Comparing thin-sheet models with 3-D multilayer models for continental collision

Abstract

S U M M A R YVarious models have been proposed to explain tectonic deformations during continent collision. A frequently applied model is the thin viscous sheet model which is however not fully 3-D and assumes a priori diffuse thickening as the dominant deformation style.We compare a fully 3-D multilayer numerical model with a corresponding thin viscous sheet numerical model for the scenario of continent indentation. In our comparison we focus on the three basic viscous deformation styles thickening, buckling (folding) and lateral crustal flow. Both numerical models are based on the finite element method (FEM) and employ either a linear or power-law viscous rheology. The 3-D model consists of four layers representing a simplified continental lithosphere: strong upper crust, weak lower crust, strong upper mantle and weak lower mantle. The effective viscosity depth-profile in the 3-D model is used to calculate the depth-averaged effective viscosity used in the thin-sheet model allowing a direct comparison of both models.We quantify the differences in the strain rate and velocity fields, and investigate the evolution of crustal thickening, buckling and crustal flow resulting from the two models for two different phases of deformation: (1) indentation with a constant velocity and (2) gravitational collapse after a decrease of the indenting velocity by a factor of 5. The results indicate that thin-sheet models approximate well the overall large-scale lithospheric deformation, especially during indentation and for a linear viscous rheology. However, in the 3-D model, additional processes such as multilayer buckling and lower crustal flow emerge, which are ignored in the thin-sheet model but dominate the deformation style in the 3-D model within a range of a few hundreds of kilometres around the collision zone and indenter corner. Differences between the 3-D and thin-sheet model are considerably larger for a power-law viscous than for a linear viscous rheology. Buckling and lower crustal flow are significant in the 3-D model with power-law viscous rheology. For example, fibre strain rates due to buckling can be several hundred per cent different to the depth-averaged strain rate and the lateral mass flow of lower crustal material can be up to six times more than the flow of upper crustal material. Our results also show that the horizontal velocity fields of the upper crust and upper mantle remain nearly identical in the 3-D model during indentation despite their mechanical decoupling due to an intermediate weak lower crust. This result questions the validity of using similarities between velocities from the surface global positioning system (GPS) and mantle shear wave splitting data as evidence for a mechanically coupled lithosphere.3-D multilayer models provide a more complete picture of continental collision than thinsheet models as they enable studying the timing, locality and relative importance of different processes simultaneously which is especially important for the hundreds of kilometre scale around the collisi...

Normal faulting from simple shear rifting in South Tibet, using evidence from passive seismic profiling across the Yadong-Gulu Rift

“…In this way, there is an obvious inconsistence of deformation direction between the lower crust and lithospheric mantle. This inconsistency leads us to infer that the upper crust and lithospheric mantle are mechanically decoupled by an intermediate weak lower crust or mantle lid, as indicated by a 3-D multilayer modeling of the continental collision (Lechmann et al, 2011) and lithospheric rheology study . With the north-south underthrusting of the Indian plate beneath Tibet, lithospheric thickening, and eastward flow in the lower crust and mantle lid to accommodate the north-south shortening (Fig.…”

Folding and necking across the scales: a review of theoretical and experimental results and their applications

“…Thin-sheet models consequently assume that vertical velocities due to folding are less than vertical velocities due to kinematic thickening and hence they assume amplification rates for folding α d < 1. Thin-sheet models are useful to explain the first-order response of the continental lithosphere due to shortening on the scale of thousands of kilometres but they are not suitable for estimating the deformation on the 100 km scale, because on this scale folding is likely to be important and may control the lateral variation of vertical velocities (Lechmann et al, 2011). For example, thin sheet models are useful to predict the average topography of Central Asia (Fig.…”

Quantifying the impact of mechanical layering and underthrusting on the dynamics of the modern India‐Asia collisional system with 3‐D numerical models

“…In this study, we investigate the impact of the mechanical layering of the lithosphere and the impact of the strength of the underthrusting Indian lower crust on the three‐dimensional (3‐D) deformation of the modern India‐Asia collisional system. Since typical thin‐sheet models are not suitable to investigate vertical variations in velocities and strain rates [ Medvedev and Podladchikov , , ; Lechmann et al ., ], we utilize full 3‐D numerical models with a similar vertical strength profile as has been used by thin viscous sheet models. Our 3‐D model elaborates on the thin‐sheet model by resolving vertical changes in strength, and therefore allowing for horizontal velocities that vary with depth.…”