Formation and Geometry of Fractures, and Related Volcanism, of the Krafla Fissure Swarm, Northeast Iceland

Formation and geometry of fractures, and related volcanism, of the Krafla fissure swarm, northeast Iceland

i Nordic Volcanological Institute, University of Iceland, 101 Reykjavik, Iceland AGUal (jUDMUNDaaON )

ABSTRACT for a quantitative geodynamic model of the rifting process. The first objective of this paper is to describe fracture geometry, new field relations During the past 12 yr, a major volcano-tectonic episode occurred between volcanism and fractures, and fracture development in the Krafla in the Krafla fissure swarm at the divergent plate boundary in north- fissure swarm. A second objective is to use these data to evaluate current east Iceland. This swarm is an 80-km-long and as much as 10-km-wide models of the rifting process. We outline qualitative explanations for the zone of tension fractures, normal faults, and volcanic fissures. The fracture formation, but a general quantitative model will be published average length of 1,083 measured tectonic fractures is about 350 m, elsewhere. the maximum length being 3.5 km, and the average estimated depth is GEOLOGIC SETTING of the order of 102 m. Most fractures strike north to north-northeast, with widths as much as 40 m and throws of as much as 42 m. Pure The structure of Iceland can be divided into four major elements, tension fractures are most common, but as they grow they commonly namely, change into normal faults. Most fractures gradually thin out at their (1) the Holocene lava formation, ends, but several exceptionally wide tension fractures end in tectonic (2) upper Pleistocene rocks belonging to the Brunhes magnetic caves, several tens of meters long, only a few meters beneath the epoch, age 0.01-0.7 m.y., surface. The total dilation measured in 5 profiles across the Krafla (3) lower Pleistocene rocks, age 0.7-3.1 m.y., and swarm reaches a maximum of at least 80 m and decreases from south (4) the Tertiary lava pile, age 3.1-16 m.y. to north along the swarm. Some 20 intrusive events and 9 eruptive The neovolcanic zones cover a fourth of the area of Iceland and are events occurred during this volcano-tectonic episode. New lavas cov- defined by volcanic rocks younger than 0.7 m.y. and by seismic activity ered many old fractures, but several new fractures were also formed (Saemundsson, 1978). Their main part is the axial rift zone that marks the and many old ones grew. New lava flowed into some of the major divergent plate boundary in Iceland, but two or three off-rift flank zones, fractures in the area, presumably forming pseudodikes. Locally, characterized by lack of extensional tectonics, also occur. magma used a part of a pre-existing fracture as a pathway to the The axial rift zone in north Iceland is a normal Iceland-type construc- surface. Small width:length ratios of the normal faults, as compared tive plate boundary (Palmason, 1973; Saemundsson, 1979). It consists of 5 with such ratios of the tension fractures, are attributed to the tendency volcanic systems with 5- to 20-km-wide and 60- to 100-km-long en of tension fractures to close as they develop into normal faults. It is echelon fissure swarms striking north-northeast (Fig. 2). Pure extension concluded that divergent plate movements with dike intrusions, or (tension) fractures, normal faults, grabens, and volcanic fissures are the pressure changes in a deep-seated magma reservoir, are viable models main structural elements of these systems. The systems of Krafla and Askja for formation of the fractures. have well-developed central volcanoes with calderas, but the systems of Fremri-Namur and Theistareykir are less developed and lack calderas. INTRODUCTION KRAFLA FISSURE SWARM The Krafla fissure swarm (Fig. 1) at the divergent plate boundary in northern Iceland is an 80-km-long, 4- to 10-km-wide zone of recent The Krafla fissure swarm (Fig. 1) extends from south of Lake Myvatn ground fissuring and volcanism. Its main structural elements are faults, to the north coast of Axarfjordur (Bjornsson and others, 1977) and bisects nested grabens, and tension fractures. A current (1975-present) volcano- the Krafla central volcano (Fig. 2). This central volcano comprises a tectonic rifting episode initiated intense research into the mechanism of diversity of rocks and a partly infilled caldera (Bjornsson and others, 1977; rifting, including geodetic measurements (Bjornsson and others, 1977, Saemundsson, 1978) with a distinctive dacitic welded-tuff layer, which 1979; Sigurdsson, 1980; Moller and Ritter, 1980; Tryggvason, 1980, marks the rim of the caldera and is supposed to be associated with its 1984, 1986a, 1986b; Johnsen and others, 1980), seismology (Einarsson formation (Bjornsson and others, 1977). and Brandsdottir, 1980), and magnetotelluric measurements (Bjornsson, Of about 35 Holocene eruptions, mostly basaltic, the majority have 1985). occurred either in the caldera or at the mountain Namafjall (Fig. 1) In addition to general geophysical measurements, detailed tectonic (Bjornsson and others, 1977). In Gjastykki (Fig. 1), north of the Krafla studies of the fractures themselves are necessary to provide data as a basis central volcano, the fissure swarm dissects primitive olivine tholeiites from the 10,000-yr-old monogenetic shield volcano Theistareykir. Small spatter »Present address: Mineralutvikling A/S, Stakkevollvn. 23, N-9000 Tromsa, cones connected to tectonic fractures have been observed in northern Norway. Gjastykki (this paper).

Geological Society of America Bulletin, v. 101, p. 1608-1622, 18 figs., 1 table, December 1989.

1608

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/12/1608/3380563/i0016-7606-101-12-1608.pdf by guest on 27 September 2021 Figure 1. Major elements of the Krafla fissure swarm. 1, Pleisto- cene/Holocene volcanic rocks; 2, lava flows from the Myvatn fires (1724-1729) and the Krafla fires (1975-present); 3, normal faults; 4, tension fractures; 5, caldera fault. T, Theistareykjabunga; E, Lake Eilifs- votn; NVZ, neovolcanic zones. Data from Saemundsson (1978) and Bjomsson and others (1984).

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Tryggvason, 1980). Subsequently, 21 rifting events, nine of which have resulted in volcanic eruptions, have occurred (Tryggvason, 1984, 1986a). 50 km A shallow magma chamber centered below Leirhnjukur at a depth of 2.5-3 km receives magma at an average rate of about 5 m3/s (Einarsson, 1978; Tryggvason, 1980). The land surface above the magma chamber slowly inflates by a few millimeters per day for several months. When a critical inflation level is reached, rifting and rapid deflation of the order of tens of centimeters occur during a few hours, and magma is injected into dike(s), some of which reach the surface as basaltic fissure eruptions (Bjornsson and others, 1977; Tryggvason, 1980, 1984,1986a). The last rifting event, accompanied by an eruption, occurred on September 4,1984, after nearly 3 yr of quiescence. The total land surface covered with fresh lava since 1975 is 36 km2, and the lava volume is estimated to be 0.25 km3 (Bjornsson and others, 1984; Tryggvason, 1986a).

I I I

Figure 2. Fissure swarms of northern Iceland. Th, Theistareykir; K, Krafla; F, Fremri-Namur; A, Askja; Kv, Kverkfjoll; TFZ, Tjdrnes fracture zone.

Nearer to the coast, in the Kelduhverfi area (Fig. 1), basaltic fissure eruptions occurred some 1,500-2,000 yr ago (Eliasson, 1979). The Krafla fissure swarm was active 1724-1729, resulting in basaltic fissure eruptions (the Myvatn fires), earthquake swarms, and movements on fractures W r 1- 1 - i T -i 1 1— i ~ E (Gronvold, 1984). 16.6 8.3 0 8.3 16

Current Volcanism Percent of total length N 527 Fractures The present rifting episode started with a volcanic eruption at Leirhn- jukur, Krafla (Fig. 1), on December 20,1975 (Bjornsson and others, 1977;

_ 16.6

Length per N -11.1 cell (%) / \ J ,_ 5.5 \ N 11 V Jr \ w

i ~ E 16.2 8.1 0 8.1 16.2 - 16.2 Percent of total length s

Length per - 10.8 556 Fractures cell (%) Figure 3. Length/orientation diagrams of 1,083 fractures meas- 5 .4 ured from maps and aerial photographs covering the whole Krafla fissure swarm. For comparison, the data set is presented using both rose diagrams and histograms. The data from the fissure swarm are divided into two sets, north and south of the mountain Mofell, located w in the middle part of the swarm (Fig. 4a).

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Figure 4. Maps a-c show the outlines and locations of the fractures measured in the field (arrows with numbers). Figure 4a shows older fractures, now completely buried by the lava flows of 1980-1984, and the location of a new fracture (A) formed in the 1984 rifting event, partly used as a conduit. Arrows without numbers (Fig. 4c) indicate spatter cones associated with tectonic fractures. Figure 4d shows the locations of Figures 4a-4c, as well as of the profiles 1-5 across the fissure swarm (Fig. 14). K, Krafla; G, Gaesafjoll; T, Theistareykjabunga; E, Lake Eilifsvotn. Data from 1960 aerial photographs, topographic maps at the scale of 1:20,000 (Technische Universität Braunschweig, 1982), and Björnsson and others (1984). (Continued on following page.)

Ground Deformation southward to zero south of Lake Myvatn. Northward, the dilation decreases gradually, being 2 m on the north coast at Axarfjordur (Tryggva- During the present rifting episode, almost the whole fissure swarm son, 1984). The flanks of the fissure swarm have contracted by an amount has dilated. Measured across the fissure swarm, the maximum dilation of 9 exceeding 0.3 m/km in some areas (Tryggvason, 1984). m occurs in a profile 10-12 km north of Leirhnjukur (Fig. 1) but decreases Each rifting event has affected only a part of the fissure swarm, and

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ii ' / /

Mfci

associated ground deformation has been largely confined to a 1- to 2-km- wide (in some instances 5-7 km) central zone of the 4- to 10-km-wide fissure swarm (Bjornsson and others, 1979; Tryggvason, 1984). This central zone is characterized by numerous fissures, commonly flanked by normal faults, forming a central graben. Rifting has occurred on pre- existing fractures but has also created new fractures. Earthquake swarms, observed to migrate laterally either northward or southward away from Leirhnjukur, may indicate lateral dike emplacement (Einarsson and Brandsdottir, 1980). Lateral dike emplacement has also been suggested as a major factor in forming fractures during rifting events (Pollard and others, 1983), as discussed below.

FIELD OBSERVATIONS

Methods

The results and observations presented below (Figs. 3 and 4) are based on field work and detailed studies of aerial photographs at the scale of 1:35,700. The width (W) and throw (T) were measured along several fractures, at intervals of 25 or 50 m (Fig. 5). Width measurements, performed with a tape, and throw measurements, performed with a tape or by hand leveling, have an estimated error of 10%, the main inaccuracy being due to surface irregularities. The width was measured at the surface, normal to the fracture strike, but where fractures were covered with scree, the width was set as zero. Strike and length of fractures were measured on maps and aerial photographs, the estimated error being 2° for strike measurements and 20 m for length measurements.

Common Features of Fractures

The length and strike of 1,083 fractures were measured from aerial photographs and maps. The average length is 350 m; the predominant Figure 4. (Continued).

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strike is N3°E-N18°E (Fig. 3). Most fractures in the fissure swarm are straight or weakly curved, but strongly curved large fractures are also w T found (Figs. 1 and 4). Many fractures split into smaller en echelon seg- a. ' > i> v /, ' > ments, particularly where the main fracture is curved (Fig. 6), but parallel w arrangement of offset fractures is common where a large fracture termi- © © nates. At their ends, fractures generally grade into several small, centimeter-scale joints, which follow the columnar joints in the basaltic lava flows. I T Normal Faults W, Because the surface irregularities along fractures are commonly of the © © order of tens of centimeters, we define normal faults as fractures having vertical displacements in excess of 1 m. The results (Fig. 7) indicate that a fracture has to attain a length of several hundred meters before any signifi- Figure 5. Fault geometries (1-4) measured at the surface and in cant vertical displacement occurs. Fault walls are subvertical and open the way shown here. near the surface (Figs. 5 and 8), and no slickensides on fault planes have been observed. Lateral movements along faults should be easily detectable, if they exist, because of matching jags and notches on the fracture walls, overestimate the "true" dilation at some points of measurement, they but none were found, indicating that the stress field favors pure normal probably underestimate the dilation at other points. faults. Eight normal faults, with lengths from 350-3,500 m, were measured Tension Fractures in the field (Figs. 4a-4c). The largest (no. 1, Fig. 4a) has a maximum throw of 42 m and a maximum width of 28 m (similar to the largest fault Pure tension fractures are by far the most common type of fractures of the Thingvellir fissure swarm, Gudmundsson, 1987a) and defines the in the Krafla fissure swarm. Some are associated with faults and grabens, western rim of a 2- to 3-km-wide central graben in the Krafla fissure either parallel to, or branching out from, the faults (Figs. 4a-4c, 6); others swarm in this area (Figs. 1,4a-4c). The fault is split along the middle part occur singly or in clusters, unconnected to faults. Long fractures tend to be of its length (Fig. 9) but can be continuously followed northward. On the wider than short fractures, but the great variations in the width: length opposite eastern rim of the central graben, 15- to 25-m-high irregular fault ratios (Fig. 7) show that this tendency is not strong. walls define the eastern graben boundary. Several very wide pure tension fractures occur west of the mountain Many parts of the central graben contain normal faults that form 30- Mofell (Fig. 4a) and at a few places farther north in the Gjastykki area. to 200-m-wide, in many cases nested, grabens. The extension measured These fractures are characterized by exceptionally high width: length ratios, across such grabens can be interpreted in several ways, but herein the from 1:20 to 1:40 (Fig. 11), and many of them end bluntly; that is, they do combined width of the fractures at the edges is considered to represent the not gradually become narrower or split into several fractures but end as total extension (Fig. 8, no. 1). Where the graben floor has collapsed (Fig. tectonic caves. These caves (Fig. 12), which are predominantly of tectonic 8, nos. 2 and 3), in which case there is no criterion to distinguish between a origin, occur at the ends of several of these fractures and vary considerably large tension fracture and a graben (Fig. 10), the total width of the graben- in size. The length is from 10 m to at least 30 m. The entrance height is like fracture is taken as the amount of extension. Although these rules may typically 2-4 m, the inside height as much as 5 m, and the width 5-10 m.

Figure 6. Aerial view of a short fault (upper part of picture), with throw of 10-15 m, splitting into fan-arranged pure tension fractures (upper right). Note the general parallel arrangement of faults and pure tension fractures.

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F10 IOOO m

F 1

^Va /V\| A A/ / W\ v"S A A

3000 4000 m

Figure 7. Width and throw in meters of fractures 1-12; locations shown in Figures 4a-4c.

500 1000 1500 m

»00 m

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/12/1608/3380563/i0016-7606-101-12-1608.pdf by guest on 27 September 2021 Figure 9. Aerial view of fracture 1 where it splits into curved en echelon segments. At the lower left, the 1980-1984 lava flows have covered many older fractures. Throw is 30-40 m in the middle left part of the picture.

Figure 8. Graben/fault geometries (1-3) encoun- tered during field work and the ways they were measured.

Figure 10. Fracture 9 where it becomes a graben-like fault similar to that in Figure 8, geometries 2-3.

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• Figure 11. Width ¡length ratios of 5 the fractures in Figures 4a-4c. The 1 pure tension fractures 3-5 from the Mofell area (Fig. 4a) have the highest • ratios. 2

•it 9

12 it 10 7

1000 2000 3000 4000 Length (m) •fr = Normal faults * : Tension fractures

All the caves are elongated parallel to the length of the associated fractures. The cave floors as well as the fracture floors are covered with large boulders and scree from the roof and side walls. The cave roofs are highly fractured, mainly along columnar joints in the pahoehoe lava flows. The terminations at these very wide fractures are generally parallel and offset by 50-300 m (Fig .13).

Profiles

Extension across the Krafla fissure swarm was measured in five east- west profiles (Fig. 14 and Table 1), at 4- to 5-km intervals north of the mountain Mofell (Figs. 4a and 4d). The extension is highest in the southern profiles, decreasing northward to a minimum south of Kelduhverfi (Fig. 14, profile 4). Although the profiles are slightly oblique to the main fracture trend (N10°E), the effect on measured dilation is very small and may be ignored for our purpose. The dilation was also measured in several profiles across the fissure swarm in 1938 (Bernauer, 1943). Except for one profile, which coincides approximately with our profile 1 (Fig. 14), the 1938 profiles are more or less covered by the new lava flows. Bernauer 1943) obtained a dilation of 58 m in profile 1, whereas we found the 1987 dilation to be 80 m. This discrepancy is partly the result of measurement differences and uncertainties and partly due to 8-9 m of actual widening of the fissure swarm in this area during the current rifting episode (Tryggva- son, 1984).

Relation between Volcanism and Fractures

During rifting in Iceland, old fractures become covered with new lava flows, whereas new fractures keep forming. This ongoing process leads to complicated structural relations. During the Myvatn fires of 1724-1729, new lavas covered many pre-existing fractures within, as well as north and south of, the Krafla caldera (Fig. 1). The recent Krafla lavas have flowed Figure 12. Tectonic cave at the southern end of fracture 11. The north into the Gjastykki area and completely buried many large fractures roof of the cave is jointed and fractured; the length of the cave is visible on 1960 U.S. Air Force aerial photographs (Fig. 4a). approximately 25 m. Rifting occurs on pre-existing fractures but creates also some new

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Figure 13. Aerial view of wide pure tension fractures west of the mountain Mofell (Fig. 4a). Note the parallel arrangement of these large fractures (middle part of the picture).

fractures. In central Gjastykki, this is easily noticeable when the prerifting large and grade into tension fractures at their ends. This indicates that 1960 U.S. Air Force aerial photographs are compared with 1982 Iceland tension fractures in the Krafla swarm have to attain a certain minimum Geodetic Survey aerial photographs of the same area. West of the moun- length and depth in order to develop into normal faults. tain Hrutafell (Fig. 4a), there occurs a 700-m-long new fracture that is not Second, the tensile strength of the Icelandic crust is very low. Most in on the 1982 Iceland Geodetic Survey aerial photographs and was most situ tensile strength estimates in Iceland range from 1-6 MPa (Haimson probably formed during the 1984 rifting event. Basaltic lava poured out of and Rummel, 1982). Provided one can estimate the tensile stress during the fracture along about 200 m, where the fracture cuts a topographic low fracture formation, it may be possible to discriminate among models that (Fig. 15). The fracture is as much as 1.5 m wide and is not connected to are based on different loading conditions. The width-.length ratios of ten- other lava-filled fractures farther west. It trends N30°E, some 20° oblique sion fractures give the average tensile stress at the time of their formation to older fractures nearby (Fig. 4a). About 2 km west of this fracture, on the (Gudmundsson, 1983a). These ratios vary by an order of magnitude in the other side of the central graben, a similar feature is found, where lava Krafla swarm (Fig. 11), giving stress estimates ranging from 12 to 134 poured gently out of a fracture. This lava is connected to the large lava MPa, with an average of 20-30 MPa. Thus, any fracture-formation model field from the 1981 and 1984 eruptions (Fig. 4a). In both cases, the must be able to generate tensile stresses of this order or, alternatively, magma apparently reached the surface through existing fractures. explain in what way these width: length ratios might be overestimated. The older volcanic activity in the Gjastykki area reveals similar de- Third, fracture toughness, that is, the critical value of the stress intensity tails. In northern Gjastykki, small spatter cones occur in a wide pure factor at which a crack starts to propagate, may be an important parameter tension fracture (Fig. 4c), similar to those west of Mofell. Four spatter during fracture formation in the Krafla swarm. Associated with a propa- cones lie at the northern end of this fracture, and one lies in the middle part gating crack is a so-called "process zone," which in rock is thought to (Figs. 16a and 16b). The cones are 5-10 m high and 5-15 m in diameter. consist of microcracks (Schmidt and Rossmanith, 1983), at the crack tip The spatter has a red-brown color and is partly welded. Spatter from the and during propagation, many microcracks in this zone link together and cone in the middle of the fracture was ejected up onto the fracture walls. form a macrocrack (Atkinson, 1987). For tensile fractures, the greater the Clearly, these spatter cones are younger than the fracture, and the magma toughness, the greater must be the minimum tensile stress at which a crack used the existing fracture as a pathway to the surface. Because these cones can propagate. are only partly covered with a thin soil layer, they are probably relatively Testing of small basaltic rock samples gives fracture toughness of young, possibly similar in age to a 1,500- to 2,000-yr-old volcanic fissure 1-2.5 MPa m14 (Atkinson and Meredith, 1987). For example, testing of an that occurs farther north (Eliasson, 1979). Icelandic tholeiite basalt gave toughness of 1 MPa m^ (Meredith and others, 1984). In situ values may be much higher; Rubin and Pollard FRACTURE FORMATION AND DEVELOPMENT

TABLE 1. DILATION AND ELONGATION ACROSS PROFILES 1-5 (FIG. 4d) Mechanical Constraints

Profile no. 1 2 3 4 5 Several mechanical factors need to be taken into account when con- Lt, in meters 4,640 4,940 6,320 6,350 5,010 sidering the formation and development of the fractures in the Krafla Ld, in meters 3,490 2,130 3,750 5,740 2,700 swarm. First, all the fractures are vertical at the surface and must there be Dilation D, in meters 80.4 32.3 28.4 14.2 35.8 generated by absolute tensile stresses. Under most loading conditions, Dilation, in % 2.30 1.52 0.76 0.25 1.33 Elongation, e 0.024 0.015 0.008 0.003 0.013 absolute tensile stresses attain their peak values at the surface. It is thus Stretch, S 1.024 1.015 1.008 1.003 1.013 likely that all fractures form as tensile fractures at or near the surface, but some subsequently change into normal faults as they propagate down- Note: Lt is the total profile length as shown in Figure 14; Ld is the length between the outermost two fractures in each profile. Ld is used when calculating dilation and elongation. ward. Compared with pure tension fractures, most of the normal faults are

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50 <0.5 1-2 4-8 >16 Fracture 0.5-1 2-4 8-16 width (m) W 500 300 100 PROFILE 1

W PROFILE 4 5 Or A/V 50 10x on faults 10x on faults 50 50 W 600 W 400 400 200 1 II 200 0

1 Km PROFILE 2 PROFILE 5 1km -W 50 50 W

10x 10x on faults on faults

50 50 w w 600 300 400 100 200 ID -100 1 km 1 km

Figure 14. Vertical displacement (upper plot) and dilation (lower plot) for profiles 1 through 5 across the Krafla fissure swarm (Fig. 4d). More data are given in Table 1.

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Figure 16. a. Spatter cone formed at the bottom of a pre-existing wide pure tension fracture (Fig. 4c). The cone is 7 m high, but the fracture is 20 m wide measured across the cone. b. Aerial view of the same fracture and the location of the spatter cone (lower arrow). Small spatter cones occur also at the end of the fracture (upper arrow).

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(1987) suggested 0-100 MPa m54 for a volcanic rift zone, and Delaney and others (1986) suggested toughness of as much as 110 MPa for sedimentary rocks. Thus, even if fracture toughness may be an important factor for fracture development in areas such as that of the Krafla swarm, the currently poor in situ constraints make it of limited usefulness in tectonic models.

Fracture Length

The strain energy release rate per crack tip, G, increases with length, as the crack propagates, according to the equation

G = dU/da = ira2aEl, (1)

where U is the elastic strain energy released per unit thickness of rock during crack propagation, a is crack half-length, a is the applied tensile stress, and E is Young's modulus. For constant a, the increase of G with increasing crack length should favor long fractures over short ones. Never- theless, short fractures are much more frequent than long fractures in the Krafla swarm (Figs. 4a-4c), as well as in other Icelandic fissure swarms (Gudmundsson, 1987a, 1987b). This length distribution of fractures in the Krafla swarm may be explained as follows. (1) In homogeneous materials, long fractures are favored if the stress Figure 17. Length versus log of maximum throw. Line F is the field is homogeneous. Because of joints, contacts, and other weaknesses, fault growth curve according to Watterson (1986). the crust dissected by the Krafla swarm is, however, heterogeneous as regards tensile strength. The tensile stress required to propagate a fracture across transverse joints is much higher than if the joints are parallel to the fracture. Transverse joints may thus stop the propagation of fractures and Fault Development may be one reason for the common occurrence of short fractures. (2) Old fractures in the Pleistocene rocks beneath the postglacial lava All faults observed in the Krafla swarm are normal faults. The throw flows may control location and length of surface fractures. Short offset or is commonly largest near the middle part of their length (Fig. 7), giving en echelon fractures (Fig. 9) may develop where these old, buried fractures many faults a roughly elliptical shape. It is not known how fault geometry strike oblique to the orientation of the postglacial maximum tensile stress. changes with depth, but in Tertiary areas in Iceland, all exposed fault Each surface fracture is then developed perpendicular to the direction of planes are steeply dipping (>60°) (Gudmundsson, 1984a). Most faults are the maximum postglacial tensile stress, but as a set, the surface fractures open at the surface but probably close at depths of several hundred meters. follow the old buried fracture. Many large faults may have been active for more than 104 yr and (3) The fracture toughness increases with fracture length, at least in thus generated by many rifting events. The number of rifting episodes in small-scale experiments. This may also apply to fractures in the Krafla the Krafla swarm during postglacial time is not known. Bjornsson and swarm, thus inhibiting growth of large fractures. others (1977) proposed that on average, rifting occurs every 100-150 yr in (4) Long fractures may release so much tensile stress that propaga- north Iceland, affecting only one of the 5 swarms during each episode. tion of neighboring, short fractures is stopped. Consequently, the time between rifting episodes in the Krafla swarm may be about 500 yr. As a rough estimate, we will assume that about 20 slip Fracture Depth movements have occurred on each large postglacial fault within the Krafla fissure swarm. In evolving tension-crack systems, it appears that crack depth should Watterson (1986) suggested that faults grow in such a way that be similar to, or less than, crack spacing (Lachenbruch, 1961). Using a provided the time interval between slip events is constant, the slip in each model wherein the applied tensile stress increases with depth, Nur (1982) event increases regularly. He proposed a relationship between fault length, obtained similar results and, furthermore, concluded that the length of L, and slip/throw, T, of the form cracks should normally be equal to, or greater than, their depths. Because the rate of increase of tensile stress increases with depth at divergent plate T= c • L2, (2) boundaries (Gudmundsson, 1988), the assumptions of Nur (1982) are probably appropriate for the Krafla swarm. There, the lateral spacing where c is a constant and depends on the previously mentioned assump- between major fractures is of the order of several hundred meters (Figs. tions. When applied to our data (Fig. 17), equation 2 underpredicts the 4a-4c), whereas the mean length is 350 m. Consequently, the depth of the throws on small (short) faults and overpredicts the throws on large faults. fractures should be of the order of 400 m or less. This is similar to the This implies that there are additional factors that affect fault growth in the results obtained from the Thingvellir swarm and the Vogar swarm (Gud- Krafla swarm. mundsson, 1987a, 1987b), both of which gave depths for most fractures of Our data show a very high correlation between fault radius a and T the order of several hundred meters. (Fig. 17). An average a! T value of 43 is calculated from the faults meas-

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only a very shallow dike would be expected to generate fractures at the

w surface, and such a dike would normally be a feeder. } H Figure 18. Develop- Second, the crustal deformation during the current rifting episode has * \w ment of a pure tension affected a 5- to 7-km-wide part of the Krafla swarm (Bjornsson and others, fracture into a fault (1-2), 1979; Sigurdsson, 1980). According to Mastin and Pollard (1988), spacing indicating how the width between outermost fissures is likely to be similar to the depth to the top of 1r decreases as the hanging the dike responsible for the fissure formation. This depth should thus be as G ® wall subsides. much as 5-7 km, which is contrary to the results above that indicate a depth of a few tens of meters in order that typical dikes should generate surface fractures in the Krafla swarm. Furthermore, it follows from the fault depth estimates above, and also from the stress field associated with a ured (Fig. 17). Barnett and others (1987) obtained similar a!rvalues for propagating dike (Rubin and Pollard, 1988), that it is particularly difficult mid-ocean-ridge faults and suggested the relation to account for large normal faults by dike intrusions. The simple plate-tectonic model assumes that tensile stress builds up 2 a/T °cM , (3) gradually within the rift zone, owing to pull from outside the zone, until new fractures form, old ones propagate, and the tensile stress is temporarily where ¿u is the modulus of rigidity, relaxed (Bjornsson and others, 1979; Bjornsson, 1985). There is no doubt that relative tensile stress builds up within the rift zone, but according to ¡x = E/2(l + v), (4) Hooke's law, the rate of increase of tensile stress varies positively with Young's modulus, which increases with depth in the crust (Gudmundsson, E is Young's modulus, and v is Poisson's ratio. Our a/7" values range from 1988). Consequently, the rate of tensile stress buildup should be higher at 26 to 82, varying by a factor of about 3. Accordingly, rock rigidity in the deep crustal levels than at the surface, so that above magma reservoirs, study area should vary by a factor of about 2.0. This is similar to the dikes would normally form at depth long before the condition for forma- variation in Young's modulus within individual Holocene lava flows in tion of tensile fractures was attained at the surface (Gudmundsson, 1988). Iceland (Gudmundsson, 1983a). Thus, a somewhat different model seems appropriate. The width:length measurements (Fig. 11) indicate that faults in general have smaller width: length ratios than do pure tension fractures. PROPOSED MODELS Our data suggest that a tension fracture must commonly attain a certain critical length before it develops into a normal fault (Fig. 7), and when it We do not think that fracture formation in the Krafla swarm, and becomes a normal fault, the tension fracture tends to close (Fig. 18). This other similar swarms, is necessarily associated with only one mechanism. probably explains the weak correlation between length and width of In some cases, nonfeeders may generate surface fractures or dilate already- fractures. formed fractures. Also, outside the totally molten uppermost part of the magma reservoir, a large part of the crustal extension may be taken up by CURRENT MODELS normal faults because there, dikes would normally not intrude and relax the tensile stress due to plate movements. Here, however, we will focus on Fracture formation in the Krafla swarm has been attributed to tensile fracture formation above the totally molten upper part of the magma stresses generated either by dike intrusions or directly by divergent plate reservoir beneath the Krafla fissure swarm (compare with Gudmundsson, movements. The dike model, initially proposed by Walker (1965), has 1986,1987c). The emphasis is on mechanisms that we consider to be most been developed into a quantitative model by Pollard and others (1983) important in the crust above the Krafla magma reservoir. This reservoir and gives a clear-cut causative relationship between dikes and surface could be associated with fracture formation at the surface by magmatic fractures. This is an attractive model, but for the following reasons, it pressure increase and a slight uplift of the crust above, by direct plate seems to be difficult to explain the formation of the Krafla swarm solely in movements together with dike intrusions, or both. terms of this model. Magma accumulation and pressure increase in the reservoir prior to First, as demonstrated in careful experiments (Mastin and Pollard, eruption can generate tensile stresses in the upper part of the crust, in a 1988), in order to generate surface fractures, the top of the dike must be region roughly half the width of the underlying reservoir, which attain a shallower than about ten times the dike thickness at its top, and these maximum at the surface (Gudmundsson, 1986). The process may repeat fractures start to develop into normal faults only when the dike top is at a itself during each volcano-tectonic event, and location of major surface depth of about five times the dike thickness at its top. We have estimated deformation may change during the evolution of the volcanic systems as a the depth of tension fractures in the Krafla swarm as of the order of several result of changes in the shape of the magma reservoir as well as a result of hundred meters, and major normal faults must attain greater depths. A plate movements. When outside the region above the totally molten upper typical 2- to 4-m-thick dike in Iceland (Gudmundsson, 1983b, 1984a) reservoir, some of the normal faults may develop large throws. This model would have to be within 20-40 m of the surface to generate tension can account for the fracture formation and development, but it needs to be fractures, and the inferred 2-m-thick dike emplaced during one of the worked out in greater detail and connected more directly to the mechanics Krafla events (Pollard and others, 1983; Rubin and Pollard, 1988) would of plate movements. have to be within 10-20 m of the surface to generate fractures. These A model that invokes plate movements in combination with dike results apply to dikes that end bluntly. Most dikes in Iceland taper away at intrusions would bypass the difficulties facing the simple plate-tectonic their upper ends (Gudmundsson, 1983b, 1984a), in which case they would model mentioned earlier. In this model, most dikes do not attain to the have to be even shallower to generate surface fractures. Consequently, surface but build up temporary high horizontal compressive stresses at

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some depth in the crust (Gudmundsson, 1988). During particular rifting REFERENCES CITED events, the uppermost 1-2 km of the crust might be free of tectonic Atkinson, B. K., 1987, Introductions to fracture mechanics and its geophysical applications, in Atkinson, B. K., ed., Fracture mechanics of rocks: London, England, Academic Press, p. 1-26. compressive stress, whereas such stress existed in the deeper layers, and Atkinson, B. K., and Meredith, P. G., 1987, The theory of subcritical crack growth with applications to minerals and rocks, in Atkinson, B. K., ed., Fracture mechanics of rocks: London, England, Academic Press, p. 111-163. relatively large tensile stress would be needed to relax the compressive Ballard, R. D„ and Van Andel, T. H„ 1977, Morphology and tectonics of the inner rift valley at lat. 36°50'N on the stress. High absolute tensile stress may thus be generated in the uppermost Mid-Atlantic Ridge: Geological Society of America Bulletin, v. 88, p. 507-530. Barnett, J.A.M., Mortimer, J., Rippon, J. H., Walsh, J. J., and Watterson, J., 1987, Displacement geometry in the volume part of the crust during relaxation of horizontal compressive stress at containing a single normal fault: American Association of Petroleum Geologists Bulletin, v. 71, p. 925-937. Bernauer, F., 1943, Junge tektonik auf Island und ihre ursachen, in Niemczyk, O., ed., Spahen auf Island: Stuttgart, deeper crustal levels. In this model, the tensile stress field associated with Germany, Wittwer, p. 14-63. plate movements is responsible for the fracture formation, but earlier or Bjornsson, A., 1985, Dynamics of crustal rifting in NE Iceland: Journal of Geophysical Research, v. 90, p. 10151-10162. Bjornsson, A., Saemundsson, K., Einarsson, P., Tryggvason, E., and Gronvold, K., 1977, Current rifting episode in north current dikes may build up horizontal compressive stress at deeper levels in Iceland: Nature, v. 226, p. 318-323. Bjornsson, A., Johnsen, G., Sigurdsson, S., Thorbergsson, G., and Tryggvason, E., 1979, Rifting of the plate boundary in the crust and neutralize the effect of increasing rate of tensile stress genera- northern Iceland 1975-1978: Journal of Geophysical Research, v. 84, p. 3029-3038. tion with depth in the crust. This model may also explain fracture forma- Bjornsson, A., Saemundsson, K., and Steingrimsson, B., 1984, The Krafla eruptions (in Icelandic): Reykjavik, Iceland, National Energy Authority, Orkustofnun, Report OF-84077/JHD-31b, 21 p. tion, but it needs to be worked out quantitatively before it can be regarded Delaney, P. T., Pollard, D. D., Ziony, J. I., and McKee, E. H., 1986, Field relations between dikes and joints: Emplace- as a satisfactory model for the formation and development of the Krafla ment processes and paleostress analysis: Journal of Geophysical Research, v. 91, p. 4920-4938. Einarsson, P., 1978, S-wave shadows in the Krafla caldera in NE-lceland, evidence for a magma chamber in the crust: fissure swarm. Bulletin Volcanologique, v. 41, p. 1-9. Einarsson, P., and Brandsdottir, B., 1980, Seismoiogical evidence for lateral magma intrusion during the July 1978 deflation of the Krafla volcano in NE-lceland: Journal of Geophysics, v. 47, p. 160-165. Eliasson, S., 1979, Kerlingarholar, old eruptive fissures in the Krafla fissure swarm (in Icelandic, with English summary): DISCUSSION Natturufraedingurinn, v. 49, p. 51-63. Gronvold, K., 1984, Myvatn fires 1724-1729. Chemical composition of the lava: Reykjavik, Iceland, Nordic Volcanolog- icai Institute, Professional Paper 8401,24 p. The regional Tertiary dike and fault swarms in Iceland, now exposed Gudmundsson, A., 1983a, Stress estimates from the length/width ratios of fractures: Journal of Structural Geology, v. 5, p. 623-626. at depths of 500-1,500 m below the initial surface of the lava pile 1983b, Form and dimensions of dykes in eastern Iceland: Tectonophysics, v. 95, p. 295-307. 1984a, Tectonic aspects of dykes in northwestern Iceland: Jokull, v. 34, p. 81-96. (Walker, 1974), make it possible to infer the infrastructure of the currently 1984b, A study of dykes, fissures and faults in selected areas of Iceland [Ph.D. thesis]: London, England, University active fissure swarms. In many profiles, dissecting more than 600 dikes in of London, 268 p. 1986, Mechanical aspects of postglacial volcanism and tectonics of the Reykjanes Peninsula, southwest Iceland: eastern Iceland, Gudmundsson (1984b) observed 19 normal faults with Journal of Geophysical Research, v. 91, p. 12711-12721. 1987a, Tectonics of the Thingvellir fissure swarm, SW Iceland: Journal of Structural Geology, v. 9, p. 61-69. throws of 0.5-8.0 m and an average of 2.7 m. In a similar study of more 1987b, Geometry, formation and development of tectonic fractures on the Reykjanes Peninsula, southwest than 400 dikes in northwestern Iceland, Gudmundsson (1984a) observed Iceland: Tectonophysics, v. 139, p. 295-308. 1987c, Lateral magma flow, caldera collapse, and a mechanism of large eruptions in Iceland: Journal of Volcanol- 68 normal faults with throws of 0.5-25.0 m and an average of 5.3 m. No ogy and Geothermal Research, v. 34, p. 65-78. 1988, Effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies: tension fractures were observed in these Tertiary swarms, as they would be Journal of Volcanology and Geothermal Research, v. 35, p. 179-194. either eroded away or intruded by subsequent dikes. Haimson, B. C., and Rummel, F., 1982, Hydrofracturing stress measurements in the Iceland research drilling project drill hole at Reydarfjordur, Iceland: Journal of Geophysical Research, v. 87, p. 6631-6649. Even if throws of as much as several tens of meters occur in the Johnsen, G. V., Bjornsson, A., and Sigurdsson, H., 1980, Gravity and elevation changes caused by magma movement beneath the Krafla caldera, northeast Iceland: Journal of Geophysics, v. 47, p. 132-140. Krafla swarm and other fissure swarms, the average throw is small. The Lachenbruch, A. H., 1961, Depth and spacing of tension cracks: Journal of Geophysical Research, v. 66, p. 4273-4292. most detailed measurements are from the Vogar fissure swarm, where the Macdonald, K. C., 1982, Mid-ocean ridges: Fine scale tectonic, volcanic and hydrothermal processes within the plate boundary zone: Annual Review of Earth and Planetary Sciences, v. 10, p. 155-190. average throw on 35 faults is 2.3 m (Gudmundsson, 1987b). This suggests 1986, The crest of the Mid-Atlantic Ridge: Models for crustal generation processes and tectonics, in Vogt, P. R., and Tucholke, B. E., eds., The geology of North America, Volume M, the western North Atlantic region: Boulder, that outside central volcanoes, most normal faults within active fissure Colorado, Geological Society of America, p. 51-68. swarms (above magma reservoirs) do not develop throws much in excess Mastin, L. G., and Pollard, D. D., 1988, Surface deformation and shallow dike intrusion processes at Inyo Craters, Long Valley, California: Journal of Geophysical Research, v. 93, p. 13221-13235. of a couple of tens of meters because it requires higher and higher tensile Meredith, P. G., Atkinson, B. K., and Hillman, N. B., 1984, in Progress in experimental petrology, 6th report, NERC Publications Series D., No. 25,1984, Natural Environmental Research Council: Swindon, England, p. 228-232. stress to propagate them deeper into the crust. So, instead of a single large Moller, D., and Ritter, B., 1980, Geodetic measurements and horizontal crustal movements in the rift zone of NE-lceland: normal fault, many small ones are formed. When the faults have drifted Journal of Geophysics, v. 47, p. 110-119. Nur, A., 1982, The origin of tensile fracture lineaments: Journal of Structural Geology, v. 4, p. 31-40. out of the volcanically active area they may, however, develop large Palmason, G., 1973, Kinematics and heat How in a volcanic rift zone, with application to Iceland: Royal Astronomical Society Geophysical Journal, v. 33, p. 451-481. throws, because there the spreading is largely taken up by faults instead of Pollard, D. D., Delaney, P. T., Duffield, W. A., Endo, E. T., and Okamura, A. T., 1983, Surface deformation in volcanic dikes. Consequently, the throws on currently active faults in the Krafla rift zones: Tectonophysics, v. 94, p. 541-584. Rubin, A. M., and Pollard, D. D., 1987, Origin of blade-like dikes in volcanic rift zones, in Volcanism in Hawaii: U.S. swarm, which are similar to those observed within the Tertiary dike Geological Survey Professional Paper 1350, v. 2, p. 1449-1470. 1988, Dike-induced faulting in rift zones of Iceland and Afar: Geology, v. 16, p. 413-417. swarms, are unlikely to increase much with time; instead, new faults may Saemundsson, K., 1978, Fissure swarms and central volcanoes of the neovoicanic zones of Iceland, in Bowes, D. R., and develop to accommodate the tensile strain. Leake, B. E., eds., Crustal evolution in northwestern Britain and adjacent regions: Liverpool, England, Seel House Press, p. 415-432. 1979, Outline of the geology of Iceland: Jokull, v. 29, p. 7-28. Many of the tectonic features of the Krafla swarm show great similar- Schmidt, R. A., and Rossmanith, H. P., 1983, Basics of rock fracture mechanics, in Rossmanith, H. P., ed.. Rock fracture ities with small-scale tectonic features of the mid-ocean ridges, in particular mechanics: Wien, Austria, Springer, CISM Courses and Lectures No. 275, p. 1-31. Sigurdsson, O., 1980, Surface deformation of the Krafla fissure swarm in two rifting events: Journal of Geophysics, v. 47, the Famous area of the slow-spreading Mid-Atlantic Ridge (Ballard and p. 154-159. Tryggvason, E., 1980, Subsidence events in the Krafla area, north Iceland, 1975-1979: Journal of Geophysics, v. 47, Van Andel, 1977; Macdonald, 1982, 1986). There is little doubt that all p. 141-153. these tectonic features, both in Iceland and on the mid-ocean ridges, are 1984, Widening of the Krafla fissure swarm during the 1975-1981 volcano-tectonic episode: Bulletin Volcanolo- gique, v. 47, p. 47-69. generated by essentially the same mechanical processes. The results from 1986a, Multiple magma reservoirs in a rift zone volcano: Ground deformation and magma transport during the September 1984 eruption of Krafla, Iceland: Journal of Volcanology and Geothermal Research, v. 28, p. 1-44. the Krafla swarm, as well as from other fissure swarms in Iceland, in 1986b, Vertical ground movement in the Krafla region 1977-1986: Reykjavik, Iceland, Nordic Volcanological combination with data from Tertiary dike and fault swarms, make it Institute, Professional Paper 8602,32 p. Walker, G.P.L., 1965, Some aspects of Quaternary volcanism in Iceland: Leicester Literature and Philosophical Society possible to develop a detailed mechanical model for the formation and Transactions, v. 49, p. 25-40. 1974, The structure of eastern Iceland, in Kristjansson, L., ed., Geodynamics of Iceland and the North Atlantic tectonic evolution of such swarms. Such a model is being developed and, area: Dortrecht, the Netherlands, Reidel, p. 177-188. when completed, should offer new insights into the general process of Watterson, J., 1986, Fault dimensions, displacements and growth: Pure and Applied Geophysics, v. 124, p. 105-115. rifting at divergent plate boundaries.

ACKNOWLEDGMENTS

We thank David D. Pollard, George P. L. Walker, and an anony-

mous reviewer for helpful comments, and the Icelandic Science MANUSCRIPT RECEIVED BY THE SOCIETY MAR™ 29,1988 R REVISED MANUSCRIPT RECEIVED FEBRUARY 23, 1989 Foundation for financial support. manuscript accepted April 12,1989 Primed in U.S.A.

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