the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Mar 2022
Control of crustal strength, tectonic inheritance and extrusion/indentation rates on crustal deformation and basin reactivation: insights from laboratory models
Abstract. Geological settings characterized by the simultaneous action of multiple tectonic regimes provide a unique opportunity to understand complex interactions among different geodynamic processes. From an experimental point of view, these contexts remain comparatively less studied than areas with more simple patterns of deformation resulting from primary plate-boundary interactions. Here, we carried out analog experiments involving simultaneous shortening and orthogonal extension under different rheological conditions, and including the effect of crustal inheritance. We performed brittle experiments and brittle-ductile experiments to simulate cases of “strong” and “weak” crusts, respectively. We present two types of experiments: i) one stage experiments with either shortening-only or synchronous orthogonal shortening and stretching, and ii) two stages experiments with a first phase of stretching and a second phase with either shortening-only or synchronous orthogonal shortening and stretching. In our models, deformation occurs by a combination of normal, thrust, and strike-slip faults with structures location depending on boundary conditions and crustal inheritance. For brittle models, we show that the three types of structures can develop at the same time for intermediate ratios of extrusion over indentation rates (1.4 < Ve/Vs < 2). For brittle-ductile models, we observe either shortening-orthogonal thrust faults associated with conjugate strike-slip faults (mod- els with low Ve/Vs and no initial extensional phase) or stretching-orthogonal normal faults associated with conjugate strike-slip faults (models with high Ve/Vs and initial extensional phase). Whatever the crustal strength, the past deformation history, and the extrusion/indentation ratio, both normal and thrust faults remain with similar orientations, i.e. stretching-orthogonal and shortening-orthogonal, respectively. Instead, strike-slip faults exhibit variable orientations with respect to the indentation direction, which may be indicative of the strength of the crust and/or of the extrusion/indentation ratio. We also show that extensional structures formed during a first stage of deformation are never inverted under orthogonal shortening but can be reactivated as normal or strike-slip faults depending on Ve/Vs. The models replicate some deformation patterns documented in nature. Independently of the crustal rheology or the presence of crustal weaknesses, conjugate strike-slips faults develop along with variable normal faulting during compression/indentation, reminiscent of tectonic escape processes along the Himalayas-Alpine chain.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-62', Nemanja Krstekanic, 22 Apr 2022
General comments
In this manuscript, the authors use crustal-scale analogue modelling to study a complex tectonic system in which indentation-driven and extrusion-driven deformation overlap in space and time and result in different coeval tectonic regimes. The topic of this research is very welcome as there is still a lack of knowledge on various controlling factors of such processes’ interplay. The manuscript is well structured and written, scientifically very valid, with a clear description of the methodology, results, interpretations and conclusions. The title is informative and reflects the content of the manuscript, while the language is good. Taking all of that into account, I consider it a nice contribution to Solid Earth.
I have a few moderate to minor comments that I’ll point out below. Also in the attached annotated pdf of the manuscript, I have smaller comments that I hope will help the authors clarify a few minor things in the text.
Specific comments
- Referencing existing publications is generally very good in the manuscript. However, I would suggest to slightly expand the comparison with existing studies of the complex interplay of different tectonic regimes, both in the Introduction when introducing the studied problem and in the Discussion when comparing to the novel results of this paper. Several relatively recent papers deal with indentation and extrusion or interplay of different tectonic regimes, using both analogue modelling and field data. See also annotated pdf.
- Section 4.1: While I generally agree with the content of this section I think it is too long and can be shortened. Also, this section would apply more to the homogeneous system, while in all models in this study there is a rheological and/or structural heterogeneity, which, in my opinion, significantly influences the deformation. It is not only the distance to the model margins (i.e., indenter and extrusion-related pull). I think the limitation of the modelling setup (i.e., relatively low amount of total shortening) has an impact on the evolution of thrusting, as thrusts will form after a certain accumulation of shortening. I think it is not only the extension/shortening ratio but the total accumulation of strain that plays an important role. This issue should be discussed more in this section. Another factor that should be taken into account is the compressional wedge that forms close to the indenter. This wedge increases the vertical load in the model, therefore increasing the vertical stress, which significantly affects the distribution of stress and strain in the model. I think all these factors should be considered and better discussed in this section. So, my suggestion is to modify section 4.1 to make it more concise and focused, while discussing all factors that affect the tectonic regime(s) in the models.
- Orientation of strike-slip faults: I made several comments in the annotated pdf about the change of strike-slip fault orientation as this is one of the important results of this study. Please consider that some of them can be boundary effects, or that some of them are indentation-driven or extrusion-driven. This last terminological distinction can be considered, but it is just a suggestion, authors do not have to accept it. Anyway, I think a bit more discussion about what controls the strike-slip fault orientation is needed in section 4.2.
- Referring to figures should be stronger in the text. I suggest to authors to refer to figures more often. This will make the connection between the text and figures much stronger and will help readers to follow the text more easily.
- Figures are generally good and informative. However, I have a few suggestions on how to improve them:
- Maybe it would be good to have an additional figure (maybe new Fig. 1) to accompany the problem statement and to illustrate the processes and some natural examples mentioned in the Introduction.
- When a figure has more than one panel, I suggest putting a letter on each panel to make it clear which part of the figure authors refer to in the text (e.g., Fig. 3c).
- Panels in figures 3, 4, 5, 8 and 13 are too small and it is difficult to read them. Try to make panels larger.
- I understand why it is important to show plots of principal stretches because they are used to derive strain type. However, these plots are not necessary here and are not discussed in the text. They also take space that can be used to make other panels larger. I suggest moving principal stretches plots from figures 3, 4, 6, 7 and 9 to Supplementary Material and maybe combining figures 3 and 4 and also figures 6, 7 and 8. This will reduce the number of figures (which is already large), while no information will be lost.
- Other smaller comments about figures I added in the annotated pdf.
Technical comment
- There are just a few typos and some technical errors I managed to see. I marked them in the annotated pdf. Otherwise, the text is technically very good.
I’m looking forward to these small corrections and the publication of this manuscript.
- AC1: 'Reply on RC1', Benjamin Guillaume, 06 Jun 2022
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RC2: 'Comment on egusphere-2022-62', Marco Bonini, 24 Apr 2022
The manuscript by Guillaume et al. presents the results of a series of analogue models addressing the role of simultaneous shortening and orthogonal extension under different rheological conditions, including the role of inherited crustal heterogeneities. The paper is concise and well written, and figure are well drafted. In addition, the aims are clearly stated, and the modeling results are analyzed through up-to-dated techniques (Particle Image velocimetry (PIV), and subsequent velocity and strain analysis). The extent of the conclusions is generally supported by the presented data, and the results may be attractive for an international readership. The manuscript is thus suitable for being published in Solid Earth (SE) after a minor/moderate revision. The issues that should be addressed during the revision are listed below and keyed to line number in the text.
General points
Conceptual simulation of indentation and lateral extrusion. In this experimental study, lateral extrusion is achieved by applying a shortening-orthogonal extension to the model. However, in this experimental procedure the system is not developing spontaneously, but its evolution is imposed by the boundary conditions (i.e., the shortening-orthogonal extension). In other models, lateral extrusion (associated with V-shaped strike-slip systems) simply resulted from a model set-up with lateral strength/thickness variations (Sokoutis et al 2000, Tectonophysics), and/or accomplished by weak lateral confinement (e.g., Ratschbacher et al., 1991, Tectonics). The authors agree on that and acknowledge the primary role exerted by a weak crust in favouring a lateral tectonic escape (Lines 403-404). On this basis, I think that some more discussion on the rationale of the modelling and its comparison with previous models would be necessary for a more in-depth comparison with the natural cases sketched in Figure 14.
Adopted terminology. The reasons of using the terms ‘extrusion rate’ (stretching velocity) and ‘indentation rate’ (shortening velocity) should be discussed in more detail. Indentation refers to a case where the colliding block is much shorter than the indented continent. However, I cannot identify this condition in the model setup of Figure 1. So why not use the terms shortening rate and extension rate? Please comment on this.
Rheology of analogue materials. The lower crust has been simulated using PDMS silicone putty (Lines 85-86). Consequently, this silicone has a lower density than the overlying Fontainebleau quartz sand that simulates the upper crust. As correctly stated by the authors, this implies an inappropriate density ratio between upper and lower crust in the models. Density contrast in the model should be equal- or at least similar - to nature. In other terms the viscous layer is too buoyant (or the sand too heavy). This may produce a strong vertical instability that may amplify the folding of the brittle-ductile interface during shortening or extension, and ultimately affect the modelling results at some extent. Please clarify the choice of the PDMS silicone (technical advantages?).
Other points
Line 83. Was the cohesion of the sand measured in this study? If not, please provide a reference.
Lines 126-130. Please report in Table 2 the Ramberg and Smoluchowsky-like numbers (Rm, Sm) for both model and nature.
Lines 147-149. Corti et al. (2006, Spec. Paper GSA) performed similar analogue models characterised by coeval shortening and orthogonal extension, which were applied to the Sicily Channel.
Lines 180-182. The convexity observed in some models could result from a high friction of the side walls. Have you adopted any technical practices to minimize this effect?
Lines 301-305. Have you considered the possibility that these anomalous faults departing from the edges of the graben could represent only boundary effects?
Lines 413-416. It is not clear why shortening-parallel thrusting should develop in this model. Has shortening-parallel thrusting been identified in any model of this series? Please clarify. Spontaneous shortening-parallel thrusting resulted in the above-mentioned model by Sokoutis et al (2000), which - differently from this experimental series - were isostatically compensated. This may represent a key difference with respect to the current series of models.
Lines 422-423. Please give more details about the characteristics of fault reactivation. From the tectonic setting (basin-parallel shortening) I would expect some component of strike-slip movement. What is the dominant kinematics of reactivated normal faults?
Figure 1. Please indicate the ‘seeds’ in the model setup.
Figure12. What does the yellow layer at the base of the model configuration in the left panel represent? This model is referred to as purely brittle, but this yellow area is equivalent to the basal ductile silicone of the models shown in the middle and right panels. Please clarify this.
Citation: https://doi.org/10.5194/egusphere-2022-62-RC2 - AC2: 'Reply on RC2', Benjamin Guillaume, 06 Jun 2022
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EC1: 'Comment on egusphere-2022-62', Ernst Willingshofer, 08 Jun 2022
Dear colleagues,
Your manuscript has been evaluated by two reviewers who provide a number of valuable suggestions to improve on the modelling aspects (setup, interpretation and discussion) that were underrepresented in the original version of the manuscript. Additionally, the reviewers provide useful suggestions to gain clarity and to incorporate additional literature that is relevant for your study. With regard to the latter, I’ like to encourage the authors to also have a look at:
Cruden, A.C., Nasseri, M.H.B., Pyskklywec, R., 2006. Surface topography and internal strain variation in wide hot orogens from three-dimensional analogue and two dimensional numerical vice models. Analogue and Numerical Modelling of Crustal-Scale Processes. S. J. H. Buiter, Schreurs, G. Geol. Soc. Lond. Spec. Publ. 253, 79–104.
Rosenberg, C.L., Brun, J.P., Cagnard, F., Gapais, D., 2007. Oblique indentation in the Eastern Alps: insights from laboratory experiments. Tectonics26, 1–23.
Though both studies are done on the scale of the lithosphere, the results are important for understanding lateral transitions form shortening to strike-slip to extensional deformation regimes and their respective structural expression and the paper by Rosenberg et al. is also relevant for the application of your findings to extrusion in the Alps.
I am looking forward to receiving the revised version of the manuscript.
Ernst Willingshofer
Citation: https://doi.org/10.5194/egusphere-2022-62-EC1
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2022-62', Nemanja Krstekanic, 22 Apr 2022
General comments
In this manuscript, the authors use crustal-scale analogue modelling to study a complex tectonic system in which indentation-driven and extrusion-driven deformation overlap in space and time and result in different coeval tectonic regimes. The topic of this research is very welcome as there is still a lack of knowledge on various controlling factors of such processes’ interplay. The manuscript is well structured and written, scientifically very valid, with a clear description of the methodology, results, interpretations and conclusions. The title is informative and reflects the content of the manuscript, while the language is good. Taking all of that into account, I consider it a nice contribution to Solid Earth.
I have a few moderate to minor comments that I’ll point out below. Also in the attached annotated pdf of the manuscript, I have smaller comments that I hope will help the authors clarify a few minor things in the text.
Specific comments
- Referencing existing publications is generally very good in the manuscript. However, I would suggest to slightly expand the comparison with existing studies of the complex interplay of different tectonic regimes, both in the Introduction when introducing the studied problem and in the Discussion when comparing to the novel results of this paper. Several relatively recent papers deal with indentation and extrusion or interplay of different tectonic regimes, using both analogue modelling and field data. See also annotated pdf.
- Section 4.1: While I generally agree with the content of this section I think it is too long and can be shortened. Also, this section would apply more to the homogeneous system, while in all models in this study there is a rheological and/or structural heterogeneity, which, in my opinion, significantly influences the deformation. It is not only the distance to the model margins (i.e., indenter and extrusion-related pull). I think the limitation of the modelling setup (i.e., relatively low amount of total shortening) has an impact on the evolution of thrusting, as thrusts will form after a certain accumulation of shortening. I think it is not only the extension/shortening ratio but the total accumulation of strain that plays an important role. This issue should be discussed more in this section. Another factor that should be taken into account is the compressional wedge that forms close to the indenter. This wedge increases the vertical load in the model, therefore increasing the vertical stress, which significantly affects the distribution of stress and strain in the model. I think all these factors should be considered and better discussed in this section. So, my suggestion is to modify section 4.1 to make it more concise and focused, while discussing all factors that affect the tectonic regime(s) in the models.
- Orientation of strike-slip faults: I made several comments in the annotated pdf about the change of strike-slip fault orientation as this is one of the important results of this study. Please consider that some of them can be boundary effects, or that some of them are indentation-driven or extrusion-driven. This last terminological distinction can be considered, but it is just a suggestion, authors do not have to accept it. Anyway, I think a bit more discussion about what controls the strike-slip fault orientation is needed in section 4.2.
- Referring to figures should be stronger in the text. I suggest to authors to refer to figures more often. This will make the connection between the text and figures much stronger and will help readers to follow the text more easily.
- Figures are generally good and informative. However, I have a few suggestions on how to improve them:
- Maybe it would be good to have an additional figure (maybe new Fig. 1) to accompany the problem statement and to illustrate the processes and some natural examples mentioned in the Introduction.
- When a figure has more than one panel, I suggest putting a letter on each panel to make it clear which part of the figure authors refer to in the text (e.g., Fig. 3c).
- Panels in figures 3, 4, 5, 8 and 13 are too small and it is difficult to read them. Try to make panels larger.
- I understand why it is important to show plots of principal stretches because they are used to derive strain type. However, these plots are not necessary here and are not discussed in the text. They also take space that can be used to make other panels larger. I suggest moving principal stretches plots from figures 3, 4, 6, 7 and 9 to Supplementary Material and maybe combining figures 3 and 4 and also figures 6, 7 and 8. This will reduce the number of figures (which is already large), while no information will be lost.
- Other smaller comments about figures I added in the annotated pdf.
Technical comment
- There are just a few typos and some technical errors I managed to see. I marked them in the annotated pdf. Otherwise, the text is technically very good.
I’m looking forward to these small corrections and the publication of this manuscript.
- AC1: 'Reply on RC1', Benjamin Guillaume, 06 Jun 2022
-
RC2: 'Comment on egusphere-2022-62', Marco Bonini, 24 Apr 2022
The manuscript by Guillaume et al. presents the results of a series of analogue models addressing the role of simultaneous shortening and orthogonal extension under different rheological conditions, including the role of inherited crustal heterogeneities. The paper is concise and well written, and figure are well drafted. In addition, the aims are clearly stated, and the modeling results are analyzed through up-to-dated techniques (Particle Image velocimetry (PIV), and subsequent velocity and strain analysis). The extent of the conclusions is generally supported by the presented data, and the results may be attractive for an international readership. The manuscript is thus suitable for being published in Solid Earth (SE) after a minor/moderate revision. The issues that should be addressed during the revision are listed below and keyed to line number in the text.
General points
Conceptual simulation of indentation and lateral extrusion. In this experimental study, lateral extrusion is achieved by applying a shortening-orthogonal extension to the model. However, in this experimental procedure the system is not developing spontaneously, but its evolution is imposed by the boundary conditions (i.e., the shortening-orthogonal extension). In other models, lateral extrusion (associated with V-shaped strike-slip systems) simply resulted from a model set-up with lateral strength/thickness variations (Sokoutis et al 2000, Tectonophysics), and/or accomplished by weak lateral confinement (e.g., Ratschbacher et al., 1991, Tectonics). The authors agree on that and acknowledge the primary role exerted by a weak crust in favouring a lateral tectonic escape (Lines 403-404). On this basis, I think that some more discussion on the rationale of the modelling and its comparison with previous models would be necessary for a more in-depth comparison with the natural cases sketched in Figure 14.
Adopted terminology. The reasons of using the terms ‘extrusion rate’ (stretching velocity) and ‘indentation rate’ (shortening velocity) should be discussed in more detail. Indentation refers to a case where the colliding block is much shorter than the indented continent. However, I cannot identify this condition in the model setup of Figure 1. So why not use the terms shortening rate and extension rate? Please comment on this.
Rheology of analogue materials. The lower crust has been simulated using PDMS silicone putty (Lines 85-86). Consequently, this silicone has a lower density than the overlying Fontainebleau quartz sand that simulates the upper crust. As correctly stated by the authors, this implies an inappropriate density ratio between upper and lower crust in the models. Density contrast in the model should be equal- or at least similar - to nature. In other terms the viscous layer is too buoyant (or the sand too heavy). This may produce a strong vertical instability that may amplify the folding of the brittle-ductile interface during shortening or extension, and ultimately affect the modelling results at some extent. Please clarify the choice of the PDMS silicone (technical advantages?).
Other points
Line 83. Was the cohesion of the sand measured in this study? If not, please provide a reference.
Lines 126-130. Please report in Table 2 the Ramberg and Smoluchowsky-like numbers (Rm, Sm) for both model and nature.
Lines 147-149. Corti et al. (2006, Spec. Paper GSA) performed similar analogue models characterised by coeval shortening and orthogonal extension, which were applied to the Sicily Channel.
Lines 180-182. The convexity observed in some models could result from a high friction of the side walls. Have you adopted any technical practices to minimize this effect?
Lines 301-305. Have you considered the possibility that these anomalous faults departing from the edges of the graben could represent only boundary effects?
Lines 413-416. It is not clear why shortening-parallel thrusting should develop in this model. Has shortening-parallel thrusting been identified in any model of this series? Please clarify. Spontaneous shortening-parallel thrusting resulted in the above-mentioned model by Sokoutis et al (2000), which - differently from this experimental series - were isostatically compensated. This may represent a key difference with respect to the current series of models.
Lines 422-423. Please give more details about the characteristics of fault reactivation. From the tectonic setting (basin-parallel shortening) I would expect some component of strike-slip movement. What is the dominant kinematics of reactivated normal faults?
Figure 1. Please indicate the ‘seeds’ in the model setup.
Figure12. What does the yellow layer at the base of the model configuration in the left panel represent? This model is referred to as purely brittle, but this yellow area is equivalent to the basal ductile silicone of the models shown in the middle and right panels. Please clarify this.
Citation: https://doi.org/10.5194/egusphere-2022-62-RC2 - AC2: 'Reply on RC2', Benjamin Guillaume, 06 Jun 2022
-
EC1: 'Comment on egusphere-2022-62', Ernst Willingshofer, 08 Jun 2022
Dear colleagues,
Your manuscript has been evaluated by two reviewers who provide a number of valuable suggestions to improve on the modelling aspects (setup, interpretation and discussion) that were underrepresented in the original version of the manuscript. Additionally, the reviewers provide useful suggestions to gain clarity and to incorporate additional literature that is relevant for your study. With regard to the latter, I’ like to encourage the authors to also have a look at:
Cruden, A.C., Nasseri, M.H.B., Pyskklywec, R., 2006. Surface topography and internal strain variation in wide hot orogens from three-dimensional analogue and two dimensional numerical vice models. Analogue and Numerical Modelling of Crustal-Scale Processes. S. J. H. Buiter, Schreurs, G. Geol. Soc. Lond. Spec. Publ. 253, 79–104.
Rosenberg, C.L., Brun, J.P., Cagnard, F., Gapais, D., 2007. Oblique indentation in the Eastern Alps: insights from laboratory experiments. Tectonics26, 1–23.
Though both studies are done on the scale of the lithosphere, the results are important for understanding lateral transitions form shortening to strike-slip to extensional deformation regimes and their respective structural expression and the paper by Rosenberg et al. is also relevant for the application of your findings to extrusion in the Alps.
I am looking forward to receiving the revised version of the manuscript.
Ernst Willingshofer
Citation: https://doi.org/10.5194/egusphere-2022-62-EC1
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Benjamin Guillaume
Guido M. Gianni
Jean-Jacques Kermarrec
Khaled Bock
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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