Fault growth and rift propagation during rotational continental rifting: Insights from an analogue modelling study

Abstract

<p>Continental rifts typically result from regional horizontal stretching of the lithosphere and in modelling studies, such rifts are typically assumed to be the result of orthogonal or oblique extension. However, in nature often V-shape rift geometries occur indicating an underlying rotational component that results in a divergence velocity gradient along plate boundaries. Consequently, the geometric, kinematic, and dynamic rift evolution in such rotational settings may significantly differ from those of orthogonal or oblique rifts. Here, we present new findings from an analogue modelling study using a crustal-scale model series with a rotational opening component to investigate the effect of such a rift-axis parallel divergence velocity gradient on fault growth and rift propagation towards the rotation axis.</p><p>We use a simplified two-layer system simulating an upper brittle and a lower ductile crust with an imposed initial mechanically weak zone on top of the viscous layer to ensure localized rifting. The experimental monitoring by means of a stereoscopic camera setup and X-Ray computed tomography (XRCT) enables a detailed and quantitative investigation of near-surface rift evolution and internal deformation, respectively. With the combination of 3D surface topography, 3D displacement fields, and XRCT, we gain a comprehensive understanding of deformation evolution in analogue models of rotational rifting. Our modelling results depict a novel characterization of normal fault growth under rotational extension and a rift evolution which is described by (1) rift propagation in two consecutive stages: A first stage showing bidirectional fault growth due to segment linkage with high rift propagation rates, and a second stage during which rift propagation occurs by unidirectional fault growth towards the rotation axis with linearly decreasing growth rates at decreasing distance to the rotation axis, (2) strain partitioning between competing conjugate normal faults with fault activity switching repeatedly from one segment of a normal fault to a segment on the oppositely dipping normal fault, and (3) active faulting migrating from the rift boundary faults inwards to intra-rift normal faults.</p><p>Our quantitative, spatiotemporal fault growth analysis reveals a characteristic segmentation of all deformation features listed above. The conclusion that the gradual decrease of the divergence velocity towards the rotation axis causes segmented deformation propagation is key and can help to understand natural examples of rotational rift settings such as the Taupo Rift Zone in New Zealand.</p>

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

“…For all slices, the displacement vectors represent the total 3D displacement, projected into the plane of the corresponding CT slice. The entire slice compilation is available in an additional data publication (T. C. Schmid, Rudolf, et al., 2022). Here, we present a representative selection of displacement data to document quantitative 3D deformation features in our XRCT‐scanned models.…”

“…As an example, in both slices the colors refer to the vertical displacement component Dz and white arrows indicate the total displacement, projected into the plane. All transects, including horizontal z slices can be found in an additional data publication on the GFZ data repository (T. C. Schmid, Rudolf, et al., 2022).…”

“…A more detailed overview of DIC and DVC parameters can be found in T. Schmid et al. (2021) and T. C. Schmid, Rudolf, et al. (2022), respectively.…”

“…Rheological measurements of the brittle and viscous materials used in this study are available in the form of open access data publications via GFZ Data Services (T. Schmid et al., 2020a, 2020b; Zwaan, Schreurs, Ritter, et al., 2018, respectively). An additional open access data publication on the GFZ Data Service (T. C. Schmid, Rudolf, et al., 2022) provides additional images and movies of our models. Links to these data sets are provided in the reference list.…”