Large Rotations of Crustal Blocks in the Tjörnes Fracture Zone of Northern Iceland

Abstract

The interpretation of uppermost crustal deformation near oceanic transform faults is based on bathymetric lineaments and earthquake focal mechanisms, and relatively little is known about the detailed kinematics within the transform tectonized zone. The Tjörnes Fracture Zone is a broad zone of deformation produced by right‐lateral transform shearing in north Iceland and is partly exposed on land providing the opportunity to study shallow‐level crustal structure of mid‐Miocene, thick, oceanic‐like crust formed by subaerial spreading. A pronounced structural curvature of lava and dike orientations near the Húsavík‐Flatey Fault within the transform zone is well documented, yet of controversial origin. In order to develop an assessment of deformation near the transform zone, samples of lavas and dikes were collected from 182 paleomagnetic sites within eight structural localities across the deformation zone on the Flateyjarskagi Peninsula. A progressive clockwise increase in locality mean remanence declinations over more than 10 km south of the fault broadly mimics the structural curvature of lava and dike orientations. Rotation estimates based on inclined rotation axes indicate significant clockwise rotation (74° ± 7° to 96° ± 9°) of multiple crustal blocks. When combined, all data from 108 sites within the deformed zone <12 km to the Húsavík‐Flatey Fault yield a best fit inclined axis rotation of 55° ± 7°. The paleomagnetic data and field relationships are consistent with a modified bookshelf faulting model, with relatively small (~1 km across) independently rotated crustal blocks with variable, and in some cases large‐magnitude rotations found within 10 km to the transform fault zone. Similar crustal deformation and comparable amounts of rotation may be present near other oceanic transforms, where accessibility and surficial deposits may limit documentation of more complex fault structures.

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

“…After reviewing the regional tectonics and controversy in more detail (sections , and ), we present new paleomagnetic data from northern Iceland (sections and ) that compliment the two recent studies by Jancin () and Horst et al (). Then, after combining our results with other regional data sets, we apply statistical techniques to paleomagnetic and structural data sets to constrain the timing of events (section ) and to quantify patterns of deformation on each peninsula (section ).…”

“…The confidence intervals overlap at FLST‐2 when we exclude the k < 50 flows but do not overlap when we keep those flows (Figure S2). Based on these reversal test results, it is reasonable to flip reversed directions to their antipodes, an approach that Horst et al () also take for data from Flateyjarskagi.…”

“…Because paleomagnetic data provide an independent way to assess rotation, several workers (including us) have sampled lavas and dikes in northern Iceland for paleomagnetic analysis. The data from Jancin () and Horst et al () show that rocks on Flateyjarskagi have deflections of paleomagnetic directions that increase near the Húsavík‐Flatey fault. These data may explain the curvature of lava and dikes in the region, although it is difficult to determine whether the magnitude of the structural and paleomagnetic curvatures are the same or different.…”

“…This classic approach has been applied successfully to rocks in other transcurrent systems (e.g., Bonhommet et al, ; Luyendyk et al, ; Ron et al, ; Sonder et al, ). Two recent studies from northern Iceland present convincing paleomagnetic evidence of clockwise deflections from lava flows and dikes on Flateyjarskagi (Horst et al, ; Jancin, ). In particular, the extensive data set from Horst et al () shows that declination deflections are largest close to the Húsavík‐Flatey fault and decrease away from the fault, strongly suggesting that off‐fault deformation plays a role in the region.…”

“…Two recent studies from northern Iceland present convincing paleomagnetic evidence of clockwise deflections from lava flows and dikes on Flateyjarskagi (Horst et al, ; Jancin, ). In particular, the extensive data set from Horst et al () shows that declination deflections are largest close to the Húsavík‐Flatey fault and decrease away from the fault, strongly suggesting that off‐fault deformation plays a role in the region. However, there are still open questions about whether lavas and dikes have experienced the same deformation history.…”