The stratigraphy and structure of the Kamasia Range, Kenya Rift Valley
The stratigraphy and structure of the Kamasia Range, Kenya Rift Valley
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肯尼亚大裂谷卡马西亚山脉的地层和结构
DOI：
10.1144/gsjgs.135.3.0265
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发表时间：
1978
影响因子：
2.7
通讯作者：
J. Martyn
中科院分区：
文献类型：
--
作者：
G. Chapman;S. Lippard;J. Martyn
卡马西亚山脉（Kamasia Range）是一个隆起的、断层的、拱形的地块，位于埃尔盖约陡崖（Elgeyo Escarpment）和巴林戈-苏加塔海槽（Baringo-Suguta海槽）之间，埃尔盖约陡崖是肯尼亚裂谷的西部边界，巴林戈-苏加塔海槽位于肯尼亚裂谷北方北纬0°15′ ~ 1°30′之间。变质基底岩石暴露在Kamasia地区的中部，在那里它们被3000米的中新世熔岩和沉积物覆盖，年龄从16 Ma到7 Ma不等。这是肯尼亚裂谷中暴露的该时代最厚的岩石序列。从这一时期开始就发生了下沉，但第一次主要的断层运动发生在大约7 Ma（中新世晚期），形成了Elgeyo和Kamasia断层崖。后来的火山活动主要局限于卡马西亚以东地区，尽管大量的洪水粗面岩熔岩几乎覆盖了埃尔盖约以东的整个地区。上新世晚期-更新世（2-0.5 Ma）发生了第二次主要断层运动，抬升了Kamasia山脉，形成了西部倾斜斜坡，并使裂谷的轴向地堑向东向下断裂。Kamasia的整体结构是一个被大断层切割的宽拱，最大位移约为4000米，就在其轴线以西，但结构细节复杂。众多断层为正倾滑型，呈雁列斜交格局，优势方向为000-040°和330-340°。在垂直于断层面的轴上的折叠产生了半圆形和盆地，这表明了一个沿着裂谷的次级压缩应力。认为大部分断层在水平岩层序中以垂直裂缝形式发育，然后在继续伸展和上倾作用下发生旋转，形成与主裂谷构造既合成又对立的复杂倾斜断块系统。
The Kamasia Range (or Tugen Hills) is an uplifted, faulted and arched block lying between the Elgeyo Escarpment, which forms the western boundary of the Kenya Rift, and the axial Baringo-Suguta trough between 0°15′N and 1°30′N in the northern part of the rift. Metamorphic basement rocks are exposed in the central part of the Kamasia area where they are overlain by 3000 m of Miocene lavas and sediments ranging in age from 16 Ma to 7 Ma. This is the thickest sequence of rocks of this age exposed in the Kenya Rift. Downwarping occurred from the beginning of this period, but the first major fault movements occurred at about 7 Ma (late Miocene) forming the Elgeyo and Kamasia fault-scarps. Later volcanicity was mainly confined to the area east of the Kamasia, although extensive flood trachyte lavas covered practically the whole area east of the Elgeyo at one time. A second period of major fault movement occurred in the late Pliocene-Pleistocene (2–0.5 Ma) uplifting the Kamasia Range, forming the western dip-slope and downfaulting the axial graben of the rift to the east. The overall structure of the Kamasia is a broad arch cut by large faults, with maximum displacement of about 4000 m, just to the west of its axis, but in detail the structures are complex. The numerous faults are normal dip-slip type and show an en echelon and obliquely intersecting pattern with dominant directions 000–040° and 330–340°. Folding on axes perpendicular to the fault planes produced half-domes and basins indicating a secondary compressive stress directed along the rift. It is proposed that the majority of the faults developed as vertical fractures in horizontal rock sequences and were then rotated by continued extension and up-arching to produce complex tilted fault-block systems both synthetic and antithetic to the main rift structure.