How do normal faults grow?

Normal faults grow via synchronous increase in displacement and length ('propagating fault model', also known as the 'isolated fault model'), or by rapid length establishment and subsequent displacement accrual (constant-length fault model). We here use time-series displacement (D) and length (L) data from natural and experimental faults to elucidate growth styles and D-L trajectories throughout fault life, and to assess the applicability of the two fault models. We show that the growth of most faults is characterized by two stages, with the first defined by fault lengthening (20-30% of fault lifespan) and the second by displacement accrual (70-80% of fault lifespan). Although broadly adhering to the constant-length model, fault growth throughout the lengthening stage, during which significant displacement (10-60% of the total end-of-life fault displacement) may also accumulate, is achieved through rapid tip propagation, relay breaching, and segment linkage, characteristics perhaps most intuitively thought to reflect growth in accordance with the propagating model. The subsequent growth stage is dominated by displacement accrual with limited lateral tip propagation, a phenomenon best described by the constant-length model. We also show that, despite being used primarily in support of the propagating model, global displacement-length (D-L) datasets are equally compatible with the constant-length model.