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 objec- tive 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). 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 1610 OPHEIM AND GUDMUNDSSON 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). 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 KRAFLA FISSURE SWARM, ICELAND 1611 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 de- creases gradually, being 2 m on the north coast at Axarfjordur (Tryggva- During the present rifting episode, almost the whole fissure swarm son, 1984).