USING STATISTICAL TOOLS TO UNRAVEL THE DEVELOPMENT OF THE HÚSAVÍK-FLATEY FAULT, NORTHERN ICELAND

The Húsavík-Flatey fault is part of a transform zone linking the mid-Atlantic Kolbeinsey Ridge to the onshore Northern Volcanic Zone in northern Iceland. This dextral fault is located offshore of the Tröllaskagi and Flateyjarskagi peninsulas and exposed on Tjörnes. Numerous workers, including us, have collected structural and paleomagnetic data from lavas and dikes on all three peninsulas. We apply a variety of statistical tools to these large data sets including fault-normal regressions to characterize patterns of deformation perpendicular to the Húsavík-Flatey fault and permutation tests to determine the significance of these regressions.

On the western peninsula of Tröllaskagi, there are hints of distributed deformation in both structural and paleomagnetic data, with slight deflections from the expected reference directions that are correlated with fault-normal distance. On the central peninsula of Flateyjarskagi, this pattern is fully developed with clockwise rotations observed over 20 km southwest of the fault. Our regressions, which are statistically significant, explain ~50% of the data variability. We predict that deformation occurred about steeply S-plunging axes with ~4° to 5° of rotation per kilometer. On the eastern peninsula of Tjörnes, the oldest rocks show significant deflections from expected directions but no spatial patterns with distance from the fault. Younger rocks on the peninsula may show continued deformation after the deposition of the Tjörnes beds (between 5.3 and 3.8 Ma). Other than some minor tilting in Matuyama-age lavas, the youngest lavas appear to be relatively undeformed.

Taken together, these analyses provide constraints for regional tectonic models. Any model must include a phase of deformation that is almost shear-zone like in its patterns of deflection based on results from Flateyjarskagi and, to a lesser extent, Tröllaskagi. This distributed deformation likely ends around the time of the Tjörnes beds. Continued translation of the Tjörnes block along a discrete fault likely brought the block to its present-day location near the Northern Volcanic Zone, where transform-related deformation would not affect the youngest rocks. Using available rock ages, we integrate these constraints into a proposed model for the rift and transform evolution in northern Iceland.