Breccia dikes and crater‐related faults in impact craters on Mars: Erosion and exposure on the floor of a crater 75 km in diameter at the dichotomy boundary

Abstract

Abstract-Environmental conditions on Mars are conducive to the modification and erosion of impact craters, potentially revealing the nature of their substructure. On Earth, postimpact erosion of complex craters in a wide range of target rocks has revealed the nature and distribution of craterrelated fault structures and a complex array of breccia and pseudotachylyte dikes, which range up to tens of meters in width and tens of kilometers in length. We review the characteristics of fault structures, breccia dikes, and pseudotachylyte dikes on Earth, showing that they occur in complex network-like patterns and are often offset along late-stage crater-related faults. Individual faults and dikes can undulate in width and can branch and bifurcate along strike. Detailed geological analyses of terrestrial craters show that faults and breccia dikes form during each of the major stages of the impact-cratering process (compression, excavation, and modification). We report here on the discovery of prominent, lattice-like ridge networks occurring on the floor of a highly modified impact crater 75 km in diameter near the dichotomy boundary of the northern lowland and southern upland. Interior fill and crater-floor units have been exhumed by fluvial and eolian processes to reveal a unit below the crater floor containing a distinctive set of linear ridges of broadly similar width and forming a lattice-like pattern. Ridge exposures range from ~1-4 km in length and ~65-120 m in width, are broadly parallel, straight to slightly curving, and are cross-cut by near-orthogonal ridges, forming a box or lattice-like pattern. Ridges are exposed on the exhumed crater floor, extending from the base of the wall toward the center. On the basis of the strong similarities of these features to terrestrial crater-related fault structures and breccia dikes, we interpret these ridges to be faults and breccia dikes formed below the floor of the crater during the excavation and modification stages of the impact event, and subsequently exhumed by erosion. The recognition of such features on Mars will help in documenting the nature of impact-cratering processes and aid in assessment of crustal structure. Faults and breccia dikes can also be used as data for the assessment of post-cratering depths and degrees of landform exhumation.

Mineralogy of the Nili Fossae region with OMEGA/Mars Express data: 1. Ancient impact melt in the Isidis Basin and implications for the transition from the Noachian to Hesperian

“…Some ridges terminate against the cap unit and no expression of these ridges can be observed in the overlying unit. Head and Mustard [2006] describe a similar type of ridge morphology beneath a competent cap unit on the floors of craters in Nilosyrtis (28.25°N 73.25°E) that they interpret to be breccia dikes formed during the impact process. The phyllosilicate‐rich surfaces are also observed along the walls of the fossae and in remnant knobs within the fossae (Figure 4).…”

“…With time, the volcanic flows from Syrtis Major evolve to dominate the surface materials south and east of the Nili Fossae region, and flow down off the plateau to the edge of the floor of the Isidis basin to the south [ Hiesinger and Head , 2004] interfingering with the Vastitas Borealis Formation that has filled the Isidis Basin [e.g., Ivanov and Head , 2003]. Degradation and erosion can expose breccia dikes and substrate [e.g., Head and Mustard , 2006] underlying the impact melt layers.…”

Layered mantling deposits in northeast Arabia Terra, Mars: Noachian‐Hesperian sedimentation, erosion, and terrain inversion

“…Moreover, unlike typical fluvial systems, the orientation of curvilinear ridges is not controlled by local or regional topography. We interpret these ridges as exposed subsurface dikes, formed by volcanic or impact processes [e.g., Head and Mustard , 2006; Head et al , 2006], and exhumed when the superposed material (the mantling unit) was removed. On the basis of the evidence that these curvilinear ridges are exhumed, exposed only where the mantling material has been removed, and locally remain partially buried, exposure of these ridges required removal of tens to hundreds of meters of overlying mantle material.…”

Assessing the mineralogy of the watershed and fan deposits of the Jezero crater paleolake system, Mars

“…Saper and Mustard [] mapped >4000 m scale ridges in the Nili Fossae region, expanding on the work of Head and Mustard [], who identified ridges with a similar morphology exposed in the floors of eroded and exhumed impact craters. These ridges are widespread in the Nili Fossae region and are associated with aqueous alteration mineral‐bearing units [ Saper and Mustard , ].…”

“…These ridges are widespread in the Nili Fossae region and are associated with aqueous alteration mineral‐bearing units [ Saper and Mustard , ]. Head and Mustard [] hypothesized that these ridges are crater‐related faults and breccia dikes associated with the impact cratering process, while Saper and Mustard [] hypothesized that the ridges are impact‐related fracture zones, possibly from the nearby Isidis basin, that have been subsequently mineralized. Widespread mineralized fracture zones in Nili Fossae are consistent with the hypothesis of pervasive alteration of the Noachian crust in this region of Mars [ Saper and Mustard, ].…”