Implications for ELD Depostion and Water Content Based on the Existence of Collinear Fold Structures

Introduction:  The occurrence of equatorial layered deposits (ELDs) across Mars are known for their association with water-altered minerals [1]. Various depositional methods have been cautiously proposed [1, 2, 3, 4],  but the precise role of water (i.e., duration and intensity) in their deposition remains a compelling and elusive geologic question. It is largely considered that the depositional period took place between the Late Noachian and Middle Hesperian [5]. However, deposition duration is difficult to constrain due to the ELDs spanning several regions across Mars and  forming vastly different sized deposits [2]. Thus, there is the possibility of numerous depositions across different regions at varying intervals [3]. Arabia Terra (AT), a heavily cratered dichotomy boundary between the southern highlands and northern lowlands, is home to a plethora of ELDs of various sizes, distributions, deformations, and erosional features [1, 2, 3, 4]. Two of these craters, Sera and Jiji, located in the southern area of AT   (Fig. 1a), host a spectacular set of ELDs [2, 3] which are unique in their layering and spatial placement within the craters.

This study examined fold structures within the ELDs of Sera and Jiji to gain a better understanding of ELD composition (specifically grain size), porosity, depositional processes, rate of deposition, and the role of water during deposition and evolution. Our efforts also showcase the potential of classical structural analysis techniques in the field of planetary science.

ELD Deposit:  The ELDs within these two craters have unique forms, with the bulk of each deposit residing in the southeast area of the craters. Deposit thickness is approx. 666 m with an average layer thickness of 10.85 m (Sera) and 9.63 m (Jiji) [2]. Garvin equations [6] have demonstrated the possibility of a 579 m thick section that predates the ELDs on top of the crater floor, residing below the observable ELDs [2].

Methodology:  A CTX mosaic registered to a HRSC composite DEM [7] forms the base dataset of our study while also utilizing four HiRISE stereo pairs for detailed measurements. A total of 223 layer attitude measurements were obtained using Orion software (Pangaea Scientific) and plotted onto Schmidt nets (Fig. 1e). Strike and dip measurements were overlain on the CTX mosaic (Figs. 1c, d).

Results: A total of eight folds were observed: three synclines, four anticlines, and one monocline (Fig. 1). Their fold axes all share a preferred northwest orientation ranging 297 - 337° (avg. 315.7°) (Fig. 1e). Hinges plunge 2 - 6° (avg. 3.3°) to the northwest. Folds are observed only within the elevation range between -2,500 and -2,300 m and each contain an average of 15 resolvable layers. Fold limb lengths range approx. 120 - 500 m and generally have a 30 – 60 m difference in elevation from the fold hinges. The entire wavelength of the folds Layer attitudes within the fold limbs do exceed 30° in one instance (Fig. 1d), but are generally shallow, ranging 7 - 25°. Fold hinges tend to be preferentially eroded (e.g., Fig. 1d).

Discussion:  Due to the collinear nature of the fold axes and the lack of offset layering, there is no evidence of a tectonic origin of these folds. The plunge of the folds might describe the attitude of bedding before folding, but clearly describes a depositional source from the southeast. Perhaps a syndepositional or immediate post-depositional (i.e. in the case of volcanic ash: immediately after deposition, but prior to heat dissipation) compaction over a pre-existing topography can explain these structures [8]. Buried faults are an attractive explanation, however they would be deep faults, as no faulting is observed in the crater wall or surrounding plateau. Furthermore, in the case of a group of deep collinear faults, they would have likely existed prior to the impact event, as they do not radiate outwards from the crater like impact induced faults [9].  However, the impact could have reactivated them which resulted in a small amount of vertical displacement for fold formation. The amount of water present during deposition, accompanied with the porosity of the material, has many implications for the past climate of Mars, particularly atmospheric pressure. Water within volcanic ash could still be present by the time it reached the craters

Preliminary and future work involve the finalization of a computer model which demonstrates the various possibilities the depositional regime can have to create these folds (i.e., porosity, rate of deposition, compaction factor, and composition), with an emphasis on the duality of aeolian loess vs volcanic ash, and wet vs dry.

References: [1] Andrews-Hanna J. C. et al. (2010) JGR, 115(E6). [2] Schmidt G. et al. (2021) JGR, 126(11). [3] Annex A. M. and Lewis K. W. (2020) JGR, 125(6). [4] Pondrelli et al. (2015) Bulletin, 127(7-8). [5] Carr M. H. and Head J. W. (2010) Earth and Planet. Sci. Letters, 294(3-4), 185-20. [6] Garvin J. B. et al. (2003) VI Internat. Conf. on Mars, 3277. [7] Heather D. et al. (2013a) Eu. Planet. Sci. Conf., 8. [8] Schmidt G. W. et al. (2018) JGR, 123, 2893-2919. [9] Kenkmann T. and Von Dalwigk, I. (2000) Met. and Planet. Sci., 35(6), 1189-1201.

Figure 1. Structural analysis of the ELDs within Sera and Jiji. a) Location of study area in the regional context of AT over a MOLA colorized DEM. White box marks location of figure b. b) CTX mosaic of Sera and Jiji. White box marks location of figures c and d. c) HiRISE stereo pair ESP_017013_1890/ESP_016657_1890 within Jiji. Fold axis locations (blue) and strike and dip measurements (black) marked by their respective symbologies. d) HiRISE stereo pair ESP_057456_1890/ESP_057245_1890 within Sera boasting the steepest dips >35°. e) Schmidt net displaying the combined attitudes from the HiRISE available in both craters (including stereo pairs PSP_002047_1890/PSP_001902_1890 and ESP_037267_1890/ESP_037834_1890).