Download The South Iceland Seismic Zone: tectonic and sismotectonic

Journal of Geodynamics 29 (2000) 211±231
The South Iceland Seismic Zone: tectonic and
sismotectonic analyses revealing the evolution from rifting
to transform motion
Franc° oise Bergerat*, Jacques Angelier
Laboratoire de Tectonique, ESA 7072 CNRS, BoõÃte 129, Universite Pierre et Marie Curie, 4, place Jussieu, 75252,
Paris, Cedex 05, France
Abstract
The South Iceland Seismic Zone (SISZ) trends E±W between two major volcanic rift zones of
Southern Iceland. It is 20±60 km wide and 70 km long, and characterized by important seismotectonic
activity. Major destructive earthquakes in the SISZ were reported throughout the centuries, from the
initial settlements in Iceland until now. We carried out a structural study in this area, including analysis
of aerial photographs, local observation of major faults and collection of minor fault slip data in
outcrops, as well as an analysis of the focal mechanisms of earthquakes. At the regional scale, the main
fault trends are approximately NNE±SSW and NE±SW. ENE±WSW, NW±SE and WNW±ESE
trending faults are also detected in aerial photographs and in the ®eld. All these faults are normal or
strike-slip in character. Some of the historical major earthquake fractures are observed in the postglacial lava ¯ows in the SISZ: most are right-lateral and trend roughly N±S. The fault trace left by the
last major single earthquake in the study area, a M7 earthquake in 1912, is mentioned as a case
example. In addition to the large-scale faults, we analyzed more than 700 minor faults at 25 sites. Most
sites are located in rocks of Upper Pliocene±Pleistocene age. Inversion of fault slip data sets enabled us
to reconstruct local paleostress tensors, hence to de®ne the major tectonic regimes which have prevailed
in the SISZ. Examples of characteristic sites are given, and the main results are presented. Two main
groups of faulting mechanisms reveal two distinct stress regimes, with perpendicular directions of
extension, NW±SE (primary) and NE±SW (secondary). Both groups, however, display inhomogeneous
data sets, related to extensional and to strike-slip faulting. The primary stress regime is in agreement
with both the general behaviour of the SISZ as a left-lateral transform zone and the opening of the rift
segments. The secondary stress regime, incompatible with the primary stress regime, is interpreted in
* Corresponding author. Tel.: +33-1-44-27-34-43; fax: +33-1-44-27-50-85.
E-mail address: [email protected] (F. Bergerat).
0264-3707/00/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 6 4 - 3 7 0 7 ( 9 9 ) 0 0 0 4 6 - 0
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F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
terms of stress permutations. A population of 231 double couple focal mechanisms …M > 1 and
depth > 2 km) was also analyzed in terms of stress states. The results show great similarity in terms of
stress directions. The present-day stress ®eld mainly inferred from analyses of earthquake focal
mechanisms is consistent with the present behaviour of the SISZ as a left-lateral transform zone.
However, the proportion of strike-slip faulting within the present-day seismic activity (71%) is
signi®cantly higher than that revealed by the geological observation of Quaternary faults (50%). This
contrast is interpreted in terms of development and evolution of the transform fault zone. The
paleostress ®elds identi®ed in the Upper Pliocene±Pleistocene formations of the SISZ re¯ect both the
previous behaviour of the area, when it was located inside the rift zone, and its present behaviour as a
transform zone, thus illustrating the local evolution from rifting to transform motion. # 1999 Elsevier
Science Ltd. All rights reserved.
1. Introduction
Iceland is located on the Mid-Atlantic Ridge, between the Reykjanes Ridge to the South and
the Kolbeinsey Ridge to the North (Fig. 1). The trend of plate separation across the MidAtlantic Ridge at the latitude of Iceland is N1048E (DeMets et al., 1990). Two major
discontinuities connect these ridges and the Icelandic rift axes located farther east: the TjoÈrnes
Fracture Zone (TFZ), a right-lateral transform zone in northern Iceland, and the South
Iceland Seismic Zone (SISZ), a left-lateral transform zone in southern Iceland (Fig. 1). Both
these zones are partly exposed on land and characterized by high levels of seismotectonic
activity. This paper presents the results of structural ®eld and earthquake focal mechanisms
studies in the SISZ. These results are used to reconstruct the evolution of this fracture zone
from the Pliocene to the Present.
The SISZ, centered at 648N, is located at the junction of three rift segments forming a
Fig. 1. Plate boundaries in Iceland and location of the South Iceland Seismic Zone. In black: main Holocene
cvolcanic systems of the rift zones onshore. Thin lines o€shore: ocean ridges north and south of Iceland. TFZ:
TjoÈrnes Fracture Zone, a right-lateral transform fault system partly exposed on land in North Iceland. SISZ: South
Iceland Seismic Zone, a left-lateral transform zone exposed on land in South Iceland. Open arrows indicate
direction and velocity of plate divergence according to Nuvel-1 data from DeMets et al. (1990). Thin rectangular
frame in South Iceland: location of Fig. 2A.
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
213
complex pattern (Fig. 2A). These segments are the Eastern Volcanic Zone (EVZ) and the
Western Volcanic Zone (WVZ) on the northern side of the SISZ, and the Reykjanes Peninsula
(RP), as the onland extension of the Reykjanes Ridge, on the southern side. Most of the
seismic activity of the SISZ concentrates in a 15±20 km wide (N±S) and 70 km long (E±W)
trending zone (Fig. 2A). However, both the distribution of the main historical earthquake
destruction areas (Einarsson and Eiriksson, 1982) and the results of recent tectonic studies in
South Iceland (Gudmundsson, 1995) suggest that the Holocene seismic zone is as wide as 60
km.
At the surface, the SISZ is partly located in Holocene lava ¯ows, partly in Upper Pliocene±
Pleistocene rocks. There are two main types of basaltic lavas. First, the ropy type is produced
by fast-¯owing and gas-depleted magma. According to the terminology based on consideration
of basalts in Hawaii, adopted in a majority of international papers and also in Iceland
(Gudmundsson, 1996; Gudmundsson and Kjartansson, 1996) this type is named Pahoehoe
(Helluhraun in Icelandic). Second, the scoria type forms out of solidi®ed viscous, gas-rich
magma. This type is named Aa from Hawaii (Apalhraun in Icelandic). The Holocene basaltic
lavas in the SISZ are mainly Aa-type ¯ows, but Pahoehoe-type ¯ows are also common. Most
rocks of the Upper Pliocene±Pleistocene are hyaloclastites (moberg, basaltic breccias),
sediments and tillites.
We carried out in the SISZ both a detailed structural study, including analysis of aerial
photographs, local observation of major faults and collection of more than 700 minor fault slip
data (Fig. 2B) and an analysis of focal mechanisms of earthquakes. The ®rst aim of this paper
is to present the di€erent aspects of faults in the SISZ and the analysis of the minor faults in
terms of stress tensor reconstructions and thus to characterize the Pliocene-Quaternary tectonic
regimes of the SISZ. A second aim is to present an analysis of the focal mechanisms of
earthquakes in this area, in order to determine the present tectonic mechanisms of this large
fracture zone. A comparison between the tectonic regimes revealed by geological and seismic
data respectively, has the potential to illustrate the evolution from rifting to transform motion.
This is because faulting has occurred almost continuously since the late Pliocene, whereas the
seismic activity recorded simply reveals the present-day behaviour.
2. Structure and seismic activity of the south Iceland seismic zone
The regional fracture pattern in South Iceland (Fig. 2B) shows a structural grain dominated
by N±S to ENE±WSW trends, with a majority of fractures trending NNE±SSW and NE±SW
(geological maps, 1/250 000, by Saedmundsson and Einarsson, 1980; Johannesson et al., 1990).
These major directions re¯ect not only the tectonic activity of the NE±SW trending rift zones,
west and east of the SISZ (the WVZ and the EVZ, respectively), but also the transform motion
between these rift segments. The main structural elements are the northeast trending dikes and
normal faults that are presumably of an age similar to that of the Pleistocene lava pile,
whereas many north to north-northeast and east-northeast trending faults (including normal
and strike-slip faults) are younger (Gudmundsson and Brynjolfsson, 1993).
In the Holocene lava ¯ows, NNE trending faults that are mainly right-lateral, as well as
ENE trending faults that are mainly left-lateral, are present. The same strike-slip fault
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Fig. 2. The South Iceland Seismic Zone (SISZ). (A) General structure and seismicity of the SISZ and surrounding
areas: Reykjanes peninsula (RP) Ð Western Volcanic Zone (WVZ), and Eastern Volcanic Zone (EVZ). Thin lines
trending NE±SW: faults of the rift segments (EVZ, WVZ and RP) and outlines of the main Holocene volcanic
systems with central volcanoes shown as roughly elliptical areas. Black dots: selected earthquakes with magnitudes
larger than 0.5 during the year 1995 (from the SIL network, see Stefansson et al., 1993; BoÈdvarsson et al., 1996).
Thin straight lines trending N±S: main seismogenic dextral strike-slip faults in the SISZ after Einarsson and
Eiriksson (1982). Thin rectangular frame: location of (B). (B) Simpli®ed fault map of the SISZ. Fault lines after the
Geological maps at 1:250,000 scale, modi®ed (Johannesson et al., 1990; Saedmundsson and Einarsson, 1980).
Asterisks: sites of brittle tectonic measurements. Rectangular frames (a to d): locations of the aerial photographs of
Fig. 3. Circled letters e, f, g and h: locations of fault examples described in Figs. 5±7 and 9, respectively.
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
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populations are observed in the Pleistocene rocks. Although some of the fault surfaces
observed in the ®eld show evidences of syn-tectonic mineral growth, indicating a creep-type,
non-seismic displacement, many other surfaces are clearly related to earthquake events
(seismic). For example, large slip occurred on several faults in the VoÈrdufell mountain, in the
central part of the SISZ, during the earthquake sequences of 1784 and 1896 (Thoroddsen,
1899). However, where fault surfaces are simple frictional surfaces, especially for the minor
ones which are the most abundant, it is generally dicult to determine whether fault motion
was seismic or non-seismic in character.
North and north-northeast trending arrays of Holocene fractures are associated to historical
major earthquakes within the SISZ (Einarsson and Eiriksson, 1982). Moreover, a map of the
destruction zones of major earthquakes that occurred in South Iceland since 1700 was
compiled by Einarsson and Eiriksson (1982). This map clearly showed that the longest axis of
each destruction area was trending approximately N±S, that is, nearly perpendicular to the
general trend of the SISZ.
In 1990, a digital seismic network was installed in the SISZ (SIL network) and is being
expanded to cover the whole country (Stefansson et al., 1993; BoÈdvarsson et al., 1996). This
system allows recording of earthquakes down to magnitude M ˆ 0. The fault plane solutions
for the recorded events are estimated at the network center (e.g., RoÈgnvaldsson and Slunga,
1993). The close spatial relationship between surface fault traces and the 3D location of the
earthquakes indicates that the microseismicity may well be related to activity of faults
extending to the surface (RoÈgnvaldsson and Slunga, 1994). The results are consistent with
previously determined fault plane solutions for larger earthquakes (Bjarnasson and Einarsson,
1991). They are also consistent with the faulting characteristics previously inferred from surface
deformation (Einarsson and Eiriksson, 1982). All these observations support the view that the
same tectonic processes govern the occurrence of small earthquakes, large events and nonseismic faulting.
3. Aspects of faults in the south Iceland seismic zone
The aspects of Plio-Quaternary and active faults in the SISZ were studied at both the
regional scale (based on aerial photographs) and the outcrop scale (based on ®eld work), in
terms of fracture patterns, attitudes and mechanisms. As already shown in Iceland for normal
faulting (Gudmundsson and BaÈcskstroÈm, 1991; Forslund and Gudmundsson, 1991; Forslund
and Gudmundsson, 1992) the expression of faulting is di€erent at the surface and at depths of
several hundred metres. Tension dominates near the surface, favouring ®ssure opening, whereas
normal shear plays a major role at shallow depths in the upper brittle crust, as shown in the
active rift of Northeast Iceland (Angelier et al., 1997).
For this reason, the aspects of the observed faults in the SISZ are di€erent depending on
their age. The Plio-Pleistocene faults have generally been eroded so that in outcrops one often
observes shear surfaces that were originally at a depth of several tens or hundred metres,
whereas the Holocene faults show in most cases their near-surface aspect. In outcrops of PlioPleistocene formations, both these types (eroded Plio-Pleistocene faults and Holocene fault
traces) are represented, although the Plio-Pleistocene faults are much more abundant. In the
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Holocene lava ¯ows, the faults are generally less abundant but their study better re¯ect the
most recent tectonic behaviour of the SISZ.
3.1. Plio-quaternary fault patterns
At a regional scale, a study of the aerial photographs available at the Icelandic Geodetic
Survey (scale: 1/50,000) enabled us to better characterize the pattern of the Plio-Quaternary
faults. We present herein, as examples, the analyses of four couples of photographs from the
central-eastern part of the SISZ (location in Fig. 2B). These photographs mainly show areas of
basaltic lavas, hyaloclastites, and acid extrusive and intrusive rocks of Upper Pliocene±
Pleistocene age (0.7±3.1 Ma). The two main fault trends detected at this scale are close to
NNE±SSW and NE±SW, in agreement with the main fault trends shown in geological maps
(Figs. 3 and 4). In aerial photographs, the longest segments of faults trend N108±208E and
N408±508E (see inset of Fig. 3: rose diagram for faults longer than 1 km). However, some
ENE±WSW, NW±SE and WNW±ESE trending faults are also detected (Figs. 3 and 4),
although they are not shown in geological maps (Fig. 2A).
In the central part of the SISZ (VoÈrdufell mountain), a detailed study of major faults was
carried out in the ®eld (Bergerat et al., 1998), allowing accurate comparison with the fracture
pattern reconstructed in aerial photographs (Fig. 3A). Especially, this ®eld study revealed not
only the strikes but also the dips of the major faults, which are often dicult to constrain from
aerial photograph studies. Such ®eld observations revealed that the dips of the major faults
generally range from 608 to 908. The slip orientations were revealed by observation of
striations on fault surfaces and other evidences where available, such as o€sets of lava ¯ows
and dikes. Two types of faults, normal and strike-slip, were thus identi®ed. The measured
displacements range from 1 to 17 m for normal faults, and from 1.5 to 5.5 m for strike-slip
faults (Bergerat et al., 1998).
3.2. Active faults
The SISZ is permanently a€ected by earthquakes (e.g., 5,006 earthquakes recorded by the
SIL network during 1994 in this area, see BoÈdvarsson et al., 1996). However, a majority of
earthquakes present low or moderate magnitudes; for instance, all the 1994 earthquakes had
magnitudes lower than 4. Historical data, however, indicate that destructive earthquakes
occurred at least 33 times during the last 800 years (Einarsson and Eiriksson, 1982). The last
two major destructive sequences occurred in 1784 and 1896. During these sequences, the
magnitudes of the largest earthquakes reached M7.1 and may have reached M7.5. The last
major single earthquake in the zone was a M7 earthquake in 1912. Some of the historical
major earthquake fractures are observed in the post-glacial lava ¯ows in the SISZ
(Thoroddsen, 1899; Einarsson and Eiriksson, 1982). These fractures generally are arranged in
en-eÂchelon arrays of open ®ssures of one to hundred metres. The arrays trend N±S to NNE±
SSW and the direction of the ®ssures themselves is NE±SW on average.
We present herein the fault of the May 6th 1912 earthquake as an example (Fig. 5). The
fault trace can be mapped over a distance of about 10 km and shows seven segments arranged
en-eÂchelon, the total rupture length being probably about 20 km long at least (Einarsson and
Fig. 3. Aerial photographs (upper half) of the Mountains VoÈrdufell, on the left (location: a in Fig. 2B) and
Skarsfjall, on the right (location: b in Fig. 2B), and corresponding interpretation in maps (lower half). Sinuous lines
indicate small rivers and edges of large rivers and lakes. Dotted lines indicate cli€s that generally correspond to
boundaries between volcanic formations. Straight continuous lines indicate main faults and dikes (dashed where
uncertain). Rose diagrams describe all fracture trends observed in aerial photographs, with weights proportional to
length. The two numbers separated by a dash successively refer to the total number of fractures observed and to the
total length of fractures, in km. Inset: rose diagram of faults longer than 1 km for the four areas of Figs. 3 and 4.
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Fig. 4. Aerial photographs (upper half) of two areas north of the ThjoÂrsa river (west, on the left and east, on the
right: locations as c and d, respectively, in Fig. 2B), and corresponding interpretation in maps (lower half).
Explanations as for Fig. 3.
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
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Fig. 5. Example of N±S trending right-lateral active strike-slip faults in the South Iceland Seismic Zone: the fault of
the May 6th, 1912 earthquake near 648N±208W (location as circled e in Fig. 2B). (A) Map of the fault trace, after
Einarsson and Eiriksson (1982) and Bjarnasson et al. (1993), showing the en-eÂchelon pattern. (B) Field sketch of a
segment of the fault near Selsund showing en-eÂchelon open fractures, strike-slip segments with minor component of
dilation, push-up structures and one pull-apart structure. Arrows indicate the sense of motion, that is consistent
with the en-eÂchelon fracture pattern. (C) Field photograph showing a strike-slip segment connected to a push-up
structure in the Pahoehoe lava ¯ows near Selsund.
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Eiriksson, 1982; Bjarnasson et al., 1993. Each of the fault segments includes shorter en-eÂchelon
fractures (Fig. 5A). Fig. 5B and C shows an example of typical arrangement of en-eÂchelon
fractures along this 1912 fault trace, with 2±4 m high push-up features located between the tips
of the individual fractures. The 1912 fault has been accurately described by Bjarnasson et al.
(1993), and interpreted as the result of a right-lateral strike-slip motion along a N±S strike by
Einarsson and Eiriksson (1982).
It is certainly easier to describe the traces of recent or active faults in the areas of Holocene
lava ®elds where the earlier deformation is small, rather than in older terranes where the preexisting structure may be complex. It is known, however, that active fault slips which occurred
over several tens of kilometres north of 648N (Johannesson and Robertsdottir, 1986;
Gudmundsson, 1995) are also observed in Pleistocene rocks. However, it is in many cases
impossible to determine whether faults in Pleistocene rocks have been active during the
Holocene. For this reason, and also because of a more rugged topography, it is more dicult
to map active fault traces as reliably and accurately as in Holocene lava ¯ows.
4. Slip data sets and tectonic regimes
The analysis of 736 minor faults (length of tens of centimetres to tens of metres) at 25 sites
(Fig. 2B) in the whole SISZ allowed us to characterize the main stress regimes. Almost all sites
were natural exposures such as the walls of river canyons, but arti®cial outcrops such as
quarries and road cuts were also used. 20 sites are located in basaltic lava and hyaloclastites,
0.7 to 3.1 Ma old. Five sites are located in lavas, acid extrusives and dolerites, younger than
0.7 Ma.
4.1. Examples of stress reconstruction
At each site, we measured all observed brittle features: dikes, mineral veins and minor faults.
For each fault, we recorded the strike and dip of the average plane, the pitch of the slickenside
lineations and the sense of motion. For reconstructing the reduced paleostress tensor, we used
fault-slip inversion technique based on (1) minimizing the angle between the theoretical shear
vector and the actual slip vector (stria), and (2) simultaneously making relative shear-stress
magnitudes as large as possible. This method is referred to as INVDIR (Angelier, 1990).
We present herein two characteristic examples of sites: one is a quarry located in a Holocene
scoria crater cone named Seydisholar (Fig. 6; location f in Fig. 2B). The other is a quarry of
basaltic rocks of age 0.7±3.1 Ma (Fig. 7) at the northern edge of the VoÈrdufell mountain
(location g in Fig. 2B).
The quarry of Seydisholar presents fault planes that are rarely observed in such kinds of
rocks. Three major normal fault surfaces could be followed along several tens of metres, with
oblique-slip striae (Fig. 6A and B); some minor faults were found, consistent with these major
structures. Two of the major faults, trending N608E and N458E, present a left-lateral
component, whereas the third fault, trending N1608E, presents a right-lateral component. The
analysis of all faults resulted in determination of a paleostress tensor (Fig. 6C) indicating an
ESE±WNW direction of extension (trend of horizontal s3 axis: N988E).
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
221
Fig. 6. Oblique-slip normal faults in Holocene scoriae at the Seydisholar post-glacial crater cone near 6484 'N-20850 'W (location as circled f in Fig.
2B). (A) Sketch showing large normal faults with opposite dips, de®ning a graben structure; stereoplots added. (B) Photograph of striated fault
surface (fault located on left in sketch A) striking N608E and dipping 658 to the south, showing oblique, normal and left-lateral slip, with a pitch
of 608 on average. (C) Stereoplot of the total population of Seydisholar faults. Caption of stereoplots: N, geographic North; M, magnetic North.
Fault planes as thin lines, slikenside lineations as small dots with single or double thin arrows (mainly normal or strike-slip, respectively). Three-,
four-, and ®ve-branched stars indicate computed s3 , s2 and s1 axes, respectively. Large arrows show the corresponding directions of extension.
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Fig. 7. Minor faults, quarry of Plio-Pleistocene hyaloclastites at the northern edge of the VoÈrdufell mountain
(location as circled g in Fig. 2B). (A) closely-spaced fractures, including many NE±SW to ENE±WSW trending leftlateral strike-slip faults (above the geologist) and also normal faults (left half of photograph) and veins. (B)
Stereoplots of these faults (caption as for Fig. 6C). From left to right: main subset of strike-slip faults indicating
NNE±SSW compression and WNW±ESE extension (a); subset of normal faults indicating NW±SE extension (b);
subset of strike-slip faults indicating NW±SE compression and NE±SW extension (c) and subset of normal faults
indicating ENE±WSW extension. Large arrows indicate directions of extension (outward-directed arrows) and
compression (inward-directed arrows).
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
223
The quarry north of the VoÈrdufell provides heterogeneous data sets of 60 minor faults
related to extensional as well as strike-slip faulting (Fig. 7A). 77% of the fault slip data set
indicate primarily strike-slip and 23% primarily dip-slip (Fig. 7B). An attempt at determining a
single stress tensor at this site was unsuccessful because it revealed unacceptably large mis®ts.
The simplest tensor determinations acceptable in terms of average and individual mis®ts
involves calculation of two distinct stress regimes. Each of these regimes includes normal and
strike-slip faulting (Fig. 7B). In general, two criteria were used to include the oblique-slip
faults, which are present not only in this VoÈrdufell quarry but also in several other sites of the
SISZ, within a given subset. The ®rst criterion is based on simple consideration of critical slip
angles that control the ratio between the strike-slip and dip-slip components of motion (e.g.,
458; note however that the ratio between lateral and transverse components also depends on
the fault dip). The second criterion is based on a mechanical separation between two or more
paleostress regimes, which results in a posteriori classi®cation of the fault slips studied as
primarily normal or strike-slip. In the VoÈrdufell quarry, the analysis of strike-slip faults in the
largest mechanically consistent subset of data provides a paleostress tensor with subhorizontal
s1 and s3 axes trending N168E and N1068E, respectively (Fig. 7B-a). This subset includes leftlateral strike-slip faults ranging from N408E to N908E, with some N1608±1808E trending rightlateral faults. The tensor determination made with the remaining subset of strike-slip data
reveals nearly horizontal s1 and s3 axes that trend N1248E and N0338E, respectively (Fig. 7Bc). All the faults but one are left-lateral and trend N1708±1908E. The direction of s1 indicated
by the strike-slip faults of the primary subset roughly coincides with the direction of s3
indicated by the secondary subset. At the same site, the analysis of the normal faults revealed
two paleostress tensors with nearly horizontal s3 axes that trend N1438E and N728E (Fig. 7Bb and d, respectively). The main trend of normal faults is N308±658E.
In summary, data collection at all sites where fractured-faulted rocks are observed, followed
by the inversion of fault slip data to reconstruct local stress tensors, provides access to
paleotectonic regimes in terms of attitudes of stress axes, hence to the main directions of
extension and/or compression. A compilation and synthesis of these local reconstructions of
tectonic regimes enables one to de®ne the main directions of paleostresses at the regional scale.
4.2. Synthesis of stress regimes
As suggested by the two examples discussed above and con®rmed by such analyses at the 25
sites studied, two main groups of faulting mechanisms revealing two opposite stress regimes
prevail in the tectonic context of the SISZ dominated by extension and strike-slip. These two
groups di€er in terms of paleostress regimes, which respectively involve compatibility and
incompatibility with the left-lateral behaviour of the SISZ. The primary stress regime is in
agreement with the general behaviour of the SISZ, the secondary stress regime is inconsistent.
These two groups also di€er in terms of the importance of related deformations, as expressed
in the number and size of brittle structures. Both groups reveal inhomogeneous data sets,
including extensional as well as strike-slip faulting modes.
The criteria of relative chronology (e.g., successions of fault slips or crosscutting
relationships between brittle structures), where available, revealed inconsistencies. For instance,
the study of chronological relationships between extensional and strike-slip faulting often
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Fig. 8. The main stress regimes identi®ed throughout the SISZ. Structural and hydrographic patterns after Fig. 2B. Upper (A and C) and lower (B
and D) halves of ®gure: normal and strike-slip types, respectively. Left (A and B) and right (B and C) halves of ®gure: primary and secondary
stress regimes, respectively, as discussed in text. At each site, the orientation of the extension (trend of s3 ), resulting from the inversion of fault slip
data sets, is shown by a couple of divergent black arrows. For the strike-slip regimes, a couple of black convergent arrows is added, in order to
also show the trend of s1 .
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
225
reveal contradictions, even in a single site. From this apparent inconsistency, we infer that the
distinct regimes did not follow each other in a simple way but alternated, probably a number
of times, during the brittle tectonic activity of the SISZ. This phenomenon has also been
evidenced by Passerini et al. (1997) in some regions of Southwest and Southeast Iceland. The
results of our study are summarized in the maps of Fig. 8. For the sake of clarity, the normal
and strike-slip types of stress regimes are separated for each group (Fig. 8A and C, B and D,
respectively).
The primary stress regime has been determined in 23 of the 25 sites. It includes two stress
states consistent with a NW±SE extension. A stress state is generally characterized by systems
of conjugate normal faults trending NNE±SSW to ENE±WSW, with horizontal s2 and s3
axes, and a nearly vertical s1 axis. Most s3 trends lie in the N1108±1308E range (Fig. 8A). This
stress state represents 35% of the total fault population measured. The secondary stress state is
generally characterized by a conjugate pattern of left-lateral strike-slip faults, trending N±S to
NNE±SSW and right-lateral faults, trending NE±SW to ENE±WSW. It is consistent with
horizontal s1 and s3 axes. The direction of s3 ranges from N1008 to N1258E (Fig. 8B). This
fault system also represents 35% of the total fault population. Considering both the normal
and the strike-slip faulting modes, which are related through simple permutations between the
principal stress axes s1 and s2 the primary stress regime reveals a N1308E average trend of
extension. It represents 70% of the total population of fault slip data that we collected
throughout the SISZ.
The secondary groups of stress regimes has been recognized at 19 of the 25 sites. As for the
primary stress regime, two faulting modes and stress states coexist. One stress state is revealed
by a system of nearly conjugate normal faults trending ESE±WNW to N±S. It corresponds to
horizontal s2 and s3 axes, with a dominant direction of s3 axes N408±558E (Fig. 8C). This
stress state is revealed by 21% of the total population of fault slip data. The other stress state
is revealed by a conjugate system of NNW±SSE to N±S trending left-lateral and E±W to NE±
SW trending right-lateral strike-slip faults. The s1 and s3 axes are nearly horizontal, and the
trends of s3 range from N208E to N408E (Fig. 8D). This stress state corresponds to 9% of the
total population. Considering both the normal and the strike-slip faulting modes, which reveal
the same direction of extension, NE±SW on average, this secondary stress regime corresponds
to 30% of the total population of fault slip data.
5. Rift tectonics and transform faulting
The structural ®eld study was completed by a study of earthquake data recorded by the SIL
network in the same area for the period 1991±1996 (Angelier et al., in preparation; Verrier,
1997), in order to compare the present-day stress regime indicated by earthquake mechanisms
and the paleostress regimes indicated by the mechanisms of the faults a€ecting the formations
of Plio-Quaternary ages (Fig. 9). The stress states reconstructed from these independent sources
of data show great similarity in terms of stress directions. However, the proportions between
strike-slip and normal motions observed within these two populations of data markedly di€er.
Thus, a comparison between the stress regimes reconstructed from tectonic data on one hand
226
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
Fig. 9. Comparison between the main stress regimes as indicated by fault slip data sets (upper half of ®gure) and
focal mechanisms of earthquakes (lower half), based on two case examples. (A) Striated faults at site KrokshoÂlt
(location as circled h in Fig. 2B). (B) Selected nodal planes and slip vectors of earthquakes throughout the SISZ for
the years 1991±1994 and 1996, at crustal depths larger than 2 km, with magnitudes larger than 1 (after Verrier,
1997). Caption of stereoplots as for Fig. 6C. (See text for discussion).
and seismic data on the other hand has the potential to reveal changes in the behaviour of the
SISZ since the Upper Pliocene to the Present.
5.1. Comparison between tectonic regimes indicated by geological and seismic data
The population considered for our study includes 3,212 events that occurred in 1991±1994
and 1996, in an area between 6388 and 6482 N and 1985 to 2185W. The magnitudes of these
events range between 0 and 2.5, and their depths range between 0.5 and 45 km (5 km on
average). Two processes were carried out: the ®rst one taking into account the whole
population of earthquakes and the second one using only the earthquakes with M > 1 and
depth > 2 km. Because (1) the di€erences are not very large in terms of stress regimes and (2)
detailed analysis is presented elsewhere (Angelier et al., in preparation.), we simply summarize
herein the main results of the second process.
A population of 231 double couple focal mechanisms …M > 1 and depth > 2 km) was
analyzed with the same inversion method used for the ®eld minor faults (INVDIR: Angelier,
1990). Particular attention was paid to the choice of the nodal plane for each double couple
(Angelier et al., in preparation; Verrier, 1997). Some criteria were de®ned, based on (1)
comparisons between the attitudes and inferred slip directions of the nodal planes and those of
the fault planes actually observed in the ®eld, and (2) the compatibility of fault plane solutions
with the regional stress states constrained using the right-dihedra method (Angelier and
Mechler, 1977).
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
227
As an example, Fig. 9 presents a comparison between the stress states inferred from faults
measured in the ®eld at a characteristic site (KroÂksholt, h in Fig. 2B) and the stress states
obtained from selected nodal planes of earthquake focal mechanisms. These stress states show
great similarity in terms of stress directions. The existence of two opposite paleostress regimes
with perpendicular directions of extension (NW±SE and NE±SW) is outlined by the analysis of
focal mechanisms, as it was by analysis of geological fault slips. The stress regimes probably
alternated several times during the considered time interval (1991±1996), which supports the
hypothesis of frequent oscillations between di€erent stress regimes during the brittle tectonic
activity of the SISZ.
Note that a similar conclusion was obtained for the faulting activity of the PlioceneQuaternary, based on consideration of relative tectonic chronology criteria as discussed before.
The existence of two opposite paleostress regimes with perpendicular directions of extension
has also been outlined elsewhere in Iceland (Bergerat et al., 1988, 1990; Bergerat and Angelier,
1998; Gudmundsson et al., 1992, 1996). The proposed explanations involve elastic rebound or
increase in horizontal pressure due to magmatic injections.
Despite this general similarity, the analysis of earthquake focal mechanisms reveal signi®cant
di€erences, relative to the analysis of fault slips based on ®eld observation. First, reverse
mechanisms are present and represent 13% of the total population (0% for faults). Second, the
proportion of strike-slip movements is larger, and represents 71% of the total population (50%
for faults). We relate this contrast to a temporal evolution of the tectonic regime in the SISZ
region (see next section).
5.2. Evolution of tectonic regimes from Pliocene to Present
Fig. 10 schematically presents a comparison between the paleostress results obtained from
faulting analysis (upper half) and the present-day activity based on focal mechanisms of
earthquakes analysis (lower half), for the SISZ region. Stress permutations between di€erent
regimes are shown as couples of open arrows. Such phenomena may result from relatively
minor variations in magnitude, provided that one di€erential stress is small relative to the
other two ones. Usual stress permutations involve exchange between the intermediate stress, s2 ,
and one of the extreme stresses, s1 1 or s3 (Angelier and Bergerat, 1983). The size of each
diagram represents the relative importance of corresponding data subsets in the whole
population.
The main di€erences between the tectonic scheme derived from earthquake focal mechanisms
and that obtained from fault slip analyses concerns the importance of strike-slip faulting: 70%
of the total population for earthquakes, and 50% for faults. It is also interesting to compare
these percentages with the lower percentages generally obtained in other areas of Iceland. For
example, in the northwestern peninsula of Iceland (an area which underwent rifting process
and then became inactive), only 20% of about 1,000 fault slip data collected correspond to
strike-slip faults (Bergerat and Angelier, 1998). Based on such comparisons, we suggest that the
high proportion of plio-quaternary strike-slip faulting and even more that of present-day
strike-slip faulting in the SISZ is related to the predominant behaviour of this area as a
transform zone.
In more detail, many of the normal faults have an age similar to that of the Upper Pliocene±
228
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
Fig. 10. The di€erent stress tensors obtained from analyses of sets of fault slip data (upper half) and earthquake
focal mechanisms (lower half). Size of diagrams proportional to numbers of data. Three-, four-, and ®ve-branched
stars: regional axes of s3 , s2 and s1 , respectively (schematic). Couples of black arrows: directions of compression
(convergent) or extension (divergent). Couples of open arrows: stress permutations …s1 =s2 or s2 =s3 modes) between
the di€erent regimes discussed in text.
Pleistocene lava pile; they were generated in the western rift zone and then drifted out of it to
the southeast (Gudmundsson, 1995; Bergerat et al., 1998). Some of these faults were
reactivated when located outside the rift zone. Fig. 9A shows an example of such NNE±SSW
trending normal faults (dipping 608 to the east or the west), reactivated as right-lateral faults
during the strike-slip event compatible with NW±SE extension. Thus, the Quaternary history
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
229
of the study area was dominated by a progressive replacement of the normal faulting mode,
which mainly highlights the structural inheritance from the rift process, by the strike-slip
related to the growing SISZ transform zone. It should be pointed out, however, that despite
this general trend of the tectonic evolution with time the normal and strike-slip faulting modes
are mixed in a complex manner at each stage. This association was revealed by our analyses of
the relative chronological relationships mentioned above. In other words, although the normal
and strike-slip faulting activities were always associated since the SISZ began to develop
approximately 2±3 Ma ago, the proportion of strike-slip generally increased from the earliest
stages until the Present.
6. Conclusion
The analysis of the brittle deformation of the SISZ (South Iceland Seismic Zone), as revealed
by both faulting and earthquake mechanisms, enabled us to reconstruct some major trends of
the temporal evolution of this rift±rift transform zone. Some of the normal faults measured in
the ®eld were probably produced during a rift-type regime. This kind of tectonic regime
resembles that already identi®ed in the northwestern peninsula of Iceland (located on the ¯ank
of an extinct rift zone) with 80% of normal fault motions and 20% of strike-slip ones
(Bergerat and Angelier, 1998). It is thus worthwhile to compare the present-day regime with
the recent regime (that prevailed during the Pliocene-Quaternary). In the SISZ, the present-day
stress-®eld inferred from analysis of earthquake focal mechanisms (71% strike-slip, 16%
normal and 13% reverse) is consistent with the behaviour of the SISZ as a left-lateral
transform zone. Therefore, the recent stress-®eld recognized in the SISZ, with 50% of normal
faulting and 50% of strike-slip faulting, presumably re¯ects both the previous behaviour of the
area, when it was located inside the rift zone, and its present behaviour as a transform zone.
The gradual change of this area from its location on the ¯ank of the WVZ (Western
Volcanic Zone) at the end of the Pliocene to its present location between the Reykjanes Ridge
and the EVZ (Eastern Volcanic Zone), was accompanied by a gradual change in the tectonic
stress ®eld, starting with rift-type pure extension and progressively incorporating the
development of preferentially strike-slip structures. This change clearly highlights the evolution
from rift kinematics to a transform motion, within the context of oceanic plate separation.
Acknowledgements
Financial support for this work was provided by the European Commission (contract
ENV4-CT96-0252) and by the French-Icelandic scienti®c-cultural collaboration program
(Iceland Ministry of Education and Culture and French MinisteÁre des A€aires EtrangeÁres). We
thank the Icelandic Geodetic Survey for permission to publish Figs. 3 and 4. We thank the
French Embassy in Iceland for its help. Invitation by Dr. G. Cello allowed us to present the
results at the International Workshop `The Resolution of Geological Analysis and Models for
Earthquake Faulting Studies' held in Camerino, Italy, and is gratefully acknowledged. Dr E.
Tavarnelli and Dr. E. Tondi are thanked for constructive reviews.
230
F. Bergerat, J. Angelier / Journal of Geodynamics 29 (2000) 211±231
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