Hello Marcos,

Could it be useful to take some oriented rock samples adjacent to the faults/joints and look for mineral-scale shear indicators (e.g., broken and offset clasts; crosscut fissures) on polished sections, thin sections, or even under the SEM? I'd expect the areas juxtaposing the faults (assuming they rather represent swarms of microfaults of the same orientation) also to show some signs of shear sense.

All the best,

Martin

Dear Mr. Pinheiro,

there may be a wide variety of possibilities offering the chance to determine the direction and the jump during displacement. You mentioned the word "neotectonics" which according to its definition extends into the part of geological history where antropogenic processes and artifacts play an ever-increasing part. I was confronted with neotectonics in areas covered with loess, a fine-grained and massive overburden on top of the "real geology". I know too little about the Parana, I worked further south in the Argentine Pampa. Maybe, for the beginning these questions and hints may bring you a bit closer to a solution of your problems. I am reluctant to flood you at the moment with a lot of individual tricks and features I suspect  thtat you might see or not. I have too little detailed information.

Best regards

Harald G. Dill

I think that the answer of Marcos is the best for such studies if there are no good indicators at the meso- scale brittle structures....

I think the answer of Martin is a good way to determine faults zones. In addition, you can use indirect indicators geometry of the conjugate structures, for example. Then, the use of model fractures or shear (like Riedel, or Anderson and even Wilcox) is a good way. This requires that you know the geometric pattern of the site by photo-interpretation of the region (this can also be done at the level of outcrop, but you'll need to study various points). One limitation of this method is the problem of overlapping families of fractures that were generated in different activation (or reactivation) of failure. This entire process should be carried out parallel to morphostructural and morphotectonics study site. As the rock that you are working is much older than the Neogene an indication that the process is neotectonic can only occur if there are changes (or deformation) in the landscape. One of the best indicators of these changes are printed in both river basins and in the river channels (drainage anomalies, for example). In the latter pointing recommend seeking information on Schumm work on rivers of response in the face of neotectonic faults.

PS: in Brazil, there is a certain consensus that the neotectonic period covers events from 10 m, that is has been udado what INQUA (International Commission on Quaternary) indicated for some time. However, the field work of several researchers have hypothesized that this period can extend up to the early Miocene or 23 m., When there was a reorganization of the South American tectonic plate.

My study area has a lot of indicatives of the tectonic control on the landforms, but it has been so difficult to support some hypothesis with field data. Unfortunately, I rarely find good outcrops. I had compared my hypothesis about the movement of the structures with the Neotectonic stress field that I determined for my area. The result has been positive for NW-SE structures. Also I had field (outcrops) and morphotectonic (river bends, knickpoints, fluvial terraces, change in the direction of ridge lines, etc) data to support it.

In the other hand, despite I have morphotectonic indicatives (river bends in the main river and its meander belt, knickpoints and large lineaments) of NE-SW structures, I couldn´t confirm them in outcrops because I haven´t seen any clear indicative of shear, although I have found large fractures. Maybe these large fractures are new strike-slip faults (Neotectonic) or reactivated old structures. Another important problem is the incompatibility between the stress-field with the movement of the structures. If these structures were active in the Neotectonic period, the stress field probably varied. These variations in the stress-field were found in nearby areas by other researchers.

Thank you for the answers and suggestions. They have showed me news possibilities.

Marcos,

one valid criterion, in the lack of clear kinematic indicators, is to look for possible pre-faulting/fracturing features that could be used as markers. In the case of a strike-slip fault, the marker appears truncated, offset and displaced.In the case of a large fracture, the marker is truncated, but not offset and displaced. I hope this could be a simple criterion to answer your question.

Best wishes,

Enrico

topography, rivers systems modified

Just check y looking to your DEM. Do not use grey, use a colour code with let's say black line 599-601m line for example and quite a bit of variations at the altitude difference between the flat lying region and you "flower structure".

Have a look to the Andean Front in Ecuador, in the Andean Amazon Basin near Lumbaqui. Forest everywhere and I can mapped fault systems using global mapper, Googlearth Pro (as a GIS system) or rivertools.

Ho that will help

Geoffrey

Dear Marcos,

I do not know how your outcrops are but if you have some assessible exposures Iwould expect to see indicative secondary fractures even in the absence of shear indicators on the fault plane itself. Check out the paper by Petit et al.:

Petit, J. P., 1987, Criteria for the sense of movement on fault surfaces in brittle rocks: Journal of Structural Geology, v. 9, no. 5-6, p. 597-608.

Some reorientation and rotation of the local stresses from the regional stress field shoud also be considered when using fault-slip kinematic indicators for neotectonic regional stress analysis.

Hope this helps. Cheers, Jürgen

Dear Marcos,

Following Geoffrey's (Hi, Geoffrey!) comment above, you may find this contribution worth checking:

http://dx.doi.org/10.1016/j.gloplacha.2012.08.005

(If you don't have access to the full paper, please contact me.)

Kind regards, Balázs

Hi Marcos-

I agree, it can be difficult to obtain sense of shear in a lithology such as fine sandstone. I suggest looking for strain markers across the planar features, such as sets of fractures or sets of unique sandstone strata that have been offset. Fault plane polish and fine-grained fault gouge can also be indicative of shearing, but is sometimes difficult to distinguish from a mineralized fracture or joint surface in the absence of kinematics indicators (e.g. slickenlines). Another shear indicator is rotated clasts that become aligned with zones of faulting/shearing. If the magnitude of shearing across a fault plane is minimal, elongate clasts may appear partially rotated in the direction of shear and can be useful for obtaining a sense of shear. If the magnitude of shearing is larger, clasts may become comletely aligned with your planar feature. This may make it difficult to determine the sense of shear (e.g. right-lateral vs. left lateral), but you can then be quite confident that the planar feature is a fault and not a fracture.

Cheers,

Scott Bennett

US Geological Survey

Thematic analysis of terrain may be useful . 

Dear Marcos

You have done a lot of work and received many useful sugestions. You may try this practical analysis as well. Map folds, because they are very reliable to set the sense of strike-slip fault motion, and since you have determined the stress field, you may use Sigma One direction to discriminate faults from joints. I would rule out joints as those fractures parallel to Sigma One. The rest of them should be conjugate faults.

Good luck, joseantonio 

My friends, I can´t answer you individually because there are many answers (and researchers), but I´m very thankful for everybody. Thank you so much for all answers.

The answers are very interesting. I have a lot of possibilities and I will consider all suggestions that you have gave me. Your opinions have pointed to diversify level approaches, from the outcrop to the regional scale. In my opinion, the multiscale approach is the better way to understand the tectonic and geomorphologic evolution.

We have people from all over the world following this topic, but I think just a few them know my study area in Brazil, so I attached some images: 1) a location map; 2) normal faults in fine aeolian sandstones of the Botucatu Formation (Cretaceous); 3) slickensides (in basalt) related to two different tectonic regimes: a) distension: normal fault (probably in the Cretaceous); b) Strike-slip fault (Neogene and Quaternary). The distension is related to the Gondwana continent fragmentation and the South Atlantic open (Aptian), while the transcurrent faults have been occurring since the Neogene due of South American Plate rotation, as it was previously pointed out by Salamuni in his answer.

Identify Neotectonic strike-slipe faults and their kinematic indicators in fine aeolian sandstone is a challenge, but many researchers, including me, have had good results and their studies and your suggestions have showed me new possibilities. Fortunately, I have a lot of work to do. When I have new information, I will share it with you.

Thank you.