Ocean basins 3

3. Ocean basinsTheir structure and evolution1

Evolution of ocean basinsPlate tectonics I• Determination of plate motions as part of thestudy of ocean basins2

Continental and oceanic lithosphere• The lithosphere (rock sphere) issolid and includes the crust andthe uppermost rigid mantle• The asthenosphere (fluid sphere)is the underneath the lithosphereand consists of the convectingmantle• Plates of lithosphere float on topof the asthenosphere3

The plates of the EarthPlate tectonics - simplification• The surface of the Earth isdivided into several plates• The plates are rigid• This allows us to describeplate motions mathematicallyThis Dynamic Earth, USGS 4

The most common plate boundaries• Mid ocean ridges (divergentplate boundaries)• Ocean-continent subductionzones (convergent plateboundaries)5

Types of plate boundaries• Divergent or “constructive”– mid ocean ridge spreading centers– continental rift systems• Convergent or “destructive”– ocean-continent subduction zones– ocean-ocean subduction zones– continent-continent collision zones• Transform or “conservative”– Transform faults6

Types of plate boundaries7

Plate boundariesSan Andreas faultWest Coast USThis Dynamic Earth, USGS 8

Magnetic anomaliesVolcano World webpage 10

Magnetic time scale• Magnetic time scale based on polarities (normal, reversal) has been establishedback to about 170 Ma• Mesozoic lineations are M0 – M27 (120 – 160 Ma)• Late Cretaceous and Cenozoic lineations to the present are 1 – 34 (0 - 120 Ma)Note: Magnetic stripes have been observed on Mars. What is your interpretation?This suggests that many of the sameforces that continue to form Earth'stopography today have shaped Mars inthe distant past. Unlike the Earth,Mars's plate tectonic system appears tohave faded about four billion years ago11

Plate motions from magnetic anomaliesFowler, Fig. 3.11 12

Transform fault azimuthsFowler, Fig. 2-9 13

Geometry of motion on a sphere; At point X, V=ωR sinΘFowler, Fig. 2.8 14

Earthquake slip vectors: focal mechanism15

Motions and earthquakesat plate boundariesPress and Siever "UnderstandingEarth", 18.16 16

Depth of earthquakes17

Hot spot model• Age and geometry suggest thatisland chains are created by a hotspot that remains stationaryunderneath the plate while theplate passes abovemelt region19

Age along seamount chains• There is an age progression alongseamount chains• The age progression correspondsto the velocity of the oceanic plateaway from the ridge20

Hot spot distributionThis Dynamic Earth, USGS 21

Direct measurementsNASA Goddard Space FlightCenter 22

Direct measurements - exampleFowler, Figure 2-10 23

Relative plate motionsPress and Siever, Figure 20.12 24

Absolute plate motionsfrom very long baseline interferometryNOVEL1A-NNR reference frameNASA Goddard Space Flight 25

For next time• Read Chapter 3 in The Ocean Basins:Their structure and evolution.• Check outhttp://pubs.usgs.gov/publications/text/dynamic.html if you haven’t done it already28

Plate tectonics II• Changes in plate motions• The Wilson Cycle• The evolution of ocean basins29

Changes in plate motions• New rifting in one place requires additional subduction inother places because the Earth size is not changing• Cessation of spreading or subduction of a ridge will decreasesubduction/collision elsewhere• Subduction can lead to new rifting in back arc setting andcontinental setting• Boundaries and poles of rotation are constantly changing30

Some obvious signs for past changes• The bend in the Hawaii-Emperor seamount chain• Direction changes of magnetic lineations and fracturezones for example in the Norwegian-Greenland Sea• Direction changes of magnetic lineations and fracturezones for example in the Arctic Ocean31

The bend in the Hawaii-Emperorseamount chainOpen University, 1989 32

Change in the Norwegian-Greenland Seaspreading• First spreading on the AegirRidge (24a-13)• Jump from the Aegir Ridgeto the Kolbensey Ridge at13• Reorganization of the JanMayen Fracture ZoneBerndt et al., 2001 33

Changes in Norwegian-Greenland SeaspreadingBerndt et al., 2001 34

Evolution of the San AndreasTransform FaultObservations:1. San Andreas Fault has increased in lengthas two triple junctions have moved apart.2. Southwestern margin of the NorthAmerican Plate has progressively changedfrom a trench boundary (with NorthAmerica the continental and therefor overridingplate) into transform fault plateboundary.3. Change from trench to transform hasproduced progressive extinguishing ofvolcanic arcs (Cascades and MiddleAmerica arcs).The Solid Earth, Fowler 35

16 typesThe Solid Earth, Fowler 37

Present day subduction of a spreading centerUSGS 38

The evolution of ocean basins40

The age of the oceansMuller et al., 1997 41

Atlantic Ocean• Rifting:– Central Atlantic 160 Ma– South Atlantic 110 Ma– NE Atlantic 55 Ma• Reorientations at– 55 Ma Labrador - NEAtlantic (perhaps due tohotspot impingement)– 35 Ma Aegir Ridge -Kolbensey Ridge42

Pacific Ocean• Complicated to reconstructbecause older than theAtlantic and subduction ofocean basins and spreadingcentres43

Indian Ocean• Abandoned ridges in theeastern and western parts ofthe Indian Ocean• Age increase to both sidesof the 90E Ridge withdifferent orientation thanpresent spreading44

Magnetic anomaliesVolcano World webpage 45

Magnetic anomalies- Pacific OceanLate Jurassic/earlyCretaceous (110-150 Ma)are clues to earlierconfiguration andorientation of the PacificPlateThe Solid Earth, Fowler 46

Magnetic anomalies- Eastern PacificGalapagos rift propagated from East to WestThe Solid Earth, Fowler 47

Magnetic anomalies- East Indian Ocean• Spreading betweenAustralia and Antarcticawas the last to commence inthe former Pangeasupercontinent• Spreading on an now extinctspreading center whichmoved India northwardThe Solid Earth, Fowler 48

The Wilson Cycle• Describes the life cycle of oceanic basins (crust) and points tothe importance of different kinds of continental margins.Named after J. Tuzo Wilson, who explained transform faults and proposed hot spots50

Ancient Oceans• The Wilson Cycle explainswhy there is no “old”oceanic crust and why it isso difficult to reconstructthe plate configurationmore than 200 Ma ago.• Older reconstructions areprimarily based onpaleomagnetism anddetailed investigations ofthe continental margins51

Plume-relateduplift• depends on:– lateral lithospheric rheology andthickness variations– buoyancy of the plume• numerical modeling predictions:– up to 1000 m of uplift forcontinents– and even more for the oceanicdomainGriffith and Campbell, 1991 52

Passive or active riftingdepends on melting process in the mantle• Solidus andliquidustemperatures• Active andpassive rifting53

Passive rifting54

Passive rifting55

Active rifting56

Active rifting57

Stages of ocean evolution1 - Embryonic Motions: crustal extension and uplift; Example: East AfricanRift Valleys2 - Young Motions: uplift,subsidence (margin starting to subside), and spreadingExamples: Red Sea, Gulf of California3 ……….4 ……….5 - Terminal Motions: compression and uplift; Example: Mediterranean Sea6 - Relict Scar Motions: compression and uplift; Example: the Indus suture inthe Himalayas58

Remember• Sequences of magnetic stripe anomalies :- Cenozoic 1-33- Mesozoic M0-M25 (ca. 108-153 Ma)• Over half of the ocean was formed less than 76Ma (based onmagnetic stripe distributions)• The oldest deep oceanic crust is late Jurassic (approx. 170 Ma)• The present oceans has been created during only 5% of theearth geological history.59

For next timeLook at animations at:http://www.es.usyd.edu.au/geology/people/staff/dietmar/Movies/pangaea.htmlhttp://www.ig.utexas.edu/research/projects/plates/plates.htmhttp://www.geol.ucsb.edu/~atwater/Animations/http://www.odsn.de/odsn/services/paleomap/animation.html60

Plate Tectonics III• The birth of an ocean• Continental margins• Mid-Ocean Ridges and oceanic crust• Hydrothermal circulation61

The birth of an ocean1. The process begins withdoming and uplift over a broadarea.2. Then the stretched and thinnedcrust subsides, eventually to befilled with seawater.3. It is followed by seafloorspreading and the creation ofoceanic crust.Open University, 1989 62

Wilson Cycle Stage A - stable continent63

Wilson Cycle Stage B – continentalriftingB64

Embryonic stage - East African rift• Wilson cycle B !• Volcanism started ca. 25 MaThis Dynamic Earth, USGS 65

Northern Red Sea• Failed rift in the Gulf ofSuez• Gulf of Aqaba transform66

Wilson Cycle Stage C - continentalbreak-up initiation of sea floorspreadingC67

A young ocean – The Red Sea• Wilson cycle C• Stretching started ~35 Ma (Oligocene)• Rifting and volcanism began ca. 25-20 Ma(early Miocene)• Proper spreading phase started ca. 5 Ma inthe south - not yet fully developed in thenorth• Dark blue - axial zone, depths greater thanabout 1000 m• The pole of rotation is located to the north-therefore the spreading rate is greatest inthe southern part of the Red Sea and thisarea has opened the mostOpen University, 1989 68

Red Sea magnetic anomalies• Well developed magnetic stripes(and thus oceanic crust) is onlyevident in the southern axial zone• Average spreading rate (full) 1.5cm/yr over the past 4.5 MaOpen University, 1989 69

Red Sea - crustal architecture• Evaporite deposition occurred in the Miocene (20 Ma to 5 Ma) as the oceanconnection (mostly with the Mediterranean at this time) was intermittentlyopened and closed.• These salt deposits are 4 km thick in placesOpen University, 1989 70

Red SeasedimentsBrines and metallic depositsKennett, 1989 71

An old ocean - the Mediterranean Sea• Ocean crust is largely of Cretaceous age, with some mid-Tertiary ocean crust (asyoung as 25 Ma) south of Italy.• The Mediterranean, like the Red Sea, also has thick evaporite sequences of lateMiocene age, but for different reasons.The Mediterranean salinity crisis:Connection to other oceans became restricted ca. 20 Ma due to the ongoingcollisionEvaporite sequences are 1 km thick and more in places, deposited ca. 5 Ma.It represents multiple, nearly complete evaporations of Mediterranean over a~700,000 year period72