A geophysical study of the Wharton Basin near the Investigator Fracture Zone

Abstract

Most of the anomalously deep portion of the central Wharton Basin in the eastern Indian Ocean appears to be the result of a 55° change in spreading direction that occurred between magnetic anomalies 34 (80 m.y.) and M‐0 (108 m.y.). An Early Cretaceous transform fault known as the Wallaby Plateau Scarp or Wallaby Fracture Zone evolved in the Late Cretaceous into the Investigator Fracture Zone during this radical change in plate motion. What is now the southern end of the Investigator Fracture Zone formed as a substantially wider and deeper feature than most fracture zones. Seismic refraction results within the anomalously deep portion of the fracture zone show an essentially normal oceanic crustal section that is slightly thinned. The western boundary of the Investigator Fracture Zone can be easily traced south of (older than) anomaly 34 with magnetic, seismic reflection, and gravity data. This boundary is marked by a magnetic anomaly edge effect whose polarity depends on the presence of normally or reversely magnetized crust adjacent to the fracture zone. The polarity reversal of the boundary edge effect is evidence that the crust within the fracture zone has a greatly reduced remanent magnetization.