Using Small-scale Structures to Constrain Distributed Deformation near the Húsavík-Flatey Fault, Northern Iceland
Seth Waag-Swift, Dept. of Geology, Carleton College, Northfield, MN 55057
Sarah Titus, Dept. of Geology, Carleton College, Northfield, MN 55057
Joshua Davis, Dept. of Mathematics and Statistics, Carleton College, Northfield, MN 55057
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The dextral Húsavík-Flatey Fault in northern Iceland is part of an active oceanic transform system connecting two rifts: the onshore Northern Volcanic Zone and the offshore Kolbeinsey Ridge. Although the fault lies offshore near the peninsula of Flateyjarskagi, rocks exposed onshore are also deformed. At the largest scale, lavas and dikes demonstrate a well-documented clockwise deflection with proximity to the fault. Far from the fault, lava bedding dips shallowly to the south and west. At intermediate distances, lavas dip west and close to the fault, the dips are towards the northwest and steepen to 40 degrees. Over the same distance, subvertical dikes strike NNE-SSW far from the fault; these become NE-striking and eventually ESE-WNW striking near the Husavik-Flatey fault. Paleomagnetic data from lavas and dikes confirm that these curved patterns result from rotations after lava deposition and dike injection. Given the strong fault-normal gradient in structural and paleomagnetic directions, rocks on the peninsula allow us a unique opportunity to characterize patterns of off-fault deformation due to an oceanic transform fault.
The mechanism of these rotations remains an open question but we believe that small-scale structures may provide some clues. Thus, we add to the significant body of structural data from the region, with new measurements of dikes, mineralized veins and small-scale faults. The faults occur both as cataclastic zones of crushed rock with localized planar displacement surfaces, as well as slickensided surfaces that curve around lozenge-shaped blocks. Our approach is somewhat different than previous studies in recognition that there has been distributed deformation. Instead of using fault-slip data to infer absolute paleostress directions, we analyze their orientations in context with other structural data types on a site-by-site basis. We allow for the possibility of structures reactivating in different modes due to simultaneous reorientation and deformation. Then we build a picture of regional deformation using sites not only concentrated along the northern coast of Flateyjarskagi, nearest the Húsavík-Flatey fault and where exposure quality is highest, but also in the interior of the peninsula to improve our understanding of spatial variability. We apply a variety of statistical tools to these structural data to support our inferences about relative orientations and spatial variability, with an ultimate goal of constraining the style and mechanisms of off-fault deformation.