Rotational Extension Promotes Coeval Upper Crustal Brittle Faulting and Deep‐Seated Rift‐Axis Parallel Flow: Dynamic Coupling Processes Inferred From Analog Model Experiments

Abstract

During continental rifting, extension in the upper parts of the crust is accommodated by brittle faulting. If stretching is distributed evenly throughout the crust (McKenzie, 1978), faulting in the upper crust is balanced by ductile deformation in the lower crust (e.g., Buck, 1991;Kusznir & Karner, 2007). However, at passive margins, subsidence analyzed from geological cross sections differs from expected values when applying the McKenzie uniform stretching model. Such observations led to the development of depth-dependent stretching models (Wernicke, 1985) that account for higher amounts of extension in lower parts of the crust, which permit larger subsidence values. More recent studies of passive margins have suggested ductile crustal flow as an alternative mechanism for removal of lower crustal material. Consequently, mass is not conserved in 2D plane-strain cross sections, which can explain discrepancies between estimated amounts of extension in the upper-and lower crust (P. D. Clift, 2015).Ductile flow in the lower crust should also occur in intracontinental rift settings. Indeed, lower crustal flow is expected in extensional settings where the crustal thickness exceeds 20 km and the upper mantle is strong (McKenzie et al., 2000 and references therein). This phenomenon has been extensively demonstrated in the Basin and Range Province, where flow of the lower crust toward the rift center compensates lateral crustal thickness

Analog Models of Lithospheric‐Scale Rifting Monitored in an X‐Ray CT Scanner

“…The result is a detailed and quantified analysis of surface model deformation over time, which we use in particular for generating incremental and cumulative maximum normal strain (MNS) data. These maximum normal strain data represent the deformation of the longest axis of the strain ellipse, and serves as a proxy for extensional deformation (Schmid et al, 2022a(Schmid et al, , 2022bZwaan et al, 2021;Zwaan, Chenin, et al, 2022).…”

“…Especially the quantification of deformation over time in these models by means of DIC analysis on CT imagery (Figure 8) allows for direct comparisons with numerical modeling studies. Additional steps could involve the expansion of our 2D DIC analysis into the third dimension (so-called digital volume correlation or DVC analysis applied on CT volumes, Adam et al, 2013;Poppe et al, 2019;Schmid et al, 2022aSchmid et al, , 2022bZwaan, Schreurs, & Adam, 2018) to fully quantify the evolution of internal model deformation in 3D. However, the resolution (512 × 512 pixels) of standard medical CT scanners poses a limitation here.…”

“…Next to 3D photography and the quantitative surface analyses it enables, the key innovation in terms of model monitoring and analysis in our study is the application of X-ray CT-scanning techniques (e.g., Colletta et al, 1991;Holland et al, 2011;Sassi et al, 1993;Schmid et al, 2022aSchmid et al, , 2022bSchreurs et al, 2003;Zwaan, Schreurs, & Adam, 2018, Figure 2b), allowing for the first time to directly trace the internal 3D structural evolution of this type of models. The X-ray CT-scanning of Models A, C and D is done in the 64 slice Siemens Somatom Definition AS apparatus at the Institute of Forensic Medicine of the University of Bern.…”

“…Another contrast to Model A is the insertion of a weakness or "seed," a rod of viscous material at the base of the brittle upper lithospheric mantle layer that runs along the whole length of the model, to localize deformation. Such seeds have been used in various previous modeling studies to simulate linear weaknesses such as crustal shear zones (e.g., Le Calvez & Vendeville, 2002;Osagiede et al, 2021;Schmid et al, 2022aSchmid et al, , 2022bZwaan et al, 2016Zwaan et al, , 2021Zwaan, Chenin, et al, 2022), but we apply them in the modeled upper lithospheric mantle instead (Figure 1c). Furthermore, Model B undergoes a total of 40 mm of divergence over a 4-hr period, where Model C, which is a rerun of Model B in the CT scanner, undergoes and extra hour of deformation for a total of 50 mm of divergence.…”

“…So far the only reliable non-destructive method to truly peek into otherwise opaque analog models materials during an experimental run is X-ray CT-scanning (e.g., Colletta et al, 1991, Sassi et al, 1993Schmid et al, 2022aSchmid et al, , 2022bSchreurs et al, 2003;Holland et al, 2011;Zwaan, Schreurs, & Adam, 2018). This technique, which basically relies on the visualization of density differences within the model materials (e.g., induced by layering and dilatation along faults) has been around for a number of decades and has been applied in various studies to analyze crustal-scale rifting models (e.g., Schmid et al, 2022aSchmid et al, , 2022bZwaan et al, 2016Zwaan et al, , 2020Zwaan, Schreurs, & Adam, 2018). Yet to our knowledge CT-scanning has so far never been used to analyze the much more complex deformation in lithospheric-scale models, providing a strong incentive to push forward the state of the art in modeling.…”