Geologic map of MTM quadrangles -15257 and -20257 of western Hesperia Planum, Mars
Farley, Melissa A.
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The volcanic edifice of Tyrrhena Patera (approximately -22°N, 252°W), Mars, stands 1800 m above the surrounding plains of Hesperia Planum, approximately 2000 km northeast of the center of the Hellas impact basin. Shallow slopes (≤1.8°) characterize the flanks, which are composed of a layered, fine-grained material heavily dissected by broad (≤15 km wide), flat-floored channels that radiate from the summit region. The gentle slopes, erosional morphology, and lack of primary flow lobes strongly suggest that Tyrrhena Patera is an ash shield formed from pyroclastic flows. The shield deposits locally extend ∼700 km from the summit. Such run-out distances are at least an order of magnitude longer than terrestrial pyroclastic flows, but results of pyroclastic flow modeling using current and ancient Martian conditions support these measurements. Recent photogeologic observations reveal that the plains of Hesperia Planum are covered by ∼300,000 km 2 of Tyrrhena Patera shield materials, an area approximately 25% larger than previously mapped. Western Hesperia Planum, once interpreted to be basaltic flood lavas similar to those found on the Moon, is actually a complex mixture of erosional, primary pyroclastic, and effusive volcanic materials associated with Tyrrhena Patera volcano. The extent of the shield materials impose significant constraints for the duration and style of volcanic activity of Tyrrhena Patera as well as the precise crater size-frequency distributions for the materials present within the region, specifically Hesperia Planum, which marks the stratigraphic base of the Hesperian Epoch.