55 research outputs found
Short-wavelength undulatory extinction in quartz recording coseismic deformation in the middle crust – an experimental study
Deformation experiments are carried out on natural vein quartz in a modified
Griggs-type solid medium apparatus to explore the preservation potential of
microfabrics created by crystal-plastic deformation at high stress,
overprinted during subsequent creep at lower stress. A corresponding stress
history is expected for the upper plastosphere, where fault slip during an
earthquake causes quasi-instantaneous loading to high stress, followed by
stress relaxation. The question is whether evidence of crystal-plastic
deformation at high stress, hence an indicator of past seismic activity, can
still be identified in the microstructure after overprint by creep at lower
stresses. First, quartz samples are deformed at a temperature of
400 °C and constant strain rate of 10<sup>â4</sup> s<sup>â1</sup> ("kick"),
and then held at 900 to 1000 °C at residual stress ("creep"). In
quartz exclusively subject to high-stress deformation, lamellar domains of
slightly differing crystallographic orientation (misorientation angle
< 2°) and a few tens of micrometres wide occur. In the
transmission electron microscope (TEM), these areas show a high density of
tangled dislocations and cellular structures. After "kick and creep"
experiments, pronounced short-wavelength undulatory extinction (SWUE) is
observed in the polarization microscope. The wavelength of SWUE is up to
10 ÎŒm, with oscillatory misorientation of up to a few degrees. TEM
inspection reveals domains with high density of dislocations and differing
diffraction contrast bound by poorly ordered dislocation walls. Only zones
with exceptional damage generated during high-stress deformation are replaced
by small new grains with a diameter of about 10 to 20 ÎŒm, forming
strings of recrystallized grains. For large original grains showing SWUE, the
Schmid factor for basal âš <i>a</i> â© glide is found to be high. SWUE
is taken to reflect high-stress crystal-plastic deformation, the modified
microstructure being sufficiently stable to be recognized after subsequent
creep as an indicator of past seismic activity
High temperature indentation creep tests on anhydrite – a promising first look
Indentation creep tests are established in materials engineering, providing
information on rheology, deformation mechanisms, and related microstructures
of materials. Here we explore the potential of this method on natural,
polycrystalline anhydrite. The tests are run at atmospheric pressure,
temperatures between 700 and 920 °C, and reference stresses between
7 and 30 MPa. An activation energy <i>Q</i> of 338 kJ mol<sup>â1</sup> and a stress
exponent <i>n</i> of 3.9 are derived. Deformation is localized into shear zones
bounding a less deformed approximately conical plug underneath the indenter.
Shear zone microstructures reveal inhomogeneous crystalâplastic deformation,
subgrains, and extensive strain-induced grain boundary migration, while
mechanical twinning appears not to be activated. Microstructure and
mechanical data are consistent with deformation by dislocation creep
K - Ar age determinations on phengites from the internal part of the Sesia Zone, Western Alps, Italy
Thermochronologic constraints on the tectonic evolution of the western Antarctic Peninsula in late Mesozoic and Cenozoic times
West of the Antarctic Peninsula, oceanic lithosphere of the Phoenix plate has been subducted below the Antarctic plate. Subduction has ceased successively from south to north over the last 65 Myr. An influence of this evolution on the segmentation of the crust in the Antarctic plate is disputed. Opposing scenarios consider effects of ridge crest â trench interactions with the subduction zone or differences in slip along a basal detachment in the overriding plate. Fission track (FT) analyses on apatites and zircons may detect thermochronologic patterns to test these hypotheses. While existing data concentrate on accretionary processes in Palmer Land, new data extend information to the northern part of the Antarctic Peninsula. Zircons from different geological units over wide areas of the Antarctic Peninsula yield fission track ages between 90 and 80 Ma, indicating a uniform regional cooling episode. Apatite FT ages obtained so far show considerable regional variability
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