Cenozoic extension in northern Kenya: a quantitative model of rift basin development in the Turkana region
Cenozoic extension in northern Kenya: a quantitative model of rift basin development in the Turkana region
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DOI：
10.1016/0040-1951(94)90187-2
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发表时间：
1994-09
期刊：
影响因子：
2.9
通讯作者：
D. Hendrie;N. Kusznir;C. Morley;C. Ebinger
中科院分区：
文献类型：
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作者：
D. Hendrie;N. Kusznir;C. Morley;C. Ebinger
最近在肯尼亚裂谷北方的图尔卡纳地区进行的地质和地球物理研究强调了一些主要沉积沉积中心的存在（莫利等人，1992年），覆盖着极薄的地壳（Mechie等人，1994年）。该地区覆盖着一个连接南苏丹和肯尼亚东部中生代-古近纪裂谷的假定结构（Ibrahim等人，1991; Bosworth，1992）。大陆伸展的弯曲悬臂模型（Kusznir等人，1991年）被用来描述图尔卡纳地区的裂谷演化，受到地震反射数据和地表地质的约束。模拟了古近纪末（25 Ma）、中新世末（5 Ma）和上新世末（2 Ma）的地壳结构、盆地几何形状和β拉伸因子剖面。我们还表明，虽然延伸估计β拉伸因子估计值为1.55-1.65，与地球物理数据一致，表明莫霍面已被变浅至20公里的延伸，该延伸的一个重要部分可能早于新近纪。我们建议，在整个图尔卡纳地区的古近纪形成了一系列的盆地，连接已知的扩展在苏丹南部的安扎/Kaisut系统和南Kerio盆地在北方和肯尼亚中部的扩展轨迹。这些盆地没有地表露头，但由重力异常和大量间接证据确定。东非裂谷系后期的持续扩张导致了这些构造在新近纪-近代的重新活化，而其他构造则被废弃。使用弯曲悬臂模型对图尔卡纳地区的裂谷盆地几何形状进行建模，得出有效弹性厚度（Te）为3.5 km。与伸展断块旋转相关的岩石圈挠曲弯曲（遵循Buck，1988）被证明会产生大的弯曲应力，引起岩石圈上部的脆性破坏和塑性变形，从而产生岩石圈弯曲强度和Te的低值。通过弯曲悬臂模型对裂谷区域局部估计的T的低值与使用重力地形相干技术确定的T的大得多的值不同（Ebinger等人，1989），该模型对区域岩石圈强度（包括裂谷外的强大陆盾）进行了采样，该模型定义的最大β拉伸因子为1.55-1.65，不足以用已发表的熔体生成定量模型（McKenzie和Bickle，1988）生成观测到的火山岩。在如此低的拉伸系数下，需要极热的地幔温度来产生任何熔体。虽然以前的作者（Karson和Curtis，1989; Latin等人，1993年）提出，大量的熔体已经侵入和岩石圈内的积水，我们表明，地壳底侵和侵入并没有显着改变地壳厚度和延伸的裂谷系统的估计。
Recent geological and geophysical studies in the Turkana region of the northern Kenya rift have highlighted the presence of a number of major sedimentary depocentres (Morley et al., 1992) overlying anomalously thin crust (Mechie et al., 1994). The region overlies a putative structure linking the Mesozoic-Palaeogene rifts of south Sudan and eastern Kenya (Ibrahim et al., 1991; Bosworth, 1992). The flexural cantilever model of continental extension (Kusznir et al., 1991) has been used to describe the rift evolution of the Turkana region, constrained by seismic reflection data and surface geology. Sections showing crustal structure, basin geometry and β stretching factor profiles have been modelled for end Palaeogene (25 Ma), end Miocene (5 Ma) and end Pliocene (2 Ma). We also show that, whilst extension estimates giving a β stretching factor estimate of 1.55–1.65 are consistent with geophysical data showing that the Moho has been shallowed to 20 km by extension, a significant portion of that extension may pre-date the Neogene. We propose that across the Turkana region during the Palaeogene a series of basins formed, linking known loci of extension in southern Sudan with extension in the Anza/Kaisut system and South Kerio basins in northern and central Kenya. These basins have no surface outcrop expression, but are defined by gravity anomalies and a number of pieces of indirect evidence. Continued extension on the later East African rift system resulted in reactivation of some of these structures in Neogene-Recent times, and the abandonment of others.Modelling of the rift basin geometry in the Turkana region using the flexural cantilever model gives a value of effective elastic thickness (Te) of 3.5 km. Flexural bending of the lithosphere associated with extensional fault block rotations is shown (following Buck, 1988) to generate large bending stresses which give rise to substantial upper lithosphere brittle failure and plastic deformation, so producing low values of lithosphere flexural strength andTe. The low values ofTeestimated locally for the rift region by the flexural cantilever model differ from the much larger values ofTedetermined using gravity-topography coherence techniques (Ebinger et al., 1989) which sample regional lithosphere strength including that of strong continental shield outside the rift.The maximum β stretching factor of 1.55–1.65 defined by the model is insufficient to generate the observed volcanics using published quantitative models of melt generation (McKenzie and Bickle, 1988). An anomalously hot mantle temperature is required to generate any melt at such low stretching factors. Whilst previous authors (Karson and Curtis, 1989; Latin et al., 1993) have proposed that large volumes of melt have been intruded into and ponded within the lithosphere, we show that crustal underplating and intrusion do not significantly alter estimates of crustal thickness and extension across the rift system.