Brittle Deformation in the Neoproterozoic Basement of Southeast Brazil: Traces of Intraplate Cenozoic Tectonics
Round 1
Reviewer 1 Report
“Brittle deformation and traces of intraplate tectonics acting in the Neoproterozoic basement in southeastern Brazil” by Pinheiro et al. presents a study of structural data (primarily faults) in the Atlantic Plateau of southeastern Brazil to characterize their paleostresses and kinematic history. The methods and topic are relevant to the scope of the Geosciences. My critiques are minor in scope.
The methodology and discussion presented by the authors are straightforward and pedestrian in scope with no serious scientific issues to address. The biggest lack of this paper is that it is devoid of larger tectonic context. The introduction largely presents itself as filling in a data gap from previous studies as opposed to presenting a scientific question to address. After reading it, I wrote in the margins of my copy “why should the reader care?” The last few paragraphs of section 2 (Geological setting) does a better job of laying out scientific questions. Much of what is said there should actually go in the introduction leaving Geological setting to be more of a laying out current consensus understanding. Even then, though, it could go further in putting this in larger tectonic context. This region has relevance to opening of the Atlantic among other orogenies, but it doesn’t come through much, likely limiting the readership of this paper.
While the authors do a fine job with their analysis, I again was left a little empty with how limited everything is with little attempt to go beyond the simple structural analysis. With an objective to determine whether the stresses are neotectonic in origin or not, at least a cursory look at seismicity, preferably microseismicity, would help the discussion. It reads as if that never even occurred to the authors.
A few other notes:
I don’t know if “rotaxes” (figure 5 caption and text) is a typo or jargon, but either way should be changed to “rotation axes”.
The legend text in figure 1 is quite tiny and will be unreadable to anyone with even slightly degraded eyesight.
It wasn’t clear to me where the data in figure 3 rose diagram came from. The text states “The polymodal Gaussian fit … is nearly ENE-WSW”. Did this come from the 1233 structural data collected by the authors? A subset? Or from the studies cited in the first paragraph of section 4?
As noted above, the paper presents a straightforward - if not terribly inspiring - study of structural data of a portion of the Atlantic plateau. Broader tectonic context would help this paper considerably, but overall I recommend publication with minor revision.
Author Response
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Reviewer 2 Report
Review of manuscript 1253296 by Pinheiro & Cianfarra
This manuscript is not acceptable for publication for several reasons:
1) it lacks maturity of presentation (enumeration of what other works have found without a sound incorporation into the topic addressed in this manuscript, repetition of certain statements regarding goal of research and conclusions)
2) the language is very poor (grammar, expression. incomplete sentences, logic in the formulation of thoughts)
3) the figures are not adequately explained
In Fig. 1 it is not clear where the detailed colored map is located in the smaller map on the lower right, and this latter is not self-explanatory
In Fig. 3 ist is not clear where the lineament map fits into the inset
The faults outlined in the photograph of Fig. 4a are hardly visible and one has no chance to see which sense of motion relates tot hem. In Fig. 4s the offset of the shallow dipping fault by a later fault is not visible.
In fig. 5B there are nearly vertical dipping slickensides which are hard to fit to strike-slip faulting.
Fig. 6 is unintelligible as presented (what are the colored stars? What is Stress_1?)
Fig. 7a: what do you mean by „fracture-to-pole planes“? is it poles to planes? In Fig. 7b the legend is too small to be readable. The fracture in 7d looks as if it was a bedding plane at the top of the conglomeratic layer.
Fig. 8: what does the abbreviation Shmax stand for? (it is nowhere explained). The angles of shear are not evident because the drawing is an oblique view onto the surface of the block. Are you referring to the maximum shear stress at 45° to the shear plane? And how was the regional stress field determined?
In Fig. 9 why are there two differing stress orientation given in the small inset at the top?
4) the terminology of tectonics is awful and wrong in some instances
5) the terminology of geologic times is not consistent (sometimes it is Tertiary, sometimes Cenozoic, and even a“Neotectonic epoch“ is advocated. It is never clear to which geologic period the authors refer to.
6) the description of the (field) data is not conclusive (the authors do not convince the reader that the faults are strike-slip and how the sense is derived). As a consequence the interpretation of the data is also not convincing at all.
7) The author contribution section could be shortened. As it stands it looks blown up.
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Reviewer 3 Report
The paper is well written and its content sounds interesting for the readers. However, I must point out that I feel little qualified to judge a paper like this because I am not a geologist.
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Reviewer 4 Report
- Inexact formulations appear in the text, e.g. in the sub-chapter of Geological Setting of Study Area. The various type of rocks that genetically related to the low- to medium-grade metamorphic grade are appointed between the lines 102 and 108. The whole paragraph needs to be reworked. Also a granite and porphyroid relationship; porphyroids arise for low-grade metamorphism of acid to intermade volcanic rocks!
- Stylistics and grammer in the text should be checked carefully. Some sentencies are very long and inaccurecies occur.
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Round 2
Reviewer 2 Report
Review of Inheiro & Cianfarra
The authors have significantly improved the quality of presentation, both regarding text as well as figures.
Line 14, 61, 126, 132 and 296: the authors still use the outdated term Tertiary for Cenozoic. In other instances they do use Cenozoic. I suggest to homogenize and use Cenozoic.
Line 325 Which „important studies“ do you mean? Please give a reference.
Line 33e The word „prosecution“ is not a good choice (one thinks of criminal prosecution). I suggest to use „continuation“ instead.
Line 369 I suggest to replace „kinematics“ with „strike-slip“ (right-lateral strike slip with transpressional component)
Line 370-371 But the Southern Atlantic has opened much earlier at around 110 Ma. This sentence needs to be reworded.
Line 377 I suggest to replace „prosecution“ by „continuation“ (see comment above)
Line 378 and 386 Replace „kinematics“ by „strike-slip“ again
Line 396 which „other studies“?
In the last part of the manuscript the statement on right-lateral „kinematics“ is repeated several times. I suggest to slightly shorten by removing repetitions.
All in all the paper has been revised thoroughly and has improved in impact. Would it be possible to expand the section on the continuation of oceanic fracture zone with your fault zone on land? This would make an important improvement regarding the plate tectonic setting and would enhance the impact of the paper. Could it be that you are dealing with the Rio de Janeiro Fracture Zone? (I think the Rio Grande Fracture zone is located farther south). And could you include active seismicity data (loci of earthquakes on land and maybe along the oceanic fracture zone)? An early work on this topic is the paper by Bird & Stuart (see below) which I think should be included in the discussion anyway. Finally there is the classic paper by Francheteau & Le Pichon 1972 which also addressed this topic for the first time.
I am looking forward to seeing this paper published.
Bird, Dale E., Hall, Stuart A.
Early seafloor spreading in the South Atlantic: new evidence for M-series magnetochrons north of the Rio Grande Fracture Zone.
Geophysical Journal International, 2016/08/01
Abstract: Recent tectonic reconstructions of the South Atlantic have partitioned the ocean basin into several segments based upon one or more proposed intraplate South American deformation zones. In several of these reconstructions, opening of the southern segment(s) by seafloor spreading prior to Aptian-Albian time is accompanied by contemporaneous strike-slip motion along an intraplate boundary extending southeastward from the Andean Cochabamba—Santa Cruz bend to the Rio Grande Fracture Zone (RGFZ). We have examined new magnetic data over the Pelotas, Santos and Campos Basins, offshore Argentina and Brazil, acquired by ION-GXT in tandem with long-offset, long record seismic reflection data, and identified seafloor spreading anomalies M4, M3, M2 and M0 (∼131, ∼129, ∼128 and ∼125 Ma). Integrating these results with our earlier work, we have been able to correlate magnetochrons M4, M3, M2 and M0 north and south of the RGFZ on the South American margin, and north and south of the Walvis Ridge on the African side. Our results are therefore inconsistent with diachronous opening models that involve substantial continental strike-slip motion north of RGFZ during M4 to M0 time. Although the ocean basin may have opened from south to north, our results indicate that seafloor spreading began north of the RGFZ earlier than previously proposed.
Francheteau, J. & Le Pichon X, 1972, AAPG 56/6
Marginal Fracture Zones as Structural Framework of Continental Margins in South Atlantic Ocean.
Abstract - A plate tectonic model of the early opening of the South Atlantic is used to describe the structural framework of the continental margins and adjacent oceanic and coastal areas on both sides of the ocean. It is proposed that major offsets of the continental margins necessarily induce the subsidence of coastal basins where fracture zones intersect the continents. The configuration of the accreting plate-margin boundary gradually developed through the opening of the ocean after Late Jurassic time.
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