 |
| Titel |
A brittle (normal?) shear zone cored in Site C0002 of Nankai Trough
Seismogenic Zone Experiment (IODP Expedition 348) |
| VerfasserIn |
Ana Crespo-Blanc, James Sample, Kevin Brown, Makoto Otsubo, Yuzuru Yamamoto |
| Konferenz |
EGU General Assembly 2016
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250131641
|
| Publikation (Nr.) |
 EGU/EGU2016-12070.pdf |
|
|
|
|
|
| Zusammenfassung |
| Integrated Ocean Discovery Program (IODP) Expedition 348, which belongs to the Nankai
Trough Seismogenic Zone Experiment, conducted riser-drilling to make deeper an existing
hole at Site C0002, up to 3058.5 meters below seafloor (mbsf). This site is located 80 km SE
of the Kii Peninsula (Japan) in the Kumano forearc basin, in turn situated on top of the
Nankai accretionary prism. Cuttings (875.5-3058.5 mbsf) and cores (2163.0–2217.5 mbsf)
were collected in the upper Miocene to Pliocene turbiditic silty claystone with few
intercalations of sandstone which characterize the accretionary prism lithological
units.
A remarkably preserved fault zone has been cored around 2205 mbsf (core section
Hole C0002P-348-5R-4). It is characterized by 34 cm of fault breccia, in which
an anastomosed cataclastic foliation is present. The rocks of the damaged zone
are formed by silty claystone with an incipient scaly fabric and scarce levels of
sandstones.
Extra-large thin sections were made along the whole core section. In the brittle shear
zone, they reveal a catalogue of deformation structures characteristic of a high structural
level. In particular, almond-type structures and arrays of microfaults cutting the stratification
are the most common structures and outline the cataclastic foliation. The occurrence of
calcite veins in the recovered cores is limited to this fault zone, which is indicative
of its role as fluid path, accompanied by carbonate cementation. Generally fault
veins have lower δ18O values than carbonate cements in the sedimentary matrix,
consistent with veins forming at higher temperatures and/or from a fluid more strongly
depleted in 18O. A continuum of the relationships between calcite veins and cataclastic
deformation is observed, from veins that precipitated early in the fault history, with
calcite grains broken during subsequent deformation, to late veins which seal the
almond-type structures within the claystones. The geometry of the calcite grains within
the veins and the relationship between the veins and the wall rock indicate that
the mechanism that actuate during the vein formation is that of crack-seal. It took
place along variable growth planes inside the vein and the wall rock (localized and
delocalized stretching veins, respectively), which result in asymmetric syntaxial
veins.
All the observed microfaults produced lengthening of the markers. Together with the
mesoscopic criteria (according to the visual core descriptions made onboard), this would
indicate that, in its present-day position, this brittle shear zone is associated with a normal
fault. Nevertheless, it is not discarded that it could be an early thrust rotated after its
development.
Acknowledgements: This research used samples provided by IODP. Grants RNM-215
and 451 (“Junta de Andalucía”, Spain) and CGL2013-46368-P (“Ministerio de Economía y
Competitividad”, Spain) supported this study. |
|
|
|
|
|
|