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The role of ridges in the formation and longevity of flat slabs


Flat-slab subduction occurs when the descending plate becomes horizontal at some depth before resuming its descent into the mantle. It is often proposed as a mechanism for the uplifting of deep crustal rocks (‘thick-skinned’ deformation) far from plate boundaries, and for causing unusual patterns of volcanism, as far back as the Proterozoic eon1. For example, the formation of the expansive Rocky Mountains and the subsequent voluminous volcanism across much of the western USA has been attributed to a broad region of flat-slab subduction beneath North America that occurred during the Laramide orogeny (80–55 million years ago)2. Here we study the largest modern flat slab, located in Peru, to better understand the processes controlling the formation and extent of flat slabs. We present new data that indicate that the subducting Nazca Ridge is necessary for the development and continued support of the horizontal plate at a depth of about 90 kilometres. By combining constraints from Rayleigh wave phase velocities with improved earthquake locations, we find that the flat slab is shallowest along the ridge, while to the northwest of the ridge, the slab is sagging, tearing, and re-initiating normal subduction. On the basis of our observations, we propose a conceptual model for the temporal evolution of the Peruvian flat slab in which the flat slab forms because of the combined effects of trench retreat along the Peruvian plate boundary, suction, and ridge subduction. We find that while the ridge is necessary but not sufficient for the formation of the flat slab, its removal is sufficient for the flat slab to fail. This provides new constraints on our understanding of the processes controlling the beginning and end of the Laramide orogeny and other putative episodes of flat-slab subduction.

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Figure 1: Reference map of the Peruvian flat-slab region, illustrating the subducting Nazca Ridge beneath the advancing South American plate.
Figure 2: Three-dimensional model of the structure of shear-wave velocities between −10° and −18°.
Figure 3: Proposed evolution of the Peruvian flat slab.

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We thank R. Clayton and P. Davies for providing the records from eight PERUSE stations. The PULSE experiment was supported by NSF grants EAR-0944184 (to L.S.W.), EAR-0943991 (to S.L.B.) and EAR-0943962 (to M.D.L.). The CAUGHT project was supported by NSF grants EAR-0908777 (to L.S.W.) and EAR-0907880 (to S.L.B.).

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Authors and Affiliations



S.K.A. generated the tomographic model. L.S.W. developed the model of temporal evolution. A.K. provided earthquake locations. S.K.A, L.S.W. and A.K. developed the ideas and wrote the paper. S.L.B., M.D.L., G.Z., H.T. and C.C. contributed to data collection and paper editing.

Corresponding author

Correspondence to Sanja Knezevic Antonijevic.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Relocated earthquakes used in this study.

Colours show the depth of events (in km) and lines indicate the slab contours in 20-km depth increments. Events below 130 km are shown in black.

Extended Data Figure 2 Events, grid and starting model used for the Rayleigh wave phase velocity inversions.

a, Teleseismic events used in the study. b, Black diamonds represent grid nodes; blue diamonds represent corners used in the two-plane wave methodology; red circles are PULSE stations; orange circles are CAUGHT stations; yellow circles are PERUSE stations; and yellow star is the permanent NNA station. c, Sensitivity kernels for periods used in our study with the one-dimensional starting shear wave velocity model.

Extended Data Figure 3 Lateral and vertical resolution.

a, Resolution for the 40-s and 58-s periods. The resolution matrix diagonal for Rayleigh wave phase velocities is indicated in grey scale. Red triangles are Holocene volcanoes. Black rectangles, circles and stars are stations used here. b, Resolution matrix diagonal values for all one-dimensional shear wave velocity inversions. Colours of the circles indicate average values of the resolution matrix diagonals for each layer. c, The r.m.s. average misfit over all periods at each point after our shear wave inversions. Colours represent the misfit in km s−1. Black rectangles represent stations used in our study.

Extended Data Figure 4 Calculated Rayleigh wave phase velocities for 40-s, 50-s, 58-s, 66-s, 77-s and 91-s periods.

Colours and contours indicate absolute phase velocities. Red triangles represent Holocene volcanoes. Black rectangles, circles and stars represent stations used in our study.

Extended Data Figure 5 Shear wave velocity maps at depths of 70 km, 95 km, 125 km and 165 km.

Colours represent velocity deviations with respect to the reference model (Extended Data Fig. 2c); contours show absolute velocities. Green stars indicate earthquakes within a depth of 20 km relocated using HypoDD24. Red triangles represent Holocene volcanoes. Black rectangles, circles and stars are stations used in our study.

Extended Data Figure 6 Checkerboard tests estimated from resolution matrix for 45 s, 58 s and 77 s.

Colours represent the recovered anomaly. Yellow stars along the northernmost profile indicate locations where we observe re-steepening of the slab, slab tear and the flat-slab remnant; the two stars along the subducting Nazca Ridge refer to locations of our (greater) inboard extent along the Nazca ridge track and a flat slab with shorter extent suggested previously12.

Extended Data Figure 7 Recovery tests for the dipping slab south of the ridge.

a, Map showing the transect and stations locations (rectangles). b, Starting model used in the shear wave velocity inversion; contour lines and colours are absolute shear-wave velocities; black inverted triangles are stations within 35 km of the transect; red triangles are Holocene volcanoes. c, Model calculated using observed data. d, Model based on our interpretations of the observed structures (see also Fig. 2d). e, Recovered model.

Extended Data Figure 8 Recovery tests for the flat-slab segment.

a, Transect. b, Dispersion curve at a location representing the shorter end of flat slab previously suggested12. c, Dispersion curve at a location representing the greater inboard extent of flat slab (proposed here); error bars represent one standard deviation of uncertainty. d, Starting model. e, Model calculated using observed data. f, Model with our (greater) inboard extent of flat slab. g, Model with shorter flat slab as suggested previously12. h, Recovered model from f. i, Recovered model from g.

Extended Data Figure 9 Recovery tests for the area just north of the Nazca Ridge, where we observe a deepening of earthquakes, and the southern end of the slab tear.

a, Reference map. b, Starting model. c, Model calculated using observed data. d, Model based on our interpretations of the torn slab (see Fig. 2d). e, Recovered model.

Extended Data Figure 10 Recovery tests for the northern profile where we observe the slab tear, a re-steepening of the currently subducting slab west of the tear, and the flat slab remnant east of the tear.

a, Transect. b, Dispersion curve at the location representing the re-steepened slab; error bars represent one standard deviation of uncertainty. c, Dispersion curve at location representing the slab tear. d, Dispersion curve at location representing the flat slab remnant. e, Starting model. f, Model calculated using observed data. g, Model with slab tear that we propose in this study. h, Model with continuous flat slab suggested previously suggested in ref. 12 and other studies. i, Recovered model from g. j, Recovered model from h. See Methods for further explanation.

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Antonijevic, S., Wagner, L., Kumar, A. et al. The role of ridges in the formation and longevity of flat slabs. Nature 524, 212–215 (2015).

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