Stratigraphy of Noachian-aged Crust in the Nili Fossae-syrtis-isidis Region

Introduction: Defining the nature of the early crust on Mars remains a challenging problem due to the breadth of time and processes that have acted upon it. Concepts of planetary evolution and geophysical modeling suggest that primary crust may have originally formed during cooling of a magma ocean or by serial magmatism [1]. Most consider this early crust to have been broadly basaltic in composition. Subsequent processes have significantly modified the original crust, through impact processes including basin formation, additional magmatism, and aqueous processes. The finding that much of the Noachian terrain on Mars exhibits evidence of phyllosilicate minerals and thus alteration [2, 3, 4] raises significant questions as to when, where, and how the alteration occurred. A significant section of wellexposed Noachian crust exists surrounding the Isidis Basin. Here we present observations that provide some constraints on the nature of the earliest crust. Regional Geology: The Isidis Basin is a 1900 km [5] diameter impact basin dated to the Late Noachian (3.96 Ga [6]). It has been significantly modified, including loss of the northeast rim through gradational processes [7], formation of radial and concentric graben due to loading and flexure [8], and emplacement of the plains volcanics that make up Syrtis Major Planum on its western rim [9]. The loading and flexure occurred, in part, in response to filling of the basin but prior to the emplacement of Syrtis Major. Syrtis Major lavas cover 10 km from the central caldera, reach the western floor of the Isidis Basin, and also cover the floor of the Nili Fossae trough. Early phases of the volcanism may have filled some of the Isidis Basin floor [10]. The opening of the fractures constituting the Nili Fossae as well as significant gradation due to fluvial processes [11, 12] provide some excellent exposures of crust along scarp walls and in erosional windows. In these exposures outcrops are observed showing spectral signatures diagnostic of mafic and phyllosilicate minerals. Mafic Composition: In exposed scarps and walls as well as materials excavated by impacts are outcrops showing mineral signatures of pyroxene and olivine. High-resolution images from the HiRISE camera show that many of these outcrops are large blocks embedded in a matrix of material that is either spectrally unremarkable or shows spectral signatures of phyllosilicate (discussed later). In HiRISE color data (IR-R, R-G, GB-B) these blocks appear green and show very sharp color contacts with the surrounding matrix. This is well illustrated in CRISM image FRT000064D9 and HiRISE image PSP_003587_2015 (not shown). CRISM spectra show the blocks to be a mixture of low(LCP) and high(HCP) Ca pyroxene with LCP dominant. Another exposure along the Nili Fossae scarp shows the LCPrich rocks to be in meters-thick tabular units embedded in a phyllosilicate matrix (FRT00009D44,