Geological history of a Floor-Fractured Crater in Gorgonum Chaos, Terra Sirenum, Mars

Introduction

Floor-Fractured Craters (FFCs) are a class of impact craters characterized by fractured and hummocky crater floors, often displaying features such as concentric or radial fractures, mesas, tilted blocks, and irregular topographies. These morphologies suggest post-impact modification processes that have been extensively studied across the Solar System. On Mars, the origin of FFCs is still debated, with several hypotheses proposed, including intrusive magmatism and associated doming, tectonic deformation, glacial or periglacial activity, and the migration or discharge of pressurized groundwater [1–4]. This study presents preliminary results from a detailed geological and geomorphological analysis of a small (~18 km diameter) floor-fractured crater located in the Noachian-aged terrain of Terra Sirenum (37°S, 190°E), within the broader Gorgonium Chaos basin (~240 km wide [5]).  The study aims to investigate the origin of the fractures and the geological evolution of the crater, and to assess the potential roles of tectonic, magmatic, and periglacial processes [6] in shaping its morphology.

Methodology

A detailed geomorphological and stratigraphic analysis was performed by integrating high-resolution datasets, including HiRISE (25 cm/px) [7], CaSSIS (4.5 m/px) [8], CTX (6 m/px), and topographic data from MOLA [9]. Crater counting was performed on the continuous ejecta blanket using CraterTools [10] and Craterstats2 [11], applying the Hartmann and Neukum chronology system [12] and Ivanov's production function [13] to constrain the formation age of the impact. A preliminary evalution of the mineralogical composition of the crater was performed using spectral data from CRISM [14]. As a first step, we used the RGB color composites (browse products) of CRISM [15], which enable a rapid, visual and qualitative multiparametric evaluation of the spectral characteristics of the surface.

Preliminary Results and Interpretation

The interior displays typical FFC features, including a polygonal fracture network, tilted blocks, and mesas of varying elevation and morphology. These morphologies are particularly developed in the southern and central portions of the floor, which appear topographically elevated compared to the northern and eastern parts. This doming-like structure, may indicate the emplacement of a shallow magmatic intrusion beneath the crater floor, causing uplift and fracturing [3]. At the same time, several indicators point toward the involvement of cryospheric processes. The crater ejecta blanket shows a double-layered morphology, a characteristic feature of many Martian craters formed in ice-rich or volatile-rich targets [16-17]. The inner ejecta lobe is more continuous and lobate, while the outer lobe is discontinuous and thinner, consistent with ballistic deposition over an icy or volatile-rich regolith. Within the crater, some mesas appear mantled or smoothed, possibly due to deposition of later ice-related material or sublimation lag deposits. The crater size-frequency distribution (SFD) suggests an early Hesperian to late Noachian formation age (3.4 ± 0.09 Ga, Fig. 1).

Fig. 1 –On the left the image shows the extent of the ejecta in red and the counting crater in yellow. The white arrows highlight the interaction of the Sirenum Fossae fractures with the crater ejecta. On the right, the plot shows the calculated age for the ejecta.

The fracture geometry and spatial distribution within the crater floor suggest at least two deformational phases: an (i) early tectonic control linked to the broader Sirenum Fossae system, and (ii) a later local doming and fracturing episode possibly due to magmatic intrusion. The CRISM-FAL (false color), MAF (mafic mineralogy), PHY (phyllosilicates), PFM (Fe/Mg-phyllo-silicates), PAL (Al-phyllosilicates) and ICE (carbon dioxide frost or ice) browse products were used to investigate the mineralogical composition of the crater and its infilling. These mineralogical maps indicate that the central/southern mesas are characterized by the presence of pyroxene, while the rim and the inner wall of the crater show signs of Fe/Mg Phyllosilicates.

Future Work

The integration of geomorphology, stratigraphy, and composition seems to support a complex interplay of tectonic, magmatic, and later cryospheric processes in the evolution of the crater. Further analysis will include (i) quantitative analysis of spectra derived from CRISM, to constrain the composition of floor and ejecta materials, and to test the possible emplacement of magmatic material and ii) the radar data from SHARAD and MARSIS, in order to assess the presence of present subsurface ice.

Acknowledgement: This work has been developed under the ASI-INAF agreement n. 2024-40-HH.0

References

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