The crustal thickness and lithospheric structure of active and inactive volcanic arc terrains in Fiji and Tonga

Abstract:

In order to investigate the crustal thickness and lithospheric structure beneath active and inactive volcanic arcs in Fiji and Tonga, we analyzed receiver functions from teleseismic P waves as well as Rayleigh waves from teleseismic earthquakes and ambient noise. The data were recorded by stations from three previous temporary seismic arrays deployed on the islands during 1993-1995, 2001-2002, and 2009-2010. Receiver functions were calculated with an iterative deconvolution in the time domain. We used an H-k stacking method to get preliminary Moho depth estimates under the island arcs, after assuming constant seismic average crustal P velocity. We also determined the shear wave velocity structure beneath each station from a 1-D combined inversion of receiver functions and Rayleigh wave phase velocity dispersion curves from ambient noise cross correlation at 8s – 20s and teleseismic surface waves at 20s-90s. The joint inversion models reveal that the Moho beneath the main islands of the Fiji plateau is 26-31 km deep, whereas the crust under the outer islands - including the Lau Ridge - is generally thinner, with Moho depths of 21-23.5 km. The thinnest crust (16 km) is found beneath Moala Island located between the Fiji Platform and the Lau Ridge. Crustal thickness beneath several Tonga islands is about 18-20 km. A relatively high velocity lithosphere (Vs of 4.4 – 4.5 km/s) extends to only about 60 km depth beneath the outer Fiji Islands and Lau Ridge, but to depths of 90 km underneath the main islands of the Fiji Plateau. The much thicker crust and lithosphere of the Fiji plateau relative to the Lau Ridge and Tonga Arc reflects its much longer geological history of arc crust building, going back to the early Miocene.