crusta si partea superioara a mantalei romaniei-demetrescu-tph-1982

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Tectonophysics, 90 (1982) 123-135 123

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

THERM L STRUCTURE OF THE CRUST ND UPPER M NTLE

OF ROM NI

CRISAN DEMETRESCU

Institute o r Earth’ s Ph ysics, Bucureqti -M Li gurele, P. 0. Box M G-.? Bucharest Romani a)

(Final version received February 8, 1982)

ABSTRACT

Demetrescu, C., 1982. Thermal structure of the crust and upper mantle of Romania. In: ES. Husebye

(Editor), The Structure of the Lithosphere-Asthenosphere in Europe and the North Atlantic.

Tectonophysics, 90: 123- 135.

A synthesis of the heat-flow data for Romania enabled a study of the thermal regime of the crust and

upper mantle to be made. This showed lateral thermal differences between various tectonic units. The

thermal structure of the crust and upper mantle appears to be mainly the result of mantle convection and

plate interaction in the studied area.

INTRODUCTION

Recent progress in deciphering the heat-flow distribution as well as the structure

of the crust in Romania has made it possible to approach the problem of the thermal

structure of the crust and upper mantle.

The geological structure of Romania is very complex, as a result of the interaction

of the Eurasian, African, and Arabian plates and of a few smaller intervening ones(Anatolian, Black Sea, Aegean, etc.). The major tectonic units are the foreland and

the Carpathians (Fig. 1).

The Carpathian foreland groups the platform areas of different ages: the East

European Platform (known as Moldavian in Romania), of Precambrian age, and the

Moesian Platform, of Epivariscan age.

The Carpathians can be divided into several major structural units which gener-

ally group together nappes of similar type and of synchronous age of tectogenesis,

the main tectogenetic phases being Cretaceous and Miocene. A Neosarmato-Plio-

cene molasse foredeep borders the folded chain outwards. Two large Neogenemolasse depressions (Transylvanian and Pannonian) cover parts of the InnerCarpathians.

Apart from ophiolitic assemblages, the Alpine magmatic activity shows three

0040- I95 i /82/~-~0/~02.75 0 1982 Elsevier Scientific Publishing Company



124

,

a

o= ‘-

5./

/

I-/_v--

i i



125

igneous periods: an ensialic predominantly alkaline period (Lower and partly

Middle Jurassic) known in the Central East Carpathians units and in the South

Carpathians and two talc-alkaline periods of Upper Cretaceous-Palaeocene and

Neogene age respectively.

The present paper gives a short account of the thermal structure of a few tectonic

units, namely: the Transylvanian Depression, the Eastern Carpathians and the

Moesian Platform, on profiles which were thought to be representative from the

geothermal point of view,

DATA AND METHOD

A new heat-flow map of Romania (Fig. 2) has recently been produced byDemetrescu et al. (1981~). The heat-flow data are based on temperature measure-

ments to depths of up to 1OOOm in 48 boreholes and on thermal conductivity

measurements on 184 sedimentary, volcanic, and crystalline rocks from 30 boreholes

in the Pannonian Depression, Transylvanian Depression, crystalline nucleus of the

Eastern Carpathians, Oag-Gut% and CIhmani-Harghita Neogene volcanic areas,

and the Moesian Platform. For most of the tectonic units on the convex side of the

Carpathian arc (Moldavian and Moesian platforms, Carpathian foredeep) the heat-

i

Fig. 2. Heat-flow map of Romania, June 1981. Units: mW mP2.



126

flow map of Demetrescu (1978/1979) has been used. This is based, for these tectonic

units, on geothermal gradient data determined by Paraschiv and Cristian (1976)

from a large number of formation temperature data (1 data point for about 1000

km’). No data are available for the Apuseni Mountains, Southern Carpathians and

Dobrudja.

The main features of the heat-flow distribution over Romania are: the high

heat-flow of the Pannonian Depression, Neogene volcanic zone, and Moesian

Platform, the low heat-flow in the Transylvanian Depression, crystalline nucleus of

the Eastern Carpathians, Moldavian Platform, and Carpathian foredeep, as well as

the large variations of the heat-flow between neighbouring tectonic units.

As regards the crustal structure, the data of Radulescu et al. (1976) Cornea et al.

(1978) Sollogub et al. (1978) and Radulescu (198 l), were used for the Conrad andMohoroviEiE discontinuities, and that of Visarion et al. (1973) and Sandulescu and

Visarion ( 1978) (Transylvanian Depression), Sandulescu ( 1980) and SanduIescu et al.

(1981) (Eastern Carpathians), Paraschiv (1974; 1979) and Barbu (1980) (Moesian

Platform), were used for the upper part of the crust.

Considering the age of the last thermal event affecting the various tectonic units it

follows that the entire crust has reached thermal equilibrium and the conduction of

heat through it is a stationary process. To support this statement we mention that,

for instance, a 30 km crust will have equilibrated to a rapid temperature change at

its base in about 8 m.y. (Lachenbruch and Sass, 1977). However, if there is heatingor cooling by a constant heat source or sink at the base of a 30 km crust the surface

heat-flow is not affected for some 8 m.y. and the time after which a steady state

conduction through the crust can safely be asssumed should be greater than about

24 m.y. (Lachenbruch and Sass, 1977); in other words only subcrustal processes

bringing or taking away heat constantly (e.g., long-term slow intrusion of a sill, or a

cooling sill after solidification), which started 24-8 m.y. ago, can affect the assump-

tion of a stationary heat conduction through the crust. Such sources or sinks might

have been active in relation to the Savian and Styrian erogenic phases affecting the

East Carpathians flysh units. The transient effect of the post-Palaeogene subsidenceand sedimentation was studied by Demetrescu and Polonic (198 1). showing negligi-

ble lateral variations in the Transylvanian Depression and Moesian Platform.

Moreover, the gross features of the heat-flow distribution over Romania are the

same after applying the necessary correction. Lack of data concerning the amount of

post-Palaeogene uplift and erosion of the Carpathians made it impossible to estimate

the apropriate correction.

Under these circumstances and treating the problem as one-dimensional, the

distribution of the temperature within the crust and the contribution of various

crustal layers to the observed heat-flow pattern can be estimated using the equation

&(K%) +A(z)=O (1)



127

where K is the thermal conductivity and A the heat production (Jaeger, 1965)

knowing the vertical distribution of K and A.

RESULTS

The thermal structure of the crust was investigated on several profiles marked

AA’-DD’ in Figs. 1 and 2. The first three profiles concern particular tectonic units.

(Oag-Gut% Neogene volcanic area, Transylvanian Basin, and Moesian Platform,

respectively), while the profile DD’ crosses several tectonic units. They were chosen

as having the densest information on the heat-flow distribution (4-20 km between

data points). The following discussion is largely based on Demetrescu et al. (1981a;

198 1 b; 198 lc), where one can find details of the chosen values of various parametersinvolved in calculations.

The profile AA’ crosses the Oag-Gutii Neogene volcanic area from NW to SE. The

heat flow along this profile takes values between 72 and 132 mW me2 (Fig. 3). The

heat-flow distribution seems to be determined by the cumulative effect of the

following factors: the distribution of volcanic rocks both at surface and at depth, the

thickness of sedimentary deposits, and the relief of the Palaeogene basement.

Therefore, in the central part of the studied profile, where the volcanic deposits of

the third cycle (Pontian-Upper Pliocene) are prevailing and the Palaeogene base-

ment is in an elevated position in comparison with adjacent areas, the heat-flowvalues are the largest ones: 125 and 132 mW m-’ (bore holes no. 607 and 602). The

somewhat smaller values of 115- 120 mW rnp2 (bore holes no. 508 and 125)

correspond to an area covered by lavas of the second cycle (Pannonian). At the NW

end of the profile the relative decrease of the heat-flow values, which are still large

(over 100 mW me2), might be accounted for by the existence of sedimentary

deposits and by the existence at depth of the volcanic masses belonging to the

04. . 8 12km

Fig. 3. Geothermal profile in OaS-Gutii Neogene volcanic area AA’ of Fig. 1). Triangles = bore holes.



1 2 8

TEMPERATURE ( ' Cl

1000 1200 1400

A ---\l o- 11

15.

-1

“-3 I0

0 gz u’

B %F rm

Y;I_- __ -_ -__--

25

- c- _ - _ _ _- - - - _ - - I - - - I + - - - - - - -

K = 4. 0 Wm“K- 1

20-111 A = 0. 21. 10-6Wm 3

3OJ, , .

Fig. 4. Step crustal model and temperature-depth variation for OaS-Gut% area. Melting relations after

Wyllie (1971). GSS-granodiorite saturated solidus; GSL-granodiorite saturated liquidus; GDS-

granodiorite dry solidus; GDL -granodiorite dry liquidus; EDS-basalt dry solidus; BDL -Basalt dry

liquidus.

[ rm7

I * Y \ \ , . - ; - - - , ' I I / * > , / - Ll

I - z , , n_ \ \.__ . ./ - .

a 9 -” Y ”

' . \,/’ ” ”

/ 9

: -J - ' l o

Y

25-0, l p0 ; O km

30-

Y

35-

Fig. 5. Geothermal model for a NNW-SSE profile in Transylvanian Depression (BB’ of Fig, 1).

I =Neogene sediments (Kx2.0 Wm-’ K-l; A= 0 . 8 pWm_‘); 2 =post-tectonic cover ‘(2.5; 0.8);

3 =ophiolite zone (2.1; 0.2); 4 =granitic layer (3.3; 2.0); 5 = basaltic layer (2.1; 0.2); 6 =units of northern

apusenides; 7 = Central-East Carpathian units; 8 = overthrust line; 9 = fault; 10 = isotherm



129

beginning of the second cycle (bore holes no. 601 and 14). The relatively low values

of 70-80 mW m- 2 found for the SE end of the profile correspond to areas covered

with old sediments-Palaeogene and Oligocene-Lower Miocene-(bore holes no.

19 and 126) and make the transition to neighbouring tectonic units.

A calculation on a step crustal model (Fig. 4) according to eq. 1 shows tempera-

tures as high as 800”-900°C at the base of the crust and the possibility that in the

absence of more efficient heat transfer through the crust, the crustal rocks of

intermediate composition are partially melted within the lower crust. About 75% of

the measured value of the heat flow is the mantle contribution.

The heat-flow distribution and the crustal structure along the profile BB’ are

shown in Fig. 5.

The Transylvanian Depression, through which the cross-section of Fig. 5 wastaken, is a post-tectonic structural element corresponding to a Neogene subsidence

area, with molasse sediments. It is superimposed on two structural complexes,

namely the folded basement and its post-tectonic cover. The former is made up of

crystalline, eruptive (of which the ophiolitic formation is of interest to us), and

Permian sedimentary formations; the latter includes Upper Cretaceous, Palaeogene,

and Eo-Miocene formations. The ophiolitic zone is the remnant of an intra-

Carpathian basin with oceanic crust, closed by the relative movement of the adjacent

units with continental crust. The ophiolitic formations are thrust towards the east

over the continental crust of the East Carpathian nappe system with crystallinerocks.

As regards the heat flow, one can see in Fig. 5 that on a background of values

decreasing from north to south from 33 to 29 mW rne2, a maximum of 45 mW rnp2

appears. Towards the borders of the basin the heat flow is increasing up to 55 mW

mA2. The minimum values, of 29-30 mW rnw2, correspond to a zone of maximum

depth of the folded basement and of reduced thickness of the granitic layer, whereas

the 45 mW rnp2 maximum corresponds to an elevated basement and a maximum

thickness of the granitic layer. The increase of the heat flow toward the borders of

the Transylvanian Depression corresponds to the increase of the thickness of thegranitic layer as the crystalline basement is rising to the surface.

The temperature variation within the crust is also given in Fig. 5, as calculated

according to. the structure described and eq. 1. One can notice the strong effect of

the structure of the upper part of the crust and the fact that the main discontinuities

in the crust are not at the same time isothermal surfaces. The temperature at the base

of the crust takes values of 250-400’C. The calculations show a value of 15 mW

rn-’ or less for the mantle contribution to the measured heat-flow. The contributionof the radioactive decay is of 15-24 mW mp2. The extra heat-flow due to the

contrast in thermal conductivity between the various crustal columns reaches 8 mWme2 in the maximum heat-flow area.

The heat-flow variation in the studied area of the Transylvanian Depression is

mainly the result of the distribution of the thermal properties of the formations



130

constituting the basin basement and its post-tectonic cover, from the first lo- 12 km

of the crust. As regards the low regional background (the mean heat-flow is about 40

mW me2), a descending limb of a convection current under the Transylvanian

Depression is not to be excluded.

The variation of the geothermal parameters along the profile ’ of Fig. 1 and the

structure of the upper part of the crust are given in Fig. 6. This profile intersects two

well known E-W faults in the Moesian Platform, the Cartojani and Videle faults.

In the central part of Fig. 6 the variation of the mean geothermal gradient,

calculated for the depth interval of O-500 m, is given. Because cores for conductivity

measurements were not available, the calculation of accurate heat-flow values was

not possible. Taking 1.2-1.4 W m-’ K-’ for the thermal conductivity of rocks, and

using comparisons with measured conductivities on similar rocks in other tectonicunits, gives heat-flow values of 30-40 mW m -’ for the northern half of the profile

and of 55-80 mW mm2 for the block limited by Cartojani and Videle faults, with a

maximum value of 70-80 mW m-*. However, taking into account that the lithology

of the 500-m thick surface layer is rather uniform along the profile, the graph of the

central part of the Fig. 6 is equivalent to that of the variation of the heat flow.

A comparison of the heat-flow profile with the crustal structure reveals the

increased heat-flow in the block bounded to the north by the Cartojani fault, an

increase that reaches 100% in the Videle fault area. As the variation of the Neogene

sediment thickness, which might affect the surface heat-flow, can contribute to the

Fig. 6. Geothermal profile in the Moesian Platform CC’ of Fig. 1). N-Neogene; Cr-Cretaceous;J-Jurassic; P-Permian; T-Triassic; C, -Carboniferous; D Devonian; 0, _Ordovician; C -Cartojani fault; V-Videle fault; triangles-bore holes.



131

0 10 km

Fig. 7. Geothermal model for the CC’ profile of Fig. 1. q-surface heat flow; q,-mantle heat flow; K,

-sediment/basement discontinuity; K-Conrad discontinuity; M-MohoroviEiE discontinuity; C-

Cartojani fault; T/-Videle fault. The values for the thermal conductivity of the sedimentary cover result

from the chosen conductivities of rocks constituting the layers shown in Fig. 6 (D, -0, and P-T = 2.5;

D - C =5.0; Cr-J=2.6; N= 1.4 W m-’ KP’)

heat-flow decrease in the northern block only to a very small extent, the observedheat-flow distribution implies differences in the geothermal regime of the two blocks,

probably for the whole crust and the upper part of the mantle. For the crustal model

of Fig. 7 the calculation gives temperature differences of up to 500°C at the base of

the crust and mantle contributions of 7-12 mW m-* and 30-54 mW m-*,

respectively. The existence of such thermal differences going down to the base of the

crust is also supported by the idea that the Ciuregti-Videle fault is a deep one, along

which linear eruptions took place as late as the Triassic. The fault is still active and

in its eastern section earthquakes with hypocenters at depths of 15-30 km are

reported (Cornea and Polonic, 1979). To account for the measured heat-flow theconvective additional heat would not have stopped after the eruptions ceased, but

continued by basalt sill-like intrusions in the upper mantle (Lachenbruch,

1978/1979).

The profile DD’ concerns an area with large lateral variations of the heat flow,

going WSW-ENE, from the central zone of the Transylvanian Depression to the

Moldavian Platform, through the Calimani-Harghita Neogene volcanic zone, crys-

talline nucleus of the Eastern Carpathians, flysh nappes, and molasse foredeep

(Fig. 8). The less certain structural elements are marked by broken lines in the

cross-section.The temperature distribution in the crust, as calculated using eq. 1 and model of

Fig. 8, shows considerable lateral variations. So does the Moho temperature, from200°C in the Moldavian Platform and Transylvaniana Depression, to 800”-900°C



132

D D’

40, /I50 1

-+ +

+ + +

\+ +- - _ v_lOOD

1 i 1 \ i i+(‘-_ _ _+_ f y‘, + n+.‘/V li\V, /

lij’ji\\,,v, I+ +

__--L_--+---

‘.t \ + ,L/\, ” ” Y

1 t n7 Tl “I’/ + +‘”

‘,+’ I\I ” \” V”

t--I+; ”I ‘b..

” ‘\&/\V/T

,v_---’ ’ 7_------ - L.,

I ” YY

\3000 ‘.

I 4000-- . - -_ _200~

Fig. 8. Geothermal model for the D D ’ profile of Fig. 1, Transylvanian Depression-Eastern Carpathians-

Moldavian Platform. I =basaltic layer K =2.1 Wrn-’ Km’; A =0.2 PW m-3); 2 =granitic layer (3.3;

2.0); 3 =Central-East Carpathian basement; 4 =green schists basement; 5 =podolic basement; 6 =

sedimentary cover of Transylvanian Depression and Moldavian Platform (2.0; 0.8); 7=flish nappes (3.5;

0.8); 8=foredeep; 9 =Neogene volcanites (2.6; 2.3); 10 =overthrust line; I1 =fault; 12 =isotherm.

in the Neogene volcanic zone. The mantle contribution to the observed surface

heat-flow presents the same type of variation, being very small under the thick crust

of the Moldavian Platform and Eastern Carpathians, as well as under the Tran-

sylvanian Depression, and very large under the Neogene volcanic area.

A parameter against which such calculations as presented above are rather

sensitive, is the heat generated in the so-called granitic layer, both through the

thickness of the layer and the heat production. The heat production is the least-knownelement and usually one considers values covering the interval of measured values.

To evaluate the possible variations we mention that for a 10 km granitic layer an

increase of 0.4 PW me3 (20%) results in a decrease of 4-5 mW m-* in the.mantle



133

heat flow and of 30” - 40°C in the Moho temperature. A value of 2 PW rnp3 was

chosen for the models above, as being more often used by various authors. A 3 PW

m -3 heat production of the granitic layer results in a negative contributions of themantle in low heat-flow areas. The lack of data concerning the vertical variation of

velocity of seismic waves makes a more accurate method of estimating the heat

production/depth relation (Rybach, 1978/1979) inoperative. However, in the case

of Transylvanian Depression (profile BB’), where such information exists (F.

Radulescu, pers. commun., 1980) and the granitic layer is thinner, calculations done

with A = 3.4 PW rnp3 and 0.6 PW m -3 for the granitic and basaltic layers

respectively, corresponding to 5.8-6.2 km SK’ (upper crust) and 6.8-7.0 km sY’

(lower crust), give an increased crustal contribution by about 15 mW rnp2, indicat-

ing a very low mantle contribution.

DISCUSSION

The data presented in the previous sections illustrate the complexity of the

heat-flow field and, related to that and to the distribution of various crustal and

upper mantle constituents, of the temperature field as well.

From the examples we have given, it is concluded that, within a given tectonic

unit, the lateral thermal variations depend on the structure and composition of the

crust, an important contribution being that of the first lo-12 km of the crust. Theconductive transfer of heat through the crust plays the main role on this scale.

On a larger, regional, scale the contribution of mantle processes becomes evident.

In these processes convection is the main mode of heat transfer.

Mantle convection might well be responsible for the Transylvanian Basin and

Carpathian foredeep heat-flow lows. Descending limbs of convection currents under

the Transylvanian Depression (Demetrescu, 1978/1979) would contradict neither

the active subcrustal currents necessary to account for the strong anisostasy of this

unit (Gavat et al., 1973; Constantinescu et al., 1976) nor a possible fossil subduction

in the area (Radulescu et al., 1976). Such a convection current could also be theresult of the active mantle diapirism under the Pannonian Depression or of the

extension of this basin (Stegena et al., 1975; Sclater et al., 1980). As regards the

well-developed minimum in front of the Carpathian bend, this seems to be the

thermal consequence of a subduction in the area; this subduction, still having

thermal effects at the end of Palaeogene (Demetrescu and Polonic, 1981) was active

at least as far back as 30-20 m.y. ago. The sinking of the cold decoupled piece of the

former subducting slab in which intermediate Vrancea earthquakes occur (Fuchs etal., 1979) might still contribute to the low heat-flow observed in the Carpathian

foredeep.For the high heat-flow areas deep seated convection is also the main heat transfer

mode, either in the Pannonian Depression, by active mantle diapirism or passive

currents induced by the extension of the basin, or in the Moesian Platform, by



134

sill-like basalt intrusions in the upper mantle, or in the Neogene volcanic areas, by

raising magma.

The negative correlation of the heat-flow and crustal thickness and the positive

one with the gravity field, rendered evident by Demetrescu (1978/1979) on a

regional scale, as well as a study of the thermal effects of post-Paleogene sedimenta-

tions in Romania (Demetrescu and Polonic, 1981) strongly support this point of

view.

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crusta si partea superioara a mantalei romaniei-demetrescu-tph-1982

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