EXTENSION ACROSS TEMPE-TERRA, MARS, FROM MEASUREMENTS OF FAULT SCARP WIDTHS AND DEFORMED CRATERS

Two independent methods, with no common assumptions, have been used to estimate the extension across the heavily deformed Tempe Terra provin ce of the Tharsis region of Mars. One method uses measurements of norm al fault scarp width with average scarp slope data for simple grabens and rifts on Mars to estimate the fault throw, which, combined with sp arse fault dip data, can be used to estimate extension. Formal uncerta inties in this method are only slightly greater than those in other me thods, given that the total uncertainty is dominated by the likely unc ertainty in the fault dip (assumed to be 60 degrees +/- 15 degrees). M easurement of normal fault scarp widths along two N25 degrees-50 degre es W directed traverses across Tempe Terra both yield about 22 +/- 16 km of extension (or similar to 2% strain across the northern traverse and nearly 3% across the southern one). About three quarters of the ex tension has occurred during the two main phases of Tharsis-related def ormation from Middle/Late Noachian to Early Hesperian and from Late He sperian to Early Amazonian, with mow extension closer to the center of Tharsis during the first phase. Extension across the region was also determined by measuring the elongation and elongation direction of all ancient Noachian impact craters without ejecta blankets, which predat e most of the deformation. Results have been corrected for initial non circularity of craters, established from similar measurements of youn g (post deformation) impact craters, yielding a statistically signific ant mean strain of 1.96 +/- 0.35% in a N38 degrees +/- 10 degrees W di rection across Tempe Terra (extension of similar to 20 +/- 4, comparab le in magnitude and direction to the average result from the scarp mea surement method). Both methods indicate an average extension for singl e normal fault scarps (and shortening across wrinkle ridges for the cr ater method) of similar to 100 m. The agreement between the results of the two independent methods in overall extension and average single n ormal fault extension argues that the average scarp slope and fault di p data in the fault scarp width method accurately represent the actual extension across the observed structures. This conclusion supports ex isting geometric and kinematic models for structural features on Mars. A preliminary estimate of the total circumferential extension around Tharsis (at a radius of similar to 2500 km) is roughly 60 +/- 42 km; t otal hoop strain is about 0.4% distributed heterogeneously (Tempe Terr a is the most highly strained region on Mars).