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| Titel |
Chemical diagenesis, porosity reduction, and rock strength, IODP Site U1480: Influences on great earthquakes at shallow depths |
| VerfasserIn |
Insun Song, Kitty Milliken, Brandon Dugan, Sylvain Bourlange, Tobias Colson, Marina Frederik, Tamara Jeppson, Mebae Kuranaga, Nisha Nair, Timothy Henstock |
| 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 |
250140827
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| Publikation (Nr.) |
 EGU/EGU2017-4267.pdf |
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| Zusammenfassung |
| International Ocean Discovery Program (IODP) Expedition 362 drilled two sites, U1480 and
U1481, on the Indian oceanic plate ∼250 km west of the Sunda subduction zone to a
maximum depth of 1500 meters below seafloor (mbsf). One of the primary objectives was to
understand the mechanism of great earthquakes such as the 2004 Sumatra earthquake (Mw
9.0) which showed unexpectedly shallow megathrust slip by establishing the initial and
evolving properties of the North Sumatran incoming sedimentary section. Core sampling and
logging from the complete sedimentary section at U1480 indicates a distinct change in
sedimentation rate from a slowly deposited pelagic system to a rapidly deposited submarine
fan system at late Miocene. Following burial, sediments of the Nicobar Fan underwent
compaction leading to porosity reduction from 66±9% near seafloor to ∼30% at the
base of the sampled Nicobar Fan section (∼1250 mbsf), representing a normal
consolidation behavior. Rock strength gradually increases with depth as the sediments are
mechanically compacted. Below the fan (1250-1415 mbsf), the pelagic sediments
are composed of tuffaceous, calcareous, and siliceous sediments/rocks and their
porosity is dependent upon lithology more than upon depth. Tuffaceous materials
exhibit high porosity ranging from ∼30-60%, even higher than that of overlying
layers. However, porosity of most calcareous samples is lower than 20% at the
same depth. The large variation in porosity depends on the degree of cementation,
which in turn is controlled by grain assemblage composition and environmental
conditions such as slow sedimentation rates and locally high temperatures related to
igneous activity as documented by local igneous intrusives and extrusives. The
minor cementation in tuffaceous sandy sediments has retained high porosity, but
strengthened their skeleton so as to bear the overburden. The low porosity in calcareous
rocks is considered to come from extensive cementation rather than mechanical
compaction. The rock strengthening by mechanical compaction is dependent on effective
stress, and does not facilitate storage of a large amount of elastic energy at shallow
depth. However, chemical diagenesis (cementation) can lead to high strength that
does not necessarily arise directly from burial. This chemical diagenesis potentially
influences sediment strengthening that localizes great earthquakes at shallow depths. |
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