Influences of stress and temperature regimes on graben subsidence—a note

Abstract

A model is presented for isostatic subsidence of a crustal graben block within a regional stress field that varies linearly with depth. In this model an idealized trapezoidal block of brittle upper-crustal material subsides along frictionless bounding normal faults into the ductile lower crust. An analytical expression is derived relating subsidence to regional stress, and the dimensions of the block. We find that deviatoric tension is required in order for graben subsidence to occur, and that greater deviatoric tension promotes greater subsidence. Deeper normal faulting, and narrower graben widths, promote deeper graben subsidence. The depth of faulting is presumably limited by the depth of the brittle-ductile transition in the crust, which is mainly a function of temperature. Extensional areas in which grabens can form tend to have elevated heat flows, implying a shallow brittle-ductile transition, shallow normal faulting, and, therefore, limited graben subsidence. Post-extension or post-rift cooling would reduce the heat flow and therefore allow deeper normal faulting and the potential for greater graben subsidence, in addition to regional subsidence. However, the stress regime in cooling continental crust tends to be compressive (e.g. passive margins), which would suppress further differential graben subsidence. In effect, graben subsidence depends on the geometry of the block, and is naturally limited by the elevated geothermal conditions associated with extension, and/or by the compressive stresses associated with cooling or cooled continental crust.