The Rhinegraben: Extension, subsidence and shoulder uplift

Tectonophyslcs.

128 (1986) 47-59

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

THE RHINEGRABEN: SHOULDER UPLIFT

EXTENSION,

SUBSIDENCE

AND

THIERRY VILLEMIN r, FRANCIS ALVAREZ a and JACQUES ANGELIER ’

’ Tectonipe

Quantitative

and ’ Gbdynamique,

D~partement

de Gkotectanique,

Universitt! Pierre et Marie

Curie, 752.70Paris Cedex 05 (France)

(Received April 26,1985; revised version accepted January 29, 1986)

ABSTRACT Villemin, T., Alvarez, F. and Angelier, Y., 1986. The Rhinegraben: extension, subsidence and shoulder uplift. Teetonophysics, 128: 41-59. We apply a numerical model of the thermal evolution of a continental lithosphere undergoing extensional deformation to the Rhinegraben. Using reasonable parameters, the results of computations are in agreement with the present configuration of the rift and of its shoulders, as well as with their evolution since the Oligocene. Contrasting evolutions are reconstructed in the northern and southern Rhinegraben: whereas the Northern history is accounted for by a simple model of homogeneous stretching of the lithosphere, the history of the southern Rhinegraben cannot be explained without introducing two main periods of heterogeneous stretching. The results are local-ty supported by structural analyses of extensional patterns. Factors of extension average I.2 in the axial portion of the Rhinegraben and 1.15 for the whole rift section, suggesting that the total amount of E-W extension reaches 6 km.

Numerous geological and geophysical data in the Rhine rift vahey (Fig. 1) are now available, including accurate information on its structure and evolution (Rothe and Sauer, 1967; Illies and Killer, 19701 Illies and Fuchs, 1974). We propose a quantitative therm~yn~c model of evolving graben subsidence and shoulder uphft, based on analyses of the thermaI evolukon of continentaf lithosphere during stretching, and accounting for the present structure and the previous evolution of this area. THE METHOD

We adopt the b~-~rnensi~~a~ numerical solution proposed by Alvarez et af. (1984) in order to reconstruct the thermal evolution of a lithosphere with extension rates that may vary in time and space, Q I986 Elsevier Science Publishers B.V.

N*

(A)

(B) .

Fig.

1. Structure

geological

map

Buntsandstein; Cenozoic location along present.

and (D):

data

I = Cenozoic

p = Permian:

(in metres);

movements sediments;

7 = pre-Permian

9 = diapirism

of type cross-sections the rift, after

on vertical

(B) and (C): average

uplift through

3 = Keuper:

Rhinegraben;

(A) average

the Rhinegraben.

8 = iso-depth

to in text. Evolution

Roll (1979). modified:

around

2 = Jurassic;

basement;

of the southern

referred

in and

contour

of the

4 = Muschelkalk;

5=

lines of the base of the

IO = Kaiserstuhl;

of sediment

thicknesses

Legend

thicknesses

of sediment

time of the western (B) and eastern

volcanics.

I and II

and vertical

motion

infill from 40 Ma to

(C) shoulders

of the rift.

.

49

Km 2.5’

2-

1.5 -

l-

0.5 -

0*

r

,2

.&.

/

,

I

-10

M.a.

0

*__~ov~H--*__-_-___

-Pm-.

1.1 -

I

-20

-30

-40

,’

/

/

/

/

@p?+.

I

, 4f -40

I

-30

OLIGOCENE

Fig. 2. Average

sediment

sediment

thickness

extension

factor

,

I

M.a.

0

-10

-20 MIOCENE

thickness

reconstructed

and maximum using

extension

Roll’s geological

(axial zone of the rift), computed

factor through data

(1979).

with the model.

time. A. Evolution B. Evolution

of average

of the maximum

50

TABLE

1

List of main parameters

used in the model

Physical parameters Thermal

conductivity

Coefficient

3.1395 W mm’ K-’

of thermal

expansion

3.28~10-5”C~’

Thermal

capacity

1.172,103

J kg-

Thermal

diffusivity

8.04.10-’

mm’

Density

of the crust at 0°C

2780kgm-’

Density

of the mantle at 0°C

3350 kg mm3

Density

of seawater

1030 kg mm ’

at 0°C

’ K-’

GrrJ sue in finite difference method

31 nodes on the X-axis (pre-rift

width:

19 nodes on the Z-axis (pre-rift

thickness:

Iteration Laws

interval

150 km) 90 km)

IO6 years

for sedimenraty layer

Density

2.62 - 0.74 exp( - Z/0.8)

Thermal

conductivity

This vertical

solution thermal

3. -l.exp(-2/0.8)Wm-’

kg m- 3 Km’

takes into account the effect of lateral cooling, in addition to exchanges already considered in various models (e.g., Jarvis and

McKenzie, 1980). Such lateral effects play a significant role in the thermal of narrow elongated rifts, (Alvarez, 1X4), such as the Rhinegraben.

evolution

In this model, the horizontal component of displacement velocity of particles remains constant with time for each vertical column. Consequently, fixing the final amounts of extension enables one to compute the different strain rates and to reconstruct

the position

of particles

at any

assumptions lead us to adopt the variation in Fig. 2B for a given final value. The boundary

conditions

time

(Alvarez

of stretching

of the model are determined

et al., 1984). These

factor with time illustrated by the temperatures

at the

top and the base of the lithosphere (1O’C and 1333°C respectively). Any initial distribution of temperatures within the lithosphere may be imposed in the model and this enables us to add particular constraints in the case of the southern ~inegraben (discussed in the following sections). The thermal effect of the presence of sedimentary layers (the graben infill) has been quantitatively taken into account in the model. The distribution with depth of densities and thermal conductivities within the sediment infill have been determined based on borehole data mentioned by Doebl et al. (1974) (see Table 1). The volumetric and thermal effects of progressive stretching of the sediments successively deposited are taken into account in this model. The thermal parameters of underlying lithosphere are those adopted by Le Pichon and Sibuet (1981); they are summarized in Table 1. The other parameters,

51

such as crustal

and lithospheric

thicknesses,

have been chosen based on the analysis

of the present configuration and will be discussed in the next section. We thus compute the values of graben subsidence (Airy’s model) and of shoulder uplift, as well as the heat flow at the surface. The subsidence and its evolution time have been computed through a simple model of local isostatic equilibrium. computed cooling

shoulder beneath

uplift

is simply

the graben.

made. The heating

of radiogenic

due to the heating

Corrections

related

related

to erosional

origin is taken into account

to lateral

with The

thermal

processes

have been

A, exp( - Z/D)

type, with

A, = 3. 1O-6 W/m3 and D = lo4 m, each lithospheric column being considered as a closed system for its radioactive sources. Radioactivity in the mantle has been neglected. We have adjusted the computed model values which are not firmly constrained, especially the rate of extension, to the present morphology and the shape of the sediment infill. This adjustment was made by fitting the computed and observed present profiles across the graben in successive attempts. These computations have been done with an accuracy of +lO m, on the complete type sections I and II located on the map in Fig. 1. GEODYNAMIC

CONSTRAINTS

In our computations, the variables are the rate of stretching and the amount of sedimentation. These variables were modified progressively, until a good fit with the present

structure

was obtained.

In practice,

choosing

a total amount

of stretching

determines in our model the variation of the stretching rate with time through simple laws (see previous section). Similarly, choosing a total amount of sedimentation

based

on the present

geological

section

determines

the sedimentation

rates

through time. This simplification introduces some discrepancies between the computed evolutions of these rates and the actual ones (where and when they are constrained by geological data), but estimating the effects of these discrepancies has shown that they do not significantly

vitiate

the Rhinegraben

model.

The other parameters have been adopted by considering the present situation to be as follows: (1) The initial crustal thickness of 29 km (before stretching) has been chosen on the basis of comparisons with weakly deformed adjacent regions, and determinations by Edel et al. (1975), who published ben region.

the Moho contour-map

of the Rhinegra-

(2) Determining the thickness of the lithosphere in the same regions is more difficult. Different values have been published (Chapman et al., 1979; Panza et al., 1980; Souriau, 1981). We have adopted a pre-stretching thickness of 90 km, which corresponds to a surface heat flow of 70 mW/m2 using a constant thermal conductivity across the lithosphere (see Table 1). Effectively, such average heat flow

52

has been measured out of the Rhinegraben (Cermak, 1979; Haenel, 1983). Note that, by definition in our model, the lower surface of the lithosphere is the 1333°C isotherm. (3) Sea-level

variations

have been

(1979), Watts and Streckler determine

in the model

incorporated,

(1979) and Kominz

whether

the upper

based

on studies

(1984). This analysis

sediment

surface

in

by Pitman

enables

one to

the rift is under

sea-level at a certain time or not, and to introduce the isostatic effect of submergence if necessary. The study of sediment facies (marine or not) thus provides a way to check these results. In the case of our study, the distributions of upper sediment surface elevations have always been found compatible with paleogeographic data within the range of allowed uncertainties. (4) After other authors (Illies, 1972: Roll, 1979) we assume that just before rifting the area was a peneplain with an average elevation of 350 m (relative to present sea-level). This is justified by the present average elevation of 350 + 100 m in areas that were not affected by significant vertical tectonic movements during the Cenozoic, such as the eastern Paris basin or the plateaus of southwestern Germany. (5) Sedimentary paleothicknesses have already been established by Roll (1979) using his decompaction method (Roll, 1974). The existence of uplift periods in the basin as well as the evolution of rift shoulder uplift have been reconstructed (Roll, 1979). These data are summarized by the curves in Fig. 1A (evolution of sediment thickness) and in Fig. 1B. C (shoulder uplift). (6) To establish the present structure, we adopt

the lower surface

of Cenozoic

sediments mapped by Doebl and Olbrecht (1974) combined with the usual topographic maps. The curves and the map contour lines of Fig. 1 summarize the main data that we have used.

THE NORTHERN

RHINEGRABEN

The Rhine valley, approximately 40 f 5 km wide, is bounded by two master normal fault zones (Fig. 1D). The structure and evolution of the Rhinegraben differ strikingly

from north

terms of the evolution

to south.

Figure

of sediment

2A illustrates

this north-south

contrast

in

thicknesses.

Fig. 3. Main results of the stretching model applied to sections I (north) and II (south), located in Fig. 1. Computed evolution between 40 Ma ago and Present (shown using curves with different patterns). Dots refer to computed values. A. Elevation of earth surface in the absence of erosion, (curves with dots 1). B. Base of graben infill (curves with dots 2). C. Adopted sea-level reference. D. Moho (curves with dots 3). E. Amount of extension (curves with dots 4). F. Surface heat flow (curves with dots 5). compared with observed one (continuous

line).

53 SECTION

II

6) E

W 40hn

In the north, the sedimentary infill process during most of the Cenozoic

is related to rather continuous subsidence (Fig. 2A). In the south, the subsidence

reconstructed through sediment thickness curves displays a similar evolution during the period between - 40 and - 30 Ma, then a much slower movement during the Late Cenozoic, the graben),

and finally which

a strong

is 10 i 2 Ma

uplift

histories have also been reconstructed the subsidence curves of Fig. 1A. Similar contrasting evolutions Strasbourg-Karlsruhe

event (creating

old (Fig.

erosional

2A). These

features

contrasting

within

subsidence

by Roll (1979) and they can be detected of

are reconstructed

vertical motion in the Rhinegraben

using

north and south of shoulders (Fig. 1B and

C): uplift and related erosion processes are limited or absent to the north, whereas strong uplift has affected the edges of the graben to the south, resulting in deep erosion that has revealed the pre-Mesozoic basement of the Vosges and Black Forest mountains mapped in Fig. 1D. The

model

of lithospheric

stretching

briefly

defined

in the previous

section

explains the present structure as well as the evolution of the northern Rhinegraben, assuming that the extensional processes have occurred regularly during the last 40 Ma (see methodological section and Fig. 2B). This hypothesis of continuous extension in the northern Rhinegraben is supported by the existence of normal fault and tilted block patterns in the Miocene as well as in the Oligocene (Schad, 1962), and for the Present by the extensional nature of most focal mechanisms of earthquakes (Ahorner, 1983). The history of subsidence since 40 Ma ago (Roll, 1979) also reveals a striking continuity north of Mannheim (Fig. 1A). The main results of the numerical model are summarized in Fig. 3. These results indicate that in first approximation the subsidence of the northern Rhinegraben is compatible with the homogeneous tectonic stretching model of the lithosphere first defined by McKenzie (1978). The fit between the model and the present structure of the northern rift leads us to determine an amount of extension /? that reaches 1.23 at the graben rift limit

axis and decreases

as shown

asymmetrical, constrained through

toward the graben

in Fig. 3E).). The distribution

like that of sediment by that of the observed

time

because

at different

thicknesses present points

edges (e.g., 1.02 near the western of extension

across

the rift is

(Fig. 3A, B). These asymmetries structure

are

of the rift and maintained

we use similar

laws

to describe

the

evolution of stretching and sedimentation. The computed amplitudes of shoulder uplift remain very small (Fig. 3A), which is in agreement with the reality (no more than 100 m of vertical uplift, the isostatic response to erosion being removed). Due to the asymmetry of the distribution of extension across the graben, the uplift of the eastern shoulder is larger (eastward increase in thermal exchanges). Finally, our quantitative model, although it was prepared as a function of graben structure (and not of the structure of adjacent areas), satisfactorily accounts for the amplitude and for the distribution of shoulder uplifts.

53

THE SOUTHERN

RHINEGRABEN

The amplitude of subsidence in the south of the Rhinegraben is smaller than in the north (Fig. 1A). This should imply smaller amounts of stretching. Accordingly, the related shoulder uplift should have been smaller. On the contrary, rough estimates of stretching based on simple subsidence models as well as on comparisons between crustal thicknesses within and around the graben, suggest that the crustal amount of extension is similar to that in the north (Villemin et al., 1984). In addition, the observed shoulder uplift is much larger in the South (Fig. 1B and C). Thus, the reconstruction of the previous section cannot explain the structure and evolution of the southern Rhinegraben. We have consequently modified the model by assuming that the lithospheric stretching has been heterogeneous and discontinuous in the southern Rhinegraben (Werner and Kahle, 1980; Villemin et al., 1984). In practice, we have been led to consider that a rectangular section of the lowest lithosphere has been replaced by the hotter asthenosphere (Fig. 4). Thus, everything happens as if a portion of lower lithosphere wider than the Rhinegraben itself had fallen in the underlying asthenosphere. This contrast between the simple evolution of the northern Rhinegraben and the more complicated one of the southern Rhinegraben is illustrated in a simple way by the diagrams in Fig. 4. Whereas in the north stretching and subsidence have been occurring in a regular way since 40 Ma in our model, a major event of removal of lower lithosphere occurs twice in the south and complicates the model. The amplitude and timing of these events have been computed in order to explain the present shoulder structure as well as the evolution of uplift reconstructed by Roll (1979) and summarized in Fig. 2A. We chose to introduce two events of this type in the model: the first is 40 Ma old (beginning of stretching), the second is 10 Ma old (Fig. 4). The rate of subsidence suddenly decreased 30 Ma ago in the southern Rhinegraben (Roll, 1979; Fig. 2A). We relate this change to the end of significant extensional processes, so that the later subsidence is mainly explained by cooling. We have consequently adopted a period of extension which is 10 Ma long (from 40 to 30 Ma ago: Fig. 2B), in agreement with geological information on rift subsidence (Fig. 2Af. The fit of this model of heterogeneous-discontinuous stretching with actual data led us to determine an average amount of extension of 1.15 in the crust (Fig. 3) as it has been done independently in the North. Although the stretching histories are quite different, the amounts of extension finally obtained are similar. In the south, the removal of lowest lithosphere over a width of 115 km and a thickness of 50 km corresponds to an average stretching factor of 6 in the upper mantle. Note that although our model cannot account for the detailed distribution of heat flow (partly controlled by shallow anomalies within the graben), the results are in rough agreement with average heat flow values measured on graben edges by Cermak (1979) and Haenel (1983), as Fig. 3F suggests.

(‘rust ,a cross pattern.

Sediments

Fig. 4. Sketch of the northern

z

0

a

I-

I

-40 M.a.

models.

::.

.,.,.

of rift shoulder\

Rhinegraben

m black. Eroawn

and southern

-I-

:::.

‘.~.‘.‘.‘.‘.‘.~.‘.~.‘.~.~. ‘.~.‘.~.~.~.~.‘.~.‘.‘.~.~.~.~.‘.’.

-30 M. a.

added.

shown Approximately

Asthenosphere

-20 M.a.

pattern. t<, scale. except fvr sediment

a$ dark hatchured

portion

uplift. rxaggerared.

dotted.

PRESENT

of lithosphere mfill and shoulder

Mantle

-10 M.a.

5-l

CONCLUSIONS

Using

a numerical

effects into account, shoulders (Figs.

Rhinegraben, proposed

of stretching

(Fig. 3) in close agreement

1 and

1.2-1.25

model

we have reconstructed

2). The

computed

that

takes vertical

the evolution

with available

amount

geological

of extension

which is now 40 km wide. The computed

in the deepest axial portion independently,

and lateral

thermaf

of the Rhinegraben

and its

and geophysical

averages amounts

data

1.15-1.2

in the

of extension

reach

of the graben (Fig. 3). Similar values have been

based on structural

reasoning

(Illies,

1972; Villemin

et al.,

1984). Although the computed amounts differences in evolution exist between

of crustal extension are very similar, the northern and southern Rhinegraben.

major First,

the extensional processes have been active during the last 40 Ma in the north, whereas in the south active extension during about 10 Ma has been followed by 30 Ma of dominating thermal subsidence. Second, we explain the north-south contrast in shoulder uplift by variations in the mantle in the south; in contrast, the evolution of the northern Rhinegraben is compatible with homogeneous stretching of the lithosphere (Fig. 4). The model used for the southern region involves much larger effects of lateral

cooling,

which explain

the intense

shoulder

uplift (Steckler,

1985).

Specifically, in the southern Rhinegraben, it is necessary to adopt a heterogeneousdiscontinuous stretching model with two events of removal of the lowest lithosphere (40 and 10 Ma ago). In turn, this model satisfactorily explains the evolution of vertical motion reconstructed using geological data. The more complicated uplift and subsidence curves of the Str~bourg-Heidelberg rift segment confirm the transitional character of this region that separates the northern and southern typical segments (Fig. 1). Finally, the more complex evolution of extensional processes and the more intense stretching of the upper mantle in the southern Rhinegraben seem compatible with the existence of Late Cenozoic volcanism (Kaiserstuhl 218 Ma), which complicates the local heat flow distribution. Our results indicate that the factor of extension averages 1.15 to 1.2 for the Rhinegraben itself, approximately 40 km wide, over a N-S distance of 250 km between E-W ago.

Mulhouse

extension

and Mainz. These computed

of about 6 km between

values correspond

to an amount

of

the master fault lines of the rift, since 40 Ma

ACKNOWLEDGMENTS

This work was supported by the French C.N.R.S. “SeismogCnQe” (11-47) and “Transferts” (15-30). Comments us to significantly improve the manuscript.

through the A.T.P. by S. Uyeda enabled

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