19 & 20 Divergent and Transform Plate Boundaries

Chpt 19 Divergent Plate Boundaries

•  Zones where 2 lithospheric plates move apart – oceanic & continental

•  Both contain: long rift valleys, normal faults (tensional stress), basaltic volcanism

•  Rift valleys: linear down-dropped blocks with steep walls and flat floors

Oceanic: Mid-oceanic Ridges

•  Divergent margin in an ocean basin – Longest mountain chain on Earth – New oceanic crust created – **Most intense and continuous volcanism

on Earth** – New crust pushes old crust aside

• Spreading a few centimeters per year

Mid-ocean Ridge System Thickness of red line (ridge) is proportional to rate of spreading.

Ridge Topography

•  Most pronounced tectonic feature – 1500 km wide with peaks up to 3 km – Total length of 70,000 km – Broad, fractured swells broken into

segments by offsets (transform faults) – Mid-ocean ridge is broad ridge with

central rift valley

Ridge Topography

• Detailed topography – profile - depends on spreading rate: •  If slow (5 cm/yr): steep, rugged, prominent

rift valley. Ex.: Mid-Atlantic Ridge •  If fast (>9 cm/yr): ridge is smooth, gentle

slope, very small, narrow rift valley if at all. Ex.: East Pacific Rise

Ridge Topography •  Cooling & subsidence

– Hot rocks expand and are less dense than cooler rocks of same composition

– New crust cools as it spreads away from ridge • Cooling crustè more denseè subsides • Age correlates with water depth, ex. At ridge

crest, water depth is 2.5 km; where rks are 2 myo, depth is 3km; 20 myo, depth is 4 km; 50 myo, depth is 5km.

Pillow basalt

Fissures

At ridge crest, only basalts - no ocean

sediments

Seismicity

•  Very narrow belt of shallow focus earthquakes associated with ridge crest (< 10 km deep), low magnitude

•  Due to movement on normal faults at crest, in rifts, and intrusions of magma

•  Along ridge crest: High heat flow (10x the average ocean crust); low density rocks

Cross section of mid-oceanic ridge

Ophiolites

•  Assemblage of mafic igneous rocks representing remnants of former ocean crust

•  Fragments of oceanic crust when accreted onto continental crust (part of rock record) – Consist of 5 layers

• Marine Sediments • Pillow Basalts • Sheeted Dike Complex • Gabbro • Tectonites (upper mantle peridotite)

Fig. 19.12. Major units in an ophiolite sequence

Continental Rifting

•  When divergent plate margins develop in continents

•  Elongate depression bounded by normal faults – Lithosphere is deformed – Crust is arched, extended, & pulled apart – Normal faults produce down-dropped

grabens (rift valley)

Continental Rifting

•  Volcanism and sedimentation – Basaltic volcanism similar to oceanic

ridges but in addition, rhyolitic magma may be present formed by partial melting of crust

– Rift valleys can contain thick layers of sediments also; conglom., sdst, mudst, evaporites

– Ex.: Basin & Range, E. Africa Rift, Rea Sea Rift

Basin & Range •  Complex rift system that extends from

Mexico to Canada – Basins (valleys) and ranges formed by tilted

fault blocks – Heat flow is 3X normal; uplifted & stretched to

2x’s original width – Crust has thinned to 25 km from ~ 50 km – Recent and active volcanism – Slower spreading rate than mid-ocean ridges

Fig. 19.29. Basin & Range Province

East Africa •  East African rift system extends 3000

km from Ethiopia to Mozambique – Thinning crust, basaltic shield volcanoes, and

rhyolitic ash flows on steep sided volcanoes – Complex volcanism throughout – Large, freshwater lakes (Lake Tanganyika)

form in isolated down-dropped blocks – Several areas below sea level

East African Rift

The Red Sea Rift Transitioning from cont. to oceanic rift

Chpt 20 Transform Plate Boundaries

- oceanic transform boundaries - continental transform boundaries

Characteristics

•  Transform boundaries are strike-slip faults where 2 plates slide past each other –  Faults are nearly vertical & parallel to movement of

plate –  Transforms are perpendicular to mid-ocean ridges –  Plates move horizontally –  No lithosphere is created or consumed –  Most associated with divergent margins

Fracture Zones •  Oceanic transform boundaries are part

of fracture zones – Large scale features up to 10,000 km long – Generally very narrow, 10’s of km at

most, but contain numerous faults – Appear as faults offsetting oceanic ridges – Transform boundary is a small portion of

fracture zone

Fig. 20.1. Major plate boundaries

Fracture Zones

•  Fractures zones are some of the largest geologic features on Earth – Vertical relief may be as much as 6 km

( 19,600 ft.)- as tall as Rocky Mts – Oceanic fracture zones not subject to

significant erosion – On continents, erosion subdues surface

expression

Oceanic Transform Boundaries

•  Active displacement occurs only between ridge crests – Only region of fracture zone with

opposite plate motion – Remainder of fracture zone is inactive – Vertical relief, ridge & trough, due to age

of crust on opposite sides of boundary

Fig. 20.4. Topography along transform boundaries

Oceanic Transform Boundaries

•  No significant amount of lithosphere is created or destroyed along boundary – Shearing and deformation are

considerable – Little or no igneous activity – Ex.: Romanche Fracture Zone;

Clipperton Fracture Zone

Romanche Fracture Zone •  Extends over the entire width of the

Atlantic Ocean – Separates the African and S. American

plates – Active transform is ~ 600 km long – Fault system is 10’s of km wide

Romanche fracture zone- 5000 km long, 100 km wide

Continental Transform Faults

•  Not as common as oceanic transform faults •  Similar in structure to oceanic t. f.

– Strike-slip motion, shallow quakes, penetrate entire lithosphere, series of faults, no volcanism

•  Unique to continental t. f.: straight narrow valleys, linear ridges, offset stream drainage, sag ponds, hazards in populated areas – Ex.: San Andreas, N. Anatolian Flt (n.

Turkey)

Fig. 20.12a. Model transform fault zone

Fig. 20.12b. Landforms along the San Andreas

San Andreas System

•  Ridge-ridge system extending ~ 3000 km – including Gulf of California – System is composed of numerous faults – Accommodate motions of Pacific and N.

American plates – Earthquakes are shallow (8-15 km deep) – 30 my old with ~ 300 km of offset; San

Andreas is 1000 km long

Fig. 20.13. The San Andreas transform fault system

End of Chapters 19 & 20

Ridge Topography

•  Cooling & subsidence – Relationship between spreading rate and

ocean depth • Fast spreading rate causes ridge to inflate • Reduces ocean basin capacity • Drop in spreading rate from 6 to 2 cm/yr

would decrease sea level by 100 m

Transform Boundary Processes

•  Thermal relationships – At ridge-ridge transforms, cold crust is

opposite a hot ridge – Conductive heating effects the cold side -

causes bulging – significant topographic feature

– Faults are not usually one vertical plane but series of faults - braided system of strike-slip faults – ridges & valleys

Fig. 20.9. Thermal structure