Oblique basin inversion leads to fold localisation at bounding faults: Analogue modelling of the Achental structure, Northern Calcareous Alps, Austria
Abstract. Within the Northern Calcareous Alps fold-and-thrust belt of the Eastern Alps, multiple deformation phases have contributed to the structural grain that localised deformation at later stages. In particular, Jurassic rifting and opening of the Alpine Tethys led to the formation of extensional basins at the northern margin of the Apulian plate. Subsequent Cretaceous shortening within the Northern Calcareous Alps produced the enigmatic Achental structure, which forms a sigmoidal transition zone between two E-W striking major synclines. One of the major complexities of the Achental structure is that all structural elements are oblique to the Cretaceous direction of shortening. It was therefore proposed to be a result of forced folding at the boundaries of the Achental basin. This study analyses the structural evolution of the Achental structure through integrating field observations with crustal-scale physical analogue models, to elucidate the influence of pre-existing crustal heterogeneities on oblique basin inversion and the prerequisites for the formation of a sigmoidal hanging wall that outlines former basin margins. From brittle-ductile models, we infer that shortening oblique to pre-existing extensional faults can lead to the localisation of thrust faults at the existing structure within a single deformation phase. Prerequisites are 1) a weak basal décollement that is offset by an existing normal fault, 2) the presence of topography in the hinterland, 3) a thin-skinned deformation style. Consequently, the Achental low-angle thrust and corresponding folds was able to localise exactly at the basin margin, with a vergence opposite to the Jurassic normal fault, creating the characteristic sigmoidal morphology during a single phase of NW-directed shortening.
Willemijn S. M. T. van Kooten et al.
Status: open (extended)
- RC1: 'Comment on egusphere-2022-1529', Anonymous Referee #1, 21 Mar 2023 reply
Willemijn S. M. T. van Kooten et al.
Willemijn S. M. T. van Kooten et al.
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The article entitled "Oblique basin inversion leads to fold localization at bounding faults: Analogue modelling of the Achental structure, Northern Calcareous Alps, Austria" submitted by Willemijn and co-authors is well written and with figures in a good style but it is also very long and a bit hard to follow.
This is in part due to the problems I had trying to link what the overwhelming text was explaining and the figures were indicating. These last statements, however, do not question the validity of the study or the results from analogical modelling.
The main objective of the work seemed to me to find the potential best solution for the structure at depth to propose solutions that better match one of these interpretations (sometimes controversial) for the hidden structure underneath the Achental structure, analysed by means of fieldwork and cross-section construction previously by (reference).
Line 52 & Figure 1
Figure 1 showing the very schematic geology of the Achental structure on a geological map that corresponds to a small box located on a small map (inset) of Austria where the Northern Calcareous Alps have been represented in black. I think that a simplified geological map of the Northern Calcareous Alps would be indispensable to understand the tectonic context in which the Achental structure is located. Figure 1b) is fine but perhaps a simple cross-section crossing the thrust zone to show the thickness distribution of the Upper Jurassic Oberalm Fm. on both blocks of the Achental thrust would also help to constrain the potential geometry of extensional fault based on thickness variations.
Line 100 & Figure 2
The description of the stratigraphy is exhaustive and complex. Figure 2 is difficult for me to understand because of the colours used, due to the lack of lithological patterns within the colours and the lack of sedimentary thicknesses. The colours do not follow the specific colours for the different ages typical and recommended from the International Chronostratigraphic Charts. The lithologies are not shown and the rheology of the sedimentary succession in not shown. Nor is the thickness of the different sedimentary successions (you could even add minimum to maximum thicknesses) if it is very variable. And an important issue is the lack of associated rheology since the shape of the stratigraphic table does not show the hardness or weakness of the material that is very important for the mechanical stratigraphy.
Also together with the mechanical stratigraphy, the potential intermediate detachment levels could be added (evaporites, shales, etc…) making a better link with the cross sections and the analogue models.
In addition, in the cross-sections of Figure 4 there are only 5 different (colours) successions and in the model set up there are only 4-5 stratigraphic units. Then, do you need this super complex stratigraphic table in Figure 2 where no direct mention is made regarding the units that are important to explain the cross-section and the model set-ups?
Line 165 & figure 3
I already understand that the map and the cross sections come from previous works and have not been modified in this study but it is very difficult for me to understand the geological-tectonic map (although it is very beautiful and well done) because everything is a bit "like painted with pastel colours" and with the main lines of the thrusts very thin. The names (labels) of the different anticlines and thrust are so small that are difficult to find them.
Line 165 & Figure 4 / 5
The cross sections, which have already been published previously and which match the geological map, need the names of the main overlaps (labels) to make them more understandable. In any case, the cross-sections do not show or allow to observe the tectonic inversion that is proposed to be modelled on this paper. This is an inconvenience together with the fact (already explained in the paper) that the fault thrust has an opposite dip to that of the supposed normal fault of the model. Although I am not discussing the presented geological cross sections, the large overturned anticline (largely eroded) in Figure 4B shows a culmination of the anticline that is almost 4 km in length (?) although it is not very significant for the results of the paper.
I am also curious on the fact that this large region of Calcareous Alps is beginning to be re-interpreted with an important component of salt tectonic while this document does not refer to it (very shortly in the lithostratigraphic section). Did you have considered incorporating some model involving large overturned domains as recently done in other re-discovered regions in the Tethyan fold belts where halokinetic processes are important and simplify some previous complicated structural interpretations?
Line 265 and Figure 5
It is not very clear to me what criteria are used to justify the geological set up presented in Figure 5 if it is not supported by the map and geological sections presented as a support and starting point for the analogical models... I also suggest that the models A to D2 (in text and Figure 6 and subsequent ones) could be labelled in addition with short label illustrating each of the models by their characteristic structure or / and characteristic stratigraphy). Otherwise it becomes difficult to remember what are their main parameters throughout the description of each of the models in the text.
Line 535 and Figure 13
Apart from the detailed description of each of the models which is long and very descriptive, in the Discussion chapter there is another very long part of the paper on analogue modelling results (pages 24 to 26) and then another section as well long (page 26 to 28) in which models are compared with the example from nature but only on a map (in plan).
The comparison of the resulting cross sections of the models with the geological cross sections of Figure 4 do not show many similarities apart from perhaps C Figure 9, model B of Figure 10, and A of Figure 12 but none of these models present inverted flanks neither in the footwall nor in the hangingwall of the east-dipping main thrust.
The conclusions' two first paragraphs only indicate the starting points for modelling. In addition, data presented only show indirect constraints that are used since neither the map nor the cross-sections show any inverted basin.
The other conclusions would be valid if the starting points were very solid. In this case in which the data only permits to prepare assumptions (which may be true), the analogue model indicates that a geometry similar to the one mapped on the surface can be achieved with a single oblique compression along the N-S normal fault trace, but do not match very well the structure illustrated by the geological cross sections...
As a summary, the methodology used in the paper is correct and it is well written and with very stylish figures, but the article is long and heavy (and it took me a lot of work to go through the detail of the paper). I think that these shortcomings that I have found can be fixed to make the paper clearer and probably shorter and with better comprehensive figures so that a wider number of readers will find it easier to read and understand the difficulties and advantages of the work done to solve this important structural feature in the Northern Calcareous Alps.