Convective upwelling in the mantle beneath the Gulf of California

Deep electrical imaging of the ultraslow-spreading Mohns Ridge

More than a third of mid-ocean ridges have a spreading rate of less than 20 millimetres a year¹. The lack of deep imaging data means that factors controlling melting and mantle upwelling^2,3, the depth to the lithosphere-asthenosphere boundary (LAB)^4,5, crustal thickness^6-9 and hydrothermal venting are not well understood for ultraslow-spreading ridges^10,11. Modern electromagnetic data have greatly improved our understanding of fast-spreading ridges^12,13, but have not been available for the ultraslow-spreading ridges. Here we present a detailed 120-kilometre-deep electromagnetic joint inversion model for the ultraslow-spreading Mohns Ridge, combining controlled source electromagnetic and magnetotelluric data. Inversion images show mantle upwelling focused along a narrow, oblique and strongly asymmetric zone coinciding with asymmetric surface uplift. Although the upwelling pattern shows several of the characteristics of a dynamic system^3,12-14, it probably reflects passive upwelling controlled by slow and asymmetric plate movements instead. Upwelling asthenosphere and melt can be traced to the inferred depth of the Mohorovičić discontinuity and are enveloped by the resistivity (100 ohm metres) contour denoted the electrical LAB (eLAB). The eLAB may represent a rheological boundary defined by a minimum melt content. We also find that neither the melt-suppression model⁷ nor the inhibited-migration model¹⁵, which explain the correlation between spreading rate and crustal thickness^6,16-19, can explain the thin crust below the ridge. A model in which crustal thickness is directly controlled by the melt-producing rock volumes created by the separating plates is more likely. Active melt emplacement into oceanic crust about three kilometres thick culminates in an inferred crustal magma chamber draped by fluid convection cells emanating at the Loki's Castle hydrothermal black smoker field. Fluid convection extends for long lateral distances, exploiting high porosity at mid-crustal levels. The magnitude and long-lived nature of such plumbing systems could promote venting at ultraslow-spreading ridges.

The initiation of segmented buoyancy-driven melting during continental breakup

Melting of the mantle during continental breakup leads to magmatic intrusion and volcanism, yet our understanding of the location and dominant mechanisms of melt generation in rifting environments is impeded by a paucity of direct observations of mantle melting. It is unclear when during the rifting process the segmented nature of magma supply typical of seafloor spreading initiates. Here, we use Rayleigh-wave tomography to construct a high-resolution absolute three-dimensional shear-wave velocity model of the upper 250 km beneath the Afar triple junction, imaging the mantle response during progressive continental breakup. Our model suggests melt production is highest and melting depths deepest early during continental breakup. Elevated melt production during continental rifting is likely due to localized thinning and melt focusing when the rift is narrow. In addition, we interpret segmented zones of melt supply beneath the rift, suggesting that buoyancy-driven active upwelling of the mantle initiates early during continental rifting.

Fossil slabs attached to unsubducted fragments of the Farallon plate

As the Pacific-Farallon spreading center approached North America, the Farallon plate fragmented into a number of small plates. Some of the microplate fragments ceased subducting before the spreading center reached the trench. Most tectonic models have assumed that the subducting oceanic slab detached from these microplates close to the trench, but recent seismic tomography studies have revealed a high-velocity anomaly beneath Baja California that appears to be a fossil slab still attached to the Guadalupe and Magdalena microplates. Here, using surface wave tomography, we establish the lateral extent of this fossil slab and show that it is correlated with the distribution of high-Mg andesites thought to derive from partial melting of the subducted oceanic crust. We also reinterpret the high seismic velocity anomaly beneath the southern central valley of California as another fossil slab extending to a depth of 200 km or more that is attached to the former Monterey microplate. The existence of these fossil slabs may force a reexamination of models of the tectonic evolution of western North America over the last 30 My.

High heat flow and ocean acidification at a nascent rift in the northern Gulf of California

The prevailing tectonic setting in the Gulf California suggests the presence of an undetermined number of short spreading centres with associated hydrothermal systems. However, to date, active seafloor spreading phenomena have been documented in only three of the eight tectonically active basins. Here we report heat flow values as high as 15,436 mW m⁻² in two of the northernmost basins of the Gulf of California, providing evidence of intense hydrothermal activity associated with the transition from continental rifting to seafloor spreading. The mean heat flow for the Wagner and Consag basins area is 1,875 mW m⁻², more than 15 times higher than the mean value for oceanic crust (105.4 mW m⁻²). Additional evidence for vigorous hydrothermal circulation and a shallow heat source includes intense gas discharge (CO₂ and CH₄), widespread low pH (average 7.7), locally high ²²²Rn concentrations in the bottom water and a high extent of organic matter maturation in the sediments.

Active seafloor spreading has been documented in some of the tectonically active basins of the Gulf of California. This work presents new geophysical and geochemical data as evidence that active seafloor spreading is also occurring in the northernmost Wagner and Consag basins of the Gulf.