Hi Umberto
This paper may not contain the answer to your question, but is a wonderful
example of the use of a food as an analogue for geology!
Zoe
-----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
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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]
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"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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