Morphometric analysis and its relation to tectonics in Macaronesia

Abstract

For an investigation of the relation between the morphology and the tectonics in the Macaronesian Islands (i.e. the geographical group of middle Atlantic Islands between 10°N and 40°N), orientation studies have been made of steeply dipping joints and, where possible, of the trends of valleys and ridges. A coincidence of the “preferred directions” of these features indicates a probable genetic relation between them. The results are: (1) Joint strikes, stream directions and ridge trends (where available) generally agree with each other on each individual island, indicating a common genetic (tectonic) origin of these features. Thus, the main result of this study is that the morphology of all Macaronesian Islands, although surficially of volcanic origin, has been significantly codesigend by tectonic processes. (2) The direction data for corresponding geomorphic features are essentially the same across each individual archipelago, certainly with regard to the joints, somewhat less so with regard to the links and ridges. (3) The Canary and the Madeira Islands Archipelago yield the same values for the direction maxima of joints and streams. (4) The compression ( σ 1)-axis indicated by the fault plane solution of the Canary earthquake of 9 May 1989 lines up with one of the bisectrices and not with one of the strike-maxima of the surrounding joints; this indicates that the joint directions may be indicative of the shear lines rather than of the principal stress directions of the neotectonic stress field in the area. (5) There is a steady anticlockwise rotation of corresponding morphotectonic directions as one moves from Africa westward from the Canary Islands through Madeira and the Cape Verde Islands to the Azores. This may be due to a general rotation of the area: whilst drifting N, Africa has been subject to an anticlockwise rotation (pole west of Madeira) owing to the drag caused by the eastward-spreading Atlantic Ocean floor. (6) The mid-Atlantic ridge seems to have no influence on the orientation of the joints/streams/ridges on the islands. In the plate-tectonic model, this lining up of joint orientations may be the consequence of the mirror reflection of the displacements away from the Ridge on its two sides. (7) The situation at the triple junction of the Eurasian, African and American plates is not clear; additional microplates (Azorean, Iberian) might even be present. The Azores might (i) have arisen around the triple junction from basaltic lava upwelling from a mantle hot spot to be carried away as islands on the moving plates; or (ii) they may represent a hot-spot trace on moving plates. Since trachytic/andesitic lava flows have been found on the islands, (iii) a tectonic control model has been proposed in the literature without recurrence to basaltic plates. The results of our morphometric studies would be able to conform to all three models.