Geology of the Thaumasia region, Mars: plateau development, valley origins, and magmatic evolution

We have constructed the complex geologic history of the Thaumasia region of Mars on the basis of detailed geologic mapping and relative-age dating of rock units and structure. The Thaumasia plateau dominates the region and co nsists of high lava plains partly surrounded by rugged highlands, mostly of Noachian and Hesperian age. Long-lived faulting centered near Syria Planum and at lesser sites produced radiating narrow grabens during the Noachian through Early Amazonian and concentric wrinkle ridges during the Late Noach ian and Early Hesperian. Fault activity peaked during the Noachian and wane d substantially during Late Hesperian and Amazonian time. Volcanism on the Thaumasia plateau was particularly active in comparison with other martian cratered highlands, resulting in fourteen volcanoes and numerous outcrops o f smooth, ridged, and lobate plains materials. A particularly extensive set of overlapping lava-flow units was emplaced sequentially from Thaumasia Pl anum to Syria Planum, spanning from the Late Noachian to the Late Hesperian ; lobate flows succeeded smooth flow at the beginning of the Late Hesperian . Deep crustal intrusion and a thickened, buoyant crust may have caused the uplift of the plateau during the Noachian and Early Hesperian, resulting i n outward-verging fold-and-thrust plateau margins. This structural style ap pears similar to that of the young ranges of the Rocky Mountains in the wes tern U.S. Within the plateau, several sites of volcanotectonic activity and valley erosion may be underlain by large and perhaps long-lived magmatic i ntrusions. One such site occurs at the headland of Warrego Valles. Here, at least two episodes of valley dissection from the Noachian to Early Hesperi an occurred during the formation of two nearby rift systems. The site also is a locus of intersection for regional narrow grabens during the Late Noac hian and Early Hesperian. However, at the site, such faults diverge or term inate, which suggests that a resistant body of rock occurs there. The overa ll volcanotectonic history at Thaumasia fits into a model for Tharsis as a whole in which long-lived Syria Planum-centered activity is ringed by a few significant, shorter-lived centers of activity like the Thaumasia plateau. Valley formation, like tectonism in the region, peaked during the Noachian and declined substantially during the Hesperian and Amazonian. Temporal an d spatial associations of single erosional valleys and valley networks with volcanoes, rift systems, and large impact craters suggest that the majorit y of valleys formed by hydrothermal, deformational, and seismic-induced pro cesses. The origin of scattered, mainly Noachian valleys is more conjectura l; possible explanations include local precipitation, seismic disturbance o f aquifers, or unrecognized intrusions. (C) 1999 Elsevier Science Ltd. All rights reserved.