Understanding seismic heterogeneities in the lower mantle beneath the Americas from seismic tomography and plate tectonic history

Ren, Yong; Stutzmann, É.; Hilst, Robert D. van der; Besse, J.

doi:10.1029/2005jb004154

Cited by 92 publications

(105 citation statements)

References 51 publications

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“…" . Farallon plate predicted at 2500 km 6 . Three study regions (ÔWÕ, ÔSÕ and ÔEÕ) are indicated by circled areas.…”

Section: Methods Summarymentioning

confidence: 99%

“…Corresponding features beneath downwellings are found in convection models of the lower mantleÑinclined deformation dipping towards the downwelling centre 29 . Regions E and S are either side of the apparent centre of the downwelling Farallon slab 6,30 (Figs. 2, 3) which strikes roughly northwest-southeast, hence we postulate northeast-southwest slip directions on inclined shear planes with an opposite sense of dip (i.e., dipping southwest for region E, northeast for region S).…”

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confidence: 99%

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Deformation of the lowermost mantle from seismic anisotropy

Nowacki

Wookey

Kendall

2010

Nature

103

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isotropy, VTI). With this assumption, SH leads SV here, corresponding to "'=±90 " in our notation (Fig. 1c). A further limitation is using only one azimuth of rays in D!: this cannot distinguish VTI from the case of an arbitrarily tilted axis of rotational symmetry in which wave speed does not vary (tilted transverse isotropy, TTI) when the axis dips towards the receivers or stations. An improvement on this situation can be made by utilising crossing ray paths in D! 10 , but this relies on having the correct source-receiver geometry, which is not possible beneath North America using only deep earthquakes.We address this issue beneath the Caribbean by incorporating measurements from shallow earthquakes in our dataset, and thus reduce the symmetry of the anisotropy which must be assumed.We measure anisotropy in D! using differential splitting in S and ScS phases using an approach described by refs. 10,11 . Both phases travel through the same region of the upper mantle (UM), but only ScS samples D! (Fig. 1a). As the majority of the lower mantle (LM) is relatively isotropic 12 , by removing the splitting introduced in the UM we can measure that which occurs only in D! (see Supplementary Information).Earthquakes in South and Central America, Hawaii, the East Pacific Rise (EPR) and the Mid-Atlantic Ridge (MAR), detected at North American stations, provide a dense coverage of crossing rays which traverse D! beneath southern North America and the Caribbean (Fig. 1b). Three distinct regions are covered (Fig. 2), each sampled along two distinct azimuths. The Caribbean (region ÔSÕ) has been previously well studied 1,4,8 , but the northeast (ÔEÕ) and southwest (ÔWÕ) United States have not. Hence nowhere are our measurements compatible with VTI, because we do not find "«=±90 " within error in both directions for any region.A likely mechanism for the production of anisotropy in D! is the lattice-preferred orientation (LPO) of anisotropic mineral phases present above the CMB such as (Mg,Fe)O, and MgSiO 3 -perovskite (pv) and -postperovskite (ppv). These may give rise to styles of anisotropy more complicated than TTI with lower symmetries, which are compatible with our two-azimuth measurements. We investigate the possibility of LPO in ppv leading to the observed anisotropy rather than other phases because of its likely Our results can differentiate between these candidate mechanisms if we assume that most of the measured anisotropy in D! is a result of deformation-induced LPO in ppv, and we have an accurate estimate of the mantle flow where we measure anisotropy.At present, such models of mantle deformation are in their infancy, but we can nonetheless make inferences from broad-scale trends in subduction and global V S models. We calculate the orientations of the shear planes and slip directions which are compatible with our measurements for the three slip systems in ppv. Aggregate elastic (001) system. These planes and directions are plotted in Fig. 3. We also produce the shear planes predicted for cases of pv and MgO ( Suppl...

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“…" . Farallon plate predicted at 2500 km 6 . Three study regions (ÔWÕ, ÔSÕ and ÔEÕ) are indicated by circled areas.…”

Section: Methods Summarymentioning

confidence: 99%

mentioning

confidence: 99%

Deformation of the lowermost mantle from seismic anisotropy

Nowacki

Wookey

Kendall

2010

Nature

103

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“…Along other profiles below the Americas, significant thickening is observed in the transition-zone slab below Peru, where it penetrates the lower mantle down to only ~1500 km depth, while slab flattening appears to occur in the transition zone below Cascadia, Mexico and southern South America (Sigloch et al, 2008;Simmons et al, 2012;Fukao and Obayashi, 2013). Slab fragments that lie in the transition zone below the United States are generally associated with previous (Laramide) subduction (Van der Lee and Nolet, 1997;Sigloch, 2011), and their connection to the prominent lower-mantle anomaly below eastern North America is debated (Bunge and Grand, 2000;Schmid et al, 2002;Ren et al, 2007;Sigloch and Mihalynuk, 2013).…”

Section: Introductionmentioning

confidence: 99%

Subduction-transition zone interaction: A review

et al. 2017

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Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACTAs subducting plates reach the base of the upper mantle, some appear to flatten and stagnate, while others seemingly go through unimpeded. This variable resistance to slab sinking has been proposed to affect long-term thermal and chemical mantle circulation. A review of observational constraints and dynamic models highlights that neither the increase in viscosity between upper and lower mantle (likely by a factor 20-50) nor the coincident endothermic phase transition in the main mantle silicates (with a likely Clapeyron slope of -1 to -2 MPa/K) suffice to stagnate slabs. However, together the two provide enough resistance to temporarily stagnate subducting plates, if they subduct accompanied by significant trench retreat. Older, stronger plates are more capable of inducing trench retreat, explaining why backarc spreading and flat slabs tend to be associated with old-plate subduction. Slab viscosities that are ~2 orders of magnitude higher than background mantle (effective yield stresses of 100-300 MPa) lead to similar styles of deformation as those revealed by seismic tomography and slab earthquakes. None of the current transition-zone slabs seem to have stagnated there more than 60 m.y. Since modeled slab destabilization takes more than 100 m.y., lower-mantle entry is apparently usually triggered (e.g., by changes in plate buoyancy). Many of the complex morphologies of lower-mantle slabs can be the result of sinking and subsequent deformation of originally stagnated slabs, which can retain flat morphologies in the top of the lower mantle, fold as they sink deeper, and eventually form bulky shapes in the deep mantle.

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“…Extensive high-resolution studies of the deepest mantle in this area have been conducted over the years (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27). The data are S waveforms from earthquakes in South America recorded in North America stations, providing a dense sampling of a narrow corridor of the lowermost mantle beneath the Caribbean and the Cocos Plate.…”

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confidence: 99%

“…The region was found to have complex structures with a S velocity discontinuity, broad fast anomalies, anisotropy, and a possible ultra-low velocity zone at the base of the mantle. Detailed studies of P-wave structure of this region have been limited (18,19,27). Here we show rapid variation of P-wave velocity in the lowermost mantle from a broad fast anomaly underneath the Caribbean and much of the Cocos Plate to a broad slow anomaly to the southwest.…”

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confidence: 99%

Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

Sun

Song

Zheng

et al. 2007

Proc. Natl. Acad. Sci. U.S.A.

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Compressional waves that sample the lowermost mantle west of Central America show a rapid change in travel times of up to 4 s over a sampling distance of 300 km and a change in waveforms. The differential travel times of the PKP waves (which traverse Earth's core) correlate remarkably well with predictions for S-wave tomography. Our modeling suggests a sharp transition in the lowermost mantle from a broad slow region to a broad fast region with a narrow zone of slowest anomaly next to the boundary beneath the Cocos Plate and the Caribbean Plate. The structure may be the result of ponding of ancient subducted Farallon slabs situated near the edge of a thermal and chemical upwelling.core-mantle boundary ͉ slab

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Understanding seismic heterogeneities in the lower mantle beneath the Americas from seismic tomography and plate tectonic history

Cited by 92 publications

References 51 publications

Deformation of the lowermost mantle from seismic anisotropy

Deformation of the lowermost mantle from seismic anisotropy

Subduction-transition zone interaction: A review

Evidence for a chemical-thermal structure at base of mantle from sharp lateral P-wave variations beneath Central America

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