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| Titel |
Mid-crustal transient stress field geometry during the seismic cycle inferred from the geological record and numerical models |
| VerfasserIn |
J.-A. Nüchter, S. Ellis |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250024876
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| Zusammenfassung |
| Interpretations of exhumed metamorphic rocks and numerical models suggest that
earthquakes on dip-slip faults impose a major stress increase in the middle and lower crust,
commonly referred to as coseismic loading. The imposed stress peak relaxes during an
episode of transient postseismic creep. Because of the low temperatures expected in the
middle crust and the resulting high viscosities there, postseismic creep in the middle crust is
expected to be hardly detectable by surface geodesy. However, a detailed understanding of
these mid-crustal processes is required to estimate crustal stress redistribution during the
seismic cycle.
During exhumation, metamorphic rocks are increasingly affected by stress cycles related
to seismic activity. These rocks may therefore provide insight into the processes and
conditions prevailing at deeper levels of the crust during the seismic cycle. Information
on the spatial variation of these parameters is typically restricted by the size of
the outcrop and the exposed surface. On the other hand, numerical techniques can
model the entire crustal stress field, but the results depend highly on the applied
boundary conditions. Therefore, we use the record of metamorphic quartz veins
hosted in the high-pressure / low-temperature metamorphic Styra-Ochi Unit of South
Evia, Greece to calibrate numerical models of the mid-crustal seismic stress cycle.
The rocks of the Styra-Ochi Unit were exhumed in the footwall of a low angle
normal fault. The veins formed from tensile fractures, indicated by the absence of
wall parallel displacement. Crosscutting relationships between the veins and all
syn-metamorphic fabrics of the host rock and the quartz microfabrics indicating crystal
plastic deformation imply that the veins formed during exhumation, but still below the brittle
ductile transition. The veins were sealed in a single stage and crystals grew into an
open cavity. Fluid inclusions trapped in the vein quartz record a time series of fluid
pressure (P) during progressive sealing, with low P at the vein walls (early stage)
to high P in the vein core (final stage). The effective stress acting normal to the
incompletely sealed fracture is controlled by the fracture normal stress (in the case of
tensile fractures, the minimum principal stress Ï3), and P . For opening of fractures,
the stress acting normal to the walls must be tensile, and P must be similar to Ï3.
If P »Ï3, fracture propagation would have re-initiated, resulting in a drop in P .
The evolution of P is therefore interpreted to reflect a major coseismic drop in Ï3,
causing a major increase in the differential stress. During the stage of sealing, the
increase in P is interpreted to reflect a restoration of Ï3, which means that the stress
peak relaxes. Numerical models with simple initial and boundary conditions are in
first-order agreement with the vein record. The results of these models suggest that
seismic activity causes: (1) a major coseismic stress peak in the middle crust; (2)
coseismic loading predominantly by a drop in Ï3 in the footwall and by an increase in
Ï1in the hanging wall by values comparable to those deduced from the veins; (3)
stress relaxation by thermally activated creep during the interseismic period; (4)
significant deflections in stress tensor orientations compared with predictions from the
Andersonian theory, which may be preserved over timescales typical for a seismic cycle. |
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