Heat flow, radiogenic heat and crustal thickness in southwest U.S.S.R.

Tectonophysics,

167

103 (1984) 167-114

Elsevier Science Publishers

B.V., Amsterdam

HEAT FLOW, RADIOGENIC SOUTHWEST

RI.

- Printed

in The Netherlands

HEAT AND CRUSTAL

THICKNESS

IN

U.S.S.R.

KUTAS

Institute of Geophysics, Academy of Sciences of the Ukrainian S.S. R., Kiev (U.S.S.R.) (Received

December

15, 1982; revision accepted

April 20, 1983)

ABSTRACT

Kutas,

RI.,

1984. Heat flow, radiogenic

L. Rybach

and D.S. Chapman

Lithosphere.

Tectonophysics,

The heat flow distribution thermal generation

energy

outflow

of a different

heat generation

The position values decrease continents

over the southwestern the deeper

Heat

U.S.S.R. during

layers. The former

in subsurface

is not an isothermal

with increasing

mantle

age. The latter

of the M-discontinuity

The M-discontinuity

thickness

Terrestrial

in southwest

Flow

Studies

U.S.S.R. and

In: V. CermBk,

the Structure

of the

103: 167-174.

in the upper Iithospheric

field in structures radiogenic

from

heat and crustal

(Editors),

crustal

shows that the crust-mantle

is dependent

tectonic

is responsible

is manifest

on two major

activation

and

for different

in the linear

factors:

the radiogenic

the heat

levels of the thermal

dependence

of heat flow on

layers of the earth’s crust. correlates

weakly

surface.

However,

thickness. interface

with heat flow and temperature within structures

An analysis cannot

of the thermodynamic

be explained

distribution.

of the same age, heat flow

by the gabbro

conditions

on the

to eclogite transition.

INTRODUCTION

In recent years it has become

clear that heat flow distribution

involves

a number

of relationships which are of great importance for understanding both the nature of the thermal field and its anomalies. These are: (a) heat flow density dependence on the age of tectonic and magmatic activity; (b) linear relationship between heat flow density and specific radiogenic heat generation in the subsurface layer of the earth’s crust; (c) heat flow density correlation with lithospheric thickness. All these relationships are far from being universal. In any given region they have specific features depending on the evolution and structure of the region. Here, we present an analysis of these features for the territory of the southwestern part of the U.S.S.R. The region considered has a complex geologic structure. Its most stable part is the southwestern slope of the East European Platform. The platform basement is composed of Archean and Proterozoic sediments and volcanic rocks metamorphosed 0040-1951/84/$03.00

0 1984 Elsevier Science Publishers

B.V.

to various degrees (from greenstones to granulite facies) and granitoid on the Ukrainian Shield. In the south and southwest, the old platform Paleozoic

units of the Donbass.

structures

of the Caucasus.

The complexity crust. Crustal number

of geologic

thickness

of interfaces km/s

evolution

ranges

Crimea

and Dobruja

among

basement

(Sollogub

is reflected

in the structure

of the earth’s

the earth’s crust, a great

which most reliably

surface and interface

and Chekunov.

and Meso-Cenozoic

and the Carpathians.

from 25 to 65 km. Within

are found.

where) are the crystalline of 6.8-7.0

Stepnoy

the Black Sea Depression

outcropping is frins:ed by

identified

(not every-

Cz with boundary

velocities

1980).

HEATFLOW

On the territory studied, the heat flow density ranges from 25 to 115 mW/m’ (Kutas, 1978). The heat flow distribution is closely related to the tectonics and geological

evolution.

with certain

Several geothermal

tectonic

zones can be distinguished

units. A zone of low heat flows (25-55

which coincide

mW/m*)

the southwestern part of the Precambrian East European Platform, Ukrainian Shield. Low heat flows are also preserved in depressions formed on the ancient Foredeep,

Peri-Black

basement

in the Mesozoic

Heat

flow peaks

(70-115

evolving

Of a special interest

during

overthrusts

are observed

the Mesozoic-Cenozoic

Eras (the Carpathian

is related

to tectonic

such as the Scythian within

the units

era (the Caucasus,

is the area of the East Carpathians.

tectogenesis,

mW/m*). This discrepancy can, evidently, be explained

mW/m’)

by the Paleozoic,

mW/m*)

intensely

to the Alpine

field (50-70

their evolution

Crimea). related

over

Sea Trough).

The next level of the thermal which had completed Donbass, Dobruja.

and Cenozoic

extends

including the and troughs

that

units Plate. were

Carpathians,

Its formation

but the heat flow here is relatively

is

low (50-65

between the heat flow density and the age of the unit by the fact that the folded area of the East Carpathians

older structure.

In the southwest of the U.S.S.R., the heat flow thus tends to decrease with the However, within individual units considerable increasing age of the structure. variations of heat flow are observed. These are caused by several factors. Firstly, the heat flow distribution reflects the duration and multiphase nature of structure evolution in the course of repeated activation periods. Secondly. the heat flow variation is affected by the processes on which the nature of tectonic activity to their effects on crustal depends and by thermal energy outflow. According processes tending evolution, these processes can be of two types: constructive towards formation of the continental crust and destructive processes directed to disintegration of the crust. Thirdly, the existence of heat flow variations are due to varying conditions of heat transfer and irregular distribution of heat flow sources, primarily radiogenic ones.

169

It is

evident

must

eliminate

heat

sources

processes, parison, clearly.

that in comparing

the heat flow density

the effects of inhomogeneities distribution

including

the dependence Figure

and analyse

the anomalies

both the constructive between

territories.

related

and destructive

the heat flow density

1 and Table I show this dependence

and the adjacent

with the structural

in the geologic

of a geosynclinal

and in the

to uniform

energetic

ones. With such a com-

and the age is revealed

for the southwest

In this case the structural

time of the active development

age one

sequence

fairly

of the USSR

age is understood

to be the

process.

In the thermal history of any region, two stages, progressive and regressive, can be marked out. These thermal history stages in different tectonic zones have their own

80

I

m.y.

80

60

8

Fig. 1. Correlation Individual

between

dots are numbered

the mean heat flow density according

and geological

to data in Table 1.

age (t) of the geological

structure:

170

TABLE

I

Mean heat flow density Tectonic

No.

of the main geologic unit

1.

Caucasus

2.

Carpathians

units in the southwest

(1 (m.y.)

of the U.S.S.R. * ,I

‘2 (m.y.)

(4,Wm

*)

80-

25

30

80+15

11

loo-

45

45

77+38

28 40

3.

Crimea

180-140

150

6X+13

4.

North DobruJa

230-180

180

64

5.

Donbass

285-260

260

51*

6.

Scythian

285-260

260

62+13

12x

7.

Hercynian

330-290

29u

60tl2

x2

450-410

420

50

Central

Plate

Central * No.-number

Europe

structure;

features,

Europe

y-mean

their physical

progressive activity

in

of point in Fig. 1; r, --time

of geological

and thermal

the earth’s

crust

conductively

and

of active evolution

heat flow density;

essence

stage of thermal

and subsequently

material

19

in

Caledonian

8.

4 1

being

unchanged.

evolution

geothermal

(Khain,

n-number

upper

mantle.

and convectively.

During

1972): f2 -age

For any type of structure,

multiple

energy from the earth’s interior,

1970: Garetsky.

of heat flow measurements.

is characterized activity,

x

by an increase transport

of tectonic

of the deep-seated

and a temperature

this stage,

increase

heat is transported

The role of one or the other

the

mechanism

in

both of heat

transport changes with depth and time. At great depths and in disturbed zones convection dominates whereas in the earth’s crust conductive energy transport prevails. The regressive

stage is characterized

energy scatter and a cooling

by a decrease of tectonic

of the earths

activity

crust and upper mantle.

and thermal

The dependence

of the heat flow density on the age of the structures reflects the regressive stage of thermal evolution. The thermal regime of geosynclinal areas is being stabilized throughout decreases

a 400-500 linearly

m.y. period. During this time interval, the heat flow density with t Ii2 . This fact is consistent with lithosphere cooling according

to the heat conduction HEAT

FLOW

law.

AND RADIOGENIC

HEAT GENERATION

A correlation between the heat flow density and heat generation in the subsurface layer should exist everywhere. It is derived from the solution of the one-dimension stationary equation of heat conduction and is written in the case of a stratified medium as: n ”

Y=,~,4w,=A,(rb,

+

c 4(z)z,

n=2

171

or in the usual form of

where q is the surface heat flow density, the heat flow penetrating

the bottom

and whose heat generation

heat flow or the density

of

is z,

is A,(z).

zr and q. are defined

The coordinates and they generally

q,, is the reduced

of the upper active layer whose thickness

by the mode of the heat source distribution

vary from point to point. They can, however, retain their constant

values within large areas where the heat distribution at depth obeys the same law. To study the relationship between heat flow density and radiogenic heat generation in the subsurface

layer of the earth’s crust below the Ukrainian

Shield,

rocks

were sampled from 50 boreholes 200-1000 m deep where heat flow density had been measured. The values of U, Th and K were determined. The heat generation values averaged

over the borehole

were compared

to the mean heat flow density

(Gerasi-

mov et al., 1982). Figure

2 gives the results of the comparison.

large and the linear

dependence

between

The scatter of data points

the heat flow density

is rather

and radiogenic

heat

generation in subsurface rocks is difficult to derive. However, such dependence is valid for individual tectonic elements, and geologic units of a similar evolution and structure

show similar

dependences.

The points

in Fig. 2 can be divided

into three

: g

0

_C--

_o- -

*-

-+

a III

6

2 4 w 2:

01

10

a.2

03

HEAT Fig. 2. Heat flow density as a function and

Volynsk

Platform

Orekhovo-Pavlograd

III-q=

23+4.2A.

Blocks;

Geosynclinal

e4

5,

Zones;

+6

+7

p W&

GENERATION,

of heat generation. 4,

x5

I = Kirovograd

Granitic

6 = Odessa-Belaya

Tserkov,

7 = Perzhansk

I-

Massif.

Massif; 2, 3 = Podolsk

Krivoy

q = 24 + 12.6A;

Rog-Kremenchug,

If - q = 24 + 8A ;

groups,

corresponding

to certain

types of structure;

for each group

the equation

y =f( A) can be written. A first dependence rift-like

zones

second

is expressed

composed

sediments

(q = 24 + 8A) describes

dependence

blocks

built of various

metamorphic

and

the heat

volcanic

formations.

flow behaviour

rocks of predominantly

(q = 23 + 4.2A) is derived

third dependence

q = 24 + 12.64; it is valid for narrow

by equation

of metamorphic

A

in platform

amphibolite

from data on a granitoid

facies. A

terrain.

Attention should be drawn to an uncertainty revealed by comparing heat flow density to radiogenic heat generation in the subsurface layer. This uncertainty is connected element

with essential content

variations

in a subsurface

high in the zone of erosion decreases density

remains

fore, the character essentially

averaged.

It is evident

heat generation

depends

1982). Searches

in the crustal supposed

On the contrary,

is generally

rocks.

m, whereas

increases

heat flow density

It often

the heat flow

with depth.

There-

and radiogenic

over which these quantities

of comparison,

to depths

been

for a correlation

concentration

dependence

the radioactive

content

of crystalline

as 150-200

on the interval

has often

of the radiogenic

thickness.

portion

and

element

are being

the heat flow density

as great as 400-500

heat and

m.

THICKNESS

assumptions direct

between

should be averaged

flow density

flow density

or it slightly

that for the purpose

HEAT FLOW AND CKUSTAL

Kutas,

as shallow

either constant

of the relation

generation

Heat

and the upper

a few times at a depth generally

of the heat

layer. The radioactive

related

to crustal

between

thickness

(cermhk,

these parameters

were based on

nature of the heat flow and a high radioactive rocks. However, between

these comparisons

the heat

a heat flow increase

flow density is observed

1979; element

did not confirm and

the earth’s

the crust

in many areas of crustal

thinning. A comparison of crustal thickness with heat flow density and temperature distribution for different age structures is given in Fig. 3. Also shown are the phase boundary

occurrence

of gabbro-eclogite

and the solidus

temperature

for dry basic

rocks. As seen from Fig. 3, heat flow density is independent on the M-discontinuity position for the region as a whole but within the structures of one type and age the plunge of the M-discontinuity is constantly accompanied by a heat flow decrease. This law is particularly pronounced within young structures of the Alpine belt. Thus, the heat flow density depends directly on the thickness of the upper active layer and is inversely proportional to the crustal thickness as a whole. This suggests. first, a very low content of radioactive elements in the lower crust and a dominating contribution of the upper mantle to the sum magnitude of heat flow and, second, a great influence of thermal energy on the formation and the evolution of the earth’s crust. The M-discontinuity

is not an isothermal

surface. At the crust-mantle

boundary,

173

TEMPERA.TURE,

“C

1500

1000

500

5t

E Y r 5 P w n x5

lO( I-

15(IFig. 3. Depth temperature mW/m*.

Curves

correspond value

M’M” thickness

3 = Paleozoic

5 = zones of young

tectonic

1966). T, -sohdus

in the same

structure

Curves A and B determine Green,

on heat flow density.

the M-discontinuity

Curve parameters

position

depending

to mean heat flow values for 1’ x lo grid in the specific

of crustal

Platform,

dependence restrict

area.

(Hercynian),

and magmatic regions

I = Ukrainian

Shield,

4 = Meso-Cenozoic

are heat flow densities

tectonic

province

versus the mean

2 = Precambrian

structures

in

on the heat flow value. Dots

of Caucasus

East-European and Carpathians,

activity.

of stability

for gabbro.

curve for dry basic rocks (Green

garnet

granulite

and Ringwood,

and eclogite

(Ringwood

and

1967).

the temperatures vary from 430’ to 930°C. However, within the structures of the same age the temperature deviations do not exceed 150”-200°C. A comparison of these temperatures with the thermodynamic conditions of the gabbro-garnet granulite-eclogite phase transition led to the conclusion that the occurrence of the M-discontinuity is not associated with that phase transition. Under the shields and ancient platforms the crust-mantle boundary is situated within the eclogite stability area, whereas it is situated within the area of the garnet granulite stability under the active areas. It may be assumed that the occurrence of an intermediate velocities of 7.3-7.8 km/s is associated with the basalt-garnet granulite

layer with transition.

174

In zones with very high heat flow the low velocity partial melting of the mantle rocks. All these features nature

of the thermal

regime

of the inverse heat flow density

hand, thermal

energy flow determines

and crustal

dependence

ascends.

This causes

products active

of active tectonic

of destruction

region.

where thermal is formed

in cool zones. Meanwhile,

and upper

mantle

leads to additional

changes.

forced

thickness

energy outflow is intense,

melting

mantle

crust. matter

of the crust.

out to the periphery

here increases.

Thus,

The

of the

in the zones

a thin crust is formed. whereas a thick crust

total thermal

conductivity

of the earth’s crust

In areas of a thin crust it is significantly

concentration

On the one

of the crust. on the

and melted

and partial

are being

the crustal

from

suggest a double

thickness.

and thickness

the heated

extension

and melting

As a result,

results

by the formed inhomogeneous

processes,

destruction,

structure

on crustal

the structure

other hand. the heat flow is being redistributed In the period

layer obviously

higher. which

of heat in these areas.

REFERENCES

cerrnik,

V.. 1979. Heat flow map of Europe.

Flow in Europe. Garetsky,

Springer,

In: V. i‘ermak

Berlin-Heidelberg-New

R.G., 1972. Tektonika

molodykh

and L. Rybach

platform

Evrazii. Nauka,

Moscow.

Gerasimov, Yu.G.. Gorlitsky, B.A. and Kutas, R.I., 1982. Sopostavleniye radiogennogo Green,

tepla na Ukrainskom

D.H. and Ringwood,

shchite.

(Editors).

Terrestrial

Heat

York, pp. 3-40. 180 pp.

(eplovogo

potoka

s generatsiey

Geofiz. Zh.. 4: 76-80.

A.E.. 1967. The genesis of basaltic

magmas.

Contrib.

Mineral.

Petrol., 15(2):

103-190. Khain, W.E.. 1970. Usloviya pkoyasa. Kutas,

zalozhenia

i osnovnye

etapy razvitiya

Sredizemnomorskogo

geosinklinal’nogo

Vestn. Mosk. Univ.. Ser. Geol., No. 2: 36-72.

R.I., 1978. Pole teplovogo

potoka

i termicheskaya

model zemnoy kori. Izd. Naukova

Dumka.

Kiev,

140 pp. Kutas,

R.I.,

1982. Thickness

Geothermics Ringwood,

A.E. and Green,

mation Sollogub,

of earth’s

and Geothermal D.H..

and some geophysical V.B. and Chekunov.

zemnoy

kori i vozrastu

Centralnoy 147-156.

i Vostochnoy

Energy.

crust

and

heat

Schweizerbart,

1966. An experimental consequences.

R. Haenel

(Editors),

pp. 27-30. of the gabbro-eclogite

transfor-

3: 385-427.

Centralnoy

In: V.B. Sollogub

Evropi po geofizicheskim

In: V. i‘ermhk,

investigation

Tectonophysics,

A.V., 1980. Rayonirovaniye eyo osnovaniya.

flow. Stuttgart,

i Vostochnoy

et al. (Editors).

issledovaniyam.

Evropi po tolshchine Struktura

Izd. Naukova

zemnoy

Dumka,

kori

Kiev. pp.