Chadwick, D. John | Search NASA

NASA NTRS1993

Extensional history of Mars' Tharsis Region

Graben structures in the outer part of Mars' Tharsis region follow calculated stress patterns that are based on Tharsis topographic and gravity anomalies and flexural response of the lithosphere. However, Tharsis extension is not evenly distributed - most extension is concentrated in the Alba, Tempe, Valles Marineris, and Syria-Thaumasia volcanotectonic provinces. Preliminary results suggest that the overall extension of Tharsis produced by loading is about 8 km (based on the assumption that the Sirenum province exemplifies the overall extension of Tharsis), whereas the total circumferential extension of the Tharsis region estimated thus far is about 33 km (an estimate was not made for the Syria-Thaumasia province, and evidence in some other areas may be buried). It is concluded that local tectonism, possibly caused by intrusion and passive and active rifting, produced most of the extension in the Tharsis region.

Tanaka, Kenneth L.↗

NASA NTRS1992

Bright crater outflows: Possible emplacement mechanisms

Lobate features with a strong backscatter are associated with 43 percent of the impact craters cataloged in Magellan's cycle 1. Their apparent thinness and great lengths are consistent with a low-viscosity material. The longest outflow yet identified is about 600 km in length and flows from the 90-km-diameter crater Addams. There is strong evidence that the outflows are largely composed of impact melt, although the mechanisms of their emplacement are not clearly understood. High temperatures and pressures of target rocks on Venus allow for more melt to be produced than on other terrestrial planets because lower shock pressures are required for melting. The percentage of impact craters with outflows increases with increasing crater diameter. The mean diameter of craters without outflows is 14.4 km, compared with 27.8 km for craters with outflows. No craters smaller than 3 km, 43 percent of craters in the 10- to 30-km-diameter range, and 90 percent in the 80- to 100-km-diameter range have associated bright outflows. More melt is produced in the more energetic impact events that produce larger craters. However, three of the four largest craters have no outflows. We present four possible mechanisms for the emplacement of bright outflows. We believe this 'shotgun' approach is justified because all four mechanisms may indeed have operated to some degree.

Chadwick, D. John↗