19 & 20 Divergent and Transform Plate Boundaries

Transcription

1 19 & 20 Divergent and Transform Plate Boundaries

2 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

3 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

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

5 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

6 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

7 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.

8 At ridge crest, only basalts - no ocean sediments Fissures Pillow basalt

9 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

10 Cross section of mid-oceanic ridge

11 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)

12 Fig Major units in an ophiolite sequence

13 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)

14 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

15 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

16 Fig Basin & Range Province

17 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

18 East African Rift

19 The Red Sea Rift Transitioning from cont. to oceanic rift

20 Chpt 20 Transform Plate Boundaries - oceanic transform boundaries - continental transform boundaries

21 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

22 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

23 Fig Major plate boundaries

24 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

25 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

26 Fig Topography along transform boundaries

27 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

28 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

29 Romanche fracture zone km long, 100 km wide

30 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)

31 Fig a. Model transform fault zone

32 Fig b. Landforms along the San Andreas

33 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

34 Fig The San Andreas transform fault system

35 End of Chapters 19 & 20

36 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

37 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

38 Fig Thermal structure