SEMINAR: MGG Student Seminar/Geotopics - TODAY, 3.15PM - SLAB Seminar Room

[Marco Bagnardi <mbagnardi@rsmas.miami.edu>]

Today's Geotopics will be a special edition of the MGG Student Seminar
Series!

Katie Inderbitzen will be presenting her doctoral research TODAY at 3:15 pm
in the SLAB Seminar Room where Geotopics Seminars normally take place:


Seafloor Uplift at Hole 857D, Middle Valley, Northern Juan de Fuca Ridge

Over the last 12 years, in-situ seafloor and basement pressures have been
continuously monitored and recorded by an ODP subseafloor hydrogeological
observatory (CORK) located in Middle Valley, Juan de Fuca Ridge. Hole 857D
was drilled in 1991 in thickly-sedimented crust to a depth of 936 mbsf and
instrumented with an original CORK that was replaced in 1996. Previous
results from this site have shown a strong formation underpressure that is
the consequence of the hydrologic structure of rift valley and the
hydrothermal state, as well as hydrologic and geodynamic responses within
basement to both local (1991, 2001, and 2004) and distal (1999) earthquake
swarms along the Juan de Fuca Ridge. We will present data from 2005-2008,
obtained via Alvin submersible operations, which show a marked change in
both seafloor and formation pressures at 857D. After a rapid increase of
~40 kPa within basement during the second half of 2005, formation pressure
peaked in early 2006, then steadily decreased by ~15 kPa over the
following 2.5 years. Seafloor pressure was relatively constant during the
first 9 years of monitoring and did not vary significantly during the 2005
rapid increase in formation pressure, but it mirrored the subsequent
decrease in formation pressure by ~18 kPa, equivalent to nearly 2 m of
uplift. Depths recorded during visits to the wellhead by the submersible
Alvin confirmed that water depth at 857D has decreased by at least 1 m
over the past three years. It is not known if this possible inflation
event is associated with deep magma injection or is tectonic in origin.
Swarms of seismic activity on the northern Juan de Fuca Ridge did occur
immediately before and after the initial pressure increase, but the uplift
iself has been largely aseismic. During the same three-year interval,
temperatures of hydrothermal fluids venting from chimneys about 800 m NE
of 857D have remained around 270°C, with no indication for any effects on
the local hydrothermal system.

Marco Bagnardi

mbagnardi@rsmas.miami.edu