RIDGE 2000: Nomination of Reykjanes and Kolbeinsey Ridges and Iceland

Summary of Previous Seismological Findings

prepared 01/18/01 by William Menke
for the RIDGE 2000 Iceland Proposal
at the request of Yang Shen (yang@island.gso.uri.edu)

The overall character of the Reykjanes Ridge is strongly influenced by the Iceland Plume. The plume itself was first imaged in the early 1980's by Trygvasson el al. (1983), who used short period teleseismic delay times to trace it to several hundred kilometers depth in the mantle. Subsequent seismic imaging by Wolfe et al. (1993) showed that the plume is quite narrow (~100 km diameter, centered in southeastern Iceland) and extends to at least 400 km depth. They quantified the compressional and shear wave velocity anomalies to be 2% and 4% respectively, corresponding to a temperature anomaly of at least 300K above the surrounding mantle. Tomography using terrestrial stations in Iceland is not able to resolve features deeper than about 400 km. Nevertheless, several lines of evidence suggest that it extends deeper. Shen et al. (1998) argue that the plume extends at least through the transition zone, because its perturbing effect on the depth of 410 and 670 km discontinuities is detected by receiver function analysis. Tomography using globally-distributed stations also provides some evidence that the plume reaches the lowermost mantle (Bijwaard and Spakman).

Tomographic images of the upper part plume have been further improved by Allen (2001), who supplemented teleseismic traveltimes with surface wave measurements in order to image top 100 km, the area in which melting is most intense. He shows that the plume widens in the very uppermost mantle. This plume head extends laterally under all of Iceland (and perhaps further). The thermal anomaly beneath Reykjanes Ridge seems, on the basis of fairly low-resolution surface wave tomography, to be limited to above 200-250 km, with a shear velocity minimum of about 2-3% occurring at about 150 km depth (Zhang and Tanimoto, 1993). The transition from a plume-dominated upper mantle beneath Iceland to a presumably narrower normal ridge anomaly to the south has not been adequately imaged.

The high melt production from the Iceland plume has lead to the formation of the Faero-Iceland-Greenland ridge, a band of extremely thick crust that crosses the north Atlantic. Crustal thicknesses generally increase from 8-10 km on the Reykjanes Ridge (Ritzert and Jacoby, 1985), to 14 km at the southern tip of the Reyjanes Pennisula (Weir et. al. 2000), to 20-25 km in south Iceland (Bjarnason et al. 1993) to 40 km beneath the center of the plume (Darbyshire et al. 1998). The crust appears to be in isostatic balance, with long-wavelength topography correlating with Moho depth (Menke, 1999). The seismic structure of the crust south of Iceland is normal. Most of the northward thickening occurs in the cummulate section of the lower crust (e.g. Layer 3) (Zehnder and Mutter, 1990).

Seafloor spreading has been very uniform on the Reykjanes Ridge, leading to magnetic anomalies that are extremely linear but oblique (by about 30 deg) to the spreading-perpendicular direction (Searle et al. 1998). The ridge has no major, first order segmentation, a fact that has been ascribed to the presumably especially hot lower crustal temperatures that allow crustal flow to smooth out bathymetry (Bell and Buck, 1992). The ridge axis is composed of en-echelon axial volcanic ridges (AVR's), oriented sub-perpendicular to the spreading direction, with average spacing of 14 km and overlap of about one third of their lengths.

As one proceeds from the Reykjanes Ridge, across the Reykjanes Penninsula and into the neovolcanic zones of Iceland, these AVR's gradually blend into Iceland's volcanic systems, which retail the same general en echelon form, but on a larger (50-100 km) scale. Each of these systems is composed of a central volcano and an associated fissure swarm. The upper crust of Iceland is formed when magma from the shallow (3-4 km) crustal magma chambers (Brandsdottor et al. 1997) of these volcanoes intrudes into the fissure swarms during lateral diking events (which are accompanied by numerous earthquakes in the upper 5 km) (Einarsson and Brandsdottir, 1980).

The origin of the lower crust is less well understood. Palmasson (1971), citing ridgeward dipping lava flows, has argues for a large component of lower crustal flow driven by primarialy shallow accretion of crust in the neovolcanic zones. Further thermal modeling by Menke and Sparks (1995) indicates that shallow cooling of lower crustal material is needed to account for the relatively low lower-crustal temperatures inferred from the relativel high (Qs=1000) shear wave atttenuation (Menke et al. 1995) and the relatively deep maximum depth of earthquakes (6 km below the neovolcanic zone, deepening to 14 km in 5 Ma crust) (Stefansson et al.). Whether all of Iceland's neovolcanic zones are fed from mantle sources located directly beneath them, or whether some are fed by lateral, near-Moho transport from the region just above the plume is not known. A band of relatively low shear velocity that extends from the plume center along the northern neovolcanic zone is arguably suggestive of the later (Allen, 2001).

Seismology has contributed little to estimates of the depth of melting beneath Iceland. Thermal models indicate that a "lid" of subsolidus mantle probably occurs (Menke and Sparks, 1995). This cool "lithosphere" may explain the observation that uppermost mantle compressional and shear velocities (i.e. from Pn and Sn) are somewhat higher in central Iceland than at the northern and southern coasts (Menke et al. 1998). Magmatic enrichment of the upper mantle in olivine cumulates (i.e. dunite bodies) may provide an alternate explanation for these higher velocities (Allen 2001).

Shallow crustal magma chambers have been tomographically imaged below several of Iceland's volcanoes (Gudmundsson et al. 1994, Brandsdottir et al. 1997) and are believed, on the basis of physiography (e.g. calderas) to occur below many more. Those that have been imaged occur in the shallow (2-4 km) crust, have lateral dimensions of 3-8 km and contain several tens of cubic kilometers of melt. These magma chambers appear to be distinctly different than the one observed 2.5 km below an AVR on the Reykjanes Ridge at 57.75 N (Navin et al. 1998; Gill et al. 2000). That feature is quite similar to the axial magma chambers observed on the East Pacific Rise, and consists of a thin (100 m) melt lens that is fairly continuous along axis, atop of a broader mush zone. Nothwithstanding this difference, the eruptive style of the AVR's, which include earthquakes swarms and lateral diking, seems quite similar to what occurs in Iceland (Crane et al., 1997).

Lateral transport of plume-derived material from Iceland southward along the Reykjanes Ridge is evident from both the ridge's bathymethry (which slopes up toward Iceland) and from the V-shaped ridges which demark pulses of especially vigorous southward transport (White et al. 1995). The V-shaped ridges themselves have both a gravity and topographic expression, and are presumed to be caused by variations in crustal thickness. Ito (2000) is able to model the overal shape of the ridges with a model in which pulses in plume flux lead elevated upper manlte temperatures that propagate outward from Iceland as rings. These rings cause excess melting, and hence crustal thickening, beneath the Reykjanes Ridge. The reason for the plume flux pulses themselves is not known.

OBS deployments along the northern Reykjanes Ridge (near 62.5 N) (Mochizuki et al., 2000) have detected numerous, mostly normal faulting events that occur on the flanks between the AVS's (implying that they are probably not associated with magmatism). Their depths are concentrated between 3 and 7 km, with some as deep as 11 km. Thus most events are observed to occur in the mid crust, with possibly a few extending as deep as the uppermost mantle. This relatively deep depth probably implies that vigorous hydrothermal cooling is taking place between the AVS's.

Kolbeinsey Ridge, the continuation of the mid-Atlantic ridge north of Iceland, is offset to the west of Iceland's northern neovolcanic zone by a tectonically complicated region that includes the subparallel Tjornes and Husavik-Flatley transform faults (Einarsson 1986, Gudmundsson, 1995). Seismicity along these faults have been investigated by using Ocean Bottom Seismometers (Mochizuki et al. 1995) and terrestrial stations on Iceland (Angelier et al. 1999). Earthquakes are generally shallow (<10 km) and have mostly strike-slip mechanisms. The two faults differ, however, in that the Tjornes fault seems to be a mateur through-going fault with left right-lateral motion, while the Husavik-Flatley fault seems to consist of numerous en enchelon segments that have left-lateral motion (i.e. bookshelf tectonics). The Husavik-Flatley is therefore interpreted as an transorm in its initial stages of formation.

Plate-tectonic spreading on Kolbeinsey Ridge is close to perpendicular to its overall axis (unlike Reykjanes, which is quite oblique) (Vogt et al. 1980, Applegate 1994, Kodaira et al. 1998). The Kolbeinsey Ridge if less linear than the Reylkanes Ridge, being offset by several transorm faults, including the major Spar Fracture Zone at 69N (Eldholm et al. 1990). Numerous earthquakes up to a magnitude of about 5 occur on these fautls and are well-recorded in Iceland (and elsewhere). The overall character of the ridge seems less influenced by the Iceland hot spot than does Reykjanes Ridge (e.g. no V-shaped ridges), which is perhaps due to asthenospheric return flow in this region being mainly from north to south (Shen 2000).

Crustal structure on Kolbeinsey Ridge at has been investigated by Kodiara et al. (1997) who perform an OBS-based refraction experiment at 70N. Crustal thickness, constrained by PmP traveltimes, is about 10 km on an off-axis line. More variability is seen on an on-axis line, with thickness varyiong between about 8 and 12 km over a horizontal scale length of 50 km. No magma chambers are detected, but lower crustal velocities are generally slower on-axis than off-axis (6.6-7.0 compared to 6.9-7.2), a fact that is attributed to elevated temperatures beneath the ridge axis.

Processes that can be explored using seismological techniques


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