Late Carboniferous foreland basin formation and Early Carboniferous stretching in Northwestern Europe: inferences from quantitative subsidence analyses in the Netherlands

Late Carboniferous foreland basin formation andEarly Carboniferous stretching in NorthwesternEurope: inferences fromquantitative subsidenceanalyses in the NetherlandsHenk Kombrink,nw Karen A. Leever,z Jan-Diederik van Wees,wz Frank van Bergen,wPetra Davidw and Theo E. WongnwnFaculty of Geosciences, Department of Earth Sciences, Utrecht University, Utrecht,The NetherlandswTNOBuilt Environment and Geosciences, Geological Survey of the Netherlands, Utrecht,The NetherlandszNetherlands Centre for Integrated Solid Earth Sciences (ISES),Vrije Universiteit, Amsterdam,The Netherlands

ABSTRACT

The large thickness of Upper Carboniferous strata found in the Netherlands suggests that the areawas subject to long-term subsidence. However, the mechanisms responsible for subsidence are notquanti¢ed and are poorly known. In the area north of the London BrabantMassif, onshore UnitedKingdom, subsidence during the Namurian^Westphalian B has been explained byDinantian rifting,followed by thermal subsidence. In contrast, south and east of the Netherlands, along the southernmargin of theNorthwest EuropeanCarboniferous Basin, £exural subsidence caused the developmentof a foreland basin. It has been proposed that foreland £exure due toVariscan orogenic loading wasalso responsible for Late Carboniferous subsidence in the Netherlands. In the ¢rst part of this paper,we present a series of modelling results in which the geometry and location of theVariscan forelandbasinwas calculated on the basis of kinematic reconstructions of theVariscan thrust system.Althoughseveral uncertainties exist, it is concluded that most subsidence calculated fromwell data in theNetherlands cannot be explained by £exural subsidence alone.Therefore, we investigatedwhether aDinantian rifting event could adequately explain the observed subsidence by inverse modelling.Theresults show that if only aDinantian rifting event is assumed, such as is found in theUnitedKingdom,a very high palaeowater depth at the end of theDinantian is required to accommodate theNamurian^Westphalian B sedimentary sequence.To better explain the observed subsidence curves, we propose(1) an additional stretching event during the Namurian and (2) a model incorporating an extradynamic component, which might well explain the very highwavelength of the observed subsidencecomparedwith the wavelength of the predicted £exural foreland basin.

INTRODUCTION

TheNorthwest EuropeanCarboniferousBasin (NWECB)is a large sedimentary basin extending from Ireland in thewest to Poland in the east (Fig. 1a), which developed northof theVariscanRheno-Hercynian belt. In aN^S direction,the width of the basin across western Germany and theNetherlands is up to 600 km. The maximum preservedsediment thickness of the Carboniferous succession isup to 6 km. In this paper, the term Central NWECB(CNWECB) will be used to indicate the English/Welsh,Dutch andWest-German parts of this basin.

During the Late Carboniferous (Namurian^Westpha-lian B), the NWECB (Fig. 1) extended over a large part ofnorthern andwestern Europe, accommodating deposition

of the source rocks of many of today’s gas ¢elds.Two sub-sidence mechanisms have been proposed for di¡erentparts of the basin. North of the London Brabant Massif(LBM), in the northern part of Great Britain, subsidencehas been explained by thermal relaxation following anEarly Carboniferous (Dinantian) rifting event (Leeder &McMahon, 1988; Coward, 1990; Fraser & Gawthorpe 1990;Maynard et al., 1997). In the Danish o¡shore, a Palaeozoicrifting event possibly during the Carboniferous was iden-ti¢ed on seismic data (Scheck etal., 2002). In contrast, sub-sidence along the southern margin of the NWECB hasbeen attributed to orogenic loading, resulting in a forelandbasin (Gayer etal.,1993;Warr,1993;McCann,1999; Burgess& Gayer, 2000; Drozdzewski, 2005; Kornpihl, 2005). TheNetherlands is located in a central position in theNWECB (Fig. 1), in between the areas where these twosubsidence mechanisms have been interpreted. However,an attempt to test whether these subsidence models can

Correspondence: Henk Kombrink, TNO Built, EnvironmentandGeosciences, Princetonlaan 6, 3508TAUtrecht,TheNether-lands. E-mail: [email protected]

BasinResearch (2008) 20, 377–395, doi: 10.1111/j.1365-2117.2008.00353.x

r 2008 The Authors. Journal compilation r 2008 Blackwell Publishing Ltd 377

adequately explain subsidence observed in the Nether-lands has never been made. A possible explanation for thismay be the relative scarcity of data; subsidence curves donot immediately reveal the dominance of one speci¢c sub-sidence mechanism.

There is no consensus on how far the Variscan forelandbasin extended to the north (Coward, 1993; Quirk, 1993;Sˇss,1996;Maynard etal., 1997; Ricken etal., 2000;Drozd-zewski, 2005). These authors present very di¡erent inter-pretations on the width of the Variscan foreland basin,ranging from a narrow [close to the present-day VariscanDeformation Front (VDF), Fig.1] to a very wide basin (upto Cleaverbank High). Burgess &Gayer (2000) presented amodelling study in the area south of the LBM inWales inwhich they reconstructed the migration of the Variscanforeland through time. However, quantitative modellingincorporating the existing ideas on the advance of theVar-iscan orogenic wedge (e.g. Oncken et al., 2000), with theaim to delineate the geometry of theVariscan foreland ba-sin inwesternGermany/theNetherlands, has not been car-ried out to date. In this paper, we present a simplemodelling study in which predictions on the geometryof the Variscan foreland basin in this area are given. Anumber of scenarios have been run to test the sensitivityof ^ among others ^ the thickness of the subducting plate(re£ected in elastic thickness,Te) and the shape of the loadon the resulting foreland basin geometry. Based on thesemodelling experiments, we show that the foreland basin

is too narrow and too far to the south to explain a signi¢ -cant part of the observed subsidence in the Netherlands.

Although there is no ¢rm evidence for an Early Carbo-niferous rifting event in the Netherlands and WesternGermany, several authors proposed this (Coward, 1990,1993; Bailey et al., 1993; Maynard et al., 1997; Martin et al.,2002; Ho¡mann et al., 2005). We show a seismic sectionfrom the central part of theNetherlands inwhich aDinan-tian carbonate build-up is interpreted. These structuresare well described in the United Kingdom, where they arepreferentially found on top of horst blocks developed dur-ing Dinantian rifting (Chadwick & Evans, 2005).The ¢nd-ing of such a structure in the Netherlands may thereforeindicate a similar structural setting for the Netherlands.The amount of Namurian^Westphalian B thermal subsi-dence that can be explained by a Dinantian rifting eventwill be discussed next. A number of rifting scenarios weretested, applying uniform (McKenzie, 1978) stretching.These scenarios have been run with di¡erent estimates ofthe late-Dinantian palaeowater depth (PWD), as this is animportant but uncertain factor. It turns out to be di⁄cultto explain the subsidence needed to accommodate thethick LateCarboniferous sedimentary sequence assuminga Dinantian rifting event alone.

To summarise, the aims of this paper are (1) to quantifythe geometry of the Variscan foreland basin and theamount of subsidence in theNetherlands that might be ex-plained by crustal loading; (2) to quantify the amount of

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Fig.1. (a) Location of Northwest European Carboniferous Basin, (b) detailed map of the study area showing the location ofwells usedfor this study and (c) stratigraphic table with global and local stages, timescale used in this study and the (tectonic) sequences recognisedin the northern part of England and in the Ruhr Basin (Germany).1, United Kingdom; 2,Midland Valley; 3, SouthWales; 4, Belgium;5, the Netherlands; 6, Germany; AD, Ardennes; CH, Cleaverbank High; DSHFZ, Dowsing-South Hewett fault zone; LBM, LondonBrabantMassif;MNSH,MidNorth SeaHigh; RB, Ruhr Basin; RM,RhenishMassif; study area of Ricken etal. (2000).Maps based onZiegler (1990).

r 2008 The Authors. Journal compilation r 2008 Blackwell Publishing Ltd,Basin Research, 20, 377^395378

H. Kombrinket al.

(thermal) subsidence that might be explained by a Dinan-tian rifting event and the comparisonwith subsidence datafrom theNetherlands and (3) to propose a number of alter-native subsidence models.These are (1) a model incorpor-ating dynamic topography and (2) one that assumes anadditional Namurian rifting event. Before presenting themodelling results, an extensive description of the structur-al and stratigraphic setting of the Late Carboniferousis given to guide the decisions made for the modelling ex-ercises.

BASIN EVOLUTION AND FILL

The crust on which the CNWECB developed mainlyforms part of (Eastern) Avalonia (Pharaoh, 1999; Verniersetal., 2002). Avalonia is aGondwana-derivedmicro-conti-nent that consolidatedwith Baltica during the Late Ordo-vician (Tait etal.,1997; Cocks &Torsvik, 2005).The IapetusOcean, separating Baltica/Avalonia from Laurentia, wasmainly closed during the Silurian (Caledonian orogeny);together, these continents formed the Old Red Continentor Laurussia.The Precambrian basement of Avalonia westof the Dowsing^SouthHewett fault zone (DSHFZ, Fig.1)is characterised by mildly metamorphosed arc-magmaticrocks (Pharaoh, 1999). This area is part of the Anglo-Brabant Fold Belt, of which the LBM is the eastern limb(Fig.1).The Cambro-Silurian shales of the LBMwere de-formed into low-grade metamorphosed slates during theLate Silurian^Early Devonian (De Vos, 1997).This defor-mation is related to the oblique accretion of easternAvalo-nia with Baltica and Laurentia (Sintubin, 1999).The LBMis probably underpinned by a granitic body, emplaced atthe end of the Ordovician, which may have caused thismassif to be a stable block through time (De Vos, 1997).From the area east of the DSHFZ (Lˇneburg terrane,Ver-niers et al., 2002), no information on the pre-Devonianbasement is available (Pharaoh, 1999).

Along the southern margin of Avalonia (more or less thesouthern rim of the basin indicated in Fig.1a), the Rheno-hercynian oceanic basin developed during theEarlyDevo-nian (Oncken et al., 1999). From that time, the southernmargin of the continent thus acted as a passive margin.This Rhenohercynian basin, which must have remainedrelatively narrow as it cannot be recognised by palaeomag-netics (Tait et al., 1997), started to close again during theLate Devonian by SE-directed subduction (Franke,2000). ByMiddle and LateDevonian times, Laurussia be-gan to convergewithGondwana in a dextral oblique, clock-wise rotational mode, with initial contact in the area ofIberia andNWAfrica during theFamenian (Ziegler,1990).

Devonian sediments in the CNWECB are mainlyfound in the Midland Valley of Scotland in the form ofthick molasse sequences, derived from the Caledonianmountain chains. Middle and Upper Devonian continen-tal sediments have been drilled in the northernDutch andBritish o¡shore (Mid North Sea High; Ziegler, 1990).Coastal plain sediments were deposited along the south-

ern margin of the continent in SouthWales and southernEngland (Cameron et al., 1992). In eastern Belgium (southof the LBM) andwestern Germany, a kilometre-thick De-vonian shallow marine sequence has been deposited onthe rifted continental margin of Avalonia (Oncken et al.,2000).

The Early Carboniferous (Dinantian) is characterisedby northward subduction of the Palaeo-Tethys Mid OceanRidge and southward subduction of the RhenohercynianBasin beneath the Mid-European terranes, which led tothe formation of the Variscan mountains (Franke, 2000;Stamp£i & Borel, 2002). Basin development during theEarly Carboniferous in the area north of the LBM (on-shore United Kingdom) area is characterised by extension(Fraser & Gawthorpe, 1990; Hollywood &Whorlow, 1993),which has been attributed to the e¡ects of a phase of litho-spheric stretching induced by the subduction of the Pa-laeo-Tethys in Brittany and Central France (Leeder, 1992).This rifting event resulted in a horst and graben topogra-phy with carbonate platforms on the horsts (e.g. Derby-shire block) and relatively deep water settings in theneighbouring grabens (e.g. Bowland and Widmerpool ba-sins). The existence of these carbonate platforms in theSouthernNorth Sea and theNetherlands still remains un-clear. There are several authors who propose a horst andgraben structure in this area (Collinson etal., 1993;Gerlinget al., 1999; Ho¡mann et al., 2005), comparable with the re-constructions from the Pennines. However, in other pa-laeogeographic reconstructions this area is interpretedto be part of one huge carbonate platform during theDinantian (Maynard etal.,1997), or as being part of a silici-clastic-dominated basin (Bridges et al., 1995). Gursky(2006) interprets the existence of mud mounds in theNetherlands. Ho¡mann et al. (2005) suggested the localpresence of Dinantian black shales (Culm facies) in north-westernGermany and the adjacent area in theNetherlandson the basis of magnetotelluric soundings. Based on theseobservations they propose a horst and graben system forthis area. The black shales are thereby thought to havebeen deposited in the grabens. In turn, Ziegler (1990,Enclosure 14) indicates a graben-like structure during theDinantian striking NW^SE trough in the Netherlands.This graben is surrounded by a carbonate platform. Alto-gether, these examples show that there is no agreement onthe Dinantian palaeogeography in the Netherlands.

A recent interpretation of onshore deep seismic lines inthe Netherlands revealed an undrilled structure that verymuch resembles Dinantian carbonate build-ups as foundin the United Kingdom (Fig. 2, compare Fig. 68 fromChadwick & Evans, 2005). Moreover, Van Hulten & Poty,(submitted) reported a similar structure in the westernpart of the Dutch continental shelf area. Althoughthese are rather small build-ups, they might point to asimilar structural setting such as described for the UnitedKingdom.

Along the southern margin of Avalonia, a series of (un-der¢lled) foreland basins (Culm) developed as a conse-quence of loading of the subducting plate (Ricken et al.,

r 2008 The Authors. Journal compilation r 2008 Blackwell Publishing Ltd,Basin Research, 20, 377^395 379

Late Carboniferous basin formation in the Netherlands

2000). The Variscan mountains acted as the sedimentsource for these basins. From these under¢lled basins inthe south of theCNWECB, the thickness ofDinantian se-diments diminishes to the north,where it can be found as acondensed sequence of black shales in the wells WSK-01in theNetherlands andMSTL-01inGermany (Ho¡mannetal., 2005, Figs1and 3).The facies of these sediments sug-gest deposition in a relatively deep water setting. Apartfrom these wells, Dinantian sediments have only been re-covered fromwells surrounding the LBM, theMid NorthSea High and onshore United Kingdom. In the remainingpart of the CNWECBwell control is absent. Based on thedata presented above, we propose a setting comparablewith the reconstruction byGursky (2006): a relatively deepwater basin with some scattered (small) carbonate build-ups such as shown in Fig. 2.

TheDinantian/Namurian transition marks the onset ofcollision between the Rhenohercynian northern passivemargin (southern margin of Avalonia) and the terranes in

the south (Oncken etal.,1999).TheLateVisean subductionof oceanic domains in the Rhenohercynian Basin was ac-companied by the intrusion of I-type granitoids in thenorthernVosges and Odenwald. However, during the Na-murian, S-type granitoids are found, which suggests thatby now substantial amounts of continental lithospheresubducted and that the Rhenohercynian Basin was closed(Ziegler, 1990). North of the LBM, in the United King-dom, the Namurian marks the onset of regional passivesubsidence, interpreted as thermally induced after the Di-nantian rifting event (Fraser & Gawthorpe, 1990). In wes-tern Germany, the change from a narrow (under¢lled) to abroad (over¢lled) foreland basin is thought to occur duringthe Dinantian/Namurian transition (Ricken et al., 2000;see below for more details), as a consequence of the sub-duction of a stronger (unrifted) lithosphere. In the areasouth of theLBM(Wales,UnitedKingdom), backstrippedsubsidence curves reveal the onset of £exural subsidenceduring Late Namurian (Burgess & Gayer, 2000). In the

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H. Kombrinket al.

CNWECB, the Namurian is mostly seen as a period inwhich steady subsidence occurred (either due to thermalor £exural foreland subsidence). However, in the centraland northern parts of theNetherlands, an intra-Namurianangular unconformity has been mapped on seismics(Fig. 2). Although this unconformity has not been recog-nised regionally, these ¢ndings suggest a more compli-cated history of Namurian subsidence.

The onset of the Namurian marks the transition from acarbonate to a siliciclastic-dominated depositional envir-onment in the CNWECB. Although sedimentation rateswere still low during the Early Namurian A, resulting inthe deposition of black shales such as the Bowland/Gever-ik shales andHangendeAlaunschiefer, in a later stage sedi-mentation rates increased dramatically. In the UnitedKingdom, the Namurian sedimentary sequence re£ectsthe draping of the Dinantian carbonate platforms and thein¢ll of the neighbouring (deep water) basins with mainlyprodelta silty claystones and turbidites (Maynard et al.,1997). This resulted in signi¢cant thickness variations(Smith et al., 2005).The few wells in the Netherlands thatpenetrate the Namurian sequence also show a silty clays-tone-dominated sequence interpreted as pro-delta sedi-ment and intercalations of (turbiditic) sandstones. Only

the very northern o¡shore area (well E12-02) shows anorthern-derivedNamurian £uvio-deltaic sequence.

Collision between Laurussia and Gondwana continuedduring the Westphalian. Subsidence curves derived fromthe area south of the LBM (Wales) suggest a northward-moving orogenic front as they show a strong convex-uppattern during this time (Burgess & Gayer, 2000). Fromthickness data, a northward movement of the Variscanforeland basin has been interpreted in the Ruhr area aswell (Drozdzewski, 2005). Moreover, Drozdzewski (2005)found the location of the forebulge, which is located intheRuhr basin.This interpretation places theNetherlandsin a back-bulge position (see below for more details).North of the LBM, thermal subsidence is thought to con-tinue (Fraser & Gawthorpe, 1990). During mid-Westpha-lian C, the tectonic regime is thought to change frompassive (thermal) subsidence to basin inversion as a resultof the increasing compression caused by the continuousmovement of Gondwana to the north with respect toLaurussia (Fraser & Gawthorpe, 1990). As a result, sedi-ments younger thanWestphalian B show a more disperseddistribution.

In general, theNamurian/Westphalian transition marksthe occurrence of the ¢rst thick coal seams in the

Fig. 3. Section from the RhenishMountains (former passive margin of the Rhenohercynian Basin) to the Cleaverbank High areashowing the thickness of the Carboniferous sediments in this area.The wells used for this study are plotted in the cross- section. Notethat theDutch part of this section is £attened anddoes not represent the actual depth (the depth below surface is indicated by the dashedline above the cross-section).The RhenishMassif section is based on Oncken et al. (2000).



CNWECB, although in the proximal areas of theCNWECB this already took place in the Namurian C(Drozdzewski, 2005). As such, the depositional environ-ment changed from a broadly shallow marine during theNamurian to a lower delta plain depositional environmentwith numerous coal layers during the Westphalian. ThePWDcan therefore be assumed to be zero fromNamuriantimes onwards.

DATA

The Namurian and Dinantian of the Netherlands are partof the lower limit of the petroleum fairway (Cameron &Ziegler, 1997).Therefore, well control is scarce.The wellsused for this study had to ful¢l two criteria: (1) they haveto be located on a cross-section that more or less runsNW^SE from theRhenishMassif because £exural model-ling has to be carried out on a line perpendicular to theVDF (Figs 1 and 3) and (2) the stratigraphic penetrationshould be as high as possible. Figure 3 shows the positionand stratigraphic penetration of the most important wellsused for this study. Subsidence curves have been made for¢ve di¡erent areas (Table 1, Fig. 4). In each area, the thick-ness of the Dinantian up to Westphalian B has been de-rived from several wells. Unfortunately, well-basedthickness data for the Dinantian are lacking for the areasNetherlands-central and north because the stratigraphi-cally deepest wells NAG-01 and TJM-02 do not penetratethe base Namurian. The thickness for the Namurian inthese areas therefore represents minimum values. How-ever, in Netherlands-central the thickness of the Namur-ian can be estimated using the seismic section given inFig. 2. On both sides of the carbonate build-up this is ap-proximately 2380m whereas the Namurian is around1830m on top of it.The thickness of theWestphalian A inthe Netherlands-central area is a minimum estimate, as isthe thickness of the Westphalian B in the area Nether-lands-north. These minimum values are caused by EarlyPermian erosion or incomplete well penetration of thesestratigraphic intervals. Finally, a dip correctionwas carriedout for the wellMSTL-01, because the strata showdips upto 801 (Richwien et al., 1963).

SUBSIDENCE CURVES

Subsidence curves can reveal important information onthe dominant subsidence mechanism. For instance, in ex-tensional regimes typical concave-up curves occur as aconsequence of decreasing subsidence related to thermalre-equilibration (McKenzie, 1978). In order to verify themodelling results as presented in the following sections,subsidence curves needed to be calculated. Basement sub-sidence has been calculated to verify the £exural modellingresults. Basement subsidence is the combination of tec-tonic subsidence and subsidence due to sediment loading,with the reference horizon being a compaction-free hori-zon (basement). Tectonic subsidence curves were neededto match the subsidence curves generated by simulationof a rifting event in the second modelling exercise.

The backstripping technique (Steckler & Watts, 1978)has been used to calculate the basement and tectonic sub-sidence curves, assumingAiry isostasy.Age constraints forbackstripping were derived from the Global time scale asused in the Devonian^Carboniferous^Permian Correla-tion Chart 2003 (Menning et al., 2006, Fig. 1c). We focuson the Dinantian, Namurian and Westphalian A/B stages.No further subdivision of stages has been made due to dat-ing uncertainties. Standard mean porosity-depth rela-tionships for di¡erent lithologies (cf. Sclater & Christie,1980) were used.The complete post-Carboniferous over-burden has been backstripped as well to account for Per-mian andMesozoic burial.

The PWDat the end of theDinantian is a great factor ofuncertainty.The only area where a reliable estimate of thelateDinantianPWDcanbemade isCleaverbankHigh.Di-nantian sediments re£ect a rather shallow PWD, whichmade us decide to use only one value of 10m.The remain-ing tectonic and basement subsidence curves have beencalculated with two ‘end-members’ of end-DinantianPWD.An estimate for the late-Dinantian PWDcan be de-rived from the carbonate build-up displayed in Fig. 2.Thecarbonate platform is approximately 550m thick. Assum-ing a sediment-starved basin during the Dinantian, thismeans that at the end of theDinantian thePWDmust havebeen 550m at a minimum on both sides of the build-up. Anumber of authors propose a long-term eustatic decline in

Table1. Thickness data used in this study

Well Dinantian (m) Namurian (m) Westphalian A (m) Westphalian B (m)

Germanyn 70 1800 1100 900Netherlands-eastw 260 1525 1380 700Netherlands-centralz 200 2150 700 470Netherlands-north‰ 200 2650 700 400Cleaverbank Highz 1000 680 520 400

nBased on:Mˇnsterland-01 (MSTL-01), Altenkamp-03 and Besenkamp-01.wBased on:Winterswijk-01 (WSK-01), Hengevelde-01 (HGV-01), Ruurlo-01 (RLO-01) and Joppe-01 (JPE-01).zBased on: Nagele-01 (NAG-01) andHardenberg-02 (HBG-02).‰Based on:Tjuchem-02 (TJM-02) and Goldhorn-01 (GLH-01).zBased on: E06-01, E12-02 and K01-02.


H. Kombrinket al.

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Fig.4. Tectonic and basement subsidence curves for selected areas for two end-members ofDinantian PWDand for two end-membersof the Dinantian thickness in Netherlands-central andNetherlands-north. Note the di¡erence in vertical scale between basement andtectonic subsidence. PWD, palaeowater depth.



sea level at the Dinantian^Namurian transition (Ross &Ross,1988, their Fig.9;Gursky, 2006).Karsti¢cation ofDi-nantian carbonate platforms is well described in theNWECB (Bless et al., 1976; Amler & Herbig, 2006). Ourminimum late-Dinantian PWD scenario therefore as-sumes such a long-term decline in sealevel (100m PWD).A maximum value of 650m is used: if larger values of thelate-DinantianPWDare adopted, the amount of sedimentthat can be accommodated during the Namurian withoutany additional tectonic subsidence exceeds the observedthickness data. It can be assumed that the PWD at theend of the Namurian and Westphalian A/B is near zero,which does not allow the assumption of a signi¢cantPWD at these times. Moreover, two additional scenarioshave been run for the Netherlands-central and north toaccount for the lack of thickness data for the Dinantian.The lower end-member is 200m, comparable with thethicknesses as found in the southeastern part of the basin.The upper end-member is 1000m, comparable with whathas been found in E06-01 in the northern o¡shore.

Most tectonic subsidence curves in the low late-Dinan-tian PWD scenarios show an overall convex-up pattern,although fromWestphalian A to B a slight decrease in tec-tonic subsidence can be seen in some areas (Fig. 4). How-ever, in the high late-Dinantian PWD scenarios, adecrease in tectonic subsidence occurs during the Dinan-tian^Namurian transition because of the mostly passivein¢ll of the deep water basin. In this case, an increase canbe seen at the Namurian^Westphalian A transition.Thereare no large di¡erences between the subsidence curvesmade for the two end-members of Dinantian strata in theareasNetherlands-central and north.Altogether, based onthe uncertainties mostly related to the late-DinantianPWD, it is di⁄cult to draw a conclusion on the dominantsubsidence type, although the convex-up patterns (lowlate-Dinantian PWD) may be more likely due to the long-term decline in sealevel proposed by a number of authors.

TWO-DIMENSIONAL (2D) FORWARDFLEXURAL SUBSIDENCE MODELLING

We investigated whether subsidence in the Dutch part oftheCNWECBcan be (partly) explained by foreland £exuredue to orogenic loading by the approaching Rheno-Her-cynian belt.This was carried out by 2D £exural modellingalong a NW^SE pro¢le (Figs 1b and 3) using an elasticthin plate approach. Before presenting the modelling re-sults, the current ideas on the kinematics of the Rheno-Hercynian belt and the geometry of the adjacent Variscanforeland basin will be reviewed brie£y.

Constraints on kinematics of the Rheno-Hercynian belt and the geometry of theadjacent Variscan foreland basin

The kinematics of theRheno-Hercynian belt arewell con-strained by the restoration of a series of sections across the

Ardennes and RhenishMassif (Oncken et al., 2000). From335 to 325Ma (LateDinantian), shortening has been calcu-lated to be 140 km, while from 325 to 305Ma, 175 km ofshortening took place (Oncken et al., 2000).The geometryof the adjacent foreland basin has been studied in particu-lar by Drozdzewski (1993) and Ricken et al. (2000). Theseauthors use di¡erent data to arrive at an interpretation.These will be outlined below. A cross-section displayingthe Rhenish Massif in the south and the German/Dutchpart of the CNWECB up to the Cleaverbank High in thenorth is given in Fig. 3.

Ricken et al. (2000), in their study area in the easternpart of the Rhenish Massif, subdivided the tectono-sedi-mentary evolution of the basins in front of the advancingRheno-Hercynian Belt in to two stages: the Rheno-Her-cynian Turbidite Basin (RHTB) and Sub-Variscan Mo-lasse Basin (SVMB).The RHTB developed in front of the(initially submerged) advancing belt as it was shortenedfrom335 to 325Ma. It is characterised by a migrating seriesof narrow under¢lled (Culm) basins. The second stage ofbasin development, the SVMB, is characterised by abroader basin (Ricken et al., 2000). It started in Namuriantime (325Ma) and lasted until the end of shortening inStephanian (305Ma). Unfortunately, Ricken et al. (2000)do not give an estimate on the width of the SVMB. The

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H. Kombrinket al.

transition between the two stages in basin evolution (fromthe narrow RHTB to the wide SVMB) is interpreted tocorrespond to the migration of the belt across the transi-tion from a weakened crust [b from1.7 to 2.4 and an e¡ec-tive elastic thickness (Te) of 4 km] to an unrifted, strongercrust (subcrustal stretching factor b51.0^1.5, Rickenet al., 2000). In this paper, we study the basin evolutionsince Namurian, i.e. corresponding to the SVMB stage.

On the basis of thickness data in theRuhr basin,Drozd-zewski (1993) found a northward-moving area of reducedsubsidence (Westphalian A and B) that was interpreted tobe a forebulge. In contrast with the conventional model ofa foreland basin (e.g. DeCelles & Giles, 1996) in which theforebulge is an erosional feature, in this case subsidencecontinues but with a slightly decreased subsidence rate

compared with the basinal areas. This implies the exis-tence of another subsidence mechanism that causes theforebulge to subside.The recognition of a forebulge meansthat the northern limit of the Variscan foreland basin canbe mapped. When this is combined with the kinematicdata fromOncken et al. (2000; Fig. 6), a rather narrow fore-land basin must be interpreted.

In summary, two contrasting geometries of theVariscanforeland basin in western Germany exist. Ricken et al.(2000) interpret a broad foreland basin developed on a re-latively strong plate whereas Drozdzewski (1993) imply arather narrow basin when their data are combined withthe kinematic data fromOncken et al. (2000, Fig. 6).Theseinterpretations can be modelled by assuming a subductingplate with a high elastic thickness (Te) resulting in a broad

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Fig. 6. (a) Comparison between modelled (£exural) basement subsidence and the basement subsidence derived fromwell data for theNamurian, (b)Westphalian A and (c)Westphalian B. Location of the wells can be seen in Fig. 3. Four model scenarios are shown (Fig. 5).The inset in (b) shows the thickness ofWestphalianA andB sediments in theRuhr basin, southeast of the wellMSTL-01.VDF,VariscanDeformation Front.



foreland basin, and a small Te, in which case a rather nar-row basinwill develop. In the next section, we will investi-gate what part of the observed subsidence in theCNWECB can potentially be explained by £exural subsi-dence, using a range of parameters.Thiswill lead to an eva-luation of the existing ideas on the geometry of theVariscan foreland basin.

Model scenarios

Reconstruction of the tectonic history of theDutch part ofthe CNWECB is hampered by deep burial of Carbonifer-ous sediments, as described above. As a consequence ofthe relative scarcity of data, especially on lithosphere para-meters like the elastic thickness and distribution of excess(mantle) loads, this modelling exercise presents a series ofscenarios of end-member estimates.Therefore, the resultsmust be seen as a sensitivity analysis for the geometry ofthe Variscan foreland basin in the CNWECB.The e¡ectsof the following parameters on the geometry of the fore-land basin were studied (Fig. 5): (1) di¡erent values of Te(5 and 60 km) in a plate of constant rigidity.This is the sim-plest scenario inwhich only the in£uence of the rigidity ofthe subducting plate is studied. Elastic thicknesses of 5and 60 km are naturally occurring end-members for a veryweak and a strong plate, respectively. (2) A strength transi-tion sensuRicken etal. (2000).The location of the strengthtransition has been derived from the reconstruction byOncken et al. (2000). (3) A late-Dinantian PWD of 650m.As mentioned above, the late-Dinantian PWD is an im-portant factor in determining the amount of subsidenceneeded to accommodate the Namurian sedimentary se-quence. For the wells MSTL-01, WSK-01, NAG-01 andTJM-02, a PWD of 650m has been assumed, shallowingto 10m for the Cleaverbank High area. (4) A di¡erent loadgeometry. When the centre of the load is moved north-wards, without changing the position of the frontal thrust,the resulting foreland basin will be wider. The e¡ects ofsuch a change will be studied in this scenario by assuminga block load.

Model set-up and parameters

We used a 2D¢nite di¡erence code,COBRA,which calcu-lates £exure of an elastic thin plate [for equations and their

derivation, seeRoyden (1988) andZoetemeijer etal. (1999)].The programme allows to change laterally the £exural ri-gidity and the application of both topographic and ‘hid-den’ loads. It takes into account di¡erent densities for theload and basin ¢ll and can be applied to both continuousand broken plates, the latter with or without an end load(‘slab pull’). Modelling was carried out along the cross-section shown in Figs 1 and 3. In each case, the predictedbasement subsidence curves will be compared with base-ment subsidence curves derived fromwell data (Fig. 6).

The total shortening amounts to 175 km in 20Ma fromNamurian to Stephanian (Oncken et al., 1999, 2000). Dur-ing this time interval, material is accreted at the thrustfront according to the critically tapered wedge concept(e.g. Chapple, 1978). Therefore, the width of the loadincreases (Tables 2 and 3) from 140 to 180 km, the topo-graphic slope (a) varying between 2.5 and 31. The wedge-shaped load stays at the end of the broken plate; we studythe pro-foreland side of the doubly vergent wedge model(e.g. Willett, 1999). Assuming a constant shortening/dis-placement rate of 9 kmMa�1, starting in Namurian, themodel was run for three time steps (Namurian and West-phalian A and B).

The density of the basin¢ll will not only a¡ect the mag-nitude of the de£ection but also its wavelength (Turcotte &Schubert 2002, Fig. 2 in Leever et al., 2006). For the Na-murian^Westphalian B time steps, the PWD is near zeroand the basinwas completely ¢lledwith sediments.To pre-vent underestimation of the sediment density, a relativelyhigh density has been used (2400 kgm� 3). For an overviewof sediment densities, see Kearey et al. (2002).

Model results

Figure 6 shows the predicted basement subsidence forthree load steps (Namurian^Westphalian B) for two end-members with respect to plate rigidities (Te 5 and 60 kmplate constant thickness). In each ¢gure, the ¢nal positionof the thrust front is indicated (present-day VDF). More-over, the forebulge positions during Westphalian A and Baccording to Drozdzewski (2005) are shown. The basin isvery narrow for the weakest plate (Te 5 km), having a widthof some 25 km (with respect to the thrust front) and abasement subsidence of �1000m. It is located at such a

Table 2. Parameters used for £exural modelling and input for models

Common parametersLoad advance rate 8.75 kmMa�1

Initial loadwidth 140 kmFinal loadwidth 180 kmDensity of load, basin ¢ll, crust, mantle 2700, 2400, 2800, 3300 kg m� 3

Model 1 Plate constant Te Te 5 and 60 kmModel 2 Transition width 200 km

Te south/Te north 10/40 and10/80 kmModel 3 Maximum late-Dinantian PWD 650^10m

Plate constant Te Te 5 and 60 kmModel 4 Block-load, constantTe Te 5 and 60 km


H. Kombrinket al.

distance from the Netherlands that none of theobserved subsidence can be explained in this case. Theamplitude is too small as well. A signi¢cantly widerbasin, with a higher amplitude, is predicted when a Te of60 km is assumed, having a width of 4400 km and abasement subsidence of �2 km. In this case, a smallpart of the observed subsidence may be explained by £ex-ural subsidence. Especially during the Westphalian B, thecomplete succession in the southeastern part of theNetherlands might be explained by £exural forelandsubsidence.

The basinwidth in relation to the load migration resultsin an additional e¡ect.The basins that develop on a weakplate (Te5 5 km) are so narrow that, in the subsequent timestep, the load has migrated past the previous basin (com-pare Figs 6a^c). Therefore, these basins will only containsediments that have been deposited during the previousperiod. In contrast, the large wavelength of the strongplates (Te440 km), will cause subsidence well in advanceof the thrust front, far enough to record sedimentationover a large period of time. In case of the strongest plates(Te � 60 km) this even takes place since the beginning ofthe Namurian.

In the next scenario, we used the same loading as in the¢rst series of experiments but introduced a 200-km-widetransition in strength in the plate (Fig. 5). The results (Tetransition10^40 and10^80 km,Fig.6) are comparablewiththe response of the continuous plate of 60 km, as thewedgehas started to advance over the transition zone from the¢rst (Namurian) load step.Only the ¢rst load step (Namur-ian) shows the signature of the weaker plate. For a lateralchange in lithosphere strength to cause a marked di¡er-ence inNamurian^WestphalianB foreland basin geometrycompared with the continuous lithosphere strength (pre-vious scenario), the transition has to be located further tothe north or the transition width has to be decreased (e.g.Leever et al., 2006).

When incorporating an initial PWDfor the late-Dinan-tian, the amount of predicted basement subsidence for theNamurian increases signi¢cantly (Te 5/60 km PWD 650m,Fig. 6). Especially in the case of a highTe, the ¢t with theobserved basement subsidence data is the best one so far(in amplitude aswell aswidth).However, it is still not su⁄ -cient to explain the high subsidence in NAG-01 andTJM-02 (Netherlands-central and north, respectively).As there is no reason to assume a PWD for the late-Na-murian and theWestphalian A and B, the results for these

stages have not been plotted, because the results are com-parable with the ¢rst modelling scenario.

The application of a di¡erent load geometry (blockload) results in a slight increase in basin width in bothcases, whereas a decrease in amplitude for the high-Teand an increase in the low-Te case can be seen. However,this increase in width is far from su⁄cient to give a muchbetter result compared with the results from the ¢rstscenarios.

Discussion of model results

The most important conclusion from the scenarios pre-sented above is that the amplitude of the (observed) subsi-dence more or less corresponds to the modelled £exuralsubsidence but the width and location of the foreland ba-sin prevent a signi¢cant part of the CNWECB from beingexplained by this mechanism, especially for theNamurian.Only when a high late-Dinantian PWD is assumed can arelatively good ¢t be seen for the Namurian, although ithas to be realised that the development of such a deep-water basin cannot be explained by £exural subsidence.For the Westphalian A, there is a slightly better matchwhen taking the high values of Te into account. However,in no case can the completeWestphalianA sequence be at-tributed to £exural subsidence. The complete basementsubsidence observed in the southeastern part of theNeth-erlands during the Westphalian B can potentially be ex-plained by £exural subsidence if a high Te of 60 km isassumed. A change in the geometry of the load to a blockload leads to a very small and insigni¢cant increase in ba-sin width. A possible scenario to obtain a better match be-tween calculated and modelled basement subsidenceduring the Namurian is to shift the deformation frontfurther to the north. However, this would be in disagree-ment with the reconstructions from Oncken et al. (2000,Fig. 3). The increase in subsidence from WSK-01 to thenorth during theNamurian is di⁄cult to explain in a fore-land basin setting. A possible explanation is the presenceof a blind thrust.We do not, however, have (seismic) datato support this hypothesis.

It is clear from the modelling results that the interpre-tation of the forebulge by Drozdzewski (1993) only ¢ts thelow-Te (5 km)model results (Fig. 6).This is in contrastwiththe interpretation of Ricken etal. (2000), who state that theforeland basin during the Westphalian developed on arelatively strong plate, which results in a broad basin.

Table3. Load displacement and geometry for the di¡erent time steps (based on reconstructions byOncken et al., 2000)

Time of base:Position of loadcenter (km)

Position of loadfront (km)

Loadwidth(half wedge, km)

Loadheight (km)

Wedgeslope

Stephanian 305 180 270 90 5 3.2Westphalian C 311 130 215 85 4.5 3Westphalian B 313.5 110 190 80 4 2.9Westphalian A 316.5 90 165 75 3.5 2.7Namurian 326.4 0 70 70 3 2.5



When the assumption is made that the forebulge inter-preted by Drozdzewski (1993) is indeed at the right posi-tion, one would expect to ¢nd a very pronouncedthickness di¡erence on both sides of the forebulge area(see the PWD 650 result forTe 5 km). However, thicknessdi¡erences display a gradual change indeed (Fig. 3 and in-set in Fig. 6b).Moreover, Fig. 6 shows that in case of aTe of5 km the foreland basins are so narrow that, in the subse-quent time step, the load has migrated past the previousbasin. Therefore, it is not expected to ¢nd sediments ofsubsequent time steps on top of each other. In the Ruhrarea, sediments ofNamurian^WestphalianB age have beendeposited on top of each other.This observation points toa relatively highTe, in accordance with the interpretationby Ricken et al. (2000). Altogether, the thickness data forthe southern part of the study area may be in favour of aquite broad foreland basin.

TECTONIC SUBSIDENCE MODELLINGAPPLYING LITHOSPHERICSTRETCHING

In the previous section, itwas shown that orogenic loadingof a foreland plate largely fails to explain subsidence ob-served in the Netherlands during the Namurian. Duringthe Westphalian A and B, the in£uence of £exural subsi-dence is limited to the southeastern part of the Nether-lands. Therefore, most of the observed subsidence mustbe explained by a second mechanism. In this section, wewill investigate whether the observed subsidence can beexplained by the simulation of a rifting event, the mechan-ism proposed in the United Kingdom to account forDinantian (syn-rift) and Namurian^Westphalian B (post-rift) subsidence (Fraser & Gawthorpe, 1990).We will focuson the areas where, according to the modelling results pre-sented above, no or only a very small amount of £exuralsubsidence took place. These areas are Netherlands-cen-tral/north and Cleaverbank High (Figs1 and 7).

Numerical Model

Various studies have demonstrated the possibility of inver-sion of subsidence curves to kinematic tectonic models(e.g.White,1994;VanWees etal.,1996;Bellingham&White,2000;VanWees&Beekman, 2000). For the modelling exer-cise described in this section, we used a (multi-) 1D prob-abilistic tectonic model. The model starts with backstripanalysis resulting in tectonic subsidence curves. Subse-quently, a best- ¢t tectonic model, matching the observedtectonic subsidence, is found using a tectonic modellinginversion procedure.The modelling approach is based onthe pure-shear lithosphere thinning model of McKenzie(1978)with the possibility of di¡erent amounts of thinningin the crustal and subcrustal parts of the lithosphere (e.g.Royden &Keen,1980). During lithospheric stretching, ve-locities are calculated assuming ¢xed strain rates. Sedi-mentation or erosional velocities are assumed to be linear

through time in accordance with backstrip analysis (e.g.Bond &Kominz, 1984).

The numerical technique to ¢nd automatically the best-¢tting stretching parameters for (part of) the subsidencedata was developed byVanWees et al. (1996). In this proce-dure, the onset and duration of the rift phases must bespeci¢ed, whereas best- ¢tting stretching values are foundby Powell’s method (Press et al., 1988), in which theminimum of the mean square root F of the deviation be-tween predicted and observed subsidence is sought as afunction of

F ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPi¼1::n ðSpi � SoiÞ2

n

s

where n is the number of subsidence data used in the ¢ttingprocedure and Spi and Soi are the predicted and observedsubsidence values, respectively. For a given rift phase, eitheruniform (i.e.McKenzie,1978), or two-layered, non-homoge-neous (b6¼d; e.g.Royden&Keen,1980), stretching can be im-plemented according to the user’s choice. For uniformstretching, the solution of the above equation requires thatat least one observed subsidence datapoint is given after theonset of rifting, whereas the two-layered stretching imple-mentation requires at least two datapoints. For multiphaselithospheric stretching, the ¢t is accomplished in sequentialorder.

Rifting historymodels

In the following section, the Late Carboniferous basinevolution will be simulated using two scenarios. As men-tioned before, the late-Dinantian PWD cannot be simplyassumed as a ¢xed value. Therefore, we have chosen tomodel the tectonic subsidence using the minimum andmaximum estimates of the late-Dinantian PWD (100 and650m, respectively). Moreover, we modelled these scenar-ios with two end-members of Dinantian thicknesses aswell, because for the Netherlands-central and Nether-lands-north these are lacking.

Estimates on the crustal thickness are given byMooneyet al. (1998, in Fischer et al., 2004) and Henk (1999), being36.3 and 30^32 km, respectively. We assumed a thicknessof 32 km in our models. Further information on the inputparameters can be found inTable 4. Like the £exural mod-elling exercise, the tectonic modelling has to be regardedas a search for the applicability of certain stretching re-gimes to the Late Carboniferous of theNetherlands ratherthan presenting a fully constrained tectonic model.

In model 1, we assume a Dinantian rifting event withuniform stretching (McKenzie, 1978), followed byNamur-ian^Westphalian B thermal sag such as found in the Uni-ted Kingdom (Leeder & McMahon, 1988, Fig. 1). On thebasis of the poor ¢t of this model with the observed data,especially in the low late-Dinantian PWD case, it wasdecided to model a scenario incorporating a Namurianrifting event (model 2). InFig.7, the observed tectonic sub-sidence from wells (see also Fig. 2) is displayed, togetherwith the best- ¢t subsidence curves.


H. Kombrinket al.

Model1 ^Dinantian rifting (syn-rift) with a uniform lithosphericextension, followed byNamurian-Westphalian B thermal sag(post-rift)

Uniform lithospheric extension implies coupling of thecrustal and subcrustal stretching factors d and b, respec-

tively (McKenzie, 1978). This is the most common andsimple model that has been applied in the United King-dom to derive stretching factors needed to explain theamount of post-rift subsidence (Leeder & McMahon1988; Coward, 1993). From the modelling results (Fig. 7),

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Fig.7. Modelled tectonic subsidence curves for models1and 2 for two end-members of late-Dinantian PWD and two end-members ofDinantian thickness in the areas Netherlands-central and north. For the Cleaverbank area only one scenario had to calculated as thelate-Dinantian PWD and thickness of the Dinantian sediments is well known.Tectonic subsidence calculated from the well data isindicatedwith squares.The stretching factors are indicated for each scenario; for model 1 this value represents stretching during theDinantian whereas for model 2 the second number represents the Namurian rifting event. PWD, palaeowater depth.



it follows that a better match between syn- and post-rifttectonic subsidence can be obtained when a high late-Di-nantian PWD is assumed, irrespective of the thickness ofthe Dinantian succession.This is not surprising as in thelow late-Dinantian PWD scenario anomalously high tec-tonic subsidence takes place during the post-rift phasewhen comparedwith the syn-rift phase. In most scenarios(high and low late-Dinantian PWD), the amount of syn-rift subsidence is overestimated by the model relative tothe observed syn-rift subsidence to obtain a best ¢t. Inother words, even in the case of the high late-DinantianPWD and a Dinantian thickness of1000m, the amount ofsubsidence in the post-rift stage is too high relative to thesyn-rift subsidence.

Model 2 ^ Dinantian andNamurian rifting (syn-rift), followedbyWestphalian A/B thermal sag (post-rift)

In an attempt to ¢nd an alternative for the poor ¢t of theprevious scenarios, in this model a second rifting phaseduring the Namurian is assumed. In this case, the modelis allowed to ¢t the Namurian tectonic subsidence withthe most suitable stretching factor instead of being re-stricted to a prede¢ned factor (i.e. thermal sag, no stretch-ing). Geological evidence for a Namurian rifting event issparse. However, Sissingh (2004) describes extrusive rocksinNAG-01that probablyhave aNamurian age.This possi-bly indicates tectonic activity during the Namurian in thisarea. Moreover, an intra-Namurian angular unconformitywas mapped in this area (Fig. 2). Although it is not a directevidence for rifting, this mechanism may result in the for-mation of unconformities as a consequence of doming orthe uplift of rift shoulders (Ziegler & Cloetingh, 2004).

In this model, the McKenzie (1978) concept (coupledcrustal and subcrustal stretching factors) is used again. Asexpected, adopting a Namurian rifting event gives a better¢t between the observed and modelled tectonic subsi-dence. This is the case for both PWD scenarios. In theCleaverbank area, the model ^ although a Namurian rift-ing event is assumed ^ cannot account for the increase in(observed) tectonic subsidence from Namurian to West-phalian A.

Discussion of model results

The most important outcome of the results describedabove is that ^ when assuming the classical Dinantian

syn-rift and Namurian-Westphalian B post-rift scenario^ the amount of sediments deposited during the post-riftstage is out of proportion comparedwith the syn-rift stagefor all areas for all scenarios. Adopting a model in whichrifting only takes place during the Dinantian (Model 1) ahigh late-Dinantian PWD is required in Netherlands-central and Netherlands-north to explain the observedLate Carboniferous subsidence.This implies the absenceof a eustatic lowering of sea level, as has been observed byseveral authors (Ross &Ross, 1988; Gursky, 2006). Only forthe Cleaverbank area does a low Dinantian PWD result ina reasonable ¢t for Model 1 because of the better ratio ofpost-rift and syn-rift deposits.

If an additional Namurian rifting event is assumed(Model 2), a better match between calculated and observedtectonic subsidence can be obtained than in the otherscenarios (Fig. 7). However, even with a Namurian riftingevent, an increase in tectonic subsidence from Namurianto Westphalian A as observed in Netherlands-central(PWD 650m scenario) is unlikely because this periodrepresents the thermal sag stage, generally characterisedby decreasing subsidence rates. When adopting anadditional Namurian rifting event, the low late-DinantianPWD scenarios are more likely, as a clearer decrease insubsidence can be observed from Namurian (syn-rift) toWestphalian A (post-rift) than in the case of the highlate-Dinantian PWD scenarios. It should be realisedthat the stretching factors needed for the Namurian incase of the low Dinantian PWD scenario are higherthan in the high Dinantian PWD scenario (Fig. 7).

Subsidence mechanisms in theCNWECB

From the modelling results presented in this paper, theconclusion can be drawn that most of the observed subsi-dence in the Netherlands during the Namurian^Westpha-lian B cannot be explained by £exural foreland subsidence,evenwhen a very highTe is assumed. Lithospheric stretch-ing is a possible alternative candidate as it has been ob-served clearly in northern England (Fraser & Gawthorpe,1990).Moreover, the ¢ndings of possibleDinantian carbo-nate build-ups in the Netherlands (Van Hulten & Poty,submitted; Fig. 2) may indicate a similar structural settingas the United Kingdom, thereby suggesting that riftingalso took place in our study area.Modelling of aDinantianrifting event using thickness data from the Netherlandsshowed that only the assumption of a very high late-Di-nantian PWD results in a ¢t between observed and mod-elled tectonic subsidence data. An additional Namurianrifting pulse results in a better ¢t.The presence of a Na-murian unconformity in the central part of the Nether-lands (Fig. 2) and the occurrence of extrusives in the samearea fromNamurian age show that this stage was not char-acterised by passive regional subsidence alone.Therefore,a subsidence model incorporating a Namurian riftingevent, next to a model of a Dinantian rifting event only, is

Table 4. Model parameters

Model parameterInitial lithospheric thickness 110 kmInitial crustal thickness 32 kmAsthenospheric temperature 1330 1CThermal di¡usivity 1 � 10� 6m2 s�1

Crustal density 2800 kg m� 3

Mantle density 3400 kg m� 3

Thermal expansion coe⁄cient 3.2 � 10� 51C�1


H. Kombrinket al.

thought to be possible. Also, a decline in sea level thatprobably took place at the onset of the Namurian (Ross &Ross, 1988; Gursky, 2006) supports this view because suchan event results in a strong increase in mis¢t when only aDinantian rifting event is assumed.

It is generally accepted that lithospheric stretching bythinning and heating results in an initial weakening of thelithosphere (e.g. Ziegler &Cloetingh, 2004 for an extensivereview). In turn, the rigidity of the lithosphere, or its e¡ec-tive elastic thickness, re£ects its integrated strength (Watts& Burov, 2003). Hence, loading of an extensionallyweakened plate will result in a narrow foreland basin.However, thermo-mechanical modelling (Braun, 1992;Cloetingh et al., 2003) has shown that even thoughrifting causes an initial weakening of the lithosphere,it will ¢nally result in a stronger lithosphere due tothermal relaxation and so-called ‘mantle healing’. Thetypical time scale for this process is some 30Ma basedon the ratio of advective- to conductive-driven heattransfer (Braun, 1992). It may vary as a function of theduration and the magnitude (crustal vs. lithospherestretching) of extension. The Eastern and WesternBlack Sea regained their pre-rift strength 10 and 50Maafter initiation of rifting, respectively (Cloetinghet al., 2003). Afterwards, their strength kept increasing todouble or even triple the initial value. On the otherhand, in a case study of the Central Iberian Basin, Ziegleret al. (1995) point out that even though the strength of thelithosphere recovers thermally, due to reactivation of faultsit never again reaches its initial value. In spite of the largedi¡erences in the predictions of the rheological models,it is likely that the lithosphere, during the Namurian,would not have fully recovered from a Dinantian riftingevent. Thus, when these mechanisms have to explainthe observed subsidence in the Netherlands, it is unlikelythat the Te of the plate was not weakened by this riftingevent.

The £exural modelling results showed that the modelproposed by Ricken et al. (2000, a broad foreland basindeveloped on a relatively strong plate), better explains theobserved thickness data because (1) pronounced thicknessdi¡erences are expected when a weak plate (Te 5 km) isassumed (Fig. 6). However, thickness di¡erences displaya gradual change indeed (inset in Fig. 6b). Moreover,(2) Fig. 6 shows that in case of a Te of 5 km the forelandbasins are so narrow that, in the subsequent time step,the load has migrated past the previous basin.Therefore,it is not expected to ¢nd sediments of subsequent timesteps on top of each other. In the Ruhr area, sediments ofNamurian^Westphalian B age have been deposited on topof each other.

The apparent contradiction between a broad forelandbasin on the one hand and a rifted basin on the other maybe explained by taking into account a third subsidenceme-chanism: dynamic topography. This mechanism is pro-posed by numerous authors to explain long-wavelength(asymmetric) subsidence in a £exural foreland setting.Long-wavelength subsidence such as that observed in the

CNWECBhas been recognised in other basins aswell: theDevono-CarboniferousWesternCanada SedimentaryBa-sin (Pysklywec & Mitrovica, 2000), the Cretaceous Wes-tern Interior Basin, United States (Pang & Nummedal,1995; Liu & Nummedal, 2004), the Silurian Baltic Basin(Lazauskiene et al., 2002) and the Middle Ordovician Mi-chigan Basin (Coakley & Gurnis, 1995). In these basins,£exural loading models could not adequately explain theobserved subsidence patterns as their wavelength consid-erably exceeds the modelled ones (as is the case in ourstudy area). These authors proposed the addition of a dy-namic component, following Gurnis (1992), who foundthat a mantle £ow-induced dynamic topography could ex-plain subsidence over such long wavelengths. This iscaused by subduction of a relatively dense slab, creating anegative buoyancy anomaly (Burgess & Moresi, 1999).However, the recognition of dynamic topography is com-plex in ancient settings. The most reliable criterion toidentify this mechanism is the observation of subsidenceto occur on a greater wavelength than expected due to £ex-ural subsidence alone (Coakley & Gurnis, 1995; Burgess &Moresi, 1999). As our £exural modelling results show thatthe wavelength of subsidence in the CNWECB exceedsthe wavelength predicted by £exural loading, such a me-chanism may be proposed for the CNWECB.The obser-vation by Gursky (2006) that the northern (passive)margin of the Rhenohercynian Basin already subsided inthe Devonian might point to an early expression of dy-namic topography. Another feature observed in theCNWECB can potentially be explained by this mechan-ism as well. Burgess &Moresi (1999) indicate that the fore-bulge may be absent or subdued as a consequence ofdynamic topography, because of the extension of the areaof subsidence beyond the forebulge. Drozdzewski (1993)reported a (northward moving) area of reduced subsidencein the Ruhr basin, which he interpreted as a forebulge. In anormal foreland basin setting, the forebulge area is anerosional feature, but incorporating a dynamic loadcreates the possibility for the forebulge area to subside.Unfortunately, there are some drawbacks on the applica-tion of dynamic topography. First, dynamic topographyis a reversible process. After a period of subsidence onecan expect uplift to take place. In general, the duration ofthe unconformity will decrease away from the orogen(Burgess et al., 1997). Despite the existence of an uncon-formity at the base of the Permian, a marked increasein duration from north to south is not evident fromthe CNWECB. Second, the direction of subduction isan issue. Most authors report dynamic topography totake place over subducting slabs (Burgess & Moresi,1999; Pysklywec & Mitrovica, 2000). However, subduc-tion of the Rhenohercynian oceanic plate was probablydirected to the south (Franke, 2000; Oncken et al., 2000),away from the CNWECB. On the other hand, northwardsubduction took place somewhat further to the southwhere the Palaeo-Tethys oceanic lithosphere subductedbelow Gondwana-derived terranes (Stamp£i & Borel,2002).



CONCLUSIONS

We modelled £exural foreland subsidence, likely to be themost important subsidence mechanism along the south-ern margin of the CNWECB, using several scenarios.Our results show that £exural subsidence largely fails toexplain the subsidence observed from well data in theNetherlands during the Namurian, even if a very high Te(60 km) is assumed, together with a high late-DinantianPWD.This is because the (predicted) basin axis is locatedtoo far to the south at these times. During Westphalian Aand B, when the deformation front has been migratingfurther to the north, part of the subsidence observed inthe southeasternNetherlands can potentially be explainedby £exural subsidence. However, this is only the case ifagain aTe of 60 km is assumed.

Lithospheric stretching during theDinantian and asso-ciated thermal subsidence is proposed to be the secondsubsidence mechanism in the CNWECB, from observa-tions in the United Kingdom. Although a very limitedamount of data is available in the Netherlands subsurface,we propose that such a rifting event took place. However, asimple modelling exercise showed that uniform stretchingpoorly explains theNamurian^WestphalianB sedimentarysequence, if only a Dinantian rifting event is assumed.Even if a high late-Dinantian palaeowater depth is takeninto account, this model still does not ¢t the observedsubsidence curves. A better match between the observedandmodelled tectonic subsidence can be achieved if an ad-ditional Namurian rifting event is assumed.Therefore, wesuggest that an additional Namurian rifting event tookplace in the Netherlands, in contrast to the observationsfrom the United Kingdom, where the Namurian is a timeof tectonic quiescence (thermal sag).This might be in ac-cordance with the ¢nding of an intra-Namurian erosionalunconformity and the presence of extrusives in the centralpart of the Netherlands. Although a rifting model poten-tially explains the Namurian^Westphalian B subsidenceobserved in the Netherlands, it does not account for thelarge-scale asymmetric shape of subsidence pro¢les.Theconcept of dynamic topography might explain the wave-length and amplitude of the observed subsidence data asit is shown that mantle- £ow-induced dynamic topogra-phy causes subsidence at wavelengths in the same order asthat observed in the CNWECB.

ACKNOWLEDGEMENTS

The research carried out for this paper is part of thePhD-project of HK on the Upper Carboniferous of theNetherlands, funded by TNO Built Environment andGeosciences/Geological Survey of the Netherlands. Thecritical proofreading of the manuscript by Kees van derZwan is highly appreciated. The constructive reviews byP. Burgess and A.Watts, as well as the comments from ananonymous reviewer greatly helped to improve the manu-

script.The editor J. Nunn is thanked for his communica-tion e¡orts.

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Manuscript received 9 February 2007; Manuscript accepted 26November 2007



Late Carboniferous foreland basin formation and Early Carboniferous stretching in Northwestern Europe: inferences from quantitative subsidence analyses in the Netherlands

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