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| Titel |
Is localised dehydration and vein generation the tremor-generating mechanism in subduction zones? |
| VerfasserIn |
Ake Fagereng, Francesca Meneghini, Johann Diener, Chris Harris |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250149147
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| Publikation (Nr.) |
 EGU/EGU2017-13473.pdf |
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| Zusammenfassung |
| The phenomena of tectonic, non-volcanic, tremor was first discovered at the down-dip end of
the seismogenic zone in Japan early this millennium. Now this low amplitude, low frequency,
noise-like seismic signal has been observed at and/or below the deep limit of interseismic
coupling along most well-instrumented subduction thrust interfaces. Data and models from
these examples suggest a link between tremor and areas of elevated fluid pressure, or at least
fluid presence. Tremor locations appear to also correlate with margin-specific locations of
metamorphic fluid release, determined by composition and thermal structure. We therefore
hypothesise that: (i) tremor on the deep subduction thrust interface is related to localised fluid
release; and (ii) accretionary complex rocks exhumed from appropriate pressure -
temperature conditions should include a record of this process, and allow a test for the
hypothesis.
Hydrothermal veins are a record of mineral precipitation at non-equilibrium conditions,
commonly caused by fracture, fluid influx, and precipitation of dissolved minerals from this
fluid. Quartz veins are ubiquitous in several accretionary complexes, including the Chrystalls
Beach Complex, New Zealand, and the Kuiseb Schist of the Namibian Damara Belt. In both
locations, representing temperatures of deformation of < 300 and < 600 ∘C respectively,
there are networks of foliation-parallel and oblique veins, which developed incrementally and
record a combination of shear and dilation. Required to have formed at differential stresses
less than four times the tensile strength, and at fluid pressures exceeding the least
compressive stress, these veins are consistent with tremorgenic conditions of low effective
stress and mixed-mode deformation kinematically in agreement with shear on the plate
interface.
We have analysed the oxygen isotope composition of syntectonic quartz veins in both
Chrystalls Beach Complex and Kuiseb Schist accretionary complexes, to unravel the
geochemical characteristics of the fluid source potentially required to produce tremor. In the
Chrystalls Beach Complex, quartz δ18O values range from 14.1 ‰ to 17.0 ‰ (n =
18), whereas in the Kuiseb schist, values range from 9.4 ‰ to 17.9 ‰ (n = 30).
In the latter, values less than 14.0‰ are associated with long-lived shear zones.
Excluding the lower values in the Kuiseb schist, the δ18O values are consistent with
metamorphic fluids in near equilibrium with the host rocks. We thus infer that the
veins that developed on the prograde path formed at a small range of temperatures
from a local fluid source. This interpretation is consistent with the veins forming in
response to a spatially localised metamorphic fluid release. If vein swarms are formed
by the mechanism geophysically recorded as tremor, this implies that tremor is,
at least in some locations, triggered by metamorphic fluid release and associated
hydrofracture and low effective stress shear activation of low permeability shear
zone rocks. If this is correct, then a corollary may be that the near-periodic nature
of tremor events is related to a regular nature in the build-up and release of fluid
pressure. |
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