Weak mantle wedge causes mantle exhumation punctuated with discrete oceanic crust in the Tyrrhenian basin.

Abstract

The Continent-Ocean Transition (COT) in the young Tyrrhenian basin documents mantle exhumation punctuated with multiple episodes of discrete oceanic crust formation. This observation challenges prevailing models of magma-poor COTs, which typically describe mantle exhumation preceding the emplacement of oceanic crust. Notably, this COT developed without the conventional conditions associated with magma-poor rifted margins, such as slow rifting velocities and chemically depleted mantle sources. A key observation is the low shear-wave velocity observed in the uppermost mantle of the Tyrrhenian basin and its adjacent onshore regions, which correlates with subduction-related volcanism, suggesting the presence of a hydrated mantle wedge with low rheological strength. Here, we show that, based on 3D magmatic-thermomechanical numerical modeling, the episodic formation of oceanic crust within the Tyrrhenian basin's COT results from the mechanical weakness of the mantle. The lithospheric mantle is exhumed to the surface through exhumation channels initiated within the weak mantle zone. The subsequent flow of partially molten mantle toward these channels leads to the development of multiple short-lived spreading centers. Our findings shed light on characteristics and mechanisms shaping the COT of marginal basins, where their opening is influenced by subduction processes.