Mantle transition zone variations beneath the Ethiopian Rift and Afar: Chemical heterogeneity within a hot mantle?

Cornwell, David G.; Hetényi, György; Blanchard, T. D.

doi:10.1029/2011gl047575

Cited by 29 publications

(39 citation statements)

References 35 publications

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“…distances than we observe and are correlated with expected temperature fluctuations. Hetényi et al (2009) andCornwell et al (2011) detected variations in the depth of the '660' at comparable length scales to those observed here using receiver functions.…”

Section: Possible Causes Of Apparent Topography On the '660'mentioning

confidence: 55%

Reconciling PP and P′P′ precursor observations of a complex 660 km seismic discontinuity

Day

Deuss

2013

Geophysical Journal International

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S U M M A R YHigh frequency precursors to P P almost invariably observe a narrow 660 km discontinuity, whereas PP precursor studies at long periods struggle to detect a reflection from the '660' despite its apparent sharpness to P P . To investigate these contradictory observations we compare PP and P P precursors in the same region. Using short period P P precursors we observe a sharp 660 km discontinuity, which appears to vary in depth substantially. The apparent topography on the '660' is too large to originate solely from thermal variations, regardless of its cause, therefore indicating chemical variations at the base of the mantle transition zone. Long period P P precursors show no '660' as they are sensitive to a larger area and thus average out the apparent topography, in agreement with long period PP precursors. Instead, we see some evidence in both long period data types for a reflection from 720 km depth, which is likely to correspond to a phase change in the garnet system. Key words: Mantle processes; Composition of the mantle; Body waves . I N T RO D U C T I O NThe 660 km seismic discontinuity (the '660') at the base of the mantle transition zone (MTZ) marks the boundary between the upper and lower mantle. The seismic signature of the '660' is attributed to the post-spinel phase transition in olivine, in which ringwoodite transforms into perovskite and periclase (Ita & Stixrude 1992). The nature of the 660 km seismic discontinuity governs the behaviour of the mantle as a whole, with its properties as a chemical and mechanical boundary controlling mantle convection and the fate of subducting slabs (Schubert et al. 2001).Observations of the 660 km seismic discontinuity in precursors to body waves have painted a complex and sometimes even contradictory picture. Despite the common observations of P 660P at short periods (Benz & Vidale 1993;LeStunff et al. 1995), longer period P660P signals are rarely seen (Estabrook & Kind 1996;Shearer & Flanagan 1999). Studies that do record a P660P signal detect an intermittent and complicated discontinuity (Deuss et al. Here, we explore the complex and potentially contradictory observations of the '660' in compressional body waves by directly comparing P P and long period PP precursor observations in the same region under South East Africa. We investigate the topography of the 660 km discontinuity and the frequency dependence of the seismic signals to reconcile the PP and P P observations. DATA A N D M E T H O D SSurface reflected body waves such as PP and P P are preceded by smaller amplitude arrivals, known as precursors. These arrivals have been reflected at discontinuities below the Earth's surface and are therefore sensitive to the impedance change at the discontinuity. P P df (also known as PKIKPPKIKP and hereafter as P P ) is a seismic phase which traverses the entire Earth before reflecting off the Earth's surface, as illustrated in Fig. 1. P P precursors are referred to as P dP , where 'd' denotes the depth of the interface at which they have been reflected. ...

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Section: Possible Causes Of Apparent Topography On the '660'mentioning

confidence: 55%

Reconciling PP and P′P′ precursor observations of a complex 660 km seismic discontinuity

Day

Deuss

2013

Geophysical Journal International

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“…Wright et al (2004) identified the 410 and 660 beneath the Kaapvaal craton slightly deeper than normal. Cornwell et al (2011) found in the Ethiopian Rift and Afar Region local thickness variations of the transition zone between 230 and 260 km. Cornwell et al (2011) found in the Ethiopian Rift and Afar Region local thickness variations of the transition zone between 230 and 260 km.…”

Section: Africamentioning

confidence: 88%

Deep Earth Structure - Transition Zone and Mantle Discontinuities

Kind

2015

Treatise on Geophysics

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“…In the case of Ethiopia, transition zone thickness, determined by receiver function analysis, shows evidence for thinning compared to the global mean; Cornwell et al (2011) cited this as evidence for high temperatures at transition zone depths, and thus connectivity between shallow low velocities imaged tomographically in Ethiopia (e.g., Bastow et al, 2008;Benoit et al, 2006;Debayle et al, 2001;Hansen et al, 2012;Pasyanos and Nyblade, 2007) and the superplume in the lower mantle beneath.…”

Section: Evidence From Broadband Seismologymentioning

confidence: 99%

“…The consensus emerging from recent studies in Ethiopia is that it is underlain by a broad low velocity anomaly, not a traditional narrow mantle plume (e.g., Bastow et al, 2008;Benoit et al, 2006;Cornwell et al, 2011;Hansen et al, 2012;Ritsema et al, 2010) (Fig. 5).…”

Section: Implications For the Thermochemical State Of The Ethiopian Mmentioning

confidence: 99%

The development of extension and magmatism in the Red Sea rift of Afar

et al. 2013

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Despite the importance of continental breakup in plate tectonics, precisely how extensional processes such as brittle faulting, ductile plate stretching, and magma intrusion evolve in space and time during the development of new ocean basins remains poorly understood. The rifting of Arabia from Africa in the Afar depression is an ideal natural laboratory to address this problem since the region exposes subaerially the tectonically active transition from continental rifting to incipient seafloor spreading. We review recent constraints on along-axis variations in rift morphology, crustal and mantle structure, the distribution and style of ongoing faulting, subsurface magmatism and surface volcanism in the Red Sea rift of Afar to understand processes ultimately responsible for the formation of magmatic rifted continental margins. Our synthesis shows that there is a fundamental change in rift morphology from central Afar northward into the Danakil depression, spatially coincident with marked thinning of the crust, an increase in the volume of young basalt flows, and subsidence of the land towards and below sea-level. The variations can be attributed to a northward increase in proportion of extension by ductile plate stretching at the expense of magma intrusion. This is likely in response to a longer history of localised heating and weakening in a narrower rift. Thus, although magma intrusion accommodates strain for a protracted period during rift development, the final stages of breakup are dominated by a phase of plate stretching with a shift from intrusive to extrusive magmatism. This late-stage pulse of decompression melting due to plate thinning may be responsible for the formation of seaward dipping reflector sequences of basalts and sediments, which are ubiquitous at magmatic rifted margins worldwide.

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Mantle transition zone variations beneath the Ethiopian Rift and Afar: Chemical heterogeneity within a hot mantle?

Cited by 29 publications

References 35 publications

Reconciling PP and P′P′ precursor observations of a complex 660 km seismic discontinuity

Reconciling PP and P′P′ precursor observations of a complex 660 km seismic discontinuity

Deep Earth Structure - Transition Zone and Mantle Discontinuities

The development of extension and magmatism in the Red Sea rift of Afar

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