Eocene-Oligocene tectonics and kinematics of the
Rhine-Saone Continental Transform Zone (eastern
France)
Olivier Lacombe, J. Angelier, D. Byrne, J. M. Dupin
To cite this version:
Olivier Lacombe, J. Angelier, D. Byrne, J. M. Dupin. Eocene-Oligocene tectonics and kinematics of the Rhine-Saone Continental Transform Zone (eastern France). Tectonics, American
Geophysical Union (AGU), 1993, 12 (4), pp.874-888. .
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TECTONICS,YOL. 12, NO. 4, PAGES874-888,AUGUST 1993
EOCENE-OLIGOCENETECTONICSAND
KINEMATICS OF THE RHINE.SAONE
CONTINENTALTRANSFORMZONE
(EASTERNFRANCE)
O. Lacombe,J. Angelier,D. Byrne,andJ. M. Dupin
Tectonique
UniversitéPierreet MarieCurie,
Quantitative,
Paris.France
Abstract. A tectonic analysis of brittle deformaton was
performed in the Rhine-Saône transform in order to decipher
and understand the deformation processes in a typical
continental hansfer zone. Cenozoic major and minor fault
patterns are described, and brittle structures are interpreted in
terms of paleostress orientations. The Eocene N-S
compressionrelated to Africa-Eurasiaconvergenceis marked
mainly by strike-slip faults. It was followed by a major phase
of crustal extension, mainly Oligocene in age, related to the
west Europeanrifting. This extensionalphaseis expressedby
normal faults, tensiongashes,and extensionalstrike-slip faults
which are especially abundant in the transform zone. These
two events are found to be quite different in terms of
distribution of horizontal shess trajectories. The trend of ol
axes is very homogeneousfor the N-S compression,whereas
o3 orientationsassociatedwith the major extensionvary from
E-W in most of the Rift system to I.&V-SE within the
transform zone. Finally, the Mio-Pliocene tectonic
emplacementof the Jura fold-and-thrustbelt is marked in the
field by strike-slip faults associatedwith a well-defined trend
of ol oriented W-IIW-E-SE to NW-SE. The preexisting
basement fracture pattern between the grabens was also
consideredin order to estimate which faults were likely to
have been reactivated during the Oligocene transform
kinematics, and the theoretical shear motion on these
inherited fault surfaces,when submitted to E-W extension,
was calculated. Left-lateraUextensionalreactivation of a
preexisting 60"N trending fault system in the brittle upper
crust is likely to have accommodatedthe shearstrain arising
between the rift segmentsin responseto crustal stretching.
Two independent numerical modelings, based on distinct
element and finite element analyses,respectively,enabledus
to verify and to refine the hypothesisof transform kinematics
related to E-W extensionand inducing regional perturbations
of extensionalstresstrajectories.The overall mechanismand
the transtensionalkinematics of the Rhine-Saônetransform
zone are thus tightly conshained. The inherited crustal
anisotropy, the large-scaleperturbationsof extensionalstress
trajectories, and the development of distributed brittle
deformation within the sedimentary cover are found to play a
major role in the tectonic evolution of continental rifttransform syst€ms.
INTRODUCTON
The development of continental rifts illushates the early
stages of continental extension and breakup. From a
Copyright1993by lhe AmericmGeophysical
Union.
Papernumber93TC00233.
o278-7407t93t93^tc-00233$I 0.00
mechanicalpoint of view, rifting is usually associatedwith
discrete transfer or transform zones which accommodate
deformation and release the shear strain arising from the
offset of the rift segmentsdue to crustal stretching.Examples
of such transform zones were identified and documented in
previous works [e.9., Illies, 1972, 1974; Tapponnier and
Yuet, 1974; Freund, 1974; Bergerat, 1977; Kazmin, 1980;
Garfunkel, l98l; Ron and Eyal, 1985; Ebinger, 1989;
Chorowicz, 19891.
Tectonic and paleostress analyses on exposed fossil
transform zones provide a good opportunity to reliably define
and interpret the stress field around continental transform
faults. Stress distribution may be further used to constrain
numerical models of the mechanicsof continental transform
zones,as is pointed out in other papers[Dupin, 1990;Byrne et
a1,,19921.In spite of somemajor differencesbetweenoceanic
and continental transform faulting, such as different
mechanicalbehavior of continental and oceanic lithospheres
and greater importance of crustal anisoûopy inherited from
earlier orogenic history within continental basement, this
approachalso may be useful in understanding stress and
deformation patterns at ridge-ridgetransforms.
During Cenozoictimes, the west Europeanplatform was cut
by a systemofapproximately N-S trending grabensextending
from the westernMeditenaneanto the North Sea.This system
is called the west European Rift. It is divided into several
segments linked by intracontinental transform zones
fLaubscher, 1970; Illies, 1972, l98l; Rat, 1974, 1918;
Bergerat, 1977: Bergerat and Chorowicz, 19811. The
"Burgundy gate" which connectsthe Saôneand Rhine grabens
is representativefor such a rift-rift transform (Figure l).
Although the natureof transformin this area is beyond doubt,
the actual kinematics of this continental transform zone and
the associatedpaleostresspattern, as well as its relation to
plate kinematics models proposed for the Africa-Eurasia
convergence[Sengôr, 1976; Tapponnier,1977; Le Pichon et
al., 19881, are still subject to contrasting interpretations
[Bergerat, 1977:Lacombe et al., 1990a;Lacombe and Dupin,
l99rl.
In the Rhinegraben,tilted block geometry, subsidencedata
as well as numerical modelings[Villemin et al., 1986] suggest
that the amount of extensiondid not exceed5 t 2 km; in the
Bresse graben, seismic reflection data collected along the
Jura-BresseECORS traverselead to a first estimateof 2 km of
crustal extension [Bergerat et al., 1990]. This means that
comparedwith 50-70 km wide zonesof extensionas seenon
the seafloorand from which the term "transform" was derived,
the Rhine and Saônerift-transform systemonly appearsas a 2
to S-km-wide zone of extension without significant crustal
creation. As a consequence,the terms "transfer zone" or
"accommodationzone" may be therefore more appropriate
than "transform zone". We decided, however, to keep the
name "Rhine-Saônetransform zone" which has been already
proposed and widely used by previous authors [e.g., Illies,
1972;Contini and Théobald, t974;Bergerat, 19771.
The aim of this paper is to present new structural data
combined with a reanalysis of previous results, in order to
provide new constraintson the Eocene-Oligocenekinematics
of the continental Rhine-Saône transform zone. We also
incorporate numerical modeling in an attempt to check our
earlier hypotheses regarding regional-scale paleoshess
perturbationsrelated to transformkinematics pacombe et al.,
l990al. First, we presenttectonic data related to the Cenozoic
Lacombe et al.: Tectonics of the Rhine-SaôneTransform Zone
geologic evolution of the transform zone. These data are
quantitatively interpreted in terms of paleostressorientations
(Figures 2, 3, and 4). Second, on the basis of the regional
paleosress field and regional structures we consûain the
875
Eocene-Oligocenetectonics and kinematics of the RhineSaône transform zone. A dynamic and kinematic model is
proposedfor tlris time interval and comparedwith numerical
and experimental models. It is important to note that as a
consequenceof the scope of the paper, tectonic features
related to post-Oligoceneevents(especiallythe Mio-Pliocene
Alpine compression associated with building of the Jura
Mountains:Figure 4) will be briefly describedbut will not be
discussed in detail further because transform kinematics
presumablyhad ceasedby then.
Fig. l. Schematic map of the westem European platform
showing the location of the area investigated(frame). Key: I,
.FoldHercynian basement; 2, Sedimentary formations; 3,
and-thrust" system of Jura Mountains (fold axes as dashed
lines); 4, Cenozoic rifts; 5, Alpine foreland basin; 6,
Stratigraphiccontacts;7, Main faults and 8, Thrusts Oarbs on
upthrustside).RSTZ: Rhine-SaôneTransformZone.
Fig. 2. Map showing orientationsof ol axesrelated to the strike-slip fault regime marking the Eocene
\renean-Alpine compression.Unpatternedareas: Mesozoic sedimentaryformations; Dotted pattem:
Cenozoic rifts. Crossedpattern: Paleozoic-Triassicformations.Nb: Noidansbasin. Key : l, Paleostess
orientationsdeterminedin this study and 2, Paleostressorientationscompiled from literature [Bergerat,
1987; Villemin and Bergerat, 1987; Larroque and Laurent, 19881.Fault slip data: diagrams (lower
hemisphere,equal area projection) with faults as thin curves and slickensidelineations as dots with
PaleosFess
double arrows (left- or right laæral) or simple ones(centrifugal-normal;centripetal-reverse).
directions as empty stars with five points (maximal compressiveshessol), four points (middle stress
o2), or three points (minimal stresso3); Direction of extensionor compressionas large black arrows.
For characteristicsof stresstensors,refer to Table l. The rose diagram indicates the frequency of o1
orientations.
876
Lacombe et al.: Tectonics of the Rhine-SaôneTransform ZonE
Fig. 3. Map showing orientâtions of 03 axes related to normal fault regime accompanying crustal
extensionwithin the Saôneand Rhine grabens.Samekey as in Figure 2. Polesto tension gashesshown
as empty squares.The rose diagram indicates the frequency ofo3 orientations.
TECTONiC ANALYSES AND PALEOSTRESS
RECONSTRUCTIONS
Fieldwork was concentratedon a detailed analysisof brittle
deformation structures around the Rhine-Saônerift system,
particularly, on tectonic features which may be used as
paleostressindicators, such as striated minor faults, stylolites
and tensiongashesat the macroscopicscale,and calcite twins
at the microscopic scale. In this section, minor fault pattems
are analyzed in order to identify and reconstruct the
successiveCenozoicpaleosEessfields (Figures2 to 5).
M ethodsfor Paleo stress Reconstr uctions
Regional paleostressorientations were determined on the
basis of fault slip data using Angelier's U984, 1989, 1990j
inverse methods. The principle of the method is to find the
best fit between observed directions and sensesof slip on
faults and theoreticalshearstressinducedon theseplanesby a
common stress tensor (the solution of the inverse problem).
For calcite twins, the inversion of crystallographic data in
order to obtain the regional stress was carried out from
oriented samples collected in Mesozoic and Oligocene
limestonesby using the computer-basedmethoddevelopedby
Etchecopar[984]. Both analysesyield the orientationsof the
three principal stressesol, o2, and o3, with o1 > o2 > o3
(compressionalstressconsideredas positive) and the .D ratio
betweendifferential stressmagnitudes[ô = (o2-o3)/(o1-o3),
with 0 S ô < ll. For a further discussionon the methodsof
determining paleostressorientationson the basis of fault slip
data and calcite twin data, the readeris referred to Carey and
Brunier [974], Etchecoparet al. [981], Angelier [1984,
19901, Etchecopar and Mattauer [988], and Etchecopar
[1984], Tourneretand Laurent [1990], Lacombe et al. [1990b,
19921,respectively. The combined use of such independent
paleostessindicatorsprovidesa densernetwork of paleostress
hajectories.
Results
For the sake of simplicity (the whole massof tectonic data
being too large to allow complete description in this paper),
herein we show only some examples of characteristic
diagramsillushating fault slip data and the method of tectonic
analysis that we have employed. These diagrams are
presentedin Figures 2,3, and 4, and the correspondingstress
tensorsare listed in Table 1. On these figures are mappedall
the paleoshess orientations determined at each site from
striated faults, tension gashesand calcite twins, or compiled
from the literature for deformation in Cenozoic times
[Villemin, 1986; Bergerat, 1987: Villemin and Bergerat,
1987;Lanoque and Laurent, 19881.The paleostressfields that
we reconstructedfrom Cenozoic fault patterns are described
below.
Cenozoic PaleostressFields in the Rhine-SaôneTransfurm
N-S "Eocene" compression.A regional paleostresssyst€m
characterizedby N-S horizontal compressionassociatedwith
Lacombe et al,: Tectonics of the Rhine-SaôneTransform Zone
877
Fig. 4. Map showingorientations
of 01 axesrelatedto the strike-slipfault regimemarkingthe MioPlioceneAlpinecompression.
Samekeyasin Figure2. Polesto tensiongashes
shownasemptysquares.
Therosediagramindicatesthefrequencyof ol orientâtions.
E-W extensionwasdeterminedin about40 sites.As shownin
Figure 2, related structuresmainly consistof left-lateral
strike-slipfaultswhosetrendsrangefrom lOoNto 60oNand
right-lateralstrike-slipfaultswhosebendsrangefrom ll0oN
to l60oN.The shearmotionon thesefaultsis purelystrikeslip or displaysa reversecomponent,The trendsof the
computedol axesare tightly clusteredaroundN-S direction
(Figure 2). This well-definedtrend is consistentwith ol
orientationsderived from stylolitic peaks.The age of this
eventis difficult to establishin the absenceof convenient
Cenozoicoutcrops,It could correspondeither to the early
Paleocenecompressionalphase which gave rise to basin
inversionin NW Europe,aswell asto breakin sedimentation
and erosionalhiatusat the boundarybetweenMesozoicand
in the areaunderinvestigation
Cenozoicsediments
[Ziegler,
1988,19901,or to the lateEoceneparoxysmof thePyrenean
compressionwhich prevailedsince the late Cretaceousuntil
the late Eocenein the Pyrenees-Provence
foreland[Mattauer
and Mercier, 1980; Letouzeyand Tremolieres,1980; De
Charpalet al., l98l; Arthaudand Seguret,l98l; Letouzey,
1986;Bergerat,1987;Lacomb€et al., 1992).ln the absence
of more precisestratigraphicdating, we proposethat the
observedcompressional
featuresassociated
with submeridian
ol orienûations
andwhichclearlypredated
development
of the
SaôneandRhinegrabensarerelatedto the stresses
exertedby
the Pyreneanand Alpine orogenson their forelandsincethe
late Cretaceousand mainly during the Eocene,in responseto
nearly N-S Africa-Euræia convergence.
E-W to nfly-SE "Oligocene" extension. A widespread
extensionalstressregime, with maximum principal stressol
vertical, was also reconstructedfrom many normal faults and
calcite-filled tension gashes(Figure 3). Superpositioncriteria
(successiveslickenside lineations on fault planes) or crosscutting relationshipsbetween faults or tension gashessuggest
that this extensionalevent regionally postdatesthe episodeof
N-S compression previously related to Eocene. Moreover, this
extension is prevalent close to the Saône and Rhine grabens,
and, where outcrop conditions permit, consistent normal
faulting is also observed within the Oligocene fill of the
grabens@gures 3 and 5a). As a consequence,it is likely that
this extensional tectonics is related to the west European
rifting event [Rat, 1976; Bergerat, 1987; Larroque and
Laurent, 19881, that started in the late Eocene within the
southern Rhinegraben [Villemin et al., 1986; Dewey and
Windley, 1988;Ziegler, 1992), and was responsiblefor major
t€ctonism and crustal shetching within the Saôneand Rhine
grabensduring the Oligocene. The computed o3 directons,
presentedin Figures 3 and 5b, are not homogeneousin trend
acrossthe area. In contrast to the overall E-W direction of
extension in the rift system (as indicated by the systematic
analysis of the growth of gypsum fibers in the Mulhouse
Oligocene basin by Larroque and Ansart tl985l), paleosFess
878
PARIS
BASIN
7:
.
.
I
b
Fig. 5. (a) Structuralmap of the Rhine-Saônerift system,with location of sitesof data collection. Key :
l, Hercynianbasement;2, Permo-Carboniferous
basinsand synclines(note the predominant60oN-70oN
fend); 3, Mesozoic sedimentaryformations;4, Main Cenozoic grabens;5, Sites of data collection; 6,
lvlain faults; 7, Thrusts (barbs on upthrust side); Lu, Luxeuilt Vi, Vittel; Mu, Mulhouse; LL-BB,
Lalaye-Lubine-BadenBaden shearzone; and Nb, Noidansbasin. (b) The o3 trajectoriesrelated to the
extensionalstressregime responsiblefor the developmentof the Saôneand Rhine grabens(coordinates
of sites and trends and plunges of o3 are listed in Table 2). The heavy dashesrepresent local o3
orientations determined from geologic shess indicators, the slight ones the averaged trajectories
(computer-basedsmoothingmethod by J.C. Lee and J. Angelier, Interpolationand Smoothingmethods
for regional distribution of directional data : paleostresstrajectories as an example, submitted to
Mathematical Geology, 1993). Note the heterogeneousdistribution of shess in the vicinity of the
presumedhansform zonecontrastingwith the homogeneousstressregime of the grabens.
Lacombe et al.: Tectonics of the Rhine-SaôneTransform Zone
TABLE 1. Main Characteristicsof PaleostressTensorsComputed
Using Fault Slip Data (Diagrarns Shown in Figures 2 to 4)
S it e
01
TrendPlunge
o2
TrendPlunge
o3
TrendPtunge
t
ll
ANG
(')
Prauthoy
Charpti tte
Scey
Dorans
Gy
Ba|J|€
Arcey
LongevetIe
Eavans
N-S Compression (Figure 2)
006-03 1?7-85 ?76-05 0.1
174-00 032-80 265-06 0.5
189-05 033-84 279-02 0.48
182-04 354-86 092-01 0.27
185-09 321-78 093-08 0.s9
350-07 110-76 259-1? 0.35
181-02 282-78 090-12 0.40
185-ti
31tt-73 093-15 0.48
184-03 072-83 275-07 0.26
30
26
28
33
17
24
14
27
24
10
Sacquenay
Prauthoy
Seuchey
Charpti tte
Vauconcourt
Scey
Dijon
Taxenne
Courtcuire
Gy
l,laiI tey
BussufeI
E-W to NWSE E*ension (Figure 3)
039-23 307-03 0.38
?10-6
035-85 221-06 131-01 0.3
076-83 210-05 301-05 0.45
136-76 040-01 310-13 0.3
310-82 202-03 T11-08 0.47
't07-03 0.22
341-85 197-04
181-83 046-05 316-05 0.31
119-75 056-01 326-15 0.3
328-83 200-05 109-06 0.4
059-74 ?09-14 301-08 0.27
309-85 196-02 106-05 0.?1
043-77 170-08 262-10 0.37
8
74
31
39
13
20
18
16
13
29
39
38
15
15
7
2?
9
20
11
6
7
15
11
21
Prauthoy
Charnpt
i tte
Ai I levans
lléci court
Tâxerne
s a i n t - v it
Gy
C h â t it t o n
Baune
SeneuiI
E-W to NIV-SECompression(Figure 4)
't13-07 005-69 206-20 0.5
17
10
290-02 182-83 020-07 0.4
165-01 054-87 255-03 0.55 1?
113-09 291-81 203-00 0.29 18
8
108'12 355'62 2Ot,-?5 0.3
290-03 031-76 199-13 0.29
9
309-04 181-83 039-05 0.29 11
?92-07
08/-82
202-04 0.32
I
326-08 ?02-76 058-12 0.34 12
335-03 ?44-23 07?-67 0.00
14
9
9
13
12
I
17
14
13
12
879
and were sealedby Pliocene sediments[Chauveet al., 1988;
Guellecet al., 19901.
Geometry and Tectonic Significance of Eocene-Oligocene
Minor and Major Fault Patterns Within the Rhine-Saône
Transformbne
tl
12
10
17
18
12
12
7
Stess axes:trend andplunge, in degrees.iDratio, definedin
text; N, numberof fault slip data; ANG, averageanglebetween
computedshear stress and observedslickensidelineation, in
degrees.
indicators yield directions of extensionranging from E-W to
NIV-SE within the transform zone @igures 3 and 5b). The
regional significanceof thesedeviationsof stressorientations
and their relation to Rhine-Saônehansform kinematicswill be
discussedbelow.
Mio-Pliocene W-NW-E-SE compression. According to relative
chronology data, a W-NW-E-SE compression characterizes
the last tectonic event that we could recognize in the field.
The trend of the ol axes computedfrom fault slip and calcite
twin data is nearly constant, W-NW-E-SE to NW-SE. This
compression is characterized by leftlateral strike-slip faults
whose hends range from 120"N to l60oN, and right-lateral
strike-slip faults with hends from 20oN to 90'N (Figure 4).
Some of lhese faults correspond to reactivaûed fault planes
inherited from the previous N-S compression and/or the E-W
to NW-SE extension, thus providing evidence that this event
postdated the main extensional phase. Although the exact age
of this event cannot be established with certainty, the relative
chronology criteria as well as the orientation of the main
direction of ol suggest that it was very probably
contemporaneous with the westward thrusting of Jura [Caire,
1974iBergerat 19871,whosefrontal thrust elementsoverrode
the eastem margin of the Bresse graben during Mio-Pliocene
The determinationof the successiveregional paleostresses
basedon fault slip datâ sets thus enabled us to decipher the
complex minor fault systemswithin the transform zone. More
particularly, it allowed us to identify and then separate
Eocene-Oligocenefault patternsfrom the Mio-Pliocene fault
patternsthat developedafter the transformmotion had ceased.
In this section, we thus only focus on Eocene-Oligocene
faulting, without consideringMio-Pliocene fault systems.
At a regional scale (Figure 5a), the sedimentary cover
between the Saôneand the Rhine grabensdisplays a dense,
complex pattem of major en échelon 35oN trending fractures
or shear zones with a predominant sinistral sense of
displacement (Figures 2, 3, 4, and 5a). Some of these
regional-scalefaults display a pure strike-slip sinishal motion,
whereas others additionally display a significant normal
motion (both left-lateral and normal), as shorvnby the contrast
ofoutcropping formationson oppositesidesof the faults, thus
cutting the Jurassic formations into small, narrow NE-SW
grabens like the shallow, Noidans Oligocene basin. Stressstrainpartitioning thus aroseand resultedin mixed families of
oblique-slip faults and pure strike-slip faults. This fault
pattern differs from tectonic features of the rift system itself
where large-scale normal dip-slip faults clearly mark the
faultedbordersof the Saôneand Rhine grabens(Figures I and
5a).
In order to allow an efficient comparison between these
major fault patternsand the minor faults that we found to be
related to the Eocene-Oligocenetectonisms,we correlatedthe
geometries of the measured individual fault planes,
slickensidelineationsand sensesof motion by plotting the dip
of each fault plane measuredin the field versusthe pitch of
the striae observed on it (Figure 6). The senses of relative
motion on a fault plane are thus easily identified as normaldexûal. normal-sinistral.reverse-dexhaland reverse-sinistral.
Three main pools of fault poles emerge from this diagram.
The first one at the center of the diagram corresponds to
normal dip-slip faults that are observed at various scales in
the area of interest; they are similar to the major faults
bordering the Saône and Rhine grabens and are related to the
main extensional phase. The two other pools correspond to
left- and right-lateral strike-slip faults. Motion on many of
these faults, however, is not purely normafdip-slip or purely
strike-slip. Rather, some strike-slip faults experience obliqueslip, which makes them "extensional strike-slip faults" as
shown by their normal component of shear (Figure 6) which is
consistentwith E-W extension.These extensional strike-slip
faults differ from the pure or compressional strike-slip faults
that clearly mark the pre-extensional Eocene N-S
compression;their geometryresemblesthat of the 35oN major
faults that crosscut the Jurassicplateaus of the transform zone.
The minor and major 35"N trending exûensional strike-slip
faults are thus very probably related to the Oligocene
evolution of the area @gure 7).
In summary, paleoshessreconshuctions and analyses of the
geometry of minor and major fault patterns unambiguously
Lacombe et al.: Tectonics of the Rhine-SaôneTransform Zonc
880
their orientationsrelative to the newly imposed stress field.
We examinedthe preexistingfracture pattern in the basement
between the Saône and Rhine grabens in order to evaluate
which faults were likely to have been reactivated during
OligoceneE-W extension.
972 FAULTS
90
d
=o
É
Variscanand Late VariscanFault Pattern
z
o.
-a
ct-
u
u
U
É
o
LEFT-LAT.
900
PITCH
RIGHT-LAT.
Fig. 6. Dip versus pitch diagrammatic representationof EoOligocene fault slip data sets. The quadrants refer to normaldextral, normal-sinistral,reverse-dextral,and reverse-sinistral
senseof relative motion. The maximum at the center of the
diagram correspondsto pure (dip-slip) normal faults. The two
other maxima correspond to sinisfral and dexhal strike-slip
faults. Note that extensionalstrike-slip faults can be detected
accordingto their normal componentof shear(seealso Figure
7).
indicate polyphasetectonicsfor Eo-Oligocenetimes, as noted
previously by Gelard [979] and Bergerat [987]. An early
phase of deformation possibly related to the Eocene
Pyreneary'Alpine compression gave rise to pure and reverse
strike-slip faulting within the Rhine-Saônetransform zone.
This compressionalphase was followed by a major phaseof
crustal extensionthat staftedduring the late Eocenewithin the
Rhinegrabenbut occurred mainly during the Oligocenein the
whole of the west European Rift and presumably controlled
the opening of the Rhine/Saône rift system. This E-W to NWSE extension expressed in the field by tension gashes and
normal dip-slip faults, but also by minor and major
extensional strike-slip faults that developed widespread
within the hansform zone. Transform kinematics and coeval
major crustal extension ceasedby the late Oligocene-early
Miocene, so that most of the younger (Miocene) subsidence
within the Saône graben and the southern Rhinegraben should
be interpreted in terms of thermal subsidence rather than in
terms of the persistenceof crustal extension.
The pre-Mesozoicfault pattern in the crust of NE France
has beendocumentedfrom geologic data [Arthaud and Matte,
1975,1977; Mùller et al., 1984;Ziegler, 1986;Wickert and
Eisbacher, 1988; Eisbacher et al., 19891, geophysical
investigationsfEdel et al., 19751'Edel and Fluck, 1989] and
satellite imagery [Bergerat and Chorowicz, l98l]. The main
characteristics of the crust in this area result from the
Variscancollisional orogeny (400 to 320 Ma) [Ziegler, 1986].
The tectonic featuresrelated to this orogeny are crustal-scale
35oN and 60oN rending strike-slip faults that developed
during the late Visean and Namurian (350-325Ma), and 60"N
to E-W striking south or north verging major thrusts [Matte,
l986a,b; Wickert and Eisbacher,1988;Edel and Ftuck, 19891.
For example, the Lalaye-Lubine-Baden Baden shear zone
(Figure 5a) is a major fault system that crops out as a steep,
southward-dipping shear zone and shows evidence of a
multiphasemovementhistory [Wickert and Eisbacher,1988].
The Variscan orogeny was followed by postorogenic
extension in late Carboniferous times (320 to 290 Ma)
[Eisbacheret al., 1989] and Permian times (290 to 250 Ma),
which induced developmentof E-NE-W-SW to E-W rrending
collapsebasins[Ziegler,1980, 1990].Thesebasinsprincipally
formed along major Carboniferousfault zones trending 60"N,
in the French Massif Central [Bles et al., 1989], the Vosges,
and in the southern Black Forest [Mùller et al., 1984]. For
example, in the southernVosges,the 60oN trending Luxeuil
fault system (see location on Figure 5a) was reactivated in
Permian times and controlled deposition of thick Permian
sandstonesand felsic volcanics[Edel and Fluck, 1989].
Thus, a complex fault system involving linked faults with
various dips (thrust sutures, wrench and normal fault zones
hending approximately60oN) occurs in the basementbeneath
the Rhine-Saône transform zone. Such 60oN-oriented
discontinuities constituted crustal weaknessescapable of
being reactivated preferentially during subsequenttectonic
events.For the sake of simplicity, this basementfault system
will be refened to as a single 60oN trending fault zone in the
following sectionsof this paper.
KINEMATIC CONSTRAINTS DERIVED
FROM OBLIQUE REACTIVATION OF
BASEMENT DISCONTINI.JITIES
Because of its complex orogenic history, continental
lithospheregenerally exhibits pervasive shength anisotropies
causedby preexisting fracturesor zonesof weakness.During
rifting of the continental crust, tensionalforces will reactivate
some of these preexisting zones of weaknessdepending on
Fig. 7. Theoretical sketch of geometrical relationships and
mechanical compatibility between extensional strike-slip
faulting and pure normal faulting under stress boundary
conditionsof E-W extension.
Lacombe et al.: Tectonics of the Rhine-SaôneTransform Zone
Oblique Reactivation of Basement Fractures Under E-W
Extension
Clay and sand laboratory experiments on continental
rift/fansform syst€ms[e.g., Elmohandes,1981] provide some
basis to interpret the regional structuralsetting of the RhineSaônetransform zone. With a main E-W extensionduring the
Oligocene (Figures 3 and 5b), a major inherited basement
fault system that trends 60oN-70oN beneath the transform
zone, and mixed sets of 35oN trending, en échelon,obliqueslip and pure strike-slip faults in the cover, the structural
pattem resembles the sand-box experiments on oblique
extension by Tron and Brun [991]. In these experiments,
uniaxial stretching was applied oblique to the external
boundariesof a brittle-ductle model (two-layer slabs of sand
and silicone above two diverging basal sheetsseparatedby a
discontinuity). For boundaryconditions involving a low angle
(about 15-25') betweenthe basementdiscontinuity (the 60oN70oN basementfault) and the applied stretchingvector (E-W)
(Figure 8), their modeling predicts formation of steeplydipping, en échelon faults in the sedimentary cover, in
responseto transtensionalreactivation of the discontinuity at
the bottom of the model (Figure 8). These en échelon faults
develop at 45o-60" from the stretching direction (i.e., not
perpendicular to it) and display norma[strike-slip or pure
strike-slip motions. The experimental models thus exhibit a
structural fracture pattern very similar to that observedwithin
the Rhine-Saône transform zone (Figures 5a and 8).
Consequently,we suggest that the fault pattern within the
Rhine-Saône transform system conforms to regional-scale
transtensional deformation, caused by the extensional
reactivation of a basement fault trending oblique to the
direction of extension.
The expectedoblique reactivation of a basementweakness
zone and the related development of extensional strike-slip
faults in the overlying cover between the Saône and Rhine
graben may be checked by the calculation of the theoretical
shear stress along available preexisting 35"N and 60oN
trending planes with various dips, submitted to E-W
Fig. 8. Top view of a laboratory sand-boxmodel on oblique
reactivation of a preexisting basementdiscontinuity showing
developmentof en échelon faults in the overlying cover, The
angle between the strerchingvector (large unshadedarrows)
and the basementdiscontinuity is about 30o [after Tron and
Brun, l99ll. This model exhibits a left-lateral extensional
fault pattern very similar to that observed in the field
(comparewith Figure 5a).
881
extension. Although frictional sliding controls the effective
reactivation of inherited faults [e.g., Celerier, 1988], friction
was neglected in a first approximation, becauseit does not
influence the orientation of the striae on the reactivatedfault
surfaces.This calculation, based on Wallace-Bott's principle
according to which slip on a fault surface occurs parallel to
the maximum shear stress,is developed in the appendix. As
one may expect, such a calculation predicts in most casesa
left-lateraVnormalmotion along 35oN as well as 60oN planes.
From the theoretical point of view, the dip of the 60'N
basement discontinuity is found to slightly influence the
orientation of the theoretical striae (appendix), and also the
possibility of the fault plane to be reactivated (frictional
sliding), but does not modify the general conclusion :
whatever its actual dip, the 60oN trending fault system is
reactivatedas a left-lateraVnormalfault zone during the E-W
extension, and the predicted orientations of oblique slip
(appendix) are compatible with the results of statistical
analysisof actual fault slip geometry(Figure 6).
Following Bergerat tl977l, we propose that the
developmentof the Saôneand Rhine grabenswas associated
with a regional wrench movementat depth along a preexisting
060o basementfault system. This fault system acted as a
transform zone during Oligocene rifting and accommodated
the resulting shear strain between the grabens.However, we
have found no evidenceof dominant N-S compressionwithin
the transform zone contemporaneouswith E-W extension
within the grabens lContini and Theobald, 19741.Bergerat,
19771.Geometrical and mechanical considerationsbased on
fault patterns in the cover and in the basement, and
reconstructionsof paleostressorientations,rather suggestthat
the overall E-W extensionproducedalong the 60oN basement
fault system a component of transverse(oriented I.[!V-SE)
extension,making the resulting transform motion extensional
strike-slip in the cover (Figure 9). The transform motion
would have been purely strike-slip if extension had trended
060o (or if the basementfault zone had trended 90'N). This
oblique extension is corroborated in the field by the
development within the transform zone of small, NE-SW
trending grabens such as the Noidans Oligocene basin
@gures 2, 3, 4, and 5a) which are clearly associatedwith
local NW-SE directed extension.A model of pure strike-slip
motion under a regional stress field dominated by N-S
compression [Bergerat, 1977] (Figure l0a) fails to explain
developmentof sucha basin.
L+
+
+ + + + + + +
Fig.9. Schematicblock diagram illustrating the interpretative
kinematic evolution of the transforrn zone, especially the
resulting transtensionaldeformation in the sedimentarycover.
Key : Rg, Rhinegraben;Sg, Saône graben and Nb, Noidans
basin.
Lacombe et al.: Tectonics of the Rhine-SaôneTransform ZonE
882
-
+
horizontal displacementswithin the cover above the basement
fault are highly dependenton the dip of this basementfault.
The absence of large vertical displacements within the
transform zone suggeststhat the reactivated inherited fault
system should rather correspondto a low-dipping fault zone
(with a presumedSE dip like most of the crustal structures
inherited from the Variscan orogeny in this arca) [Matte,
l986al, but the subsurfacedata are unfortunatelytoo scarceto
allow to constrainthis dip precisely.
In conclusion, this study emphasizes that regional
deformation mode and fracture directions were not only
controlled by stress orientations at the boundaries of the
system (i.e., N-S confining pressureand E-W extension) but
also by the heterogeneityand the anisotropy of the brittle
crust.
KINEMATIC CONSTRAINTS DERIVED
FROM PERTI.]RBATIONS OF EXTENSIONAL
STRESSTRAJECTORIES
Fig. 10. Comparison of the previous basement model of
Bergerat |977) and the model proposedin this paper for the
Rhine-Saône transform zone. (a) Graben formation associated
with pure strike-slip motion along a 60"N trending basement
fault, under regional N-S to 20'N compression.(b) Graben
formation associatedwith wrench /extensionalmotion along a
60oN trending basementfault as a consequenceof its oblique
reactivation under actual E-W extension.The N-S part of the
model is fixed (confining pressure)but not dominatedby N-S
compressionas in (a).
In terms of rigid block kinematics, the model that best fits
the structural d^ta thus involves oblique left-lateral
reactivation of a 60"N hending basement fault under E-W
extension @gure lOb), resulting in extensional sinistral
strike-slip faulting in the cover (Figure 9). The model suggests
that in addition to the predominantsinistal wrench movement
that accommodatescrustal extension within the Saône and
Rhine grabens,a transverse(NW-SE) extensionalcomponent
of motion is likely to have occurred (Figure 10b). This
oblique component of motion in the basementis responsible
for the development of (l) the Noidans Oligocene basin
@gurc 5a) and (2) the mixed oblique-slip and strike-slip
35"N "Riedel" faults at nearly 55" to the E-W stretching
vector. Taking into account the 60oN fiend of the basement
fault (at 30" to the direction of extension)and the :rmountof
extension within the Saôneand Rhine grabens(maximum 5
km), the maximum amount of hansferred motion in the 60"N
direction (left-laæral slip) is expected to reach 4.3 km,
whereas the amount of opening in the NW-SE direction (i.e.,
perpendicular ûo the transform) is 2.5 km maximum. Here the
dip of the reactivated basement fault system plays a major
role, b€cause it conhols the way these 2.5 km of oblique
exûension are accommodated within the cover, even though
the transform motion is in all cases extensional left-laûeral
(see appendix). More particularly, lhe vertical versus
Kinematic constraintsalso arise from the existenceof stress
deviationsrelated to the main extensionalphase.The trendsof
ol axesrelated to the Eocenecompressionaldeformation are
very homogeneous (Figure 2). In contrast, the rend
distribution of the 03 axesrelated to the extensionalphaseis
more complex (Figures 3 and 5b). Angular deviations from
the E-W direction, ranging from 0o @-W) to 45o clockwise
(NI!V-SE),have been detectedas indicated by the analysisof
minor normal faulting and other paleostressindicators (see
rose diagram, Figure 3). As most of the small-scalenormal
faults, which developed during this extension, are
newlyformed (Figure 3), they should correspondto the local
preferreddirection of fracture within the sedimentarycover in
response to local stress fields. The distribution of these
deviations relative to the overall regional E-W trend of o3
consists of a progressive clockwise change from 90oN to
135'N north of the Saônegraben.Although this distribution of
paleostresstrajectories is reported herein only for the northern
side of th€ transform zone (Figure 5b), a consistent
extensionalpaleostressfield has been identified to the South
in the extemal plateaus of the French-SwissJura Mountain
ll-acombe,1992].
Figure 5b illustrates 03 trajectories interpolated from local
paleostressreconstructions(solid bars)(Table2) using a power
function of distance weighting (J.C. Lee and J. Angelier,
Interpolationand Smoothingmethodsfor regional distribution
of directional data : paleostress hajectories as an example,
submitted to Mathematical Geology, 1993).lt shorvsthat the
o3 directions arc sûongly deviated from the E-W directon in
the wesûernsegment of the transform zone north of the Saône
grabn, so that they become almost perpendicular to the
presumed 60"N trending basement fault. In contrast, these o3
trends remain approximately E-W along the eastern segment
of the transform zone in the southern part of the Rhinegraben
(Figure 5b). Using simple models of sûess distribution and
perturbation along shear fractures fRicou, 1978; Segall and
Pollard, 1980; Rispoli, l98l; Xiaohan, 19831, we interpret
these deviations of extensional sFess as perturbations of the
overall E-W extension due to regional-scale hansform motion
fi.acombe et at., l990a].
Two independent numerical modelings of shess hajectories
within the Rhine-Saône rift system were carried out in order
to check and refine our interpretation of sûess deviations
Lacombe et al.: Tectonics of the Rhine-SaôneTransform Zone
related to hansform kinematics, in a general stress field
dominated by E-W extension. These models, discussedin
more detail in other papers [Dupin, 1990; Byrne eta1.,1992J,
involve two different and complementary approaches: a
distinct element analysis, which is appropriatefor modelling
discontinuous deformation in the basement (i.e., stless
patterns and deformations associated with slip on â discrete
TABLE 2. l-ocationof SitesShownin Figure5a andTrendsand
Plungesof theMinimumHorizontalStresso3 Relâtedto
OligoceneExtension
site
1
?
3
4
5
6
7
I
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
?4
?5
26
?7
28
29
30
31
32
33
34
35
36
37
38
39
40
41
4?
13
44
45
16
47
48
49
50
51
52
53
5{
55
56
57
site Nâme
o3
Latitude Longitude
TrendNord
Est
PIunge
Tatant
47"201
Aittevans 17'351
seuchey
47"121
sacquenây 47"351
Clervât
47'23'
Angirey
47'271
Gy
47'24'
Taxenne
47'131
l{ontagney 47'171
Prauthoy 47"421
St Geosmes 47'49r
châuûcnt 48'101
Is / Titte 47'32'
t
Gevrey
t
N u it s
Beaune
Chagny
t
*
5 . 0 0'
6"?51
5 . 3 1'
5.18'
6 . 3 0'
5 . 4 5'
5 . 5 0'
5 . 4 1'
5 . 4 0|
5 . 1 8'
5'211
5 . 1 0'
5.07'
t
*
t
*
*
*
*
Chanptitte 47'37'
t
Fouvent
Efotte
Vauconcourt
SceY
Fretigney
47'381
47"361
47'39r
17"411
17"291
t
5 ' 3 0'
t
5.421
5 . 4 5'
5 . 4 9'
5.59'
5.57'
Couîtcuire 47'20t
Traitié
47'241
17'331
Echenoz
5 . 5 1'
6'5?l
6.08'
lfaittey
saint-vit
seneui I
Longevette
Bussurel
Dorans
Denney
47'331
47"111
47'221
47'27t
17"321
17"351
17'391
6'0eI
5.191
6.321
6.39'
6.471
6 . 5 1|
6 . 5 5|
47'381
17'101
47'451
47"581
48'10'
48'181
48'31 t
48'50t
48'431
48'51 '
48'08'
7'131
7.30 '
7'231
7.401
7.151
7.551
7'?51
7'351
7 . 0 3'
7 ' 0 5'
9.10'
*
Attkirch
Lôrrach
l,futhouse
Freibourg
Ribeawitté
Lahr
Saverne S
Saverne ll
Saarburg S
Saârburg [
Ebingen
3 1 6 .- 0 5 .
3 0 1 .- 0 7 .
3 0 1. - 0 5 '
3 0 7 .- 0 3 '
Reference
this paper
this paper
this paper
this papef
this paper
this pap€r
this pâper
this paper
this paper
this paper
this paper
this papel
Bergerat [1987J
E e r g e r a t[ 1 9 8 7 t
Bergerat [1987I
Bles tl9891
Btes t19891
Bergerat 119871
Bergerat[198n
Couton tl9881
Couton tl9881
Coulon t1988I
Coulon t19881
coulon t19881
Coulon t1988I
this paper
Bergerat [1987I
this paper
this paper
this paper
this pâper
this paper
Bergerat [1987]
this Fper
this paper
this pâper
Bergerat [19871
Bergerat [1987J
this paper
this pâpef
this paper
this pâper
this papec
Lacroque[19881
Lafroquet1988I
Larroque[19881
this paper
Bergerat [19871
Larroque[19881
Bergerôt [1987I
Bergerat [19871
Sergerat n987I
Eergerat [19871
Bergerat [1982t
Bergerat [19871
Bergerat [19871
Eergerat [19871
Reference pspers : Bergerat [987], Bles et al. [989], Coulon
and Frizon de Latnotte [988], Lacombe et al, [1990b], Larroque
and l:urent [1988]. The "+" indicates the values not given by the
authors (o3 tends and site locations only shown by arrows),
883
basementfault) [Dupin, 1990], and a finite element analysis,
suitable for modeling stressesand distributed displacements
within the overlying, approximately continuous, sedimentary
cover [Byme et al., 1992]. The two-dimensional horizontal
elastic models we used are physically simplified compared
with the actual rheology distribution; also, they do not take
into account coupling at depth between plate kinematics and
thermomechanicaldeformationswithin the lithosphereduring
rifting fFleitout and Froidevaux, 1982, 1983]. Despite these
simplifications, however, our models are valid in terms of
distribution and orientationof the shessfield within the upper
crust, becausewe consider only horizontal tectonic stresses
resulting from plate divergence without implying anything
about the mechanism (the driving forces) for lithospheric
extensionand rifting (i.e., passiveor active rifting).
A numerical modeling based on the "distinct element
technique" was used to investgate the relationshipsbetween
the stress perturbations and the Rhine-Saône transform
kinematics. This modeling technique is appropriate for
studying the behaviorof discontinuitiesin media with uniform
or nonuniform rheologies, such as the faulted basement
[Dupin, 1990]. The two-dimensionalmodel takesinto account
the main stnctural characteristicsof the rift-rift transfer: two
20oN trending grabens linked by a 60oN basement fault
experiencing discrete slip and acting as a lransform. As a
consequenceof the two-dimensionalmodeling, the influence
of the dip of the basementfault could not be considered,and
everything happened as if the fault plane were vertical.
However, as horizontal stress perturbationsobserved in the
field worldwide are mainly due to the strike-slip components
of fault motion [Ricou, 1978;Rispoli, l98l; Xiaohan, 1983],
the results obtâined in two-dimensional models in terms of
horizontal stressperturbationsare still valid. Note that as a
consequenceof the modeling process, using the distinct
element technique (especially the need for maintaining the
model cohesion along the basement discontinuity at each
time-step),the two-dimensionalmodel does not enable us to
checkdirectly our mdel of oblique openingin the basement.
Numerical characteristicsof this modeling and detailed
results are discussedin Dupin [1990]. The rheology of the
basementwas assumedto be elastic, and fault displacements
were assumedto follow a Mohr-Coulomb failure criterion.
The model was loaded under displacement boundary
conditions involving E-W divergencealong tlre east and west
sides, whereas the north and south boundaries were submitted
to constant confining pressure. Figure ll displays stress
hajectories for this basic model. The modeled direction of
extension (minimum horizontal stressaxes) is homogeneous
in large blocks and near the border faults of the grabensand is
consistentwith the major E-W extensionderived from shess
indicators.However, large deviationsof o3 from E-W to NWSE occur iuound the transform fault and near the ends of the
grabens (Figure 11). This supports the possibility that the
Rhine-Saônetransform zone, acting during the major crustal
extensionwithin the grabens,induced perturbationsof the o3
trajectoriesvery similar to thosereconstructedfrom field data
[Dupin, 1990;Dupin andLacombe,1991](Figure5b).
An additional modeling based on the "finite element
technique"was performed to extend our understandingof the
mechanics of the transforn zone and to enhance the
interpretationof the paleostressdata in the sedimentarycover.
This technique assumes that continuous infinitesimal
deformation occurs throughout the region, which can include
884
Lacombe et al.: Tectonics of the Rhine-SaôneTransform Zone
Fig. 11. Distinct element model showing horizontal principal
stressaxeswithin the centerof the grid. The stressaxesdo not
deviate significantly from the edgesof the region shown out
to the edge of the grid. For each computed stresstensor, the
long and the short axes conventionally represent the
maximum and the minimum horizontal sfresses,respectively.
Note the deviation of extensionalstress03 from E-W to NIWSE in the westem pafi of the transform,For more details, see
text and Dupin [990j.
rotation of principal stressaxes reconstt:uctedfrom field data
within the transform zone. Figure 12 shows the stress
trajectories for the symmetric model. The direction of
extension as indicated by the o3 axes remains nearly E-W
across the weaker grabens but experiences significant
perturbations around their ends and throughout the transform
zone. This simple model shows a general NW-SE directed
extensionwithin the transform zone quite similar to the field
observations(Figure 5b), and thereforereproducesthe general
change in shess directions in the vicinity of the transform
zone. More elaborate models involving additional details of
the hansform and graben structure, produce a shess field that
fits better the paleoshessdistribution [Byrne et al., 1992]. We
conclude from these finite element models that the stress
orientationsreconshuctedin the Rhine-Saôneregion may be
interpreted in terms of a regional E-W extension with
significant perturbationsin the hansform zone between the
two weaker grabens. It is interesting to note that models
dominated by N-S compression failed to produce such a
perturbed stress field, which confirms our model based on
geometricaland structuraldata (Figure l0b).
Both numerical modelingsfinally confirm that the observed
deviations of o3 orientations may be simply related to the
structure and the kinematics of the Rhine-Saône grabentransform system, involving combined discrete slip in the
basementand asymmetric distribution of rheologies in the
coyer, under a regional shess field dominated by E-W
extension.
'+
+:1-.'J-+
..+:,+-++i+*
*-**-+ - i - . -r"*'
;*:t*iï
regions of different rheology or shearstrength.The details of
this modeling and related experimentsare describedby Byme
et al. OCn2l. An elastic rheology, appropriate as a first
approximation for the brittle upper crust, and a simple
structural pattern involving two 20oN grabens and a 60oN
trending transform zone were considered.It was assumedthat
the pervasive faulting within the transform zone resulted in
increasingweaknessrelative to the adjacentplatform areaand
that thinning of the crust below the grabensmakesthem also
weaker. The differencesin shearstrengthwere approximated
by applying different elastic constants for each of these
regions (these constantswere taken to be consistentwith the
known amount of crustal stretching within the grabens; in
practice, Young's Moduli of 5 GPa within the grabens,l0 to
30 GPa within the ûansform zone and 50 GPa for the adjacent
platform were adopted in calculations).Our models are thus
physically simplified, because the strength, as normally
understood,refers to the onset of nonelasticlrchavior and is a
different phenomenon from lowered elastic constânts.
However, the overall conclusionsof the modeling will be still
valid becausea similar (though larger) deformation may be
expected with nonelastic rheology (M.H.P. Bott, written
communication,1992).
The finite element model shown in Figure 12 used
displacementboundary conditionsof E-W extensionalong the
east and west sides and of zero along the north and south
boundaries(see Byrne et al, |9921 for details). Remarkably,
other models using N-S compression in addition to E-W
extension were found to be unable to produce the significant
-
-t*
#_
.+*
;]-*:'-=|i-{;
Lffi
.-s++-rll
=+l::-:1..-:'=:Ë+;i-5-ii1f
:- j - . _ + 1 . + - - - + -++'+*
.-r+
FF
-J=t-- -+tI|+.
+-
*
r
Fig. 12. Finite element model showing horizontal principal
stressaxeswithin the centerof the grid. The stressaxesdo not
deviate significantly from the edgesof the region shown out
to the edge of the grid. Arrowheadsdifferentiate compressive
and extensivestressaxes. Lengths of axes are proportional to
stressmagnitudes.This simple model reproducesthe essential
features observed in the paleostressdata, most notably the
rotation of the minimum horizontal stress axes from E-W
trends outside the hansform zone to NW-SE within it. For
more details, seetext and Byrne et al. ll992l.
Lacombe et al.: Tectonics of the Rhine-SaôneTransform Zone
DISCUSSION AND CONCLUSIONS
During the main Cenozoic subsidenceof the continental
Rhine and Saône grabens, in responseto E-W lithospheric
extension, a major Permo-Carboniferous basement fault
system, trending 60oN, was reactivated as a leftlaterafextensional hansform zone connecting the two
grabns. This oblique reactivation induced transtensive
distributed deformation in the overlying sedimentarycover
along numerous oblique-slip en échelon faults. Following
previous works, which already emphasizedthe influence of
ancient structureson rifting [e.g., Illies, l98l; Daly et al.,
19891,this provides further evidencethat preexistingfractures
and zones of weakness in the continental crust can be
reactivated during rifting and control localization of
continental rift-transform systems. The dependence of
continentâl transform zones on inherited structuresexplains
why they tend to form larger and more complex shearzones
than oceanic transform faults and may not be perpendicularto
the rift segments and parallel to the direction of major
extension.
The trend of ol orientationsrelated to the initial PaleoceneEocene Pyrenean-Alpine compressional phase is very
homogeneousin the area investigated, and no significant
large-scaleperturbationscould be detected(Figure 2). On the
other hand, the Rhine-Saône transform kinematics was
associated with regional-scale perturbations of extensional
stress Eajectories (Figures 3 and 5b). Numerical modeling
indicates that boundary conditions of actual E-W extension
are required to account for these perturbations, This leads to
the conclusion that no significant bansform motion occurred
during the preextensional N-S Eocene compression. It also
suggeststhat the shessregime prevailing during the whole of
the development of the Rhine-Saône rift-hansform system
was predominantly extensional rather than compressional.
This portion of the west European Rift consequently did not
entirely originate in response to N-S compression due to
convergence between Africa and Eurasia (Alpinel$nenean
compression).Although this convergencemay have played an
indirect role [Sengôr, 1976; Tapponnier, 1977: Illies and
Greiner, 19781, the Rhine-Saônerift system rather resulted
from regional tensional stresses and coeval E-W extension
along preexisting zonesof crustal weakness.This conclusion
is in agreement with the recent plate kinematics
reconsûuctions of Europe implying that the entire west
European Rift developed under pure extension due to E-ril
divergent motion between westernmost Europe and the rest of
Eurasia during the Oligocene pe Pichori et al., 19881.This
dominant E-W extension also accountsfor the abundanceof
extensionalfeaturesof Oligoceneagethat can be found within
and away from the west EuropeanRift system [e.g., Coulon
and Frizon de Lamotte, 19881.
The model we finally propose for the Oligocene tectonics
and kinematics of the Rhine-Saônetransform zone takes into
account most of the geometric constraints raised from
structural and paleostressanalyses.It accountsfor the nearly
synchronousopening of the Saôneand Rhine continentalrifts,
the large-scaleperturbationsof extensionalstresstrajectories,
and the development of a typical en échelon wrench
/extensional fault pattem in the sedimentary cover of the
Rhine-Saône transform zone, and takes into account the
influence by structural heterogeneities in the brittle
contin€ntal crust. The Rhine-Saône transform zone thus
885
appearsto be a major, Hercynian-inherited,zone of crustal
weaknessthat localized deformation and reoriented tectonic
stressduring Oligoceneextension.
APPENDD(
A simple geometricalmodel of stress-sliprelationshipsfor
a preexistingfault plane can be usedin order to determinethe
slip vector, following the assumptionof Wallace [951] and
Bott [959] that slickenlines on a fault plane are parallel to
the maximum shearstress.Let us considera single fault plane
of given orientation submitted to homogeneousstress.In our
case, the minimum stress03 is horizontal and trends E-W.
Consideringthe Oligocenetectonism(seetext), the maximum
compressionalstressol is generally vertical (normal faulting
regime). The orientation of shear (and slip) on preexisting
faults not only dependson the fault orientation with respectto
the new stressfield but is also a function of the ratio ô - (o2o3)/(ol-o3) between principal stresses.As pointed out by
Angelier [1984], substituting values of l, ô, and 0 for
principal stressmagnitudesol, o2, and o3, respectively,do€s
not affect the orientation and senseof shearon any plane. In
the case considered herein, o = Q corresponds to
multidirectionalextension(o1 vertical, with o2 = o3), whereas
<D = I corresponds to uniaxial extension (o3 horizontal
trending E-W, with ol = o2). Values between 0 and I
correspondto normal fault regime (ol vertical, o3 horizontal
trendingE-W).
The stresstensorT being thus defined, with principal values
I (vertical stress), O (N-S horizontal axis) and 0 (E-W
horizontal axis), one may easily compute the stressd exerted
on any fault plane whose unit normal vector is ;' (ô = T n ).
Then the normal stress,i , is given by i = t (6.t ),
and finally the shear sûess, t, is given by ô = V + t.
Components of 6 along reference axes @-W, N-S and
vertical) are thus obtained as a simple linear functions of (D
(Figure Al).
l. For a fault plane which strikes 30oN and dips 80o to the
S-SE (upper row ofFigure Al), the expectedslip varies from
dip-slip (for ô = 0) to oblique-slip (normal left-lateral) with a
decreasingpitch of 35"N for o = 0.5 and of lToN for ô = L
This situation is tlpical for steeply dipping faults inherited
from the Eocene compressional event (former leftlateral
strike-slip faults). Our calculationsuggeststhat such faults are
reactivated as oblique-slip faults (normal sinisûal) with
pitches of slickenside lineations ranging from 90o to 35o to
the north as o2 varies from o3 (ô = 0) to 0.5(o1 + o3) (ô =
0.5). Values of ô ranging from 0 to 0.5 are common in the
Rhine-Saônetransform zone, as stresstensor determinations
indicate (seeTable l).
2. For a fault plane which strikes60oN and dips 30' to the
S-SE (secondrow of Figure Al), similar results are obtained
in terms of senseof motion, but critical pitch values differ.
For a ô ratio varying from 0 to 0.5 as before, the pitch of the
expected slip vector remains very large (90' to 78'1.0,
indicating that the left-lateral component of motion on such
faults should be expected to be very small. For values of (D
ranging from 0.5 to l, the pitch decreasesrapidly, from 78o to
27"N (FigureAl).
3. For a fault plane that also slrikes 60oN but dips steeper,
60o to the S-SE (lower row of Figure Al), similar results are
Lacombe et al.: Tectonics of the Rhine-SaôneTransform Zone
886
OT VERTICAL
o-0
A
o-0.5
o-l
Strike
N030'8.
Dip
80'ssE.
Strike
N060'8.
Dip
30'ssE.
Strike
N060'8.
Dip
60'ssE.
Fig. Al. Direction and senseof theoretical slip motion along
fault planeswith various strikes and dips as functionsof the o
ratio.
obtained:the pitch of normal-sinistralmotion varies from 90o
to 55"N and l6"N as o varies from 0 to 0.5 and to I (Figure
At).
The last two determinations suggest that for faults that
trend 60"N, variations in dip within the approximate range
30o-60ohave little effect on fault slip direction (however, dip
variation may strongly influence friction). For reasonable
values of o (between 0 and 0.5, as indicated by stress
determinations),fault motion is predominantlynormal, with a
minor but significant left-lateral component. This result is
crucial, becausethe real nature of inherited basementfaults,
hencetheir dip, remains unknown. The 30o dips are common
for reverse faults, whereas the 60" dips are common for
normal faults. The present knowledge of the Hercynian
structure shows that both types may exist along common ENE-W-SW trends.
Acknowledgmens. Field work and data analysis were
supported by the Institut Français du Pétrole. The authors
thank Lee Jian-Cheng, who kindly provided the spatial
averaging progriln used in Figure 5b. G. Eisbacher, P.A.
Ziegler, and C. Ebinger are also thanked for their thorough
reviews and their constructive comments which allowed
improvementof the manuscript.
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