Three-dimensional crustal structure and magma chamber geometry at the intermediate-spreading, back-arc Valu Fa Ridge, Lau Basin-results of a wide-angle seismic tomographic inversion

Abstract

Summary In this paper the results of a tomographic analysis of a 3‐D wide‐angle seismic refraction data set acquired at the Valu Fa Ridge (VFR) in 1995 are presented. The VFR is an intermediate‐spreading ridge located in the southern Lau back‐arc basin in the southwest Pacific. The ridge comprises three morphological segments, the Southern, Central and Northern Valu Fa Ridges, separated by overlapping spreading centres (OSCs). Previous seismic experiments have identified a robust axial magmatic system beneath the central segment (CVFR) and the OSC with the northern segment (NVFR). The experiment described in this paper aimed to resolve details of the structure of this magma chamber and the adjacent post‐rift crust. A regularized inversion scheme that minimizes model roughness was applied to the first‐arrival traveltime picks made from the wide‐angle data. A quantitative approach for determining data uncertainties is described based on the signal‐to‐noise ratio of the arrivals. Several initial model assumptions were tested, including one with a thin melt lens, representing a seismic reflector identified in previous studies, explicitly included in the initial model. The inversion results suggest that crustal layer 2 exhibits northward thickening, which mirrors a similar northward thickening of the whole crust. In addition, local thinning of layer 2 is identified in the vicinity of the boundary between pre‐ and post‐rift crust, which is thought to represent thinning of the crust prior to the onset of rifting. Axial low‐velocity anomalies are identified in layer 2B/C and layer 3. The models are consistent with a continuous ∼ 6 km wide negative velocity anomaly in layer 3 with an amplitude of ∼ 0.7–0.9 km s−1 relative to off‐axis post‐rift crust. This anomaly is consistent with the presence of an axial mush zone comprising a small percentage (< 1 per cent) of partial melt. The negative velocity anomaly in layer 2B/C is modelled with its largest amplitude (∼0.5 km s−1) beneath the northern OSC. Possible origins for this anomaly include locally thicker crust or locally higher porosity near the OSC, or a high‐temperature anomaly associated with the axial magmatic system.

Seismic Traveltime Tomography of the Crust and Lithosphere

“…This type of scheme is also popular (e.g. Sambridge, 1990;Zelt & Barton, 1998;Day et al, 2001). Often, however, a mixture of both frameworks is used, in which case one seeks a physically reasonable model that contains no unnecessary structure, is not highly perturbed from the initial model and satisfies the data.…”

“…Ritsema et al (1998) used several different synthetic models rather than checkerboards and analyzed the accuracy of their recovery. This kind of analysis is not commonly used in wide-angle studies, although recently, Zelt (1998), Zelt et al (1999Zelt et al ( , 2001 and Day et al (2001) implemented checkerboard tests for analyzing the resolution of velocity structure , also use it for interface structure) derived from wide-angle traveltime inversion. In local earthquake tomography, checkerboard tests have been implemented by Chiarabba et al (1997) and Graeber & Asch (1999); Walck & Clayton (1987) and Walck (1988) used synthetic reconstructions with anomalies positioned in key localities (i.e.…”

“…Also using finite-difference techniques, Riahi et al (1997) invert wide-angle reflections for Moho structure beneath the Gulf of Bothnia and Zelt et al (1996) invert both reflection and refraction traveltimes for velocity and interface structure beneath the southwestern Canadian Cordillera. Other studies to use finite difference solutions of the eikonal equation in 3-D wide-angle traveltime inversions include Zelt & Barton (1998), Zelt et al (1999Zelt et al ( , 2001 and Day et al (2001). Parsons et al (1996) use a finite difference approach in the inversion of wide-angle traveltimes for the 2-D crustal structure of the Colorado Plateau.…”

Aspects of marine geoscience: a review and thoughts on potential for observing active processes and progress through collaboration between the ocean sciences

“…This transition was finally sampled by drilling at IODP Site 1256D [16]. Seismic refraction experiments have been used to map out the velocity structure and thickness of oceanic crust [17][18][19][20][21][22][23][24][25]. Some of those data suggest that oceanic crust produced at ridges of extremely slow spreading rate is anomalously thin, owing to reduced melting of a colder mantle beneath the ridge.…”

Modified Finslerian spaces for more effective ray-tracing. Application to seismic travel-time tomography

“…[1] were originally designed for deep crustal subsurface imaging and have been applied in several interesting case studies [2][3][4][5][6][7][8][9]. In crustal studies, the velocity contrasts are low with smooth gradients so that the algorithms function without major difficulties and bugs.…”