Contribution to the thermal regime of the Provençal Basin based on Flumed heat flow surveys and previous investigations

Tecfonoph_ysics, 128 (1986) 303-354 Elsevier Science Publishers

CONTRIBUTION PROVENCAL FLOW

303

B.V.. Amsterdam

TO THE THERMAL BASIN

SURVEYS

BASED

in The Netherlands

REGIME

ON FLUMED

AND PREVIOUS

’ and J.P. FOUCHER

J. BURRUS

- Printed

european

OF THE

HEAT

INVESTIGATIONS

’

’ Institur Franqtts du PPtrole. BP 311. 92506 Ruetl-Malmaison Cede-x (Frunce) -’ lnstitut Fran@

geotraverse

pour rExploitation de la Mer, Centre de Brest, BP 337,

79273 Brest Cedex (France) (Received

November

22. 1985; accepted

March

23, 1986)

ABSTRACT

Burrus,

J. and Foucher,

Flumed

J.P., 1986. Contribution

heat flow surveys and previous

European We present

original

heat flow determinations

of the Proveqal

A total of 121 thermal determinations

along

gradients depth

and 37 conductivities

sections

based

where results are ambiguous).

Lions) to 85 f 14 mW m-* (lower Corsican whether

margin),

this asymmetry

suggesting could

cause (sedimentation,

subcrustal

heat flow is probably

east in the abyssal flow relations magmatic

activity.

The

Further

or attenuated remains

refraction

investigations

the heat flow on the upper margin

investigations.

continental

are necessary

heat flow

The mean

(expect

observed

for the Toulon-Calvi

from 55-65

mW mm’ (Gulf of

above

salt structures

be accounted lithosphere.

constrained, to elucidate

for by the standard The geodynamic be related

the apparent

of the Gulf of Lions and on the Provencal

or by any other distribution

of the

heat flows observed

but could

’

heat flow. We examine

that an asymmetrical

regime. The elevated

cannot

poorly

Nice-Calvi).

with previous

of the observed

etc.) and conclude

for sedimentation,

by the CEPM

mW mm’ (Var Basin) to 103 - 108 mW m

distribution

the cause of this thermal

surveys

Toulon-Ajaccio;

together

heat flow increases

by thermal

topography,

origin

the Flumed

Sardinia;

geophysical

and from 55-65

an asymmetrical

for oceanic

subcrustal

out during are examined

on previous

The observed

be caused

plain, corrected

established

of this speculative

Basin based on

and St. Mueller (Editors).

from NW to SE along the profiles

(West Sardinia)

superficial

carried

Basin (Gulf of Lions-West

heat flows are clearly shown to increase transect,

regime of the ProvenCal

In: D.A. Galson

Part 2. Tectonophysics, 128: 303-334.

Geotraverse,

along three transects

to the thermal

investigations.

to the

age/heat significance

to post-rifting

high local variability

margin

of the Ligurian

of Sea

INTRODUCTION

During the last decade, numerous studies have been devoted to the formation of passive margins, Experimental data as well as quantitative reconstructions have suggested that passive margins form after a rifting stage characterized by brittle extensional tectonics in the upper crust, ductile attenuation in the lower crust and

0040-1951/86/$03.50

0 1986 Elsevier Science Publishers

B.V.

44

FRANCE

Fig. 1. Morphologic O.B.-oceanic

map of the Proveqal

boundaries

(Burrus,

Basin,

including

the Gulf

of Lions

and the Ligurian

Sea.

1984).

upper mantle, and upwelling of hot asthenospheric material, causing initially, high flow and fault-controlled subsidence (McKenzie 1978; Steckler and Watts, 1978, 1980; Royden et al., 1980, 1983; Royden and Keen, 1980; Le Pichon and Sibuet, 1981; Beaumont et al., 1982; etc.). Following this rifting stage, the margin undergoes a phase of general subsidence and decreasing heat flow induced by passive cooling of the lithosphere. Despite numerous restrictions, this concept has been widely applied as it permits a simple relation between crustal geometry, heat flow and subsidence history. Of particular interest are young basins, where heat flow and subsidence are still in a transient stage: predictions are then more sensitive to geodynamic assumptions. The Provenqal Basin (northwestern Mediterranean) (Fig. 1) is a well-documented example to apply this kind of analysis. This deep, intra-erogenic basin, formed after a rifting stage during Late Oligocene-Early Miocene times, is covered by more than 8 km of sediments and 2.7 km of water in its deepest part. Its opening is generally related to the Apenninic subduction, somewhere east of Sardinia, and subsequent collision during the Lower Miocene in the Apennines (Biju-Duval et al, 1978; Dercourt et al, 1985), following eastwards rotation of Corsica and Sardinia (Burrus, 1984). The sedimentary cover includes four main sequences (see the recent review by Rehault et al., 1984). (1) Synrift deposits are sandy shales of Rupelian-Aquitanian age (30-24 Ma), drilled in the past (Cravatte et al., 1974; Hsii et al., 1978a). (2) Lower Miocene (24-7 Ma) deposits are turbiditic shales. (3) Following isolation of the Mediterranean from the Atlantic, Messinian evaporites and salt were deposited and affected by halokinetic restructuring, while the upper margins eroded (7-5 Ma, Hsii et al., 1978b). (4) Deltaic and turbiditic shales have been deposited since the Pliocene transgression (5-O Ma). The seismic structure of the pre-Messinian sediments remains poorly constrained in the center of the Basin, but has been precisely described in the Gulf of Lions

30s

(unpublished structures Hinz,

CEPM

Ligo 1 and

and synrift

deposits.

1972; Leenhardt,

2 MCS

surveys),

revealing

typical

There is ample seismic (Fahlquist

half-graben

and Hersey,

1972; Recq et al., 1979: Le Douaran

1969:

et al., 1984), gravity

/

Fig. 2. Location

map of Flumed

al., 1984): 3 = Flumed 6 = previous

thermal

thermal

and previous gradient:

gradient;

7 = previous

evaporites;

Y = limit of Messinian

protruding

diapirs

between

in the Gulf

Languedoc

superposed measurements

and Sardinia,

on the transect (JA-JG).

thermal

4 = Flumed

salt;

The two segments

gradient of thick

12 = oceanic

is divided

investigated

thermal

IO = limit

of Lions;

measurements. conductivity;

by Jemsek

gradient

+ conductivity;

8 = limit of Lions:

(A-E).

et al. (1985)

(Burrus,

1984).

The profile which

of Fig. 10 and 11 are located

within

of Mesainian II = limit of

The profile

CC’ (Nice-Calvi)

included region

et

+ conductivtty:

salt m the Gulf

boundaries

in seven regions

I = well; 2 = ESP (Le Douaran 5 = Flumed

seven A.

groups

AA’.

is of

306

(Morelli, 1975; Morelli et al., 1977) and magnetic (Bayer et al., 1973; Galdeano and Rossignol, 1977; Burrus, 1984) evidence that the center of the basin is underlain by oceanic crust surrounded by attenuated continental margins. Despite considerable previous investigation, only few heat flow data had been collected, and mainly in the Ligurian Sea. For this reason, the CEPM (Comite d’Etudes Petrolieres Marines), grouping IFP, IFREMER and SNEA(P), carried out in 1981 and 1982 the Flumed (FLUX in Mediterranean surveys to investigate the distribution of heat flow along transects located between the Gulf of Lions and Sardinia, between Toulon and Ajaccio, and between Nice and Calvi. The surveys were completed on the R.V. “Suroit” from IFREMER (Fig. 2). Mainly based on Flumed results, this paper is devoted to our present knowledge of the thermal regime of the Provenqal Basin, and is divided into two sections. First, Flumed data (120 heat flow determinations} are presented together with previous results; uncertainties on both conductivities and gradients are evaluated for Flumed data. Second, referring to the geophysical and geological setting, Flumed and previous thermal data are discussed: superficial perturbations are examined, in particular thermal refraction due to the Messinian diapir field. Previous geodynamic interpretation in terms of crustal attenuation and standard oceanization processes are critically discussed, and an alternative model of asymmetrical subcrustal heat flow is proposed. FLUMED AND PREVIOUS AND ONSHORE

HEAT FLOW DETERMINATION

IN THE PROVENGAL BASIN

Using mainly bottom-hole temperatures, mean heat flows (corrected for paleotemperature variations) have been previously determined in Corsica, in Sardinia, and in Provence. In Hercynian Corsica, the mean heat flow is 76 + 6 mW mm2 (8 values), but is only 60 k 5 mW rnw2 (5 va 1ues ) on the Maures-Esterel substratum (Lesquer et al., 1983). In Sardinia (Loddo et al., 1982), high heat flows are observed in the Sardinian rift and Campidano graben (24 values, 100 mW me2), but the heat flow from the Hercynian substratum is significantly lower (55 mW mM2). In Languedoc, the heat flow is variable (80-100 mW m-2-Vasseur et al., 1980) and influenced by the Massif Central thermal anomaly. The radiogenic heat pr~uction is still poorly constrained. It is 1.8 & 0.2 FW rnv3 for the crystalline Maures-Esterel basement (Lesquer et al., 1983), but the mean value for the Provenqal basement, made of sedimentary thrust sheets, is probably lower. A value of 3.1 + 0.2 PW me3 has been reported (Lesquer et al., 1983) for Corsica; no r>diogenic data are available for Sardinia. The highest heat flow is apparently correlated with the highest radiogenic contribution, and the radiogenic heat generation can probably not explain the apparent difference between Provence and Corsica, as pointed out by Lucazeau et al. (1985). In contrast, Mailhe et al.

307

(1986) proposed

that this difference

could result from erosion

not very clear whether

a significant

the Corsica-Sardinian

block and the Provenc;al

basement

is characterized

(Lucazeau

et al., 1985).

Gulf of Lions-Sardinian

difference

by a mean

substratum.

equilibrium

heat

in Corsica.

It is thus

heat flow exists between However,

the Hercynian

flow of 55-65

mW

rn-’

margin

Previous results Few previous heat flow determinations and the southern Ligurian Sea. Erickson values

of corrected

have been published for the Gulf of Lions et al. (1977) mentioned two uncorrected

of 50 and 59 mW m- ’ for the deep fan of the Phone,

and 104 mW me2 (uncorrected)

in the Southern

Ligurian

and two values of 87

Sea. From

bottom-hole

temperatures, an uncorrected heat flow of 102 mW rn-’ has been calculated base of the East Menorca Rise (Erickson and Von Herzen, 1978). Hutchison

at the et al.

(1985) have measured uncorrected heat flows of 822117 mW m -’ in the vicinity of DSDP 372 and explained this variability by the effect of salt structures. The mean heat flow after correction for sedimentation would be 92 + 10 mW m-‘. These determinations are shown on Fig. 2 and summarized in Table 1.

TABLE Previous

1 heat flow determinations

Station

Dl-1.1

in the Gulf of Lions and Baiearic Basin

Lat.

Long.

Penetration

Gradient

Conductivity

Observed

(“N)

(OE)

m

(“C

(W mm’ Cm’)

heat flow

probes

km-‘)

Ref. *

(mW mm’) 1

40 02.9

05 00.7

-

5

87

f

3

1.12+0.05

975

7

1.2

40 03.6

04 58.8

_

5

71

f

3

1.12+0.05

79+

6

I

1.3

40 02.6

04 57.7

_

5

70

f

2

1.12*0.05

79*

6

1

1.4

40 02.5

04 56.1

5

76

k28

1.12kO.05

85*34

I

1.5

40 02.6

04 54.9

_

5

73

*

1.12?0.05

s2+11

1 1

Dl-2.1

7

40 00.4

04 54.0

_

6

74

*

7

1.15+0.07

86215

2.2

40 01.6

04 53.6

_

6

70

+

9

1.15+0.07

82k16

1

2.3

40 02.4

04 52.9

_

5

71

*13

1.15+0.07

82+19

1

2.4

40 02.8

04 52.1

_

5

96

+13

1.15f0.07

110*

2.5

40 03.8

04 51.5

5

101

511

1.15kO.07

116i_16

1

2.6

40 04.6

04 51.4

5

102

+11

1.15*0.07

117+16

1

2.7 DSDP

Leg 42

40 05.8

04 51.8

40 01.9

04 47.8

_

5

85

+

9

1.15io.07

75

!I

2

1.30

6

C9-135

41 37.

05 18

6.3

2

56

i

c9-134

41 59.

05 54

11.5

4

75

i15

c9-133

41 33

08 01

10.1

4

63.5+

CH 21: 19

42 14

07 09

* References: (1977),

4-Lister

I-Hutchison (1963).

3

2-Erickson

7

2

14

3

1.04

58*

1.16

87122

1.28

81+

and Von

Herzen,

I

1

9x*13 103i

_~

et al. (1985).

17

3 8

3

104 + 26

4

(1978).

3-Erickson

et al.

30x

Flumed results A complete

thermal

transect

Gulf of Lions to western data reduction

are described

to 0 = questionable) on the number

has been

Sardinia.

in Appendix

has been ascribed

of sensors

obtained

Experimental

1. A quality

margin

of the

of the determinations

and

factor Q (from 3 = very good

to each heat flow determination

used and linearity

tary cover and the deep structure

from the upper

aspects

of the thermal

gradients.

of the crust are well constrained

depending The sedimenin the western

part of the transect, but only post-Messinian sediments are clearly observed in the eastern part, where the total thickness of sediments is poorly constrained and the crustal

structure

has not been investigated.

The thermal gradients (86 values) and conductivities (27 values) are presented with uncertainty, location, and experimental characteristics in Appendix 3. Seven different geographic groups have been distinguished (A-G, Fig. 2) and the mean gradients and heat flows are given in Table 2; questionable determinations (Q = 0) were not considered for the calculation of these means. The observed heat flows have been superposed on the geological section in Fig. 3; previous heat flow determinations are also plotted in Fig. 3. The geological section is based on a compilation geophysical Ligurian

of unpublished CEPM seismic lines in the Gulf of Lions and published interpretation (Burrus, 1984; Le Douaran et al.. 1984).

Sea

Previous investigations Numerous heat flow

determinations

have

been

carried

out

in the

northern

Ligurian Sea along the Nice-Calvi transect. Foucher et al. (1976) and Rehault (1981) reported heat flows of 100-110 mW me2 (after correction for sedimentation)

TABLE

2

Mean Flumed standard Group

heat flow determinations

between

the Gulf of Lions (G of L) and Sardinia

(uncorrected);

deviations Mean gradient

*

Mean conductivity

(‘Ckm-‘)

n

(W m-’

C-‘)

*

Mean heat flow

Location

(observed)

n

(mW m-*) A

49+13

B

46rt

C

73f15

D

47*

E

54+

20

1.35kO.12

16

14

1.36&0.10

5

3

1.40*0.14

3

103 f 27

east of G of L

7

18

1.3210.07

1

64&10

western oceanic

domain

5

14

1.10f0.15

1

725

eastern

oceanic

domain

6

upper G of L margin

66*19 62*

8

6

lower G of L margin

F

76519

10

(1.10*0.12)

0

84*21

lower Sardinian

margin

G

77*13

6

1.08kO.12

1

84*14

upper Sardinian

margin

* n = number

of values

309

/

Fig. 3. Geological

section and observed

into seven groups

of measurements

of the profile, The location

D

j

E

the Gulf of Lions and Sardinia. divided heat flow determinatic .ms in the vicmity and triangles represent previot IS deterrnlll~iti~~ns.

heat flow profile between

(A-G).

open circles are projected of the ESP’s (Le Douaran

Full circles represent measurements.

et al.. 1984) are also indicated.

mW rn-’ (after in the centrat part of the Basin; higher values of 165-175 correction) were observed at the top of salt structures. whereas lower (corrected) values of 80 mW mS2 were recorded on the Var and Roya deep fans. More recently,

TABLE

3

Previous Group

mean gradient, of

fz *

stations

conductivity

and heat flow determinations

in the northern

Ligurian

Location

Conductivity

Observed

Corrected

(Fig. 2)

(W rn-

heat flow

heat flow

(mW mm2)

(mW rn- I)

5

Var Basin

5

Central

I y-i)

79-82

Ligurian

_

_

101-107

Sea Ref. **

I_____-

I 1

(except salt domes) JA

11

Var Basin

I.20 kO.13

65k24

77

7

JB

10

Var Ridge

1.26 i: 0.04

56rfr 7

73

2

JC

15

West Central

1.3orto.10

71*13

103

2

JD

14

Central

1.29,0.15

78113

95

2 2

Basin

Basin

JE

8

NE Central

Basin

1.151kO.10

1032

3

121

JF

11

SW Central

Basin

1.28 i: 0.09

105 ._c21

147

2

JG

13

Lower Corsican

1.09 * 0.02

86+11

104

2

* n = number

of stations.

** References:

l-Rehault

(1981): 2-Jemsek

et al. (1985)

310 TABLE Mean

4 Flumed

rected);

heat flow determinations

standard

between

Toulon

and Ajaccio

(southern

Ligurian

Sea. uncor--

deviations ___-

Group

Mean gradient (“Ckm-‘)

Mean conductivity

* II

(Wm-‘“C-r)

*

Mean heat flow

Location

(observed)

n

(mW m-‘) TAl

65+12

5

1.14,O.lO

74+1g

Proveqal

TA2

95124

8

1.12rirO.07

7

86_+20

oceanic-lower

TA3

63&

3

1.15io.10

1

71*

Corsican 7

8

margin margin

upper Corsican margin

* n = number

of values.

Jemsek et al. (1985) reported an increase of the corrected heat flow from 73-77 mW me2 off Monaco to 104 mW rn-’ off Calvi; higher (corrected) values (121-147 mW m-‘), were observed on the eastern part of the transect. These previous determinations are shown in Fig. 2, and summarized in Table 3.

+ ++ ++ + +i +++* * + +++*+++ ++*+r+ *+++++++++ -+++++r+ + l

it++++ +++++t ++++++ +++++++ +L++++t +++

Fig. 4. Geological unreliable

section

and observed

(Q = 0) determinations,

are observed

on the Provenqal

heat flow between

triangles

margin,

represent

making

Toulon

previous

it difficult

and Ajaccio.

determinations.

to outline

Open circles represent High and low heat flows

the mean trend.

311

Flumed determinations

Toulon-Ajaccio. Toulon-Ajaccio located

Twenty transect

in the abyssal

indicated

thermal

gradients

have

(Fig. 2) and are presented plain.

The

in Table 4: three regional

mean

thermal

been

determined

in Appendix gradients

groups have been considered

along

the

3. All stations and

heat

were

flows

(Provencal

are

margin,

oceanic basin and Corsican margin). The heat flow profile is superposed on the deep structure derived from Burrus (1984) Le Douaran et al. (1984), the CID10 seismic profile (Mauffret et al., 1982) and seismic data collected during the Flumed experiment in Fig. 4. Owing to penetration problems. data are concentrated to the east and west, and little information is available for the central part of the profile.

The deep structure along the Nice-Calvi transect (Fig. 5) is conNice- C&i. strained by the MS 47 seismic profile (Finetti and Morelli, 1974) and ESP data (Le Douaran et al., 1984). Many of the gradient determinations carried out along the

Fig. 5. Geological determinations

section

and observed

heat Row between

with Q z 1, open circles are unreliable

mean heat flows observed by Jemsek et al. (1985).

Nice and Calvi. Full circles represent

determinations

(Q = 0). triangles

(JA - JG)

Flumed are the

312 TABLE

5

Mean Flumed Group

heat flow determinations

Mean gradient (“C km-‘)

between

Nice and Calvi (northern

Mean conductivity

* n

(W 111-‘“c-l)

*

Mean heat flow

Sea. uncorrected)

Location

(observed)

f,

(mW m

NC1

50*10

4

1.18*0.07

3

59+10

NC2

51*20

3

1.23&0.11

62+

NC3

91f56

3?

1.20 i 0.23

1 1

* n = number

Llgurian

‘) Provenqal

3

oceanic

108 f 6X

Corsican

margin

domain margin

of values.

profile are of poor quality owing to penetration difficulties. Only nine reliable determinations could be obtained, on the Provenqal margin (deep fan of the Var), in the oceanic domain, and on the Corsican margin (Table 5). To the east, the apparent variability is extremely high (Table are unreliable (Q = 0, two sensors).

5, Appendix

3), and half of the determinations

DISCUSSION

One of the purposes of heat flow experiments in young margins is to constrain the deep geodynamic evolution; this goal can be achieved only if perturbations of the crustal heat flow by superficial phenomena can be removed. The presence of a recent and thick sedimentary layer causes considerable thermal blanketing and reduces the heat flow by up to 30% in the abyssal plain (Jemsek et al., 1985; Lucazeau and Le Douaran, 1985; Burrus and Bessis, 1986) and represents such a first-order regional perturbation. A second kind of perturbation is caused by thermal refraction around high-conductivity structures such as crystalline blocks (substratum), salt domes and ridges. This kind of perturbation is due to a lateral transfer

of heat,

negative

(lower

and

Erickson

et al. (1977), salt structures

than

causes

both

positive

normal

heat

flow)

(higher thermal

than

normal

anomalies.

alter the local distribution

heat

As pointed

flow) out

and by

of the heat flow, but

do not cause any shift in the mean heat flow; they have been de’scribed as a major cause of thermal perturbation in the Western Mediterranean (Erickson et al., 1977; Rehault et al., 1984; Hutch&on et al., 1985; Jemsek et al., 1985). A third kind of perturbation is related to convective effects in the sediments, whether driven by regional discharge or by free convection. These convective effects are generally difficult to quantify; they involve a negative thermal anomaly (recharge area, downward water flow) and a positive anomaly (discharge area, upward flow). In the case of the Provensal Basin, large-scale regional water flows have not been investigated, but are not likely for two reasons. First, the Messinian layer, composed of salt and evaporites, is probably relatively impermeable, as are to a lesser degree the Plio-Quaternary shales in the abyssal plain; therefore, vertical water flow would

probably consist

be severely hindered. mainly

Maures),

of crystalline

which are unfavorable

perturbations geodynamic

to the heat context

Discussion

Second, rocks

the basements

(except

aquifers.

flow. Then

Before analyzing the thermal

We examine we discuss

and to the passive-margin

of the superficial perturbations

of the surrounding

in the region

continents

to the northeast

below the possible the heat

of the

superficial

flow referring

to the

model.

to the heut flm

in detail the Flumed

thermal

profiles,

we examine

theoretically

effect of salt structures.

Theoretical effect qf suit diupirisrn on the surface heat flm’ Selig and Wallick (1966) have quantitatively demonstrated that the heat flow is enhanced above or in the vicinity of salt structures. However, in their calculation. the base of the salt layer was considered as a boundary at a constant temperature, which seems unrealistic to apply to the Western Mediterranean case, where the base O-3 km deep. Therefore, we have used a 2-D numerical code

of the salt is only

(Doligez et al., 1986) to quantify the perturbation, assuming constant temperature at the base of the sediments (at a depth of 8 km below sea level), which is more than two times the depth to rectangular shapes and low conductivity (1.3 series. We studied the

the salt-shale interface. We assumed that salt structures have a high conductivity (5.5 W m - ’ OC-’ ) contrasting with the W m- ’ OC-‘) of the Plio-Quaternary and infra-Messinian effect of the width and elevation of individual and twin salt

structures (see Appendix 2). Isolated m/t structures. Positive and negative anomalies salt structures (Fig. 6). The positive perturbation appears

are induced concentrated

by isolated above the

6km

Fig. 6. Computed

perturbation

represents

the perturbation

protruding

at sea level.

to the surface for a burial

depth

heat

flow above

of 0.5 km,

a single salt structure. The solid line

the dotted

lines eorrespond

to a diapir

314

HEAT

SURFACE 125 Fo

Fig. 7. Computed of the perturbation instead

FLOW

perturbation is reduced

of the surface with respect

of 1 km) and to the presence

heat flow above two juxtaposed to Fig. 6 owing

to the greater

salt diapirs.

The amplitude

width of each structure

(5 km

of the two structures.

rectangular structure and its magnitude depends mainly on the burial depth of the top of the salt structure. For a depth of 0.5 km, the perturbation is 30% of the steady-state heat flux (Fig. 6), and reaches a maximum of more than 100% for structures protruding at or above sea level. If the width of the structure is 10 km instead of 1 km (which are reasonable bounds in the case of the Western Mediterranean-Mauffret et al., 1973; Pautot et al., 1984), the perturbation is slightly attenuated; negative heat flow anomaly is visible around the structure, but is much less concentrated than the positive anomaly, and its amplitude is 4 to 5 times lower. Twin structures. If several structures are juxtaposed (Fig. 7), the effects of single structures add and the contrast between positive and negative anomalies is reduced. From this discussion, we can derive some practical rules for the case of the Western Mediterranean: (1) shallow salt structures amplify the surface heat flow by a factor of 1.3 (0.5 km burial) to 2.2 (0 km burial), which is roughly in agreement with measurements above the diapir field of Angola (see Von Herzen et al., 1972), and (2) these positive perturbations are concentrated above the top of the structures. In_ contrast, outside the structure, the surface heat flow is less than normal. This negative thermal anomaly is less concentrated, and is observed across a distance two or three times greater than the width of the structure. its amplitude is several time smaller than the positive one. For this reason, a random distribution of heat flow measurements is likely to yield the mean, unperturbated heat flow.

Superficial

perturbations

in the Gulf of Lions

The mean value of heat flow (uncorrected for sedimentation) increases (Fig. 3, Table 2) from the lower margin of the Gulf of Lions (group B) towards the east (groups D, E, F and G). Maximum values are encountered between the eastern flank of a possible paleo-oceanic ridge (Burrus, 1984) and the tilted blocks of the Sardinian margin (groups F and G). Could this trend be explained by the presence of salt structures?

315

Four different

styles of salt structures

and 12). Group

A is characterized

mostly

by elastic

replaced

corresponds eastern

are encountered

by the near absence

equivalents.

to discontinuous,

The second

shallow

thick (0.7-1.0

km), nearly

flat-lying

style (western

half-domes

part of group B and the western

from west to east (Figs. 3 of salt structures,

limited

half of Group

salt. In contrast,

by normal

D correspond

prominent,

salt being

half of group faults.

B) The

to continuous.

massive

structures.

often protruding at sea level, are observed at the edge of the deep fan of the Rhone (eastern part of groups D and E). Finally, to the west, the bathymetric contours show the presence of prominent structures to the northwest and west of the Asinara Islands (Fig. 3, group F, Fig. 12). The halokinetic structuring is much weaker between Menorca and Sardinia, profiles are caused by “normal” tions, and not by massive domes The elevated mean heat flows structures for the following four

where many of the diffractions visible on seismic structures at the top of the shallow salt undula(Figs. 9 and 12). (groups F and G) are probably not related to salt reasons:

(1) Groups F and G appear scattered enough, but twelve values lie between 80 and 115 mW m-* and only three values are between 45 and 65 mW m-‘, two of which are very low (45-50 mW m-*) and probably aberrations (Fig. 3). Some of these twelve values are probably enhanced by salt effects, but certainly not all of them. According to the previous discussion, this would require that all stations are located exactly at the top of salt structures, which is unlikely; consequently, some of the twelve values are probably less than the normal heat flow. As the theoretical negative perturbation is several time smaller than the positive one, the normal (uncorrected for sedimentation) heat flow could be 80-85 mW mPZ for groups F and G; we note that the 85 mW m-’ value fits with the trend observed from groups D and E.

Fig. 8. Seismic time section The total width

is about

value of the mean gradient.

and thermal

gradients

100 km. The presence

for group

of prominent

E on the Gulf of Lion-Sardinia salt diapirs

transect.

does not cause a shift in the

316

(2) In the case of group prominent

salt

(P102-P104

structures

E (Fig. X), where excellent are observed

and P%-P89)

overlie

continuous,

ference in the mean heat flow between salt structures theoretical

is to increase

considerations

below

F and

analyses

control

P93-P99;

is available. other

salt. There

the most significant

of the gradients

and from previous

of the high heat flows of groups

unstructured

the two groups;

the scattering

seismic

stations

stations is no difeffect of

(Fig. 8) as inferred

(Hutchison

from

et al., 1985). If all

G were due to salt structures,

this would

require that the burial of diapirs is significantly reduced to the east. which is apparently not the case, and that all stations are located at the top of salt structures (which is unlikely). (3) Some of the high heat flows from groups F and G are observed in areas where salt structures are absent (upper Sardinian margin: P73, S85, 86, 87 and 88). An alternative speculation would be to relate the high heat flows of groups F and G

1

Stack (without

Migrated

.lr.a...“.-

migration)

-m-1.*..

.,

1

seismic profile 5km

Fig. 9. Unpublish~

seismic reflection profile in the Ligurian Sea (ODlO profile). The migration reveals

that many of the diffractions are not due to salt diapirism, but to fracturing of the Plio-Quaternary swuence above subtle salt structures. Similar phenomena are observed west of Sardinia. (Courtesy of J. Letouzey.)

317

SE

NW

Fig. 10. Observed

heat flow for the northeastern

heat flow is abnormally

high to the northwest,

segment

of

which cannot

group A (Fig. 2) in the Gulf of Lions. The be explained

simply by topographic

effects.

local hydrotermal effects, associated with possible fracturing observed above salt domes (Fig. 9). This effect is very difficult to quantify. However, once again, positive and negative anomalies would probably be detected, and no shift of the mean heat flow would be expected. (4) A more questionable feature is the scattering of the heat flow for group A (Figs. 10 and 11) in the upper Gulf of Lions. Values for two near-parallel segments differ by 20-25 mW m-2 (see location on Fig. 2). More precisely, four abnormally high values are observed (Fig. 10) and are located above a deep, narrow graben.

with

Could the 20-25 mW m-2 difference be explained by. topographic effects? We observe no clear correlation between the topography of either the substratum or the bathymetry and the heat flow profile. However, a 3-D component is likely as the segment is oblique to the structures. Local water circulation in the trough could be argued: no salt barrier is observed here and upward or downward flow would not be hindered. Finally. as the extension of the thermal anomaly cannot be assessed (in particular towards the upper slope and platform), further investigation is necessary prior to interpretation. Similarly, high uncorrected heat flows of group C (Fig. 2) (103 + 30 mW m ’ ) are questionable; thermal refraction above salt or basement structures appears insignificant, however, because such structures are not visible on seismic profiles. Nonetheless, the geodynamic setting is peculiar here: group C is close to the extremity of the N140 Marseille-Asinara fracture zone (Fig. 2), which guided the opening of the Basin (Burrus, 1984; Rehault et al., 1984). Although the crust is extremely thin here (Le Douaran et al., 1984, ESP 211-212), more data are required

SE

Fig. 11. Observed heat flow for the southwestern segment of group A (Fig. 2) in the Gulf of Lions. The mean heat flow is 20-25 mW mm2 lower than that of Fig. 10.

before establishing setting.

a relation between locally high heat flow and the structural

Superficial perturbations in the Ligurian Sea Toulon-Ajaccio transect. The uncorrected heat flow (Fig. 4) increases rapidly from east (63 mW m-2) to west (79 mW rne2) across the Corsican margin. To the west, 6 uncorrected heat flows ranging from 80 to 140 mW rnp2 are observed between ESP 220 and ESP 232 across the Corsican oceanic boundary (ref. above). No reliable determinations are available for most of the oceanic domain (ESP 220 to ESP 230). However, three gradients have been determined with only two sensors (Q = 0) in the central oceanic domain: one value appears very low (53 mW mw2), but the two others (100 and 150 mW mV2) apparently support the high heat flows encountered on the eastern oceanic boundary. Similarly, Erickson et al. (1977) measured two heat flows of 87 and 104 mW m-’ in the central oceanic basin (Table 1). Near the western oceanic boundary, high uncorrected heat flows (90 k 15 mW m-*) are also observed. The variability of the uncorrected heat flow for the Provensal margin appears questionable: two high values (90 f 15 mW m-‘) and two low values (56 f 15 mW m-*) are observed. The low values are similar to those obtained to the northeast on the Nice-G&i transect by Jemsek et al. (56 f 7 mW me2) and during the Flumed experiment (59 + 12 mW mw2). According to seismic data, salt is absent from most of this segment, and only small salt structures are observed below the most eastern

319

station

(P52);

ahyssal

plain.

the substratum

argued.

Obviously,

Neither

thermal

here is smooth. refraction

more data are required

and all stations

effects nor topographic to precisely

variability of heat flow on this part of the margin. The thermal profile between Toulon and Ajaccio unequal

distribution

of the stations.

of the heat flow on the Provencal

are located

in the

effects can thus be

determine

the magnitude

could be asymmetrical.

and

but the

more dense to the east. as well as the scattering side, do not make it possible

to demonstrate

this

unambigu~~usly (Fig. 4). The number of determinations in the apparent “warm” anomaly is too few to discuss the possible role of salt structures clearly present in the central part (seismics, bathymetry), as was done for the Lions-Sardinia transect. Nice-C‘crir*i trlitzscct.

Heat

flows

were

obtained

during

the

Flumed

survey

are

c~)mpiled with previous measurements reported by Jemsek et at. (1985) along a close transect. The uncorrected heat flow (Fig. 5) increases regularly from west (deep fan of the Var. 55-65 mW ms2) to east (eastern flank of a circular basement structure discussed below, ESP 223, 80-100 mW rn--‘). The thermal asymmetry is much less ambiguous here than on the Toulon-Ajaccio transect. The area where the highest heat flows (> 100 mW nrm2) are encountered is narrow (25 km) and is clearly displaced towards Corsica, as already noted (Rehault et al., 1984). The variability of Flumed results for the Corsican margin (Table 5) is not in agreement with previous measurements (Jemsek et al., 1985), and is believed to be related to penetration problems. Jemsek et al. (1985) suggested that the high heat flow observed in region 2) could be explained by nearby undetected salt diapirism. Detailed seismic tion of the eastern flank of the circular structure (unpublished IFP indicates (Fig. 2) that region JF is indeed located within the salt basin; regions JD, JE and JG are mainly outside the limit of salt. if not outside of evaporites (region JE). The uncorrected high heat flow from region JE

JF (Fig. cxploraprofiles) however. the limit (103 mW

m ~~-. Table 3), can thus not be explained stations P61 (138 mW mm’. Q= 0) and

by salt structures. Similarly. Flumed P63 (180 mW m ‘. Q- 1) present

apparently

and are located

high heat flows (of poor quality)

outside

the salt basin

(Figs. 2 and 5). Owing to insufficient data, the extension of the thermal anomaly cannot be assessed. However, high heat flows are observed perpendicularly to the Nice-Calvi transect (region JE. stations P61-P63, region JF), suggesting no relation with the circular basement high (ESP 223).

All three thermal profiles discussed above present an interesting similarity: they suggest an increase of the uncorrected heat flow from west (Provensal margin) to east (Corsica-Sardinia). This trend is readily visible for the Gulf of Lions-Sardinia transect and for the Nice-Calvi transect. It is more ambiguous for the Toulon-Ajaccio transect. Furthermore, measurements suggest that elevated heat

320

Fig. 12. Bouguer Flumed

stations

with abnormally Corsica

anomaly

map (Morelli

and the extension high ( > 80-90

et al., 1977) of the Provenc;al

of the protruding

salt diapirs.

mW m ~‘) heat flow approximatively

Basin showmg

The 200 mGal coincide

contour

the locati and the zone

and are displaced

towards

and Sardinia.

flows are observed perpendicularly to the thermal profiles (in a NE-SW direction) between Nice and Calvi, and on the Sardinian margin, suggesting the existence of a regional thermal anomaly that extends from northwestern Corsica to western Sardinia (Fig, 12). For the Nice-Calvi transect as well as for the Gulf of Lions-Sardinia transect, there are reasonable indications that the elevated mean heat flow cannot be attributed to salt structures. Owing to an inadequate distribution of the measurements, this is unclear for the Toulon-Ajaccio transect. Further

321

experiments

are absolutely

of the heat

flow. We examine

anomaly

necessary

with regard to other possible

Blunketing

effect.

is known

to cause

The sedimentation a significant

to precisely

below

determine

the possible

the regional

significance

variability

of the proposed

surface perturbations. effect (De Braemecker,

perturbation

of the heat

1983; Hutchison, flow.

For

1985)

the central

Provencal Basin, Lucazeau and Le Douaran (1985) Jemsek et al. (1985) and Burrus and Bessis (1986) have mentioned that 30% of the crustal heat flow is absorbed by the sediments. Could the proposed thermal asymmetry be the result of unequal sedimentation rates? This is unlikely for two reasons. (1) High and low heat flows are observed along the Gulf of Lions-Sardinia transect for apparently similar sedimentary thicknesses (Fig. 3). (2) Between Provence and Corsica, where sediment thickness is variable, Jemsek et al. (1985) have shown that regions JE and JF still yield the highest heat flows after correction and that the magnitude of the anomaly is preserved after correction (Table 3). Erosional effects. Messinian erosion, which did not exceed 1000 m (Ryan. 1976; Bessis. 1986) has increased the heat flow; the magnitude of this effect has been evaluated as lo-15 mW rn-’ for the upper Gulf of Lions (Burrus and Bessis. 1986). However. there is clearly no correlation between the location of the Messinian erosional surface and heat flow. Topographic effects. Except for the upper margin of the Gulf of Lions, where rough topography and high, variable heat flows are observed, most stations are located in the abyssal plain, where topographic effects are negligible. From this discussion. we conclude that even if more data are required, there are reasonable indications that the ProvenGal Basin is characterized by a thermal asymmetry. which is probably not the consequence of surface perturbations such as thermal

refraction

above

salt diapirs,

topographic

effects,

Consequently, it is interesting to speculate on whether the geodynamic history of the basin, and whether subcrustal phenomena. Geodvnamic

significance

or thermal

blanketing.

this asymmetry is related it is caused hy crustal

to or

of the thermal regime

Before examining the possible geodynamic significance of the heat flow, we briefly review major aspects of the kinematics and geodynamic evolution of the basin, and previous interpretations of the thermal regime. Kinematics and geodynumic evolution Dating of rifting and oceanization in the Provenqal Basin is constrained by various results. First, according to paleomagnetics (Montigny et al.. 1981). Oligo-

322

Miocene dated

lava record

between

a 30” anticlockwise

21 f 1 Ma and

rotation

19 F I Ma using

of Sardinia, K--Ar

which

I:, prectsel~

radiochronology.

No such

method can be applied to the Cenozoic rotation of Corsica. Second. drilling of synrift deposits (Cravatte et al.. 1974: Hsii et al., 1978) on the Provencal and Balearic Third,

margins detailed

gives an age of ChattianAquitanian field

studies

on the Sardinian

for the synrift

rift (Cherchi

sediments.

et Montadert.

1982)

indicate that synrift faults sealed in the middle of the Aquitanian. Fourth. absolute ages of dredged volcanic rocks vary between 29 Ma and 18 + 0.5 Ma (Rehault et al., 1984). There is a consensus and

the

end

of

on the dates of the beginning

oceanization

(19 _t 1 Ma,

Lower

of rifting

(Chattian.

Burdigalian,

30 Ma)

according

to

paleomagnetics). For the end of rifting, two dates have been proposed: 21 k 1 Ma, as indicated by the onset of the paleomagnetic record of the rotation of Sardinia (Rehault et al., 1984). and syn-Aquitanian. as indicated by the end of activity along tensile faults in Sardinia (Burrus, 1984). In the latter case, the absolute age of the end of rifting depends on the chronostratigraphic scale for the Aquitanian: 245 19 Ma (Lowrie and Alvarez, 1981). 22.0-18.5 Ma (Odin, 1975), or 24.0-22.0 Ma (Vail and Hardenbol, 1979). Burrus (1984). using the latter time scale, proposed that oceanization began 23-24 Ma ago: the 2 Ma time span (23-21 Ma) preceeding the rotation could be accounted for by the translation (or rotation with a distant pole) of Corsica and Sardinia. In conclusion, according to previous authors, the discrepancy on the age of the oldest oceanic crust is 2 Ma, which corresponds to a difference of almost lo-15 mW m ’ on the standard oceanic heat flow curve (Parsons and Sclater, 1977). The duration of the oceanization was 2-4 Ma; the corresponding difference in heat flow is IO-25 mW m ml+ reduced to 6-16 mW m ’ by thermal blanketing in the abyssal plain. Nature of the crust. In the Gulf of Lions, Le Douaran et al. (1984) have suggested that a continental “plateau” made of greatly thinned continental crust (attenuation factor of p = 4 to 7) extends below the abyssal plain. Burrus (1984) has reconstructed

the prerift position of Corsica and Sardinia and suggested that a conjugate present west of Sardinia. These continental domains, char“ plateau” is probably acterized by extremely attenuated crust, are probably intruded as oceanization is considered to begin as soon as the crustal attenuation factor exceeds 3.5 (Le Pichon and Sibuet, 1981). In the Ligurian Sea, Le Douaran et al. (1984) and Burrus (1984) have suggested that the oceanic domain could be narrower than previously proposed (Rehault, 1981; Rehauli et al., 1984). In particular, on the Nice-Calvi transect (Fig. 5) the basement high (ESP 223) has been considered as an oceanic seamount or oceanic ridge owing to the dredging of a tristanite (Rehault et al., 1984), while the deep seismic structure (ESP 223) has been interpreted as continental (Le Douaran et al.,

1984). Similarly,

on the Toulon-Ajaccio

terms of continental,

attenuated

the Corsican

is more “gradual”

Rehault,

margin

transect.

than the Provenqal,

1981. p. 100) some kind of asymmetrical

seismic data are required

ESP 232 has been interpreted

crust (Fig. 4). The deep seismic results indicate

before this structural

rifting asymmetry

suggesting process.

(as noted

in that by

However,

further

can be precisely

defined

further to the south. The complex magnetic pattern cannot be directly used for dating the oceanic spreading. However, assuming absolute dating for the oceanic stage. Rehault et al. (1984) identified the prominent, ubiquitous positive magnetic anomaly as An 5D. whereas Burrus (1984) proposed An 6. in better agreement with the Cenozoic time scale based on DSDP Leg 73 results (Hsii et al.. 1984). In the absence of drilling. the magnetic pattern was interpreted by modeling (e.g.. Bayer et al.. 1973; Burrus. 1984): these attempts demonstrated that the weak magnetic signature is due to the depth of the oceanic source. It has been suggested (Burrus. 1984) that the magnetic pattern is well accounted for by an asymmetrical spreading model spreading rates towards Corsica and Sardinia than towards Provence. authors suggested previously that the magnetic pattern was confused. spreading centers were mentioned. Previous geodvnamic interpretations of the htwt fhv ad subsidence Despite the paucity of previous heat flow and subsidence studies. geodynamic interpretations have been proposed. Gulf flow have (p =

with higher Most other and diffuse

numerous

OJ Lions. Several authors have tried to relate the subsidence history. the heat and the deep crustal structure in the Gulf of Lions. Steckler and Watts (1980) backstripped the Autan well (Fig. 2) and found a very high thinning factor 10). incompatible with geophysical data. Rehault (1981, p. 95) suggested that

this discrepancy was mainly due to a wrong assumption about the beginning of the thermal subsidence (25 Ma). and pointed out that a 20 Ma age (probably too young) would have resulted in a more reasonable p factor of 2 to 4. Bessis (1986) has shown that the subsidence history of the horst of Autan is very peculiar. and not representative for the whole margin: the vertical displacements are probably controlled by the brittle behavior of the upper crust. and not by the deep internal evolution. Several authors (Chenet. 1984: Foucher and Tisseau. 1984; Lucazeau and Le Douaran. 1985) have suggested that the first-order magnitudes of subsidence and heat flow for the whole margin could be reasonably well accounted for by a simple. uniform, lithospheric thinning model. Analyzing the subsidence, however, reveals significant discrepancies. The continental platform should at the present time be several hundred meters above sea level (Burrus and Bessis, 1986). In addition, the abyssal plain should be 500 m less deep (Rehault et al., 1984; Hutchison et al., 1985: Burrus and Bessis. 1986). and its tectonic subsidence rate appears very constant over the last 24 Ma. which is not

324

compatible

with any cooling

model

that predicts

exponential

decay

(Burrus

and

Bessis, 1986). Ligurian

Sea.

asymmetrical

As noted distribution

flows to the east, This

above.

Rehault

of the heat trend

has been

(1981, p. 100) has proposed

flow in the Ligurian tentatively

explained

a regional

Sea, with higher assuming

heat

an earlier

spreading age near Corsica than Provence; however, the duration of the oceanic phase appears too short to explain the difference in heat flow observed, and the highest heat flows (on the Nice-Calvi transect) are probably not observed on oceanic substratum (Le Douaran et al., 1984). The lower heat flows below the Roya and Var deep fans have been interpreted by either 35 Ma oceanic crust (which is much too old and contrary to refraction results) or by “thinned continental margin” (Rehault et al., 1984). More recently, Jemsek et al. (1985) have refined this interpretation. The basin-wide heat flow (corrected for sedimentation) has been related to 21 Ma oceanic crust, in apparent agreement with paleomagnetic results. I-Iowever. alternative seismic interpretation suggests that this mean heat flow consists of high values (120-140 mW observed on continental basement to the east and lower m -’ after correction) corrected values (95-103 mW m ‘) observed on oceanic crust to the west. Moreover, Jemsek et al. (1985) have mentioned that the subsidence appears at least several hundred meters greater than typical oceanic subsidence. Rehault (1981, p 104) had already noted the abnormally great depth to the substratum in both the Ligurian Sea and the Gulf of Lions. Considering the paleomorphology of eroded margins at the end of the Messinian, he estimated that the subsidence became abnormal only after the Messinian; this late discrepancy was related to the PlioQuaternary

compressive

trend.

Thermal regime, crustal thickness,

age of oceanization

Gulf

of Lions-Sardinia transect. The uncorrected 60-65 mW rn-’ (80-87 mW m-l assuming 30% sedimentation correction) heat flow measured on the deep margin of the Gulf of Lions is in reasonable agreement with the measured crustal attenuation

(p = 4 to 7), and for the upper margin,

two-dimensional

modeling

suggests

that the

heat flow is well accounted for by the observed crustal thinning (Burrus and Bessis, 1986). The oceanic heat flow (uncorrected, 65-75 mW rnm2; 84-97 mW mm-’ after correction for sedimentation) satisfies reasonably the standard oceanic curve on the Proveqal side, assuming oceanization between 23 and 19 Ma. This method is, however, too unprecise owing to uncertainties in measured heat flow, in corrections for sedimentation, and in the variability of the standard curve. On the Sardinian side, the 85-90 mW rnp2 heat flow (110-117 mW m-2 after correction) could be accounted for by oceanization between 16 and 19 Ma but is too high to be accounted for by 30-24 Ma rifting, even for high crustal thinning

(/3 > 10). The 16619 Ma. see previous that the Sardinian does

extend

inferred Burrus.

Ma, time span is not very far from the oceanic

discussion), thermal

and owing to the uncertainties,

anomaly

over continental

for group

F)

and

is of non-oceanic

basement the youngest

1984) does not appear

reasons, the Sardinian thermal tion or rifting processes.

to coincide anomaly

origin.

(definitively oceanic

However,

demonstrated

crust

stage (19-23

does not demonstrate

(inferred

this anomaly for group

from

G‘.

magnetics,

with the highest heat flow. For these two cannot

be explained

by standard

oceaniza-

Non-uniform attenuation of the crust and mantle (Royden and Keen. 1980: Royden et al., 1980, 1983). in which the mantle is more attenuated than the crust. has been applied to the Gulf of Lions (Burrus and Bessis, 1986: Bessis and Burrus. 1986). The predicted heat flow was shown to be 15-20 mW rn-’ higher than the measured mean after correction for sedimentation. Such a model could thus apply to the deeper part of the Sardinian margin. However. the non-uniform model could not account for the rapid and constant subsidence rate observed in the abyssal plain off the Gulf of Lions (see above) or for the 500 m depth excess. For these reasons. it probably does not apply to the Sardinian margin. Nic,e-Cahi transect. Following the alternative seismic interpretation discussed above, the oceanic heat flow (85--105 mW n-’ after correction) would be well accounted for by 23-19 Ma oceanization. The more eastern heat flows ( > 120 mW m ‘. after correction) would require more recent oceanization or crustal attenuation ( -c 15 Ma), both of which are unreasonable. These more eastern, high heat flows can probably not be explained by a higher crustal radiogenic contribution. as the thickness of the crust is not very different for the Provenqal and Corsican segments. A deep, as~~mnetrical, internal contribution. 7 There are several reasons proposed

thermal

asymmetry

by an asymmetrical

internal

to explain

contribution.

First.

the the

Proven$al Basin has tentatively been characterized by asymmetrical crustal attenuation on the Nice-Calvi transect, with an asymmetrical magnetic pattern in the oceanic domain. Second, the regional geodynamic setting is asymmetrical: to the east, the Apenninic subduction and collision zone contrasts with the stable European platform to the northwest. However, there is no simple relation between this regional (300-500 km) asymmetrical structural context and the thermal anomaly of smaller scale observed (30 km to the north and 60 km to the south). Third. the Bouguer map (Morelli et al., 1977. fig. 12) shows that the strongest gravity anomaly is shifted to the east, and extends approximatively across the eastern oceanic boundary reconstructed by Burrus (1984). This location corresponds roughly to the thermal anomaly proposed here. Such a fit is unexpected (positive gravity anomalies should be associated with dense, cold material), except if the light crustal material has been locally replaced by dense mantel material. This gravity signature supports the idea of post-oceanic (?) magmatic activity as a possible cause of the asymmetrical thermal pattern.

326

Thermal anomalies and magmatism have been related in the Tyrrhenian (Della Vedova et al., 1984). An extension of this relation to the Provencal however, around

raises a major the Provenqal

signs of recent thermally

increases

abnormal

difficulty:

Late Cenozoic

magmatic

and volcanic

Basin does not show any clear asymmetry, in magmatic

zone. However.

activity

in the last million

a major change

activity

and there are no years beneath

in the magmatic

occurred since Plio-Pleistocene time. The talc-alkaline Sardinia between 30 and 13 Ma, offshore eastern Corsica,

Basin Basin,

processes

the has

volcanism observed in and in the region of Nice

(35-33 Ma) has been related to the SW-dipping subduction (Bellon. 1981). In contrast, the alkaline activity that followed (in Sardinia, 4-2 Ma; in the Valencia trough, 7-1 Ma) has been related to the recent N-S oriented compression (Bellon. 1981). At present, there is no intelligible relation between this complex magmatic history, the present thermal state, the regional stress, and the peculiar subsidence pattern of the Provenqal Basin. It has been suggested in the past that marginal basins and mature oceans are formed by similar oceanization processes (Karig, 1970). The same heat flow/age relation has been verified for the two cases (Sclater et al., 1980); however, the depth of marginal basins is typically 0.5 to 1.0 km greater than for oceans of the same age (Louden, 1980) and asymmetrical evolution (with lower opening rates towards the subduction (Weissel, 1981). All of these characteristics still in an “immature

oceanic

zone) has been proposed to explain this apply to the Tyrrhenian Basin, which is

stage” (Hutchison

et al., 1985)

and to the Provenc;al

Basin, except in the area of the proposed thermal anomaly. For the Provencal Basin, the geodynamic context is complex: the Provenqal Basin can be considered as a marginal basin, as an intra-erogenic basin, and as a mature, rifted basin associated We speculate that the asymmetrical thermal with widely developed extension. pattern proposed here is a major clue to an understanding of its complex geodynamic

history.

ACKNOWLEDGEMENTS

We thank

our colleagues

from IFP, F. Bessis, P.Y. Chenet.

J. Letouzey

and L.

Montadert, and S. Le Douaran (from SNEAP) for constructive discussions on the thermicity and geodynamic evolution of the ProvenCal Basin. The manuscript has been critically reviewed by J. Jemsek, F. Lucazeau, and one anonymous reviewer. Appendix 1: EXPERIMENTAL

DETERMINATION

AND REDUCTION

OF FLUMED

DATA

Thermal gradients have been determined during the Flumed surveys using two probes with 4 m and 10 m penetrations (Von Herzen and Maxwell, 1959). The quality of the measurements was generally not very good owing to penetration

327

problems

in the upper sandy mud, especially

Var. where 30% of the stations The number varied

of outrigged

(2 to 7) depending

determined into account

a subjective the number

thermistor

sensors used for each gradient

on the penetration quality

in the deep fans of the Rhone

and the

were unsuccessful.

factor

of sensors

depth

(between

Q for each gradient

used to calculate

determination

3 and 7 m). We have

determination

the gradient

G. taking

and the linearity

of the gradient. The factor Q ranks are 3 (very good: 4-7 sensors used and good linearity). 2 (good), 1 (poor) and 0 (questionable: only 2 sensors or non-linear gradient). The uncertainty AC due to the non-linearity of the gradient has been calculated (Bevington, 1969). Conductivities K (51 values) have been measured on core samples and, in some cases, during in-situ experiments (harmonic mean). The uncertainty AK has also been evaluated. and the resulting uncertainty AF on the heat flow is AF = A( KG) = K. AC + G. AK. The uncertainties on individual measurements have been replaced by standard deviations in the case of mean determinations (Tables 3, 4 and 5). The experimental uncertainty on the heat flow. which represents f 5 mW m ’ in some (rare) best cases and k 30 mW rn-’ or more in the worst one, can be very important, as it represents up to generally between 40 and 60% of of the conductivity; this effect gradients. All Flumed stations have been

30% of the mean values. It is worth noting that this uncertainty is due to imprecise determination is as important as the uncertainty on thermal superposed

on previous

seismic lines or on seismic

profiles shot during the Flumed survey using onboard facilities (6-traces, towed array and 2 light sources). This seismic control. although limited to the PlioQuaternary sequence, was essential to try to avoid salt structures, known to cause a first-order perturbation to thermal gradients. However. such control is limited to two dimensions. and lateral structures cannot be recognized.

Appendix 2: THEORETICAL

PERTURBATIONS

OF THE SURFACE

HEAT

FLOW

BY SALT

DOMES

The steady-state heat equation has been solved using a finite-difference code on a two-dimensional rectangular grid including 20 x 30 rectangular elements (Doligez et al., 1986). The thickness of the elements increases with depth from 10 to 500 m: their width is 500 m in the vicinity of the salt structures, and increases up to 2000 m towards the lateral vertical boundaries. The total thickness of the grid is 8 km, its total width is 110 km (case of a salt dome 5 km in width). The conductivity contrast between salt and shales is assumed to be 2.5. The upper boundary is at a constant temperature (T= 10°C), the lower boundary (8 km below the bottom of the sea) is at T = 250°C. The base of the salt layer is assumed to be at a depth of 1.5 km below the bottom of the sea.

328 Appendix

3: FLUMED

(Symbols

are defined

vicinity Station

DATA in Appendix

1. Conductivities

are set between

brackets

if extrapolated

from the

and not locally determined.) Location

K

lat.

long

(W m

(“NJ

(OF)

Gulf of Lions - Sardinia

'C')

F

G

(mWm_‘)

(“Ckm~‘)

Q

Penetration n (sensors)

meters --_

-- Group A

PI4

42 41.6

4 34.4

43+13

32+

7

P-l5

42 41.9

4 34.0

36i_13

27k

7

P76

42 42.4

4 33.9

58+13

43&

7

P77

42 42.0

4.33.4

49+13

37+

7

P71

42 36.6

4 36.2

56i

6

42+

1

PI2

42 37.9

4 35.6

59i-

6

44+

1

P67

42 29.4

4 39.2

70113

52+

8

P68

32 31.1

4 38.5

56+13

42k

8

P69

42 32.5

4 38.0

7Ok1.7

52~

8

P70

42 35.2

4 36.7

45113

34k

8

P40

42 34.2

4.37.2

51+13

38zt

8

P12

42 27.0

4 40.1

6Ort13

45i

8

_

_

KS7

42 22.6

4 41.8

1.32+0.05

KS30

42 48.7

4 42.8

1.39*0.13

78+12

56+

3

3

4

5.7

KS28

42 42.2

4 41.8

1.49*0.14

104+ 16

70f

4

3

5

3.8

KS29

42 47.8

4 43.0

1.40*0.14

105*13

75*

2

3

5

5.7

KS24

42 46.6

4 44.3

1.37*0.28

89*23

65+

3

3

4

4.8

KS26

42 46.2

4 44.2

1.38kO.14

89&18

65+

6

3

5

4.5

KS25

42 43.9

4 45.6

1.38 i 0.07

745 16

54+

9

3

4

5.7

KS27

42 38.1

4 48.6

1.34 4 0.01

58rt

43+

2

3

4

5.7

KS22

42 31.4

4 50.6

1.35*0.11

77+13

57+

5

3

6

X.1

Gulf

3

of Lions - Sardinia - Group B

P31

42.17.2

4 43.9

1.34kO.16

60* 14

45+

5

P32

42 18.7

4 43.2

1.3440.16

75+14

56k

5

P33

42 20.6

4 42.8

1.34kO.16

63+

47+

5

P34

42.22.3

4 41.9

1.34kO.16

56ztl4

42i_

5

P35

42 24.1

4 41.3

1.34kO.16

70+ 14

52j1

5

Pi

42 07.1

4 47.6

1.34kO.16

68112

51$:

4

4

P2

42 08.7

4 46.9

1.34&0.76

66*12

495

4

P25

42 04.6

448.7

1.34*0.16

68k12

51+

4

P27

42 09.7

4 46.8

1.34*0.16

56k12

42j,

4

KS3

42 11.5

4 45.9

1.36 + 0.07

_

KS2

42 02.2

4 49.6

1.35*0.15

_

P78

42 10.5

5 03.8

1.34&0.16

49*14

36+

P79

42 09.3

5 05.9

1.34+0.16

51+14

37*

5

P80

42 08.1

5 07.7

1.34rtO.16

57*14

42f

5

P81

42 06.3

5 09.5

1.34kO.16

56+14

41*

5

P82

42 04.8

5 11.5

1.34*0.16

61+14

5

KS5

42 10.3

5 03.7

1.32&0.07

491 _

_ 5

_

-. _

,-

329

Station

Location lat.

long

(“N)

(OE)

Gulf of Lion.r ~ Sardmia -

K

F

G

(W mm’ C-‘)

(mW m-*)

(“C

Q km-‘)

Penetration n

meters

(sensors) Group C

KS1 8

42 38.2

5 28.0

1.51+0.13

KS21

42 44.3

5 29.3

1.31 kO.16

KS31

42 31 .o

5 37.0

1.38kO.14

Gulfof 1.ron.sSardiniu -Group

121+1X

so*

5

3

4

3.6

72115

55*

5

2

4

4.9

117522

83+

X

2

4

3.0

D

P60

41 51.3

5 22.4

(1.3610.15)

38120

2X*10

P61

41 57.9

5 22.2

(1.36i_0.15)

57k20

42+_10

P54

41 47.9

5 36.6

(1.36kO.15)

70113

51*

4

P55

41 49.2

5 34.8

(1.36*0.15)

56k13

41+

4

P56

41 50.5

5 32.7

(1.36kO.15)

56&13

41*

4

P57

41 52.3

5 30.4

(1.36*0.15)

64*13

41+

4

P58

41 53.4

5 28.7

(1.36+0.15)

57*13

42i

4

P59

41 55.0

5 26.4

(1.36i0.15)

66k13

4x*

4

P44

41 37.5

5 51.2

(1.36+0.15)

64+15

47*

5

P45

41 39.2

5 50.0

(1.36k0.15)

77+15

56*

5

P46

41 40.2

5 48.6

(1.36kO.15)

70+15

51*

5

P47

41 41.1

5 46.7

(1.36kO.15)

77+15

56k

5

P48

41 42.0

5 44.X

(1.36+0.15)

72+15

53*

5

P49

41 42.7

5 43.2

(1.36i0.15)

h8_t15

50+

5

P50

41 44.0

5 41.9

(1.3650.15)

71+15

52+

5

PSl

41 45.3

5 40.4

(1.36+0.15)

72115

53*

5

P52

41 46.3

5 39.0

(1.36*0.15)

70*15

51*

5

P53

41 47.3

5 37.4

(1.36+_0.15)

55+15

40*

5

KS6

42 00.8

5 17.7

1.43i_0.12

_

KS7

41 47.7

6 2X.7

1.3s+o.13

_

Gulf of L~on.s- Srrrdiniu -Group

_

_

_

E

P97

41 47.8

6 29.0

(1.34kO.18)

X2*20

61i

6

..

P98

41 48.3

6 26.4

(1.34+0.18)

63k20

47+

6

_

P99

41 48.X

6 23.5

(1.34iO.18)

74520

55k

6

P93

41 45.1

6 41.5

(1.34+0.18)

7X+19

5X*

5

P94

41 45.7

6 39.5

(1.3410.18)

59*19

44+

5

P95

41 46.3

6 36.5

(1.34*0.1x)

72+19

541

5

PX6

41 42.3

1 00.2

(1.34i_0.18)

74117

55f

3

P87

41 42.5

6 58.0

(1.34+0.18)

71+17

531

3

P88

41 42.9

6 55.1

(1.34*0.18)

76k17

57*

3

P89

41 44.4

6 52.8

(1.34&0.18)

67+17

3

KS8

41 47.8

6 28.8

1.34*0.1?3

5oi _

P102

41 50.7

6 13.5

(1.34*0.18)

68k20

51i

6

P103

41 51.0

6 10.7

(1.34kO.18)

72+20

54+

6

P104

41 52.0

6 07.8

(1.34kO.18)

72+20

54*

6

P105

41 52.1

6 05.0

(1.34+0.1X)

79*20

59+

6

_

330

Station

K

Location lat.

(ON)

(OR

Gulf of Lions - Sardinra -

Q

G

I(mWm

I)

(OCkm

‘)

Penetration -. w (sensors)

- -I_.-

meter\ -__--

Group f 93*:11

4.6

KS78

41 14.8

6 49.7

(1.10+0.12)

85*1x

2

5

KS74

41 21.5

6 28.9

(1.10+0.12)

105-

96-

1

3

3.5

KS75

41 22.7

6 28.8

(l.lOkO.12)

73-

66-

0

5

6.2 3.6

KS76

41 22.8

6 27.8

(1.10*0.12)

44& 10

40+

5

3

4

KP84

40 57.6

7 40.5

(1.10+0.12)

86+19

78+

9

2

3

1.9

KS83

41 02.1

7 25.4

(1.10+0.12)

88+11

80+10

2

4

4.1

KS22

41 04.5

7 20.1

(1.10~0.12)

53+11

48+

5

3

5

3.7

KP81

41 06.9

7 15.4

(1.10*0.12)

82k

74*

6

2

4

3.4

KS79

41 10.9

6 59.9

(1.10+0.12)

84+28

1

4

4.6

KP80

41 11.6

6 55.7

(l.lOrtO.12)

76kl5

7oi

7

2

4

4.6

KS89

39 51.8

6 08.3

(1.10+0.12)

118i15

107+

2

3

4

4.0

Gulf of Lions - Sardinra -

16

76*18

Group G

KP85

40 53.0

7 45.7

(1.08kO.12)

80+11

72&

2

3

5

5.7

KS72

40 57.6

7 44.9

1.12+0.14

63+17

6Ok

8

2

3

3.3

KS73

40 54.8

7 39.1

1.06kO.11

go+

73+

1

2

3

2.8

KS86

39 53.0

7 26 2

(1.08kO.12)

83k14

76*

5

3

4

5.8

KS87

39 52.8

7 24.1

(1.08+0.12)

94+11

85+

1

3

4

8.0

KS88

39 51.6

6 14.4

(1.08+0.12)

107+ 27

2

3

3.6

9

97+14

Toulon - Ajaccro - TAI - Provenfal Margin KS50

42 51.6

6 48.2

1.19+0.07

s3*13

69zk

7

3

5

3.8

KS51

42 50.7

6 49.9

(1.15+0.12)

55516

48k

9

1

6

4.7

KS49

42 48.5

6 52.8

1.14+0.11

96+14

84k

4

3

4

3.8

KP52

42 47.6

6 55.6

(1.15+0.12)

57+13

50+

6

3

6

4.7

KS48

42 42.4

7 06.6

797t15

71+

3

2

3

3.8

90+

5

3

4

4.2

2

3

3.0

0

3

2.1

49-

0

2

0.4

71+10

1.11 kO.16

Toulon - Ajaccro - TA2 - Oceanic Domain 99116

KP46

42 40.6

7 11.8

(1.11 kO.12)

KP39

42 20.4

7 49.0

1.12+0.15

112130

100+13

KS42

42 29.4

7 32.1

1.25 *0.3

278 + 80

223rt

KS41

42 28.6

7 33.7

1.09+0.11

KP67

42 22.8

7 43.2

(1.12+0.15)

3

4

2.1

KS43

42 28.2

7 33.1

1.12+0.15

150-

134-

0

2

2.4

KS44

42 28.3

7 32.8

(1.12+0.15)

99-

88-

0

2

1.8

KS36

42 21.5

7 47.3

1.17kO.22

143*36

3

4

2.9

101 + 23

3.1

KS35

42 20.4

7 48.3

1.09f0.08

KS68

42 20.1

7 47.7

(1.12$-0.15)

KS38

42 20.8

7 49.1

KS34

42 20.5

7 49.5

KS37

42 20.8

7 49.2

1.05 rt 0.26 1.08 + 0.08 (1.12&0.15)

5379522

122*

8

8

92+14

1

3

73-

65-

0

2

1.2

89+40

84_+17

1

4

2.9

79k22

73*15

2

3

2.1

63+14

56+

3

4

3.9

5

___

Station

K

Location lat.

long

(ON)

(“E)

C-‘)

(W m’

F

G

(mW mm’)

(“C km-‘)

Q

Penetration meters

n (sensora)

Toulon - Apccio - TA 3 ~ Corsican Margin KP69

42 00.2

7 50.4

(1.12*0.15)

79115

71*

4

3

6

4.x

KP70

41 53.4

7 56.3

(1.12+0.15)

70*15

63*

5

3

6

5.0

KP71

41 51.6

8 01.7

1.12+0.16

63k13

565

4

2

4

3.6

Nice -C&r

~ NC1 -

Prouenpl

Basin

KP60

43 24.1

7 18.9

(1.18_+0.07)

47_+ 6

401

3

3

5

4.x

KS59

43 20.6

7 17.2

1.11+0.05

65+

59*

4

3

4

2.5

KS58

43.21.2

7 25.6

1.18+0.06

49*15

41+11

0

7

7.7

KS57

43 20.1

7 30.6

1.25+0.10

65k14

525

7

1

7

6.2

Nice -C&I

7

~ NC,7 - Oceanic Domarn

KS56

43 1X.7

7 34.9

1.23+0.11

52+

2

3

7

6.2

KP55

43 16.4

7 39.4

(1.21 kO.15)

63+16

52+

7

3

6

4.7

KP54

43 14.3

7 46.5

(1.21 kO.15)

5x*14

4X+

6

3

6

4.7

85*

8

N11.e- Cukv -

64*

8

NC3 - Corsican Margin

KP53

43 16.X

7 59.6

(1.2OkO.15)

KP65

43 07.4

8 21.2

(1.20+0.15)

KP64

43 07.7

8 25.3

KP61

43 05.4

8 28.6

KP63

43 05.1

KP62

43 00.0

102 & 23

2

6

4.7

78-

65

0

2

0.7

336-

250-

0

2

1.35

(1.20+0.15)

13x-

115-

0

2

1.30

8 31.5

(1.2OkO.15)

180*42

150+17

1

3

1.30

X 41.1

(1.20+0.15)

3

4

4.5

pattern

in the Western

1.20 k 0.23

44& 10

37*

4

REFERENCES

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