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2022.05.26- Dr. Bailey Lathrop: How do normal faults grow?

Continental extension is accommodated by normal faults, and fault growth is achieved by increases in displacement (D) and length (L). Before 3D seismic data became widely used, it was assumed that faults maintain a constant D/L ratio from initiation to cessation, growing steadily via sympathetic increases in displacement and length. Since the advent of 3D geophysical seismic data, it has been suggested that faults reach their maximum length before accruing significant displacement. Several uncertainties remain, namely, how different geological factors affect fault geometry and growth. To address this, I first assemble and interrogate a global database of normal faults that includes displacement and length, as well as tectonic setting, host rock lithology, fault maturity, and D/L through time where available. I next present a case study that uses growth faults imaged in 3D seismic data from offshore NW Australia to track fault lengthening, displacement, and changes in slip rate through time. Observations on fault growth from the global database and seismic-based interpretations were then tested using physical analogue models, designed to investigate fault displacement and length through time in higher resolution than seismic data allows. Results demonstrate that ‘one-size-fits-all’ D/L scaling laws are not accurate because the fault geometry and growth is affected by fault size, host rock lithology, fault reactivation, and fault maturity in particular. Universal D/L scaling laws are inherently problematic because the relationship between length and displacement is ever-changing throughout a fault’s life. Individual normal faults follow the following growth pattern: a lengthening stage (10-30% of a fault's active life), displacement stage (30-75% of a fault's active life), and possible tip retreat stage (final 25% of a faults’ active life). On a fault array scale, faults follow a cyclical growth pattern, where faults alternate between the lengthening and displacement stages.