Mercurian Tectonics

Virginia Pasek

Tectonics definedAlso 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 agoChange in the shape of the lithosphereChange in radiusCaloris related events

Only on Earth

Found on Mercury

The Despinning Model

Tectonics of a despun planetLineaments found at N

50°, N 130° and weaker N - S trends

Similar to MoonN 60°, N 120°

2500 images studiedDifference between

theoretical and observed grids

Mercurian GridPattern of fractures, or weak zones, in the

lithosphereMost ancient of all tectonic evidenceOccurred before any recognizable

geological featuresSimilar to the Moon

Shortfalls of despinning aloneLobate scarps are just as abundant in the

polar regions as the equatorialThe polar regions do not contain normal,

or tension, faultsLobate scarps have pseudorandom

orientationsThe lineament system is post-dated by

some relatively young features

Simultaneous processesSecular cooling and despinning

Fault distribution

Surface featuresAncient 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 MercuryBorealis basin,

located near the north pole is 1560 km

Basin boundaries

Antipodal effectsCovers at least 336,000 km2

Hills, depressions, and valleys that disrupt pre-existing landforms

5-10 km wide and up to 2 km in heightSmooth terrain within some craters

indicate that volcanic activity occurred after Caloris impact

Effects enhanced due to Mercury’s large iron core

Antipodal effects

Additional processesReactivated tectonic trends due to large

impactLocal processes

Kalidasa - Milton areaThe Phildias areaTolstoj - 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.

ReferencesR.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)