Spreading rates, rift propagation, and fracture zone offset histories during the past 5 my on the Mid-Atlantic Ridge; 25°–27°30′ S and 31°–34°30′ S

Abstract

Abstract. The southern Mid-Atlantic Ridge (MAR) is spreading at rates (34-38mmyr -l) that fall within a transitional range between those which characterize slow and intermediate spreading center morphology. To further our understanding of crustal accretion at these transitional spreading rates, we have carried out analysis of magnetic anomaly data from two detailed SeaBeam surveys of the MAR between 25~176 and 31~176 S. Within these areas, the MAR is subdivided into 9 ridge segments bounded by large-and short-offset discontinuities of the ridge axis. From two-dimensional Fourier inversions of the magnetic anomaly data we establish the history of spreading within each ridge segment for the past 5 my and the evolution of the bounding ridge-axis discontinuities. We see evidence for the initiation and diminishment of small-offset discontinuities, and for the transition of rigid large-offset transform faults to less stable short-offset features. Individual ridge segments display independent spreading histories in terms of both the sense and amount of asymmetric spreading within each which have given rise to changes through time in the lengths of bounding ridge-axis discontinuities. Over the past 3-5 my, the short-offset discontinuities within the area have lengthened/shortened by approximately the same amount (~ 10 km). During this same time period, larger-offset transform faults have remained comparatively constant in length. A shift in plate motion at anomaly 3 time may have given rise to change in the length of short-offset second-order discontinuities. However, the pattern of lengthening/shortening short-offset discontinuities we see is not simply related to the geometry of the plate boundary in these regions which precludes a simply relationship between plate motion changes and response at the plate boundary. We document a case of rapid (minimum 60 mm yr-t) small-scale rift propagation, occurring between 2.5 and 1.8 my, associated with transition of the Moore transform fault to an oblique-trending ridge-axis discontinuity. Propagation across the Moore discontinuity and similar propagation within the 31~176 " S area may be associated with the reduced age contrast in lithosphere across second-order discontinuities.Total opening rates within our northern survey area decreased from anomaly 4' to 2 time and rates within both areas have increased since the Jaramillo. Total opening rates measured for anomaly intervals differ along the plate boundary significantly, more than expected with changing distance to the pole of rotation. These differences imply a degree of short-term non-rigid plate behaviour which may be associated with ridge segments acting as independent spreading cells. Magnetic polarity transition widths from our inversion studies may be used to infer a zone of crustal accretion which is 3-6 km wide, within the inner floor of the rift valley. A systematic increase of transition width with age would be expected if deeper crustal sources dominate the magnetic signal in older crust but this is not observed. W...

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

“…From anomaly 3 time to anomaly 2 time, the offset length increased to a maximum offset of 14 km and it is over this time interval that clear, through-going slip vector parallel lineaments are observed along the western extension of the discordant zone (see Carbotte et al, 1991). From anomaly 2 time to the present the magnetic anomaly data indicate that the magnitude of the offset remained the same but the anomaly pattern is broad, lacks character and is difficult to interpret unambiguously.…”

“…(1982) examine plate behavior in a central region of the MAR south of the Azores and find variations in spreading rates from ridge segment to ridge segment. Fox et al (1991) and Carbotte et al (1991) report that the ridge from 31~176 also shows variations in opening rates as well as differences in direction and magnitude of asymmetry between spreading segments during the last 5 m.y. The changing magnitude of asymmetry from ridge segment to ridge segment implies that each segment can have an independent spreading history.…”

“…An analysis of the regional magnetic lineations establishes an average spreading rate of 35 mm yr-(full rate) for the MAR between 25 ~ and 38~ (Cande et al, 1988;Carbotte et al, 1991). Given this opening rate, the age offsets of the seafloor across the rigid discontinuities range from 2.4 m.y.…”

“…This structural fabric suggests that during this phase in the evolution of the discontinuity strains associated with the shear zone were accommodated along a focused zone of strike-slip faulting. The magnetic data (Carbotte et al, 1991) document that 4Ma (anomaly 3 time) the offset was 40 km. From this time to anomaly 2 time (1.77 Ma) the offset magnitude decreased to 33 km.…”

“…Long (180 km) east-west profiles spaced every 20 km along the plate boundary define the magnetic anomaly pattern for each ridge segment and permit a reconstruction of the spreading rate history for the last 4-5 m.y. (Carbotte et al, 1991;Fox et al, 1991). The survey area between 25~176 was mapped during the winter of 1986-87 aboard the R/V Robert Conrad Grindlay et al, 1987;Grindlay et al, in prep.…”