Devonian–Mississippian Collapse and Core Complex Exhumation, And

Solid Earth Discuss., https://doi.org/10.5194/se-2019-200-AC2, 2020 SED © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Interactive comment Interactive comment on “Devonian–Mississippian collapse and core complex exhumation, and partial decoupling and partitioning of Eurekan deformation as alternatives to the Ellesmerian Orogeny in Spitsbergen” by Jean-Baptiste P. Koehl Jean-Baptiste Koehl [email protected] Received and published: 4 June 2020 Dear Dr. Piepjohn, thank you very much for responding to my invitation to submit addi- tional comments to my manuscript. Your input on the manuscript is highly appreciated. Printer-friendly version Thank you very much for taking the time to send your comments. Here is my reply to your comments. I hope that the changes implemented improve the shortcomings of Discussion paper the manuscript highlighted by your comments and suggestions. Please do not hesitate C1 to contact me shall this not be the case for some comments. Please note the figures of this discussion were attached as supplements. Please do not hesitate to contact me SED should you require high-quality versions of the figures. Introduction The concept of Ellesmerian/Svalbardian deformation event was first men- Interactive tioned by Vogt (1938) to explain moderately–steeply dipping Devonian sedimen- comment tary beds below flat-lying uppermost Carboniferous–lowermost Permian strata of the Wordiekammen Formation in Dickson Land. This concept was brought up to fill a need, i.e., the need to explain the tilted and folded character of Devonian sedimentary rocks of the Andrée Land Group and Mimerdalen Subgroup below presumably un- deformed uppermost Devonian–Mississippian strata of the Billefjorden Group and uppermost Carboniferous–lowermost Permian strata of the Wordiekammen Formation. The present manuscript shows that strata of the Billefjorden Group and Wordiekam- men Formation are indeed deformed in places and uncovers structures localized at important lithological (rheological) boundaries, which may have partly/locally decou- pled and partitioned Eurekan deformation. The newly evidenced structures, among which Eurekan bedding-parallel décollements, thrusts and duplexes, show that deformation patterns and differences between Devonian rocks of the Andrée Land Group and Mimerdalen Subgroup and overlying uppermost Devonian–Mississippian Billefjor- den Group and uppermost Carboniferous–lowermost Permian Wordiekammen Forma- tion can be simply explained by local strain partitioning and decoupling during Eurekan deformation possibly in association with Devonian extensional collapse (e.g., detachment folding, core complex exhumation), thus not requiring an extra phase of con- tractional/transpressional deformation in the Late Devonian. Although Dr. Piepjohn’s studies also present abundant structural measurements made on actual outcrops at the bottom of valleys and along riverbeds, his regional models (e.g., Piepjohn et al., 1997a; Printer-friendly version Piepjohn, 2000) build on regional structural relationships, which are based on observations and interpretations made from great distance and on regional outcrop tran- Discussion paper sects partly–mostly covered by screes and partly inaccessible (see Figure 2–Figure 25). Dr. Piepjohn’s studies are, therefore, inappropriate to discuss local geological C2 processes and mechanisms such as the presence of local bedding-parallel duplexes and décollements (e.g., between the Wordiekammen Formation and Wood Bay For- SED mation/Mimerdalen Subgroup and between the Billefjorden Group and the Wood Bay Formation/Mimerdalen Subgroup) that may affect sedimentary successions in central Spitsbergen and that are the object of the present manuscript (see Figure 1 for loca- Interactive tions). The author of the present manuscript does not attempt to undermine the value comment The main issue raised by the present manuscript targets the episode of shallow-crustal to near-surface contraction/transpression inferred by Vogt (1938) and further discussed by Dr. Piepjohn and others (e.g., Dallmann, 1992; Piepjohn et a., 1997a; Kempe et al., 1997; Michaelsen, 1998; Piepjohn, 2000; Bergh et al., 2011; Dallmann and Piepjohn, submitted) in Dickson Land, Andrée Land, Blomstrandhalvøya, and southern Spitsber- gen. It is long known that field observations are not necessarily reflecting the utlimate truth (Bardossy and Fodor, 2001). The present manuscript shows that evidences in favor of shallow-crustal to near-surface Ellesmerian/Svalbardian deformation in Devo- nian sedimentary rocks in Spitsbergen involve high amounts of uncertainty related to (1) over-interpretation of poorly exposed (because partly–mostly covered by screes) and mostly inaccessible (because of the steep slopes) regional outcrop transects (e.g., in Piepjohn et al., 1997a; Piepjohn, 2000), (2) erroneous field interpretation of low- angle extensional detachments, and/or tilted stratigraphic unconformities, and/or Eu- rekan thrusts as Ellesmerian thrusts (e.g., in Michaelsen, 1998), (3) poorly preserved spore assemblages (Buggisch et al., 1994) and erroneous palynological interpretation (Schweitzer, 1999; Piepjohn et al., 2000), and (4) omission/non-discussion of al- ternative hypotheses such as that of extensional detachment folding as discussed in Chorowicz (1992), Roy (2007, 2009) and Roy et al. (unpublished). The present discussion includes actual field photographs of regional transects in central Spitsbergen documenting the very low quality of regional outcrop transects in this area and the Printer-friendly version high amount of uncertainty associated to the interpretation of supporters of Ellesme- Discussion paper rian/Svalbardian deformation (Figure 2–Figure 25). The photographs show that the field observations made by Dr. Piepjohn and his team are, at most, “permissive” to C3 “suggestive” in the scale established by geoscientists and psychologists (Wilson et al., 2020) indicating “that a particular idea or interpretation [e.g., shallow Ellesmerian SED deformation in Svalbard] cannot be ruled out, but it is also not the only available so- lution” or “that there is positive evidence for a particular interpretation [e.g., shallow Ellesmerian deformation in Svalbard], but that the evidence also allows the possibility Interactive for other interpretation [e.g., continuous Devonian–Mississippian extensional collapse comment and Eurekan strain partitioning]”. The present manuscript and present discussion em- phasize the large amounts of uncertainty related to the method of acquisition of field observations in studies where field observations and regional structural relationships were established from great distance on partly–mostly covered (by screes) and inaccessible outcrop transects partly–mostly consisting of loose material (e.g., Piepjohn et al., 1997a; Piepjohn, 2000; Figure 2–Figure 25) by comparison with studies based on actual observations made looking at (and physically stepping onto) specific outcrops directly correlated to the structures discussed (present manuscript). It is well known that strain decoupling and partitioning may have a significant impact on the distribu- tion of deformation and localization of structures in various lithostratigraphic units and throughout an area as shown, e.g., by recent studies by Janecke et al. (2018) and Bergh et al. (2019) along the San Andreas fault in California, but also in Spitsbergen where strong strain partitioning and decoupling is advocated during early Cenozoic contraction–transpression (e.g., Maher et al., 1997; Braathen et al., 1999a, 1999b; Bergh et al., 2000; Leever et al., 2011a, 2011b). In particular, Braathen et al. (1999a), who concluded that the West Spitsbergen Fold-and-Thrust Belt consists of “compos- ite wedges” (referring to critical wedge taper), even wrote that this “translates into in- creased decoupling toward a weaker foreland” (central Spitsbergen is indeed relatively close to this foreland basin, the Central Tertiary Basin). Such decoupling is discussed further in the present study, but also in the studies of Koehl and Muñoz-Barrera (2018; Printer-friendly version presence of early Cenozoic bedding-parallel décollements within shales of the Billefjor- Discussion paper den Group) and Koehl et al. (in prep. a; décollements parallel to stratigraphic boundaries, especially between Devonian rocks of the Andrée Land Group and Permian sed- C4 imentary rocks of the Wordiekammen Formation). The revised version of the present manuscript also discusses the possible occurrence of deep, late Caledonian contrac- SED tion tectonism in western Spitsbergen in the Late Devonian–Mississippian (Kosminska et al., 2017, 2020), which was comparable in intensity to late Caledonian deformation along the conjugate margin of Svalbard in northeastern Greenland (Gilotti et al., 2004; Interactive McClelland et al., 2006). In northeastern Greenland, Devonian–Mississippian eclogite- comment facies metamorphism was coeval with near-surface extensional collapse and deposi- tion of thick Devonian–Mississippian sedimentary basins along low-angle extensional detachments (Stemmerik et al., 1991; Strachan, 1994; Larsen et al., 2008), which is common in phases of late–post-orogenic collapse (Platt, 1986; Rey et al., 2001). The author of the present manuscript is open to adding selected parts and/or figures of the discussion below to the submitted manuscript, should it be advised by the referees and/or editor. Comments from Dr. Piepjohn Comment 1: General

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