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| Titel |
Damage, permeability and sealing processes of an exhumed seismic fault zone; The Gole Larghe Fault Zone, Italian Alps |
| VerfasserIn |
Thomas Mitchell, Marieke Rempe, Steven Smith, Joerg Renner, Giulio Di Toro |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250083895
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| Zusammenfassung |
| The Gole Larghe Fault Zone (GLFZ) in the Italian Southern Alps has been extensively
studied as a natural laboratory for seismic faulting. Ancient seismicity is attested by the
widespread occurrence of cataclasites associated with pseudotachylytes (solidified frictional
melts) formed at 9 - 11 km depth in tonalite host rock at ambient temperatures of 250 – 300Ë
C. Here we sythesise systematic meso- and micro-structural data with permeability and
ultrasonic velocity laboratory measurements from samples collected along fault transects, in
order to define the damage structure, fluid flow properties and sealing history of a
seismogenic source in the crystalline basement.
The GLFZ (~600m wide) and surrounding tonalite wall rocks have a broadly symmetric
damage structure and can be divided in to five distinct zones, distinguished by large variations
in fracture density, distribution of pseudotachylyte, volume of fault rock materials, and
microfracture sealing characteristics. The ~80 m wide central zone has pervasive fracture
damage in the form of dense cataclastic fault-fracture networks, and is bordered by two
unusually (2 m) thick and continuous cataclastic horizons. This central zone is flanked by
outer damage zones ~250 m wide where the fracture density is lower, and individual fault
surfaces surround relatively intact blocks of tonalite. In the southern damage zone,
macroscopic fracture density (faults + joints) increases gradually from background wall-rock
values towards the central zone where it remains relatively high throughout. The
boundary between the wall rocks and the southern damage zone is defined by an
abrupt transition from joints to cataclasite- and pseudotachylyte-bearing faults.
Fracture density drops off sharply within the northern damage zone. Within and
immediately surrounding the central zone, the syn-tectonic sealing of both micro-
and macro-fractures by epidote, K-feldspar, and chlorite minerals was pervasive,
resulting in low permeabilities (~10-21m2). Here the fault-fracture networks were
associated with pervasive fluid-rock interaction, defining a c. 200 m wide alteration zone
bounded by fluid infiltration fronts with irregular geometry. Fracture density is lower
in the damage zones, and partial healing results in higher sample permeabilities
(~10-18m2). Laboratory P-wave velocities correlate well with both the architecture and
sealing characteristics of the fault zone. P-wave velocities are uniformly high (up
to 6km/s) both within and immediately surrounding the central zone, consistent
with pervasive sealing of fractures and low sample permeability. In the damage
zones P-wave velocities are much lower (3-4km/s) due to the presence of open
fractures.
Our field and laboratory measurements highlight the close interplay between fracturing,
fluid flow, mineralization, and the strength of large fault zones. Importantly, they demonstrate
that seismic wave velocities and permeability depend on both fracture density and the degree
of fracture sealing, which has implications for the interpretation of active fault zone structure
based on geophysical data. |
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