Effects of lateral friction on the structural evolution of fold-and-thrust belts: Insights from sandbox experiments with implications for the origin of landward-vergent thrust wedges in Cascadia

The stacking of thrust sheets and mass transfer of sediment during fold and thrust belt accretion imposes a load on the basement and underlying mantle. This load induces an isostatic adjustment through a flexural response, which may also contribute to the overall architecture of the fold and thrust belt. Whereas plate kinematics imposes its tempo to evolving fold and thrust belts, the rheology of the mantle controls the tempo of the isostatic flexure. Using two-dimensional high-resolution numerical experiments, we explore how the interplay between the tectonic compressional rate and the isostatic flexural rate influences the structural evolution and final architecture of fold and thrust belts.&#160;We run a suite of numerical experiments using the well-tested code Underworld. Our geological model is mapped over a 42 km by 16 km numerical grid, with a cell resolution of 80 m. The geological model consists from top to bottom of&#160; &#8216;sticky air&#8217;, 4 km of sediment that alternates in competence at 500 m intervals, a 3 km thick basement, and a basal layer which - in combination with a basal kinematic boundary condition - controls the amount of isostatic flexure. Materials have a mechanical behavior that results from elasto-visco-plastic rheology. The pressure at the base of the model is held constant, and the vertical velocity is updated at each timestep. The amount of material entering or exiting the model at each point along the base scales with the density of the basal layer, which is used to control the isostatic rate. Sedimentation and erosion are self-consistent through mechanical erosion and a hillslope diffusion law. Our models show that as the ratio between tectonic and flexural rates decreases (i.e. flexure gets faster), fold and thrust belts become narrower, lower in elevation, and structurally more complex. We compare these results with natural analogs including the Cordilleran and Jura fold and thrust belts.

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A discussion of the (re)activation of basement structures during multi-phase shortening in thin- and thick-skinned styles

10.5194/egusphere-egu21-4162 ◽

2021 ◽

Author(s):

Nicolas Molnar ◽

Susanne Buiter

Keyword(s):

Structural Evolution ◽

Shear Zones ◽

Fault Reactivation ◽

Swiss Alps ◽

Lateral Strain ◽

Thrust Belts ◽

Experimental Apparatus ◽

Analogue Models ◽

Fold And Thrust Belts ◽

Multi Phase

Shortening in fold-and-thrust belts can be accommodated with little or substantial basement involvement, with the former, thin-skinned, style arguably being the more common (Pfiffner, GSA Special Paper, 2006). Experimental studies on thin-skinned fold-and-thrust belts have confirmed critical taper theory and have highlighted the roles of bulk rheology, embedded weak layers, d&#233;collement strength, and surface processes in structural evolution. However, analogue models of thick-skinned fold-and-thrust belts are less common, which may be related to practical challenges involved in shortening thick layers of brittle materials. Here we focus on basement fault reactivation, which has been suggested for several fold-and-thrust belts, such as the Swiss Alps, the Laramide belt in North America and the Sierras Pampeanas in South America, which show evidence of deep-rooted thrust systems, pointing to a thick-skinned style of shortening.Within an orogenic system, the shortening style may change between thin- and thick-skinned in space (foreland to hinterland) and time. This raises the question how inherited structures from one shortening phase may influence the next. We aim to use analogue experiments of multi-phase shortening to discuss the effects of deep-seated shortening-related inherited structures, such as thrusts and basement topography, on the structural evolution of fold-and-thrust belts.We employ a push-type experimental apparatus that can impose shortening in both thick- and thin-skinned style. The device has two independently moving backstops, permitting to change between these shortening styles over time, allowing the simulation of multiple contractional scenarios. We start with an initial stage of thick-skinned shortening, followed by either thin- or thick-skinned reactivation. We use quartz sand to simulate crustal materials and microbeads for embedded weak (sedimentary) layers. Surface and lateral strain, as well as topography, is quantified using a high-resolution particle imaging velocimetry and digital photogrammetry monitoring system.We will present preliminary results of this innovative experimental approach with the objective of discussing to what extent pre-existing conditions in the basement control the geometric, kinematic, and mechanical evolution of thick-skinned and basement-involved thin-skinned tectonics. In this presentation, we hope for a discussion of mechanisms of localisation of shortening in brittle analogue models, of sequences of thin- and thick-skinned deformation expected during multi-phase shortening, and comparisons to ongoing research and natural observations. Questions we aim to discuss are: Can weaknesses and anisotropies within the basement influence and control later structural evolution? Are pre-existing structures, such as thrusts or shear zones within the basement, responsible for subsequent fault nucleation, thin-skinned folding or basement uplift? What role does the rheology of the basement-cover interface play in the reactivation of basement thrusts? Can we model these reactivations with an analogue setup?

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A discussion on the validation of structural interpretations based on the mechanics of sedimentary basins in the northwestern Mediterranean fold-and-thrust belts

BSGF - Earth Sciences Bulletin ◽

10.2113/gssgfbull.187.2.83 ◽

2016 ◽

Vol 187 (2) ◽

pp. 83-104 ◽

Cited By ~ 3

Author(s):

Josselin Berthelon ◽

William Sassi

Keyword(s):

Cross Section ◽

Cross Sections ◽

Numerical Models ◽

Structural Evolution ◽

Sedimentary Basins ◽

Structural Style ◽

Thrust Belts ◽

Fold And Thrust Belts ◽

Thin Skin ◽

Northwestern Mediterranean

Abstract Using the geologist’s interpretation of 6 published balanced cross-sections in the fold and thrust belts of the northwestern Mediterranean, a comparative analysis of the interpreted subsurface structural architecture is used to address the links between the structural style and the mechanics of fold and thrust emplacement. For each cross-section example, the geo-dataset and the methods used by the interpreters are different in quantity and quality. Here we have examined how useful is the content of information of each cross-section to constrain the structural evolution scenario. Each interpretation is examined according to considerations of the mechanics of sedimentary basin deformation and how uncertain is the extrapolation of fault trajectory at depth. It is shown that each case reveals a particular type of structural style: thin-skin or thick skin tectonics, fault-related folding, pre-existing fault pattern. The present structural analysis is used to determine for each cross-section the nature of the mechanical problem to address that will reduce uncertainty on the geologic scenario reconstruction. The proposed mechanical boundary conditions could serve to develop analog or numerical models that aim at testing the mechanical validity of the structural scenario of fold and thrust emplacement.

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