The long-wavelength mantle structure and dynamics and implications for large-scale tectonics and volcanism in the Phanerozoic

• Mantle structure is related to large-scale gravity anomalies, tectonism, and magmatism.
• Present-day degree-2 mantle structure results from the last 100-150 Myrs subduction.
• Pangea assembly and breakup may imply different mantle structure in the Phanerozoic.
• African and Pacific LLSVPs may display significant mobility over time.
• Degree-1 and -2 mantle convection is generated dynamically self-consistently.

The Earth's lower mantle structure, as revealed by seismic tomography studies, is best characterized by two large low seismic velocity provinces (i.e., LLSVP) beneath Africa and Pacific and their surrounding, circum-Pacific seismically fast anomalies. This mantle structure, sometimes called degree-2 structure, has been the most robust feature of all the seismic tomography models for the last 30 years. The dominantly degree-2 mantle structure explains the long-wavelength geoid anomalies including the geoid highs over Africa and Pacific. The LLSVPs are suggested to be the source regions for hotspot volcanism and large igneous province (LIP) events that are compositionally distinct from that for mid-ocean ridge basalts, thus holding the key to understanding mantle geochemistry. The degree-2 structure and LLSVPs have also been used as a reference frame to reconstruct paleogeography and true polar wander (TPW) history of the Earth for the Phanerozoic (i.e., for the last 500 Ma). This paper presents a comprehensive review of studies on the degree-2 mantle structure. While seismic tomography inversion and models are discussed, the main focus of the paper is on the dynamics of long-wavelength mantle convection, plate tectonics and large-scale volcanism. Important topics in the paper include: 1) the long-wavelength seismic structure for the present-day mantle, its possible relation to the long-wavelength geoid anomalies, volcanism and magmatism, and plate motion history, 2) the time evolution of the long-wavelength mantle structure in the Phanerozoic and its relations to Pangea assembly and breakup and history of volcanism/LIP events, and 3) the physics that controls the dynamics of long-wavelength mantle convection. The paper also provides a critical assessment on the validity of the hypothesis of spatially stationary Africa and Pacific LLSVPs since the early Paleozoic and its implications.

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