Re-evaluation of magnetic chrons in the north Atlantic between 35ºN and 55ºN: implications for the development of the Azores

Joaquin F. Luis¹ & J.M. Miranda²

¹University of Algarve, CIMA, Campus de Gambelas, 8000 Faro, Portugal, jluis@ualg.pt

²University of Lisbon, CGUL, IDL, Lisboa, Portugal, jmmiranda@fc.ul.pt

We present a new magnetic compilation for an area of the north Atlantic between 35°N and 47°N and up to anomaly 34. Magnetic anomalies, when accurately mapped, still represent the best source of information about the time evolution of complex geological domains, and this is the case for the Azores Triple Junction. To pick magnetic isochrons and compute finite rotation poles we applied a slightly heterodox approach, enhancing magnetic anomaly identification with a continuous reduction-to-the-pole technique. This takes into account the magnetic latitude variation within the study area and the direction of remnant magnetization, assuming that all magnetic rocks were generated at the ridge and acquired magnetization according to the geocentric dipole model. This allowed us to “pick” a large number of magnetic anomalies and to compute a new set of rotation poles that describe well Iberia-North-America relative motion. The final result is displayed in Figure 1, on which the anomaly picks made in this work are overlain.

Figure 1: Magnetic anomalies continuously reduced to the pole. Overlain are the polylines whose vertices correspond to the anomaly picks along the magnetic isochrones. White dashed line encompasses the area where new magnetic data have been added to the existing compilation. Click here or on Figure for enlargment.

The first question we address concerns the location, in the Eurasian plate, of the newly formed Azores lithosphere. In principle, the easiest way to answer this question would be to map magnetic anomalies in the plateau and identify magnetic striping sub-parallel to the Azores trend. This has was looked for by Krause & Watkins (1970) but not found. Our compilation shows why: the Azores plateau displays two different magnetic zones, one north of ~ 38°N where there are "Azorean" magnetic anomalies close to the topographic highs and with no systematic organization (the Azores Domain–AD) and the other south of this (the South Azores Domain–SAD) where magnetic anomalies are MAR-like. The obvious conclusion is that the newly formed Azores lithosphere is presently located in the AD.

The second question concerns the relationship between the Pico Fracture Zone and the East Azores Fracture Zone between Chrons 22 and 5. The similarity between the trends of the magnetic anomalies in the AD and the SAD suggests that they correspond to the same spreading regime. This means that homologues of each of North American Chrons 5 to 22 can be found in the Eurasian plate, split by Azores extensional processes into two branches, one now located in the AD and the other within the SAD. Figure 2 shows successive configurations of the Azores plateau, where it can be (quantitatively) seen that at the time of Chron 13 both the Pico Fracture Zone and the East Azores Fracture Zone had spatial continuity.

Figure 2: Magnetic reconstructions at Chrons 18, 13, 6 and 5. The set of rotation parameters used for Africa and the SAD are taken from Klitgord & Schouten (1986). Anomalies of the SAD are rotated using the same set of parameters as Africa, which presumes that any extension between the SAD and the northern border of the African plate is not compensated.

The third question concerns the onset of Azores spreading. To understand better the extension imposed on the Azores by differential motion between the Iberian and African plates we computed the successive dimensions and strikes of a segment that connects the southern end of Iberian Chron 21 with the northern end of African Chron 21. The time evolution of this segment, that presently crosses S. Miguel Island, gives an interesting “Lagrangian” description of the changes in stretching and direction of extension. The results are plotted in Figure 3, where it can be seen that only after Chron 6 did significant extension take place in the Azores. Its average value can be simply estimated from the data plotted in Figure 3 as close to 4.1 mm/yr for the period Chron 6 - Chron 5 and 3.5 mm/yr for the period Chron 5 - Chron 5. The direction of extension can also be estimated as close to 50°N for the period Chron 6 - Chron 0 and 68°N for the period Chron 5 - Chron 0. Thus, the onset of the Azores extensional regime appears to follow the welding of Iberia to Eurasia, corresponding to a sharp change of extensional velocity.

Figure 3:  Time evolution of a segment that links two points fixed to the Iberian and Africa plates for the period between Chrons 21 and 0. The length is indicated in km and the azimuth in degrees.

The very slow development of a large lithospheric block did not generate the conventional magnetic striping that is characteristic even of very slow spreading regimes (e.g., the Southwest Indian Ridge). From the magnetic compilation it is clear that only the most recent magnetic epochs (the Brunhes and Matuyama) are observable in the Azores domain and mainly in its northern limb. This favours an interpretation whereby the evolution of the Azores plateau had two separate phases:

1. where no coherent discrete magmatic features were created and predominantly extensional motion occurred. This was most probably related to the development of the deep basins along the whole central and southern parts of the Azores domain, and
2. a very recent phase, concentrated on the northern limb of the Azores domain, that is associated with the presently active axis, seismicity, large topographic masses, and coherent  magmatic processes that create sharp magnetic anomalies.

The change from one phase to the other could be related to small changes in the Eurasia-Africa spreading direction or velocity, or changes in magma supply.

The attempt by Luis et al. (1994) to establish a rigid Eulerian description of the behaviour of the small Azores block has inherent limitations due to the small size of the study area, but nevertheless its main conclusions are robust. Stable Eurasian plate behaviour can today be found north of the Azores domain, and stable African behaviour south of the East Azores Fracture Zone–Gloria Fault. Within the Azores area, the now-slowly developing spreading regime affects a large area that encloses newly formed lithosphere (the Azores domain) and previous Eurasian lithosphere split by Azores extension and dragged by the stable African plate. To describe the motion of the Azores domain by a set of rigid Eulerian rotations we need a set of parameters that is different from the stable Iberian and African plates.

This interpretation is similar to the interpretative sketch of Searle (1980) (Figure 4) but the time evolution is more constrained, the build-up of the Azores domain occurs off the mid-Atlantic Ridge, and it is connected not by a discrete transform boundary but by a more complex tectonic domain. The lack of evidence for a discrete triple junction already suggested (Searle, 1980; Luis et al., 1994) and the amount of extension in the Azores domain, either magmatic or amagmatic, supports the model of Lourenço et al. (1998) and Vogt & Jung (2003) of the Azores axis as a plate boundary acting both as an oblique, ultraslow spreading center and a transfer zone accommodating dextral differential Eurasian-African shear motion.

Figure 4: Interpretive sketch of Searle (1980) illustrating the evolution of the Azores triple junction.

References

• Krause, D. and N. Watkins (1970), North Atlantic crustal genesis in the vicinity of the Azores, Geophys. J. Royal Astron. Soc., 19, 261–283.
• Lourenço, N., J.F. Luis, J.M. Miranda, A. Ribeiro and L.A. Mendes Victor (1998), Morphotectonic analysis of the Azores Volcanic Plateau from a new bathymetric compilation of the area. Mar. Geophys. Res., 20, 141-156.
• Luis, J.F., J.M. Miranda, P. Patriat, A. Galdeano, J.C. Rossignol, L. Mendes Victor (1994), Açores Triple Junction Evolution in the last 10 Ma from a New Aeromagnetic Survey, Earth Planet. Sci. Lett., 125, 439-459.
• Searle, R. (1980), Tectonic pattern of the Azores spreading centre and triple junction, Earth Planet. Sci. Lett., 51, 415-434.
• Vogt, P.R., W.Y. Jung (2003), The Terceira Rift as hyper-slow, hotspot-dominated oblique spreading axis: A comparison with other slow-spreading plate boundaries. Earth Planet. Sci. Lett., 218, 77-90.