Combined dynamic and geochemical evidence for convergent melt flow beneath the East Pacific Rise

Episodic intraplate magmatism fed by a long-lived melt channel of distal plume origin

In the past decade, marine geophysical observations have led to the discovery of thin channels at the base of oceanic plates with anomalous physical properties that indicate the presence of low-degree partial melts. However, mantle melts are buoyant and should migrate toward the surface. We show abundant observations of widespread intraplate magmatism on the Cocos Plate where a thin partial melt channel was imaged at the lithosphere-asthenosphere boundary. We combine existing geophysical, geochemical, and seafloor drilling results with seismic reflection data and radiometric dating of drill cores to constrain the origin, distribution, and timing of this magmatism. Our synthesis indicates that the sublithospheric channel is a regionally extensive (>100,000 km²) and long-lived feature that originated from the Galápagos Plume more than 20 Ma ago, supplying melt for multiple magmatic events and persisting today. Plume-fed melt channels may be widespread and long-lived sources for intraplate magmatism and mantle metasomatism.

Continental lithospheric thinning and breakup in response to upwelling divergent mantle flow: application to the Woodlark, Newfoundland and Iberia margins

Depth-uniform stretching is not the dominant deformation process for thinning continental lithosphere leading to breakup; it cannot explain the observed depth-dependent lithosphere stretching and mantle exhumation at rifted continental margins. Depth-dependent lithosphere thinning, in which stretching of the lower crust and lithosphere mantle greatly exceeds that of the upper crust, has been observed at many non-volcanic and volcanic rifted continental margins including conjugate margin pairs. Passive continental margins show a paucity of brittle deformation in the upper crust during continental lithosphere thinning leading to breakup and sea-floor spreading initiation. A new model of rifted continental margin formation has been developed that assumes that deformation and thinning of continental lithosphere leading to breakup occurs in response to an upwelling divergent flow field within continental lithosphere and asthenosphere, and that this deformation evolves into sea-floor spreading. The new model successfully predicts depth-dependent stretching of continental margin lithosphere for both non-volcanic and volcanic margins and mantle exhumation at non-volcanic margins, both of which are observed, but are not explained, by existing depth-uniform continental lithosphere stretching models. The new model provides a balance of extensional strain, supplies an explanation for the paucity of synrift brittle deformation, and offers a simple transition from prebreakup lithosphere thinning to sea-floor spreading. The observed diversity of rifted continental margin structure and width of the ocean–continent transition can be explained by variability in the form of the upwelling divergent flow field. The new upwelling divergent flow model of continental lithosphere thinning leading to continental breakup successfully predicts the observed bathymetry and margin geometry for the most recent segment of sea-floor spreading initiation in the Woodlark Basin in the western Pacific, and the observed bathymetry and free air gravity anomaly for the Newfoundland and Iberian margins.

Timing and magnitude of depth-dependent lithosphere stretching on the southern Lofoten and northern Vøring continental margins offshore mid-Norway: implications for subsidence and hydrocarbon maturation at volcanic rifted margins

Subsidence analysis on the southern Lofoten and northern Vøring segments of the Norwegian rifted margin shows depth-dependent stretching of continental margin lithosphere in which lithosphere stretching and thinning at continental break-up at ~ 54 Ma greatly exceeds that of the upper crust within 100 km landward of the COB. For the southern Lofoten margin lithosphere β stretching factors approaching infinity are required at 54 Ma west of the Utrøst Ridge to restore the top Basalt (inner lava flow) reflectors and top Tare formation (54 Ma) to presumed sub-aerial depositional environments, while for the northern Vøring margin lithosphere β values of 2.5 are required. In contrast, upper crustal extension by faulting shows little stretching with β < 1.1 at break-up or immediately preceding break-up in the Paleocene and Late Cretaceous. The presence of lithosphere depth-dependent stretching and the absence of significant Paleocene and Late Cretaceous upper crustal extension imply that depth-dependent stretching of the southern Lofoten and northern Vøring margins occurred during sea-floor spreading initiation rather than during pre-break-up intra-continental rifting. Depth-dependent stretching, where upper-crustal extension is significantly smaller than whole-crustal or whole-lithosphere extension within 75–150 km of the COB, has been observed worldwide for both volcanic and non-volcanic rifted continental margins. Temperature and hydrocarbon maturation modelling show that the inclusion of depth-dependent stretching has an important effect on temperature and hydrocarbon maturation evolution in depth and time. Failure to include the large β factors for the lower crust and lithospheric mantle (below the less stretched upper crust) leads to a serious under-prediction of temperature and hydrocarbon maturation. While the effect of emplacing thick sill intrusions or magmatic underplating at continental break-up has an important effect on predicted temperature and %VR, their effects can be small in comparison with that of depth-dependent stretching.

Spreading-rate dependence of melt extraction at mid-ocean ridges from mantle seismic refraction data

A variety of observations indicate that mid-ocean ridges produce less crust at spreading rates below 20 mm yr(-1) (refs 1-3), reflecting changes in fundamental ridge processes with decreasing spreading rate. The nature of these changes, however, remains uncertain, with end-member explanations being decreasing shallow melting or incomplete melt extraction, each due to the influence of a thicker thermal lid. Here we present results of a seismic refraction experiment designed to study mid-ocean ridge processes by imaging residual mantle structure. Our results reveal an abrupt lateral change in bulk mantle seismic properties associated with a change from slow to ultraslow palaeo-spreading rate. Changes in mantle velocity gradient, basement topography and crustal thickness all correlate with this spreading-rate change. These observations can be explained by variations in melt extraction at the ridge, with a gabbroic phase preferentially retained in the mantle at slower spreading rates. The estimated volume of retained melt balances the approximately 1.5-km difference in crustal thickness, suggesting that changes in spreading rate affect melt-extraction processes rather than total melting.

Constraints on melt movement beneath the East Pacific Rise from 230Th-238U disequilibrium

We report 230Th-238U disequilibrium data on mid-ocean ridge basalts recovered 5 to 40 kilometers off the ridge axis near 9 degrees 30'N of the East Pacific Rise. These data indicate near-symmetrical eruptions of normal mid-ocean ridge basalts (NMORBs) and incompatible element-enriched mid-ocean ridge basalts (EMORBs) as far as 20 kilometers off axis. Our results suggest large-scale subsurface lateral transport of NMORB melt at 19 to 21 centimeters per year and also provide constraints on the petrogenesis of EMORBs of off-axis origin.