Deformation of intrasalt competent layers in different modes of salt tectonics

Abstract

Abstract. Layered evaporite sequences (LES) comprise interbedded weak layers (halite and, commonly, bittern salts) and strong layers (anhydrite and usually non-evaporite rocks such as carbonates and siliciclastics). This results in a strong rheological stratification, with a range of effective viscosity up to a factor of 105. We focus here on the deformation of competent intrasalt beds in different modes of salt tectonics using a combination of conceptual, numerical and analog models, and seismic data. In bedding-paralell extension, boudinage of the strong layers forms ruptured stringers, within a halite matrix, that become increasingly isolated with increasing strain. In bedding-parallel shortening, competent layers tend to maintain coherency while forming harmonic, disharmonic, and polyharmonic folds, with the rheological stratification leading to buckling and fold growth by bedding-parallel shear. In differential loading, extension and the resultant stringers dominate beneath suprasalt depocenters while folded competent beds characterize salt pillows. Finally, in tall passive diapirs, stringers generated by intrasalt extension are rotated to near vertical in tectonic melanges during upward flow of salt. In all cases, strong layers are progressively removed from areas of salt thinning and increasingly disrupted and folded in areas of salt growth as deformation intensifies. The varying styles of intrasalt deformation impact seismic imaging of LES and associated interpretations. Ruptured stringers are often visible where they have low dips, as in slightly extended salt layers or beneath depocenters, but are usually not imaged in tall passive diapirs due to steep dips. In contrast, areas of slightly to moderately shortened salt typically have well imaged, mostly continuous intrasalt reflectors, although seismic coherency decreases as deformation intensifies. Similarly, wells are most likely to penetrate strong layers in contractional structures and salt pillows, less likely in extended salt because they might drill between stringers, and unlikely in tall passive diapirs because the stringers are near-vertical. Thus, both seismic and well data may be interpreted to suggest that diapirs and other areas of more intense intrasalt deformation are more halite rich than is actually the case.

Deformation of Intrasalt Beds Recorded by Magnetic Fabrics

“…Three‐dimensional seismic data from NW Europe (Van Gent et al, ) and offshore Brazil (Fiduk & Rowan, ; Jackson et al, ) show complex geometries of folds and boudins, which only partly reflect the overall geometry of the surrounding salt structures. These data demonstrate that thick intrasalt beds commonly form independent mechanical units within the salt bodies (Rowan et al, ). Numerical models demonstrate the sensitivity of thick intrasalt beds to geometrical and mechanical variables, such as the initial position and thickness of the beds, and the viscosity of the encasing rock salt (Chemia & Koyi, ; Li, ; Li et al, ).…”

“…Observations (e.g., Borchert & Muir, 1964;Jackson et al, 2014;Li et al, 2012;Strozyk et al, 2012), supported by numerical models (e.g., Koyi, 2001;Rowan et al, 2019) show that thick intrasalt beds typically boudinage and/or fold during passive diapiric processes. Li (2016) shows that for shallow intrasalt beds positioned close to the top of sections, the first breaks and separation of boudins occur soon after the start of the downbuilding process.…”

“…Thin centimeter‐scale intrasalt beds tend to reflect complex flow patterns of the host rock salt (Burliga, , ; Sans et al, ), whereas thick intrasalt beds, of meters to hundred‐meter scale, form independent mechanical units as imaged on high‐resolution seismic (Jackson et al, ; Rowan et al, ; Van Gent et al, ). Observations (e.g., Borchert & Muir, ; Jackson et al, ; Li et al, ; Strozyk et al, ), supported by numerical models (e.g., Koyi, ; Rowan et al, ) show that thick intrasalt beds typically boudinage and/or fold during passive diapiric processes. Li () shows that for shallow intrasalt beds positioned close to the top of sections, the first breaks and separation of boudins occur soon after the start of the downbuilding process.…”

“…Studies have shown that the deformation of intrasalt beds depends on their thickness. Thin centimeter-scale intrasalt beds tend to reflect complex flow patterns of the host rock salt (Burliga, 2014(Burliga, , 2018Sans et al, 1996), whereas thick intrasalt beds, of meters to hundred-meter scale, form independent mechanical units as imaged on high-resolution seismic (Jackson et al, 2014;Rowan et al, 2019;Van Gent et al, 2011).…”

“…Numerical models demonstrate the sensitivity of thick intrasalt beds to geometrical and mechanical variables, such as the initial position and thickness of the beds, and the viscosity of the encasing rock salt (Chemia & Koyi, ; Li, ; Li et al, ). Moreover, conceptual models founded on natural observations and analog and numerical models suggest that the style and extent of deformation of thick intrasalt beds varies significantly for different modes of salt tectonics, that is, passive or active diapirism (Rowan et al, ).…”