Second-order ridge axis discontinuities in the south Atlantic: Morphology, structure, and evolution

Abstract

Atmtract. Continuous along-axis Sea Beam coverage of the slowintermediate spreading (34-38 mm yr 1 full rate) southern MidAtlantic Ridge (25~176 and 31~ ~ S) shows that the ridge axis is segmented by both rigid and non-rigid discontinuities. Following the model of Macdonald et al. (1988b), a hierarchy of four orders is proposed for ridge axis discontinuities based on a continuum of relative age and distance offset across the discontinuites. This paper discusses the characteristics associated with five second-order discontinuities found in the areas surveyed. Firstorder discontinuities represent rigid offsets, transform faults, whereas non-rigid discontinuities fall into the second, third and fourth orders. Like transform fault boundaries, second-order discontinuities have distinctive morphologic signatures both on and off-axis-discordant zones-and therefore are better defined than third-or fourth-order discontinuities. Second-order discontinuities are offsets that range in distance from less than 10 km to approximately 30 km and vary in age offset from 0.5 to approximately 2.0 m.y. The variable morphotectonic geometries associated with these discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike-slip tectonism and that the geometries are unstable in time. Three characteristic geometries are recognized: (1) en echelon jog in the plate boundary where ridge axis tips overlap slightly, (2) en echelon jog in the plate boundary where ridge axes are separated by an extensional basin whose long axis is oriented parallel to the strike of the adjoining ridge axes, and (3) oblique offset characterized by a large extensional basin that is oriented approximately 45 ~ to the strike of the ridge axes. In the case of the third type, evidence for short strands of strike-slip tectonism that link an obliquely oriented extensional basin flanking ridge tips is often apparent. Analysis of the detailed bathymetric and magnetic data collected over the second-order discontinuities and their off-axis terrain out to 5-7 m.y. documents that second-order discontinuities can follow several evolutionary paths: they can evolve from transform fault boundaries through prolonged asymmetric spreading, they may migrate along strike leaving a V-shaped wake, and they may remain in approximately the same position but oscillate slightly back and forth. In addition, a small change in the pole of relative motion occurring 4-5 Ma is thought to have resulted in the Marine Geophysical Researches 13: 21-49, 1991. 9 1991 Kluwer Academic Publishers. Printed in the Netherlands. initiation of at least one second-order discontinuity in the survey area. A geologic model is proposed which involves the interplay of lithospheric thickness, asymmetric spreading, temporal and spatial variability of along-axis magmatic input and changes in the poles of relative motion to explain the origin, morphology and evolution of second-order ridge axis discontinuities.

Relationship of the Central Indian Ridge segmentation with the evolution of the Rodrigues Triple Junction for the past 8 Myr

“…As the mantle upwelling beneath the CIR and SEIR axes is continuous but changes strike at the triple junction [West et al, 1995], the creation of a new CIR segment may be associated with the creation of a new magma reservoir which is not in a vertical line with the mantle upwelling on the CIR but rather is aligned with the SEIR axis. Then the observed asymmetry could correspond to displacements of this magma source with respect to the CIR mantle upwelling [Palmer et al, 1993], and/or by relocations of the neovolcanic zone [Grindlay et al, 1991 ]. In the first case, the migration of the magma source would have the effect of heating the overlying lithosphere and creating new zones of weakness where spreading and crustal accretion relocate [Palmer et al, 1993].…”

“…In the first case, the migration of the magma source would have the effect of heating the overlying lithosphere and creating new zones of weakness where spreading and crustal accretion relocate [Palmer et al, 1993]. In the second case, the melt freezing in shallow crustal level magma chambers, between phases of magma supply, creates a barrier for the ensuing magmatic pulse which may be deflected to the older and faulted crust, resulting in a relocation of the neovolcanic zone [Grindlay et al, 1991 ]. Whatever the origin of the asymmetry, shallow magmatic processes probably play a dominant role in controlling the initiation of the CIR segmentation at the triple junction.…”

Numerical modelling of non-transform discontinuity geometry: Implications for ridge structure, volcano-tectonic fabric development and hydrothermal activity at segment ends

“…Ocean ridge discontinuities partition and offset spreading centres into spatially and temporally independent segments at a range of scales, which are observed to www.elsevier.com/locate/epsl migrate, lengthen or shorten over time [1,2]. Large scale and structurally organised discontinuities are synonymous with first-order transform faults which can offset a ridge by 10's-100's km.…”

“…NTDs may initiate by sustained differential, asymmetric spreading occurring between two spatially independent segments [1,2]. Ultimately discontinuities, with the exception of transforms have a common origin linked to upwelling and segregation of mantle melt beneath the mid-ocean ridge which leads to propagation of magmatic pulses along the ridge [1].…”

The evolution of lithospheric deformation and crustal structure from continental margins to oceanic spreading centers

“…Spreading concentrates magmatic activity at a narow ridge axis, which becomes the source of new oceanic crust. Mid-ocean ridge systems at slow spreading rates are documented to be highly three-dimensional in nature, with segmentation observed both at active spreading centers and in aged oceanic crust by the presence of fracture zones and off-axis traces of nontransform offsets (Grindlay et al, 1991, Sempéré et aI., 1993Tucholke and Lin, 1994) (see Figure 2-1). One topic of considerable research in recent years has been the relationship between segmentation and magma supply at the axis of a mid-ocean ridge.…”