The Melt Redemption

The spreading center axis of a mid-ocean ridge is always believed to be the only exit for the upwelling melts beneath it to erupt. However, recent observations suggest that those seemingly abiding melts have probably already found a secret passage to escape. Throughout the global mid-ocean ridge system, mid-ocean ridges are offset by transform faults. These transform faults, with efficient conductive cooling, may produce a thickened lithosphere that directs melts away, but recent research showed that, the thickened crust along transform faults might indicate efficient melt extraction toward transforms.

Laura and Laurent (2011) apply a 3D model of melt migration and extraction to investigate the unusual thickened crust along an oceanic transform at fast spreading ridges, and suggest that a melt extraction zone (MEZ) should be responsible for the melt redemption. Melt migration at mid-ocean ridge system can be modeled by a 2-D process with two steps: (1) vertical upwelling under buoyancy within the asthenosphere; (2) lateral migration along a low-permeability boundary (permeability barrier) inclined towards the ridge axis within the thermal lithosphere. However, in 3-D, the thermomechanical  structure of the transform will affect the melt pathway to the ridge axis and result in thickened crust. Laura and Laurent solve 3-D model based on a thermal structure that incorporates rheological feedbacks associated with brittle deformation and hydrothermal cooling, and emphasize the potential structural controls on melt extraction. Their results show that an MEZ, interpreted as structural damage such as faults and dikes, intersects the permeability barrier, redistributes excess ridge crust to the transform domain, and accounts well for the unusual crustal thickness at intermediate and fast slipping transform faults observed.

Conceptual geometry of melt extraction zone (MEZ) (A) ssociated with a fast-spreading mid-ocean ridge, (B) associated with a transform fault.

(A) Isoviscous model results for crustal thickness for the simplified ridge-transform cases and varying the distance of lateral extraction; (B) schematic representation of crustal accretion at ridge segments with no shunting; (C) Schematic representation of crustal redistribution into the transform domain when shunting is allowed.

However, at ridges with slow and ultraslow spreading rate, no thickened crust has been observed along transform faults. Thus, more questions emerge. If there is any MEZ around slow to ultraslow slipping transform? What prevents these MEZs from acting efficiently? What’s the relationship between the depths of MEZ & permeability barrier and the spreading rate? My research will be focusing on these questions and I will be modeling the mid-ocean ridge systems with different spreading rate, and finally find out the reasons for the different crustal thicknesses among transform faults with different slipping rates.

Reference

Hebert, L. B., and L. G. J. Montési (2011), Melt extraction pathways at seg- mented oceanic ridges: Application to the East Pacific Rise at the Siqueiros transform, Geophys. Res. Lett., 38, L11306, doi:10.1029/2011GL047206.