Lateral Displacement in
Northern Marius Regio, Ganymede:
Evidence from Galileo SSI Data.


Constantine Thomas1, Jim Head, and the Galileo SSI Team
(1): Planetary Science Research Group, Environmental Science Dept., I.E.N.S., Lancaster University, Lancaster, LA1 4YQ, U.K.

*: This poster was originally presented at the 29th Lunar and Planetary Science Conference, March 16-20 1998, Houston, TX. as 'The Morphology of Byblus Sulcus, Ganymede from Galileo SSI Data'.

Abstract: This poster describes the morphology of a groove lane on Ganymede known as Byblus Sulcus that was imaged by the Galileo Orbiter during the G2 and G8 fly-bys through the Jovian system. Small-scale surface features imaged at high resolution during the G2 fly-by are described in detail and lineament trends analysed and plotted. The regional setting (G8) is then considered. A hypothesis is presented to explain the features seen along Byblus at both high and low resolution, suggesting that lateral movement has taken place along Byblus, displacing features across it by ~ 60 km in a sinistral manner. Implications for this hypothesis are then discussed briefly.

Introduction: Byblus Sulcus is a bright groove lane identified in Voyager 2 images, located at 39°N 201° W in northern Marius Regio on Ganymede. It was imaged at high resolution (86 m/pxl) and high sun angle on September 6th 1996, during the G2 orbit of the Galileo Orbiter. This region was targeted in order to provide high resolution images of an impact feature named Nergal with a distinctive dark/bright ejecta blanket - also in the imaged region is a roughly east-west trending groove lane named Akitu Sulcus. Another (near-terminator) image of north Marius Regio containing this area was taken at a 940 m/pxl resolution during the later G8 orbit to provide regional context for the G2 image; this also provides useful topographic information. This region was mapped previously by Murchie and Head [1] on a 1:5,000,000 scale using Voyager images as a base. They defined three geological units in this area - dark furrowed terrain (df) containing eastward-trending furrows, light grooved material (lg) consisting of high albedo sets of conspicuous grooves, and bright or partly degraded craters (c1,c2) [1]. A preliminary analysis of this area based only on G2 Galileo data presented by Head et al. [2] is expanded here.

Data: For this analysis the G2 images were mosaicked, projected onto the G8 context image and reprojected as a Lambert equal area projection centred at 39N 201W. Layering both images over each other (Figure 2) proved to be a very powerful yet simple aid to image interpretation - by rendering the G2 image partially transparent over the G8 image, it was possible to directly correlate albedo features in the high sun G2 image with topography observed in G8. The whole G8 Regional Context image (Figure 1) was also used when examining features at a larger scale. A map of the lineaments in the G2 image (Figure 3) was also constructed and orientation histograms (Figure 3b) were derived in order to highlight dominant trends.

Figure 1: G8 Regional Context image showing Northern Marius Regio and Byblus, Akitu, Philus, and Nippur Sulci (Galileo image c0394517800).
[Click image for larger version (382K)]
Regional Context: Byblus Sulcus is located in a block of dark terrain on Ganymede known as Northern Marius Regio, shown in the G8 image (Figure 1, north at top). Northern Marius Regio is the dark region occupying the bottom-left half of the image; the complex region of bright terrain in the top-right half of the image is Nippur Sulcus. The region of bright terrain in the NW corner of the imaged part of Marius Regio is the easternmost extent of Philus Sulcus. Byblus Sulcus is the NNW-SSE lenticular feature crossing down the left-hand third of the image. It thins to the north and south and is widest (~ 30 km across) in the area imaged at high resolution during G2. The southern apex of the sulcus ends in a deep crack that shallows and curves eastwards at its termination, and its albedo at this resolution is higher than that of the surrounding dark terrain. Ridges and scarps run roughly parallel to but are not centred along the axis of the sulcus - these have a wavelength of approximately 10 km that can be readily observed in both G2 and G8 images using Fourier transform analysis [3]. This topography is especially noticeable in the northern half of Byblus (which includes the G2 image) nearer the terminator - in the southern half the topography consists of more subdued sub-parallel ridges with comparatively lower relief. Nergal crater is distinctly visible in G8 in central Byblus Sulcus, but due to the low-sun imaging geometry the albedo differences visible in the G2 image between its immediate environs and more distant bright terrain cannot be distinguished.

Two other small groove lanes trend roughly E-W and NE-SW from central Byblus on its eastern side; the northern one is Akitu Sulcus. These features do not continue from the corresponding location on the opposite side of Byblus, although two groove lanes are located in similar orientations approximately 60 km to the south along the western edge of Byblus.

G2 Morphology: The G2 mosaic (Figure 2) is divided here into three main areas based on the Voyager images - highly tectonised dark terrain in the west, the morphologically distinct Byblus Sulcus (trending N145E) in the central third of the image, and a second region of dark terrain in the east that contains the westernmost portion of Akitu Sulcus.

Figure 2: 'Merged' G2 (high resolution) and
G8 (low resolution) images of Byblus Sulcus.
[Click image for larger version (244K)]

Figure 3: Map of lineaments in G2 Byblus Sulcus image
(colours correspond to histograms in Figure 3b).
[Click image for larger version (59K)]
Figure 3b: Orientation histograms of lineaments in G2 Byblus Sulcus images.
[Click image for larger version (26K)]

Fracture trends: Lineaments were mapped in the G2 image by measuring the orientations of their strike (Figure 3). Based on their morphologies, the lineaments in Byblus Sulcus were interpreted as ridge crests while those in the Dark Terrain were interpreted as fractures or troughs. These lineament sets are not visible in the lower resolution G8 image. Lineament trends were measured in the Western and Eastern Dark Terrains, and also in Byblus Sulcus in the G2 image; these are presented graphically for each region in the histograms of Figure 3b. Lineaments are binned at five-degree intervals and orientations measured clockwise (eastwards) from north. Errors in angle measurements are estimated at ± 2 degrees.

The Dark Terrain histograms suggest a marked difference in the orientation of fractures on either side of Byblus Sulcus. The WDT contains two distinctly separable sets of lineaments; one between N41-100E associated with the System I furrow and the other set between N110-165E corresponding to the fractures parallel to Byblus. Cross-cutting relationships indicate that the System I set is older than the set parallel to Byblus. The EDT shows a different, more dispersed set of orientations that are not clearly divisible into two separate groups; however, both Dark Terrains appear to contain a preponderance of fractures oriented between N131-140E.

The Byblus Sulcus histogram shows a marked peak in orientations between N136-150E, parallel to the overall trend of Byblus. A slight peak in lineaments at high angles (> N160E) represents ridges that are found only on the eastern side of the sulcus, beyond the large axial ridges seen at G8 resolution.

Evidence for lateral displacement?: The dissimilarity in dark lineament orientation trends, surface morphologies and textural character on either side of Byblus Sulcus (BS) at G2 resolution cannot be easily explained if one assumes a priori that continuous terrain is represented on each side. While not impossible, a sudden in situ change in morphological character over the width of Byblus is highly unlikely. The two groove lanes (Akitu and the unnamed NW-SE trending sulcus) visible in the G8 image are unique on both sides along the length of Byblus - no other groove lanes are present that cross or cut Byblus - yet they do not connect across its length in their current configuration. In addition, a linear feature that is similar in appearance to the System I furrow identified within the WDT in the G2 mosaic is located ~ 60 km to the north in the EDT (beyond the northern limit of the G2 mosaic).

Given this similarity in their morphologies, it is not unreasonable to hypothesise that these features were originally continuous across this region and that they were displaced along the axis of Byblus during or after its formation to create the configuration seen today. Since cross-cutting relationships indicate that Akitu Sulcus and the unnamed groove lane are intermediate in age between Byblus (itself one of the youngest features in the region) and the older furrows that they cut in the Dark Terrain, any such hypothetical displacement should affect all the linear features (i.e. both groove lanes and furrows) that are cut by Byblus to an equal extent. Therefore, for this hypothesis to be a valid explanation of the observed features, other lineaments would also have to realign across Byblus after the terrain has been reconstructed.

Since the groove lanes across Byblus are the most easily identifiable linear features that cross it, this 'displacement hypothesis' was tested by reconstructing the terrain on either side of Byblus in such a way that at least one of the groove lanes lined up with no lateral displacement relative to its counterpart on the opposite side of Byblus. For this initial test, it was assumed that the displacement took place along a single straight axis that connected the apices of Byblus.

When this realignment was attempted, no less than four furrows (including the System I furrow identified in the G2 image), the two groove lanes, an oval fracture system at the southern apex of Byblus, and a bright band visible in Voyager images that crosses the oval fracture system (noticeable in the G8 image only as a slight change in surface texture) were found to realign across Byblus if one side was sinistrally displaced relative to the other over a distance of ~ 60 km. In addition, several other furrows and lineaments could be tentatively identified that may line up if the terrain is reconstructed in this manner.

Figure 1: Northern Marius Regio today
(G8 Regional Context image before reconstruction).
[Click image for larger version (382K)]
Figure 4: Possible original configuration of Northern Marius Regio before lateral displacement along Byblus Sulcus.
[Click image for larger version (358K)]

Figure 1 (reproduced above) shows Northern Marius Regio as seen today, while Figure 4 shows the results of this reconstruction and represents a possible original configuration of the area. The nine features identified above as realigning after the reconstruction (but not aligned before) are also highlighted in white in Figure 4. The ?'s in Figure 4 indicates terrain that was not accountable for after the reconstruction when this poster was originally presented - this is discussed in the concluding section.

The Dark Terrain block to the east of Byblus was translated in this test because it contains a clearly defined zone of complex structures (visible in the southeast corner of the G8 image, also imaged at higher resolution during G2) that may be interpretable as an extension accommodation zone. The area to the west of Byblus (as observed in Voyager images) does not contain such features and is therefore likely to have remained static and stable, or at most not significantly translated.

The evidence presented here supports the hypothesis that the terrain on either side of Byblus can be reconstructed by translating them along the axis of the sulcus. More precisely, a sinistral displacement of ~ 60 km is required to realign the linear features across Byblus. However, the exact mechanism of that displacement is very unclear and is discussed briefly in the next (concluding) section.

Conclusions and problems: The dissimilarity in dark lineament orientation trends, surface morphologies and textural character on either side of Byblus Sulcus at G2 resolution can most easily be explained if one assumes that features on either side are not directly correlative as they stand. Analysis of the G8 regional context image strongly suggests that a sinistral displacement of approximately 60 km has occurred along the strike of Byblus Sulcus, that readily explains these discrepancies and realigns many linear features that are cut by Byblus. It is more likely that the Dark Terrain to the east of Byblus was translated northwards and that the Dark Terrain to the west remained relatively static due to the presence of complex structures in the eastern block that could be explained as extensional features.

However, while the case presented here for the existence of significant lateral displacement along Byblus is very strong, it is by no means clear how this displacement is accommodated in the north and south; the exact manner that the displacement occurred in is also subject to debate. The linear axial strike-slip movement suggested here is but the simplest explanation - preliminary trials involving rotation of the eastern DT block as well as lateral displacement can also result in the realignment of the same features. In addition, it is possible that terrain in this region may have been so compressed, rendered unrecognisable by fracturing, or otherwise deformed that it cannot be reconstructed in a linear manner. It is likely that at least some of the suggested extension may have been accommodated in the complex structures in the south-eastern region of the G8 image; however, the compression expected in the north is much more problematic (though not impossible) to explain.

The features within Byblus Sulcus itself may be explained by a combination of compression and shear as the wedge-shaped eastern DT block moved northwards. The compression could explain the broad ridges seen at G8 resolution within Byblus Sulcus since the stress would be applied perpendicular to the displacement direction (i.e. parallel to the ridge axes), while the shear could explain the small-scale ridges seen at G2 resolution within the sulcus. The implication is that Byblus Sulcus may have formed as a result of the displacement itself, possibly through melting in the fault zones, such that it was emplaced as smooth bright material that was then later deformed as the displacement continued. It should be stressed, however, that all of these suggestions for the tectonic history of this region should be considered tentative at best.

Clearly, the exact tectonic history of this region is very convoluted and cannot be explained in a simple way - it is likely that compression, rotation, extension, and lateral displacement all played a part in defining the terrain seen today - though any proposed history of the region should take into account the strong evidence for lateral displacement suggested in this poster. It is hoped that further examination of the Voyager images, the G8 regional context image, and the other G2 images covering this region will serve to clarify the history of northern Marius Regio.

[1] S. L. Murchie & J. W. Head (1989), Geological map of the Philus Sulcus quadrangle of Ganymede, USGS Misc. Geol. Investigation, Map I-1966
[2] J. W. Head et al. (1997) LPSC 28, 539-540
[3] J. G. Patel et al., Wavelengths of Ganymede Grooved Terrain determined from Fourier Analysis of Galileo Images, LPSC 29, 1998
[4] S. L. Murchie & J. W. Head (1988), JGR 93: pp. 8795-8824
[5] S. L. Murchie et al. (1990), JGR 95: pp. 10743-10768.

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