demetrescu-tph-1994

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TECTONOPHYSICS

ELSEVIER

Tectonophysics 230 (1994) 265-276

On the thermal regime of some tectonic units in a continental

collision environment in Romania

Cri%an Demetrescu, Maria Andreescu

Institute of Geodynamics 19-21 J.L. Calderon St. R-70201 Bucharest-37 Romania

(Received January 26, 1993; revised version accepted September 16, 1993)

Abstract

An analysis of the evolution of the main tectonic units in Romania shows that the thermal regime of the

lithosphere should be derived according, on the one hand, to the particular tectonic interactions the various tectonic

units have been involved in and, on the other hand, to the investigated depth interval. A steady-state conduction

model of the crustal temperature field based on the heat flow distribution and information on the structure is

presented for the Romanian territory. It shows large lateral thermal variations between tectonic units, as a result of

different geological and thermal histories. Within the frame of a complex modelling of the thermal evolution of the

lithosphere in the East Carpathians, as proposed in this study, the zero-order thermal effects of the pre-Miocene

oceanic subduction of the Eurasian plate are evaluated. The deep thermal structure of the subducted slab is derived

and shown to be compatible with the velocity structure of the lithosphere and the intermediate-depth seismicity of

the Vrancea area.

1. Introduction

The heat flow distribution on the Romanian

territory shows a complex pattern with high heat

flow in the Pannonian Depression, the Neogene

volcanic zone and parts of the Moesian Platform,

with low heat flow in the Transylvanian Depres-

sion, the Crystalline-Mesozoic and Flysch zones

of the East Carpathians, the Carpathian foredeep

and the Moesian Platform, and with large varia-

tions of the heat flow between neighbouring tec-

tonic units. A contour map, recently updated

(Demetrescu et al., 1991b), is presented in Fig. 1.

The tectonic units we refer to are described in

Fig. 2. The heat flow map is based on available

heat flow data (167 heat flow values by the end of

19881, supplemented, in areas of poor coverage,

with estimations from temperature and thermal

gradient information from oil industry boreholes.

Most of the 167 heat flow values were obtained

by temperature measurements down to 500-1000

m in thermally stabilized boreholes and conduc-

tivity measurements or estimations for the same

depth interval (Demetrescu et al., 1991b). The

heat flow contours are shown by broken lines in

areas for which no (Southern Carpathians) or

very poor (e.g. Apuseni Mountains, Western

Transylvanian Basin) geothermal information is

available.

The heat flow distribution gives a preliminary

idea on the geothermal regime of different tec-

tonic units in the Romanian territory. The next

0040-1951/94/ 07.00 0 1994 Elsevier Science B.V. All rights reserved

XSDI 0040-1951(93)E0205-9


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266

C. Demetr escu, M . Andreescu / Tect onophysi cs 230 1994) 265-276

step in characterizing the thermal regime is to

infer the depth-distribution of temperature. For

certain areas and certain depths, for which a

thermal steady-state can be assumed, the usual

way of doing this is by means of steady-state

conduction models. In the next section we pre-

sent such a model for the crust.

or still ongoing tectonic processes and the at-

tempts to model their thermal effects are briefly

reviewed in Section 3. A model for the deep

thermal structure of the East Carpathians is given

in the Section 4.

When the entire lithosphere is considered, one

has to take into account thermal effects of large-

scale tectonic processes involving the whole litho-

sphere. In the Romanian territory the lithosphere

structure is the result of the complex interaction

between the Alpine and older Europe. Some past

2.

The thermal regime of the crust

As far as the crust alone is concerned, steady-

state conduction models for defining the geother-

ma1 regime of various tectonic units are justified

for most of the Romanian territory (Demetrescu,

p

2 o

2 o

20

26O 280

I

I

I

90

I

‘“.‘r.

I

I

I

*t

,. \ U KRAINE

Fig. 1. Heat flow distribution on the Romanian territory. Contours in mW m

-*. Dots represent surface heat flow data points (after

Demetrescu et al., 1991b). EC = East Carpathians; SC = Southern Carpathians; AM = Apuseni Mountains; EEP = East European

Platform; MP = Moesian Platform; PD = Pannonian Depression; TD = Transylvanian Depression; NV= Neogene volcanites;

A-A ’ = possible direction of the cross-section of Fig. 5 (see text); O-O’ = trench line; rectangle = epicentral area of intermediate-

depth earthquakes.


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C. Demetr escu, M . Andreescu / Tectonophy sics 230 1994) 265-276

261

Fig. 2. Structural sketch of central and southeastern Europe (after Siindulescu (1984)). I = the major Tethyan suture: Transyl-

vanides, Vardar, South-Pannonian suture, Pienine klippes (a = units with ophiolites,

b =

associated units); 2 = Internal Dacides

(Austroalpine); 3 = Median Dacides b = Serbo-Macedonian Massif); 4 = Outer Dacides; 5 = Marginal Dacides (Danubian);

6 = Moldavides; 7 = Carpathian foredeep; 8 = Internal Dinarides; 9 = Outer Dinarides; 10 = North Dobrudjan orogen; 11 =

Neogene volcanites; 12 = East European Platform; 13 = Moesian Platform; 14 = epicentral area of intermediate-depth Vrancea

earthquakes: PD = Pannonian Depression; TD = Transylvanian Depression; A-A ’ = possible direction of the cross-section of Fig.

5 (see text): O-O’ = trench line of Fig. 5; rectangle = study area.

1982; Demetrescu and Veliciu, 1991). To support

this statement we mention (see Lachenbruch and

Sass, 1977) that the conductive time constant

(T = h’/K h = layer thickness, K thermal diffu-

sivity) for a 30-km crust is 7-8 Ma, which means

that steady-state conduction through the crust

can safely be assumed as at N 24 Ma after the

initiation of heating or cooling by some subcrustal

processes bringing or taking away heat constantly.

Such sources or sinks might have been active in

relation to the Savian (Early Miocene) and Styr-

ian (Middle Miocene) erogenic phases affecting

the East Carpathian flysch units, to the Walachian

(Pleistocene) phase affecting the Subcarpathians,

and in the case of the Pannonian Depression

which has been undergoing extension since the

Badenian (Middle Miocene). The transient effect

of the cooling of the Neogene igneous systems of

the East Carpathians was studied by Veliciu and

Visarion (1984) and was found to be negligible at

present due to the relatively old age (3-12 Ma).

The study of the transient effect of the post-

Palaeogene subsidence and sedimentation showed

negligible lateral variations in the Transylvanian

Depression and Moesian Platform (Demetrescu

et al., 1983). According to Royden and Burchfiel


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268 C.

Demetr escu, M . Andreescu / Tect onophysi cs 230 1994) 265-276

(1989) rapid uplift and deep erosion, likely to

produce strong thermal effects, were generally

lacking in the Carpathians. If we accept an ero-

sion of 5 km in 20 Ma, the effect on the original

heat flow could be only about 10% (Powell and

Chapman, 1988). And last, but not in the least,

we have neglected a possible important cause for

the non-steady-state thermal conditions, namely

disturbances by water flow, for the following rea-

sons: (1) large crustal-scale water flow systems in

the crust are not known in the study area; (2)

basin-scale {a few kilometr~s) water flows do not

show up in the map because the density of data

points can not resolve the characteristic lateral

distribution of the heat flow in that case; (3) the

consistency of the heat flow values at the regional

scale of this study shows that water flow distur-

bances do not control the temperature field at

this scale.

Geotherm families, vertical cross-sections and

maps of the lateral variation of temperature at

various depths, based on steady-state conduction

models, have been presented. The results (see a

complete reference list in Demetrescu and Veli-

ciu, 1991) show that: (1) within a given tectonic

unit, the lateral thermal variations depend on the

structure and composition of the crust, the most

important contribution being that of the first

lo-20 km; (2) large lateral thermal variations

appear between different tectonic units, including

horizontal gradients of 200-3OO”C/IOO km; this

seems to be a consequence of different Moho

heat flows (e.g. lo-20 mW rnw2 in platform ar-

eas, 15-30 mW m-’ in the external units of the

East Carpathians, 50-60 mW m-’ in the Neo-

gene volcanic zone, 15-20 mW m-’ in the Tran-

sylvanian Depression, and 40-60 mW rnp2 in the

Pannonian Depression), characterizing tectonic

units with very different geological histories (Fig.

2).

In this section we present a steady-state 1-D

conduction model for the entire Romanian terri-

tory, calculated in a grid of 12 X 20 minutes of

latitude and longitude. The heat flow map of Fig.

1, information on crustal structure (Radulescu,

19791, and a model of the vertical variation of

thermal properties of rocks (Table l> were used

in deriving the grid data. FolIowing Cermak (1982)

Table 1

Thermal properties of crustal rocks (partly after Cermik,

1982)

Crustal layers

K, c

Ao

D

(Wm-’ K“) (K-l) (PWm-s) (km)

Neogene

1.3

0

0.8 0

sediments

Pre-Neogene

2.5

0

0,s 0

sediments

Upper crust

3.0

0.0008

* *

Lower crust

2.0

0

* *

* According to C-,(O)of the province and Eqs. (4) and (9) with

C, = 89~900, C, = 2.17 (Rybach and Buntebarth, 1984). :,@I

= 5.9 km s-r for the East European Platform; 6.0 km s _ ’ for

the Pannonian Depression, Apuseni Mountains and Neogene

volcanic arc; 6.2 km s- ’ for the Transylvanian Depression,

Carpathians, Foredeep and Moesian Platform (F. Radulescu,

pers. commun., 1989).

the thermal conductivity was considered as tem-

perature-dependent according to:

K=K,(l t-CT)_’

(I)

A single A(z) relationship (A = heat produc-

tion: z = depth) for the entire investigated depth

interval has been derived for each heat flow

province (Demetrescu et al., 1991a), making use

of the existent information (F. Radulescu, pers.

commun., 1989) on seismic velocities characteriz-

ing the crystaiiine basement of various tectonic

units.

Briefly, combining the exponential depth-de-

pendence of the heat production (Lachenbruch,

1968):

A(Z) =A(O) exp( -z/D)

(2)

with the seismic P-wave velocity-dependence of

the heat production (Rybach and

1984):

A( 2) = C, exp[ -C,v,( z)]

where C, and C, are empirically

constants, and z*r is the velocity of

subs in:

Buntebarth,

(3)

determined

P-waves, re-

A(z) = C xp[ -C+,(O)] exp( --z/L))

(4)

where u,(O) is the seismic velocity in the crys-

talline basement.

For a given heat flow province we then com-

bine two well known empirical relationships, pro-


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C. Demetr escu, M . Andreescu / Tectonophy sics 230 1994) 26.5-276

269

posed by Birch et al. (19681, Roy et al. (1968) and

by Pollack and Chapman (1977):

9=qr+flA,

(5)

q, = 0.6~

(6)

with a modification by CermQk (1982) of Eq. (5):

Q = 4, + LSi;,

(7)

to obtain:

D 0.4@&,

(8)

In Eqs. W-(8), q is the surface heat flow, 4,

the reduced heat flow, A, =A(01 the surface heat

production, and the bar indicates averages over

the province.

Taking into account Eq. (31, we obtain:

0.49

L, = C, exp[ -C,i-j,(O)]

(9)

which defines a single A(Z) relationship, Eq. (41,

for the heat flow province, provided the mean

surface heat flow and the mean seismic velocity

in the basement could be estimated.

The calculation resulted in a quasi 3-D model

of the crustal temperature field for the entire

Romanian territory. The lateral distribution of

the temperature field in the study area is illus-

trated by maps at different depths, of which

give in Fig. 3, as an example, the map for 20

-I

UKRAINE

Fig. 3. Temperature distribution at a depth of 20 km. Contours in “C. EC = East Carpathians; SC = Southern Carpathians;

AM = Apuseni Mountains; EEP = East European Platform; MP = Moesian Platform;

PD = Pannonian Depression; TD =

Transylvanian Depression; NV = Neogene volcanites.


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270 C. Demetr escu, M . Andreescu / Tect onophysi cs 230 1994) 265-276

depth. At this depth, temperatures of 200-400°C

characterize the platform areas, 400-500°C the

external units of the East Carpathians, 600-800°C

the Neogene volcanic zone, 300-500°C the Tran-

sylvanian Depression, and 500-700°C the Pan-

nonian Depression.

The model obtained confirms previous models

concerning individual tectonic units. The large

lateral thermal variations between tectonic units

seem to be a consequence of

heat flows.

3.

Tectonic processes involving

sphere in the study area

different mantle

MED. DACIDES

TRANSYLVANIDES

the entire litho

INT. DACIDES

A structural sketch of central and southeastern

Europe (Fig. 2) shows the major Carpathian units,

which group together nappes of similar type and

of synchronous age of tectogenesis, and their

relation to the East European and Moesian plat-

forms of the foreland. Two main ophiolitic su-

tures are present in the study area (Trans-

ylvanides and Outer Dacides).

In the Alpine evolution of the Carpathians two

major periods, each with paroxysmal intervals,

can be distinguished following Sandulescu (1984;

1988), Royden and Burchfiel (19891, and Csontos

et al. (1992): one in which extension prevails and

one in which compression is the main stress char-

acteristic. The extensional period is characterized

by the opening of the Tethys in the Middle Trias-

sic and its evolution by spreading processes, ac-

companied by the appearance of extensional

basins with stretched continental or oceanic crust

on both the Apulian and the European margins

(the Outer Dacides on the latter). The change to

the compressional regime was determined by the

opening of the Atlantic and the subsequent inter-

action of the African and European plates along

the mobile Tethyan area.

Two main tectogenetic phases (mid-creta-

ceous and Miocene) are responsible for the

mountain building in the Carpathians: the

Dacides, Transylvanides and the Danubian as the

result of the first phase, and the Moldavides as

the result of the second phase (Fig. 4). The Mol-

davidic tectogeneses (Styrian-Middle Miocene,

MOLDAVIDES

EXT DACIDES

I

125 100

75

50

25

0

TIME BEFORE PRESENT 1M.Y.

Fig. 4. Main tectogenetic phases in the Carpathians (after

S5ndulescu (1984)). .T3= Late Jurassic; K, = Early Creta-

ceous;

K, =

Late Cretaceous;

PAL =

Paleocene; EOC =

Eocene; OLI = Oligocene; MI0 = Miocene; PLI = Pliocene.

Moldavian-Late Miocene) implied also shorten-

ing and consumption of the crust, processes linked

to the formation of the Neogene volcanic arc

superimposed on the internal areas of the Carpa-

thians. Minor deformations in the bend area of

the East Carpathians were produced during the

last tectogenetic phase-the Walachian (Pleisto-

cene). The East European and Moesian plat-

forms are underthrust with respect to the Carpa-

thians.

Two large post-tectonic Neogene molasse

basins (Transylvanian and Pannonian) are super-

imposed on the folded units and a molasse fore-

deep (of Sarmato-Pliocene age in the study area>

borders the folded chain outwards.

At present, in the Romanian territory, the

Eurasian plate is in contact with two (Con-

stantinescu et al., 1976) or three (Airinei, 1977)

lithospheric fragments (sensu Burchfiel, 19801,

namely: the Inter-Alpine, the Moesian, and the

Black Sea. Various models of plate-tectonics ap-

plied to the Carpathian area tried to include the

intermediate-depth seismicity of the Vrancea

area. The earthquakes of this type occur in a very


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271

narrow epicentral area (Fig. 2) of about 60

X

20

km’, to a depth of about 200 km. The seismo-

genie volume is almost vertical; in the NE-SW

direction the hypocentres deepen at an angle of

about 65”. The quasi-verticality of the T-axes and

the quasi-horizontality of the P-axes characterize

the focal mechanism (mainly reverse type) of

these earthquakes (see a review on the seismicity

and source models in Trifu, 1991). Historically

two types of tectonic models were proposed: (1)

active subduction of the oceanic part of the

Eurasian plate in the East Carpathians (Radules-

cu and Sandulescu, 1973; Herz and Savu, 1974);

(2) active SE-NW quasi-vertical subduction in

the East Carpathians bend area (Roman, 1970;

McKenzie, 1972; Constantinescu et al., 1973). For

the East Carpathians, more recent ideas favour a

Miocene continental collision with a type A sub-

duction following the episode of oceanic subduc-

tion of the Eurasian plate (Burchfiel, 1980;

Sandulescu, 1988; Royden and Burchfiel, 1989).

Better-constrained seismological data called for

variants and combinations of the two basic mod-

els, namely: (a) palaeo-subduction of the Eurasian

plate in the East Carpathians with a decoupled

lithospheric block in its southeastern extremity

sinking gravitationally into the mantle (Fuchs et

al., 1979; Oncescu, 1984a); and (b) active SE-NW

subduction in the Carpathian bend area interact-

ing with the palaeo-subducted slab in the East

Carpathians (Oncescu, 1984b). For the latter case,

a model has recently been presented (Kiratzi,

19931 of the deformation attributed to the occur-

rence of intermediate-depth earthquakes.

The compressional tectonics characterizing the

folded units of the Carpathians, changed into

extensional tectonics in the Pannonian Depres-

sion. The relationship between the evolution of

the lithosphere and its thermal regime for these

two areas was studied by Veliciu and Visarion

(1984), and, respectively, Demetrescu and Polonic

(1989).

For the East Carpathians, Veliciu and Visar-

ion (1984), following Radulescu and Sandulescu

(19731, considered a Miocene oceanic subduction

of the Eurasian plate and, adopting the 2-D model

of Lubimova and Nikitina (19781, concluded that

both the high heat flow and the build-up of the

Neogene-Quaternary volcanic chain are conse-

quences of this process.

However, according to the tectogenetic scheme

presented above, we suggest an alternative more

complex approach which consists of modelling a

pre-Miocene oceanic subduction followed by a

Miocene underthrusting of a continental litho-

sphere in the process of continental collision

leading to the Moldavidic tectogeneses. In the

next section we model only the zero-order ther-

mal effects of the pre-collisional (pre-Miocene)

subduction of an oceanic lithosphere followed by

an interval (Miocene-Present) of immobility and

show that the deep thermal regime of the litho-

sphere is compatible with available data on the

velocity structure (Oncescu et al., 1984) and, for

the Vrancea area, on the intermediate-depth seis-

micity (Trifu, 1991). The thermal effects of the

Miocene interactions, to be studied elsewhere as

a next step, would be superimposed on the ther-

mal field described in the next section. A Miocene

subduction of a continental crust and upper man-

tle would give thermal effects of the same type as

those described in the next section (van den

Beukel, 19921, attenuating to a certain extent the

recovering effects of the Miocene-Present inter-

val of immobility supposed in the present model.

Parts of the crust will be consumed in the short-

ening process and contribute to the formation of

the volcanic arc, with its characteristic high sur-

face heat flow, superimposed on the low heat

flow background produced by the subduction in

the corresponding area. The thermal effects of

the thrusting of the Moldavide nappes would

concern mostly the upper part rather than the

deeper sections of the subducted slab (C. Deme-

trescu et al., in prep.).

For the Romanian sector of the Pannonian

Depression, Demetrescu and Polonic (1989),

based on subsidence data from 75 boreholes fairly

uniformly distributed over the area, and on heat

flow data interpreted in terms of the instanta-

neous homogeneous lithospheric stretching model

of McKenzie (1978), concluded that the extension

in the study area amounts to 40%, that the con-

vective transfer of heat by lithospheric material

ascending during extension contributes 15-30

mW m-2

to

the observed surface heat flow and


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272


that the deep thermal structure of the litho-

sphere, including the thermally defined thickness,

differs by 15-20% from the current estimates

based on steady-state conduction models.

4.

Subduction model for the deep thermal struc

ture of the East Carpathians

In this section we present an order-of-magni-

tude 2-D calculation of the thermal structure of a

lithosphere involved in the subduction process

possibly responsible, at its southeastern extrem-

ity, for the Vrancea intermediate-depth earth-

quakes.

According to Enescu et al. (1989), based on

arguments relating the magnitude of the strongest

2

1

0

O 1 km

Or

sw

NE

I

earthquakes to the subduction velocity and the

age of the subducting plate, the subduction is of

the oceanic type and the age of the oceanic

lithospheric slab subducted in the Vrancea zone

is 140-160 Ma. The velocity of subduction was

estimated to be 2-5 cm yr-’ (Enescu, 1985) or

5.7-8.4 cm yr-* (Enescu et al., 1989).

The calculations were made with the method

of Hasebe et al. (1970) for a 120~km-thick oce-

anic lithosphere (thermal conductivity K = 4.18

W m-i K-‘, heat production A = 0.2 pcl.wm-3

in the 5-km-thick basaltic crust, 0.009 PW me3 in

the upper mantle) allowed to subduct with 2 cm

yr-‘, dipping 65” from northeast to southwest, for

50 Ma and kept immobile for another 20 Ma. The

calculation would be representative for any

cross-section cutting roughly perpendicularly the

b

5 TOkm

NE

Fig. 5. Thermal regime of a subducted lithospheric slab: (a) after 50 Ma of continuous subduction with a rate of 2 cm yr-I; (b) after

another 20 Ma of immobility. Crosses represent the lithosphere/asthenosphere limit; dots indicate projections of hypocentres (see

text). Upper plot = surface heat flow of the model; lower plot = temperature.


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273

structures of the East Carpathians. One possible

direction of the cross-sections is marked in Figs. 1

and 2 (A-A’) and it was chosen to be perpendicu-

lar to the trench line, O-O’, which we placed at

the eastern margin of the Neogene volcanites, in

agreement with the position of the subduction

inferred from magnetotelluric studies (StatricH et

al., 19861, and parallel to the main horizontal

strike of the Vrancea seismogenic volume.

The mesh used in the finite-difference calcula-

tions was made up of 60 grid points in the hori-

zontal direction (600 km) and 20 in the vertical

one (380 km). In the initial state the tempera-

tures at the lithospheric top and bottom were 0

and 1333°C respectively, and the thermal gradi-

ent below the lithosphere was taken as 3 mK

m-l. Zero horizontal heat flux at the lateral

margins and zero vertical heat flux at the bottom

of the model have been maintained.

According to a recent review (Davies and

Stevenson, 1992), the frictional heating has been

shown not to be a major heat source in subduc-

tion zones. However, a small value of 25 mW rnp2

has been considered at the upper face of the

subducted slab, which corresponds to a shear

stress of about 40 MPa for a subduction rate of 2

cm yrr’

(or smaller for higher rates). The heat

from this source conducted into the slab is of no

importance in establishing the temperature distri-

bution in the descending slab, in comparison with

the sink represented by the downward movement

of the cold lithospheric material during subduc-

tion.

The major source of heat in the back-arc area,

namely the induced flow generated in the mantle

wedge by viscous coupling to the subducted plate

(Davies and Stevenson, 1992), which advects heat

above the slab, was modelled considering veloci-

ties of 0.2 cm yr-

’

in the vertical direction and

0.1 cm yr-’ in the horizontal direction. These

values were suggested to us when applying our

approach to the heat flow data analysed by

Hasebe et al. (1970) for the Japan trench and

back-arc system, and they were used here only to

simulate high heat flow in the back-arc area of

the subduction model. The induced flow in the

mantle wedge above the slab is not capable to

heat up the slab/mantle interface (Davies and

Stevenson, 1992) and, consequently, will not in-

fluence the temperatures in the slab. As we con-

centrate our attention only on the subducted slab,

the actual values of parameters characterizing the

heat sources and the heat transfer above the slab

are not important in this paper. The back-arc

heating during subduction and the presence of

such a thermal anomaly after subduction stopped

are possible elements of the thermal evolution of

the Pannonian and Transylvanian depressions.

This is still to be studied.

The results of this two-dimensional calculation

are shown in Fig. 5a (temperature distribution

after 50 Ma of continuous subduction) and Fig.

5b (temperature distribution after another 20 Ma

of immobility). The calculated surface heat flow

of the model is given as well. Similar to other

subduction zones (Hasebe et al., 1970; Minear

and Toksiiz, 1970; Toksiiz and Bird, 1977; Fur-

long et al., 19821, the typical drag of isotherms by

the subducting slab, low heat flow above the

trench and high heat flow in the back-arc area, is

the main characteristic of the thermal regime of

the subducted lithosphere and the overriding

plate. It is to be mentioned here that the subduc-

tion velocity and the exact duration of subduction

are not critical parameters in this case, because

after some time of continuous subduction (about

36 Ma for 2 cm yr-’ and shorter times for larger

velocities) a steady-state is reached in which tem-

peratures within the plate do not change until the

slab stops, in accordance with the results of Mol-

nar and England (1990) concerning the thermal

regime near major thrust faults.

The 20 million years of immobility results in a

partial recovering of temperatures in the slab and

of the surface heat flow. The slab can still be

identified in the surrounding mantle down to

depths of about 300 km, in accordance with the

existence of “fast blocks” in the Carpathian area

in the depth intervals SO-180 km and 180-250

km, rendered evident by a 3-D model of the

velocity structure of the Romanian territory

(Oncescu et al., 1984).

The hypocentres of 268 Vrancea intermediate-

depth earthquakes which occurred between Octo-

ber 1981 and August 1986 (Trifu, 19911, were

projected in Fig. 5b on a NE-SW plane. It is


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274


interesting to note that the intermediate-depth

seismic activity, reaching 180-200 km, is confined

within the isotherm of 800°C which approxi-

mately marks the brittle-ductile transition for

ultramafic materials (Marquis and Hyndman,

1992). Of course, having in view the very narrow

seismogenic volume (about 20 km), a 3-D mod-

elling would seem more appropriate (Jones et al.,

1994).

On the other hand, the absence of intermedi-

ate-depth seismicity elsewhere in the East Carpa-

thians gives way to the question whether the

seismogenic volume in the Vrancea area is in-

deed genetically linked to the subduction process

in the East Carpathians. A thermal model of the

compressive type interaction of the Black Sea

fragment with the other tectonic fragments and

plates in the Romanian territory, which includes

the Vrancea seismogenic volume, might be rele-

vant. The more general geodynamic models tak-

ing into account the eastward flow of the mantle

with respect to the lithosphere, which have been

suggested to apply also in the case of the Carpa-

thians and which have recently been forwarded

(Ricard et al., 1991; Doglioni, 1991, 19921, should

also consider and explain local features such as

the confined intermediate-depth seismicity of the

Vrancea area before any attempts to thermal

modelling are undertaken. As a matter of fact,

thermal modelling of subduction cannot distin-

guish among various tectonic models, the same

cold lithospheric volume at depth being obtained

in either case, due to the downgoing of the litho-

spheric material. For the East Carpathians, if the

seismogenic volume were linked genetically to the

subduction process, we would be able to distin-

guish between continuous subduction and one

which stopped 20 Ma ago, taking into account the

depth of the brittle-ductile transition inferred

from the thermal model; in this respect, it seems

that the latter tectonic model is to be preferred.

The model calculation predicts a high heat

flow of 60-90 mW m-* in the back-arc area

beginning some 120 km west of the trench line

which we identify with the Apuseni Mountains-

Pannonian Depression high in the heat flow map

of Fig. 1, and a low heat flow of 25-60 mW m-2

in a 120-km wide zone west of the trench line

which is partly coincident with the Transylvanian

Depression low in the heat flow map of Fig. 1. As

no standard measured surface heat flow data for

the Vrancea area are available (Fig. 11, a direct

comparison of the model results with measured

values is not possible for this particular area.

When discussing the model surface heat flow,

one should take into account that in this section

we modelled only one stage (the pre-Miocene

subduction) of a complicated tectonic process;

however, this stage is crucial in defining the ther-

mal regime of the deepest part of the East Carpa-

thians and adjacent areas. The measured surface

heat flow distribution is, of course, the result of

the superposition of thermal effects of several

processes at different depths and lateral-extent

scales. In this respect, the narrow belt of high

heat flow corresponding to the Neogene volcanic

arc seems to be the result of the much shallower

process of shortening and the consumption of

parts of the continental crust during the colli-

sional stage.

5. Conclusion

The analysis of the thermal evolution of the

main tectonic units in Romania, considering that

the formation of the Carpathians, their relation-

ship with the foreland and the development of

sedimentary basins inside the orogen have been

the result of complex interactions of tectonic

plates in the mobile Tethyan area and of subse-

quent continental collision following the closure

of Tethys and related basins, shows that the study

of the thermal regime of the lithosphere in Ro-

mania should be differentiated according to the

particular tectonic interactions between the vari-

ous tectonic units on the one hand, and to the

investigated depth interval on the other hand.

As far as the crust alone is considered, simple

steady-state conduction models for defining the

geothermal regime of various tectonic units, start-

ing from the surface heat flow distribution, are

justified for most of the Romanian territory. A

steady-state conduction model of the crustal tem-

perature field for the entire Romanian territory

confirms previous models concerning individual

tectonic units. The large lateral thermal varia-


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215

tions between tectonic units, as illustrated for

example by the temperature map at a depth of 20

km, seems to be a consequence of different man-

tle heat flows, which in turn can only be a result

of different geological histories with past or ongo-

ing tectonic processes involving the whole litho-

sphere.

In this study a complex modelling is proposed

of the thermal evolution of the lithosphere in the

East Carpathians area, namely of the pre-Miocene

subduction of an oceanic lithosphere followed by

a Miocene underthrusting of the East European

continental margin in the process of continental

collision leading to the Moldavidic tectogeneses.

The zero-order thermal effects of the pre-

Miocene subduction, which were evaluated in this

paper, seem to characterize the deep thermal

regime of the lithosphere in the area, which is

compatible with available data on the intermedi-

ate-depth seismicity in the Vrancea area and on

the seismic velocity 3-D structure of the litho-

sphere. In our view, the thermal effects of the

collisional stage concern shallower depths, affect-

ing mainly the crustal temperature field and the

lateral distribution of the surface heat flow in the

East Carpathians area.

6.

Acknowledgements

Thanks are due to Drs. T. Lewis, F.W. Jones

and B. Bodri for useful discussions during the

preparation of this paper, and to Prof. H. Wil-

helm, Dr. F. Horvath and two anonymous review-

ers for comments and suggestions for improving

the first version. This study has been supported

by the Institute of Geodynamics, Bucharest (Pro-

jects 7/1991, 4/1992X The revised version was

prepared during a CEC “Go West” fellowship at

the Geophysical Institute of Karlsruhe University.

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