the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Feb 2025
Characterizing some major Archean faults at depth in the Superior craton, North America
Abstract. The geometry of ancient faults at depth can only be mapped by high-resolution geophysical surveys such as seismic reflection profiling. Recent deep (35–48 km) reflection profiles acquired across the Archean southern Superior craton of North America provided such data with which to map in 3-D some major shear zones, many of which are associated with significant orogenic gold or VMS deposits. Most faults are (re)interpreted as thrusts; a few appear as sub-vertically aligned breaks in prominent reflectors. Sub-vertical faults possibly originated as syn-volcanic transform faults. Thrusting probably relates to the dominant phase of folding and horizontal shortening strain that occurred during the regional crustal deformation, mineralization and peak metamorphism at 2.72–2.66 Ga, associated with the Kenoran orogeny. Most deformation after this orogenic event resulted in modest lateral movement. Coincident magnetotelluric (MT) surveys indicate pervasive conductive minerals such as graphite/carbon and sulfides, exist within the mid-crust and in near-vertical channels within the more brittle and resistive upper (greenstone) crust. Many such channels, but not all, coincide with fault zones and mineral deposits. Palinspastic and paleomagnetic-based reconstructions suggest many faults had multiple periods of activity with changing vertical to horizontal offsets. Some faults appear paired, partitioning normal and oblique strains on vertical shear zones and dipping thrust zones, respectively.
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RC1: 'Report on “Characterizing some major Archean faults at depth in the Superior craton, North America” by David Snyder et al.', Anonymous Referee #1, 20 Mar 2025
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This study employs high-resolution seismic reflection profiling to map major shear zones and faults in the Archean southern Superior craton. The manuscript provides a consistent interpretation of new deep seismic reflection profiles to constrain the architecture of ancient major faults and reconstruct their tectonic history. These findings significantly enhance our understanding of the geometry and evolution of ancient fault systems in the southern Superior craton. The discussion of fault geometries, crustal deformation, and tectonic implications is thorough and well-supported by seismic data.
Their analysis suggests that two tectonic boundaries exhibit lithospheric-scale significance, possibly representing ancient suture zones. These sutures likely coincide with the southern boundary of the North Caribou Terrane and the northern boundary of the Abitibi Terrane. The study successfully reconstructs the geometry of these two major faults before displacement, restoring the deformation that occurred between 2.72 and 2.66 Ga (D3). Additionally, the authors estimate the rotation of the Kapuskasing structure between 2.07 and 1.87 Ga.
Minor Revisions and Suggestions:
- Line 175: The paragraph describes the color coding of reflectors based on their respective crustal layer. Blue is stated as the chosen color for the lowermost crust, but red is used in the figures instead. Please correct this inconsistency.
- Line 395: Add "Ga" after "2.72-2.66" for clarity.
- Line 401: Figure 13 is referenced with "a" and "b," but these subdivisions are not present in the figure. Please revise accordingly.
- Figure 1: Consider including an inset map of North America to help orient the reader regarding the study area's location. Additionally, clarify the meaning of the black dashed lines near the Kapuskasing Structural Zone.
- Figures 8, 10, 11: Add directional indicators or labels (e.g., N-S, A-A’) to clarify the orientation of the sections.
- Figure 11: There is a typo in the strip geology (a): "Deformation zone seperating …" should be corrected to "Deformation zone separating …."
- Figure 7: The caption refers to (a) and (b), but these labels are missing from the figure.
Overall, this article is well-written and informative. It provides a valuable contribution to our understanding of tectonic structures in the Superior craton. With minor revisions, it will be an excellent addition to Solid Earth.
Citation: https://doi.org/10.5194/egusphere-2025-390-RC1 -
RC2: 'Comment on egusphere-2025-390', Anonymous Referee #2, 24 Mar 2025
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The paper by Snyder et al. attempts to provide an overview of several crustal faults crossed by various seismic and magnetotelluric surveys acquired in the Archean Superior craton as part of the Metal Earth project; much of the material is taken from previously published publications or the Metal Earth Atlas (Simmons et al., 2024). As presented, the paper comes across as a disjointed set of observations with quite poorly supported interpretations. My concerns fall into four categories: (a) fundamental misconceptions on the nature of Superior craton crust, (b) no clear well justified thesis to tie the interpretations together, (c) poor justification of the many individual seismic interpretations, and (d) a disorganised written presentation of the results with many paragraphs containing a jumble of disparate ideas.
Superior crust: The authors assume that the crust is made up of three layers with an upper resistive (mafic) greenstone layer above a moderately conductive gneissic middle crust (lines 172-179); the greenstone layer is shown in the figures to be 12-18 km thick, based on the assumption that greenstones exhibit high electrical resistivity values. In fact on line 230, the upper crust is noted as extending to 25 km depth. Surely not in 40 km thick Archean crust! The fundamental flaw here is the association of high resistivity with the upper crust. This general assumption is incorrect, because granitoids can also exhibit high resistivities, as shown in the North Caribou terrane by Roots et al. (2024) and in the Pontiac by Roots et al. (2022). Figure 5 from the Malarctic transect also contradicts the three-layer model, showing a resistive upper crust where it is mostly granitic (La Motte and La Corne plutons). There is also absolutely no reason to believe that mafic greenstones are 18 km thick in Fig. 2 where the surface geology is shown to be mostly granitic, which is also indicated by seismic velocities of ~6.0 km/s in Fig. 2c. Furthermore, in Fig. 11 the greenstone layer is shown to include the English River metasedimentary belt, which exhibits amphibolite and granulite facies metamorphism. Though greenstones may be as thick as 10 km or so in the Abitibi belt where their surface expression is more extensive, the idea that the south Superior craton everywhere includes a >10km thick greenstone layer is fundamentally incorrect.
Fault geometries: Although it’s not well articulated in the confused text, it seems that a major point of the paper is to argue that sub-vertical crustal faults originated as (oceanic?) transform faults at the time the greenstones formed, and the low-angle thrusts are related to convergence during the various orogenic phases that affected the Southern Superior craton. No evidence is presented to support the former hypothesis other a vague allusion (lines 198-200) that subvertical faults in the Superior craton look like oceanic transform faults. Both oceanic crust and continental crust can support transform faults, e.g. the San Andreas fault in California, but that does not mean such continental faults have an oceanic origin. If the (unstated) motivation is that some key Superior craton faults originated in volcanic rocks in a subaqueous (oceanic?)setting, then it becomes important to distinguish between autochthonous and allochthonous greenstones; in the latter case, there is no reason why a fault would still be active and cut through the underlying continental crust after, say, obduction; in the former, why would a fault cut through underlying continental crust created and modified by multiple episodes of TTG magmatism? An argument supporting a long-lived fault history such as this needs evidence and justification, rather than a simple statement that some Archean faults look like those in a modern oceanic setting. In the case, of low-angle thrusts, it is likely that many are related to shortening across the Superior craton, but they can also be created in strike-slip regimes with varying stress regimes, so characterising the faults’ vergence becomes important. If the bigger argument, is that the strike-slip and thrust faults are two elements of a strain-partitioning regime, then this needs to be clearly documented in terms of the fault geometry and timing of motion, which has not been presented.
Seismic interpretation: There are numerous problems with interpretation of the seismic reflection images, which are documented in more detail below. In general, seismic reflections do not stand out well from the background noise, which has been laterally smeared out into a subhorizontal fabric by the processing. Faults seems to be interpreted through laterally continuous reflections (Fig. 2a, 8a) and along the edge of migration artefacts (Fig. 4b), given a strange zig-zag geometry (Fig. 9b, 11a) intended to indicate wedging, and presented with two conflicting interpretations (Fig. 8a, 8b). In addition, the Moho is interpreted at unusually low depths (9.5 s or 30 km in Fig. 3b), at the bottom of the seismic data (Fig. 4b), and offset by 2 s (7 km) from the reflection Moho (Fig. 11a). None of this inspires much confidence in the seismic interpretation, or inferences there from.
The CLLF is presented as a convincing example of a subvertical fault extending to 30 km depth. However, in Fig. 4b, the fault does not continue upward to its surface location, because continuous shallow reflections are interpreted above the fault at 1-2.5 s, in contrast to the resistivity model that shows a steeply dipping conductor that is not discussed. I suspect that the truncation of reflections at >5 s is simply the edge of a migration artefact arising from amplitude variations in the unmigrated data. Note how deeper reflections exhibit greater lateral smearing. It’s worth noting that Roots et al. (2022) also presented this seismic line together with Lithoprobe line 23 just to the south and showed apparently continuous subhorizontal reflections at this location below 4-5 s depth. Also why is the Moho interpreted at the bottom of the seismic data at 11.5 s? The seismic data are hardly convincing as they don’t exist here!
Apparently, the purpose of Fig. 8 here is to show the layered middle crust, but the figure also reveals two contradictory interpretations of the Porcupine-Destor fault. Is the PD fault meant to be a high-angle “synvolvanic” fault like CLLF? If so why is it in this section? Note that in Fig. 8a the PD fault cuts through a laterally continuous interpreted reflection at ~ 1 s, which seems contradictory.
In the P-S section in Fig. 10a, the interpreted CLLF cuts through a continuous body at ~10 km depth, and the interpretation seems problematic given that there are south-dipping artefacts subparallel to the CLLF in both crust and mantle. In Fig. 10b, the footwall to the PD fault is described as more conductive, which is clearly not the case below 15 km.
Written presentation: Though it may be challenging to provide an overview of the geology of the entire southern Superior craton, the section on the Geologic Setting makes no attempt to describe all the areas where the seismic lines are located. In paragraph 1, there is a summary of greenstone assemblages in the Abitibi belt, but no information is provided on greenstones in the western Superior. This paragraph, which presumably focusses on the granite-greenstone domains, also repeats seismic velocities described in the preceding paragraph, giving the impression of a confused organisation of the paper. The next paragraphs on plutonic and gneissic domains are better written, but there is no clarification of the complexity found by Mole et al (2021), which perhaps provides an alternative to the successive accretion model(?).
Since the different stages of deformation are key to interpretation of the seismic images and the paper’s conclusions, these stages need to be better described. Percival et al. (2006) noted 5 deformation stages, but are these the same phases across the entire south Superior craton, or do they just refer to individual orogens: North Superian, Uchian etc. If there is a model of successive southward accretion, how can D1 shortening affect both the North Caribou and Minnesota River Valley terranes that have not yet been accreted? Is there an accepted framework for timing of the polyphase deformation affecting the entire south Superior craton where the seismic lines are located? For example, is deposition of the English River sedimentary rocks in the west really considered coeval with the Porcupine basin in the east, as stated?
Paragraph 1 of the “Faults observed…” section is also confused, as it starts discussing the seismic interpretation approach, but ends with very general statements about MT surveys. In fact, the resistivity models appear to have vey little influence on the interpretation of the seismic images, even though the resistivity models frequently appear clearer. Shouldn’t the paragraph in lines 162-166, be part of the motivation in the introduction rather than buried here? The paragraph at lines 167-179 is also confused with a description of how form lines are used in the interpretation, and then moves into characterizing the geology of the Kapuskasing Uplift.
Syn-volcanic faults section: Line 187-188 essentially stating that there are high-angle or low-angle faults within greenstones has no citation, but this is clearly a key observation, if correct. (What is a fault-related structure?). Presumably, this comment is here because both fault types are syn-volcanic, but on lines 228-229 in the Syn-tectonic section low-angle faults are described as post-volcanic, which contradicts the earlier statement. Also why is there a discussion of the upper-middle crustal boundary here? Shouldn’t this be in an earlier section on crustal layering together with some of the geological observations?
Syn-tectonic faults section: Why is a description of the metasedimentary belts here and not in the Geological Setting section? On lines 216-217, can all these sedimentary units record D1 in a successive accretion regime? Deformation needs to be better described (see above). Extensional collapse of the metasedimentary belts is stated here to be at ~2.75 Ga, but Percival et al. (2012) in his Fig. 13 show the Winnipeg River and more southerly terranes belts to be apart at 2.72 Ga, with the Minnesota River Valley terrane apart at 2.69 Ga! So what’s really going on?
Specific comments:
Fig. 5: There is a >5 km offset between the Moho on the reflection section and the velocity-depth profile.
Fig. 6: Three seismic lines are shown on the geology map. Which one corresponds to Fig 8?
What is the purpose of introducing the palinspastic reconstruction in Fig. 7 and the associated geological discussion, which seems to come out of nowhere? This section just seems to be unnecessary and confusing.
Line 290: You don’t know these conductors are fractures.
Line 290-291: What does it mean to say that these faults are akin to the PD fault, which has both senses of dip and is more steeply dipping? What is the purpose of this comment?
Line 292-296: Note the publication by White et al of seismic data from this area. The metasedimentary rocks are described as conductive, but Fig. 9 clearly shows rocks under the metasedimentary Quetico belt are resistive above 4 s. Where are the three low angle-faults? Is this something to do with the zig-zag feature?
Line 299-300: This comment is not true and contradictory. The reflection Moho can commonly be inferred from good quality deep crustal reflection sections, though clearly it is a challenge with a lot of the Metal Earth seismic data! Note the clear reflection Moho on the Lithoprobe line in Fig. 11.
Fig. 11. Why is the Moho interpreted at 14 s? Note the incorrect depth scale, which implies that the velocity in the upper crust is the same as the upper mantle, because the 5 km depth interval is the same when the time scale is linear. English River is a metasedimentary belt, not a greenstone layer, as implied by the left hand annotation. Another poorly justified zig-zag fault!
Line 312: Abiitibi spelling
Line 345: Where is the downward extension of the Sydney Lake fault? Does it follow the zig-zag trajectory? How is this justified as a fault? How is strike-slip motion of the Sidney Lake fault accommodated along one of these zig-zag structures?
Line 361-364: There is no need to introduce fluid movement here. It’s just an unnecessary distraction.
Line 372-373: This is unnecessary as the modification is not described.
Line 378: If these are all “crust-only” terranes, what is the nature of the underlying upper mantle?
Line 403: 10c not 10b
Line 405: Suggesting the CLLF and PD are syn-volcanic faults controlling lithospheric strain is wild speculation with absolutely no justification in this paper, as far as I can see.
Line 407: How was the amount of underthrusting inferred?
Line 409-410: What does this mean?
Line 432: I have seen no convincing seismic evidence that the CLLF cuts through the entire crust. See Roots et al. (2022). There has been no real justification presented for the initial formation of the CLLF as synvolcanic.
Line 460-479: This section appears to largely be a reproduction of previous MT results.
Line 491: Kenoran orogeny is mentioned twice, once in the abstract and once in the Conclusions, but the term is never defined.
Citation: https://doi.org/10.5194/egusphere-2025-390-RC2
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