EPSL ELSEVIER Earth and Planetary Science Letters 146 (1997) 18 1- 193

Thermomechanical evolution of the South Alpine rifted margin ( North Italy) : constraints on the strength of passive continental

margins

G. Bertotti aq * , M. ter Voorde a, S. Cloetingh a, V. Picotti b

’ Department of Sedimentary Geology, Vrije Unicersiteit. de Boelelaan 1085, 1081 HV Amsterdam, The Netherlands b Dipartimento di Science Geologiche. cia Zumboni 67, l-40127 Bologna. Italy

Received 31 July 1996; accepted 28 October 1996

Abstract

We have reconstructed the kinematic, thermal and rheological evolution of the Triassic-Cretaceous South Alpine passive continental margin (Northern Italy) and estimated lateral strength variations associated with rifting. Rifting initially caused the formation of a roughly symmetrical, ca. 100 km wide graben located 150-200 km away from the future break-up site. 30

Myr after the onset of rifting, the rift axis shifted laterally and eventually led to oceanic crust formation. The thermal evolution of the margin was first controlled by the waning of a thermal anomaly, which was emplaced shortly before rifting

and was cooling during the first extensional stages. Numerical modelling of rifting and drifting, constrained by field data, is

used to trace thermal and rheological changes affecting the rifted margin. The effects of the thermal anomaly are restricted to the first lo-20 Myr of rifting. Subsequently, isotherms are slightly affected by lithospheric thinning and show very little distortion after break-up. Rheological modelling demonstrates that the South Alpine lithosphere preserved substantial strength even shortly before break-up and became stronger thereafter. The modelling also shows that, at break-up and 100 Myr later, thinned domains of the margin are stronger than the less thinned ones.

Kqvwords: Piedmont Alps; passive margins; continental margin; extension tectonics: rift zones

1. Introduction

The mechanical configuration of the lithosphere

resulting from rifting and passive continental margin formation processes is poorly understood. Geophysi- cal studies based on gravity anomalies on passive

margins, along, for example, the North Atlantic, yield low effective elastic thickness values (Te) of the order of 5 km (e.g., [1,2]). The low Te values

* Corresponding author. Fax: +31 20 6462457. E-mail: bert@geo.vu.nl

obtained have led to the concept of a “frozen effec- tive elastic thickness (Te)“, according to which the

lithosphere is weak at the end of rifting and remains

so for tens of millions of years thereafter. However, high Te values of 20-28 km on other margins, such

as that of New Zealand [3], suggest that low values are not ubiquitous.

The concept of a lithosphere remaining weak during drifting also contrasts with the intuitive no- tion that, as thermal perturbations relax, a lithosphere

with thin continental crust should be stronger than one with thick crust [4-61. To support this notion,

0012-821X/97/$12.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved. PI1 SOOl2-821X(96)00214-2

182 G. Bertotti et al./Earth and Planetary Science Letters 146 (1997) 181-193

only simple 1-D thermorheological modelling has been presented so far, neglecting processes such as

deformation kinematics and sediment blanketing ef-

fects. Furthermore, no “real” case has been used to test model predictions.

To constrain the thermal and rheological evolu-

tion of lithospheric strength during and after rifting,

we combine field data with numerical modelling. To

this purpose, we use a transect of the Late Triassic to Middle Jurassic Adriatic passive continental margin

which is presently exposed in the Southern Alps of

North Italy (Fig. 1). The South Alpine margin is

particularly well suited since intermediate to deep Mesozoic crustal levels are directly accessible to

investigation and its kinematics and thermal evolu-

tion during and after rifting are well constrained.

Several parameters of this margin, such as duration

of rifting and drifting and amount of extension are

representative for other rifted margin systems. A further useful feature of the South Alpine margin is

that, 100 Myr after break-up, it was loaded by

Alpine thrusting. The analysis of the associated flex- ure allows an important, independent test of the

strength results obtained.

We firstly reconstruct the crustal kinematic evolu- tion of the South Alpine transect. We apply numeri-

cal forward modelling techniques to derive the ther-

mal evolution of the margin from the onset of exten-

sion to break-up and drifting and study the rheologi-

cal configurations at the end of rifting and during

drifting. We then test the modelling results with

field-derived data. The approach adopted here is substantially different from other modelling studies

of passive margin evolution (e.g., [7] and references therein) in that we explicitly apply the model to a

“real” case with its rifting geometry and evolution-

ary history.

2. The Southern Alps from rifting to contraction

2.1. Kinematic eoolution

A large body of sedimentological, stratigraphic

and structural data collected over several decades

allows for a well constrained reconstruction of the

rifting and drifting evolution of the South Alpine

margin. Both timing of fault activity and the amount of displacement accommodated by a single fault are well known from detailed field studies ([9] and refer-

ences therein). This has allowed a palinspastic restoration of the upper crustal profile through the

South Alpine passive continental margin back to its

pre-rift situation, thereby constraining the amount

and timing of displacement accommodated by each

major fault [9]. The tectonic units presently exposed in the South-

em Alps underwent Late Triassic to Middle Jurassic

. . I L :i syn-erogenic I syn- and post-rift

Fig. 1. Tectonic outline of the Southern Alps with main Mesozoic extensional faults. Most of the Alpine features have been omitted for

clarity. LMF = Lago Maggiore fault; LF = Lugano fault; SF = Sebino fault; GF = Garda fault; SZ = Sesia Zone; IZ = Ivrea Zone. A and

B = traces of the profiles in Fig. 10.

G. Bertotti et al. /Earth and Planetap Science Letters 146 (19971 181-193 183

extension in connection with relative movements between Europe and Adria, culminating in the late

Middle Jurassic formation of oceanic crust (Fig. 2) [8,9]. Rifting, leading to passive margin formation,

started in the Norian and was first localized some 150-200 km east of the site of future break-up (Fig.

2). Extension was mainly controlled by a set of four,

roughly N-S trending normal faults symmetrically delineating a strongly subsiding region, the Lombar-

dian basin. The two western faults, the Lago Mag-

giore and Lugano faults, dipped to the east while the

two eastern ones, the Sebino and Garda faults, dipped

to the west. Field data demonstrate that faults were listric [3,10] and can, in some cases, be followed

down to a paleo-depth of 12- 15 km [ 111. An overall

pure shear geometry for the extension in the Lom-

bardian basin (lfina,/linit,a, = 1.5) is compatible with the observed geometry and amount of displacement

of normal faults and with the subsidence pattern recorded by the sediments [ 1 I]. The Lombardian

basin was surrounded by two less extended regions: the Trento Plateau in the east and the Biellese to the

west. Stretching in the Lombardian basin gradually

ceased during the Toarcian and extension shifted westward affecting previously less stretched domains

and eventually leading to the opening of the Ligu-

ran-Piemont ocean. The shift in the site of extension has been associated with the progressive hardening

of the lithospheric column undergoing extension [9]. Low strain rates of the order of lo- I6 set- ’ and abnormally high initial Moho temperatures, such as

those reconstructed for South Alpine rifting ([9] and

[12], respectively) favour this behaviour (e.g., [7]). Extension during this second stage, which led to the

W canavese biellese lombar

Middle-Late Jurassic formation of oceanic crust,

was probably controlled by a major, possibly west- dipping lithospheric fault (e.g., [ 13,141). For most of

the subsequent drifting stage the margin was starved

and received a very small amount of sediments. Drifting continued until the Late Cretaceous, when it

was interrupted by the onset of convergence between

the Adria and Europe [ 151 and S-vergent thrusting of

the South Alpine margin ca. 100 Myr after break-up.

The South Alpine fold-and-thrust belt loaded the

E-W extended margin, thereby allowing for the deposition of a thick sedimentary wedge (South

Alpine foredeep).

2.2. The thermal evolution

Recently acquired petrographic and isotopic data have provided crucial constraints for the thermal

evolution of the South Alpine crust before, during and after rifting [12,16,17].

The thermal history of the South Alpine crust is mainly controlled by the emplacement of a thermal

anomaly in the Early-Middle Triassic and its subse-

quent waning [12,18]. The thermal anomaly can be quantitatively explained by the presence at lower

crustal depths of a large magmatic intrusion [ 12,191.

Waning of the thermal anomaly began at ca. 220 Ma, overlapping with the initial stages of rifting [ 121. The

subsequent crustal cooling caused the closure of

radiometric and fission track clocks and explains the Late Triassic to Jurassic absolute ages obtained from

several localities of the South Alpine basement (e.g.,

[ 12,16,19]). The upper 15 km of the Mesozoic South Alpine crust were below the zircon fission track

dian b a s i n trento E

0

Okm

50 km

Fig. 2. Field-derived upper crustal section across the South Alpine passive continental margin (Early Cretaceous). The main paleogeographic

domains and faults can be recognized in Fig. 1. The star beneath the Lugano fault shows the locality where samples were taken for absolute

dating (see text). The water column has been neglected. Inset: plate tectonic configuration at the end of the Jurassic showing the position of

the South Alpine transect across the Adriatic passive continental margin (from (391).

184 G. Bertotti et al. /Earth and Planetary Science Letters 146 (1997) 181 -I 93

annealing temperature during the Early Cretaceous; that is, before Alpine contraction.

3. A quantitative model for the thermokinematic and rheological evolution

3.1. The thermokinematic numerical model

The kinematic model we use [20] allows for a complete description of crustal extension and meets the condition of volume preservation (Fig. 3). It consists of an upper part where extension is accom- modated by discrete faults and a lower layer with distributed stretching [20]. Our model for the descrip tion of fault-controlled extension is much closer to reality than other models, which assume, even in the case of finite rifting, contemporaneous activity of all normal faults. In the numerical calculations pre- sented here, on the contrary, timing of fault activity and the amount of accommodated displacements can be independently attributed to each fault. The fault- controlled basins formed during extension accommo- date the observed sediment thicknesses; the remain- ing accommodation space is filled with water. The wavelength of thinning and the horizontal position of the maximum extension site in the lower layer, where distributed deformation takes place, can be varied.

Adopting the kinematic scheme derived for the upper to middle crust and assuming an initial crustal thickness, we have reconstructed the complete kine- matic evolution (stretching and thinning) of the South Alpine rifted margin.

The temperatures in the model are calculated by a finite-difference method on a rectangular but non-

Table 1 Parameters used in the thermokinematic modelling ([20] and

references therein)

T at surface 25°C

T at base lithosphere 1330°C

Thickness of heat-producing layer 15km

Heat production in layer 2.3 X 10-s W/m’

Specific heat llOOJkg-’ K-’

Thermal diffusivity in sediments 0.75 X lo-” mz set- ’ Thermal diffusivity in basement 10e6 m’ set- ’

uniform grid. Thermal properties on each point are obtained by interpolation from a second, moving grid representing the extending basin [20]. The thermal model includes heat production, and allows for varia- tions in thermal conductivity between the crust and the sediments. This makes the model suitable for studying sediment blanketing effects. The model pa- rameters used are given in Table 1.

3.2. Rheological modelling and effective elastic thickness

Extracting thicknesses and thermal gradients from the numerical model, we have constructed yield strength envelopes (YSE) [21-231 for five represen- tative domains across the margin at break-up and 100 Myr later (Fig. 5). The following compositional layers have been used for the construction of the profiles: (1) water, with a constant temperature T, and zero strength, but contributing to the increase of pressure with depth; (2) unconsolidated sediments, also with zero strength, but imposing a temperature gradient; (3) consolidated sediments with a finite strength; (4) granites (wet or dry) for the upper crust; (5) diorite and diabase for the lower crust to repre-

Fig. 3. The setting of the numerical model. Deformation in the upper layer, above the detachment level, is accommodated by discrete faults.

Deformation in the lower layer is distributed. The model preserves surfaces (in 2-D).

G. Bertotti et al. / Earth and Planetary Science Letters 146 (1997) 181-193 185

&G_-___-______-’ <-------7 .-_____ 40-~----- -

-“-.---__--_~~-__-- / ’ \ /

/ i \

60 -&- --_________-----’ ‘-

‘---_--__68

-s---_______

a) at the onset of extension

Okm

20

40

Canavese Lombardian basin Trento plateau

___-_----

60

b) at break-up

Okm

60

c) 100 Myr after break-up 0 1OOkm I I

Fig. 4. Results of thermokinematic modelling including the effects of the Middle Triassic thermal anomaly. (a) situation at 223 Ma, when

normal faulting began and the thermal anomaly started to cool. (b) situation at break-up (160 Ma). (c] situation before the onset of

contraction (60 Ma). Dashed lines in (a) indicate the future normal faults, triangles in (c) give the locations where the geothermal gradient

was extracted to construct the rheological profiles shown in Figs. 5 and 6.

sent wet and dry rheologies, respectively; and (6) olivine for the mantle. The thicknesses of the various

layers after rifting are visible in Fig. 4. We use wet

rheologies and geologically reasonable thermal diffu- sivity values for the sediments of 0.75 X

10m6 m’/sec (e.g., [20]).

To constrain the YSE we use “yield strength”

equations, relating the stress needed to cause defor-

mation to depth. For the brittle domain, we use a

modification of Byerlee’s law [21,24]:

uh=cypg(l -h)z (9

where p is the density; g is the gravitational accel-

eration; z is the depth; and A is the pore fluid factor.

(Y can be given in terms of the principal effective

stress ratio, R, and is dependent on the mode of

Table 2

Parameters used for the construction of strength profiles [28]

Upper crust

Quarzite (w) Quartzite (d)

Lower crust

Diorite (w) Diabase (d)

Mantle

Olivine

A (PaaN s-‘) 1.26 x IO- ” 6.03 x lo-?’ 1.26 x IO- I4 3.16 x lo-?’ 7.0 x lo- ‘j E &J/mol)

n E. &J/mol)

A, (s-l)

(To (Gpa)

172.6 134 212 276 510

1.9 2.72 2.4 3.05 3.0 _ - 535

_ 5.7 x lo- ” _ _ 8.5

186 G. Bertotti et al. /Earth and Planetary Science Letters 146 f f997) 181-193

faulting. R is related to the coefficient of friction /1

as R=((p’+ 1) ‘I* + pj2. For thrust faulting, (Y = (R - 1); A is = 0 for zero pore pressure; and A = 0.4

for hydrostatic pore pressure. Following Ranalli [25], commonly the value for p is assumed to be 0.75.

For the “ductile” domain we use “Power law

creep” [26]:

,d=A-i/“k-i/” exp( E/nRT) (2)

or Dom law creep at high stresses where the Power

law creep breaks down [27,28]:

(3)

E is strain rate; A, n and E are empirically derived material constants; R is the gas constant (= 8.3 14

J/mol . K); and T is the absolute temperature. The

values of the parameters are given in Table 2. We are concerned with gross lateral variations

rather than precise absolute values (e.g., [291). In order to avoid problems related to phenomena such

as crustal-scale strain localization and changes in

deformation mechanisms, rheological modelling is

performed only for tectonically “quiet” times such as the drifting stage. We adopt therefore a low strain

rate of 1 X lo-” set-‘. Integrated strength values equivalent to the force

needed to lithospheric rupture [30] have been derived

from the rheological profiles, as shown in Fig. 5.

The YSE can also be translated in Te values [23]. We use the formulation for detached layers, since the

300km

Fig. 5. Integrated strength and Te values across the South Alpine

margin at break-up (dashed lines) and 100 Myr later (solid lines).

0 = the position of the rheological profiles (see Figs. 3 and 4). IS

values are given in 10” N/m.

extremely weak lower crust we obtained allows for

total upper crust-mantle decoupling:

where n is the number and h the thickness of the

depth intervals where the strength exceeds 20 MPa

[231.

3.3. The input of the model

The scheme we adopt for kinematic modelling is

robustly constrained by the field-derived data on timing of fault activity and on the amount of dis-

placement accommodated by the single faults. We

have adopted an initial crustal thickness of 40 km

along the entire section resulting from the Variscan

orogeny. We set at 20 km depth the transition be- tween the upper layer, where deformation is fault-

controlled, and the lower one, where deformation is distributed. This is compatible with the field record,

shows localized shear down to paleo-depths of at

least 15 km [l 11 and is also similar to what is

observed in several deep seismic profiles across non-inverted passive continental margins (e.g., [311).

We adopt the time scale of Gradstein et al. [32].

In the model input, extension starts at 223 Ma.

From 223 Ma to 186 Ma it was accommodated by a set of four major normal faults defining the Lombar-

dian basin (Fig. 3). Specifics about the amount and timing of extension can be found in [9]. In accor-

dance with the pure shear geometry envisaged for this stage, the site of maximum thinning in the “distributed deformation” layer lies beneath that of

the upper layer. At 186 Ma the site of extension

shifted westward where crustal separation took place. Kinematic calculations are stopped before actual

break-up. The geological input for the thermal model in-

cludes an underplated magmatic body emplaced at 230 Ma which starts to cool at 223 Ma. The intrusion temperature is 1000°C. The dimensions of the intru- sive bodies are hypothetical but, as we will show later, bear little influence on the final result. The thermal evolution after the deactivation results from the interplay of the waning thermal anomaly with the

G. Beriotti et al. / Earth and Planetary Science Letters 146 (1997) 181-193 187

stretching-related mass movements and the blanket-

ing effects of sediments.

4. Thermal evolution, rheology and strength of

the South Alpine margin

4.1. Thermal euolution

The crustal and thermal configurations at the on- set of rifting and at break-up are shown in Fig. 4a,b.

Fig. 4c shows the thermal field 100 Myr later, shortly before the onset of convergence between the

African and European plates. The modelling results show that the thermal evolution of the South Alpine

lithosphere during the initial rifting stages is mainly controlled by the waning of the Middle Triassic

thermal anomaly. Extensional movements of fault

blocks do not substantially influence this pattern [20]. The model predicts that most of the thermal

perturbation had disappeared by the end of rifting: 100 Myr after break-up, the isotherms were practi- cally flat. Isotherms under the sedimentary basins are

slightly “pulled up” because of the blanketing effect of the basin-fill sediments (e.g., [20]).

The results of the model can be tested by compar-

ing the thermal distribution in depth with absolute

age data. The model predicts, for rocks lying at a paleo-depth of 14- 15 km along the Lake Como basement (star in Figs. 2 and 4), temperatures below

250°C at the end of rifting. This result is compatible with new Ar-Ar measurements on micas [ 121 and

preliminary zircon fission track data, which have produced Late Triassic to Middle Jurassic ages.

w

Canavese

Temperatures below 300°C have also been estab-

lished for the lower crustal rocks of the Ivrea zone which were part of the most distal section of the

preserved margin [33].

4.2. Strength euolution

To model the strength evolution of the South

Alpine rifted margin, we assume wet rheologies,

geologically reasonable thermal diffusivity values for the sediments of 0.75 . 10-6m2/s, and a strain rate of lo- ” s- ’ . Other parameters are given in Table 1.

A sensitivity analysis aimed at studying the influence of these parameters on the strength values will be

presented in a following section. Yield strength en-

velopes (YSE), integrated strength and Te values

have been obtained from rheological models for five representative localities along the passive margin for

the moment of break-up as well as 100 Myr later. Integrated strength (IS) curves at break-up (Fig.

5) show relatively constant values across the margin.

The IS values 100 Myr after break-up have increased while there is further thermal relaxation. However,

the cooling, and therefore the increase in strength, is slightly suppressed by the insulating effect of the sediments. Due to the combined effect of further

thermal relaxation leading to cooling in the lower

crust and mantle, and extra heating, due to heat production and sediment blanketing in the uppermost

crust, the lower crust and mantle gain strength in time after rifting, whereas the upper crust looses

strength.

Te values for the break-up stage are between 22 and 26 km, with the highest values in the stretched

E

Lombardian basin trento plateau

0 103Ull I

Fig. 6. Yield envelopes for five selected sites along the South Alpine margin 100 Myr after break-up.

188 G. Bertoffi et aL/Earrh and Planetary Science Letters 146 (1997) 181-193

Lombardian basin. Values in the most distal part of the margin are relatively low because of the strong thermal anomaly associated with break-up. Te values 100 Myr later have increased in the stretched seg- ments. The largest increase is produced for the distal part of the margin, which at this point is the strongest segment of the system. A clear correlation can be observed between the higher Te values and thinned parts of the margin.

Yield strength envelopes across the margin (Fig. 6) 100 Myr after break-up demonstrate that most of the predicted lithospheric strength is carried by the mantle. The upper crust provides a limited contribu- tion, whereas the lower crust is systematically very weak.

4.3. Strength evolution: sensitivity analysis

To investigate the importance of the choice of the assumed model parameters, we constructed inte- grated strength profiles along the South Alpine rifted margin with various values for the pre-rift Moho depth and the sediment conductivity, both for wet and (stronger) dry crustal materials, and for strain rates of lo-l7 set-I and lo-l5 set- ’ . The composi- tion of the mantle is assumed to be the same for all cases. The results are shown in Fig. 7. The models were constructed for the moment 100 My after break-up.

Two rift-related phenomena have counteracting effects: the replacement of crustal material by mantle

Moho = 31 km Moho = 41km

A) e =I 0-I 7secm1

Fig. 7. Sensitivity analysis for the integrated strength values across the South Alpine margin 100 Myr after break-up for two different initial crustal thicknesses. Dashed and solid lines are dry and wet rheologies respectively. In all cases, mantle lithology is olivine. Numbers near

lines are different sediment conductivity values. (a) Strain rate = IO-” set- ‘. (b) Strain rate = 10-l ’ set- ‘.

G. Bertotti et al. /Earth and Planetary Science Letters 146 (1997) 181-l 93 189

material causes lithosphere strengthening, whereas the replacement of the upper part of the crust by water or sediments causes weakening. This weaken-

ing might (temporarily) be increased by lithospheric heating at high stretching rates, or on a longer time-

scale by sediment blanketing. In Fig. 7a the follow- ing features can be observed:

M&o depth: Typically, an increase in Moho depth

is associated with a decrease in strength; this is simply related to the larger amount of crustal mate-

rial. Larger lateral variations along each profile are

observed in the deep Moho case than in the shallow

one. This is explained by the fact that, since the condition of area balancing is met, a deeper pre-rift

Moho results in a larger increase in mantle material during extension, and thus in an extra increase in

strength due to rifting.

Srrain rate: Higher strain rates are associated with higher strengths. The pattern of lateral variations

along each profile remains the same.

W

Sediment conductivity: A decrease in sediment

conductivity (K~,~) causes a general decrease in strength: due to the stronger insulating effect of the

sediments, the upper crust gets warmer and weaker. At very low diffusivity values, this weakening effect

can even overprint the strengthening caused by the

upwelling mantle material. The weakening is

strongest for sites 1 and 4, where the sediment pile is thickest. This can lead to an inversion of the general

pattern with sites representative of extended litho-

sphere (such as 4) being weaker than the surround-

ings.

Wet us. dry rheologies: Not surprisingly, dry rhe- ologies produce a higher lithospheric strength than

wet ones. The difference decreases with decreasing sediment conductivity and becomes very small for

extremely (unrealistically) low conductivity values

Of K,,d of 0.25 . 10m6 m/s2. The insulating effect of

the sediments is for these values so strong that the crust preserves only limited strength. Despite the

E O~m__~~~~~~_~~__~~~~~~~~_~__~____~__~~_~~~~_

~_~____---/-_~-_-----z ,-J--,__- 7-_1--____2*

20 - 2-_ _-G_L5d n z=.Yz_ --c-z--_~~--- zTz%-zd=--__= _ Z=e

L4____------___----__---------------___4

40-~---- Lo____--------------_----------------__666Qc

60-A 3

a) a the onset of extension

Okm

20

Canavese Lombardian basin Trento plateau

60

b) at break-up

Okm

60

c) IOOMyr after break-up IOOkm I

Fig. 8. Results of thermokinematic modelling of the South Alpine lithosphere without including the Middle Triassic thermal anomaly. (a)

Situation at 223 Ma, when normal faulting began and the thermal anomaly started to cool. (b) Situation at break-up (160 Ma). (c) Situation

before the onset of contraction (60 Ma). Dashed lines in (a) indicate the future normal faults, triangles in (c) give the locations where the

geothermal gradient was extracted to construct the rheological profiles and for which integrated strength and Te values were calculated.

190 G. Bertotti et al. /Earth and Planetary Science Letters 146 (1997) 181-193

different absolute values, wet and dry rheologies

produce the comparable pattern of lateral strength distribution. This, however, does not hold for very

low sediment conductivities.

Lateral effects of heating: Strength values at site 2 are typically lower than at site 5, despite the fact

that both sites are on non-extended lithosphere. This

is caused by the 2-D thermal calculations. The ef- fects of heating caused by sediment blanketing in the

neighbouring areas is noticed at site 2 but not at site

5.

4.4. The influence of the thermal anomaly on the strength distribution

Although we know that the thermal evolution of

the Mesozoic Southern Alps is controlled by the presence of a thermal anomaly in the initial rifting

stages, the dimensions and residence time of this

anomaly is largely speculative. To examine the influ-

ence of our assumption of a rather large and long- term intrusion, we also studied the other end-mem-

ber; that is, thermal evolution without any preceding magmatism. We thus run the model without consid-

ering the Middle Triassic thermal anomaly. The results of this thermokinematic modelling are

shown in Fig. 8 and demonstrate a much “quieter”

evolution than if an anomaly is included. Block

movements along the faults do not cause any notice- able disturbance of the isotherms. These are dis-

placed by lithospheric thinning but tend to regain their equilibrium situation relatively quickly: 100

Myr after break-up recovery is complete and the isotherms are practically flat.

Applying the same procedures described above,

we have constructed yield strength envelopes and

obtained integrated strength and Te values 100 Myr after break-up for the same localities as for the “intrusion model”. The numerical values are < 2%

off from the ones derived for the “intrusion model” and the yield strength profiles are practically identi-

cal.

5. Cross-check with other data sets

Our rheological modelling applied to the well constrained South Alpine case led to two major

conclusions:

(1) The South Alpine lithosphere preserved sub-

stantial strength even during the last stages of rifting immediately preceding break-up. This holds also for

the most distal segments of the margin, the areas closest to the site of ocean opening.

(2) 100 Myr after break-up the stronger domains of the margins are those which have undergone more

extension. This is a result of the nearly flat position

of the isotherms after break-up. Thinner areas have a larger proportion of mantle than less thinned ones

and are therefore stronger. In the following we check these predictions with

the data derived from other, mainly field-derived data.

5.1. Lithospheric strength at break-up

The prediction that the South Alpine lithosphere preserved substantial strength during rifting and at

break-up is compatible with the analysis of plate

movements presented in Bertotti et al. [9]. Based on the amount of extension accommodated by the ob- served normal faults they derived an estimate of few

millimetres/year for the relative movement of the Adriatic plate away from the European plate (Fig. 9).

This estimate was found to be compatible with the

one proposed by Savostin et al. [32], based on the analysis of magnetic anomalies in the Central At-

8 1 2 7 I

--____

200 175 150 time (Ma)

Fig. 9. Rate of displacement between the Adriatic and the Euro-

pean plates during rifting and after break-up. The solid line gives

rates of movement based on the extension accommodated by

South Alpine normal faults. The dashed line shows velocities

derived by Savostin et al. 1341 from the central Atlantic magnetic

anomalies. They are used under the assumption that Adria was

part of the African plate until the Middle Jurassic. From Bertotti et

al. [9].

G. Bertotti et ul. / Earth and Planetu~ Science Letters 146 (1997) 181-193 191

lantic. According to Savostin et al. [32], velocities increased by an order of magnitude to a few centime-

tres/year around 160 Ma, which corresponds to the Adria-Europe break-up (Fig. 9). We interpret this

increase in velocities to be the result of the cessation

of the resistance opposed by the lithosphere to ten- sional forces. This is compatible with our predictions

of the South Alpine lithosphere preserving part of its

strength during the final stages of rifting.

5.2. Lateral uariations in lithospheric strength dur- ing and qfter dr$ing

The prediction that lithospheric segments which have undergone thinning (such as the lithosphere

underlying the Lombardian basin) are stronger than those which suffered less deformation is tested by

analyzing the shape of the Tertiary foredeep basin

which developed on top of the South Alpine litho- sphere, in front of S-vergent Alpine thrusting (Fig.

1). The analysis of foredeep geometries is in general considered to be a powerful tool in constraining Te values of loaded plates. Bertotti et al. [35] have

studied the geometry of the foredeep basin along N-S profiles crossing different domains of the older passive continental margins. Profile A (Fig. 1) is

mainly located in a domain which underwent limited extension. Profile B, in contrast, runs through the Lombardian basin, the most extended structure of the

margin. The position of the base foredeep along the

two profiles is shown in Fig. 10 and demonstrates that it was steeper in less extended regions (profile

distance (km)

6L1, _I-1 - 0 100 200 300

distance (km)

Fig. 10. Sections across the Tertiary South Alpine foredeep [35].

The trace of the sections is given in Fig. I. Mountain ranges are

shown only for geographical reference. Dashed lines are paleo-

bathymetries before loading. Triangles are data points for the

depth of base foredeep derived from seismic sections. Vertical

scale in kilometres. Note how the base foredeep is steeper in

profile A than in profile B.

A) than in the more extended ones (profile B). This is consistent with the predictions made by our study.

6. Discussion and conclusion

The results of this study demonstrate that the

thermal structure of the lithosphere at break-up is mainly a function of the geometry and rate of exten-

sion. Block movements associated with normal fault-

ing do not cause major disturbances. Magmatic events, on the contrary, can have profound influence

but their effect is only felt for time intervals of a few

millions of years. This holds also for large and deep intrusions. Magmatic events occurring in the initial

stages of rifting, bear no major influence on the

thermal and mechanical structures of the lithosphere at break-up and during drifting.

The South Alpine lithosphere preserved substan-

tial strength throughout rifting and drifting. Even at break-up, Te values of the most distal parts of the

future margin were > 15 km. Effective elastic thick-

ness values increase thereafter. The preservation of strength at break-up can have important conse-

quences on plate movements. Even during the last stages of rifting. the thinner

parts of the margin tend to be stronger than the less

thinned ones [4.5]. After a time span long enough to allow for thermal relaxation the lithospheric strength is controlled by the lithological composition of the

lithospheric column [6]. Thinned parts of the margin

have a higher proportion of strong mantle than crustal material compared with non-thinned segments and

are, therefore, stronger. Our numerical model shows

that thermal perturbations associated with rifting are mostly recovered within a time span of few tens of

million years. In the case of slow rifts, which most

passive margins are, this recovery can even happen

during or shortly after rifting, which appears to be the case for the Southern Alps.

Since the lithospheric strength drifting is mainly a

function of the composition of the lithologic column, the often made assumption of the effective elastic

thickness being controlled by an isotherm is not correct. In fact, our Te values at the end of rifting correspond to temperatures of 340”, 400”, 370”, 340” and 300°C from the most proximal (eastern) to the more distal (western) profile. These values remain very similar 100 Myr later, except for the most distal

192 G. Bertotti et al./Earth and Planetary Science Letters 146 (1997) IN-193

part of the margin where the Te corresponds to the and the evolution of foreland basins, Basin Res. 4, 169-178,

400°C isotherm. 1992.

The inferred Te values of the South Alpine litho- sphere during rifting were between 25 and 35 km and are therefore higher than those obtained from fitting deflection and gravity data on elastic wedges at passive margins (e.g., [1,36-381). Our Te values, in contrast, correlate well with those obtained with geophysical methods by Holt and Stem [3] on the western New Zealand passive continental margin. The New Zealand margin, similarly to the Southern Alps, remained starved of sediments for most of its drifting stage and only some 70 Myr later, with the onset of convergence, became loaded by a thick sedimentary wedge.

[3] W.E. Holt and T.A. Stem, Sediment loading on the Western

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[4] P. England, Constraints on extension of continental litho-

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[5] GE. Vink, W.J. Morgan and W-L. Zhao, Preferential rifting

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[6] R.E. Oxburgh, Heterogeneous lithospheric stretching in the

early history of erogenic belts, in: Mountain Building Pro-

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1986.

[7] G. Bassi, Relative importance of strain rate and rheology for

the mode of continental extension, Geophys. J. Int. 122,

195-210, 1995.

The rheological profiles across the South Alpine margin support the notion that most of the litho- spheric strength both during the last stages of rifting and during drifting is carried by the mantle. The crust is, on the whole, much weaker. Particularly weak appears to be the lower crust, which even during drifting has very low strength. This can strongly affect subsequent tectonic events, such as renewed rifting or tectonic loading, when the af- fected lithosphere would behave in a decoupled rather than coupled mode [23].

[8] H.P. Laubscher and D. Bernoulli, Mediterrenean and Tethys,

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Plenum, New York, NY, 1977.

[9] G. Bertotti, V. Picotti, D. Bernoulli and A. Castellarin, From

rifting to drifting: tectonic evolution of the South-Alpine

upper crust from the Triassic to the Early Cretaceous, Sedi-

ment. Geol. 86, 53-76, 1993.

Acknowledgements

G.B. acknowledges financial support by AWON (project 75 1.356.026A). V.P. benefitted from MURST for his stay in Amsterdam. M.t.V. was supported by the IBS (Integrated Basin Studies) pro- ject, part of the Joule II research program funded by the Commission of European Communities (contract No. JOUZCT 91-010). J.-P. Burg and F. Beekman are thanked for valuable suggestions. M. Diament and F. Neubauer are thanked for their reviews. Publi- cation Nr. 961102 of the Netherlands Research School of Sedimentary Geology. [UC1

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