--> Abstract: Flexural Modeling of Normal Faulting: Effects of a Stratified Rheology, by M. Voorde, R. T. van Balen, G. Bertotti, and S. Cloetingh; #90933 (1998).

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Abstract: Flexural Modeling of Normal Faulting: Effects of a Stratified Rheology

Voorde, M. ter; R.T. van Balen; G. Bertotti; and S. Cloetingh - Vrije Universiteit

In the past decades, numerical models have proven to be a powerful tool for the analysis of extensional basin formation. Dynamical models have provided more insight in the physical processes acting during extension, whereas kinematic models have been increasingly used for regional studies. In most of these kinematic forward models, the isostatic response on extension is calculated by assuming that the lithosphere reacts on a load in the same way as a thin elastic plate. The thickness of this (imaginary) plate is estimated by comparing the model predictions with the observed basin geometry and/or gravity data.

The effective elastic thicknesses (EET's) resulting from this approach show large differences. The obtained values depend not only on the modeled region, but also on the kind of model that is used. For example, for Mesozoic rifting in the Viking Graben, estimated EET values vary from 1.5 to 45 km. This discrepancy is often attributed to the possibility that the crust is decoupled from the mantle, due to vertical variations in the strength of the lithosphere. This would allow for independent deformation of the resulting "sub-plates". Whether this is possible depends mainly on the viscosity of the lower crust: In order to enable complete decoupling during extension, the rate of compensation by ductile flow of the lower crust has to keep pace with the rate of upper crustal deformation. In general this is not the case. Apparently, fully decoupled behavior is associated with anomalous circumstances, and the lithosphere is more likely to be in a "partly decoupled" mode. The degree of decoupling depends on the rheology and temperature of the lithosphere, the stretching rate and the time passed since the cessation of rifting.

We developed a two-layer finite difference model for the flexural response of the lithosphere to extensional faulting. The model allows for three modes of flexure: (1) fully coupled, with the upper crust and mantle welded together by the lower crust, (2) fully decoupled, with the upper crust and mantle behaving as independent layers, and (3) partly decoupled, signifying that the response of the upper crust to small-wavelength loads is superimposed on the response of the entire lithosphere to long-wavelength loads.

Modeling results indicate that the major observable differences between the coupled and the decoupled mode of stretching are the amount of fault block rotation and the Moho topography (Fig. 1). A high degree of decoupling yields large amount of footwall uplift and a relatively flat Moho.

The Baikal Rift zone, with its high rigidity, provides a typical example for the coupled mode of flexure. A fully decoupled lithosphere, related to anomalous high temperatures in the lower crust, is derived for the Basin and Range province. The most common case, the partly decoupled lithosphere, is inferred for example for the Bay of Biscay margin.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil