UC San Diego

Rifted margins and plate boundary tectonics dominate the surface morphology of the ocean basins, in particular the processes that occur in rifted margins and transform fault zones remain poorly understood. Northern Iceland is an ideal area to investigate distributed extensional deformation and the evolution of transform fault zones because of the repeated rift jumps eastward back to the Iceland plume. Studying this continually evolving system allows us the opportunity to understand better the complicated nature of evolving transform faults. Using CHIRP subbottom, MCS, multibeam bathymetry, side-scan sonar, and towcam images we imaged the three basins that comprise the Tjörnes Fracture Zone : Eyjafjördur, Skjálfandi, and Öxarfjördur bays. In the Tjörnes Fracture Zone, there appears to have been an initial stage of distributed extension accommodation : graben faulting with normal faults accommodating the extension between the newly formed North Volcanic Zone and the southern extent of the Kolbeinsey Ridge. This appears to have been followed and overprinted by more traditional oceanic transform faulting, with the development of the Húsavík- Flatey Fault and then the Grímsey Lineament that was subsequently formed with the northward propagation of the North Volcanic Zone. Pockmarks are increasingly being commonly identified along margins, and our data allowed us to investigate controls over their occurrence, morphology and location in Skjálfandi Bay. In this area they have been observed in a band 15-22 km offshore, and in along the Húsavík-Flatey fault scarp near shore. Pockmarks along the fault scarp are easily attributed to fluid migration along the fault plan from depth, and we observed an increasing biota density and abundance within these pockmarks. This is likely due to either an increased nutrient flux from depth or increased detrital deposition within the pockmarks. Offshore pockmarks occur in a well- defined band determined by the interplay of the distribution of source material and the thickness of the overlying glacial sediments. The westward dipping layers at depth act as fluid flow pathways, resulting in linear pockmark chains on the seafloor. They are further modified by sea floor currents creating an asymmetric profile with the long axis orientated with the current moving clockwise around the basin. Through time as the North Volcanic Zone becomes more organized the extensional deformation was replaced by dextral strike-slip faults giving rise to the Húsavík-Flatey fault. The deformation along the Húsavík- Flatey fault is ongoing, albeit decreased from the maximum that occurred in the Miocene. This decrease in activity is the result of the North Volcanic Zone propagating past the Húsavík-Flatey fault and transferring a portion of the deformation to the Grimsey Lineament. Based on GPS data, the deformation is approximately equal between the Húsavík -Flatey fault and the Grimsey Lineament. This northward migration of the North Volcanic Zone is accompanied by the northward retreat of the Kolbeinsey Ridge imparting a clockwise rotation that reactivates the N-S normal faults as oblique slip sinistral strike-slip faults in the manner of bookshelf faulting