Mountainous units in the Martian Gusev Highland region: volcanic, tectonic, or impact related? T.E. Zegers 1 , M. van Kan 1 , B., Foing 1 , R. Pischel 1 , K. Gwinner 2 , F. Scholten 2 , S. Werner 5 , G. Neukum 5 and the HRSC Co- Investigator Team, 1 ESTEC, ESA, Noordwijk, The Netherlands, tzegers@rssd.esa.int , 2 Institute of Planetary Research, German Aerospace Center (DLR), Berlin, Germany, 3 Remote Sensing of the Earth and Planets, Freie Universität, Berlin, Germany Introduction: Gusev crater is situated at the boundary between the southern Noachian Highland terrains and the northern Lowlands. Geological map- ping of this region was carried out, combining THEMIS and HRSC image data and HRSC digital terrain models (DTM), based on HRSC stereo capa- bilities. Three major geological units can be distinguished in the Southern Highlands. These units, their relation- ships, and the structural elements, provide a record of the earliest part of Martian geological evolution. Gusev Highlands: A typical Highland terrain is well preserved to the south and east of Gusev Crater. The larger region was previously geologically mapped by Kuzmin et al. [1] based on Viking data. The High- lands form a heavily cratered plateau at an elevation of approximately –1500 m (DTM HRSC referenced to Mars ellipsoid). The plateau is transected by the Ma’adim Vallis river system cutting approximately 1500 m into the highland plateau. The majority of the surface outcrop is formed by the Gusev Highland Unit. The surface of this unit is typically irregular on a 100m scale and shows a large number of craters and ero- sional features such as channels and grooves. Crater count statistics suggest an age of ~4.04 Ga for this unit, based on the algorithm by Hartmann and Neukum [2]. Although the details of crater-retention ages are controversial, the crater-count statistic ages presented here were obtained using a uniform data set and can therefore be used as absolute ages within this dataset. In local, relatively low lying areas, the Gusev Highland Unit is covered by a younger and relatively smooth unit called the Modified Gusev Highland Unit. The crater count age of this unit is ~ 3.73 Ga. Since most erosional features are absent in the Modified Gusev Highland Unit, the erosional activity in the highlands is largely constrained between 3.73 and 4.04 Ga. Although there is no layering visible in the (Modi- fied) Gusev Highland Units, it is likely that the two units have remained in their original position and ori- entation. There is no indication for tectonic disruption or tilting. The oldest unit of the highlands occurs in three relatively small (<30 km diameter) areas. In these ar- eas the “mountainous“ unit surfaces through the Gusev Highland Units as mountainous terrains of higher to- pography. Figure 1 shows the HRSC based anaglyph of one of these mountainous terrains. Figure 1. Anaglyph of mountainous terrain, based on HRSC nadir and stereo channel (orbit 283). This terrain occurs approximately 50 km southeast of Gusev Crater. North is to the left of the image. Mountainous units: Topographic profiles based on HRSC DTM’s through these mountainous terrains show that these terrains rise up to 2500 m above the surrounding Highland plateau. The topography is ir- regular with ENE-WSW oriented ridges. Images show that the mountainous units are embayed by the Gusev Highland Units, indicating that the mountainous units may have a more extended volume in the subsurface. This age relationship also indicates that the Mountain- ous Unit is older than 4.04 Ga, the crater count age of the Gusev Highland Unit. Images show no fine recog- nizable layered structure. Topographic profiles per- pendicular to the ridges typically show a rugged and irregular northern slope, and a smooth planar southern Lunar and Planetary Science XXXVI (2005) 1651.pdf