Recent (Late Amazonian) enhanced backweathering rates on Mars: Paracratering evidence from gully alcoves

Mars is believed to have been exposed to low planet-wide weathering and denudation since the Noachian. However, the widespread occurrence of alcoves at the rim of pristine impact craterssuggests locally enhanced recent backweathering rates. Here we derive Late Amazonian backweathering rates from the alcoves of 10 young equatorial and midlatitude craters. The enhanced Late Amazonian Martian backweathering rates (10(-4)-10(-1)mmyr(-1)) are approximately 1 order of magnitude higher thanpreviously reported erosion rates and are similar to terrestrial rates inferred from Meteor crater and various Arctic and Alpine rock faces. Alcoves on initially highly fractured and oversteepened crater rims following impact show enhanced backweathering rates that decline over at least 10(1)-10(2)Myr as the crater wall stabilizes. This paracratering backweathering decline with time is analogous to the paraglacial effect observed in rock slopes after deglaciation, but the relaxation timescale of 10(1)-10(2)Myr compared to 10kyr of the Milankovitch-controlled interglacial duration questions whether a paraglacial steady state is reached on Earth. The backweathering rates on the gullied pole-facing alcoves of the studied midlatitude craters are much higher (approximate to 2-60 times) than those on slopes with other azimuths and those in equatorial craters. The enhanced backweathering rates on gullied crater slopes may result from liquid water acting as a catalyst for backweathering. The decrease in backweathering rates over time might explain the similar size of gullies in young (<1Ma) and much older craters, as alcove growth and sediment supply decrease to low-background rates over time.