Geol 320, Week 6 Friday

Topics: Plate tectonics and continental margins, Passive Continental Margins

Book: Chap. 7, p.155-158

Plate tectonics generates three kinds of continental margins / interactions:

- continent passively linked to ocean crust: divergent margin

- continent moving relative to ocean crust: convergent margin

- continental margins moving relative to each other: collision margin

Divergent Margins:

The development of an ocean basin via the splitting of a continent and the evolution of passive continental margins on either side of the split.

In class, we did a series of diagrams showing the splitting of a continent and the development of margins on the split

Steps:

(1) Heating below continent causes local warming of the crust, lowering its density and causing uplift.

(2) Uplift causes fracturing in upper crust where crust is flexing upward

(3) Uplift deforms upper surface of continent so that Gravity pulls two halves of continent away from uplift area - develops tension across the uplift area.

(4) Tension causes faulting, vertical dropping of blocks of crust into the developing openning between the two halves of the continent that are pulling apart. A Grabben Valley develops.

(5) As the valley widens, more crust slides into the widening gap and the more movement there is along the faults dropping crustal blocks into the valley. The feature accumulates continental sediments (alluvial fans, lakes) and can have associated continental volcanism (magma coming up fault lines in the crust).

(6) When the base of the valley drops below sealevel, it may be invaded by the ocean, shift to marine sediments - shallow water environments. If are in a carbonate setting may develop reef complexes that grow upwards as the valley floor subsides (drops in elevation).

(7) The margins of the Grabben (or Rift) Valley begin to subside as they draw away from the hot spot heating the crust - they cool and become more dense, sinking.

(8) When the valley floor drops to around 2.5km below sealevel it reaches a level of isostatic equilibrium which allows ocean floor to begin developing from lavas rising along the rift. Sea floor spreading initiates and takes up the spreading between the two continental halves on either side of the rift.

(9) The continental margins continue subsiding as they spread apart and cool.

(10) The rate of subsidence slows down. So the early history of the continental margin is rapid thermal subsidence, and the later history is slower sediment loading subsidence.

(11) The development of the margin takes about 80 million years.

(13) The sedimentary signature of the passive continental margin is a lower (older) section which is continental followed by a continuous series of relatively shallow marine sediments. The overall sequence gets thicker as you go from the continent out onto the continental passive margin. Thickness is controlled by subsidence history.

Example: a cross section view of the U.S. east coast