Geothermal profiles of the lithosphere in Central Asia

Tectonqhysics,

164 (1989) 165-173

Elsevier Science Publishers

165

B.V., Amsterdam

Geothermal

- Printed

in The Netherlands

profiles of the lithosphere R.P. DOROFEEVA

and S.V. LYSAK

Institute of the Earth’s Crust, Siberian Branch of the U.S.S.R. (Received

November

30,1987;

in Central Asia

Academy

of Sciences, Irkutsk

revised version accepted

664033 (U.S.S.R.)

May 27,1988)

Abstract Dorofeeva,

R.P. and Lysak,

S.V., 1989. Geothermal

Rybach

and E.R. Decker

(Editors),

We present profiles profiles. platform.

generalized

in Central

heat flow data

Asia. Geological

The 12OO’C

isotherm

control

and estimated along

is estimated

The depth of this isotherm

profiles

of the lithosphere

Heat Plow and Lithosphere

decreases

deep temperatures

these profiles

to be at depths to 120-150

Structure.

is provided of 180-200

in Central Tectonophysics,

Asia.

in the lithosphere by boreholes

L.

along

two geothermal

and three seismic reflection

km in the southern

km in the Trans-Baikal

In: V. term&k,

164: 165-173.

part

area, and to 60-80

of the Siberian km in the Baikal

rift zone.

Introduction

mentary cover, and with the heterogeneity of the thermophysical properties of the rocks. Heat

The southern part of East Siberia includes three tectonically different regions: (1) the southern part of the Siberian platform in the west, (2) the

transfer by groundwater movement under the Angara-Lena artesian basin also affects the flux:

Tram-Baikal area of moderate orogenesis in the east, and (3) the Baikal rift zone in the central part (Fig. 1). Formation

of continental

Briefly, lower values occur in recharge areas of the basin (marginal uplifts), whereas highest values occur in the discharge areas (anticlinal uplifts and faults) (Lysak, 1983). The regional heat flow in the southern part of

crust in the southern

part of the Siberian platform was completed by or before the Riphean (Yanshin, 1980). The last tectonic activity there occurred in the PermianTriassic, with extensive intrusion of Siberian traps.

the Siberian platform is determined by relatively low geothermal gradients (13 + 1 mK m-‘) and the high thermal conductivity (3.0 + 0.1 W m-’ K-t)

which is mainly due to the halmeic carbonate

The mean heat flow in this area is 39 f 2 mW rnp2, based on 93 measurements. In detail, the flux is considerably lower at the Nepa dome and in the marginal uplifts, but higher (up to 43-45 mW rnd2 ) in the Sayan-Yenisei and the Angara

section in the sediments. Geothermal data are compatible with other geological and geophysical observations; e.g., a slightly dissected relief, absence of seismicity in the region, a thick magnetic layer (about 32.5 km), a very conductive layer

syneclises and in the Zhigalovo swell. The zones with highest heat flow coincide with terranes with widely developed trap volcanism or with anticlinal uplifts in the platform sedimentary. cover. The heat flow distribution varies with the geological and structural characteristics of the sedi-

being at great depth (about 100 km), normal Pwave velocities (8.1 km s-‘) just below the Moho, and a 40-km thick crust (Golenetskiy, 1977; Krylov et al., 1981). All this testifies to the tectonic stability of the area, and to a steady-state thermal regime.

0040-1951/89/$03.50

0 1989 Elsevier

Science Publishers

B.V.

Fig. 1. Heat flow distribution Regional

values

8 = plateau Sayan-Yenisei

of heat

basalts

in the southern flow

(Cenozoic).

syneciise,

Muysk

depressions, Basal-~lok;

(Ma

part of East Siberia.

m-‘):

9 = Main

Ic = Angara

rift zone and rift zone boundaries and

(mW

3 = < 25; tectonic

synechse,

zones:

and

5 = 50-75;

I = southern

Id = Zhigalovo

swell);

= Aldan shield, IIfb = Chara

ZZIe = Nizhneangarsk D-E

1 = Heat flow determination

4= 25-50;

Crustal stabilization in the moderately erogenic Trans-Baikal area was probably accomplished by late Palaeozoic-early Mesozoic times, although there is evidence for minor rejuvenation of the crust in the late Mesozoic and partly in the Cenozoic (Yanshin, 1980). The heat flow here is higher than in the Siberian platform: from 57 measurements the average is 51+ 5 mW rnm2. The lower heat flow values in this region are mainly confined to uplifts, and average values of 57-75 mW me2 occur in intramontane depressions filled with Mesozoic and Cenozoic volcanogenic rocks. Higher heat flow values (> 75 mW m-*) characteristically occur in areas of Cenozoic volcanism,

of the Siberian

I = Trans-Baikal

depression,

Severobaikalsk

= Ust-Kut-N~~ng~sk-Chara;

part

6 = > 75.

on land;

platform

zone of moderate

IIIc = Severe-Muysk

depressions). F-K

points

7= Siberian

10 = profile

2 = thermal

traps

sources.

(Permo-Triassic);

(la = Nepa orogenesis;

dome,

Zb =

121= Baikal

ridge, IIId = Verkhneangarsk lines:

= part of the Yamal-Kyakhta

A-B

= Ust-Uda-Lake

profile.

in the vicinity of the fault zones which discharge thermal waters. Compared to the southern part of the Siberian platform, the values of geothermal gradients in the Trans-Baikal area are almost twice as large (22 f 3 mK m-l). However, the thermal conductivity of the section composed mainly of crystalline rocks is significantly lower (about 2.5 W m-’ K-‘). Other geological and geophysical properties also change. For example, there is an almost fourfold increase in relief, seismic activity begins to occur, and the magnetoactive and the high electrical conductivity layers are thinner (19.5 km and 40 km respectively). However, P-wave velocities of the upper

167

mantle remain the same (8.1 km s-l), and the crustal thickness (up to 45 km) increases by a small amount. A steady-state geothermal regime prevails in a considerable part of the Trans-Baikal area (e.g., the western and central parts of the area). In some of the southeastern regions, however, especially those subjected to the Cenozoic activity, the thermal field has a pronounced non-equilibrium character. The thermal field is most variable in the central part of the studied region, where formation of the crust ended in the Late Devonian. In the Cenozoic, this territory was subjected to significant thermal and tectonic activity and the Baikal rift zone was formed. The rift zone is still active as indicated by its increased seismic activity and the well-defined geothermal anomalies (Lysak and Zorin, 1976; Florensov, 1977). Heat flow in the region ranges from 15 to 100-400 mW m-* or more (Golubev, 1982; Lysak, 1984). The low heat flow values occur in the mountains that border the rift depressions. Within depressions, the flux exceeds 50-75 mW m-*. The average heat flow is 57 f 23 mW m-* on the sides of the Baikal depression, based on nine measurements. Within the lake, the mean of 206 measurements is a higher value of 78 f 9 mW m -*. The most anomalous regions are the underwater Akademic ridge (southern Lake Baikal) with 88 f 8 mW m-*, and the southern- and northern depressions of the lake with 79 f 14 mW m-*, where geothermal anomalies are concentrated on the eastern (in the south) or western (in the north) margins. The lowest heat flow regions face the Selenga delta (63 f 6 mW m-*), and are also to be found in the central depression of Lake Baikal (54 f 8 mW m-*). On the basis of the narrow zones of large increases in geothermal gradients (up to 60-90 mK m-l or more) the highest heat flow zones are usually confined to the fault zones. These regions are often characterized by considerably higher temperatures in the bottom waters of the lake (Duchkov et al., 1977; Golubev, 1982), a phenomenon that seems to be caused by increased hydrothermal circulation and subaqueous discharge near local fault zones. Sedimentation, sub-

sidence and landslides contribute greatly to the decreased heat flows in the near-slope parts of the rift depressions, especially in the Baikal depression. Close agreement of the main geothermal parameters indicate that the non-equilibrium thermal regime of the Baikal rift zone is closely associated with the youngest tectonic activity of the area. This area has greatly dissected relief, increased seismic activity, a thin (18.5 km) magnetoactive layer, thinner crust (36-40 km) and lithosphere (X 75-40 km), anomalously low P-wave velocity in the upper mantle (7.7-7.8 km s-i), and an upwarping of the high-conductivity layer to a depth of 20 km (Golenetskiy, 1977; Krylov et al., 1981). Temperature-depth

modeling along profiles

Lithosphere temperatures were modeled for two profiles that cross the southern part of East Siberia and Lake Baikal. Three deep seismic sounding profiles were used for geological and structural control: the Ust-Uda-Lake Baikal-Khilok profile, the Ust-Kut-Nizhneangarsk-Chara profile, and a third profile that crosses the area from northwest to southeast (Fig. 1). Geothermal values for all profiles were estimated using the “sliding window” method. The window width, oriented along the profile, was 40 km (corresponding to the average crustal thickness), the window length, perpendicular to the profile, was 80 km (40 km on each side of the profile). Along both profiles in the Lake Baikal region the size of the “sliding window” was half as great because the geometry of the structure is more effectively revealed with this size. Radiogenic heat modeling

Heat production values for crustal rocks along the sections crossing the various tectonic areas of Southeastern Siberia were estimated on experimentally determined radiogenic elements in the core material (100 samples) or on samples collected from crystalline outcrops (1300 samples). Heat production values (A, PW m- 3, were calculated using the following formula from

168

Rybach

and Buntebarth

files. The logarithmic

(1982):

be 10 km. From

decrement

the use of exponentially

A = 0.1325 p (0.718 C, + 0.193 C,, + 0.262 C,)

ing heat production,

all crustal

where p is density

quite small at depths

of 20-30

are the uranium

(g cmp3) and thorium

sium (%) contents. the magmatic values

correspond

and

units.

data

For the southern

values

and C, of A for

1982;

identical along

Haak,

surface, rocks

in the study region was from

the individual

part of the Siberian

near

the base

each profile,

understood

to the published

Buntebarth,

1983). Surface heat generation using

Cr,

(10m4 W) and potas-

The obtained

rocks

(Rybach

determined

and C,,

abrupt

become

km, and are nearly

of the crust. within

and the transition in the lower

crust

to

decreas-

A-values

Therefore,

most of the radiogenic

to be generated

decrease

was assumed

heat is

20 km of the

to basic and ultrabasic is accompanied

in radiogenic

by an

heat.

rock plat-

Thermal conductivity

models

form, the mean A-value (1.3 pW mp3) for the sedimentary cover is based on lithological data from deep drilling. This is close to the mean

Experimental data from Dorofeeva (1982, 1983, 1984) were used to estimate the thermophysical

A-value

properties

(1.13 pW m-3)

for the sedimentary

of the whole of the Siberian al., 1979).

For

the Baikal

platform rift zone

cover

(Smyslov and

et

Trans-

Baikal areas, surface heat generation was calculated using separate sections that corresponded to the size of the window of estimated values in the line of the profiles. When experimental data were not available, information from adjacent massifs was used and account tent, age and tectonic Heat generation

of the local material

con-

position was taken. along the Ust-Uda-Lake

of the sedimentary,

magmatic

metamorphic

and

rocks along each profile.

Archaean and Proterozoic underlie sedimentary cover

crystalline basement in the Siberian plat-

form. Most of the sediments are early Palaeozoic formations. In the areas of the profiles the sediments are 3.5-5.5 km thick, as derived from deep drilling and deep seismic sounding data. A study was made of 230 core samples representing halmeic carbonate and terrigenous rock stituents

of the sedimentary

the con-

cover, and, to a lesser

Baikal-Khilok profile ranges from 1.87 to 2.03 pW m-3. The best A-value for the rift zone is considered to be 2.0 pW rnm3, and that for the

degree, the crystalline basement rocks. Over 400 and 100 rock samples were collected from boreholes in the Baikal rift zone and the Trans-Baikal

sediment cover in the rift basins is 0.6 pW me3 (Duchkov and Sokolova, 1974). The A-values for the Trans-Baikal area range from 1.2 to 4.0 pW m-3. The mean value of A is taken as 2.6 pW

area, respectively. Most of the core material is of magmatic and metamorphic rocks. The average thermal conductivity of the sedimentary cover in the southern part of the Siberian

mm31

platform is 3.0 + 0.1 W m-i erately erogenic Trans-Baikal

The Ust-Kut-Nizhneangarsk-Chara also crosses part of the Siberian platform,

profile traverses

the northeastern edge of the Baikal rift zone, and ends on the Aldan shield. An A-value of 1.3 pW rnp3 was used for the sedimentary cover of the platform because the sediments are similar to those discussed above. The approximate A-value for the rift zone along this profile was determined to be 1.64 pW mM3, a value lower than that for the rift zone of the southern profile (2 pW m-‘), and lower still than that for the Trans-Baikal area (2.6 pW me3). Crustal heat production was assumed to decrease exponentially with depth below both pro-

conductivity

K-‘. In the modarea the average

of the rocks is 2.5 k 0.2 W m-l

K-‘,

and in the Baikal rift zone and Baikal depression the deepest sediments have an average value of 1.2 f 0.6 W m-i mal conductivity structures. Such

K-‘. In all of these areas, therdata delineate large geological differences are not apparent in

the southern part of the Siberian platform (2.7-3.2 W m-’ K-‘) and in the Trans-Baikal area (2.4-2.6 W m-l K-i), although they are pronounced in the rift zone (0.8-2.7 W m-l K-l). The shallow sediments in the Baikal depression in the zone of Cenozoic volcanic activity have the lowest thermal conductivity (0.8 * 0.06 W m-i K-l). On the

shores of Lake Baikal the conductivity increases to 2.0 f 0.4 W m-’ K-i, and in the other depressions it ranges from 1.8 to 2.4 W m-l K-‘. Thermal conductivity increases to approximately 2.7 + 0.2 W m-i K-’ in the rift depression adjacent to the bordering mountains. Briefly, the thermal conductivities of upper crustal rocks depend on the major minerals in the structural-facies complex and on the geological and thermotectonic histories of a region. Deep thermal conductivity values were indirectly estimated by considering likely pressures at 20 km (5 kbar) and at the Moho (10 kbar) (Ma&en and Betcher, 1979), and seismic velocities and other geophysical data. Estimated thermal diffusivity, thermal capacity and density variation relationships from seismic velocities along the cross sections were also used, as were likely P-T conditions in the lower crust and upper mantle (Leonidov, 1967; Seipold and Gutzeit, 1974; Semenova, 1978; Dorofeeva, 1986). Density-velocity profiles based on deep seismic sounding data in the area were also used. (Krylov et al., 1981). Throughout the sedimentary cover the results show that deep thermal conductivity gradually increases with depth to depths of up to 3 km. In the granitic layer, conductivity decreases slightly to the 20-km depth, and in the upper part of the basalt layer it remains constant at about 2.4-2.5 W m-’ IS-‘. From the depth of 30 km to the Moho and into the upper mantle, thermal conductivity sharply increases to 3.5 W m-’ K-‘. This increase is caused by the sharp change in seismic velocities near the Moho and a change in composition, namely the increased predominance of ultrabasic rocks (eclogite and amphibolite) with hip-conducti~ty minerals. Deep temperature ca~cu~atio~~ For southern regions of the Siberian platform and a considerable part of the Tram-Baikal area lithospheric temperatures were calculated with the estimated conductivities and radiogenic heat values assuming steady-state heat transfer. For the Baikal rift zone, equilibrium models produced the lowest temperatures. This probably reflects a non-equi~b~um temperature field here,

and other models such as those devised by Zorin and Osokina (1981) were considered. In this twodimensional, time-dependent model both regional (large asthenospheric diapirs) and local {rift intrusion in the crust beneath the Baikal depression) heat sources at depth were assumed. Published data on thermophysical parameters and heat content were also used (Dortman, 1976), and well-defined boundary conditions were specified (0 o C at the surface and 1200 “-1250 o C at the base of the lithosphere). The distribution of isotherms in the crust in the southern part of the region was calculated using computer programs employing finite difference methods and estimated thermal properties. The resulting transient model temperatures are presented in the figures, together with the calculated steady-state temperature-d~th models. Results

The Ust-Uda-Lake

Baikal-Khilok

profile (Fig. 2)

The calculated temperatures range from 40 o to 70°C at the bottom of the predo~nantly halmeic carbonate sedimental cover in the southern part of the Siberian platform, where the average sediment thickness is 4.25 km. The average thickness of terrigenous sediments is 5 km in the Baikal depression, and temperatures at the base of the sedimentary cover are above 200” C. Along this profile, sediments do not occur in the Trans-Baikal area. Beneath the western part of this profile the first reflector in the crust (I) occurs at a depth of 20 km. The temperature at this depth is close to 230° C. This reflector is 6-7 km shallower in the region where the profile crosses the rift zone. The mean temperature at this depth is 320” C. In Zabaikalie, horizon I is found at a depth of 13-15 km, where the temperature decreases to 250 o C. The second reflector beneath the platform is at a depth of 24-28 km, where the temperature is calculated to be approximately 230°C. Beneath the rift zone and the Trans-Baikal area, this reflector is at a depth of 13 km, and the temperatures are 450” and 300°C, respectively.

170

T,“C ?ZOU

mo

i,mWd I 120ao-

-800 -600

400

-400 -200

) x-+w _~:.~?-y&-lT~rw. -+ .. l

THE SIBERIAN PLATFORM

THE

BAIKAL

RIFT

ZONE

THE TRANWAIKAL MODERATE

0

100

200

300

400

500

AREA OF

OROGENESIS

600

700KM

M

100 1.50 200

ilOO"

_y='200x c'

liO0'

Fig. 2. Tbe Ust-Uda-Lake B~~-~lok profile. Depth profile (Krylov et al., 1981): I = Siberian platform basement surface; 2 = intercrust boundaries I and II (n = established through deep seismic sounding data, b = assumed); 3 = low-velocity layer in the crust; 4 = Moho; 5 = low-velocity Iayer in the upper mantle (7.7-7.8 km s-l); 6 = terrigenous sediments in the Baikal depression; 7 = near-surface faults; 8 = deep fault zones. Geothermal profile (Golubev, 1982; Dorofeeva, 1983; Lysak, 1983; etc.): 9 = isotherms obtained for the steady-state geothermal field; IO = presumed temperature (” C) at the base of the lithosphere; II = heat flow distribution plot in the profile zone (dots indicate measurements made on the bottom of Lake Baikal); I2 = temperatures calculated with the help of the stationary model at the base of the crust; 13 = temperatures at the base of the crust projected with the help of the non-steady-state model (Zorin and Lepina, 1984).

The crust is about 40 km thick beneath the western part of the profile. The temperature at the Moho here is close to 375O C. The crust thins to 36 km in the rift zone, but the steady-state temperature at the Moho increases to 700” C. The average crustal thickness in the Trans-Baikal area is 44.5 km, and the temperatures there decrease to about 5OO’C at the Moho. From steady-state regime calculations, the 1200” C isotherm is at 180-200 km beneath the platform part of the profile, at 80 km in the Baikal rift zone, and at 150 km in the Trams-Baikal area. From the tw~d~ension~, non-eq~b~um geothermal model of the Baikal rift, it can be assumed that the 1200°C temperature in the rift occurs at the base of the crust, i.e., at a depth of 40 km. Steady-state and transient models agree with each other in the upper crust (down to a depth of lo-20 km). At the base of the crust,

however, a 400 *C difference between these models is observed. This difference can be regarded as a “geodynamic addition” to the steady-state field. The Ust-Kut-Nizhneangarsk-Chum

profile (Fig. 3)

Most of this profile traverses zones of increased seismicity, e.g., the depression uplands and especially the Severo-Mujsk ridge. In crustal areas below the central parts of the profile, low-velocity zones have been detected by deep seismic sounding. These low-velocity layers occur as separate lenses in the crust. The western part of the profile embraces part of the Siberian platform, where the average thickness of the sediments is 3 km. The calculated temperatures on the surface of the platform basement range from 30” to 75°C. Calculated temperatures do not exceed 35 o C beneath the north-

171

$mWtT?

1 16012080*

. -;“,

#w””

Q

3uH H

,I,,

x-x--*, . --_

I- ---_I- S- x-i,.,

l

THE BAlKAL

THE SIBERIAN PLATFORM

*--*_

l

RiFT ZONE

/ /

/ 400

i5a

500

X--.X\ *: . :. . * * t

*_x_x-x--x--x-~. .* *l*

600

100

THE ALDAN

- IZOD ” llxm - 800 - 600 -400 _200

SHIELD

800

900

I

h-0 200

H,KM

Fig. 3. ‘Ihe Ust-Kut-Nizhneangarsk-Chara profile. Depth profile (Krylov et al., 1981): 1 = Siberian platform basement surface; 2 = intercrust boundaries I, II and III (a = established through deep seismic sounding data, b = assumed); 3 = low-velocity layers in the crust; 4 = Moho; 5 = low-velocity layers in the upper mantle (7.7-7.8 km s-l); 6 ‘5 terrigenous sediments in the rift depressions; 7 = deep fault zones. Geothermal profile (Golubev, 1982; Dorofeeva, 1983; Lysak, 1983; etc.): 8 = isotherms obtained for the steady-state geothermal field; 9 = presumed temperatures (“c> on the lithosphere foot; IO = heat flow distribution plot in the profile zone (dots indicate drilling measurements, circles indicate measurements made on the bottom of Lake Baikal); If = temperatures calculated with the help of the steady-state model on the base of the crust: 12 = temperatures at the base of the crust projected with the help of the non-steady-state model (Zorin and Lepina, 1984).

eastern part of the Baikal rift zone at the base of the sedimentary cover in the Verkhneangarsk and Mujsk depressions. Beneath the Nizlmeangarsk and Severobaikalsk depressions these temperatures increase to 40”-80 o C. At the base of the sediments in the Chara depression the temperature is about 40 o C. Horizon I beneath the platform part of the profile occurs at the depth of 11-12 km. The calculated temperature here is about 14O’C. Beneath the Baikal depression, this horizon and horizons II and III, are delineated by “intelligent guesses” and the estimated temperature is 200 OC. Horizon I is determined with greater certainty beneath the remainder of the northeastern part of the Baikal rift zone, and the temperatures along it

range from 100 o to 375’ C. On the Aldan shield, this reflector accurs beneath the Chara depression at depths ranging from 2.25 to 4.25 km, and temperatures in these intervals are probably 40”~80°C. Regional deep seismic soun~ng surveys did not detect horizons II and III beneath the western part of the profile. Reflector I, however, was located at depths ranging from 15 to 24 km (see Mujsk and Verkhneangarsk depressions). The temperatures at these depths may be 250 O-475 o C, values that are about 150’ C higher than at the same depths in the platform. Reflector III occurs at depths of 21 (Mujsk depression) to 29.5 km (Ver~eang~sk depression), where the crustal temperatures may exceed

172

375 O-550” C. This horizon depth

of about

26 km

where the temperature Average part

crustal

36.5-42

km or more beneath The steady-state continuity and beneath

The

Dorofeeva,

the

temperature

Geofiz.,

depressions

to

and to 45-47 uplands.

on the Moho Severobaikalsk exceed

the Mujsk depression

disand

750 “-890 ’ C, they range from

700 O-800 o C, despite

the large

increase

crustal thickness to 45 km. The 1200 o C isotherm in the platform the northern

profile

(and likewise

in

isotherm

Teplofizicheskie

4: 111-113

Dorofeeva,

R.P.,

svoistv

1983. Rezultaty

kartirovaniya.

porod

Dorofeeva,

geoter~cheskogo B.P. Dyakonov

Primenenie

Geotermii

i Poiskovo-Razvedochnykh Sverdlovsk,

amfiteatra.

and v Re-

Issledovaniyakh.

pp. 76-80

R.P., 1984. Teplofizicheskie

Irkutskogo

Geol.

teplofi~~heskikh

tselei

Bulashevitch,

(Editors),

Ural Sci. Cent.,

osnovnykh

amfiteatra.

izucheniya

dlya

In: Yu.P.

Hatchai

svoistva

Irkutskogo

(in Russian).

gornykh

Yu.V.

porod

(in Russian).

svoistva gornykh

Geol. Geofiz..

10: 123-126

porod (in Rus-

sian). Dorofeeva.

R.P.,

Vostochnoj Dortman,

1986.

Teploprovodnost

Sibiri. Geol. Geofiz.,

N.V. (Editor),

Porod

i Poleznykh

zemnoj

10: 85-94

1976. Fizicheskie

Iskopaemykh.

kory

Svoistva

Nedra,

yuga

(in Russian). Gornykh

Moscow,

527 pp.

(in Russian).

part

of

in the southern

profile) occurs at depths of 180-200 km. Beneath the Baikal depression it rises to 60 km, and perhaps to 40 km. In the bordering areas, the depths of the 1200°C

1982. gornykh

gionalnykh

600 o to 700 o C. Severo-Mujsk ridge is anomalous because temperatures along the Moho there are only

R.P.,

raznovidnostei

the intradepression

temperatures

References

the western

the crust thickens

the rift basin,

beneath

NiThneangarsk

at a

Baikal,

from 360 o to 380 o C. In

part of the profile,

km beneath

beneath

is 39 km.

the Moho ranges

the central

occurs North

exceeds 600 o C.

thickness

of the profile

along

probably beneath

lower to SO-120 km.

Duchkov,

A.D.

and

Issledovaniya

Sokolova,

v Sibiri.

LX,

Nauka,

1974.

Geotermicheskie

Novosibirsk.

279 pp.

(in

Russian). Duchkov,

A.D.,

Kazantsev,

S.A., Golubev,

S.V., 1977. Geotermicheskie Geol. Geofiz.. Florensov,

N.A.

Stroeniyu

6: 126-130

Lysak,

na ozere Baikal.

(in Russian).

(Editor),

1977.

Baikalskogo

V.A. and

issledovaniya

Rifta.

Ocherki Nauka,

po

Glubinnomu

Novosibirsk,

153 pp.

(in Russian). Golenetskiy.

Conclusion

S.I., 1977. Analiz

cheskaya

aktivnost.

cheskoe

Raio~rova~e

Geofizicheskie

Lithospheric temperatures below the southern part of East Siberia were calculated using heat production and thermal conductivity models, geological control from boreholes, and three seismic reflection profiles which adequately sample the region.

Average

temperatures

of the upper

(the Moho) beneath the Siberian about 400 ’ C, with values exceeding

mantle

platform are 500 o C in the

Trans-Baikal area. ii Normal” lithospheric thicknesses are in the range 150 to 180-200 km, and thicknesses as low as 40-60 km may occur in the Baikal rift zone. The profiles and deep temperature calculations should be regarded as preliminary, especially for northern parts of the region. This is because of the heterogeneous distribution of thermal and geological data, and the varying reliability of the geothermal data. These parameters will be defined more precisely as more data are collected and as calculated techniques are updated.

epizentralnogo

In: V.P. Solonenko Vostochnoj

Osnovy.

Nauka,

polya.

Seismi-

(Editor),

Seismi-

Sibiri

i ego Geologo-

Novosibirsk,

pp. 162-184

(in Russian). Golubev,

V.A.,

1982.

Geotermiya

Baikala.

Nauka,

Novosi-

birsk, 150 pp. (in Russian). Haak,

U.. 1983. On the content

Th, U in the continental

and vertical

crust.

Earth

distribution

Planet,

of K,

Sci. Lett., 62:

360-366. Krylov,

S.V.,

Mandelbaum, Petrik

M.M..

enkina,

Z.R.,

Baikala

(po Seismicheskim

Mishenkin,

G.V. and Seleznev, Dannym).

VP.,

Mish-

V.S., 1981. Nedra

Nauka,

Novosibirsk,

104 pp. (in Russian). Leonidov,

V.Ya.,

1967.

porod pri vysokikh

Teployo~ost

nekotorykh

temperaturakh.

Geokhimiya,

gornykh 4: 470-472

(in Russian). Lysak,

S.V.,

1983.

kartirovaniya B.P. Dyakonov Geotermii

Metodika

territorii and

Yu.V.

v Regionalnykh

sledovaniyakh.

Ural

i rezultaty

geotermicheskogo

yuga Sibiri. In: Yu.P. Bulashevitch. Hatchai

(Editors),

Primenenie

i Poiskovo-Razvedochnykh

Sci. Cent.,

Sverdlovsk,

Is-

pp. 55-60

(in

Russian). Lysak,

S.V., 1984. Terrestrial

Siberia. Lysak,

Tectonophysics,

S.V. and

Baikalskoi sian).

Zorin,

heat flow in the south

of East

103: 2055215. Yu.A.,

1976.

RiFtovoj Zony. Nauka,

Geotermicheskoe Moscow,

Pole

90 pp. (in Rus-

173

Maisen,

B. and Betcher,

Mantii. Pollack,

A., 1979. Plavlenie

Mir, Moscow,

H.N.,

1965. Steady

heat condition

the half space and sphere. J. Geophys. Rybach,

L. and Buntebarth,

the petrophysical generation

and

Vodosoderzhaschej

123 pp. (in Russian). in layered

Res., 70: 5645-5648.

G., 1982. Relationships

properties

density,

mineralogical

media:

seismic

between

velocity,

constitution.

Earth

heat

Planet.

V. and Gutzeit,

kabhandigkeit

der

Gerl. Beitr. Geophys., Semenova, dolnykh

S.G.,

In: A.V. Chekunov

tatsiya

Geofizicheskikh

Kiev, pp. 3-10 Smyslov,

W., 1974. Untersuchungen Warmeleitfahigkeit

einiger

der DrucGesteme.

1978. Sootnoshenie porod

plotnosti zemnoj

i skorosti

pro-

kory i verkhnej

(Editor),

Metodika

Issledovanij.

i Interpre-

Naukova

Dumka,

(in Russian).

Moiseenko,

U.I.

and

Rezhim i Radioaktivnost

Chadovich,

T.Z.,

Zemli. Nedra,

1979.

Leningrad,

188 pp. (in Russian). Yanshin, Zorin,

A.L.

(Editor),

Moscow,

Zorin,

Nauk

Tectonika

Sevemoj

Evrazii.

S.V., 1981. Model nestatsionarnogo

polya zemnoj kory Baikalskoj SSSR., Fiz. Zemli, 7: 3-14

Yu.A. and Lepina,

rift lithosphere.

1980.

222 pp. (in Russian).

Yu.A. and Osokina,

temperatumogo Izv. Akad.

83: 4988504.

voln dlya gomykh

A.A.,

Teplovoj

Nauka,

Sci. Lett., 57: 367-376. Seipold,

mantii.

riftovoj zony. (in Russian).

S.V., 1984. Model field of the Baikal

Tectonophysics,

103: 193-204.