WSAG
TectonoTorsdag 001
The image below was one of the first images released during the Magellan mission to Venus and is referred to as the Gridded Plains of Venus. What do you think this tells us about the tectonics of Venus?
Back in the day, we (I was a NASA–funded Research Fellow at SMU from 1994–1996) had to do significant image processing to tile images, reproject, etc. Now you can use Google Map in Space to view imagery from Venus and other planets and moons. Load Google Maps. Click Layers and then More. Select Satellite View and click Globe View near the bottom. Then scroll out as far as possible and eventually that triggers a side bar where you can select the body you want to view. The link below can take you to the general vicinity of the main image so you can scroll around and look for yourself.
Scroll down for interpretation.
Figure 1. The gridded plains of Venus, Guinevere Planitia, 30°N, 333°E. Magellan image, left-look synthetic aperture radar. Banerdt, W. Bruce, Sammis, Charles G. (1992) Small-scale fracture patterns on the volcanic plains of Venus. Journal of Geophysical Research, 97. 16149. doi:10.1029/92je01892.
TectonoTorsdag 001 Interpretation
Both sets are not extensional and are asynchronous. Orthogonality is coincidental.
In the gridded plains, two sets of orthogonal features are present, a NW–SE set of radar bright events with slight en echelon offsets and a NE–SW set of faint features. The typical interpretation is that the two sets are both extensional and due to their orthogonal nature indicates that they are likely synchronous in time. Unfortunately, the limits of the Magellan radar systems places both sets at the limit of decisive interpretation in terms of their individual details. Most workers, including myself, would agree that the radar bright set is extensional with a slight component of shear to generate the slight echelon pattern. Their patterns are consistent with examples elsewhere that do exhibit classic graben/gash with dark-bright paired trends, one scarp in shadow (left side with left-look direction) and the other illuminated (right side with left-look direction) indicating a downdropped or open systems. In the plains (or planitia), the radar dark nature of the lava plains often limits definitive observation of the shadowed scarp.
The faint NE–SW set is likewise at the limits of resolution. Details cannot be seen other than their general appearance and orientation.
Determining stress and tectonic history can be dangerous. For example, orthogonality due to essentially synchronous formation is confirmed in many places on earth–set 1 opens to release stress in one direction and set 2 follows perpendicular to the other as stress continues to build in the orthogonal direction. Set 2 typically abuts agains set 1, but regionally or even in the same outcrop area, local stress deviations can occur (one direction more stressed in one area but orthogonal direction more stressed in another area), where set 2 abutting against set 1.
The danger however is seeing two sets that are orthogonal and then assuming that both sets are extensional and essentially formed at the same time. Additional context must be determined to help understand their nature, otherwise the interpretation can become model driven (“They intersect at 90 deg and therefore must be this.”) Hence, again, looking at more detail to confirm their mutual relationship is needed, how do the two sets interact with one another, can you see abutting relationships? can you see offsets? etc. Otherwise, the orthogonal model just becomes a predictive tool, that of course may actually be wrong! Being at the limit of resolution doesn’t help because we’re unable to see the details. But we can look at their regional trends. Does the orthogonality hold regionally or not? Are there additional features that may shed light on origins? Etc.
In the original image (Figure 1), the orthogonal nature dramatically stands out, or does it? In the SW portion of Figure 1, the faint set changes trend to become more southerly causing non-orthogonality. Now, note that the orthogonal nature in known systems does not have to be perfectly 90 deg. Stress deviations due occur but this looks to be a more systemic change. Thankfully, we can look at additional data to determine the relations and this is where the orthogonal model fails. Regionally the NE–SW set maintains a fairly consistent pattern, whereas the faint set exhibits regional changes. Additionally examination to the NE and E especially reveals that there are actually two faint sets, each consistently about 50 deg apart from one another even as the orientation changes regionally (Figure 2).
Figure 2. Northeast of Figure 1, a second faint set is present (NNE–SSW) in addition to the NE–SW set originally seen in Figure 1. Note that in Figure 1 although the NE–SW set is pervasive, subtle indications of local occurrences of the NNE–SSW set can be seen. Even as the NE–SW set changes orientation slightly regionally, the NNE–SSW set maintains the same ~50 deg intersection angle to the NE–SW set.
Reiterating the caution about using orientations to help understand formations, the two faint sets in Figure 2 and elsewhere exhibit a remarkably consistent orientational relationship. They maintain ~50 intersection angle no matter what the orientation of the original NE–SW set trends. This is very different than the relationship between the radar-bright NW–SE set in Figure 1 and the faint NE–SW set—yes, they are mutually orthogonal in Figure 1 but elsewhere that relation changes to non-orthogonality, implying that they are not mutually related to one another in their formation. The two faint sets in Figure 2 and elsewhere maintain their intersection angle across large swaths of Guinevere Planitia, suggestion mutual relationship.
So how did they form? We can effectively dismiss that they are contractional elements as they look nothing like contractual features elsewhere on Venus wrinkle ridges/reverse faults). Therefore, we’re left with two other models to explain their relation. Model 1 is that both are extensional elements and model 2 is that both form a conjugate set of shear fractures (which do collectively record contraction approximately parallel to the acute bisectrix of the intersection angle). Again unfortunately limits of resolution preclude key details like abutting relationships or offsets. But, we can consider plausibility of the two models. If both sets are extensional (model 1), the first set would form, generally for extensional features, perpendicular to the minimum horizontal stress. Formation of the other set as extensional elements would require that the minimum horizontal stress to reorient at ~50 deg to the orientation necessary. That stress reorientation would be required everywhere, including including even where the trend of the first set changes, yet somehow the second set maintains the same intersection angle. This is problematical for various reasons. It is just frankly difficult to determine a causal mechanism to change stress orientation consistently at a 50 deg angle.
A 90 deg change is trivial—stress magnitude flip flops are common. This is why the original orthogonal model was proposed. We have seen documented cases in many cases on Earth. Release of tensile stress in one direction can cause the orthogonal stress to become the minimum principal stress. However, consistent non–90 deg intersections over large swaths of land are non-trivial and require specific causal mechanisms to explain why intersection angle is maintain even as orientations change. The model of two extensional sets intersecting at 50 deg just doesn’t seem plausible. It requires two tectonic events that must somehow be related to maintain the intersection angle (and really three when the original NW–SE radar-bright set is added).
Model 2 is the conjugate shear fracture, whereby both of the faint sets form simultneously (at least at time scale of the single tectonic event). This again has been observed in numerous environments on Earth. The definitive description to confirm a conjugate interpretation is that both sets must mutually cross-cut one another (e.g., members of set 1 offset set 2 and members of set 1 are offset by set 2, sometimes even along the same fracture, AND the offsets must be mutually opposing where, say, set 1 exhibits left-lateral offset and set 2 exhibits right-lateral offset, or vice versa). Again, due to resolultion, we can not see those details, similar to how we can see abutting relations if it were an extensional system. But, maintaining consistent intersection angle over large areas of land can be more easily explained in an overall contractional environment by the formation of a conjugate shear fracture set. Can we definitely state that these are conjugate shear (strike-slip) fractures? NO! That would be a model-driven interpretation. But, we can say that model is plausible and perhaps in the future with higher resolution we can observed the key details to confirm their formation.
Occurrence of two faint fractures in an apparent conjugate relationship is pervasive on Venus. In just the first week of my Venus work way back when, perusing the image dataset (we had it all digitally but we had multiple binders of the C– and F–MIDR prints to flip through), I observed over 100 occurrences were identified, including several that exhibited predictable relationship is specific structural environments. For example, Figure 3 shows a volcanic dome. A basic model of dome formation predicts a central zone of radial extensional elements (tension cracks/normal faults), an intermediate zone of conjugate strike-slip elements (with radial acute bisectrix), and an outer zone of concentric contractional elements (folds/reverse faults). Figure 3 shows this exact relationship. We cannot say the intersecting set are definitive conjugate strike-slip elements as that’s again the model driven the interpretation. But, the fact that it all fits neatly together within a well-defined model sure does predict that they are a conjugate set. Another great example that I may post when I locate the original image was a collapse basin that showed the probable conjugate strike-slip set exhibiting classic Archimedian spirals, exactly as basin models predict.
My bet is on the conjugate shear fracture model for these faint sets.
Figure 3. Volcanic dome in Atla Regio, Venus, showing various structural elements that match model predictions of dome formation. A central area of radiating extensional elements, a central area of conjugate strike-slip elements (with radial acute bisectrix), and an outer zone of concentric thrust faults (forming wrinkle ridges). The normal fault and wrinkle ridges can both be definitively interpreted as such due to observation of specific characteristics. Resolution limits confirmation of actual strike-slip displacements but domal models predict their occurence in same configuration that is observed.