Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands

GEOTHERMAL INVESTIGATIONS IN SOME REGIONS OF WESTERN SIBERIA

E.E. FOTIADI, U.I. MOISEENKO, L.S. SOKOLOVA and A.D. DUCHKOV

Institute of Geology and Geophysics, Academy of Sciences, Novosibirsk (U.S.S.R.)

(Received April 15, 1970)

SUMMARY

Data on heat flow and the possible temperatures of the lower (the Moho area) boundary of the crust are discussed. In the limits of the west Siberian lowland the heat flow values vary from 1.23.10-6 to 0.87.10-6 cal./cm2.sec. In the Altay-Sayan region the heat flow varies from 0.70.1+6 to 1.50.10-6 cal./cm2. sec. A conclusion is made about the correlation of the heat flow with the age and the structure of the crust. Some estimations of temperature values on the Moho boundary are given. The temperature on the Moho surface seems to reveal a sharp difference in separate dis- tricts.

INTRODUCTION

At the laboratory of the Physics of the Earth’s Crust of the Institute of Geology and Geophysics of the Siberian Branch of the U.S.S.R. Academy of Sciences, geothermal investigations in the western Siberian lowland and its southern mountain borders have been in progress for several years. In this paper the measured heat flow data are presented, and the temperature of the upper crust (the cover of the folded basement) and lower crust (the layer above the Moho) are discussed. Calculations and discussion of the lower crust also include seismic profile data and the indications of the structure and thickness of the crust from magnetic and gravitational data (Krylov et al., 1966, 1968; Moiseenko, 1967).

WESTERN SIBERIAN LOWLAND

Geology

Tectonically, the western Siberian lowland constitutes the greater part of the young platform of the same name. The sedimentary cover of the lowland, its third structural stage, is formed by sediments (sandstone, aleurolite, clay, argillite) of the Mesozoic and Cenozoic eras. The thickness of sediment increases from the periphery to the centre and also to the north of the lowlands. Below the sediments, there are usually deposits of various genesis and grades of metamorphism of the second structural stage.

Tectonophysics, 10 (1970) 95-101 95

The folded basement of the lowland, its first structural stage, is composed of folded complexes of different ages (Paleozoic and Precambrian).

Up to the present time, temperature measurements in the sedimentary cover have been carried out in drill holes in several areas of oil and gas exploration. Within the lowland a decrease of temperature within the sedi- ments is apparent from west to east. The geothermal gradient follows the same trend. The gradient varies from 10 to 45°C/km. Compared with normal geothermal gradients of 25-30°C/km, regions of relatively high values reaching 4’i’°C/lun and low values of Il°C/ km have been distin- guished.

In order to obtain the values of terrestrial heat flow, preliminary measurements of thermal conductivity have been made on core specimens from deep holes in 11 areas in the central, aouthern and southeastern areas (Fig. 1). More than 260 samples have been measured. The measure-

Fig. 1. Schematic arrangement of the regions of the we& &b&an lowland and its southern mountain frame in which geothermal inva#&&ations were carried out. 1 = profiles A.&P.; 2 = bounda

%y lowland; 3 = line of the heat flow equal to I.O* 10* of the ;e& S%erian

cal. /cm . set; 4 = values of the heat flow X low6 cal. /em2.sec; and 5 = the numerator - the tempera- tures at the surface of folded foundation; the denominator - the tempera- tures at the Moho boundary (in “C); 6 = Heat flow stations in the ~ltay- Sayan folded region.

96 Tectonophysics, 10 (1970) 95-101

ments were made by a steady state method. The measured conductivities ranged from 2.5 to 4.3 mcal./cm. sec. OC. The widest variation was ob- served in sandstones which cover the whole of the above range. The con- ductivity of the aleurolites varies from 2.9 to 3.8 mcal./cm.sec.OC. For the argillites it is around 3.1 mcal./cm-sec.°C. Most of the conduc- tivities fall within the range 2.8-3.8 mcal./cm.sec.°C.

Using the temperature gradients and conductivities from each indivi- dual hole, the terrestrial heat flow was determined in 11 areas. On th% edges of the lowlands the heat flow varies from 0.87 to 1.23 qal./cm . set, which is somewhat below the world mean for continents. The maximum heat flow was found in the Alexandrovsk area and the minimum in the Beloyarsk area. The general tendency was for heat flow to decrease from the center of the lowlands to the borders but there was considerable scatter in the central areas.

ALTAY-SAYANSK REGION

In the mountainous areas to the south of the lowlands, heat flow measurements have been made in four different tectonic districts of the Altay-Sayansk region: the Shoriya Mountains (stations NN I-III), Kuznietsk Alatau (IV, V), the south Minusinsk depression (VI-IX), eastern Sayan (X, XI).

Geology

Geologically the mountains form part of the Caledonian and Salairid systems. The Minusinsk depression is filled with Devonian and Carboni- ferous deposits. In other districts metamorphic rocks of Proterozoic, Cambrian, Ordovician age (crystalline, schist, sandstone, volcanic rock), and magmatic rocks of different tectonic stages are widespread. The Altay- Sayan region is a series of branching mountain crests divided by valleys, with relative relief of 200-700 m.

In calculating the heat flow, topographic corrections have been made by the Jeffreys-Bullard method (Lubimova, 1968), and corrections were made for inclinations of the holes. Sections where temperature was noticeably disturbed by ground-water motion were excluded.

The measured geothermal gradient in the metamorphic and volcanic rocks varied from 12.5 to 21,3OC/km. In the sedimentary rocks of the Beya basin of the south Minusinsk depression, the gradient is in the range 33-54.3O C/km.

Tectonophysics, 10 (1970) 95-101 97

The thermal conductivity of the rocks has been determined by both steady state and transient methods. Conductivities range from 4.5 to 8.0 mcal./cm. sec. OC. The highest conductivities are found in the dolomites; magnetite ores showed conductivities around 7 mcal./cm.sec.V, albite porphyry about 5.7 meal./ cm. sec. “C, crystalline schist about 6.4; sand- stone about 6.5 mcal./cm. sec. “.C. The conductivities of the sedimentary rocks of the south Minusinsk depression are considerably lower; argillites about 2.6 mcal./cm.sec. OC; sandstones about 4.4; aleurolites about 3.9 and coal about 0.4 meal./ cm. set * OC.

Heat flow

Heat flow values in these regions have been determined from measure- ments in several holes from each. The values are shown in Table I.

The calculated heat flows at 12 sites in the Shoriya Mountain area vary from 0.87 to 1.17 pcal./cm2 . set and the average is 1.02 wal./cm2. sec.

In Kuznietsk Ala-Tau, heat flow has been measured at two holes and has given results of 0.76 and 0.93 ncal./cm2 *sec. At five holes in the south Minusinsk depression, the heat flow has been found to be 1.07-1.63 pal./cm2 . set (Moiseenko et a1.,1967). The average in this district is 1.37 wal./cm2 *sec. In eastern Sayan heat flow has been measured at six holes in two dis- tricts. In the eastern part, three holes gave results of 1.42, 1.49 and 1.50 ncal./cm2.sec. In the western part the results were 0.7, 0.73 and 0.82 wal./cm2. sec.

The average heat flow for the whole region of Altay-Sayansk amounts to 1.11 ncal. /cm2.sec, which corresponds to the average found in areas of Caledonian folding (Smirnov, 1968).

ESTIMATION OF THE TEMPERATURE AT THE MOHO BOUNDARY

These heat flow data in the western Siberian lowland and southern mountain fringe, taking into account the dependence of the thermal conduc- tivity on the temperature (Moiseenko et al., 1965, 1967), have been used for the evaluation of temperature in the lower crust. Seismic data on profiles along the Ob and Irtish rivers have been used as well as gravimetric and magnetic data on the structure of the earth’s crust in the mountains (Krylov et al., 1966, 1968; Moiseenko, 1967).

The seismic profiles reveal differences in the stratification of the earth’s crust. In places, one can distinctly distinguish granitic and basaltic layers in the crust, sometimes in very detailed form. In other places, no such differentiation is apparent and separate pieces of crust show varia- tions of velocity characteristics. The Moho can be traced in all profiles with certainty.

In Table I estimates of the extrapolated temperature at the Moho are shown. Three basic models have been used.

(1) The maximum concentrations of radioactive heat products have

98 Tectonophysics, 10 (1970) 95-101

e : T

AB

LE

I

s g E

stim

atio

ns

of

hea

t fl

ow

and

tem

pera

ture

va

lues

on

th

e fo

lded

fo

un

dati

on

surf

ace

and

at t

he

base

(M

oho

bou

nda

ry)

of

.o

3

con

soli

date

d ea

rth

cr

ust

fo

r so

me

regi

ons

of

the

wes

t S

iber

ia

C.

G:

NN

R

egio

ns

and

Dep

th

of

occu

r-

Dep

th

of

Moh

o T

empe

ratu

re

Tem

pera

ture

at

Moh

o V

alu

es

of

area

s re

nce

of

fo

lded

bo

un

dary

in

km

on

th

e su

rfac

e bo

unda

ry (

TM

) h

eat

flow

g fo

un

dati

on

(km

) fr

om

A.S

.P.d

ata

of

fold

ed

fou

nd-

(1

0-6

Cal

./ cm

2-

sec.

) gr

avim

etri

c at

ion

in

O

C

anom

alie

s ca

l-

Tm

in

Tm

ax

TB

-D

cula

tion

s

%

I G

Th

e w

est

Sib

eria

n

Pla

tfor

m

+J

1 A

lexa

ndr

ovsk

3.

0 2

Bel

oyar

sk

2.0

3 Z

avya

lovs

k

3.6

4 U

st-B

alyk

sk

3.0

5 N

ovol

ogin

ovsk

2.

6 6

Tob

olsk

2.

4 7

Om

sk

2.7

8 M

ikh

ailo

vsk

2.

0 9

Ch

eln

okov

sk

2.5

10

Tu

isk

2.5

11

Boc

hk

arev

sk

3.5

Alt

ay-S

ayan

fo

lded

re

gion

1 M

oun

t S

hor

iya

0 2

Ku

znie

tsk

A

la-T

au

0 3

Eas

tern

S

ayan

0

4 S

outh

ern

M

inu

sin

sk

depr

essi

on

4

46I

290

290

1,65

0 68

0

401

110

820

1,50

0 82

0 1.

37

z ’

Dep

th

of

Moh

o bo

un

dary

fr

om

grav

imet

ric

anom

alie

s da

ta

(Fot

iadi

, 19

63,

Moi

seen

ko,

19

67).

44.0

11

0 87

0 1,

480

870

1.23

44

.0

56

200

1,00

0 60

0 0.

87

37.4

93

31

0 1,

080

540

0.94

35

.0

100

470

1,02

0 63

0 1.

12

34.8

6’

7 27

0 1,

040

480

0.88

34

.5

87

360

1,07

0 53

0 0.

94

32.O

I 70

36

0 85

0 46

0 0.

94

38.5

I 84

60

0 1,

400

700

1.22

39

.01

86

530

1,20

0 67

0 1.

12

38.O

I 80

42

0 1,

100

650

1.13

38

.5

110

460

1,36

0 63

0 1.

09

1.02

0.

83

1.12

been assumed for each rock type. The seismic gravimetric and magnetic methods have been used to delineate the rock types at depth. Estimates of radioactive heat products have been taken from Birch (19551, who gave for granites 70 pcal./cm . set, intermediate rocks 40, basalt and gabbros 11 pcal./cm3*sec. These assumptions give a minimum value for the tempera- ture at the Moho denoted by the T,in.

(2) The complete absence of heat sources in the crust is assumed, the upward heat flow being attributed solely to heat conductivity from the mantle. In this case, the temperature at the Moho is a maximum, Tm.

(3) It is assumed that approximately a quarter of the observed heat flow comes from the mantle (Birch, 1955).

Assumptions 1 and 3 give rather similar results. The first model gives temperatures very similar to those calculated by Lubimova (1968). Model 2, it is interesting to note, gives temperature values on the Moho similar to those obtained from Tozer’s model for the stationary tempera- ture distribution under the Precambrian shield.

DISCUSSION

At present, the small number of heat flow measurements give no grounds for any serioiis conclusions or correlation with geological struc- ture, with recent ground movement or other factors, but some speculations can be made. For example, the variation of heat flow in the lowland might prove consistent with variations of the age of its folded basement. The high value of heat flow in the Alexandrovsk area is ~ndoub~~ly connected with the Koltogorsk Rift. The small thicknesses of granitic rocks, from the seismic data in this area accounts for the high temperature value quoted for the Moho in this area. This feature also occurs in the southern Minusinsk depression.

In other areas some correlation of heat flow with the anomalous geomagnetic field can be observed. Associations may also be observed with the faulting and morphology of the upper surface of the folded base- ment and with irregularities in the Moho. Calculated temperature on the Moho reveals large differences in adjacent areas which cannot be entirely accounted for by errors of calculations.

REFERENCES

Birch, F., 1955. Physics of the crust. Geol. Sot. Am., Spec. Papers, 62: 101-118. Fotiadl, E.E. and Karatayev, G.J., 1963. The structure of the earth’s crust of the

Siberia and Far East from the results of the regional geophysical investi- gations. Geof. i Geofiz., Akad. Nauk S.S.S.R., Sibirsk. Otd., 1963 (10): 5-19.

Krylov, S.V., Krylova, A.L., ~~s~~in, B.P., ~ish~~~na, Z.R. and Samoilovich, A.%, 1966. The structure of the earth’s crust in the central and south-eastern parts of the west Siberian lowland from piece seismic sounding data. Geol. i Geofiz., Akad. Nauk S.S.S.R., Sibirsk. Gtd.,1966 (1): 10-19.

Krylov, S.V., Krylova, A.L., mishear, BP., Mishenkina, Z.R., Rudnitski, A.L., Suvorov, V.D. and Janushevich, T.A., 1968. The deep seismic iny~~~~tions in the south-eastern part of the west Siberian lowland and Altay&ayan region. Geol. i Geofiz., Akad. Nauk S.S.S.R., Sibirsk, Otd.,1968 (4): 3-14.

100 Tectonophysics, 10 (1970) 95-101

Lee, W.H. and MacDonald, G.J.F., 1963. The global variation of terrestrial heat flow. J. Geophys. Res.,68: 6481-6492.

Lubimova, E.A., 1968. Heat of the Earth and Moon. Nauka, Moscow, 279 pp. Moiseenko, F.S., 1967. The structure of the earth’s crust of the Siberian southern

mountain frame. In: V.N. Gaiski, G.I. Karatayev, A.A. Kaufman, F.S. Moiseenko, H.H. Puzyrev and V.S. Surkov (Editors), The Regional Geophysical Investiga- tions in Siberia. Nauka, Sibirsk. Otd., Novosibirsk, pp. 123-136.

Moiseenko, U.I. and Sokolova, L.S., 1967. The earth’s heat flow in the bore holes of the south Minusinsk Depression. Geol. i Geofiz., Akad. Nauk S.S.S.R., Sibirsk. Otd., 1967 (1): 75-82.

Moiseenko, U.I., Soloviova, Z.A. and Kutolin, VA., 1965. The thermal conductivity of the granite at high temperature. Tr. Geol. Inst., Akad. Nauk S.S.S.R., 165: 670-671.

Moiseenko, U.I., Soloviova, Z.A. and Kutolin, V.A., 1967. The thermal conductivity of the eclogite and dolerite at high temperature. Tr. Geol. Inst., Akad. Nauk S.S.S.R., 173: 669-671.

Smirnov, Ja.B., 1968. Connection of the thermal field with the structure and develop- ment of the earth’s crust and the upper mantle. Geotectonics, 1968 (6): 3-25.

Tihonov, A.N., 1937. About the effect of the radioactivity decay on the temperature of the earth’s crust. Izv. Akad. Nauk S.S.S.R., Geogr. Geofiz. Ser., 1937 (3).

Tectonophysics, 10 (1970) 95-101 101