Fine Structure of En Echelon Ridge Axes and Crustal Deformation at Constructive Plate Boundaries

摘要

Where oceanic segments of the world rift system are exposed at the surface in Iceland and the Afar Depression of Ethiopia, the neovolcanic zones are observed to die out along their length and are offset en echelon to one side or the other. In Iceland the exposed crust is composed of a lava pile of Late Tertiary to Holocene age which in detail is built up of overlapping lenticular units. These units together with their feeder dike swarms are focused about silicic centers. During crustal growth the units move to one side or the other of the neovolcanic zone where they are preserved intact and only partially covered by later centers. There are two neovolcanic zones in Iceland. The western zone commences at Reykjanes and overlapping sections extend en echelon for 40 km to the east; it continues northeastward to near 64°55' N. where it changes to a northerly strike and volcanism dies out at 65° 10' N. The continuation of this line to the north coast is an ancient volcanic line. The eastern volcanic zone overlaps the western one along its southern half and is active across the whole length of Iceland. In the course of crustal growth, volcanic lines have decayed and have been replaced by new lines to one side. Anticlines and synclines are formed by this process within the lava pile and define the location of the ancient axes of crustal growth. A situation similar to that observed in Iceland is found in Ethiopia where the Wonji fault belt within the main Ethiopian rift is formed of en echelon fault lines. In the northern part of the Afar Depression two well-defined neovolcanic lines overlap one another and the ridge axis is further offset to join the Red Sea ridge. These neovolcanic lines are not connected by transform faults and the lengths of the ridge segments are a function of their offsets. They are transient features which during their active lives are accommodated by deformation of the crustal plates.