3‐D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle: III. Thermochemical tomography in the Western‐Central U.S.
Juan Carlos AfonsoNicholas RawlinsonYingjie YangD. SchuttAlan G. JonesJavier FulleaWilliam L. Griffin
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Abstract We apply a novel 3‐D multiobservable probabilistic tomography method that we have recently developed and benchmarked, to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times, Rayleigh wave dispersion data, Bouguer anomalies, satellite‐derived gravity gradients, geoid height, absolute (local and dynamic) elevation, and surface heat flow data. The temperature and compositional structures recovered by our inversion reveal a high level of correlation between recent basaltic magmatism and zones of high temperature and low Mg# (i.e., refertilized mantle) in the lithosphere, consistent with independent geochemical data. However, the lithospheric mantle is overall characterized by a highly heterogeneous thermochemical structure, with only some features correlating well with either Proterozoic and/or Cenozoic crustal structures. This suggests that most of the present‐day deep lithospheric architecture reflects the superposition of numerous geodynamic events of different scale and nature to those that created major crustal structures. This is consistent with the complex lithosphere‐asthenosphere system that we image, which exhibits a variety of multiscale feedback mechanisms (e.g., small‐scale convection, magmatic intrusion, delamination, etc.) driving surface processes. Our results also suggest that most of the present‐day elevation in the Colorado Plateau and surrounding regions is the result of thermochemical buoyancy sources within the lithosphere, with dynamic effects (from sublithospheric mantle flow) contributing only locally up to ∼15–35%.Keywords:
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The existence of continental roots and the gradual thickening of the cooling oceanic lithosphere give rise to large‐scale rheological heterogeneities in the uppermost mantle. The effect of these heterogeneities on the long‐wavelength geoid is investigated using a 3‐D mantle flow model involving a low‐viscosity asthenosphere beneath the oceanic lithosphere and the tectonically active continental regions and a thick highly viscous lithosphere beneath ancient continents. Below 400 km the mantle viscosity is laterally homogeneous with a lower mantle more viscous than the overlying layer. The mantle circulation is driven by imposed surface velocities NUVEL‐1 HS2 (Gripp & Gordon 1990) and by the density anomalies inferred from the tomographic models P16B30 and S16B30 (Masters 1996). The geoid heights both due to plate motion and due to internal loading differ by as much as 30 per cent between the models with and without lateral viscosity variations. In contrast to what was suggested in previous studies, spherical harmonics 2 and 3 are strongly affected by the lateral viscosity variations. These differences in the forward problem suggest that the response to the inverse problem that consists of finding the profile of viscosity as a function of depth providing the best fit to the geoid should be considerably affected by the lateral viscosity variations.
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Abstract Geoid anomalies offer crucial information on the internal density structure of the Earth, and thus, on its constitution and dynamic state. In order to interpret geoid undulations in terms of depth, magnitude and lateral extension of density anomalies in the lithosphere and upper mantle, the effects of lower mantle density anomalies need to be removed from the full geoid (thus obtaining the residual “upper mantle geoid”). However, how to achieve this seemingly simple filtering exercise has eluded consensus for decades in the solid Earth community. While there is wide agreement regarding the causative masses of degrees >10 in spherical harmonic expansions of the upper mantle geoid, those contributing to degrees <7–8 remain ambiguous. Here we use spherical harmonic analysis and recent tomography and density models from joint seismic‐geodynamic inversions to derive a representative upper mantle geoid, including the contributions from low harmonic degrees. We show that the upper mantle geoid contains important contributions from degrees 5 and 6 and interpret the causative masses as arising from the coupling between the long‐wavelength lithospheric structure and the sublithospheric upper mantle convection pattern. Importantly, the contributions from degrees 3 < l < 8 do not show a simple power‐law behavior (e.g., Kaula's rule), which precludes the use of standard filtering techniques in the spectral domain. Our upper mantle geoid model will be useful in studies of (a) lithospheric structure, (b) dynamic topography and mantle viscosity, (c) lithosphere‐asthenosphere interactions and (d) the global stress field within the lithosphere and its associated hazards.
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Core–mantle boundary
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Abstract We apply a novel 3‐D multiobservable probabilistic tomography method that we have recently developed and benchmarked, to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times, Rayleigh wave dispersion data, Bouguer anomalies, satellite‐derived gravity gradients, geoid height, absolute (local and dynamic) elevation, and surface heat flow data. The temperature and compositional structures recovered by our inversion reveal a high level of correlation between recent basaltic magmatism and zones of high temperature and low Mg# (i.e., refertilized mantle) in the lithosphere, consistent with independent geochemical data. However, the lithospheric mantle is overall characterized by a highly heterogeneous thermochemical structure, with only some features correlating well with either Proterozoic and/or Cenozoic crustal structures. This suggests that most of the present‐day deep lithospheric architecture reflects the superposition of numerous geodynamic events of different scale and nature to those that created major crustal structures. This is consistent with the complex lithosphere‐asthenosphere system that we image, which exhibits a variety of multiscale feedback mechanisms (e.g., small‐scale convection, magmatic intrusion, delamination, etc.) driving surface processes. Our results also suggest that most of the present‐day elevation in the Colorado Plateau and surrounding regions is the result of thermochemical buoyancy sources within the lithosphere, with dynamic effects (from sublithospheric mantle flow) contributing only locally up to ∼15–35%.
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The seismic velocity anomalies resolved by seismic tomography are associated with variations in density that lead to convective flow and to dynamically maintained topography at the Earth’s surface, the core-mantle boundary (CMB), and any interior chemical boundaries that might exist. The dynamic topography resulting from a given density field is very sensitive to viscosity structure and to chemical stratification. The mass anomalies resulting from dynamic topography have a major effect on the geoid, which places strong constraints on mantle structure. Almost 90% of the observed geoid can be explained by density anomalies inferred from tomography and a model of subducted slabs, along with the resulting dynamic topography predicted for an Earth model with a low-viscosity asthenosphere ( ca . 10 20 Pa s) overlying a moderate viscosity ( ca . 10 22.5 Pa s) lower mantle. This viscosity stratification would lead to rapid mixing in the asthenosphere, with little mixing in the lower mantle. Chemically stratified models can also explain the geoid, but they predict hundreds of kilometres of dynamic topography at the 670 km discontinuity, a prediction currently unsupported by observation. A low-viscosity or chemically distinct D" layer tends to decouple CMB topography from convective circulation in the overlying mantle. Dynamic topography at the surface should result in long-term changes in eustatic sea level.
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geodynamics
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This study addresses the question of post-collisional magmatism and its production mechanisms, addressing especially the mantle processes involved. Numerical experiments are conducted to examine the effects of viscosity weakening by subduction related water content increase in the upper mantle and the resulting sub-lithospheric small-scale convection. The models presented incorporate parameterized and thermodynamic melting models, and take into account variable relationships between mantle water content, mantle strength, water extraction by partial melting and related depletion stiffening.
The results demonstrate the possible importance of so called ”hydrous activation” of the lithosphere-asthenosphere boundary: The post-collisional loss of the lithospheric mantle can be initiated and augmented by the elevated upper mantle water contents that enhances the sub-lithospheric small-scale convection, increases heat flow into the lithosphere, and produces localized lithosphere thin- ning. The irregular spatial and temporal melting patterns and the mantle melt volumes correspond to typical post-collisional mantle-derived magmatism. The small-scale convection can be localized into an edge-driven convection by significant lithosphere thickness gradients, e.g. craton edges. This helps to understand the uplift and volcanism observed in intraplate orogenic settings and implies the importance of these processes at other locations of lithosphere thickness gradients, e.g. recent collision zones.
The lithospheric thinning produced by small-scale convection can initiate whole lithosphere mantle loss via positive feedback mechanisms: gradual thinning of the lithosphere causes partial melting in the lowermost crust, weakening the crust-mantle boundary and providing a detachment mechanism for the lithospheric mantle, leading to stronger lithosphere thinning and, finally, exposure of the lower crust to the hot asthenosphere.
Small-scale convection and processes related to or initiated by it offer new insight and future research possibilities in studies of continental collision magmatism.
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