| A chronology of the Syrtis Major Quadrangle on Mars has been devised using topographic, structural, gravimetric, and magnetic data along with recent narrow-angle images from the Mars Orbiter Camera.; The dichotomy boundary and Arabia Terra are two of the major structural features within the quadrangle. Arabia Terra appears to be the basement at least on the southern side of the dichotomy boundary. Recent reports of an andesite-like composition similar to the northern lowlands may indicate that Arabia Terra is a part of the lowlands rather than the southern highlands, but other interpretations of the data appear more likely. Arabia Terra is relatively low topographically, especially to the west of Syrtis Major Quadrangle, but it is usually shown on the southern side of the north-south dichotomy, which corresponds closely to the edge of the thinnest crust.; The Isidis Basin was formed at the end of the Noachian along with the other large impact basins of Hellas, Argyre, and Chryse. The basin is topographically degraded especially on its northern and eastern rims. However, it has a strong gravity signature, positive within the basin and negative in a wide ring surrounding it. The basin is located at a bend in the dichotomy boundary. Its strong gravity signature may be a reflection of the very thin underlying crust, which in turn may indicate that it formed after the dichotomy was in place.; The Nili Fossae are concentric to the Isidis Basin rim and are essentially contemporaneous with the formation of Isidis or very slightly younger if the fossae formed in reaction to the initial impact. They occur within Arabia Terra-type basement material.; Syrtis Major volcanics superpose the main trough of the Nili Fossae at its southern extremity. Lobes of Syrtis Major volcanics also superpose Arabia Terra to the west and Isidis to the east and probably contributed to the masking of the dichotomy boundary. The inception of Syrtis Major volcanics may have preceded or been contemporaneous with any of the other four major structural features in the quadrangle, but the most widespread lavas are the youngest of the major features. |
