Impactite dykes in impact crater central uplifts: Insights from Negril crater, Mars

Abstract

Negril crater is a 52 km complex crater located north of Syrtis Major, Mars. The central uplift of Negril crater is remarkably well-exposed and contains one of the best examples of dykes associated with a Martian crater. This study aims to characterize the dykes within the central uplift and determine if they are impactite dykes that formed as a part of the impact process or whether they are pre-existing mineral veins or volcanic dykes that were uplifted. This study also attempts to put the geology of the uplift into the proper stratigraphic context of the regional geology in the vicinity of Jezero crater. The uplift consists of two major geomorphologic units: the uplifted fractured and brecciated massive bedrock crosscut by linear ridges and dykes, and a dark-toned clast-rich to clast-poor unit that coats the central uplift and crater floor, interpreted as impact melt rock. The uplift is quite diverse with respect to its morphology and spectral characteristics, exhibiting bedrock with spectral signatures consistent with a low-calcium pyroxene, Fe/Mg-bearing phyllosilicates and a few occurrences of a plagioclase feldspar, dykes with an olivine-bearing signature, and additional Fe/Mg-bearing phyllosilicates associated with pervasive light-toned ridges and mounds. The ridges crosscut the impact melt rock and are consistent with impact-generated hydrothermal alteration. Two varieties of dykes occurring within the uplift that have a complex cross-cutting relationship, indicating they formed syngenetically with respect to each other. The morphology, orientation and composition of the dykes suggest they may represent impact melt dykes rather than pre-existing dykes that were uplifted. This interpretation is also based on comparisons made with terrestrial observations. Finally, the uplifted fractured bedrock is potentially related to the extensive Fe/Mg phyllosilicate-bearing basement units in the Nili Fossae region that may be related to the formation and evolution of the Isidis Basin.