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Deep Mantle Structure As a Reference Frame for Movements in and on the Earth Deep mantle structure as a reference frame for movements in and on the Earth Trond H. Torsvika,b,c,d,1, Rob van der Voob,e, Pavel V. Doubrovinea,b, Kevin Burkeb,d,f, Bernhard Steinbergera,b,g, Lewis D. Ashwald, Reidar G. Trønnesa,b,h, Susan J. Webbd, and Abigail L. Bulla aCentre for Earth Evolution and Dynamics, University of Oslo, 0316 Oslo, Norway; bCentre for Advanced Study, 0271 Oslo, Norway; cGeodynamics, Geological Survey of Norway, 7491 Trondheim, Norway; dSchool of Geosciences, University of the Witwatersrand, WITS 2050 Johannesburg WITS 2050, South Africa; eDepartment of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109; fDepartment of Geosciences, University of Houston, Houston, Texas 77204; gHelmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany; and hNatural History Museum, University of Oslo, 0318 Oslo, Norway Edited by John Suppe, National Taiwan University, Taipei, Taiwan, and approved May 8, 2014 (received for review September 27, 2013) Earth’s residual geoid is dominated by a degree-2 mode, with el- remain nearly stationary from the early Cambrian (540 Ma) in evated regions above large low shear-wave velocity provinces on the large-scale convection within the Earth’s mantle. the core–mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to Plume Generation Zones the Earth’s surface, correlate with the reconstructed positions Previous work (1, 2, 5, 10) and numerical models (9, 11) suggest of large igneous provinces and kimberlites since Pangea formed that the most likely candidates for the plume generation zones about 320 million years ago. Using this surface-to-core–mantle in the lower mantle are those areas that correspond to the largest boundary correlation to locate continents in longitude and a novel lateral gradients of the shear-wave velocity directly above the iterative approach for defining a paleomagnetic reference frame core–mantle boundary. Although the distribution of plume corrected for true polar wander, we have developed a model for generation zones depends on the particulars of the seismic to- absolute plate motion back to earliest Paleozoic time (540 Ma). For mography model used to define them, the differences between the Paleozoic, we have identified six phases of slow, oscillatory alternative definitions are typically small (5). Torsvik and col- true polar wander during which the Earth’s axis of minimum mo- leagues (1) used the 1% slow-velocity contour in the lowermost ment of inertia was similar to that of Mesozoic times. The rates of EARTH, ATMOSPHERIC, layer of the mean Shear-wave tomographic model SMEAN (6) AND PLANETARY SCIENCES Paleozoic true polar wander (<1°/My) are compatible with those in to define the plume generation zones. This contour corresponds the Mesozoic, but absolute plate velocities are, on average, twice to the steepest lateral gradients of shear-wave velocity, and 80% as high. Our reconstructions generate geologically plausible sce- of reconstructed large igneous provinces of the past 300 My plot narios, with large igneous provinces and kimberlites sourced from within 10° of it (Fig. 1B). the margins of the large low shear-wave velocity provinces, as in A perhaps more robust definition of the plume generation Mesozoic and Cenozoic times. This absolute kinematic model sug- zones can be deduced from the recently published cluster anal- ’ gests that a degree-2 convection mode within the Earth s mantle ysis of five global shear-wave tomography models (7). In this may have operated throughout the entire Phanerozoic. work, a “voting” map was produced that described whether a geographical location was above a seismically slower-than- plate reconstructions | thermochemical piles average velocity region in the mantle below 1,000 km depth. The voting map (Fig. 1B) shows how many of the five tomographic wo equatorial, antipodal, large low shear-wave velocity prov- models agree on the classification of the data point. Within Tinces (Fig. 1) in the lowermost mantle (1) beneath Africa contour 5, all five tomographic models show slower-than-average (termed Tuzo) (2) and the Pacific Ocean (Jason) are prominent in all shear-wave tomographic models (3–7) and have been ar- Significance gued to be related to a dominant degree-2 pattern of mantle convection that has been stable for long times (3). Most recon- Since the Pangea supercontinent formed about 320 million structed large igneous provinces and kimberlites over the past years ago, plumes that sourced large igneous provinces and 300 My have erupted directly above the margins of Tuzo and kimberlites have been derived from the edges of two stable Jason, which we term the plume generation zones (1, 2, 5). This thermochemical reservoirs at the core–mantle boundary. We remarkable correlation suggests that the two deep mantle struc- test whether it is possible to maintain this remarkable surface- tures have been stable for at least 300 My. Stability before Pangea to-deep Earth correlation before Pangea through the de- (before 320 Ma) is difficult to test with plate reconstructions be- velopment of a new plate reconstruction method and find that cause the paleogeography, the longitudinal positions of continents, our reconstructions for the past 540 million years comply with and the estimates of true polar wander are uncertain (8). It known geological and tectonic constraints (opening and clo- is similarly challenging to reproduce such long-term stability in nu- sure of oceans, mountain building, and more). These results merical models (9). However, if the correlation between the erup- have important implications for Earth history, including the tion sites of large igneous provinces, kimberlites, and the plume style of mantle convection in the deep past and the long-term generation zones observed for the past 300 Ma has been main- stability of mantle reservoirs. tained over the entire Phanerozoic (0–540 Ma), it can provide a crucial constraint for defining the longitudinal positions of con- Author contributions: T.H.T., R.v.d.V., and P.V.D. designed research; T.H.T. and R.v.d.V. – performed research; T.H.T., P.V.D., and B.S. contributed new reagents/analytic tools; tinental blocks during Paleozoic time (250 540 Ma). T.H.T., P.V.D., K.B., B.S., L.D.A., R.G.T., S.J.W., and A.L.B. analyzed data; and T.H.T., R.v.d.V., Here we show that a geologically reasonable kinematic model P.V.D., K.B., B.S., L.D.A., R.G.T., S.J.W., and A.L.B. wrote the paper. that reconstructs continents in longitude in such a way that large The authors declare no conflict of interest. igneous provinces and kimberlites are positioned above the This article is a PNAS Direct Submission. A plume generation zones at the times of their formation (Fig. 2 1To whom correspondence should be addressed. E-mail: [email protected]. SI Appendix and , Fig. S2) can be successfully defined for the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. entire Phanerozoic. This model requires that Tuzo and Jason 1073/pnas.1318135111/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1318135111 PNAS Early Edition | 1of6 Downloaded by guest on September 29, 2021 Sumatra Paleozoic Plate Model A W Indian S Upper mantle & ridge transition zone The Early Paleozoic (8) was dominated by the great continent of New Guinea Gondwana. Other continents included Laurentia and Baltica (Fig. 660 km Sinking 2A), which fused together with the Avalonia microcontinent to mantle form Laurussia, the second largest Paleozoic continent, after the pPv PGZ ∼ Plume generation closure of the Iapetus Ocean in the Silurian ( 430 My). By the Africa zones (PGZ) late Carboniferous (∼320 My), Gondwana and Laurussia had Outer, liquid core amalgamated, forming the supercontinent of Pangea. Relative fits P within Gondwana, Laurussia, and later, Pangea are reasonably V P geoid S A L C well known; the sources of these reconstructions have been L geoid I F I A Tuzo Inner, solid C Jason documented in a recent review by Torsvik and colleagues (8). C I core L R L In contrast, absolute Paleozoic reconstructions have remained F S A V P uncertain because longitudes of continental blocks cannot be ULVZ derived from paleomagnetic data (although latitudes and azi- Mid-Atlantic ridge PGZ Ultra-low velocity muthal orientations can). Our plate model is mainly based on zones (ULVZ) apparent polar wander paths for Gondwana, Siberia, Laurentia/ pPv PGZ Baltica (Laurussia after 430 My), and their later combinations Sinking into Pangea (8). Of these paths, the Gondwana path during the mantle Mid-Paleozoic is probably the most controversial. Euler poles were calculated from the apparent polar wander paths, and con- S-America tinents were reconstructed in latitude and azimuthal orientation. East-Pacific rise Paleogeographic reconstructions relate the past configurations B Siberian Traps of continents to the Earth’s spin axis (8). However, correlating Columbia River Basalt Perm the reconstructed positions of large igneous provinces and kim- berlites to the plume generation zones requires reconstructions relative to the Earth’s mantle. The two reference frames (pa- leogeographic, which we also refer to as “paleomagnetic,” and the mantle frames) generally differ because over time, the solid Jason Tuzo PGZ Jason Earth (mantle and crust) can slowly rotate with respect to the spin axis, driven by the redistribution of density heterogeneities within the solid Earth, resulting in changes of the planetary moment of inertia. This process is known as true polar wander (13). The estimates for Cenozoic and Mesozoic times (8, 14–16) suggest that the direction of true polar wander is largely con- 5 3 1 0 Cluster analysis (1000-2800 km) 1% slow contour SMEAN (2800 km) trolled by the mass of the two antipodal large low shear-wave velocity provinces associated with persistent degree-2 residual Fig.
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