M. C. Sinha

Seismic mapping of a magma chamber beneath the Valu Fa Ridge, Lau Basin

We analyze a dense grid of seismic normal incidence and wide-angle data collected over an active back arc spreading center in the Lau Basin of the southwest Pacific. Our survey area covers the whole of a 35-km-long morphological segment and a small overlapping spreading center. A bright reflector, which is coincident with a velocity inversion at a depth of 3.2 ± 0.2 km below the seafloor, is observed on every one of the 40 across-axis profiles. We interpret this reflector as being the roof of a crustal magma chamber. The widest magma chamber reflector occurs beneath the overlapping spreading center, where it extends up to 4 km, being imaged beneath both ridges and, in places, beneath the overlap basin. Elsewhere the width of the reflector varies between 0.6 and 2.3 ±0.4 km. The narrowest reflectors are observed beneath deviations (devais) of the ridge axis. In addition to being observed on each of the kilometer-spaced across-axis profiles the reflector is also seen as a continuous event for at least 10 km on a profile along the ridge axis. The reflection coefficient of the event is between −0.34 and −0.65. This requires the melt at the top of the chamber to have a P wave velocity less than 2.7 km/s if the interface is planar or less than 3.8 km/s if the interface is layered.

On the electrical nature of the axial melt zone at 13° N on the East Pacific Rise

The first controlled source electromagnetic experiment directly on a ridge, with the potential to identify the presence of an axial melt body beneath a fast-spreading center, was conducted at 13oN on the East Pacific Rise (EPR) in 1989. Transmission for 36 hours was achieved by a deep towed horizontal electric dipole source, of moment 6000 Am, operating at frequencies between 1/4 and 8 Hz. Signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100,000-year-old crust. Data above ambient noise levels were obtained at ranges of up to 10 kin. The results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low (,,,1 tim), indicating a heavily fractured, high-porosity surficial layer. Below this topmost layer, the upper 2 km of crust is found to be moderately resistive (,,,100 tim). We find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust. If a partial melt body is present, which is continuous along strike and which comprises a connected, and therefore conductive, melt texture, it must be of very limited volumetric extent. This picture is consistent with recently proposed models of a thin sill-like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent. The larger region below the sill, characterized by low seismic velocities, must contain at best a very small melt fraction distributed in isolated pockets, providing further evidence that the EPR at 13oN is currently in a state of relative magmatic quiescence.

Upper crustal resistivity structure of the East Pacific Rise near 13°N

An active source electromagnetic (EM) sounding has been conducted on the axis of the East Pacific Rise (EPR) at 13° 10′N. 1D inversion and modelling techniques, seeking resistivity as a function of depth, have been applied to 8 Hz amplitude data collected along the ridge crest. Resistivity is seen to increase monotonically between 50 m and 1 km below the seafloor, increasing from ∼1Ωm to around 90Ωm. We observe no intrinsic difference in upper crustal resistivity structure between the rise axis and 100,000 year old crust. Inferred surface porosities of 20% are larger than those recorded in 5.9 my old crust in DSDP hole 504B. Our data do not require, and lack sufficient information for, the reliable inclusion of a conductive termination to the model below 1.2 km.

The Valu Fa Ridge: The pattern of volcanic activity at a back-arc spreading centre

Abstract The Central Valu Fa Ridge, an active back-arc spreading segment bounded by small overlapping spreading centres, can be divided into three regions, each 8–12 km long, with distinct bathymetric, morphological and magnetic characteristics. These observations suggest that each region has a different volcanic history, driven by variations in melt supply along axis. Two of the three regions are centred on small deviations of the ridge axis, which appear to be the local foci of extrusion. There appears to have been a recent re-organisation of the magmatic supply to the ridge. In the past the highest magmatic budget was at the southern end of the ridge but it has now switched to the northern end, where it coincides with a small overlapping spreading centre. This re-organization may be associated with island are activity. Beneath the ridge a bright reflector, which is interpreted as being from the roof of a magma chamber, is laterally more continuous that the inferred volcanic segmentation. This implies that mixing of magma within the chamber must be limited along strike. The magmatic budget at various parts of the ridge, inferred from axial morphology, can be correlated with variations in the width and brightness of the reflector. This correlation suggests that the size of the chamber is dynamic and changes in response to variations in supply from below or delivery upwards.

A three-dimensional study of a crustal low velocity region beneath the 9°03′N overlapping spreading center

Overlapping spreading centers (OSCs) play a key role in models of magma distribution at fast spreading ridges. To investigate the relationship between ridge-axis discontinuities and magma supply, we conducted a three-dimensional seismic reflection and tomography experiment at the 9°03′N OSC along the East Pacific Rise. Tomographic analysis imaged a broad mid-crustal low velocity zone (LVZ) beneath parts of the overlapper and the associated overlap basin, demonstrating that it is magmatically robust. The complementary datasets reveal a complex storage and tapping of melt: the LVZ and melt sill at either end of the overlap basin are not simply centered beneath the rise crest but are skewed inwards. The subsequent focussing of the LVZ and sill beneath the axis of the eastern limb appears to be due to melt migration toward the tip. The OSC western limb is less magmatically robust and may be in the process of dying.

Gephysical Constraints upon the Thermal Regime of the Ocean Crust

Geophysical measurements and models constrain the total rate of production of crustal material and the flux of thermal energy over the global ridge system. Flux estimates based on basin-scale compilations of heat-flow measurements or on 1-dimensional geodynamic models of melt generation and plate cooling provide a useful, but only a partial view of the crustal thermal regime. The rate of heat supply to the crust depends both on complex patterns of flow in the mantle, and on how this flow supplies magma (a major carrier of advective heat flux) to the crust. Much progress is still needed if we are to understand the thermal regime at and close to the crust-mantle boundary, and hence the extent to which segmentation and other variations in crustal structure may be inherited from the mantle. Within the lower and middle crust, the thermal regime is dominated by the presence (or otherwise) of crustal magma chambers. Over the last decade, geophysical data have provided a progressively more sophisticated understanding of these features, at all spreading rates. Correlations and quantitative links between new models of magma chamber structure and what is known from other disciplines about the overlying hydrothermal circulation system remain weak. Significant unknowns also still remain regarding the patterns and pathways of hydrothermal circulation within the crust. High resolution geophysical data are now beginning to provide quantitative constraints on the physical structure (overall porosity, and interconnectedness of pore spaces) of the permeable crust. The same observations and methods are also beginning to allow us to detect in situ variations in the properties of the fluids themselves, to depths equivalent to the base of layer 2, and on horizontal scales of several kilometres. In one case this has provided glimpses of what may be two-layer hydrothermal convection, related to phase separation. How flow patterns are influenced by key tectonic parameters such as spreading rate and ridge morphology remains an open issue. Also unknown is the extent to which shallow circulation may be driven by newly injected dikes, and the spatial and temporal scales of the resultant thermal perturbations. Lastly, we must consider the case where high and low temperature hydrothermal circulation is occurring in the absence of any significant crustal magma body. Are such systems related to the cooling of rocks that have recently crystallized from basaltic magmas? Do serpentinization reactions play a significant role? And how widespread are such circulation regimes?

Segmentation and rift propagation at the Valu Fa Rridge, Lau Basin: Evidence from gravity data

Gravity and bathymetry data from a detailed ship-based survey of a 40-km length of the Valu Fa ridge allow an analysis of the fine-scale segmentation of this young, back arc spreading center, where the primary morphological segmentation takes the form of small, overlapping spreading centers (OSC) separating ridge segments typically 30 km long. The gravity survey covers one complete segment and the overlap at its northern end and part of the overlap at its southern end. The mantle Bouguer anomaly shows a small ( 10 mGal), subcircular negative anomaly. The pattern of anomalies implies that the northern OSC is the site of particularly vigorous magmatism, with either increased crustal thickness or density reduction in the mantle (or both) accounting for the observed gravity low. This interpretation confirms the results of seismic analyses of this spreading center, which also imply that the OSC is the site of enhanced magmatism. The observations appear to contrast with widely accepted models of segmentation at mature, ocean basin spreading centers, where morphological offsets are believed to coincide with a relatively weak supply of magma. On a longer (130 km) gravity profile crossing the ridge, the mantle Bouguer anomaly shows a large (∼50 mGal), longer- wavelength (∼100 km) positive anomaly centered on the spreading axis. This can be modeled by an increase in crustal density and/or a decrease in crustal thickness over a 40- to 50-km-wide area centered on the axis. This is consistent with recent models for the evolution of the basin, involving recent southward propagation of seafloor spreading through preexisting, but stretched, thinned, and intruded island arc lithosphere.

A SEA BOTTOM MULTICHANNEL HYDROPHONE ARRAY

The PUMA (Pull-Up Multichannel Array) is a sea-bottom instrument for remotely recording data from a 12 channel hydrophone array. Its purpose is to achieve (i) denser data coverage, leading to (ii) improved velocity analysis and (iii) multichannel processing of wide angle seismic data collected on the continental shelf. The instrument consists of a 1.2 km array terminating with a pressure case in which 8 FM cassette recorders, a power supply, microprocessor controller and internal clock are housed. It can be pre-programmed to switch on during shot windows for a total of four hours recording time.The PUMA was successfully used in an experiment west of Lewis, Outer Hebrides, U.K. in August–September 1984. We show an example of PUMA data from this experiment. Indications are that the instrument will provide improved constraints on seismic velocities in the lower continental crust and uppermost mantle.

A fixed receiver for recording multichannel wide-angle seismic data on the seabed

Previous experiments to record seismic data at wide angle on the continental shelf have generally been unsuccessful in determining velocity structure in the lower crust; either the lines were too short or shot-receiver density too sparse to identify lower crustal arrivals. In contrast, deep normal incidence profiles show good structural resolution in the crust and uppermost mantle. A sea-bottom multichannel instrument has been developed to record datasets containing closely spaced traces, in order to improve the resolution of reversed wide-angle experiments on the continental shelf.