Publication Date:
2017-04-10
Description:
The interrelation between deformation styles and behavior of
fluids in accretionary prisms is under debate, particularly the possibility
that overpressuring within the basal decollement may enable
mechanical decoupling of the prism from the subducting material.
Anisotropy of magnetic susceptibility (AMS) data from
sediments spanning the basal decollement of the Barbados accretionary
prism show a striking progression across this structure that
strongly supports the hypothesis that it is markedly overpressured.
In the accretionary prism, above the decollement, the minimum
AMS axes are subhorizontal and oriented nearly east-west, whereas
the maximum AMS axes are oriented nearly north-south and shallowly
inclined. At the top of the decollement, the minimum AMS
axes orientations abruptly change to nearly vertical; this orientation
is maintained throughout the decollement and in the underthrust
sediments below. The AMS orientations in the prism sediments
above the decollement are consistent with lateral shortening
due to regional tectonic stress, as the minimum axes generally parallel
the convergence vector of the subducting South American plate
and the maximum axes are trench-parallel. Because the orientations
of the AMS axes in deformed sediments usually parallel the
orientations of the principal strains, the AMS results indicate that
the incremental strain state in the Barbados prism is one dominated
by subhorizontal shortening. In contrast, the AMS axes
within and below the decollement are consistent with a strain state
dominated by vertical shortening (compaction). This abrupt change
in AMS orientations at the top of the decollement at Site 948 is a
direct manifestation of mechanical decoupling of the off-scraped
prism sediments from the underthrust sediments. The decoupling
horizon occurs at the top of the decollement zone, coinciding with
the location of flowing, high-pressure fluids.
Type:
Article
,
PeerReviewed
Format:
text
DOI:
10.1130/0091-7613(1996)024〈0127:SDATDO〉2.3.CO;2
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