Planetary Tectonics

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Planetary Tectonics

• PtyS 554, Spring 2009

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E. M. Andersons 1951 theory relates stress and
fault type, assuming that faults are shear
fractures
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The most ancient structures on the moon are
outlined by lineaments--usually polygonal
crater walls. Called the lunar grid it
reflects fractures deep within the lunar crust
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But are these lineaments real? Apollo 15 taught
us a lesson about low sun angles
AS15 Mount Hadley 4.5 km away
10 cm high cement pile Obliquely illuminated
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Small-scale experiments give warning!
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These fractures may have formed as the moon
slowly retreated from the Earth and its tidal
bulge relaxed--a closely related tectonic pattern
is believed to be present on Europa, due to tides
induced by its eccentric orbit around Jupiter
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Mercury also seems to have an ancient grid of
tectonic features
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In Mercurys case, these lineaments may have been
formed when the planet spun down early in solar
system history. If so, they are strike slip
faults with very small offsets.
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Extensional Processes--Grabens in Canyonlands
National Park
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Rocks were stretched apart as they slid towards
the Colorado River
ERTS-A image, 70 km wide
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Grabens are also common on Mars
Parallel--regional extension
Intersecting--no preferred direction
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The moon is traversed by many grabens--many
radial to large basins, and some concentric
Serenitatis basin S. rim
Rima Cauchy
The Straight Wall
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Graben-topography intersections allow estimates
of fault dip
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Updoming leads to radial graben sets
Salt Dome, S. Russia
Humboldt crater
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Grabens on Mercury formed inside the 1300 km
diameter Caloris basin impact
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Surprisingly, these look very much like the
polygonal terrain on the Northern Plains of Mars
Mercury
Mars
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Extensional grabens appear nearly everywhere in
the solar system--even on Uranus tiny moon
Miranda
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Compressional features seem less
common--Mercurys famous scarps are the best
known.Note the compressed craters!
Vostok Scarp, Mercury
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Because of lithospheric burial and remobilization
at depth, Ios lithosphere is in a permanent
state of compression. The result is tilted,
slab-like mountains that may be uplifted along
thrust faults
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The ubiquitous mare ridges may be compressional
in origin
Meckering, Australia fault scarp
Mare Serenitatis
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Up close, Mare Ridges are very complex
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but bear an uncanny resemblance to terrestrial
models of Transpression (compression and shear)
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Known as flower structure this may hint at some
degree of strike-slip as well as compression
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Imposed surface loads--mascons--have a
characteristic tectonic style on the Moon
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A central zone of compression is surrounded by an
annulus of grabens. Humorum and Serenitatis are
the best examples.
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Mars tectonics is dominated by the Tharsis upbulge
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Strong radial and a few concentric features are
common
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An interesting possibility is that the earliest
phases of Martian tectonics may have been
influenced by a changes in the orientation of
Mars lithosphere
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This would produce a characteristic pattern of
faults--which is not seen, at least at the
present day!
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Europa is the only body other than the Earth, so
far, to show clear evidence for strike slip
motion (although small strike slip offsets can be
seen in places on Venus)