Growth of Fault‐Cored Anticlines by Flexural Slip Folding: Analysis by Boundary Element Modeling

Abstract

Fault‐related folds develop due to a combination of slip on the associated fault and distributed deformation off the fault. Under conditions that are sufficient for sedimentary layering to act as a stack of mechanical layers with contact slip, buckling can dramatically amplify the folding process. We develop boundary element models of fault‐related folding of viscoelastic layers embedded with a reverse fault to examine the influence of such layering on fold growth. The strength of bedding contacts, the thickness and stiffness of layering, and fault geometry all contribute significantly to the resulting fold form. Frictional contact strength between layers controls the degree of localization of slip within fold limbs; high contact friction in relatively thin bedding tends to localize bedding slip within narrow kink bands on fold limbs, and low contact friction tends to produce widespread bedding slip and concentric fold form. Straight ramp faults tend to produce symmetric folds, whereas listric faults tend to produce asymmetric folds with short forelimbs and longer backlimbs. Fault‐related buckle folds grow exponentially with time under steady loading rates. At early stages of folding, fold growth is largely attributed to slip on the fault, but as the fold increases amplitude, a larger portion of the fold growth is attributed to distributed slip across bedding contacts on the limbs of the fold. An important implication for geologic and earthquake studies is that not all surface deformation associated with blind reverse faults may be attributed to slip on the fault during earthquakes.

Post‐Earthquake Fold Growth Imaged in the Qaidam Basin, China, With Interferometric Synthetic Aperture Radar

“…In the two examples, co‐seismic slip is translated into permanent deformation during the post‐seismic phase through localized signals that match young anticlines observed in the morphology, and that cannot be simply explained by slip along faults driving elastic deformation of the medium. The results demonstrate, along with other observations and models (e.g., Ainscoe et al., 2017; Bonanno et al., 2017; Johnson, 2018), that it not always valid to assume that fold growth above blind reverse faults is co‐seismic or that it is appropriate to model it by slip on a fault, because a significant part of the finite shortening may occur as distributed off‐fault deformation (flexural slip, interbed flow, shearing) during the post‐seismic or inter‐seismic phases. The study highlights that the contribution of distributed deformation and slip‐partitioning between secondary structures needs to be considered in earthquake cycle models, analyses of fault‐related folds, and the assessments of earthquake hazard from surface measurements.…”

“…However, the importance of fold buckling and distributed strain to the component of shortening and fold growth is not clear (e.g., Ainscoe et al., 2017; Gonzalez‐Mieres & Suppe, 2006; Veloza et al., 2015; Yonkee & Weil, 2010). In other words, it is also recognized in the literature that there is not a simple relationship between slip and fold growth (e.g., Bonanno et al., 2017; Huang & Johnson, 2016; Johnson, 2018).…”

Geodetic evidence for shallow creep along the Quito fault, Ecuador

“…Despite some variability among modelling approaches, models of fold evolution usually predict that the width of the fold roughly scales with the depth of the root of the underlying causative fault (e.g. Savage & Cooke 2003;Bernard et al 2007;Johnson 2018). If true, for the Quito fault, the characteristic width of the different segments is about 7 km, a value also consistent with our modelling results and the seismicity distribution with depth.…”

A Unified Framework for Earthquake Sequences and the Growth of Geological Structure in Fold‐Thrust Belts

“…We first use a Boundary Element Method (BEM) framework to model a ramp-décollement system in an elasto-plastic medium, incorporating the anisotropy of sedimentary layers MALLICK ET AL. (Huang & Johnson, 2016;K. M. Johnson, 2018).…”