Evolution of pull‐apart basins and their scale independence

TLDR

Pull‑apart basins or rhomb grabens and horsts along major strike‑slip fault systems are generally associated with horizontal slip, and their formation and growth involve interaction among new and preexisting fault strands on a larger scale. A simple model posits that rhomb width is set by initial fault geometry while length grows with displacement; testing 70 basins shows width increases with fault offset, likely due to coalescence of neighboring grabens or formation of parallel fault strands, so fault zone width expands as displacement rises. Contrary to the model, the basins maintain a roughly constant length‑to‑width ratio of about 3, indicating that they widen proportionally as they lengthen with fault displacement.

Abstract

Pull‐apart basins or rhomb grabens and horsts along major strike‐slip fault systems in the world are generally associated with horizontal slip along faults. A simple model suggests that the width of the rhombs is controlled by the initial fault geometry, whereas the length increases with increasing fault displacement. We have tested this model by analyzing the shapes of 70 well‐defined rhomb‐like pull‐apart basins and pressure ridges, ranging from tens of meters to tens of kilometers in length, associated with several major strike‐slip faults in the western United States, Israel, Turkey, Iran, Guatemala, Venezuela, and New Zealand. In conflict with the model, we find that the length to width ratio of these basins is a constant value of approximately 3; these basins become wider as they grow longer with increasing fault offset. Two possible mechanisms responsible for the increase in width are suggested: (1) coalescence of neighboring rhomb grabens as each graben increases its length and (2) formation of fault strands parallel to the existing ones when large displacements need to be accommodated. The processes of formation and growth of new fault strands promote interaction among the new faults and between the new and preexisting faults on a larger scale. Increased displacement causes the width of the fault zone to increase resulting in wider pull‐apart basins.