特集 火山活動と地殻応力場 中生代玄武岩岩脈の岩脈連結断層と火山性地震の関係 群発地震の化石か？

Abstract

In the coastal region of the State of Rio de Janeiro, Brazil, Jurassic (120 Ma) basaltic dykes are present intruding in late Precambrian (600 Ma) gneissic rocks. They are typically 10 m wide, constituted by thoreiitic gabbro, and frequently have sinistral en-echelon setting. Intrusion depth of the present surface exposure is estimated to be 4 km based on apatite fission track dating and regional uplift history. The en-echelon dyke terminals are often linked by a short oblique path, showing extended “Z” -shaped dyke appearance on outcrops. Some of the link paths of wide dykes (>4 m) are accompanied by well-consolidated narrow (<1 m) brecciated zone on both sides, composed uniquely of angular host gneiss fragments. The fragment size is relatively fine (1 cm) on dyke link path side (inner), and gross (10 cm) on host rock side (outer). Large fragments are very angular, and show auto-brecciated texture. The lithologic characteristics of the breccia correspond to those of fault breccia.Geometric setting and kinematic relation between these fault breccia zones and the dykes are similar to those of mid-ocean ridges and transform faults. Dyke dilatation due to magma injection may cause shear stress at the dyke junction zones. If the stress is sufficiently great, the host rock will rupture to form fault fracture zones, and they are successively filled by magma to result the link paths. If the stress is insufficient, there small cracks may be created. Such type of faults, namely dyke linkage faults, are primary and monogenic ones, which are generated in uncracked host rock body with no repeated stick slips.The movement mechanism of dyke linkage faults is similar to the Hill's model, but differs a little from it especially in case of the faults with link path filled by magma. Total displacement of dyke linkage fault is defined by dyke width, Even after the magma fill on the fault plane, fault displacement will continue aseismically. The basaltic dykes in this region commonly have well-developed horizontal columnar joints, but the cooling joints at link paths are pertubated and platy-like joints take place, indicating that the link paths were submitted to shear deformation during magma cooling. In this sense, the earthquakes are more likely to occur only at the top of the intruding dykes. The fault length is defined by en-echelon dyke off-set at junction zones, being several times more than the dyke width. In comparison with tectonic faults and earthquakes, the diplacement is very large relative to the length.The earthquakes generated by dyke linkage fault formation are expected to have strike-slip focal mechanism, linear epicentre trend, nodal plain oblique to the trend, and a high frequency spectrum. Some Japanese volcanic earthquake swarms have such characteristics.