The Geometry of Listric Growth Faults in the Devonian Basins of Sunnfjord, W Norway
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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 Sunnfjord, W Norway 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 & Gloppen 1980) have Hornelen, HBsteinen, Kvamshesten and Solund 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 conglomerates contain 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 the importance 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 Bergen 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 probably continuous 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 be expected 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 younger fault. thrust sheet. In a regional 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.