Interpretation of heat flow density of the Apennine chain, Italy

Tectonophysics, 164 (1989) 267-280

267

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

interpretation

of heat flow density of the Apennine chain, Italy F. ~ONGELLI,

G, ZITO, N. CIARANFI

and P. PIER1

Dipartimento di Geologia e Geofisica, Universittl di Bar& Ban’ (Italy)

(Received April 9,1988; revised version acdepted May 23,1988)

Abstract Mongelli, F., Zito, G., Ciaranfi, N. and Pieri, P., 1989. Interpretation of heat flow density of the Apennine chain, Italy. In: V. t?erm&k, L. Rybach and E.R. Decker (Editors), Heat Flow and Lithosphere Structure. Tectonop~y~ic~, 164: 267-280. The Apennine chain is a consequence of the continental collision between Europe and Africa characterized by several compressive phases with events of intense shortening. During one of these phases, from Early Oligocene to Early Miocene, the continental crust broke into various segments which became packed one over the other, forming an Adriatic-verging structure of imbricated listric wedges. This event is thermally modeled in the Southern Apermines by the successive overthrust of two slabs of 7 km total thickness and explains the low heat flow density (HFD) (< 40 mW m-*) observed there. In the Northwestern Apennines, modeling is on the basis of the overthrusting of a slab 10 km thick, which reduced the HFD of the steady lithosphere (63 mW m-‘) observed on the Iblean platform to 50.4 mW m-‘. From the Late Miocene onwards, tensional tectonism developed in the internal areas of the chain. Application of McKenzie’s “simple stretching” model shows that the high regional heat flow (> 100 mW m-‘), obtaining by filtering the observed data, is explained by a stretching of the ~thosphere of a factor fi = 3.1.

Introduction

General geodynamic setting of Italy

Many heat flow measurements made in Italy in the past 20 years have allowed the preparation of various maps which have so far only qualitatively been interpreted (Loddo and Mongelli, 19X,1979; Haenel, 1979; Mongelli, 1983). The aim of this work is to attempt a quantitative inte~retation of the heat flow density (HFD) data for the Apennine chain. This interpretation is made within the framework of the current geodynamic model of Italy based on the existing geological and geophysical data, and with the intention of providing further information and quantitative details for this model.

The circum-Mediterranean Alpine chains originated as a consequence of the continental collision between Europe and Africa, and form a part of a complex structure taking the form of a Meso-Cenozoic megasuture. This geodynamic framework is the result of a complicated tectonic history characterized by several compressive phases with events of intense crustal shortening which caused the closure of the palaeo-Tethys and then the collision between the European and African continental margins (Fig. la). During the Alpine orogeny, beginning from the Late Cretacegus, this collision produced the Alps and the Apennine-Dinaride chain, and then the Italian Peninsula was formed (Fig. lb). The most advanced margin of the African plate (the African promonto~), on which the structural units of the

~-1951/89/SO3.50

Q 1989 Elsevier Science Publishers B.V.

268

b al

-

1

bl

0

*

113 nlnllm4 -L-

5

-6 -7 -8

cl

A-A

9

y

IO

I

'1 \\

12

V

13

1

Fig. 1. a. Geodynamic of the present 5 = transcurrent

evolution

of the Italian

state. c. Plio-Quaternary fault;

6 = normal

fault;

Peninsula

tensional

areas.

in the Mediterranean I and 7 = thrust

Sea from Palaeogene front;

2 = Appennine

8 = allochthonous front; 9 = Ancona-Anzio 12 = graben; 13 = recent volcanism.

line;

to Neogene. chain;

10 = profile;

b. Schematic

3 = foredeep;

picture

4 = foreland;

I1 = neotectonic

tension;

269

chains with African vergence are overthrust (among them the Apennine units), plays a very important role in the palaeotectonics of the peninsula. Finally, from the Late Miocene, tensional tectonism developed in the internal areas of the Apennine chain (Fig. lc). Structural geology

The Apennine chain can be divided structurally into the northern and central-southern segments. The latter is linked to the south with the Calabrian orocline (Calabrian-Peloritanian Arc). The boundary between the two Apennine segments corresponds to the Ancona-Anzio line, along which transc~ent movements (Late Miocene) and overthrusting (Early-Middle Pliocene) towards the east took place. The Northern Appenmnes are formed by largely allochthonous tectonic units separated by overthrust surfaces with Adriatic vergence. In this segment erogenic activity took place from Oligocene to the Early-Middle Pliocene. Similarly, the structure of the Central-Southern Apennines is characterized by large wedges formed by several all~hthonous units with Adriatic vergence. The regional trend of the structures, although parallel to the chain axis, is much more irregular and discontinuous here. The orogenie activity took place mainly from Early Miocene to Early-Middle Pliocene. Along the external front of the Apennine chain, from the PO Plain to the Taranto Gulf, the Adriatic Trough developed. This area is occupied by Pliocene-Quatemary deposits which are locally weakly deformed. In these sediments, fronts of the allochthonous Apennine sheets are inserted at various levels. The trough extends eastward to the Adriatic Sea and southward to the Ionian Sea, offshore from the Calabrian-Peloritanian Arc. The longitudinal continuity of the Adriatic Trough is interrupted by a series of buried structural highs located along the segments of important, active, transverse structures of the chains (Casnedi et al., 1982; Ciaranfi et al., 1983); this permits the identification of a series of basins in the trough (Fig. lb). In a more external position, Mesozoic terrains occur; these are part of the Apulian carbonate

platform (Apulian microplate) and the Iblean platform (southeastern Sicily). They continue at depth beneath the whole of the Adriatic and beneath part of the northern Ionian Sea, up to Cefalonia. These terrains constitute a variously deformed foreland characterized by simple tectonics with large open folds and systems of normal, longitudinal and transverse faults producing an en-echelon structure. Orogenesis

The long orogenetic process can be divided into two stages according to the modes of subduction. The first (oceanic stage) is characterized by the subduction of the oceanic ~thosphere and the second (ensialic stage) is marked by the subduction of the continental lithosphere (Boccaletti et al., 1980, 1982). The oceanic stage developed in two main phases (Fig. 2). During the first phase (Early-Late Cretaceous), both oceanic lithosphere and sedimentary cover were subducted along an E-dipping plane, resulting in structures verging towards the European continental block. In the second stage (Late Cretaceous-Middle Eocene), a W-dipping subduction is hypothesized. This implies thrusting of the oceanic lithosphere beneath the CorsoSardinian massif (or European plate) until the complete closure of the oceanic area. The ensialic stage to which the origin of the Apennine chain is attributed, is characterized by the collision of the two continental margins. It developed in several phases, which are differentiated on the basis of the structural levels where the shortening took place (Fig. 3). In the first phase, which took place from the Early Oligocene to the Early Miocene, the structural levels are represented by the lithospheric mantle, which continued to greater depths, and by a thinned continental crust which supposedly detached from the lower level corresponding to the low-velocity channel (LVC). Thus, while the mantle continued.its downward movement, the continental crust, under compression, broke into various segments which became packed one over the other forming an Ad~atic-verging structure of imbricated listric wedges.

270

(1) (2)

i

Lsa

Confinental

[=I

Apennine conk-daf

Corso-Sardinian block

al sedinen~ary

margin :

(3)

m

Oceaniccrust

(4)

hK%

Intraoceanic

mimcrah

cover

bl basement Fig. 2. The subduction mechanism during the oceanic stage. A. Lithospheric

subduction of the Corso-Sardinian

block. B. Oceanic

lithosphere subduction.

During the successive tectonogenetic phases (from Late Oligocene to Late Tortonian-Early Messinian), the third structural level developed corresponding to the sedimentary cover detached from the basement by subduction. Through this mechanism the sedimentary covers (not subducted) were overthrust towards the east (also as a result of gravity sliding) on to the tectonic units which originated in the preceding period, in agree-

Fig. 3. The ensialic subduction which generated the Apennine chain from Oligocene (A) to the Late Miocene-Tortonian and C) (after Boccaletti et al., 1980,1982).

(B

The symbols are the

same as in Fig. 2, except 4 which indicates mantle.

ment with the migration of the tectonic front towards the Adriatic. Furthermore, as a result an area of tectonic denudation to the west, and an area of accumulation to the east developed. The ensialic subduction migrated successively further to the east and involved the peri-Adriatic area. Here, the most important phase occurred in the Middle-Lower Pliocene (Middle Pliocene tectonic phase). With the ovation of the compressional front towards the external areas, beginning from the Late Miocene, tensional tectonism developed in the internal areas of the chain. This tectonism, which also affected crustal zones, appears to be connected with the opening of the Tyrrhenian Sea, which dates back to about 6 m.y. ago (Fig. 4). The effects at the surface are represented by predominantly vertical movements of rigid blocks along normal faults with the formation of tectonic grabens separated by ridges which developed in a NW-SE direction. At depth, there was again gradual crustal thinning promoted by the rotational “ sliding” of crustal segments (the most western ones) formed during the preceding compressional phases. It is hypothesized that the sliding occurred along the original listric planes. Intense magmatic activity, both intrusive and extrusive, which is very diffused in the pet-i-Tyrrhenian zone from Tuscany to Campania (Barberi and Innocenti, 1980) accompanied this new neot~to~c regime.

271

m

m”,

ml 1

2

Fig. 4. Evolution chain during

4

of the Tuscan-Latial

Mio-Pliocene

area of the Apennine

time due to a tensional

event. The symbols

geodynamic

are the same as in Fig. 3.

on the study of surface wave dispersion, Calcagnile and Panza (1980) estimated the lithospheric thickness L for this area to be between 30 and 50 km (Fig. 5). From DSS experiments, Nicolich and Pellis (1979) and the IESG (1983) deduced a crustal thickness in the order of 20-25 km. These results show that the tensional processes on the surface were accompanied by lithospheric thinning. Assuming that the thickness of the thermally steady lithosphere is 125 km, and estimating L = 40-45 km for the Tuscan region, one obtains a stretching factor of /? = 125/L = 3.1-2.8. Interpretation of the heat flow pattern Southern Apennines

The present structure of the lithosphere in the pre-Apenninic zone was determined from the results of seismic and seismological research. Based

Figure 6 is a HFD map compiled on the basis of the data corrected for Pliocene sedimentation.

T

46”

64”

8”

c3 8” Fig. 5. Lithospheric

thickness

(km) in Italian

territory

for the

and surrounding

areas (after Calcagnile

and Panza.

1980).

internal

palaeogeographic

ternal

domains.

into (-

two

time

22-17

domains

The main

phases

intervals-Aquitanian-Langhian

Ma) and Tortonian-Messinian

Ma). During

the formation

overthrusting following

processes

relaxation

of a mountain

contribute

(2) production on the thrust

chain

heat by radioactive

by

the three (1) thermal

from one stratum

of heat

plane,

10-7

to the establish-

equilibrium:

by heat conduction

another,

(-

of one layer onto another,

ment of the new thermal

friction

onto more excan be grouped

to

as a result

of

and (3) production

decay. Various researchers

dealt with this problem (see Molnar for an exhaustive bibliography).

of have

et al. (1983)

In the case of the Center-Southern Apennines, the heat of radioactive origin can be disregarded since

the overthrust

rocks (Ciaranfi

strata

comprise

sedimentary

et al., 1983).

Brewer (1981) discussed a theoretical case where a slab overthrusts a half-space at constant velocFig. 6. Surficial

HFD map of the Central-Southern

Apennines

(mW m-‘).

The map shows lower than normal values of 30-40 to the Apenmne chain mW mP2 corresponding and the Adriatic-Bradanic Trough, and higher values towards the Apulian platform and the Tyrrhenian Sea. Local anomalies are located on the structural highs of the trough basement. Thus, these may be due to both disturbance of the heat flow by the structures and possible local ascent of warm waters. The low heat flow values for the trench may result from the insufficient correction for sedimentation, whereas those for the Apennines may be due to the compressional overthrust origin of the chain. The maxima of compressional activity in the Southern Apennines developed in several phases: each of them was characterized by the overriding of successions of basins and platforms from more

[dTf,ict/dz],+,=

[-D/b

cpKt,)]

[-TD/(PCpKtl)](erfc[H/(2(Ktl)1'2)]

ity, assuming that the heat is produced only as a result of friction and thermal relaxation. The HFD at the ground surface in this case is given by: Q =

Qfrict+ Qre,ax=

- K(dT,,,,/dz)

- K

(dT,c,ax/dz> where (see eqn. (1) below): and : dT,,,,,/dz

= -AT/AZ

[exp( - H2/4d)]

( H/( nKt)1’2 x - 1)

(2)

The time t is counted from the start of the overthrusting, which lasts for a period of time equal to t,. Additionally, T =fpgH, representing the shear stress along a fault plane, f= friction coefficient, p = density, g = acceleration due to gravity, D = amount of displacement of the overthrust slab, H = thickness of the overthrust slab,

erfc( H/[2(Kt)1’2] ); t5

t, =

-errc[H/[2(K(t-t,))“‘]]);t>tl

(1)

273

The

4

1 2

3

original

volved estimated

a)

thickness

in the overthrusts

of the

sediments

in the basinal

to have been around

in-

areas

3 km, whereas

in the platformal

areas is about

1978). Let us assume

that in the first tectonic

6 km (Pescatore, phase a

&km thick and 50-km wide slab was overthrust km towards slab, ments,

the east, and in the second

together

with

a 3-km

moved another

is

that

thick

layer

50

phase the of sedi-

50 km in the same direction

(Fig. 7). b)

II

In our region,

the steady

lithosphere

is pres-

ently represented by the Apulian microplate and the Iblean platform, where the HFD is = 63 mW mm2. This III Fig. 7. Structural the Apennine tectonic

evolution

chain.

a. The situation

ment of the most internal domains

model evolution

cp =

of the palaeogeographic

events. The arrow indicates

the external

corresponds

30° K km-‘.

specific

heat,

before

(after

Pescatore

thermal

of the move-

domains

phases (I-III).

diffusivity,

K =

to a gradient to study

of about

the variation

in

heat flow Q at the surface during the first phase let us assume that t = 0, corresponding to 21 Ma, t, = 4 Ma,

towards

et al., 1978). b. block

of the main tectonic

K =

of

the Aquitanian

the direction

palaeogeographic

units

In order

f=0.2,

0.4

and

0.6 (from

Brewer,

1981), p = 2500 kg rne3, g= 9.8 m sw2, km, D = 50 km, cp = 1.024 kJ kg-’ K-‘, K m2 sT1, K= 2.1 W m-l K-’ and AT/AZ km-’ (To= 0°C; T, = 18OOC).

H =6 =

10m6

= 30°K

Using eqns. (1) and (2), one obtains the curves shown in Fig. 8 which represent the variation of Q

thermal conductivity and AT/AZ = initial thermal gradient. It is probable that the original extent (before the Aquita~~) of the succession of basins and platforms was about 150 km. Since at present the

with time. As one can see, changing f leads to a significant initial variation in the curves, but after approximately 8 m.y. from the time zero, the

chain is approximately 50 km wide, we deduce shortening in the order of 100 km.

differences become insignificant. Furthermore, one can observe that the effect of friction dies out

a

. 10

Fig, 8. Surficial

HFD

Qt values (continuous

line) due to the overthrust

constant velocity for different friction coefficients Qr (dashed

line) (the HFD produced

of a &km thick and 50-km wide slab towards the east at a

f. Q, is the sum of Q, (dotted line) (the HFD due to the thermal

by the friction).

Q, is the undisturbed

HFD.

The overthrusting

lasted

relaxation)

t, = 4 m.y.

and

214

ago

M.Y. 20

15

10

5

0 L

_______~j_____________________--------

60

40 ;

o.s-

0.6 23

u 1 0+ 0

5

10

15

3 z 3 rl,

20

M.Y. Fig. 9. Surficial

HFD time variation

caused by two successive computation

overthrusts

which involved

the Apennine

are given in the text. Q, is the undisturbed

rapidly after the overthrust event, whereas only the effects of relaxation remain. After about 6

son,

1977;

chain. The values used for the

HFD.

England

and

Thompson,

reduction is faster and stronger at depth. When the thermal

1984).

The

at the surface than perturbation pro-

m.y., the variations become extremely slow, and the normal HFD is restored in a time period of about 100 m.y. In this long period, thermal conditions can be considered quasi-stationary. In particular, the surface gradient after 11 m.y. from the

As regards the Central-Southern Apennines, we estimate that in the interval between the first

beginning of the overthrusting is about 25’K km-‘, i.e., slightly more than 10% below the stable

and second phase of overthrusting, 2 km of sediments were eroded. To study only the effect of the

value. For these reasons, we consider the second phase of overthrusting as thermally independent from

overthrusting,

the first, and we assume that t = 0, corresponding to 10 Ma, t, = 3 Ma, H = 9 km and AT/AZ = 25°K km-’ (T,=OOC; T,=225“C). Leaving the other parameters unchanged we again apply eqns. (1) and (2). Figure 9 shows the variation in HFD at the surface for the first and second present

phase, successively. the effect of friction

As one can see, at has declined, and the

HFD is about 38 mW m-‘. This is the value one would observe today in the Central-Southern Apennines if there had not been any influence of other processes on the heat flow regime. In this context, in reality, a process that could have considerably influenced the above-described thermal regime is erosion. This process removes heat from the surface and reduces the internal temperatures of the relief (England and Richard-

duced by erosion dies out, the original gradient is restored.

let us assume

geothermal

that at the start of the

second phase the thermal effect of the erosion died out, and that the gradient in the first 7 km was again equal to 25°K km-’ (To = 0°C; TH = 175 o C). Using

these values

and recalculating

the HFD

at the surface after the second phase, one obtains for the present day a HFD of about 42 mW rne2. This is the value one should observe today in the Apennines after the correction for erosion. Another process to be considered in interpreting heat flow values is the infiltration of meteoric water. This can reduce the temperatures, sometimes down to depths of a few kilometres, depending on the permeability of the rocks. As a result, the surface gradient declines significantly to a few degrees per kilometre. Generally, with the progression of erosion, the permeable zone extends. This seems to be the case in the Apennines, the highest part of which predominantly comprises highly

215

Fig. 10. HFD map of the Tuscan-Latial

region and the facing Tyrrhenian Sea. Contours = mW mm2

fractured

rocks. This can explain

observed

heat flow values are below 40 mW m-2.

the fact that the

Interpretation of the Northern Apennines heat flow values Figure 10 shows a HFD basis of the observed data.

map compiled on the The map reveals that

on average, the Tuscany-Latium pre-Apennine zone is characterized by very high values. The values decrease rather abruptly towards the chain, and more slowly towards the Tyrrhenian Sea. The pre-Apennines zone is particularly noted for its locally exceptionally high (5: 1000 mW mm2) as well as very low (= 50 mW m-*) values.

These oscillations are attributed to the infiltration of meteoric waters into the permeable areas (Calamai et al., 1977), topography, geometry of the carbonate basement structure (Galeone and Mongelli, 1982) and magmatic intrusions. These factors, although sarily superficial.

of local character,

are not neces-

All these effects overlap on those character related to the lithospheric

of regional shortening

and extension processes which are the focus of this study. In order to facilitate this study we filtered the HFD map using several cut-off wavelengths (CW) equal to 50, 75, 100, 125, 150, 200 and 250 km. Filtering to CW = 100 km the map (Fig. lla) reveals the presence of two large

276

a Fig. 11. Regional

HFD after filtering

b with a cut-off

wavelength

anomalies of wide extent: one of these anomalies is located over Tuscany and the other in the Tyrrhenian. For CW = 150 km (Fig. llb), the first anomaly disappears, whereas the second one still remains after filtering with CW = 200 km. The latter anomaly can be explained as due to the oceanization of the Tyrrhenian area, a process which has been studied from a thermal viewpoint by Hutchinson et al. (1985). In contrast, the Tuscany anomaly is of continental nature and undoubtedly linked to the extension processes at the western margin of the Apulian platform. Since the boundaries of the Tuscany basin are approximately defined by the lOO-mW me2 isoline

of CW = 100 km (a) and CW = 150 km(b).

12. Block

model of the compressive (a) and phase (b) of the brittle crust.

tensional

= mW rne2,

and the highest value isoline crossing the area is 140 mW rnp2, we assume a value of 120 mW me2 as the average value of HFD for the basin. Also in this region, the steady lithosphere is represented by the Apulian microplate and Iblean carbonate platform characterized by a HFD of about 63 mW rne2, with a surplus of 30 mW rnp2 with respect to a reference sublithospheric heat flow at 33 mW me2 (McKenzie, 1978). As already stated above, the Oligocene (30 Ma) compressional phase broke the brittle continental crust into various segments which, afterwards, piled one over the other, causing crustal thicknening (Fig. 12a). Thermal relaxation of the overlying strata occurred in the thickened lithosphere; in addition, a certain quantity of heat was generated by friction during

Fig.

Contours

thrusting.

The extension process occurred in the Late Miocene-Pliocene (7 - 4 Ma). The brittle crust extended and thinned so that blocks slid along the pre-existing listric faults and rotated in such a way that these faults took up a more horizontal position (Fig. 12b). This rotational motion involved a

211

0 Ma

Fig. 13. Time variation

of the surficial

HFD

due to relaxationand friction in tensional

heat transfer to the surface by advection. The remainder of the lithosphere (ductile crust and lid) extended plastically and ascended together with the asthenosphere. Presumably also during this phase some heat was generated due to friction on the listric faults. Therefore, it is necessary to consider the variation in the initial value 63 mW mm2 as being due to the above-mentioned processes. In order to calculate the thermal perturbation produced by the compressional and overthrusting phase, disregarding the heat generated by radioactive decay, we apply Brewer’s (1981) model again. In our case, it is possible to assume that’ the entire compressional event, which began 30 m.y. ago and developed in four phases, was completed in approximately 24 m.y. The Tyrrhenian and Adriatic crustal blocks were the first and the last, respectively, to be formed. Ignoring the dip of the blocks and considering the interaction between the two initial Tyrrhenian blocks only (initial phase), we assume the following (Fig. 12): t = 0, corresponding to 30 Ma, t, = 6 Ma, f = 0.2, 0.4 and 0.6 (Brewer, 1981), p = 2500 kg mF3, g = 9.8 m s-‘, H=lO km, D=20 km, c,=1.024 kJ kg-’ K-‘, K = IO-6 m2 s-1 , K= 2.1 W m-i K-i and AhT/Az=30*K km-’ (T,=O”C;’ T,=3OO”C). AT/AZ represents the HFD before the compression, assumed to be 63 mW rn-=.

the compressional

phase. and to friction

only in the

phase.

Applying eqns. (1) and (2), we obtain the curves shown in Fig. 13. It can be seen that the friction effect becomes negligible after 12 m.y., and that after 23 m.y., the HFD, being still in the restoration phase, is around 50.4 mW m-*. This is the time of the beginning of the extensional phase (Fig. 12b), which most probably took place between 7 and 4 Ma. The cont~bution of the heat generated by friction to the HFD at the surface in this phase can be calculated using eqn. (2) and assuming 10 km of thrusting in 3 m.y. Figure 13 shows that the thermal effect generated by friction during the extensional phase is small; hence, after 30 m.y., i.e., at the present, only the effects of the thermal relaxation due to the compressional phase are important. The resulting HFD, assuming lack of variation in the asthenosphe~c cont~bution to the heat, should be 55.2 mW mm2 of which 33 and 22.2 mW m-’ are of asthenospheric and lithospheric origin, respectively. The stage of tensional tectonics in the lithosphere manifests itself at the surface through the presence of rift systems which range from well-defined grabens to very large, less distinct regions. Sengijr and Burke (1978) suggested two general mechanisms for the formation of rifts. The first, an active mechanism, is a result of thermal upwelling of the asthenosphere (&onvection or mantle

278

plumes). This causes thinning of the lithosphere, especially in its subcrustal part, by melting or diapirism. The second, a passive mechanism, is a passive response to a regional field of pre-existing tensional forces generated at the plate margins by ridge push or trench pull; the lithospheric thinning, both crustal and subcrustal, is quasi-uniform. In the light of the above-mentioned geodynamic data and of the structural characteristics of the ~thosphere in the pre-Apennine TuscanyLatium zone, we are strongly inclined to favour the mechanism of passive extension in our case. The simplest quantitative model of this process (McKenzie, 1978) assumes that the stable lithosphere stretches uniformly, plastically and instantaneously, while the asthenosphere rises and the heat flow increases instantanously by advection. The stretching is followed by cooling and re-thickening of the lithosphere with a subsequent decline in the HFD at the surface. In this model, the heat of crustal origin is ignored, and only heat flow 4 which originates from the base of the lithosphere is considered. According to McKenzie (1978), the HFD at the surface in the declining phase is: q(t)=KT,/a

1+2~[(B/nn)sin(nl;/P)1 i

X

1

exp( - n2t/7) 1

where K is the thermal conductivity, T = 1333” C is the temperature of the asthenosphere, a = 125 km is the lithospheric thickness, KT,/u = 33 mW rnw2 is the HFD at the surface of a steady lithosphere, p = the extension and thinning factor, t = the time after extension, 7 = a2/(lr2k) = 62.8 Ma and K = thermal diffusivity. Subsequently, Jarvis and McKenzie (1980) demonstrated that an extension can be considered instantaneous if it lasts less than 20 m.y. Sclater et al. (1980) and Royden and Keen (1980) proposed to increase the thermal contribution during an extension, envisaging a smaller thinning for the upper crust and a greater thinning for the remainder of the lithosphere.

200

~

of 0

,

I

70

20

‘0

30 Ma

Fig. 14. Mean regional HFD value of the Tuscan-Latial region (cross) compared with the cooling curves for different stretching values /3 from McKenzie’s tensional model.

Before applying McKenzie’s model, we must subtract the amount of the HFD of crustal origin (22.2 mW m-‘) from the regional heat flow of the Tuscany-Latium region (120 mW me2). Thus, we obtain about 98 mW m-‘. Figure 14 shows several curves derived from eqn. (3) using different values of #3. The small cross in the figure, located just above the curve calculated for /I = 3 gives /I = 3.1 for the Tuscany basin. This is in agreement with the lithospheric data of Calcagnile and Panza (1980) which are representative of the present geodynamic situation, which gave p = 3.1-2.8. This is provided we keep in mind that (1) the /3 value used in our interpretation refers to the stage of extension, i.e., at about 6 Ma, and (2) at the end of the extension the lithosphere is thickened due to cooling. Finally, it could be concluded that the heat flow data are in agreement with the model of passive extension, and indicated that in the Tuscany region during the tensional phase the thinning and extension factor for the lithosphere was in the order of fl = 3. Conclusions

The geological history of the Apermine chain has allowed the modeling of its geothermal evolution both in time and in space. As far as the Central-Southern ApemGnes are concerned, the model takes into account the overthrusting phases (occurring at various times) of

219

the

several

heat

sedimentary

production

tunately,

most

by

nificant. HFD

radioactive

chain

(30-40

cannot

be strictly

duced

from

comparison

mP2)

compared

the model

in

is sig-

these

(38 mW

m-2).

areas de-

A close

only if observations

as the Northern

Apenmnes

are con-

applying McKenzie’s model, the value to which the HFD was reduced (55.2 mW mP2) has first been estimated, i.e., it has been considered that the brittle crust does thins by sliding and

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