Gravity interpretation of the jotun nappe of the norwegian caledonides

Tectonophysics, 22 (1974) 205-222 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

GRAVITY INTERPRETATION OF THE JOTUN NAPPE OF THE NORWEGIAN CALEDONIDES”

SCOTT B. SMITHSONl,

IVAR B. RAMBERG

and GISLE GR’O’NLIEZ

‘Department of Geology, University of Wyoming, Laramie, Wyo. (U.S.A.) 21nstitute for Geology, University of Oslo, Oslo (Norway) (Accepted for publication

November 5, 1973)

ABSTRACT Smithson, S.B., Ramberg, LB. and Groinlie, G., 1974. Gravity interpretation nappe of the Norwegian Caledonides. Tectonophysics, 22: 205-222.

of the Jotun

The Jotun nappe in the Norwegian Caledonides consists of a body of dense, mafic granulite-facies Precambrian rocks overlying Cambro-Silurian rocks. Hypotheses to explain the emplacement of the nappe include: (1) an intrusion; (2) low-angle thrust; (3) decollement nappe; (4) fold nappe with Precambrian core; (5) upthrust from below. Hypotheses 2-4 imply a distant root zone, hypothesis 5 implies a local root zone, and hypothesis 1 is generally discredited. Density determinations show that rocks of the nappe have a positive density contrast of approximately 0.12 g/cm’ with the surrounding rocks; therefore, gravity interpretation should provide useful information concerning the geometry of the nappe. Gravity stations were largely put in by helicopter in order to locate stations as favorably as possible with respect to topography. Bouguer gravity anomalies show a maximum residual anomaly of +58 mGa1 situated over the Jotun complex. This large anomaly results in a gravity model for the nappe with maximum thickness of 16 km that decreases to the southeast, and this model can only be made to fit the residual anomaly by increasing the density below the surface. Even anomalies over the two northeastern lobes of the nappe can only be simulated by a gravity model lo-15 km thick. The possibility of a dense mass underneath the nappe contributing to the anomaly is considered but ruled out, because of the coincidence of the gravity high with the nappe outcrop. Other possible interpretations indicate that the thickness of the nappe must be at least 8 km. Mineralogical data suggest that initial crystallization of the Jotun complex took place at depths around 25 km followed by a sharp reduction in pressure. Most geologists favor the distant-nappe hypothesis but the shape of the nappe is more consistent with a local root zone. The relationof the nappe to a plate-tectonic framework depends on the position of a Caledonide subduction zone and the position of the root zone. The nappe could represent a slice of the lower crust upthrust from below, similar to the dense rocks found along the Ivrea zone of the Alps. INTRODUCTION

The Jotunheimen area, part of an Early Paleozoic (Caledonide) erogenic belt, still represents one of the least understood and most debated problems in * Contribution

No. 80 to the Norwegian Geotraverse Project.

206

Upper

l

.,*

MIddie

,_Y’

\-

,*,

\

nappes

’

LOW@r

nappts nODptS

I i

\ \, Faults \ \

Old Red

’

1- e

* /

Besot

gneist

Fig. 1. Geologic map of southern Norway. Jotun nappe is &own as several different units according to older interpretations. Recent interpretations combine these units into one nappe.

207

Norwegian geology. Massifs of apparent Precambrian age occupy areas of more than 10,000 km ’ above deformed ~ambro-Sil~ian rocks of the Caledonian mountain belt through southern Norway (Fig. 1). This area constitutes part of the nappe region (Strand, 1960, 1972). The massif itself which forms the Jotun nappe consists of a variety of ultrabasic-monzonitic granulite-facies rocks of the Bergen-Jotun kindred (Goldschmidt, 1916). Although the nappe theory is the most generally accepted explanation for the Jotun complex, this has not always been the case. Bragger (1893) regarded the Jotun rocks and equivalents as laccoliths intruded into sediments. The underlying sedimentary rocks were thus thermally metamorphosed, and Bragger’s idea would then explain the fact that in the Jotunheimen area and surrounding parts of the nappe region, the metamorphic grade increases from greenschist- to granulite-facies upwards in the sequence. The later discovery of a persistent mylonite sole below the whole complex (Bjorlykke, 1905, and later authors) and minor signs of thrust planes within the Jotun complex proper were more in favor of the nappe theory, and in 1936 Holtedahl concluded that the whole massif had been emplaced as a folded-nappe overthrust from the northwest. For a more detailed survey of the various and changing views of the genesis of the Jotun complex and a more complete reference list the reader is referred to Strand (1960,1972). The nappe theory implies a thrust distance of possibly more than 200 km, a distance well in accord with other assumed nappe movements in the Scandinavian Caledonides (Tornebom, 1896; Kautsky, 1953; Kulling, 1955, 1962; Asklund, 1961), and also well in accordance with plate-tectonics theory that in zones of merging plates, large slabs of shelf rocks are supposed to override the inner region of stable continental crust. Geophysical data could be decisive in determining the origin of the nappes, and preliminary analyses of gravity data have indicated that the Jotun complex penetrates deep into the crust and has a shape that does not agree very well with the nappe theory (Smithson, 1964a; Ramberg and Gronlie, 1969; Smithson and Ramberg, 1970). Additional data have recently become available and results in a refined gravity anomaly map, and new seismic data in neighboring regions allow a better interpretation. This article will refine the gravity studies and compare the conclusions with recent geological and mineralogical studies, GEOLOGY

OF THE JOTUN NAPPE AND SURROUNDINGS

The Jotun nappe is situated in a synclinal depression (Faltungsgraben of Goldschmidt, 1916) that has a NNE-SSW trending axis (Fig. 1). The depression continues to the SSW into the War-danger area at the west coast of Norway and to the NNE into the Trondheim region, both composed of deformed Cambro-Silu~~ rocks. The axis marks a zone of Caledonian intrusive and extrusive rocks of gabbroic-monzonitic composition (the trondhjemiteopdahte kindred of Goldschmidt, 1916). On both sides, Precambrian rocks of the Fennoscandian Shield occur. This feature continues far northward into North Norway; the Cambro-Silurian rocks are found in elongate tectonic

208

troughs, between geanticlinal regions of mobilized basal gneiss in the west and Precambrian rocks in tectonic windows or “highs” partly covered by a relatively thin series of ~lochthonous met~edimen~ rocks to the east, Whether these troughs only represent secondary troughs and the Caledonian geosyncline occurred farther to the west (as generally believed, Roberts, 1971; Nicholson, 1971; Naterstad et al., 1973), or they in fact also represent the depositional troughs is not as yet settled. Below the Jotun complex, sedimentary rocks always occur. They belong to the Valdres Group (Strand, 1964) which are inverted (Loeschke and Nickelsen, 1968) and consist of Cambro-Ordovician shales (Melsenn Formation) and reddish arkose (Valdres Sparagmite) of assumed Eocambrian age. Similar Eocambrian deposits are known all along the Caledonian mountain belt and in marginal, faulted basins which are probably connected with the main depositions basin (Geosyncline?) (En~und, 1971; Ramberg and Englund, 1969). Primary contacts between basal Eocambrian conglomerates and Jotun rocks are found (Loeschke and Nickelsen, 1968; Englund, 1971). It is therefore possible that the Jotun rocks were exposed in Precambrian time and that they formed part of the floor of the main sedimentary trough in which thick Eoc~brian and C~bro-Silurian sediments were deposited. Numerous small sheets of Jotun rocks are also found to the southeast outside the Faltungsgraben, and probably represent erosional remnants. To the west-southwest however, similar rocks are found close to the coast in the Bergen arc district (Kolderup, 1921) and in the Dalsfjord area (Skjerlie, 1969); these areas are separated from the Faltungsgraben by an anticlinal area (Kildal, 1973). From the Mare Gneiss region, occu~ences of ~trab~i~ anorthositic, and monzonitic rocks have tentatively been related to the Jotun complex as possible remnants in a root zone for the Jotun nappe (Bryhni, 1966; Bryhni and Grimstad, 1970). In the Jotun complex, Goldschmidt distinguished between the characteristic Bergen-Jotun kindred rocks and a series of true intrusives of noritic and gabbroic composition. The first series range from ultra-m&c to felsic composition. Peridotitic bodies occur as irregular bodies and lenses. Monomineralic “end-member” rocks like pyroxenite, anorthosite and microperthite rocks occur, while intermediate types like Jotun-norite (jotunite of Hddal, 1945) and monzonite are the most frequent. Hypersthene-syenites occur and “normal” biotite granite and alkali granite are found as large batholiths or as sills in the more mafic representatives of the Jotun kindred. THE NAPPE PUZZLE

Some of the principal hypotheses regarding the formation of the Jotun nappe are illustrated in Fig. 2. From top to bottom they are: (1) intrusion; (2) low-angle thrusting of rigid plates; (3) de’collement nappes above mobilized basement rocks in the central zone; (4) fold nappe with core of Precambrian rocks (Jotun complex and equivalents); (5) upthrust from local root zones.

b

--. *’

I’

4

5

L ,

Fig. 2. Different hypotheses for development of the Jotun nappe.

The idea of Bragger (1893) that the Jotun complex was emplaced by intrusion into sedimentary rocks was also the conclusion of Goldschmidt (1916) even though he recognized the presence of mylonite zones at the base of the complex. This feature was explained by later small-scale movements, and they both assumed that the complex had its root below the complex proper (1). This intrusion hypothesis has been abandoned mainly because of the recently established Precambrian age of the complex (Priem, 1968; Battey, 1971) and also because of the total lack of intrusive contacts between the Jotun rocks and their surroundings. The overthrust hypotheses (2 and 4) have been advocated by a number of workers for the Hardanger and Trondheim regions SSW and ENE of the Jotun area respectively (Holtedahl, 1936; Bailey and Holtedahl, 1938; Strand, 1961; Roberts, 1971; Naterstad et al., 1973) as well as for the Jotun area. The Jotun complex might have been preserved in the Faltungsgraben because of secondary folding (Holtedahl, 1936) or because of gravity sinking of the dense Jotun rocks (N. Spjeldnaes, written communication, 1970)

combined

with the buoyant

rise of the surrounding

basal gneiss (H. Ram-

berg, 1967) thus causing the deep, rootlike shape of the complex suggested by preliminary gravity interpretations (Smithson and Ramberg, 1970). Compression of para-autochthonous or allochthonous sedimentary rocks sliding on a basement is a well known phenomenon; e.g., the Alps. Similar de’collement nappes, accompanied by an extreme metamorphism and mobilization of the basement along a central zone, have been proposed for the Trondheim region northeast of the Jotunheimen (Bugge, 1954; F.C. Wolff, oral communication, 1970). Bugge emphasized that the axis of the anticlinorium lies exactly on the northeastern extension of the Faltungsgraben. The central part of the Trondheim region has, however, been interpreted as a synclinal zone by others (Goldschmidt, 1916; Vogt, 1922). Because it is compatible with recent gravity interpretations, alternative 5 was suggested as a possible model (Smithson and Ramberg, 1970). The gravity data show that a large mass excess is present in the Faltungsgraben below the Jotun complex. If this mass anomaly belongs to the Jotun complex, it can not readily be explained by any of hypotheses 1, 2, or 4, but more likely implies a local root zone. Additional gravity and density data have now been collected to better define the mass anomaly and geometry of the Jotun complex. New gravity measurements have been made especially in the More Gneiss region northwest of the Jotun nappe. These data are particularly useful for determination of the regional anomaly, as are the recent seismic refraction data and offer a new and more sound basis for geophysical interpretations. ROCK DENSITIES

The great petrographical variation within the Jotun complex is reflected in rock densities. Variation of mean density with depth is difficult to estimate because the relative proportions of various rock types may well change with depth if these rocks are part of a layered igneous complex. We obtained a mean density of 2.86 g/cm3 for 103 samples of the Jotun complex. Barkey has obtained the same value horn 81 sampIes (personal communication, 1971). Battey obtains a somewhat higher mean density of 2.93 g/cm3 for 65 samples that include numerous ultramafic rocks, but if a mean density weighted according to areal extent is used, it is near 2.86 g/cm3 (personal communication, 1971). A mean density of 2.86 g/cm3 is therefore used for the Jotun complex; however, an increased proportion of mafic or ultramafic rocks at depth would raise the mean density of the nappes. For the areas of gneiss outside the nmpe, 36 samples of Precambrian gneiss south of the nappe have a mean density of 2.76 g/cm3, and 15 samples of basal gneiss north of the nappe have a mean density of 2.73 g/cm3. These values agree with mean densities of other rocks from the Precambrian Shield of southern Norway. A mean density of 2.74 g/cm3 is used for the gneiss surrounding the nappe.

211 GRAVITY INTERPRETATION

Gravity measurements were carried out principally with helicopter transportation and gravity observations were reduced by standard methods (Dobrin, 1962). Rock densities of 2.86 and 2.74 g/cm” were used for the Bouguer correction to stations in the Jotun nappe and the surrounding gneisses, respectively. Terrain corrections, which were minimized by careful location with respect to abrupt relief for many of the stations, were carried out to a distance of 21 km. Average terrain corrections were from 5 to 20 mGa1 with a maximum value of 39.5 mGal, and precision of gravity anomaly values is largely determined by the terrain correction. The Bouguer gravity anomaly map (Fig. 3) shows that the Jotun nappe is marked by a large positive gravity anomaly whose maximum value is --25 mGa1. This gravity anomaly follows the Faltungs~aben closely in the NESW direction; the anomaly has a higher gradient along its northwestern flank than along its southeastern flank. South of the Jotun nappe, another large positive gravity anomaly whose maximum value is -20 mGa1 is attributed to mafic intrusion in the Precambrian basement (Smithson, 1964b). North of the nappe, anomaly values decrease to -75 mGal, and south of the nappe the minimum value is -95 mGa1. The regional Bouguer gravity anomaly map over South Norway (Ramberg and Smithson, 1971) shows that anomalies approach zero at the coast in More north of the nappe and that anomalies reach -90 mGal over a large area south of the nappe. A positive anomaly of smaller amplitude continues northeast from the Jotunheim area to the ~ondheim area and an elongate negative anomaly parallels this gravity feature along its southeastern flank. Gravity interpretation of the Jotun nappe is critically dependent on resolution of the regional (background) gravity field because this will determine the amplitude of the residual gravity anomaly associated with the Jotun nappe. New gravity data from inaccessible areas and a seismic crustal refraction profile Trondheim~slo (Kanestrom and Haugland, 1971) have helped in determining a better regional gravity trend than the simple linear trend applied earlier (Smithson, 1964a; Smithson and Ramberg, 1970). Because the regional anomaly is of such importance in the interpretation, we have used two different approaches. Recent seismic work east of the Jotunheim (Kanestr~m and Haugland, 1971) has been modified and is applied to our profile AA’. We assume that the Jotunheim rocks and the Moho discontinuity are the two primary contributors to the gravity anomalies, and we have calculated the gravity effect of the Moho and compared it with the observed anomaly (Fig.4). The other approach is to assume complete isostatic equilibrium. We have used an average crustal thickness of 33 km and a mantle density of 3.32 g/cm3 (Woollard, 1969). We have also assumed that the isostatic compensation is achieved within the upper 40 km of the crust and mantle, and assuming a standard crust with a mass per unit area of 11,840 kg/cm* at a depth of

212

Bougucr

Grovit;fAnomoly

Mop 710

Jotunhtimtn

and

More,

Norwoy

,’

,

-

-.*._

-..* . *. . 62 *._-a5

. ..

Fig. 3. Bouguer gravity anomaly map of the Jotunheimen and surroundings.

:-6s

70’ \.

2-13

1

A*r”m.d

rlg,onalgrovlty - - -

Km

Fig. 4. Regional gravity profiles through Jotunheimen area based on refraction seismic profile northeast of the Jotunheimen, and on Airy isostatic hypothesis with topography averaged over 100-200 km.

40 km we get an average value of 2.88 g/cm3 for the crust (Woodside and Bowin, 1970). Two isostatic two-dimensional models are calculated (Fig.4). One uses heights averaged over 100 km and one over 200 km (this is similar to Woollard’s 1” X 1” and 2” X 2” averages (Woollard, 1969)). The gravity effect of the models was adjusted to have values about zero at the sea coast. All these models show, however, that the regional trend in the gravity has to be curved downward underneath the Jotunheimen area, thereby making the positive residual anomaly greater than previously assumed for any given linear approximation for the regional anomaly (Smithson, 1964a; Smithson and Ramberg, 1970). While the regional gravity anomaly based on the seismic profile is displaced to the south, regional anomaly based on the 100 km averages is somewhat low and displaced to the north, and the regional based on 200 km averages is too high. We have, therefore, used a regional gravity anomaly in between these two curves and the observed gravity profile. The main point is

214

that these calculations demonstrate conclusively that any reasonable regional trend must be curved downward and that our selection of a regional does not make the residual gravity anomaly over the Jotun nappe unre~onably large. Two profiles across the Jotun nappe have been studied in more detail, and are presented in Figs.5 and 6. The positions of the profiles are shown in Fig.3. The calculations have been made based on two-dimensional bodies (Talwani et al., 1959). For an elongate outcropping body, the difference between a two-dimensional and a three-Dimensions inte~retation will in general be quite insignificant. Applying Nettleton’s (1940) “end correction” on the actual case and assuming a thickness of the anomalous mass of about 16 km, then the profile closest to the end of the nappe (profile AA’) will differ from the ideal three-dimensional solution by about 7%. That is, the two-dimensional solution will underestimate the solution by about one kilometer. In profile AA’ (Fig.5) the anomaly shows a steep gradient toward the northwest and a smaller gradient toward the southeast where it merges into the Slidre gravity high. This overlap makes resolution of the Jotun residual anomaly difficult at the southeastern end of this profile. In profile AA’, an increase in the southeastern gradient can also be distin~ished above the central part of the Jotunheimen. This increase marks a local high superimposed on the broad gravity high. The local high coincides with an area characterized by more common ultrabasic rocks than in the surrounding nappe area. This area has been separated from the remaining nappe by two fault zones, the Gjende and Utladalen faults (Battey, 1965). The maximum residual anomaly in profile AA’ is about 58 mGal. Using the observed density contrast of 0.12 g/cm3 between the Jotun rocks and adjacent gneisses, a body of a maximum thickness of 25 km (7 km below the southeastern flank) has to be assumed to fit the residual. Still the peak value in the central area is

m

8

JOTUNHEI~~N

at

PROFILE

_20 Coast rise -40

A

/a

Grotli

Fig. 5. Gravity profile and model along proftie AA’. Circles and crosses give computed points for corresponding models. Dense shallow body was added to fit the observed gravity profile.

215

mgal O-

JOTUNHEIMEN

PROFILE

B-B’

-2OOslo

40

L km

0

B&YI, ISkrn,

.

Body I 8 Ultroborlc Body II,

21 km.

&m=O I2

.J/eml’

body,

OBkm.

ap so.12

p/cnP

Fig. 6. Gravity profile and model along profile BB’. This profile negates the possibility of a dense mass underneath the Jotun nappe.

too low (Fig.S), nor can it be reached simply by increasing the thickness since the flank values will be too high. An alternate model is therefore suggested, reducing the thickness of the main body to 16 km (below the sea level), and introducing a hypothetical body of ultrabasic composition (p = 3.20 g/cm3 ) from sea level and 1.3 km downwards. This gives a good fti, but readers should note that the ultrabasic body is arbitrarily chosen. An increased thickness in the hypothetical ultrabasic body will effectively reduce the thickness of the main body. If all the nappe below the sea level consists of ultrabasic rocks, this extreme solution will result in a thickness of about 7 km. This value consequently represents the minimum possible thickness of the anomalous body below the central part of the Faltungsgraben. In profile BB’ (Fig,G) the interference with the Slidre and Grotli gravity highs is negligible. The two Jotun gravity highs are clearly associated with the two lobes of the nappe and reveal that no anomalous buried mass exists in the anticlinal area between the two lobes. The regional trend used is about as high as is permissable; therefore, the residual anomalies with peak values of about 49 and 31 mGa1, respectively, are regarded as minimum values. In order to explain the residual anomalies again two principal models were considered. Firstly, if the observed density contrast of 0.12 g/cm3 is applied, the anomalous bodies reach down to about 21 km and 11 km below the two separated lobes. Like in AA’ the bodies have to have an inward dip, and the peak value is not reached in the base of the main branch in the Faltungsgraben. By assuming a hypothetical ultrabasic body 0.8 km thick, the fit between residual and calculated gravity is better and the thickness of the main body is reduced to 15 km. No ultrabasic body had to be assumed in the case of the southeastern lobe. Similar to the situation in profile AA’, an increased amount of ultrabasic rocks may be assumed, leading to a reduction in the total thickness to a minimum of 6 km below sea level. It has to be

216

pointed out, however, that an increased density with depth is not supported by observations in the 2,500 m of vertical section that is exposed. Whatever subsurface model is chosen, we will end up with a deep slab of considerable thickness, of the order of 10 km. Although innumerable model solutions are ideally possible, the outcropping nappe and the observed density contrast put strong constraints on the variability of the models. Also, to fit the gravity gradients one has to assume inward dipping contacts, and without inferring unreasonable subsurface densities, the models have to extend to great depths. DISCUSSION

The results developed here present a very different geometrical interpretation of the Jotun nappe than that which has usually been proposed by geologists. Because of this and because of the ambiguity of gravity interpretation, implications, assumptions and weaknesses of our conclusions must be carefully considered. This interpretation suggests a maximum calculated thickness of 16 km for the nappe and, even more significantly, the two northeastern lobes of the nappe show a thickness of 15 km and 10 km, respectively. Even so, it was necessary to assume a higher density for the nappes below the surface in order to match the observed gravity field. The nappe has an interpreted thickness that is approximately one half that of the continental crust; even if they were composed solely of peridotite, they would have a thickness of 6-7 km below sea level plus the 2 km above sea level. The facts that the density had to be arbitrarily increased below the surface and that large amounts of dense rocks are found in the Precambrian south of the Jotun nappe (Smithson, 1964b) suggest the possibility that the nappe might be a thin slab overlying dense unrelated rocks in the basement below it. Gravity contours and the positive gravity anomaly closely follow the surface exposure of the nappe and together with the measured higher rock density of the nappe suggest that the nappe, itself, is causing this anomaly. Anomalies over the nappe decrease strikingly to the southwest, where the composition changes from the intermediate rocks to less dense anorthosite, as is clearly shown in Fig.7. Profile BB’ in Fig.6 also shows that gravity anomalies decrease sharply in between the two lobes of the nappe and that the lobes are thick. Dips of fault planes below the nappe generally agree with the geometry suggested by the gravity model. Thin klippen of mafic rocks underlain by flat-lying thrust phmes southeasterly of the nappe show small gravity anomalies. All of the above observations suggest that it would be a remarkable coincidence if much of the gravity anomaly associated with the Jotun nappe were caused by an unrelated underlying body. We therefore conclude that the most plausible interpretation is that the nappe is indeed thick, because of the above reasons and because a nappe of peridotite represents the extreme case that is unsubstantiated by observa-

0

mgol

Jotun

growty

high

b - Slidre

a-

gravity

high

c -FIG

gravity

low

Fig. 7. Regionaf gravity profiles 25 km apart across the entire Jotun nappe region (from Ramberg, in preparation). The amplitude of the positive anomaly over the nappe decreases to the southwest where the rocks become more anorthositic and less dense. Profile I is to NE and profile VI is to SW.

tions. For the distant-nappe hypothesis, which is supported by most geologists, the nappe must have been greater than 15 km or have undergone secondary thickening and it must have been down-folded after arriving at its present position. The geometry of the nappe is more compatible with its having been upthrust from below its present position. In this way it resembles the large positive gravity anomaly along the Ivrea zone of the Alps and the geophysical interpretation of this anomaty (Berckhemer, 1968; Kaminski and Menzel, 1968; Mueller and Talwani, 1971), as a dense mass lobate in cross-section that was thrust up from the lower crust or upper mantle (Gilluly, 1971). The Jotun nappe could thus represent a wedge of dense rocks thrust up from below, such as the Ivrea zone, but eroded to a deeper level. On the basis of mineralogical studies of coronas in Jotun rocks, Griffin (1971) concluded that initial crystallization took place at depths of 2530 km and that this was followed by a sharp reduction in pressure and limited recrystallization to form coronas around some minerals. Battey (1960,1965) and Dietrichson (1958) both concluded that the Jotun complex represents an original layered intrusion. The mineralogical data suggest two stages: (1) fo~ation in the lower crust; and (2) emplacement in the

218

upper crust which corresponds to resetting of the radioactive clock during the Caledonide deformation. Gabbroic rocks that have been correlated with Jotun rocks are found with a depositional contact against Eocambrian arkose (sparagmite) east of the Jotunheimen (Englund, 1969). If these are the same as the Jotun rocks, then the rocks of the Jotun complex must have been transported to the surface during the Late Precambrian, been partly eroded, and had the arkosic rocks deposited on top of them. In this case it seems unlikely that these same arkoses could have been deposited off the coast of Norway as those which are now found in the great Eocambrian arkose basin east of the Jotunheimen. It seems that either the Jotun complex was a nappe with its root zone off the coast of Norway or that it formed part of the Precambrian basement upon which Eocambrian sediments were deposited near its present location, but not both. The relation of the Jotun nappe to a plate-tectonic framework is determined by the position of a suture in the Scandinavian Caledonides. Many authors (e.g., Strand, 1961) stress the allochthonous nature of Cambro-Silurian micaschists, greenstones, and marbles. A subduction zone located in the Norwegian Sea west of the present coast of Norway has been suggested (Roberts, 1971; Nicholson, 1971). They stress the lack of exposed roots for these nappes. Other workers (Dewey, 1969) propose that a suture runs along the zone of nappes and is marked by the presence of ophioli~s. If the subduction zone was off the coast of Norway, then the rocks of the Jotun nappe were probably forced up to the surface near the subduction zone and then slid off a rising dome by gravity sliding to their present position. If the subduction zone ran along the site of the present nappe and ophiolites, then the Jotun nappe was pressed up from almost directly below its present position. The shape of the nappe seems to be more in line with the latter possibility. But, one might note that in the Hardanger region southwest of the Jotun nappe the Precambrian basement is exposed in a “window” in the central part of the Faltungsgraben. Also the possible establishment of similar geochronological provinces in the basement across the F~tung~~n does not support the idea of a suture line through this part of the Caledonides. Several pieces of evidence suggest that the Jotun complex may represent a segment of the lower crust,that has been squeezed to the surface, The high pressure of formation (Griffin, 1971), and the granulite-facies mineralogy (Ringwood and Green, 1965) are indicative of the lower crust. Besides a thickness that is about right for the lower crust, the rocks have appropriate density for lower crustal material - althou$ the density is slightly low - and should have seismic velocities ranging from 6.5 to 6.9 km/see. as found in the lower crust. On the basis of seismic reflections,.Clowes and Kanasewich (1970) have suggested that the lower crust is made of aRemating sills of highand low-velocity material, a model that resembles the layered rocks of the Jotun complex. One ~~rpre~tion would calI the nappe lower crustal

219

material that has been pushed up from the base of the crust at its present position like the Ivrea zone of the Alps (Berckhemer, 1969), another interpretation would call these rocks lower crustal material that has moved as nappes over a 100 km from a source underneath the sea off the coast of Norway. CONCLUSIONS

This study has shown that the Jotun nappe is associated with a large mass excess. Although this mass excess could be caused by a dense body that underlies the nappe and is unrelated to it, overall relationships, particularly the striking correspondence of the positive gravity anomaly with the exposure of the nappe, makes this possibility seem remote. We must, therefore, conclude that an unusu~ly large mass excess is associated with the mafic rocks of the nappe; this is an unambiguous fact that is directly determined from the gravity anomalies. The thickness of the nappe could be as great as 16 km or more in the Faltungsgraben and probably no lower than 6-7 km with any plausible density contrast. The shape of the nappe together with its composition suggests that it could be a wedge of lower crust, thrust up from below similar to the Ivrea zone of the Alps. Most geologists, however, prefer the hypothesis that the Jotun rocks were emplaced as a nappe whose root zone lies underneath the Norwegian Sea off the present coast of Norway. Geophysical measurements at sea show a zone of dense rocks with a Caledonide trend in the Norwegian Sea off the coast of More (Gronlie and Ramberg, 1970; Talwani and Eldholm, 1972). In this case the nappe must have moved over 100 km as a thick sheet, but thickening of the nappe by downfolding in its present position may have taken place. A depositional contact between the Jotun rocks and Eocambrian sparagmite does not support the distant-nappe hypothesis. Geophysical results show that the Jotun nappe is much thicker than has previously been believed; more geological studies are needed in order to choose between these two hypotheses. ACKNOWLEDGEMENTS

We wish to thank the Geographical Survey of Norway (Norges geografiske oppm~ing) for abundant gravity data that they furnished us. Partial support, for Smithson was received from a G. Unger Vetlesen Post-doctoral Fellowship at the Geologisk Museum, University of Oslo, Norway. Ramberg received partial financial support from NTNF and NAVF (in Norway) and from a post-doctoral fellowship at the Department of Geology, University of Wyoming. Mr. Thoralf Nordby is thanked for help with the field work. REFERENCES Asklund, B., 1961. The extension of the Serv nappe in the Scandinavian Sver. Geol. Unders., Ser. Ca., Avh. Uppsat., 584, 28 pp.

mountain

chain

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