Lateral H2O variation in the Zealandia lithospheric mantle controls orogen width

Mantle xenoliths exhumed from beneath the South Island of New Zealand offer an opportunity to investigate the role of lithospheric mantle water in influencing deep lithosphere deformation and formation of the narrow Southern Alps mountain range during orogeny along the Australia-Pacific plate boundary. Fourier transform infrared spectrometry (FTIR) water measurements on clinopyroxene (cpx), orthopyroxene (opx) and olivine of peridotite xenoliths from seven localities across the Zealandia continent reveal a lateral variation of H2O within the Oligocene-Miocene lithospheric mantle. There is a “wet” domain beneath what is now the Southern Alps mountain range with average measured H2O concentrations of 610 ppm for cpx and 298 ppm for opx. In contrast, dominantly “dry” domains with average H2O concentrations of 110 ppm for cpx and 49 ppm for opx reside beneath East Otago, Chatham Island and Auckland Islands. Contrasting H2O concentrations within olivine (calculated to be 60 to 100 ppm for the wet domain versus <10 ppm for the dry domains) indicate a lateral gradient in mantle effective viscosity of up to one order of magnitude beneath the Southern Alps and the surrounding regions. The narrow width of the Southern Alps orogen, ∼80 km, contrasts with most of its broad counterparts worldwide, such as the North American Cordillera where the mantle was hydrated and weakened by the subduction of the Farallon plate. However, those developed orogens were often complicated by subsequent tectonic overprinting. Our findings show 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 width of the developing Southern Alps orogen.