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Global seismic tomography has greatly progressed in the past decades, with many global Earth models being produced by different research groups. Objective, statistical methods are crucial for the quantitative interpretation of the large amount of information encapsulated by the models and for unbiased model comparisons. Here we propose using a rotated version of principal component analysis (PCA) to compress the information in order to ease the geological interpretation and model comparison. The method generates between 7 and 15 principal components (PCs) for each of the seven tested global tomography models, capturing more than 97 % of the total variance of the model. Each PC consists of a vertical profile, with which a horizontal pattern is associated by projection. The depth profiles and the horizontal patterns enable examining the key characteristics of the main components of the models. Most of the information in the models is associated with a few features: large low-shear-velocity provinces (LLSVPs) in the lowermost mantle, subduction signals and low-velocity anomalies likely associated with mantle plumes in the upper and lower mantle, and ridges and cratons in the uppermost mantle. Importantly, all models highlight several independent components in the lower mantle that make between 36 % and 69 % of the total variance, depending on the model, which suggests that the lower mantle is more complex than traditionally assumed. Overall, we find that varimax PCA is a useful additional tool for the quantitative comparison and interpretation of tomography models.

Global seismic tomography has greatly progressed in the past decades, with many global Earth models being produced by different research groups. Objective, statistical methods are crucial for the quantitative interpretation of the large amount of information encapsulated by the models as well as for unbiased model comparisons. We propose here to use a rotated version of the Principal Component Analysis (PCA) to compress the information, in order to ease the geological interpretation and model comparison. The method generates between 7 to 15 principal components (PC) for each of the seven tested global tomography models, capturing more than 97 % of the total variance of the model. Each PC consists of a vertical profile, to which a horizontal pattern is associated by projection. The depth profiles and the horizontal patterns enable examining the key characteristics of the main components of the models. Most of the information in the models is associated with a few features: Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle, subduction signals and low velocity anomalies likely associated with mantle plumes in the upper and lower mantle, and ridges and cratons in the uppermost mantle. Importantly, all models highlight several independent components in the lower mantle that make between 36 % and 69 % of the total variance, depending on the model, which suggests that the lower mantle is more complex than traditionally assumed. Overall, we find that the varimax PCA is a useful additional tool for the quantitative comparison and interpretation of tomography models.

The goal of this paper is to identify the fate of the continental lithosphere along the Iberia-Eurasia convergent plate boundary marked by the formation of the Pyrenean orogenic belt. The present-day volumes of crust and lithosphere beneath the Pyrenees and the volume of eroded crust redistributed in neighbo ring basins are evaluated based on a synthesis of available geological and geophysical data. The volumes that are expected to have transited across the former plate boundary are modeled taking into account Iberia-Eurasia convergence and making assumptions regarding the initial lithospheric and crustal structure of the Iberia-Eurasia plate boundary at the onset of continental collision (~83 Ma). Despite large uncertainties, the difference between the initial and present-day lithospheric structures suggests that at 83 Ma, either the Iberia-Eurasia plate boundary was marked by a zone of thinned lithosphere (oceanic and/or continental), or the lithosphere having transited across the plate boundary has for the most part been recycled into the mantle. At the crustal-scale, the volume of tectonically accreted crust is estimated by adding the volume of crust currently present in the Pyrenean orogenic belt to the volume of sediments deposited in neighboring basins, and by subtracting the initial volume of crust at the onset of continental collision considering two end-members, namely (i) a continental rift or (ii) a 35 km wide oceanic basin. In both cases, this tectonically accreted crustal volume is not enough to match the calculated volume of crust that has potentially transited across the plate boundary as a consequence of convergence since 83 Ma. As a result, our computation suggests that at least 30% (and as much as 63%) of the continental crust has subducted with the Iberian lithospheric slab and has been recycled into the mantle. In addition, the synthesis of topographic and geophysical (gravity and seismic tomography) reveals a peculiar crustal and lithospheric scale structure for the current day Pyrenees characterized by (i) an elliptical-cone-shape Pyrenean mountain range underlain by an elliptical-cone-shaped crustal root pointing down, and (ii) two tongues of lithospheric mantle in the central part of the belt. These features are interpreted as reflecting redistribution of the lithospheric mantle and of the orogenic crust by ductile flow after subduction and tectonic accretion. We propose that following a period of subduction/collision from 83 to 35 Ma, the decrease in the convergence rate between Iberia-Eurasia favored thermal relaxation of the Iberian slab promoting ductile flow and the development of gravitational instabilities. We suggest that the orogenic root has been dragged down by the dense lithospheric root and that part of it has been recycled into the mantle. In this view, the current-day lithospheric tongues represent the remnants of the lithospheric root after thermal relaxation and recycling by convective removal.