Proterozoic Tectonics and the Growth of Continents

I. Objectives

1. To examine the accreationary nature of continents.
2. To discuss evidence for a Proterozoic change in the style of plate tectonics and the resultant increased stability of the continents.

The Proterozoic represents some 42% of geologic time (~2.5 B.Y. to 570 m.y. ). During this interval, the character of plate tectonics is changed from that which we discussed for the Archean. The continents grew significantly and more closely approximated their present size. In class we will :
1. examine the growth of continents vs. the Archean
2. look in detail at the North American tectonic age provinces and discuss the processes involved in the assembly of the craton. Note that your book presents this subject in detail. I will illustrate and summarize main text conclusions on this subject.
3. examine the Proterozoic sedimentary record and interpret its paleoenvironmental significance.

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III. WHAT DO SEDIMENTARY ROCKS TELL US ABOUT PROTEROZOIC CONTINENTS?

• quartz-rich graywackes, arkoses, and pure quartz sandstones
• greater extent of textural maturity
• reworking
• sorting rounding
• high percentage of quartz
• successive cycles of weathering with enrichment of quartz over less stable feldspar and mafic minerals

• earliest significant amounts

• sandstone characteristics suggest
1. larger more stable continents
2. large broad, relatively flat shelves and basins

IV. PROTEROZOIC CONTINENTS

• The first continents exhibiting tectonic stability
• seen are large deposits of weakly deformed, unmetamorphosed Proterozoic sediments overlying deeply eroded basement of deformed and metamorphosed Archean rocks
• Two main types of terranes:
1. slightly deformed stable regions
2. highly deformed mobile areas
• Two general types of deformed belts:
1. multiple deformed regions which are rich in volcanic rocks
2. areas with thick linear belts of sedimentary sequences (once linear troughs, former continental margins).