$$^{40}Ar/^{39}Ar$$Age Calibration of the Litho- and Paleomagnetic Stratigraphies of the Ngorora Formation, Kenya
$$^{40}Ar/^{39}Ar$$Age Calibration of the Litho- and Paleomagnetic Stratigraphies of the Ngorora Formation, Kenya
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$$^{40}Ar/^{39}Ar$$肯尼亚恩戈罗拉组岩地层和古地磁地层的年龄校准
DOI：
10.1086/629425
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发表时间：
1990
期刊：
影响因子：
--
通讯作者：
R. Drake
中科院分区：
文献类型：
--
作者：
A. Deino;L. Tauxe;M. Monaghan;R. Drake
用激光熔融单晶40 Ar/39 Ar法测定了肯尼亚裂谷中新世中期恩戈罗拉组火山岩中再加工和污染的新生火山长石的精确喷发年龄。这些年龄范围从底部的13.06 Ma到Kabarsero附近的模式剖面顶部的10.51 Ma。当地的古地磁地层与地磁反转时间尺度的相关性产生的磁年代学年龄估计是年轻的同位素年龄平均为0.18马。如果推断的海底扩张速率变化发生在13 Ma或更早，而不是先前提出的10.42 Ma，则可以消除大部分差异。根据$^{40}Ar/^{39}Ar$$结果计算的卡巴尔塞罗的沉积速率从初始值约25 cm/1000 yr下降到剖面顶部的约5 cm/1000 yr。最初的快速沉降速率表征了最初的0.1至0.3百万年。这表明Baringo盆地在这个时候可能不存在作为一个裂谷由伸展构造创建，而是可能已经形成了沉降功能，在响应从岩石圈中移除大量的岩浆。一颗前臼齿，初步鉴定为原执政官种。在Bartabwa村附近的Ngorora地层中发现的一个化石，其年代为12.42 Ma，可能是化石记录中最后一个已知的该属化石。
Precise eruptive ages have been determined by the laser-fusion, single-crystal $$^{40}Ar/^{39}Ar$$method for juvenile volcanic feldspars from reworked and contaminated volcaniclastic rocks of the middle Miocene Ngorora Formation, Kenya Rift Valley. These ages range from 13.06 Ma at the base to 10.51 Ma toward the top of the type section near Kabarsero. Correlation of the local paleomagnetic stratigraphies with the geomagnetic reversal time scale yields magnetochronologic age estimates that are younger than the isotopic ages by an average of 0.18 Ma. Much of the discrepancy can be eliminated if an inferred change in sea-floor spreading rate occurred at 13 Ma or earlier, rather than at 10.42 Ma as previously suggested. Sedimentation rates at Kabarsero calculated from the $$^{40}Ar/^{39}Ar$$results decrease from initial values of ~25 cm/1000 yr to ~5 cm/1000 yr toward the top of the section. The initial rapid sedimentation rates characterize the first 0.1 to 0.3 m.y. following emplacement of the underlying, voluminous, basin-filling Tiim Phonolites, indicating that the Baringo Basin at this time may not have existed as a rift valley created by extensional tectonics, but instead may have been a subsidence feature formed in response to removal of large volumes of magma from the lithosphere. A premolar tentatively identified as Proconsul sp. indet. found in the Ngorora Formation near the village of Bartabwa has been dated at ~12.42 Ma, representing perhaps the last known occurrence of this genus in the fossil record.