Structural Response of the Menard Fracture Zone to Changes in Pacific-Antarctic Plate Motion

We present the results of a reconnaissance survey of swath MultiBeam and magnetic data of the Menard Fracture Zone. These data were mainly collected by the NSF (Office of Polar Programs) operated R/VIB Nathaniel Palmer during transits between Lyttelton, New Zealand and Punta Arenas, Chile between 1997 and 2005. Based on these data we analyzed the tectonic development and structural features of the Menard Fracture Zone. The data enable us to look at the evolution of the structures along the fracture zone between about 45 Ma (anomaly 20y) and the present. The Pacific-Antarctic spreading ridge at this latitude nucleated within a propagating rift system that birthed the Hudson microplate and formed the conjugate Henry and Hudson Troughs, which flank the fracture zone on either side (Eakins, 2000). Development of the Menard Fracture Zone resulted from tectonic realignment of the conjugate rifted margins. We used the trends of the Menard Fracture Zone and magnetic anomalies to determine a record of plate-motion changes. The Menard Fracture Zone experienced adaptations to changes in plate motion. We interpret the swathmap bathymetry data to examine how the Menard Fracture Zone evolved in response to these changes. The medium to fast spreading Pacific-Antarctic ridge generated about 1450 km of ocean crust on the east and 1550 km on the west limb. At its initiation there are 2 splays forming a narrow corridor of about 25 km. In this oldest part of the fracture zone volcanic ridges formed by transpression are clearly visible. Between 28 and 23 Ma (anomaly 10 to 6b) an adjustment in spreading direction narrows the corridor to roughly 10 to 15 km width. Roughly at 4 Ma the two splays apparently coalesce into a single offset. The northern fracture zone on the west limb and the fracture zones on the east limb of the Menard Fracture Zone are often barely or not visible on the bathymetry, due to burial by younger volcanism from overshot ridges on the inner volcanic highs. Small adjustments in direction of plate motion are also marked by occasional cross-cutting faults that offset the fracture zone splays up to 5 km.