Distributed Deformation in an Oceanic Transform System: Applying Statistical Tools to Structural and Paleomagnetic Data Near the Húsavík‐Flatey Fault, Northern Iceland

Abstract

The right-lateral Húsavík-Flatey fault is part of the Tjörnes Fracture Zone, which links the offshore and onshore rift axes in northern Iceland. There has been debate about whether rocks near this fault have accommodated distributed off-fault deformation, which is testable using paleomagnetic data. Recent studies from Flateyjarskagi show clockwise declination deflections that are largest near the fault. We augment these data with new structural and paleomagnetic measurements from 106 lava flows across three peninsulas-Flateyjarskagi, Tröllaskagi, and Tjörnes-also finding clockwise deflections that vary with distance from the fault. To test whether the deflections could be caused by off-fault deformation, we combine our measurements with other regional data sets, applying several statistical tools including regressions of structural or paleomagnetic directions versus fault-normal distance. To evaluate the significance and uncertainties of the regressions, we use permutation tests and bootstrapping. For Flateyjarskagi, our analysis suggests that lavas and dikes were deformed together; the regression results predict 4 ∘ -6 ∘ of rotation per kilometer about a steep, but not vertical, axis. Rocks on Tröllaskagi hint at similar spatial patterns with fault distance, but the data quality precludes a full analysis. Rocks on Tjörnes show no spatial patterns, but they preserve a temporal history, where rotation seems to have ceased after deposition of the Pliocene-age Tjörnes beds. Using constraints from our statistical analyses, geochronology, and comparisons with the transform system in southern Iceland, we propose several modifications to models for the evolution of axial rift zones in northern Iceland.

Problems With Net Tectonic Rotation for Dikes and Suggestions for Alternative Approaches

“…(2018) as well as a broader structural data set from the peninsula compiled in Titus et al. (2018). One confusing tectonic complication is that the rocks on Flateyjarskagi did not initiate from the NVZ but rather from a rift axis to the west that is no longer active (Garcia et al., 2008; Sæmundsson, 1979).…”

“…(This process is outlined in more detail in Text S1 as well as Titus et al. (2018). That latter paper also includes permutation tests, which give evidence that the results are statistically significant.…”

“…While it has long been known that dikes on the peninsula change their strike as a function of distance from the transform fault (Fjäder et al, 1994;Young et al, 1985), paleomagnetic studies only recently documented similar changes on the peninsula (Horst et al, 2018;Titus et al, 2018;Young et al, 2020). Here, we analyze structural and paleomagnetic data from 70 dikes primarily from Horst et al (2018) as well as a broader structural data set from the peninsula compiled in Titus et al (2018). One confusing tectonic complication is that the rocks on Flateyjarskagi did not initiate from the NVZ but rather from a rift axis to the west that is no longer active (Garcia et al, 2008;Saemundsson, 1979).…”

The Iceland Palaeomagnetism Database (ICEPMAG)

“…Palaeomagnetic techniques have also been used to understand the tectonic history of the island (Jancin 2010;Horst et al 2018;Titus et al 2018;Young et al 2018), e.g. palaeomagnetic directions combined with structural data have been used to quantify varying clockwise rotation with distance from the Husavík-Flatey fault in northern Iceland (Titus et al 2018).…”

“…It would not be advised to include data collected to understand, e.g. the influence of tectonic rotation in northern Iceland (Jancin 2010;Horst et al 2018;Titus et al 2018;Young et al 2018) in statistical analyses of palaeosecular variation. This is a compromise in the construction of the ICEPMAG database.…”