Continental Rifts Flashcards
1
Q
What is the 1st stage of the Wilson Cycle?
A
- Continental rifting
2
Q
Wilson Cycle
A
- Continental craton
- Narrow rift
- Seafloor spreading
- Subduction initiation
- Contracting ocean
- Continental collision and orogeny
3
Q
Passive Rifting
A
- Initial stretching, rifting, crustal subsidence, then magmatism
4
Q
Active Rifting
A
- Initial doming and magmatism, then rifting
- Magma thought to force rift open
5
Q
Failed Rift
A
Aulacogen
6
Q
General rifting initiation
A
- Doming
- Early volcanism, not likely linear
- Mercedes Benz configuration w/ 3 rift arms at 120 degrees, axial dykes
- RRR junction
- One arm likely fails (aulacogen), rift valley crust
7
Q
Examples of aulacogens
A
- North Sea, complex history triassic - tertiary
- S. Oklahoma, Paleozoic, prior to closing of proto-atlantic
- Benue Trough, Nigeria, Early Cretaceous, newly formed ocean closed again
8
Q
Types of Continental Rift
A
- Rift associated w/ continental break up
- Collisional Rift
9
Q
Rift associated w/ continental break up
A
- Site of continental splitting and development of new ocean
10
Q
Examples of rifting associated w/ continental breakup
A
- Red Sea
- East Africa Rift System
- Basin and Range province (Western U.S.)
11
Q
Red Sea
A
- Continental rift
- > 2000km long, 100-300km wide, fault scarps 3km high
- Opened in early tertiary
- Deepest part, seafloor magnetic anomalies back to 5Ma
- Afar TJ
12
Q
East Africa Rift System
A
- Continental rift
- 6000km long, 50km wide
- Active 25Ma to present
- Afar TJ
- Small central relative Gravity high, from cooled Mafic intrusions?
- Broad gravity low from thermal expansion, hot, low-density upper mantle, low density seds
- Volcanism along rift axis
13
Q
Afar TJ
A
- RRR TJ
- East Africa
- Continental breakup rifting
- Cradle of mankind
- Many fossils
- Rifts still active, may create/be new plates
- Extension, thinned continental crust, flood basalts (buried under volcanics and seds)
- Eventually oceanic-type basalts, new ocean crust
14
Q
Basin and Range province
A
- Western U.S.
- Continental rift
- 500-800km wide
- 250-300km extension since 16Ma
- Heterogeneous thinning of hot, weak, previously thickened lithosphere
15
Q
Collisional Rift
A
- Extensional at approximately right angles to collisional margin
16
Q
Examples of Collisional Rift
A
- Rhine Graben, North of Alps
- Baikal Rift, Siberia
17
Q
Rhine Graben
A
- North of Alps
- Rifting began 48Ma prior to 40Ma full alpine collision
- Several sequences of lava eruption and graben subsidence until 2-4 Ma
- Aulacogen
18
Q
Baikal Rift
A
- Siberia
- Related to Himalayan collision
- Squeezing in N-S causes extension in E-W, especially w/ the Western pinpoint that moves less
19
Q
Characteristics of continental rifts
A
- High heat flow
- Domal uplifts
- Rift valley flanked by normal faults
- Thin crust or lithosphere
- Significant volcanism, alkaline, heterogeneous
- Horsts and grabens
- Shallow extensional seismicity, normal faults
20
Q
Graben
A
- Down-dropped block
21
Q
Airy isostasy
A
- Moho rises to maintain isostatic equilibrium
22
Q
Extensional Rift Basins: Formation Models
A
- Uniform stretching model, McKenzie
- Shear zone model, Wernicke
23
Q
Uniform Stretching model
A
- Pure shear
- Symmetrical deformation
- Upper crust: brittle deformation on listric normal faults
- Lower crust and mantle: Symmetrical ductile deformation
