Investigation of Ancient Volcanism in Valles Marineris: Evidence for Effusive Activity and Possible Plutonic Intrusions

The ratio of effusive to explosive volcanism from the late Noachian to the early Amazonian remains a knowledge gap in understanding the volcanic evolution of Mars. Valles Marineris, a 4000 km canyon system in the Tharsis region, exposes up to 7 km of stratigraphy that records billions of years of geologic history, allowing for the investigation of the changes in volcanic styles over a large swath of martian geologic history. In this work, we performed a morphologic investigation to identify and characterize stacks of exposed lava flows in order to quantify the relative contribution of effusive volcanism. We initially selected four sites across Valles Marineris:  Candor, Ophir, West Ius, and East Melas ta, located at [-5.95679, 282.70], [-3.00, -287.50], [-7.34, 273.11], and [-10.98, 293.63], respectively.  All sites exhibited clear exposures of at least 1 - 2 km of stratigraphic section and sufficient data coverage.  We performed analysis using high resolution visible image data from the High-Resolution Imaging Science Experiment (HiRISE) (~0.3 m/px) and the Context Camera (CTX) (~6 m/px); moderate and high-resolution digital terrain models High Resolution Stereo Camera (HRSC) DTM (~60 m/px) and CTX DTM (~15 m/px).  Data analysis was conducted in the Java Mission-planning and Remote Sensing (JMARS) GIS.  To quantitatively assess morphology, we measured the lateral extent of exposed marker beds and estimated bed thickness by measuring the distance between marker beds.  We also measured cliff slopes by producing transects at multiple locations within each study site. Preliminary observations in Candor Chasma show a clearly defined transition between upper competent units and the lower talus-rich zone. In Ophir Chasma, we observed deposits consistent with mass-wasting events that have exposed lower competent and layered units.  Whereas in Ius and Melas , while exposed competent rock is present, the competent cliffs are mostly obscured by talus. In West Candor Chasma, marker beds (i.e., competent rock layers) exhibit a mean vertical spacing of 8.4 m and a mean lateral extent of 22.3 m, in Northern Ophir Chasma (Site 2) layers show a mean vertical spacing of 5.81 m and a mean lateral extent of 15.8 m, while in West Ius Chasma the mean vertical spacing is 6 m. Assuming the distance between marker beds represent individual flow units, we interpret this sequence of layers as massive thick (~6 – 8 m) lava flows. Our preliminary results are consistent with previous literature that the upper and middle walls of Valles Marineris preserve horizontal lava stacks, which suggests that effusive volcanism has dominated in the region in recent martian geologic history.  Deeper layered deposits observed in Ophir Chasma may belong to magmatic intrusions, consistent with previous literature that subsurface magmatism occurs under extensional tectonic regimes and has played an active role either before or during the formation of Valles Marineris.