Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.t24a..05f&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #T24A-05
Mathematics
Logic
6225 Mars, 7230 Seismicity And Tectonics (1207, 1217, 1240, 1242), 7245 Mid-Ocean Ridges, 8010 Fractures And Faults, 8149 Planetary Tectonics (5475)
Scientific paper
Two rifting-related seismic events in 1975 and 1978 along the Mid-Atlantic Ridge near the northern coast of Iceland produced an array of surface deformation features in Holocene basalt flows and overlying unconsolidated sediments. New field mapping and aerial photograph interpretation is coupled with analysis of maps of seismic activity and level-line survey results to constrain the timing, style, and magnitude of this deformation. Fault scarps and fissures in basalts can be traced laterally down a gentle northward dip projecting into unconsolidated braided stream deposits, providing an impressive view of the deformation style in the two contrasting mechanical layers. We report on detailed field mapping of two of these laterally traceable structures conducted in the summer of 2008 and analysis of a suite of aerial photographs from 1958 to 1998. Map-scale structures in the basalts with little or no sedimentary cover include (i) fault scarps, (ii) fissures, and (iii) locally-developed gentle dip away from the related normal fault. Dilation of faults and extension fractures in the basalt has led to rock toppling and rock fall causing widening of fissures. Wedging of toppled rock blocks at the tops of fissures has locally produced keystone arches and bridges across the tops of open fissures. Different stages in the progression of fissure formation and collapse, including (i) fissure, (ii) widened fissure with cavern, (iii) localized collapse pit, and (iv) elongate collapsed fissure, can be observed over along-strike distances of 10's of meters. Where unconsolidated sand and gravel deposits >3 m thick cover the basalts (200 m to the north along strike) structural geomorphologic features are dominated by (i) grabens, (ii) pit craters, and (iii) elongate troughs. Graben-bounding normal faults cutting the sedimentary cover in many cases have displacements >1 m. Pit craters have cone to bowl shapes, commonly occur within grabens, and have depths up to 2.8 m. The mapped surface structures in the braided stream deposits formed during the 1975 and 1978 rifting events and were likely triggered by reactivation of faults and fissures in the underlying basalt. Fissures, caverns, pits, and troughs in the basalt and braided stream deposits are partially water filled, extending below the local water table. We interpret that pit craters formed in basalt by collapse into caverns formed by fissure-widening rock fall, and in sediments by draining of unconsolidated material downward into fault or fissure-related voids in the underlying basalt. In sediments, troughs appear to be produced by a combination of graben formation and pit crater amalgamation. A sequence of development from pit crater chain (or pit chain within graben) to trough to fissure occurs with increasing dilational fault displacement or fissure width, and as a function of sedimentary cover thickness. This field based study demonstrates that pit craters are readily explained by draining or falling of poorly consolidated material downward into subterranean cavities produced by coseismic fault and extension-fracture dilation in underlying cohesive material (basalt). Directly analogous geomorphologic patterns on Mars, clearly visible in high resolution surface imagery, suggest that similar mechanisms of deformation and surface collapse may be at work on Mars.
Ferrill David A.
Smart Kevin J.
Wyrick Danielle Y.
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