The Mid-Miocene East African Plateau: a pre-rift topographic model inferred from the emplacement of the phonolitic Yatta lava flow, Kenya
The Mid-Miocene East African Plateau: a pre-rift topographic model inferred from the emplacement of the phonolitic Yatta lava flow, Kenya
复制标题
DOI：
10.1144/sp357.15
复制
发表时间：
2011
期刊：
影响因子：
--
通讯作者：
Henry Wichura;R. Bousquet;R. Oberhänsli;M. Strecker;M. Trauth
中科院分区：
文献类型：
--
作者：
Henry Wichura;R. Bousquet;R. Oberhänsli;M. Strecker;M. Trauth
东非裂谷高原地区的高地形通常由两种不同的机制来解释：（1）由于机械和/或均衡松弛引起的裂谷侧翼隆起和（2）由于地幔柱的撞击引起的岩石圈隆起。东非的高地地形对该区域的大气环流系统以及降雨量和分布有着深远的影响。虽然东非高地形的气候和古环境影响已被广泛接受，但地形变化的时间、幅度和时空特征仍不清楚。这种困境源于缺乏可确定年代的、在地貌上有意义的、可以明确记录地表隆起的参考层位。在这里，我们报告在新生代裂谷之前，在东非高地形的形成。我们推断地形隆升的东非高原的基础上的侵位特征的c。300公里长和13.5马Yatta响岩熔岩流沿着以前的河谷，排水高地形，在今天的裂谷为中心。熔岩流沿着一条古老的河床流动，这条河床曾经是高原东侧的径流通道。使用的成分和温度依赖的粘度模型与随后的冷却和调整的雅塔熔岩流的尺寸和覆盖的古地形（坡度角），我们使用的流量作为一个“古倾斜仪”。根据这些观测结果和我们的模拟结果，我们确定了肯尼亚穹隆在裂陷之前的古坡度至少为0.2°，并推断出最低高原海拔为1400 m。我们认为，这种高地形是由热膨胀的岩石圈相互作用与地幔柱产生的热源。有趣的是，推断中中新世隆起与基本的古生态变化相吻合，包括东非草原的两步扩张以及热带非洲的重要辐射和物种形成事件。
Abstract High topography in the realm of the rifted East African Plateau is commonly explained by two different mechanisms: (1) rift-flank uplift resulting from mechanical and/or isostatic relaxation and (2) lithospheric uplift due to the impingement of a mantle plume. High topography in East Africa has far-reaching effects on atmospheric circulation systems and the amount and distribution of rainfall in this region. While the climatic and palaeoenvironmental influences of high topography in East Africa are widely accepted, the timing, the magnitude and this spatiotemporal characteristic of changes in topography have remained unclear. This dilemma stems from the lack of datable, geomorphically meaningful reference horizons that could unambiguously record surface uplift. Here, we report on the formation of high topography in East Africa prior to Cenozoic rifting. We infer topographic uplift of the East African Plateau based on the emplacement characteristics of the c. 300 km long and 13.5 Ma Yatta phonolitic lava flow along a former river valley that drained high topography, centred at the present-day rift. The lava flow followed an old riverbed that once routed runoff away from the eastern flank of the plateau. Using a compositional and temperature-dependent viscosity model with subsequent cooling and adjusting for the Yatta lava-flow dimensions and the covered palaeotopography (slope angle), we use the flow as a ‘palaeo-tiltmeter’. Based on these observations and our modelling results, we determine a palaeoslope of the Kenya dome of at least 0.2° prior to rifting and deduce a minimum plateau elevation of 1400 m. We propose that this high topography was caused by thermal expansion of the lithosphere interacting with a heat source generated by a mantle plume. Interestingly, the inferred Mid-Miocene uplift coincides with fundamental palaeoecological changes including the two-step expansion of grasslands in East Africa as well as important radiation and speciation events in tropical Africa.