Passive Versus Active Salt Diapirism

Existing definitions for passive and active salt diapirism are some-what overlapping and ambiguous. These terms are also equated to downbuilding and upbuilding, respectively, which are problematic concepts as originally conceived and even as subsequently modified. This results in conflicting usage and lack of consensus in the literature, creating confusion and decreasing the utility of the terms. For example, a diapir with a thin roof is defined as active by some but passive by others.In this short contribution, we first review historical definitions and then focus on several inherent problematic aspects including (1) the applicability of the concepts of downbuilding and upbuilding, (2) using the absence or presence and thickness of a diapir roof as criteria to classify diapirs as active or passive, and (3) the related concept of halokinetic sequences. We offer three suggestions in the hope of improving understanding and communication. First, we suggest that the terms downbuilding and upbuilding should be abandoned. Second, we argue that passive diapirism should be used sensu lato in that the presence or absence of a thin roof should not be the key defining factor. Third, we propose that active diapirism should be reserved for two specific scenarios: (1) uplift and folding of a thick roof during diapir rejuvenation and (2) the single brief stage when salt in a precursor structure (e.g., reactive diapir, salt-cored contractional fold, or salt pillow) breaks through its roof to subsequently rise as a passive diapir.