The Microstructural Record of Synseismic Loading and Postseismic Creep in the Uppermost Plastosphere
Abstract
Failure of the upper continental crust in major earthquakes is suspected to cause short-term deformation at very high differential stress and high strain rates in the uppermost plastosphere, at temperatures of 300 to 350° C, and with marked fluctuations of pore fluid pressure. The microstructural record of such events has been identified in exhumed metamorphic rocks of the Sesia Zone, Western Alps [1, 2], and provides insight into the deformation processes related to synseismic loading and postseismic creep. Garnet crystals embedded in a quartz matrix are intensely fragmented, with the fragments displaced by sliding along the fractures, and finally slightly pulled apart with quartz and feldspar precipitated from the fluid phase in the open cracks. Jadeite crystals reveal mechanical twinning. The quartz microstructures indicate a transient stage of deformation in the low-temperature plasticity field, followed by deformation in the dislocation creep regime during stress relaxation with a very small recrystallized grain size, and finally minor static grain growth in some samples. The peak magnitudes of differential stress achieved during synseismic loading exceed 0.5 GPa, as indicated by the orientation distribution of mechanically twinned jadeite [2]. Apparent flow stresses of 0.2 to 0.4 GPa are suggested by the dynamically recrystallized grain size of quartz, related to the subsequent stage of non-steady state deformation by dislocation creep, with rapidly decaying strain rates during stress relaxation. The mesoscopic structures reflect a highly inhomogeneous strain field. Strain is concentrated in cm- to m-scale zones (mylonites, ultramylonites, pseudotachylytes) and remarkably low in the intervening rocks. Branching quartz veins developed from open fissures reflect brittle failure, with subsequent plastic deformation and recrystallization of the precipitated quartz. These fractures propagating into a temperature range of 300 to 350° C during synseismic loading are suggested to cause a drop of pore fluid pressure to markedly sublithostatic values, which is prerequisite for plastic deformation at extreme differential stress. Restoration of a near-lithostatic pore fluid pressure in the final stage of postseismic creep is reflected by slight opening and sealing of the cracks in garnet. In summary, the peculiar microstructural record of the exhumed rocks indicates episodic plastic deformation in the uppermost plastosphere at temperatures of 300 to 350° C, with (1) very high peak stresses exceeding 0.5 GPa at the stage of synseismic loading, (2) concomitant drop of pore fluid pressure due to fractures propagating downward, followed by (3) stress relaxation primarily controlled by power law creep of quartz, and (4) eventual restoration of a near-lithostatic pore fluid pressure, reconciled with a major earthquake in the overlying schizosphere. References: [1] Küster, M. and Stöckhert, B. (1999): Tectonophysics 303:263-277. [2] Trepmann, C. and Stöckhert, B. (2001): Int J Earth Sc 90:4-13.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.T51A0841T
- Keywords:
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- 3902 Creep and deformation;
- 7221 Paleoseismology;
- 7230 Seismicity and seismotectonics;
- 8030 Microstructures;
- 8123 Dynamics;
- seismotectonics