mound-like feature located between the inner and outer SDR packages. It is usually characterized by high-amplitude top reflectors and a rather chaotic internal reflection pattern.
– Lava flows: these are traditionally categorized into subaerial and submarine (hyaloclastite). They can also be distinguished as inner and landward flows: the inner flows are structurally located below the landward flows and lava delta and the landward flows are located landward from the inner SDRs. These are the flows that supply lava to the lava delta (Figure 1.35): the landward flows are subaerial, while the lava delta is subaqueous, marking the limit from where the lava enters water.
– The escarpment is a structural cliff observed along some distal margins. It is related to depositional/erosional processes, marking the transition from the subaerial to the submarine environment. The escarpment is interpreted to record the shoreline at the time of delta progradation (Wright et al. 2012). It is thus an important geometry that can give crucial information on the paleogeography of the margin and its topographic evolution. Structurally, when the lava enters water, it undergoes fragmentation into hyaloclastic beccias that can be transported downslope by gravitational processes to form the overall progradational foreset pattern with the escarpment.
– The lava delta corresponds to the structure built by the progradation of the lava flows outboard. The delta grows by the addition and stacking of new flows and hyaloclastite debris/breccias, resulting in the progradation of the shoreline oceanwards. The thickness of the delta may be used as a parameter to estimate the paleo-water depth, as it gives insights into the accommodation space available at the time of deposition.
Additional features can be defined and mapped, such as hyaloclastic mounds, various types of sills (e.g. saucer-shape), plugs, plutons, vents, dikes and volcanoes, volcanic-derived sediments, slumps and mass wasting. For more information, the reader is referred to the recommended publications below (on p. 58).
The source of the magma is very poorly understood, both locally and regionally. Locally, it is often proposed that dikes (called “feeder dikes”) bring the magma to the sills and lava flows to build the various magmatic features. However, these dikes can be near-vertical structures and thus are extremely difficult to image and identify on seismic reflection data. Additionally, it has been proven that magma can be transported over very significant distances (hundreds to thousands of km) within widespread sill complexes, promoting the development of magmatic features that do not overlie the melt source (Magee et al. 2016). Therefore, the source location can be a great distance from the observed structures, adding huge uncertainties on the identification and characterization of the physical sources.
Figure 1.35a. Summary of the main geometries related to magmatic activity encountered in rifts and rifted margin studies. The figure is from Calvès et al. (2011), based on the study of the West Indian margin, and adapted from Symonds et al. (1998). Schematic volcanic margin transect at the top panel illustrating seismic facies units associated with extrusive volcanic deposits
Figure 1.35b. Seismic facies chart, morphometric data and interpretation of the volcanostratigraphic elements identified on the West Indian margin (source: from Calvès et al. 2011)
Further reading.– The above descriptions are abbreviated and often simplified. If interested in reading and learning further, the reader is referred to the following list of publications and references:
– General: (Mitchum et al. 1977; White et al. 1987; White and McKenzie 1989; Eldholm 1991; Saunders et al. 1997; Symonds et al. 1998; Planke et al. 2000; Calvés et al. 2011; Magee et al. 2016).
1.6. References
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