How Do Normal Faults Grow?

Presented as part of the Geological Society of American (GSA) Distinguished Lecturer Tour 2017.

How Do Normal Faults Grow?

Normal faults grow via a sympathetic increase in their displacement and length (‘isolated model’), or by rapid establishment of their near-final length prior to significant displacement accumulation (‘constant-length model’). The isolated model has dominated the structural geology literature for >30 years, although some recent studies provide support for the constant-length model. Because they make different predictions regarding rift development and related earthquake hazards, these models must be tested. Here I outline several techniques that can constrain the kinematics of synsedimentary normal faults and thus test competing fault growth models. I then apply these techniques to three seismically imaged faults, showing that, in general, they grew in accordance with the constant-length model. Analysis of growth strata represents the best way to test competing fault growth models; most studies utilising this approach support the constant-length fault model, suggesting it may be more applicable than currently assumed. It is plausible, however, that the very early development of such large faults is characterised by the development of faults that, pre-linkage, grow in accordance with the isolated model; we may simply lack the data resolution, especially in the subsurface, to resolve this very early stage of fault growth.