TRANSFORM FAULTS IN THICK, ANISOTROPIC OCEANIC CRUST OF ICELAND: EVOLUTION OF STRUCTURES ASSOCIATED WITH OBLIQUE TO PARALLEL PLATE MOVEMENT

Transform faults in oceanic crust are fundamental components of Earth’s lithosphere plate boundary system, yet little is known of their fault structures and kinematics. Subaerial exposures of transform fault zones in Iceland provide a unique perspective that may be relevant for transform faults along mid-ocean ridges and other settings. Active rift zones transecting Iceland are offset from the regional trend of the Mid-Atlantic Ridge by the Tjörnes Fracture Zone (TFZ) in the north and the South Iceland Seismic Zone (SISZ) in the south. These transform systems migrate as rift zones propagate to the north and south, away from the center of the Iceland Hot Spot. Active and abandoned transform faults show a range of structures corresponding to different amounts of displacement in thick, anisotropic oceanic crust. Based on these relationships the transform faults evolve as follows: 1) Formation of an oblique rift (or leaky, transtensional transform) with basins and magmatic centers in pull-aparts, linked by bookshelf fault zones (antithetic strike-slip faults at a high angle to the spreading direction) (Grimsey Fault Zone, youngest part of the TFZ); 2) broad zone (tens of km) of conjugate faulting and dike intrusion (Reykjanes Peninsula and Hreppar Block N of the SISZ); 3) narrower (<20 km) zone of bookshelf faulting aligned with the spreading direction (SISZ); and 4) mature, narrow (~1 km) through-going principal transform displacement zone cutting previously deformed crust (bookshelf faulting and block rotations) distributed over 10 km to either side (Húsavík-Flatey Fault Zone in the TFZ). With progressive slip, the active transform zone becomes narrower, more closely aligned with the spreading direction and less influenced by pre-existing structures. These Icelandic transform faults provide analogs for migrating transforms along mid-ocean ridge spreading centers and other places where new transform faults form in anisotropic crust.