Alexander James Coleman, Oliver B. Duffy, Christopher Aiden-Lee Jackson 

Growth folds above the upper tips of normal faults are ubiquitous in extensional settings, especially during the early phases of extension and in salt-rich basins. As slip accumulates on the underlying normal fault, the geometry and size of the fold changes. These changes reflect the dip, throw, displacement and propagation rate of the underlying normal fault, as well as the thickness and rheology of the overlying cover. These changes also have a marked impact on the architecture and distribution of synkinematic sediments, as well as the styles of secondary deformation accommodating strain within the growing fold. Here, we analyse a large dataset of natural, and physically- and numerically-modelled growth folds to: (i) characterise their diagnostic features; (ii) investigate the controls on their geometry, size and differences; and (iii) describe how they grow with increasing extensional strain. We demonstrate that larger fault throws and a thicker and weaker cover are associated with larger growth folds. In contrast, small fault throws as well as thin and strong brittle cover are associated with smaller growth folds. We show that the geometry and size of growth folds vary through time; the width (and thus, the wavelength) of the fold is established relatively early during fold growth, whereas fold amplitude increases gradually with increasing fault throw. Fold width and amplitude become increasingly similar during fold evolution, until the fold is breached by the underlying normal fault. We also derive a number of preliminary empirical relationships between readily observable structural and stratigraphic parameters in our dataset that may help estimate the geometry and size of poorly exposed (i.e. in the field) or imaged (i.e. in the subsurface) growth folds. In addition, we discuss how fault growth models (i.e. constant-length vs. propagating) may impact the three-dimensional evolution of growth folds. Finally, our work shows that growth folds are likely more common than previously thought. For example, although they are well-documented in areas characterised by weak, ductile cover strata and low strain rates, our dataset illustrates that growth folds may also occur in brittle, relatively strong rocks and in regions with high strain rates. However, the underlying controls on fold occurrence remain elusive.