Abstract
Geomechanical restoration methods are dependent on boundary conditions to ensure geological consistency of the restored model in terms of geometry and strain. Classical restoration boundary conditions, such as flattening a datum horizon, may lead to inconsistent displacement and strain fields. We restore a laboratory structural sandbox model with known deformation history to develop guidelines for definition of boundary conditions that produce improved results from geomechanical restorations. The sandbox model has a basal silicone layer, includes synkinematic deposition, and is characterized by structures analogous to those found in suprasalt extensional environments. The deformed geometry is interpreted from three-dimensional tomography imaging, and a time series of cross section tomography images provides a benchmark to quantify restoration error and inform boundary conditions. We confirm that imposing a lateral displacement equal and opposite to far-field tectonic shortening or extension provides a more accurate restoration. However, the amount of displacement may not be known in real cases. We therefore test several established methods, using only the unrestored geometries, to assess the amount of shortening that should be used to guide geomechanical restorations. An accurate estimation is provided by the area–depth method and potentially by a dilatation analysis. Additionally, novel fault-compliance boundary conditions produce improved results in the vicinity of crossing and branching faults. Application of similar methods should produce improved restoration of natural geologic structures.
Highlights
Et al (2012) show a simple example in which classical boundary conditions applied to a geomechanical restoration lead to unphysical strain elds and that a dierent set of boundary conditions signicantly changes the resultant strain eld (see Figure 1 in Lovely et al (2012))
We describe the structural sandbox model and our tests of boundary conditions with the goal of restoring deformed geometries and related fault slip that are consistent with the reference model paleogeometries
A shortening boundary condition was applied to obtain a good t with reference paleo-geometries
Summary
Structural restoration is a valuable tool to investigate the geometries of geological structures through time, assess the validity of structural interpretations, and analyze strain elds (e.g., Chamberlin, 1910; Dahlstrom, 1969; Gratier et al, 1991; Léger et al, 1997; Williams et al, 1997; Rouby et al, 2002; Griths et al, 2002; Dunbar and Cook, 2003; Muron, 2005; Groshong, 2006; Maerten and Maerten, 2006; Moretti, 2008; DurandRiard et al, 2010, 2013b; Maerten and Maerten, 2015; Vidal-Royo et al, 2015; Stockmeyer et al, 2017). Durand-Riard (2010); Lovely et al (2012) and Durand-Riard et al (2013b) suggest that these classical boundary conditions may be insucient to restore geologically consistent and physical strain They show on synthetic models that a lateral displacement boundary condition along a boundary wall is necessary to recover the expected strain in compressive, extensional, and strike-slip and oblique-slip contexts. The main challenge for dening these additional constraints is that they require knowledge of the deformation history, which is rarely accessible and, ideally, should be an output of the mechanics-based restoration These studies of boundary conditions were applied to numerical or synthetic models, which are typically idealizations of natural geologic structures, and present additional uncertainties and assumptions (structural interpretation, deformation path, etc.). We propose methods to dene lateral displacement boundary conditions without detailed knowledge of the forward deformation path, improving the viability of the 3D geomechanical restoration method for use with natural geologic structures
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