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Basal continental mantle lithosphere displaced by flat-slab subduction

Abstract

During the subhorizontal (flat) subduction of an ocean–lithosphere plate, the overlying continental plate is deformed far inland of the plate boundary. In addition, arc magmatism, which is caused by melting in the asthenospheric wedge during steep subduction, wanes or ends when the subduction is subhorizontal. The observed upper-plate deformation patterns have been explained by an end load exerted at the plate boundary or by stress transmitted into the overlying plate along the top of the flat slab. Here we present numerical thermal–mechanical models of flat-slab subduction that show the flattening of the slab results in a compression of the continental plate through end loading. The advancing flat slab scrapes off the lowermost 20–50 km of continental mantle lithosphere. The displaced continental mantle lithosphere fills the asthenospheric wedge, ending arc-type melting. If the displaced material is buoyant, it accumulates in a growing keel that migrates ahead of the slab; if it is dense, the displaced material sinks with the slab. Flat-slab removal renews the asthenospheric wedge and arc magmatism, and leaves a step in the lithosphere–asthenosphere boundary and/or a keel that consists of displaced continental mantle lithosphere. A fossil keel and a fossil step, formed during Laramide flat subduction, are preserved below the western United States.

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Fig. 1: Laramide tectonic setting of western North America and tectonic features of flat-slab subduction.
Fig. 2: Evolutionary steps during flat-slab subduction.
Fig. 3: The perspective block diagram (view to northwest) shows the inferred Laramide flat-slab geometry at the maximum extent (Fig. 1).

Data availability

The authors declare that the data supporting the findings of this study are available within the article, its Supplementary Information and citations therein.

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Acknowledgements

Numerical models were run using computational resources from Compute Canada (WestGrid). This work was supported by NSF awards EAR-1348076 (G.J.A. and J.v.W.) and EAR-1015250 (J.v.W.), and NSERC Discovery Grant funding (C.A.C.).

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G.J.A. and J.v.W. developed the bulldozed CML wedge concept. C.A.C. modified the numerical code and ran numerical experiments. All the authors contributed to the text and figures.

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Correspondence to Gary J. Axen.

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Supplementary information

Supplementary Information

Supplementary Discussion and Figures 1–3.

Supplementary Video

Video of the flat slab development and advance.

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Axen, G.J., van Wijk, J.W. & Currie, C.A. Basal continental mantle lithosphere displaced by flat-slab subduction. Nature Geosci 11, 961–964 (2018). https://doi.org/10.1038/s41561-018-0263-9

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