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)