Causes and Consequences of Diachronous V‐Shaped Ridges in the North Atlantic Ocean

Abstract In the North Atlantic Ocean, the geometry of diachronous V‐shaped features that straddle the Reykjanes Ridge is often attributed to thermal pulses which advect away from the center of the Iceland plume. Recently, two alternative hypotheses have been proposed: rift propagation and buoyant mantle upwelling. Here we evaluate these different proposals using basin‐wide geophysical and geochemical observations. The centerpiece of our analysis is a pair of seismic reflection profiles oriented parallel to flow lines that span the North Atlantic Ocean. V‐shaped ridges and troughs are mapped on both Neogene and Paleogene oceanic crust, enabling a detailed chronology of activity to be established for the last 50 million years. Estimates of the cumulative horizontal displacement across normal faults help to discriminate between brittle and magmatic modes of plate separation, suggesting that crustal architecture is sensitive to the changing planform of the plume. Water‐loaded residual depth measurements are used to estimate crustal thickness and to infer mantle potential temperature which varies by ±25°C on timescales of 3–8 Ma. This variation is consistent with the range of temperatures inferred from geochemical modeling of dredged basaltic rocks along the ridge axis itself, from changes in Neogene deep‐water circulation, and from the regional record of episodic Cenozoic magmatism. We conclude that radial propagation of transient thermal anomalies within an asthenospheric channel that is 150 ± 50 km thick best accounts for the available geophysical and geochemical observations. Plain Language Summary In the North Atlantic Ocean, immense amounts of hot material rises up beneath Iceland from deep within Earth's mantle, forming a gigantic pancake‐shaped upwelling. This upwelling, known as the Iceland mantle plume, is the largest on Earth and plays a key role in determining the depth and shape of the North Atlantic Ocean over thousands of kilometers. A pattern of distinctive V‐shaped ridges and troughs that are hundreds of kilometers long and tens of kilometers wide occur on the seabed south of Iceland. These V‐shaped ridges are thought to have been generated by waxing and waning of the plume, but their precise origin is hotly debated. Here we use an acoustic (i.e., seismic) survey, spanning the North Atlantic Ocean to image these features. We assess competing hypotheses for their formation and argue that they are indeed an indirect record of plume activity through time. Pulses of hot material appear to be generated every 3 to 8 Ma. As they spread beneath adjacent tectonic plates, these pulses cause vertical movements that trigger changes in ancient oceanic circulation. Key Points Seismic reflection images of oceanic crust south of Iceland reveal geometry of V‐shaped ridges Thermal pulsing accounts for V‐shaped ridge structure, regional volcanism, and geochemical observations Iceland plume is largest on Earth and pulses every 3–8 Ma