Crustal and lithospheric structure of inactive volcanic arc terrains in Fiji

Abstract

The crustal and lithospheric velocity structure of the Fiji area may place important constraints on the evolution of island arcs in the region. Here, we obtain receiver functions from one permanent seismic station and three temporary networks on the Fiji Platform, the Lau Ridge, and other small islands to develop point estimates of crustal thickness under the networks. We obtain preliminary estimates of crustal thickness and Poisson's ratio from H-κ stacking of the receiver functions. We then perform an inversion for shear-wave velocity structure using a joint inversion of receiver functions and Rayleigh wave dispersion curves from ambient seismic noise and teleseismic events. The joint inversion results show an average crustal thickness of ~30 km beneath Viti Levu, the largest Fiji island, suggesting an extensive period of arc crustal formation and thickening. In contrast, we find shallower Moho depths averaging ~22 km beneath the smaller Fiji islands and the Lau Ridge. The limited thickness of the high velocity lid obtained by inversion indicates that the lithosphere is thinned beneath the marginal Fiji islands, which may be due to the early rifting event forming the Lau Basin together with the crustal thinning of this area as well, and to thermal lithospheric erosion processes associated with adjacent back-arc spreading. A relatively slow velocity middle crustal layer, roughly of 11–16 km thickness, exists throughout Fiji, suggesting that remnant arcs contain significant volumes of felsic crustal rocks necessary for continental crust formation.