the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Dec 2024
Insight into the tectonostratigraphy of the historic Kefalonia island (Greece): a reflection seismic survey
Abstract. Kefalonia island, in front of the Greek west coast, is placed in a peculiar tectonic setting characterized by a transition from an oceanic subduction contact to a continental collision. This tectonic setting results in strong tectonic activities and seismicity in the area making the island a testbed for geological, geophysical, and archeological studies. To improve the subsurface knowledge and shed light in the top 100s of meters, we acquired three seismic profiles in the isthmus connecting the main part of the island to the Paliki peninsula, in the Thinia valley, where the presence of a possible channel has been disputed to make Paliki the Homer’s Ithaca (home of Odysseus). A total of approximately 3.5 km of seismic data was acquired using 5 m receiver and shot spacing and a 25 kg accelerated weight-drop as the main source. The sharp topographic changes and morphological features of the valley made the survey challenging, limiting the spread, precluding uniform shot points, and resulting in strongly crooked profiles. The acquired data, however, show visible reflections with variable quality down to 0.5 s and occasionally to 1 s. First-break traveltime tomography and 3D reflection traveltime modelling were performed to complement the seismic reflection processing work together with lithological columns from three boreholes present along the profiles. Results show a low-velocity zone with no reflectivity from the surface to approximately 100 m depth probably related to the presence of loose material, under which two main east-dipping reflections are imaged. With the help of surface geology and tectonic history of the valley, we interpret these features as the same lithological boundary displaced by three highly east-dipping thrust/reverse faults probably part of the Hellenide thrusts. These findings further constrain the local recent tectonic history and thus, the long-debated presence of an historic water channel in the valley.
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RC1: 'Comment on egusphere-2024-3783', Edward Marc Cushing, 17 Jan 2025
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Dear authors, dear editor,
I have carefully reviewed the proposed preprint: “Insight into the tectonostratigraphy of the historic Kefalonia island (Greece): a reflection seismic survey.” My comments and observations are based on regional knowledge and the relevant literature. As seismic processing is not my area of expertise, my feedback focuses primarily on the geological interpretation proposed in this preprint. I do not assess the validity of the hypotheses that motivated this work, being aware of its potentially contentious nature.
Nonetheless, I note that the paper has the merit of providing valuable geophysical and structural information on the Thinia area and, as such, deserves to be published. This study complements the extensive and remarkable work presented in Kirsten Hunter's dissertation, which I strongly recommend the authors consider carefully, particularly regarding the use of her illustrations, as they would greatly enhance the clarity of the paper.
I therefore encourage the authors to improve and refine the figures, maps, and cross-sections at appropriate scales, enabling readers to better locate the investigations conducted and to better understand the structural interpretations they propose.General Comments
The authors present a geophysical study based on seismic reflection and seismic tomography methods. After briefly describing the geological, seismic, and structural environment of the island of Cephalonia, the authors focus on a specific area within the Thinia Valley. This area has been the subject of numerous studies since the 2000s aimed at exploring a theory linked to the myth of Homer’s Odyssey, namely the location of Odysseus’ palace.
The main body of the text provides a fairly detailed description of the geophysical methods used to image the studied structures. Using seismic reflection and tomography techniques, the study reveals deep structures up to 500 meters beneath the surface. The results highlight intense tectonic activity characterized by thrust faults and complex deformations. Although the data is limited by the variable quality of the surveys, it indicates the presence of unconsolidated sedimentary material, likely originating from landslides, and east-dipping structures. The valley is described as a syncline, emphasizing the need for further research to refine geological models and understand tectonic interactions.
The scope of the article aligns well with the theme of the Special issue: « Seismic imaging from the lithosphere to the near surface » and represents its most developed aspect. Not being a specialist in seismic data processing, I do not address this section in detail, but I highlight a few points that might need verification.
The imaging obtained from the geophysical measurements appears to be of relatively good quality, considering the complex tectonic context of the area. It reveals duplicated thrust structures, which is consistent with the structural framework of Cephalonia’s thrust system (see Sorel; Underhill, Cushing). The additional value of the study lies in the interpretation of the upper layer velocity (Vp), which corroborates the presence of slipped formations as suggested in previous studies.
The authors remain cautious in interpreting this seismic imaging with respect to the "archaeological" hypotheses.
Some figures are too small or lack sufficient detail. The paper would benefit from improved figures (maps, legends, etc.), as detailed below.Individual Scientific Questions/Issues ("Specific Comments")
Title
The title is somewhat ambiguous. I would suggest focusing on the specific study area, as the current title may lead readers to expect a general study of the island. Additionally, I recommend emphasizing the contribution of geophysical methods to the structural understanding of this area. The term “Historical” is ambiguous. The “archaeological” investigations aim to confirm a hypothesis derived from the interpretation of a text referencing a myth (Odysseus’), which is not strictly historical.
• L24: Do you have any information about the age of the presumed landslide? It seems that Hunter’s study did not confirm any activity younger than 1.8 Ma. The dating from coring appears to be highly questionable.
• L40: You should cite the following references: Stiros et al. (1994), Sorel (1988, 1976).
• L41: You mention "dynamic" motions (translational from GPS) and rotational movements. Please clarify further.
• L55-57: The relationship between the studied fault structure and the presumed landslide is unclear. Better understanding the tectonic history alone is not sufficient. The key question seems to be: what is the structural (and potentially causal) relationship between the fault structure (folds and faults) and the overburdened landslide? You should also explain how this study contributes beyond the comprehensive work of Kirsten Hunter (2013).
• L60: Indicate that these investigations build upon and complement the comprehensive studies performed in Hunter’s thesis, which is detailed extensively. You may list all methods and datasets used in Section 3.
• L64: The "Aeanos Thrust" is a structural feature identified by Underhill (1989), but other names and traces exist in the literature (e.g., Kontogourata-Agon Fault). Refer also to Lekkas et al. (2016), Lagios et al. (2012), Stiros (1994), and Sorel (1976). Note that Sorel (1976), had already identified this structure as a reverse fault as many faults on Kefalonia Island.
• L66: “The area is approximately 773 km²”. This value is “accurate” and not approximative.
• L70 (Figure 1): While the text includes a topographic description, an altimetric map is clearly missing. Consider using a DEM or a topographic map from Hunter’s thesis (e.g., Figure 4.1).
• L74: In the Poros town area and eastern coast.
• L83: Figure 1b appears to be missing or is not easily located.
• L89: These faults are interpreted as ancient normal faults within the Preapulian-Paxos zone, which underwent tectonic inversion during the lower Pliocene and lower Pleistocene (Sorel, 1976, 1988).
• L97 (Figure 1): As previously noted, there is no hypsometric map. The red inset is very small and difficult to read.
• L100: Explain in the text why this specific zone was chosen. It is one of the locations studied by Hunter (paleolake of Katokhori) and the site of other investigations, such as drill holes, gravimetry, ERT, seismic refraction, and airborne magnetism. This study builds on and complements previous investigations.
• L114 (Figure 2): This figure does not provide information on the profile locations. There is no way to locate these profiles. Include a detailed, large-scale map with orientation, scale, contour lines, and coordinates, similar to Figure 2.9 in Hunter’s thesis.
• L167: Why not consider velocities in the range of 2400 to 3300 m/s, as previously mentioned?
• Figures 3 and 4: It is difficult to clearly identify R1 and R2.
• L187-188: These velocities differ again from those mentioned earlier. Please explain this discrepancy or modify.
• L280 (Figure 8): A comparative geological cross-section and a detailed, large-scale geological map are missing. The figure is too small. Consider using the geological map from Figure 3.29 in Hunter’s thesis and the cross-section from Figure 5.15, which includes two of the three boreholes (C5C, C4a). Indicate the location of the landslide referenced in Line 311 on both the cross-section and geological map.
• L318: Based on Hunter’s thesis conclusion, the hypothesis of a covered channel is ruled out by her data analysis. The only remaining possibility seems to be that a large landslide, encompassing the entire slope, could have carried and possibly destroyed the supposed channel. Refer to Section 462 of Hunter’s thesis for further details.Technical corrections
References:
• L 39: (Gaki-papanastassiou et al., 2010 or 2011 ?) see ref listCitation: https://doi.org/10.5194/egusphere-2024-3783-RC1
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