Mohns Ridge, Norwegian Sea. O. Dauteuil and ... Norwegian-Greenland Sea following a N060 ° trend from the .... Map of the ship tracks of leg 1 of the cruise.
TECTONICS,VOL. 15,NO. 4, PAGES 870-884,AUGUST 1996
Deformation partitioning in a slow spreading ridge undergoing oblique extension: Mohns Ridge, Norwegian Sea O. Dauteuil
and J.-P. Brun
GdosciencesRennes, UPR 4661, CNRS, Rennes,France
Abstract. Although oceanic spreading is often perpendicularto the ridge trends,in somecasesthe angle between these two directions can be significantly less than 90ø (40ø-50ø).This occursbecauseof eithera bendof the ridge trendor a changeof the spreading direction.We heredescribe
length(seeexamplesin studiesby Bonatti and Crane [1982], Lonsdale [1989], Embleyand Wilson[1992] andMacdonald et al. [1992]). Such changes in the rift-transform
configuration can produce abnormal structures (transverse ridges,medianridges) due to local compression or extension. However, in casessuchas the ReykjanesRidge [Searle and oblique spreading in the Mohns Ridge, resulting in Laughton, 1981], Mohns Ridge [Dauteuil et al., 1990; deformationpartitioningbetweenthe valley walls, which are Dauteuil and Brun, 1993], or North Fiji Basin [Auzendeet al., dominantlyaffected by strike-slipdisplacements, and the 1990], reorganization does not occur after a change of axial valley which is subjectto nearlypure extension.The axial valley walls are characterizedby en dchelonnormal spreading direction. In the above examples, the spreading ratesrangefrom medium(northFiji) to slow (Reykjanesand faultsaffectingthe walls, while the axial valley is affectedby parallelfaultsgroupedintoobliquesets.Thesefaultsetsdefine Mohns). At theseridges,new material is accretedalongto the spreadingridge, but the crust with the same age remainsas different structures,horst or tilted blocks, that are regularly spacedinsidethe axial valley.Moreoversomeridgesegments bandsparallel to the ridge. Such nonconventionalpatternsof spreading ridge raise a number of basic kinematic and mainlyundergopureextension,whereasothersare affectedby questions.The presentpaperis focusedon obliqueextension.We explainthis faultingpattern,including thermomechanical the analysis of deformation and kinematics of the Mohns the along-strikeand transverse variations,as a consequence of Ridge. depth variationsof the brittle-ductiletransition.
Geological Setting
Introduction
Oceanic ridges are commonly seen as trending perpendicularor nearly perpendicularto the spreading direction. However, recent studies have described ridges
undergoinga marked oblique extensionduring lengthly geological time spans [Searle and Laughton, 1981; Macdonald, 1986; Gudmusson, 1987; Auzendeet al., 1990; Dauteuil and Brun, 1993]. In these ridges, the angle between
the ridgeandthe spreading directioncanbe as smallas40ø. It is necessary, however,to distinguish obliquespreading ridges that exhibita fairly lineartrendon a regionalscale(from tens to hundredsof kilometers) from thosethat only correspondto small-scale anomalies within ridges trending nearly
perpendicularto the spreadingdirection. This latter case corresponds to secondorder discontinuities mostlydescribed in the Mid-Atlantic Ridge [Macdonald, 1986; Sempdrdet ai.,1990; Grindlay and Fox, 1992; Sempdrdet al., 1993] which are due to ridge offsets locally modifying the extensional pattern.
The ridge geometry undergoingoblique spreadingon a regionalscalemay stronglymodifiedby a significantchange in the spreadingdirection.The ridge systemcan reactrapidly giving rise to a new ridge (ridge jump) or a change in transformdirection or an extensionor shorteningof the ridge Copyright1996 by the AmericanGeophysicalUnion.
The Mohns Ridge extends for over 500 km in the Norwegian-Greenland Sea followinga N060ø trendfrom the JanMajen FractureZone in the south,to the KnipovichRidge in the north(Figure l a) [Perry, 1986]. This ridgewascreated perpendicular to the spreading direction,duringtheNNW-SSE
opening of theNorwegian-Greenland Sea,53 Ma [Talwaniand Eldholm, 1977; Vogt et al., 1981; Vogt, 1986]. During the Oligocene,a major kinematicchangeoccurredin this area where spreading in the Labrador Sea ceased and a counterclockwise rotation of the Greenland Plate relative to
the EurasianPlate (anomaly7) occured.In the NorwegianSea,
the spreadingdirectionbecameorientedN110ø-120 ø, andthe full spreadingrate sloweddownfrom 2.5 to 0.5 cm/yr.Later on, the spreadingrate slowly increasedto the presentday value of 1.8 cm/yr, with a period of reducedspreadingrate from 9 Ma (anomaly5) to 5 Ma (anomaly3a) [Gdli, 1991, 1993]. The spreadingdirectionremainsN110ø closeto the trendof Jan Mayen FractureZone The width of the axial valley ranges from 8 to 15 km,
beingpoorlydefinedat oneplace(72ø20N- 2øE)wheretheSE rift wall disappears for 30 km (Plate1 andFigure2). The rift shoulders are asymmetric, the shallowerNW shoulder having a complexbathymetricpattern,and the deeperSE shoulder having a simple geometry.This asymmetryis regionally persistantand is not a consequent of second-order segments. It has been observed constantly thoughout the whole
Paper number 95TC03682.
NorwegianSeaareaby Talwaniand EMholm[1977]andVogt
0278-7404/96/95TC-03682512.00
et al. [ 1981] and can resultfrom a large-scalephenomenon. 870
DAUTEUILAND BRUN:OCEANICOBLIQUERIFFING
40øW
Department of Geosciences Marines [Renard et al., 1989; Ggli, 1993] (Figure lb). The axial valley was surveyedwith 100% coveragerate, but the shoulderswere coveredby more widely spaced profiles. Some 170-km-long profiles were operatedperpendicularto the ridge. The resolution of the gridded data is better within the axial valley than on the shoulders where interpolation artefacts between ship tracks affect the bathymetry (see shadedrelief image; Figure 2). These artefacts,which appear between two ship tracks, are marked by rough bathymetry and generate alternationsof wavy bands, trending N-S and E-W. The main consequence is that the bathymetry appears more detailed within the axial valley, a fact which should be born in mind when comparing the distribution of structures in the axial valley and on the shoulders.
o 20øE
80øN.
70
N
MOHNS
RIDGE
60oN IND
Fault
40oW 20oE
20øW
0o
Figure la. Location of the Mohns Ridge in the rift system (gray line) of the northernmost part of the North Atlantic.The main magnetic anomalies (thin lines) and the spreading direction path (thick lines with arrows) have been drawn showing the kinematic change which occurred 27 Ma (anomaly 7).
The mean depth of the axial valley increases toward the northeast,which is due to the proximity of the Icelandic hot spot [Talwani and Eldhorm, 1977; Vogt et al., 1981]. The axial valley floor is characterizedby oblique topographic highs which were interpretedas volcanicridgesby Renard et al. [1989] and Ggli [1993] or as faultedblocksby Dauteuil and Brun [1993]. These oblique structures are the sites of significant seismicity being the loci of oblique swarms of earthquakes [Vogt, 1986]. Deep basinsreaching a depth of 3500 m alternatewith oblique highs which culminateat 1500 m above the basin floor. A local tomographicstudy focused on one of these structures [Lecomte,
871
Identification
and Analysis
of Fault
Pattern
The fault pattern is extractedfrom the gridded bathymetry by means of image processing,using methodsexplained by Dauteuil [1995]. Seafloor imagesprovidedby systemssuchas SeaMARK or synthetic aperatureradar (SAR) are evidently more accurateand efficient for this purposethan bathymetry. Three basic treatmentswere usedfor this study:the local slope calculation which indicates the general trend of the bathymetry, shadedrelief simulationswith different artificial lighting orientations,and stereoscopicview simulations.The fault pattern is obtained by interpreted these three types of images. Only the shadedrelief image, which is best suited for structural interpretation, is presented here. Quantitative methods such as slope values or automatic methods [Sauter, 1988; Sauter et al., 1988] can distinguish all types of structures.In this interpretation,we have to keep in mind that the faults are inferred from topographicscarpsand drawn with
2oW
0o
2oE
4oE
73øN
73oN
72øN
72oN
1990] revealed that
seismic velocities are higher beneath the rift shouldersand the oblique high than beneath the axial valley and surroundingdeepbasin.Dredgescarriedout by the R/V Meteor indicatethat oblique structuresare coveredby recentvolcanic sheet flows while the basins are floored by altered volcanic rocks [Hirschleber et al., 1988]. Although major transform faults are absent for 400 km,
the Mohns Ridge is characterizedby transfer zones trending nearly parallel to the spreading direction [Dauteuil et al., 1990; Dauteuil and Brun, 1993]. These transfer zones have
lengths ranging 10 to 40 km and correspondto bands with smoother bathymetry bounded by short large faults, which can be observed on the NW wall (Figure 2). They separate different areasof the axial valley. Methods
The fault pattern was extracted from the gridded bathymetry. The SeaBeam data set was acquired during a French cruiseon the R/V Jean Charcot, in 1988 by IFREMER,
2oW
0o
2ø-
4oE
Figure lb. Map of the ship tracks of leg 1 of the cruise carriedout by R/V Jean Charcotin 1988 and organizedby the GeosciencesMarine department of IFREMER. This leg surveyedthe axial valley of the Mohns Ridge along 250 km and mapped170-km-longprofilesperpendicularto that ridge. The axial valley has 100% coverage,while the ship tracksare more spacedout on the shoulders.
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DAUTEUIL AND BRUN: OCEANIC OBLIQUE RIFFING
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Figure 2. Artificially shadedrelief of the Mohns Ridge. The light comes from NW with a 40 ø inclination. This image processingapplied to the bathymetryhighlightswith great detail all the featuresaffecting the rift and allows an accurate interpretation by distinguishingtectonic from volcanic structures[Dauteuil, 1995]. The alternation of different roughnessbands on the shoulderscorrespondsto interpolation artefacts between ship tracks.
a sign which only indicates the fault scarp. This does not imply that all the drawn faults are pure normal faults, but that they have a normal component. Strike-slip faults can be identified only when topographic features are affected by
of magnitude need to be taken into account. Vertical offsets have been classifiedby 500-m range values.The fault pattern is also described by azimuth distribution, and by spatial distribution
of azimuth.
lateral offsets. Moreover, due to the limited resolution on the
seafloor, the structures probably correspondto fault zones rather than single fault planes. The fault pattern is digitized and superposed on bathymetricdata allowing a rough estimateto be made of the vertical offset on each fault. This parameter is estimated by measuring the depth differences between two points perpendicularto the fault track and corresponding to a change of sign of the slope. The real calculatedvalue is significant within the limits of depth accuracyobtainedby the SeaBeam (10 to 20 m [Renard and Allenou, 1979] so that only the order
Kinematic
Interpretation
of Fault
Pattern
Kinematic interpretation of the fault pattern provides a basis for interpretation of the style of deformation of an oceanicrift. Although the fault patterncannotbe mappedwith the same accuracy as in continental areas, the type of deformation
must be determined
in order to better constrain
the tectonic processes.This analysis is carried out in two steps: first determination of the fault geometry, and second analysis of the fault pattern. Two end member modes of
874
DAUTEUIL AND BRUN:OCEANICOBLIQUERIPTING GEOMETRY
OF SINGLE
FAULTS
work in progress, 1996). Its kinematic and mechanical significancehas been tested through laboratoryexperiments on small-scale models [Allemand, 1988; Tron and Brun,
b: wavy
a' linear
c' curved
1991; Dauteuil and Brun, 1993]. Single fault geometries fall into four categories: linear (Figure 3a), wavy (Figure 3b), curved (Figure 3c), and sigmoidal (Figure 3d). Curved and sigmoidal faults are correlatedwith oblique deep heterogeneitieswhich disturb the fault generation and propagation perpendicular to the extension [Tron and Brun, 1991; Dauteuil and Brun, 1993].
d' sigmoidal
ELEMENTARY
FAULT
ASSOCIATIONS ,,
f: anastomosed
e: parallel
Five basic patternsof faulting are identified: parallel (Figure 3e), anastomosed(Figure 3f), en •chelon composedeither of linear or sigmoidal faults (Figure 3g), spindle(Figure 3h) and "Y"-type (Figure 3i). Laboratoryexperiments[Tron and Brun, 1991; Dauteuil and Brun, 1993] show that parallel and anastomosedpatternsresult from nearly pure extension,while en 6chelon patternsindicate a strike-slip component.
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Morphology
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Axial
Valley
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The axial valley is characterizedby obliquehighs and basinsspacedat 20 to 45 km apart.Well-developped highs and basinsare locatedin the centralpartof the studyarea. From northeastto southwest (Plate 1), four obliquehighs (OH1 to OH4) havebeenstudiedin detail(Figure4). OH1
g: en chelon
trendsat N020ø, with a lengtharound14 km anda width of 6
h:s
•indle
i: "Y"type connection
km. It corresponds to two ridgesrisingup 600 m abovea nearlyflat depressionat a meandepthof 3000 m. The two ridges are bounded by steep walls toward the inner topographic depression and smoother flanks toward the
Figure 3. Geometry and pattern of faults in areas submitted to extension(pure or oblique).This classification wascarried out after a synthesisof fault patternscorresponding to natural cases(Afar, Iceland) and to small-scalemodels[Allemandand Brun, 1991; Tron and Brun, 1991; Dauteuil and Brun, 1993]. The models were used to determine
the kinematic
context of
the fault. The (a) linear and (b) wavy faults represent commonlyobsevedfaults. The (c) curvedand (d) sigmoidal faults indicate the presenceof a deep heterogeneity,the (e) parallel or (f) anastomosed fault set a pure extension,and the (g) en •chelon fault set a strike-slip deformation.The (h) spindle and (i) "Y" type faults correspondto connections between faults. All these structureswere observedin examples subjettedto one singlephaseof deformation.
valleyshoulders, givingan overallasymmetric shapeto each ridge.Volcanoesare locatedon the bottomof thedepression. OH2 has steepflanksand a plateauculminatingat 1000 m abovethe axial floor. It is 22 km longand9.5 km wide. The extremitiesrun parallel to the rift wall, formingan overall sigmoidalshape.The centralplateauis cut by smallfaultsand manyvolcanoescoverthe flanks.OH3 is morecomplex:it is affectedby a principalcurvedfault zonewhichstartson the SE valley wall and nearly reachesthe northwesternwall. It is 14.5 km long and 5.8 km wide, and it culminatesat 800 m above the axial floor. Volcanoesare mainly locatedon its
northwestern flank. OH4 is slightlyexpressed in the valley topography with a maximum elevation of 500 m above the
axial floor. it has a roundedshape,being 12.5 km long and 5.1 km wide and is coveredby severalsmallvolcanoes.
The deepbasinswhich separatethe obliquehighsdisplay variablegeometriesand sizes. The deepest(< ~3500 m) are locatedto the east and to west of OH I (Plate 1). The western
basinhasa sigmoidalshapewith a southernboundarythat is poorly defined because of the disappearance of its southeasternwall near 72ø20N-2øE. On the contrary, its northernwall is the steepestpart of the study area, thus
deformationcan be identified: strike-slipand pure extension. The preliminary classificationproposedby Dauteuil et al. [1990] is simplified here by consideringseparatelythe fault geometry and the spatial arrangement of faults (Figure 3). This classificationof fault types and spatial organizationhas been comparedwith well-exposednaturalexamplessuchas in
are interpreted eitheras smallnormalfaultsor as old scarps
the Afar depression [Souriot, 1993], or Iceland (Dauteuil,
coveredby lava flows (Figure 2). The easternbasinhas a more
separatingthe deepestzone of the valley from the shallowest zone of the shoulder. Its smooth and approximatelyflat bottom,possiblycomposedof lava flows [Lecomte,1990], is affectedby smallundulations trendingN020ø to N040ø. They
DAUTEUILAND BRUN'OCEANICOBLIQUERIFFING 3D BLOCK DIAGRAM
CONTOUR MAP (150 m)
875 INTERPRETATION
-2000• ..... -3500m
OBLIQUE
HIGH 1'
-2100m
,,,,3400m
OBLIQUE'-IG.2' Figure 4. Different kinds of oblique highs located inside the axial valley. (left) A contour map with isobathsspacedevery 150 m. (middle) A 3-D view of eachstructure,with the grey valuescorresponding to artificial lighting coming from the east. The 3x vertical exaggerationemphasizesthe volcanoesand the surfacedip. (right) The tectonicinterpretationof eachhigh. The interpretationof eachstructureis explained in detail in the text: OH1 may correspondto a grabenseparatingtwo tilted blocks,OH2 to a horst,OH3 to a tilted block and OH4, to a small dome or horst. Solid blobs indicate the volcanoes, the solid lines with ticks the faults with down sliding blocks.
complex shape due to normal faults of OH1 limiting its western
border.
The other
southwestern
basins have similar
flat floors; in all cases, the volcanoes are mainly located at the boundary between the basins and the oblique highs, but few are observed inside the basins (Figure 4). No significant sediment has been observed in the basins [Lecomte, Gdli, 1993]
Rift
1990;
Shoulders
Thevalleywallsdisl•lay a strong asymmetry. TheNW wall have steep slopes and high unevennessforming sharp and irregularanglesin differentplaces,whereasthe SE wall is more rectilinear and with a less conspicuous.As previously mentioned, the shouldersare highly asymmetric (Plate I and Figure 2). The northwesternshoulder,which is the shallow, has a complex topography and displays three main morphological trends. Topographic highs observed close to
the valley trend N060ø, parallel to the axial walls. A second morphologicaltrend, roughly oriented N l10 ø, is located at 72ø30'N-2øE. It offsets a N060 ø topographictrend located to the north. A third trend is oriented N030 ø (Plate 1), roughly parallel to the oblique highs of the valley. These N030ø topographicfeaturesform a band whosecenter is locatedat 72ø20'N-0ø30'E and which is limited at the north by N110 ø morphological trend (Plate 1). The wall bordering this northwestern shoulder has a complex geometry with trends oriented N100 ø and N030 ø. It exhibits the steepest slopes observedin Mohns Ridge area. The southeasternshoulder of the axial valley is strikingly different, with a simpler and smoother morphology. To the southwestern,it is made up of two topographicstepsparallel to the axial valley, the shallower step being locatedfar away from the valley (18 km). This pattern disappearstoward the northeast, where the southeastern valley wall totally disappears.The shallowesttopographicfeature is made up of small en ficheIonstepstrending N030 ø.
876
DAUTEUIL AND BRUN: OCEANIC OBLIQUE RIFFING
-2250m
-3100m
....
OBLIQUE
OBLIQUE
HIGH 3
HIGH 4
Figure 4. (continued)
Fault Pattern Analysis
correspondingto the oblique highs. They have not been detectedin the deep basins;this suggeststhat theseareasare
The interpretation of different bathymetric treatments allowed three kinds of tectonicsto be distinguished:faults with a significant
vertical
throw, minor faults and
undeterminedstructures(Figure 5). Although faults with a vertical throw are mapped as normal faults, this does not necessarilyimply that they are solely dip-slip normal faults, but simply that their throw has a vertical component(as explained before). Minor faults are features with weak throw
(lessthan 50 m) well observedin the bathymetry. Faulting along the valley walls is complex with short faults slightly oblique to the wall trend as observedon the northeasternand southwesternparts of the rift walls (Figure 5). In the central part, the faults are longer and have a more complex shape, particularly at the intersection between valley walls and oblique highs:they display "T" intersections (OH1, OH2), 90ø bend(OH1, OH3). The inwardfacingfaults display different types of shapesillustratedin Figure 3 but linear and wavy shapes predominate.The faults mainly
displaya left-lateralen 6chelonpattern,with a few parallel patternsconcentratenear the oblique highs.
Inside the axial valley, the fault patternis simpler.Faults are roughlyparalleland trendfrom N020ø to N045ø, except close to the walls where they becomecurved and oriented N060ø (parallelto the walls).Thesefaultshavemainlylinear or wavy shapesand form parallel patterns,groupedinto sets
less affected by vertical displacementor coveredby lava flows.
The oblique highs are characterizedby differents fault patterns(Figure 4). The structureof OH1 is mainly controlled
by two inward facing faults that limit blocks resembling outward
tilted
blocks.
The
whole
structure
is therefore
interpretedas a small grabenseparatingtwo tilted blocks. OH2 is affectedby outwarddippingfaultsroughlyparallelto the trend of topographichigh, as well as by a few smaller faultsparallelto the valley trend.Fracturating on the summit is parallel to the structural trend. The border faults are partially masked by volcanoes and lava flows. The whole
structure is interpretedas a horst.The OH3 structure is mainly affectedby a nearlycontinuous andinwarddippingfault zone located on the western flank, while the eastern flank is limited
by an inward dipping fault belongingto the southeastern valley wall. The summithas a gentleslopedippingtowards the southeastern wall, so the wholestructurecorresponds to a tilted block. A small obliqueand slightlytilted block can be distinguishedon the oppositeside of the valley. OH4 does not display any major faulting. All faults have a minor vertical offset and trend parallel to the high. The structure ressemblesa small dome or horst slightly fractured on the top.
The distributionof fault strike (Figure 5, inset) indicatesa
DAUTEUIL AND BRUN:OCEANICOBLIQUERIFHNG
AXIAL SPREADING
VALLEY
DIRECTION
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