 |
 |
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: )