ABSTRACT: Computer Modeling of Block Rotation Mechanics

Sarah Saltzer

A large number of extensional basins are underlain by fault blocks that have rotated domino style through angles up to several tens of degrees. Serious spatial consequences and strain incompatibilities that arise at the base of the blocks as rotation proceeds are not well understood. In addition, little is known about the driving and resisting forces or the constitutive laws of the blocks and bounding faults. The distinct and finite element methods have been combined to provide a tool for tectonic reconstructions in extensional environments based on mechanics. Specifically, the distinct element method describes the deformation of a system of blocks driven by tectonic and gravitational loads and moving according to Newton's laws of motion. Force-displacement relations base on friction and noninterpenetration govern the interactions between blocks. The finite element method analyzes the stress state within individual fault blocks to determine conditions and locations for failure.

These numerical techniques have been used to model deformation in the Basin and Range Province where locally extension may reach 100% or more. The modeling results confirm field studies suggesting that fault blocks can rotate up to 55° before the frictional resistance across the blocks exceeds the shear stress and the blocks lock. In the computer model, blocks rotate under gravitational loading, and brittle failure at the base of the blocks provides a mechanism for filling potential voids. After locking, new faults are introduced so that deformation can proceed. This sort of mechanically based modeling provides a computational tool for helping to understand the dynamics of block rotation. These results should help to constrain the mechanical development of extensional basins.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990