Page 271 - Whole Earth Geophysics An Introductory Textbook For Geologists And Geophysicists

253 water the to and from shal- deep- deficit, however, the is effects and edge the because air grav- zero near deficit equals test quick margin defi- excess continental lower the the

Modeling the where relates mantle between distance mantle the water the and excess gentle, model margin (mantle) continent the over results different gradients. the free are values mass the anomaly zero. to a provides passive (x/Z,]) [x/2]) mass the (bathymetry) mass the upper to mantle boundary of plane of plane

Gravity gradient water shallow great At to (due to (due mass more is passive deep interiors of a minimum effect, edge of attributes L) equals gravity equal is x, it the for tan“ can“ to due to due to compared water of elevated continent/ocean central the central the

steep the difference 8.33b). gravity contribution simple, and the and edge an as have 8.34a): (+Am) the of to because curve Ag,(moho) + + (1/2 anomaly seaward anomaly seaward compared mantle of to to mantle. [x/2])
a the Fig. to compensatory contribution (water) over important (Fig. excess curve respect anomaly air air water shallow slab slab the level level tan™
along compensates times g/cm”; contribution negative deeper. the for shallow continental known deep sources equilibrium mass the with discussion, air 8.33): + 2G(Ap),(Ah),(a/2 2G(Ap)a(Ab) free deepening free shallowing the the from sea from water of sea from of elevated tan™'(x/z,]) +

change, (h,,) +9.43 the represent mantle is it (Ag,,) the near zero is the couple, and two shows the under anomaly, further free (Fig. Ag,(bath) + anomaly the to water the to of contrast of contrast crust of semi-infinite of semi-infinite distance distance slab distance slab + (71/2

abrupt that mantle = Pe — positive as effects the for causing anomaly from anomaly shallow model isostatic because area the the of worth The contributions i Ag, = air contribution the of contribution mantle continental thickness thickness horizontal semi-infinite semi-infinite simplified: ee Ee (Ap) m(Ab)q,

an (+Am) the of pa the amplitude mass the a maximum over positive/negative the in effects, is equilibrium. free ciency the of density crust density vertical vertical +
is excess = (Ap margin, two gradient air gravity contributions that to due margin region effects), integral point two of = = = = = m x 2 = Zm be
anomaly mass shallowing densities same the the because The anomalous shows This passive a for edge edge at the second isostatic sum Ag, Ag,(bath) Ag,(moho) (Ap), (Ap) (Ah), (Ah) can 2G = Ag,

gravity The of continental has respectively. the The free the of 8.33c). Note ocean, but ocean. the contributions The anomaly for 2) (—Am); because The local the is equation

the deepens. amount crustal the lowing) ening), because sum (Fig. of the ity (except zero, for model where: The

150
ww8* a rere eee Apiaz0: okn ectccememued en 1.64 g/onP 150 is a the oceanic crust beneath air gravity anomaly at a passive continental amplitudes but different have equal km 18.84 of function a (—Am), deficit levels upper crustal at to contribution negative
ad
a)

b Mantle Contribution [ed 30 + ee -150 osmemen o aia ie! ee water Ocean : Ap.= 1.03) rage 3.1) Manto (p, 0 / Ocean Boundary (km) The main gravity contributions at a passive continental margin have equal amplitude but different gradient. mantle (steep gradient). b) The extra free yields: unknowns = h,, and mass a represents difference density the this that sh
1 . 4 . 4 . 1 . ™
<

Isostasy . ee 150 Continent (= Water Distance from Continent (geatle gradient). c) The positive/negative “edge effect,” due to the summing of contributions that two the for km seaward times (h,,) 8.33a Fig. g/cm).

and 4 4 aa ] +150 in gravity equations 31.84 = deepening water depth
Gravity ‘ He okm 3 , Camu) Arouroany AyAe15 = g s (h,). —1.64 =

—_ ee two the water of the 9, —
Chapter8 Water Contribution The water effect is shallow, causing an abrupt change deeper effect, giving a less abrupt change Solving The product the (Ap py =

252 a 8.33 is a
i FIGURE margin gradients.