Quantifying dyke-induced graben and dyke structure using 3D seismic reflection data

Abstract

During dyke intrusion, tensile stresses concentrated within the overlying rock may lead to the formation of normal faults. These faults typically form graben-bounding pairs that are sub-parallel to, and dip toward, the upper tip of their underlying dyke. Many studies use geometric properties extracted from the surface expression of such dyke-induced faults to estimate the geometry of subsurface dykes. These methods assume dyke-induced faults are planar and nucleate at the surface. However, recent investigations of the 3D structure of dyke-induced faults using seismic reflection data confirm they can be non-planar and have complex growth histories. Here, we use 3D seismic reflection surveys from offshore NW Australia to: (1) examine how the surface expression of dyke-induced faults relates to subsurface dyke geometry and depth; and (2) test whether subjective bias may influence the quantitative analyses of dyke-induced faults. We show displacement and dip vary across dyke-induced faults, supporting previous suggestions that faults nucleate between dyke upper tips and the free surface. We also find that depths of dyke upper tips predicted using graben width and area of loss measurements, whilst sensitive to fault dip variations and interpretation biases, are often similar to measured dyke depths. Both measured and predicted dyke depths vary by several hundred metres along-strike, which we relate to the preservation of dyke heads, segmentation, and/or magma density changes. Overall, we show reflection seismology provides a better understanding of the 3D structure of dyke-induced faults and their relationship to the geometry and emplacement dynamics of their causal dykes.

Quantifying fault interpretation uncertainties and their impact on fault seal and seismic hazard analysis

“…Analysis of fault cut-offs suggests that areas of low image quality are associated with large uncertainty, leading to increased uncertainty with depth (Alcalde et al, 2017b;Schaaf and Bond, 2019). Moreover, cut-offs on faults with low displacement near the limit of separability (Magee et al, 2023) and the hanging wall cut-off of large displacement faults, which are deeper and due to additional accommodation space often show changes in seimic stratigraphy compared to the footwall (Alcalde et al, 2017b), are prone to higher uncertainties. Continuous cut-offs require the regional dip of the horizon to be projected onto the fault plane (Figure 1biv).…”

“…Continuous cut-offs require the regional dip of the horizon to be projected onto the fault plane (Figure 1biv). In cases of small-displacement faults where continuous deformation comprises a significant portion of the displacement, the interpreter must choose where the fault intersects the deflected horizon (Faleide et al, 2021;Magee et al, 2023). This introduces uncertainty as there are multiple feasible locations for projecting the horizon onto the fault plane, and the position of the fault plane itself becomes more uncertain (Fig 1b).…”

“…However, limited attention has been given to the consistency of an individual's interpretation. Magee et al (2023) conducted a study where an individual made repeat picks on the same horizon of a low-displacement fault, revealing variations in fault cut-off positions that affected the extraction of throw and heave. Nevertheless, the datasets were found to be statistically equivalent and exhibited lower uncertainty compared to another interpreter's interpretation of the same horizon.…”

“…Repeatability, which is the ability to replicate the data and interpretations of a study, is recognised as a crucial aspect of any experiment (e.g., Goodman, 2016). Geology, in particular, is susceptible to subjective uncertainty due to incomplete datasets and the lack of consensus within the research community regarding key concepts and research methods (Frodeman, 1995;Bond, 2015;Pérez-Díaz et al, 2020;Steventon et al, 2022;Magee et al, 2023;Robledo Carvajal et al, 2023). For seismic reflection datasets, subjective uncertainties can lead to multiple interpretations being drawn from the same seismic image (e.g., Bond et al, 2007;Alcalde et al, 2017).…”

“…For seismic reflection datasets, subjective uncertainties can lead to multiple interpretations being drawn from the same seismic image (e.g., Bond et al, 2007;Alcalde et al, 2017). Previous work has suggested that fault properties extracted from seismic reflection data should have an error associated with then of between ±5% (Magee and Jackson, 2020a) and ±10% (Magee et al, 2023), however, no parametric studies have been undertaken to date to test these essentially qualitative values.…”