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

Only on Earth Found on Mercury

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

Surface features  Ancient tectonic grid, called “Mercurian Grid”  Lobate and arcuate scarps

Such an impact!  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  Caloris is not the largest impact on Mercury  Borealis basin, located near the north pole is 1560 km

Basin boundaries

Antipodal effects  Covers at least 336,000 km 2  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) Thomas  Two troughs which can not be explained as coalescent secondary impacts

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 (1997)  J.W. Head et al., Space Sci. Rev. 131, pp (2007)