J. geol. Soc. London, Vol. 141, 1984, pp. 62e637, 7 figs. Printed in Northern Ireland.

The geometry of listric growth faults in the Devonian basins of , W

John R. Hossack

SUMMARY: Thick Devonian clastic sequences accumulated in fault-bounded basinsin W Norway at the close of the Caledonian orogeny and mark the beginning of a phase of crustal extension. Traditionally, the eastern boundary fault to the Devonian basins has been regarded as a thrust but cross-sections based on the use of a branch-line reveal it to be anextension fault. There is clear omission of structural section across the boundary and bymatching up metamorphic rocks in the hanging- and foot-walls of the boundary fault, over 40 km of extensional dip slip can be measured at theN end of the fault. The boundary fault is a flat-lying listric fault with rollover anticlines in the hanging-wall. The fault also cuts up and down section along strike to form lateral ramps parallel to the movement direction. These ramps appear as wrench faults in outcrop. The ramp-flat geometry of the fault may have been inherited from a precursor Caledonian thrust fault which reversed its sense of slip in early Middle Devonian times to form the basins. Over 25 km of onlapping sediment accumulated in the largest basin during faulting in 14 Ma, with an average slip rate on the fault of 3 mm yr-'.

The group of four Devonian basins from the Sunni- sented a late tectonic Svalbardian phase (Vogt 1929). fjord area weredescribed by Kolderup (1923, 1925, Steel and his co-workers (Steel 1976; Steel & Aasheim 1926, 1927) (Fig. la). Theseare from N to S the 1978; Steel et al. 1977; Steel & 1980) have Hornelen, HBsteinen, Kvamshesten and suggested that the basins were formed by downwarp- basins, respectively. These isolated basins, in spite of ing on dip-slip (extension) faults which formed during their small area of outcrop, have thick onlapping dextralstrike-slip faulting with a curved or braided sequences of early or middle Devonian conglomerates, fault trace. This phase was then followed by Svalbar- sandstones and siltstones dated by fish and plant dian contraction, thrusting and uplift. remains.Because the conglomeratescontain pebbles However, none of the previous authors have been of mylonite and metamorphic rock derived from the ableto demonstrate or measure off-sets across any nearby Caledonian rocks (Hoisaeter 1971) the deposi- of the presumed normal, wrench or thrust faults. Some tion of the basin-fill clearly postdates most or all of of the explanations require several different tectonic the Caledonian deformation and metamorphism. events in sequence and other explanations violate the The stratigraphy, structureand sedimentology of simple rules of structural geology by which faults can these Devonian basins have subsequently been elabo- be identified. For instance, except in special cases, rated by several authors (Bryhni 1964; Skjerlie 1967, thrust faults repeat part of the structural succession 1971; Nilsen 1968, 1973; Hoisaeter 1971; Bryhni & and extensionfaults omit part of the structural Skjerlie 1975; Steel 1976; Steel et al. 1977; Steel & succession (Dahlstrom 1970). Although I have not Aasheim 1978; Steel & Gloppen 1980). However, visited the area, I believe that a study of the published there is no clear agreement on the major structureof maps, especially the compilation map of MBby (Kilda1 the basins and the tectonic sequence which controlled 1970) can be used to demonstratethat there is an their development. All these authors are agreed that extensional offset or omission of the structural sequ- the basin formation and sedimentationwere fault- ence across the main boundary fault of the basins. I controlled but have variously appealedto normal, hope to show that the basins formed during one event thrust and wrench faults to explain the formation of by syn-faulting accumulation of sediments in exten- the basins. N.-H. Kolderup (1921), Skjerlie (1971) and sional basins which had ageometry inherited from Hoisaeter (1971) suggested that the Devonian basins older Caledonian thrust faults. are thrust boundedand pointed to the presence of mylonites within the Devonian on theboundary faults, Regional geology and structure and the truncationof fold structures inside and outside the basins by the boundary faults. Bryhni (1964) and The Devonian basins of Sunnfjord lie above the most Nilsen (1968) emphasized theimportance of normal northwesterlyexposed part of the Caledonian faults controlling the sedimentation of the basins but orogenic belt (Fig. la). On theE side of the basins, in alsodescribed thrusting on a local scale. Hoisaeter theM@re area, is alarge dome of Precambrian (1971) and Bryhni & Skjerlie (1975) suggested that the basement and on the SE side of the dome is the main Devonian sediments originally accumulated in normal outcrop of the Caledonian thrust sheets in S Norway. fault-bounded basins andthat the thrustingrepre- Thesesheets extend in thearea of Fig. la from

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021 630 J. R. Hossack

[7 Devonian basins R Bergsdalen thrust sheets thrust gradesheetsmetamorphic / thrust fault ...****'buried branch-line High grade $l$ry[2; 0Valdres thrust sheets PreCambridnbasement Textensionfault .H*)eroded branch-line

FIG. la. Regional geological map of the southern Norwegian Caledonides showing the location of the Devonian basins and the branch-lines of the Bergsdalen and Valdres thrust sheets.

FIG.lb. Cross-section from the Hornelen basin (see Fig. 4) through the Cale- donian thrust belt.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021 Listricfaults, growth W Norway 63 1 Valdres WSW toBergen and are preservedfrom stratigraphic cut-off lines.Branch-lines and fault erosion in a large synformal depression adjacent to the cut-off lines can be recognized and drawn on geologic- basement dome. Within this synform is astacked al maps(Elliott & Johnson 1980; Hossack 1983)by sequence of thrust sheets which individually wedge out plotting points of intersection of two reference faults towards the NW. The lowest thrustsheets are on the map and drawing curves passing through these composed of low grade Rphean-Vendian-Cambrian- points. The curves (the branch-lines or the fault cut-off Ordovician sediments which have been thrust in from lines) must contain all known sub- or out-crops of the the NW overthe foreland of the orogenic belt rocks between the faults on the one side of the line (Hossack et al. in press). The highest thrust sheet in and not on the other. It is possible to distinguish the the synformalsection is composed of high grade areas of the branch-lines and the fault cut-off lines Precambrain pyroxene gneisses, mangerites, anortho- which exist in the air (i.e.eroded lines) (Elliott & sites and gabbros which form the Jotunheim and the Johnson 1980) from those that exist underground (i.e. Jotun thrust sheet. Towards the NE, the rocks of the buriedlines). But how can branch-lines andfault Jotun thrust sheet change across a steep contact zone, cut-off lines be distinguished?.They can be disting- which may be an extension fault downthrowing to the uished on good geological maps because a branch-line NE,into metasediments and volcanics of probable produced by genetically related faulting is aunique Cambro-Ordovician age. line onthe mapwhereas two separate intersection At the SW end, the Jotun thrust sheet is separated, lines are produced on the younger fault in the case of via an erosion gap, from identical rocks of the fault cut-off lines. One line is the line of intersection arcs. Originally the Bergen andJotun rocks were on the hanging-wall and the other on the foot-wall of probablycontinuous in a single thrust sheet. Similar the younger fault. On a map these two fault cut-off rocks also occur beneathand within the Devonian lines would beexpected to have similar shapes that basins of Sunnfjord (Fig. la) and these in turn were could be fitted back together again with few holes or probably originally attachedto the Bergen-Jotun overlaps by removing the slip on the youngerfault. thrust sheet. In aregional view, the Jotunheim, the This of course would be a wayof estimating the Bergen arcs and the W coast mangerites can be traced amount of fault slip. in an almost continuous arc around theSW end of the Two examples of branch-lines are drawn on Fig. la. M@rewindow and by inference prior to erosion must These arethe branch-lines between the Bergsdalen have originally continued over the top of the window and Valdres thrusts, respectively, with the immediate- (Fig. lb). The thrusted crystalline rocks immediately ly overlying Jotun thrust. I am confident that these are beneath and within the Devonian basins represent the branch-lines and not fault cut-off lines because there trailing edges of theJotun thrust sheet. Associated are no matching second lines in each case. Both have with thistrailing edgeare Cambro-Ordovician characteristicU-shaped patternson the map.This metasediments and volcanic rocks which can be pattern arises from the complicated ramp-flat geomet- correlated with the similar rocks at the NE end of the ry that exists in thrust planes (Butler 1982; Hossack Jotun thrust sheet (Fig. la). 1983). Thrusts climb upsection in the direction of The geometric patterns produced by the intersection thrust transport as well as up and down section along of faultsurfaces arepertinent to our discussion. strike (Dahlstrom 1970) (Fig. 2). The step features are Although the terminologyhas been derived and definedfor contraction faults, thesame terms and ideas must equally apply to extension faults. Boyer & Elliott (1982) define the line of intersection of two thrust faults as a branch-line. This line is the structural equivalent of a zero isopach (Hossack 1983) and in a section crossing a branch-line the rocks between two faults will exist on one side of the line and not on the other. Also at the point of intersection, the lower and the higher faults will merge. Boyer & Elliott (op. cit.) clearly imply that the branch-line is a line from which a younger fault is born by splaying off an older fault. However, if the faults are not genetically related but merely happen to betwo unrelated faults that cross oneanother there will be two lines of intersection FIG. 2. Thrust with the hanging-wall removed to between them.The youngerfault will displace the show the foot-wall of the thrust with two opposed older fault and thereby form anintersection line in the lateral ramps connected by faults and a frontal hanging-wall andanother in the foot-wall of the ramp. A younger thrust developing by branching younger. I propose that in this case the lines should be off the foot-wall ramp produced a trailing edge called fault cut-off lines and be treatedstructurally like branch-line in the ramp which is pecked.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021 632 J. R. Hossack the ramps and those that formnormal to the thrust slip non-metamorphicDevonian sediments sit on direction are frontal ramps, those parallel to the slip metasediments and volcanics of presumed lower direction are lateral ramps.Finally, there is a third set Palaeozoic age which contain upper Ordovician fossils. of ramps, not figured here, which are oblique to the The basin infill consists of alluvial-fan conglomer- slip direction and are oblique ramps. The branch-lines ates at the fault contacts and above the unconformity are constrained to lie within theseramps and hence (Fig. 3), with alluvial channel sands infilling the centre theoutcrop pattern of the branch-lines define the of the basin. Steel andothers (Steel 1976; Steel & shape of their associated ramps (Hossack 1983). The Aasheim 1978; Steel et al. 1977; Steel & Gloppen sides of the U-pattern of the Bergsdalen and Valdres 1980) have shown that the sedimentation accompanied branch-lines, because they are parallel, mirror-image fault movements andthat debris-flow fansformed structures,represent the foot-wall cut-offs of the along the northern fault margin and stream-flow fans lateral ramps and define the slip direction for the two on the southern fault margins. The centre of the basin thrustsin question.It is clear thatthe Caledonian is made up of over 25 km of onlapping alluvial sands thrusts of S Norwayhave complicated ramp-flat which were deposited longitudinally from an eastern geometries. source. In general, this basin infill consists of numer- ous minor and major coarsening-upwards cyclothems. These cyclothems are basin-wide, affecting both Structure of the Sunnfjord margins and centre and are regarded as tectonically- basins induced cyclothems. The sediments in the basin centre show longitudinal facies changes from proximal allu- The most extensively described basin is the most vium in theE to distal alluvium in the W. This change, northerlyone, the Hornelen basin (Fig. 3) (Bryhni and the onlapping nature of the marginal alluvial fans, 1964; Steel 1976; Steel & Aasheim 1978; Steel et al. led Steel (1976) to suggest that the Hornelenbasin was 1977; Steel & Gloppen 1980). The basin is surrounded formed by strike-slip faulting. He has suggested two on three sides by high grade metamorphic rocks of the possible models. Firstly, that the Hornelen basin could More window. Theboundary fault onthe northern haveformed by transtensionalmovement (Harland and southern sides of the basin has a steep dip but on 1971) with a sinistral sense of strike-slip movement on the eastern side is a low angle fault. The faults have a curved fault (Steel 1976). Secondly, that the basin mylonitic zones which formed from both theDevonian could have formed by dextral strike-slip displacement sediments and the country-rock gneisses. The western on the northern boundary fault which Steel has named margin of the basin is an unconformitywhere the Nordfjordfault. This fault is shown in Steel’s

FIG.3. Geological map of the Hornelen basin (see Fig. 4) after Steel & Gloppen (1980).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021 Listric growthListric faults, W Norway 633

diagrams to have an S-shape. Movement on this fault The branch-lineintersects the section at the eastern was supposed to induce a series of dip-slip extension end at a point where the sole thrust has to merge with faults tothe S of thebend in the fault which theMBby fault.Between these two faultsin the accumulatedsediment to form the basin. Dextral footwall of the Mbl0y fault there is anarea, now strike-slip onthe faultcould explain the eroded, which originally contained the samemetamor- eastwards migration of the locus of sedimentation and phic rocks that occur beneath the Kvamshesten and the source areawith a westwards direction of sediment Solund basins (Fig. 5, c & d). The rocks have simply transport in the basin centre. Unfortunately for Steel’s been projected along the branch-line on to the line of model, a dextral strike-slip displacementshould sectionA fromtheir present area of outcrop.The produce transpression (Harland 1971) on the S side of same metamorphic rocks occur in the hanging-wall of the S-bend in the Nordfjordfault and should have the Mil0y fault over 40 km to theW and there is clear formed contraction faults and not the extension faults omission of structural sequence along the fault. Steel figures by Steel (1976) and Steel & Gloppen (1980). & Gloppen (1980) estimated that thereis over 25 km of Although the remaining basins to the S of Homelen onlapping Devonian sediment in the hanging-wall of are smaller, they have similar geometries (Fig. 4). The the Mil~yfault and this gives one estimate of the sediments within the basins sit unconformably on amount of slip onthe fault. Using the offset of older metamorphiclower Palaeozoic rocks or mangeri- metamorphic rocks suggested by the branch-line tic Jotun rocks in the W and are surrounded on the another estimate of over 40 km of dip separation can other three sides by faults which dip either gently or be suggested. The basin can be explained as the result steeply. On a regional scale, the boundary fault zone of accumulation of Devonian sediment on the down- to all four Devonian basins has a sinuous trace with thrownside of a listric extension growth fault. The re-entrants. I propose to name the boundary fault the easterly dip and onlap of the sedimentsresult from Miby fault. The Devoniansediments with the basins, the formation of a rollover anticline in the hanging- as well asdipping eastwards, are folded on E-W wall which accompanies the listric fault shape.The trends. In the Solundbasin, the metamorphic rocks flat-lying geometry of the MBby fault in this area can underlying the Devonian sediments are brought up in be explained by the deep level of erosion reaching and thecore of a NEtrending anticline. Also the exposing the level at which the MBlOy fault ‘soles out’ Kvamshesten basin contains an E-W thrust which into a basal detachment (Bally et al. 1981). carries metamorphic rocks in the hanging-wall up over Sections B and Cat HBsteinen and Kvamshesten its normal cover of Devonian sediments. respectively (Fig. 5) show similar listric extensional Thesouthern basins of Solund and Kvamshesten geometrybut theamount of dip separation of the also have a slice of thrusted metamorphic rock which metamorphic rocksin the hanging- and foot-walls dips westwards beneaththe Mil0y faultand the decreases. In the mostsoutherly section at Solund basins. Regionalcorrelations (Fig. la) suggest that (Fig. 54 theamount of dipseparation of the these metamorphic rocks, which are identical to the metamorphic rocks on the MB10y fault is almost zero. metamorphic rocks inside the Devonian basins, repre- In fact the section shows a small amount of structural sentthe trailing edge of the Jotun-Bergen thrust repetition and the fault here shows thrust geometry. sheets. The thrust at thebase of this trailing edge rests The Miley fault appears to change southwards from directly on the Precambrian gneisses of theMore an extensional fault into a thrust. window and probably represents the sole thrust to the The transverse section E (Fig. 6) demonstrates the whole Caledonian thrust belt in this area (Fig. 1). role that the faults have played in forming the basins. The sole thrust merges with the Miby fault in three The present day transverse section (Fig. 6a) is based points of intersection (Fig. 4) and a well-defined line, onthe section published by NorgesGeologiske which I believe is a branch-line, can be drawn through Undersakelse on the Mil0y map sheet (Kilda1 1970). these points. There is no other corresponding parallel In this section, the base of the Devonian appears to be line in the area to suggest that it is a fault cut-off line. folded and repeated by the southerly-directed thrust However, the distinction is immaterial in the argument within the Kvamshesten basin which brings up the I will present below. I merely use the line to project underlying metamorphic basement over the Devonian the geology on to thelines of section and demonstrate sediments. The section appearsto have undergone that there is omission of structural section across the some N-S contraction which has been removed by Mil0y fault. restoring the base of the Devonian in the line of A series of longitudinal sections (Figs 4 & 5) and a section to a flat datum (Fig. 6b). The Mil~yand sole transversesection (Figs 4 & 6) havebeen drawn faults unite on the N side of the Solund basin in the through the basins. The branch-line between the sole branch-line. The continuation of the united fault thrust and the Mil0y fault has beenused to project the ramps up and down to the N to form the depressions line of intersection of these two faults on each of the of the basins. Note that in the line of this section the longitudinalsections. The first section (Fig. 5a), Histeinen and Kvamshesten basins are united into a through Hornelen, is perhapsthe most instructive. single basin in the centre of the section. This section is

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021 Listric growth faults, W Norway 635

Cambro-Silurian and meta-mangerite-syenites ......

...... sea-level + +

...... (a)

0 10 km B FIG.6. Transverse section E of Fig. 4. a, Present-day section; b, base of the Devonian restored to datum. Thrusts have carets and extension faults have circles.

displaying ramp and flat geometry but in this orienta- geometry of Fig. 2. If the hanging-wall of a fault with tion the ramps are lateralramps which run closely two mirror-image lateral rampsconnected by a frontal parallel theto faultmovement direction. ramp undergoesextensional displacement, the under- There-entrant shape of the Devonianbasins can be lying foot-wallgeometry will be exposed during explained by the complicated three-dimensional ramp- faulting because of the rule of omission of structural flat geometry of the M5lOy fault. These box-like basins succession. Hence the box-like shape of the foot-wall arethe extensionalequivalents of thethrust ramp will appear duringfaulting (Fig. 7). Lateral ramps

& V=H

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021 636 J. R. Hossack usually appearon geological mapsas wrench faults soles out into the older Lewis thrust and has caused a with anormal component of slip (Dahlstrom 1970; later reversal of movement onthe thrust down-dip Butler 1982). The northern and southern fault bound- from the line of intersection. Likewise the Mil0y fault aries of the Hornelen basin are such examples. With merges with the Caledonian sole thrust (Fig. 5) in a an extensional offset on the M%My fault underneath branch-line and seemsto cause a reversalof slip on the the Hornelen basin, the northern boundary fault (the sole thrust down-dip from the branch-line. Nordfjord fault of Steel 1976) has to have a dextral The commonfault down-dip from the branch-line sense of displacement (Figs 3 & 4). This is the sense of also shows complicated ramp-flat geometry which I offset described by Steel & Gloppen (1980) based on believe was inherited from the Caledonianthrust the apparent offset of the marginal alluvial fans from geometry.Prior tothe deposition of the Devonian theirsource areas. Steel & Gloppen (1980) further rocks theM20y fault could have existed in the estimated that therewas a component of dip-slip on the metamorphic Caledonian rocks as a Caledonian thrust Nordfjord fault which was two or three times smaller with ramps and flats. During the early Middle than the strike slip component. The southern bound- Devonian phase of crustal extension, the shoebox-like ary fault to the Hornelen basin, if it is a lateral ramp depressions between the Caledonian lateral ramps of on the extensional M30y fault, will havea sinistral this precursor thrust would open up if a reversal of sense of offset (Figs 3 & 4). Steel & Gloppen (1980) movement on the Caledonian thrust occurred (Fig. 7). estimated that this fault had a smaller component of Devoniansediments could then accumulate in the dip-slip thanthe mirror-image northern fault.This depressions which became the individual Devonian could arise in the present rampmodel if the underlying basins and the Mil0y fault propagated up into these flat on the Miby fault, beneath the Hornelen basin, accumulatingDevonian sediments as an extension was uneven or sloping. Likewise all theother fault. The original Caledonianlateral ramps of the Devonian basins can be explained in terms of shoe-box precursor thrust reversed their sense of displacement geometry madeup of opposed mirror-imagelateral to form the wrench fault-like boundaries on the N and rampsconnected by frontal ramps and underlain by S side of the basins andthe internal folds in the flats. Inmap view thesteep northern and southern Devoniansediments could bethe accommodation boundary faults or lateral ramps will show dextral and folds that always accompany thrust orextensional fault sinistraloffset, respectively (Fig. 4). Themore ramps(Dahlstrom 1970). If the lateral ramps were flat-lying boundary fault atthe eastern end of each inclined towards each other on opposite sides of the basin correspondsto the connectingfault flat. The basin, the hanging-wall rocks would have to contract connecting frontal ramp in the foot-wall of the MbMy as they dropped down into the basin depression. This fault originally lay fartherto the E beyond the contraction would tighten up the folds and could have branch-line and has now disappeared through erosion. formed the E-W thrust within the Kvamshesten basin (Figs 4 & 6).These fold-and-faultstructures are The origin of the MIley fault usually attributedto the Svalbardianevent of Vogt (1929) but such a separate tectonicevent is not I have suggested that the M%My fault has a compli- required. The structures can form at the same time as cated ramp-flat geometry and has clearly undergone a the extensionalfaulting. The presence of mylonites large amount of extensionaldisplacement in the N within Devonianrocks on the extensionalboundary which decreasessouthwards until the samefault fault is not unique because mylonites also occur on the displays a small thrustcomponent. Several authors flat-lying extensionalfaults above the metamorphic have described internal E-W folds and thrusts which core complexes of the N American Cordilleras (Davis theyascribe to severalphases of deformation.How & Coney 1979). can the thrusting, folding and extensional faulting be Theamount of earlyMiddle Devonian extension integrated? Most previous explanations (Bryhni 1964; slip was sufficient in the Hornelen area to completely Skjerlie 1971; Steel 1976) haveextensional faulting remove the proposed Caledonian overthrust geometry followed by a later phase of Svalbardian folding and onthe precursor MWy fault. Herethe amount of thrusting(Vogt 1929). I believe that Caledonian Devonian extension is estimated to be over 40 km in a thrusting followed by earlyMiddle Devonian exten- down-dipdirection. Theamount of slip appears to sional faulting can just as easily explain the origin of decreasesouthwards to the Solund basin where the the basins. The Mil0y fault merges with the Caledo- extensionalreversal was not enough to completely nian sole thrust, and the Devonianbasin infill contains remove the overthrustgeometry of the precursor fault. pebbles of Caledonian metamorphic rocks and mylo- The Milpry fault still retains in part its pre-Devonian nites. Hence the Mgl0y fault postdates most or all of thrust geometry. The decrease of extensional slip to the Caledonian thrusting. The Miloy fault appears to the S helps to explainachange in facies of the be identical in geometry to the Flathead fault of the Devoniansediments from N to S (Reading,pers. CanadianRockies (Dahlstrom 1970) which is a comm.). The Hornelenbasin in the N (Figs 3 & 4) has post-thrusting listric extensionfault that flattens or mainly a sandstone infill. The two central basins of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021 Listric growth faults, W Norway 637 Hhsteinen and Kvamshesten havesandstone-con- Hornelen basin accumulatedsediment over 14Ma glomerate infills and the Solund basin in the S, where (Friend 1969). If there has been >40 km slip on the the extensional slip was aminimum, has mainly a Mil0y fault during this period the time averaged slip conglomeratic infill (Steel 1976; Steel et al. 1977; Steel rate on the fault is on the order of 3 mm yr-'. 8z Gloppen 1980). I suggest that the southern area, because of the minimum amount of extensional slip, ACKNOWLEDGEMENTS.I would like to thankthe following was consequently closer to its source area in the E and for helpful comments on the ideas contained in this paper: accumulated the coarsestsediment. Incontrast, the Bert Bally, Ron Steel, Geoff Milnes, HaroldReading, Hornelen basin was removed by extensional slip DebbySpratt, Tony Spencer,Trevor Elliott and Mark farther away fromits source areaand accumulated Cooper. Dave Elliott gave me the most encouragement for more distal sediments. It has been estimated that the my ideas and this paper is dedicated to his memory.

References

BALLY,A. W.,BERNOULLI D., DAVIS, G.A. & MON- skart Mll0y. 1: 250 000, Norges geologiske Undersokelse. TADERT,L. 1981. Listricnormal faults. Oceanol. acta. KOLDERUP,C. F. 1923. Kvamshestens devonfelt. Bergens Actes 26" Congr.Int. Geol., Colloque des marges Mm. Aarbog. 1920-21,4, 1-96. continentales. 87-101. -. 1925. Haasteinensdevonfelt. Bergens Mus.Aarbog. BOYER,S. M. & ELLIOTT,D. W. 1982. Thrust systems. Bull. 1923-24, 11, 1-32. Am. Assoc. Petrol. Geol. 66, 1196-230. -. 1926. Solunds devonfelt. Bergens Mm. Aarbog. BRYHNI,I. 1964. Relasjoner mellom senkaledonsk tektonikk 1924-25, 8, 1-73. og sedimentasjon ved Hornelens og Hasteinens devon. -. 1927. Hornelensdevonfelt. Bergem Mm. Aarbog. Nor. geol. Unders. 225, 10-25. 1926, 6, 1-56. - & SKJERLIE,F. J. 1975. Syndepositional tectonism in KOLDERUP, N.-H. 1921. Der Mangeritesyenite und the Kvamshesten district (Old Red Sandstone), western umgebende Gesteine zwischen Dalsfjord und Stavfjord Norway. Geol. Mag. 112,593-600. in westlischen Norwegen. Bergens MW. Aarbog. 1920- BUTLER,R. W. H. 1982. The terminology of structures in 21, 5, 1-71. thrust belts. J. struct. Geol. 4, 239-45. NILSEN,T. H. 1968. The relationship of sedimentationto DAHLSTROM,C. D. A. 1970. Structural geologyin the tectonics in the Solund Devonian districtof southwestern eastern margin of the Canadian Rocky Mountains. Bull. Norway. Nor. geol. Unders. 259, 1-108. Can. Petrol. Geol. 18, 332-406. -. 1973. Devonian(Old Red Sandstone) sedimentation DAVIS, G. H.& CONEY, P.J. 1979. Geologic development of and tectonics of Norway. In: PITCHER,M. G. (ed.)Arctic the Cordilleran metamorphic core complexes. Geology Geology. Mern. Am. Assoc. Petrol. Geol. 19, 471-81. 7, 120-4. SKJERLIE,F. J. 1969. The pre-Devonian rocks in the ELLIOIT, D. W. & JOHNSON,M. R. W. 1980. The structural - area and adjacentdistricts, western evolution of the northern part of the Moine thrust zone. Norway. Nor. geol. Unders. 258, 325-59. Tram. R. Soc. Edin. Earth Sci. 71, 69-96. -. 1971. Sedimentasjon og tektonisk utvikling: Kvamshes- FRIEND, P.F. 1969. Tectonic features of Old Red Sedimenta- tensdevonfelt, Vest Norge. Nor.geol. Unders. 270, tion in North Atlantic Borders. In: KAY, M. (ed.). North 77-108. Atlantic-Geologyand Continental drift. Mem. Am. STEEL, R. J. 1976. Devonian basins of ; Assoc. Petrol. Geol. Tulsa, 12, 703-710. sedimentary response to tectonism and varying tectonic HARLAND,W. B. 1971. Tectonic transpression in Caledonian context. Tectonophysics, 36, 207-24. Spitzbergen. Geol. Mag. 108, 27-42. -. & AASHEIM,S. 1978. Alluvial sand deposition in a H~ISAETER,T. 1971. ThrustDevonian sediments in the rapidly subsiding basin (Devonian, Norway). In: MIALL, Kvamshesten area, Western Norway. Geol.Mag. 108, A.D. (ed.) Mem.Can. Soc. Petrol. Geol. Alberta, 5, 287-92. 385-413. HOSSACK,J. R. 1983. A cross-section through the Scandina- -. & GLOPPEN, T.G. 1980. Late Caledonian (Devonian) vian Caledonides constructed with the aid of branch-line basin formation,western Norway: signs of strike-slip maps. J. struct. Geol. 5, 103-11. tectonicsduring infilling. In: BALLANCE,P. F. & -, GARTON,M. R. & NICKELSEN,R. P. (in press). The READING,H. G. (eds) Sedimentation in Oblique-Slip geological sectionfrom the forelandup to the Jotun Mobile Zones. Spec. Publ. Int. Assoc. Sediment. No 4, thrust sheet in the Valdres area, south Norway. In: GEE, 79-103. D. G. & STURT,B. A. (eds) The Caledonian orogen- VOGT, T. 1929. Den norskefjellkjedes revolusjonshistorie. Scandinavia and related areas. John Wiley, New York. Norsk geol. Tiddsk. 10, 97-115. KILDAL,E. S. 1970. Geologisk kart over Norge, berggrunn-

Received 29 January 1983; revised typescript accepted 3 October 1983. J. R. HOSSACK,Geology Department, City of LondonPolytechnic, Walburgh House, Bigland Street, London El 2NG.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/4/629/4888061/gsjgs.141.4.0629.pdf by guest on 01 October 2021