Horizontal and vertical crustal movements in Iceland

Tectonophysics, 29 (1975) 223-231 0 Elsevier Scientific Publishing Company,

HORIZONTAL

KARLHEINZ Geologisches

(Accepted

AND VERTICAL

223 Amsterdam

CRUSTAL

- Printed

in The Netherlands

MOVEMENTS

IN ICELAND

SCHAFER Institut

der Universitiit,

for publication

Karlsruhe

(Germany)

June 18, 1975)

ABSTRACT Schafer, K., 1975. Horizontal and vertical crustal movements in Iceland. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 223-231. Based mainly on geodetic and geological investigations, an attempt is made to demonstrate the kinematic bearing of the Icelandic crust while drifting apart from its birthplace in the axial rift zone to the 12 m.y. old east coast of the island. The 40 km wide rift ‘valley’ floor is subjected to horizontal and vertical crustal movements that are in a ratio of 1. The ground deformation within the rift zone is predominantly confined to scattered narrow grabens whose width does not exceed 5 km. It is suggested that the layer 2/3-boundary plays a similar role for oceanic rifting as the Moho does for continental grabens. A maximum subsidence of superficially deposited flood basalt to a depth of more than 10 km within the limits of the axial rift zone can be expected. Outside the axial rift zone , a continuous uplift of the crust attains 1300 m along a 100 km distant culmination axis, as shown by the distribution of zeolite zones which have been secondarily formed within deeply buried, reheated flood basalts within the axial rift zone. The state of crustal stress changes from mainly tensional within the rift ‘valley’ to predominantly compressive along the rift flanks. A change from normal to reverse faulting when going across the margins of the axial rift zone can be expected. The discrepancy of the topography of Iceland with a submarine part of the Mid-Atlantic Ridge is probably due to more lava production within its ‘median valley’ and also due to an erosional degradation of its ‘rift mountains’.

INTRODUCTION

Detailed studies of the central Mid-Atlantic Ridge( Van Andel and Bowin, 1968; Luyendyk and Macdonald, 1974) and of the Gorda rise (Atwater and Mudie, 1968, 1973) reveal a median valley bordered by rift mountains as a steady-state result of slow-spreading ridges. The rift-valley floor is suggested to be normal faulted while the valley walls are subjected to reverse faulting. Similarities with continental grabens and bordering uplifted flanks appear to be obvious, since an antithetic fault pattern and a low-angle outward-directed tilt of the graben shoulders are compatible for both systems. Based mainly on data of deep-ocean investigations, a number of kinematic models of oceanic rifts both of slow (with a central graben) and fast (with a central horst) spreading velocities have been presented. (Cann, 1968; Deffeyes, 1970; Osmaston, 1971; Piper and Gibson, 1972; Anderson and Noltimier,

224

1973; Harrison, 1974). In accordance with the major conclusions of those models, one should expect that relatively slow spreading in Iceland would create a rift valley and rift mountains rather than to provide Iceland with a topography that is similar to an overturned bowl. Although the particular orographic situation of Iceland, which is an oceanic spreading centre above sea level, could account for that. There is, however, no reasonable explanation for a lack of a rift valley along the submarine Reykjanes ridge. Another great difference between a slow-spreading ridge structure and the tectonics of Iceland is based on synthetic faulting within the Icelandic axial rift and a general dip of basalt layers towards the spreading centre. PAlmason (1973) tested his kinematic model of t,he Icelandic rift zone by a great number of data, predominantly by such as the dip of basaltic layers, the increase of dykes with depth and by heat-flow discharge. The present author agrees with Pilmason’s (1973) kinematic model applied to the axial rift zone, but intends to contribute some other findings that may be of importance for the knowledge of the tectonic behaviour of the Icelandic crust outside the axial rift zone. The purpose of this paper is also to review briefly geodetic investigations that have been carried out during the last decade, and to apply those data of horizontal and vertical movements of the Icelandic crust for a further test of the model. GEODETIC

Horizontal

MEASUREMENTS

movements

Decker et al. (1971) reported a widening of the eastern axial rift zone, 25 km north of Hekla volcano by 6-7 cm. The dilatation occurred between 1967 and 1970 along a 25 km long survey line which was normal to the rift axis. Another survey that they carried out during the same time interval in the Thingvellir area (Fig. 1) did not show any horizontal length change. Repeated measurements of horizontal movements in 1973 (Decker et al., 1974) showed that a contraction occurred along the profile line of the eastern axial rift zone, so that no net horizontal displacement for the entire period of 1967--1973 could be recorded. The western branch of the axial rift zone, however, revealed a widening of 2 cm along the survey line where no horizontal movements have been recorded from 1967 to 1970. They measured an extension of 4 cm across the Thingvellir graben (Fig. 1) for the entire period of 19671973. Gerke (1974) measured an extension of 5.7 cm along a 3.2 km long survey line north of Lake Thingvellir during the period 1967-1971. Vertical movements The 5 km wide Thingvellir graben area was also investigated recently for determinations of vertical crustal movements within the western axial rift

225

Fig. 1. Iceland is traversed from south to north by the axial rift zone of the Mid-Atlantic Ridge which is bipartite in the southern part of the island. Active horizontal and vertical crustal movements are predominantly confined to fissure systems and narrow grabens that develop at various locations within the axial rift zone (e.g. Th.(ingvellir)-Graben and H.(rafnagjl)-Graben). Despite the relatively slow spreading rates (1 cm/yr), no morphological rift valley and no rift mountains have developed. The E-W section across the axial rift zone and the eastern flank area is shown in Fig. 3.

zone. Tryggvason (1968) calculated that from 1966 to 1967 the central and the extreme eastern part of the graben floor subsided between 0.5 and 1.0 mm. He also measured subsidence during the same period of time in an area south of Reykjavik where the central part of a 2 km wide graben in the Burfellshraun area has been downthrown 0.6 mm. A few kilometers further southwestward towards Cape Reykjanes, within the western axial rift zone, another 5 km wide graben was subjected to subsidence, particularly after an earthquake swarm hit that area in 1967. The graben floor near the northwestern marginal graben fault (Hrafnagji, Fig. 1) subsided 6 mm from 1966 to 1969 and 8 mm about 180 m southeast from there in the direction of the centre of the graben (Tryggvason, 1970). Spickernagel(l968) reported annual subsidence rates of 4-7 mm of the GjMikki area in northern Iceland, as result of an investigation that he conducted in 1965 and 1967. Tryggvason (1974) finally estimated that the axial rift zones of Iceland

226

are subsiding 0.5-1.0 cm/yr relative to the flanks of the rifts which he determined as tilting with a rate of 0.2-0.7 microradians annually. Thus, it is concluded that Recent horizontal and vertical crustal movements are predominantly confined to fissure swarms and narrow grabens within the axial rift zones. They seem to accumulate for an indeterminable period of time most of the crustal deformation that has been estimated to be operative across the entire rift zone. GEOLOGICAL

OBSERVATIONS

The Thingvellir graben attains a width of 5 km between its marginal main faults (Almannagja and Hrafnagji) and it is traversed by numerous open fissures. Bernauer (1943) measured the gaping space of all those fissures which resulted in 75 m of the total extension. Since absolute age determinations of the Postglacial lava that has been horizontally stretched by 75 m during the formation of the Thingvellir graben, reveal an age of 9130 f 260 years, the annual horizontal dilatation rate amounts to about 8 mm/yr. The same value can be estimated for the annual subsidence rate of the floor of the Thingvellir graben, since the deepest part of the graben floor is about 70 m below the lava surface beyond the bordering main faults of the graben. The age of Burfellshraun lava-flow south of Reykjavik is estimated to be 2000 years (Tryggvason, 1968). Referring to the earlier-mentioned graben formation in that area, which has downthrown the lava surface to about 15 m, an annual subsidence rate of 7.5 mm for the last 2000 years can be averaged. Older Postglacial lavas of 10 000 years maximum age surround Burfellshraun and are subjected to vertical displacements of 65 m, hence resulting in a 6.5 mm subsidence rate per year. The width of the HrafnagjCgraben also does not exceed 5 km. This graben is supposed to have commenced with subsidence not earlier than 10 000 years ago. The maximum downthrow of the graben floor attains 70 m which corresponds to an average rate of vertical displacement of 7 mm/yr. All normal faults and fissures that have been observed at the surface of the axial rift zone are perpendicular to the layering of the lava flows and this tendency appears to continue to a greater depth. Dykes are suggested as filling all the fissure swarms with increasing depth. Apart from the axial rift zone in eastern and western Iceland, where erosion has removed several hundred meters of the original top of the crust, many dyke swarms can be observed cutting perpendicularly across the flood-basalt layers. Only the deepest stockwerk of the Icelandic crust which is exposed at the southeast coast of the island, reveals numerous dykes, or rather sheets, that traverse obliquely the flood-basalt pile. Additional evidence that the narrow grabens of the axial rift zones are enclosed by marginal dip-slip faults which bend with increasing depth towards the graben centre, is also given by Tryggvason’s (1970) observations. According to the amounts of subsidence that he measured along the Hrafnagji fault and 180 m southeastward of it in the direction of the graben

227

centre, he calculated the Hrafnagja fault to bend with increasing depth from a vertical fault or fissure to a 65”-southeast dipping normal fault. In summarizing these observations one may conclude that: (1) Recent horizontal and vertical crustal movements within the axial rift zone are predominantly confined to fissure swarms, normal faults and narrow grabens. (2) The Recent distribution of fissures, major normal faults and grabens within the axial rift zones of Iceland reveals that the area of visible tensional tectonics attains a maximum width of about 40 km. (3) The grabens’ width does not exceed 5 km, but their length may attain more than 50 km. The marginal graben faults change their dip with increasing depth from vertical to about 65” converging towards the graben centre. The faults, thus, intersect theoretically at the layer 2/3-boundary. (4) The rates of horizontal and vertical crustal movements measured across active miniature grabens are equivalent, their ratio is 1. This is also in agreement with continental grabens, where subsidence and dilatation rates also depict a ratio of 1, and where marginal faults are suggested as intersecting at the Moho. (5) Surficial rock masses close to the centre of the axial zone may subside more than 10 km, before they are able to leave the rift zone by simultaneous lateral drift. PaImason (1973) has calculated the trajectories of surface masses that were at different distances from the rift centre. PETROLOGICAL

EVIDENCE

No geodetic investigations have yet been conducted outside the axial rift zone to obtain evidence as to whether there are specific crustal movements or not. Also, it is difficult by geological means, that cannot be discussed here but will be more extensively discussed elsewhere, to observe differential vertical crustal movements outside the rift zone. We know about horizontal crustal movements, of course, by evaluating absolute-age measurements and magnetic mapping. These results agree with the drift velocities of the Reykjanes ridge and the Kolbeinsey ridge that continue south and north of Iceland. Walker (1960) described the occurrences of amygdale minerals within the Tertiary basalts of eastern Iceland. Of particular interest is a secondary mineralization of zeolites, which, according to Walker could be mapped as westdipping zones characterized by a distinct assemblage of zeolite species. This zonation can be followed from the Icelandic east coast to the west until Quaternary hyaloclastites and tillites cover the zeolite-bearing Upper Tertiary basalts. The zeolites were formed after the cooling down of the lavas subsequent to eruption. The deepest zone contains zeolites that require a higher temperature to be formed than those which follow in the next zone above. The uppermost zone encloses zeolites that could cristallize under the lowest temperature conditions that were sufficient for zeolite formation. Thus, each

228

Fig. 2. Two sections across the axial rift zone of Iceland and its eastern flank (for location of sections refer to Fig. 1). The upper section shows the trajectories of two surface elements originating at different distances from the centre of the spreading zone. Based on observed Postglacial subsidence and dilatation rates within the axial rift zone and on the distribution of zeolite zones outside the axial rift zone, the crustal kinematics can be demonstrated by the two trajectories (heavy lines with arrows) from the central spreading zone to the east coast. A culmination of crustal uplift is attained about 120 km off the spreading centre. The lower section reveals schematically the composition of layers 1 and 2 with lava flows and dykes that increase with depth. Lava flows are only extruded at the surface of the axial rift zone. They are either split by the rift process and accreted to both the eastern and the western plate (stippled signature) or they join one of the diverging plates. Thus, one can observe up-dip and down-dip thinning of lava flows along the eroded flank.

of the zeolite zones can be attributed to a distinct geoisotherm. Another interesting observation is that tilting of the basalt layers must have occurred earlier than the zeolitization, since the zoning crosses the flood basalt stratification obliquely (Fig. 3). There is only one location, namely the axial rift zone, where a secondary zeolitization of the flood basalts could have occurred. Only there do conditions exist that allow cold surface rocks to subside while being covered with new lava flows and to dive to depths where temperature and water were sufficient to fill their vesicles and joints with zeolites. A general tilting of the flood basalts toward the rift axis must have occurred also in the axial rift zone since the zeolite zones cut across the lava stratification, according to the surface parallelism of the geoisotherms (Fig. 3). An uplift of the Icelandic crust outside the axial rift zone is responsible for the fact that the zeolite zones could emerge and slowly cool, conserving their individual mineral assemblages. The crustal uplift outside the axial rift zone is continuous, attaining about 1300 m total throw from the margin of the rift zone to a 100 km distant culmination close to the east coast. From this culmination further to the east, one can observe a slight downwarping of the crust, indicating that no

Fig. 3. Crustal section from central Iceland across the eastern axial rift zone to the east coast. Flood hasalts are only extruded at the surface of the axial rift zone (rift ‘valley’) and are subsequently buried there beneath younger lava flows. Zeolites that have formed within subsided and reheated flood basalts indicate a continuous crustal upwarp outside the axial rift zone attaining 1300 m at a 100 km distant culmination axis. The tectonic bearing of the eastern rift flank is also demonstrated by the original top of the crust and the layer 2/3-boundary, both of which are parallel to the zeolite zonation.

Fig. 4. The upper profile is a detailed record across Escanaba trough and bordering ridges of the Gorda rise obtained by Atwater and Mudie (1968). The lower profile is drawn without exaggeration from the upper one. Topography, low-angle faulting and corresponding sediments on the valley floor and on several ridge blocks, suggest normal faulting within the axial valley and reverse faulting at both sides outside of it (as shown by arrows and by the trajectory of a surface element originating from the central axial valley).

230

isostatic disequilibrium due to an erosional weight reduction was responsible for the uplift (Figs. 2 and 3). Since the Tertiary flood basalts in the culmination area near the east coast are about 10 m.y. old, one can calculate an average crustal uplift of 0.13 mm/yr. That is slightly more than Menard (1969) calculated for annual subsidence rates (10-Z cm/yr---0.25 . 10-Z cm/yr) for seamounts drifting down the outer flanks of mid-oceanic ridges. An attempt to apply the kinematic model of Iceland to submarine rifts appears to be difficult, since no appropriate location is available at the first glance which fulfills the necessary requirements. Slow-spreading rifts, like the Gorda rise, depict simularities on the grounds of their tectonic inventory but not of their topography (Fig. 4). The latter is likely to be explained by more lava production within the Icelandic axial rift zone compared to submarine rift valleys and is also due to an erosional degradation of the Icelandic ‘rift mountains’. The lower profile of Fig. 4 is drawn without exaggeration from the Escanaba trough and the bordering ridges of the Gorda rise. From that, one is attempted to deduce that normal faulting may only occur in the valley, but the low-angle faults at the ridge areas are doubtless the results of reverse faulting - a change of the tectonic regime that also can be expected when going across the margins of the Icelandic axial rift zone.

REFERENCES Anderson, R.N. and Noltimier, H.C., 1973. A model for the horst and graben structure of midocean ridge crests based upon spreading velocity and basalt delivery to the oceanic crust. Geophys. J.R. Astron. Sot., 34: 137-147. Atwater, T.M. and Mudie, J.D., 1968. Block faulting on the Gorda Rise. Science, 159: 729-731. Atwater, T. and Mudie, J.D., 1973. Detailed near-bottom geophysical study of the Gorda Rise. J. Geophys. Res., 78: 8665-8686. Bernauer, F., 1943. Junge Tektonik auf Island und ihre Ursachen. In: 0. Niemczyk (Editor), Spalten auf Island..Wittwer, Stuttgart, pp. 14-64. Cann, J.R., 1968. Geological processes at mid-ocean ridge crests. Geophys. J.R. Astron. sot., 15: 331-341. Decker, R.W., Einarsson, P. and Mohr, P.A., 1971. Rifting in Iceland: New geodetic data. Science, 173: 530-532. Decker, R.W., Einarsson, P. and Plumb, R., 1974. Rifting in Iceland: Measuring horizontal movements. Symp. on the Geodynamics of Iceland and the North Atlantic Area, Reykjavik, Iceland (abstract). Deffeyes, K.S., 1970. The axial valley: A steady-state feature of the terrain. In: H. Johnson and B.L. Smith (Editors), The Megatectonics of Continents and Oceans. Rutgers University, New Brunswick, NJ., pp. 194-222. Gerke, K., 1974. Crustal movements in the Myvatn and in the Thingvallavatn area, both horizontal and vertical. Symp. on the Geodynamics of Iceland and the North Atlantic Area, Reykjavik, Iceland (abstract). Harrison, C.G.A., 1974. Tectonics of mid-ocean ridges. Tectonophysics, 22: 301-310. Luyendyk, B.P. and Macdonald, K-C., 1974. A near-bottom geophysical study of the MidAtlantic Ridge near 37”N (FAMOUS area). Symp. on the Geodynamics of Iceland and the North Atlantic Area, Reykjavik, Iceland (abstract).

231 Menard, H.W., 1969. Growth of drifting volcanoes. J. Geophys. Res., 74: 4827-4837. Osmaston, M.F., 1971. Genesis of ocean ridge median valleys and continental rift valleys. Tectonophysics, 11: 387-405. Palmason, G., 1973. Kinematics and heat flow in a volcanic rift zone, with application to Iceland. Geophys. J.R. Astron. Sot., 33: 451-481. Piper, J.D.A. and Gibson, I.L., 1972. Stress control of processes at extensional plate margins. Nature Phys. Sci., 238: 83-86. Spickernagel, H., 1968. Erdkrustenbewegungen in Island. Umschau, 68: 379. Tryggvason, E., 1968. Measurements of surface deformation in Iceland by precision levelling. J. Geophys. Res., 73: 7039-7050. Tryggvason, E., 1970. Surface deformation and fault displacement associated with an earthquake swarm in Iceland. J. Geophys. Res., 75: 4407-4422. Tryggvason, E., 1974. Vertical crustal movement in Iceland. Symp. on the Geodynamics of Iceland and the North Atlantic Area, Reykjavik, Iceland (abstract). Van Andel, T.H. and Bowin, C.O., 1968. Mid-Atlantic Ridge between 22” and 23” North latitude and the tectonics of mid-ocean rises. J. Geophys. Res., 73: 1279-1298. Walker, G.P.L., 1960. Zeolite zones and dike distribution in relation to the structure of the basalts of eastern Iceland. J. Geol.. 68: 515-528.