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.

Critical Wedges and Penetrative Strain: How Does Penetrative Strain Alter the Concept of a Critical Wedge?

“…This rapid decrease in surface angle, up to 10° for several models, occurred over a 1% to The penetrative strain sequences that were calculated for the 1:2 ratio, 1:3 ratio, and 1:4 ratio models, shown in Table 2 and Figure 7a, each had a distinctively different trend. These trends are compared to the penetrative strain sequences found in the 1:3 silicon to sand ratio experiments performed in Lathrop and Burberry (2017) and the fully sand experiments performed in Burberry (2015). It is apparent that the 1:2 ratio calculated penetrative strain sequence is closer to the ductile scenario that Lathrop and Burberry (2017) calculated (Figure 7b).…”

“…These trends are compared to the penetrative strain sequences found in the 1:3 silicon to sand ratio experiments performed in Lathrop and Burberry (2017) and the fully sand experiments performed in Burberry (2015). It is apparent that the 1:2 ratio calculated penetrative strain sequence is closer to the ductile scenario that Lathrop and Burberry (2017) calculated (Figure 7b). The 1:2 ratio model setup is unstable due to its high proportion of silicon (salt) this is observed through its extremely high calculated percent penetrative strain start values (71.3% and 73.3%) and the rapid decrease to 35.8% calculated percent penetrative strain at 11% total shortening.…”

“…Penetrative strain in the model series. a) average penetrative strain in the three model series from this study, with (b) figure 10 fromLathrop and Burberry (2017) reproduced for comparison.…”

Response to Revisions from A. Bauville

“…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).…”

“…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.…”