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 39°N 201°W. 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)]

Western Dark Terrain (WDT): This region is characterised by a low albedo background surface interspersed with bright rounded higher-albedo hills several kilometres across, and two orthogonal sets of dark lineaments (fractures and troughs) that divide the terrain into polygonal units a few kilometres across. One set of these lineaments trends roughly parallel to the groove lane (N145-135E) and cross-cuts the other trend (N65E), which is itself sharply cut by Byblus Sulcus. Although the NW trend continues all the way along the image, the easternmost lineaments curve in towards Byblus and are cut by it; this is especially noticeable near the centre of the image.

The N65E trend is centred around three large sub-parallel ridges separated by two troughs, with smaller scale sinuous dark and bright lineaments run roughly parallel and through them. The topographic relief is difficult to discern using the G2 image alone, and was distinguished using the G8 low-sun image. While these features can be separated at high resolution, at lower resolution they can be grouped as a single feature and were classed as such by Murchie et al. [5], who interpreted them as a single System I furrow.



Byblus Sulcus (BS): The central N145E band in the G2 image is comprised of sets of parallel or subparallel densely packed flat-topped and 'knobbly' ridges approximately 1-3 km wide and one kilometre high. Individual ridges frequently widen out or thin along their length, to the point of becoming wide plateaux up to 5 km across or small linear knobs a kilometre or so across. Like other groove lanes on Ganymede, at lower (i.e. G8, Voyager) resolutions Byblus is distinguishable from the surrounding dark terrain by its higher relative brightness. At G2 resolution however, a significant contributor to the brightness of the sulcus appears to be local features such as the bright ejecta blankets of small craters, high-albedo material on sunward facing slopes and ridges, and small-scale surface texture rather than the actual base albedo of the sulcus material itself which instead appears not too dissimilar to that of the dark terrain.

The overall trend of the ridges within Byblus is unidirectional, parallel to the axis of the sulcus; however, closer examination reveals that ridges are oriented along smaller curvilinear/arcuate sub-trends that lie concordant to the main trend with most lying between N136-150E. Of particular note is a lenticular region on the northeastern border of the sulcus in which ridges bend around a straight central axis that is parallel to the nearby Byblus/Eastern Dark Terrain boundary. The borders of several of these sub-trends frequently correspond to major topographic changes (usually west-facing scarps) running along the axis of the sulcus. In addition, these small-scale ridges are clearly superimposed on both the flat regions and the slopes of the large-scale G8 topography. Two distinct broad high-relief ridges approximately ten kilometres apart run along the strike of Byblus just east of its axis that stand out visually only in the G8 image; the region of the sulcus east of these features contains the highest density of small-scale ridges that do not run parallel to the main Byblus trend (N136-150E).




Eastern Dark Terrain (EDT): Tectonism is more complex in this part of the dark terrain - the 'furrow' visible in the Western Dark Terrain does not appear to correspond to any features visible in the east. A densely-packed set of dark lineaments (fractures) and bright lineaments (ridges) abuts against the northeastern border of Byblus near the lenticular region described previously. The orientation of these lineaments rotates from N145E (parallel to the Byblus trend) to N90E, and the area covered by them widens towards the central eastern part of the image; terrain is highly disrupted in this zone and some large craters near the Byblus are heavily tectonised. This region corresponds to Akitu Sulcus. To the north of this sulcus, a set of arcuate fractures defines a semicircular region that is comparatively fracture-free at G2 resolution.

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Dark Terrain impact features: Craters in the dark terrain range in size from ~500 m to 12 km in diameter and can be found in varying states of degradation. The north-facing rims are generally the brightest parts of these craters (even though solar illumination is from the southeast with the sun 37° above the horizon), and ejecta are not generally obvious due to low albedo contrast. Dark terrain craters range from those with a morphologically fresh appearance and sharply defined bright rims to degraded craters with poorly defined circumferences and shallow bowl-shaped interiors (often with more recent craters superposed) and heavily tectonised craters modified by fracturing.




Byblus Sulcus impact features: The craters in Byblus Sulcus range from ~300 m to 8 km in diameter. All the visible craters are fresh, with sharp rims; there are no degraded or partially destroyed (and by implication 'old') craters visible in this area at all. The morphology of these small fresh craters strongly resemble those in the Dark Terrain - in both instances the crater outlines are generally asymmetrical with most having a pronounced triangular or square plan. Bright ejecta is also visible around some of the craters here.

The main features of note in Byblus Sulcus are the two craters in the centre of the G2 image surrounded by dark lobate structures and an irregular bright ejecta blanket beyond. No secondary craters have been unambiguously identified; either they never formed, are below the limit of resolution, or they have been removed or covered by some unrecognised process. The larger crater (Nergal) is ~8 km in diameter with a polygonal rim, dark floor, and a broad cruciform central peak structure. The eastern rim is highly modified and appears to follow the trend of impinging lineaments in Byblus - it appears that it has collapsed eastwards into the sulcus and formed a wide (albeit short in radial extent - just over a kilometre in length) debris fan. Lineaments can also be seen continuing through the crater, modifying the eastern crater floor and central peak structure; these are not as apparent in the western part of the crater. The smaller (unnamed) crater to the west of Nergal is ~3 km in diameter with a circular bowl-shaped appearance (though it does have a small central peak). It is not modified in any way by the lineament structure in Byblus; indeed, a pronounced bright lineament stops at the outer boundary of the southeastern wall of this crater. These craters are both surrounded by thick dark lobate structures that are in places outlined by what appears to be a brighter levee-like structure, probably emplaced as a consequence of the impacts; close examination suggests there may be multiple lobe sets, though it is unclear which sets are associated with which crater.

Both craters are in turn surrounded by an asymmetric bright deposit that appears confined to Byblus Sulcus only - the eastern border appears to follow the BS / EDT boundary. The outline of the deposit is highly irregular and does not correspond to that of the dark lobes immediately surrounding the craters - indeed, there is a thin triangular extension of this deposit extending northwards along one of the major G8 ridges in the eastern portion of Byblus. A small crater to the NW of Nergal may contribute in part to this ejecta blanket. The lineaments in the underlying sulcus appear to continue through Nergal itself (and its lobate ejecta) but not through the unnamed smaller crater - however, the smaller crater is located in a smoother, comparatively lineament-free area that continues approximately 40 km either way along the strike of Byblus. Either the ridges in this area have been subdued somehow (perhaps by cryovolcanism) or they never formed here.

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).

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 (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.

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.

References:
[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.