Thermo‐Mechanical State of Ultraslow‐Spreading Ridges With a Transient Magma Supply.

The thermal structure of mid‐ocean ridge axes is a critical control on the mechanical properties of young lithosphere and modulates the faulting styles that shape the Earth's seafloor. Models that balance a steady input of magmatic heat with a vigorous hydrothermal output are generally successful at explaining the thermal state of magmatically robust mid‐ocean ridges. However, the magma supply of ultraslow spreading ridges is often subdued and highly episodic, and the depth‐extent and vigor of hydrothermal circulation in these settings remain poorly known, requiring modifications to the standard magmatic‐hydrothermal paradigm. We develop models that couple repeated magmatic intrusions with hydrothermal convection in a permeable domain whose boundaries are controlled by the extent of grain‐scale thermal cracking. Our simulations reveal decreasing trends of venting temperatures with increasing intrusion periodicity, as well as with increasing permeability. Our model explains the seismically inferred depth (∼10 km) to the brittle‐ductile transition and the depth of MORB crystallization at the Mid‐Cayman Spreading Center by invoking repeated intrusion of sills every 20–50 kyrs, depending on their size. Doubling this periodicity predicts a thicker (∼15 km) seismogenic layer and lower‐temperature venting, as observed at the Southwest Indian Ridge (SWIR) at 64˚E. However, if fluid convection is not possible below ∼6 km, our model requires that high‐temperature venting at ultraslow ridges (e.g., Longqi at SWIR 49.7˚E) be fueled by shallow episodic magma intrusions into the long‐term convective region, which maintain high‐output circulation for at most a few tens of kyrs. Key Points: New model for the thermal structure of ultraslow ridges couples hydrothermal circulation with episodic sill emplacementHydrothermal convection in uppermost 6 km of lithosphere with thick boundary layer can explain cold thermal regimeHigh‐output venting at ultraslow ridges likely requires transient shallow magmatic intrusions [ABSTRACT FROM AUTHOR]