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

火山地貌是地球上普遍存在的特征，但它们在地幔和地壳之间的质量传递以及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，西南向东北的y形火山中心和岩体链，在到达喀麦隆-乍得边境的阿达马瓦高原时分成两个分支。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的海上部分由比奥科岛、普林西比岛、圣多美岛和帕加卢岛组成，从西北到东南。在这条线的海洋和大陆部分，年轻的玄武岩熔岩的组成是相同的，表明母岩浆相似，地壳岩石的污染很少，然而，形成比奥科岛和普林西比岛的最早火山活动的年龄之间存在显著差异，并且从大陆-海洋边界开始，沿着这条线的任何方向，放射性成因铅比率都有系统的下降（Halliday et al., 1990）。这表明比奥科岛和普林西比岛之间的性质和/或过程发生了重大变化。CVL缺乏明显的年龄进展（Fitton and Dunlop, 1985; Meyers et al., 1998），这使得一些作者将其称为“热线”，并援引上地幔中的瑞利-伯纳德对流来解释其奇怪的时间、地球化学特征和明显的火山活动（Meyers et al., 1998; King and Anderson, 1998; King and Ritsema, 2000）。然而，一种修正的羽流模型，可能涉及多个羽流或沿着岩石圈底部地形的物质横向输送，也可能满足这些限制条件（Burke et al., 1971）。我们将对喀麦隆计划于2005年开始的大规模临时部署宽带地震仪进行一项补充性的海上地震研究。将CVL岛屿的地震研究结果与喀麦隆的结果相结合，将有助于区分CVL成因的各种模型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.