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CONTINENTAL RIFTS AND RIFTED MARGINS  159



                 NW                                                                            SE
             (a)  SP11  SP12    SP13         SP14         SP15/25    SP16     SP17            SP18
                                                              3.3
                                     5.0                                      5.2
                0                                        4.8    6.13
                             6.10                        6.08  6.51          6.07
                                                                             6.15
               10                                        6.24  6.52  6.23       6.31
                             6.16
                                                            6.54                6.38
                               6.33
               20
              Depth (km)  30  6.64  6.40  7.38             7.70 6.73   8.05     6.63
                                                            6.65
                             6.82
                                                                                6.83
               40
               50
               60
                  0        50       100       150       200       250      300       350
                                                    Distance (km)



                             Rift volcanic and
                (b)          sedimentary rock      Rift valley


                    0
                         Pre-rift basalt and
                   10    sedimentary rock
                  Depth (km)  20                  Mafic intrusion Sills

                                                     Lower crustal reflectors
                   30
                             High velocity lower crust
                                                 ?  Hot mantle         Moho
                   40         Igneous underplate                      Normal mantle
                                   Moho
                   50

                       0      50     100     150     200     250    300     350
                                                 Distance (km)

            Figure 7.5  (a) P-wave velocity model and (b) interpretation of the Main Ethiopian Rift (after Mackenzie et al., 2005,

            with permission from Blackwell Publishing). Location of profile (B–B′) shown in Fig. 7.4c.



               4  High heat flow and low velocity, low density   as in East Africa, domal uplifts and
                 upper mantle. Heat fl ow measurements         pervasive volcanism result. Nevertheless,
                                   −2
                 averaging 70–90 mW m  and low seismic        there is a large degree of variability in
                 velocities in many rift basins suggest       temperature and volcanic activity among
                                             −1
                 temperature gradients (50–100°C km )         rifts. The Baikal Rift, for example, is much
                 that are higher than those in the adjacent   cooler. This rift displays low regional heat
                                                                              −2
                 rift fl anks and nearby cratons. Where the    flow of 40–60 mW m  (Lysack, 1992) and

                 asthenosphere is anomalously hot, such       lacks volcanic activity.
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