Two- and three-dimensional inversions of magnetic anomalies in the MARK area (Mid-Atlantic Ridge 23° N)

Abstract

Magnetic data collected in conjunction with a Sea Beam bathymetric survey of the Mid-Atlantic Ridge south of the Kane Fracture Zone are used to constrain the spreading history of this area over the past 3 Ma. Two-dimensional forward modeling and inversion techniques are carried out, as well as a full three-dimensional inversion of the anomaly field along a 90-km-long section of the rift valley. Our results indicate that this portion of the Mid-Atlantic Ridge, known as the MARK area, consists of two distinct spreading cells separated by a small, zero-offset transform or discordant zone near 23°10′ N, The youngest crust in the median valley is characterized by a series of distinct magnetization highs which coalesce to form two NNE-trending bands of high magnetization, one on the northern ridge segment which coincides with a large constructional volcanic ridge, and one along the southern ridge segment that is associated with a string of small axial volcanos. These two magnetization highs overlap between 23° N and 23°10° N forming a non-transform offset that may be a slow spreading ridge analogue of the small ridge axis discontinuities found on the East Pacific Rise. The crustal magnetizations in this overlap zone are generally low, although an anomalous, ESE-trending magnetization high of unknown origin is also present in this area. The present-day segmentation of spreading in the MARK area was inherited from an earlier ridge-transform-ridge geometry through a series of small (∼ 10 km) eastward ridge jumps. These small ridge jumps were caused by a relocation of the neovolcanic zone within the median valley and have resulted in an overall pattern of asymmetric spreading with faster rates to the west (14 mm yr−1) than to the east (11 mm yr−1). Although the detailed magnetic survey described in this paper extends out to only 3 Ma old crust, a regional compilation of magnetic data from this area by Schoutenet al. (1985) indicates that the relative positions and dimensions of the spreading cells, and the pattern of asymmetric spreading seen in the MARK area during the past 3 Ma, have characterized this part of the Mid-Atlantic Ridge for at least the past 36 Ma.