THE IGNEOUS AND ALTERATION HISTORY OF THE GREATER THARSIS REGION, MARS

Identification and correlation of distinct mineralogical units with topography and morphology can provide key constraints on the geologic history and formation of Valles Marineris, the Tharsis Rise, and the greater Thaumasia Plateau region. Results from detailed mineralogic mapping indicate a wide range of primary and alteration minerals throughout the greater Thaumasia region. Distinct mineral assemblages that occur at similar longitudes on both the north and south walls of eastern Coprates Chasma suggest the genetic mechanism for Valles Marineris did not include large east-west offset. Mapping also reveals localized uplift of basement rock suggesting thrust faulting in western Coprates Chasma and supports massive basin formation or down-dropping of the lithosphere west of Melas/Candor Chasma. East-west facies changes in metamorphic grade within the walls of Coprates Chasma indicate regional differences in late Noachian crustal heat flow related to the formation of the Thaumasia highlands. The primary mineralogy exposed in the walls of Valles Marineris indicates the thinning of igneous units away from Tharsis Rise, consistent with a constructional origin of the Tharsis Plateau. Our work suggests the following formation timeline for the greater Thaumasia region: 1) the formation of a broadly uniform Noachian basement that was 2) exposed to laterally discontinuous alteration likely related to subsurface (impact or groundwater related) hydrothermalism. 3) The onset of the Thaumasia Plateau uplift and formation of the Thaumasia highlands provided a higher thermal gradient that transformed smectites in the E. Coprates walls to higher metamorphic grade minerals. As Tharsis continued to grow, 4) a compressional regime that would have existed produced the thrust faults that offset compositional boundaries in W. Coprates. 5) Eventually, thickening of volcanics to the west caused tectonic subsidence of the material west of Coprates Chasma.