the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 13 May 2022
Structural characterization and K-Ar illite dating of reactivated, complex and heterogeneous fault zones: Lessons from the Zuccale Fault, Northern Apennines
Abstract. We studied the Zuccale Fault on Elba Island, Northern Apennines, to unravel the complex deformation history that is responsible for the remarkable architectural complexity of the fault. The ZF is characterized by a patchwork of at least six distinct, now tightly juxtaposed Brittle Structural Facies (BSF), that is, volumes of deformed rock characterized by a given fault rock type, texture, color, composition, and age of formation. ZF fault rocks vary from massive cataclasite to foliated ultracataclasite, from clay-rich gouge to highly sheared talc phyllonite. Understanding the current spatial juxtaposition of these BSF’s requires tight constraints on their age of formation during the ZF life span to integrate current fault geometries and characteristics over the time dimension of faulting. We present new K-Ar gouge dates obtained from three samples from two different BSF’s. Two top-to-the E foliated gouge and talc phyllonite samples document faulting in the Aquitanian (c. 22 Ma), constraining E-vergent shearing along the ZF already in the earliest Miocene. A third sample constrains later faulting along the exclusively brittle, flat-lying principal slip surface to < c. 5 Ma. The new structural and geochronological results reveal an unexpectedly long faulting history spanning a ca. 20 Ma long time interval in the framework of the evolution of the Northern Apennines. The current fault architecture is highly heterogeneous as it formed at very different times under different environmental conditions during this prolonged history. We propose that the ZF started as an Aquitanian thrust which then became selectively reactivated by early Pliocene out-of-sequence thrusting during the progressive structuring of the Northern Apennines wedge. These results require the critical analysis of existing geodynamic models and call for alternative scenarios of continuous convergence between the late Oligocene and the early Pliocene with a major intervening phase of extension in the middle Miocene allowing for the isostatic re-equilibration of the Northern Apennines wedge. Extension started again in the Pliocene and is still active in the innermost portion of the Northern Apennines. In general terms, long-lived, mature faults can be architecturally very complex. Their unraveling, including understanding the dynamic evolution of their mechanical properties, requires a multidisciplinary approach combining detailed structural analyses with dating the deformation events recorded by the complex internal architecture, which is a phenomenal archive of faulting and faulting conditions through time and in space.
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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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RC1: 'Comment on egusphere-2022-229', Anonymous Referee #1, 23 Jun 2022
Viola and coworkers present the results of a combined structural and geochronological study of the Zuccale Fault (Island of Elba, Northern Apennines Hinterland, Italy), an iconic structure subject of several studies over the years which have also led to very different interpretations.
The manuscript is generally well-written and provides a valuable contribution in understanding the time-constrained structural and mechanical evolution of architecturally complex fault zones. The internal structure of the Zuccale Fault was addressed by means of the “Brittle Structural Facies - BSF” approach recently proposed by Tartaglia et al., (2020), and by the direct dating of some of the juxtaposed BSF using a well-consolidated methodology.
As well as being an interesting example of how to unravel the deformation history of a long-lived fault, the research is also very interesting because proposes new time constraints for the reconstruction of the structural evolution of the Northern Apennines hinterland, a crucial area for understanding the structural and geodynamic evolution of the Apennines, which is currently much debated.
Main specific comments:
- Authors suggest that the preservation of Aquitanian ages within the fault zone excludes significant thermal overprinting possibly associated with the intrusion of the Porto Azzurro Pluton. This is quite astonishing due to the location and geological evolution of the area, and I think it would need a more in-depth discussion (i. e. addressing what would be the P-T conditions that could eventually prevent a dating in the frame of the thermal evolution of the area, thus defining the thermal window that affected the area)
- it would be useful to try to insert in the discussion, as well as in the cartoon of figure 9, the effects of the middle Miocene extensional phase. A brief discussion of how this LANF phase may have (or not) reactivated the pre-existing thrusts (negative inversion tectonics) as well as the subsequent OOSTs (including the ZF) may also have (or not) reactivated the previous LANFs (negative inversion). Definitely, it would be pretty useful to briefly discuss these possible scenarios.
- the proposed reconstruction once again brings attention to the possible emplacement of magmatic bodies during shortening in southern Tuscany. This seems to confirm what has already been proposed for the Gavorrano area (Mazzarini et al., 2004), but also for the Larderello-Travale geothermal field (Sani et al., 2016). This is consistent with the argument that the Island of Elba could be considered an exhumed analogue of the deep roots of the Larderello geothermal system, with huge implications for the evolution of the well-known Tuscan geothermal systems. I believe that a brief discussion focused on these issues would be useful in the frame of the ongoing debate, also for the important associated implications.
- Some figures need to be implemented, see comments in the annotated version of the ms.
Overall, the topic and the results are certainly of interest to a wide readership, the scientific methods and assumptions are effective and clearly outlined, the results adequate to support the interpretations and conclusions, and thus the manuscript is appropriate for being published in Solid Earth after minor revision.
Please also note the supplement to this comment with detailed minor points to be addressed
References:
Mazzarini, F., G. Corti, G. Musumeci, and F. Innocenti (2004), Tectonic control on laccolith emplacement in northern Apennines fold-thrust belt: The Gavorrano intrusion (southern Tuscany, Italy), in Physical Geology of High-Level Magmatic Systems, edited by C. Breitkreuz, and N. Petford, Geol. Soc. Spec. Publ., 234, 151– 161.
Sani, F., Bonini, M., Montanari, D., Moratti, G., Corti, G., & Del Ventisette, C. (2016). The structural evolution of the Radicondoli–Volterra Basin (southern Tuscany, Italy): Relationships with magmatism and geothermal implications. Geothermics, 59, 38-55.
Tartaglia, G., Viola, G., van der Lelij, R., Scheiber, T., Ceccato, A., and Schönenberger, J.: “Brittle structural facies” analysis: A diagnostic method to unravel and date multiple slip events of long-lived faults, Earth and Planetary Science Letters, 545, 10.1016/j.epsl.2020.116420, 2020.
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AC1: 'Reply on RC1', Giulio Viola, 25 Jul 2022
Bologna, July 23, 2022
We thank Reviewer 1 for the constructive inputs to our manuscript. We appreciate that our methodological approach, which builds on- and further develops the work by Tartaglia et al. (2020), is considered sound and useful when dealing with the time-constrained structural and mechanical evolution of architecturally complex fault zones. This is indeed our main goal and hope that the community may wish to adopt and further refine it in similar future studies.
The points raised by the Reviewer deal, instead, with the geological implications that our study (hence, the Brittle Structural Facies approach - BSF) bears on the understanding and on the possible revisiting of some aspects of the regional geological evolution of the Northern Apennines.
In the following we address these points individually and anticipate the changes that we will be implementing in the resubmitted text to improve it as per these useful inputs.
In Italic font are the original comments, while our replies/rebuttal are in normal font.
Authors suggest that the preservation of Aquitanian ages within the fault zone excludes significant thermal overprinting possibly associated with the intrusion of the Porto Azzurro Pluton. This is quite astonishing due to the location and geological evolution of the area, and I think it would need a more in-depth discussion (i. e. addressing what would be the P-T conditions that could eventually prevent a dating in the frame of the thermal evolution of the area, thus defining the thermal window that affected the area).
This is an interesting, yet rather complex, issue, the solution of which (if ever possible) requires considering elements of regional geology, structural considerations and analytical constraints on the K-Ar dating method applied to fine-grained clays.
Dating generally assumes that no- or little isotopic re-equilibration has occurred since the dated minerals formed. However, exposure to temperatures at or above the formation temperature of the dated clay in the studied gouge (for example, to the high-temperature effects of the cooling Porto Azzurro pluton) for considerable time intervals may cause volume diffusion of radiogenic 40Ar, leading to the partial or complete resetting of the system and, thus, to mixed ages that are devoid of geological meaning. It is, therefore, correct to wonder (as the Reviewer does) about the preservation of ages as old as Aquitanian within the complex patchwork of Brittle Structural Facies of the Zuccale Fault.
Finding such old ages, however, is perhaps, not at all incompatible with the idea of complex evolution of the internal architecture of long-lived fault zones.
When distancing ourselves from the static vision of a fault architecture as we see it in the field and when instead trying to integrate such architecture and its dynamic evolution over the time dimension of localization of faulting, then it becomes easier to understand and accept that (as the BSF approach predicts) different structural facies that are now side by side may have formed very far apart and at very different times.
In the specific case of the Aquitanian ages of the ZF, those ages may thus reflect the fact that the very specific BSF’s preserving them made it to their current structural position only after the waning of the resetting effects of the thermal aureole of the Port Azzurro pluton. Those BSF’s, therefore, would reflect deformation and isotopic records acquired elsewhere more to the west, with their final translation to the current structural position occurring only after final cooling.
We actually had already commented on this in the original text, but we will certainly better stress this possibility in the amended version, highlighting even more clearly the implications that this possibility has upon the structuring of the ZF complex architecture.
A different (yet complementary) take on this point relates to the systematics of K-Ar of fine-grained clays. In more detail, one may wonder how, if the BSF’s containing Aquitanian ages were not translated to their current location after final cooling of the pluton, those ages could “escape” thermal resetting by the Porto Azzurro. After all, the estimated pressure-temperature (P-T) conditions of its contact aureole are reported as ranging from 300 °C (biotite zone) to 650 °C (andalusite–K-feldspar zone and wollastonite zone), with Pmax <0.18–0.2 GPa (Duranti et al., 1992; Caggianelli et al., 2018; Papeschi et al., 2019). These P-T conditions are diagnostic of a low-pressure/high-temperature (LP/HT) contact metamorphism and indicate that the Porto Azzurro pluton was emplaced at a very shallow crustal level. These conditions would almost certainly suffice to reset the Aquitanian isotopic signature, if maintained over long-enough time spans.
We stress in here, however, the nature of complex fault zones such as the ZF, whose transient behavior (both mechanical and thermal) represents a significant difference to the “static” environments that are generally used when conceptualizing Ar diffusion in coarse mica grains, and, thus, partial or total resetting. To explore possible resetting scenarios of the Ar signature within clays, we can refer to the study by Torgersen et al. (2014), who tried to evaluate the impact of thermal pulses of different duration upon different grain-size clay fractions. Their calculations assumed a cylindrical grain geometry and modeling was repeated for a range of grain-sizes (10, 2 and 0.1 μm), peak temperatures (190-370° C) and duration of thermal episodes (5 and 10 Ma). They concluded that during heating-cooling pulses of 5 and 10 Ma (that is, very long compared to the duration of contact metamorphism by the Porto Azzurro of only a couple of Myrs) to temperatures of 230-240° C (quite lower than those of contact metamorphism), even very fine-grained illites (< 0.1 μm) would not experience more than a 10% resetting of their initial K/Ar age. At 300-310°C, on the other hand, the Ar isotopic system of the < 0.1 and 2 μm grains would be completely reset. In summary, although Ar diffusion should not be completely ruled out, we feel confident that our internally consistent data do not reflect a significant influence of radiogenic 40Ar diffusion and this is well supported by diffusion modelling done with the most recent and relevant diffusion parameters for clays. These results, therefore, indicate that the Aquitanian ages did escape thermal resetting and are thus geologically meaningful, pointing to a discrete faulting event (recorded by two samples in different portions of the fault zone) that is well preserved in both the measured isotopic signature and the structural framework.
Only a BSF approach can unravel these complexities.
We will elaborate further on this important point in the revised version of the text, by also clarifying the prevalent P-T conditions during the thermal anomaly associated with the Porto Azzurro pluton.
It would be useful to try to insert in the discussion, as well as in the cartoon of Figure 9, the effects of the middle Miocene extensional phase. A brief discussion of how this LANF phase may have (or not) reactivated the pre-existing thrusts (negative inversion tectonics) as well as the subsequent OOSTs (including the ZF) may also have (or not) reactivated the previous LANFs (negative inversion). Definitely, it would be pretty useful to briefly discuss these possible scenarios.
We agree and thank Reviewer 1 for pointing this out. We originally abstained from doing it because we thought it could excessively complicate both the discussion and the figure. After all, that part of the story is studied and thoroughly discussed in Massa et al. (2017), which we extensively quote in our work. The comment by the Reviewer, however, has convinced us to explicitly address this important step of the local evolution in our work and the amended version of the text will, therefore, contain an add-on on this and Figure 9 will be amended accordingly.
The proposed reconstruction once again brings attention to the possible emplacement of magmatic bodies during shortening in southern Tuscany. This seems to confirm what has already been proposed for the Gavorrano area (Mazzarini et al., 2004), but also for the Larderello-Travale geothermal field (Sani et al., 2016). This is consistent with the argument that the Island of Elba could be considered an exhumed analogue of the deep roots of the Larderello geothermal system, with huge implications for the evolution of the well-known Tuscan geothermal systems. I believe that a brief discussion focused on these issues would be useful in the frame of the ongoing debate, also for the important associated implications.
We agree with this suggestion and will insert a short discussion on this, even though this aspect of the regional geological evolution is not central to the mostly methodological flavour of our contribution. Already here we point out that several scenarios contemplating different emplacement mechanisms have been proposed in the last few decades for the Porto Azzurro pluton. Pluton emplacement was initially attributed to extensional structures accommodating local gravitational instabilities (Trevisan, 1950; Pertusati et al., 1993). Subsequently, The Porto Azzurro Pluton emplacement has been interpreted as being coeval with extensional faulting of the Northern Apennines upper crust (e.g., Smith et al., 2010) or with- and driven by oblique, transtensional tectonics (Liotta et al., 2015). Recently, according to Spiess et al (2021), the Porto Azzurro Pluton has been interpreted as a syn-kinematic intrusion emplaced in the footwall of the active Zuccale Fault. The results of the geological mapping of the entire Calamita Peninsula by some of our research group, along with meso- and micro-structural analysis of the host rock fabrics, however, suggest to us that the Porto Azzurro pluton was emplaced into the Ortano and Calamita units during overall crustal shortening (Mazzarini et al., 2011; Musumeci and Vaselli, 2012; Papeschi et al; 2017; Papeschi et al; 2021; Papeschi et al., 2022), and certainly before the last recorded early Pliocene activity of the ZF (Musumeci et al., 2015; Viola et al., 2018).
Some figures need to be implemented, see comments in the annotated version of the ms.
Please also note the supplement to this comment with detailed minor points to be addressed
We will attend to all this in the next version of the manuscript.
Giulio Viola
Citation: https://doi.org/10.5194/egusphere-2022-229-AC1
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RC2: 'Comment on egusphere-2022-229', Anonymous Referee #2, 07 Jul 2022
Paper: Structural characterization and K-Ar illite dating of reactivated, complex and heterogeneous faut zones: Lessons from the Zuccale Fault, Northern Apennines
Authors: Viola G. et al.
General comment
In this work Viola and co-authors provide insights on a new analytical approach in studying complex fault zones with a polyphasic activity. The authors combine multiscalar structural studies and fabric analysis to identify different structural-mineralogical facies within the fault zone. Relative chronological relationships defined among the occurring facies have then been used as the base to identify suitable samples for K-Ar dating of fault gouges. Age data relative to fault activity have been then framed in the orogenic evolution of the Northern Apennines.
The paper is well written and provide an integration to an already previously described methodological approach in studying brittle fault zones (Tartaglia et al., 2020; Vignaroli et al., 2021). The designed fault is well exposed and object of study of several works since 1990 (Keller and Pialli) and has been considered has as a reference example of a low angle normal fault in recent works (e.g. Smith and Faulkner, 2010; Collettini et al., 2011) due to its flat to shallow E-ward dipping and top-to-E transport direction.
The data here provided suggest a different interpretation, following the one proposed by Musumeci et al. (2015), and consider the Zuccale Fault as a thrust fault active at different times from the Aquitanian to the Miocene-Pliocene boundary. The provided interpretation is quite well supported by presented data but I feel that it needs a more detailed description of the relationships among fault activity and the intrusion of the Porto Azzurro pluton. In figure 9, the one that summarizes the tectonic significance of the Zuccale Fault, the evolution step in which the granite intruded the already formed stack is missing and I strongly suggest the authors to consider it in the in the figure.
I feel the paper will be ready to be published after a MINOR REVISION. Minor comments on text and figures are listed below.
Line 19: Add the acronym ZF after “Zuccale Fault”. In other parts of the text “fault” is written within the initial cap: please select a form and use it in the whole text, or simply use the “ZF” acronym.
Line 31: “ambient” conditions instead of “environmental conditions”
Line 190: I will move here the discussion on the normal vs. thrust kinematics of the Zuccale Fault
Line 200-203: Please express here also the thickness and its lateral variability of the foliated domain
Line 235: 40 and 39 should be formatted as superscript font.
Line 241: Is there any information related to the depth of emplacement of the Porto Azzurro pluton
Line 316: The “Triassic Verrucano quartzite and metapelite” are named in several way throughout the text (the Verrucano Formation, the Verrucano Fm., the Verrucano….), please choose a form and always use it.
Line 388: Please add a brief description of the Calanchiole Shear Zone
Line 532: in “1Md” and “2M1” d and 1, respectively, should be written as subscript. Please check the whole text for this.
Line 534-539: This section is to my opinion too much speculative. I suggest to discuss in a more detailed manner this point.
Line 635-642: I feel this paragraph may be obscure to readers not accustomed to the Apennines geology. The extensional phase that affected the Northern Apennines and its timing need to be described in brief to better support the interpretation proposed by the authors.
Tables
Table 1: In the L4 box of the central column “phylonite” should be “phyllonite”
Table 2: Substitute “Whole-rock composition” with “Mineraogical composition “ or “Rock mineralogy”. The term “whole rock composition” usually refers to the chemical composition expressed in oxides weight %.
Table 3: Please express the significance of “40Ar*” in the table caption.
Figures
Figure 1: The color used for the Porto Azzurro pluton make it poorly visible. As it represents a key geological marker I strongly suggest to change color and make the pluton more visible. In the Italy inset I would add the label “Apennines”. Font size of the legend in figure (a) should be increased. Figure (b) lacks of topographic labels, please add some.
Figure 2: Please add a vertical scale to the Fence diagram. I suggest to include in the legend also the granitic sills within the Cretacopus flysch, or at least add them in the text near the color box.
Figure 3: I suggest to add a scale bar in figure (a) and (b)
Figure 6: Add a scale bar in figure (a) and (b) or described the width of view in the caption.
Figure 8: It would be useful I order to ameliorate the figure to use for dated samples the same color code used for the BSF’s in figure 2 and table 1. Please explain I the caption the significance of the error bars.
Figure 9: As already explained in the general comment, I feel that another evolutionary step, placed between the second and the third, that show the intrusion of the Porto Azzurro pluton is mandatory. This step would serve to show which are the structures cross cut by the intrusive bodies and which tectonic units underwent thermal metamorphism close to the intrusion.
Citation: https://doi.org/10.5194/egusphere-2022-229-RC2 -
AC2: 'Reply on RC2', Giulio Viola, 25 Jul 2022
Bologna, July 23, 2022
We thank Reviewer 2 for the constructive inputs provided to our manuscript. We are pleased to see that the Reviewer appreciated the proposed methodological approach to the study of long-lived, complex fault systems. Also, we appreciate the fact that, although our data seem to confirm that alternative takes from those commonly reported by the majority of the community on the evolution of the Zuccale Fault (ZF) are partly necessary, the Reviewer thinks that our model is quite well supported by the new data. This is indeed an important point, in our opinion. To discriminate between existing models, it inevitably becomes necessary to produce and make available new results and data that can assist in stepping forward in a specific direction. The results presented here are analytically sound “hard numbers”, which require us to take them into account and use them while refining the understanding of this spectacular structure.
As in the case of Reviewer 1, Reviewer 2 only has minor comments and requests of improvements on the part of the study that deals with the regional framework of the ZF and on how this needs to be partly revisited in light of the new dates.
In the following we address those comments individually and anticipate the changes that we will be implementing to the resubmitted text to improve it as per these useful inputs.
In Italic font are the original comments, while our replies/rebuttal are in normal font. We do not comment in here on all editorial aspects that will certainly be attended to in the revised version.
The data here provided suggest a different interpretation, following the one proposed by Musumeci et al. (2015), and consider the Zuccale Fault as a thrust fault active at different times from the Aquitanian to the Miocene-Pliocene boundary. The provided interpretation is quite well supported by presented data but I feel that it needs a more detailed description of the relationships among fault activity and the intrusion of the Porto Azzurro pluton. In figure 9, the one that summarizes the tectonic significance of the Zuccale Fault, the evolution step in which the granite intruded the already formed stack is missing and I strongly suggest the authors to consider it in the in the figure.
A similar comment has been proposed by Reviewer 1. Both Reviewers commenting on the same issue with basically the same suggestion and request for improvement has convinced us about the necessity to comply with this request. As already written in the reply to Reviewer 1, we originally abstained from elaborating in detail on the relationships between tectonics and plutonism because we thought it could excessively complicate both the discussion and the figure. After all, that part of the story is studied and thoroughly discussed in Vaselli and Musumeci (2012), Massa et al. (2017) and Papeschi et al. (2017) which we extensively quote in our work. This notwithstanding, the amended version of the text will be shortly expanded to also contain an add-on on this aspect and Figure 9 will be amended accordingly.
Line 241: Is there any information related to the depth of emplacement of the Porto Azzurro pluton
The estimated pressure-temperature (P-T) conditions of the Porto Azzurro contact aureole are reported as ranging from 300 °C (biotite zone) to 650 °C (andalusite–K-feldspar zone and wollastonite zone), with Pmax <0.18–0.2 GPa (Duranti et al., 1992). Recently, fluid-inclusion studies (Caggianelli et al., 2018) and detailed investigations of the migmatitic layering of the hosting Calamita Schist unit (Papeschi et al., 2019) showed that these P-T conditions are diagnostic of low-pressure/high-temperature (LP/HT) contact metamorphism and indicate that the Porto Azzurro pluton was emplaced at a very shallow crustal level. The revised text will explicitly report these constraints.
Line 388: Please add a brief description of the Calanchiole Shear Zone
We will be happy to comply with this request in the new version of the manuscript.
Line 635-642: I feel this paragraph may be obscure to readers not accustomed to the Apennines geology. The extensional phase that affected the Northern Apennines and its timing need to be described in brief to better support the interpretation proposed by the authors.
We are happy to comply with this request in the new version of the manuscript. We will specifically address the Mid-Miocene evolution of the Northern Apennines orogenic prism in terms of the significant extensional phase that affected it. We will also amend Figure 9 so as to introduce this further step of the complex local tectonic evolution (see also specific comment to Figure 9).
As to the other requested changes to the figures we will implement them all.
Giulio Viola
Citation: https://doi.org/10.5194/egusphere-2022-229-AC2
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AC2: 'Reply on RC2', Giulio Viola, 25 Jul 2022
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2022-229', Anonymous Referee #1, 23 Jun 2022
Viola and coworkers present the results of a combined structural and geochronological study of the Zuccale Fault (Island of Elba, Northern Apennines Hinterland, Italy), an iconic structure subject of several studies over the years which have also led to very different interpretations.
The manuscript is generally well-written and provides a valuable contribution in understanding the time-constrained structural and mechanical evolution of architecturally complex fault zones. The internal structure of the Zuccale Fault was addressed by means of the “Brittle Structural Facies - BSF” approach recently proposed by Tartaglia et al., (2020), and by the direct dating of some of the juxtaposed BSF using a well-consolidated methodology.
As well as being an interesting example of how to unravel the deformation history of a long-lived fault, the research is also very interesting because proposes new time constraints for the reconstruction of the structural evolution of the Northern Apennines hinterland, a crucial area for understanding the structural and geodynamic evolution of the Apennines, which is currently much debated.
Main specific comments:
- Authors suggest that the preservation of Aquitanian ages within the fault zone excludes significant thermal overprinting possibly associated with the intrusion of the Porto Azzurro Pluton. This is quite astonishing due to the location and geological evolution of the area, and I think it would need a more in-depth discussion (i. e. addressing what would be the P-T conditions that could eventually prevent a dating in the frame of the thermal evolution of the area, thus defining the thermal window that affected the area)
- it would be useful to try to insert in the discussion, as well as in the cartoon of figure 9, the effects of the middle Miocene extensional phase. A brief discussion of how this LANF phase may have (or not) reactivated the pre-existing thrusts (negative inversion tectonics) as well as the subsequent OOSTs (including the ZF) may also have (or not) reactivated the previous LANFs (negative inversion). Definitely, it would be pretty useful to briefly discuss these possible scenarios.
- the proposed reconstruction once again brings attention to the possible emplacement of magmatic bodies during shortening in southern Tuscany. This seems to confirm what has already been proposed for the Gavorrano area (Mazzarini et al., 2004), but also for the Larderello-Travale geothermal field (Sani et al., 2016). This is consistent with the argument that the Island of Elba could be considered an exhumed analogue of the deep roots of the Larderello geothermal system, with huge implications for the evolution of the well-known Tuscan geothermal systems. I believe that a brief discussion focused on these issues would be useful in the frame of the ongoing debate, also for the important associated implications.
- Some figures need to be implemented, see comments in the annotated version of the ms.
Overall, the topic and the results are certainly of interest to a wide readership, the scientific methods and assumptions are effective and clearly outlined, the results adequate to support the interpretations and conclusions, and thus the manuscript is appropriate for being published in Solid Earth after minor revision.
Please also note the supplement to this comment with detailed minor points to be addressed
References:
Mazzarini, F., G. Corti, G. Musumeci, and F. Innocenti (2004), Tectonic control on laccolith emplacement in northern Apennines fold-thrust belt: The Gavorrano intrusion (southern Tuscany, Italy), in Physical Geology of High-Level Magmatic Systems, edited by C. Breitkreuz, and N. Petford, Geol. Soc. Spec. Publ., 234, 151– 161.
Sani, F., Bonini, M., Montanari, D., Moratti, G., Corti, G., & Del Ventisette, C. (2016). The structural evolution of the Radicondoli–Volterra Basin (southern Tuscany, Italy): Relationships with magmatism and geothermal implications. Geothermics, 59, 38-55.
Tartaglia, G., Viola, G., van der Lelij, R., Scheiber, T., Ceccato, A., and Schönenberger, J.: “Brittle structural facies” analysis: A diagnostic method to unravel and date multiple slip events of long-lived faults, Earth and Planetary Science Letters, 545, 10.1016/j.epsl.2020.116420, 2020.
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AC1: 'Reply on RC1', Giulio Viola, 25 Jul 2022
Bologna, July 23, 2022
We thank Reviewer 1 for the constructive inputs to our manuscript. We appreciate that our methodological approach, which builds on- and further develops the work by Tartaglia et al. (2020), is considered sound and useful when dealing with the time-constrained structural and mechanical evolution of architecturally complex fault zones. This is indeed our main goal and hope that the community may wish to adopt and further refine it in similar future studies.
The points raised by the Reviewer deal, instead, with the geological implications that our study (hence, the Brittle Structural Facies approach - BSF) bears on the understanding and on the possible revisiting of some aspects of the regional geological evolution of the Northern Apennines.
In the following we address these points individually and anticipate the changes that we will be implementing in the resubmitted text to improve it as per these useful inputs.
In Italic font are the original comments, while our replies/rebuttal are in normal font.
Authors suggest that the preservation of Aquitanian ages within the fault zone excludes significant thermal overprinting possibly associated with the intrusion of the Porto Azzurro Pluton. This is quite astonishing due to the location and geological evolution of the area, and I think it would need a more in-depth discussion (i. e. addressing what would be the P-T conditions that could eventually prevent a dating in the frame of the thermal evolution of the area, thus defining the thermal window that affected the area).
This is an interesting, yet rather complex, issue, the solution of which (if ever possible) requires considering elements of regional geology, structural considerations and analytical constraints on the K-Ar dating method applied to fine-grained clays.
Dating generally assumes that no- or little isotopic re-equilibration has occurred since the dated minerals formed. However, exposure to temperatures at or above the formation temperature of the dated clay in the studied gouge (for example, to the high-temperature effects of the cooling Porto Azzurro pluton) for considerable time intervals may cause volume diffusion of radiogenic 40Ar, leading to the partial or complete resetting of the system and, thus, to mixed ages that are devoid of geological meaning. It is, therefore, correct to wonder (as the Reviewer does) about the preservation of ages as old as Aquitanian within the complex patchwork of Brittle Structural Facies of the Zuccale Fault.
Finding such old ages, however, is perhaps, not at all incompatible with the idea of complex evolution of the internal architecture of long-lived fault zones.
When distancing ourselves from the static vision of a fault architecture as we see it in the field and when instead trying to integrate such architecture and its dynamic evolution over the time dimension of localization of faulting, then it becomes easier to understand and accept that (as the BSF approach predicts) different structural facies that are now side by side may have formed very far apart and at very different times.
In the specific case of the Aquitanian ages of the ZF, those ages may thus reflect the fact that the very specific BSF’s preserving them made it to their current structural position only after the waning of the resetting effects of the thermal aureole of the Port Azzurro pluton. Those BSF’s, therefore, would reflect deformation and isotopic records acquired elsewhere more to the west, with their final translation to the current structural position occurring only after final cooling.
We actually had already commented on this in the original text, but we will certainly better stress this possibility in the amended version, highlighting even more clearly the implications that this possibility has upon the structuring of the ZF complex architecture.
A different (yet complementary) take on this point relates to the systematics of K-Ar of fine-grained clays. In more detail, one may wonder how, if the BSF’s containing Aquitanian ages were not translated to their current location after final cooling of the pluton, those ages could “escape” thermal resetting by the Porto Azzurro. After all, the estimated pressure-temperature (P-T) conditions of its contact aureole are reported as ranging from 300 °C (biotite zone) to 650 °C (andalusite–K-feldspar zone and wollastonite zone), with Pmax <0.18–0.2 GPa (Duranti et al., 1992; Caggianelli et al., 2018; Papeschi et al., 2019). These P-T conditions are diagnostic of a low-pressure/high-temperature (LP/HT) contact metamorphism and indicate that the Porto Azzurro pluton was emplaced at a very shallow crustal level. These conditions would almost certainly suffice to reset the Aquitanian isotopic signature, if maintained over long-enough time spans.
We stress in here, however, the nature of complex fault zones such as the ZF, whose transient behavior (both mechanical and thermal) represents a significant difference to the “static” environments that are generally used when conceptualizing Ar diffusion in coarse mica grains, and, thus, partial or total resetting. To explore possible resetting scenarios of the Ar signature within clays, we can refer to the study by Torgersen et al. (2014), who tried to evaluate the impact of thermal pulses of different duration upon different grain-size clay fractions. Their calculations assumed a cylindrical grain geometry and modeling was repeated for a range of grain-sizes (10, 2 and 0.1 μm), peak temperatures (190-370° C) and duration of thermal episodes (5 and 10 Ma). They concluded that during heating-cooling pulses of 5 and 10 Ma (that is, very long compared to the duration of contact metamorphism by the Porto Azzurro of only a couple of Myrs) to temperatures of 230-240° C (quite lower than those of contact metamorphism), even very fine-grained illites (< 0.1 μm) would not experience more than a 10% resetting of their initial K/Ar age. At 300-310°C, on the other hand, the Ar isotopic system of the < 0.1 and 2 μm grains would be completely reset. In summary, although Ar diffusion should not be completely ruled out, we feel confident that our internally consistent data do not reflect a significant influence of radiogenic 40Ar diffusion and this is well supported by diffusion modelling done with the most recent and relevant diffusion parameters for clays. These results, therefore, indicate that the Aquitanian ages did escape thermal resetting and are thus geologically meaningful, pointing to a discrete faulting event (recorded by two samples in different portions of the fault zone) that is well preserved in both the measured isotopic signature and the structural framework.
Only a BSF approach can unravel these complexities.
We will elaborate further on this important point in the revised version of the text, by also clarifying the prevalent P-T conditions during the thermal anomaly associated with the Porto Azzurro pluton.
It would be useful to try to insert in the discussion, as well as in the cartoon of Figure 9, the effects of the middle Miocene extensional phase. A brief discussion of how this LANF phase may have (or not) reactivated the pre-existing thrusts (negative inversion tectonics) as well as the subsequent OOSTs (including the ZF) may also have (or not) reactivated the previous LANFs (negative inversion). Definitely, it would be pretty useful to briefly discuss these possible scenarios.
We agree and thank Reviewer 1 for pointing this out. We originally abstained from doing it because we thought it could excessively complicate both the discussion and the figure. After all, that part of the story is studied and thoroughly discussed in Massa et al. (2017), which we extensively quote in our work. The comment by the Reviewer, however, has convinced us to explicitly address this important step of the local evolution in our work and the amended version of the text will, therefore, contain an add-on on this and Figure 9 will be amended accordingly.
The proposed reconstruction once again brings attention to the possible emplacement of magmatic bodies during shortening in southern Tuscany. This seems to confirm what has already been proposed for the Gavorrano area (Mazzarini et al., 2004), but also for the Larderello-Travale geothermal field (Sani et al., 2016). This is consistent with the argument that the Island of Elba could be considered an exhumed analogue of the deep roots of the Larderello geothermal system, with huge implications for the evolution of the well-known Tuscan geothermal systems. I believe that a brief discussion focused on these issues would be useful in the frame of the ongoing debate, also for the important associated implications.
We agree with this suggestion and will insert a short discussion on this, even though this aspect of the regional geological evolution is not central to the mostly methodological flavour of our contribution. Already here we point out that several scenarios contemplating different emplacement mechanisms have been proposed in the last few decades for the Porto Azzurro pluton. Pluton emplacement was initially attributed to extensional structures accommodating local gravitational instabilities (Trevisan, 1950; Pertusati et al., 1993). Subsequently, The Porto Azzurro Pluton emplacement has been interpreted as being coeval with extensional faulting of the Northern Apennines upper crust (e.g., Smith et al., 2010) or with- and driven by oblique, transtensional tectonics (Liotta et al., 2015). Recently, according to Spiess et al (2021), the Porto Azzurro Pluton has been interpreted as a syn-kinematic intrusion emplaced in the footwall of the active Zuccale Fault. The results of the geological mapping of the entire Calamita Peninsula by some of our research group, along with meso- and micro-structural analysis of the host rock fabrics, however, suggest to us that the Porto Azzurro pluton was emplaced into the Ortano and Calamita units during overall crustal shortening (Mazzarini et al., 2011; Musumeci and Vaselli, 2012; Papeschi et al; 2017; Papeschi et al; 2021; Papeschi et al., 2022), and certainly before the last recorded early Pliocene activity of the ZF (Musumeci et al., 2015; Viola et al., 2018).
Some figures need to be implemented, see comments in the annotated version of the ms.
Please also note the supplement to this comment with detailed minor points to be addressed
We will attend to all this in the next version of the manuscript.
Giulio Viola
Citation: https://doi.org/10.5194/egusphere-2022-229-AC1
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RC2: 'Comment on egusphere-2022-229', Anonymous Referee #2, 07 Jul 2022
Paper: Structural characterization and K-Ar illite dating of reactivated, complex and heterogeneous faut zones: Lessons from the Zuccale Fault, Northern Apennines
Authors: Viola G. et al.
General comment
In this work Viola and co-authors provide insights on a new analytical approach in studying complex fault zones with a polyphasic activity. The authors combine multiscalar structural studies and fabric analysis to identify different structural-mineralogical facies within the fault zone. Relative chronological relationships defined among the occurring facies have then been used as the base to identify suitable samples for K-Ar dating of fault gouges. Age data relative to fault activity have been then framed in the orogenic evolution of the Northern Apennines.
The paper is well written and provide an integration to an already previously described methodological approach in studying brittle fault zones (Tartaglia et al., 2020; Vignaroli et al., 2021). The designed fault is well exposed and object of study of several works since 1990 (Keller and Pialli) and has been considered has as a reference example of a low angle normal fault in recent works (e.g. Smith and Faulkner, 2010; Collettini et al., 2011) due to its flat to shallow E-ward dipping and top-to-E transport direction.
The data here provided suggest a different interpretation, following the one proposed by Musumeci et al. (2015), and consider the Zuccale Fault as a thrust fault active at different times from the Aquitanian to the Miocene-Pliocene boundary. The provided interpretation is quite well supported by presented data but I feel that it needs a more detailed description of the relationships among fault activity and the intrusion of the Porto Azzurro pluton. In figure 9, the one that summarizes the tectonic significance of the Zuccale Fault, the evolution step in which the granite intruded the already formed stack is missing and I strongly suggest the authors to consider it in the in the figure.
I feel the paper will be ready to be published after a MINOR REVISION. Minor comments on text and figures are listed below.
Line 19: Add the acronym ZF after “Zuccale Fault”. In other parts of the text “fault” is written within the initial cap: please select a form and use it in the whole text, or simply use the “ZF” acronym.
Line 31: “ambient” conditions instead of “environmental conditions”
Line 190: I will move here the discussion on the normal vs. thrust kinematics of the Zuccale Fault
Line 200-203: Please express here also the thickness and its lateral variability of the foliated domain
Line 235: 40 and 39 should be formatted as superscript font.
Line 241: Is there any information related to the depth of emplacement of the Porto Azzurro pluton
Line 316: The “Triassic Verrucano quartzite and metapelite” are named in several way throughout the text (the Verrucano Formation, the Verrucano Fm., the Verrucano….), please choose a form and always use it.
Line 388: Please add a brief description of the Calanchiole Shear Zone
Line 532: in “1Md” and “2M1” d and 1, respectively, should be written as subscript. Please check the whole text for this.
Line 534-539: This section is to my opinion too much speculative. I suggest to discuss in a more detailed manner this point.
Line 635-642: I feel this paragraph may be obscure to readers not accustomed to the Apennines geology. The extensional phase that affected the Northern Apennines and its timing need to be described in brief to better support the interpretation proposed by the authors.
Tables
Table 1: In the L4 box of the central column “phylonite” should be “phyllonite”
Table 2: Substitute “Whole-rock composition” with “Mineraogical composition “ or “Rock mineralogy”. The term “whole rock composition” usually refers to the chemical composition expressed in oxides weight %.
Table 3: Please express the significance of “40Ar*” in the table caption.
Figures
Figure 1: The color used for the Porto Azzurro pluton make it poorly visible. As it represents a key geological marker I strongly suggest to change color and make the pluton more visible. In the Italy inset I would add the label “Apennines”. Font size of the legend in figure (a) should be increased. Figure (b) lacks of topographic labels, please add some.
Figure 2: Please add a vertical scale to the Fence diagram. I suggest to include in the legend also the granitic sills within the Cretacopus flysch, or at least add them in the text near the color box.
Figure 3: I suggest to add a scale bar in figure (a) and (b)
Figure 6: Add a scale bar in figure (a) and (b) or described the width of view in the caption.
Figure 8: It would be useful I order to ameliorate the figure to use for dated samples the same color code used for the BSF’s in figure 2 and table 1. Please explain I the caption the significance of the error bars.
Figure 9: As already explained in the general comment, I feel that another evolutionary step, placed between the second and the third, that show the intrusion of the Porto Azzurro pluton is mandatory. This step would serve to show which are the structures cross cut by the intrusive bodies and which tectonic units underwent thermal metamorphism close to the intrusion.
Citation: https://doi.org/10.5194/egusphere-2022-229-RC2 -
AC2: 'Reply on RC2', Giulio Viola, 25 Jul 2022
Bologna, July 23, 2022
We thank Reviewer 2 for the constructive inputs provided to our manuscript. We are pleased to see that the Reviewer appreciated the proposed methodological approach to the study of long-lived, complex fault systems. Also, we appreciate the fact that, although our data seem to confirm that alternative takes from those commonly reported by the majority of the community on the evolution of the Zuccale Fault (ZF) are partly necessary, the Reviewer thinks that our model is quite well supported by the new data. This is indeed an important point, in our opinion. To discriminate between existing models, it inevitably becomes necessary to produce and make available new results and data that can assist in stepping forward in a specific direction. The results presented here are analytically sound “hard numbers”, which require us to take them into account and use them while refining the understanding of this spectacular structure.
As in the case of Reviewer 1, Reviewer 2 only has minor comments and requests of improvements on the part of the study that deals with the regional framework of the ZF and on how this needs to be partly revisited in light of the new dates.
In the following we address those comments individually and anticipate the changes that we will be implementing to the resubmitted text to improve it as per these useful inputs.
In Italic font are the original comments, while our replies/rebuttal are in normal font. We do not comment in here on all editorial aspects that will certainly be attended to in the revised version.
The data here provided suggest a different interpretation, following the one proposed by Musumeci et al. (2015), and consider the Zuccale Fault as a thrust fault active at different times from the Aquitanian to the Miocene-Pliocene boundary. The provided interpretation is quite well supported by presented data but I feel that it needs a more detailed description of the relationships among fault activity and the intrusion of the Porto Azzurro pluton. In figure 9, the one that summarizes the tectonic significance of the Zuccale Fault, the evolution step in which the granite intruded the already formed stack is missing and I strongly suggest the authors to consider it in the in the figure.
A similar comment has been proposed by Reviewer 1. Both Reviewers commenting on the same issue with basically the same suggestion and request for improvement has convinced us about the necessity to comply with this request. As already written in the reply to Reviewer 1, we originally abstained from elaborating in detail on the relationships between tectonics and plutonism because we thought it could excessively complicate both the discussion and the figure. After all, that part of the story is studied and thoroughly discussed in Vaselli and Musumeci (2012), Massa et al. (2017) and Papeschi et al. (2017) which we extensively quote in our work. This notwithstanding, the amended version of the text will be shortly expanded to also contain an add-on on this aspect and Figure 9 will be amended accordingly.
Line 241: Is there any information related to the depth of emplacement of the Porto Azzurro pluton
The estimated pressure-temperature (P-T) conditions of the Porto Azzurro contact aureole are reported as ranging from 300 °C (biotite zone) to 650 °C (andalusite–K-feldspar zone and wollastonite zone), with Pmax <0.18–0.2 GPa (Duranti et al., 1992). Recently, fluid-inclusion studies (Caggianelli et al., 2018) and detailed investigations of the migmatitic layering of the hosting Calamita Schist unit (Papeschi et al., 2019) showed that these P-T conditions are diagnostic of low-pressure/high-temperature (LP/HT) contact metamorphism and indicate that the Porto Azzurro pluton was emplaced at a very shallow crustal level. The revised text will explicitly report these constraints.
Line 388: Please add a brief description of the Calanchiole Shear Zone
We will be happy to comply with this request in the new version of the manuscript.
Line 635-642: I feel this paragraph may be obscure to readers not accustomed to the Apennines geology. The extensional phase that affected the Northern Apennines and its timing need to be described in brief to better support the interpretation proposed by the authors.
We are happy to comply with this request in the new version of the manuscript. We will specifically address the Mid-Miocene evolution of the Northern Apennines orogenic prism in terms of the significant extensional phase that affected it. We will also amend Figure 9 so as to introduce this further step of the complex local tectonic evolution (see also specific comment to Figure 9).
As to the other requested changes to the figures we will implement them all.
Giulio Viola
Citation: https://doi.org/10.5194/egusphere-2022-229-AC2
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AC2: 'Reply on RC2', Giulio Viola, 25 Jul 2022
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Giovanni Musumeci
Francesco Mazzarini
Lorenzo Tavazzani
Espen Torgersen
Roelant van der Lelij
Luca Aldega
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