SZO Seminar Notes – June 3, 2015: Learning from the
geologic record.
Discussion leader: Darrel Cowan
Paper: Brandon, M.T.,
2004, The Cascadia
subduction wedge: the role of accretion, uplift, and erosion.
In In:
"Earth Structure, An Introduction to Structural Geology and
Tectonics", by B.A. van der Pluijm and S. Marshak, Second
Edition,
WCB/McGraw Hill Press, p. 566-574
The discussion focused on the structure and
evolution of
accretionary wedges, particularly in Cascadia and geologic
evidence available
from the Olympic mountains.
Darrel showed cross sections of well-studied
and
well-constrained subduction zone accretionary wedges in Japan,
highlighting
their characteristic features.
These
include very distinct imbricated thrust faults near the
deformation
front/trench that become less clearly defined with distance from
the front,
until the internal structure becomes transparent to seismic
imaging and
deformation appears to be accommodated via ductile/plastic
mechanisms. Accretionary
wedges also are found in other
settings (e.g. Montana), but in these the imbricated thrusts seem
to occur
throughout.
Most of the discussion focused on the
deformation styles and
transport of materials from the trench to the mountain tops of the
Olympics in
Washington. The paths
of sediments
deposited at the trench, carried down during subduction,
metamorphosed, and
then brought up to the surface during exhumation have been mapped
using
thermochronology constraints.
Alison Duvall explained what and how
geochronology and
thermochronology may be used to date the ages of rocks with
isotope methods
(i.e. measuring the ratio of daughter to parent materials in
rocks). Basically
each type of measurement relies on
knowledge of the decay rate (parent to daughter) and whether
‘closure’ occurs
upon crystallization of the rock (geochronology) or below a
particular known
temperature (thermochronology).
The
closure properties (whether upon crystallization or at what
temperature) depend
on rock type. Thus in the latter, measurement of the
daughter/parent ratio
provides estimates of when the rock was at a particular
temperature, which may
be a proxy for its depth/location at that time.
Thermochronology measurements have been used to
map the
trajectories of rocks that have been exhumed (brought to the
surface) in the
Olympics. The Brandon
(2004) paper
describes one such study, which remarkably infers that “rocks at
the summit of
Mount Olympus started out as sand at the trench”.
The transport of material in these systems has
been, to
first order, well explained by critical wedge theory. Ken Creager suggests
that critical wedge
theory and the varying dip of the slab along strike likely
explains what is
observed in Cascadia, particularly the exhumation of deep rocks
only in the
Olympics. That is,
the slope of the
wedge top surface and its thickness is controlled by the dip of
its bottom
surface, in a manner consistent with what is known about the
geometry of the
plate interface and wedge structure in Cascadia.
Additional thermochronology studies might
improve our
understanding of the rates of accretionary wedge processes. Questions also were
raised about the style of
deformation within wedges, and in particular that there is little
evidence of
brittle deformation within the Olympics – both in the rocks and
the lack of
seismicity.