Lithospheric structure and melting processes in southeast Australia: new constraints from joint probabilistic inversions of 3D magnetotelluric and seismic data
Maria ManasseroSinan ÖzaydınJuan Carlos AfonsoJoshua SheaStephan ThielAlison KirkbyIlya FominK. Czarnota
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Abstract:
The thermochemical structure of the lithosphere exerts control on melting mechanisms in the mantle as well as the location of volcanism and ore deposits. Imaging the complex interactions between the lithosphere and asthenospheric mantle requires the joint inversion of multiple data sets and their uncertainties.In particular, the combination of seismic velocity and electrical conductivity with data proxies for bulk composition and elusive minor phases is a crucial step towards fully understanding large-scale lithospheric structure and melting.We apply a novel probabilistic approach for joint inversions of 3D magnetotelluric and seismic data to image the lithosphere beneath southeast Australia. Results show a highly heterogeneous lithospheric structure with deep conductivity anomalies that correlate with the location of Cenozoic volcanism. In regions where the conductivities have been at odds with sub-lithospheric temperatures and seismic velocities, we observe that the joint inversion provides conductivity values consistent with other observations. The results reveal a strong relationship between metasomatized regions in the mantle and i) the limits of geological provinces in the crust, which elucidates the subduction-accretion process in the region; ii) distribution of leucitite and basaltic magmatism; iii) independent geochemical data, and iv) a series of lithospheric steps which constitute areas prone to generating small-scale instabilities in the asthenosphere. This scenario suggests that shear-driven upwelling and edge-driven convection are the dominant melting mechanisms in eastern Australia rather than mantle plume activity, as conventionally conceived. Our study offers an integrated lithospheric model for southeastern Australia and provides insights into the feedback mechanism driving surface processes.Keywords:
Asthenosphere
Magnetotellurics
Low-velocity zone
Lithospheric flexure
geodynamics
The plate tectonic theory requires a rigid lithosphere floating over a weak asthenosphere, separated by the lithosphere-asthenosphere boundary, which has been sometimes interpreted as the Gutenberg discontinuity. Using a deep seismic reflection technique, we report the presence of two continuous reflections covering 27 Ma to 58 Ma oceanic lithosphere in the Atlantic Ocean. We find that the upper reflection deepens with age and follows the ~1250°C isotherm, whereas the deeper reflection lies at a constant depth of ~75 km. We suggest that the upper reflection represents the thermally controlled lithosphere-asthenosphere boundary, whereas the lower reflection is the Gutenberg discontinuity, a frozen-in dehydration boundary separating the dry mantle melting region above from the hydrated mantle below formed at the ridge axis. We also find that thermal mantle anomalies rejuvenate the lithosphere, uplift the lithosphere-asthenosphere boundary, and destroy the Gutenberg discontinuity.
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Lithospheric flexure
Low-velocity zone
Core–mantle boundary
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Low-velocity zone
Lithospheric flexure
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Lithospheric flexure
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[1] Oceanic lithosphere constitutes the bulk of Earth’s tectonic plates and also likely represents the building blocks of the continental lithosphere. The depth and nature of the oceanic lithosphere‐asthenosphere boundary are central to our understanding of the definition of the tectonic plates and lithospheric evolution. Although it is well established that oceanic lithosphere cools, thickens, and subsides as it ages according to conductive cooling models, this relatively simple realization of the tectonic plates is not completely understood. Old (>70 Ma) ocean depths are shallower than predicted. Furthermore, precise imaging of the lower boundary of the oceanic lithosphere has proven challenging. Here we directly map the depth and nature of a seismic discontinuity that is likely the lithosphere‐asthenosphere boundary across the Pacific plate using a new method that models variations in the shapes of stacked SS waveforms from 17 years of seismic data. The depth to the discontinuity varies from 25 to 130 km and correlates with distance from the ridge along mantle flow lines. This implies that the depth of the oceanic lithosphere‐asthenosphere boundary depends on the temperature of the underlying asthenosphere, defined by a best fitting isotherm at 930°C with a 95% confidence region of 820–1020°C, although the sharpness of the observations in some locations implies a mechanism besides temperature may also be required.
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Lithospheric flexure
Low-velocity zone
Convergent boundary
Ridge push
Seafloor Spreading
Discontinuity (linguistics)
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Lithospheric flexure
Low-velocity zone
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Lithospheric flexure
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Abstract Earth's cratonic mantle lithosphere is distinguished by high seismic wave velocities that extend to depths greater than 200 km, but recent studies disagree on the magnitude and depth extent of the velocity gradient at their lower boundary. Here we analyze and model the frequency dependence of S p waves to constrain the lithosphere‐asthenosphere velocity gradient at long‐lived stations on cratons in North America, Africa, Australia, and Eurasia. Beneath 33 of 44 stations, negative velocity gradients at depths greater than 150 km are less than a 2–3% velocity drop distributed over more than 80 km. In these regions the base of the typical cratonic lithosphere is gradual enough to be explained by a thermal transition. Vertically sharper lithosphere‐asthenosphere transitions are permitted beneath 11 stations, but these zones are spatially intermittent. These results demonstrate that lithosphere‐asthenosphere viscosity contrasts and coupling fundamentally differ between cratons and younger continents.
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Lithospheric flexure
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We applied data on kimberlite-hosted mantle xenoliths from Lahtojoki, Kaavi, in eastern Finland for thermal, rheological and seismic velocity modeling of the lithospheric mantle in the central part of the Fennoscandian Shield. We also report petrographic evidence for decrepitated fluid inclusions indicating a presence of fluids in the upper mantle. Our data and models suggest that the thermal and rheological lithosphere is very thick (230–250 km), contains small amounts of fluids, and follows wet (but not fluid-saturated) olivine rheology. The lithospheric part of the mantle where heat transfer is mainly conductive and which does not participate in convection, changes into a mechanical asthenosphere in a solid state. Mantle viscosity shows a weak minimum at the mechanical lithosphere–asthenosphere boundary. The lithosphere–asthenosphere transition does not require partial melting, and very probably there is no partial melt-bearing asthenosphere beneath the lithosphere at all. This also precludes an electrical asthenosphere in the sense of high conductivity due to partial melting. Seismic velocity models calculated using data on mineral composition of the xenoliths and mineral elastic parameters indicate no low-velocity layer at the lithosphere–asthenosphere boundary, instead only a downward increase in the vertical velocity gradients. The velocity model indicates velocities higher than iasp91 (up to 2%) at depths above 250 km, and it is in good agreement with seismic body-wave tomography results.
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Xenolith
Lithospheric flexure
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Thermal models of the oceanic and continental lithosphere are presented. Variation of heat flow as a function of crustal age is used to compute the temperature distribution in the lithosphere. The lithosphere thickness is determined as a depth where the temperature reaches the melting temperature, which is estimated on the basis of high-pressure experiments under water-deficient conditions. The lithosphere-asthenosphere boundary is thermodynamically an open system in which H2O and CO2 behave as mobile components. The melting temperature is roughly 1100°C. The oceanic lithosphere thickens rapidly during the first 30–50 m.y. and grows little after 80–100 m.y. The continental lithosphere grows much more slowly but becomes thicker than does the oceanic one. The density of the asthenosphere under the ocean is calculated on an assumption that isostasy is maintained at the lithosphere-asthenosphere boundary. The computed results require that the lower part of the lithosphere must be denser than peridotite. A mechanism to produce a dense layer in the lower part of the lithosphere is proposed. The dense layer is considered to consist of garnet-rich eclogite. The heat flow from the asthenosphere to the lithosphere is twice larger, and the temperature of the asthenosphere at a same depth is significantly higher under the oceans than under the continents. Geophysical implications of this contrast are discussed.
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Lithospheric flexure
Low-velocity zone
Delamination
Thermal subsidence
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Low-velocity zone
Lithospheric flexure
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