QUANTITATIVE EVALUATION OF THE PROPOSED ANALOGY BETWEEN LARGE-SCALE MARTIAN POLYGONAL TERRAIN AND TERRESTRIAL POLYGONAL FAULT SYSTEMS

Planetary landforms commonly are interpreted in terms of terrestrial analogs that should be evaluated using quantitative and objective criteria. In this study, the normal fault arrays that define Large-Scale Martian Polygonal Terrain (LMPT) were compared to terrestrial Polygonal Fault Systems (PFS) (see Moscardelli et al., 2012, GSA Today). For LMPT, a fault-trace map was drawn from a mosaic of Context Camera (CTX) images of LMPT in southeastern Acidalia Planitia, Mars. This map was compared to a fault-trace map of a PFS in the North Sea published by Goulty (2008). Both the characteristics of the fault arrays (fault metrics) and the shapes and sizes of fault-bounded blocks (patch metrics) were compared. The fault metrics included spacings between and relative dip directions of adjacent faults, both of which were measured from the midpoint of each fault segment. In the LMPT, fault spacings are bimodal with small interfault distances (mean = 82m) in the fault down-dip direction compared to interfault distances in the up-dip direction (mean = 3320 m). In the fault down-dip direction, the nearest fault dips toward ~72% of faults. These characteristics indicate that faults in LMPT tend to bound narrow symmetric graben between broader horsts. In contrast, PFS fault spacings are unimodal and nearly normally distributed, and fault spacing differs little with dip direction (means are 27 m down-dip, 29 m up-dip). The nearest fault in the down-dip direction dips away from ~55% of the faults. PFS faults thus tend to form arrays of domino-style tilted blocks. Patch metrics for the fault-bounded blocks were measured using the program Fragstats. LMPT and PFS have a mean Related Circumscribing Circle values of 0.72 and 0.60, respectively. This indicates that the LMPT contains a high proportion of linear, irregularly shaped blocks while PFS blocks are more equidimensional. Significant length scale differences between LMPT and PFS make the interpretation of patch area and perimeter distributions unclear. Fault and patch metrics both indicate that LMPT is dominated by narrow graben that bound broad polygonal horsts, whereas the North Sea PFS seems to be dominated by arrays of nearly equidimensional tilt blocks. These observations suggest that PFS are not an appropriate terrestrial analog for LMPT.