epi-Variscan basement. This zone includes the Black Sea region, the

Buntebarth (1984) is based on numerous laboratory measurements on a set of rocks varying in

Crimean

composition

plains

epi-Variscan peculiar

structure:

Caspian

basins

mediate absent

and

the Cis-Caucaus

Donetz-Caspian

crust

area.

system

the Black Sea and are

characterized

type, where

or reduced

by

The

has

a

the South an

the “granite”

interlayer is

in thickness

and the lower crust

is covered with an enormously

thick layer of sedi-

ments. The entire Alpine greatly

varying

crustal

area is characterized thickness

attaining

and more in the roots of some mountain and

being

reduced

to as little

by a

from granite

lower heat generation

ference

to be independent

seems

the geological studies.

of the k-value, parame-

in the conversion

Only a small part of this difference

attributed has

As older

and this dif-

age is likely to be another

ter which has to be incorporated to the natural

reduced

the

50 km,

rocks, but a certain

ranges,

of the near-surface

as 25 km in the

to ultrabasites.

rocks showed

evolution

heat

radioactive

decay which

production

in the oldest

degree of uranium rocks

(Neuerburg,

can be

throughout

enrichment the earth’s

1956) has to be taken

into

Pannonian Basin and the Black Sea region. Heat flow is generally elevated; however, there are

account. Rybach and Buntebarth (1984) therefore divided the rocks into Precambrian and Phanero-

strong local variations

of the

zoic groups.

struc-

posed: In A = 13.7 - 2.17 or, for Phanerozoic and In A = 12.6 - 2.17 up for Precambrian

observed

and the relationship

heat flow field to the local tectonic

ture may be different

in the various

parts

of the

The

following

formulae

were

prorocks rocks.

( PW m 3 ) and LJ~is

system as a whole. The Carpathian curved arc provides a typical example by a heat flow increase

A represents

from the outer

Laboratory measurements of the seismic velocity used to establish the above mentioned experimental relationships were performed at room tem-

The extensive Carpathians.

towards

Pannonian represents

the inner

tectonic

units.

Basin, situated inside the an imposing feature on

the heat flow map, with high to very high heat flow (80-100 mW rn- 2). The number of reliable heat flow data from the Balkan peninsula does not allow definitive heat flow isolines to be constructed, but it seems that the heat flow pattern here is broken into a number of local anomalies. Low heat flow is characteristic

of the Moldavian

and Moesian platforms and surprisingly enough, also of the Transylvanian depression. Local high heat flow zones are probably

connected

the seismic velocity

(km s .‘) (Fig. 2).

perature ( = 20 o C) and at 100 MPa. Before applying these relationships it is therefore necessary to make “in situ” and “laboratory” data comparable (Rybach and Buntebarth, 1984). The correction function B(z) was introduced: u,(20,100)

= u,(T,

P)

1+

B(z) u,(T,

P)

where

with hy-

drothermal features and thus difficult to relate to the deep crustal structure. Increased geothermal activity can be observed within most of the Scythian plate and quite prominent is the Stavropol uplift anomaly with a heat flow of over 90 of all of mW rn--‘. Low heat flow is characteristic the Black Sea area; however, due to the enormous thickness of sediments, the measured data are problematic as yet, for the magnitude of the appropriate correction for sedimentation is still questionable. Conversion of seismic velocity to heat production The relationship and heat production

heat production

between seismic velocity up A described by Rybach and

B=$T+$AP AT=20-T AP = 100 - P This function

includes

the temperature

and pres-

sure derivatives of the seismic velocity. The procedure proposed here, contrary to the originally proposed correction factor f of Rybach and Buntebarth (1984), f= 1 + B/u,, enables the correction function B(z) to be expressed directly as a function of depth. For this purpose the temperature and pressure dependences of the corresponding derivatives of up were assessed and then combined with the temperature and/or pressure-depth distributions, respectively.

199

3

2 iRANIt

E

1

g U

GRANULITE

0

2 s n.

BASALT

K

-1

% !i! 5

-2

I z

-3

AN’

PRECAMBh

% -I -4

-5

I

I

5

6 SEISMIC

Fig. 2. Heat production lines represent conditions.

compared

the above-mentioned

For comparison

Seismic velocities

to seismic velocity. relationship

7 VELOCITY,

Thin lines represent

after application

the mean heat production

after Kern (1982); heat production

rw,

I

\I

\

8 km-i’

Rybach

of temperature

compared

to the mean

data after Rybach

and Buntebarth’s and pressure seismic

and cermak

(1984) “laboratory”

corrections

velocity

values

(1982). Error

data; heavy

and are valid for “in situ” for selected

bars bound

rocks is shown.

the range of existing

values.

1

Seismic velocity increases with increasing pressure. At low pressures of up to 100-200 MPa the increase may be quite pronounced (au,/ SP = 10-2-10-3 km s-’ MPa-‘), but most of it can be attributed to closures of pores and microcracks; experimental data are thus very unsuitable for in situ conditions. At pressures above 100-200 MPa, the velocity increase with pressure is smaller and smoother, and reaches 2-6. lop4 km s-l MPa-‘. In the literature, a large number of laboratory velocity determinations at pressures up to 1 GPa are available and Fig. 3 shows the characteristic course of aup/ SP compared to pressure, statistically fitted to ample material compiled by Gebrande (1982, pp. 41-74). There are no significant differences in the behavior of different rock types and all reported data covering various crustal rocks were therefore treated as a single set. In the

tl

IT

0

500

1000

PRESSURE,

Fig. 3. Pressure pressure. material

The

derivative curve

compiled

was

1500

MPa

of seismic velocity

as a function

statistically

to experimental

by Gebrande

fitted

(1982). The range

values is shown by shading.

of

of existing

200

earth,

pressure

the vertical gional

increases

with depth

distribution

departures

are practically The general

form:

27 MPa km

’

beyond

O-10 km. 30 MPa km-’

temperature

below 30 km.

compiled

three

the seismic veloc-

Appreciable by

experimental

Kern

our conclusions

(1982,

are based

of these

data

pp.

vent the opening

pressure

on the

of cracks during

MPa,

the decrease

mea-

z w

n

basic),

which

may better

vidual

crustal

layers. With increasing

summarized

difference formula,

among

derivative

However,

for ultra-

to the indibasicity,

smaller

the differences

the

for higher found

for so

1982). The major factor calculation is the value

EFFECT

I 200

of seismic velocity

by Kern (1982). Dots-acid individual

the “all-data”

is generally

of 350 “-800 o C (Cermak, for the deep temperature

1 400 TEMPERATURE,

data

value

correspond

and

-12-

0

Fig. 4. Temperature

basic

the crust. Towards the base of the crust the temperature gradient decreases; however, at the Moho the temperature may still vary in a broad interval

is thus

POSSIBLE OF THERMAL

(acid,

the temperature gradient can be as low as lo-15°K km~ ‘, while in young orogenea. it may attain 50 o K km ’ and more in the upper part of

showed a sharp decrease of velocity at temperatures above 500 o C; as these data were obtained at of 400-600

types

temperature

were computed

The temperature-depth distribution within the crust is closely related to the tectonic history and may vary considerably. In Precambrian terrains

surements was estimated to be about 100 MPa per 100” C (Kern, 1982). Most of the laboratory data

pressures

velocity

500°C

the individual rock types are not so pronounced the least-squares fit was applied to all material.

to pre-

laboratory

rock

temperatures.

(see Fig. 4).

needed

specific

extrapolated below

The characteristic

of seismic

derivative

teresis in velocity-temperature curves indicating microfracturing. In contrast, a near linear slope and a good reversibility are obtained at a higher The minimum

this limit.

derivatives

Laboratory measurements at low pressures usually show a non-linear slope and a significant hys-

pressure.

600 MPa,

we have therefore

decreases.

processing

at depths

well above

for temperatures

found

of

are generally

the trend

km and 33 MPa km-’

statistical

of 500°C by pressures

for our purposes

lo-30

and

a temperature characterized

adopted

within

118-127)

setting. pressure

in the following

was

structures

by the above effect of

Because in the crust the regions

distribution

the depth interval

material

more likely to be produced microcracking.

by the increasing

within

With increasing

to

the re-

of the tectonic

pressure-depth

ity generally

and

due to local density

independent

here is characterized with depth

according

of density,

groups

of rocks,

as a function

of temperature.

1

I

600

800

“C Calculated

relationships

rocks;

squares-basic rocks; triangles-ultrabasic “laboratory” data before applying for correcting

curve was used statistically

fitted to experimental data within the interval for T> 500°C.

are based on experimental

rocks. As there is no substantial Rybach and Buntebarth’s (1984) O-500 o C and linearly

extrapolated

of the surface heat flow. Using a simple model of radioacti~ty decreasing exponentially with depth, A(z) = A, exp( -z/O), with A,, = 2 PW mm3 and D = 10 km, and a uniform thermal conductivity of 2.5 Wm-“’ K-l, we calculated one-dimensional steady-state preliminary conductive temperature-depth curves for all major tectonic units in Europe (Cermak, 1982), which are now applied to the evaluation of the magnitude of the correction factor B(z) as a function of depth. With regard to the opposite signs of pressure and temperature derivatives of the seismic velocity, both parameters partly compensate each other. At a shallower depth, and especially in relatively “cold” terrains, pressure may dominate, while at a greater depth the temperature effect finally exceeds the pressure effect. To demonstrate the correction procedure used and to give the magnitude range of the correction function B(z), two examples are shown here: (1) For the conditions existing at a depth of 10 km in the Precambrian terrain characterized by a relatively low surface heat flow of 44 mW mw2, i.e., for a pressure of 270 MPa and a temperature of 150°C, the correction function B(z) = (Sup/ 6T) AT+ (6u,/ 6P) AP = (-2.9 x 10-4) (- 130) + (6 X 1om4)( - 170) = -0.064 km s-l. (2) For a depth of 25 km in a young terrain of a high surface heat flow of 80 mW m-‘, where the temperature attains 680” C, and the pressure is 720 MPa, the correction function B(z) = ( - 7.8 X lO-4)( -660) + (2.3 x 10-4)( -620) = +0.372 km SC’.

While in the former case the effect of pressure dominates and the converted heat production is greater, in the latter case it is the temperature that prevails and the converted heat production is lower. Such a correction is definitely important. For a velocity up of 6.8 km s-‘, typical of the respective depth intervals, it gives heat production of 0.15 FW rnm3, i.e., less than half of 0.35 FW rnm3, which would correspond to the uncorrected velocity. The family of B(z) -functions is shown in Fig. 5; individual curves are labelled with the value of surface heat flow. For the 2-D deep temperature calculation performed along the five traverses (Cermhk and Bodri, 1986), mean Precambrian (44

t 0

1

10

30

20

DEPTH, Fig.

5. Correction

Indi~dual

curves

parameter are labelled

B(r)

40

50

km

as a function

with surface

heat

of depth. flow values

(mW m-‘).

mW mp2) and Phanerozoic (68 mW mp2) heat flows were used together with the mean seismic velocity distributions in both tectonic realms. In this way it was possible to “correct” the Rybach and Buntebarth (1984) relationship (see Fig. 2). However, for the following considerations, to convert one-Dimensions seismic profiles into heat production profiles, we treated each profile individually and the correction factor B(z) was applied with respect to the specific depth of each layer. Upper ten kilometer layer

The geological as well as the tectonic structure of the uppermost part of the crust is most complicated. Moreover, the presence of microcracks dominates the physical properties of rocks at a pressure of up to a few hundred mega-Pascals and the uppermost crust is thus characterized by a highly variable value of the pressure derivative of seismic velocity, which produces great uncertainty in determining the value of the correction function B(z). Furthermore, according to Costain (1978) the network of microcracks may facilitate the redistribution of U and Th by migrating underground waters and the original radioactive content of the rocks may have been considerably altered here. The depth at which the microcrack system closes and prevents deeper water migration depends on pressure and this was also believed to be in the region of a few hundred mega-Pascals (Cos-

202

tain,

1978). The existence

by groundwater demonstrated

of alteration

in the deep-reaching experimentally,

produced cracks

for example,

was

at dep-

(1977) formula, we get 2, = 0.4 c&/D. As the surface heat flow data used in the following calculations

were taken

ths of 10 km in the deep hole in the Kola peninsula,

inherently

Northern

observed

USSR (Borevskiy

There

is a possibility

radiogenic could

heat sources

be evaluated,

the interpretation tween

surface

that

the distribution

in the near-surface

heat

flow Q,

of layer

and this can be provided of the linear

A,, (Lachenbruch, Q, + DA,,

et al., 1984).

relationship

by be-

and heat generation

1968; Roy et al., 1968):

where the intercept

Q, =

value Q, (reduced

heat flow) corresponds to the heat flow from below a certain depth, and the slope value D gives the depth scale of heat source distribution. This relationship was proved to be valid in both plutonic and metamorphic terrains and seems to be universally are invoked

valid. As a rule, two crustal to explain

(Lachenbruch, rithmic

it: (1) an exponential

1968) in which D defines

decrement

models model the loga-

of A(z) = A, exp( -z/D),

being the surface heat production,

A,,

and (2) the step D defines

from the heat flow map, they

represent

a certain

istic of a smaller

assume

hence

it follows

Q, = Q,.

and

also

A,, = 0.4 Q&D.

we can forA, = A,,, and

In this case the

surface heat production

can be directly

from

flow pattern,

the surface

heat

the site of

Even if these areas are not

to any heat flow provinces,

mally

of the

as character-

or larger area around

the profile investigated. identical

smoothing

values and can be regarded

considered and

for the

exponential heat production distribution and for a fixed value of D = 10 km, the heat production within

the UTK layer can be assessed.

This distri-

bution resembles Lachenbruch’s (1968) model, and gives A,, = 0.04 Q, and A,,, = 0.015 QO. The contribution of this layer is AQ = A,,D(1 - e ’ ). which for D = 10 km gives AQ = 0.253 Q,,. (2) The obtained

A,)-value

from the above case

can be regarded as the mean heat production within the UTK layer. Then, A,, = (l/D)

tween 4 and 16 km (Morgan and Sass, 1984) and its typical value is about 10 km. For all the reasons given above, the application

/,:‘A; exp( -z/D)dz, which gives Ah = A,,/(1 e ‘) for the actual surface heat production. For D = 10 km, then, A; = 0.063 Q,). the heat production at the depth of 10 km, A;,, = Ai,. e ’ = 0.023 Qo- and the contribution of the UTK layer to the surface heat flow is AQ = A,,D= 0.4 Qo, which

of the u,-A

was actually

model

(Roy et al., 1968), in which

the

thickness of the radioactively enriched near-surface layer. The value of the D-parameter varies be-

the uppermost

relationship

may be problematic

in

part of the crust and this zone was

therefore treated separately. The value of 10 km was formally taken to determine its thickness in this paper. Here, for the evaluation of heat source distribution the linear relationship between surface

assumed.

This distribution

is a com-

bination of an exponential and a step model and in a way corresponds to the idea of Roy et al. (1968). Two proposed

heat

generation

for the UTK

profiles layer

were

therefore

of each profile

heat flow and heat production was used, complemented with another empirical relationship be-

vestigated corresponding to the exponential with a fixed value of D = 10 km and surface

tween the mean surface heat flow (Q,) and the reduced heat flow (Q,): Q, = 0.6 &, (Pollack and

production

Chapman, 1977). Actually, two distributions of heat production in the upper ten kilometer layer (UTK layer) that correspond to two possible interpretations are proposed here: (1) Let the mean surface heat flow (0,) be related to the mean surface heat production within the heat flow province (a,) as in the above relationship, i.e., Q0 = Q, + Da,. Combining, then, this relationship with Pollack and Chapman’s

inlaw heat

A, proportional to the observed heat A, = 0.4 flow Q,,, i.e., A, = 0.4 Qa/D and Q,/D(l - em-‘), respectively, describing two likely cases. The former version of the heat production within the UTK layer gives smaller radioactivity and thus corresponds to higher Moho heat flows. The difference between both models equals about 15% of the Q,, i.e., 6-12 mW rnp2 for the surface heat flow ranging from 40 to 80 mW m ‘. At this stage no preference is shown for either model; the existing values are believed to be within the interval bounded

by the above limits.

203

Application of the 9-A

relationship to actual data

stant

and the problem of seismic inversion zones Figure 6 shows how a u,(z) into A(z)

distribution

profile is converted

by using

Buntebarth

(1984)

summarizes

all specific

relationship.

value

of logarithmic

km. The surface

to be proportional

heat

flow Qa, i.e., A, = 0.4 Q,/D

and

Q,/D(l

This

also

section).

cases which may occur in

- e-l ), respectively

were converted

(1) Some areas within characterized

by large sedimentary

U,(Z) profile investigated a sedimentary

the territory

basin

studied basins.

was situated

and the thickness

are

Rybach

If the

within

to the observed

and Buntebarth

accounted

of the sedi-

by using the

(1984) formulae, and pressure

for. The general

of the heat production

surface A, = 0.4

seismic velocities

into heat production

effect of temperature

such

and

(see end of previous

(3) Below 10 km the reported

practice.

of D = 10

A, was consid-

ered the Rybach figure

decrement

heat production

conditions

exponential

decrease

after the was

character

with depth

was

mentary cover exceeded 1 km, this fact was taken into account. The radiogenic heat production of

assumed, and the value of the logarithmic decrement was calculated for each layer. The obtained

sediments pth and

D-values

Phanerozoic tively, Haack

was presumed to equal 1.0

to be constant and 1.2 PW

and Precambrian

according (1982).

to the

(2) Generally,

sediments,

mean

the heat

with dem-3 for

source

respec-

values

given

distribution

varied

within

kilometers to about this issue).

by

(4) Certain the conversion

in

duction

the UTK layer (except for the sedimentary section) was assumed to be exponential with a con-

channels) existence

SEISMIC VELOCITY

the interval

50 km (Cermak

from

problems may be connected with of seismic velocities into heat pro-

within

the inversion

zones

(low-velocity

observed in some up(z) profiles. The of low-velocity layers in the crust was

, km. i’ UPPER - TEN -KILOMETERS

SEDIMENTARV HEAT GENERATION

3 ,

COYER

UNIFORM

5

7

9

I

I

I

TWO MODELS PROPOSED - SEE TEXT VALUE OF D-PARAMETER FIXED fD=/Okm)

0

T

HEAT GENERATION PROPORTIONAL TO SURFACE HEAT FLOW

BEL ow IOkm VARIES __ .,_ .._ _- D-PARAMETER -~ . II> “A‘UL IV BE CALCULATED FROM A (1)-~~0~~~~

\ TWO POS! ilISLE ON P,:’ DEPENDING

,TlONS ‘IONS

c

I

SUBCRUSTAL LITHOSPHERE UNIFORM HEAT GENERATION

I

I

I

I

I

I

I

0.004

0.02

0.l

0.4

1

2

5

HEAT

GENERATION

a few

and Rybach,

, pW.ti3

Fig. 6. Exampleof how the seismic velocity-heat production technique is used (see text).

204

first suggested by Gutenberg proposed

by several

(1955) and later again

authors,

e.g., Giese

Mueller

and Landisman

velocity

zones in the crust fundamentally

on temperature ture

has

yet

been

Large sialic bodies geosynclines higher

and mountain

prominent

described

satisfactorily

their naexplained.

in troughs.

together

with

seem to favor the occur-

zones (Meissner,

low-velocity

in mountain

ranges,

Lowdepend

however,

as they are found

heat flow values

rence of low-velocity most

(1966) and others.

and chemistry;

not

(1966)

zones

underthrusting of the upper crust under the higher density lower crust (Giese and Prodehl, 1976) but it may also prevail so conspicuous.

view may be the low-velocity some

of the

cambrian

profiles and treated

of

layers

in

reported

located

in the

Pre-

in this work (pro

files 2-3. 2-5, 4-5, 4-6 and 4-7. see Fig. 1) (Sollo-

1976). While

12-20). sner

in the

up(z)

terrains

which are not

from this point

gub et al., 1978. p. 140; Sollogub

were mostly

belts, those observed

in other regions

Quite puzzling

In spite of the above

(1976)

and

these profiles

Pavlenkova’s

et al., 1980, pp.

statement

by Meis-

(1969)

suggestion.

must be used as originally

proposed.

margins of fold belts are less significant and usually smaller. It was also claimed that no reliable

The relatively shallow layer together with

low-velocity

high-density rock composition with less sialic material do not indicate that their nature may be

zones existed in the platform

regions

(Meissner, 1976). Pavlenkova (1969) suggested that the apparent low-velocity sections found in the

position of the low-velocity the low heat flow and

East European platform may simply represent an anisotropy where data from refraction and

ascribed to the temperature effect. Obviously there is no uniform treatment for converting seismic velocities into heat production

wide-angle observations showed a higher velocity than those obtained from near-vertical reflection

in all the low-velocity layers reported ranging in age from the Precambrian

methods.

tiary.

A certain decrease in up can be associated with the alpha-beta transition of the quartz crystals at a

Rybach and Buntebarth relationship to the lowvelocity layer in. for example, the Alpine realm (profile 1-l) would give unrealistically high crustal

temperature of about 600 o C (Fielitz. 1976; Kern. 1982); additional effects contributing to the velocity reversal

may be partial

melting

facilitated

by

the existence of compressed water in pores and dehydration reactions connected with pore geometry reconstitution, etc. (Theilen and Meissner, 1979; Kern, 1982). In all these cases the low velocity of the rock material does not correspond to the change in its chemical composition and thus to higher radioactivity within interval. Nevertheless, the structure

the respective

depth

of the crust may be

quite heterogeneous and one cannot exclude the possibility that a material with a higher velocity is underlain by a material of a lower velocity and of a different composition. While in the former case the corresponding heat generation value in the converted A(z) profile should be found by interpolation, in the latter case the seismic reversal may correspond to higher heat production. This may be not only the case of some specific anomalous tectonic units, such as the Ivrea zone where a pronounced low-velocity layer ( = 5 km s ‘) under a high-velocity layer ( > 7 km s ‘) is the result of

While

the

mechanical

for localities to the Ter-

application

of the

heat production and thus very small mantle heat flow, this procedure applied to u,(z) profiles within the East European platform is very applicable and no substantial differences in the mantle heat flow were observed no matter which profile in the shield area was interpreted. In the present work, the following simple criterion was applied: if the pressure

(MPa)

was lower than or approxi-

mately equal to the temperature ( o C) at the top of the corresponding inversion zone, this zone was interpreted conditions

as being produced by the physical and the heat production value was

calculated by interpolation from the data in the neighboring layers. If the pressure was substantially greater than the temperature, the inversion zone was interpreted as a layer of geological material of a different composition with a lower velocity and thus also of higher heat production than the material of the upper layer. Even though this is a purely speculative approach. it seems to correspond well to Kern’s (1982) empirical knowledge acquired during laboratory measurements of elastic parameters of rocks, particularly to the

205

necessity

of increasing

pressure

by a minimum

100 MPa per each 100 ’ C temperature order

to prevent

also separated taining

thermal

cracking.

was considered continental

profiles

(middle

that the latter

in the general

crust proposed

by Smithson

km) located

to lower crust)

group

case

includes

relatively

low-velocity

velocity

over a thickness

m

and De-

-2

7-11

than the overlying

metamorphic

seismic velocity

rocks containing

granitic

the results

traverses

layer underlying

the

shading

show the preferred

surface greenstone layer of low heat production in some Archean shields. While the Phanerozoic

velocity

in each layer;

cates the presumed

group

polation,

generally

includes

more

pronounced

low-

7-2

r-7

sites

show

in

km) located

in mW

of converting

to heat production

49

granitic

less pro-

reduction

by a heat flow of 40-50

Hy-

radioactivity

of surface

and

of 2-8

of a

higher

depths

shallow

Further,

intrusions.

admitted

at greater

layers (1.5-5s

the existence

et al. (1969)

over a

.

Figures

zone is

more radioactive ndman

in velocity

and in regions

nounced

regions characterized

model of the stable

cker (1974), in which the felsic migmatite

of 5-11

reduction

heat flow of over 60 mW rnp2, the Precambrian

con-

the above speculation.

to mention

layers (7-15%

thickness

in

layer from the Phanerozoic

ones and thus substantiated It is interesting

increase

This division

well the Precambrian

a low-velocity

velocity

of

reported (shown

along

profiles

five

East

forms of

interpretation

the NE-SW

phase change

of low

shading

indi-

and A(z)

inter-

shading

corresponds

to

7-5

7-4

l-3

European

in Fig. 1). The various

and the NW-SE

for all

o10 -

10 -

28 32

30 -

40-

, pW. ni’

HEAT GENERATION SEISMIC VELOCITY,

km.<’

o10 -

20 -

30 -

40-

1-6

so-’

7-8 Fig. 7. Seismic velocity-depth were taken from numerous generation

values

correspond

to inversion

ature caused

conditions,

(pW

authors

me3)

l-l

are shown

zones (low-velocity change,

to l-10 and their conversion

listed in Sollogub

and the heat generation

by compositional

uppermost

profiles

with

ordinary

layers):

NE-SW

figures.

was deduced

corresponding

Dotted

shading--low

value was calculated

and the heat generation

10 km layer the heat production

into heat generation-depth

et al. (1978). Seismic velocity areas

by interpolation;

value was converted from the observed

to two possible

correspond

velocity

interpretations

profiles.

values (km ss’)

to sedimentary

is presumed NW-SE from

to be produced

shading-low

observed

seismic

heat flow on the surface; (see text).

Seismic velocity

are shown

data

in italics, and heat

cover.

Shaded

areas

by pressure-temper-

velocity

is believed

velocity

(see text). In the

two distributions

to be

are given

206

l-11

7-12

7-74

r-13

7-75

a

ia 20

30

40

42

50

.02

.02 -1 .4 1 2 1I-1-L

.l

4 1 2

,02

.l .4 1 2

HEAT ,

,

, SEISMIC VELOCITY

3

5

7

o-

9

.02

.l

,02

4 1 2

GENERATION

.l

4 1 2

, pWi3

, km .i'

3m

3F--rT-i

JF--T-EI

3FT---Ty

2.3

10 -

20-

30 -

40-

50-

1-76

7-78

Fig. 8. Seismic velocity and heat production

the preferred compositional low-velocity layer.

change

within

profiles

the

Tables

I and

7-19 For explanations

see Fig. 7.

Ukrainian shields (39 mW me2); slightly higher heat flows (46 mW rn- ‘) characterize the remaining part of the craton, the East European platform. The whole Precambrian area (Table 1) is thus typified by a mean surface heat flow of

Moho heat flow

calculated

I-11 to 1-19 and 2-

2 summarize

geothermic

parameters

the observed

and

of the studied

area. The tables and the following discussions relate to the first-version (see p. 202) of heat production in the UTK layer (unless specifically mentioned otherwise). The UTK contribution is proportional to the surface heat flow Q, and is = 0.25 Q,, which is about 15% less than the contribution in the case of the second version (see p. 202). Moho heat flows in the first version are thus 15% higher than in the second version. The area studied is dominated by the Precambrian European craton with a generally good data coverage, while the profiles belonging to the Phanerozoic units are less numerous and rather scattered (see Fig. 1). The lowest surface heat flows are observed in both the Baltic and

41.4 + 5.4 mW rnmm2,which is in good agreement with the data reported for other continental cratons (see Jessop et al., 1976). The individual calculated Moho heat flows vary from about 77 to 30 mW rn--*, again with minimum values in both shields: a mean value of 20 mW me2 in the Baltic shield, and of 23 mW rn- * in the Ukrainian shield. The slopes of the shields may have slightly higher Moho heat flows; e.g., see profiles l-11 and 4-11. These values thus approach the characteristic Moho heat flow of the platform (26 mW m -‘). While there is practically no difference in the surface heat flows observed in both shields, the lower crust of the Ukrainian shield seems to be less radioactive than the crust of the Baltic shield and consequently the mean Moho heat flow of the Baltic shield is 3 mW m -’ lower than in the case

207

TABLE

1

Heat flow characteristics

of Precambrian

areas Contribution

in UTK layer

lower crust

heat flow

N-r)

(mW m-*)

(mW m-*)

(mW me2)

Surface

Moho

Thickness

heat flow

depth

sedimentary

(mW m-*)

(km)

of cover

of

Moho

Heat flow

Profile number

Baltic shield l-11

46

29.5

0

11.6

12.6

22

1-12

40

42

0

10.1

10.2

20

1-13

39

40

0

9.8

10.0

19

l-14

39

42.5

0

9.8

11.8

17

1-15

37

38

0

9.3

8.2

19

1-16

37

41.5

0

9.3

9.1

19

l-17

37

35.5

0

9.3

9.6

18

I-18

35

40

0

8.9

6.3

20

l-19

40

40

1.5

10.9

9.4

20

2-4

38

35

0

9.6

5.3

23

2-5

39

38

0

9.8

8.2

21

Means:

38.2 * 2.8

38.4 f 3.8

9.2 + 2.1

19.9 + 1.7

Ukrainian shield 3-3

40

4s

10.1

4.9

25

4-6

39

50

9.8

8.5

21

4-7

42

43

10.6

7.7

24

4-8

36

55

9.1

1.5

19

4-9

34

30

8.6

4.6

21

4-10

38

45

9.6

5.0

23

4-11

46

38

11.6

6.9

27 *

Means:

39.3 f 3.9

43.7 f 8.1

6.4 + 1.6

22.8 f 2.7

13.2

6.3

30

11.6

7.5

21

East European platform 8

1-9

50

42

l-10

40

45

2-3

40

42

0

10.1

7.0

23

3-4

50

42.5

18

12.0

11.9

26

3-5

50

42.5

15

12.0

15.0

23

3-6

40

42.5

2.5

11.5

8.3

20

3-7

50

47.5

13.9

6.3

30

4-4

55

54

6

14.4

3.5

37 *

4-5

40

42

1

10.7

6.1

23

Means:

46.1 rt 6.0

44.4 f 4.0

8.0 + 3.4

25.9 f 5.5

* Problematic

or less reliable

value.

of the Ukrainian shield. It is difficult to decide whether this difference is structurally important, but the larger and more stable Baltic shield forms the oldest part of the original huge craton ( > 1750 Ma), while the smaller Ukrainian shield in its southern sector may have experienced a certain degree of rejuvenation in the late Proterozoic when graben structures, now dividing both shields, were formed. There are no considerable regional differences in the cont~bution of the lower crust (derived

from seismic velocity) between the platform and the shields. The calculated Moho heat flow values within the platform vary from 20 to 30 mW rnp2, the higher values corresponding to depressed structures such as the Dnieper-Donetz aulacogen (profiles 3-4 and 3-5) and to the Pachelmskiy aulacogen (profile 3-7), and to border zones of the platform (profile l-9). A Moho heat flow of 37 mW m-* calculated for profile 4-4, even if located close to the southern rim of the platform seems to be too high and may be less reliable.

2-2

2-3

2-4

2-5

3-1

8.2

35 41 42

60-

sstk--&5 .02

I

, ,

.02

.l -4 I,, 1 2

.02 ,

.I , .4 ,,‘ 1 2

-1 .4 1 2

HEAT

I,,,

SEISMIC VELOCITY,

.02 ,

1

,

.4 1 2

.02 ,

,,c

GENERATION

,1

,4 1 2 ,,L

, pW.m-’

km.<'

SO-

3-3

60-

3-4

3-5

3-6

3-2 Fig. 9. Seismic velocity and heat production

All Phanerozoic (Table 2) generally ranging

profiles

units surrounding the craton show higher Moho heat flows

in a broad interval

of less than 20 to more

than 50-60 mW me2. For these data a considerable scatter is typical. Usually only one or a few profiles are available for each specific tectonic unit, and thus any detailed statistical conclusions may be questionable. Two profiles in this set must definitely be excluded from further considerations. Profile l-8 (calculated Moho heat flow of over 50 zone, mW me2), covering the Teisseyre-Tornquist is characterized by high surface heat flow (75 mW by m- ‘), which is based on the data published Majorowicz (1975). However, such a high geothermal activity was not confirmed by more recent observations and is believed to be more probably the result of near-surface phenomena such as salt tectonics. Nevertheless, even if the contact zone between the craton and the Paleozoic platform is

2-2 to 2-5 and 3-1 to 3-6. For explanations

see Fig. 7.

characterized by a lower surface heat flow of 60-65 mW mp2, the Moho heat flow of about 40 mW rnp2 may still be too high. Another questionable

profile

(4-3, covering

the Holy Cross

Mountains) gave an unrealistically high crustal heat contribution, and thus negative Moho heat flow. As no unsteady-state effect can be assumed in this area, the u,-A conversion technique completely failed in this case. The outer Eastern Carpathians profile (2-2) gave a low Moho heat flow of only 17 mW m 2, which is of platform magnitude, but all other Phanerozoic profiles clearly confirmed the increasing outflow of heat from the upper mantle in younger terrains. Variscan units in Central Europe represent a complex structure, which includes the Bohemian Massif (profiles 1-2, 1-3, l-4 and 5-l), the Paleozoic platform in Poland (profiles 1-5, 1-6, l-7 and

209 TABLE 2 Heat flow characteristics of Phanerozoic areas Profile number/locality

Surface heat flow (mW m-‘)

Moho depth

50

37.5 37.5 38 30 32 30 34 34 40 50 36 40 50 45 31 50 55 30 29.5 26 28 28

(km)

Thickness of sedimentary cover

Heat flow in UTK layer (mW m-‘)

Contribution of lower crust (mW md2)

Moho heat flow (mW m-‘)

12.7 14.1 12.7 16.6 15.2 15.6 16.6 16.8 10.0 10.0 12.1 14.8 10.0 11.3 12.0 10.0 10.0 16.4 18.3 21.5 17.1 8.0

24.8 22.3 18.2 9.5 17.4 12.2 21.0 23.3 12.2 11.6 41.1 ** 18.8 21.7 20.8 5.7 18.7 23.1 16.5 17.9 11.3 7.1 13.1

13* 20 19 404 27 34 28 29 53 * 53 ** 0 ** 24 28 28 37 21 17 * 37 39 57 66 24 *

(km) l-2 Bohemian Massif l-3 Bohemian Massif 5-l Bohemian Massif 1-4 Ditto, Cretaceous basin l-5 Paleozoic platform l-6 Paleozoic platform 4-2 Paleozoic platform l-7 Fore-Sudetic zone 4-l East Labe Massif l-8 Teisseyre-Tomquist zone 4-3 Holy Cross Mountains 3-8 Vorkuta foredeep 3-2 Scythian plate-Crimea 5-5 Scythian plate-Crimea 5-4 Scythian plate 5-6 Scythian plate 2-2 East Carpathians (outer) 5-2 West Carpathians (inner) l-1 Alpine molasse 2-l Pannonian Basin 5-3 Pannonian Basin 3-l Black Sea depression

56 50 66 60 62 66 68 75 15 48 58 60 60 55 50 50 70 75 90 90 45 *

0 0 0 1 0 2 2 2.5 11 12 0 2.5 10 9 8 22 11 4.5 2.5 2.5 6.5 8

* Problematic or less reliable value. * * Unreliable value (excluded).

4-2) and the East Labe Massif profile (4-l). Both the crustal structure and the surface heat flow vary considerably and the latter can be affected by various near-surface phenomena. Local anomalies may be detected here as being produced by underground water circulation in sedimentary basins (cerm&k and Jetel, 1985), which may be the case for the increased geothermal activity of the Bohemian Cretaceous basin (Table 2, profile l-4). Salt tectonics (Creutzburg, 1964) or other structural effects may have produced local high heat flow zones in the North German Lowland (Hurtig and Oelsner, 1979), which may be the case for the East Labe Massif (profile 4-l). However, the mean surface heat flow of 61.4 + 8.4 mW rnp2 in Variscan structures, the lower crust heat contribution of 18 mW m-* and the calculated Moho heat flow of 29 mW m-* correspond well to the increased values expected for the territory surround-

ing the craton area in view of its tectonic age and history. An increasing outflow of heat from the mantle in the area framing the craton is further confirmed by the profile in the Vorkuta foredeep (profile 3-8) in the northern part of the Ural belt (QM = 24 mW m-‘), as well as by the data from the Scythian platform encompassing the Crimea Mountains to the south of the craton (profiles 3-2, 5-4, 5-5, 5-6) where QM = 29 mW rnm2. The Alpine structures complete the picture. Alpine molasse (profile l-l) is characterized by a Moho heat flow of 39 mW rnp2, and the inner Western Carpathians (profile 5-2) by QM 2: 37 mW m -2. The Pannonian Basin wedged in between two branches of the Alpine mountain range has an anomalously thin crust and a very high heat flow; the calculated Moho heat flow is 50 mW mm2 or more (see also Horv&th et al., 1979).

210

3-7

.O2 I

3-8

-1 ,4 1 2 I11k

--

,02

.l

4-r

.4 1 2

02 I,

1,412 ,,I

4-2

.02 I,

.l

4-3

4 12 I,,

HEAT GENERATION SEISMIC VELOCITY 3rz77

,02 1,

.l

.4 1 2 11,

, pW.m-’

, km. i'

JY--TT-i

3Fr-7-23

3-

3;

.2 *5

50-

4-5

bO-

4-4 Fig. 10. Seismic velocity and heat production

The application

3-7 and 3-8 and 4-1 to 4-X. For explanations

see Fig. 7

corrections

to the

the evaluation

of the

yet been satisfactorily explained, and there exist rocks which will not fit to the reported relation-

effects of rapid erosion, uplift and sedimentation. Due to their geological histories, practically all the

ship. Rybach and Buntebarth’s (1984) work was criticised (Fountain, 1986) from this point of view.

structures within Central and Eastern Europe (except for some Alpine units) are free of such corrections or the correction is negligible (Cermak,

In their reply Rybach and Buntebarth (1986) stressed the stochastic character of the up-A dependence and emphasized the need to understand it within comprehensive petrophysical systematics where seismic velocity, heat production and also density are interrelated through the cation packing

measured

of geological

profiles

heat flow includes

1979). The only really problematic region is the Black Sea area where the observed heat flow may be too low due to the uncompensated effect of rapid sedimentation (Erickson, 1970). The present calculated Moho heat flow for profile 3-1 is 24 mW m-2; if substantial correction is to be applied to the surface heat flow, it will be necessary to increase the corresponding outflow of heat from the upper mantle for this area. The crucial point for any uncertainty assessment of the calculated Moho heat flow values involves above all the up-A conversion technique used. The nature of the u,-A dependence has not

index. The up-A relationship corresponds to the existing characteristic trends in the earth’s crust of generally increasing seismic velocity with depth and decreasing heat production with depth. This relationship was independently confirmed by other authors (Gordienko, 1980; Glaznev et al., 1985; Stegena and Meissner, 1985) who derived it on the basis of correlating surface heat flow with structural models of the crust. However, the actual heat production values calculated from the formulae

.02 1-M

.t

.4 1 2 HEAT

SEISMIC VELOCITY, 3---F-l 5

g

5-2

5-1

4-v

4-10

4-9

02

.l

.4 1 2

GENERATION

,02

,1

.4 1 2

, pW.m“

km s-1

3rF--FzJ

O-

10 -

20 -

30 40 50 -

Fig. 11. Seismic velocity and heat production profiles 4-9 to 4-11 and 5-1 to 5-6. For explanations see Fig. 7.

proposed by these authors may substantially differ. As all the above authors somewhat pre-estimated the mantle heat flow in their studies, their formulae are not suitable for verifying the present results. The possible range of characteristic urand A-values and their scatter for major upper lithosphere rocks is shown together with Rybach and Buntebarth’s (1984) relationships in Fig. 2. This range may thus well illustrate the reliability interval. Another source of uncertainty in the estimated Moho heat Row values relates to the indefinite nature of the seismic velocity profiles analyzed. Depending on the techniques used to investigate the crustal structure, deep seismic sounding or other seismological data were processed and the representative up(z) profile calculated. This is usually the most Iikely profile and the simplest of many possible solutions. The real velocity-depth function may vary considerably or may even oscillate on a small scale and the characteristic v,-value for a depth interval a few

kilometers thick requires a certain amount of smoothing and averaging. While in thick, geologically uniform and homogeneous depth sections the uncertainty of the determination of up is smaller than 0.1 km s-r, in highly tectonized intervals, which may also include low-velocity layers, the vel~ity-depth space within which any particular velocity-depth function may exist may reach 1 km s-’ (Miller and Gebrande, 1976). Simiiarly, the determination of the depths to the interfaces ranges from a few hundred meters at shallow depths, to more than 1 km at greater depths and in intricate terrains. An error of 0.1 km s-t in seismic velocity introduces a deviation of approximately 0.15 FW m-3 (at up = 6.0 km s-l), 0.02 PW mm3 (at up = 7.0 km s-‘) and 0.002 PW me3 (at up = 8.0 km s-l) in the heat production-depth profile in the Precambrian platform and about two and a half as much in the Phanerozoic realm. If the respective layer is located in the upper crust and is about 5 km thick, this

212

deviation may produce a departure of almost 1 mW mm2 and slightly over 2 mW m l, respectively, in the total crustal

heat flow contribution.

In the lower crust, no significant for this reason. an incorrect pends

The magnitude

depth

of interface

on the respective

on this interface. s ’ o,-contrast interface,

crust.

area

vertical

dekm

shift in the

to less than 0.5 mW

and

in a Phanerozoic

again be of minor

arise

contrast

for a 6.0/6.5

a 1 km vertical

this error amounts

a similar

determination

heat production

For example, and

m ’ in a platform mW rn-*

departures

of an error due to

approximately

terrain.

1.3

In the lower

shift in the interface

will

significance.

creased

outflows.

Less

agreement,

been reached in the specification istic values of the Moho heat tectonic

units. The discussions

all on the reliable flow in the stable key parameter

however,

has

of the characterflow in specific

have focused above

assessment

of the Moho

continental

crust,

for any model

heat

which is the

of the continental

lithosphere. As the surface tion,

observed

heat

linear

flow and

relationship

near-surface

Q,, = Q, + DA,, (Lachenbruch,

al.. 1968)

is presumed

have two constraints upper mantle:

1968; Roy et

to be generally on the heat

between

heat producvalid.

we

flow from

the

Of course, flow estimate

the uncertainty in the mantle heat is further directly proportional to

(1) The Moho heat flow must be equal to or less than the reduced heat flow Q,, and (2) the

any

surface

mantle simple

incorrect

heat

flow observations,

well as to failure or improper sponding

assessment

as

of corre-

heat flow corrections.

The conversion

of seismic

data provides

velocities

a unique

into

heat

opportunity

for

and the limits of validity of the proposed conversion technique, and the scatter of the present data, to obtain

valid

For the sources,

in the whole

crust,

the reduced and mantle heat flows are sometimes presumed to be equal. Heat production decreases

assessing the distribution of the radiogenic heat sources within the crust and may thus significantly help in the improvement of our knowledge on the outflow of heat from the upper mantle. Notwithstanding a degree of uncertainty in the reliability

it was possible

cannot be negative. distribution of heat

A(z) = A,, exp( -z/D),

Discussion and conclusions

production

heat flow exponential

reasonably

good informa-

rapidly with depth and the lower crust is believed to represent only a small contribution to the surface geothermal activity. It was this case together with the discovery of the empirical relationship between reduced heat flow and the mean n surface heat flow Q. within heat flow provinces (Q, = 0.6 Q, (Pollack and Chapman, 1977)) that enabled the construction of maps of mantle heat flow on a global scale. The equalization of reduced and mantle heat flows leads to the relatively high value of the mantle heat flow of 25-28 mW m * in continental shield areas. Similarly, the

tion on the Moho heat flow values in various tectonic units over a large area. It has also been

value of 27 mW m -* was considered by Vitorello and Pollack (1980) as a background heat flow, i.e.,

proved that the Moho heat flow is generally low and stable over large regions of the Precambrian crust and that its value increases towards younger

heat flow arising from below the zone of crustal radioactivity enrichment and within or below the zone of tectonothermal mobilization. Likewise,

units. Many authors have tried to estimate the value of the heat flow from the upper mantle. As opposed to earlier ideas of the relative constancy of the mantle heat flow over the entire continental area (21-25 mW m-*) (Clark and Ringwood, 1964; Hyndman et al., 1969), it is now generally accepted that there are some regional variations in its distribution and that the younger and tectonically more active areas are characterized by in-

Sclater et al. (1981) believed that the best estimate of mantle heat flow through a craton lies between 25 and 29 mW rn-‘_ and in view of a possible error of 4 mW m-* they widened this range to 21-33 mW m ‘. Contrary to the above rather high Moho heat flow estimates some other authors arrived at considerably lower Moho heat flow values. Based on realistic estimates of metamorphic rock volumes and H,O contents together with the existing

213

seismic

velocities

within

the crust

to specific rock types, a general continental Decker m-*,

the

For

authors

a surface specified

by Smithson heat

mW m-*.

suggested

the low temperatures

model

ruled

out low-velocity

effects. The measured

duction

of rocks

vertical

crustal

zones

rocks

section

flow

produced

thick

originally

latter group of models.

However,

rather

too low and therefore

a

structure

version

for the 2-D

crustal

traverses

the reality tremes

mW m-*

for

version. UTK

between

proportional

The

u,-A

which are

we preferred

the first

calculation

for

and Bodri, 1986). Thus,

be somewhere

between

the ex-

here by the first and the second

conversion

layer,

the latter distri-

temperatures

temperature

(Cermak

may

proposed

together with two models for heat production of the lower crust restrict the Moho heat flow to et al.,

(p. 202) to

the calculated

Moho heat flows will drop to 14-17

by of

version

production,

may lead to crustal

profile

(Nicolaysen

the second

heat

a shield area and to 19-20 mW m-* for a platform area. These data are more consistent with the

in South Africa (surface heat flow of 46 mW m-*)

12 and 17 mW m-’

layer

bution

forming

of the Vredefort

UTK

a

in such

values of heat pro-

of a 15 km

crystalline

heat

At the same time they

thermal Archean

and

flow 50 mW

a Moho

range of 12-21 that

the

model for a stable

crust was proposed

(1974).

(1981). If we apply

corresponding

where

heat

cannot

be used

contribution

to the observed

in the

was

taken

heat flow. It amounts

1981). The former group of models is above all based on correlating surface data on heat flow, heat production and mean heat flow within the corre-

to approximately 0.25 Q, (first version) or 0.4 Q. (second version). This means that for a surface heat flow of 40-50 mW rnp2, typical of a stable

sponding

lo-12

province,

and extrapolating

these results

downwards. The vertical distribution of heat sources is described by a simple exponential function and thus relates to a uniform crustal evolu-

continental

crust,

to 16-20

tial distribution

the UTK

layer

contribution

is

mW rnp2, which for an exponenof heat sources and for D = 10 km

corresponds to A, ranging from 1.6-1.9 to 2.5-3.2 PW rne3. This is a rather broad interval, but it

tion by the differentiation of the primordial magma. The latter models employ stratified crustal

corresponds

structures and compute tion using characteristic

(Kutas, 1979). The heat production measured on rock samples from the Kola deep borehole in the Baltic shield varies from 0.4 to 1.7 PW me3

the crustal heat producrock properties according

to the presumed petrological composition. The function describing the vertical distribution of heat sources may be more complex and eventually more difficult to express in an analytical form. While there are no substantial differences in surface conditions (both groups of models have similar values of A, and 0) or in the heat production of the lower crust, the middle part of the crust may differ significantly as mentioned by Nicolaysen et al. (1981). It was suggested that the core of discrepancy between the Moho heat flow calculated for both groups of models was produced by a heat generation hump beneath an upper zone of exponential decline. Our data gave Moho heat flow values of 20-23 mW m-’ for the shield and 26 mW rnp2 for the platform areas (the first version of the UTK layer, p. 202), which is slightly less than the values proposed by, for example, Sclater et al. (1981), but more than those reported by Nicolaysen et al.

Ukrainian

(Kozlovskiy, contribution case (lo-20 tributions production

well

to the

shield ranging

data

reported

for

the

from 0.6 to 3.35 r.lW me3

1984, p. 346). The estimated UTK of the stable continental crust in our mW rn-*) also agrees well with conbased on characteristic data

0.92 PW me3

reported

near-surface

by other

for the average

authors;

upper

heat e.g.,

continental

crust (shield)(Clark and Ringwood, 1964) 1.25 PW rnp3 (Sclater and Francheteau, 1970) and 2.22 PW rnp3 (Roy et al., 1968) proposed for continental crust, and 1.64 PW rnp3 (Shaw, 1967) and 1.52 PW m-3 (Jessop and Lewis, 1978) reported for the average Canadian shield. The contribution

of the crustal

rocks below 10

km (here referred to as the lower crust) was converted from seismic data and is shown in Tables 1 and 2. The Precambrian crust of Eastern Europe gives 6-9 mW rnp2, but younger terrains of Central and Southeastern Europe are characterized by a contribution of 15-20 mW mp2. If this amount is

214

to be produced by simple exponential distribution, ,oj35 A, exp( -z/D)dz, then for D = 10 km it

CermBk, V., 1982. Crustal

would require

Cermik,

PW m-j,

A, of 1.8-2.7 PW mm3 and 4.5-6.1

respectively.

compatible

While

the former

with the preceding

an ancient

value

considerations

crust, the latter &-values

is for

are definitely

too high for both the first and the second versions of heat production observed

(see p. 202) in relation

surface data.

to heat

middle

flow is negligible,

part

it must

of the be the

crust which seems to be more radioactive

than one would expect

from the simple

exponen-

tial distribution. Our data thus support the idea of a hump in heat source distribution (Nicolaysen et al., 7981), which we shall locate just beneath UTK layer. This hump

the

may be more pronounced

in younger terrains, but it cannot be excluded in the craton. The crust immediately below the UTK layer may have higher production than the base of the UTK layer, this inversion being result of the redistribution of uranium ing groundwaters

(see eermak

and

issue). This fact is demonstrated mental material in several profiles 7--l 1: e.g., profiles

V. and Bodri.

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mainly the by migratRybach,

this

by the experishown in Figs.

1-2 and 1-3.

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