Download 9.3 Mb pdf - IRIS

very forearcs lowincluding surface heat Cascadia flow 10,11 (30–40 . In mWm Fig. 2b 22 ) we observed plot a in thermal
most
from distance ,40 of to ,50–100 .80 mWm km. 22 However, over ,20 a landward km signals increase an abrupt in heat increase
flow
model forearcs for including central Oregon Cascadia 12 corresponding 10,11 . In Fig. to 2bthe we teleseismic plot a thermal profile,
from in deep ,40 temperatures. to .80 mWm In particular, 22 over ,20 Moho km signals temperatures an abrupt beneath increase the
model for central Oregon 12 corresponding to the teleseismic profile,
arc in deep and temperatures. backarc are significantly In particular, higher, Moho temperatures above 800 8C. beneath Thus ser-
the
pentine arc and should backarcexist arein significantly that portion higher, of the mantle above 800 forearc 8C. contained
Thus ser-
within pentinethe should dashed exist square in that in portion Fig. 2b, of but the it will mantle not forearc be stable contained beneath
the within arc the and dashed backarc. square The in degree Fig. 2b, of but serpentinization it will not be stable in the beneath forearc
will the sional arc depend and travel upon backarc. time thetomography The amount degree of H of methods for about the last
2 O serpentinization that chemically interacts the forearc with
will forearc 10 years depend mantle, and upon can which, the easily amount inexploit turn, of Hdepends the 2 O that expanded chemically on H 2 instrument O flux interacts frombase
with the
forearc subducting mantle, slab which, and the inpermeability turn, depends structure on H
(e.g., Hole, 1992; Zelt and Barton, 1998).
2 of O the fluxslab, fromplate
the
interface subducting and slab mantle.
and the permeability structure of the slab, plate
interface Serpentinite and mantle. exhibits elastic properties that are unique among
commonly
Theoretical Serpentinite occurring
developments exhibits rock elastic types,
and properties notably
advances
low that
in
elastic
computational
are unique wave velocities
among
and commonly high Poisson’s occurring ratio. rockThe types, very notably low S-velocity low elastic of wave serpentinite velocities is
central and capabilities, highto Poisson’s thecoupled interpretation ratio. with Theseveral very of our low community results. S-velocity In particular, of efforts serpentinite its S-
is
velocity central
methodological to(v the S ) is interpretation significantly exploration, of lower and our
in than results.
software that In of standardiza-
particular, its peridotitic
its S-
protolith velocity (v(dv S < 2 2 km s 21 S ) is significantly) lower and commonly than that of occurring its peridotitic lower-
protolith crustal tion and lithologies (dv dissemination (dv (e.g., S < 1 km s Computational 21 S < 2 2 km s 21 ) and) 13 commonly . Figure 3 shows Infrastructure
occurring the S-wave
lowercrustal
velocity for Geophysics lithologies of mantle [CIG] (dv peridotite and Seismic S < 1 kmsamples s 21 ) 13 . wave Figure as a Propagation function 3 showsof the degree and
S-wave of
serpentinization velocity of mantle at a peridotite pressure of samples 1 GPa, which as a function is appropriate of degree for the
of
base serpentinization Imaging
of a ,35-km-thick Complex at a pressure media
continental of[SPICE]) 1 GPa, crust which are
as
facilitating is presented appropriate in
the
ref. for the 14.
Correction base increased of a ,35-km-thick application from room of temperature continental waveform crust tomography 400–500 as presented 8C(i.e., will infull
shift ref. this
14.
curve Correction downward from room to velocities temperature 0.1–0.2 tokm 400–500 s wavefield inversion) to two-dimensional, 21 lower. 8C will This shift information
downward allows us to interpret velocitiesthe 0.1–0.2 image kmin s 21 Fig. lower. 2a quantitatively
This infor-
in mation land, terms marine, allows of degree us and to ofonshore-offshore serpentinization interpret the image ininvestigations the forearc Fig. 2amantle. quantitatively to provide As in a
previous in
this
controlled-source
curve
extremely terms of study degree
high-resolution 3 , we ofinterpret serpentinization
images the change of in
velocity the in forearc dip of and the mantle.
density subducting
As a
plate previous by 45 study km 3 depth , we interpret to indicate the the change onset inof dip eclogitization of the subducting of the
oceanic plate variations bycrust, 45 km in leading, crust. depthAlthough to eventually, indicatestill to thea a onset 15% computational increase of eclogitization in density challenge,
of and the a
pronounced oceanic
three-dimensional crust, reduction leading,
waveform eventually, in the seismic inversion to a 15% contrast increase
of large with in
data density underlying sets and
for
a
oceanic pronounced mantle reduction 15 .
in the seismic contrast with underlying
oceanic controlled Although mantle and a continuous 15 . natural sources dehydration is within of downgoing sight. oceanic crust
and Although entrained a continuous sediments is dehydration expected, the of downgoing water released oceanic by eclogi-
crust
tization and entrained (between sediments 1.2 and is3.3 expected, wt%; ref. the 16) water is especially released important
by eclogitization
expulsion (between into 1.2the and overlying 3.3 wt%; mantle ref. 16) wedge, is especially where important it causes
hydration for teleseismic expulsion andwavefields, into serpentinization, the overlying and three-dimensional and mantle significantly wedge, active-source
where diminished it causes velocities.
hydration The
Enhanced experimental developments, well-sampled
for
arrays, coupled and horizontal serpentinization, boundary
with theoretical and near
and significantly 32 km depth
data processing
diminished and between
vel-
2122.6 ocities. The and 2123.38 horizontal longitude boundary that near juxtaposes 32 km depth high- and (or neutral-)
between
velocity 2122.6 advances, and material are 2123.38 significantly above longitude withadvancing low-velocity that juxtaposes geological material high- and below (orgeody-
neutral-) is thus
inferred velocity
namic insight to material manifest into above the Earth highly with
history low-velocity unusual and occurrence processes. materialof Increasingly
below an ‘inverted’
is thus
continental inferred to manifest Moho separating the highlylower-crustal unusual occurrence rocks from of an underlying,
‘inverted’
continental detailed seismic Moho structural separating imaging lower-crustal now rocks permits frominterpreta-
underlying,
tion of chemical- and phase-change boundaries, and more
useful inferences on the presence or absence of free fluids
or hydrated materials. This advance, in turn, allows the
seismological community and an increasing diversity of
collaborators in geology, geodynamics, and geochemistry to
Figure 23. Comparison Scattered of wave scattered image wave made inversion using results a generalized with thermal Radon model. a, S-
advance understanding of fundamental Earth processes.
velocity Figure Transform 2perturbations Comparison inversion below of of scattered the P-to-S array, wave converted recovered inversion from waves results the from inversion with thermal a dense of scattered model. a, waves S- in
the velocity broadband P-wave perturbations coda array of 31 across below earthquakes the the array, Oregon recorded recovered Cascadia at from teleseismic thesubduction inversion distances. of scattered zone, The image
waves in
represents the superimposed P-wavea coda bandpass-filtered of on 31a earthquakes thermal version model recorded of of true the at teleseismic perturbations subduction distances. to zone, a one-dimensional,
The and image an
smoothly represents interpretation varying a bandpass-filtered of reference the image. model. version The Discontinuities loss of theof true signal perturbations present from the where to continental
a one-dimensional,
steep changes in
perturbation smoothly Moho in varying the polarity mantle reference occur. forearc b, model. Thermal is Discontinuities attributed model of Cascadia to aremantle present subduction serpentinization wherezone steep corresponding
changes by
approximately perturbation fluids released polarity to the
from occur. profile
the b, in
subducting Thermal a. cool model subducting
plate. of Cascadia (From
plate subduction Bostock
depresses zone et
isotherms
al., corresponding 2002.
the
forearc, approximately Field experiment
rendering to the serpentine profile described instable a. The in
within
Nabelek coolthat subducting portion
et al.,
of plate 1993.)
the mantle depresses encompassed isotherms in by the
dashed forearc, rectangle; rendering serpentine solid lines stable indicate within locations that portion of subducting of the mantle oceanic encompassed crust and
by the
continental dashed rectangle; Moho. solid Note lines temperature indicatecontour locations interval of subducting is 200 8C. oceanic c, Interpretation crust and of
structure continental in Moho. a. High Note degrees temperature of mantle contour serpentinization interval is where 200 8C. the c, subducting Interpretation oceanic
of
crust structure enters in the a. High forearc degrees mantle ofresults mantlein serpentinization an inverted continental where the Moho subducting (high-velocity oceanic crust
on crust Discovery
low-velocity enters themantle), forearc mantle which results gradually in an reverts inverted eastward continental to normal Moho polarity (high-velocity by 2122.38
crust Figure 3 S-velocity of altered peridotite as a function of degree of serpentinization. Data
longitude. low-velocity The signature mantle), which of the gradually subducting reverts oceanic eastward Moho to diminishes normal polarity with depth by 2122.38 as a
from Figure ref. 3 14. S-velocity Bold line of altered shows peridotite best-fit linear as aregression function ofwith degree ^1j of error serpentinization. bounds. The
Data
result longitude. progressive The signature eclogitization of the subducting below 45 oceanic km. Inverted Moho triangles diminishes in with a and depth c show
as a
from predicted ref. 14. velocity Boldcontrast line shows at the best-fit wedge linear corner regression suggests with degrees ^1j error of serpentinization bounds. The as
instrument result Over of85 progressive locations. institutions eclogitization contributed below 45 km. “one-pager” Inverted triangles research in a and csum-
show
high predicted shaping as 50–60%. velocity current contrast v S , S-wave scientific at the velocity.
wedge discussions corner suggests about degrees Earth of serpentinization processes.
as
instrument
maries, attributed locations.
to PASSCAL instrumentation, to the 2005
high
Because as 50–60%.
seismic v S , S-wave
exploration velocity.
of the Earth has never before
NATURE | VOL 417 | 30 MAY 2002 | www.nature.com © 2002 Nature Publishing Group
537
NATURE IRIS proposal. | VOL 417 | 30These MAY 2002 summaries | www.nature.com provide a representative
© 2002 Nature Publishing been Group undertaken at the spatial and temporal scales of the
537
overview of the variety of seismic experiments currently GSN and the aggregate of PASSCAL experiments, serendipitous
discoveries are quite common.
being fielded that is more comprehensive than space allows
in this review. Here, we note a few key themes that are