Continental Rifts

Question 1

What is the 1st stage of the Wilson Cycle?

Answer 1

• Continental rifting

Question 2

Wilson Cycle

Answer 2

• Continental craton
• Narrow rift
• Seafloor spreading
• Subduction initiation
• Contracting ocean
• Continental collision and orogeny

Question 3

Passive Rifting

Answer 3

• Initial stretching, rifting, crustal subsidence, then magmatism

Question 4

Active Rifting

Answer 4

• Initial doming and magmatism, then rifting

- Magma thought to force rift open

Question 5

Failed Rift

Answer 5

Aulacogen

Question 6

General rifting initiation

Answer 6

• 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

Question 7

Examples of aulacogens

Answer 7

• 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

Question 8

Types of Continental Rift

Answer 8

• Rift associated w/ continental break up

- Collisional Rift

Question 9

Rift associated w/ continental break up

Answer 9

• Site of continental splitting and development of new ocean

Question 10

Examples of rifting associated w/ continental breakup

Answer 10

• Red Sea
• East Africa Rift System
• Basin and Range province (Western U.S.)

Question 11

Red Sea

Answer 11

• 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

Question 12

East Africa Rift System

Answer 12

• 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

Question 13

Afar TJ

Answer 13

• 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

Question 14

Basin and Range province

Answer 14

• Western U.S.
• Continental rift
• 500-800km wide
• 250-300km extension since 16Ma
• Heterogeneous thinning of hot, weak, previously thickened lithosphere

Question 15

Collisional Rift

Answer 15

• Extensional at approximately right angles to collisional margin

Question 16

Examples of Collisional Rift

Answer 16

• Rhine Graben, North of Alps

- Baikal Rift, Siberia

Question 17

Rhine Graben

Answer 17

• 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

Question 18

Baikal Rift

Answer 18

• Siberia
• Related to Himalayan collision
• Squeezing in N-S causes extension in E-W, especially w/ the Western pinpoint that moves less

Question 19

Characteristics of continental rifts

Answer 19

• 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

Question 20

Graben

Answer 20

• Down-dropped block

Question 21

Airy isostasy

Answer 21

• Moho rises to maintain isostatic equilibrium

Question 22

Extensional Rift Basins: Formation Models

Answer 22

• Uniform stretching model, McKenzie

- Shear zone model, Wernicke

Question 23

Uniform Stretching model

Answer 23

• Pure shear
• Symmetrical deformation
• Upper crust: brittle deformation on listric normal faults
• Lower crust and mantle: Symmetrical ductile deformation