Chapter 20 Multi-stage collapse events in the South Soufrière Hills, Montserrat as recorded in marine sediment cores

We present new evidence for sector collapses of the South Soufrière Hills (SSH) edifice, Montserrat during the mid-Pleistocene. High-resolution geophysical data provide evidence for sector collapse, producing an approximately 1 km³ submarine collapse deposit to the south of SSH. Sedimentological and geochemical analyses of submarine deposits sampled by sediment cores suggest that they were formed by large multi-stage flank failures of the subaerial SSH edifice into the sea. This work identifies two distinct geochemical suites within the SSH succession on the basis of trace-element and Pb-isotope compositions. Volcaniclastic turbidites in the cores preserve these chemically heterogeneous rock suites. However, the subaerial chemostratigraphy is reversed within the submarine sediment cores. Sedimentological analysis suggests that the edifice failures produced high-concentration turbidites and that the collapses occurred in multiple stages, with an interval of at least 2 ka between the first and second failure. Detailed field and petrographical observations, coupled with SEM image analysis, shows that the SSH volcanic products preserve a complex record of magmatic activity. This activity consisted of episodic explosive eruptions of andesitic pumice, probably triggered by mafic magmatic pulses and followed by eruptions of poorly vesiculated basaltic scoria, and basaltic lava flows.

本研究报道了更新世中期蒙特塞拉特岛南部苏弗里耶尔山（South Soufrière Hills, SSH）火山锥发生侧翼崩落的新证据。高分辨率地球物理数据为该侧翼崩落事件提供了证据，其在SSH南侧形成了体积约1立方千米的海底崩落堆积物。对沉积物岩芯采集的海底沉积物开展沉积学与地球化学分析后发现，这些堆积物系SSH陆上火山锥发生多期大规模侧翼垮塌并坠入海中所形成。本研究基于微量元素与铅同位素组成，在SSH火山序列中识别出两套截然不同的地球化学组合。岩芯中的火山碎屑浊积岩保留了这些化学组成不均一的岩石组合。然而，海底沉积物岩芯中的陆上化学地层序列发生了倒转。沉积学分析表明，火山锥垮塌形成了高浓度浊积岩，且崩落事件分多期发生，首期与第二期垮塌的间隔至少为2 ka。结合扫描电子显微镜（Scanning Electron Microscope, SEM）成像分析的详细野外与岩相学观测结果显示，SSH火山产物保留了复杂的岩浆活动记录。该岩浆活动包括多期安山质浮石爆炸性喷发——此类喷发可能由基性岩浆脉冲触发，后续则发育了低泡玄武岩火山渣喷发以及玄武岩熔岩流。