Accommodation of Penetrative Strain During Deformation Above a Ductile Décollement

Abstract

The accommodation of shortening by penetrative strain is widely considered as an important process during contraction, but the distribution and magnitude of penetrative strain in a contractional system with a ductile décollement are not well understood. Penetrative strain constitutes the proportion of the total shortening across an orogen that is not accommodated by the development of macroscale structures, such as folds and thrusts. In order to create a framework for understanding penetrative strain in a brittle system above a ductile décollement, eight analog models, each with the same initial configuration, were shortened to different amounts in a deformation apparatus. Models consisted of a silicon polymer base layer overlain by three fine-grained sand layers. A grid was imprinted on the surface to track penetrative strain during shortening. As the model was shortened, a series of box fold structures developed, with a zone of penetrative strain in the foreland. Penetrative strain in the foreland decreases away from the fold belt. Restoration of the model layers to the horizontal indicates that penetrative strain accounts for 90.5%-30.8% of total shortening in a brittle system with a ductile décollement, compared to 45.2%-3.6% within a totally brittle system. Analog model geometries were consistent with the deformation styles observed in salt-floored systems, such as the Swiss Jura. Penetrative strain has not been accounted for in previous studies of salt-floored regions and estimates of this type could help resolve concerns of missing shortening highlighted by global positioning system data.

Determining finite strain: how far have we progressed?

“…Compaction and stratigraphic thickening due to deformation can be estimated and incorporated in the construction of balanced cross sections (Woodward et al, 1986;Protzman and Mitra, 1990;Mitra, 1994). Despite Layer Parallel Shortening (LPS) or internal deformation (compaction, collapse of pore space, dissolution or cleavage formation) being shown to accommodate significant shortening in balanced cross sections from carbonate duplexes (27%; Cooper et al, 1983), gravity driven thrust systems (18-25%; Butler and Paton, 2010), and analogue models (15-30%; Koyi et al, 2004;Burberry, 2015;Lathrop and Burberry, 2017), strain analysis techniques are rarely applied to balancing cross sections. Hobbs and Talbot (1966) highlighted a few of the limitations of strain analysis a year before Ramsay published his seminal text, and the majority of these seem to prevail today: the initial shapes of many strain markers cannot be measured accurately enough to yield highly accurate estimates; and homogeneous strain is typically assumed.…”

The influence of detachment strength on the evolving deformational energy budget of physical accretionary prisms

“…Recent analytical analyses indicate that the solutions governing the stability and critical taper of noncohesive accretionary wedges do not differ in subaerial (dry) and submarine (normally pressured) conditions (Lehner and Schöpfer, 2018), suggesting that insights from dry physical accretion experiments are applicable to submarine, crustal accretionary prisms. Physical accretion experiments reveal distributed layer-parallel shortening associated with detachment slip prior to the localization of slip along thrust faults (e.g., Mulugeta and Koyi, 1992;Koyi, 1995;Burberry, 2015;Lathrop and Burberry, 2017;McBeck et al, 2017). In drysand accretionary wedges, inclusion of a ductile detachment greatly increases the overall distributed internal strain associated with folding and thrusting (Lathrop and Burberry, 2017).…”

“…Physical accretion experiments reveal distributed layer-parallel shortening associated with detachment slip prior to the localization of slip along thrust faults (e.g., Mulugeta and Koyi, 1992;Koyi, 1995;Burberry, 2015;Lathrop and Burberry, 2017;McBeck et al, 2017). In drysand accretionary wedges, inclusion of a ductile detachment greatly increases the overall distributed internal strain associated with folding and thrusting (Lathrop and Burberry, 2017). Incremental displacement fields of the sides of wedges indicate that short-lived shear bands episodically develop until strain localizes onto a single frontal forethrust (e.g., Bernard et al, 2007;Dotare et al, 2016).…”

“…In accretionary prisms, margin-perpendicular convergence produces distributed layer-parallel compaction ahead of the wedge until strain localizes as slip along discrete frontal thrust faults (e.g., Koyi, 1995;Burberry, 2015;Ghisetti et al, 2016;Lathrop and Burberry, 2017). The frictional strength of the basal detachment and rheology of the wedge control the internal deformation throughout the wedge prior to thrust fault development.…”

Comparison of fold-thrust belts driven by plate convergence and gravitational failure

“…The measured 8.1-14 km 2 extensional area from the three horizons is much larger than that of the contractional area, 2.8-9.5 km 2 . This difference is attributed to layer-parallel shortening, lateral compaction, and ductile deformation (Sans et al, 2003;Koyi et al, 2004;Butler and Paton, 2010;Şengör and Bozkurt, 2013;Lathrop and Burberry, 2017). The strain across the extensional and contractional domains is also measured as the difference between restored and current bed length for each selected horizon (Fig.…”