Re: Fold nappe upside down limb
width
John,
They are good questions.
Most of the west-verging nappe is within a regional down-to-west
mylonitic shear zone (roof of the Ruby Mts. core complex), and the
recumbent-syncline hinge lines beneath the Soldier Creek nappe
describe a horseshoe shape indicative of top-to-W sheath folding
directly along the mylonitic shear direction. Only the upright
root zones of this nappe and adjacent syncline are below
the mylonitic shear zone. The original age of the upright folds
could be Cretaceous or possibly Tertiary. While it is conceivable that
the upright fold curved upward into an asymmetric or even recumbent
shape before the Tertiary mylonitic shearing was superimposed, it
seems compelling that the greatly attenuated (20:1?) inverted limb of
the nappe is sheared out (and partly detached) in the
extensional mylonitic shear zone to which its attenuated shape links
kinematically . The vergence of the upper nappe , by the
way, is away from the foreland; it relates to post-contraction
extension.
The lower (Lamoille Canyon) nappe has vergence instead
toward the foreland, and stratigraphic units are attenuated to
as little as 5 % original thickness on the upper limb and in part both
limbs where overprinted by the extensional shear zone. Two other
nappes in the core complex have vergence north or south, so you can
find any direction of nappe vergence you want here. To complicate
matters further, MacCready et al. presented evidence of some axial
shear flow in the Lamoille Canyon nappe.
As to your question about how nappes end, one end of this lower
nappe is pinned to near-zero displacement, where mapped and
interpreted curvilinear hinge lines bounding the inverted limb
converge in a zone of small thrusts.
Keith
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)
--
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
