Geology Reference
In-Depth Information
Fig. 7.1
Pre - folding, syn - folding and post - folding magnetization geometries.
an orogenically driven fl uid event in the Appalachians
during the Late Paleozoic (Oliver 1986; McCabe &
Elmore 1989). The clastic rocks were partially remag-
netized and the carbonate rocks were completely
remagnetized, so paleomagnetists approach carbonate
rocks in mountain belts with a good deal of apprehen-
sion and suspicion (or at least they should).
The types of strain that would be expected for the
rocks typically sampled by paleomagnetists in moun-
tain belts are those that result from relatively low-
pressure and low-temperature conditions. As rocks are
exposed to higher temperatures and pressures that
would cause metamorphism at greenschist grades or
higher, magnetizations are reset either thermally or by
the growth of secondary magnetic minerals; these
high-grade rocks are therefore usually avoided by pale-
omagnetists. The grain-scale deformation mechanisms
of the low-grade rocks, particularly folded rocks, are
spaced cleavage (axial planar) formation, pressure
solution and particulate fl ow (i.e. grain boundary
sliding). Flexural slip/fl ow and tangential-longitudinal
strain geometries are particularly important for the
low-grade folded rocks sampled by paleomagnetists.
The deformation of folded rocks is ultimately the result
of pure and simple shear strain. Pure shear strain
causes a complicated vector displacement fi eld (Ramsay
& Huber 1983) but results in co-axial strain (Fig. 7.2).
The strain ellipse created by pure shear strain does not
change the orientation of its axes as strain increases
(Fig. 7.2). Simple shear strain has a relatively simple
vector displacement fi eld (Ramsay & Huber 1983), but
causes non-coaxial strain. The axes of the strain ellipse
created by simple shear rotate as the amount of strain
increases.
Flexural slip/fl ow is the strain most associated with
folded sedimentary rocks and should be considered by
paleomagnetists whenever they work in deformed
rocks. Flexural slip and fl exural fl ow are the end
members of a continuum of behavior. In fl exural slip
the strain is discontinuous at the grain scale and is
concentrated at slip surfaces in the fold, usually
bedding planes (Fig. 7.3); in fl exural fl ow however the
strain is distributed continuously throughout the rock.
The observation of slickensides on bedding plane sur-
faces in folded rocks is good evidence that fl exural slip
has occurred. If some of the simple shear strain that
caused the fl exural slip was also distributed through
the rock, then fl exural fl ow also occurred. It is diffi cult
to know, without making strain measurements, how
these different types of strain were partitioned in the
rock. Flexural slip/fl ow is caused by bedding-parallel
simple shear so it is non-coaxial strain and is the type
of strain that could cause problems for the interpreta-
tion of the fold test. Facer (1983) points out that con-
centric or parallel folds in which bed thickness stays the
same around the fold are typically affected by fl exural
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