| 07 Oct 2025
Mineralogic controls on fault displacement-height relationships
Abstract. Understanding the distribution and geometry of subsurface faults is critical for predicting fault penetration and associated leakage of fluids such as groundwater, hydrocarbons, and injected anthropogenic waste through sealing intervals. Fault dimensions are often underestimated due to the resolution limits of seismic reflection data, which only image portions of faults with sufficient displacement to offset seismic reflectors. To address this fault underestimation problem, we quantify relationships between host rock composition and fault displacement gradients using a well-exposed outcrop of normal faults in mechanically layered sedimentary rocks in the footwall to the west branch of the Moab Fault, Utah. We integrate high-resolution digital photogrammetry, structural mapping, X-ray diffraction (XRD) mineralogy, and Schmidt rebound measurements to analyze how mineralogy and mechanical properties influence fault displacement vs. height relationships. Our results indicate that normal fault displacement gradients tend to be higher in less competent beds and lower in more competent strata, and that fault displacement gradient is positively correlated with clay content and negatively correlated with strong minerals (e.g., quartz, feldspar, dolomite). Outcrop-derived relationships are used to build a predictive framework that uses fault displacement and mineralogy to predict fault height. We apply this framework to a worked seismic interpretation example and demonstrate that fault dimensions are likely substantially underestimated in conservative seismic interpretations. Our results highlight the importance of mechanical stratigraphy in controlling fault geometry and provide a data-driven approach for estimating sub-seismic fault dimensions, with implications for reservoir characterization, fluid containment, and geohazard assessment.
Dear Editor,
Dear Authors,
I have reviewed the manuscript entitled “Mineralogic controls on fault displacement-height relationships” submitted to Solid Earth by Cawood and co-Authors.
The paper presents a methodology for predicting fault dimensions (specifically, fault height) from mineralogical (XRD) and mechanical (Schmidt Hammer) data in siliciclastic rocks. The study integrates structural and mineralogical datasets collected along a well-known cross-section of the west branch of the Moab Fault to develop statistical relationships between fault displacement gradient and XRD mineralogy. The Authors further apply the outcrop-derived relationship to a seismic interpretation of a kilometre-long section affected by normal faults from offshore Newfoundland.
I recommend minor revision. Some issues need to be addressed before manuscript publication in Solid Earth. I outline major and minor comments below.
Major comments:
To understand the paper, it is fundamental the reader understands how the fault dimension parameters (i.e. displacement gradient and fault tip distance) are calculated. Although the (simple) calculations are clearly presented in the Methods section, I recommend including a graphical explanation of these parameters and equations by adding a second column to Figure 3. In this expanded figure, the Authors could illustrate an example of calculation of displacement gradient between two measurement points and an example of T_dist calculation. For T_dist, it is important to graphically show that the distance is calculated from the point where the measured displacement is maximum and that the total height corresponds to 2*T_dist. In this new figure, adjustment factors used in the seismic profile (lines 355-365, Table 2) could also be included.
In my view, a limitation of the paper is that 9 faults out of 15 lack observable tips in the outcrop. Although the selection criteria stated by the Authors are reasonable (e.g., exclusion of smaller faults from the displacement analysis), the absence of exposed tips does not allow to verify the proposed relationship between fault tip distance and total clay content (Fig. 10a) undermining the applicability of the proposed method. The application of the prediction method to the seismic reflection profile is interesting, but subsurface can not replace outcrop validation.
In Figures 10a and 10c (core business of the manuscript), the displacement gradients exhibit a wide range of values for any given mineralogical composition (e.g., for ~25% total clay, displacement gradients span 0.01–0.4), resulting in very weak correlations. The Authors have used median displacement gradients to improve correlations and reduce the influence of outliers and local heterogeneity (line 275-276). However, the scatter appears to reflect more than just heterogeneities or outliers and may instead relate to additional factors influencing fault propagation in mechanically layered media. These factors are briefly listed in the Discussion (lines 411–422), but I recommend expanding this section in light of the presented data.
Conversely, the first part of section 5.3 of the Discussion (lines 431-449) can be shortened or removed since it adds little to the manuscript in its current form.
In the final part of the Discussion (lines 423-429), it is important the Authors more fully address the implications of assuming a uniform clay content throughout the entire stratigraphic sequence in the seismic profile.
Minor comments: