A model for the structure, composition and evolution of the Kenya rift
A model for the structure, composition and evolution of the Kenya rift
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肯尼亚裂谷的结构、组成和演化模型
DOI：
10.1016/s0040-1951(97)00097-8
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发表时间：
1997
期刊：
影响因子：
2.9
通讯作者：
S. J. Gaciri
中科院分区：
文献类型：
--
作者：
J. Mechie;G. R. Keller;C. Prodehl;M. A. Khan;S. J. Gaciri
1985年和1990年在肯尼亚裂谷进行的地震折射-广角反射实验（KRISP '85和KRISP '90）表明，沿沿着和横跨裂谷的地壳厚度变化很大。沿着裂谷轴，地壳厚度从南部肯尼亚穹隆下的35公里到北部图尔卡纳地区下的20公里不等。根据裂谷两侧地壳厚度的分布，可以说，地壳厚度沿着裂谷轴的主要变化是由第三纪裂谷幕引起的。地壳厚度向北减少与地表地形（向北减少）、裂谷宽度（向北增加）、地表延伸估计（南部5-10公里，北部35-40公里）和布格重力的变化相关，区域性向北增加完全可以用地壳厚度的变化来解释。在750 km长的轴向裂谷剖面以下，上地幔Pn速度较低，为7.5- 7.7km/s。而在裂谷北方下部，低速地幔物质中分别包埋有两层速度为8.1 km/s和8.3 km/s的地幔物质，深度分别为40-45 km和60-65 km。相比之下，广角数据显示，在肯尼亚穹隆下方，裂谷南部，低地幔速度发生在至少65公里的深度。这种地幔速度结构表明，在裂谷北方部分之下至少有65公里的深度是开始熔融的，因此并不比在南部肯尼亚穹隆之下开始熔融的深度（45-50公里）浅。横跨肯尼亚穹隆以北、巴林戈湖纬度裂谷的剖面表明，裂谷地表以下存在7.5- 7.7km/s的低上地幔P_n速度和5-10 km的地壳减薄。莫霍面深度和P_n速度的突变发生在裂谷边界的交叉。在裂谷侧翼之下，出现8.0-8.2 km/s的正常Pn速度。热地幔物质的存在下的肯尼亚圆顶，因为这里的火山活动在15-20马的发病仍然是兼容的地幔P波速度的突变，因为裂谷边界的交叉。地震速度的岩石学解释表明，除了在裂谷北方部分下速度大于8.0 km/s的两个层外，裂谷下方的地幔中有几个（高达5%）玄武岩熔体，其中一些晶体取向（各向异性）是必要的。在裂谷南部约45-50公里深度以下，岩浆可能以就地部分熔融的形式存在。上述结果，结合岩石学、地球化学和地表地质学的研究结果，表明在肯尼亚裂谷现今位置的下方，大约在20-30 Ma时出现了异常热的地幔物质。从那时起，这种异常热的地幔物质的活跃上升，引起了沿着整个裂谷的广泛火山活动，并通过镁铁质火成岩底侵和侵入，特别是侵入基底地壳层，改变了裂谷下面的地壳。伴随着异常热的地幔物质的上升，在裂谷南部的肯尼亚穹隆下发生了轻微的地壳伸展（5-10 km），那里的地壳厚度很大（35 km）。在裂谷北方的图尔卡纳地区，发生了更大的伸展（35-40公里），地壳厚度很小（20公里），尽管裂谷北方开始熔化的深度要比裂谷南方大。
The seismic refraction-wide-angle reflection experiments carried out in 1985 and 1990 in the Kenya rift (KRISP '85 and KRISP '90) show major crustal thickness variations both along and across the rift. Along the rift axis crustal thickness varies from 35 km in the south beneath the Kenya dome to 20 km in the north beneath the Turkana region. Due to the distribution of crustal thickness beneath the rift flanks, it can be stated that the major amount of variation in crustal thickness along the rift axis is due to the Tertiary rifting episode. The northwards decrease in crustal thickness can be correlated with changes in surface topography (northwards decrease), rift width (northwards increase), surface estimates of extension (5–10 km in the south and 35–40 km in the north) and Bouguer gravity, the regional northwards increase of which can be explained entirely by the change in crustal thickness. Below the 750 km long axial rift profile, uppermost mantle Pnvelocities are low, being 7.5–7.7 km/s. However, under the northern part of the rift two layers with velocities of 8.1 km/s and 8.3 km/s are embedded in the low-velocity mantle material at 40–45 km and 60–65 km depth, respectively. In contrast, the wide-angle data show that beneath the Kenya dome, in the southern part of the rift, low mantle velocities occur down to at least 65 km depth. This mantle velocity structure is indicative of the depth to the onset of melting being at least 65 km beneath the northern part of the rift and thus not being shallower than the depth (45–50 km) to the onset of melting under the Kenya dome to the south. A profile across the rift north of the Kenya dome at the latitude of Lake Baringo shows that the low uppermost mantle Pnvelocity of 7.5–7.7 km/s and crustal thinning of 5–10 km is confined to below the surface expression of the rift. An abrupt change in Moho depths and Pnvelocities occurs as the rift boundaries are crossed. Beneath the rift flanks, normal Pnvelocities of 8.0–8.2 km/s occur. The presence of hot mantle material beneath the Kenya dome since the onset of volcanism here at 15–20 Ma is still compatible with the abrupt change in mantle P-wave velocities as the rift boundaries are crossed. Petrological interpretation of the seismic velocities indicates a few (up to 5) percent basaltic melt in the mantle below the rift except in the two layers with velocities greater than 8.0 km/s under the northern part of the rift where some crystal orientation (anisotropy) is necessary. Below about 45–50 km depth beneath the southern part of the rift the magma could exist as in situ partial melt. The above results, taken together with results from teleseismic studies, petrology and surface geology, indicate anomalously hot mantle material appearing below the present site of the Kenya rift at about 20–30 Ma. The active uprising of this anomalously hot mantle material since this time has given rise to widespread volcanism along the whole length of the rift and has modified the crust beneath the rift by mafic igneous underplating and intrusion, especially into the basal crustal layer. Accompanying the uprise of the anomalously hot mantle material minor crustal extension (5–10 km) has occurred beneath the Kenya dome in the southern part of the rift where crustal thickness is large (35 km). Under the Turkana region in the northern part of the rift, a greater amount of extension (35–40 km) has taken place and the crustal thickness is small (20 km), although the depth to the onset of melting under the northern part of the rift is, if anything, greater than under the southern part of the rift.