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Hawaii Institute of Geophysics and Planetology

Department of Geology and Geophysics

SOEST

UH Manoa

Iceland

Click on the above free air gravity image to see a full-size version.

Click on the above figure to see a full-size version.

Click on the above Vestmannaeyjar bathymetry image to see a full-size version.

Click on the above Reykjanes Ridge EM300 backscatter data to see a full-size version.

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 based on a large-scale survey of the Reykjanes 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) 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. The computer graphics movies of Reykjanes Ridge evolution Ásdís created, & her new magnetic anomaly modeling program, will be available soon on her website.

The involvement of rift propagation in VSR formation suggests 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.

Publications

Oceanographic Expeditions

• ** Chief Scientist
• * Co-Chief Scientist

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Students involved in this research

• Ásdís Benediktsdóttir
• Tryggvi Bjorginsson
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Comments or questions about this page go to illust@soest.hawaii.edu. Last update Wed 14 Sep 11.