Thermo-mechanical control of axial topography of intra-transform spreading centers

The Quebrada/Discovery/GoFar fracture zone region on the East Pacific Rise at ~ 4°S spreads at a “fast” half-rate of ~ 70 mm/yr and includes 8 intra-transform spreading centers with lengths ranging from 5 to 70 km. The longer spreading centers have uniform axial highs typically associated with fast spreading ridges. However, the Quebrada and GoFar fracture zones each contain 2–3 short spreading centers with axial depths as great as 4800 m, reminiscent of axial valleys at “slow” spreading centers such as the Mid-Atlantic ridge. Residual mantle Bouguer gravity anomalies (RMBA) indicate that these anomalously deep spreading centers are not isostatically compensated by thin crust. Instead, there must be a dynamic component to the axial topography similar to that invoked to explain the uncompensated topography at axial highs and median valleys for normal fast and slow spreading ridges. We show that a simple “rheological” parameter combining inferred crustal thickness and predicted depth of the 1000 °C isotherm correlates well with the axial depth at all spreading centers in the Quebrada/Discovery/GoFar region. Spreading centers with cold or thin crust tend to be deeper than those with hot and/or thick crust, with relief apparently amplified by stresses in the lithosphere. In addition, we note that the Quebrada and GoFar fracture zones are regions of normal to positive RMBA, contrary to a recent suggestion that negative RMBA and thicker crust is typical of fracture zones on fast spreading ridges.