Hi (fold-) nappe freaks
I'm very concerned with nappes of deep origin , small in size, but
repeating constant patterns of "cylindrical" thermal imprints along
strike in belts 1000 km long.
Adherence of numerical models of lithosphere scale deformation -able
to reproduce fold nappe geometric configurations- to fold nappes,
as conceived historically on "geologic" evidence, is being tested by
numerical models of ocean subduction-continental collision. Some of
them display comparisons of particle trajectories tracing P-T
histories (T. Gerya, A. M. Marotta).
Apparently cahotic configurations, in which an artist may repeatedly
see formation of nappes, are constructed by particle trains at various
levels of the upper mantle and lithosphere. What we see at surface may
honestly be a sequence of natural artifacts of a complexity far beyond
"geological" based imagination.
Experimental petrology helps since 35 yrs (not really nothing!) to
cast ways of approach to a more physical treatment of such
petrological and mechanical complexity.
Experience in the European Alps deals since 1955-57 (!! ) with
lithosphere scale location of cyclic coupling-decoupling of mantle and
crustal materials (papers of P. Bearth on eclogitised Mesozoic pillow
lavas, van del Plas on Alpine Adula "nappe" HP-LT rocks, A. Michard on
Jadeite-quartz first Alpine occurrence in Permian sequences of
"Briançonnais Zone" , W.G. Ernst, J. Platt, U.Ring , Dal Piaz and so
on.... (just quoted the pioneers who indicated deeper sites of
potential nappe forming mechanisms, as VHP and UHP environments ).
The concept of tectonic unit might first to be semantically analysed:
are we doing anything natural when we feel obliged to start with a
discrete block and envisage his tectonic trajectory (and internal
strain) as a " tectonic unit" until the end of a material path?
Coupling-decoupling rock sheets and repeated stages of mineral
equilibria (more dis-equil.!) seem to be the dominant repeating
imprints in subduction zones (telescoping of W:G.Ernst , 1971).
Folding of coupled sheets that retain deep signatures happens at any
level of a Subd. Z. as "post-nappe" deformation appears to show
(coupled different-P-peak sheets refolded during upwards trajectory:
retrogressive glaucophane eclogites and lawsonite after bi-mineralic
eclogites),
Thermal gradients, field metamorphic gradients, dominant metamorphic
imprints (England, Thompson, Spear, Peacock) are concepts that may
connect physics of rocks with our poorly interpretable "geological
evidence".
Thanks for eventual help, and to John Dewey who lifted up the lid of a
static melting pot
Guido Gosso
### quotation marks " " open way to your imagination
Guido GOSSO, Univ. di Milano, Dipart. di Scienze della Terra "A.
Desio",Via Mangiagalli 34, I 20133 MILANO. tel ++39 02 5031 5555; fax
++39 +2 5031 5494; e-mail [log in to unmask] * Let's do Science as
we make Love: not for the effects we will obtain, but simply because we
like it! *Science is our best defense against what we prefer to
believe in.
http://users.unimi.it/DRT2007/ http://www.gp.terra.unimi.it/
index.htm http://users.unimi.it/geodid/
Il giorno 03/mar/09, alle 22:41, Rebecca Jamieson ha scritto:
> Hi all -
>
> In response to John's request to keep the discussion coming, I'd like
> to
> draw your attention to recent papers from the Dalhousie Geodynamics
> Group
> (references below). These show what happens in 2D, vertical
> cross-section,
> numerical models involving convergence of continental crust with
> lateral
> variations in lower crustal strength (weak interior to strong
> exterior).
>
> The models exhibit a diachronous 3-stage convergence: 1) Progressive
> shortening
> and thickening of upper and lower crust, producing dominantly upright
> structures; 2) progressive thermal relaxation leading to weakening of
> lower and
> middle crust; 3) activation of lower crustal flow, including formation
> and
> expulsion of fold nappes, in response to underthrusting by a strong
> lower
> crustal indentor. The models produce a ductile infrastructure beneath
> older
> upright structures in the superstructure; phases 1-3 overlap in time
> and
> propagate laterally towards the foreland as the orogen grows.
> Corresponding
> strains in ductile and middle crust are variable but can be very high,
> although
> I don't have numbers to hand. Work in progress shows that
> post-convergent
> ductile flow (phase 4) enhances structures formed during phase 3 and
> leads to
> extension and thinning in the orogenic core and thrusting on the
> flanks.
>
> We have applied these models to the western Grenville Orogen and
> Superior
> Province, and it is likely that they also apply to parts of other
> large hot
> orogens (e.g., Variscan, Canadian Cordillera) at some stage in their
> evolution.
>
> We would be interested in feedback from the list as to whether the
> model
> structures (at some stage) are appropriately referred to as fold
> nappes, and if
> this mechanism is consistent with field observations, particularly of
> large-scale nappe structures. We are also interested in other good
> examples of
> this type of progressive orogenic evolution.
>
> References:
> Culshaw et al. (2006) Geology 34, 733-736.
> Beaumont et al (2006) Geol. Soc. Lond. Special Publication 268
> (channel flow
> volume), 91-145.
> Jamieson et al. (2007) Tectonics 26, TC5005, doi:10.1029/2006TC002036.
> (an
> animation of model GO-3 from this paper is available)
>
> Thanks to Maarten for getting a good discussion going. I'm looking
> forward to
> further comments -
>
> Becky Jamieson
>
> Department of Earth Sciences
> Dalhousie University
> Halifax, NS
> Canada B3H 4J1
> [log in to unmask]
>
>
>
>
>
> Quoting "John F. Dewey" <[log in to unmask]>:
>
>>> Cees,
>>
>> The question that I ask is: an upright fold is generated probably as
>> part of an array by bulk layer-parallel buckling. What, then gives
>> the array a vergence by overturning and shearing them? To answer my
>> own question, perhaps by the over-riding of a nappe moving towards
>> the foreland? If one starts with upright folds with, say, 2km
>> amplitude, and then flattens and shears them to a, say 20 km nappe
>> width, we have extensional strains of 10 on both limbs. Do we see
>> this anywhere? Keep the arguments coming! It keeps a retiree "off the
>> streets" and his brain engaged!
>>
>> John
>>
>>
>>
>>> John,
>>> To come back to your queries concerning the mechanism responsible
>>> for the formation of foldnappes. I mapped in detail beautifully
>>> exposed (nearly continuous exposure in a recent uplift above a
>>> blind, seismically active Andean thrust described by Victor Ramos
>>> with up to 2. km of vertical relief) Palaeozoic foldnappes in the
>>> foothills of the Argentinean Andes, immediately east of the town of
>>> San Juan. Progressive rotation of small scale parasitic upright
>>> folds to recumbent structures both on the upright and inverted limbs
>>> -the process being frozen in locally in relatively low strain
>>> pockets- suggest that these nappes formed by shearing of once
>>> upright folds as suggested by Tim and Yvette. The width of the
>>> inverted limb of the nappes is on the order of 5-10 km. This
>>> doesn't mean that all foldnappes have to form like this, but my
>>> experience in the infrastructure of several mountain belts suggest
>>> that this process is probably common.
>>>
>>> Cees van Staal
>>>
>>>
>>> From: Tectonics & structural geology discussion list
>>> [mailto:[log in to unmask]] On Behalf Of John F. Dewey
>>> Sent: Tuesday, March 03, 2009 10:24
>>> To: [log in to unmask]
>>> Subject: Re: Fold nappe upside down limb width
>>>
>>>> Dear Keith,
>>>>
>>>
>>> How do you know that the small nappe was initiated as an upright
>>> fold. On a general note, I think that Maarten has opened a
>>> fascinating "can of worms" and one that should be pursued. Another
>>> question, how are fold nappes terminated? By relays or by
>>> zero-displacement pinning? How much vertical axis rotation do fold
>>> nappes show. We might put together a list of questions that need to
>>> be addressed and the, communally, address them. I bet that that
>>> something interesting would emerge.
>>>
>>> I have thought, for a long time, that geo- and indeed many other
>>> questions could be addressed by multi-input through websites like
>>> this, including social and political. There is a tyranny of ideas,
>>> publication, grants, tenure, and promotion exercised by the
>>> established order which is organized as a controlling bureaucracy,
>>> which loves review, assessment and arid "paperwork". Geologists are
>>> mostly iconoclasts, study one of the most important disciplines for
>>> mankind, and should confidently promote our subject and oppose the
>>> bureaucrats who are wrecking our subject at all levels.
>>>
>>> John Dewey
>>>
>>>
>>>> Not as large but beautifully exposed in 3D in a succession of deep
>>>> glaciated canyons is the Lamoille Canyon nappe in the Ruby
>>>> Mountains, Nevada. The nappe has a maximum overturned limb width 9
>>>> km perpendicular to strike (length >22 km). Structurally above it
>>>> (with opposite vergence) is an example of a small nappe derived by
>>>> rotation of an initially upright fold: the Soldier Creek nappe,
>>>> the upright root of which is sheared out upward into the
>>>> sheath-shaped nappe where caught up in extensional shear zone
>>>> (overturned limb 4 km wide perpendicular to transport).
>>>>
>>>>
>>>> The inverted limb of the basement-cored, thrust-floored Scanlon
>>>> nappe in the Mojave Desert of California tracks >45 km along
>>>> strike.
>>>>
>>>>
>>>> Keith
>>>>
>>>>
>>>> Howard, K.A., 1980, Metamorphic infrastructure in the northern Ruby
>>>> Mountains, Nevada, in Crittenden, M.D., Jr., Coney, P.J., and
>>>> Davis, G.H. eds., Cordilleran metamorphic core complexes:
>>>> Geological Society of America Memoir 153, p. 335-347.
>>>>
>>>>
>>>> Howard, K.A., 1987, Lamoille Canyon nappe in the Ruby Mountains
>>>> metamorphic core complex, Nevada, in Hill, M.L., ed., Cordilleran
>>>> section of the Geological Society of America: Geological Society
>>>> of America Centennial Field Guide v. 1, p. 95-100.
>>>>
>>>>
>>>> MacCready, Tyler, Snoke, A.W., Wright, J.E., and Howard, K.A.,
>>>> 1997, Mid-crustal flow during Tertiary extension in the Ruby
>>>> Mountains core complex, Nevada: Geological Society of America
>>>> Bulletin, v. 109, p. 1576-1594.
>>>>
>>>>
>>>> Howard K.A., John, B.E., and Miller, C.F., 1987, Metamorphic core
>>>> complexes, Mesozoic ductile thrusts, and Cenozoic detachments: Old
>>>> Woman Mountains - Chemehuevi Mountains transect, California and
>>>> Arizona, in Davis, G.H. and Vandendolder, E.M., eds., Geologic
>>>> diversity of Arizona and its margins: Excursions to choice areas:
>>>> Arizona Bureau of Geology and Mineral Technology Special paper 5,
>>>> p. 365-382.
>>>>
>>>>
>>>> Howard, K.A., 2002, Geologic map of the Sheep Hole Mountains 30' x
>>>> 60' quadrangle, San Bernardino and Riverside Counties, California:
>>>> U.S. Geological Survey map MF-2344, 2 sheets,
>>>> http://pubs.usgs.gov/mf/2002/2344/, (1:100,000).
>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Keith A. Howard
>>>> Scientist Emeritus
>>>> U.S. Geological Survey, MS 973
>>>> Menlo Park, CA 94025
>>>> U.S.A.
>>>> phone 1-650-329-4943
>>>> fax 1-650-329-5133
>>>>
>>>
>>>
>>> --
>>>
>>> -----------------------------------
>>> John F. Dewey FRS, M.R.I.A., Distinguished Emeritus Professor
>>> University of California.
>>>
>>> Sherwood Lodge,
>>> 93 Bagley Wood Road,
>>> Kennington,
>>> Oxford OX1 5NA,
>>> England, UK
>>>
>>> University College,
>>> High Street,
>>> Oxford OX1 4BH
>>>
>>> Telephone Nos:
>>> 011 44 (0)1865 735525 (home Oxford)
>>> 011 44 (0)1865 276792 (University College Oxford)
>>
>>
>> --
>>
>> -----------------------------------
>> John F. Dewey FRS, UC Distinguished Emeritus Professor of Geology
>> Department of Geology
>> UC Davis
>> One Shields Avenue
>> Davis CA 95616
>>
>> Telephone Nos:
>> 530 752 5829 (UC Davis)
>> 011 44 (0)1865 735525 (home)
>> 011 44 (0)1865 276792 (University College)
>> 530 752 0915 (Fax: )
>
>
|
