Subsidence analysis of salt tectonics‐driven carbonate minibasins (Northern Calcareous Alps, Austria)

Abstract

Subsidence analysis study for several Triassic carbonate platforms from the eastern Northern Calcareous Alps shows that salt expulsion allowed for the growth of thick isolated depocentres (>1.5 km) at rates faster than those tectonic subsidence alone can provide. Our results, in addition to independent regional geological evidence, argue against previous models of thickskinned extension controlling accommodation space. Differential sedimentary loading and stretching of the salt layer can explain the development of the Triassic isolated carbonate platforms in the Northern Calcareous Alps, with salt expulsion being proportional to the growth potential of the carbonate producers. Aside of topographic loads, early diageneses of carbonates allow for the density inversion between sediment and salt, with differential loading by carbonate aggradation leading to a self-sustained feedback cycle of density-driven and gradient load subsidence; stretching of the salt layer and extensional deformation of its overburden, as constrained by cross-section restoration, also contributed to diapir initiation and salt expulsion. Our model can: (a) explain the occurrence of isolated Middle Triassic carbonate platforms in the eastern Northern Calcareous Alps, and (b) differentiate between accommodation space controlled by (local) salt expulsion and by (regional) tectonic subsidence. The Triassic Neo-Tethys shelf of the studied area constituted therefore a salt wall and minibasin province. This contribution and methods herein can also be applied to other carbonate platform systems developed on salt basins, especially where the transition from rift to drift remains unclear.

Stretching and Contraction of Extensional Basins With Pre-Rift Salt: A Numerical Modeling Approach

“…The NCA of Austria constitute a 700 km long E-W striking salt-detached fold-and-thrust belt characterized by thick (up to ∼4500 m) successions of Neo-Tethyan Triassic passive margin carbonates (Mandl, 2000). The NCA stratigraphy documents the diachronous growth of Middle Triassic carbonate platforms over a Permian-Triassic layered evaporitic sequence (Strauss et al, 2020) deposited during the late-thinning phase of rifting (Leitner et al, 2017). In the proximal domain of the Neo-Tethys passive margin, the salt likely constituted an extensive layer overlaying those basement faults responsible for crustal extension, whereas toward the developing ocean, the salt may have been bound by active extensional faults accounting for salt thickness variations.…”

“…In the first case study -the Moroccan Central High Atlas-there is still some debate as to the degree of decoupling between the presalt basement and its sedimentary cover (i.e., whether the Triassic evaporites represented a thin-skinned detachment or not). The second case study is the Austrian Northern Calcareous Alps fold-and-thrust belt, for which the origin of thick carbonate sequences is debated as to either related to syn-rift thick-skinned extensional faulting or post-rift tectonics purely developed by downbuilding and thin-skinned extension on salt Strauss et al, 2020).…”

“…In the proximal domain of the Neo-Tethys passive margin, the salt likely constituted an extensive layer overlaying those basement faults responsible for crustal extension, whereas toward the developing ocean, the salt may have been bound by active extensional faults accounting for salt thickness variations. The role of such faults in controlling the accommodation space of the carbonate platforms is still a matter of debate, with some authors proposing salt evacuation and saltdetached extension in a thermally subsiding basin Strauss et al, 2020), whereas others still suggest thick-skinned extension as the main subsidence and accommodation space driver Considerations on regional geological evidence, facies distribution across the belt, and subsidence analysis indicate a dominating post-rift setting, where accommodation space was provided by thermal subsidence largely assisted by salt evacuation (Strauss et al, 2020). Since Alpine shortening completely sheared off the NCA along the Permian-Triassic evaporites from their pre-salt rifted basement (Linzer et al, 1995), the original relationships between the post-salt stratigraphy and the basement structures responsible for crustal thinning cannot be directly observed.…”

Comment on Strauss et al., 2020: Subsidence analysis of salt tectonics‐driven minibasins (Northern Calcareous Alps, Austria)

“…The methodology for subsidence analysis applied by Strauss et al. (2020), is excellent. They use formation thickness, absolute age of the top and bottom of formations, bathymetry, plus assumed values for what they call "tectonic subsidence" without salt.…”

Jurassic Salt Tectonics in the SW Sub‐Alpine Fold‐and‐Thrust Belt

“…Pre-tectonic salt can trigger salt tectonics from the time of the deposition such as in the Zagros fold-and-thrust belts, Iran (e.g., Callot et al, 2007Callot et al, , 2012Fernandez & Kaus, 2014), but more generally is triggered by a pre-compressional tectonic phase. If the salt predates the formation of the sedimentary basin from which the fold-and-thrust belt is built, early extension results in reactive salt tectonics with potential structural and sedimentary consequences within the sedimentary pile; Morocco, the Spanish Pyrenees, or the Northern Calcareous Alps are examples (Ducoux et al, 2019;Granado et al, 2019;Lopez-Mir et al, 2014;McClay et al, 2004;Saura et al, 2014;Strauss et al, 2020;Vergés et al, 2017). Moreover, physical and analog models have also illustrated the importance of early salt structures in later compression (e.g., Callot et al, 2007;Duffy et al, 2018).…”