Coseismic, dilational-fault and extension-fracture related pit chain formation in Iceland: Analog for pit chains on Mars

Abstract

Pit crater chains are common topographic features on Mars and several other planetary bodies, and a wide range of mechanisms has been proposed for their origin. Two rifting-related seismic events in 1975-1976 and 1978 along the Mid-Atlantic Ridge near the northern coast of Iceland, associated with the Krafl a volcanic eruptions to the south, produced an array of pit chains in unconsolidated sediments overlying Holocene basalt fl ows. Fault scarps and extension fractures in basaltic lava fl ows are traceable laterally into overlying unconsolidated fl uvial deposits, revealing contrasting deformation styles in the two mechanical layers. Map-scale structures in basalt with little or no sedimentary cover include (1) fault scarps, (2) extension fractures and fracture swarms, (3) faulted monoclines, (4) widened fractures with caverns, and (5) localized circular or elongate collapse pits. Where unconsolidated fl uvial sand and gravel deposits >3 m thick cover the basaltic lava fl ows, structural geomorphic features are dominated by (1) grabens bounded by normal faults with ~1 m displacement, (2) cone-to bowlshaped pit craters with depths up to 2.8 m, and (3) elongate troughs. Formation of these structures in fl uvial sediment was triggered by reactivation of faults and extension fractures in the underlying basalt. Pit craters are readily explained by downward "draining" of poorly consolidated material into subterranean cavities produced by fault and extension fracture dilation in underlying cohesive material (basalt). High-resolution imagery on Mars shows geomorphic patterns that are directly analogous to these Icelandic pit chains, suggesting similar processes have occurred on Mars.

Changes in structural style of normal faults due to failure mode transition: First results from excavated scale models

“…Depending on the amount of dilatancy this process forms either pit craters or large along-strike/elongated collapse grabens. These pit craters were described by Ferrill et al (2004Ferrill et al ( , 2011b on Mars and in Iceland.…”

“…Van Gent et al (2010) observed dilatant faults in competent carbonates of the United Arab Emirates. Several authors have studied basalts in Hawaii and Iceland which show massively dilatant extensional faults (Gudmundsson and B€ ackstr€ om, 1991;Angelier et al, 1997;Acocella et al, 2000;Holland et al, 2006;Ferrill et al, 2011b). Soden and Shipton (2013) investigated dilational faults on Gran Canaria and found mechanical property variations to be a controlling factor for the development of extension fractures.…”

Chaotic breccia zones on the Pembroke Peninsula, south Wales: Evidence for collapse into voids along dilational faults

“…Woodcock et al, 2006;Ferrill et al, 2011;Walker et al, 2011). This collapse mechanism grades into other processes: confined implosion at pressure-release zones along faults (e.g.…”

Geomorphology and structural geology of Saturnalia Fossae and adjacent structures in the northern hemisphere of Vesta

“…12f and h). Pit crater chains on Earth and Mars are proposed to form by unconsolidated material draining down into extension fractures and/or dilational normal faults in a more consolidated material at depth (Ferrill et al, 2011;Wyrick et al, 2004). In this scenario the crater chains are pit crater chains and are the surface expression of funnels of down-draining material.…”