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Two Deep-Mantle Sources for Paleocene Doming and Volcanism in the North Atlantic

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Two Deep-Mantle Sources for Paleocene Doming and Volcanism in the North Atlantic Two deep-mantle sources for Paleocene doming and volcanism in the North Atlantic Petar Glišovic(Петар Глишовић)a,1 and Alessandro M. Forteb aGeotop, Université du Québec à Montréal, Montréal, QC, Canada H3C 3P8; and bDepartment of Geological Sciences, University of Florida, Gainesville, FL 32603 Edited by Barbara A. Romanowicz, University of California, Berkeley, CA, and approved May 20, 2019 (received for review September 19, 2018) The North Atlantic Igneous Province (NAIP) erupted in two major extent of the mantle and thus allows us to obtain explicit links pulses that coincide with the continental breakup and the opening between mantle dynamics and emplacement of the NAIP. To of the North Atlantic Ocean over a period from 62 to 54 Ma. The date, it has been mostly assumed that this mantle dynamic con- unknown mantle structure under the North Atlantic during the nection could be entirely modeled in terms of a mantle plume Paleocene represents a major missing link in deciphering the geo- under the ancestral Iceland hotspot. As we show below, there is an dynamic causes of this event. To address this outstanding challenge, equally important upwelling below the ancestral Azores hotspot we use a back-and-forth iterative method for time-reversed global that is a major contributor to North Atlantic surface topography convection modeling over the Cenozoic Era which incorporates and mantle melting in the Paleocene and Eocene. models of present-day tomography-based mantle heterogeneity. We find that the Paleocene mantle under the North Atlantic is Validating Backward Convection Solutions characterized by two major low-density plumes in the lower mantle: Before embarking on a detailed consideration of the recon- one beneath Greenland and another beneath the Azores. These structed evolution of mantle heterogeneity under the North strong lower-mantle upwellings generate small-scale hot upwellings Atlantic, it is important to verify its reliability and geodynamic and cold downwellings in the upper mantle. The upwellings are consistency. A detailed discussion of this verification is presented dispersed sources of magmatism and topographic uplift that were in SI Appendix and we summarize here the main points. active on the rifted margins of the North Atlantic during the In the first test, when the reconstructed mantle heterogeneity formation of the NAIP. While most studies of the Paleocene evolution at 70 Ma is used as the starting model for a convection simula- EARTH, ATMOSPHERIC, of the North Atlantic have focused on the proto-Icelandic plume, our tion integrated forward to the present day, we obtain strong AND PLANETARY SCIENCES Cenozoic reconstructions reveal the equally important dynamics of a correlations to present-day heterogeneity given by the global hot, buoyant, mantle-wide upwelling below the Azores. tomography models (SI Appendix, Fig. S1). This test also dem- onstrates that mantle evolution is sensitive to the differences in mantle plumes | North Atlantic Igneous Province | time-reversed mantle radial viscosity variations (SI Appendix, Fig. S2A), and the V2 convection viscosity model (12) yields the highest global correlation to the tomography models. he Paleocene separation of Greenland from northwest In the second test, we verify whether the present-day hetero- TEurope and from North America is closely linked to the geneities predicted by the (forwardly integrated) reconstructions at massive outburst of igneous activity (the total crustal volume is 70 Ma yield a fit to a wide range of convection-related datasets that ∼6.6 × 106 km3; ref. 1) that is now deposited in a region that includes Baffin Island, the western and eastern margins of Significance Greenland, the mid-Norwegian margin, the Faeroe Islands, and the British Isles (2, 3). It has been hypothesized that this volca- nism played a role in triggering the Paleocene–Eocene Thermal A vigorous debate has raged over the past decades on the Maximum (4, 5). The emplacement of the North Atlantic Igne- mantle plume origin of the North Atlantic Igneous Province ous Province (NAIP) occurred in two main events (5, 6), phase 1 (NAIP). This debate has persisted for such a long time because magmatism (∼62 to 59 Ma) and phase 2 magmatism (∼56.5 to the 3D structure of the mantle under the North Atlantic during 54 Ma). These phases are most often thought to be linked to the the Cenozoic was, until now, unknown. This paper presents emergence of anomalously hot structure inside the stretched and thermodynamically consistent tomography-based reconstruc- tions of two deeply seated hot mantle upwellings (plumes) rifted lithosphere produced by a plume that is now centered under Greenland and the Azores whose evolution is tracked beneath the present-day position of the Iceland hotspot (2, 3, 6). A from 70 Ma to the present day. We show how these dual up- recent appraisal of NAIP geochronological data suggests that re- wellings gave rise to the small-scale mantle melt sources and gional volcanism progressed without significant pauses and that topographic changes of the NAIP. We believe that this discov- associated magmatic activity continued until about 20 Ma (7). It ery will contribute to resolving the long-standing debate on has been also suggested that the dispersed distribution of NAIP the origin of the NAIP. created in several discrete events requires the presence of either another mantle plume (8), south of the Iceland hotspot (9), or Author contributions: P.G. and A.M.F. designed research; P.G. performed research; P.G. multiple phases of mantle upwelling (6). While the mantle-plume and A.M.F. analyzed data; and P.G. and A.M.F. wrote the paper. hypothesis is considered a viable candidate for explaining the The authors declare no conflict of interest. NAIP petrogenesis (10), there are a number of competing theories This article is a PNAS Direct Submission. and models, including delamination, meteorite impact, small-scale Published under the PNAS license. rift-related convection, and melting of a fertile mantle (a review Data deposition: All codes and data reported in this paper have been deposited in the may be found in ref. 11). Here we apply a recently developed data Geotop, Research Centre on the Dynamics of the Earth System, Université du Québec à assimilation method that yields a quantitative geophysical re- Montréal (https://www.geotop.ca/en/recherche/donnees/geophysique/manteau). construction of the Cenozoic evolution of 3D thermal structure 1To whom correspondence may be addressed. Email: [email protected]. below the North Atlantic employing a tomography-based mantle This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. convection model. This modeling provides a detailed spatiotem- 1073/pnas.1816188116/-/DCSupplemental. poral mapping of 3D thermal anomalies that span the entire depth Published online June 13, 2019. www.pnas.org/cgi/doi/10.1073/pnas.1816188116 PNAS | July 2, 2019 | vol. 116 | no. 27 | 13227–13232 Downloaded by guest on October 1, 2021 constrain the 3D density structure and dynamics of the mantle (SI plumes is resolved in all tomography-based flow predictions tested Appendix,TableS1). This test shows (perhaps not surprisingly) that here and that they remain active today (SI Appendix,Fig.S5). mantle reconstructions based on the joint (seismic–geodynamic) At 55 Ma, strong low-density anomalies (δρ/ρ < −0.2%) and tomography models yield the best overall fits to the present-day high vertical flow rates spanned the upper half of the mantle geodynamic constraints. (depth < 1,445 km) below the Azores hotspot and below a region On the basis of these tests, in the following discussion we will extending from South Greenland (SG) to the Iceland hotspot mainly focus on mantle reconstructions obtained with the V2 (Fig. 1B). The upwelling below the Azores hotspot injects a large viscosity and GyPSuM tomography model (13). The predictions volume of hot material in the upper mantle that is directed to- obtained with the other tomography models are nonetheless im- ward the north and west (SI Appendix, Fig. S4 A and B). This portant and for this reason we included them in cluster analyses of finding suggests the “Azores plume” was a major contributor to predicted vertical flow (SI Appendix) and mantle melting predic- the NAIP, along with the SG upwelling (Fig. 1B). About 10° east – C tions (presented below). Furthermore, the reconstructed mantle from the Iceland hotspot, the west east cross-section (Fig. 1 ) evolution under the North Atlantic obtained with the S40RTS shows an additional low-density anomaly and localized upwelling tomography model (14) is shown in SI Appendix,Fig.S3. in the upper mantle beneath the nascent North Atlantic ridge. This same cross-section shows that the deeply rooted Greenland Reconstruction of Paleocene Mantle Flow Under the North plume under the ancestral Iceland hotspot appears to be located Atlantic at the confluence of two large-scale convection rolls below the Maps of the reconstructed Paleocene mantle flow show an up- North American and Eurasian plates. We note that our re- welling below Greenland characterized by a locus of maximum construction of mantle structure and flow at 55 Ma also reveals a ′′ localized upper-mantle upwelling beneath Ireland and Scotland vertical flow that emerges from the D layer below the northern D portion of Greenland and shifts southward at shallower depths (Fig. 1 ) located in the eastern margin of the NAIP (4). A comparison of the Iceland plume reconstructions obtained before impacting the lithosphere mostly below the Greenland SI Appendix margins (Fig. 1 and SI Appendix,Fig.S4A and B). The north– with the GyPSuM and S40RTS tomography models ( , Fig. S3) shows that even though the latter provides a less strong southaxisofthiselongated“Greenland upwelling” or “plume” is expression in the deep mantle, the Cenozoic evolution of this plume centered under the ancestral Iceland hotspot (Fig.
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