Mineralogical Evidence of Sedimentary Volcanism for the Formation of the Thumbprint Terrains in Acidalia Planitia, Mars.

Introduction:  The Thumbprint Terrains (TT) are intriguing geological formations that can be observed in the northern plains (NP) of Mars. TT are commonly described as an alternation of parallel alignments of 10s of meters high high-albedo ridges and dome-like mounds, separated by shallow depressions with contrasting low albedo^[1]. These smooth surface deposits are composed of fine-grained loose materials^[2]. TT mounds have been tentatively proposed as being formed by: explosive volcanism^[3], glacial processes^[1,4], sedimentary volcanism^[2,5,6], or tsunami-driven events formed after giant impacts^[2,6,7]. TT formation may be recent (Late Hesperian-Early Amazonian), period overlapping with the outflow channel activity timing and a possible transient Hesperian Ocean on Mars^[7]. However, our lack of knowledge about the nature of these deposits persists, not least because no mineralogical clue has been found^[8]. This study proposes to provide constraints on the nature of these deposits and their possible subsurface layering, thanks to the recent observation of hydrated silica (HySi) and sulfates (PHS) on the TT of Acidalia Planitia^[9].

HySi is systematically associated with TT’s mounds [Fig. 1A,B,C,D]. HySi is only observed in these mounds and seems to be directly linked to the activity of these. We calculated spectral criteria on HySi CRISM detection to decipher the type of silica and to infer its possible geological origins^[10]. Silica possibly occurs as altered volcanic glasses, or as dehydrated opals. These criteria also suggest that the silica was formed by low-T fluid-rock interactions and/or late-stage water-limited alteration. We propose that HySi in the Acidalia’s TT cones represents weathered, Si-rich fine materials. Possibly in the form of low-density volcanic ashes, HySi was extruded to the surface by sedimentary volcanism involving low quantity of fluids/volatiles (no surface flow observed). Such type of mounds could be indicative of sand volcano-like mounds and/or hydrodynamic blowouts whose sources, constrained by the thickness of the TT (less than 100m). This low-lying origin is confirmed by the results obtained by calculating the estimated extraction depths of such HySi-rich sources by sedimentary volcanism using a buoyancy-driven model in subaerial environment^[11]. HySi-rich materials seem to be sourced from several 10s of m underground (between -10 to -80 m), from subsurface layers that are internal to the TT.

PHS are mainly associated with impact craters with diameters less than or close to ~1 km, and mostly located in their (sometimes lobate) ejecta [Fig.1A;B]. As such small impacts can excavate materials up to 100-150 m depth, PHS presence in these may indicate the presence of sulfate-bearing lithologies in the TT’ subsurface or in the underlying units. This is inferred from observations such as the one presented in Fig.1B where two impacts excavate PHS in two different settings. Further evidence of the presence of sulfates within the TT (or in deeper underlying units) is the observation of PHS in one large 165 m high mound that also display HySi at its base [Fig. 1C]. Applying the same buoyancy-driven model to this PHS mound, it returns that the PHS sources can be several hundred meters deeper (-200 up to -600 m), possibly out of TT’ stratigraphy. However, it is also possible that the sulfates present here have an origin directly within the TT. Indeed, when sedimentary volcanism occurs, it is common to have remobilization and incorporation of the crossed layers by the injected fluids or materials. This hypothesis of co-presence of PHS and HySi in the TT could explain the observation made of such high-mounds (Fig.1C).

Implications for Subsurface Stratigraphy. The detection of hydrated minerals confirms the sedimentary volcanism hypothesis for the origin of TT mounds. HySi-rich mounds indicate that subsurface localized pockets of hydrated materials are present several 10s of m deep within the TT’ stratigraphy. The nature of surface materials suggests that the TT are partly composed of Si-rich volcanic ashes, that, mixed with moderate quantities of over-pressurized fluids, were able to migrate to the surface through (explosive?) sedimentary volcanism. Highest mounds, enriched with PHS, can have sources that lie several hundred meters below the TT. In such a configuration, these sulfates-rich sediments are older than the “mostly Amazonian” Si-rich lithologies making up the TT and may be an inherent part of the stratigraphy composing the underlying “mostly Hesperian” Vastitas Borealis Formation (VBF), possibly in the form of buried evaporites.

 Ongoing Perspectives. This study is currently being extended to numerous sites where sedimentary volcanism has been proposed on the basis of geomorphological observations^[12] and where hydrated minerals have been detected^[9]. Applied across all the NP, our study will enable us to better characterize the aqueous materials in the Martian subsurface. It will also provide constraints on the nature of the sedimentary volcanism-related edifices in various regions, but also on the origins of the VBF, for which numerous studies suggest a potentially “not so ancient” aqueous origin^[13].

Acknowledgments. We acknowledge the support from the Agence Nationale de la Recherche (ANR, France) under the contract ANR-20-CE46-0013 entitled “PaleoSilica”, the Centre National d’Études Spatiales (CNES, France), the Centre National de la Recherche Scientifique (CNRS, France), and the French government under the France 2030 investment plan, as part of the Initiative d'Excellence d'Aix-Marseille Université - A*MIDEX AMX-21-RID-O47.

 References. ^[1]Lockwood et al. (1992) 23^rdLPSC. ^[2]Di Pietro et al. (2021) Icarus. ^[3]Frey & Jarosewich (1982) JGR Solid Earth. ^[4]Souček et al. (2015) EPSL. ^[5]Salvatore & Christensen (2014) JGR Planets. ^[6]Costard et al. (2017) JGR Planets. ^[7]Costard et al. (2019) JGR Planets. ^[8]Oelher & Allen (2010) Icarus. ^[9]Carter et al. (2023) 54^thLPSC. ^[10]Pineau et al. (2020) Icarus. ^[11]Hemmi & Miyamoto (2018) Geosciences. ^[12]Broz et al. (2023) Earth Surf. Dyn. ^[13]Kreslavsky & Head (2002) JGR Planets.

 Figure 1. A. to D. PHS (green) and HySi (cyan) CRISM detections in four sites within the Acidalia’s TT over CTX background, North is up. For each, coordinates of CRISM cubes are provided. Scale bars indicate 2000m.