Page 284 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

km
to 8.34). and there is no
nearly identical to
the earlier passive margin
lithosphere/asthenosphere
$
fundamental boundaries and resulting
Contributions of three
-
wile
=
local isostatic
of
7.5 km
thickness of water column
deficiency
through
8.45)
gravity
gabbro
controlled,
the
267
the
conti-
km)
and
depth
part

167.5 km.
| lithosphere:
gravily anomalies for a passive
;
=
beneath
Normal oceanic lithosphere:
of
thickness of crust 12.5
air
=
thickness of mantle
z
.
(Fig.
Parameters
passive
therefore
in
free
Expressions
and
=
from
cools
5
Density contrasts are
in
=
(Moho depth
:
density
ridge
change
continental margin
lithosphere
eis
thermally
compensation
mantle
3 The
(Figs. 8.32
results
km.
basin,
region
kin
| contil
in
8.44
‘
the
equilibrium.
2.3, 4.9):
mid-ocean
are
are:
147
used
h:=33
(Fig. 8.42). The
Gravity
=0
relief on
Substantial
FIGURE
extra
ocean
=
model
column
high
=
=
(Fig. 2.14c). The
h, =
margin
ie
those
the
h,
h,,
h,
h,
1984).
N
(Figs.
the
gravity
on
isostatic
their
evolved
:
a
water

passive

of

for
al.,
left
boundary
excess
and
8.44).
rift zone
situation
et
superimposed
relief
low-density
8.35;
(Serpa
fully
Settings
evolved
ridge
ie
mass
(Fig.
significant
to
In_a
chemical
continental
isostatic
axis
mid-ocean
8.33
ty
Tectonic
>
between
boundary
fully
x
overlying,
Margin
rift
\
\
Figs.
Z
a °
‘S
the
a
basins;
a
The
;
see ememrmsmen, ame
Temomemememememes
no
en is
at
balance
for a
in
a
along
which
Continental
be
anomalies
shown
from
lithosphere/asthenosphere
Ridge
the
depicted
rift
may
crust
Moho,
far
and
from
‘.
of
amplitude
there
Mid-Ocean
are
infill
crust
the
gravity
result
-200
4
7
4
a
4
0-
a
that
Passive
margins
the
+400
0
5 intruded
F
that
sedimentary
g
oceanic
g
3S
similar to
on
anomalies
and
Gravity Anomalies
Bouguer
so
remains
4.
3.
g
nental
<
form
time,
that
thin
Bathymety
Contributions
Bouguor
Free Alr
LthAsth
is
7
Moho
.
wae
momen»
wrmemes
——
ww
: boundary; free air and Bouguer gravity anomalies are essentially the same as in a: (Ap = na athymetry . the earlier model. -1.64) Continental Margin

rifts, along with and Basin California G: R: Satius'and with Geophysical treaihe Low values supports the weight north the Rift Paper. Society of to high asthenosphere lowa Rift in States. R. Meyer. Journal of Geophysical prominent high-density extending through rift b).
Bouguer gravity at density models, a) continental Nevada. Frais 14, vol. © 1995. of the American Union, Washington, D.C. Observed curve anomaly. whereby ciiecixot Bouguer anomaly, —100 mGal reflect shallow, hot a eo Special \ 1972. Redrawn with Geological America, Boulder, Colorado, USA.A maximum, duc intruding the crust. is the Keweenawan United 1973. Redrawn with American relatively material th
8.43 Range Province {rom southern Tectonics, pp. 1235-1244, Redrawn festual Gosoenic hacins werésema ied that higher topography on side of the section. b) East African and J. Williams, GSA pp.. © of the Bouguer gravity material superimposed on c) Nebraska. central and L. Ocola Geophysical Research. vol. 78, pp. 5173-5194, © of the Union. Washington. D.C. The results from 1.1 this with
FIGURE anomalies jilierpreted locentval Thompson, permission ig's complete around asthenosphere of the 136. 67 No. permission density ini ENE and From permission high (3.08 g/cm’) within (compare
oe yor) (2.2 Crust (2.9 g/crP) Obsaned cas -: Calculated : somes
Isostasy Valle y Bouguer Anomaly
and Bouguer Anomaly Rift >. oe ‘ : J
Gravity Province Observed [20 pe)
Chapter 8 Range Rift =—— Crust (2.9 g/cnP) a 7 ZB 2 -60-
and
266 East African WSW
Basin -140 -200- -260
a) b) eous