Ahmed Alghuraybi, Rebecca E. Bell, Christopher Aiden-Lee Jackson 

Observations of how faults lengthen and accrue displacement during the very earliest stages of their growth are limited, reflecting the fact that the early syn-kinematic sediments that record this growth are often deeply buried and difficult to image with geophysical data. Here, we use borehole and high-quality 3D seismic reflection data from offshore Norway to quantify the lateral propagation (c. 0.38 – 3.4 mm/year) and displacement accumulation (c. 0.0062 – 0.025 mm/year) rates (averaged over 6.2 Myr) for several long (up to 43 km), moderate displacement (up to 155 m), layer-bound faults that we argue provide a unique, essentially ‘fossilised’ snapshot of the earliest stage of fault growth. We show that lateral propagation rates were up to 300 times faster than displacement rates during the initial ~25% of fault lifespan, suggesting that these faults lengthened much more rapidly than they accrued displacement. Our inference of rapid lengthening is also supported by geometric observations including: (i) low Dmax-Lmax (<0.01) scaling relationships, ii) high (>5) length/height aspect ratios, iii) broad, bell-shaped throw-length profiles, and iv) hangingwall depocenters forming during deposition of the first seismically detectable stratigraphic unit spanning the length of the fault. We suggest that the unusually high ratio between lateral propagation rate and displacement rate is likely due to relative immaturity of the studied fault system, an interpretation that supports the ‘constant-length’ fault growth model. Our results highlight the need to document both displacement and lateral propagation rates to further our understanding of how faults evolve across various temporal and spatial scales.