THE CONTROL OF ENVELOPING STRUCTURES ON VEIN FORMATION

It is a common observation that veins are more prevalent in regions of increased dilation. Nevertheless, work on veins has emphasized internal textures and their geometrical evolution without the mechanical context of the surrounding structures. Recent studies of the link between veins and structures at the grain scale has emphasized the need to examine vein formation in a broader context. Veins within the Ordovician Mazarn Formation, Benton Uplift, Ouachitas and preferentially localized in the necks of boudins. The veins consist of swarms of veinlets (5-25 µm thick) that individually are one-grain diameter in thickness but collectively form spanning crystal fibers using host grains as crystallographic templates. Each fiber is composed of multiple overlapping veinlets, each episode mimicking the crystallographic orientation of the last. As the vein matures, the intervening host fragments become a small fraction of the entire vein. Late stage recrystallization has further obscured the earliest stages of vein formation. The repeated localization of vein formation is intimately controlled by the mechanics of the boudined layer. The boudin neck or pinched region is an area of reduced mean stress leading to layer-parallel extension. Macroscopic vein width is controlled by the extent to which beds extended after the individual neck regions were broken and separated by subsequent deformation of the larger structure. This and examples from the outer arcs of folds and releasing bends of faults indicate that understanding the mechanics of the enveloping structure is a crucial step in explaining the origin of syntectonic veins.