Transform zone migration: Implications of bookshelf faulting at oceanic and Icelandic propagating ridges

Abstract

At a propagating ridge a migrating transform zone (MTZ) offsets the propagating and retreating spreading axes. We examine several kinematic models of MTZ deformation to determine which models can generate observed seafloor fabric orientations, vertical tectonics, and faulting patterns within the 95.5°W Galapagos propagating ridge (PR) system. Models that involve transform‐parallel simple shear within the MTZ can fit observed seafloor fabric rotation patterns within the 95.5°W MTZ. However, these models predict pervasive transform‐parallel (strike‐slip) faults within the MTZ that are not observed and do not predict the dramatic shear‐concurrent deepening and post‐shear uplift that is seen in this propagating ridge system. Since we only measure the amount of structural rotation of the abyssal hill fabric within the MTZ and do not measure extensional or compressional deformation perpendicular or parallel to abyssal hills, this structural information can only determine the rotational component of the finite strain tensor. Thus this simple shear family of kinematic deformation models is actually only a small subset of MTZ deformation patterns that can produce the same finite rotation patterns, but with different amounts of shortening and/or extension parallel or normal to abyssal hills. There is strong seismic and surface fault‐break evidence in Iceland that the South Iceland Seismic Zone MTZ propagates along an array of en echelon, “transform”‐perpendicular strike‐slip faults. A likely reason for this is that if deformation were accommodated through transform‐parallel strike‐slip faults, then migration of the transform zone would require the creation of new transform‐parallel strike‐slip faults for each increment of transform migration. Instead, if transform migration can be accommodated through slip on preexisting faults, then transform migration would only require the incremental growth of and slip along preexisting faults for each increment of transform zone migration. We examine the kinematic consequences of finite MTZ deformation within this generalized “bookshelf faulting” deformation scenario. We find that our kinematic realization of generalized bookshelf faulting predicts structural rotation and vertical tectonic patterns that are consistent with observations at the 95.5°W Galapagos PR system. Vertical tectonics are an almost inevitable by‐product of bookshelf shearing on dipping faults because of the geometric strain incompatibility associated with this mode of deformation. Thus local extension within the abyssal hills of De Steiguer Deep may occur because these abyssal hills do not join a spreading axis where the geometric strain incompatibilities associated with bookshelf slip can be accommodated. We conclude that generalized bookshelf faulting is a promising candidate for the dominant deformation process within a MTZ since it can explain not only the structural rotations observed at the 95.5°W propagating ridge but can also relate these rotations to the uplift history recorded there.