Observation of manganese deposits on the East Pacific Rise during the CYAMEX-RITA expedition

A submersible study has been conducted in February - March 1978 at the axis of the East Pacific Rise near 21°N. The expedition CYAMEX, the first submersible program to be conducted on the East Pacific Rise, is part of the French-American-Mexican project RITA (Rivera - Tamayo), a 3-year study devoted to detailed geological and geophysical investigations of the East Pacific Rise Crest. On the basis of the 15 dives made by CYANA in the axial area of the Rise, a morphological and tectonic zonation can be established for this moderately-fast spreading center. A narrow, 0.6 to 1.2 km wide zone of extrusion (zone 1), dominated by young lava flows, is flanked by a highly fissured and faulted zone of extension (zone 2) with a width of 1 to 2 km. Further out, zone 3 is dominated by outward tilted blocks bounded by inward-facing fault scarps. Active or recent faults extend up to 12 km from the axis of extrusion of the East Pacific Rise. This represents the first determination from direct field evidence of the width of active tectonism associated with an accreting plate boundary. Massive sulfide deposits, made principally of zinc, copper and iron, were found close to the axis of the Rise. Other signs of the intense hydrothermal activity included the discovery of benthic fauna of giant size similar to that found at the axis of the Galapagos Rift. We emphasize the cyclic character of the volcanicity. The main characteristics of the geology of this segment of the East Pacific Rise can be explained by the thermal structure at depth below this moderately-fast spreading center. The geological observations are compatible with the existence of a shallow magma reservoir centered at the axis of the Rise with a half-width of the order of 10 km.

1978年2月至3月，研究团队在北纬21°附近的东太平洋海隆（East Pacific Rise）轴部开展了潜水器考察研究。CYAMEX考察航次是首个在东太平洋海隆实施的潜水器科考项目，隶属于法-美-墨西哥三国合作的RITA（Rivera-Tamayo）项目——该项目为期三年，旨在对东太平洋海隆轴部开展精细地质与地球物理调查。基于CYANA潜水器在海隆轴区完成的15次下潜作业，可针对该中等快速扩张中心构建形态构造分带方案：宽0.6至1.2千米的熔岩喷发带（1区）以年轻熔岩流为主体特征，其两侧为宽1至2千米的强裂隙-断层拉伸带（2区）；再向外延伸的3区则以内向断层崖围限的向外倾伏地块为主要特征。活动或近期形成的断层最远可从东太平洋海隆的喷发轴部向外延伸至12千米，这是首次通过实地直接观测证据，明确与增生板块边界相关的活跃构造活动的宽度。研究团队在海隆轴部附近发现了以锌、铜、铁为主要成分的块状硫化物矿床；强烈热液活动的其他佐证还包括发现了与加拉帕戈斯裂谷轴部类似的大型底栖生物群落。本研究着重强调了火山活动的周期性特征。该段东太平洋海隆的主要地质特征，可通过该中等快速扩张中心下方深部的热结构得到合理解释。地质观测结果与“存在以海隆轴部为中心、半宽约10千米的浅部岩浆房”的推论相符。