Interpretation of some magnetic anomalies over Iceland

Tectono@zysics, 16 (1973) 163-187 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

INTERPRETATION OF SOME MAGNETIC ANOMALIES OVER ICELAND J.D.A. PIPER Department

of Earth Sciences.

University of Leeds, Leeds (Great Britain)

(Accepted for publication May 24, 1972)

ABSTRACT Piper, J.D.A., 1973. Interpretation

of some magnetic anomalies over Iceland. Tectonophysics,

16: 163-l

Simulated dike models with rectangular and normal probabilities are used to examine the distribution of volcanism implied by the magnetic anomalies over the Reykjanes ridge. The linearity and symmetry of the anomalies are lost once the area of Iceland is approached. This is attributed to a greater spread of the volcanism in space about the neovolcanic zone, lateral variations in thickness of the lava polarity groups, the introduction of weakly magnetic acid rocks at intervals, and variable amounts of remagnetisation and metamorphism within the lava pile. The source bodies of the magnetic anomalies over Iceland are complex and include shallowly dipping lava flows, and near vertical dikes and other intrusions. The earlier-formed lavas within the pile are probably completely remagnetised by subsequent dike injection towards the layer 2 - layer 3 seismic discontinuity and below this the crust is believed to be entirely intrusive. Most Icelandic volcanics have comparable magnetic intensities to other basaltic rock collections but the susceptibilities are an order or so higher than submarine lavas. The anomalies are not significantly affected by plausible magnetic decay rates or tilting of the lava pile. Rock magnetic properties show no regular change with distance from the neovolcanic zone. Magnetic polarity maps are presented for the two flanks of the neovolcanic zone in northern Iceland, the lava pile in eastern Iceland, and southwestern Iceland. The lava stratigraphy can be correlated with the aeromagnetic anomalies in the latter two areas, and models involving the top few kilometers of crust can be fitted to the observed data and set controls on its probable structure. The combined geologic and magnetic anomaly data allow a spreading rate to be deduced for southwest Iceland which is closely comparable to that inferred for the Reykjanes ridge. High amplitude and short wavelength anomalies correlat closely with the distribution of young volcanic rocks and they indicate that differential spreading is taking place between north and south Iceland.

INTRODUCTION

Iceland is the only large landmass exposed about the mid-ocean ridge system and the largest portion of undoubted ocean crust available for surface sampling and observation. In spite of this it has not figured prominently in the developments which have taken the original ocean-floor spreading concepts of Dietz (196 1) and Hess (1962) from hypothesis to well substantiated theory. This is largely due to the geological complexities which result from Iceland’s supramarine nature, and there is still dispute over a proper interpretation of the structure of the island (see for example the differing explanations provided by Einarsson ( 1967) and Saemundsson (1967)). Two of the strongest arguments for crustal spreading fror

164

J.D.A. PIPER

mid-ocean

ridges are the high degree of bilateral symmetry

of the anomalies about the

ridges, and the agreement of the pattern with the known polarity changes (Vine, 1966). Magnetic surveys over Iceland, however, have failed to identify

either of these features here,

and the present paper will examine the reasons for this. The exposed crust in Iceland is almost entirely volcanic in origin and more than 80% basaltic in composition. The products of recent volcanism are concentrated along a single zone up to 50 km wide in the north of the island. In the south there are two parallel volcanic zones between 30 and 80 km wide. The eastern and western parts of the island are composec of piles of sub-area1 lavas of Upper Tertiary and Pleistocene age (Moorbath et al., 1968). The lavas have acquired a shallow inclination which in the east is consistently towards the active belt, but in the west the structure is more complicated and although the bulk of the lavas dip towards the active zone, there are departures from and even reversals of, this trend. The lava pile was first studied in detail by Walker (19.59, 1960) in eastern Iceland and his observations there were combined with the geophysical data to produce a model for the evolution of the crust in Iceland which allowed for considerable spreading at the base of the lava pile if it continues downwards to the Moho (Bodvarsson and Walker, 1964). Unfortunately this model does not agree with the seismic data for Iceland (Einarsson, 1965a) which reveal a contact between layer 2 (average velocity 5.08 km/set) and layer 3 (average velocity 6.35 km/set) at depths generally between 2 and 5 km below sea level (Palmason, 1967a, 1970). Gibson (1966) attempted to reconcile the data and showed that the lenticular nature of the lava pile permitted it to be terminated at the seismic discontinuity. Considerable information already exists on the magnetic stratigraphy of the lava pile in Iceland and an attempt can be made to interpret the magnetic anomalies observed here using this ground control. This is the subject of the present paper which examines anomalies found during the first regional aeromagnetic study of Iceland by Serson et al. (1968) in the light of ground information. Since the neovolcanic zone in Iceland is a direct landward continuation of the Reykjanes ridge the subject of magnetic anomaly interpretation will first be approached by consideration of the anomalies about the ridge and comparisons will then be made between this area and the Icelandic mainland. THE REYKJANES

The Reykjanes

RIDGE

ridge was one of the first lengths of mid-ocean

ridge to be investigated

by

a magnetic survey (Heirtzler et al., 1966) and Vine (1966) interpreted the anomalies observed here in terms of the ocean-floor spreading hypothesis and the known reversal history of the earth’s magnetic field for the last 3 to 4 m.y. The agreement between observed profiles and a model assuming a spreading rate of 1 cm/year on each side of the ridge axis is remarkable and leaves little doubt that the main anomalies correspond to epochs of the earth’s magnetic field. The interpretation of the magnetic anomalies can be taken a step further to examine the distribution of volcanism about the ridge implied by the anomalies. Simulated dike-injection models have been used for this purpose by Matthews and Bath

MAGNETIC

ANOMALIES

OVER

165

ICELAND

(I 967) and Harrison (1968). They assume that material is injected with a normal probability of some standard deviation about the ridge as vertical dikes, and between two planes approx mating to the upper crustal layer. Such models have been criticised as artificial but their importance lies in the fact that since they represent the ideal case, they give limiting condition! and the actual dispersion of volcanism must be smaller because of a variety of smoothing effects to be expected in practice. If the lateral spread of the extrusive volcanic products on the ocean floor is less than or of the order of the depth of the ocean, then the sharpness of the anomaly pattern is not diminished (Watkins and Richardson, 197 1). The retreat of the Curie-point isotherm within the crust leads to inclined interfaces between normal and reversely magnetised

bodies and diminishes the amplitude

of the anomalies.

Further sup-

pression of event anomalies will take place from the effects of faulting, topography

and an

irregular base to the magnetic layer. Simulated models are examined here for the case of the Reykjanes ridge using both normal and rectangular probability distributions. The method followed is essentially that used by Harrison (1968) in which vertical dikes 20 m thick are injected with a frequency appropriate to the spreading rate parallel to the ridge axis and infinite in this direction.

The in-

jection sequence is reversed to give the pattern on the other side of the ridge axis and the ‘magnetisation averaged over successive groups of twenty dikes for the anomaly calculation. The model is terminated at the mean depth of the sea floor over the Reykjanes ridge and extends 2 km below this. The observed field at a height of 0.47 km above sea level has been computed so that the profiles are directly comparable with those obtained by Heirtzler et al. (1966) and slightly smoothed relative to those obtained by Godby et al. (1968) at 0.3 km above sea level. Fig. 1 shows the simulated anomalies for a rectangular distribution with several widths of injection (0) and a normal distribution with several standard deviations

Fig. 1. Results

of simulated

dike-model

studies with rectangular

and normal

probabilities.

166

J.D.A. PIPER

(T). The magnetisation

is assumed to be entirely remanent

with an intensity

of 0.0065 G

and oriented along the dipole field. Several of the observed profiles over the Reykjanes ridge have an axial anomaly modified by a minor anomaly which can be attributed to the Jaramill event and on the models this anomaly is practically suppressed at T values of 0.5 km and completely at D values of 2.0 km, suggesting that if this Jaramillo anomaly is real then actual distributions must be narrower than this. The next control is obtained from the Mammoth-Kaena event anomaly, which can be isolated on a few of the published profiles, and which, where present, restricts T values to less than about 2.0 km and D values to less than about 3 km. The Olduvai-Gilsa anomaly is universally present, and on the models this is nearly lost at D = 3.0 km and T = 5.0 km although the amplitude

and definition

of

the anomalies over the Reykjanes ridge is such as to imply values smaller than this. The model studies suggest that if volcanism takes place about the ridge with a normal distribution, it must have a maximum standard deviation of between 0.5 and 3 km. If the volcanism is more uniformly distributed and approaches a rectangular distribution, it can be spread acres widths of 5 km or possibly more before critical event anomalies are suppressed. The case in reality probably lies somewhere between the two models with volcanism being confined within a certain distance of the ridge axis but reaching a maximum intensity near the axis. MAGNETIC ANOMALIES OVER ICELAND

Ground magnetic surveys in Iceland show short-wavelength

high-amplitude

anomalies

comparable to deep-towed profiles over the ocean floor, and maximum depth estimates indicate that they are controlled by near-surface effects. With increasing height these effects are smoothed to reveal anomalies due to thicker sections of crust. Several medium-height surveys over Iceland are being interpreted at the present time (T. Sigurgiesson, personal communication, 1968; Dennis et al., 1970). The high-level survey (3.0, 3.7 or 4 km above sea level) of Serson et al. (1968) has smoothed out topographic effects and is useful for studying the bulk crustal structure; this survey data is considered here. The vertical Z-component of this survey after the removal of a third degree polynomial residual field is shown in Fig. 2 superimposed on the zones of active rifting and volcanism in Iceland. It is immediately

apparent that the linearity parallel to the ridge and the sym-

metry about the axis, which are observed over the Reykjanes ridge, are lost once the latitude of Iceland is reached. There is in fact little correlation between adjacent flight lines but the neovolcanic zone is clearly associated with high amplitude, predominantly positive anomalies. These axial anomalies are complex, particularly where the lines cross thermal areas, and the local minima are attributed to leaching of the magnetic minerals by hot solutions and the rise of the Curie-point isotherm to near the surface. In spite of this it is generally possible to define the centre of the axial anomaly precisely and the line joining the centres along the eastern active zone can be seen to follow very closely the dotted line which marks the centre of the Mid-Atlantic Ridge proposed by Ward et al. (1968) on the basis of microseismic observations. The western branch of the neovolcanic zone, which is also de-

167

MAGNETIC ANOMALIES OVER ICELAND

Fig. 2. Magnetic anomalies over the Icelandic area after Serson et al. (1968) superimposed on the zone of young volcanism (shaded area) and axis of the Mid-Atlantic Ridge defined by Ward et al. (1968) on microseismic evidence. The rectangles enclose areas discussed in the text. The stars and circles mark dike and lava paleomagnetic sampling areas respectively, the magnetic properties of which are discussed in the text.

fined by high amplitude Ward et al. (1968) and have postulated Reykjanes

anomalies,

ends abruptly

have failed to find significant a transform

fault further

in the vicinity

of the Langjokull

microearthquake south

activity

to connect

the eastern

observed

over Iceland

icesheet.

along this line branch

of the

ridge.

ANOMALY CONTROLS The factors discussed

which control

under

the magnetic

anomalies

will now be

six headings.

Rock magnetic properties An assessment

of the magnetic

properties

of the rocks making

up the crust in Iceland

is

168

J.Y.A. PIPER

TABLE I

___”

X~-_-~_~~._~-__-_-._,,._______-_

Jo

-l_-~_-

k

ho0

_l__-l__~_l_~_--~__~.--“._---

On

&SO0

61.61(101)

30.66(101)

favasamples from intragIaciaI volcanic bodies, SW Iceland

0.0127(1,44)

Pillow broccia samples from iritra-&cial valcar& bodies, SW &eland

0.0181(77)

Samples of subarea1 lavas of Plio-Pleistocene age, SW f&and

~.~~6~~~ 15)

3.90fll5)

Submarine basaftic voicaaics from dredge hauls at seven localities in the Atlantic and Pacific oceans

~.~~9~~~84~

8.40ff 73)

Samples from Tertiary dike swarms in east and west lcsland

0.0050(5 I I)

Piifow

_-I

87.32(63)

0.0032(494)

1.15(494)

0.1580(571)

,,

---

--

* fe and k-are in c.g.s. units. &OQ and &m are defined by Ade-RaU et al. (1968). The figures in bracket: are the numbers of samytes.

fundamental

to any reasoned interpretation

Icelandic rock collections

of the magnetic anomalies. Properties of several

have been reported by Kristjansson

(1970) and the properties

of

others measured by the present author are given in Table 1. The sample localities are given in Fig, 2 and the properties of a submarine parison. The rna~et~~~~

fxetd

vofcanic rock co&ction

ofeach sample

are included for com-

is assumed to be dipolar and for ~om~arjso~

the intensities of magnetisation are reduced to their paleoequatorid value by dividing by (1 3 co&)+ where q!~is the paleolatitude of the sampling locality. Where this last parameter is not known, as in the case of the submarine collection, the present latitude is used, and since the material is Upper Tertiary or Pleistocene in age the error involved is not likely to be v-cry large. As ;f ~e~~~~~ of tie ~~~~~~~~~ of the magnet~sat~on~ .&e parameter .Yzm has been used (Wilson et al., I%%), Within the coIIections, variations in Jo (total [email protected]) and k (low field susceptibility) of two orders of magnitude are present and to re-

169

MAGNETIC ANOMALIES OVER ICELAND

Fig. 3. Logarithmic normal curves fitted to intensity of magnetisation (Jo) and low field susceptibility (k) distributions for dike, pillow lava, pillow breccia, submarine volcanic rocks and sub-areal lava collections. The mean and standard deviation of gabbros given by Kristjansson (1970) is also included in the diagram. The shaded area is the distribution of susceptibilities for 27 samples of intra-glacial vitric tuffs.

alise a comparison

between them, normal probability

curves have been fitted to the distri-

butions of logloJo and log,,k, and the resultant curves are shown in Fig. 3. The mean log,OJO and loglok values for 21 fresh gabbro samples reported by Kristjansson (1970) have also been included for comparison. Only one of these samples has a Q, (Koenigsberger ratio value less than unity and Kristjansson’s and remanent

magnetisms

observation

that the gabbros have higher induced

than typical lavas and dikes is confirmed

The sub-areal lavas and dikes have closely comparable distributions.

remanent

by the present data. intensity

and susceptibili

The mean Jo value for the dikes is slightly less and the mean k value slightly

more than that of the lavas, which results in a rather greater percentage of the dikes having Q, values less than unity than do the lavas; this effect probably results from dikes tending to have lower oxidation states than the lavas (Ade-Hall and Lawley, 1969). The intra-glacial volcanics (pillow lavas and pillow breccias) have intensities somewhat higher than the lavas and dikes, and susceptibilities an order or so lower and comparable with the submarine volcanics. This quenched material contains fine-grained, predominantly unoxidised, titanomagnetite and the magnetism is presumed to be largely a T.R.M. rather than predominantly a C.R.M. as in the case of the sub-area1 lavas and dikes. Intra-glacial volcanics, formed of thiz material together with mantling deposits of practically non-magnetic vitric tuffs, occur at intervals within that part of the lava pile which is younger than about 3 m.y. old. The question of viscous magnetisation (V.R.M.) has been considered by Kristjansson (1970). In the

170

J.D.A. PIPER

case of the collections under consideration the arithmetic mean value of the stability index (S,,,,,) for the intra-glacial pillow lava and submarine collections lies between that of the dikes and the lavas, but only the dike sample is significantly level from the other collections.

different

The pillow breccias have exceptional

have a random fabric and a low susceptibility

they cannot contribute

at the 95% confidence stability but since the) significantly

to mag-

netic anomalies. Comparison of the distributions shows that the main difference in magnetic properties to be expected between the Icelandic basalt pile and an area of submarine volcanics is in the susceptibility, and consequently the induced magnetisation will make up a greater proportio of the measured total magnetic field. The contrasts between bodies of normal and reverse remanent

magnetisations

will then be reduced and may be completely

nullifield.

Shape of the causative bodies

Dredge hauls from the crests of the mid-ocean

ridges have shown that basaltic lava is

erupted about the ridge in the form of pillow lavas. Examination of sub-aqueous eruptions in Iceland suggests that these build up ridges of triangular cross-section elongated along the eruptive dike and parallel to the ridge axis. Unlike sub-area1 basaltic lavas, which spread out on either side of the eruptive fissure as thin sheets which may cover several hundred square kilometers, pillow lavas pile up around the eruptive orifice at angles of up to 30”. Apart from the intra-glacial volcanics and minor amounts of submarine material, the Icelandic lava pile is entirely sub-areal. It dips at shallow angles, mostly less than 15”, and is made up of alternating sequences of normal and reverse polarity (Dagley et al., 1967). Hence observations of the exposed geology indicate that the source bodies giving rise to the magnetic anomalies will be shallowly dipping sheets of lavas of alternating polarity centred about the eruptive dikes. Influence

of the central volcanoes

Mapping of the lava pile in eastern Iceland by Walker (1959, 1960) has established the lavas are in general fed by the dikes and these tend to concentrate

that

as swarms about the

central volcanoes which are loci of silicic volcanic activity within the lava pile. Detailed stratigraphic (Gibson, 1966) and paleomagnetic (Piper, 1971) studies have shown that individual lava groups thin away from these centres. Two complications are thus introduced into the magnetic anomaly interpretation by the central volcanoes. Firstly, quantities of silicic volcanics with lower magnetic remanent intensities and susceptibilities than typical basalts are present, and secondly, the basaltic products erupted during any one polarity epoch are most heavily represented in the areas of central activity current at the time. Silicic and basaltic intrusions are found within the central volcanoes, and the proportion of minor intrusives appears to increase with depth in the lava pile. Considerations of geometry, heatflow data and rates of extrusive volcanism (Gibson and Piper, 1972) suggest that the lava

MAGNETIC

ANOMALIES

171

OVER ICELAND

pile cannot extend far below the contact between seismic layers 2 and 3 (Pilmason,

1967a,

1970). T%edistribution of volcanism Because of the complexity

of the source bodies and the general absence of event anoma-

lies, the magnetic anomalies over Iceland cannot be used to decide the maximum spread of volcanism here. Inspection of the present neovolcanic zone shows that postglacial (younger than about 10,000 years) open and eruptive fissures are spread across widths of 15-50 km across these zones and clearly have a much wider dispersion about the ridge axis than is implied by the magnetic anomalies over the Reykjanes ridge. Heating and remagnetisation of the lavapile From his studies in eastern Iceland Walker (1960) has found that after the lava pile has been tilted, it undergoes a reheating which produces a series of quasi-horizontal

zeolite zones

which are related to the depth of burial beneath the original surface of the pile. The heating is due to intrusion of the earlier formed lava pile by subsequent dikes, and if the temperature reached exceeds the blocking temperatures (Nagata, 1961) of the intruded lavas, they will be effectively remagnetised in the same direction as the dike when they cool through the Curie point. The distance to which the 100, 200,300 and 400°C isotherms migrate from the centre of dikes of variable width inruded at 1000°C is shown in Fig. 4A. The effects of conduction alone are considered using equations given by Lovering (1935) and assuming a thermal diffusivity of 0.010 cm/set. The country rock adjacent to a dike will never be heated up to a temperature

more than one half that of the magma. Icelandic dikes average about

3 m in thickness, and assuming a uniform distribution across the lava pile it is a simple matter to compute the percentage of crust of a given Curie point which will be remagnetised for any dike density (Fig. 4B). The temperatures of remagnetisation are likely to be very variable. Ade-Hall et al. (1965) have studied the Curie points of a large number of basaltic lavas and have found that for basalts showing homogeneous titanomagnetites they range between 50 and 600°C but most tend to lie in the region 300-450°C. Young lavas from the neovolcanic zone in Iceland have low Curie points in the range lOO-250°C (author’s measurements)

but these will increase as the lavas become buried (Ade-Hall et al., 1970).

Dike frequencies of up to 40/km are observed at sea level through central volcanoes exposed within the Tertiary lava pile, but across most of the pile there are less than 10 dikes/km exposed at this level and any remagnetisation effects must be small. The dikes are effectively confined within cupolas about the central volcanoes and increase in number with depth (Walker 1960). Walker (in Bodvarsson and Walker, 1964) has also found that the lava pile is intruded by numerous sheets where it is exposed at depth in eastern Iceland. From general geological considerations it seems likely that the lava pile will be largely remagnetised around the central volcanoes and towards the seismic discontinuity.

172

J.D.A. PIPER

20 Dikes

per

km

Fig. 4. A. Distance of migration of the 100, 200, 300 and 4OO’C isotherms from the margin of a dike intruded at 1000°C. B. The percentage of crust remagnetised by the intrusion of dikes of 3 m thickness as a function of the density of intrusion.

Thermal metamo~hism Accompanying

of the lava pile

the effect of remagnetisation

by subsequent

intrusion

will be a thermal

metamorphism at depths in the lava pile at which the temperature and available water pressure is sufficient to partially or completely reconstitute the mineralogy of the rocks. According to Walker (1960) the zeolitisation results from the mobilisation of water contained in the scoriaceous and vesicular lava flows below the water table. Below the lava pile the rocks are likely to be less permeable (Pilmason, 1967b) but may contain up to several percent of nla~atic water (Ade-Hall et al., 1968) to facilitate met~orphic reactions. Within the thermal areas in Iceland temperature gradients of up to 260aC/km have been recorded and elsewhere values lie between 34 and 165”C/km (P$mason, 1967b). If continued in depth these gradients imply that the zeolite, greenschist, and amphibolite metamorphic facies fields of Turner (1968) are reached before the base of crust. In addition the higher values intercept the minimum melting curve of “granite” at depths of about 6 km, and the magnitude of the temperature gradients implies that conditions appropriate to ~phibolitisation and partial melting will be reached within the crust. The consequences of a greenschist metamorphic zone reaching to within 3 km of the surface and an amphibolite metamorphic zone

MAGNETIC

ANOMALIES

-

173

OVER ICELAND

0.015 e.m.u / c. c.

- - - - 0.010 0.005

Fig. 5. Estimates in Fig. 2.

of the maximum

0

,

depth

to the source bodies

1

i

km

’

along the aeromagnetic

coming to within 5 km of the surface must be considered.

’

50

1

profile AB shown

Ade-Hall et al. (1968) have de-

fined an alteration process of titanomagnetite granulation from the examination of the opaque mineralogy of basalts from the lower zeolite zones in eastern Iceland. These authors conclude that this process does not result in remagnetisation. Mid-Atlantic

Ridge have very weak remanent

Basalts altered to greenstones

intensities

about the

caused by complete replacement

of

the titanomagnetite by non-magnetic minerals, and serpentinised peridotites have intensities a fraction of those of unaltered basalts which is carried by magnetite formed at low temperatures (Haggerty and Irving, 1970). According to Verhoogen (1959) metamorphic

rocks

which have recrystallised and cooled slowly should have small T.R.M.‘s of low stability because of the low number of dislocations in the titanomagnetite crystal structure. These considerations

suggest that the depth of the source bodies of the magnetic anomalies will be

controlled

by metamorphic

effects which replace basaltic titanomagnetites

by assemblages

with weak magnetic properties. The maximum depth to the base of the source bodies can be estimated from the magnetic anomalies by using the criteria of Smith (196 1) which are relationships that do not require any assumptions concerning the shape of the bodies. Estimates of the maximum depth made along the aeromagnetic line AB in western Iceland (Fig. 2) are shown in Fig. 5 for several magnetic intensity values. If the anomalies result from contrasts in the magnetisation of normal and reversely magnetised bodies, then using typical Icelandic lava intensities shows that the source bodies must lie within about 8 km of the surface. Across the neovolcanic zone it is necessary to assume intensities in excess of 0.03 G to make the base of the anomalies lie above inferred Curie-point isotherm depths, and an explanation of this involves consideration of the high-amplitude axial anomalies which will be made in the next section.

174

J.D.A.

MAGNETIC

PIPER

MODELS

The magnetic anomalies observed over Iceland will result from lava piles of shallow inclinations

with normal and reversed polarities,

portion of the country isotherm

and dikes, which make up an increasing pro-

rock with depth. Consideration

of the retreat of the Curie-point

as the crust grows and spreads shows that vertical interfaces

cannot

normally

be produced between the bodies. This produces a complication which cannot readily be evaluated and in this discussion the assumption will be made that the interfaces between bodies are either vertical or defined by lava interfaces. Two simplified models are illustrated in Fig. 6a and 6b; the first is a simple sequence of lavas symmetrical about and dipping towards an axis at 8”, and the second introduces vertically sided blocks 1 km below the surface. The bodies are taken as 4 km thick and striking north-south;

the observation

height

is 3 km. Curve A is the computed field for a reman&t magnetism of 0.005 G and a Q, value of 4 and the amplitude is seen to depend strongly on the shape of the body, being much greater for the model involving vertical interfaces than for the model involving shallow interfaces. The shape of the anomaly at the latitude of Iceland where the inclination is steep (76-78”) is relatively independent of body shape. Curve B has been computed by assuming that the remanent magnetisation of 0.005 G has decayed to one half of its original value by

-1 * ---.. -

Fig. 6. Magnetic models of the crustal structure of Iceland.

:

175

MAGNETIC ANOMALIES OVER ICELAND

the time it has moved 100 km from the axial zone. For a spreading rate of 1 cm/year this would be equivalent to a relaxation constant of 2.10-16 c.g.s. units or twice that suggested by Nagata (1960). It can be seen that over the distances involved in these profiles (40 km from the axis) the effect of such a decay on the amplitude Curve C shows the resultant

field when the remanent

of the anomalies is very slight.

vector is tilted 10” away from the axis

as would happen when the lava pile is tilted as it moves away from the neovolcanic resultant effect is to move the remanent

component

zone. The

towards the induced component

side of the axis and subtract it on the other, and again the effect on the amplitude

on one

of the

anomalies is small. Consideration of these models introduces the question of the amplitude of the anomaly over the neovolcanic zone, since it can clearly not be explained in terms of plausible spontaneous decay rates of the remanent magnetism or by the observed tilting of the lavas. For the case of submarine mid-ocean ridges the amplitude of the axial anomaly has been attributed to several factors, in particular a high-magnetic-field intensity during the Bruhnes epoch, demagnetisation by a reversing earth’s field (Harrison, 1968) and maghematisation of primary titanomagnetite (Haggerty and Irving, 1970). While the first two effects may apply to the case of Iceland the problem is not clearly resolvable here because the great lateral spreading of the sub-area1 lavas about the neovolcanic zone will result in an axial anomaly of exceptional amplitude. This is illustrated in Fig. 6c which is a structure striking nortlsouth at the latitude

of Iceland and attempts to take into account the rise of the Curie-

point isotherm beneath the neovolcanic zone. The heat flow across a 30 km wide neovolcanic zone is assumed to be 7 cal cm-* set-’ dropping to 1.5 cal cm-* set-’ at 60 km from the axis. A regional minimum of 800 gammas is produced over the axial zone if all rocks above the 400°C isotherm are assumed to be uniformly magnetised with an intensity of 0.005 G (curve D), and this depression of the regional could result from the induced field alone. However, curve E which is the field over a dipping lava model using the polarity time scale to 4 m.y and making the zone of Bruhnes epoch lavas 55 km wide, shows that in spite of the rise of the Curie-point

isotherm to 3 km from the surface, the axial anomaly is still dominant

be-

cause of the great width of the wedge of Bruhnes epoch lavas. Finally a plot of the mean magnetic properties

of Icelandic subareal lavas and dikes against

distance from the axis of the neovolcanic

zone (Fig. 7) does not suggest any real change in

properties with distance. The remanence

of the bulk of these collections

is probably

acquired

at the stage of deuteric oxidation shortly after eruption and is largely chemical in origin. The exceptional magnetic properties of the intra-glacial pillow lavas are similar to those reported for submarine pillows, and although they may contribute significantly anomaly they make up only a small proportion of the lava pile. PALEOMAGNETIC

STRATIGRAPHY

to the Bruhnes epoch

AND MAGNETIC ANOMALIES

The magnetic stratigraphy of the lava pile in Iceland has now been mapped over considerable areas. Einarsson (1962) examined polarities in several parts of the country using a com-

176

J.D.A. PIPER

2 LoqoJo *

*

3 1

-I

2 Log,,k i

I

,

I

50 Distance

from

100 neovolcanic

150

km

zone

Fig. 7. Logarithmic mean values of intensity of magnetisation (Jo) and low field susceptibility (k) for lava and dike collections plotted as a function of distance from the axis of the neovolcanic zone.

pass, and Dagley et al. (1967) have made a complete palaeomagnetic collection for laboratory measurement through the succession in eastern Iceland mapped by Walker and his coworkers. Further paleomagnetic mapping has been undertaken by Wensink (1964) and Piper (197 1, 1973). Three areas will be considered where data on both aeromagnetic anomalies and paleomagnetic

stratigraphy

exist. The locations of these areas are given in Fig. 2.

Eastern Iceland The lava pile in this area has been mapped in detail by Walker (1964) and his colleagues; it is made up of a thick sequence of basaltic lava flows with central volcanic products at intervals dipping westwards towards the neovolcanic which is considered here lies between m.y. to 12.4 m.y. in age (Moorbath

zone. The portion of the succession

13”3O’W and 14”3O’W and varies from about 9.5 et al., 1968). The lava polarities within this succession

fall into five main groups lettered Nx, Rx, Ny, Ry and Nz and their outcrop is given in Fig. 8 and is derived from the polarity zones obtained by Dagley et al. (1967) and the stratigraphy mapped by Walker (1959, 1960, 1963, 1964) and Gibson et al. (1966). The polarity zones of Dagley et al. (1967) are also compared with the geomagnetic

time scale obtained

by

Heirtzler et al. (1968) from the South Atlantic marine magnetic profiles for the interval 9.0-12.5 m.y. The predominantly normal sequence of lavas Nx can be correlated with a positive magnetic anomaly observed on three parallel profiles (Fig. 2) and may further be correlated with the 9-10 m.y. event proposed by Heirtzler et al. (1968). Below this is a largely reversed sequence of lavas (Rx) which can account for the negative anomaly observed

177

MAGNETIC ANOMALIES OVER ICELAND

Fig. 8. Magnetic polarity map of part of eastern Iceland showing the correlation of the magnetic stratigraphy with the paleomagnetic succession of Dagley et al. (1967) and the geomagnetic time scale of Heirtzler et al. (1968). The location of this area is shown in Fig. 2.

on the three aeromagnetic lines over this area. A comparable anomaly has been found on marine magnetic profiles by Heirtzler et al. and dated 9.94-10.77 m.y. The strike of thes anomalies is between 0 and 20” east of north or comparable to the strike of the lavas and dikes observed on the ground. In Fig. 9 the observed profile along the line FG (Fig. 2) is compared with the computed graphy continues

anomaly made on the assumption

that the outcropping

strati-

to 1 km below sea level. The profile is oriented 57” west of geographic

north, and the magnetic bodies are assumed to extend to infinity with a strike of 10” east of north. The observed anomalies can be closely reproduced along this line using the magnetic stratigraphy

of the lava pile alone.

Northern Iceland The localities concerned in this section lie between 14”2O’W and 17’4O’W and 65”40’N and 66”25’N. The succession and magnetic stratigraphy in the Tjornes peninsula area of northern Iceland is known largely through the work of Einarsson (1958). It includes glacial vestiges throughout and is characterised by great lateral changes in lithology. Normally magr tised volcanics

belonging

to the present polarity epoch and underlying

lavas belonging to

178

J.D.A.

IOkm

Fig. 9. Computed field over a model compared with the observed profile.

PIPER

,

structure for the aeromagnetic line FG (Fig. 2) in eastern Iceland

the last reversed event of the Matuyama epoch are upfaulted

along the eastern side of this

peninsula by a system of north-south trending normal faults. A thick sedimentary sequence outcrops below these lavas in the northwest of the peninsula. These sediments, which have been described in detail by Einarsson (19.58), evidently represent a considerable time span since sedimentation was interrupted by the eruption of both normally and reversely magnetised lavas at intervals. Further south this predominantly sedimentary sequence grades laterally into a succession which is predominantly volcanic, though much of it appears to have been erupted underwater (Einarsson, 1965b). The lavas along most of this side of the neovolcanic zone dip at very low angles towards the zone but the rocks of northeastern Tjornes have had a more complex tectonic history and are an exception to this rule since they dip to the northwest. On the eastern side of the neovolcanic zone reversely magnetised lavas provisionally correlated with the Matuyama epoch are first located 70 km east of Tjornes and successively older groups of lavas outcrop eastwards (Fig. 1OB). A correlation

with the known polarity time scale has been made in Fig. 11 by match-

ing the thickness and position of polarity groups to the geomagnetic southwards

along the strike, the correlation

timescale. Continued

agrees well with the paleomagnetic

stratigraphy

established by Wensink (1964) which has been absolutely dated (McDougall and Wensink, 1966). The aeromagnetic anomalies have been superimposed on the outcrop of the rocks Correlated with the polarity epochs in Fig. 11. There is evidently little correlation in this area between the outcrop of lava polarity groups and magnetic anomalies and it is seen that the positive anomalies centred over the neovolcanic zone are much narrower than the width of the country occupied by the Bruhnes epoch lavas. Similarly the first negative anomalies to the east of this are found up to 35 km from the Matuyana outcrop. The reason for this is not clear but the lava dips in this area are low and the depth of erosion of the lava pile is very shallow. The observed anomalies could then be explained if the bulk of the lava pile is concealed downdip and the eastern part of the cover of Bruhnes volcanics is very thin.

179

MAGNETIC ANOMALIES OVER ICELAND

Fig. 10. Magnetic polarity maps of the eastern and western flanks of the active zone in northern Iceland.

Sou thwestem

Iceland

The structure of this area has been outlined by Saemundsson-(1967); it consists of lava piles of Late Tertiary and Pleistocene age which dip towards the western neovolcanic zone on both its east and west sides. The normally magnetised volcanics erupted during the Bruhnes magnetic epoch can be conveniently

subdivided into lavas erupted in Postglacial

times (younger than about 10,000 years old), lavas erupted within the glaciers that have existed at intervals during this epoch (intra-glacial been glaciated (inter-glacial

volcanics), and sub-aerial lavas which have

volcanics). On either side of a belt 40-60

these volcanics (Fig. 12) older inter-glacial

km wide covered by

and intra-glacial volcanics belonging to the Ma-

tuyama magnetic epoch are exposed. These volcanics have shallow inclinations

towards the

neovolcanic

shows

zone and have been uplifted relative to it. The paleomagnetic

the outcropping

map

polarity groups closely parallel the trend of the present neovolcanic

and the Reykjanes

ridge (Fig. 2); a provisional

correlation

of this stratigraphy

that

zone

with the geo-

magnetic time scale has been given by Piper (1971). Magnetic models have been compiled from the magnetic stratigraphy along the aeromagnetic profile lines CB and DE of Fig. 2. In the first model the normal and reversed lava groups mapped on the surface have been projected down to the seismic discontinuity which lies at about 2.5 km below sea level in the area. A second model has been derived in which a block model based on the geomagnetic time scale for a spreading rate of 1.15 cm/year has been projected upwards to intercept the downward projection of the lava units. This would have some geological reality if the downdip extension of the lavas was remagnetised by subsequent dike injection. The computed

J.D.A. PIPER

1

I? Kay

f

i

16O

1

15*

.

Gilbert

km 0

do

Fig. 11. Correlation of the magnetic stratigraphy in northern Iceland with the geomagnetic time with the aeromagnetic lines of Serscm et al. (2 968) superimposed.

scale

profiles over these models are compared with the observed profiles in Fig. 13 and 14. The bodies are assumed to strike infinitely in a direction 40’ east of north and to have a remanertt magnetic intensity of 0.065 C and a Q, value of 4. The anomalies due to the Matuyama, Gauss and Gilbert valcanics match the observed anomalies along the profile CB; the Jaramillo Lavas are we11 represented in the area crossed by this he (Piper, 1971) and an anomaly at-

ii i

IDA.

PIPER

Fig. 14, Cwmputed field OPBX model structures along the aeromagnetic line DE (Fig. 2) in sauthwest Iceland compared with the obstl~ed profile.

t~b~tab~~ lo these favas is kmnd on rhe crlrservedprofile. It cannotbe reproducedby the model involving shallowly dipping lava interfaces only, but is clear on the profIle over the second model which involves vertical interfkces. That this second model is mure! realistic is also suggested by the better agreement in position between the observed and campured ~at~y~~~ and Gauss anomalies. The agreement between commuted and obs&~ed pro&x for the iine DE is less good. The ~a~~~~ anom&es are r~~~~~c~b~~ bit there is a nrinintt over the neovolcanic zone, This line crosses the neovdcanic zone over the silicic centre at

183

MAGNETICANOMALIESOVERICELAND Hengill which is also a high-temperature

thermal area; temperatures

recorded here at depths as shallow as 300 m (Palmason, rise in the Curie-point Considering distinguished

of 200°C have been

I967b) and it seems likely that the

isotherm in this area may explain the mirlimum.

anomalies over this area as a whole it is clear that negative anomalies can be on either side of the axial anomalies which correlate with the Matuyama

crops or its lateral continuation

on both sides of the neovolcanic

out-

zone, but it is not possible

to isolate anomalies over the older rocks. Both the anomaly system and the neovolcanic zone are displaced en echelon to the west across the Reykjanes Peninsula to meet the contin uation of the submarine

Reykjanes ridge. The spreading rate of 1.15 cm/year from each side

of the axis of the neovolcanic

zone which was employed in the second model for lines AB

and CD gives a plausible geological model which matches the observed profiles well. It is close to the rate of 1 cm/year deduced by Vine (1966) for the adjacent Reykjanes ridge by matching a block model derived from the polarity time scale with the observed anomalies. Extending the base of the lava wedges in Fig. 9, 13 and 14 down-dip has the effect of shifting the maxima and minima in this direction, but the effect becomes less marked as the extension continues because of increasing distance from the point of observation. Unfortunately this effect cannot be used to decide the depth to the base of the lava pile because of remagnetisation and metamorphic effects which have been discussed in the section “Anomaly controls”. The better agreement of model B in Fig. 13 suggests some control of the anomalies by steep-sided magnetic bodies at depth, and since it is possible to correlate anomalies with lava polarity groups of the right relative thickness to account for their magnitude, remagnetisation by dikes must be approximately spatially coincident with the lava outcrop. A correlation has thus been found between some high-level magnetic anomalies and the outcropping ground magnetic stratigraphy in eastern and southwest Iceland. This correlation is not universally found however and we must conclude in these cases that the anomalies are controlled by effects deeper than can be inferred from the surface outcrop. Kristjansson (1970) has given evidence to suggest that high-level gabbro intrusions may explan steep local anomalies which bear no relationship

to the magnetic stratigraphy

of the

lavas. DISCUSSION Unlike the case of the Reykjanes

ridge, the magnetic anomalies over Iceland cannot be

used to reconstmct the history of crustal growth here since they cannot generally be correlated unambiguously with magnetic intervals. However, they do permit a definition of the zone of Bruhnes epoch volcanism and in some cases define the areas of Matuyama crust as well. Combined with other observations these data permit a summary of knowledge concerning the development of the crust in Iceland. Models for the growth of the lava pile in Iceland (Bodvarsson and Walker, 1964; Gibson and Piper, 1972) account for a general dip in towards the neovolcanic zone of the lava pile as it migrates outwards. This inward dip is observed along all the flanks of the present neo-

184

J.D.A.

volcanic zones, and in eastern, southwestern, sistently towards the nearest neovolcanic

and northwestern

growth of the pile here was controlled

Iceland the dips are con-

zone, In north-central

however, the lavas dip away from the present neovolcanic

and part of western Iceland

zones, and they imply that the

by an ancient active zone on the downdip side which

has since become inactive. These ancient zones occur in the Snaefellsnes peninsula area between Langjokull

PIPER

and the

and the Skagi area at the north coast. Mapping at the north end

of the Reykjanes-Langjokull

neovolcanic

zone (Einarsson,

1962; Piper,

1973) has

shown that this zone dies out along its length and is in continuity with a synclinal axis continuing to the north. The observed aeromagnetic anomalies confirm this wedging-out of the neovolcanic line in western Iceland, and large positive anomalies are observed as far north as the Langjokull area but are not found north of this. Fig. 15 shows the distribution of Postglacial and late intra-glacial eruptions as it is known at the present time. The areas of young volcanism correlate well with the observed’high-amplitude anomalies (Fig. 2). They demonstrate

and short-wavelength

that there has been a greater amount of recent volca-

nism along the two neovolcanic zones in southern Iceland than along the single neovolcanic zone in northern Iceland, and the amount of crustal growth and probably spreading has apparently

been greater here.

r’

Fig. 15. Distribution of anticlines and synclines jokull syncline (B), the cline (E). The icesheets

SRUNHLS/YAlUVAhlA

Postglacial and young intra-glacial central and fissure eruptions. The arrows show within the lava pile. They are the Eyafjordur anticline (A), the Hunafloi-LangHreppar anticline (C), the Borgarfjordur anticline (D) and the Snaefellsnes synare: Langiokull (L), HofsjokulI (H), Myrdalsjokull (M) and Vatnajokull (v).

MAGNETIC

ANOMALIES

Sykes (1965) has defined an east-west which has been interpreted with the Iceland-Jan

185

OVER ICELAND

as a transform

earthquake

epicentral

zone at latitude 66.4”N

fault linking the active zone in northern

Mayen ridge. Disturbance

in the magnetic lineations

Iceland

characteristic

old fracture zones have been found by Avery et al. (1968) on the continuation

of

of this line

for over 400 km to the east. In southern Iceland a similar zone of earthquake activity is not reflected in the observed geology and the magnetic anomalies (Ward et al., 1968). The Snaefellsnes

area of western Iceland is a zone of marked central silicic and basaltic volcanism

superimposed on older plateau basal& and is apparently aseismic. These features together with the differential spreading across Iceland cannot be explained in terms of a simple ridge system offset by transform

faults, and the crust grows in Iceland in a more complex fashion

from overlapping volcanic lines with deformation

of the plate margins (Piper, 1972).

ACKNOWLEDGEMENTS

I would like to thank Professor R.G. Mason and Dr. A. Richardson Natural Environmental

for their help and the

Research Council for a Research Studentship.

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