Lateral Mantle H2O Variation Controls Orogen Width

How the lithosphere deforms to accommodate convergence at continent-continent plate boundaries remains a long-standing geodynamic question. The South Island of New Zealand straddles the boundary between the Pacific and Australian continental plates and has long been considered a premier locality to study this issue.The research paper (Li et al., 2018) directly addresses the role of the lateral H2O variation in the lithospheric mantle in affecting the continental deformation and hence bears on a general problem of how processes in the mantle and crust are related or coupled to each other during orogeny.

There are two contrasting mantle domains, based on the water content measurements of minerals in mantle peridotites: a restricted “wet” and rheologically weak mantle domain beneath what is now the Southern Alps orogeny adjacent to the Australian-Pacific plate boundary prior to the convergence,and the dominantly “dry” and relatively rigid mantle domains beneath East Otago, Chatham Island and Auckland Islands. Contrasting H2O concentrations indicate a lateral gradient in mantle effective viscosity of up to one order of magnitude beneath the Southern Alps and the surrounding regions. The findings provide direct evidence that the pre-existing rheologically weak zone of the mantle lithosphere, most likely resulting from hydration by the subduction of the oceanic lithosphere attached to the Australian plate beneath the South Island of New Zealand at ~ 25 Ma, affects the continental deformation at or near the surface and controls the lateral width of the developing orogen.

Reference:

Li, P., Scott, J.M., Liu, J., Xia, Q.-K. 2018. Lateral H2O variation in the Zealandia lithospheric mantle controls orogen width. Earth and Planetary Science Letters 502, 200-209.