Seismic Study of the Offshore Portion of the Cameroon Volcanic Line, Gulf of Guinea, West Africa

火山线状构造是地球上普遍存在的一种特征，但它们在地幔和地壳之间的物质转移以及在板块构造框架中的作用仍存在争议。喀麦隆火山线(CVL)是一条1600公里长的从西南到东北Y形的火山中心和火山岩链，在到达喀麦隆和乍得边境的阿达马瓦高原时分裂成两个分支。大约一半的CVL总长度位于非洲大陆；另一半向东南延伸到几内亚湾和大西洋。CVL的近海部分从西北到东南由比奥科岛、普林西比岛、圣多美岛和帕加卢岛组成。在这条线的海洋和大陆部分，年轻的玄武岩熔岩的成分是相同的，表明类似的母质岩浆，很少受到地壳岩石的污染，但在创造比奥科岛和普林西比岛的最早火山活动的年龄之间存在着巨大的差异，从大陆-海洋边界的线的任何一个方向上的放射性成因铅的比率都有系统地下降(Halliday等人，1990年)。这表明，比奥科岛和普林西比岛之间在性质和/或工艺方面发生了重大变化。CVL缺乏明显的年龄进程(Fitton和Dunlop，1985；Meyers等人，1998)，这使得一些作者将其称为一条“热线”，并引用上地幔中的Rayleigh-Bernard对流来解释其奇怪的时间、地球化学特征和明显重新激活的火山作用(Meyers等人，1998；King和Anderson，1998；King和RitSema，2000)。然而，一个修正的羽流模型，也许包括多个羽流或沿岩石圈底部地形的横向物质输送，也可能满足这些限制(Burke等人，1971)。我们将进行一项海上地震研究，以补充定于2005年在喀麦隆开始的大规模临时部署宽带地震仪。将CVL群岛的地震研究结果与喀麦隆的结果结合起来，将有助于区分CVL成因的各种模式及其与地幔对流的关系。

Volcanic lineaments are a ubiquitous feature of our planet but their role in the mass transfer between mantle and crust and in the plate tectonic framework is still debated. The Cameroon Volcanic Line (CVL) is a 1600-km-long, SW to NE, Y-shaped chain of volcanic centers and plutons that splits into two branches as it reaches the Adamawa Plateau on the Cameroon-Chad border. Roughly half the total length of the CVL lies on the African continent; the other half extends southeastward into the Gulf of Guinea and the Atlantic Ocean. The offshore portion of the CVL is comprised, from northwest to southeast, of the islands of Bioko, Principe, Sao Tome, and Pagalu. The composition of young basaltic lavas is identical in the oceanic and continental portions of the line, indicating similar parental magmas and little contamination from the crustal rocks, yet there is a significant disparity between the ages of earliest volcanism that created the islands of Bioko and Principe and there is a systematic decrease in radiogenic Pb ratios in either direction along the line from the continent-ocean boundary (Halliday et al., 1990). This suggests that an important change in properties and/or processes occurs between the islands of Bioko and Principe. The lack of an obvious age progression in the CVL (Fitton and Dunlop, 1985; Meyers et al., 1998) has led some authors to refer to it as a "hotline" and invoke Rayleigh-Bernard convection in the upper mantle to explain its odd timing, geochemical characteristics and apparently reactivated volcanism (Meyers et al., 1998; King and Anderson, 1998; King and Ritsema, 2000). However, a modified plume model, perhaps one that involves multiple plumes or lateral transport of material along topography on the base of the lithosphere, might also satisfy the constraints (Burke et al., 1971). We will conduct a complementary seismic study offshore to the large-scale, temporary deployment of broadband seismographs scheduled to begin in Cameroon in 2005. Integrating the results of seismic studies on the CVL islands with the results from Cameroon will help distinguish between various models for the genesis of the CVL and its relationship to convection in the mantle.