The RAMESSES experiment-V. Crustal accretion at axial volcanic ridge segments-a gravity study at 57°45′N on the slow spreading Reykjanes Ridge

Abstract

Summary Presented in this paper are the results of a two‐stage analysis of gravity data acquired during a multidisciplinary geophysical survey of a magmatically active axial volcanic ridge (AVR) segment located at 57°45′N on the Reykjanes Ridge, part of the slow spreading Mid‐Atlantic Ridge south of Iceland. Modelling of the free‐air anomaly in 2‐D shows that, across‐axis, the observed anomaly results largely from density and layer thickness variation in the mid‐lower crust. Although seismic control on crustal thickness along‐axis is limited, modelling also suggests that both crustal density and thickness also vary towards AVR tips. Using the 2‐D modelling results as crustal reference, the residual mantle Bouguer anomaly (RMBA) is calculated to assess whether magma‐related density anomalies are present in the mantle and to investigate the structure of, and relationship between, adjacent AVRs along‐axis. RMBA lows are associated with both an along‐ridge trend encompassing a number of adjacent AVRs and with individual, more topographically robust AVRs. Modelling of the RMBA low associated with the 57°45′N AVR further suggests that along‐axis density variation is confined to the central region of this AVR and that the anomaly can largely be accounted for by density variation within Layer 3 and a degree of crustal thinning towards AVR tips. The nature of the along‐axis variation in crustal density further suggests that it may result from repeated phases of magma supply to the crustal system from the mantle. Within the resolution of the RMBA, modelling does not confirm or preclude the presence of a subcrustal density anomaly associated with retention of a small percentage of melt in the mantle. However, a melt‐free model for at least the top 40 km of the mantle is preferred as this is consistent with the results of modelling a coincident magnetotelluric data set. The ridge‐trend characteristics of the RMBA also suggest that magma delivery may take place along this trend, and in an episodic fashion that controls initiation of an AVR tectonomagmatic cycle. AVRs may, therefore, be created along a spreading‐orthogonal direction by tapping of a recent magma injection into the ridge‐trending crustal system, with flow of this magma laterally along AVR‐parallel faults and fissures accommodating spreading and crustal accretion.

Temporal and spatial cyclicity of accretion at slow-spreading ridges-evidence from the Reykjanes Ridge

“…For the MBA, the approach of Prince & Forsyth (1988) was adopted as described in detail in Peirce & Navin (2002). To enable the MBA calculation to be performed on a regular grid of nodes (see Table 2), the bathymetry grid was rotated though 36° to align the oblique trend of the ridge in a ‘north–south’ direction.…”

“…The gravity models presented in this section support the theory that tectono‐magmatic cycles influence the crustal structure of individual AVRs. In addition, a ridge‐trending RMBA low is observed, suggesting that upwelling occurs in this orientation but that melt migrates laterally filling spreading‐normal fissures orientated oblique to the ridge direction and forming AVRs (Peirce & Navin 2002). AVRs appear to young and become shorter towards second‐order discontinuities, suggesting that AVR growth is initially focussed at the centre of the upwelling and that crustal production gradually extends to the segment extremities.…”

Morphology and genesis of slow-spreading ridges-seabed scattering and seismic imaging within the oceanic crust

“…Navin et al 's (1998) across‐axis velocity model shows that this magma chamber contains approximately 20–40 per cent partial melt, which is periodically refreshed from the mantle on the order of every 20 000–60 000 yr (MacGregor et al 1998). The crust is thickest beneath the shallowest AVR topography, as evidenced by the along‐axis velocity model of Navin et al (1998), free‐air gravity anomaly modelling (Navin et al 1998; Peirce & Navin 2002; Gardiner 2003; Peirce et al 2005) and a residual mantle Bouguer anomaly low (Gardiner 2003; Peirce et al 2005). Layer 2A is also thicker at the centre of the AVR, as evidenced by a magnetic intensity high (Fig.…”

“…Heinson et al (2000) used MT data to show that the melt source region lies at a depth of 50–100 km below sea level and that accumulation and transport to the crust is rapid and episodic, there being no evidence from the MT data of any clear conduit or connection between the mantle melt source and the crustal magma body. Peirce & Navin (2002) showed from RMBA modelling that magma delivery from the mantle to the crust takes place along the ridge trend and that individual AVRs tap this supply at the crustal level, initiating a tectono‐magmatic cycle and accommodating crustal accretion. Continued amagmatic spreading later in the cycle is accommodated by AVR‐parallel faulting and fracturing.…”

Kinematic models for the thickness of oceanic crust at and near mid‐oceanic spreading centers

“…However, there are exceptions to the systematics of ridge topography, the most famous being the slow‐spreading MAR segments of Iceland and the Reykjanes Ridge south of it, which are influenced by a ridge‐centered hot spot: the Reykjanes Ridge has structural features generally typical of a fast spreading ridge, but also an anomalous crust, whose thickness rises northward from 7 to 21 km [ Searle et al , 1998; Weir et al , 2001; Peirce and Navin , 2002] and reaches more than 40 km in Iceland itself [e.g., Darbyshire et al , 2000; Fedorova et al , 2005]. On the Reykjanes Ridge, the volcanically active zone has also been found to be wider, probably due to the nearby hot spot, whereas the strain zone narrows northward, and on Iceland it is thought to be narrower than the volcanic zone [ Appelgate and Shor , 1994; Searle et al , 1998; Peirce and Navin , 2002; Pálmason , 1980].…”