The position of Iceland over the Mid-Atlantic Ridge is an ideal setting in which to investigate mid-ocean ridge processes and the effects of hotspots on these processes. Iceland is one of only two places on Earth where a seafloor spreading center rises above sea level, which allows work on the ridges to be placed in the extensive geological and geophysical context established for subaerial Iceland. The reorganizing plate boundaries in Iceland show some characteristics of propagating rifts and microplate tectonics
In July–August 2003 we conducted brief marine geophysical investigations of both Vestmannaeyjar and the Reykjanes Ridge. This was a collaborative effort with Ármann Höskuldsson of the University of Iceland, Neal Driscoll of the Scripps Inst. of Oceanography, and Bob Detrick of the Woods Hole Oceanographic Inst. We used the SIO SUBSCAN chirp seismic system to collect seismic profiles and sidescan sonar swaths, and the Iceland EM300 system to acquire swath bathymetry and backscatter data. Vestmannaeyjar, including the recently active eruptive centers of Surtsey and Heimaey, may be forming at the tip of the reorganizing North America-Eurasia plate boundary system, where the seafloor spreading on Iceland appears to be shifting from the Western Volcanic Zone to the Eastern Volcanic Zone (EVZ). A linear chain of small islands and seafloor eruptions is consistent with Vestmannaeyjar being the southern extension of the EVZ. The seafloor volcanic pattern shows large spatial variability, with only a few indications of tectonic rift structures seen in either the seafloor or subsurface data. If this is the tip of a propagating rift, the rifting has not yet focussed the volcanic eruptions into a single coherent eruptive segment. In contrast to Vestmannaeyjar, a brief EM300 survey of the first Reykjanes Ridge segment offshore of the Reykjanes Peninsula shows that seafloor spreading here has been active long enough that the volcanism has coalesced into a focussed volcanic system. Few rift structures are visible here either, perhaps because shallow explosive eruptions tend to bury them. The approximately 20 degree change in azimuth between the en echelon volcanic system trends on the Reykjanes Peninsula and those observed on the Reykjanes Ridge occurs within this first offshore segment.
Our major present research effort is focused on the Reykjanes Ridge , beginning with a large-scale survey of that ridge system we conducted in June-July 2007 to determine how this system has evolved and responded to the known plate boundary reorganizations on land. Our goal is a seamless history of the plate boundary geometry both at sea and on land, an essential step toward the full understanding of Iceland and the geodynamic influence of the hotspot or mantle plume on the mid-ocean ridge system. Our initial results (Hey et al., 2010) showed that the conventional plate tectonic wisdom about this area, symmetric seafloor spreading on the Reykjanes Ridge and V-shaped ridges, troughs and scarps (VSRs) symmetric about the spreading axis, is not correct. The VSRs have an asymmetric geometry consistent with a rift propagation origin instead of the presumed symmetric geometry that had been considered compelling evidence that Iceland was a pulsing plume. Although we had previously noted that plume pulses might drive the propagators away from Iceland, a significant new result (Ásdís Benediktsdóttir, 2011; Benediktsdóttir et al., 2012,2013) is that excellent magnetic anomaly fits can only be achieved if some rift propagation toward Iceland has also occurred. These newly identified propagators toward Iceland can’t be driven by plume pulses even if the ones propagating away from Iceland are. As part of her thesis work Ásdís created computer graphics movies of Reykjanes Ridge evolution, and a new magnetic anomaly modeling program. Her rift propagation evolution model for the RR can be viewed schematically in a YouTube video. In this animation the horizontal axis is the distance from ridge and the vertical axis is the distance from the Reykjanes Peninsula. Green lines connect the pseudofaults of northward propagators, blue lines connect the pseudofaults of southward propagators (L, Loki; F, Fenrir; S, Sleipnir; H, Hel), and red dots are failed rifts. This evolution overlain on gravity derived from satellite altimetry (Sandwell & Smith, 2009) is also on YouTube. The pseudofaults (black circles) are connected by solid and dashed lines for southward- and northward propagators, respectively. Red circles are failed rifts. For each time the gravity is gridded in the area bounded by 19 Ma, the current time, and profiles 17 and 25 on each ridge flank. The two areas are then rotated toward each other to close the space between the ridge and the areas. For further details see a summary of Benediktsdóttir et al., 2012.
The evident involvement of rift propagation in VSR formation suggested that this is also a possible explanation for the major ongoing diachronous reorganization of North Atlantic seafloor spreading occurring at present south of Iceland, from an orthogonal ridge/transform geometry to the present oblique spreading geometry without transform faults on the Reykjanes Ridge (Hey et al., 2011). This reorganization is presently interpreted as a thermal phenomenon, with a pulse of warmer mantle expanding away from the Iceland plume causing a progressive change in subaxial mantle rheology from brittle to ductile, so that transform faults can no longer be maintained. Given that this is certainly the most obvious and arguably the type-example of active plate boundary reorganization, it is somewhat surprising that a thermal mechanism has near universal acceptance here whereas most if not all other seafloor spreading reorganizations are equally universally thought to result from the tectonic rift propagation mechanism. If propagating rifts are the mechanism causing this change in plate boundary geometry, the tip of the reorganization would presently be near the first transform fault south of Iceland, the Bight transform near 56.8N, rather than in the extensively surveyed area 200 km farther north where the thermal reorganization model predicted the reorganization tip should be.
R. Hey, F. Martinez, A. Hoskuldsson, Á. Benediktsdóttir, Propagating Rift Origin of the V-Shaped Ridges South of Iceland. IAVCEI, General Assembly, 2008.
Á. Benediktsdóttir, T. Björginsson, R. Hey, Magellan, a new program to model magnetic anomalies, IAVCEI, General Assembly, 2008.
Á. Höskuldsson, R. Hey, F. Martinez, Á. Benediktsdóttir, E Kjartansson, Á. Vésteinsson, N. Driscoll, Hafsbotnsrannsóknir fyrir Sudur- og Sudvesturlandi, HaustrÁdstefna Jardfrædafélags íslands, 23 October, Reykjavik 2009.
Á. Höskuldsson, R. Hey, F. Martinez, Á. Benediktsdóttir, Njördur central volcano, first direct evidence of shallow magma chambers on the Reykjanes Ridge, 29th Nordic Geological Winter Meeting, January 11-13, Oslo, 2010.
Hey, R., F. Martinez, Á. Höskuldsson, and Á. Benediktsdóttir, Propagating rift model for the V-shaped ridges south of Iceland, Geochem. Geophys. Geosyst., Vol. 11, No. 3, Q03011, doi: 10.1029/2009GC002865, 2010.
Hey, R., The pulsing Iceland plume: A personal evolution from believer to agnostic, www.mantleplumes.org/Disclosure.html, 2010.
Benediktsdóttir, Á. (2011), Detailed tectonic evolution of the Reykjanes Ridge during the past 15 Ma using Magellan, a new tool for modeling magnetic anomalies, M.S. Thesis, Univ. Hawaii, 109 pp.
Hey, R.N., Seafloor Spreading, in Encyclopedia of Solid Earth Geophysics, ed. H. Gupta, Springer, Dordrecht, ISBN 978-90-481-8701-0, 1055-1059, 2011.
Benediktsdóttir, Á., R. Hey, F. Martinez & Á. Höskuldsson, Detailed tectonic evolution of the Reykjanes Ridge during the past 15 Ma, Geochem. Geophys. Geosyst., Vol. 13, No. 2, Q02008, DOI:10.1029/2011GC003948, 2012.
Benediktsdóttir, Á., R. Hey, F. Martinez & Á. Höskuldsson, (2013), Evolution of the Reykjanes Ridge during the past 15 Ma, www.mantleplumes.org/RR_VSRs.html
Hey, R.N., Propagating Rifts and Microplates at Mid-Ocean Ridges, in Elsevier’s Science Direct online reference program, Earth Systems and Environmental Sciences, ed. S. Elias, in press, 2013.
Benediktsdóttir, Á., T. Bjorginsson, F. Martinez & R. Hey (2011), Magellan: A new magnetic anomaly modeling program, preprint for G-cubed.
**2007 June-July, Seabeam, magnetics, and gravity survey of the Reykjanes Ridge south of Iceland, Reykjavik- Reykjavik, SOEST, University of Hawaii, chief scientist R.N. Hey, R/V KNORR