JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B5, 2103, /2001JB000150, 2002

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B5, 2103, /2001JB000150, 2002 Strike-slip faulting, normal faulting, and lateral dike injections along a single fault: Field example of the Gljúfurá fault near a Tertiary oblique rift-transform zone, Borgarfjör*ur, west Iceland Maryam Khodayar 1 and Páll Einarsson Science Institute, University of Iceland, Reykjavík, Iceland Received 1 May 2000; revised 14 May 2001; accepted 19 May 2001; published 31 May [1] The crustal block of Borgarfjör*ur in western Iceland was caught between the propagating Reykjanes-Langjökull Rift Zone (RLRZ) and the receding Snæfellsnes Rift Zone (SRZ) during a Tertiary rift jump at 7 Ma. This Tertiary rift jump configuration presents an analogy with the presently unstable plate boundary in south and southwest Iceland. Field examination of the Gljúfurá fault in the Tertiary lavas of Borgarfjör*ur reveals a complicated history of movements and magmatic activity. During its evolution since Tertiary time, this N-S fault acted as follows: (1) The fault acted as a possible dextral strike-slip fault, as judged from Riedel fractures in the oldest associated fault breccia. (2) Then the fault acted as a normal fault, resulting in vertical displacement, new brecciation of part of the oldest breccia, hydrothermal activity, and mineralization dominantly in N-S mode I veins. (3) The fault acted again as a dextral strike-slip fault, when the fault was injected laterally by dikes. Magma was injected mainly into the N-S strike-slip fault and secondarily into adjacent NNE normal faults. Evidence of this stage are the en échelon geometry of the northerly dikes, cooling cracks oblique to dike edges, flow lineation, elongated vesicles, and soft striations on dike edges, as well as bending and displacement of preexisting dikes. (4) After the emplacement and partial erosion of the northerly dikes, the fault acted dominantly as a strike-slip fault, with possible reactivation as an open fissure. The Gljúfurá fault strikes obliquely to the NNE trending rifting structures active during crustal formation in this area. Its activity was initially related to a transform zone or an oblique rift connected to the SRZ. Then a shift in the plate boundary occurred around 7 6 Ma when the RLRZ became active. During this shift the Gljúfurá fault probably played a role similar to the N-S strike-slip faults of the presently active plate boundary within the South Iceland Seismic Zone and the oblique rift of the Reykjanes Peninsula. These faults may, similar to the Gljúfurá fault, change mode and be injected by dikes if they are sufficiently close to a magma source. INDEX TERMS: 8010 Structural Geology: Fractures and faults; 8150 Tectonophysics: Plate boundary general (3040); 3035 Marine Geology and Geophysics: Midocean ridge processes; 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: Iceland, plate boundary processes, dike injections, rift jump 1. Introduction 1 Also at National Energy Authority of Iceland (Orkustofnun), Reykjavík, Iceland. Copyright 2002 by the American Geophysical Union /02/2001JB000150$09.00 [2] Faulting at oblique oceanic plate boundaries has received increased attention in recent years. Crustal deformation at oblique plate boundaries tends to be distributed on many faults, each one taking up only a part of the total plate movement. In addition, these boundaries tend to be unstable, migrating with time or even jumping. Individual faults therefore never accumulate much finite displacement, and their topographic expression tends to be subdued. Detection on the ocean floor by acoustic techniques is difficult. [3] The interaction between the Icelandic mantle plume and the mid-atlantic plate boundary has led to a series of oblique rifts and unstable transform zones [Einarsson, 1991]. The Reykjanes Ridge becomes progressively more oblique as it approaches Iceland. On land, on the Reykjanes Peninsula (RP), the boundary is at an angle of to the spreading direction (Figure 1a). During magmatic episodes, faulting takes place here on fissure swarms or spreading segments that are at an angle of 40 to the boundary. Between magmatic episodes the deformation appears to take place by strike-slip faulting along faults striking at high angles to the boundary [Erlendsson and Einarsson, 1996; Hreinsdóttir et al., 2001]. Farther east along the boundary, within the South Iceland Seismic Zone (SISZ), the rifting structures disappear, and all the transform deformation seems to be accomplished on N-S faults by bookshelf faulting, i.e., right-lateral faulting and rotation of the fault blocks [Sigmundsson et al., 1995]. [4] Similarly, north of Iceland, highly oblique rift zones or transforms are formed where the Kolbeinsey Ridge merges into the Tjörnes Fracture Zone (TFZ) to join the main rift zone of north Iceland. Also, here active faulting takes place along faults striking at a high angle to the zone as shown by fault plane solutions and relative locations of earthquakes [Rögnvaldasson et al., 1998]. [5] In order to address the issue of high-angle faults and their role in the plate boundary deformation we have selected a fault, the Gljúfurá fault, in the eroded Tertiary lava succession of Borgarfjör*ur in west Iceland (Figure 1b). In this paper, we focus only on the qualitative analysis of the Gljúfurá fault, and we show how this fault has evolved with time and in the context of unstable Tertiary ETG 5-1

2 ETG 5-2 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY Figure 1. (a) Simplified tectonic map of Iceland modified from Sæmundsson [1986]. The location of the Snæfellsnes Rift Zone is from Jóhannesson [1980]. SVZ, Snæfellsnes Volcanic Zone; SRZ, Snæfellsnes Rift Zone; RLRZ, Reykjanes-Langjökull Rift Zone; ERZ, Eastern Rift Zone; RP, Reykjanes Peninsula; SISZ, South Iceland Seismic Zone; TFZ, Tjörnes Fracture Zone. The box indicates the area of Borgarfjör*ur covered by Figure 1b. (b) Geological map of western Iceland, modified from Jóhannesson [1994]. Central volcanoes: L, Laugardalur; R, Reykjadalur; H, Hallarmúli; HS, Hafnarfjall-Skar*shei*i. Legend for Figures 1a and 1b: 1, Quaternary Snæfellsnes Volcanic Zone; 2, Tertiary and Pleistocene formations; 3, acidic rocks of Hallarmúli dated 6.7 Ma (Snæfellsnes Rift Zone); 4, acidic rocks and central volcanoes of the Reykjanes-Langjökull Rift Zone; 5, unconformity; 6, Borgarnes anticline; 7, normal faults; 8, dikes; 9, active rift zones (Pleistocene to recent); 10, active faults in the SIZS, RP, and TFZ; 11, coast lines.

3 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY ETG 5-3 plate boundaries. We use structural analysis of faults, fault breccia, dikes, flow lineations, striae, mineral veins, and fracture crosscutting relationships to reconstruct the history of the Gljúfurá fault. We show that the fault changed from a strike-slip fault to a normal fault, then again to a strike-slip fault and was repeatedly injected by dikes. [6] The Gljúfurá fault has a N-S strike and is located south of 65 N, where Icelandic fissure swarms and normal extensional structures have a NNE trend [Sæmundsson, 1978]. It therefore had a similar geometric relationship to the rifting structures of its time as the presently active N-S faults on the RP and in the SISZ [Einarsson and Eiríksson, 1982; Erlendsson and Einarsson, 1996] have to the present rifting structures. Both our exhumed fault from the Tertiary of western Iceland and these presently active faults are N-S trending segmented fractures acting dominantly as dextral strike-slip faults, with comparable length and relatively small magnitude of fault displacement. We thus argue that the Gljúfurá fault is an analogue structure to these presently active faults at the plate boundary. The Gljúfurá fault is related to the shift of activity from the old Snæfellsnes Rift Zone to the presently active Reykjanes-Langjökull Rift Zone. During this transfer, which took place around 7 Ma [Jóhannesson, 1975, 1980; Kristjánsson and Jónsson, 1998], the area was active as a transform zone and resembled the presently active SISZ and the oblique rift of the RP. We argue that active N-S striking dextral faults in the SISZ and the RP may be taken as the closest present-day analogies of the Gljúfurá fault. Our study of this analogue exhumed fault sheds light on the mechanism of the migratory plate boundary and is a contribution to the better understanding of the complex mantle plume/ridge system in Iceland. 2. Regional Setting [7] The Tertiary and Quaternary crust is heavily fractured in western Iceland and shows a complicated tectonic pattern at different erosional levels (Figure 1b). These fractures have gradually been formed since western Iceland began to form at the plate boundary at 15 Ma at least [Jóhannesson, 1980]. The main steps of this history are the following: An early rift zone (Snæfellsnes Rift Zone, SRZ) was active from 15 to 6 Ma. It has been speculated that a transverse fracture zone around the latitude of 65 N was active during Tertiary rifting [Sigur* sson, 1967; Jóhannesson, 1975; Jancin et al., 1985]. Around 7 6 Ma, active rifting jumped eastward from the SRZ to the Reykjanes-Langjökull Rift Zone (RLRZ). It then propagated southwestward [Jóhannesson, 1980] and has been active from 6 Ma to the present [McDougall et al., 1977]. Two overlapping rift segments were active, one propagating (RLRZ) and the other (SRZ) receding [Kristjánsson and Jónsson, 1998]. Beginning at 6 Ma, the area west of the RLRZ evolved as an intraplate block flanking the new rift zone. Finally, a part of this block became an active transverse volcanic zone (Snæfellsnes Volcanic Zone, SVZ) during the past 2 Myr [Sigur* sson, 1970; Sæmundsson, 1978]. According to Sigur*sson [1970] and Sæmundsson [1978] the SVZ was generated by the dextral shear along a WNW transcurrent fracture. Intraplate earthquakes up to magnitude 6 occur in the easternmost part of western Iceland and indicate that the area is still tectonically active [Einarsson et al., 1977]. Fault plane solutions, however, show normal faulting, indicating crustal extension at the present time. The volcanic products of the SVZ are of alkali type [Jakobsson, 1972], and the youngest products are erupted both at the western and the eastern end of the chain. In spite of the highest activity being in the west, however, the SVZ has recently been interpreted as a volcanic zone propagating southeastward and approaching the RLRZ [Gu* mundsson, 1996]. [8] The main Tertiary tectonic feature of western Iceland is the Snæfellsnes syncline, which is the surface expression of the SRZ, and has a kink shape in map view. In the west it trends NE [Sæmundsson, 1967; Sigur*sson, 1967], then E-W, and finally N-S [Jóhannesson, 1975, 1980] (Figure 1a). This type of syncline results from the tilt of the lava toward the active rift zones due to loading and extension [Walker, 1963; Sæmundsson, 1967; Pálmason, 1973; Grellet, 1983]. The Borgarnes anticline is the other NE trending extensional structure produced by the tilting of the lava, first toward the SRZ to the NW, then from the local retilting of the lava toward the RLRZ to the SE [Jóhannesson, 1975]. The dip of the lava pile ranges regionally from 2 to 14 [Jóhannesson, 1994]. [9] The bulk of the lava pile consists of tholeitic flows with scoria and interbedded red soil or dust layers, containing secondary minerals such as zeolites, quartz, chalcedony, and calcite [Jóhannesson, 1975]. A period of erosion separates the lava of the two rift zones with an accumulation of reworked sedimentary horizons mapped as an unconformity [Jóhannesson, 1975, 1994; Franzson, 1978]. Acidic rocks are confined to the main central volcanoes of the rift zones, namely, Hallarmúli (6.7 Ma), Laugardalur (age obscure), Reykjadalur (5.8 Ma) [Jóhannesson, 1975], and Hafnarfjall-Skar*shei*i (6 4 Ma) [Franzson, 1978]. Except for Hallarmúli, these central volcanoes have developed a collapse caldera. K/Ar dating suggests ages of 13 ± 2 Ma [Moorbath et al., 1968] and 9.4 ± 0.7 Ma [Aronson and Sæmundsson, 1975] for the oldest rocks of western Iceland. These rocks are found in the crestal area of the Borgarnes anticline structure, below the unconformity. [10] The general orientation of the fissure swarms in Iceland changes near 65 N (Figure 1b). North of this latitude, the swarms trend N-S, but to the south they trend NNE [Sæmundsson, 1978]. The Gljúfurá fault is located south of 65 N, in the area of NNE trending structures, but it has a N-S orientation (Figures 1a and 1b). Its relation to the surrounding tectonic elements is, however, rather complex. The fault and dike swarms of Laugardalur central volcano are not clearly expressed at the surface on either side of the volcano, but those of Hafnarfjall-Skar*shei*i have a dominant NE trend. By contrast, Reykjadalur has a well-developed swarm trending N-S north of the volcano and NNW south of the volcano. Thus the Gljúfurá fault, which crops out midway between Laugardalur and Hafnarfjall-Skar*shei*i, appears as one of the few N-S faults below the unconformity, without an apparent relation to a particular fissure swarm (Figure 1b). [11] The Gljúfurá fault is also a part of a complicated tectonic pattern where NNE and WNW faults are common (Figure 1b), and where normal and strike-slip motions were recognized along the N-S, NNE, and WNW faults [Schäfer, 1972; Jóhannesson, 1975, 1980; Bergerat et al., 1990; Passerini et al., 1997; Khodayar, 1999]. However, a general agreement has not been reached, neither on the fault types and the sense of fault motions nor on the significance of this pattern. The Gljúfurá fault itself has been mapped by Jóhannesson [1994] as a single westward dipping normal fault injected by a N-S dike (Figure 1b). On the other hand, strike-slip faulting along the above three fault trends were interpreted either as Riedel fractures formed during the dextral shear along the Quaternary SVZ [Schäfer, 1972], or treated separately from normal faults, and seen as result of compression parallel or perpendicular to the rift zones [Bergerat et al., 1990; Passerini et al., 1997] which lasted until late Quaternary [Bergerat et al., 1990]. Recent analysis of a large amount of data on tectonic pattern and its chronological evolution [Khodayar, 1999] supports Jóhannesson s [1980] suggestions that this fracture pattern dates from Tertiary time. Additionally, the results indicate that dikes and strikeslip and normal-slip faults of the three fracture trends alternated through time [Khodayar, 1999]. In this paper, we only focus on the qualitative analysis and significance of the Gljúfurá fault. 3. Field Observations [12] The Gljúfurá fault and the immediately adjacent structures were mapped in detail. A total of 243 measurements were made on various features of faults (14), individual dikes (31), mineral veins

4 ETG 5-4 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY Figure 2. Aerial photograph showing the NNE rifting structures cut by the northerly trend of the Gljúfurá fault (open arrow). Published with the permission of the National Land Survey of Iceland (# landmælingar Íslands, permission L ). (168), and striated planes (30), along the river and the 30-m-high wall rock on both sides of the gully. Two types of fault breccia were observed along the length of the structure, and these are analyzed with respect to the evolution of deformation here. Lateral propagation of dikes was determined from the plunge of flow lineations on dikes edges. [13] The northerly trend of Gljúfurá cuts the old Tertiary basement, which is dominated here mainly by NNE fractures (Figure 2). The Gljúfurá fault is analyzed over 2.3 km length in the field, of which only 1.8 km is continuously well exposed. It contains at least four N-S en échelon dike segments A, B, C, and D (Figure 3a), all injected into preexisting mineralized fault breccia (Figures 4a, 4c, 4d, and 4e). To the south the Gljúfurá fault disappears beneath Holocene fluvial sediments and to the north under fluvial and marine clay sediments with a minimum thickness of 6 m. The total length of the fault is unknown. It is, however, likely that it exceeds 2.3 km and that the number of en échelon segments is higher than four, especially toward the north. [14] The N-S fault in Gljúfurá shows evidence of an early strike-slip faulting without magma injection, normal faulting and hydrothermal activity, and strike-slip faulting with magma involvement. Strike-slip faulting persisted after dike emplacement. Detailed structural analysis, mapping, and determination of the chronological relation between fractures suggest an evolution for deformation at Gljúfurá. [15] The outcrop presents tectonic striae (both on dike edges and in the host rock) postdating dike emplacement, as well as a few soft striations and flow lineations on dike edges. These latter structures can be distinguished by their features. Flow lineations often occupy a larger surface on the observed dike edges. The lineations are dense, flat, and impregnated on dike edges. They lack, however, kinematic indicators to determine the sense of slip. Soft striations on dike edges are less frequent. The striae are short, have larger relief, and present kinematic indicators that allow slip determination. The determination of the sense of motion in our case is checked with other field evidence examined below General Aspects [16] Measured faults along the Gljúfurá trend mostly NNE, whereas dikes, mineral veins, and striated planes strike dominantly

5 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY ETG 5-5 Figure 3. (a) Simplified map of the Gljúfurá fault and the adjacent structures. The thickness of the dikes and the fault breccia are exaggerated on the map. Note that not all the reported 31 measured injections are reported because of the scale of the map. Legend: 1, dikes of type a; 2, dikes of type b 1 ; 3, dikes of type b 2 ; 4, dikes of type c; 5, dikes of type d; 6, Holocene marine and fluvial sediments; 7, Holocene fluvial sediments; 8, fault breccia 2; 9, basaltic lava pile dated 13 ± 2 Ma; 10, normal faults; 11 and 12, dextral and sinistral strike-slip faults, respectively; 13, strike and dip value of the lava; 14, location of Figure 4c. (b) Rose diagrams of the measured fractures (the interval on the rose diagrams is 10 ). N175 E and secondarily NNE, NE, and WNW (Figure 3b). The general aspects of the measured faults, fault breccia, mineral veins, dikes, and striated planes are described below Faults. [17] The faults mapped are mostly normal slip, though some are strike slip. Reverse faults were not found in Gljúfurá. The N-S faults have normal and dextral slip. The major ones coincide with planes of the N-S dikes, but a few are in the breccia zone (normal slip) and in the host rocks (dextral strike slip). Other mapped faults trend NNE to NE (dip slip) or WNW (sinistral strike slip). The magnitude of vertical (throw) and horizontal (offset) displacements positively identified ranges from 0.32 to 30 m for 12 normal faults and from 0.30 to 13.5 m for three strike-slip faults. The maximum values are obtained along the N-S trend in segment A. Faults are within 10 of vertical, and strike-slip faults dip as steeply as normal faults. A breccia zone is associated with the N-S faults and also with two NE and NNE normal faults. The total length and the full throw of the faults are unknown due to lack of exposure Fault breccia. [18] Fault breccia are of two distinguishable types of different significance. Breccia type 1 is the oldest one; it is well exposed at the bottom of the gully on both sides of the river mainly to the north and seems to extend horizontally beyond the limit of the actual outcrop. The matrix is a basalt or crushed scoria. Organized fracturing gives angular rock fragments that range in size from 50 to 60 cm down to millimeters (Figures 4a and 4b). While the trend of the fissure swarm of the SRZ is dominantly N30 E (Figures 2 and 3a), the breccia presents Riedel shears trending dominantly N10 20 E,

6 ETG 5-6 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY Figure 4. (a) A dike of segment C injected into breccia 2 and cutting breccia 1. Note the shear fractures in breccia 1, which is partly ground and included in fault breccia 2. (b) Detail of breccia 1 and the shear fractures (thin arrows). (c) Photograph and sketch of the northerly dikes of segment A, injected into altered and mineralized fault gouge of breccia 2. Part of this breccia (left side of the photograph) is slightly tilted toward the center of the segment. Note the absence of breccia in the host rock in contact with the dike to the right. (d) Variety of breccia 2 with angular rock fragments (type c) oriented NNE and a few rotated elements (black arrow) in a red clay matrix injected by secondary minerals (stars). (e) Fault breccia 2 cut by network of mineral veins. The dike to the right is neither brecciated nor intruded by mineral veins.

7 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY ETG 5-7 Table 1. Lithology of Dike Segments Type a Type b 1 Type b 2 Type c Type d Dike lithology very fine to fine-grained, with or without small phenocrysts (feldspar and pyroxen); few millimeter-scale vesicles fine-grained with higher percentage of small phenocrysts (feldspar); rare vesicles locally up to 4 cm in the dike edge massive fine-grained with an even higher percentage of phenocrysts (feldspar), without vesicles coarse-grained with abundant and locally orientated phenocrysts; abundant vesicles distributed throughout the whole dike coarse-grained porphyritic (feldspar) to sparse microporphyritic, with locally small vesicles near the dike edge Segments A, B, C, D A A A A secondarily N45 50 E, and less commonly N65 75 E and N150 E (Figure 4b). The long axes of rock fragments smaller than cm long are oriented parallel to these fractures. The features of breccia 1 show a striking analogy with Riedel structures and cataclastics described in association with transform faulting in northern Iceland [Young et al., 1985; K. Sæmundsson, personal communication, 1998]. Fracturing in breccia 1 may be interpreted in terms of Riedel shear [e.g., Twiss and Moores, 1992], respectively as (R), (T) without much mineralization, (R 0 ) and (P) fractures, where (R) and (T) are the most pronounced shear. These shear fractures are consistent with the northerly trend of Gljúfurá. They suggest an early strike-slip motion along this fault, possibly dextral, without, however, evidence of magma involvement at this stage. Similar breccia, with at least one or two of the shear fractures developed, was found occasionally in the old lavas of the SRZ, and particularly some 8 km north of Gljúfurá and associated with a N-S structure. Although its history cannot be traced with accuracy, this oldest activity of Gljúfurá predates the sequence of normal faulting, strike-slip faulting, and dike injections detailed below. [19] Breccia 2 stretches along the length of the Gljúfurá fault (Figure 3a), grinding part of breccia 1 (Figure 4a). The en échelon dikes are injected into breccia 2. The thickness of this breccia ranges from 4.5 to 16.5 m, the maximum being in segment A (Figure 4c). Riedel fractures are absent, and joints and mineral veins trend dominantly N-S in breccia 2. Breccia 2 presents three varieties: (1) Basaltic fragments are angular without external matrix, ranging from 4 m down to millimeters (fault gouge) in segment A (Figure 4c). (2) Basaltic fragments are subangular and without external matrix in segments B and C. These fragments range from 2 mm up to 35 cm; in segment B, 5 10% of rock fragments are sparse in a 90 95% fine-grained gray matrix corresponding to a crushed vesicular basalt with few mineral veins. (3) Basaltic fragments are angular, ranging from a few millimeters up to 4 5 cm wide and cm long in a red clay matrix. Rock fragments are not in contact, and a few are rotated (Figure 4d). [20] Breccia 2 is younger than breccia 1, grinding part of the latter. It is therefore younger than the early strike-slip motion along the Gljúfurá fault and results from the normal-slip of this fault. However, subangular breccia and fault gouge in breccia 2 indicate that the initial angular fault breccia 2 has been reworked later by fault movements along the N-S trend. [21] Part of breccia 2 in segment A is in turn reworked. A meter-scale block of this preexisting mineralized breccia is detached from the western edge of the breccia zone and slightly tilted toward the center of the segment. This implies that segment A has been reactivated as an open fissure some time after the sequence of normal faulting and magma injection. [22] The outcrop also contains a third type of breccia that consists of boulders of basalt (25 30 cm) and basaltic angular fragments (few millimeters up to a few centimeters), cemented in a hard yellow matrix of sedimentary type. This material crops out locally along a N170 E trend on the western side of segment B and is cut by few northerly joints and by rare thin and short mineral veins. The breccia has a morainic origin and covers both the eroded lava and breccia 2. Its age is unknown. It may have accumulated in a fissure and thus indicates that segment B also has been reactivated as an open fissure Secondary minerals and mineral veins. [23] Quartz, calcite, and zeolites (analcime, stilbite, chabasite, scolecite, and mesolite) are the chief constituents of amygdules in lava and scoria here. Paleoburial depth is inferred from mesolite/scolecite boundaries and stilbite, which are found in the uppermost 1 km in the crust [Kristmannsdóttir and Tómasson, 1978]. According to Walker s [1960] vertical zeolite zonation, scolecite and stilbite mineralize in tholeitic lavas from around 1.4 km below the original top of the lava pile. The initial depth of the Gljúfurá s lava before erosion is therefore likely to be km. An intensive network of mineral veins (Figures 4c, 4d, and 4e) intrudes breccia 2 and the host rock. A total of 168 veins were measured, mainly in mode I fractures, and trend dominantly N-S (Figure 3b). Their thickness ranges from 0.1 to 11 cm, and the thickest vein strikes N10 E. They are infilled by stilbite, quartz, and calcite, but a relation between type of infilling versus vein trends is not observed. Only 4% of measurements were from the dike edges, infilled by millimeter veins of stilbite. [24] Mineral deposits were found in one to a maximum four rows, indicating several episodes of mineralization. They are partly well-crystallized perpendicular to the fracture walls. About 89% of the measured veins are in mode I fractures, dominantly N-S trending, consistent with a minimum compressive stress (s 3 ) acting E-W. The growth of crystals and deposition in rows implies that during episodes of mineralization most of these cracks remained open under pure extension. Only 11% of measurements show dextral and sinistral short en échelon cracks or dominoes. Their widely different trends suggest two sets of stress fields, where the minimum (s 3 ) and the maximum (s 1 ) compressive stresses trend NE and NW, respectively, in one set and ENE to E-W and NNW to N-S in the other set. The chronological relationship between these subordinate sets and with the dominant E-W extension cannot be established. However, as dikes (except three, cut by short thin veins) are not intruded by the intensive network of mineral veins, we conclude that the hydrothermal system was active mainly under pure extension prior to the N-S dike emplacement Dikes. [25] Each dike segment consists of one to four injections, separated by fault breccia, host rock, or chilled margins (Figures 3a, 4a, and 4c). Individual injections consist often of a single and, less commonly, of multiple pulses of magma within the same fracture. The thickness of 31 individual injections ranges from 0.06 to 4 m, and the thickest dikes trend N-S. Chilled margins from a few millimeters up to 1 2 cm were observed on most dike edges. Except for two dikes dipping as shallowly as 65 in segment A (Figure 4c), the dikes dip within 10 of vertical. Most injection boundaries are sharp. Occasionally, they are sinuous in fault breccia and very locally in the host rock. Features such as bend, apophyse, and ends of dikes in vertical and horizontal sections were observed and are presented in section 3.2. [26] The Gljúfurá dikes are all basaltic and present four lithologies recognizable by eye. We distinguished them as dikes of type a, b 1, b 2, c, and d. Table 1 summarizes their features. Generally, horizontal columnar jointing is well developed in the N-S dike segments (Figures 4a, 4c, 5a, and 5c). The maximum of

8 ETG 5-8 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY 1 m of red contact metamorphism in breccia 2 near the dike edges clearly indicates that the dikes are posterior to fault breccia (Figures 4c and 5a) Striated planes. [27] A total of 30 striated planes were measured on mesoscopic planes (Figure 3b). More than half are from the edges of the N-S dikes, the rest from the host rock, and very few from the fault breccia. The trends and polarities of striated planes collected in the host rock are similar to those measured on the N-S dike edges on the chilled margins. In segment A the fault gouge of breccia 2 contains slickensided surfaces covered by clay mineral and hematite. These are on centimeterscale blocks loosened by fracturing in the breccia and were not measured due to their unstable position in the outcrop. Tectonic striations on the dike edges are on widely varying planes, and their trends and sense of motions are similar to those measured in the host rock. These striations postdate the injection of the dikes and thus are not relevant to the mechanism of dike emplacement. They are discussed further in section 5. Figure 5. (a) Northerly dike of segment B and the associated N-S fractures cutting preexisting NNE fractures. (b) View south on segment C showing two dikes of northerly trend (white arrows) and only one injection of NNE trend injected into a NNE striking normal fault (thick black arrow). (c) Cooling cracks on a dike s edge (arrows) in segment C. The cracks are oblique to the northerly trend of the dike. They suggest drag and further evidence of strikeslip motion of the Gljúfurá fault during dike emplacement Field Pattern and Deformation in Each Dike Segment [28] The Gljúfurá fault is located on the eastern flank of the Borgarnes anticline very near the crest of this structure (Figure 1b) and has consequently been subject to little or no tilting or subsidence. The fault cuts the lava pile that on both its sides maintains a 5 9 dip toward the SE (toward RLRZ). Field evidence does not permit to assess the initial geometry of the Gljúfurá fault prior to dike injection, but the pattern observed today consists of N-S en échelon dike segments injected into the fault breccia of the Gljúfurá fault. We define a segment when a single or a group of dikes present the same geometrical features, that is, the bend from N-S (when injected into the Gljúfurá fault) to SSW (when injected into faults or joints of the adjacent basement) in their southwestern parts. The en échelon system thus presents at least four such segments (A-D), and the features and contents of each dike segment are analyzed here. [29] The stepping of the northerly en échelon system is clearly to the left, which, with other criteria described below, indicate dextral strike-slip motion along the Gljúfurá fault during magma injection (Figure 3a). Because of poor exposure it is unknown how the dikes terminate in their northern parts. There evidence of offshoots to the east or injections into NNE faults on the eastern side of Gljúfurá were not observed. By contrast, near their southern extremities, the dikes bend away from the northerly fault(s) to the southwest where their trend changes systematically from N-S to SSW (Figure 3a). Dikes of segments B, C, and D were injected into the NNE striking normal faults, and dikes of segment A show a SSW horse tail geometry infilled by magma. Generally, the combined thickness of dikes in each segment is thicker along the northerly trend. Individual dikes bending from N-S to SSW become slightly thinner as they turn into SSW and some die out there after m. Along their northerly trend, the minimum length of dikes in segments A, B, C, and D are about 428, 522, 754, and 4 m, respectively Segment A. [30] Segment A is the widest and contains most of the field evidence regarding faulting and repeated northerly dike injections (Figures 3a and 4c). In its northern part this segment cuts and displays a NNE rift parallel dike indicating that Gljúfurá fault became active at some stage of rifting after its surrounding area was subject to regional WNW least compressive stress (s 3 ). [31] Among several lavas cropping out along this part of Gljúfurá, there are two porphyritic flows at different stratigraphic levels, one in each wall. Both contain plagioclase and altered olivine phenocrysts. The one to the east is 3 m thick with analcime in the cavities. About 9 m lower, the other porphyritic layer to the west has a thickness of 15 m and contains calcite, clay minerals, and chabasite in the cavities. In more complete stratigraphic successions in this area, the thick layer is tens of meters below a

9 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY ETG 5-9 number of thin ones (M. Khodayar, unpublished data, ). On the basis of this stratigraphy, the Gljúfurá fault has a throw of 30 m (height of the outcrop) or more, down to the east, and is consequently expected to be eastward dipping. Consistent with this dip direction, the northerly dikes and parallel fractures in the breccia 2 are eastward dipping, within 10 of vertical. The N-S fault seems to continue to the south beyond the SSW bend of the dikes, as the lava series remain uncorrelatable on both sides of the N-S structure. Dikes of segment A are injected into breccia 2 or in the host rock, and brecciation due to dike injections was not observed. The absence of brecciation due to dike injection is well demonstrated by the host rock in contact with one of the northerly injections, indicating that brecciation is due to faulting prior to dike emplacement (Figure 4c). Thus breccia 2 is associated with the normal-slip movement and reaches its maximum in this segment with a thickness of 16.5 m, not counting the dikes. The hydrothermal alteration was of low temperature, chlorite is absent, smectite was found in the fault gouge, and stilbite was mainly observed in veins but also in amygdules along with mesolite and scolecite. Alteration, brecciation, and the network of mineral veins did not affect the dikes (Figures 4a, 4c, 4d, 4e, and 5c), indicating that normal slip, brecciation and the main hydrothermal activity occurred prior to the N-S en échelon dikes. The stress field consistent with normal faulting along the N-S trend had a s 3 acting E-W. This result fits the analysis of mineral veins which, as explained earlier, show episodes of mineralization mainly in northerly striking mode I fractures and under extension. [32] The ENE normal fault in the central part of segment A has a throw 23 m down to the NW. About 8.7 m fault breccia 2 is associated with this fault. The ENE trending fault crops out only in the western wall of the Gljúfurá fault but not in the eastern wall. It may thus be suggested that the ENE normal fault is a splay or belongs to a row of splays associated with the N-S fault. The widest fault breccia 2 and the most pronounced hydrothermal alteration in the fault gouge are expressed around this splay, indicating that during normal faulting phase, the maximum of opening, brecciation, and upflow occurred near the intersection of the secondary fault orientated at a high angle to the main N-S normal fault (Figure 3a). Other studies in rifting areas suggest that the hydrothermal activity is primarily located in the areas of fault interactions and less commonly in fault tip lines and fault intersections [Curewitz and Karson, 1997]. Furthermore, Georgsson et al. [1985], and Franzson [1998] suggest that hydrothermal flow occurs along fractures with the upflow at the intersection of faults, dikes, or open fissures crosscutting each other at a slight angle. [33] Segment A contains mainly dikes of type b and secondarily two injections of type a, one of 1.3 m and the other of maximum 1.2 m and minimum 0.35 m thickness. However, dikes of type a are much thinner here than in the other segments. Thus magma type a is much less pronounced here than in the other segments. Injections of type b 1 consist of three northerly dikes with a total thickness of 8.8 m (2.5, 2.2, and 3.1 m, respectively). There are four injections along the SSW bend to the west with a total thickness of 7 m (0.9, 1.1, 3.2, and 1.8 m, respectively). The injections of type b 2 consist of one northerly dike (1.8 m thickness) between those of type b 1 to the north (Figure 4c) and of two other dikes farther south on the eastern side of Gljúfurá fault, one striking SSW with a bend to N-S (1.27 m thick) and the other striking NNE (0.38 m thick). Although discontinuous from south to north, dikes of type b 2 also show the same geometrical bend in their trend (Figure 3a). Of these two dikes of type b 2 to the south, one continues upward while the other ends bluntly in the vertical section without a noticeable change in thickness (Figure 6a). No offshoots connect the two dikes, and the blunt end occurs at the contact between two lava flows with <10 cm of interbedded sediments. Above the blunt end, except for a thin mineral vein trending parallel to the dike, the lava is not vertically displaced by any fault. Figure 6. (a) Cross section (view oblique) of the two dikes of type b 2 in segment A. The dike on the left changes its direction from NNE to northerly and continues upward. The dike on the right has a NNE trend and ends bluntly in a thin scoria layer at the contact of the two lavas. (b) Map of the apophyse (and two associated injections of cm length) in segment B ending in the mineralized fault breccia 2. [34] Dikes of both b types present a few soft short striations (Figure 7a), flow lineations and elongated vesicles (Figures 7b and 7c) on their northerly edges. Soft lineations dip systematically to the south. Their average pitch is 25 S inside of the dike and very close to the margin and 2 S on the dike s walls, implying that these dikes propagated laterally and were emplaced subhorizontally (Figure 7b). Because of local magma turbulence inside the fracture, curved and swirling soft lineations appear locally in breccia 2, at few millimeters of the dike s chilled margin Chronological relationships in segment A. [35] Dike crosscutting in segment A indicates that dikes of type a are older than type b (Figure 3a). Of the dikes of type b, the type b 2 were injected first. This is deduced from the shallower dip (65 ) of the dike of type b 2 in the northern part of segment A (Figures 3a and 4c). If it is assumed that the dike of type b 2 was initially closer to vertical, like the other dikes of Gljúfurá, then it can be suggested that this dike was slightly tilted due to later emplacement of dikes of type b 1. [36] The time gap between normal and strike-slip faulting of the Gljúfurá fault is unknown. From a geometrical point of view, the en échelon arrangement of dike segments is suggestive of dextral motion of the fault during magma injection. In addition, several examples of deformation support dextral motion. Two preexisting NE-NNE dikes, one of type a and the other of type c, all located in

10 ETG 5-10 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY Figure 7. Soft structures on the edges of the northerly dikes of type b 1 in segment A. (a) Soft striae dipping 25 S (arrow). (b) Elongated vesicles dip on average 25 S inside of the dike very close to the margin (arrow). On the dike s wall, flow lineations have 2 dip toward south (arrow) (see text for explanation). (c) Elongated pipe vesicles dipping 18 S (arrow). the western side of segment A, show a bend in their trends as they approach the Gljúfurá fault (Figure 3a). Their trends change respectively from NE to ESE and from NNE to E-W, consistent with dextral shear during the emplacement of dikes in segment A. Furthermore, the counterpart of the dike of type a has a N-S trend on the eastern side of the Gljúfurá fault (Figure 3a), but the dike cannot be matched on both sides of the Gljúfurá fault without a dextral displacement of at least This shift indicates the minimum magnitude of dextral motion along the Gljúfurá fault during or after dike emplacements in segment A Segment B. [37] This segment contains three northerly dikes of type a, all injected into fault breccia 2 and cutting the network of mineral veins (Figure 3a). As in segment A, the lava flows on both sides of the gorge along segment B do not match, but a reliable marker horizon was not found to estimate the throw of Gljúfurá fault here. The dikes consist of one main injection (2.3 m thickness), a 0.30-m-thick dike northwest of it, and an apophyse visible east of the main injection when the water level is low. The apophyse is 40 cm thick at the junction with the main injection, 1 m in the middle, and 30 cm toward north. To the north, it ends in a horizontal section with only 8 cm thickness, presenting a bend perpendicular to the main dike (Figure 6b). Red contact metamorphism in breccia 2 lies along the length of the main injection (Figure 5a) and the apophyse. [38] The main injection changes its northerly trend to SSW in its southern extremity where it is injected into a NNE trending normal fault. This fault has a throw 22 m down to the NW (Figure 3a) and 6-m-thick associated fault breccia. Field observations show that only the N-S and the NNE faults were injected by dikes. The simultaneous activity of the N-S (dextral) and NNE (normal) faults can be explained by a s 3 orientated at WNW during dike emplacement. However, if the magma pressure is high enough during dike emplacement, preexisting fractures of any orientation can be dilated by magma regardless of the orientation of s 3 [Delaney et al., 1986]. [39] The thin dike in the northwest of segment B is injected into a variety of breccia 2, which contains a few rotated fragments (Figure 4d). Breccia 2 was formed during the normal faulting phase; consequently, these fragments were rotated later, likely during the dextral motion of the Gljúfurá fault at the time of dike emplacements. A total of 83 soft lineations were measured on the edge of the thin dike. Of these, 12 are elongated vesicles with a pitch between 10 and 40, all dipping toward north. The remaining 73 measurements were mainly flow lineations and a few individual short soft striae, 83% of which dip northward with a pitch between 0 and 30. The average pitch of the flow lineations suggests that dikes of type b too propagated laterally and were emplaced subhorizontally. [40] Regarding the crosscutting relations, the main injection of segment B cuts preexisting NNE and NE faults and joints (Figure 5a) but, in turn, is cut by a WNW sinistral strike-slip fault (Figure 3a). The WNW trending fault, visible when the water level is low, clearly displaces the red contact metamorphism and the main dike by 30 cm, indicating strike-slip faulting after the emplacement of the northerly dike Segment C. [41] Segment C has the largest combined thickness of dikes. There are four injections of type a separated by breccia 1 and/or 2 (Figure 4a) or by host rock. The same series as in segment B continues here without being vertically correlatable on both sides of the northerly segment. From west to east, the dikes of segment C have thicknesses of 2.5, 4, 2.6, and 0.50 m. The westernmost dike changes direction to SSW in its southern part and is injected into a NNE normal fault with a throw 9 m down to the SE (Figure 5b). There the dike thins to 0.4 m over 3 4 m distance, and its trace could not be found farther than m. The next two injections to the east also reduce their thickness. They seem to disappear in breccia 2 in the river just south of the SSW bend. The easternmost injection overlaps with segment B with a clear shift to the east, however, without a connecting offshoot. [42] One of these injections shows cooling cracks on its easterly edge (Figure 5c) trending N E, oblique to the northerly trend of dike. If cooling cracks develop perpendicular to dike walls emplaced as a fracture mode I, then these oblique cooling cracks suggest drag and are further evidence of strike-slip motion of the

11 KHODAYAR AND EINARSSON: WESTERN ICELAND TERTIARY PLATE BOUNDARY ETG 5-11 Gljúfurá fault during the emplacement of dikes of type a. If it is assumed that magma pressure was not high enough during dike emplacement to dilate preexisting fractures of any orientation, in this segment also a WNW s 3 explains the simultaneous activity of N-S dextral and NNE normal faults during magma injection Segment D. [43] This segment is the thinnest segment and is observable only over a short distance (Figure 3a). This segment is type a and consists of two injections of 0.84 and 0.18 m thickness, separated by fault breccia, but the thinnest injection crops out only locally and disappears quickly. The thickest injection shows a clear bend from N-S to SW in its southern extremity, where it is injected into a normal fault with a throw of 10 m down to the NW. There this thickest injection disappears after 10 m, but it is unknown how it ends. The dike becomes thinner and disappears very likely to the north, parallel to segment C, in breccia 2 in the river. Neither an offshoot nor a bend to the right of this dike cut the adjacent segment C (Figure 4a). [44] We interpret the bulk of data from the four segments as evidence of dextral motion of Gljúfurá fault during dike emplacement, for which a steady WNW s 3 acted during different phases of magma injections. 4. Diking Along Planes of Shear Faulting [45] It is frequently assumed that dikes form perpendicular to the minimum compressive stress, s 3, [Anderson, 1951; Gu* mundsson, 1990] and occupy purely extensional cracks, provided the magma propagates in its self-generated fissure. Thus the orientation of dikes is often used to infer the direction of paleostresses. Conventional views on this relationship between the regional stress field and the orientation of dikes have been challenged, however. Examples of dikes propagating in self-generated fissures [Anderson, 1938; Currie and Ferguson, 1970; Delaney et al., 1986] or in preexisting ones [Beccaluva et al., 1983; Delaney et al., 1986; Baer et al., 1994; Jolly and Sanderson, 1995] both exist. Numerous examples show that dikes may be emplaced at oblique angles to the principal stresses [Beccaluva et al., 1983; Delaney et al., 1986; Gautneb, 1988; Tokarski, 1990; Geoffroy and Angelier, 1995]. The conditions of dike injection into preexisting fractures, misaligned with respect to the principal stress directions, are discussed theoretically by Ziv et al. [2000]. [46] Our field example is a case of dike injection along a preexisting fault, with offshoots along NNE faults. It is likely that magma pressure was not high enough to dilate preexisting fractures of all directions around Gljúfurá, but only fractures of these two trends. The northerly fault acted as a dextral strike-slip fault during intrusions, as judged from the cooling cracks, a few soft striae, and crosscutting and right-lateral displacement of preexisting dikes, as well as from the en échelon geometry of the northerly dikes. The offshoots intruded reactivated NNE normal faults. [47] Our example also demonstrates a case of dike injection oblique to the direction of the minimum compressive stress, s 3. Dextral strike-slip motion along N-S faults and normal faulting along NNE faults are both consistent with a WNW trending s 3. This implies that the main N-S fault was oblique to the orientation of s 3 and thus was misaligned with respect to the principal stress directions, although it is difficult to determine the extent of this obliquity. [48] WNW is the direction of regional minimum compressive stress acting both in the rift and the transform zone south of 65 N in Iceland. If the magma pressure was less than the WNW regional compressive stress, it is possible that only strike-slip motion along the N-S fault allowed the dike to intrude the NNE faults. We emphasize the analogy with the presently active faults along the transform zone of South Iceland Seismic Zone and the oblique rift of the Reykjanes Peninsula [Einarsson and Eiríksson, 1982; Erlendsson and Einarsson, 1996]. These are N-S striking dextral strike-slip faults with WNW trending s 3. These faults may, similar to the Gljúfurá fault, change mode and be injected by dikes if they are sufficiently close to a magma source. [49] The offshoots along the NNE faults in Gljúfurá resemble those discussed by Ziv et al. [2000]. Both results show dike injection into preexisting faults at shallow crustal levels. Because of low confining pressure the dike was able to break away from the misaligned N-S crack only when it encountered an existing crack of a different orientation. This is possible if the stresses were sufficient for the dike to break intact rock near the tip. 5. Deformation Posterior to N-S Dikes [50] While the major mapped faults in Gljúfurá show mainly normal-slip movement, the pitch of striae indicates mostly strikeslip (0 29 ), and less commonly oblique-slip (30 59 ) and dipslip (60 90 ), with only three values indicating reverse slip. As already mentioned, these striations very likely occurred posterior to the northerly dikes, and slip data clearly indicate the dominance of strike-slip faulting after dike injection. Striae crosscutting is rarely observed and a chronological evolution cannot be established through such relations. Interpretation of striae is not always straightforward. This is particularly well demonstrated in Borgarfjör*ur, where different studies of the same area and the same fracture pattern have led to widely divergent interpretations in terms of stress field [see, e.g., Sæmundsson, 1978; Bergerat et al., 1990; Khodayar, 1999]. Our study of Gljúfurá has shown that apparently contradictory data may be obtained from the same outcrop. Striated planes have widely different trends and sense of motions. Northerly striking striated planes (N E) show dominantly strike-slip (dextral and sinistral) and a few normal and reverse motions. Other trends also present similar variations. The slip data are clearly not compatible with a single stress pattern (Figure 8a) and thus reflect polyphase tectonics. Of the total of 30 measured striated planes (Figures 3b and 8a), only 57% (17 planes) could be separated in three sets of compatible faults (Figures 8b 8d). This includes the WNW sinistral fault that cuts dike segment B. The rest of the measurements do not group into compatible sets. The three compatible sets are presented below (their order does not reflect their relative ages): (1) NE dextral, WNW sinistral and N-S reverse planes, fit with an extension roughly N-S and E-W compression, (2) northerly sinistral, NW normal, and NE reverse planes, consistent with a NE extension and NW compression, and (3) conjugate NW dextral and NE sinistral strike-slip and northerly normal-slip planes, reflecting an extension roughly E-W and compression oriented N-S. If the local tilt of part of breccia 2 in segment A and the distribution of the morainic material along segment B corresponded to fissure infilling, they would indicate an extension compatible with this set and thus the reactivation of both segments as an open fissure long time after dike injection. [51] A more regional interpretation of the tectonic pattern was offered by Khodayar [1999] based on a large amount of data on major structures and mesoscopic faults, as well as comparison with the results of other workers. Results from the analysis of the Gljúfurá fault emphasize three points. First, although striated planes show reverse slip, none of the major mapped fault has a reverse polarity. Generally, field evidence of severe compression by shortening (fold or major reverse fault) is not observed in Gljúfurá, nor in the area at a regional scale [Khodayar, 1999]. Reverse-slip striation may be of local significance, for instance, because of the local bend of a steeply dipping normal-slip plane. Second, striation may not necessarily reflect the state of regional stress. Some of the mesoscopic shear planes may simply correspond to the local adjustment of deformation associated with a single reactivated major structure. Finally, striation on mesoscopic planes does not always correspond to a significant amount of displacement. Examples of slickensided planes on a meter scale were observed with conspicuous tectonic striation, while the