Strain Localization within Fault Arrays over Timescales of 100-107 Years

Statistical characterization of fault networks, combined with an analysis of geodetic data and the location of historical earthquakes, is a method commonly used to quantify the degree of strain localization in a given tectonic setting. However, such analyses do not address the fundamental questions of why, how, and when (i.e., after what percent total strain) does localization occur on a lithospheric scale. Many studies suggest that the initial phase of crustal deformation is characterized by distributed strain accumulation and structural complexity and that the system evolves towards highly localized deformation on a small number of discrete fault zones. What controls the transition from one regime to the other within a rheologically layered lithosphere? Observations of the evolution of fault networks over a range of spatial and temporal scales may help us to understand the underlying controls on the localization process. Dip-slip faults, especially moderate to high-angle extensional structures, inherently provide the best conditions for preserving such temporal information over geological time because they generate adjacent sedimentary depocenters that usually remain undeformed by subsequent movement on the fault. The aim of this paper is (a) to review recent observations of extensional fault growth, (b) to summarize conclusions drawn from these observations concerning the underlying controls on strain localization in extensional settings, and (c) to discuss the relevance of these observations to other tectonic settings. In particular, several of the ideas that have been derived from studies of strain localization in extensional settings are used to reexamine existing theories concerning strike-slip fault evolution.