Hugues Raimbourg @(n
f―ΘwκU)
Mechanisms of ductile deformation in the lower crust:
microstructures and rheological evolution of a high-temperature
shear
zone in Hokkaido, Japan
2007N1031ϊ
The rheology of the continental lower crust, relying mainly on creep mechanisms,
is a major control on the deformation of the whole continental crust. From cm- to
the km- scale, ductile deformation observed in natural rocks is usually strongly
partitioned within mylonite zones cutting through virtually undeformed host rock.
A major challenge of petrological and structural geology is to understand and quantify
the processes at the origin of this very localizing behaviour of ductile deformation.
With this objective in mind, we have studied the microstructures of a high-temperature
mylonite zone of the Hidaka Metamorphic Belt on Hokkaido to determine what processes
are operative in the mylonite development and how the rheology varies along with
accumulated strain.
The first part of this presentation focuses on the description of the microstructures.
The mylonite incorporates both large porphyroclasts inherited from its undeformed
state as well as much finer grains formed during its progressive deformation. The
analysis of orthopyroxene crystal orientations by EBSD (electron back-scattered
diffraction) unraveled a very different distribution pattern between large inherited
porphyroclasts with a strong crystal-preferred orientation (CPO) and the fine-grain
fraction with a random fabric. This change in orientation distribution is taken as
the evidence of a switch in deformation mechanism from dislocation to diffusion-
accomodated creep. Furthermore, we show that the small grains of orthopyroxene are
formed by subgrain rotation on the porphyroclasts rims.
In a second part we develop simple ideas to quantify the mylonite rheology and the
deformation conditions operative during mylonite development. The grain size dependence
of the switch in deformation mechanism provides upper bounds on the stress conditions.
Besides, we observed folding of the elongated opx porphyroclasts, which we explained
using classical models of folding of viscous multilayered media under compression.
Such an approach yielded rheological contrasts between porphyroclasts and matrix of
a surprisingly low amplitude.
Finally, as future prospects, we show ongoing research on the microscopic mechanisms
of porphyroclasts dislocation deformation, which are essential in the generation of the
very fine grains and associated rheological weakening.