Tectonic analysis of the Husavik-Flatey Fault {northern Iceland)

Transcription

1 TECTONICS, VOL. 19, NO. 6, PAGES DECEMBER 2000 Tectonic analysis of the Husavik-Flatey Fault {northern Iceland) and mechanisms of an oceanic transform zone, the Tj/Jrnes Fracture Zone. Franqoise Bergerat, Jacques Angelier, and Catherine Homberg Laboratoire de Tectonique, CNRS-UPMC, Paris, France Abstract. The inversion of fault slip data collected in the Flateyjarskagi Peninsula allows reconstruction of four main tectonic regimes. These include normal and stfike-slip faulting modes and are related to the general behavior of the Husavik- Flatey Fault (HFF), a major structure of the Tjfmes Fracture Zone connecting the Kolbeinsey Ridge and the North Icelandic Riff. The two most important regimes (E-W and NE-SW extensions), consistent with the right-lateral motion along the Husavik-Flatey Fault, constitute the main tectonic group. The two others (NW-SE and N-S extensions), forming the subordinate tectonic group, are incompatible and result from drastic stress permutations. The relationships between these stress regimes imply not only c /c 2 and IJ2/IJ3 stress permutations but also c /c 3 reversals. A critical review of other data available, such as lava bedding, dike, and major fault attitudes, allows us to complete the structural pattern of the Flateyjarskagi peninsula and to highlight the mechanism of the transform zone. The complex pattern of dikes and faults in the northern part of Flateyjarskagi can be explained by the superposition of several processes: (1) a transform-perpendicular extension (E-W to ESE- WNW trends), (2) a simple shear (NNE-SSW to NE-SW trends), and (3) a stress perturbation due to the transform motion (NW- SE trends). An important factor controlling the transform mechanism is the variation of coupling along the HFF. The obliquity between the direction of transform motion and the trend of extension for the two main regimes may vary between 20 ø and 90 ø, reflecting repeated changes of the coefficient of friction along the HFF. Such change from very low mechanical coupling (weak fault) to moderate friction may occur very rapidly since it takes place several times in a few years, as shown by focal mechanisms of earthquake analysis. 1. Introduction Iceland is located on the Mid-Atlantic Ridge, at the junction between the Reykjanes Ridge to the south and the Kolbeinsey Ridge to the north. Since the middle Miocene or even earlier, the plate boundary has moved westward with regard to a large hot spot located below Iceland, resulting in successive jumps of the axial riff zone and creating an unstable tectoni configuration [Ward, 1971; Saemundsson, 1974; Helgasson, 1984, 1985]. Presently, the Icelandic axial riff zone is offset eastwards by- Copyright 2000 by the American Geophysical Union Paper number 2000TC /00/2000TC km with regard to the general axis of the Mid-Atlantic Ridge (Figure 1). In northern Iceland the offset of the rift zone occurs along a 120 km long and km wide transform zone (Figure la) referred to as the Tjfmes Fracture Zone (TFZ) and active since - 9 Ma. This zone trends N120øE on average and forms an acute angle to the rift zone of roughly 60 ø. It is a zone of high seismicity (Figure lb) where earthquake mechanisms indicate right-lateral motion [Einarsson, 1991; Rognvaldsson et al., 1998]. This zone includes three main seismic lineaments (Figures lb and l c) that are, from north to south, the Gfimsey lineament, the Husavik-Flatey Fault and the Dalvik lineament. These tectonic lines trend WNW-ESE on average, parallel to the transform zone. Most seismic faults along the Husavik-Flatey Fault also trend WNW-ESE to NW-SE and present right-lateral motion. The Dalvik and Grimsey lineaments include mostly N-S to NNE-SSW left-lateral seismic faults [Rognvaldsson et al., 1998]. The Dalvik lineament is now the less seismically active, showing morphological traces such as fiver canyons but no major fault observable in the field [Bergerat et al., 1992; Bergerat and Angellet, 1999]. The main feature of the TFZ is the Husavik- Flatey Fault, which is partly exposed on land. It can be followed as a distinct morphological feature over a distance of 25 km across the Tjfrnes peninsula [Saemundsson, 1974; Gudmundsson et al., 1993] and is marked by a zone of crushed rocks associated with tilted blocks of lava pile and numerous minor faults on the northern coast of the Flateyjarskagi peninsula [Fjttder et al., 1994; Gudmundsson, 1993]. The first aim of this paper is to present a detailed structural analysis in the Flateyjarskagi peninsula south of the Husavik- Flatey Fault. This study includes accurate field measurements of minor faults, mineral veins, and dikes at 20 sites (Figure 2). The fault slip data are analyzed in terms of stress tensors. The second aim is to discuss the reconstructed stress patterns in accordance with the mechanisms of the Transform Zone. 2. Geological and Structural Setting The Husavik-Flatey Fault (HFF) runs 1-3 km offshore north of the Flateyjarskagi peninsula, between Flatey and the main land (Figure 2). To the east the HFF is marked by a Nl15øE oriented graben in the Skjalfandi area, shown by bathymetric data [Johnsson, 1974; McMaster et al., 1977] and associated with a negative gravity anomaly [Palmason, 1974]. Then, farther to the east, the HFF can be traced as a 25 km long distinct morphological feature across the Tjfmes peninsula (Figure 1 a) The peninsula is composed of two sequences of flood basalts of Ma and Ma in age [Jancin et al., 1985]. Two 1161

2 1162 BERGERAT ET AL.' TJORNES OCEANIC TRANSFORM ZONE (ICELAND) 21 øw 20øW 19'W 18øW 17øW 16'W 67'N 67øN 66øN 66øN 65øN 65øN 21øW 20øW 19øW 18øW 17øW 16øW Figure 1. Seismotectonic pattern of the TjOrnes Fracture Zone. Position of the Icelandic riff axes on the Mid- Atlantic Ridge (inset). Nuvel-1 plate velocities are after DeMets et al. [1990]. RR, Reykjanes Ridge; KR, Kolbeinsey Ridge; TFZ, TjOrnes Fracture Zone; SISZ, South Iceland Seismic Zone. Black pattern shows the Icelandic riff. (a) Seismotectonic map. Thin solid lines indicate faults mapped by direct field observation or seismic reflection methods. Thick solid lines indicate active fault segments [after ROgnvaldsson et al., 1998]. Darkest gray pattern shows the zone of the diffuse deformation. Shear couples of arrows indicate the sense of transform motion. NVZ:, northern Volcanic Zone; KR, Kolbeinsey Ridge; TFZ, TjOrnes Fracture Zone; DL, Dalvik Line; HFF, Husavik-Flatey Fault; GL, Grimsey Line, FP, Flateyjarskagi peninsula, TrP, Tr611askagi peninsula, TP, Tj6rnes peninsula. (b) Seismicity of the Tj6rnes Fracture Zone, with earthquakes as small open dots [ROgnvaldsson et al., 1998]. Bathymetry and topography show the geomorphology of the area.

3 .. ß BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) 1163 Flatey 18ø30 $kjmfandi Major fault Regional unconformity Anticline axis Flexure '- Strike and dip of lava beds Site of measurements 3'112 Figure 2. Main structural elements of the Flateyjarskagi peninsuland location of the data collection sites. HFF is the offshore trace of the Husavik-Flatey Fault shown as dashed line. ICDZ is the southern limit of the intense crustal deformation zone shown as shadedashed line [after Gudmundsson, 1993]. GLL is the Gil-Latur line shown as shadedashed line, regional unconformity is given as a dotted line, flexure axis is given as a dashed line, anticline axis is given as a thick shaded line, and main faults are given as thin lines [after Young et al., 1985]. Places known as Gil and Latur are indicated. FCV, Flateyjardalur central volcano; NCV, Nattfaravik central volcano. Simplified strikes and dips of lava are after Young et al. [1985], Fj ider et al. [1994], and authors' measurements. Dots indicate measured microtectonic sites. central volcanoes have developed in its central part, Flateyjardalur and Nattfaravik (Figure 2) and are interbedded the oldest and the youngest flood basalt piles, respectively. The younger sequence rests unconformably on flexured rocks of the The interpretation of all these geological features remains a matter of debate [Young etal. 1985; Fj' ider et al., 1994; Gudmundsson and Fj ider, 1995; Jancin et al., 1995]. We discuss this aspect in section 5 of this paper. None of the authors older lava pile [Young et al., 1985]. This unconformity cited, however, have carried out a detailed analysis of minor represents a major lithological and structural boundary [Aronson and $aemundsson, 1978; Jancin et al., 1985]. In the western part of Flateyjarskagi, the Tertiary lava sequence dips 5ø-15 ø SW to WSW. The strike and dip progressively change to the north of the peninsula (Figure 2). On the northern cost, the lava inclination reaches 45 ø to the NW [Young et al., 1985; Gudmundsson, 1993; Fj ider et al., 1994]. In faulting in the Flateyjarskagi peninsula. Fjader et al. [1994] measured nearly 300 striated surfaces, but their analysis merely consisted of a statistical presentation of the fault plane geometry. The present work focus on the observation, data collection and mechanical analysis of minor faults in the Flateyjarskagi peninsula in order to reconstruct stress regimes and to illustrate the kinematic behavior of the Husavik-Flatey Fault. the southern part of the peninsula the main fault set trends N-S to NNE-SSW. To the north the fault pattern becomes complex, and the major fault trends change to NE-SW (Figure 2). The 3. Brittle Structure of Flateyjarskagi Peninsula trend of the Tertiary basaltic dikes in the southern and central and Main Paleostress Regimes. part of the peninsula is mainly N-S to NNE-SSW (the dikes having intruded through acidic rocks of central volcanoes being In order to determine tectonic stress states we collected left apart). In the northern part of the peninsula most dikes strike ENE-WSW to ESE-WNW [Young et al., 1985]. measurements on all observable brittle features (dikes, mineral veins, and minor faults) at 20 sites in the Flateyjarskagi

4 1164 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) peninsula. The locations of these sites are shown in Figure 2. Natural exposures along the northern and western coasts provided the majority of the sites, but outcrops along river valleys inside the peninsula were also used. For each fault we recorded the strike and dip of the average plane, the pitch of the slickenslide lineations (striae), and the sense of motion. The instrumental uncertainty was in the range +1 ø, much smaller than the observation uncertainty that usually ranges between 4 ø that a simple succession of brittle regimes was unlikely to account for our observations. For reconstructing the reduced paledstress tensors we used an analytical inversion method (INVDIR) developed by Angelier [1990] and already described in detail and discussed in previous papers [e.g., Angellet, 1994]. The reduced paledstress tensor contains all the information defining the trends and plunges of the three principal stress axes as well as the ratio ci)=(c 2-c 3)/(c - and 10 ø. At all places where polyphase fault slips or crosscutting ix3) between the principal stress differences. relationships between faults could be observed, the succession of Because this method aims at (1) minimizing the angles movements was noted in order to establish the relative between theoretical shear stress and actual slip vector and (2) chronology of brittle tectonism. simultaneously having relative magnitudes of shear stress large 3.1. Distribution of Minor Faults and Veins enough to induce slip despite rock cohesion and friction, it is particularly appropriate for sets of mixed nedformed and Figure 3 summarizes the results of the statistical analysis of the 992 minor fault planes and 266 mmeralized veins and dikes inherited fault slips, such as in the studied area. To evaluate the validity of the results, consideration of uncertainties on the four measured in the Flateyjarskagi peninsula. For the total variables of the reduced tensor is crucial. These uncertainties population of dikes and mineralized veins, the strikes and dips are shown. Most dikes and mineralized veins (Figure 3e) trend N0ø-20øE, but distinct N30ø-50øE and N80ø-95øE trends exist as well. For the total population of faults (Figure 3a) and for each subgroup of faults (Figures 3b-3d), the distribution of fault strikes, fault dips, and pitches of motion vectors are shown (diagrams a, b, and c, respectively). The distribution of pitches of slickenslide lineations within this population of faults clearly shows that two contrasting types, normal and strike-slip, are present (Figures3b-3d, diagram c) and deserve separate analyses. Most normal faults (Figure 3b) trend N0ø-40øE, but N120 ø- 145øE trends are also abundant. Their dips range from 45 ø to 80 ø, while striations reveal oblique to pure dip-slip motion vectors (pitches of 40 ø to 90ø). The strike-slip faults (Figures 3c- 3d) dip generally ø, and the pitches of the striations range mainly from 0 ø to 30 ø. The left-lateral faults are very scattered in directions with predominant strikes ofn15ø-50øe, N75øE and N165øE (Figure 3d). The major strike angles of the right-lateral faults (Figure 3c) are N150øE, N35øE, N60øE, and N100øE. The abundance of oblique striations may result from the tilting of part of the fault planes, but also from the successive slips on a given plane under different stres states. The variety of faulting modes, normal and strike-slip, as well as the large range of fault plane orientations for each type and the abundance of oblique striation suggest polyphase tectonism in the Flateyjarskagi peninsula. Field observation confirms that in many cases, brittle structures have been active at different times with contrasting mechanisms, as shown, for instance, by vary with the number of data used,in each local inversion and the angular accuracy of measurements mentioned above. They also depend on the distribution of misfits indicated by the a posteriori reviews of the individual results following the inversion. Keeping in mind the data angular uncertainties already mentioned, our inversion aimed at fulfilling two conflicting requirements: using all the measurements collected (that is, no datum was ignored) and obtaining acceptable individual misfit ranges (in the range 0-75% for the ratio RUP as defined by Angelier [1990], and in the range ø for the computed shear stress observed slip angle; for each inversion, the average misfits are, of course, smaller, as Table 1 shows). Other parameters, such as the three-dimensional (3-D) distribution of the shear data used, also constrain the accuracy of the paledstress determinations. For the sake of simplicity and according to these factors, we summarized the quality of the results with a four-letter scale from A, very good, to D, low (Table 1). For principal stress axes with shallow plunges the azimuthal uncertainty thus commonly ranges between +4 ø (quality A) and +16 ø (quality D). In all the sites of the Flateyjarskagi peninsula we had to deal with an important aspect in paledstress determinations: the presence of heterogeneous data sets, resulting from the occurrence of two or several distinct tectonic regimes. A simple and straightforward approach of the problem consists of examining the subsets of conjugate fault slips because of their simple geometrical relationships with the stress axes. However, because many fault slips have occurred on inherited fault surfaces (so that the orientations of fault surfaces do not depend superimposed dip-slip and strike-slip striations on fault surfaces, on the orientations of stress axes), this geometrical approach was dikes crosscut and offset by faults, veins reactivated as faults, or shear surface showing evidence of later opening and mineral infill. The geometrical relationships between the fault-fracture patterns and the attitude of faulted-tilted blocks also provided a key to decipher the chronology of faulting relative to tilting. The fault-slip geometry itself indicates that many fault slips are inconsistent with any simple model of conjugate faulting, numerous faults having undergone oblique slip reactivation. While collecting brittle data, we paid particular attention to all individual evidences of relative chronologies between brittle movements in order to define later the succession of the different tectonic regimes. Even at this early stage of our analysis, obvious contradictions between reliable chronological criteria not sufficient. As a result, both the separation of subsets consistent with distinct stress regimes and the determination of the related paledstress tensors were carried out within iterative processes involving comparisons between the individual misfits obtained with the different tensors. Although few data may remain ambiguous (i.e., equally consistent with two or several tensors), such an iterative analysis generally results in the optimum mechanical separation between the fault slips of an inhomogeneous data set [Angelier, 1984]. It is worth noting that this separation into different subsets was exclusively based on the search for mechanical consistency and, by definition, ignored the relative chronology data observed in the field as mentioned before. The a posteriori consideration were often noticed at several sites of data collection, suggesting of the independent relative chronology data confirmed our initial

5 BERGERAT ET AL.' TJORNES OCEANIC TRANSFORM ZONE (ICELAND) 1165 ENTIRE FAULT POPULATION b c g..z f A NORMAL FAULTS N RIGHT-LATERAL STRIKE-SLIP FAULTS a b c C (265) N LEFT-LATERAL STRIKE-SLIP FAULTS c o 9o N VEINS AND DYKES b (266) Figure 3. Rose diagrams of minor faults, dikes and veins measured at the Flateyjarskagi peninsula. From top to bottom, a complete set of minor faults (A), subsets defined according to the main fault types (normal faults (B), right-lateral strike-slip faults (C), and left-lateral strike-slip faults (D)) and veins and dikes (E) are given. Diagrams a and b correspond to strikes and dips of the fault planes, respectively; diagram c corresponds to pitches of striae. Number of structures is indicated between parentheses.

6 1166 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) Table 1. Paleostress Tensor Computations Based on Fault Slip Data Analyses in Flateyjarskagi. Site SR N o ß tx RUP Q Fl 1 S C Fl 1 S A Fl 1 N D FI 2 S B FI 2 N D FI 2 S C Fl 2 N D Fl 2 S D Fl 2 S B FI 3 S C FI 3 N A FI 3 S B FI 3 S B FI 3 N C FI 5A S D FI 4 S C FI 4 N C FI 4 S B FI 4 S C FI 5A S D FI 5A S A FI 5B N D FI 5B S B FI 5B S C FI 6 N D FI 6 S A FI 6 S C FI 7 N C FI 7 S B FI 8A S B FI 8A S C FI 8A S C FI 8A N B FI 8A S C F1 8B N D FI 8C N A FI 8C S D FI 8C S D FI 9 S C FI 9 S D FI 9 S D FI 9 N C FI 10 S D FI 10 N D FI 10 S B FI 10 N B FI 10 S C FI 11 S D FI 11 N D FI 11 S D F1 11 S D FI 12 S D FI 12 S B FI 12 S C FI 12 N C FI 13 N D FI 13 S B FI 13 S C SK N B EY 6 S B EY 6 N B Columns, from left to right, are as follows: reference number as located in the map of Figure 2 (site colurn). SR, Stress regime; N, number of fault slip data; trends and plunges of principal stress axes o, o 2 and 3 (in degrees); ratio (I)=( 2-3)/( - o3) between the principal stress differences; average angle between computed shear stress and observed slickenslide lineation tx (in degrees); ratio "upsilon" (RUP, ranging between 0-200%) of the INVDIR method langeher, 1990] (average RUP values below 50% indicate good fits between actual fault slip data and computed shear distributions); quality of the computed stress tensor Q (A, very good; B, good; C, fair; D, low).

7 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) 1167 observation: several brittle tectonic regimes have occurred in the these normal faults revealed two paleostress tensors with nearly Flateyjarskagi peninsuland can be reliably separated, but no horizontal cr 3 axes that trend N36øE and N128øE, respectively simple succession of regimes can account for our observations. (Figures 4d and 4e). The complexity of the site F1 3 (Figure 4) is not exceptional Example of Stress Reconstruction All but a few sites studied in the Flateyjarskagi peninsula In order to illustrate how the different stress states have been contain several fault subsets, revealing a high level of computed we present in this section detailed analysis at a heterogeneity. In the whole peninsula we identified eight characteristic site (site F1 3 in Figure 2) located on the different brittle tectonic regimes. Because these regimes are northeastern coast of the peninsula (Figure 4). This site provides evident in most of the sites (Figures 5-8), they correspond to a a good example of fault separation because a heterogeneous data variety of regional stresstates not to local variations of a single set of 92 minor faults related to extensional as well as strike-slip regime. Four of these regimes are strike slip in type (Si), and faulting was collected there. Consistent with geometrical four are normal in type (Ni). These regimes differ in importance, evidence of incompatible senses of strike-slip motions on faults based on consideration of numbers of sites and data (see Table with similar trends, attempts at determining a single stress 1), and certain geometrical relationships can be established tensor at this site were unsuccessful owing to unacceptably large between some of them. We will come to these aspects in the sections 3.3 and 3.4. misfits. The tensor determinations that could be considered acceptable in terms of average and individual misfits involved calculation of at least five stress states Synthesis of Stress States Three of these stresstates are related to strike-slip tectonics (Figures 4a-4c), and the other two reveal normal-slip tectonics (Figures 4d and 4e). The separation between these subsets (and stress states) is justified because it maintains all individual misfits within the acceptable bounds discussed before, and results in the smallest average misfits. For instance, an inversion Because of the regional occurrence of eight stress states the mechanism of the Husavik-Flatey Fault seems complex at first sight; however, obvious geometrical relationships exist between some of these stress states. The most prominent links are between strike-slip and normal regimes displaying a common carried out with all normal faults would result in unacceptable direction of cr3, a relationship that can be viewed as a tx /cr2 individual misfits, whereas the separation between Figures 4d and 4e optimizes the misfits. As mentioned before, we searched stress permutation. All the strike-slip and normal stress states characterized in the Flateyjarskagi peninsula can be related for the best separation through iterative processes involving pairwise in this manner. They are noted herein Si-Ni and different distributions of data within subsets. In more detail, the represent the four main stress patterns. Another kind of strike-slip fault subset in Figure 4a corresponds to a paleostress relationship is represented by orthogonal normal regimes (with tensor with nearly horizontal cr] and cr 3 axes thatrend N172øE perpendicular direction of cr3), that is to say, a c 2/cr3 and N83øE, respectively. This subset includes right-lateral permutation. Finally, the last kind of relationship implies drastic strike-slip faults ranging in strike from N100øE to N150øE and a few N10ø-30øE left-lateral trending faults. The subset of strikepermutation between maximum and minimum stresses ch and cr3. One can expect that all these relationships are not slip faults in Figure 4b reveals nearly horizontal cr] and cr 3 axes coincidences but result from the transform mechanism itself. that trend N41øE and N135 ø, respectively. The faults of this Among the stress patterns that we could reliably determine in subset trend N 20øE for the right-lateral ones and N50ø-70øE for the Flateyjarskagi peninsula, two groups (each including two the left-lateral ones. The last strike-slip fault subset, Figure 4c, stress patterns) can be distinguished based on both the indicates horizontal cr and cr 3 axes that trend N124øE and importance of the related deformations, as expressed in the N27øE ø, respectively. The left lateral faults trend N160øE on number and size of brittle structures as well as in their average, and the right lateral faults trend N80ø-I 10øE. The normal faults show large scattering strikes, most of which range, however, from N10øE to N80øE. The analysis of compatibility with the right-lateral behavior of the HFF. The main group is in agreement with the general behavior of the HFF and the subordinate group results from drastic stress Figure 4. Example of data analysis from a representative site on the northeastern coast of Flateyjarskagi (site F1 3, Figure 2). Stereoplots are Schmidt's projection, lower hemisphere (N is geographic north, and M is magnetic north). Fault planes are shown as thin lines, silkenside lineations (striae) are small dots with single or double thin arrows (mostly normal or strike slip, respectively). Three-, four-, and five-branched stars are computed axes cr3, cr2. and tx, respectively. Large solid arrows are corresponding directions of horizontal compression and extension. Numerical results are given in Table 1. Detailed explanations are given in text.

8 1168 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) F15B N3 Fl12 FI13 Figures 5. The S3-N3 stress regime, as characterized by analyses of normal faulting (N3, E-W extension) and strike-slip faulting (S3, E-W extension and N-S compression). Bars indicate the trends of minimum stress (cr3) for each site where the stress regime has been determined. Bar size increases with quality of paleostress tensor determination. Some examples of characteristic stereoplots are given (legend as for Figure 4). See Table 1 for numerical results. permutations and is incompatible with the general behavior of the HFF. For a better understanding, we describe hereafter the different stress states grouped pairwise (Si-Ni). The relationships among them will be developed in the sections 3.4 and 4.3. The main group includes two stress patterns: one is consistent with an E-W extension (S3-N3), and the other is consistent with a NE-SW extension (S4-N4). The S3-N3 stress regime has been identified at 17 of the 20 sites. It includes two stress states, a strike-slip one and a normal one, that are consistent with an E-W extension (Table 1). They are related through simple permutations between the principal stress axes cr and ( 2: 1. The stress state N3 is characterized by systems of conjugate normal faults trending N150øE to N240øE (Figure 5). The cr 2 and cr 3 axes are horizontal and the direction of cr 3 (Figure Figures 6. The S4-N4 stress regime, as characterized by analyses of normal faulting (N4, NE-SW extension) and strike-slip faulting (S4, NE-SW extension and NW-SE compression). Legend and explanations are as for Figure 5. See Table 1 for numerical results.

9 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) 1169 S1 Figures 7. The S1-N1 stress regime, as characterized by analyses of normal faulting (N1, NW-SE extension) and strike-slip faulting (S 1, NW-SE extension and NE-SW compression). Legend and explanations are as for Figure 5. See Table 1 for numerical results. 5, Table 1) ranges from N87øE to N102øE. This stress state represents 7% of the entire fault population and is manifest in E- W extension via normal faulting. Hereafter such a stress regime will be referred to as "pure extension." 2. The stres state S3 is characterized by strike-slip faults, trending N20øE to N60øE for the left-lateral ones and N130øE to N160øE for the fight-lateral ones. The cr and cr 3 axes are horizontal and the direction of cr 3 ranges from N072øE to Nøl12øE (Figure 5 and Table'l). This stresstate represents 32% of the total fault population and causes E-W extension accommodated by strike-slip faulting. Hereafter such stress regime will be referred to as "strike-slip extension." The S4-N4 stress regime has been determined at 13 of the 20 sites. The two stress states are consistent with a NE-SW extension (Table 1). As for S3-N3, they are related through simple permutations between the principal stress axes Crl and or2: S2 F14 N2 FL0 Figures 8. The S2-N2 stress regime, as characterized by analyses of normal faulting (N2, N-S extension) and strike-slip faulting (S2, N-S extension and E-W compression). Legend and explanations are as for Figure 5. See Table 1 for numerical results.

10 1 170 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) 1. The stress state N4 is characterised by systems of normal faults with relatively scattered strike directions, ranging from N90øE to N230øE. The cr 2 and cr 3 axes are horizontal and the direction of '3 lies in the range N14øE to N61øE (Figure 6 and Table 1). This stres state represents 10% of the total fault population and causes a NE-SW pure extension. 2. The stress state S4 is characterized by strike-slip faults, trending N150øE to N180øE for the left-lateral ones and N90øE with contrasting directions of extension, or are they apparent and simply resulting from a variety of horizontal block rotations? This problem is discussed extensively in sections 4.3 and 5 of this paper; however, a preliminary answer is given by simply considering that the eight calculated stress regimes with contrasting directions have been found in many sites of the peninsula (Figures 5-8), regardless of their position relative to the major shear zone where large rotations may have occurred for the right-lateral ones. The c and c 3 axes are horizontal, and (Figure 2). Although it has much less potential to drastically the direction of c 3 ranges from N15øE to N44øE (Figure 6 and modify the paleostress trends than the horizontal rotation, the Table 1). This stres state represents 13% of the total fault block tilting was also considered in relation to the computed population and consists in a NE-SW strike-slip extension. stress axes and failed to account for the large azimuthal variations observed. The subordinate group also includes two stress patterns: one is consistent with a NW-SE extension (S1-N1), and the other is 3.4. Complex Stress Patterns and Tectonic Regimes consistent with a N-S extension (S2-N2). The S1-N1 stress regime has been determined at 12 of the 20 sites. The two Fault slip analysis has led us to identify eight stress states, as presented in the section 3.3. Similarities in extensional corresponding stres states, strike slip and normal in type, are related through simple permutations between the principal stress directions allowed us to combine these eight regimes in four axes c and c 2 and are consistent with an NW-SE extension: couples (Figures 5-8). For each couple Si-Ni, a simple 1. The stress state N1 is marked by systems of conjugate permutation of c and c 2 is required. The most representative stress regimes, S3-N3 and S4-N4, indicate E-W and NE-SW normal faults trending N20øE to N40øE. The cr 2 and '3 axes are extensions, respectively. Perpendicular relationships exist horizontal and the direction of c 3 varies from N115 øe to N141 øe between the directions of extension of the couples S3-N3 and (Figure 7 and Table 1). This stresstate represents 16% of the S2-N2 on one hand, and those of the couples S4-N4 and S l-n1, total fault population and consists in a NW-SE pure extension. 2. The stres state S 1 is generally characterized by left-lateral on the other hand [Angelier et al., 2000]. The relationships strike-slip faults, trending N70øE to N90øE, and right-lateral between N3 and N2 involve a simple c 2/c permutation, and the ones, trending N10øE to N40øE. The cr and or3 axes are relationships between N4 and N1 involve a cwc 3 permutation as well. Such c /-2 and c 2/c 3 permutation phenomen are common horizontal and the direction of c 3 ranges from N122øE to N146øE ø (Figure 7 and Table 1). This stress state represents in extensional tectonics [e.g., Angelier and Bergerat, 1983]. The 10% of the total fault population and consists in a NW-SE relationships between S3 and S2 and between S4 and S1 imply strike-slip extension. drastic reversals of c and c. The origin of this phenomenon The S2-N2 stress regime has been determined at 9 of the 20 probably lies in elastic rebound, fault block accommodation and sites. The two stress states are consistent with a N-S extension magma injection phenomena as already mentioned elsewhere through simple permutations between the principal stress [Angelier et al., 2000]. Similar permutations (cry/or2 and axes c and c 2: and reversals (c /c ) were recognized in the E-W trending South 1. The stress state N2 is characterized by very few normal Iceland Seismic Zone langelief et al., 1996; Bergerat et al. faults trending N90øE to N135øE. The or2 and cr 3 axes are 1998, 1999], using similar fault slip analysis, and in the Tjfmes horizontal and the only two computed c trends are N160øE and Fracture Zone [Garcia, 1999], based on analyses of present-day N180øE. (Figure 8 and Table 1). This stresstate represents 2% focal mechanisms of earthquakes, which sufficed to rule out the of the total fault population and consists in a N-S pure hypothesis of polyphase tectonism. That the earthquake focal extension. mechanism data of the TjOmes Fracture Zone cannot be 2. The stres state S2 is characterized by a system of strike- interpreted in terms of homogeneous regional stress but belong slip faults, trending N90øE to N120øE for the left-lateral ones to several distinct stress states requires extensive presentation of and N15øE to N60øE for the right-lateral ones. The c and c 3 seismic data and is documented in a forthcoming paper. Considering the N120øE trend of the transform fault zone and axes are horizontal and the direction of cr 3 ranges from N 162 ø to N189øE (Figure 8 and Table 1). This stresstate represents 9% its right-lateral sense of motion, two major sets of regimes can of the total fault population and consists in a N-S strike-slip f'mally be distinguished: (1) an E-W extension characterized by extension. the S3-N3 couple but also by the reversed S2-N2 couple and (2) a NE-SW extension characterized by the S4-N4 couple but also Considering the number of sites where a given stress pattern by the reversed S1-N1 couple. Therefore each major set of (Si-Ni) has been characterized and the corresponding number of minor faults, we can demonstrate that the S3-N3 stress regime regimes contains one of the main Si-Ni stress regime, consistent with right-lateral transform motion (S3-N3 and S4-N4) with (E-W extension) controlled most of the deformation of the respective angles relative to the transform trend of 20o-30 ø and Flateyjarskagi peninsula: it is observed at 17 sites (on a total of 80o-90 ø. Each major set of regimes also includes the 20) and represents 39% of the total fault population. The main corresponding reversed stress regime Sj-Nj' (S2-N2 and S1-N1, group (including S3-N3 and S4-N4) includes 63% of the faults, and the subordinate group (S1-N1 and S2-N2) includes 37% of respectively). Each set of regimes is thus basically defined by the faults. This distribution suggests that the S3-N3 and S4-N4 the angular relationship between -3 and the transform trend stress regimes (E-W and NE-SW extensions, respectively) play a (Figure 9). majorole in the mechanisms of the transformotion along the 4. Mechanisms of the Transform Motion HFF. The following questionaturally arises when identifying such Some structural data had been collected in the Flateyjarskagi a large number of stresstates, Do they reflect distinct regimes peninsula prior to our study. They did not deal with fault slip

11 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) ø 212 c 3 Main regime Reversed regime _90 ø T Main regime Reversed regime S3 S2 N3 N2 4 Figure 9. The tectonic stresstates characterized in Flateyjarskagi (modified after Angelier et al. [2000]). Each major set of regimes contains one of the main stress regime and the corresponding reversed stress regime. Divergent arrows indicate the average trends of extension (c 3 axes) for S- and N-type regimes, and convergent arrows indicate the average trends of compression (c axes) for S-type regimes. Arrow size increases with relative importance of stress regime. Thick line (N120øE) represents the average trend of right-lateral transform fault zone. Angles between transform direction (T) and average trends of extension (c 3) of the mains regimes are indicated. analysis but were concerned essentially with the strikes and dips of the lava flows, of some of the major faults, and of many dikes and mineral veins. Because the interpretation of these data was controversial and the data themselves have been discussed, it is necessary to briefly present and discuss these aspects before using them to complete the results of our fault slip analysis. Taking into account both these earlier data and the new ones presented herein, we aim at improving the understanding of the tectonic behavior of the Tjrrnes Fracture Zone. 6 ø to 19 ø SW. In the northern part of the peninsula gradual but noticeable change in attitude occurs, resulting in a clockwise change in strike and in increasing dip (Figure 2). Along the northern coast the strike of the lava beds is NNE-SSW to NE- SW, and their dips may reach 45 ø NW. In the central part of the peninsula, east of the flexure axis, the lava beds dip 20o-30 ø ESE on average. Interestingly, the attitudes of the major faults observed by Young et al. [1985] also gradually change in strike from south to north, with the same clockwise variation as for the 4.1. Lava Bedding, Major Faults, and Dikes Attitudes lava flow attitudes: N-S to NNE-SSW near Grenivik and NE-SW on the northern coast (Figure 10b). Some WNW-ESE fault Two major surveys have been carried out in Flateyjarskagi strikes have also been observed near the northern coast [Young peninsula, one by Young et al. [1985] throughout the whole et al., 1985]. peninsuland another by Fjader et al. [1994] mainly along the The most interesting and debated features are the dikes. For northern coast, the western coast and three river canyons. Their Young et al. [1985] the southern and central parts of interpretations of the structural features differ, so that their Flateyjarskagi are dominated by N-S and NNE-SSW trending tectonic significance has been the subject of controversy (dancin dikes (excluding the dikes intruded through acidic rocks of the et al. [1995] defending the position of Young et al. [1985] and Flateyjardalur central volcano); north of a Gil-Latur line the Gudmundsson and Fjader [1995] defending the position of orientation is ENE-WSW to WNW-ESE (Figure 10a). The dike Fj ider et al. [1994]). trends measured by Fjttder et al. [1994] are nearly the same: N-S The geological structure is mainly known on the basis of the to NNE-SSW in the southern part of the peninsul and E-W to strikes and dips of the basalt lava flows (Figure 2), which were NW-SE along its northern coast. The controversy, which mainly presumably horizontal prior to tectonic deformation. There is no deals with the interpretation of these dike patterns in terms of controversy as far as the general attitudes of lava flows are dike swarms, is summarized below. concerned. The flood basalt pattern is better documented in the 4.2. Divergent Interpretations of the Structural Bend work of Young et al. [1985], who studied a larger area than Fjader et al. [1994]. Typical attitudes are found south of Latur, Despite their general agreement on the overall structural in the western part of Flateyjarskagi where the lava strike NNW- pattern, Young et al. [1985] and Fjader et al., [1994] deeply SSE to NW-SE. Their dips increase, from south to north, from disagreed and proposed contrasting tectonic interpretations.

12 1172 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) A c? C \1\11!//i I u // D Figure 10. Dike trends (a) and major faultrends (b) in the Flateyjarskagi peninsula (modified after Young et al. [1985]). Figures 10c and 10d represent our attempt to separate (c) the N-S to NE-SW curvedike set and (d) the transform-parallel dike set. Few NW-SE trending dikes (not represented in a separate box) are visible in the northern part of the peninsula Figure 10a. The Gil-Latur Line of Young et al. [1985] is indicated as dashed line in Figures 10c and 10d. Young et al. [1985] consider that the clockwise rotation of the northern Flateyjarskagi was original and hence did not result strike of lava, faults and dikes resulted from block rotations due from subsequent tectonic rotation. Fjader et al. [1994] also to a heterogeneous simple shear occumng within an 11 km wide pointed out that the arithmetic mean thickness of the dikes at the shear zone bounded by the HFF to the north (Figure 2). In, western coast is 5.4 m, whereas it is only 4 m at the northern contrast, for Ffiider et al. [1994], intense deformation has been coast. This difference in thickness led them to suggest that two concentrated within a 3-5 km wide zone along the north coast of separate dike swarms exist in the peninsula (referred to as the the peninsula (Figure 2). More important, according to these Grenivik and Flatey swarms, respectively). Subsequent authors, the clockwise change in the strike of lava beds in discussions [Jancin et al., 1995; Gudmundsson and Fjader,

13 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) ] following the publication of the Fj/ider et al.'s work concentrated on the interpretation of the dikes swarms. The main discrepancies between these interpretations dealt to our N3 (and maybe partly N1) stress regimes. Despite some local ambiguities between the two azimuthal subsets, this separation was found statistically valid. with (1) the location of the southern limit of the deformed zone, 3. Despite their disagreement concerning the interpretation, (2) the number of swarms within the dike population, and (3) the authors mentioned above agree that a certain clockwise the tectonic interpretation of the major bend in the structural change of some major features occurs in the northern part of the grain of the peninsula in terms of either a clockwise rotation or a Flateyjarskagi peninsula. For Young et al. [1985] the primary feature influenced by a curved stress field. On the basis reorientation of the lava beds and of the whole dike population of both the critical lecture of the aforementioned papers and our results from clockwise block rotation due to a right-lateral own observations we discuss these three points as follows. shearing in a large zone parallel to the HFF. This rotation would 1. To Ffiider et al. [1994, p. 115], there is "little evidence for be accommodated internally by left-lateral movement on a regional crustal deformation south of a 3-5 km wide on-land antithetic (R') Riedel faults. For Fj ider et al. [1994] the zone." Discussing this point is made difficult by the variable structural elements formed approximately in their present density of structural observations, since Fj/ider et al. did not positions, and the clockwise change in orientation does not provide observation and measurement profiles immediately reflect block rotations. They distinguish different dike swarms. south of this coastal area. Although the amount of deformation is Concerning the main one that trends N-S to NE-SW, they much larger along the northern coasthan inside the peninsula, it consider that the change in trends is due to fluctuations in the is difficult to define a real southern boundary of the deformation stress field occumng during the period of activity of the HFF, zone. On the basis of our own observations, it seems that the particularly to the t 3 trend at the time of the dike emplacement. deformation diminishes more or less gradually to the south. The Jancin et al. [1995] pointed out that, in this case, the curvature Gil-Latur Line of Young et al. [1985] is defined as the line north of stress trajectories, as suggested by the trends of dikes near the of which the dike trends and the strikes of tilted lava flows show HFF, would have been incompatible with the right-lateral a noticeable clockwise curvature [dancin et al., 1995, Figures 1 motion but would have better fitted with a left-lateral motion. In and 2]. The northern area is peculiar in that a specific dike fact, the demonstration of Gudmundsson and Fj ider [1995], swarm, parallel to the transform zone, is present [Gudmundsson based on boundary element modeling, supports their and Fj'ader, 1995]. It is indisputable that north of the Gil-Latur interpretation provided that the right-lateral transform motion Line both the trajectories of lava bed strikes and the trends of was concentrated west of Flateyjarskagi peninsula. Aware of this major faults and dikes show a clockwise change. This does not requirement, Gudmundsson and Fj ider [1995] mentioned the imply that this line is a tectonic line, but at least it represents the Hrisey dike swarm, west of the peninsula, as a main site of southern boundary of the domain of large deformation related to opening south of the transform zone. the HFF. 2. dancin et al. [1995, p. 1628] claim that Fj ider et al. [ We propose that fight-lateral shear deformation and associated clockwise rotation effectively took place in the 1994] "have not sufficient areal distribution to their dike northern part of the peninsula but with a magnitude smaller than observations to have gleaned (or tested) the progressive pattern that proposed by Young et al. [1985]. According to our shown in Young et al. [1985]." It is true that Gudmundsson and Fj'eider [1995] had no observation in the area between their interpretation, the bending of the dikes may reach an angle of- 60 ø instead of the 110 ø that they propose. The main reason for "deformation zone" and the Gil-Latur Line and that the work of this difference of- 50 ø in the amount of inferred clockwise Young et al. [1985] document a very dense network of dikes in the whole peninsula (Figure 10a); however, Gudmundsson and Fjader [1995] carried out very detailed analysis on the dike profiles that they measured, and they reliably showed that there are different dike swarms in this area (the dike swarms of Grenivik, of Flatey, and of Hrisey). They pointed out that Young et al. [1985] had not distinguished these different swarms and questioned the calculation of average dike strikes. The dike map of Young et al. [1985] is the only map that reveals dike orientations between the Gil-Latur Line and the intense crustal deformation zone (ICDZ of Figure 2). It shows that north of the Gil-Latur Line, two main dike trends coexist (Figure 10a): NE- SW on one hand, and E-W to WNW-ESE on the other hand. On the basis of azimuthal analysis we have distinguished these two sets (Figures 10c and 10d). The set of dikes trending E-W to WNW-ESE, restricted to the northern part of the peninsula, strikes more or less parallel to the HFF (Figure 10d). Because most dikes in Iceland are pure extension fractures (at right angle to t 3), this set of dikes presumably developed under N-S to rotation lies in our interpretation of the dike pattern in terms of two swarms (Figures 10c and 10d), rather than the single set (Figure 10a) considered by Young et al. [ 1985]. Moreover, we consider that this 60 ø angle of clockwise rotation represents a maximum value because the ENE-WSW trending dikes in the northern peninsula (Figure 10c) may be interpreted in two ways: as dikes of the N-S set (and hence consistent with the S3-N3 regimes) rotated by 60 ø clockwise or as dikes of the NE-SW set (consistent with the S 1-N 1 regimes) that underwent a rotation of - 20 ø clockwise. The latter hypothesis finds some support in the change of the major fault trends, which averages 20 ø clockwise from south to north (Figure 10b). It is also supported by the moderate clockwise change in the calculated trends of extension for the S3-N3 regimes (Figure 5 and Table 1). However, no definite answer can be given with the data presently available because the miscellany of dike trends in the northern peninsula may also partly result from injections that occurred before, during and al er the inferred rotation. In addition, there is little geological evidence for rigid crustal NNE-SSW extension. This can be consistent with the N4 and/or blocks bounded by faults as in the model of Young et al. [1985] N2 stress regimes inferred from fault analysis. The other set (Figure 10c) seems to involve dikes in the whole peninsula. It has a roughly N-S trend south of the Gil-Latur Line and changes gradually to NE-SW near the northern coast. It may correspond because of the lack of large left-lateral faults (although this lack may be only apparent, resulting from sparse geological observations inside the peninsula). At the present time, north striking left-lateral strike-slip seismic faults exist [Rognvaldsson

14 1 174 BERGERAT ET AL.: TJORNES OCEANIC TRANSFORM ZONE (ICELAND) et al., 1998] south and north of the HFF (along the Dalvik and Grimsey Lines, respectively; see Figure 1). Nevertheless, the shear deformation has probably been partly accommodated by some faults but essentially as a diffuse shear process in a zone of km wide south of the HFF. This deformation took place at least in the first stage of the transform process before the HFF acted as a real transform fault Interpretations of the Various Tectonic Regimes The interpretation of the complex tectonic regime along the HFF may be done in terms of variable coupling [Angelier et al., 2000]. This interpretation is favored for several general reasons. First, such a phenomenon is not exceptional and has already been described and documented along major faults [e.g., Mount and Suppe, 1992; Zoback et al., 1987; Zoback, 1991] where maximum and minimum stress axes trend parallel and perpendicular, rather than oblique, to the fault trend. In the case of the TjOrnes Fracture Zone, the angle between the c 3 axis of the main tectonic regimes S4-N4 and the transform directions is 80o-90 ø on average (Figure 9). This situation resembles that described along the Dead Sea transform [Garfunkel, 1981] or the Kane transform fault [Wilcock et al., 1990]. Second, consideration of the fluid pressure in the brittle crust, especially in major fault zones, is known to play a prominent role that provides a consistent mechanical explanation in terms of effective normal stress versus shear stress and hence in terms of friction. The relationship between fluid overpressure and stress drop has been pointed out in the case of the Husavik-Flatey Fault Zone [Gudmundsson, 1999 and 2000]. In contrast, all other hypotheses may account for some particular aspects of the paleostresses reconstructed in the TjOrnes Fracture Zone but fail to explain the complexity of the whole distribution. For instance, the normal-type faulting extensions cannot be explained as the result of near-surface gravitational phenomena because they are intimately related to strike-slip faulting, dike injection and earthquakes at various depths in the shallow crust (according to focal mechanisms [Rognvaldsson et al., 1998]). Third, not only do the main regimes S4-N4 indicated by fault slip analyses reveal extension across the transform zone, but the presence of numerous transform-parallel dikes (Figure 10d) confirms that this transform-perpendicular dilation, which contradicts the conventional model of shear stress relationships, cannot be neglected at the crustal scale. As Figure 9 shows, the minimum horizontal stress C } mm of the S4-N4 regimes is nearly perpendicular to the transform fault trend, whereas the maximum horizontal stress c } m x is nearly parallel to this trend, so that the normal stress acting on the fault zone is small, the friction is low, and the fault is extremely susceptible to shear (and still more in the presence of fluid pressure). This behavior of the transform zone indicates very little mechanical coupling. In contrast, the angle cwtransform direction for the S3-N3 tectonic regimes is ø (Figure 9). When these regimes S3-N3 prevailed, the maximum and minimum stress axes were trending oblique to the main fault. The minimum horizontal stress m min of the S3-N3 regimes is oblique at an acute angle to the transform fault trend (in contrast with common situations involving an obtuse angle), whereas the maximum horizontal stress Cmm x is oblique at a large angle to this trend, which implies that in a dry situation the normal stress acting on the fault zone would be large, the friction would be high and the fault would be very resistanto shear. However, because the transform fault has moved under the S3-N3 regime, we infer that the presence of fluids (water and/or magma) resulted in a lower effective stress and hence made slip possible. We conclude that this behavior reflects moderate mechanical coupling, equivalent to limited but significant friction across the transform zone. Such variations in mechanical coupling in the vicinity of the HFF transform fault may explain the spectacular changes in nature and orientation of tectonic regimes, inducing strong S4-N4 Figure 11. Scheme of the main brittle deformation and associated stresses in the TjOrnes Fracture Zone after the Flateyjarskagi results (modified after Bergerat and Angelier [1999]).Solid lines, strike-slip faults (arrows indicate the sense of shear); thin double lines, normal faults and tension fracture; shaded arrows, directions of compression and extension (thin lines indicate the perturbation of 3 trajectories close to the HFF). The dark shaded fault line corresponds to the situation of low coupling (very weak fault, on the right) and the light shaded one indicates the case of moderate coupling (on the left).

15 BERGERAT ET AL.: TJ RNES OCEANIC TRANSFORM ZONE (ICELAND) 1175 perturbations in stress trajectories, even though the far-field normal faults, and dike swarms that trend parallel with the pattern remain stable (Figure 11). Evidences for intermediate fracture zone has been emphasized by Gudrnundsson [1993] and situations are few, suggesting that changes in coupling were interpreted by this author in terms of tensile stress field. The probably abrupt rather than progressive during the rifting- WNW-ESE graben-like structure that marks the I-[FF offshore transform history [Angelier et al., 2000]. between the peninsul and Flatey [McMaster et al., 1977] is also Garcia [1999] completed this study by analyzing earthquake compatible with such a stress field. For Gudrnundsson [1993, focal mechanisms of the years 1995, 1996, and This work 1995] this ridge-parallel tensile stress field appeareduring the demonstrated that the present stress regimes determined from expansion of the migrating mid-ocean ridge. Both the major the seismicity analysis are the same as those reconstructed from structures and the earthquake focal mechanisms characterized an fault slip data analysis. This confirms that the complexity of the extensional axis perpendicular to the trend of the transform, tectonic paleostress field may be interpreted terms of a single implying that this transform is under extension. Such tectonic tectonic phase (with variations of stresses in time and space) and structures and strain partitioning can be compared to those not in terms of numerous consecutive tectonic phases. Because occurring across the Kane transform fault, also considered as a of the obliquity of the transform zone versus the riffs zones fault mechanically weak relative to the surrounding lithosphere (Northern Icelandic Rift and Kolbeinsey), Garcia [1999] [I/Vilcock et al., 1990]. For I/Vilcock et al. [1990] the origin of suggests the existence of "accommodation" triangular zones weakness is not known, but potential sources of extension across south and north of the Tjfrnes Fracture Zone, where extension the transform can include thermal stress in the young oceanic occurs. In such a model the S4-N4 regime (extension lithosphere, topographic loading, and a small component of plate subperpendicular to the transform trend) is a response to the divergence normal to the transform. The fault zone may also be necessity of extension in the triangle. The S3-N3 regime weak because of the presence of abnormally high fluid (extension at- 25 ø of the transform trend) would be a pressures, as pointed out by Zoback et al. [1987] for the San transtensional motion accommodating both the right-lateral Andreas Fault. movement of the HFF and the extension within the triangle. 2. Most of the remaining set of dikes (the dikes that trend Finally, Garcia [1999] considers that the S1-N1 regime more or less parallel to the HFF being left apart), N-S in the (extension subparallel to the transform trend) may be due to the southern and central part of the peninsula, show a general locking of a part of the fault (such as that presently observed in curvature of - 45 ø clockwise in its northern part (Figure 10c). the eastern part of the I-[FF since the last rifting crisis of the This curvature seems inconsistent with a stress reorientation Krafla [e.g., Rognvaldsson et al., 1998]). during the right-lateral transform motion. For this reason the These two interpretations are mechanically similar, both clockwise curvature rather results, as suggested by Young et al. being based on the important role of the S4-N4 and S3-N3 stress [1985], from a right-lateral simple shear occurring a narrow regimes in the kinematic behavior of the transform zone and zone (- 10 km) south of the HFF. However, this shear is implying variations of friction along the HFF. The two other probably accommodated by numerousmall structures rather stress regimes (S1-N1 and S2-N2) are more enigmatic; they may than by large left-lateral faults that could not be clearly be related to several phenomena such as elastic rebound, fault identified. This conclusion is supported by the observation of block accommodation, and magma injection [Angelier et al., numerous crushed zones along the northern coast. 2000], partial locking of the transform fault [Garcia, 1999], or 3. Nevertheless, examining in more detail this northern area hydrofracturing [Gudrnundsson, 1999]. of the peninsula (Figure 10a), one observes that some dikes do not belong to the transform-parallel set and do not follow the 5. Discussion and Conclusion curvature mentioned above. Instead, these dikes trend NW-SE and thus are mechanically compatible with the right-lateral Our analysis of fault slip data, combined with a critical motion. Such dikes as well as some large normal faults with the review of previous interpretations of other features such as same NW-SE trend have been also reported by Fjtider et al. major faults, lava bedding, and dikes, allows us to highlight [1994] and by Langbacka and Gudrnundsson [1995] and some major aspects of the mechanism of the TjOrnes Transform Gudrnundsson [1995] in the northermnost Tr611askagi peninsula Zone. We point out that an important factor is the superposition (Figure 1). During our field study we also measured NW-SE of structural patterns due to differentectonic processes. Taking trending minor normal faults (Figure 3b). All these NW-SE into consideration both the different stress regimes characterized trending structures may belong to the main N-S trending in our faulting analysis and the curved features described by structural grain and correspond to a counterclockwise rotation of Young et al. [1985] and Fjtider et al. [1994] allows us to the stress trajectories associated to the fight-lateral motion of the conclude that the complexity of this area cannot be explained by HFF, according to numerical modeling (as documented a single process. elsewhere, for example, by Hornberg et al. [1997]). Concerning the dikes, three different sets can be Therefore the complex pattern of dikes and faults in the distinguished and linked to some of the characterized stress northern part of Flateyjarskagi can be explained by the regimes: superposition of several processes: (1) a transform-perpendicular 1. A distinct set of dikes parallel to the I-I F [Fjtider et al, extension (E-W to ESE-WNW dike and fault trends), (2) a 1994; Crudrnundsson and Fjtider, 1995] was not recognized by simple shear (NNE-SSW to NE-SW dike and fault trends), and Young et al. [1985] as a separate set, although it clearly appears (3) a stress perturbation due to the transform motion (NW-SE in their map of the dikes in the Flateyjarskagi peninsula (Figure dike and fault trends). In this paper, we principally highlighted 10d). The occurrence of these dikes is in good agreement with the coexistence of different brittle processes in the Tjfrnes the main stress regime S4-N4 determined from fault slip data Fracture Zone and its relation to variations in the mechanical analysis (Figures 3b and 5). The occurrence of tension fractures, behavior of the transform zone. One should keep in mind,

16 1176 BERGERAT ET AL.' TJORNES OCEANIC TRANSFORM ZONE (ICELAND) however, that because the transform process began- 9 My ago Various works may bring to light some answers conceming and evolved from the stage of overlapping spreading centers to the respective roles of the different processes. One is a that of a transform zone [Bergerat and Angelier, 1999], some paleomagnetic study. Preliminary work has been carried out by evolution of the brittle tectonics with time is likely to have Orkan et al. [1984], who claimed that the paleomagnetic study occurre duhng this period. supported the hypothesis of a simple shear, but this work has not Our brittle tectonic analysis inversion of fault slip data in the been developed subsequently. In fact, there are considerable Flateyjarskagi peninsula shows that the existence of numerous difficulties in using paleomagnetic directions to infer tectonic tectonic regimes by no means implies that numerous distinct rotations in Iceland, first, because of the steep inclination of the phases have occurred. The analysis of focal mechanisms of average field (- 77 ø) and, second, because the between-lava earthquakes indicates that all these regimes currently occur, scatter in the primary remanence directions is very large, with an invalidating as well the hypothesis of polyphase tectonism. It angular standar deviation of- 23 ø in the last 15 My (L. suggests that transfor motion along an oceanic fault zone may Kristjansson, personal communication, 1999). It seems that only induce a variety of tectonic regimes. Therefore a simple very detailed studies on hundreds of lava flows could bring kinematics may be associated with several tectonic regimes reliable information. alternating time. The occurrence of reversed tectonic regimes Another necessary aspect, which is being addressed now, (i.e., sense opposite to transform motion) may be related to involves radiometric dating of the dikes. Only very few dikes stress drop, fault block accommodation, elastic rebound, and have been dated [Jancin et al., 1985] in the Flateyjarskagi magma injection. A major point of the transform mechanism peninsula. The dating of dikes belonging to different-trending the variation of coupling along the I-I F. The obliquity between groups may certainly help to establish the age of their the direction of transform motion and the trend of extension for emplacement and a chronology of the different magmatic the two main regimes (S3-N3 and S4-N4) may vary in the range injections. 20o-90 ø (Figure 11), reflecting repeated changes of the Because some sites show most of the different stress states, coefficient of friction along the I-[FF [Angelier et al., 2000]. we inferred that all these stres states exist regionally. However, These changes may correspond, at least partly, to the necessity even for the best documented stres states (e.g. the S3-N3 stress of extension in this area because of the obliquity of the state, Figure 4), it is still difficult to say if the stress trajectories transform zone versus the rifts zones of north Iceland and of for a given stress state rotate significantly coming close to the Kolbeinsey, but the occurrence of magmatism at depth probably HFF. This difficulty originates in the heterogenenous geographic also plays an important role in such a phenomenon. distribution of the sites, with a concentration of sites along the In spite of the complexity of the mechanisms having occurred northern coast of the peninsul and a nearly empty area just during the transform motion the contradictions between the south of it (Figure 2). A target for future research is to increase relative chronologies observed in the field as well as the variety the denseness of the network of sites in order to be able to draw of focal mechanisms of earthquakes at the present time precise stress trajectories. demonstrate that there is probably no definite succession of the different processes in time. The originality of the I-[FF versus other weak transform faults (such as San Andreas or Kane) lies in alternating periods of very low mechanical coupling (weak Acknowledgments. Financial support was provided by the European Commission (contracts ENV4-CT and ENV4-CT ), by the fault) and of moderate friction on the same transform. Moreover, IFRTP (Arctic Program 316), and by the French-Icelandic scientific-cultural this alternation may occur very rapidly since it takes place collaboration program (Iceland Ministry of Education and Culture and several times in a few years, as shown by focal mechanisms of French Minist re des Affaires Etrang res). We thank the French Embassy in Iceland for its help. Careful reviews by J. Townend and C. Baslie allowed us earthquakes analysis for the period. to greatly improve this paper. References Angeher, J, Tectonic analysis of fault slip data sets, Geophys. 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