The development of multiple phases of superposed rifting in the Turkana Depression, East Africa: Evidence from receiver functions
The development of multiple phases of superposed rifting in the Turkana Depression, East Africa: Evidence from receiver functions
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东非图尔卡纳凹陷多阶段叠加裂谷的发展：接收函数的证据
DOI：
10.1016/j.epsl.2023.118088
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发表时间：
2023
影响因子：
5.3
通讯作者：
Rooney, T.O.
中科院分区：
文献类型：
--
作者：
Ogden, C.S.;Bastow, I.D.;Ebinger, C.;Ayele, A.;Kounoudis, R.;Musila, M.;Bendick, R.;Mariita, N.;Kianji, G.;Rooney, T.O.
东非图尔卡纳洼地将南部的东非高原和北部的埃塞俄比亚-也门高原分隔开来。目前尚不清楚是否该凹陷缺乏动力地幔的支持，或者整个东非地区的动力支持和均衡补偿的低气压薄地壳。同样知之甚少的是，中新世-新近纪伸展是如何跨越凹陷发展的，连接了埃塞俄比亚和肯尼亚在空间上分离的岩浆裂谷带。接收函数分析用于约束莫霍面深度和块体地壳的VP/VS比低于新的地震仪网络在凹陷，并在北方坦桑尼亚克拉通。地壳厚度在乌干达北方之下约40公里，埃塞俄比亚南部之下约30-35公里，但在大部分洼地之下约20-30公里，质量平衡计算表明，低海拔地区仅用地壳减薄就能充分解释。尽管事实上，岩浆活动已经发生了45马整个凹陷，超过15马前东非裂谷（ESTA）的延伸开始，散装地壳VP/VS整个埃塞俄比亚南部和图尔卡纳凹陷（ESTA 1.74）是类似的观察到的地区未受新生代裂谷和岩浆活动。因此，缺乏大量下地壳侵入和/或熔融的证据，广泛分布在埃塞俄比亚裂谷和埃塞俄比亚高原以北。这些观察结果，当根据高拉伸因子（β≤ 2.11）进行审查时，表明新生代伸展直到最近才被断层和板块拉伸所主导，而不是岩浆侵入，岩浆侵入可能是一个初期过程，直接在地震活跃的图尔卡纳湖下方运作。图尔卡纳湖以西的早期拉张盆地，其相关拉伸因子为β δ 2，形成于早期裂陷幕期间仅适度变薄的地壳中。相反地，在Kino Sogo断层带（KSFB）下方的23公里厚的地壳有小的偏移断层和薄的沉积地层，这表明几乎所有观察到的拉伸都发生在中生代。尽管KSFB标志着南北集中伸展带之间的最短路径，但地震活动和GPS数据显示，现代伸展位于图尔卡纳湖以西。失败的中生代裂谷带，现在的特点是地壳变薄和地幔岩石圈相对难熔，正在绕过，而不是利用复裂。
Abstract The Turkana Depression in Eastern Africa separates the elevated plateaus of East Africa to the south and Ethiopia-Yemen to the north. It remains unclear whether the Depression lacks dynamic mantle support, or if the entire East Africa region is dynamically supported and the Depression compensated isostatically by thinned crust. Also poorly understood is how Miocene-Recent extension has developed across the Depression, connecting spatially separated magmatic rift zones in Ethiopia and Kenya. Receiver function analysis is used to constrain Moho depth and bulk-crustal V P/V S ratio below new seismograph networks in the Depression, and on the northern Tanzania craton. Crustal thickness is∼ 40 km below northern Uganda and 30–35 km below southern Ethiopia, but 20–30 km below most of the Depression, where mass-balance calculations reveal low elevations can be explained adequately by crustal thinning alone. Despite the fact that magmatism has occurred for 45 Ma across the Depression, more than 15 Ma before East African Rift (EAR) extension initiated, bulk crustal V P/V S across southern Ethiopia and the Turkana Depression (∼ 1.74) is similar to that observed in areas unaffected by Cenozoic rifting and magmatism. Evidence for voluminous lower crustal intrusions and/or melt, widespread below the Ethiopian rift and Ethiopian plateau to the north, is therefore lacking. These observations, when reviewed in light of high stretching factors (β≤ 2.11), suggest Cenozoic extension has been dominated until recently by faulting and plate stretching, rather than magma intrusion, which is likely an incipient process, operating directly below seismically-active Lake Turkana. Early-stage EAR basins to the west of Lake Turkana, with associated stretching factors of β≈ 2, formed in crust only moderately thinned during earlier rifting episodes. Conversely,∼ 23 km-thick crust beneath the Kino Sogo Fault Belt (KSFB) has small offset faults and thin sedimentary strata, suggesting almost all of the observed stretching occurred in Mesozoic times. Despite the KSFB marking the shortest path between focused extensional zones to the north and south, seismicity and GPS data show that modern extension is localized below Lake Turkana to the west. Failed Mesozoic rift zones, now characterized by thinned crust and relatively refractory mantle lithosphere, are being circumnavigated, not exploited by EAR rifting.
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