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

Abstract

SUMMARY A unifying model of oceanic crustal development at slow spreading rates is presented in which accretion follows a cyclic pattern of magmatic construction and tectonic destruction, controlled by along‐axis variation in melt supply and coupled to along‐axis variation in spreading rate and across‐axis asymmetry in spreading. This study focuses on the Reykjanes Ridge, Mid‐Atlantic Ridge south of Iceland, which is divided along its entire length into numerous axial volcanic ridges (AVR). Five adjacent AVRs have been analysed, located between 57°30′N and 58°30′N and south of any strong Iceland hotspot influence. The seabed morphology of each AVR is investigated using sidescan sonar data to determine relative age and eruptive history. Along‐axis gravity profiles for each AVR are modelled relative to a seismically derived crustal reference model, to reveal the underlying crustal thickness and density structure. Correlating these models with seabed features, crustal structure, ridge segment morphology and relative ages, a model of cyclic ridge segmentation is developed in which accretion results in adjacent AVRs with a range of crustal features which, when viewed collectively, reveal that second‐order segments on the Reykjanes Ridge have an along‐axis length of ∼70 km and comprise several adjacent AVRs which, in turn, reflect the pattern of third‐order segmentation. Tectono‐magmatic accretion is shown to operate on the scale of individual AVRs, as well as on the scale of the second‐order segment as a whole.

Crustal structure of the Reykjanes Ridge near 62°N, on the basis of seismic refraction and gravity data

“…A weak trend of increasing Vp values may be recognized from ∼7.8 km/s near Iceland to >8 km/s near ∼57 • N. The gravity interpretation of Peirce et al (2005) suggested a continuous southward thinning of the cust of approximately 10-7 km from the region of the present study to ∼57 • N and an accompanying decrease of the uppermost mantle density of 3230-3000 kg/m 3 . Gravity is thus important for the question of what crust and Moho mean in the vicinity of the Iceland plume.…”

“…Travel time interpretation, as presented here, is not sensitive to stronger or weaker gradients, and the moderate gradients found by travel time inversion are probably affected by the initial assumptions of weak gradients. Bunch (1980), Smallwood and White (1998), Weir et al (2001) and also Ritzert and Jacoby (1985) and Peirce et al (2005) emphasized velocity gradients and calculated synthetic seismograms by which also the amplitudes are taken into account. The results favour stronger gradients in the upper part of the crust such that a discerete boundary between the upper and lower crust layers becomes insignificant.…”

“…It has the potential of constraining density and temperature and thus highlighting dynamic processes of mantle plume flow and crustal formation, as shown, e.g. by Peirce et al (2005) for a profile extending along the Reykjanes Ridge axis from ∼57 • N to ∼62 • N near the present stady area. Though principally ambiguous, the combination with seismic a priori information in gravity inversion sheds light on the interpretation of the seismic refraction and wide-angle reflection observations.…”

“…Further south, the plume influence tapers out, the central horst is replaced by a median valley south of ∼57 • N. Multidisciplinary geophysical studies (wide-angle seismic reflection-refraction, electromagnetic sounding and magnetotellurics, gravity, magnetics) between 57 • and >58 • N (Sinha et al, 1998;Peirce et al, 2005) show that under an axial water depth of 1.5-2 km, the upper crust is ∼2.5 km thick, and the lower crust (6.5-7 km/s) is ∼5 km thick or its base at ∼9 km below sea level (bsl). Subcrustal P-wave velocities are calculated to be 7.9-8.2 km/s, rising to 8.4 km/s at 50 km depth.…”

Constraints from melt inclusions on depths of magma residence at intermediate magma supply along the Galápagos Spreading Center

“…At rates of magma supply less than 0.3 × 10 6 m 3 /yr/km, seismic reflections from axial melt lenses are absent along most ridge segments studied (e.g., Detrick et al, 1990;Peirce et al, 2005;Sinton and Detrick, 1992), although two studies that specifically targeted ridge segments thought to be magmatically active did detect melt lenses (Singh et al, 2006;Sinha et al, 1997). At these lower rates of magma supply, there does not appear to be a global correlation between the time-averaged rate of magma supply and the depth of magma residence, although detected magma reservoirs are consistently deeper than at high-magma-supply ridges.…”