[X] My Assistant

[djellison]

http://www.unmannedspaceflight.com/doug_im...aving_larry.jpg (476kb)

Larry is the outcropping to the right - Methuselah is the layered rock on the left.

As an interesting point to judge distances - assuming zero slip the marks made by the hold-down-brackets on the wheel tread are 80cm apart - or about 2ft 7inches

Doug

[tty]

http://www.unmannedspaceflight.com/doug_im...aving_larry.jpg (476kb)

Larry is the outcropping to the right - Methuselah is the layered rock on the left.

As an interesting point to judge distances - assuming zero slip the marks made by the hold-down-brackets on the wheel tread are 80cm apart - or about 2ft 7inches

Doug

The layers seem to be pretty steeply inclined suggesting there has been active tectonics in the area since they were deposited. I can understand they are interested.

TT

[dvandorn]

The layers seem to be pretty steeply inclined suggesting there has been active tectonics in the area since they were deposited. I can understand they are interested.

Your thinking is a bit Earth-centric, I'm afraid, TT. By far and away the more likely mechanism for upthrusting and tilting of bedrock layers in this area have to do with impact processes, not tectonics.

Since the Columbia Hills rise above the plains of lava that were deposited within Gusev after its formation (and likely inundation by water), they were probably formed as rims of ancient craters. Impacts will tilt flaps of bedrock all around. I'm pretty sure that's the process that tilted these slabs of bedrock.

A detailed analysis of the rock layers won't tell us a whole lot about the upthrust / tilting mechanism, anyway -- that's not what the science teams are so excited about. What's exciting is that Spirit has *finally* found an exposed face of layered sedimentary rock. The layers will tell a story of the conditions under which they were laid down, and *that* will tell us a lot about the history of Mars...

-the other Doug

[dvandorn]

Another thing to consider in re these beds of sedimentary rock within Gusev -- you need to think in terms of stratigraphic sequence.

First, you had the pre-impact rock beds that existed before Gusev was formed. The odds are fair to good that these rock beds were already jumbled and brecciated by earlier impacts -- Gusev lies at the northern border of what's called the ancient cratered terrain that dominates most of Mars' southern hemisphere. So the odds are good that the landforms in which Gusev was originally formed were highly brecciated, jumbled, shocked rocks already. The Gusev impact probably shocked and brecciated them some more.

Then you had a long period when it would seem that the crater left by the Gusev impact was filled with water. At least, trhere are a number of features that hint of this. During this time, water-borne deposition could have laid down layers of sedimentary rock on the floor of the crater.

Finally, there was a volcanic flooding of Gusev that created a new crater floor, made up of relatively unaltered lavas -- basalts, mostly. And from what Spirit has seen, these lavas don't seem to have been inundated by water. Yes, they show a tiny bit of alteration from a small amount of frost or upwelling groundwater. But they don't appear to have ever been covered by liquid water. Water hasn't had a big role in their history.

So, if we see uplifted beds of layered sedimentary rock, that rock was almost certainly formed *after* Gusev was formed and *before* the lavas inundated the crater. And considering how the lavas seem to embay against the hills, I would highly suspect the hills were formed well before the lavas came.

I will say that, on the side of possible tectonics, it *is* true that Gusev lies very close to the boundary between the ancient cratered terrain of the southern hemisphere and the uplifted Tharsis Bulge. But for the most part, the tectonic features that have grown out of the Tharsis development all seem to be much larger -- huge rift valleys, hundreds-of-kilometer-cracks in the surface, that sort of thing. I have a hard time picturing how these tectonic processes could uplift a very small range of hills within an existing impact crater without obviously deforming the entire crater and the surrounding craters.

-the other Doug