Читать книгу Continental Rifted Margins 1 - Gwenn Peron-Pinvidic - Страница 44

Magmatism contributes to rift evolution and final margin architecture to various degrees, and depends on various parameters, such as extension rate and different characteristics of the lithosphere and asthenosphere (White and McKenzie 1989; White 1992). A series of typical volcanic geometries often observed, interpreted and/or modeled in rifted margins are summarized below and exemplified in Figure 1.35. These are traditionally distinguished into three main types: intrusives, extrusives and underplates.

Intrusive magmatic features refer to magmatic rocks emplaced in rocks that were previously in place. Depending on the emplacement geometry relative to the previous rocks, these intrusions can be concordant or discordant. Sills are tabular intrusions, emplaced between older layers, in a concordant intrusive sheet, while dikes are defined as discordant intrusive sheets that will crosscut older rocks. Sills are commonly fed by dikes. Additional intrusive magmatic bodies include the following: pluton, which refers to large intrusive bodies; batholiths, which are intrusive complexes of several magmatic bodies of large dimension (typically several plutons) and laccoliths, which are concordant plutons and form when the magmatic rocks are intruded between rock layers with high pressure, forcing an upward doming/folding of the overlying strata and giving rise to an overall mushroom-like form with a generally planar base.

Extrusive magmatic features are magmatic rocks emplaced above the Earth’s surface, at the seafloor or at the surface. In rifted margins, these include a long series of possible features such as lava flows, lava deltas and volcanoes. Secondary geometries can also be generated, such as mass wasting events and volcanic-derived sediments. Their seismic reflection facies are often very well defined with typically high-amplitude reflectors; however, the conditions leading to their emplacement are still very much debated.

Underplates refer to rocks that are emplaced as uniform bodies at the base of pre-existing rocks – usually below the lower crust in extended rifted margin settings. Field outcrops and deep-sea drilling observation prove the existence of underplated magmatic bodies. In modern rifted margins, however, these structures cannot be directly observed. Their presence has been proposed based on the interpretation of geophysical models, making them indirect observations. HVLC and LCB are widely used acronyms that refer to these particular bodies. HVLC is an abbreviation for “high velocity lower crust” and “LCB” for “lower crustal body” (see Chapter 2 for further explanation and discussion around these terms).

Given the wide range of geometries and contexts of emplacement, the study of magmatic features in rifts and rifted margins is a specialized research topic: seismic volcanostratigraphy, a concept inherited from Mitchum et al. (1977) and Symonds et al. (1998). Seismic volcanostratigraphy uses seismostratigraphy and sequence stratigraphy concepts to identify and map magmatic sequences, based on their seismic facies. The study of seismic reflection geometries associated with volcanic successions is used to constrain the emplacement context, depositional environment and potential subsequent remobilization and transport. For many decades, these geometries remained unclear and unconstrained due to seismic imaging issues. However, the recent advances in seismic processing have greatly improved imaging and resolution. Some deep-sea drilling campaigns that are specifically dedicated to the sampling of the volcanic successions of the NE Atlantic rift have been able to place crucial constraints on their lithologies and ages (DSDP Leg 38, 104 and 152, 163; Eldholm et al. 1987; Saunders et al. 1998; Larsen et al. 1999). These modern advances allow for highly detailed studies of the temporal and spatial evolution of the magmatic activity of the rifted margin, notably in terms of paleogeography and morphology at breakup time.

We briefly list below the major magmatic geometries often encountered in distal rifted margin settings (Figure 1.35). We will follow the classification built on the West Indian margin case (Calvès et al. 2011) and descriptions of the Voring mid-Norwegian margin (Abdelmalak et al. 2016). Both are based on the early descriptions made by Symonds et al. (1998) and Planke et al. (2000):

– SDRs (seaward dipping reflectors): these are a specific feature identified on seismic reflection profiles by a wedge-shape/fan-like geometry, opening and dipping oceanward with medium- to high-amplitude seismic reflectors (Mutter et al. 1982). The top continentward termination has been drilled (e.g. in the NE Atlantic; Eldholm et al. 1989) and the related strong reflectors have been proven to correspond to subaerially-erupted flood basalts. Thus, when identified in distal rifted margin settings, SDR-like seismic geometries are often interpreted as fully basaltic structures. However, no constraints exist on their internal lithologies, notably the ratio between sediments and magma. Based on their position, the packages are often distinguished into inner SDRs and outer SDRs, depending on whether they are located landward or oceanward from the outer high, respectively:

 - The outer high is a 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).