Geologic Analysis of Deformation in the Interior Region of Artemis (venus,

Introduction: Artemis measures approximately 2600 km in diameter making it the largest known circular structure on a terrestrial planet. Artemis' most distinctive feature is Artemis Chasma, a ~25-200 km wide, ~1-2 km deep, ~2100 km diameter circular trough surrounding an interior topographic high. Ve-nus' surface abounds with circular to quasi-circular features at a variety of scales including, from smallest to largest, small shield edifices, large volcanic edifices, impact craters, coronae, volcanic rises and crustal plateaus , however Artemis defies classification into any of these groups. Artemis dwarfs Venus' largest impact crater, 270 km diameter Mead, as well as the smaller volcanic edifices. Topographically Artemis resembles some coronae, however Artemis is an order of magnitude larger than the average corona and is more than twice the size of the next largest corona, Heng-O. Ar-temis resembles volcanic rises and crustal plateaus in planform, but differs greatly in topography. Debate during the past decade resulted in the proposal of four hypotheses for Artemis' formation. Artemis represents: 1) the surface expression of a bolide impact [1]; 2) a composite structure with the interior marking the exposure of ductily deformed deep crustal rocks analogous to a metamorphic core complex [2]; 3) a zone of northwest directed convergence and subduction [3-6]; or 4) the surface expression of a mantle plume [7-10]. We evaluate these hypotheses based on detailed geologic mapping of the interior of Artemis using NASA Magellan SAR imagery and topography data. Observations: Artemis records a rich history of deformation and volcanic activity (figure 1). Basal terrains A and B (bta, btb), interpreted as deformed basal units, host a remarkably penetrative fabric that displays a consistent northeast trend across the interior. Radial and concentric fractures that postdate the pene-trative fabric trace back to three centers of tectonic and magmatic activity, each hosting distinctive thin low-viscosity lava flows in their centers (cfm) that delicately inter-finger with the adjacent units. Composite flows that surround the centers (efb, efc) are pockmarked by shield volcanos and host numerous mag-matic troughs that indicate subsurface magmatic transport. Flows along the eastern margin of the interior (efc) are both cut by and bury chasma structures, implying generally synchronus emplacement of flows and chasma formation. Artemis Chasma formed as a coherent entity, covering nearly 300 degrees of arc and