Mercurian Tectonics Virginia Pasek Tectonics defined Also known as crustal deformation tectonics is the result of stresses in the outer layers of a planet that are produced by thermal and or mechanical processes Occurs on terrestrial planets and the Moon It happened long long ago Change in the shape of the lithosphere Change in radius Caloris related events Found on Mercury Only on Earth The Despinning Model Tectonics of a despun planet Lineaments found at N 50 N 130 and weaker N S trends Similar to Moon N 60 N 120 2500 images studied Difference between theoretical and observed grids Mercurian Grid Pattern of fractures or weak zones in the lithosphere Most ancient of all tectonic evidence Occurred before any recognizable geological features Similar to the Moon Shortfalls of despinning alone Lobate scarps are just as abundant in the polar regions as the equatorial The polar regions do not contain normal or tension faults Lobate scarps have pseudorandom orientations The lineament system is post dated by some relatively young features Simultaneous processes Secular cooling and despinning Fault distribution Ancient tectonic grid called Mercurian Grid Lobate and arcuate scarps Surface features Such an impact Caloris is not the largest impact on Mercury Borealis basin located near the north pole is 1560 km Review Mercury s diameter is 4878 km Caloris basin is 1550 kilometers in diameter 32 of the size of Mercury Approximately 336 000 km2 on antipodal side affected by impact Basin boundaries Antipodal effects Covers at least 336 000 km2 Hills depressions and valleys that disrupt pre existing landforms 5 10 km wide and up to 2 km in height Smooth terrain within some craters indicate that volcanic activity occurred after Caloris impact Effects enhanced due to Mercury s large iron core Antipodal effects Additional processes Reactivated tectonic trends due to large impact Local processes Kalidasa Milton area The Phildias area Tolstoj Zeami area Kalidasa Milton Area U1 U2 Two troughs which can not be explained as coalescent secondary impacts Thomas Phidias Area K Absence of secondaries central peak and wall terraces Thomas proposes that Phidias depression is due to a tectonic subsidence of a nearly circular area References R G Strom A L Sprague Exploring Mercury The Iron Planet Springer New York 2003 P G Thomas Planet Space Sci 45 pp 3 13 1997 J W Head et al Space Sci Rev 131 pp 41 84 2007