Crustal taper and topography at passive continental margins 2011/Taper paper.pdf · Crustal taper...

Crustal taper and topography at passive continental margins

P. T. Osmundsen1,2 and T. F. Redfield1

1Geological Survey of Norway, N-7491 Trondheim, Norway; 2Department of Arctic Geology, University Centre in Svalbard, N- 9171

Longyearbyen, Norway

Introduction

Tectonic unroofing during continentalextension and breakup causes kilo-metres-scale rift-flank uplift alongpassive margins (Buck, 1988; Weisseland Karner, 1989). Although theresultant topography is countered bypost-rift erosion and thermal subsi-dence (McKenzie, 1978; Watts, 2001),many passive margins display impres-sive ocean-facing escarpments thatcontinue to control sediment routingand supply for tens of millions ofyears after rifting has ceased (e.g.Gilchrist and Summerfield, 1990;Gilchrist et al., 1994; Gunnell andFleitout, 1998; Lidmar-Bergstromet al., 2000; Moore et al., 2009; Gabri-elsen et al., 2010). The cause of eithercontinuity or rejuvenation of suchtopography is not well understood,and consequently is hotly debated(e.g. Lidmar-Bergstrom and Bonow,2009; Moore et al., 2009; Nielsenet al., 2009, 2010; Gabrielsen et al.,2010). As the geological processesbehind the long-term evolution of apassive margin are fundamental tothe evolution of hydrocarbon-bearingbasins as well as to landscape evolu-tion and geohazards, they may be ofsignificant importance to society.Several mechanisms have been pro-

posed to explain post-rift uplift in thecircum-Atlantic region (e.g. Faleideet al., 2002). However, no current

model is completely satisfactory. Forexample, it is difficult to link the 1 kmof Neogene uplift in Norway proposedby Riis and Fjeldskaar (1992) to thethermal effects of breakup at c. 54 Ma.Likewise, Neogene glacial erosion can-not explain the proposed Palaeogeneuplift in the circum-North Atlanticregion, nor can it explain present-dayescarpments at non-glaciated marginssuch as those of Namibia or Brazil.Numerical models of mantle-relatedtopography under areas such asGreen-land, Scandinavia, Brazil or southernAfrica (e.g. Steinberger, 2007) tend toproduce long-wavelength, low-ampli-tude and morphologically symmetricuplifts that peak in the continentalinterior. Nevertheless, the heights ofthe asymmetric escarpments that runalong the coasts of these margins canapproach or even surpass 2 km.Although it is likely that external

processes add their topographic sig-nature to passive margins, the globallywidespread distribution of post-riftescarpments indicates that they relyon a primary control rooted in theprocesses related to rifting and break-up. One such fundamental process iscrustal thinning.In this contribution, we review data

and recent observations from offshoreand onshore the Norwegian marginand a compilation of published geo-seismic cross-sections from otherpassive margins to demonstrateempirically that a robust control onescarpment topography is associatedwith the location of the first highlythinned crust in the distal margin. We

first review some recent concepts forcrustal thinning at passive margins.

The architecture of extendedmargins

Recent publications have introducednew perspectives to our knowledge ofcrustal thinning at passive margins,and as a result, a new conceptualframework has emerged (e.g. Manats-chal et al., 2001; Perez-Gussinye andReston, 2001; Manatschal, 2004; Res-ton et al., 2004; Reston, 2009; Peron-Pinvidic and Manatschal, 2009).These works show that many passivemargins can be divided into proximaland distal domains, where parts of thelatter are characterized by continentalcrust thinned to <10 km (e.g. Reston,2009; Peron-Pinvidic and Manatschal,2009). For a number of margins, it isnow well demonstrated that crustalthinning in their distal parts wasfacilitated by low-angle normal faultswith displacements in the order of tensof kilometres (e.g. Boillot et al., 1987;Reston et al., 1995, 2004; Manatschalet al., 2001; Whitmarsh et al., 2001;Thinon et al., 2003; Reston, 2005;Tucholke et al., 2007; Osmundsenand Ebbing, 2008; Peron-Pinvidicand Manatschal, 2009). These faultsbecame directly responsible for theexhumation of serpentinized mantlerocks to the seafloor outboard of themost highly extended continentalcrust (e.g. Boillot et al., 1987; Man-atschal et al., 2001; Perez-Gussinyeand Reston, 2001; Manatschal,2004). For such complete excision to

ABSTRACT

The long-term evolution of kilometre(s) high, seaward-facingescarpments at passive margins is linked directly to the crustalthinning gradient. The development of ‘post-rift’ faults in theonshore parts of the margin and the associated distribution ofdrainage patterns, landscape types and sediment dispersalpatterns reflect this linkage. For the seismically well-imagedNorwegian margin as well as for a number of passive marginsworldwide, we identify a scaling relationship that correlatesescarpment height with the distance to the highly extendeddistal margin or, more specifically, the taper break. The highest

escarpments and the most asymmetric margin topography arefound inboard of sharply tapering crystalline crust, independentof margin age. Conversely, escarpments are lower where thetaper is gentle. Thus, the topography of passive margins doesnot primarily reflect age, magmatism, climate or mantleconvection, but more probably the response to loading of thelaterally variable, fault-controlled architecture of the crystallinecrust.

Terra Nova, 00, 1–13, 2011

Correspondence: P. T. Osmundsen, Geolog-

ical Survey of Norway, N-7491 Trondheim,

Norway. E-mail: [email protected]

� 2011 Blackwell Publishing Ltd 1

doi: 10.1111/j.1365-3121.2011.01014.x

occur, normal faults must cut theentire crust. On the Iberian margin,this became possible only when thecrust had already been made brittlethroughout by an earlier reduction toa thickness of �10 km or less. Thus,previous thinning is required to havefirst reduced the continental crustfrom its �normal� pre-extensionalthickness (Peron-Pinvidic and Man-atschal, 2009; Mohn et al., 2010).The �necking zone� at a passive

margin has been defined as the areawhere the crust tapers off from athickness of �30 km to <10 kmthickness (Peron-Pinvidic and Man-atschal, 2009; Mohn et al., 2010).Recently, it has been shown thatlarge-magnitude normal faults playan important role also in the forma-tion of the necking zone, and that the�hyper-extension� style of deformationis not exclusive to so-called magma-poor margins, but has occurred alongmagmatic margin segments as well(Osmundsen and Ebbing, 2008; Lun-din and Dore, 2011).

The passive margin offshoreNorway

Based on seismic reflection, refrac-tion ⁄OBS and potential field data, ithas been shown that the fundamentalstructure of the Norwegian extendedmargin changes significantly alongstrike from south to north, and thatthe Møre and Vøring sectors displayvery different crustal thinning gradi-ents (e.g. Mjelde et al., 2005; Ebbinget al., 2006; Faleide et al., 2008; Figs 1and 2). Changes in the thickness of thecrystalline crust relate mainly to dif-ferences in pre-breakup thinning and,in particular, to the distribution anddisplacement magnitude of very largenormal faults (Osmundsen et al.,2002; Gomez et al., 2004; Osmundsenand Ebbing, 2008).The crustal taper is sharp in the

Møre margin area, where up to200 km of highly extended crust underthe Møre Basin is separated from theunrifted continent by a narrow neck-ing zone. The necking zone is definedby an extensional detachment com-plex that thinned the crust from 30 kmdown to <10 km over a horizontaldistance of <100 km in the LateJurassic–Early Cretaceous (Fig. 2;Osmundsen and Ebbing, 2008). Incontrast, the taper is relatively gentle

inboard of the Vøring Basin, wherehighly extended crust under the basinis separated from the continent bymoderately thinned, up to 200 kmwide platform and terrace areas.Large-magnitude normal faults andflexures separate western and northernedges of the Trøndelag Platform fromthe Halten Terrace and from the deepCretaceous basins farther offshore(Blystad et al., 1995; Brekke, 2000;Osmundsen et al., 2002). The depthsof flanking Cretaceous basins arecomparable to those in the MøreBasin (10–15 km; Ebbing et al.,2006). North of the Trøndelag Plat-form, in the Lofoten–Vesteralen (Lof)

and Troms (Lyn) areas, the taper ofthe crystalline crust once againbecomes relatively sharp. In the caseof Lofoten, the extension is distrib-uted over 3–4 large normal faults ofpre-Cretaceous and Early Cretaceousage (Blystad et al., 1995; Tsikalaset al., 2005; Bergh et al., 2007).We propose that after Permian-

Triassic rifting, the most importantvariations in crustal structure weredetermined by the large-magnitudeJurassic-Cretaceous faults, whichreached displacement values in theorder of 20–30 km. The location of,and interference pattern between thesestructures determined the position of

Fig. 1 Map of the Norwegian passive margin and adjacent Fennoscandia, showingmain elements of offshore architecture and onshore topography and drainagepatterns (simplified from Blystad et al., 1995; Sandwell & Smith, 1997; Mosar et al.2002 and from Faleide et al., 2008). Profile lines f1–f6 refer to geoseismic sectionspresented by Faleide et al. (2008); and lines (or1) through (or6) documenttopographic cross-sections prepared for this paper. The merged sections are presentedin Fig. 3. Red dots indicate the taper break as constrained by the sections of Faleideet al. (2008). Green dots indicate the maximum elevation of the seaward-facingescarpment (see Fig. 3). Dashed black line indicates the inferred map-plane trajectoryof the taper break. SSB, Slørebotn Sub Basin; MB, Møre Basin; VB, Vøring Basin;HT, Halten Terrace; TP, Trondelag Platform; VG, Viking Graben; MTFC, Møre-Trøndelag Fault Complex; KFC, Klakk Fault Complex; Lof, Lofoten Archipelago;Lyn, Lyngen �alps�; TB, Taper Break; COB, Continent-Ocean boundary after Faleideet al. (2008); GBTZ, Gulf of Bothnia Transition Zone, which is the point of commonorigin for profile coordinates shown in Fig. 3.

Crustal taper and topography • P. T. Osmundsen and T. F. Redfield Terra Nova, Vol 00, No. 0, 1–13

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2 � 2011 Blackwell Publishing Ltd

the necking zone with respect to therelatively �undeformed� continent, theposition of the most distinct crustalthickness minima and the location ofpost-rotational �sag� basins (i.e. theMøre and Vøring basins) that begandeveloping after the main phase ofLate Jurassic–Early Cretaceous crus-tal thinning, but prior to magmaticbreakup in the Eocene. Closer to theline of breakup, a Late Cretaceous–Palaeogene phase of crustal thinningaffected areas that were already veryseverely thinned, such as the GjallarRidge (Blystad et al., 1995; Ren et al.,1998; Gernigon et al., 2006). In theSkagerrak Sea area south of Norway,the Triassic and Jurassic strata arewarped, eroded and overlain uncon-formably by the Middle CretaceousChalk Group, consistent with upliftand south-eastwards rotation of thatpart of Scandinavia in the Late Jurassic–Early Cretaceous (e.g. Gabrielsen etal., 2010). Offshore western Norway,

clastic wedges of Late Jurassic agewere sourced in the Norwegian main-land and deposited outboard of pres-ent-day fjords, supporting the viewthat mainland Norway was beinguplifted at this time (Gabrielsen et al.,2010). The turbidites of the Danian–Maastrichtian Ormen Lange Fan ofthe Møre Basin requires substantialonshore topography in the drainagearea (see Sømme et al., 2009).Some components of uplift clearly

occurred during the Cenozoic (Riisand Fjeldskaar, 1992; Riis, 1996; Lid-mar-Bergstrom et al., 2000; Gabriel-sen et al., 2010). Offshore along theNorwegian coast, a �hinge zone� isdefined by warped Mesozoic andCenozoic strata consistent with post-Oligocene uplift (e.g. Faleide et al.,2002; Gabrielsen et al., 2010). Off-shore evidence includes clastic influxof sediments into the Møre Basin inthe Palaeogene (e.g. Faleide et al.,2002) and in the Norwegian–Danish

Basin in the Oligocene (Rasmussen,2009). The Molo Formation, a pro-grading deltaic unit of Late Miocene–Early Pliocene age has been taken asevidence for uplift of the mainlandduring Neogene time (e.g. Eidvinet al., 2007). Later Neogene glacia-cions are represented offshore by theLate Pliocene–Pleistocene sedimen-tary wedge (e.g. Ottesen et al., 2009).

The Norwegian margin onshore

In Norway, the highest, most deeplyincised, most alpine and most asym-metric onshore topography occurs inthe Møre, Lofoten–Vesteralen andTroms areas (Figs 1 and 3; e.g.Etzelmuller et al., 2007; Osmundsenet al., 2010), directly inboard of themost sharply tapering segments ofthe passive margin. Conversely, in-board of the more gently taperingcrystalline crust of the TrøndelagPlatform (Figs 1 and 3), the onshore

Fig. 2 (a) Seismic expression of major seaward-dipping, top-basement detachment fault (white arrows) at the Møre margin,offshore Mid Norway (Osmundsen and Ebbing, 2008). The detachment became responsible for thinning the crust from ‡30 kmdown to an average of <10 km in the Late Jurassic–Early Cretaceous. Thus, the dramatic crustal thinning gradient (taper) in theMøre area depends largely on this fault system. Detail from seismic reflection line GMNR 94-103. (b) Depth-convertedinterpretation of part of long-offset seismic reflection line GMNR 94-103 (Osmundsen and Ebbing, 2008) showing fault in (a) andthe hangingwall cutoff for the closest fault-block in the highly thinned continental crust under the Møre basin (red vertical line),conceptually the location of the taper break along this profile (see main text). LC, Lower Cretaceous; LCB, Lower Crustal Body;PC, Pre-Cretaceous; SB, seismic basement; SD, Slørebotn detachment; Te, Tertiary and Quaternary; UC, Upper Cretaceous.

Terra Nova, Vol 00, No. 0, 1–13 P. T. Osmundsen and T. F. Redfield • Crustal taper and topography

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topography is lower, exhibits a re-duced escarpment and is less asym-metric. Thus, a first-order correlationbetween the crustal taper and theheight and shape of the onshoreescarpment is evident. A correspond-ing correlation is observed betweentaper and the distribution of relictsurfaces, drainage patterns and major

reactivated faults (Osmundsen et al.,2010).Palaeosurfaces are widely preserved

in Scandinavia east of the watershed,where they partly define the gently east-to south-east sloping topography (e.g.Lidmar-Bergstrom et al., 2000, 2007).Their westwards fanning pattern hasbeen interpreted in terms of erosional

bevelling punctuated by episodes ofuplift (Lidmar-Bergstrom et al., 2007).Their ages of formation have been thematter of much debate (e.g. Riis, 1996;Lidmar-Bergstrom et al., 2000, 2007;Japsen et al., 2006; Lidmar-Bergstromand Bonow, 2009; Nielsen et al., 2009,2010; Gabrielsen et al., 2010), but theoldest ones are constrained by apatitefission-track (AFT) data (Norway) andfossil material (Finland) to haveformed between the Late Jurassic andthe Eocene (Rohrman et al., 1995;Ebert, 2009). Late Jurassic AFT agesare widespread at sea level in coastalNorway, indicating 2–3 km of denu-dation since 150–160 Ma (Rohrmanet al., 1995; Hendriks et al., 2007).Fission-track age minima of 110 Maor less mainly occur inboard of sharpcrustal tapers in the Møre and Lofo-ten–Vesteralen areas. Herein, sea-levelAFT (2r) age-jumps in combinationwith structural data also provide evi-dence for kilometre-scale, Late or post-Cretaceous reactivation of faults (Red-field et al., 2004; Hendriks et al., 2007,2010; Leighton, 2007; Osmundsenet al., 2010). Normal-sense reactiva-tion appears to have occurred afterc. 100 Ma along the Møre-TrøndelagFault Complex (MTFC; Redfieldet al., 2004, 2005a,b), and at or laterthan 81 ± 7 and 72 ± 5 Ma alongbasin-bounding normal faults inLofoten and Vesteralen respectively(Hendriks et al., 2010). Normal-sensereactivation is supported by structuralwork carried out along the tectoniclineaments that bound high-relief, lin-ear alpine ranges in Møre, Lofoten–Vesteralen and Lyngen areas (Bering,1992; Redfield et al., 2005a,b; Redfieldand Osmundsen, 2009; Osmundsenet al., 2010). These alpine ranges mostprobably evolved, as glacial erosionwas allowed to exploit the pre-glacialfootwall drainage patterns (Osmund-sen et al., 2010). In Lofoten, relictsurfaces that occur at a height of afew hundred metres appear to havebeen tilted towards the south-east, in amanner consistent with uplift androtation of the footwall of the WestLofoten Border Fault (Fig. 4; seeBergh et al., 2007, 2008; Hendrikset al., 2010; Osmundsen et al., 2010).In addition to the kilometre-scalenormal faults, a set of small-scale,recent stress-release reverse faults wereobserved in roadcuts in Mid Norwayby Roberts and Myrvang (2004).

Fig. 3 Cross-sections through the Scandinavian passive margin based on offshoresections by Faleide et al. (2008) and onshore sections constructed by ourselves usingan 800-m resolution DEM in combination with the Crust 2 Moho model (Bassinet al., 2000) and Moho models by Svenningsen et al. (2007), Stratford et al. (2009),and England and Ebbing (2008). Note vertical exaggeration of topography above sealevel. A direct relationship appears to exist between the crustal thinning gradient(taper) and the present-day onshore topography. When quantified (see Fig. 6 below),this relationship becomes quite clear. Sharp tapers, such as those of the Møre area(Section 1), are flanked by high, fault-bounded alpine topography (Osmundsen et al.,2009, 2010) and a pronounced topographic asymmetry (Redfield et al., 2005a).Inboard of the Trøndelag Platform (TP), which is characterized by a much gentlercrustal taper, topography is less elevated, less escarpment-like, and much lessasymmetric. Thus, on the Norwegian margin, there is a relationship between thegross-scale offshore structure and the adjacent onshore topography. Lof, LofotenArchipelago; TR, Troms.


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Quantifying the influence of crustalarchitecture on topography

The consistent correlation betweencrustal taper, topography and geo-morphology along a margin whosebreakup occurred everywhere atc. 54 Ma suggests that the variationin crustal taper exerts a long-termcontrol on both the tectonic andgeomorphic evolution of the hinter-land. To test how much control thetaper exerts over passive margintopography on a more general level,we have defined some benchmarklocations in the generalized marginarchitecture that can be readily iden-tified and used to quantify aspects oftaper and topography. We next applythis approach, first to the Norwegianmargin, and then to a set of otherpassive margins distributed across theglobe.

One obvious benchmark is the gen-eralized location of the Continent-Ocean boundary (COB), which definesthe continental margin per se. Davisand Kusznir (2002) noted a spatialrelationship between crustal thinningfactors and distance to the COB. How-ever, the relationship observed betweenthe height of the escarpment and thedistance from the escarpment to theCOB is inconclusive (see Fig. S3 inSupporting Information). Uncertain-ties commonly associated with locatingthe COB and the statistical limitationsto our dataset may have precluded amore conclusive resolution.In light of both recent models for

passive margins and the empiricalrelationship observed along the Nor-wegian coast (Fig. 1), another bench-mark point appears as equally ormore important than the COB in thepresent context. This is the �taper

break�, defined here as the point near-est the coast where the crustal thick-ness is reduced to 10 km or less(Fig. 5a,b). Conceptually as well asin the well-imaged Norwegian exam-ple, the taper break is located close tothe hangingwall cutoff for the firstfault-block in the distal margin. Atmargins with an even taper, the taperbreak is placed at the 10 km mark(Fig. 1c). An inflection point in thebasinward-shallowing Moho may alsobe commonly located at or in closevicinity of the taper break. Thus, thedistance between crust of �normal�thickness and the taper break (thetaper length) relates to the width ofthe necking zone and to the meantaper angle for the crystalline crust inthe area, where it is thinning fromnormal down to <10 km.A measure of the taper length

would require that we could confi-dently measure the distance from thetaper break to the first occurrence ofcrust of normal pre-rift crustal thick-ness under the continent. However,the taper length is generally notdirectly recoverable. Global data suchas the CRUST2 model (Bassin et al.,2000) are of too coarse resolution toaccurately identify the transition tounrifted crust. The mean thickness ofemergent and non-orogenic continen-tal crust in the CRUST2 model isabout 38 km, and thus the �30-kmcrustal thickness resolved at the land-ward end of a number of deep seismictransects should probably not be usedas a proxy for unrifted crustal thick-ness, as it would underestimate thetaper length. This is well illustrated byour two best-constrained transectsfrom Norway (i.e. the transects withthe best definition of the subcontinen-tal Moho, see Figs 2 and 3).As the true taper length can only

rarely be measured with confidence, weare obliged to use another, more mea-surable entity as a substitute for thetaper length when extending our con-cept to a global dataset. The maximumtopographic elevation encountered atthe passivemargin escarpment (definedhere as the highest peak within a 20 kmradius moving window swath) does notnecessarily overlie the point where thecrust regains normal thickness. In thetwo best-constrained sections of Fig. 3,the highest point on the topographicescarpment lies close to and directlyoutboard of the thickest crust. Such

(a)

(b)

Fig. 4 Examples of fault-controlled topography and landscape contrasts inboard ofsharply tapering crystalline crust. (a) Topographic and landscape contrast acrosssound that hosts Jurassic half-graben basin and its bounding faults in Sortland,north-east of the Lofoten archipelago (Figs 1 and 3). The gently dipping topographyin the foreground is located in the hangingwall of the bounding fault(s), whereas thealpine topography in the background is located in the footwall. Apatite fission-trackdata indicate reactivation of the half-graben bounding fault(s) at or later than72 ± 5 Ma (Hendriks et al., 2010). The faults probably represent the continuation ofthe West Lofoten Border Fault of Bergh et al. (2007) and crop out at the base of thealpine range in the background (Osmundsen et al., 2010). (b) South-eastwardsdipping palaeosurfaces in the Lofoten archipelago (Bergh et al., 2008; Osmundsenet al., 2010). South-eastwards rotation of the surfaces is consistent with uplift alongthe NW-dipping West Lofoten Border Fault (op. cit. and Bergh et al., 2007). Apatitefission-track ages constrain the latest phases of uplift in the footwall of this faultsystem to be Late Cretaceous or younger (Hendriks et al., 2010).


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positioning may not be coincidental;although rift-related escarpments canbe replaced by erosional escarpmentswithin a few million years (Gilchristet al., 1994), Matmon et al. (2002)concluded that the positions of thenew escarpments are controlled pri-marily by deeply rooted crustal struc-tures. Our empirical observations fromthe Norwegian margin (Figs 3 and 6)suggest that the location of the highestpoint of the escarpment relative to thetaper break holds an important tec-tonic significance.The relationship in Fig. 6 invites

comparison with other passive mar-

gins. In the following, we test theabove relationship on a global dataset.We also assess the relative effects ofother parameters frequently invokedin models for margin uplift, such asbreakup age, magmatic character, gla-cial history and computed mantle-induced uplift.

A simple test of taper andtopography

We have spliced 40 published, depth-converted cross-sections (includingthose from Norway) through the off-shore parts of passive margins with

onshore topographic data to build adataset from which the apparent taperlength and escarpment elevation canbe measured directly. Our ability tomeasure correctly and confidentlyrelies on the quality of the data andon the previous interpretations usedin our compilation. Also, measureddistances sometimes become a ques-tion of definition. We have compiledour measured sections in Section 2 ofthe Supporting Information thataccompanies this article.In Fig. 7, we have plotted escarp-

ment elevation against apparent taperlength for the entire dataset. We

Fig. 5 Schematic cross-sections of some generalized passive margin geometries showing subdivision into proximal and distalmargin (see Peron-Pinvidic and Manatschal, 2009) and the definition of taper break, taper length and apparent taper length used inthis paper. Measuring crustal thickness away from the escarpment, the taper break is defined to be at the first location where thecontinental crust is thinned down to a thickness of 10 km or less. This location corresponds roughly to the outer limit of the�necking zone� and the first crustal thickness minimum outboard of the proximal-distal margin boundary as defined in Peron-Pinvidic and Manatschal (2009). For some margins with good seismic definition at depth, this location has been shown tocorrespond to the hangingwall cutoff for the first rotated fault-block in the distal margin. The principal difference between themargins in (a) and (b) is the crustal thinning gradient: in (b), more gradual thinning resulted in a platform area with intermediatecrustal thickness and a gentler taper. The taper break is readily defined in both (a) and (b), but the actual reduction in crustalthickness at the taper break is much less in (b). In the case of even tapers (c), the taper break is defined as the first instance ofcontinental crust of 10 km thickness or less. The greatest uncertainty in the definition of taper length lies in the definition of normalcontinental crustal thickness, and where it is achieved under the continent. We thus must instead measure the apparent taperlength, which is the distance from the taper break to the highest point on the onshore margin escarpment. (d) Shows outline ofescarpments, coastline, taper break and COB on a hypothetical margin with platform and deep basin areas. Profiles a–a¢ and b–b¢constructed normal to the main domain boundaries of the margin may resemble profiles in (a) and (b), respectively, and documentvariations in taper along the margin.


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conclude that the relationship betweenapparent taper length and topographyis a global phenomenon: the highestmaximum elevations are found formargins with short apparent taperlengths. Conversely, where the appar-ent taper length is longer, the maxi-mum elevation of the onshoreescarpment is less. The implication, inour view, is that the ratio betweenapparent taper length and escarpmentheight reflects a fundamental relation-

ship between (syn-rift) crustal thinningand (post-rift) escarpment evolution.Active rifts or margins that are

very young (e.g. the Red Sea ⁄Afarescarpments of Fig. 8 and the Tran-santarctic Mountain rift shoulder)display the greatest maximum eleva-tions. Morocco, the margin with theoldest age of breakup (175–190 Ma),exhibits a very low maximum eleva-tion. Thus, at first glance, an age-function that describes a slow decay

of syn-rift topography appears rele-vant. Between these end-members,old margins such as that of Namibia(breakup at c. 127 Ma) present high-relief, high-elevation escarpments,whereas the much younger South-ern Australia margin (breakup atc. 85 Ma) escarpment is as low orlower than that of Morocco (Fig. 7).Also, where the age of breakup isconstant, wide ranges in escarpmentelevation exist. For instance, both theNorwegian and East Greenland mar-gins display great along-strike topo-graphic variation, which in the case ofNorway correlates directly with thesharpness of the crustal taper. Otherconjugate margins clearly showstrongly contrasting topography, suchas those of South Africa ⁄Namibia(high escarpment, short taper) andArgentina (no escarpment, very gentletaper). Thus, the long-term topo-graphic evolution of passive marginsdoes not appear to be dominated bya simple age of breakup function(Figs 7, S3 and S6).Our sections include �old� and

�young,� glaciated, non-glaciated,magmatic and non-magmatic mar-gins, over regions of both high andlow mantle velocities (Figs S6–S12).When published data are considered,elevation appears to correlate betterwith the crustal taper than with any ofthe abovementioned properties. Theonly truly global relationship revealedby our test appears to be that betweenescarpment elevation and apparenttaper length.

Discussion

We have identified a global relation-ship that links the height of a passivemargin escarpment to its offshorestructural architecture via the locationof the taper break. As we cannotconfidently identify the innermostlimit of extended crust for manymargin segments, the relative impor-tance of the true crustal thinninggradient upon escarpment elevationcannot be entirely resolved in thiscontribution. However, the observedrelationship between escarpment ele-vation and apparent taper length mustbe to a large degree associated withthe seawards tapering of crystallinecrust, as the apparent taper length inall our examples covers the regionwhere the crust is thinned from �30 to

Fig. 6 Plot of apparent taper length vs. escarpment elevation for sections through theNorwegian margin presented in Fig. 3. The highest escarpments are associated withshort apparent taper lengths (and sharp crustal tapers), whereas more subduedescarpments occur inboard of long apparent taper lengths.

Fig. 7 Graph showing onshore maximum escarpment elevation against apparenttaper length for 35 post-breakup margin segments (Information about offshorecrustal structure compiled from Gladczenko et al., 1998; Schlindwein and Jokat,1999; Contrucci et al., 2004; Franca and Assumpcao, 2004; Schmidt-Aursch andJokat, 2005; Direen et al., 2007; Hansen et al., 2007; Afhilado et al., 2008; Antobrehet al., 2009; Blaich et al., 2008; Faleide et al., 2008; Hirsch et al., 2009; Klingelhoeferet al., 2008; Reston, 2009 after Chian et al., 1995; Dragoi-Stavar and Hall, 2009) andfive pre-breakup margins (Makris et al., 1975; von Frese et al., 1992; Seber et al.,2001; Hansen et al., 2007; Huerta, 2007). Horizontal error bars reflect uncertainties indistance measurements made along cross-section profiles. This graph shows that theevolution of onshore topography at passive margins follows a nonlinear, butpredictable trend during both the syn-rift and post-rift phases. A logarithmic best-fitline has R2 value 0.72 (see Fig. S3).


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(a)

(b)

(c)

(d)

Fig. 8 Four examples of cross-sections that show the relationship between escarpment elevation and apparent taper length fromnon-glaciated African margins. (a) Section off Morocco after Klingelhoefer et al. (2008). The age of breakup is c. 175–190 Ma.The apparent taper length is long, the escarpment virtually non-existent and its elevation very low. (b) Section off Namibia afterHirsch et al. (2009). Although the age of breakup is relatively old (c. 127 Ma), the escarpment is sharp, its elevation is high and thecrustal taper is very pronounced. (c) Section from the Red Sea rift in Arabia after Hansen et al. (2007) illustrating first-order riftmorphology, sharp taper and high elevation. (d) Composite section from the Ethiopian margin crossing the Afar Depression fromdata by Makris et al. (1975), Seber et al. (2001) and Redfield et al. (2003) This section shows a margin that is essentially in itssyn-rift phase (i.e. when the lithospheric rigidity is at its weakest).


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<10 km. In our view, this correlationsuggests that a commonality of pro-cess applies to all passive margins withrespect to the creation, maintenanceor rejuvenation of topography in thepost-rift phase.Although crustal taper forms pro-

gressively as the crust is thinnedduring rifting, it is probably thelarge-magnitude detachment faultsthat form during the margin�s �thin-ning� phase that are the most funda-mental agents in the reduction ofthickness of the crystalline crust (e.g.Lavier and Manatschal, 2006; Man-atschal et al., 2007;Mohn et al., 2010).Their three-dimensional arrangementtherefore probably controls the lateralvariations in crustal thinning gradientas well as the location of the taperbreak along the resultant passivemargin.In Scandinavia, copious data docu-

ment fundamental differences betweenhow the onshore portions of themargin evolved (Redfield et al., 2004,2005a,b; Osmundsen et al., 2009, Red-field and Osmundsen, 2009; Hendrikset al., 2010; Osmundsen et al., 2010).Normal faulting or normal-sense reac-tivation of pre-existing faults appearsto have occurred in the Latest Creta-ceous or Cenozoic inboard of the mostsharply tapering crystalline crust (seeRedfield et al., 2005a,b; Hendrikset al., 2010). Numerical modellingappears to support the role of inher-ited structures, such as the MTFC, asmajor stress boundaries in the post-rift deformation of the margin (Pascaland Gabrielsen, 2001).Cenozoic compressional deforma-

tion is observed offshore passive mar-gins such as that of Norway andIberia (Løseth and Henriksen, 2005;Peron-Pinvidic et al., 2008; Dore et al.,2008) as well as in some roadcutsonshore Norway (Roberts and Myrv-ang, 2004; Pascal et al., 2006). Thesecompressive structures represent verysmall strain magnitudes and appearout of phase with the processesresponsible for kilometre-scale nor-mal-sense fault reactivation.Onshore normal faulting in the

post-rift phase is not solely a Norwe-gian phenomenon. In western Green-land, normal faults displaced androtated an uplifted, relict surfaceincised into Palaeogene volcanic andsedimentary rocks, inboard of an areacharacterized by a sharp crustal thin-

ning gradient (Japsen et al., 2006). InBrazil, Palaeogene and Neogene sed-imentation has taken place alongfaults that strike parallel to the southBrazilian escarpment and to theBrazilian margin, and that have apronounced topographic expressionat present day (Zalan and Oliveira,2005). Viola et al. (2005) describednormal faults from the south-westernAfrican escarpment. Although theAFT age-jumps across these Africanfaults were not large enough to doc-ument �post-rift� fault activity, Violaet al. (2005) interpreted from generalgeological relationships that Meso-zoic–Cenozoic and neotectonic activ-ity was responsible for the observedmargin-parallel faulting. We suggestthat the association of sharp taper,�out of sequence� normal faulting,resultant topographic and landscapecontrasts and, in some cases, fault-bounded basin formation, is under-explored on several margins. Perhapssurprisingly, it appears from the Nor-wegian and Brazilian examples thatalthough fault activity migrates sea-wards during formation of a passivemargin (e.g. Peron-Pinvidic et al.,2007), it migrates inboard in thepost-rift phase, restricted mainly tothe areas of uplift, inboard of wherethe crustal taper is sharp.The observation that maximum

escarpment elevations follow a simplefunction related to crustal thicknessgradients, independent of breakupage, magmatism, glaciations or mod-elled mantle effects suggests thatcrustal and lithospheric factors dom-inate the evolution of an extendedmargin throughout the remainder ofits Wilson Cycle. Processes such aserosion and deposition, relative buoy-ancy and upwarping commensuratewith the flexural rigidity of the marginsegment probably constitute the prin-ciple parameters that govern thedevelopment and evolution of thetopography, structure and geomor-phology of a margin segment. Theobserved elevation limit of 3.5–4.5 km approached by the young,essentially syn-rift Ethiopian escarp-ment and the Royal Society range ofthe Transantarctic mountains followsdirectly. As unstretched, �normal�continental crust can only be un-loaded so much, the amount ofupwarping that is possible must alsobe limited.

Numerical models invoking an elas-tic thin plate with minimal couplingbetween the continent and the margin(Stuwe, 1991; Gunnell and Fleitout,1998) have successfully simulated thegeometry of rift-flank uplift, mainte-nance of post-rift topography, step-likeescarpment shapes and their propaga-tion. This situation is probably bestapproximated by margin segmentswith short distances from normalcrust to the taper break and a pene-tratively faulted, mechanically weak-ened distal margin – in other words,sharp tapers. As a significant part ofthe flexural rigidity of the lithosphereis determined by the brittle layer ofthe crust, brittle faulting will have animportant impact on the ability of thelithosphere to support imposed loadsor to bend. It follows that the taperbreak, which is located at the bound-ary between two domains of verydifferent mechanical properties, mustbe an important boundary also withrespect to flexural rigidity. As faultedcrust is weaker than undeformed crust(e.g. Buck, 1988; Watts, 2001), thenecking zone must represent the tran-sition from rigid, less faulted strongcrust to crust that is highly thinned,entirely broken by faults and thus veryweak. Such relations may be transient,and admittedly, it is not obvious thatreduction in crustal strength ismatched by a corresponding reduc-tion in lithospheric strength. Indeed,previous models have argued thatextended lithosphere becomes stron-ger after rifting and thermal relaxa-tion, thereby explaining the commonobservation of seaward fault migra-tion on passive margins (e.g. Zieglerand Cloething, 2004). However, therheological structure of the litho-sphere is currently under debate (e.g.Jackson, 2002). Some recent observa-tion-based works have argued that themost highly extended areas of themargins must remain weak for a longtime to explain, for instance, concen-tration of much younger compres-sional structures over areas withexhumed or nearly exhumed mantle(e.g. Peron-Pinvidic et al., 2008; Lun-din and Dore, 2011).Crustal thinning gradients control

gradients in thermal subsidence andthus the location of post-rift depo-centres. Footwall uplift will be largestin the area adjacent to the faultsthat facilitate the largest amount of


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tectonic unroofing (e.g. Kusznir andEgan, 1990), and coincides with theareas that undergo the deepest erosionafter rifting has ceased. Taper andload effects may therefore represent apositive feedback mechanism on manypassive margins, consistent with re-peated uplift in the areas that weremost uplifted in the syn-rift phase,and consistent with repeated or long-lasting sediment input from the samecatchment systems. For instance,some of the fjords in western Norwayrepresent areas of repeated input ofsediments to the North Sea basin fromthe Jurassic onwards (e.g. Gabrielsenet al., 2010).After the main phase of crustal

thinning, processes such as breakup-related underplating may theoreticallymodify the taper by adding crustalmaterial to the section. In Fig. S13and Table S1, we show the effect ofincluding high-velocity bodies, com-monly interpreted as underplate, inthe crustal section when defining thetaper break. The effect does notappear as significant on our globaltaper vs. elevation curve.

Conclusions

We conclude that the topographic fateof a passive margin is determined bythe end of the rift phase, and that thisis directly related to the pattern oflarge-magnitude extensional faultsthat created the crustal taper anddecided the location of the taperbreak. Crustal taper and its attendantstrength gradient (essentially, thelocation of the associated taper break)exercise the most fundamental andlong-lasting controls over escarpmenttopography and on the asymmetriccharacter of the onshore parts of therifted margin. Taper and load effectsmay comprise a positive feedback onmany passive margins, consistent withrepeated exhumation in the areas thatwere most uplifted in the syn-riftphase, and also consistent with re-peated or long-lasting sediment inputfrom the same catchment systems.Although thermal effects and climaticchanges may affect rates and mag-nitude of uplift, the shape and local-ization of uplift appears to bepre-determined and, in some areas,affected by post-rift normal faults.For passive margins in general, theimplications with respect to drainage

evolution and landscape formation,and the associated erosion, transportand deposition of sediments are pro-found.

Acknowledgements

We thank the Geological Survey of Nor-way for financial support and SusanneBuiter, Jorg Ebbing, Bart Hendriks andErik Lundin for stimulating discussions.The interpretations made in this paperremain, however, the sole responsibility ofthe authors. Gianreto Manatschal and ananonymous referee are thanked for theirinsightful and constructive reviews.

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Received 17 August 2010; revised versionaccepted 7 June 2011

Supporting Information

Additional Supporting Informationmay be found in the online versionof this article:Appendix S1. Electronic supplement

file for Osmundsen, P.T. and Redfield,T.F.: Crustal taper and topography atpassive continental margins.Table S1. Parameters used in con-

struction of the diagrams in Figs S3–S13.Please note: Wiley-Blackwell are

not responsible for the content orfunctionality of any supporting mate-rials supplied by the authors. Anyqueries (other than missing material)should be directed to the correspond-ing author of the article).


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