An integrated model for the deep structure of the Chyulu Hills volcanic field, Kenya

Novak, O.; Ritter, Joachim; Altherr, Rainer; Garasic, V.; Volker, F.; Kluge, Christina; Kaspar, T.; Byrne, G.F.; Sobolev, S. V.; Fuchs, Karl

doi:10.1016/s0040-1951(97)00104-2

Cited by 19 publications

(12 citation statements)

References 28 publications

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“…The ratio between P and S wave velocities, V p / V s or γ, has proven a useful discriminator of different possible lower crustal compositions [e.g., Kern , 1993; Christensen , 1996; Musacchio et al , 1997; Novak et al , 1997; Chevrot and van der Hilst , 2000]. Kay et al [1999a] calculated γ for the whole crustal column (γ c , shown in Figure 11) from vertical travel time to Moho ratios determined on wide‐angle reflection P and S wave stacks for lines 1 and 2 discussed in this paper.…”

Section: Other Geophysical Constraintsmentioning

confidence: 99%

Lithospheric structure and composition of the Archean western Superior Province from seismic refraction/wide‐angle reflection and gravity modeling

Musacchio¹

2004

J. Geophys. Res.

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[1] In 1996, LITHOPROBE acquired two $600 km long seismic refraction/wide-angle reflection profiles oriented parallel and orthogonal to the regional geologic strike of the Archean western Superior Province of the Canadian Shield. We present results from ray-based travel time inversion of these data, combined with modeling of the longwavelength gravity field and independent estimates of V p /V s (g), all of which constrain the structure and composition of the lithosphere to depths of 120 km. Large variations in lower crustal V p (6.7-7.5 km s À1 ), lower crustal g (1.72-1.86), upper mantle V p (8.0-8.8 km s À1 ), and Moho depth (32-45 km) are determined. Velocities along orthogonal profiles indicate 8% azimuthal V p anisotropy (fast propagation normal to strike) in part of the lower crust and >6% azimuthal V p anisotropy (fast propagation parallel to strike) in a 15-20 km thick layer (layer H) in the upper mantle which dips northward at $10°from a minimum depth of 48-50 km. Wide-angle reflections identify a deep boundary at $110-120 km depth along both lines. Lower crustal chemical compositions estimated from V p , g, and density are generally intermediate (57-66% SiO 2 ) requiring the presence of 25-60% granitic rocks, which is significantly higher than previous estimates (30% intermediate or felsic granulite) for average Archean crust. The anisotropic lower crustal layer characterized by anomalously high V p , high g lc , and low (À7%) relative density values is interpreted as an amphibolite (basic-intermediate composition) having a subhorizontal lineation to account for 8% azimuthal V p anisotropy. The high V p and intermediate anisotropy of upper mantle layer H requires a harzburgite peridotitic composition with the a axis of olivine aligned E-W. We suggest that both of these layers are relic oceanic lithosphere accreted at the base of the crust during the final stages of lithospheric assembly in the western Superior Province at $2.7 Ga.

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Section: Other Geophysical Constraintsmentioning

confidence: 99%

Lithospheric structure and composition of the Archean western Superior Province from seismic refraction/wide‐angle reflection and gravity modeling

Musacchio¹

2004

J. Geophys. Res.

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“…The Moho was detected at a depth of 44 km, with a thickness of ca. 20 km assigned to the lower crust (Novak et al 1997b;Ritter and Kaspar 1997). The lithosphere-asthenosphere boundary was (2000), the distribution of Cenozoic volcanic and sedimentary rocks in the region is after Williams (1970) and Baker et al (1971) not detected seismically.…”

Section: Geological Settingmentioning

confidence: 99%

“…The Chyulu Hills include several hundred volcanic cones forming a~100 km long chain aligned in a NW-SE direction. The Chyulu Hills have been the target of petrological studies on mantle xenoliths (Henjes-Kunst and Altherr 1992; Altherr et al 2006, submitted) and on alkaline volcanics (Haug and Strecker 1995;Spä th et al 2001) as well as of structural and geophysical studies (Haug and Strecker 1995;Ritter and Kaspar 1997;Novak et al 1997aNovak et al , 1997b.…”

Section: Geological Settingmentioning

confidence: 99%

Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro–troctolitic cumulates subjected to lithospheric foundering

Ulianov

Kalt

Pettke

2006

Contrib Mineral Petrol

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Whole rock major and trace element abundances in aluminous garnet-spinel websterite, sapphirine-bearing Mg-Al granulite and hibonite-bearing Ca-Al granulite xenoliths from the Chyulu Hills volcanic field, Kenya, suggest that the samples represent a metaigneous suite linked by fractionation. The incompatible major element contents increase from the websterites to the Mg-Al granulites and further to the Ca-Al granulites. High bulk rock Mg#s and very low concentrations of most incompatible trace elements indicate that the rocks are cumulates rather than crystallized melts. Elevated Ni abundances, impoverishment in Cr and HFSE and high contents of normative plagioclase and olivine in the granulites indicate that their protoliths were similar to troctolite. The textures and metamorphic reaction paths recorded in the granulites suggest igneous emplacement in the crust and cooling from igneous to ambient crustal temperatures accompanied or followed by compression. For the websterite xenoliths, there is an apparent contradiction between the results of P-T calculations that suggest high P and T of crystallization of early generation pyroxenes and elevated P-T conditions during final equilibration (1.4-2.2 GPa/740-980°C) on the one hand and the positive Eu anomaly that suggests shallow-level plagioclase accumulation on the other hand. This contradiction can be reconciled by a model of compression of a plagioclase-bearing (gabbroic) protolith to mantle depths where it recrystallized to an ultramafic assemblage, which requires foundering of dense lower crustal material into the mantle.

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“…In KRISP 90 three profiles were acquired across and along the rift, targeting the north and central part, an axial line from Lake Turkana in the north, to Lake Magadi in the south (Mechie et al, 1994a;Keller et al, 1994); a cross-rift line from Lake Victoria in the west to Chandlers Falls in the east Masotti et al, 1997); and an eastern flank line, from Chandlers Falls to Lake Turkana (Prodehl et al, 1994b). In KRISP 94 two profiles were acquired in the southern part of the rift, a cross-rift line running from Lake Victoria, crossing the rift at Lake Magadi and terminating north of the Chyulu Hills (Birt, 1996;Birt et al, 1997;, and a profile ran from the eastern flank of the rift near Nairobi, to the Indian Ocean at Mombasa, crossing the Chyulu Hills (Novak et al, 1997a;Novak et al, 1997b). …”

Section: 21mentioning

confidence: 99%

“…The high conductivities have limited the MT penetration depth and mask the structure of the deeper layers. A low-velocity zone beneath the Chyulus was attributed to the presence of 2-5% of partial melt (Novak et al, 1997b) and elevated temperatures (Ritter and Kaspar, 1997) and interpreted as a sign for the active magmatic processes related to the rifting in East Africa; the MT data failed to penetrate these depths. Novak (1997) which is also supported by Macdonald (1994).…”

Section: Chaptermentioning

confidence: 99%

Combined transient electromagnetic and magnetotelluric study of the southern Kenya rift valley

Σακκάς¹

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An integrated model for the deep structure of the Chyulu Hills volcanic field, Kenya

Cited by 19 publications

References 28 publications

Lithospheric structure and composition of the Archean western Superior Province from seismic refraction/wide‐angle reflection and gravity modeling

Lithospheric structure and composition of the Archean western Superior Province from seismic refraction/wide‐angle reflection and gravity modeling

Aluminous websterite and granulite xenoliths from the Chyulu Hills volcanic field, Kenya: gabbro–troctolitic cumulates subjected to lithospheric foundering

Combined transient electromagnetic and magnetotelluric study of the southern Kenya rift valley

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