[meteorite-list] Chelyabinsk - slickensides or shock planes?

[Bernd V. Pauli]

Hello All,

We had a similar discussion many, many years ago
(September 2001). Here's a short overview of our results:

Summary:

- broken surface is covered with glossy striations
- slickensides are identified by shiny mirror like surfaces
  on an otherwise rough rock

- they are the product of faulting in a rock body (as the crust
  shifts, even slightly, the roughness of the rock tends to smooth)

- slickensides are formed from the movement of rocks relative to each other
  along fracture planes in fault zones

- rub your finger along the grooves which make up the slickensides:

* they feel rough when you move your finger in the direction opposite
  to which the adjacent rock moved to form the slickensides

+ they feel smooth when you rub in the same direction the adjacent rock
  moved because it sheared off any microscopic projections or rough
  edges as it moved

 => Not found in shatter cones!

- slickensides are formed when opposite sides of rock faults
  move in different directions

- extreme pressure generates frictional heat as the rock faces are forced
  past each other partially melting a thin veneer of rock at the interface
  (result: smoothing of rough edges and a polished looking surface)

- they are not formed by explosive breakup in the earth's atmosphere
  (in such a breakup pieces would be flying apart from each other
  whereas in slickensides the opposite is happening: the rock faces
  are being forced against each other) but: see below **

- possible formation scenario: an impact event in space results in
  movement of two adjacent parts of  the stony meteorite relative to
  each other along a preexisting fracture plane thus creating grooves

- slickensides are polished, grooved surfaces that occur along shear planes

- slickensides form along internal shear planes as opposite parts
  move past one another

- opposite parts rub against each other, their surfaces become smoothed,
  lineated, and grooved

- slickensides are formed when two planar sides grind past each other

- slickensides can be created at the moment of breakup (not by the explosive
  part of this breakup but rather when two parts of the meteorite grind past
  each other along a pre-	existing fracture – so-called shear rupturing) **

and here are some of the listees that participated in the discussion:

Charlie Devine (started the discussion), Eric Olson, Robert Verish,
... to name a few.

Cheers,

Bernd