Tectonic stress and magma chamber size as controls on dike propagation: Constraints from the 1975–1984 Krafla rifting episode

Abstract

[1] The best-studied dike intrusion events on a divergent plate boundary occurred along the Krafla segment of the northern rift zone in Iceland from [1975][1976][1977][1978][1979][1980][1981][1982][1983][1984]. Seismic and geodetic measurements there showed that a central magma chamber fed dikes that propagated laterally many times the thickness of the lithosphere. The patterns of dike length, dike width, caldera subsidence, and lava extrusion strongly suggest that dike propagation is affected by tectonic stresses that change with each dike intrusion event and that magma pressures are linked to the dike opening. These observations have inspired us to develop a quantitative model for the lateral propagation of basaltic dikes away from a magma chamber. We assume dikes propagate as long as there is sufficient driving pressure, defined as the difference between magma pressure and tectonic stress at the dike tip. The opening dike and the magma chamber are treated as a closed system for a given dike intrusion event. During an event, magma pressure is reduced linearly with the magma volume withdrawn from the chamber. Relative tectonic tension in the lithosphere is reduced linearly as the dike width increases. A dike begins propagation when the driving pressure equals the ''breakout'' pressure needed to force the magma out of the chamber. It stops when the driving pressure reaches a minimum value. Generally, the dike width is proportional to this ''stopping'' pressure, and a reasonable value gives a width of 1 m. Besides the breakout and stopping pressures, the propagation distance depends on the initial distribution of tectonic stress and the thickness of the lithosphere cut by a dike. The intrusion of a dike changes the tectonic stress distribution so that subsequent dikes may propagate different distances and directions than the first dike. After a period of magma chamber refilling, a new dike can initiate if the breakout pressure is reached. For an idealized spreading segment the tectonic stress field evolves to produce a sequence of dikes propagating in one direction followed by a sequence of dikes propagating in the opposite direction. The first dike in each sequence should be the longest followed by successively shorter dikes. When tectonic stresses close to a magma chamber have been largely relieved, then extrusion of magma may start. The model pattern of dike propagation and extrusion is consistent with data from the Krafla episode. Magma chamber size should have a major effect on magmatic systems in other tectonic settings with larger magma chambers producing longer characteristic dikes.Citation: Buck, W. R., P. Einarsson, and B. Brandsdóttir (2006), Tectonic stress and magma chamber size as controls on dike propagation:

Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland

“…The strike of this dislocation was set to N12¡E, to be about perpendicular to direction of plate movements predicted by global plate motion models. We assume the tectonic stress throughout the depth interval of the crust considered does not vary with depth, similarly to the approach of Buck et al 3 . The value of stress we use is that calculated at 10 m depth in the dislocation model.…”

“…Our approximate model has two dipslip faults at the boundary of the caldera and an underlying magma source; either a spherical or a flat top chamber. In approach (i) the best-fit models have a spherical chamber centred at 1.3-1.5 km depth below sea level and volume change of 0.26-0.29 km 3 , or a flat-topped chamber stretching from 3.4-3.6 km downwards and volume change of 0.24-0.31 km 3 (Extended Data items 5 and 6, Supplementary Figs. 5 and 6).…”

Interaction between Transform Faults and Rift Systems: A Combined Field and Experimental Approach

“…About 2.4 ka BP, the latest products of this eruptive center were emplaced, forming the "Theistareykjahraun" lava flows, located between the shield and the HFF (Saemundsson et al, 2012). The latest major event that occurred nearby the studied area was the rifting episode at the Krafla volcanic system from 1975 to 1984 (Bjornsson, 1985;Buck et al, 2006), resulting in several surface meter-large horizontal and vertical offsets along nearby fractures (Tryggvason, 1980(Tryggvason, , 1984(Tryggvason, , 1986. Such rifting episode was capable of inducing stress changes in the nearby fault systems within the TFZ; in particular, the 1975 dyke intrusion triggered the 1976 Köpasker earthquake as a consequence of induced Coulomb stress on the Grímsey Oblique Rift (Maccaferri et al, 2013).…”

Geophysical constraints on the dynamics of spreading centres from rifting episodes on land

“…The dyke was associated with a minor eruption within the Krafla caldera, which subsided by up to 2 m, consistent with a pressure drop in the shallow magma chamber. The initial dyke was followed by a sequence of smaller intrusions that began in September 1976 and occurred in irregular sequences until 1984 29,39 (Fig. 2).…”

“…Subsequently, eruptive activity increased with six of the final seven dykes breaching the surface and volumes increasing. It is likely that the change in activity reflects a reduction in extensional stresses to a level that did not allow dykes to propagate over long distances 39 .…”