%0 Journal Article %1 prodehl_etal:1997a %A Prodehl, C. %A Ritter, J. R. R. %A Mechie, J. %A Keller, G. R. %A Khan, M. A. %A Jacob, B. %A Fuchs, K. %A Nyambok, I. O. %A Obel, J. D. %A Riaroh, D. %D 1997 %J Tectonophysics %K geophysics seismics %N 1-4 %P 121--147 %R 10.1016/S0040-1951(97)00098-X %T The KRISP 94 lithospheric investigation of southern Kenya --- the experiments and their main results %U http://dx.doi.org/10.1016/S0040-1951(97)00098-X %V 278 %X Following two previous experiments in 1985 (KRISP 85) and 1989-1990 (KRISP 90), a series of geophysical experiments was undertaken in 1993-1995 (KRISP 94) to study the lithospheric structure of the southern Kenya rift down to depths of greater than 100 km, with special emphasis on the Chyulu Hills, a complex of volcanic centres on the eastern flank of the rift. KRISP 94 involved a teleseismic tomography experiment of the Chyulu Hills area in July and August 1993, a seismic refraction-wide-angle reflection survey across southern Kenya from Lake Victoria to the Indian Ocean in February 1994, seismicity studies of southern Kenya from 1993 to 1995, a special seismicity study of the Lake Magadi area in February 1994, a gravity study along the seismic-refraction lines before and after the seismic-refraction study, and a magnetotelluric study of southern Kenya in February 1995. Major scientific goals of the project were to reveal the detailed crustal and upper-mantle structure under the southern Kenya rift and its flanks for several 100 km to the west and to the east and their evolution, to study the relationship between deep crustal and uppermost mantle structure, to learn more on the development of sedimentary basins and volcanic features on the flanks and its relation to the Kenya rift, to obtain information on the temperature conditions underneath the rift and its flanks, to perform a particular integrated and calibrative study of seismological and petrological data in the Chyulu Hills, and to understand the processes which are producing extension, uplift, and extensive magmatism. This report is an introduction to a series of subsequent papers. It focuses on the technical description of the main seismic surveys of the KRISP 94 effort and summarizes the key results. During the teleseismic survey an array of 31 seismographs was deployed to record teleseismic, regional and local events for a period of about 3 months from June to August 1993. The elliptical array covered an area about 150 km (N-S) x 100 km (E-W) and spanned the central portion of the Chyulu Hills and its surroundings, with an average station spacing of 10-30 km. The seismic refraction-wide-angle reflection survey was carried out in a 2-week period in February 1994. It consisted of two profiles: one extending from Lake Victoria across the western flank and the southernmost Kenya rift at Lake Magadi, the other extending from Athi River near Nairobi across the eastern flank of the rift, traversing the Chyulu Hills and terminating at the Indian Ocean near Mombasa. A total of 204 mobile seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to 730 km. Key results are as follows: (1) The crust reaches a maximum thickness of up to 44 km under the Chyulu Hills. (2) Only a minor upwarping of the crust-mantle boundary is seen under the rift proper in the Lake Magadi area. (3) To the west the crust shallows to about 34 km near Lake Victoria, in contrast to the thickening of the crust further north from the central part of the rift near Lake Baringo towards the west. (4) There is a steep rise of the Moho east of the Chyulu Hills towards the Indian Ocean. (5) P-wave velocities in the uppermost mantle are above 8 km/s except under the rift proper and under the Neogene volcanic centre of the Chyulu Hills, where the velocity is 7.9-8.0 km/s. (6) Under the Chyulu Hills, the Moho is replaced by a gradual crust-mantle transition, and the low velocities near the crust-mantle boundary extend to greater depths as evidenced by teleseismic tomography which indicates a velocity decrease of 3-5\%, i.e. from 8.1-8.2 km/s to at least 7.9 km/s. Both effects are probably caused by the local recent volcanic activity, but cannot be interpreted as due to plume activity which is assumed to be present under the Nyanza craton further west. (7) Gravity modelling and first preliminary results of the magnetotelluric measurements support the seismic-refraction and tomographic results. Furthermore, under the western flank the magnetotelluric and gravity data indicate increased conductivity and decreased density in the uppermost mantle below 60-80 km depth.

@article{prodehl_etal:1997a, abstract = {Following two previous experiments in 1985 (KRISP 85) and 1989-1990 (KRISP 90), a series of geophysical experiments was undertaken in 1993-1995 (KRISP 94) to study the lithospheric structure of the southern Kenya rift down to depths of greater than 100 km, with special emphasis on the Chyulu Hills, a complex of volcanic centres on the eastern flank of the rift. KRISP 94 involved a teleseismic tomography experiment of the Chyulu Hills area in July and August 1993, a seismic refraction-wide-angle reflection survey across southern Kenya from Lake Victoria to the Indian Ocean in February 1994, seismicity studies of southern Kenya from 1993 to 1995, a special seismicity study of the Lake Magadi area in February 1994, a gravity study along the seismic-refraction lines before and after the seismic-refraction study, and a magnetotelluric study of southern Kenya in February 1995. Major scientific goals of the project were to reveal the detailed crustal and upper-mantle structure under the southern Kenya rift and its flanks for several 100 km to the west and to the east and their evolution, to study the relationship between deep crustal and uppermost mantle structure, to learn more on the development of sedimentary basins and volcanic features on the flanks and its relation to the Kenya rift, to obtain information on the temperature conditions underneath the rift and its flanks, to perform a particular integrated and calibrative study of seismological and petrological data in the Chyulu Hills, and to understand the processes which are producing extension, uplift, and extensive magmatism. This report is an introduction to a series of subsequent papers. It focuses on the technical description of the main seismic surveys of the KRISP 94 effort and summarizes the key results. During the teleseismic survey an array of 31 seismographs was deployed to record teleseismic, regional and local events for a period of about 3 months from June to August 1993. The elliptical array covered an area about 150 km (N-S) x 100 km (E-W) and spanned the central portion of the Chyulu Hills and its surroundings, with an average station spacing of 10-30 km. The seismic refraction-wide-angle reflection survey was carried out in a 2-week period in February 1994. It consisted of two profiles: one extending from Lake Victoria across the western flank and the southernmost Kenya rift at Lake Magadi, the other extending from Athi River near Nairobi across the eastern flank of the rift, traversing the Chyulu Hills and terminating at the Indian Ocean near Mombasa. A total of 204 mobile seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to 730 km. Key results are as follows: (1) The crust reaches a maximum thickness of up to 44 km under the Chyulu Hills. (2) Only a minor upwarping of the crust-mantle boundary is seen under the rift proper in the Lake Magadi area. (3) To the west the crust shallows to about 34 km near Lake Victoria, in contrast to the thickening of the crust further north from the central part of the rift near Lake Baringo towards the west. (4) There is a steep rise of the Moho east of the Chyulu Hills towards the Indian Ocean. (5) P-wave velocities in the uppermost mantle are above 8 km/s except under the rift proper and under the Neogene volcanic centre of the Chyulu Hills, where the velocity is 7.9-8.0 km/s. (6) Under the Chyulu Hills, the Moho is replaced by a gradual crust-mantle transition, and the low velocities near the crust-mantle boundary extend to greater depths as evidenced by teleseismic tomography which indicates a velocity decrease of 3-5\%, i.e. from 8.1-8.2 km/s to at least 7.9 km/s. Both effects are probably caused by the local recent volcanic activity, but cannot be interpreted as due to plume activity which is assumed to be present under the Nyanza craton further west. (7) Gravity modelling and first preliminary results of the magnetotelluric measurements support the seismic-refraction and tomographic results. Furthermore, under the western flank the magnetotelluric and gravity data indicate increased conductivity and decreased density in the uppermost mantle below 60-80 km depth.}, added-at = {2012-09-01T13:08:21.000+0200}, author = {Prodehl, C. and Ritter, J. R. R. and Mechie, J. and Keller, G. R. and Khan, M. A. and Jacob, B. and Fuchs, K. and Nyambok, I. O. and Obel, J. D. and Riaroh, D.}, biburl = {https://www.bibsonomy.org/bibtex/20c7932604a9f45d77992f1b6ece37790/nilsma}, day = 15, doi = {10.1016/S0040-1951(97)00098-X}, interhash = {5f8c7656de7c80590927602542c31698}, intrahash = {0c7932604a9f45d77992f1b6ece37790}, issn = {00401951}, journal = {Tectonophysics}, keywords = {geophysics seismics}, month = sep, number = {1-4}, pages = {121--147}, timestamp = {2021-02-09T13:27:42.000+0100}, title = {The KRISP 94 lithospheric investigation of southern Kenya --- the experiments and their main results}, url = {http://dx.doi.org/10.1016/S0040-1951(97)00098-X}, volume = 278, year = 1997 }