Dear all,
we are working on the active deformation pattern which sits above a
well-known offshore deformation belt, known to have existed at least since
Jurassic times.
This belt has been interpreted as being either a very long normal fault
and/or an inverted normal fault or a major reverse fault system, yet most
authors think it has exhibited strike-slip kinematics for a large part of
its history. Finally, given the seismotectonic context of an adjacent long
structure which could be the onland prolongation of the offshore shear
zone, one can reasonably assume that the latter is affected by the same
regional stress field, Namely, such field would cause reactivation with
right-lateral motion, given the geometry and orientation of the shear zone.
Using very high resolution seismics (ascertained vertical resolution is
0.3 m), we recognized a remarkable set of faults and folds, running
parallel to the main shear zone and virtually on top of it. All these
faults affect the Middle-Late Pleistocene (down to data resolution) and
some rupture through the Holocene and the seabed - so, really no doubt
about active deformation here. We also performed a test concerning
differential compaction on and off of the faulted fold system and the
values were far lower than displacement ones, so we do believe we are
studying tectonic deformation.
These sub-vertical faults, whose extent ranges from one to a few km per
segment, form a set of two main fault and fold systems, parallel to one
another, their kinematics being essentially normal. Locally, we believe
there can be a horizontal component, due to the arrangement of strata on
each wall.
Apart from reasoning about the regional present-day kinematics of the
entire shear zone (be it right-lateral, as we think, or else), our concern
is that the ~50 km long structural pattern we observe is long enough to
suggest deep-seated faulting, but our data only resolves down to 150-200
m. Published and unpublished multi-channel seismic data from the 70’s and
80’s resolve down to only a few km; their interpretation hints at a
reverse fault system.
In other words, seismic data that could resolve the crustal extent
(whatever) of the shear zone is not available and, since we are trying to
characterize the size of the fault system we described at surface, we are
faced with a sheer interpretation constrain.
So, our current issues are:
- How is normal faulting we observe compatible with the plausible
kinematics of a large and long-lived shear zone at depth?
- Normal faulting at the 200-500m scale requires only and solely normal
faulting at (seismogenic ?) depth, or
- Could such shallow normal faulting be the surface expression of any
other larger-scale kinematics, as long as the latter is compatible with
the regional stress field (which caused right-lateral reactivation on a
parallel fault system onshore) ?
Many thanks in advance for any hints, suggestion of similar cases, etc. !
Cordially,
Umberto Fracassi
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Istituto Nazionale di Geofisica e Vulcanologia
Via di Vigna Murata, 605
00143 Roma
Italy
Tel: +39-06-51860557
Fax: +39-06-51860507
Email: [log in to unmask]
Skype: umbogroove
WWW: http://roma1.rm.ingv.it/personal.asp?id=100&kuki=en&action=Cv_en
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