Crustal structure of central Japan and its petrological implications

Abstract

The crustal structure of central Honshu, Japan, has been influenced by several tectonic events: a series of collision processes during the late Tertiary, extensive faulting, highly compressive deformation and Quaternary volcanism. The southern part of the study area is covered by various granitic rocks which intrude into old Palaeozoic formations. Quaternary volcanoes occupy the northern part and cover the basement, which is mainly composed of granitic rocks and Tertiary formations. A traveltime inversion for velocity and interface depth was applied to the seismic data, together with constraints from amplitude forward modelling to produce a seismic velocity model of the crust in central Honshu. In this experiment both refracted P and S waves are clearly recorded along a 110 km profile. These provide constraints on the 2-D P- and S-wave velocity structure down to a depth of about 10 km. Moreover, clear P-wave reflections from intracrustal interfaces provide constraints on deeper parts of the crust down to about 25 km. For the P-wave velocity model, the velocities of near-surface layers show large lateral variations from 3.7 to 4.4 km s-1, increasing towards the south. Underlying layers have velocities ranging from 4.85 to 5.6 km s-1. Velocities in the range 5.85-6.2 km s-1 characterize the upper crust that extends to a depth of about 15 km. For the S-wave velocity model, the near-surface layer velocities change from 1.7 to 2.55 km s-1, showing remarkable lateral heterogeneity, whilst the upper crust is characterized by velocities of 3.4-3.6 km s-1. The deeper parts are modelled by two prominent reflectors in the mid-crust. The shallower one, which delineates the bottom of the upper crust, dips southwards from 14 to 16 km. The deeper reflector lies between 20 and 24 km depth and dips northwards. The velocity in this mid-crustal layer is 6.35 km s-1 at the top and 6.7 km s-1 at the bottom. The crustal thickness under the profile could not be determined because of a lack of P(n) and/or P(m)P phases. This study also presents the V(p)/V(s) ratio and the Poisson's ratio distribution and gives the probable composition of the upper crust. Poisson's ratio varies from 0.29 to 0.34, which can be ascribed to a porous and water-saturated structure for the near-surface layers in the northern part of the region. Poisson's ratios of about 0.24-0.25 in the southern part and 0.22-0.24 in the upper crust, together with the geological data, show that the upper crust has a felsic content and is formed by granite-granodioritic rocks with a high quartz content. The changes in the quartz content explain the lateral variations of V(p)/V(s) and Poisson's ratio in the model. The higher velocities in the middle crust suggest a more mafic structure, probably composed of diorites.