 | |||
| IODP publications Expeditions Apply to sail Sample requests Site survey data Search | |||
|
doi:10.2204/iodp.sp.340.2011 Scientific objectivesGenerally, the “Lesser Antilles Volcanism and Landslide” project is designed to understand the constructive and destructive processes occurring along island arcs using the Lesser Antilles arc as a prime example. This project involves drilling, coring, and logging along one transect with three sites southeast of Montserrat, one site southwest of Montserrat, one site southwest of Dominica, one site northwest Martinique, as well as one transect with three sites southwest of Martinique (Fig. F1). The record of eruptive activity and volcanoclastic sedimentation obtained during coring and logging will be used to accomplish the following primary goals (three main topics [1–3] and two additional ones [4 and 5]). 1. Identify the mechanisms controlling processes and timing of potentially tsunamigenic, large volcanic debris avalanches emplacement. Volcano flank collapses are an integral part in the lifetime of a volcano (Ida and Voight, 1995; McGuire, 1996; Voight, 2000) and are a large geohazard since they produce large debris avalanches and, in oceanic settings, tsunamis. However, up to now it is generally unclear what factors control the timing of large flank failures, how such failures evolve, and what are the emplacement mechanisms of the debris avalanches associated with these collapses (Voight, 2000; Voight and Elsworth, 1997). For example, understanding whether significant substrate erosion occurs during such processes is crucial for determining the mobility of debris avalanche and for including realistic parameters in numerical simulations of flow processes (Heinrich et al., 2001; Le Friant 2003b; Kelfoun and Druitt, 2005). Deplus et al. (2001) proposed that submarine debris avalanches in the Lesser Antilles erode significantly into underlying sedimentary layers, incorporating large amounts of marine sediment as well as disturbing the underlying stratigraphy. Such erosion and sediment deformation is apparent in some seismic profiles. In addition, the volume of deposits deduced from seismic data (several hundreds of cubic kilometers) is typically one order of magnitude larger than the estimated collapsed volume on land (Le Friant et al., 2003a). Cores will document the internal facies architecture and stratigraphy of debris avalanche deposits and reveal the degree to which given debris avalanche deposit volumes result from erosion and entrainment during emplacement. Identification of subunits within debris avalanche deposits will indicate multiple episodes of emplacement. In addition, deposits of longer run-out turbidity currents generated during debris avalanche emplacement may provide some of the best records of emplacement dynamics (Wynn and Masson, 2003). For instance, large-scale flank-collapse events on the Canary Islands and Hawaiian Islands have generated distinctive turbidites that comprise multiple fining-upward subunits (Wynn and Masson, 2003; Garcia and Meyerhoff Hull, 1994), which suggest that flank collapse occurred in a number of stages separated by days to weeks. Thus, with the cores recovered during this expedition we will investigate whether specific flank collapse events are random in time or if they are linked to some external or internal forcing as well as the controlling mechanisms of debris avalanche emplacement being triggered by such collapses. We will specifically try to answer the following questions:
2. Characterize the eruptive history to assess major volcanic hazards and volcano evolution. It has to be emphasized that our knowledge of volcano history is mainly founded on the shore-based geological record. However, deciphering a complete eruption record from onshore geology is commonly problematic, due to burial by deposits from younger events, erosion, or removal of deposits by catastrophic events such as flank collapses. Marine sediment cores typically preserve a much more complete record of volcanism. However, this improvement from regular piston cores is still not sufficient to characterize the evolution of volcanic systems that can extend to a few million years and is also insufficient to diagnose the return periods of very large magnitude, infrequent but very high consequence volcanic events, such as explosive eruptions and major flank collapses. Drilling will allow us to get a complete eruptive history of a volcano and thus to address several important but yet unanswered questions:
As each of the volcanic islands along the arc has erupted magmas with a distinctive mineralogy and geochemistry (Sigurdsson et al., 1980; Lindsay et al., 2005a), we are sure that the questions raised above can be answered from the material we core since the distinguishing of the sources of the tephra layers in the cores is straightforward. 3. Characterize the magmatic cycles and long-term magmatic evolution of the arc. This third objective shares some common objectives with those aimed at elucidating volcanic history and behavior. Volcanism along the Lesser Antilles arc is characterized by large variability in magmatic activity, magma composition, and eruptive activity in space and time as summarized above. Even though we have acquired an enormous amount of information on this system and also have a great deal of knowledge on magma generation and evolution processes in general, this knowledge has given rise to reams of questions on the different controlling mechanisms of magmatism and eruptive activity in settings like this. Thus, we will use the time-series and spatial records of variations in magma composition (mineralogy, major and trace element composition, and isotopic signatures) and volume to be encountered at the different sites to characterize the processes governing magma composition (composition of the primary material, ascent rates, production rates, and differentiation processes), associated eruption mechanisms, and eruption frequencies. In particular, we will try to answer the following questions.
4. Characterize nondebris avalanche-related sedimentation processes in the deep ocean around the arc. The majority of detrital material resulting from the erosion of the islands of the arc is transported into the surrounding ocean (e.g., Sigurdsson et al., 1980; Le Friant et al., 2004; Picard et al., 2006). Volcanogenic sediments are channeled by debris flows, turbidity currents, and persistent ocean currents through deep submarine canyons located west of the volcanoes and which, for Guadeloupe and Dominica, lead into the northern part of the Grenada Basin (Fig. F1). In addition, around Montserrat there are examples of single, or multiple stacked, carbonate turbidites that contain reworked shallow-water sediment and fauna, the volume of which exceeds that of, for example, volcaniclastic deposits associated with the more recent (<100 ka) eruptions of the Soufrière Hills volcano. The source of these carbonates is most likely the large carbonate platforms associated with islands such as Antigua or Redonda (Fig. F1). Apparently, these turbidites have not been triggered by volcanic eruptions but either by platform instabilities during rapid sea level rises at the end of major glaciations or by major regional earthquakes. This points out that the sedimentation processes occurring along the Lesser Antilles arc might be more complex than previously thought. Thus, with the cores obtained during this expedition we will contribute to the understanding of the sedimentary facies on the submarine flanks and in the basins that surround arc volcanoes, characterize the sedimentation processes, and estimate local sedimentation rates in the northern and southern parts of the arc as well as the relative fraction of volcanogenic material in the sediment. Furthermore we will answer the following questions:
5. Determine the processes and element fluxes associated with submarine alteration of volcanic material. The processes associated with submarine alteration of magmatic matter are of fundamental importance on a global as well as on a regional scale. For example, (1) the composition of ocean water is largely buffered by alteration of magmatic material in the ocean basins, (2) the composition of the Earth’s mantle is influenced by the subduction of altered oceanic crust and seamounts, and (3) major elements, trace elements, or isotopes are used to model the magmatic history of volcanic settings, requiring knowledge about which of the geochemical patterns encountered are of primary magmatic origin and which are not (Palmer and Edmond, 1989; Palmer et al., submitted). Nonetheless, systematic studies of natural alteration processes reflecting the diversity of magmatic systems on our Earth are generally rare (e.g., Gardner et al., 1986; Gérard and Person, 1994; Martin, 1994; Stroncik and Schmincke, 2001; Utzmann et al., 2002). Generally, submarine alteration processes (including, e.g., element fluxes and alteration rates) are controlled by the following parameters: (a) the structure, composition (e.g., glassy versus crystalline, microfracture density, and basaltic versus silicic), and grain size of the parent material, (b) the physical emplacement mechanism and resulting internal structure of the deposit (e.g., thin air fall deposits versus thick debris flows), and (c) temperature. Continued coring and logging at the proposed sites will allow us to systematically study alteration processes as a function of those different parameters in a magmatically relatively diverse system and will allow us to answer the following questions:
|
||
