Structure and Tertiary tectonic history of the Mulhouse High, Upper Rhine Graben: Block faulting...

Structure and Tertiary tectonic history of the Mulhouse High,

Upper Rhine Graben: Block faulting modified by changes

in the Alpine stress regime

Y. Rotstein1,2 and M. Schaming

Institut de Physique du Globe de Strasbourg, CNRS/ULP, Strasbourg, France

S. Rousse

Centre de Geochimie de la Surface, CNRS/ULP, Strasbourg, France

Received 25 March 2004; revised 1 October 2004; accepted 27 October 2004; published 23 February 2005.

[1] The Mulhouse High (Horst) is situated in thesouthernmost Upper Rhine Graben, and changes inits tectonic history record the interaction with theadjacent Alpine front. Recently released seismic datashow that the Mulhouse High consists of theSierentz and the Altkirch tilted blocks, separatedby the Ferrette fault zone with a throw of severalhundred meters. During the Eocene-early Rupelianmain rifting phase, the Mulhouse High was theelevated flank of the Dannemarie basin withsedimentation affected by the nearby southernboundary of the Rhine Graben. The Miocene Juracompression extended into the Mulhouse High,creating a series of semiparallel buried folds withno evidence for thin skin tectonics. The change to aNW-SE regional compressive stress reactivated themain graben faults as wrench faults. It also resultedin transpression on the NE-SW trending faults,particularly the northern segment of the Illfurth faultzone. Consequently, the northern part of the MulhouseHigh was uplifted. Strike-slip motion along the Ferrettefault zone is shown to be small. Citation: Rotstein, Y.,

M. Schaming, and S. Rousse (2005), Structure and Tertiary

tectonic history of the Mulhouse High, Upper Rhine Graben:

Block faulting modified by changes in the Alpine stress regime,

Tectonics, 24, TC1012, doi:10.1029/2004TC001654.

1. Introduction

[2] The area south of Mulhouse, France, has long beenknown to be elevated relative to the western and easternsides of the Upper Rhine Graben (URG, Figure 1). Itappears to bifurcate the URG at its southern end to twoseparate basins, the Dannemarie and Sierentz basins [Van

Wervecke, 1913; Theobald, 1950; Sittler, 1969; Raymondand Theobald, 1975; Theobald et al., 1977a]. Althoughrecently it was referred to as the ‘‘Mulhouse Bulge’’ orthe ‘‘Mulhouse Swell’’ [Schumacher, 2002], the centralpart of the southernmost URG is usually known as the‘‘Mulhouse Horst’’ [e.g., Van Wervecke, 1913; Sittler,1969; Theobald et al., 1977a; Vogt, 1992; Liaghat etal., 1998; Lutz and Cleintuar, 1999; Lopes Cardozo andGranet, 2003]. Niviere and Winter [2000] discussedrecently the structure of this area and noted that it isnot a horst. Instead, they suggested that it was uplifted bycompression related to the propagation of Jura decollementfurther north into the URG. In this work we will use the termMulhouse High (MH), which does not infer a specificgeologic structure. Surface geology of the MH was mapped[Ruhland and Blanalt, 1973; Theobald et al., 1976, 1977b];however, in the compilations of regional Tertiary geology ofthe URG it is sometimes considered as an area of no data[e.g., Doebl and Olbrecht, 1974]. A more recent compi-lation of Alsatian geology [Wannesson, 1998] includedalso the MH, but concentrated on the Mesozoic history ofthe graben and did not describe its development duringthe Tertiary.[3] The location of the MH, along the boundary

between the URG and the Alpine deformation front,makes it particularly interesting for the understanding ofthe interactions between these two dominant tectonicprovinces. Moreover, this general area is known to beseismically active [Ahorner, 1970; Bonjer et al., 1984;Deichmann, 1992; Bonjer, 1997; Plenefisch and Bonjer,1997] and is adjacent to the sites suggested for the 1356Basel earthquake [Mayer-Rosa and Cadiot, 1979; Meyeret al., 1994; Meghraoui et al., 2001], the largest earth-quake in central Europe during the last millennium.[4] In general, the internal structure of the URG was

mostly determined using borehole and seismic reflectiondata, collected over several decades in the course of oilexploration campaigns. However, most of the availableseismic data were collected in earlier times, and in manyinstances yielded poor data that left room for a widerange of interpretations. Newly released industrial seismicdata from the southernmost URG (Figure 1), acquired in1985 (Euromin) and 1987 (Enterprise Oil), are of superiorquality. Two lines from the 1987 data set were used

TECTONICS, VOL. 24, TC1012, doi:10.1029/2004TC001654, 2005

1Also at Geophysical Institute of Israel, Lod, Israel.2Now at United States-Israel Binational Science Foundation, Jerusalem,

Israel.

Copyright 2005 by the American Geophysical Union.0278-7407/05/2004TC001654$12.00

TC1012 1 of 15

earlier by Niviere and Winter [2000]; however, thecomplete set has never been discussed before. It revealsimportant details of the internal structure of the southern-most URG and throws light on the relations between theURG and the Alpine front. It does not include data fromthe entire MH but it covers well the southern part, whichis the more significant for understanding the response of

the URG to the changes in the regional stress regimeimposed by changes in the Alpine front.

1.1. Geologic Setting

[5] The URG is part of the European Cenozoic riftsystem, which extends for over 1100 km from the Mediter-ranean to the North Sea [e.g., Ziegler, 1992]. It was the

Figure 1. Location map superimposed on the digital terrain map of the area, showing the 1985 (Blines), 1987 (C lines) and 1988 (D lines) seismic surveys that are discussed in the paper. Also shown arethe locations of the older seismic lines (A lines) and some of the deeper wells that were used forcorrelation (KNO 1, Knoeringue 1; SUN 201, Sundgau 201; SUN 203, Sundgau 203; SPE1,Spechbach 1; GAL 1, Galfingue 1). Seismic sections that are shown in this work are denoted by boldlines; newly released lines are denoted by thick lines, and older lines are denoted by thin lines.Ferrette, Le Glaserberg, Leymen and Blauen are Jura folds. The inset shows the location of the areawith respect to the major regional units. Abbreviations are as follows: BG, Bresse Graben; VM,Vosges Mountains; URG, Upper Rhine Graben; BF, Black Forest Mountains.

TC1012 ROTSTEIN ET AL.: TECTONICS OF THE MULHOUSE HIGH, UPPER RHINE GRABEN

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target of numerous geological and geophysical investiga-tions for over a century and the results were summarized inseveral reviews [Illies, 1974; Teichmuller and Teichmuller,1979; Pflug, 1982; Huttner, 1991] and more recently bySissingh [1998, 2003], Lutz and Cleintuar [1999], andSchumacher [2002].[6] The MH usually includes the triangle-shaped area

between the town of Mulhouse, in the center of thesouthernmost part of the URG, and the Jura Mountainsto the south (Figure 2). To the west is the Dannemariebasin separated from the MH by the large and well-known Illfurth fault zone with a throw of over 1000 m atsome places [Niviere and Winter, 2000; Le Carlier deVeslud et al., 2004]. To the east, the boundary of the MHis considered as an active fault [Rouland et al., 1980]along the Quaternary Rhine river terrace. A smallerSierentz graben [Doebl and Olbrecht, 1974], or a tiltedblock [Breyer, 1974], was suggested to lie east of theRhine terrace. However, Niviere and Winter [2000] notedthat such a normal fault along the terrace is not observedin the seismic data. Thus, in an E-W section, thesouthernmost URG now appears to consist of two tiltedblocks, leading to the large and deep Dannemarie basin.[7] Stratigraphic information is available from the sev-

eral wells in the area (Figure 1) and is summarized inFigure 3. The deepest well is KNO 1, located in thecenter of the MH, which reached basement at a depth of2150 m. The well penetrated an unusually thick Jurassicsection of some 700 m that includes almost 150 m of theGrande Oolithe limestone. This sequence is well knownthroughout the graben and is usually observed as anexcellent seismic marker also in the data presented byLutz and Cleintuar [1999] further north. The Jurassicrocks are exposed on the surface in the Jura folds,immediately to the south of the MH. The top of theJurassic is an unconformity overlain by marls and con-glomerates of the Eocene (Lutetian; 47–44 Ma) Side-rolithic formation, which is usually considered as theearliest graben deposits [e.g., Sissingh, 1998]. Followinga late Lutetian and Bartonian hiatus [Sissingh, 2003], it isoverlain by early Rupelian (Sannoisian; 35–31 Ma) sand-stones, limestones and marls, mostly deposited in lacus-trine environment [Duringer, 1988], which represent themain phase of rifting and deposition in the southern URG[Sissingh, 1998; Schumacher, 2002]. Lately, Sissingh[2003] compiled the available data and suggested thatthe main rifting phase started already in the Priabonian(37 Ma).[8] Following the main rifting episode the URG sub-

sided thermally, except for a period of pronounced upliftin middle Miocene (14–10 Ma [Roll, 1979]). This upliftwas accompanied by an isolated burst of volcanism,which formed the Kaiserstuhl shield volcano, about50 km north of the MH. Postrift sediments includemiddle and late Rupelian (Grey series; 31.5–28.5 Ma)and, at places, Chattian (28.5–24 Ma). However, Chattiansediments are mostly absent together with some of theunderlying Rupelian rocks, because of erosion induced bysubsequent Miocene uplift [Sissingh, 1998; Schumacher,

2002]. This uplift and associated erosion is considered asthe result of Alpine compression [Sinclair et al., 1991;Laubscher, 1992], mantle uplift [Illies, 1977; Villemin etal., 1986] and at places transpressional reactivation of theBurgundy Transform Zone [Schumacher, 2002].

1.2. Seismic Data

[9] Both seismic surveys that are discussed in this workwere collected using similar parameters (Table 1),designed to illuminate the depth interval equivalent tothe thickness of the graben sediments observed in theKNO 1 well. The use of 30 m group interval and sweepfrequency range of 12–96 Hz facilitated the collection ofdata with quite high vertical and horizontal resolutions.Near stations were at 60–75 m, enabling the acquisitionof relatively shallow data, as observed in many of thesections. It is generally possible to trace the seismicmarkers up to a two way time of less than 0.1 sec,equivalent to a depth of approximately 100 m. Thestandard commercial land processing that was applied tothe data at the time yielded good seismic sections, partic-ularly for the unmigrated sections. Two seismic markersare used to analyze the Tertiary history of the MH: theerosional unconformity at the top of the Jurassic sequence,separating it from the Eocene rift sediments (J), and thetransgressional unconformity in the middle Rupelian (R)that marks the end of the main rifting event in the southernURG [Sissingh, 2003]. Identification of the J marker onthe MH is based on the surface exposure of the Jurassicsequence on the northern slopes of the Jura anticlinesand the apparent unconformity that is associated with it(Figures 2 and 4). The R marker is identified by theexposed boundary between the synrift and postrift sedi-ments further north on the MH (Figure 5) and its charac-teristic appearance throughout the southern and centralURG [see Lutz and Cleintuar, 1999]. Additional Mesozoicmarkers are shown in the sections for the sake ofcompleteness. Thicknesses of the Mesozoic layers areknown from the KNO 1 well, but identifying them onthe seismic sections requires knowledge of the seismicvelocities. None of the wells on the MH itself hadvelocity information, but the Bellemagny 1 well (BLY-1,Figure 2) in the adjacent Dannemarie basin, had suchinformation. Prerifting sequences are mostly Mesozoicplatform sediments and they have been known to be quiteconstant across much of the URG [Lutz and Cleintuar,1999]. Consequently, the BLY-1 data proved very usefulfor the identification of the Mesozoic markers. Thevelocities that were used to convert the time to depthwere 2200 m/s for the postrift sediments and 3500 m/s forthe synrift sediments, based on the BLY 1 data. The KNO1 well on the MH was used to further confirm theidentifications of the markers, but as it is in a fault zone(Figures 5 and 6), it was not used as the main identifi-cation tool.[10] Additional, older seismic data are also available

from this area (Figure 1). These are usually also 24 folddata, but are of a much lower quality in terms of reflectorcontinuity and resolution. Thus the older data were used


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only in areas where new data were not available and insuch areas, only to extend structures that were identifiedin the new data.

2. Observations

2.1. Rift Structures

[11] Since the MH is at the boundary between the URGand the Jura fold system, its structure is likely to reflect bothof these tectonic episodes. The formation of the N-Strending URG was the earlier event and the structuresassociated with it are best observed in E-W sections. Severalsuch seismic sections are available, describing a history ofblock faulting as expected in a rift environment. Figure 5 isa composite of three lines, partially overlapping, thattogether cross the entire MH at its center. The main featurethat is observed in the eastern part is a tilted block that we

name the Sierentz block, with an apparent dip to the east. Asthis composite line extends to the river Rhine it crossed theRhine river terrace. No fault is associated with the terrace,as was noted by Niviere and Winter [2000], but can be seenmore clearly in the present data. We note also that theBouguer gravity data compiled by Edel et al. [2002] showsthat no gravity gradient is associated with the Rhine riverterrace, further indicating the absence of a normal faultthere.[12] The Sierentz block does not extend all the way to the

Illfurth fault zone and the Dannemarie basin. Instead, itappears that the MH is crossed at its center by a large fault,semiparallel to the Illfurth fault zone, which we name theFerrette fault zone. In the composite section, the Ferrettefault zone is masked by an old river channel; nevertheless, itis seen in other seismic sections from the area (Figure 6),and in older seismic lines further south (Figure 7). TheFerrette fault zone down-faults a second tilted block that

Figure 2. Simplified geological map of the MH and the southern URG. Note the exposure ofsyntectonic sediments on the NW part of the MH and the bend in the Illfurth fault zone. Also shown is theFerrette fault zone, the Altkirch block, and the Sierentz block, which is discussed in the text. Pre-Tertiarysediments in the mapped part of the Jura are of Jurassic age.


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occupies the western part of the MH, and is named here theAltkirch block. The Ferrette fault zone appears to be a highangle fault zone and the vertical throw on it can beestimated from the base Tertiary map (Figure 8), to be over400 m in some parts of the southern MH. Farther south, theFerrette fault zone is observed in all the old seismic linesthat cross it, as a series of large and mostly high angle faults,with a normal component. It extends along the western frontof the Ferrette fold (Figure 2), indicating that the stronglycurved arc shape of this Jura fold is controlled by thepresence of the older fault (Figures 1 and 7). Thus, thispart of the Ferrette fault zone, along the Ferrette fold, islikely to have presently been inverted, or have a thrustcomponent superimposed on the original normal fault. Thesouthern extent of the Ferrette fault zone inside the Jura foldsystem is not known. However, the unusual concentration ofsurface faults with the same overall trend that are knownsouth of the Ferrette fold (Figure 2) may be related to thesouthward extension of this Eocene/early Oligocene fault.[13] Toward the north, the throw on the Ferrette fault

zone reduces and is in the order of 200 m in the center of theMH. To the north of the mapped area, where information isavailable only on the Altkirch block, the contour pattern in

the base Tertiary map suggests that the throw decreasesfurther in this direction. Some 10–15 km north of Altkirch,dip slip component may be absent completely. In thenortheastern corner of the MH, some faults are observed,but there is no indication of a large Ferrette fault with a dipslip component (Figure 9). Around station 300 of line D1we interpret a vertical fault having a simple flower structureand with a different reflector pattern across it, suggestingthat it is a strike-slip fault. This fault may be the extensionof the Ferrette fault northward. However, even so, the little

Table 1. Seismic Acquisition Parameters of the Data Used in This

Work

Parameter Value

Number of channels 48VP Interval 30 mStation interval 30 mFold 2400%Source vibratorsFrequencies 12–96 HzGeometry symmetrical split (65–1355 m)

Figure 3. Simplified stratigraphic section of the MH, based on the KNO 1 well. A generalized Tertiaryrecord for the MH is shown in more detail.


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Figure 4. Migrated seismic section of lines C2 and C3 across the northernmost Jura and the southernMH. Zero time (reference elevation) is at 350 m. Letter labels are as follows: R, middle Rupelian (end ofrifting); S, base of upper salt sequence; J, base Tertiary/top Jurassic (start of rifting); O, Grande Oolithe;LT, Liassic or top Triassic; M, Mushelkalk formation; P, top Permian; B, basement. See color version ofthis figure at back of this issue.


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deformation that is associated with it indicates that this isnot a large fault as further south. Thus, the Ferrette faultzone appears to die out in the northern part of the MH andthe division into two blocks characterizes only the southernand central parts of the MH and its extension into the Jura.[14] The western boundary of the MH is the well-

known Illfurth fault zone. The main fault appears to be alistric normal fault at Mulhouse (Figure 9). To the south,in the MH and Dannemarie basin, the Illfurth fault zoneis crossed by numerous, closely spaced new lines, whichshow it to be a wide and large fault zone, as is presentlywell known. We note that in the northern part of theDannemarie basin and the MH, the fault is trending NE-

SW and is associated with compression. This is evidentby the folding in the Dannemarie sediments adjacentto the fault, as well as in the positive flower structureand other compression features in the fault zone itself(Figure 10). In some sections the compression appears tobe so intense that the original and characteristic normalfault features are hardly observed (Figure 10b). Thiscompression is not a secondary feature of the originalnormal faulting since the folding is apparent in thepostrift sediments. Farther south, the well-known normalfault characteristics of the Illfurth fault are clearly seenand there is no evidence for superimposed youngercompression (Figures 5 and 11).

Figure 5. A composite seismic section across the MH, showing the present tilted block structure of theMH. For locations of the three parts of the lines that are used to construct the figure (B27, C5, and B3),see Figure 1. Note that whereas the Illfurth fault zone is quite apparent, the Ferrette fault zone is notclearly observed on this section, as it is masked by a river channel and because in this area the faultappears to partially heal. Other sections showing this fault are presented in Figures 5 and 6. Note also thewedge of synrift sediments on the eastern part of the Altkirch block that is discussed in the text. Fordetails see Figure 4. See color version of this figure at back of this issue.


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[15] In the southernmost part of the MH, new data aboutthe Illfurth fault is not available. However, as this fault is amajor feature in the southernmost URG, it was alreadytraced in the older seismic lines and using well correlation[e.g., Doebl and Olbrecht, 1974; Breyer, 1974; Sittler,1974]. The details of this part of the fault zone are generallynot agreed upon and while it has generally an NNE-SSWtrend, it is sometimes divided to several faults, including aprominent N-S trending fault [e.g., Niviere and Winter,2000; Edel et al., 2002]. Farther south, in the tabular Jura,the extent of the fault is not known but surface faults withthe same trend have been mapped (Figure 2).

2.2. Jura Structures

[16] The Alpine push in the Miocene created the Jurafold system that abuts the MH to the south. The effect ofthis tectonic episode in the MH can be best examinedthrough N-S lines. Two of these lines that extend deep intothe MH from the Jura proper, where they cross the Ferretteand Le Glaserberg folds, are shown in Figure 4. Someof these data were partially described by Niviere andWinter [2000] and Rotstein and Schaming [2004], whoconcluded that Jura compression extends deep inside theMH. However, these works mostly discussed the roleof thin/thick skin tectonics in the MH and did not makea full use of the available data for the study of the Jurafolds on the MH. We have mapped the depth to the R

Figure 6. A part of migrated seismic section B1, showingthe wide Ferrette fault. Note the opposing dips across thefault, which is similar to a flower structure architecture,suggesting strike slip. For location see Figure 1 and fordetails see Figure 4. See color version of this figure at backof this issue.

Figure 7. Older seismic section A2 across the Ferrette fault zone and the Ferrette Jura anticline. Forlocation see Figure 1.


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reflector, which marks the end of the rifting period and isthe uppermost and youngest marker that can be reliablymapped (Figure 12). This marker is well determined on theSierentz block and in the Dannemarie basin. It is not asaccurately determined in some of the lines on the Altkirchblock, where it was uplifted to a shallow position and, atplaces, can only be estimated. However, even with theinherent approximation of the shallow structure on parts ofthe Altkirch block, the overall structural pattern is quiteclear. The same pattern is observed also in the map of thedepth to the base Tertiary (Figure 8). The Jura relatedcompression is expressed by open folds, as well as someminor thrusts (Figure 4) that are much less intensive thanin the Jura itself. The folds appear to be semiparallel to theJura folds and have no correlation with the surface ramps,suggested by Niviere and Winter [2000].[17] The internal reflections in the postrift Rupelian/

Chattian sequence in the N-S lines are mostly not clear.However, wherever postrift reflections above the Rupelianmarker are identified, they show a similar dip as the synriftand prerift sediments. This suggests that the observedfolding is a post Chattian event, probably related to theJura buildup as is also indicated by their lateral continuitywith the Jura folds. However, the present data does notresolve the age of this post Chattian folding.

2.3. Sedimentary Record

[18] Generally, basement is not clearly observed in theseismic sections and, possibly, can be speculated only at

some places, where a strong seismic reflector is observedat depth (Figure 5). Thus, it is not presently clear whetherthe thickness of the Paleozoic sequence is uniform overthe MH. The gravity map [Edel et al., 2002] shows agradient toward an apparent gravity low in the Jura. Thislow is likely to be associated with deepening of thebasement, possibly toward the center of the large Paleo-zoic graben, which is known from northern Switzerland[e.g., Laubscher, 2001]. The Triassic is observed in theseismic data to have a constant thickness throughout theMH as has most of the Jurassic, which overlies it in aconcordant fashion. The Cretaceous is missing in wellson the MH and the Dannemarie basin, either because ofnondeposition or to erosion [Schumacher, 2002]. Thepeneplained Jurassic is overlain by the Eocene and earlyOligocene synrift sequences. On the Sierentz block thesesequences are characterized by very strong and somewhatirregular reflectors (Figures 4 and 5). The thickness of thesynrift sediments on the Sierentz block is nonuniform,thickening toward the north and locally toward the east(Figure 13). We note that the thickness is mostly reducedsouthward, as the Jura is approached, and not toward thewest and updip of the tilted block, as might have beenexpected. This thinning is not related to postrift erosioninduced by Jura uplift. The Jura uplift is a younger event,which did not result in the erosion of the entire postriftsediments that protect the underlying synrift sediments(Figure 4). Where the synrift sequence is thicker, somepossible onlaps suggest that these areas were somewhatbasinal. Moreover, at some places, upward truncationappears in the elevated areas, suggesting some possiblesynrift erosion. The strength of some of the reflectors inthis sequence and their irregularity also suggests thatthese may be erosional unconformities. Finally, the re-duced thickness of the early synrift sediments (lowerSaliferous zone; 35–34 My) in the KNO 1 well, ascompared to its thickness in wells elsewhere on theblock, also suggests synrift erosion in parts of theSierentz block.[19] On the Altkirch block a detailed analysis is not

possible since the data is scarcer, and is estimated atplaces. However, the emerging pattern is quite differentthan in the adjacent Sierentz block, suggesting that inthis part of the Altkirch block, there are no largechanges in synrift sediment thickness, which is some300–400 m. In the northern part of the Altkirch block,this thickness is similar to the observations from theadjacent part of the Sierentz block, suggesting that thenormal component of the Ferrette fault dies out. Towardthe south, data are limited; however, the availableseismic data and the SUN 203 well suggest thatthe synrift sedimentary thickness is larger than on theequivalent part of the Sierentz block. We note that thesynrift sediments on the Altkirch block appear to bequite concordant in an E-W direction, with no noticeableonlaps and unconformities. If any dip of the block ispresent, it may actually be toward the west, as notedfrom the wedge shaped sequence on the Atkirch block(Figure 5). Thus, in spite of the apparent tilting which is

Figure 8. Map showing the depth of the base Tertiarymarker (J). Reference elevation is sea level and deeper areasare positive while negative values denote areas above sealevel. Thin lines denote the available seismic lines.


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Figure

9.

Migratedseismic

sectionD1,showingtheIllfurthfaultin

theMulhouse

area.Alsoshownis

thepossible

extensionoftheFerrettefaultto

theNEpartoftheMH.Notethelack

ofsimilarityin

thesectiononthetwosides

ofthis

fault,suggestingstrikeslip.ForlocationseeFigure

1andfordetailsseeFigure

4.


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presently observed on this block, the synrift thicknessacross the block appears to be constant.

3. Discussion

[20] The new seismic data from the MH constitute animportant step toward incorporating the MH in models of thestructure and history of the southern URG. It confirms thatthe MH is not a horst, but consists of two tilted blocks, themost fundamental building units of grabens [Rosendahl et al.,1986; Rosendahl, 1987; Ebinger, 1989]. These blocks, to-gether with the block between the Rhine Valley flexural faultand the Allschwill fault (Figure 2), form a series of tiltedblocks leading to the main early Tertiary depocenter in theDannemarie basin. The blocks all appear to have an apparentdip to the east, and thus each is presently associated with asmall Tertiary basin at its eastern side.[21] Several lines of evidence suggest that during rifting,

both the Sierentz and the Altkirch blocks were nearly

horizontal in an E-W direction, perpendicular to theirapproximately N-S trend. The northward thickening of thesynrift sediments on the Sierentz block is in the strikedirection of the URG and is not related to conventionalblock tilting. In addition, the postrift sequence on theSierentz block appears to be tilted together with the under-lying sediments (Figure 5), also suggesting that the tilting isa postrift event. On the Altkirch block, in an E-W section,the reflectors do not show the onlaps that are expected witha syntectonic block tilting. If any thickening is observed it isin the opposite sense, with an increased thickness towardthe uplifted western part of this block (Figure 5).[22] A schematic E-W cross section of the synrift and

postrift geometry of the blocks is shown in Figure 14. Thelowest block is the Dannemarie basin where most sedimentswhere deposited. The extension across the rift was accom-modated differently in the north and south of the MH. In thenorth, the MH was not faulted much internally, and theFerrette fault zone had only a small dip slip component, if

Figure 10. Migrated seismic sections across the Illfurth fault zone (IFZ), showing the compressionfeature. For location see Figure 1 and for details see Figure 4. (a) Line C19 showing folded sediments inthe Dannemarie basin next to the fault. (b) Line C20 across the Illfurth fault zone. Note a small synclinein the Dannemarie basin and what appears to be a positive flower structure in the fault zone itself. Seecolor version of this figure at back of this issue.


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any. Consequently, all of the extension and dip slip occurredon the Illfurth fault zone. In the south, we suggest that muchof the extension was accommodated by the Ferrette faultzone and there was a stepwise drop from the MH, into thedeep Dannemarie basin.[23] The syntectonic sediments in the URG are mostly

lacustrine sediments deposited in shallow water [Duringer,1994]. Thus it is likely that during the rifting episode, theelevated MH, or large parts of it, were above water someof the time. During these periods the sediments that werepreviously deposited, must have been eroding. Conse-quently, the original cumulative thickness of sedimentsdeposited on the MH is likely to have been larger andsome reworked sediments of the MH must have found theirway to the lower parts of the region. The unconformitiesthat are associated with these periods of erosion alsoaccount, most probably, for the strong and irregular seismicreflectors in the synrift sequence.[24] Following the end of the Eocene-early Rupelian main

rifting period, sedimentation continued into the Rupelianwith the Grey series, the first marine transgression into theURG [Duringer, 1988; Sissingh, 1998]. On the Sierentzblock, the synrift and postrift sedimentation appears to bemostly concordant suggesting that there was no majortectonic event at the end of the main rifting episode in thesouthern URG. Where data is available on the Altkirchblock, much of the same sediments were subsequentlyuplifted and eroded.[25] The next stage in the tectonic development of the

region started with the development of the Jura fold system

Figure 11. Migrated seismic section C11 across theIllfurth fault zone, where it is not under compression. Forlocation see Figure 1 and for details see Figure 4. See colorversion of this figure at back of this issue.

Figure 12. Depth map of the middle Rupelian (R) marker,associated with the end of the rifting period and grabenformation. Depth values are in meters. Reference elevationis sea level, with positive values below sea level andnegative values above sea level. Thin lines denote theseismic lines used for constructing the map.

Figure 13. Thickness map of the synrift sediments (inmeters). Thin lines denote the seismic lines used toconstruct the map.


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and the associated change in the regional stress regime,resulting from changes in Alpine tectonics. Initially, thegraben was formed under an E-W [Villemin and Bergerat,1987] to WNW-ESE [e.g., Laubscher, 2001] extension, orN-S compression in Priabonian, which changed into nearlyE-W extension in the Rupelian [Schumacher, 2002]. Thelargest principal stress s1 may have changed in the Chattianto NE-SW and started counterclockwise rotation in the earlyMiocene, first to NNW-SSE and later to the present trend ofNW-SE [Schumacher, 2002]. The exact timing of theMiocene change in the stress regime is still somewhat inquestion. Thus it is not clear if it is related to the onset of theJura folding that was recently determined to be 10.5 Ma inthis area [Giamboni et al., 2004]. The Jura compressionresulted in the development of several folds on the MH,trending mostly E-W, parallel to the prominent Ferrette, LeGlaserberg and other Jura folds further south (Figure 1). Thefolds on the MH are subsurface structures that are lessintensely folded and faulted than the known surface folds inthe Jura proper. With the present data, we can documentonly several buried Jura folds, but additional folds may bepresent further to the north.[26] Jura folding and faulting is usually associated with

thin skin tectonics [e.g., Buxtorf, 1907; Laubscher, 1961;Tschanz, 1990; Jordan, 1992; Hindle et al., 2000]. Sedi-ments overlying thick Triassic evaporates in the Jura aredisplaced on a low-angle detachment fault in these evapo-rites, resulting in a repetition of much of the overlyingsedimentary sequence. The same mechanism was recentlysuggested by Niviere and Winter [2000] to control also thetectonism in the MH. They used the previously available

seismic reflection data and the topography to support adecollement beneath the MH. We find no indication in thepresent data for thin skin tectonics in the MH. We also notethat the topography in the MH is a poor indicator for thesubsurface structure and we find no subsurface ramps in thelocations that were predicted and used by them to suggestdecollement. In contrast to the central Jura, the observedfaults are mostly high angle faults and appear to cross theTriassic with no apparent change in character. We also notethat the Triassic in the KNO 1 well does not contain massiveaccumulation of evaporites, but rather thin beds distributedthroughout large parts of it. It is doubtful that the amountand mode of distribution of these evaporites make it asuitable lubricant for the horizontal transport of the overly-ing sediments. The seismic data also suggests that thefaulting pattern in the Ferrette and Le Glaserberg Jura foldsis not consistent with thin-skin tectonics there [Rotstein andSchaming, 2004].[27] The NW-SE compressive stress that presently char-

acterizes the URG [e.g., Becker, 1989; Schumacher, 2002]is consistent with the seismic source studies. These showleft-lateral strike-slip motion to be the dominant faultmechanism on the N-S trending faults in the graben[Ahorner, 1975; Villemin and Bergerat, 1987; Plenefischand Bonjer, 1997]. Strike-slip faulting is difficult to identifyin seismic sections. The most common approaches are tosearch for flower structures [Harding, 1985, 1990] and forvertical faults, which are more often characteristic of strike-slip faults, rather than of dip slip faults. In the present dataset from the MH, many faults are nearly vertical (Figures 5,6, and 10), suggesting some strike-slip faulting. In addition,

Figure 14. Schematic cross section across the MH, showing the stages in the development of theSierentz and Altkirch tilted blocks. For location see Figure 2. Not shown is the Miocene Jura foldingphase, as it is observed mainly in a perpendicular section.


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in several northernmost lines that cross the Illfurth fault, wefind a record of local compression associated with the faultzone, while lines further south do not show such compres-sion. The compression is unrelated to the rifting, affectingalso the postrifting sediments. It is also not related to Jurapush, because we observe it only locally in part of theIllfurth fault zone, and not in other parts. We note thatthe compression is most evident in the northern part of theIllfurth fault zone that appears to have a NE-SW trend, ascompared with the N-S trending segment further south. Asimple explanation for the compression results from thepresently accepted idea that the large faults of the URGhave turned into strike-slip faults sometime in the Miocene.Left-lateral strike-slip motion on the Illfurth fault zone musthave resulted in transpression on the NE-SW trending,northern segment of the fault. Transpression across strike-slip faults is, of course, a common phenomenon, as thefaults are seldom completely straight. Recently, Schumacher[2002], who has described the general tectonic setting andthe development of the URG, predicted the existence ofrestraining bends and transpression in the URG. There is noneed for a large horizontal slip on the fault in order to causeprominent compressional structures. For example, the smallleft-lateral slip on the Carmel fault in northern Israelresulted in an uplift of several hundred meters of Mt.Carmel at a restraining bend [Rotstein et al., 1993]. In theMH, the transpression resulted in the uplift of the NW partof the MH, creating the tilted block morphology. The upliftwas accompanied by an increased erosion, leading to thesurface exposure of the synrift sediments on the NW part ofthe MH (Figures 2 and 14). Further north, near the town ofMulhouse, the trend of the Illfurth fault zone is not apparentand the structure may be affected by other tectonic ele-ments. Similarly, the details of the southern part of theIllfurth fault zone are not clear and similar analysis there isnot possible.[28] The nearly vertical geometry of most of the faults in

the Ferrette fault zone suggests that it is currently a wrenchfault zone. There is little doubt that the Ferrette fault zonewas an important element in the Oligocene rift. However,unlike the Illfurth fault zone, which appears to be animportant element also in the present tectonic setting, thepostrifting role of the Ferrette fault is not clear. It is likely tobe an active element in the southern part of the MH, but inthe northern part it is a small feature, if it extends there atall.[29] Although strike-slip faulting in the URG is widely

accepted, there is little quantitative information about thehorizontal slip on the individual faults. One way to measureit may be through an accurate mapping of the axis ofthe folds on the MH. The present map of the R marker(Figure 12) suggests that the folds on the MH are notdisplaced by the Ferrette fault. Thus the horizontal slip onthis fault is small.[30] The horizontal extent of the blocks in the southern-

most URG during rifting is not clear yet. The decrease in thethickness of the synrift sediments on the Sierentz blocktoward the south, to a mere 100 m, indicates that thesouthern boundary of the rift is likely to be quite close. It

also suggests that the southern end of the URG in this areawas not controlled by a major fault and that the presentelevation difference between the Jura and the MH is notentirely due to Miocene Jura compression and buildup.Instead, it is likely to have been partially inherited, as isalso indicated by the northward flow of synrift paleocur-rents in the Ferrette region [Duringer, 1988].[31] The analysis of seismic hazard in the area is closely

linked to the understanding of the active tectonics, and inparticular, to the active fault distribution and rate of slip.The larger earthquakes are expected to occur on the largerfaults in this area, mainly the Illfurth and, possibly, also theFerrette fault zones. In addition, restraining bends tend toinhibit creep by increased friction on the faults because ofthe compression and, thus, are associated with fewer, butlarger earthquakes. There is evidence that the MH wasuplifting as late as the Quaternary [Schumacher, 2002].Such uplift is likely to have occurred mostly as a by-productof strike-slip faulting. If so, it suggests that there was recentslip on the Illfurth fault and that it constitutes a seismichazard.[32] Finally, we note the presence of deep river channels

in some locations, with the most obvious ones coincidingwith the Ferrette fault zone in the center of the MH(Figure 5). The age of this river cannot be determined,but it appears to cut Rupelian rocks. The observation thatit appears in the fault zone and in an E-W sectionsuggests that it flowed along the fault and that, at thattime, there was no vertical offset across the Ferrette faultzone and that it was not active any more as a normal fault.

4. Conclusions

[33] We find that the MH was a simple, relativelyuncomplicated part of the URG, which was later affectedby the changing Alpine stress regime. Rift related structures,the main building units of the MH, consist of two tiltedblocks, the Sierentz and Altkirch blocks that were notdeformed much internally. The Sierentz block is the easternand largest of the two, occupying most of the MH anddipping presently to the NE. It is separated from theAltkirch block by the large Ferrette fault zone, which is adominant structure in the southern MH, but may be insig-nificant in the northern part of the MH. The presentgeometry of the blocks appears to have been acquired morerecently, by the Jura folding and subsequent strike-slipfaulting. Jura compression extended from the Alps intothe MH, resulting in a series semiparallel buried folds.The recent NW-SE compression in the area resulted instrike-slip motion and local transpression. The transpressionaffected most strongly the northern segment of the Illfurthfault zone, and may have caused the uplift of the NW part ofthe MH, setting set the stage for the extensive erosion there.

[34] Acknowledgments. The authors wish to thank Euromin andEnterprise Oil for making their seismic data available to us. The authorsalso thank P. Duringer and M. Lutz for valuable discussions. Y. R thanksCNRS for financial support. This work was carried out as part of theEUCOR-URGENT project. This is EOST contribution 2004.019-UMR7516.


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Figure 4. Migrated seismic section of lines C2 and C3 across the northernmost Jura and the southernMH. Zero time (reference elevation) is at 350 m. Letter labels are as follows: R, middle Rupelian (end ofrifting); S, base of upper salt sequence; J, base Tertiary/top Jurassic (start of rifting); O, Grande Oolithe;LT, Liassic or top Triassic; M, Mushelkalk formation; P, top Permian; B, basement.

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Figure 5. A composite seismic section across the MH, showing the present tilted block structure of theMH. For locations of the three parts of the lines that are used to construct the figure (B27, C5, and B3),see Figure 1. Note that whereas the Illfurth fault zone is quite apparent, the Ferrette fault zone is notclearly observed on this section, as it is masked by a river channel and because in this area the faultappears to partially heal. Other sections showing this fault are presented in Figures 5 and 6. Note also thewedge of synrift sediments on the eastern part of the Altkirch block that is discussed in the text.


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Figure 6. A part of migrated seismic section B1, showing the wide Ferrette fault. Note the opposingdips across the fault, which is similar to a flower structure architecture, suggesting strike slip. For locationsee Figure 1 and for details see Figure 4.


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Figure 10. Migrated seismic sections across the Illfurth fault zone (IFZ), showing the compressionfeature. For location see Figure 1 and for details see Figure 4. (a) Line C19 showing folded sediments inthe Dannemarie basin next to the fault. (b) Line C20 across the Illfurth fault zone. Note a small synclinein the Dannemarie basin and what appears to be a positive flower structure in the fault zone itself.


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Figure 11. Migrated seismic section C11 across the Illfurth fault zone, where it is not undercompression. For location see Figure 1 and for details see Figure 4.


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Structure and Tertiary tectonic history of the Mulhouse High, Upper Rhine Graben: Block faulting...

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