Giant seabed polygons and underlying polygonal faults in the Caribbean
Sea as markers of the sedimentary cover extension in the Grenada Basin
Abstract
Based on an extensive seismic and multibeam dataset, 1-5 km wide giant
polygons were identified at the bottom of the Grenada basin, covering a
total area of ~55000 km². They represent the top part of
an active underlying polygonal fault system due to the volumetric
contraction of clay- and smectite-rich sediments during burial. To date,
this is the widest area of outcropping polygonal faults ever found on
Earth. The seabed polygons are bounded by rectilinear
~1000-1500 m wide and ~10-60 m deep
furrows, depending on the location in the basin. They are relatively
regular in the north Grenada Basin, whereas they are getting longer and
more elongated in the south Grenada Basin. The polygonal faults consist
in a set of discrete normal faults affecting a 700 to 1200 m thick
interval, initiated in the shallow sub-surface at the transition between
Early to Middle Pliocene and then having propagated both upward and
downward during sedimentation. The centre-to-centre method has been
applied to determine the local ellipse of strains, providing a major
orientation for extension needed for polygons to initiate. In the north,
the minor axes are oriented N40°, indicating a general NE-SW extension
of the upper part of the sedimentary cover consistent with the
forearc/backarc regional extension. In the south Grenada Basin, minor
axes are progressively turning towards the south, pointing out the
actual maximum subsidence point. This implies that seabed polygonal
faults could thus be indicative of the present-day (or recent) strain
state within the upper sedimentary column.