Low stress deformation along the plate interface in subduction zones

253 Pages·2012·36.42 MB·English

by  Wassmann, Sara

Ruhr-Universit¨at Bochum
Institute of Geology, Mineralogy and Geophysics
Low stress deformation along the plate interface
in subduction zones - the microstructural
rock record
Doctoral thesis
by Sara Wassmann
born in Essen
Bochum, 2012

Ruhr-Universit¨at Bochum
Institute of Geology, Mineralogy and Geophysics
Low stress deformation along the plate interface
in subduction zones - the microstructural
rock record
Doctoral thesis by Sara Wassmann
Faculty of Geosciences
Ruhr-Universit¨at Bochum
Thesis committee Prof. Bernhard St¨ockhert
Prof. Claudia A. Trepmann
Prof. Ju¨rgen Schreuer
Defense of doctoral thesis April 19, 2012

Declaration of Authorship
I hereby declare in lieu of an oath that I have written
this thesis independently and autonomously using only
the sources indicated.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Location, Date Signature

Many fine books have
been written in prison.
Raoul Duke

Summary
The present study investigates microstructures of high pressure - low temper-
ature (HP-LT) metamorphic rocks from geological settings that are understood to
represent ancient subduction channels. Deformation mechanisms activated under
specific conditions leave an unequivocal imprint reflecting stress history and ma-
terial behaviour at depth. Interpretation of these microstructures, combined with
petrological evidence of metamorphic conditions during burial and exhumation, al-
lows for inferences on the mechanical state of modern subduction zones at depth,
following the principle of uniformitarianism. In this case implications are made for
the rheology of the plate interface in subduction zones using the microstructural
record of three different rock types. Analytical methods include polarization mi-
croscopy (transmitted and reflected light), scanning electron microscopy (SEM) in
backscattered mode, SEM combined with an energy dispersive X-ray (EDX) analyti-
cal system, SEM combined with electron backscattered diffraction (EBSD), electron
microprobe analysis, and X-ray powder diffraction.
Schistose and compositionally layered HP metamorphic serpentinites (P ≈ 2 ±
(cid:176)
0.5 GPa, T ≈ 550 ± 50 C) from the Zermatt-Saas zone, Western Alps, are investi-
gated in the first case study. Serpentinites being believed to play an essential role
for the development of a subduction channel, the microstructural record provides
information on large scale flow processes and rheology at inaccessible depth. The
serpentinites exhibit small scale folding, and crenulation cleavage with depletion of
olivine in the fold limbs and precipitation of olivine at dilatants sites in the fold
hinges. Prismatic strain shadows formed in between clinopyroxene porphyroclasts
broken and drifted apart and wedge-shaped strain shadows flank the sides of these
porphyroclasts.Togetherwiththeabsenceofevidencefordeformationbydislocation
creep,thesemicrostructuresindicatethathighstrainwasessentiallyaccumulatedby
dissolution precipitation creep. This implies low viscosity and a Newtonian rheology
for hydrated mantle material in deeper levels of subduction zones.
A highly deformed jadeite-blueschist from the Franciscan Complex (P > 1 - 1.4
(cid:176)
GPa, T ± 300 - 450 C) represents matrix material of a block in matrix structure
from a tectonic m´elange. While blocks of varying size floating in a weak matrix re-
main undeformed, the matrix is thought to take up the deformation and to control
the rheology. The investigated specimen exhibits a transposed foliation that is char-
acterized by dismembered fold hinges and boudinage structures. Strain shadows and
strain caps developed at large jadeite grains. Microstructures of veins in fragmented
jadeite grains are sealed by quartz and healed by new jadeite epitactically grown
on the broken host. This indicates that the fractures were open before healing and
sealing occurred at HP metamorphic conditions. The shape of small veinlets that
formed at HP metamorphic conditions and crosscut specific layers, indicates dis-
tributed host rock deformation during and after sealing. Monophase glaucophane
aggregates consist of extremely thin (down to 100 nm) undeformed needles and ex-
hibit open pores indicating the presence of an aqueous fluid, which is also preserved
as abundant fluid inclusions in quartz. The predominant deformation mechanism at
HP-LT metamorphic conditions is dissolution precipitation creep. Low flow strength
and a very low viscosity of material in the interplate shear zone are inferred.
A graphitic micaschist from the Tauern window, Eastern Alps, contains large
poikiloblastic garnet crystals grown during maximum burial at pressures of 2.0 -
(cid:176)
2.5 GPa and temperatures close to 600 C. Microstructures developed during burial
are preserved inside the garnet poikiloblasts. Strain shadows flanking these garnets
(cid:176)
formed in a narrow P-T field spanning from ca. 1.7 GPa at 570 C to ca. 0.9 GPa at
(cid:176)
470 C,indicatingongoingdeformationbydissolutionprecipitationcreepduringearly
exhumation. Dissolution seams, in places stylolitic, developed between converging
garnets. The less soluble material, passively enriched in these seams (mainly white
mica and graphite), was originally included in the dissolving garnet. The preserved
delicate microstructures suggest that garnet dissolution proceeded at low stress.
Independent of burial depth and type of material, the microstructural record of
rocks exhumed from ancient subduction channels imply that the overall deforma-
tion is governed by dissolution precipitation creep. An aqueous fluid must have been
present throughout. Deformation by dissolution precipitation creep permits defor-
mation to be localized into narrow shear zones and allows for viscous flow at low
temperatures. As shear heating is understood to be negligible for material with low
viscosity, this correlates well with the cool geotherms inferred for subduction zones.
The three case studies on HP metamorphic rocks presented here imply low differ-
ential stress, low flow strength and a Newtonian rheology for material undergoing
deformation to high strain along the interface in subduction zones.