Im. Turner et al., Seismic imaging of the axial region of the Valu Fa Ridge, Lau Basin - the accretionary processes of an intermediate back-arc spreading ridge, GEOPHYS J I, 138(2), 1999, pp. 495-519
In 1995, a multidisciplinary geophysical experiment targetted the intermedi
ate spreading Valu Fa Ridge (full rate 60 mm yr(-1)), which is centred on 2
2 degrees 20'S, 176 degrees 40'W in the Lau Basin. As part of this experime
nt, wide-angle and normal-incidence seismic profiles were collected both al
ong- and across-axis to determine the crustal structure of the Central Valu
Fa Ridge (CVFR) and its overlap with the Northern Valu Fa Ridge (NVFR). Co
ntrolled-source electromagnetic profiles and underway gravity, magnetic and
bathymetry data were also collected.
In this paper we describe the results of forward modelling of the along- an
d across-axis wide-angle and normal-incidence seismic data. An axial low-ve
locity block and its underlying slightly broader zone of depressed seismic
velocities (low-velocity zone) have been identified, and these features are
interpreted as corresponding to a melt lens and underlying magma chamber.
The low-velocity block is 1-2 km wide and has a first-order upper boundary,
from which large-amplitude reflections are observed; amplitude analysis of
these indicates an interconnected melt fraction. The nature of the lower b
oundary is more poorly constrained, as no reflection event corresponding to
the base of the low-velocity block is observed. Modelling indicates that v
elocities similar to those observed at the base of layer 2 within the axial
region (similar to 5.5 km s(-1)) are achieved by 250 m below the upper bou
ndary, possibly suggesting a gradational lower boundary with high velocity
gradient. The low-velocity zone (LVZ) is interpreted as an similar to 4 km
wide magma chamber delineated by a seismic velocity anomaly of -0.2 km s(-1
), extending down through layer 3 to within 1.5-2 km of the Moho. The veloc
ity anomaly and dimensions of the LVZ are generally smaller than those obse
rved at the East Pacific Rise (EPR) and Mid-Atlantic Ridge (MAR).
The observed alone-axis continuity of the low-velocity block is remarkable,
extending from the southern tip of the CVFR to the overlapping spreading c
entre (OSC) with the NVFR. A low-velocity block is modelled beneath the ins
ide flanks (i.e. the slopes that dip into the overlap basin) of both ridges
at the OSC, although the existence of a single low-velocity block beneath
the overlap basin itself cannot be ruled out. The identification of a singl
e LVZ centred on the overlap region, rather than two merged LVZs beneath ea
ch segment, implies that the material in each low-velocity block originates
from the same crustal magma source. A reflection event from the Moho is ob
served from directly beneath the axis on both across-axis profiles, which i
ndicates that a distinct crust-mantle boundary may be formed within the axi
al region. Many of the observations at the Valu Fa Ridge are consistent wit
h those at the EPR and the Reykjanes Ridge (MAR), which implies that, regar
dless of spreading rate, crustal accretionary processes at mid-ocean ridges
with similar magmatic budgets are also broadly similar.