46 Origin and Morphology of Ocean Margins
The initial phase of this process can be studied in the East African Rift Valley,
where the sea has not entered the rift. The pulling apart (and therefore thinning) of the
continental crust opens a window for mantle material (Fig. 2.3 a, b) intruding from
the asthenosphere. Heat flow further increases and there is a bulging upward from the
rising mantle material, much as at the Mid-Ocean Ridge. However, between the
separating edges of continental crust there is an increasing gap now. Pieces of the
continental crust break off along "listric faults". This process continues to thin and
strech the continental crust. ("Nonvolcanic" margin type as in the Northern Biscay or
at Georges Bank, see Fig. 2.4).
Also, the central graben receives large amounts of sediment from the wall-like
mountains surrounding it. These mountains - first uplifted by the rising mantle material - continue to rise as erosion unloads them. The central valley, which started as
1--50- 150km----,--t I ..... .---50 - 300+km - -..oj
A
B
I'
IOO~.
~<::,::: : • • •
~
~~·~I·~--~@r----~·~ I ·~~
Fig. 2.4 a~. a Comparison of typical structural elements of "volcanic" (A) and "nonvolcanic" (8)
continental margins. I Normal thickness oceanic crust; 2 seaward dipping units; 3 structural high in
continental crust. often occurring adjacent to 2; 6 thinned. subsided continental crust; 7 un stretched
continental crust. Parallel signatures Sediments; douhle line Moho. Mantle underneath. [J. C . Mutter et a. 1987 and I. C. Sibuet and Z. Mascle 1978 in European Science Foundation. Cosod II Report.
Strasbourg. 1987: 92.] b Rifted continental margins in the North Atlantic. "Volcanic" type (A) and
" nonvolcanic" type (8). Iceland as hot spot on the Mid-Atlantic Ridge. [R. S. White et al. . 1987,
Nature. 330: 439.] c Continuous seismic reflection profile across the "volcanic" type of a passive
continental margin, with proposed drill sites for deep-sea drilling and the final drill hole 642 .
(Voring plateau off Norway. see Fig. b). Dipping reflectors between horizons E and K. E Top of
Lower Eocene basalt flows; K base of seaward-dipping reflector sequences; M and 0 Tertiary
unconformities (M Middle/Upper Miocene , 0 Middle Oligocene). [Data from K. Hinz in O. Eldholm et aI. , Proc. ODP Initial Repts. 104, 12.]
The initial phase of this process can be studied in the East African Rift Valley,
where the sea has not entered the rift. The pulling apart (and therefore thinning) of the
continental crust opens a window for mantle material (Fig. 2.3 a, b) intruding from
the asthenosphere. Heat flow further increases and there is a bulging upward from the
rising mantle material, much as at the Mid-Ocean Ridge. However, between the
separating edges of continental crust there is an increasing gap now. Pieces of the
continental crust break off along "listric faults". This process continues to thin and
strech the continental crust. ("Nonvolcanic" margin type as in the Northern Biscay or
at Georges Bank, see Fig. 2.4).
Also, the central graben receives large amounts of sediment from the wall-like
mountains surrounding it. These mountains - first uplifted by the rising mantle material - continue to rise as erosion unloads them. The central valley, which started as
1--50- 150km----,--t I ..... .---50 - 300+km - -..oj
A
B
I'
IOO~.
~<::,::: : • • •
~
~~·~I·~--~@r----~·~ I ·~~
Fig. 2.4 a~. a Comparison of typical structural elements of "volcanic" (A) and "nonvolcanic" (8)
continental margins. I Normal thickness oceanic crust; 2 seaward dipping units; 3 structural high in
continental crust. often occurring adjacent to 2; 6 thinned. subsided continental crust; 7 un stretched
continental crust. Parallel signatures Sediments; douhle line Moho. Mantle underneath. [J. C . Mutter et a. 1987 and I. C. Sibuet and Z. Mascle 1978 in European Science Foundation. Cosod II Report.
Strasbourg. 1987: 92.] b Rifted continental margins in the North Atlantic. "Volcanic" type (A) and
" nonvolcanic" type (8). Iceland as hot spot on the Mid-Atlantic Ridge. [R. S. White et al. . 1987,
Nature. 330: 439.] c Continuous seismic reflection profile across the "volcanic" type of a passive
continental margin, with proposed drill sites for deep-sea drilling and the final drill hole 642 .
(Voring plateau off Norway. see Fig. b). Dipping reflectors between horizons E and K. E Top of
Lower Eocene basalt flows; K base of seaward-dipping reflector sequences; M and 0 Tertiary
unconformities (M Middle/Upper Miocene , 0 Middle Oligocene). [Data from K. Hinz in O. Eldholm et aI. , Proc. ODP Initial Repts. 104, 12.]