SMALL-SCALE FAULTING AND BLOCK ROTATION NEAR THE HÚSAVÍK-FLATEY TRANSFORM FAULT IN NORTHERN ICELAND

The dextral Húsavík-Flatey transform fault is exposed across two peninsulas in northern Iceland. These on-land exposures provide a rare opportunity to characterize deformation around an oceanic transform fault. Previous studies present paleomagnetic and structural data from dikes and lava flows indicating that crustal blocks within 10-15 km of the fault have rotated clockwise by variable magnitudes. Rotation magnitudes form a gradient with proximity to the fault and exceed 100° in the nearest on-land areas.

We investigate the extent to which this rotation gradient is recorded by small faults in the deformed zone. Our data set includes more than 1000 measurements of small faults with slip directions that are generally left-lateral, right-lateral, or normal. Small faults far away from the transform fault are used as a benchmark with which to compare faults from the deforming zone. We use orientation statistics to quantify the rotation gradient and produce predictions for the orientations of rotated faults. Some faults conform to the patterns observed in the far-field data set, while others are consistent with predictions based on the rotation gradient. The presence of both rotated and unrotated faults introduces a temporal variability that has previously been attributed to variations in the stress field around the transform fault.

Patterns in small faults are consistent with a modified bookshelf geometry, which could have accommodated the large-scale block rotations recorded by paleomagnetic data. The magnitudes of the rotations suggest approximately 10 km of fault-parallel displacement has accumulated on antithetic faults in the deformed zone south of the fault scarp. It is likely that a bookshelf-like system developed with the formation and propagation of the adjacent ridge segment around 7-9 Ma. Therefore, the distributed deformation recorded adjacent to the Húsavík-Flatey fault may represent an early phase in transform development along the mid-Atlantic ridge.