The transition from salt diapir to weld and thrust: Examples from the Northern Flinders Ranges in South Australia

Abstract

The interactions between salt diapirs, thrust welds and thrusts in contractional belts are poorly understood due to, first, the inability of seismic data to distinguish between thrusts and welds or resolve associated sub‐resolution deformation, and second, the paucity of good field examples. The Warraweena area in the Northern Flinders Ranges of South Australia contains examples of Neoproterozoic to Early Cambrian squeezed diapirs linked by steep reverse faults formed during the Delamerian Orogeny. Benefiting from good field exposures, we use geological mapping, cross‐section construction and conceptual structural models to assess the three‐dimensional geometry and evolution of the structures, the lateral transition from diapirs to linking faults and the variability of associated meso‐ and small‐scale deformation. Three discrete diapirs consist of narrow outcrops of Callanna Group megabreccia (Willouran in age) up to 5‐km long. Their diapiric origin is confirmed by local development of caprock, steepening of flanking strata in composite halokinetic sequences and reworked diapir and roof debris in adjacent strata. The surrounding rocks display only background levels of small‐scale deformation. In contrast, the linking faults show no evidence of precursor diapirism, have fault‐related anticlines up to 100s of m in wavelength in their hanging walls, and an associated increase in small‐scale deformation (i.e. millimetre to metre scale folds, fractures and shear fabrics). The transitions from diapirs to faults occur within less than 200 m as short thrust welds at the diapir terminations. The exposed structures are analogous to those found on the subsurface of other salt basins such as the Gulf of Mexico and the South Atlantic conjugate margins. The results of this work can aid geoscientists evaluating three‐way traps against squeezed diapirs, welds or faults, and can help them to predict the style and abundance of both halokinetic and small‐scale structures that are below seismic resolution.

From Salt‐Bearing Rifted Margins to Fold‐And‐Thrust Belts. Insights From Analog Modeling and Northern Calcareous Alps Case Study

“…In contractional systems, salt is either preserved as squeezed diapirs (e.g., Callot et al., 2012), along thrust welds (e.g., Hossack, 1995; Vidal‐Royo et al., 2021), at the base of the thrust sheets (e.g., Davis & Engelder, 1985; Hudec & Jackson, 2006), and under certain conditions, as allochthonous salt sheets (e.g., Hudec & Jackson, 2006; Rowan, 2017). Upon shortening, the existence and rejuvenation of pre‐existing salt bodies represent efficient channels to evacuate salt from the bodies themselves and the source layer to the surface (Callot et al., 2012; Ferrer, 2012; Nilsen et al., 1995; Roca et al., 2006; Rowan & Vendeville, 2006; Santolaria, Ferrer, et al., 2021).…”

“…World‐class examples of thrust‐related salt sheets are the Quele sheet in the Kuqa basin (e.g., Li et al., 2014), the Lufilian Arc of the Democratic Republic of Congo (Jackson et al., 2003), or the Salt Range in the Himalayan foreland of Pakistan (Baker et al., 1988), amongst others. Salt in fold‐and‐thrust belts can occur as squeezed diapirs (e.g., Callot et al., 2012), along thrust welds (e.g., Hossack, 1995; Vidal‐Royo et al., 2021), and at the base of the thrust sheets (e.g., Davis & Engelder, 1985; Hudec & Jackson, 2006) (Figure 1b). But in general, contractional structures in salt‐influenced fold‐and‐thrust belts tend to show limited amounts of salt.…”

Linked detachment folds, thrust faults, and salt diapirs: Observations and analog models

“…Salt rising in passive diapirs flows from beneath subsiding minibasins. These commonly display multiple-km scale wedge-shaped minibasin tectonostratigraphic successions (Rowan and Giles, 2021) that record the subsidence history and salt-flow kinematics. However, models suggest that differential loading does not always result in minibasin thickness changes (e.g., Nilsen et al, 1995;Koyi, 1998;Rowan and Vendeville, 2006).…”

“…show dips up to 65-70º (Jahani et al, 2009;Hassanpour et al, 2018). A better analog might be the Flinders Ranges of South Australia, where hanging wall strata dip very steeply and thus constrain thrust faults connecting diapirs to dips attaining 75º or even more (Vidal-Royo et al, 2021).…”

“…In this short paper, we use 3-D, depth-migrated seismic data from the Sureste Basin in the southern Gulf of Mexico to examine the usefulness and limitations of analog models in understanding the interaction between diapirs, detachment folds, and thrust faults. Note that we deal primarily with passive diapirs (ones that grow at or just beneath the surface as sediment is deposited; Rowan and Giles, 2021), whether in the form of salt stocks, salt walls, or salt sheets. Moreover, we focus on so-called isolated-diapir provinces (i.e., diapirs largely surrounded by minibasin strata) rather than isolated-minibasin provinces (Duffy et al, 2017).…”