Volatile-rich Upper Crust of Mars Constructed during the Impact Cataclysm

1. Introduction/Overview: Escarpments along Noachian cratered plateaus that form the margins of the outflow channels, chasmata and chaotic terrains [1] commonly display layered sequences appearing to contain populations of buried impact craters [2,3]. We refer to these materials as the Noachian Basal Layered Unit (NBLU), which resides stratigraphically at the base of the Hesperian and above the Noachian or pre-Noachian basement. The NBLU, in places several kilometers thick, is interpreted to consist of intercalated sedimentary rocks, impact breccia, volcanic rocks, and in some regions, volatile condensation deposits. The NBLU was produced during an early period in Martian history characterized by elevated erosional/depositional rates and frequent impacts [3-5]. Destabilized zones of the NBLU are thought to have led to subsidence, collapse and outflow activity in eastern circum Chryse and fretted canyon development in northern Arabia Terra. Elsewhere, stable remnants may exist still today [6]. This model explains the extreme volatile enrichment of the NBLU [7]. The uniqueness of this stratigraphic unit and the prevailing conditions during its formation are best explained in the context of cataclysmic bombardment [8]. Variations in regional geothermal gradient may have led to complex upper crustal chemical zonation and heterogeneous regional resurfacing by mechanical and chemical weathering, clastic and chemical deposition, and volatile sublimation processes. 2. Evidence for volatile enrichment within the NBLU Rodriguez et al [6] propose that the Noachian highland plateau situated north of the Aureum Chaos, east of the Hydraotes chaos, and south of the Hydaspis chaos forms part of the NBLU. This region shows evidence for extensive crustal subsidence and collapse that has been attributed to the partial loss of massive deposits of subsurface volatiles and entrained debris [6]. Enhanced and preferential surface subsidence and collapse over and nearby shallow buried impact craters indicates that buried impact craters represent volatile enriched zones within the cryosphere and that the loss of subsurface volatiles utilized their tectonic fabrics [3]. The northern margin of Arabia Terra also seems to be volatile enriched. Fretted canyons, lineated valley fill, and lobate debris aprons are interpreted as due to ductile failure and creep of the crust itself in response to crustal heating [7]; this creep is similar to but distinct from Earth-style climate-driven glacial flow. 3. NBLU volatile integration and time of formation Key issues that we seek to explain include: (1) How did the NBLU become highly enriched in volatiles? (2) When did the NBLU form? We present …