Unsolved Questions in the Study of Passive Margins
51
2.4 Unsolved Questions in the Study of Passive Margins
The unraveling of the exact origin and evolution of each stretch of passive margin
poses its own problems. The commonly used analogy of the evolution of rifting -
from the East African Rift to the Red Sea, to the Gulf of California, and finally the
Atlantic - provides guidance as to which processes may be at work. Was there
stretching before, and erosion of a crustal bulge at the site of future rifting? How wide
was the original rift valley? How does the sinking of the outer parts of the continental
edge affect landward crustal blocks? What are the rates of uplift and subsidence, and
of erosion and sedimentation in time and space? With regard to the history of subsidence, what is the relative role of "floating in the mantle" (isostatic equilibrium) of
the blocks, versus gravitational sliding? What are the forces provoking the very long
lasting uplift of certain parts of the continental margin (as off South Africa), and the
formation of long deep-seated barrier ridges along some margins? What is the significance of the lack of sediments of a certain age, in many margins? Was the lack
caused by erosion? By nondeposition? By huge landslides?
One question of fundamental interest is whether and where the thick sediment
stacks piling up on the passive margins will eventually be found in the geologic
record, on land. After all, the Atlantic cannot just go on rifting apart - sooner or later
it runs out of space. One suggestion by J. T. Wilson is that a proto-Atlantic once was
formed by rifting, and then closed again, running the previously passive margins into
each other. The presumed product of this process: the chain of mountains from
Norway through Scotland through Newfoundland and to the Appalachians. Check
their positions on the "Bullard Fit" (Fig. 1.19) - Wilson's suggestion makes good
sense.
If Wilson's hypothesis is correct, the passive margins would have turned into
active margins when colliding with the trench that must have been there to make the
proto-Atlantic disappear. What does such a collision margin look like? Would we be
able to go to the mountains to check for the signs of collision?
To answer this question we must study the Pacific-type margins, that is, the collision margins.
2.5 Pacific-Type (= Active) Margins
We have previously alluded to the collision of a continent with a trench, focusing on
the evidence for subduction (Sect. 1.5). Actually, there are at least three types of
collision margins which we need to consider: those produced by continent-continent
collision as in the Himalayas, by continent-ocean collision as at the Peru-ChileTrench, with a shallowly dipping subduction zone (Fig. 2.7a) and those where the
subduction takes place along island arcs, as along the Marianas, for example (Fig.
2.7b). This type has a deeply dipping subduction zone.
Perhaps the most important characteristics of collision margins are the folding and
shearing of sediments, and especially the addition of volcanic and plutonic material,
from the active vents sitting on top of the downgoing lithosphere. Also, the fractio-
51
2.4 Unsolved Questions in the Study of Passive Margins
The unraveling of the exact origin and evolution of each stretch of passive margin
poses its own problems. The commonly used analogy of the evolution of rifting -
from the East African Rift to the Red Sea, to the Gulf of California, and finally the
Atlantic - provides guidance as to which processes may be at work. Was there
stretching before, and erosion of a crustal bulge at the site of future rifting? How wide
was the original rift valley? How does the sinking of the outer parts of the continental
edge affect landward crustal blocks? What are the rates of uplift and subsidence, and
of erosion and sedimentation in time and space? With regard to the history of subsidence, what is the relative role of "floating in the mantle" (isostatic equilibrium) of
the blocks, versus gravitational sliding? What are the forces provoking the very long
lasting uplift of certain parts of the continental margin (as off South Africa), and the
formation of long deep-seated barrier ridges along some margins? What is the significance of the lack of sediments of a certain age, in many margins? Was the lack
caused by erosion? By nondeposition? By huge landslides?
One question of fundamental interest is whether and where the thick sediment
stacks piling up on the passive margins will eventually be found in the geologic
record, on land. After all, the Atlantic cannot just go on rifting apart - sooner or later
it runs out of space. One suggestion by J. T. Wilson is that a proto-Atlantic once was
formed by rifting, and then closed again, running the previously passive margins into
each other. The presumed product of this process: the chain of mountains from
Norway through Scotland through Newfoundland and to the Appalachians. Check
their positions on the "Bullard Fit" (Fig. 1.19) - Wilson's suggestion makes good
sense.
If Wilson's hypothesis is correct, the passive margins would have turned into
active margins when colliding with the trench that must have been there to make the
proto-Atlantic disappear. What does such a collision margin look like? Would we be
able to go to the mountains to check for the signs of collision?
To answer this question we must study the Pacific-type margins, that is, the collision margins.
2.5 Pacific-Type (= Active) Margins
We have previously alluded to the collision of a continent with a trench, focusing on
the evidence for subduction (Sect. 1.5). Actually, there are at least three types of
collision margins which we need to consider: those produced by continent-continent
collision as in the Himalayas, by continent-ocean collision as at the Peru-ChileTrench, with a shallowly dipping subduction zone (Fig. 2.7a) and those where the
subduction takes place along island arcs, as along the Marianas, for example (Fig.
2.7b). This type has a deeply dipping subduction zone.
Perhaps the most important characteristics of collision margins are the folding and
shearing of sediments, and especially the addition of volcanic and plutonic material,
from the active vents sitting on top of the downgoing lithosphere. Also, the fractio-