Understanding the nature of continent-ocean transition zones along continental margins and classifying them is key to building robust plate models. Together these can lead to a better understanding of the evolution of sedimentary basins and depositional systems along continental margins.
One tool to help illustrate margin structure is to look at analog geophysical signatures of ‘typical’ margin types. In this article we investigate the character of the continent-ocean transition zone in potential field data at a number of Atlantic passive margins. We also consider whether common elements are observed in equivalent environments.
Margin types are classified in general terms as magma-rich or magma-poor (see 'Terminology Explained: Margin Classification', below); in reality, a spectrum of variations exists between these end-members. Nevertheless, with appropriate independent support, analog examples can significantly support this classification.
To look at continental margin classification on a broad scale and to allow a sufficient pool of comparison material requires excellent, internally consistent data coverage. The gravity and magnetic datasets developed at Getech enable the problem to be addressed in a truly global perspective. The company’s continental margins’ gravity data are derived from the latest, post-2010 satellite altimetry missions, including multiple data collection cycles from the Cryosat-2 and AltiKa satellites.
Together with enhanced processing techniques, this has enabled the production of a high-quality, self-consistent dataset at a previously unachievable resolution. The consistent source and resolution of the data are vital features that enable the interpreters to have confidence that their observations are directly related to geology, as opposed to variations in survey specification and processing parameters. Terrestrial magnetic data are patchier in coverage, but the reduction to common processing parameters and nominal reference elevation increases their usability in this context.
Margin Classification Examples
The following examples describe a number of Atlantic passive margin locations that illustrate the nature of 'end-member' margin styles within the Getech classification system.
Iberian Continental Margin
The first example is from the Iberian Atlantic margin, which we consider to be a magma-poor margin. Figure 1 shows examples from the vicinity of the Tagus Abyssal Plain, offshore Portugal. To highlight the textural character, the analytic signal (also known as total derivative) of the gridded gravity and magnetic data is presented. Figure 1a shows that the magnetic data are relatively subdued (low amplitude) over the transitional domain. The gravity data (Figure 1b) are also relatively subdued, with the exception of two areas that show high-amplitude responses that are associated with the presence of the Gorringe Bank (an uplifted and tilted block of exhumed mantle at the boundary between the Eurasian and African Plates) and the Estremadura Spur (which contains pre-rift volcanics). In the vicinity of the Tagus Abyssal Plain, the character and amplitudes in potential field data are also subdued.
We modeled the crustal structure (Figure 2) of the margin with several 2D gravity and magnetic models (supported by seismic observations where possible), and consistently observed the characteristically subdued magnetic and gravity response (Figure 3). It is suggested that this character supports the interpretation of a magma-poor transitional margin, with the hypothesis of exhumed mantle flooring the basin being reinforced by the low-amplitude magnetic and gravity responses.
West Ireland/UK Continental Margin
The second example is from the West of Ireland/ UK Atlantic margin. We interpret this as a magma-rich continental margin, as evidenced by both the dense crust that was formed by the intrusion and addition of igneous material, and the seismic interpretation of the seaward-dipping reflector sequences indicative of tilted volcaniclastic flows. To highlight the margin’s textural character, the analytic signal of the gravity and magnetic maps is again shown. Figure 4a indicates that the magnetic data of the western Rockall margin display significantly higher amplitude in the transitional domain than the magnetic data observed in the Tagus Abyssal Plain; the gravity data (Figure 4b) are of similar nature.
It is important to be aware that the characteristic signature is not solely due to the magma-rich crust associated with break-up as pervasive Cenozoic basalt flows also contribute to the observed signal. A representative 2D model from the margin highlights the high-amplitude, high-frequency character that we consider diagnostic of this type of margin (Figure 6).
Figure 7 (below) shows the same derivatives of potential field data applied to both the southern West African continental margin in the vicinity of the Orange Basin and Figure 8 (opposite) shows a corresponding example from its conjugate margin in offshore Argentina.
In these examples, we see high-amplitude responses to the north of the Cape-Colorado Fracture Zone, with generally smoother, lower-amplitude responses to the south, between the Cape-Colorado and Agulhas Fracture Zones. Extending the analog approach based on the observations from Iberia and Rockall leads us to an initial interpretation of a volcanic, magma-rich margin being present to the north of the Cape-Colorado Fracture Zone. The margin then sharply transitions to a magma-poor setting to the south of this fracture zone.
This interpretation requires further testing with 2D modeling and ground truthing against independent data.
Consistent Data and Good Coverage
Defining crustal types based solely on the character of potential field data is ambitious, and an overarching typical character is unlikely to be consistently observed globally; however, when ground truthed against independent observations, these data allow the users to extend their initial interpretations along the margins to develop crustal architecture models. Key to this process is the use of self-consistent gravity and magnetic datasets with good regional coverage.
Some elements that are consistent with magma-rich and magma-poor transitional domains are clearly observed, particularly in the more direct crustal response of the magnetic data. Additionally, the gravity data can also be used for these interpretations owing to both the bulk density contrast between intruded and unintruded crustal blocks and the long-wavelength response generated by an elevated Moho at hyperextended margins.
Initial observations and geological models should be further tested by detailed 2D gravity and magnetic modeling; ideally, this modeling should be constrained by seismic data and ‘pegged’ at both ends in the margin-bounding oceanic and undeformed continental crust domains.