We report new
- NSF-PAR ID:
- 10196405
- Date Published:
- Journal Name:
- Seismological Research Letters
- ISSN:
- 0895-0695
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract P andS wave velocity models of the upper mantle beneath southern Africa using data recorded on seismic stations spanning the entire subcontinent. Beneath most of the Damara Belt, including the Okavango Rift, our models show lower than average velocities (−0.8% Vp; −1.2% Vs) with an abrupt increase in velocities along the terrane's southern margin. We attribute the lower than average velocities to thinner lithosphere (~130 km thick) compared to thicker lithosphere (~200 km thick) immediately to the south under the Kalahari Craton. Beneath the Etendeka Flood Basalt Province, higher than average velocities (0.25% Vp; 0.75% Vs) indicate thicker and/or compositionally distinct lithosphere compared to other parts of the Damara Belt. In the Rehoboth Province, higher than average velocities (0.3% Vp; 0.5% Vs) suggest the presence of a microcraton, as do higher than average velocities (1.0% Vp; 1.5% Vs) under the Southern Irumide Belt. Lower than average velocities (−0.4% Vp; −0.7% Vs) beneath the Bushveld Complex and parts of the Mgondi and Okwa terranes are consistent with previous studies, which attributed them to compositionally modified lithosphere resulting from Precambrian magmatic events. There is little evidence for thermally modified upper mantle beneath any of these terranes which could provide a source of uplift for the Southern African Plateau. In contrast, beneath parts of the Irumide Belt in southern and central Zambia and the Mozambique Belt in central Mozambique, deep‐seated low velocity anomalies (−0.7% Vp; −0.8% Vs) can be attributed to upper mantle extensions of the African superplume structure. -
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Abstract The along‐strike variations of the velocity, thickness, and dip of subducting slabs and the volcano distribution have been observed globally. It is, however, unclear what controls the distribution of volcanoes and the associated magma generation. With the presence of nonuniform volcanism, the Aleutian‐Alaska subduction zone (AASZ) is an ideal place to investigate subduction segmentation and its relationship with volcanism. Using full‐wave ambient noise tomography, we present a high‐resolution 3‐D shear wave velocity model of the AASZ for the depths of 15–110 km. The velocity model reveals the distinct high‐velocity Pacific slab, the thicker, flatter, and more heterogeneous Yakutat slab, and the northeasterly dipping Wrangell slab. We observe low velocities within the uppermost mantle (at depth <60 km) below the Aleutian arc volcanoes, representing partial melt accumulation. The large crustal low‐velocity anomaly beneath the Wrangell volcanic field suggests a large magma reservoir, likely responsible for the clustering of volcanoes. The Denali volcanic gap is above an average‐velocity crust but an extremely fast mantle wedge, suggesting the lack of subsurface melt. This is in contrast with the lower‐velocity back‐arc mantle beneath the adjacent Buzzard Creek‐Jumbo Dome volcanoes to the east. The back‐arc low velocities associated with the Pacific, the eastern Yakutat, and the Wrangell slabs may reflect subduction‐driven mantle upwelling. The structural variation of the downgoing slabs and the overriding plate explains the change of volcanic activity along the AASZ. Our findings demonstrate the combined role of the subducting slab and the overriding plate in controlling the characteristics of arc magmatism.
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Abstract Shear wave splitting is often assumed to be caused by mantle flow or preexisting lithospheric fabrics. We present 2,389 new SKS shear wave splitting observations from 384 broadband stations deployed in Alaska from January 2010 to August 2017. In Alaska, splitting appears to be controlled by the absolute plate motion (APM) of the North American and Pacific plates, the interaction between the two plates, and the geometry of the subducting Pacific‐Yakutat plate. Outside of the subduction zone's influence, the fast directions in northern Alaska parallel the North American APM direction. Fast directions near the Queen Charlotte‐Fairweather transform margin are parallel to the faults and are likely caused by the strike‐slip deformation extending throughout the lithosphere. In the mantle wedge, fast directions are oriented along the strike of the slab with large splitting times and are caused by along‐strike flow in the mantle wedge as the slab provides a barrier to flow. South of the Alaska Peninsula, the fast directions are parallel to the trench regardless of sea floor fabric, indicating along strike flow under the Pacific plate. Under the Kenai Peninsula, the complex flat slab geometry may cause subslab flow to be parallel to Pacific APM direction or to the North America‐Pacific relative motion.
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Abstract Shear‐wave splitting observations can provide insight into mantle flow, due to the link between the deformation of mantle rocks and their direction‐dependent seismic wave velocities. We identify anisotropy in the Cook Inlet segment of the Alaska subduction zone by analyzing splitting parameters of S waves from local intraslab earthquakes between 50 and 200 km depths, recorded from 2015–2017 and emphasizing stations from the Southern Alaska Lithosphere and Mantle Observation Network experiment. We classify 678 high‐quality local shear‐wave splitting observations into four regions, from northwest to southeast: (L1b) splitting measurements parallel to Pacific plate motion, (L1a) arc‐perpendicular splitting pattern, (L2) sharp transition to arc‐parallel splitting, and (L3) splitting parallel to Pacific plate motion. Forward modeling of splitting from various mantle fabrics shows that no one simple model fully explains the observed splitting patterns. An A‐type olivine fabric with fast direction dipping 45° to the northwest (300°)—aligned with the dipping slab—predicts fast directions that fit L1a observations well, but not L2. The inability of the forward model fabrics to fit all the observed splitting patterns suggests that the anisotropy variations are not due to variable ray angles, but require distinct differences in the anisotropy regime below the arc, forearc, and subducting plate.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1785/0220200214
