Faults (shear zones) in the Earth's mantle

Continental transforms: A view from the Alpine Fault

http://meetingorganizer.copernicus.org/EMC2012/EMC2012-194.pdf

Deformation and hydration of the lithospheric mantle beneath the Kaapvaal craton, South Africa

There has been long debate on the asthenospheric versus lithospheric source for numerous intraplate basalts with ocean island basalt (OIB) and high time-integrated U/Pb (HIMU)-like source signatures that have erupted through the Zealandia continental crust. Analysis of 157 spinel facies peridotitic mantle xenoliths from 25 localities across Zealandia permits the first comprehensive regional description of the subcontinental lithospheric mantle (SCLM) and insights into whether it could be a source to the intraplate basalts. Contrary to previous assumptions, the Oligocene-Miocene Zealandia SCLM is highly heterogeneous. It is composed of a refractory craton-like domain (West Otago) adjacent to several moderately fertile domains (East Otago, North Otago, Auckland Islands). Each domain has an early history decoupled from the overlying Carboniferous and younger continental crust, and each domain has undergone varying degrees of depletion followed by enrichment. Clinopyroxene grains reveal trace element characteristics (low Ti/Eu, high Th/U) consistent with enrichment through reaction with carbonatite. This metasomatic overprint has a composition that closely matches HIMU in Sr, Pb ± Nd isotopes. However, clinopyroxene Hf isotopes are in part highly radiogenic and decoupled from the other isotope systems, and also mostly more radiogenic than the intraplate basalts. If the studied spinel facies xenoliths are representative of the thin Zealandia SCLM, the melting of garnet facies lithosphere could only be the intraplate basalt source if it had a less radiogenic Hf-Nd isotope composition than the investigated spinel facies, or was mixed with asthenosphere-derived melts containing less radiogenic Hf.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014GC005300

Metasomatized ancient lithospheric mantle beneath the young Zealandia microcontinent and its role in HIMU‐like intraplate magmatism

The geologic evolution of the New Zealand microcontinent was characterised by intermittent Cretaceous to Quaternary episodes of intraplate volcanism. To evaluate the corresponding mantle evolution beneath New Zealand with a specific focus on the tectonic evolution, we performed a combined major and trace element and Hf, Nd, Pb, Sr isotope investigation on a suite of representative intraplate volcanic rocks from both main islands and the Chatham Islands. Isotopically, the data set covers a range between “HIMU-like” end member compositions (206Pb/204Pb: 20.57, 207Pb/204Pb: 15.77, 87Sr/86Sr: 0.7030, eHf: + 3.8, eNd: + 4.2), compositions tending towards MORB (206Pb/204Pb: 19.01, 207Pb/204Pb: 15.62, 87Sr/86Sr: 0.7028, eHf: + 9.9, eNd: + 7.0) and compositions reflecting the influence of subducted sediments (206Pb/204Pb: 18.99, 207Pb/204Pb: 15.67, 87Sr/86Sr: 0.7037, eHf: + 4.4, eNd: + 3.9). Whereas volcanism on the Chatham Islands constitutes the HIMU end member of our data set, intraplate volcanic rocks from the North Island are dominated by MORB-like compositions with relatively radiogenic 206Pb/204Pb signatures. Volcanic rocks from the South Island form a trend between the three end members. Assuming a polybaric melting column model, the primary melt compositions reflect variations in the degree of melting, coupled to variable average melting depths. As the three isotope and trace element end members occur throughout the volcanic episodes, the “HIMU-like” and the sediment influenced signatures most likely originate from a heterogeneous subcontinental lithospheric mantle, whereas an asthenospheric origin is inferred for the MORB-like component. For the South Island, affinities to HIMU wane with decreasing average melting depths whereas MORB and sediment-like signatures become more distinct. We therefore propose a polybaric melting model involving upper asthenospheric mantle and a lithospheric mantle source that has been modified by subduction components and veins of fossil “HIMU-like” asthenospheric melts. The proportion of asthenospheric versus lithospheric source components is controlled by variations in lithospheric thickness and heat flow, reflecting the different tectonic settings and rates of extension. Generally, low degree melts preferentially tap enriched vein material with HIMU signatures. The widespread occurrence of old Gondwana-derived lithospheric mantle beneath intraplate volcanic fields in East Gondwana is suggested by overall similarities between New Zealand intraplate volcanic rocks and volcanic rocks in East Australia and Antarctica. The petrogenetic model proposed here may therefore serve as a general model for the petrogenesis of Cretaceous to Recent intraplate volcanic rocks in former East Gondwana.

Intraplate volcanism in New Zealand: the role of fossil plume material and variable lithospheric properties

SUMMARY We constrain the extent of lithospheric and asthenospheric deformation beneath New Zealand by coupling measurements of shear wave splitting of teleseismic waves and petrophysical analysis of mantle xenoliths. SKS splitting for the central South Island and the eastern part of the North Island confirms earlier observations. The fast S-wave polarization directions are roughly NE/SW, which is parallel to the Alpine Fault in the South Island and to the strike of the Hikurangi subduction zone in the North Island. This suggests that flow parallel to the plate boundary extends up to 160 km away from the boundary. Departures from this pattern are restricted to the southernmost station and the two northernmost stations and indicate changes in the mantle flow or a more complex anisotropic pattern, such as a dipping axis of symmetry or heterogeneous anisotropy. Analysis of mantle xenoliths from the Raglan (North Island), Dunedin (South Island) and Chatham Island regions allows us to constrain the lithospheric contribution to the observed seismic anisotropy. The intrinsic S-wave anisotropy is 1.5 per cent higher for the South Island samples (∼5.0 per cent versus ∼3.5 per cent for the North Island), indicating a strong lithospheric deformation that could explain the major part of the observed splitting. In the North Island, the intrinsic lithospheric anisotropy is too low to explain the SKS splitting times. This indicates that asthenospheric deformation probably plays a major role there. For the Chatham Island, the anisotropy pattern remains unresolved as the xenolith suggests strong lithospheric anisotropy but no present splitting is measured.

/pdf/mantle-tectonics-beneath-new-zealand-inferred-from-sks-ojnp8nuzio.pdf

Mantle tectonics beneath New Zealand inferred from SKS splitting and petrophysics

Seismic anisotropy is one of the few tools for assessing the orientation of mantle deformation. We use it to study the plate boundary through New Zealand, which includes two oblique subduction zones of opposing polarity linked by the transpressional Alpine Fault. The mantle under South Island has strong, coherent deformation as evidenced by shear-wave splitting of SKS phases with fast polarizations oriented subparallel to the plate boundary, and with delay times averaging 1.7 s. SKS delay times are consistent through most of South Island and southern North Island, and fast polarizations are independent of distance from the plate boundary across the landmass. A fundamental change in anisotropy is nevertheless observed, which does not correspond in a simple way to changes between subduction and transform faulting. Instead, the change corresponds roughly with the northward narrowing of South Island (the "Waistline"). North of the Waistline, SKS splitting is parallel to the plate boundary, and the region of anisotropy is narrow (60-100 km wide) in the uppermost mantle as evidenced by Pn wave speeds and by shear wave splitting of local events. Lithospheric strain with dynamic recrystallization and sub-slab asthenospheric flow may both be contributing to the anisotropy. South of the Waistline, SKS splitting averages 20°, about 25° to the shear plane, and along the Waistline, Pn anisotropy extends about 200 km across the plate boundary. Lithospheric shearing without dynamic recrystallization, appropriate for smaller strain in this region, or asthenospheric flow directed at 20° or 200° could form the anisotropy.

Seismic Anisotropy in South Island, New Zealand

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Mantle tectonics beneath New Zealand inferred from SKS splitting and petrophysics

Seismic Anisotropy in South Island, New Zealand