TECTONO-VOLCANIC HISTORY OF NORTHERN NOACHIS TERRA, MARS: IMPLICATIONS FOR MAGMATISM AND CLIMATE IN THE LATE NOACHIAN

Large expanses (10-100 km diameter) of in-place rock units in Noachis Terra, detected using Mars Odyssey THEMIS nighttime radiance images, permit investigation into the processes that formed the crust in this region. We use high-resolution spectral and imaging data sets to better understand the origin of these units and the relative timing of volcanic and tectonic events. A previously unrecognized compositional stratigraphy is present within some of the bedrock units, and can be mapped discontinuously over a ~30,000 km² area. The lowermost unit is distinguished by a lower feldspar/pyroxene ratio relative to the younger unit. The derived surface emissivity spectra for these units are distinct from other units observed on Mars to date, and represent new lithologies. CRISM spectral data show no evidence for aqueous alteration products, and modeled abundance of high-silica alteration products from TES data is below 15%, suggesting little influence from chemical alteration. The occurrence of this stratigraphy over such a wide area, along with embayment relationships, high thermal inertia, and rugged, massive texture suggesting structural competence, point to a volcanic origin. The extensive area and rarity of flow fronts implies low viscosity, high effusion rates, and high magma volumes, analogous to large igneous provinces on Earth. The study region includes a number of ridges and fault-bounded, elongate troughs up to ~100 km wide and ~1000 km long that trend NE-SW, roughly parallel with the curvature of western Hellas Basin. Cross-cutting relationships indicate that the bedrock units were emplaced after the formation of the troughs but prior to ridge formation. Some bedrock units are found on the floors of the troughs or partially burying the troughs. The association of some bedrock units with these features suggest that extensional tectonics, perhaps related to the formation of Hellas, played an important role in the generation of melt in this region. At ~3.7-3.9 Ga old, these are some of the oldest known volcanic units exposed at the surface of Mars, and thus provide important data points for comparisons with younger volcanic terrains such as Thaumasia. Extrusion of these lavas would have been accompanied by significant volatile release, and may have influenced the Martian climate during the late Noachian time period.