n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

east+entral Alaska, & Coonrad, W. L., and
Elliott, R. L., eds., The United States Geological
Survey In Alaska: Accomplishments during
1981r U.S. Geological Survey Circular 868, p. 60-
62.
Holdsworth, B. K., and Jones, D. L., 1980, A provision-
al Radiolaria biostratigrnphy, Late Devonian
through Late Permian: U.S. Geological Survey
Open-Pile Report 80-876, 32 p.
Mertie, J. B., Jr., 1937, The Yukon-Tanana region,
Alaska: U.S. Geological Survey Bulletin 872, 276
P.
Strueturd obematio~ in the CireIe qmdmgle,
Yukoa-Taneaa Upland, Al&m
By Grant W. Clrshing and Helen L. Foster
particularly conspicuous; these zones are typically
characterized by complexly folded large torlike out-
crops. One such zone has been traced southwestward
in the Circle B-4, 55, and A-6 quadrangles; other,
smaller zones have also been recognieed in the higher
grade rocks to the southeast. The fold styles and
orientations of planer features in the highly folded
zones do not significantly differ from th- of rocks in
the adjacent areas. A possible expbation for these
highly folded zones is that they occur on the noses of
large recumbent folds, where minor folds are abundant
owing to thickening of the fold hinge. The less folded
sections are on the limbs of large structures, where
minor folds are less abundant and the gently dipping
foliation results from the recumbence of largescale
folds. This Interpretation does not preclude the possi-
bility of tectonic movement between the lower Umb
and nose of the recumbent fold.
Folds and associated planar structures observed
in the metamorphic rocks of the Circle quadrangle
south of the Tinttna fault (area 5, fig. 23) during the
course of reconnaissance geologic mapping are being
analyzed to help determine the deformatIana1 history
of the metamorphic terrane of the Yukon-Taaana
Upland. The study has concentrated on minor folds,
ranging in wavelength and amplitude from a few cen-
Axial.plane schistosity
timeters to several meters, which are believed to be
developed
Schistosity folded
related to large-scale regional structures throughout
most of the quadrangle.
D4 / /
me metamorphic terrane consists of interlayered
quartzite, quartzitic schist, pelitic schist and
gneiss, marble, amphibolite, and greenschist. These
rocks range in metamorphic grade from middle greenschist
to upper arnphibolite facies. The protoliths
/
were dominantly sedimentary rocks and probably
Refolded
Open folding
Precambrian and (or) Paleozoic.
Pour distinct deformational events are recoec
nized in-the metamorphic rocks on the besis of fzd Figwe 39.4chematic representation of results of four
styles and the type and orientation of p W and lineur deformationgl events in the Circle quadrangle,
features (fig. 39). The first reaognized deformational Alaska. Solid lines represent compositional Layering,
event (Ill) produced a penetrative schistosjty that is dashed lines sliow schistosity, dash and double dot lines
parallel or subparallel to gently dipping adal planes of indicate cleavage subparallel to axial plane of l&
rarely observed ti ht to isoclinal recumbent folds. folds.
This schistosity (gl k corn monly parallels compositional
layerlng and is everywhere prevalent in the
metamorphic rocks.
The folds associated with the second deforma- Structural features of a third deforrnational
tionttl event (D ) are ubiquitous and vary considerably event (3) are difficult to distinguish from features of
both in style &?I size. FoM styles range fmm tight to the I& event because the fold styles and orientations
isoclinal recumbent and include rounded and chevron- of final phes are similar. The folds of the D generfold
hinges; folds range in amplitude end wavelength ation are recumbent, tight to isoclinal, and, dere obfrom
microscopic to several meters. A second schist- served, have a horizontal or gently dipping axial
osity ($) has developed locally owing to mechanical plane. These folds, like folds, rarely have an
rotation of the preexisting El schistosity. In the axial-planar cleavage. Because the style of both the
pelitic rocks, this 3 Schistoslty is observed only g2 ~d D folds are so similar, the two folds ore disrarely,
as an axial planar fabric, and in quartzite and tinyshde only in a few fortuitously oriented outcarbonate
rocks it is incipient to absent. 'Ihe orienta- crops. Commonly, two sets of recumbent folds are
tion of the S scNstositg is generally subhorizontal to observed, but their relative age cannot everywhere be
horieontal thus esocntidly parallels the gl fabric. determined. In one outcrop in the Circle 5 2 quad-
At some fold hinges, these two schistosities can be dif- rangle, one set of fold axes trend from N. 30' to 50'
ferentiated; the folds mciated with this second E. and plunge from 0' to 5' SW.; the other set of fold
event deform the gl schistosity and compositional axes trend from N. 30' to 60' W., with no significant
layering U ig. 3 9).
Plunge.
Several zones have been recognized throughout In many outcrops, the 9 deformational event is
the quadrangle where folds of this generation (g2) are evidenced by convergent foliation surfaces, but very