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> -----Original Message-----
> From: Tectonics & structural geology discussion list
> [mailto:[log in to unmask]]On Behalf Of Zoe Shipton
> Sent: 02 August 2006 18:33
> To: [log in to unmask]
> Subject: Re: Triangular facets
>
>
> Oops, attached paper too large for JISCmail!
>
> Here's the ref http://www.sciencemag.org/cgi/reprint/275/5298/369.pdf
>
> Hillslope Evolution by Bedrock Landslides
> Alexander L. Densmore, Robert S. Anderson, Brian G. McAdoo,
> Michael A. Ellis
>
> Science 17 January 1997:
> Vol. 275. no. 5298, pp. 369 - 372
> DOI: 10.1126/science.275.5298.369
>
>
>
> -----Original Message-----
> From: Tectonics & structural geology discussion list
> [mailto:[log in to unmask]] On Behalf Of Umberto Fracassi
> Sent: 02 August 2006 17:35
> To: [log in to unmask]
> Subject: Triangular facets
>
> Dear all,
>
> first of all, apologies for cross-postings..
>
> I have a simple (I'm joking..) issue to pose: how do triangular facets
> form/develop? Or, in fact, how much does (active ?) faulting
> interfere with
> their very inception?
>
> Besides the seemingly schoolbook-like question, the issue
> here is subtle.
> Let me make some introduction to the 'where' and 'why'.
> We're studying an intermontane basin sitting on top of the Southern
> Apennines here in Italy. In fact, it seems not to be a basin
> at all, i.e.
> the sedimentary cover is at best very thin and does not match
> at all the
> very complex structural pattern bounding the elongated
> valley. In other
> words, there does not seem to be a master fault controlling
> the valley.
> The main river, in several places, runs on the bedrock and
> along the eastern
> flank of the valley. On that eastern side, a scarplet can be
> found at the
> bottom of the gentle slope, with triangular facets above it.
>
> The scarplet, however, may not be of tectonic origin at all,
> since it seems
> to be almost continuous parallel to the course of the river,
> even when this
> bends ca. 90° (the bend is gentle, probably bevelled, yet the
> curvature
> radius is small). The triangular facets are very gently
> inclined (10-20°,
> compared to the steep ones commonly described - especially
> those associated
> with tectonic control. The same facets seem to be remnants of
> the cumulative
> alluvial wedge, in turn incised by the tributaries of the
> main river. All
> terrains outcropping here are mainly clays, marls and sands,
> so they are
> certainly well erodible, and one may not rule out a case of two
> morphogenetic processes leading to comparable morhological features.
>
> So, here again the question: do triangular facets always need
> a fault to be
> born, assuming the fault will create enough offset and
> uncover a fault plane
> - to be incised? Conversely, can triangular facets be formed
> entirely by a
> drainage system responding to the lowering of the base level
> that controls
> the main river in the valley floor, thus developing V-shaped
> valleys whose
> interface with thee valley shoulder will result in a
> "triangular facet"?
>
> My "sensation" is that the former may well apply to the case
> we're studying,
> particularly since the master fault might either not be there
> at all (we're
> conducting shallow geophysical survey right now) or be old.
> Old enough that
> it no longer plays any role in the landscape evolution.
>
> Any comment, hint (or reprisal! I shall humbly accept them
> too..) will be
> greatly appreciated. Many thanks in advance to all. Have a
> nice summer,
>
>
>
> Umberto Fracassi
>
> --------------------------------------------------------
> Istituto Nazionale di Geofisica e Vulcanologia Via di Vigna
> Murata, 605
> 00143 Roma
> Italy
> Tel: +39-06-51860557
> Fax: +39-06-51860507
> Email: [log in to unmask]
> --------------------------------------------------------
> *********************
> "This is what you should do: Love the earth and the sun and
> the animals,
> despise riches, give alms to everyone that asks, stand up for
> the stupid and
> crazy, devote your income and labor to others, hate tyrants, argue not
> concerning god, have patience and indulgence toward people,
> take off your
> hat to nothing known or unknown, or to anyone or number of people
> ....reexamine all you have been told at school or church, or
> in any book,
> dismiss what insults your soul, and your very flesh shall be
> a great poem."
>
> Walt Whitman
> *********************
>
> -------------------------------------------------
>
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