Oceanic crustal thickness from seismic measurements and rare earth element inversions

Abstract

Seismic refraction results show that the igneous section of oceanic crust averages 7.1±0.8 km thick away from anomalous regions such as fracture zones and hot‐spots, with extremal bounds of 5.0–8.5 km. Rare earth element inversions of the melt distribution in the mantle source region suggest that sufficient melt is generated under normal oceanic spreading centers to produce an 8.3±1.5 km thick igneous crust. The difference between the thickness estimates from seismics and from rare earth element inversions is not significant given the uncertainties in the mantle source composition, though it is of the magnitude that would be expected if partial melt fractions of about 1% remain in the mantle and are not extracted to the overlying crust. The inferred igneous thickness increases to 10.3±1.7 km (seismic measurements) and 10.7±1.6 km (rare earth element inversions) where spreading centers intersect the regions of hotter than normal mantle surrounding mantle plumes. This is consistent with melt generation by decompression of the hotter mantle as it rises beneath spreading centers. Maximum inferred melt volumes are found on aseismic ridges directly above the central rising cores of mantle plumes, and average 20±1 and 18±1 km for seismic profiles and rare earth element inversions respectively. Both seismic measurements and rare earth element inversions show evidence for variable local crustal thinning beneath fracture zones, though some basalts recovered from fracture zones are indistinguishable geochemically from those generated on normal ridge segments away from fracture zones. This is consistent with a model where the melt generated beneath spreading ridges is redistributed to intrusive centers along the ridge axis, from where it may flow laterally along the axis at crustal or surface levels. The melt may sometimes flow into the bathymetric lows associated with fracture zones. Oceanic crust created at very slow‐spreading ridges, and in regions adjacent to some continental margins where rifting was initially very slow, exhibits anomalously thin crust from seismic measurements and unusually small amounts of melt generation from rare earth element inversions. We attribute the decreased mantle melting on very slow‐spreading ridges to the conductive heat loss that enables the mantle to cool as it rises beneath the rift.

Vp/Vs-ratios and anisotropy on the northern Jan Mayen Ridge, North Atlantic, determined from ocean bottom seismic data

“…This indicates that the major contribution to the crustal anisotropy comes from dikes in the upper crust, which is in agreement with general models for oceanic crustal formation (e.g. White et al, 1992). …”

Crustal structure and kinematics of the TAMMAR propagating rift system on the Mid-Atlantic Ridge from seismic refraction and satellite altimetry gravity

“…We therefore take 6.5 km to be the average crustal thickness in all discussions that follow. This is within the limits of 'average oceanic crust' (7.1 ± 0.8 km) as observed globally by White et al (1992), albeit on the thin side.…”

Why no anomaly is visible over most of the continent–ocean boundary in the global crustal magnetic field

“…This is because of our choice of crustal model, which includes two broad subdivisions of oceanic crust, based on its geologic age. The crust having age <34 Ma is classified as young crust while >34 Ma is classified as old crust (White et al, 1992). Following White et al (1992), the old crust is assumed thicker by 400 m from the young crust.…”

“…The crust having age <34 Ma is classified as young crust while >34 Ma is classified as old crust (White et al, 1992). Following White et al (1992), the old crust is assumed thicker by 400 m from the young crust. The important parameter, however, for the oceanic region is the VIS value of the old crust (0.405 SI km) which engulfs most of the continents except the western coast of North and South American craton (CGMW, 2000), where the VIS value of the young oceanic crust (0.385 SI km) is more important.…”

“…All Precambrian and Phanerozoic provinces of the world are assigned susceptibility values according to the modelling procedure described in Hemant and Maus (2004). To model the oceans, the oceanic crust is divided into three layers following White et al (1992). Layer 1 (0.0 SI), 0-1 km thick, is sediment.…”