Contemporary, Holocene, and Quaternary deformation of the Asal Rift, Djibouti: Implications for the mechanics of slow spreading ridges
Contemporary, Holocene, and Quaternary deformation of the Asal Rift, Djibouti: Implications for the mechanics of slow spreading ridges
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吉布提阿萨尔裂谷的当代、全新世和第四纪变形：对缓慢扩张的山脊力学的影响
DOI：
10.1029/91jb02118
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发表时间：
1991
影响因子：
--
通讯作者：
F. Gasse
中科院分区：
文献类型：
--
作者：
R. Stein;P. Briole;J. Ruegg;P. Tapponnier;F. Gasse
由于大洋中脊裂谷事件的频率和特征在很大程度上是未知的，因此此类事件的重复如何产生裂谷结构尚未被探索。非洲吉布提阿法尔洼地的阿萨尔裂谷为年轻的缓慢扩张的洋中脊提供了世界上最好的地面模拟。对 1978 年阿萨尔地震火山事件的地震、大地测量和现场观测得出了断层和岩脉位置、几何形状、位移和裂谷事件中玄武岩挤压体积的估计。 Asal 的一个 6-9 kyr 历史的湖岸高台已向下弯曲 70 m，提供了全新世裂谷垂直变形的测量结果。裂谷地形提供了一个更古老的基准，我们使用全新世变形率推断其年龄为 34±6 kyr。我们发现整个裂谷的断层都是活跃的；距离裂谷轴超过 4 公里处，全新世滑移率逐渐减弱；晚第四纪速率在 6-7 公里之外下降。全新世滑动速率以1978年滑动的断层位移为特征，估算重复次数；我们发现单个断层上的构造事件每 200-300 年就会发生一次。在 1978 年的事件中，一半的裂谷断层一起滑动。如果这是典型情况，那么每隔 100-150 年就会激活一组断层。我们认为一半的事件发生在裂谷轴并伴有火山挤压；其余的发生在新火山带的外围，仅涉及断层滑动，这两个事件的重复时间为 200-300 年。考虑到裂谷宽度为 10 公里，扩张速度为 16 毫米 yr−1，裂谷中物质的平均年龄应为 ∼350 kyr，比推测的裂谷地形形成年龄要老一个数量级。过去35 kyr裂谷轴的沉降速率为8~9 mm yr−1，火山挤压充填速率<1 mm yr−1。由此产生的净沉降率大约等于裂谷的半扩张率，如果没有熔岩的显着填充，就无法维持 300 kyr。因此，这两项观察都表明裂谷的长期垂直变形并不是稳定状态。相反，我们认为阿萨尔存在裂谷/充盈周期，最近的充盈事件结束于~35 kyr。
Because the frequency and character of rifting events along mid-ocean ridges are largely unknown, how the repetition of such events gives rise to rift structures is unexplored. The Asal rift in the Afar depression of Djibouti, Africa, provides the world's best subaerial analogue for young slow spreading mid-ocean ridges. Seismic, geodetic, and field observations of a seismovolcanic event in 1978 at Asal yield estimates of the fault and dike locations, geometry, displacement, and volume of basalt extruded in a rifting event. A 6–9 kyr-old lake shore highstand at Asal has been warped downward by 70 m, providing a Holocene measure of the vertical deformation across the rift. The rift topography furnishes an older datum, which we infer to be 34±6 kyr old using the Holocene deformation rate. We find that faults throughout the rift valley are active; Holocene slip rates diminish beyond 4 km from the rift axis; late Quaternary rates decrease beyond 6–7 km. The Holocene slip rates are used to estimate repeat times by taking the displacement on the faults which slipped in 1978 as characteristic; we find tectonic events on individual faults recur every 200– 300 years. Half the rift faults slipped together in the 1978 event. If this is typical, then groups of faults are activated every 100–150 years. We suggest that half the events take place in the rift axis accompanied by volcanic extrusion; the remainder occur peripheral to the neovolcanic zone and involve fault slip only, both events having a repeat time of 200–300 years. Given the 10 km width of the rift and its 16 mm yr−1 spreading rate, the mean age of the material in the rift should be ∼350 kyr, an order of magnitude older than the inferred age of the formation of the rift topography. The subsidence rate of the rift axis during the past 35 kyr is 8–9 mm yr−1, with the rate of infilling by volcanic extrusion <1 mm yr−1. The resulting net subsidence rate, about equal to the half-spreading rate of the rift, could not be sustained for 300 kyr without significant infilling by lavas. Thus both observations suggest that the long-term vertical deformation in the rift has not been steady state. Instead, we suggest that there is a rifting/filling cycle at Asal, with the most recent filling episode ending ∼35 kyr.