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265
       Expressions
       Gravity
       their
       and
       Settings
       Tectonic
                          “8
                         Gravity Anomalies
    ee ATT
           Gravity  expression  of     Local  isostatic  model,   8.42  FIGURE  showing  free  air  and  Bouguer  gravity  Contributions   model  are  (see  anomalies  at  a  continental  rift  zone.   Topography   ——   for  +2.67  Density  contrasts,  in  g/cm’,  are:   Moho   wemeee   the   0   =   to  air;  relative   level  mass  above  sea   Lith/Asth   sun     relative  lithosphere  mantle  +0.4  for  the   km   33   =   for  the  —0.04  lower  crust;  and   the   to  thickness
           8.41  normal  continental  lithosphere.   Parameters  for  the.   height  of  topography   thickness  of  crust   lithosphere   air  and   reality,  the   few   a  hundred  level,  resulting  in  slightly  negative  Bouguer  anomalies).   of  compensation   the   figures.   the   crust   an   as   in   and,   the   positive   +0.4   far   result   crust   compensatory   broader   anomaly   effect   the   Bouguer   States   8.42.  The   indicated   (Fig.   partial   that
           FIGURE   Fig.  8.40):   =   ‘-h,   =   h,   =   h,,   The   configuration   free   (In   tens  to   throughout   next   few   both   8.42),   (Fig.   viewed   be   can   elevated   mantle   contrast  across   far   the   than   less   =   (Ap  Moho   thus   is   km)   (130   km).   thin   the  beneath   The  gradient.   much   a  giving  effect,   gravity   air   lows.  When   removed,   the   United  western   Fig.   in  shown   as  lithosphere,   anomalies  Bouguer   pre
               All  Contributions   0   =            Ee oe Bogue  7 Anomalles   =0           SS   ('Craton")   Moho       continental   rift   asthenosphere   of  weight   the   density  negative   is  g/cm?)  —0.04   and  g/cm*)   the   boundary   boundaries   (4.5   elevated  mantle   relatively   steep   deep   a   is   amplitude.   free   The   flanking   with   rift,   is  contribution)   asthenosphere.   the   in   gravity  anomalies   little  very   mantle   low   and   such








       Isostasy   ew             Level   Continental  Crust   =:     a   At   Elevated   =   +2.67   other   the   of   asthenosphere   the   elevated   Province   is   there   4.19a)   depth   flat


       and                                                           -   Rift   supporting   the   that   (Ap   =   (Ap  lithosphere/asthenosphere   and   full   over   (topographic   the   Range   Bouguer   that   4.15,   shallow   new,   a
       Gravity              +          :                               thin.   boundary   relief  of  the   contributions   elevated   reach   not   high   a   and   and   air   (Fig.   at   (Figs.  2.7,  2.24b).  The

       =   +200   4   g   &   al   =   -200   +200  +   0   z   -200  Topography  at  Sea   0   50   100   Continental   plate   buoyancy   Note   topography   the   topography   the   does   is   level   reflecting   Basin   free   shallow   so   velocities   establishing
       Chapter8                                                      2.   lithospheric   loon,”   its   topography.   asthenosphere   the   on   relief   combined   The   gravity   tive   of  deficit   that   low   three  effects)   above   sea   low  broad   The   of  pattern   is  sphere   seismic   asthenosphere   occurs   crust,


            Contributions   ——   Moho  asmees   wus   ——   sememe  “Bouguer
       264     Topograpty   LithVAsth   Gravity Anomalies   Free Alr   Continental   Craton   for   the        tal
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