Tectonic significance of the South Iceland Seismic Transform Zone

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B8, PAGES 17,96%17,980, AUGUST 10, 1997 Tectonic significance of the South Iceland Seismic Transform Zone Pascal Luxey, Philippe Blondel, and Lindsay M. Parson Southampton Oceanography Centre, Southampton, England Abstract. The subaerial expression of the Mid-Atlantic Ridge on Iceland comprises two overlapping spreading axes, referred to as the West Volcanic Zone (WVZ) and the East Volcanic Zone (EVZ), respectively. The way the spreading rate is distributed on both volcanic zones has an important impact on the stress pattern in the overlap area. Our field data from the area trace the evolution of the stress direction as recorded by slip motion on fault planes. We found four different strike-slip stress phases. An early N-S compression phase (A) preceded a NE-SW compression phase (B). This phase was followed by a SE-NW compression phase (C). However, we cannot date an E-W compression, phase (D), relative to the other phases. Numerical modeling based on the assumption that the WVZ has been permanently active during the last 3 Myr and that the EVZ is propagating southward confirms that stress directions have rotated clockwise by more than 140 ø. These results fit perfectly with our field analysis, and we propose that phase A corresponds to initial EVZ ridge-tip propagation, phase B to emplacement of the EVZ southern tip near Torfaj6kull, and phase C to an extreme southern location near the Surtsey Islands of the EVZ southern tip. Phase D could correspond to an intermediate stage between phases B and C. We suggest that this sequence of tectonics, recorded in a regional overlapping ridge-tip setting, is directly analogous to smaller-scale and more common phenomenat second-order ridge discontinuities throughouthe global mid-ocean ridge system. Introduction The Mid-Atlantic Ridge (MAR) in Iceland follows a complex pattern as its location is influenced by the Icelandic hotspot nearby. It has been shown that the MAR, during the last 6-7 Myr, has been unstable and relocated a number of times [Palmason, 1973, 1980; Helgason, 1984, 1985; Oskarsson et al., 1985; Bott, 1985; Steinthorsson et al., 1985; Ribe et al., 1995]. At 3 Ma, the southern end of the North Volcanic Zone (NVZ) started propagating southwards, leading to the development of the East Volcanic Zone (EVZ) (Figure 1). The corresponding northern tip of the West Volcanic Zone (WVZ) did not retreat southward, and geological data presented by Forslund and Gudmundsson [1991] show that the WVZ remained active during the whole period. Currently, in southern Iceland, the MAR comprises two parallel and overlapping ridge segments: the East and the West Volcanic Zones (Figure 1). Morphologically, the MAR in south Iceland forms an overlapping spreading center (OSC) (Figure 1). We examine the way spreading has been accommodated and distributed on these parallel ridges over the past 3 Myr, and we focus on the evolution of paleostress related to the evolution of the OSC. The plate geometry, with propagating and retreating ridge segments, is familiar to researchers. Although at a different scale, it is very reminiscent of overlapping spreading centers (OSCs) on faster ridges such as the East Pacific Rise [e.g., Searle and Hey, 1983]. Previous workers have established much of the current geological setting of Iceland. The principal characteristics of the ridge can be summarized as follows: 1. In the Iceland region, the plate boundary between North America and Eurasia can be defined by linear array of Copyright 1997 by the American Geophysical Union. Paper number 97JB /97/97JB earthquake hypocenters corresponding to ridge spreading [e.g., Tryggvason, 1973; Einarsson, 1979, 1989, 1991; Foulger, 1988a, 1988b] and by the location of postglacial volcanism [e.g., Kristjansson, 1967; Saemundsson, 1974, 1978a; Helgason, 1984, 1985]. This boundary corresponds to the Reykjanes Peninsula, the South Iceland Seismic Zone, both the West and East Volcanic Zones, the Mid-Iceland Volcanic Zone, and the North Volcanic Zone (Figure 1). 2. Most major recent earthquakes in south Iceland have occurred in the South Iceland Seismic Zone (SISZ). As the focal mechanisms at these events exhibit a significant strike-slip component, the SISZ is considered to be the locus of transform deformation between the northern tip of the Reykjanes Ridge and the southern tip of the EVZ. This geometry suggests that the spreading along the northern WVZ is decaying or has already stopped, although locally some seismic activity is still recorded along with some postglacial to historic (10,000 to 2000 years) volcanic activity [e.g., Bjarnason and Einarsson, 1991; Bjarnason et al., 1993; Einarsson, 1991; Gudmundsson, 1995; Slunga et al., 1995] (Figure 1). 3. Geodetic measurements, as well as fault and fissure measurements, carried out across the East and West Volcanic Zones show that the average spreading during the last 10,000 years is about 1 cm/yr across both ridges [Gudmundsson, 1987a; Johannesson et al., 1990] and that most of the spreading occurred in the EVZ during the last 1000 years [Sigmundsson et al., 1995]. However, other geodetic results indicate that the total spreading during the past 30 years on the ridge north of the Hekla-Katla volcanic system in the EVZ is not significant [Jonsson et al., 1995], implying that spreading across both ridges is not a uniform phenomenon. 4. According to finite element models the three-dimensional volume represented by the SISZ, as it is defined by its seismicity, cannot account for all the transform deformation released by the earthquakes occurring in the area [Stefansson 17,967

2 17,968 LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE Figure 1. Present-day geological setting of the MAR along the Iceland region. B, Bardarbunga; EVZ, East Volcanic Zone; G, GrimsvOtn; H, Hekla; He, Hengill; Ho, HofsjOkull; KR, Kolbeinsey Ridge; L, LangjOkull; MIL, Mid-Iceland volcanic zone; NVZ, North Volcanic Zone; OV, Overlap zone; RR, Reykjanes Ridge; S, Surtsey Islands; SISZ, South Iceland Seismic Zone; T, TorfajOkull; TFZ, Tjornes Fracture Zone; WVZ, West Volcanic Zone; White arrows show relative North America and Eurasia plate motion. Solid dots are locations of hypocev_tral depths of earthquakes occurring in Iceland since 1940 (magnitude above 4; source is International Seismological Centre (ISC) and courtesy of British Geological Survey 0BGS). Focal mechanism of earthquakes are from the centroid moment tensor database; lower hemisphere, compressional quadrants are shaded. and Halldorsson, 1988; Stefansson et al., 1993]. To take into account all the energy released, either (1) the seismogenic part of the crust would have to be thicker, which is not the case as demonstrated by Stefansson et al. [1993], or (2) the number and / or length of seismic surface ruptures in the SISZ must be larger than observed [Hackman et al., 1990], which is plausible but could also mean that a part of the transform deformation might be released aseismically along pre-existing or neo-formed faults elsewhere north of the SISZ. We used remotely sensed data, SPOT, Seasat and Landsat Thematic Mapper (TM) satellite images to study the overlap area located between the West and the East Volcanic Zones. We combined these data into tectonic maps, incorporating new data collected in the field as observations on principal faults. These data allow the quantification of different stress phases, an estimation of their relative chronology, and the development of a new model for the magmatic/tectonic evolution of the region. From our observations we suggest that the West Volcanic Zone plays an important role in the location of deformation in response to North America-Eurasia spreading and that it must be taken into account to explain the tectonic evolution of the southern Iceland region [Gudmundsson, 1994, 1995; Sigmundsson et al., 1995]. Present-Day Ridge Organization in Southern Iceland West Volcanic Zone The West Volcanic Zone is a N25 ø E striking lineament located between the Langj6kull glacier in the north and the Hengill fissure swarm in the south. The latter is popularly considered as the northeasternmost fissure swarm of the Reykjanes Peninsula [Saemundsson, 1978a; Tryggvason, 1973; Gudmundsson, 1987b] (Figure 2a). Its gross form is a

3 : LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE 17,969 22øW 22 o30 ' W 21 øw 20ø30 ' W I i I I...: ::...:: :.... :: :. : :... ' '" ' ' '(' :' ' ' :?': "'"':"' ':..'-'G ::..-.:. : :...: j ' i... '... '... ::.:':. '-..L' ::: T' ;.. - " ' ",-... ':..::-" :-...,: : : ; :: : :: : ::.: ::: : :: ::. :. : :: :::... :: :.. :.::: :: / o....:... :....,,... :f-.% : -- 22øW 21ø30'W 2iøW 'W... Kilometers 20 0 Figure 2 a. Satellite image (Landsat TM 5) of the Reykjanes Peninsula (bottom left) and the West Volcanic Zone (middle right). White is ice and clouds; Dark is sea and lakes. half-graben with a right-stepping wall in the west. It is characterized by numerous volcanoes and eruption sites and is cut by many sub-parallel fissure and fracture systems. Most of the rocks presently outcropping in the West Volcanic Zone are 10,000-9,000 years old, and only a few presently active volcanoes are located on the eastern part of the depression [Gudmundsson, 1987a; Johannesson et al., 1990]. Most of the fractures of this area consist of normal faults and eruptive fissures and strike about N25øE (Figure 2b) [Gudmundsson, 1987a, b; Johannesson et al., 1990]. Over the past 1000 years, no major eruption has occurred in the WVZ except the 1789 Hengill eruption [Saemundsson, 1978b], and no significant rifting episode is known here other than the 1789 Thingvellir rifting episode, north of Thingvallavatn. This event led to crack and fissure formation and the initiation of hot springs. Subsidence of Thingvallavatn lake reached about 2 m [Saemundsson, 1978b; 1992] (Figure 2b). On a larger timescale, however, the WVZ spread about 100 m, based on summing the widths of faults, fissures, and cracks mainly from the Thingvellir fissure swarm [Gudmundsson, 1987a]. These faults cut 9000-year-old lava, and thus the spreading rate, when averaged over this period, is 1 cm/yr, almost half the spreading rate between the North American and Eurasian plates.

4 17,970 LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE 64 0 N 64 N Figure 2 b. Geological sketch map of the West Volcanic Zone. East Volcanic Zone During the last 3 Myr, the East Volcanic Ridge propagated southward at a rate of 5 to 10 cm/yr from the presumed location of the hotspot underneath the Vatnaj6kull glacier (Figure 3a) [Einarsson and Eiriksson, 1982; Foulger, 1988a], and we suggest that the recent activity in the vicinity of Surtsey Island could mark the southernmost limit of associated volcanism. The present activity along this volcanic zone is mainly characterized by earthquake swarms often related to eruptions [Tryggvason, 1973; Einarsson, 1991]. The main sites of activity are located in the Vatnaj6kull area where the earthquake occurrences have been related to inflation and deflation of the magma chamber [Einarsson, 1991] and where focal mechanisms show reverse faulting, unusual in an extensional context (Figure 1). Einarsson [1991] explained the compressional mechanisms as resulting from a decrease of pressure in the magma chamber 10 km below the brittle crust, inducing a relative increase of horizontal stress in the chamber roof during deflation. Another important site of activity is the Torfaj6kull-Hekla area (Figures 3a and 3b). Its surface expression is a major fissure swarm on which volcanic constructions are developing. Further field investigations are. necessary to describe more precisely the relationship between faults or fissures and volcanic constructions. Nevertheless, seismicity here is related to, or influenced by, the locus of the transform area further to the west. Magnitude of events reaches Ms 5 to 5.5, and Bjarnason and Einarsson [ 1991] showed that the 1987 Vatnafj611 earthquake (located a few kilometers south of Hekla) had a strike-slip mechanism. Similarly, Hekla's 1991 eruption was followed by intense seismic activity, with mostly associated strike-slip focal mechanisms [Stefansson et al., 1993]. The Torfaj6kull fissure swarm could easily representhe present junction between the SISZ and the EVZ. When averaging the total amount of spreading related to the main volcanic and rifting episodes in the East Volcanic Zone, 20 m of extension can be estimated over the last 1000 years [Sigmundsson et al., 1995]. This corresponds to a 2 cm/yr spreading rate during the last 1000 years, representing the total spreading component between the American and Eurasian plates. However, over a shorter timescale it seems that the behavior is less uniform, and small perturbations hidden by averaging are common. Recent Global Positioning Syste m

5 LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE 17,971 19øW -, i::. 5:,: :-:..,.:.; ;.;':.:';.' :<""' "'*"...' '; ::i'.:/ %,{.,'..o.;'.' -;-...,..:.. -.!' , 18øW 17øW!... :! -':... :.... ;;;,'.,:, :-'--':'-< '7.:" '}$... :.'.;'.:... '. --%; ;. -.-:,; :',.:.....,?f': ::':..' /-':"..:.:..... '*' ::.;.-.:.. :..:. }'...-&.. *."*' :... ::.-:-?,... ;;.? :*." :..-,. --;..::...;,.'.-* '*;;%" "/"-?;*':""q" " *:' ' :e,c. ;::"X, -' ½... ':. t.: ;: a'" *?-.:.-' : --.:',.;'; ; 20øW 17ow Figure 3 a. Satellite image (Landsat TM 5) of the East Volcanic Zone. White is ice and clouds; Dark is sea and lakes. (GPS) surveys across the EVZ show that no regular spreading occurred in the vicinity of Hekla and that it is characterized by periods of dilation followed by periods of contraction, resulting in insignificant spreading [Jonsson et al., 1995]. South Iceland Seismic Zone This seismically active zone, 70 km long and 20 km wide, lying between the East Volcanic Zone and the Reykjanes Ridge is centered on latitude 64øN (Figure 1). It was proposed to be a transform fault zone by Ward [1971] on the basis of its geometric relationship to the Reykjanes Ridge and the East Volcanic Zone. This zone is the locus of the most significant earthquakes in Iceland during historic time [Einarsson and Eiriksson, 1982; Einarsson, 1989]. Seismicity since 1988 includes more than 7000 earthquakes with magnitudes between 0 and 4 [Stefansson et al., 1993]. Most of the events areassociated with right-lateral faulting on N-S striking faults arranged en echelon within the E-W trending zone. This geometry leads to the interpretation of the South Iceland Seismic Zone tectonics as dominated by "book shelf" tectonics with small blocks rotating in a counter clockwise sense to accommodate the main left-lateral slip motion, despite a lack of field evidence indicating that previous faults

6 ... 17,972 LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE 19 W 18 W 17 W,. ** "?;... z ' :: :..:...::-::...:......,.: '. -==========================....'-":.2 :. z 20 W..-','.,,a.. "... ::.,... }...??:;... 7o < ee.,.. I":":'::':':'":'::" ".. MYRDAL KULL ' :..,...,.:: i0 < < 70 <3,;, ß...? -, :....'- :..."- :::::;.::: : - ::. o < < o hi, :--::-.**:%... : ::.'%. ;? ,, ;:.. :..:...,. : :;::... ::::::::::::::::::::::::::::::.:::::-:: "! :::*:,,::,::::::::::: "... z 19" W 18" W 17 W Figure 3 b. Geological sketch map of the East Volcanic Zone. have rotated [Hackman et al., 1990; $tefansson et al., 1993; Sigmundsson et al., 1995]. Stefansson and Haldorsson [1988] proposed that the left-lateral shear stresses responsible for the motion along the SISZ are a consequence of the upwelling of mantle material linked to the presence of the hotspot farther northeast. They suggest that the stresses are caused by the friction of horizontal sub-crustal radial flow away from the hot spot conduit at a depth about 100 km. This could explain the build up of horizontal compression leading to strike-slip faulting in the area of the South Iceland Seismic Zone. We propose that during the past 10,000 years, % of the spreading occurred along the WVZ and % along the EVZ [$igmundsson et al., 1995], and that the excess shear deformation is released aseismically, or by creep, along preexisting faults in the area north of the SISZ between the EVZ and the WVZ. We constrain the suggestions with the following observations: (1) the South Iceland Seismic Zone as described above cannot be older than a few thousand years, as the cumulated slip along faults does not exceed 15 m [Gudmundsson, 1987a; Sigmundsson et al., 1995]; (2)the seismic moment released in the SISZ corresponds to 80%, at maximum, of the moment release calculated from estimated magnitudes of historic earthquakes [$tefansson and Halldorsson, 1988; Hackman et al., 1990], and that the seismic moment release cannot be higher than 80% as the depth of the planes of seismic rupture are not deeper than 5-6 km in the west and 14 km in the east near Torfaj6kull [Bjarnason and Einarsson, 1991; Flovenz and Gunnarsson, 1991]; and (3) the measured displacements across the South Iceland Seismic Zone accommodate % of the plate motion [Sigmundsson et al., 1995]. The dimensions of the transform area and the direction of the shear stresses might have rotated as the EVZ was propagating southward (3 Myr) and as the spreading rate in the WVZ was decreasing (10,000 years). Thus our measurements of fault motions in the overlap area (Figure 4a) were planned to determine the deformation history, both for the initiation of the southward propagation of the EVZ and for the more recent evolution of the present ridge geometry. Field Data and Stress Analysis The field area, located between both the EVZ and the WVZ, is dominated by Pliocene to Pleistocene basalts (younger than

7 .. LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE 17,973 20ø30 V ß. ß t _-. --.l :. ' : ::':i. i ,.,. ;? *'" " 20øW I..: :..?... : -.. a ::... '. ;..... ':.:. :: :'" '. ½?..;;½.. -?: , 4 ;:':.i'.. :: ' ':':":'"Tg::':'? ; ½; :;.. ;;;.:.,: ' :::;'.':' a:.,:::. %/'..,.-' ':'"'"'.::;;5 -:.,:,t :,4:;......'....:,,:::..'..,:,?:.; e::,-... ½.: : :...:-:... -, : :::.. :.. i ;. :;. '"..:....:.: :. 4.. lo o lo Figure 4 a. Satellite image (Landsat TM 5) of the South Iceland Seismic Zone. White is ice, muddy water, and clouds. 3 Ma but older than 10,000 years) (Figure 4b). Commonly lengths ranging between 100 m and 4 km, but the population within this area, faults do not strike parallel or normal to the with the most continuous faults strikes parallel to the West and main spreading centers. These faults are numerous and mostly East Volcanic zones. strike-slip. They can be organized in different populations Many of the faults are weathered and slickensides are rarely according to their orientation (Figures 5a and 5b) [Blondel and observed. More than 300 fault planes, however, were Luxey, 1995]. Each population corresponds to faults with measured. Forty percent are left-lateral strike-slip (pitch

8 . 17,974 LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE Ka < Age < 3 Ma Fau i: LANGJ6KULL...,.:{... :}i :' "':: :':'::':"'""i':.: :. ; ::'" ß :".:'. :::::' i! ' $0..:.:.:: - :::.: Ii:':' 5';' ' ::i: : ':}..:..? :... ' j....'.*' :.<:.. :. :::: N,'... :... : '. ::.:.-?... :.. ':'. :..:::: ½ ' 11 '1 :.:- 1 ' '18 :-?' 31,2 ß 0... '>' 15.[ 6, ; % ::-..::"-:..-.. :..:,: :. : : :":f '7 ::.... ::...:., :{.:.. :. :../: :-::...:: ß. :.:.../ :.:-.-. '::- ;"'... -:... ':.-:...,...- :-... :..:.... :. " : N 20 N,..:.....:..,:...:... 19a3o N.i 64 N Figure 4 b. Geological sketch map of the South Iceland Seismic Zone showing the location of the sites where field data have been collected. between-44 ø and +44ø), 30% are right-lateral (pitch between -136 ø and +136 ø) and 30% are normal (pitch between 45 ø and 135 ø) (Figure 5a). Most faults trend between N40øE and N60øE or between N225øE and N240øE (Figure 5b). Both trends represent the same orientation, and following our convention (Figure 5b), faults trending between N40øE and N60øE are SE dipping and the others. are NW dipping (Figure 5b). This population of faults includes 50% left-lateral strike-slip faults, 30% right-lateral strike-slip faults, and the rest are normal faults (Figure 5c). The right-lateral strike-slip faults have E-W trending conjugates as illustrated at sites 4 and 14 (Figure 6b) or trending N60øE to N75øE as in sites 1, 2, and 3 (Figure 6b). In site 5 (Figure 6c) the right-lateral strike-slip faults have their conjugates trending N170øE, but this site shows a very broad distribution of the right-lateral faults. They are distributed between N45øE to N100øE. The left-lateral strike- slip faults have their conjugates trending N150øE to N200øE, as illustrated at sites corresponding to phase A (Figure 6a). No pure reverse faults have been measured, but a few right-lateral or left-lateral strike-slip faults exhibit a reverse component (Figure 5c). These reverse strike-slip faults are less numerous than the strike-slip faults carrying a normal component (Figure 5c). The frequency of left-lateral strike-slip faults is greater than right-lateral strike-slip ones and this may be due to the fact that the N40øE-N60øE trend, along which we found most of them, is more prominent than the others directions (Figure 5c). The method that we used to determine the paleostress regime history consists of finding the best fit between observed direction and sense of slip on faults and the theoretical shear stress induced on these planes by a common stress tensor. We computed a stress tensor from a homogeneou set of fault slip data using the direct inversion method developed by Angelier [1990]. Most sites are characterized by polyphase deformation allowing the calculation of two or more paleostress tensors which can be distinguished by preliminary selection of faults

9 ... o 345 ;:...-: :..:.:.,.. '...:.?:..: : :..:: ::,½ ':,.- ;:!,-.:. ;?½.' '.; ½ :-:-::.-,:,,,.,...;..:. :.:-,.: 3O 45 :;-5517g:, -,..,:4: - 75 :;?,' ".-':: :....!;..'-.½-.' i.. ;"½"135 Nb of faults (in %) 180 ø B)... -s-::: :...o: ::½??:.?? :5:??;,: :-::;-'?:':.?½t: ;??:(???;:.?:.??>;::: :-:;':?...-i-:-.::..-,-:?:.:--: -.-.: ' """"'""""""'"'"'"' '... " " l:' "'" '"'""*'"'"' "' " '"'' '-" ' '; ' ; a ' : '½'* ' i;; ; '! IS:': Pitch 0... ::.-: ::;,;:;-;,.;::.;;;: :.... :?.-::-.'-- 1 re.l:!,... ;,,. -.,:-...;;..,:' '-..- ½.'... -,... ½5,' -180 ø Nb of fault (in %) 3 o o 1 oo 200 C) Strike (in-ø/n) Figure 5. (a)block diagram showing how we describe the distribution of the faults with respect to their signed strike (O), dip (3) and pitch (p) instead of the five classically used parameters which are the strike, dip, dip direction, pitch of striae on the fault plane, and pitch direction. Here p is between 0 and 180 ø or between 0 and -180 ø according to the presence of a normal or reverse component, respectively. (b) Rose diagram of all the field measured faults (310). (c) Diagram showing the relationship between strike and pitch. The strike and pitch are also represented on both axis by histograms. ß

10 17,976 LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE

11 LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE 17,977 or, more often, by the separation of fault mechanisms and the paleostress.regimes to the boundary conditions at the related tensors [Angelier, 1984, 1989]. Each paleostress overlap zone. Numerical models of stress regime along the tensor is described by four variables: the spatial orientation of three principal stresses, referred to as the maximum (c l), intermediate (c 2) and minimum (c 3) stresses and q which is the ratio between the principal stress differences: (c 2-c 3)/(c 1-c 3). We grouped a set of stress tensors in a single phase (or paleostress regime) when these stress tensors calculated from different fault populations measured at different sites have the same spatial orientation and a homogenous q ratio value. South Iceland Seismic Zone provide a good basis for discussing the geological significance of our results. Gudrnundsson and Brynjolfsson [1993] and Gudrnundsson [1995] modeled the overlap zone in the last 3 Myr (Figure 7). They consider the West Volcanic Zone as an extensional fracture. The East Volcanic Zone is modeled similarly but propagatesouthward. On these models, the overlapping area is characterized by a stress tensor with both maximum and Four different paleostress regimes have been calculated from minimum stress axes horizontal. Gudmundsson and our data. We referred to the first paleostress regime as phase A: a N340 ø to N-S horizontal compression associated with horizontal extension has been recognized throughout the overlapping area at sites 1, 2, 5, 7, 14, 18 (Figure 6a). This regime corresponds to left-lateral motion along N-S to N40øE oriented faults. Conjugate fault orientations are between N150øE and N-S. At sites 1, 2, 5, 7, 14, 18, the J1 direction varies from N350øE to N20øE. At site 18, {J3 is vertical, but as the q ratio is close to zero, the difference between c 2 and c 3 is very small and the tensor can still be attributed to a strike-slip regime. A second paleostress regime, which we refer to as phase B, corresponds to N50øE horizontal compression associated with a horizontal extension at sites 1, 2, 3, 4, 14, 18 (Figure 6b). The c 1 trend is constant at all these sites. Here, faults oriented N30øE are identified as right-lateral although at two sites (2 and 18), both right-lateral and left-lateral senses were measured on the same fault plane. This fault population also contains faults which were previously left-lateral strikeslip (phase A) and subsequently inherited structures. The third paleostress regime, phase C, corresponds to N145øE horizontal compression associated with a N55øE extension and was recognized at sites 5, 7, and 14 only. At site 7, most of the Brynjolfsson [1993] and Gudrnundsson [1995] predict the main stress directions and also the motion along faults in the area located between the tips of the WVZ and the EVZ (Figures 7a to 7d). The effect of the EVZ propagation is a clockwise rotation of the stress trajectory by more than 145 ø, from the period where the southern tip of the EVZ was located centrally with respect to Iceland, and to the period where it reaches the Surtsey Islands (compare Figures 7a to 7d). Each stage of the Gudmundsson's model can be correlated with a phase of our calculated paleostress regime. In Figure 7, we represented both Gudmundsson's model and our new results. The black lines show the minimum stress directions (modeled c 3) corresponding to the boundary conditions determined by Gudrnundsson [1995]. Our determination of the paleostress regimes is sketched in a Schmidt lower hemisphere projection. The average calculated minimum stress direction (calculated c 3) is shown as a pair of black arrows. One of our calculated paleostress regime should correspond to one of the model phases when both the calculated and the modeled c 3 directions are parallel. According to this, phase A corresponds to the earliest stage and phase C to the latest. In between, phase B is followed by phase D (Figure 7). faults are normal, but as the ratio q is close to zero, c 2 and c 3 have the same magnitude and thus could be interchanged Discussion and Conclusion (Figure 6c). A fourth regime, phase D, is characterized by a N-S extension which is in some cases related to vertical The ridge evolution, as described above, corresponds to a uniform phenomenon related to a regular southward propagation of the EVZ southern tip from VatnajOkull to Surtsey and considering the WVZ as permanently active, leading to an orientation of c l N130øE. However, from earthquake focal mechanisms [e.g., Einarsson, 1991], we know that the present orientation of c l is NE-SW in south Iceland. We suggest two options to explain the stress regime change: 1. From the geometry of the ridges only, the change of paleostress since phase C (Figure 7d) could be related to a change in the distribution of spreading along the ridges in compression (pure normal faulting, e.g., at sites 8 and 18) and in some cases to E-W horizontal compression (such as at sites 9 and 20) (Figure 6d). Where polyphase motion has taken place on faults, superposition of slickensides or destruction of earlier surfaces often prevents the observation of relative chronologies. However, it is possible to identify temporal relationship at three sites. At site 2, superposition of slickensides show that phase A is older than phase B. At site 18, similar evidence shows that phase B is older than our stress regime phase D. At site 5, slickenside lineation superposition show that phase A is older than phase C. It is thus only possible to determine that phase A is older than phase B which is itself older than phase D. Phase C is younger than phase A but no evidence has been found to date it relatively to phase B or phase D. The global evolution of c 1 orientation between phase A and phase D is a clockwise rotation from N-S to E-W. If phase C is older than phase D then c l orientation rotates clockwise by 145 ø on average. If phase C is older than phase B but younger than phase D, then our conclusion is that the orientation of {J1 has rotated first clockwise Interpretation and then counter clockwise. The correlation between our data and the geological evolution of the overlap zone can be understood if we correlate south Iceland (Figure 7e). This means that the EVZ southern tip would have retreated between the period of spreading in Surtsey (Figure 7d) and the present day (Figure 7e), and that the WVZ would have retreated from LangjOkull to Hengill during the same period. This retreat of both the East and West Volcanic Zones implies a counter clockwise rotation of c l by 90 ø. We note that the retreat of the WVZ is discussed widely in the literature, but there is no consensus about the corresponding retreat of the EVZ from Surtsey to TorfajOkull. This maybe because it seems difficult to explain such a great change in a volcanic process occurring in such short amount of time. 2. Taking into account not only the effect of spreading but also the effect of the hotspot as proposed by Stefansson and Halldorsson [1988] and Stefansson et al. [1996], the flow of mantle material from the hotspot conduit produces a

12 17,978 LID(EY ET AL.: TECTONICS OF SOUTH ICF SEISMIC ZONE a) c)., e) Figm-e. Each part is a sketch map of the neovolcanic zones and the main volcanic centers of Iceland (gray shades) on top of which we have.represented the active spri ading centers (thick black lines) and the main -dire.c,tions of the minimum stress (thin black lines) used in the Gudmundsson's [1995] model. The Schmidt lower hemisphere projection represents the average direction bf the main stress for each phase determined from our field data. The bl,ack arrows represent o l; the white arrows represents o3. The Conjugated fault planes are the most likely activated fault planes for each phase. They average the directions of the measured fault planes. (a) 3 Myr ago, (b)and(c) intermediate stages, (d) recent past (few thougands years ago), and (e) present-day situation.

13 LUXEY ET AL.: TECTONICS OF SOUTH ICELAND SEISMIC ZONE 17,979 compression stress oriented NE-SW over the SISZ area and oriented N-S over the EVZ. If this compressional stress is on the same order of magnitude as the extensional stress related to the America-Eurasia spreading, then the stress tensor is likely to produce strike-slip faulting in the SISZ area with the same ridge geometry as described in Figure 7d. Thus there is no need to involve a retreat of the EVZ from Surtsey to Torfaj6kull to explain the recent counter clockwise rotation of {31. It is then possible that some of the earthquakes occurring in the SISZ might be related to the hotspot "push" and not only to the spreading. If we assume that the long-term model of Gudmundsson [1995] can explain the global process of the ridge evolution across south Iceland and that our data describe the different stages of this evolution, we then conclude that the stress tensor regime is mainly related to spreading between America and Eurasia and to the ridge geometry in South Iceland without taking into account the hotspot effect as described by Stefansson and Halldorsson [1988] and Stefansson et al. [1996]. As our data show only the sum of different processes, it is impossible to differentiate the magnitude of each of the processes involved in the evolution of the MAR across Iceland for the last 3 Myr. Nevertheless, taking into account the hotspot "push" as suggested by Stefansson et al. [1996] could also be a good explanation for the recent counter clockwise rotation of {31 as it does not imply any rapid evolution of the ridge geometry. The proposed conclusions from our studies are (1) the four paleostress regimes which characterize the overlap area correspond to a strike-slip regime; (2) these paleostress regimes are organized in the following order: phase A, phase B, phase D, and phase C; (3) the orientation of {31 has rotated clockwise from an N-S orientation to N 145øE and then counter clockwise from N145øE to N30øE; and (4) The WVZ, EVZ, and SISZ are a valid model for overlapping spreading centers described widely on fast spreading ridges. Acknowledgments. This work was funded through the European Community "Human Capital and Mobility" program, contract ERBCHBGCT Philippe Blondel's participation to the field program was funded by the Natural Environment Research Council (Great Britain). We would like to thank Raymond Russo and an anonymous reviewer whose helpful comments greatly improved the quality of this paper. 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