ERWEITERTE ZUSAMMENFASSUNGEN DER PLENARVORTRÄGE VON DER 61. DGG-JAHRESTAGUNG (FRANKFURT/M)Structure and Evolution of the Iceland Hotspot
P. Einarsson, Science Institute, University of Iceland
Introduction
The Iceland mantle plume and its super-position on the plate boundary has pronounced effects on the geological structure and history of Iceland. One of the main effects is that crustal generation is by no means a steady-state process in Iceland, not even when viewed on a long time scale. The fluctuating discharge of the plume has produced a relatively thick crust of variable thickness. The relative movement of the plate boundary with respect to the plume has led to several jumps of the boundary. The transient nature of the boundaries is expressed in complicated structure, particularly in the transform zones.
Rift jumps
Plate boundary has been unstable in the Iceland area since the initial separation of North America from Eurasia. When ridge jumps occur above sea-level, as in Iceland, their symptoms are (1) antiform structures separating the old and new rift (Pálmason, 1986), (2) unconformities between lava sequences formed by the old and new rifts, sometimes associated with a sedimentary sequence (Jóhannesson, 1980), (3) complex fault pattern showing variable crustal stress fields during the jump (Khodayar, 1999).
The main ridge jumps are:
1. Shift from the Labrador Sea to the Greenland and Norwegian Seas at 59-84 Ma (Srivastava and Tapscott 1986).
2. Ægir Ridge to Kolbeinsey Ridge 26-36 Ma (Srivastava and Tapscott 1986).
3. Vestfirðir rift to Snæfellsnes rift at15 Ma (Harðarson et al. 1997).
4. Snæfellsnes Rift to Reykjanes-Langjökull Rift Zone at 6-7 Ma (Kristjánsson and Jónsson, 1998).
5. South Iceland, Western Rift to Eastern Rift at 0-3 Ma (Sæmundsson 1986).
Temporal change in the plume activity
V-shaped ridges.
One of the most remarkable features of the topography of the flanks of the Reykjanes Ridge SW of Iceland are basement ridges that cut obliquely across lines of equal age (magnetic anomalies). They are arranged symmetrically about the spreading boundary and form a V-shaped pattern (Vogt, 1971). A crustal thickening of about 2 km beneath these ridges has been suggested from their gravity signature. From this, it can be deduced that a pair of ridges is formed by a locus of enhanced crustal formation that has moved along the Reykjanes Ridge away from Iceland. The speed of propagation is of the order of 10-20 cm per year, or 100-200 km per million years. It has been determined that pulses of this type moved away from the center of the hotspot at 13-17 Ma and 6-7 Ma (Vogt, 1971). This correlates remarkably well with the timing of rift jumps. It may be concluded that the plume interacts with the plate boundary, attracts or "anchors" it. A ridge jump occurs when the plate boundary has moved a certain critical distance from the plume center. A jump may be triggered when the plume temporarily increases its activity and a new rift zone propagates away from its center.
A pair of V-shaped ridges is presently being formed at the Reykjanes Ridge at 59°30´-61°30´ N (Searle et al., 1998). This part of the ridge crest shows evidence of high volcanic activity, and has low seismicity compared to adjacent segments. From the length of the active segment and the apparent propagation speed the duration of the magmatic pulse can be determined to be about one million years.
Episodic volcanic activity
It has been pointed out by several authors that the volcanic activity within the different volcanic zones of Iceland tends to be clustered in time. Periods of high activity are separated by long intervals of quiescence. Since the plate movements are likely to be continuous in time, the magma supply rate is the most plausible agent for this episodicity, that occurs on several time scales. Examples:
Structure of presently active plate boundariesThe plate divergence in South Iceland appears to shift between the Western and Eastern Volcanic Zone on a time scale of thousands of years (Sigmundsson et al. 1995), most likely responding to the availability of magma in the crust. Thus the rifting activity in historical times is limited mostly to the Eastern Zone, whereas Postglacial rifting is divided almost equally between the rifts. Magmatic activity within the Reykjanes Peninsula´s oblique rift appears to occur in episodes of a few hundred years' duration, separated by quiet intervals of about thousand years, during which the plate separation takes place by bookshelf faulting and high seismicity (Hreinsdóttir et al., in press). Activity in the central area of the plume Vatnajökull is episodic or even semi-periodic, with a period of about 140 years (Larsen et al., 1998). Several of the volcanic systems are active during the active periods. It appears that the plume area is presently entering the active phase of the cycle after a very pronounced quiet interval lasting most of the twentieth century. Recent unrest and eruptions at Bárðarbunga, Grímsvötn and the Loki Ridge are the indications of this increased activity. Seismicity of the Reykjanes and Kolbeinsey Ridges
Seismicity at mid-oceanic ridges shows generally a distinct negative correlation with spreading rate. Fast-spreading ridges have low seismicity and slow-spreading ridges, such as the Mid-Atlantic Ridge, have much higher seismicity. The maximum moment of earthquakes follows the same general correlation. At least two factors have been suggested as being responsible for this, i.e. availability of magma and temperature of the crust. High magmatic productivity will lead to a larger proportion of the spreading being taken up by dyking. Higher temperatures in the crust will lead to a thinner brittle layer and consequently smaller earthquakes. The ridges next to Iceland, the Reykjanes Ridge and the Kolbeinsey Ridge, are notable exceptions to the generally high seismicity of the Mid-Atlantic plate boundary. Both ridges show some of the characteristics of fast-spreading ridges. The central graben disappears, the topography becomes smoother and the seismicity lower as Iceland is approached. This is generally ascribed to the proximity to the hotspot.
Oblique rifts and unstable transforms
The plate boundary is expressed on land by a series of seismic and volcanic zones. Two transform zones connect the presently active Northern and Eastern Volcanic Zones to the ridges off shore. The structure of these transforms is not simple. They are unstable and respond to changes in the configuration of the rifts (Einarsson, 1986).
The South Iceland Seismic Zone
Plate divergence in the southern part of Iceland is accommodated by two subparallel rift zones, the Western and the Eastern Volcanic Zone. The gap between them is bridged in the south, near 64°N, by a zone of high seismic activity, the South Iceland Seismic Zone, which takes up the transform motion between the Reykjanes Ridge and the Eastern Volcanic Zone. It has been argued that rifting is dying out in the Western Zone, and is being taken over by the Eastern Zone, or that the partition of rifting between the rift zones may be uneven and change with time.
The South Iceland Seismic Zone has been defined by destruction areas of historical earthquakes, Holocene surface ruptures and instrumentally determined epicenters. It is oriented E-W and is 10-15 km wide. Destruction areas of individual earthquakes and surface faulting show, however, that each event is associated with faulting on N-S striking planes, perpendicular to the main zone. The over-all left-lateral transform motion along the zone thus appears to be accommodated by right-lateral faulting on many parallel, transverse faults and counterclockwise rotation of the blocks between them, "bookshelf faulting". This mechanism of plate movements was demonstrated in June 2000 when two magnitude 6.5 events occurred on two parallel strike-slip faults in the central part of the zone.
The Reykjanes Peninsula oblique rift
The Reykjanes Peninsula in SW Iceland is a structural continuation of the Reykjanes Ridge. The tectonic structure of the northern Reykjanes Ridge and the Reykjanes Peninsula is characterized by volcanic systems that are arranged en echelon along the plate boundary. The plate boundary is marked by a narrow zone of seismicity. The fissure swarms of the volcanic systems are oblique to the boundary and extend a few tens of kilometers into the plates on either side. Less conspicuous, but probably equally important, are strike-slip faults that cut across the plate boundary at a high angle and resemble the "bookshelf faults" of the transform zone farther east. Earthquake fault plane solutions show a consistent direction of the least compressive stress, horizontal and NW-SE. Maximum compres-sive stress is variable in space and time, from being horizontal, leading to strike-slip faulting on N-S or E-W striking faults, to vertical, leading to normal faulting on NE-SW striking faults.
Magmatic activity on the peninsula is quite episodic. Active episodes of a few hundred years' duration are separated by quiet periods on a time scale of a thousand years. The latest major magmatic episode occurred in the tenth to thirteenth centuries and no eruptions have taken place on the peninsula since 1240 AD. No seismic evidence has been found for intrusive magmatic activity in recent decades in any of the volcanic systems on the peninsula.
The fissure swarms on the Reykjanes Peninsula occupy a much wider zone than the epicentral belt. They have the general structure of shallow grabens, and are structurally identical to other fissure swarms of the divergent plate boundaries in Iceland, including the Krafla fissure swarm. It is therefore natural to interpret them in a similar way, as the result of repeated dyke injection into the crust.
Crustal deformation along the plate boundary on the Reykjanes Peninsula thus appears to occur in two different modes: (1) dry or seismic mode, and (2) wet or magmatic mode. Deformation in the dry mode occurs during periods when magma is not available to the crust in any appreciable quantity.
The Tjörnes Fracture Zone
The other major fracture zone of Iceland, the Tjörnes Fracture Zone, is located near the north coast. It is a broad zone of seismicity, transform faulting and crustal extension that connects the southern end of the submarine Kolbeinsey Ridge to the volcanic zone in North Iceland. The transform motion appears to be taken up by several parallel NW-striking seismic zones. The northernmost one, the Grímsey zone, which is entirely off shore, has an overall NW-SE trend but N-S structural elements are also prominent. A series of graben-like troughs with this trend have been identified, indicating crustal dilation. The larger earthquakes, on the other hand, seem to be associated with horizontal shear and transform motion. The Grímsey zone joins the Northern Volcanic Zone in the Axarfjörður Bay, where it merges with the Krafla fissure swarm. The intimate relationship between rifting activity in the fissure swarm and transform faulting along the Grímsey zone was demonstrated in January 1976, when a large transform earthquake (MS 6.5) occurred at the junction, immediately following a major rifting episode in the Krafla fissure swarm.
The second seismic zone is about 40 km south of the first one, and is well defined by the seismicity near its western end, near the mouth of Eyjafjörður. The fault zone can be traced on the ocean bottom to the coast in the Húsavík town, continuing on land into the volcanic zone, where it merges into the Theistareykir fissure swarm. A major earthquake sequence with at least two events of M 6-6.5 occurred in this zone in 1872 . A third zone, the Dalvík zone, is indicated about 30 km south of the Húsavík faults. Earthquakes in this zone include the damaging earthquake in the town of Dalvík in 1934 (M 6 1/4) and the M 7 earthquake of 1963 in the mouth of Skagafjörður. In spite of rather clear alignment of epicenters, the Dalvík zone is not seen as a throughgoing fault on the surface. A common feature of all three seismic zones is the occurrence of earthquakes on transverse structures (Rögnvaldsson et al., 1998), similar to that observed in the South Iceland Seismic Zone and the Reykjanes Peninsula. The similarity of the Reykjanes Peninsula zone and the Grímsey zone is particularly striking. The two zones show a high degree of symmetry with respect to the plate separation vector. The similarity is seen in the overall trend, the en echelon fissure swarms, transverse bookshelf faults, and the occurrence of geothermal areas.
In summary:
The presence of the Iceland mantle plume near the Mid-Atlantic plate boundary leads to the formation of a thick crust and a complicated structure of the active zones. Ridge jumps, rift propagation, bookshelf faulting, oblique rifting and subdued seismicity are a few of the associated phenomena.
The activity of the Iceland plume fluctuates with time on many time scales. The time scales of fluctuations are:
106 years: V-shaped ridges, rift jumps;
At present the hotspot seems to be going from a quiet state to an agitated state. At least six volcanoes have been in a state of unrest during the last 5 years: Bárðarbunga, Grímsvötn, Hekla, Katla, Eyjafjöll and Hrómundartindur. 104 years: Shift in activity within rifts, and between E- and W-rift; 103 years: Magma production within rifts, Reykjanes Peninsula;
102 years: Activity of the Vatnajökull area, seismicity;
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