Late Amazonian continual volcanic eruption, contemporaneous tectonics and pit chain formation in the central Tharsis region, Mars: Implications for long-lived magmatism and mantle plume

The Tharsis bulge hosts some of the largest shield volcanoes on Mars. Among these, Arsia Mons, Pavonis Mons, and Ascraeus Mons are more prominent volcanoes in the central Tharsis region. The spatial and temporal evolution of these volcanoes and associated tectonic features and pit chains during the Late Amazonian period is important for understanding the nature of geodynamic processes that shaped them. Therefore, we conducted a detailed geological study of the large shield volcanoes, surrounding small shield volcanoes, graben and pit chains in the central Tharsis region. Our dating of 500 stratigraphically young lava flows from these volcanoes revealed a wide-spread and continual Late Amazonian volcanism. The data showed that the volcanic activity in the caldera region of these three large volcanoes ceased around 46–55 Ma. The volcanoes showed lateral migration of volcanic activity from caldera to scalloped walls during Late Amazonian period, indicating movement of magmas along a deep-seated giant dike that marks the NE-SW oriented axial spreading zone. The lateral migration of volcanic activity, in the form of small shield volcanism and eruptions from vent fields, also occurred across the axial spreading zone, especially towards the southeast of these volcanoes in the last 200 Ma. A major set of small shield volcanoes formed linear clusters in NE-SW direction to the east of the large shield volcanoes, possibly indicating the development of another axial spreading zone, parallel to the existing one. Some of these small shields erupted lavas about 17–29 Ma ago as well. The large shield volcanoes are dissected by several curvilinear graben, possibly defining a broad circular ring dikes around the summit region. Some of these graben have contact relations with the dated lava flows, revealing their contemporaneous development along with the Late Amazonian volcanism. Our mapping of ∼10,000 pits in Tharsis showed spatial distribution of both simple and complex pits, and their relative ages with respect to the dated lava flows. The spatial variation of size and aspect ratio of the pits provides insights into the dimension of underlying magmatic dikes and the magma withdrawal processes. The contact relations between the pits and the dated lava flows suggest that the pits were formed contemporaneously with the volcanoes and graben during Late Amazonian period. Some of the pit chains also define a NE-SW trend, exhibited by the small shield volcanoes. The long-lived and continual volcanism in the central Tharsis region would require long-lived and sustained magma chambers underneath the central Tharsis volcanoes, which are possibly recharged by long-lived individual underplated magma bodies that are present beneath the up-warped base of the crust. A long-lived large mantle plume may also be present beneath the Tharsis lithosphere sustaining magmatic and volcanic activities in the central Tharsis region.