Mid-Ocean Ridges

364 Pages·2013·41.92 MB·English

by  Searle R.

Mid-OceanRidges
Theworld’smid-oceanridgesformasingle,connectedglobalridgesystemthatispartof
everyocean,andisthelongestmountainrangeintheworld.Geologicallyactive,mid-ocean
ridgesarekeysitesoftectonicmovement,intimatelyinvolvedinseafloorspreading.This
coursebook presents a multi-disciplinary approach to the science of mid-ocean ridges –
essentialforacompleteunderstandingofglobaltectonicsandgeodynamics.Designedfor
graduate and advanced undergraduate students, it will also provide a valuable reference
forprofessionalsinrelevantfields.Backgroundchaptersprovideahistoricalintroduction
andanoverviewofresearchtechniques,andfollowingchapterscoverthestructureofthe
lithosphereandcrust,andvolcanic,tectonicandhydrothermalprocesses.Asummaryand
synthesischapterrecapsessentialpointstoconsolidatenewlearning.Accessibletostudents
and professionals working in marine geology, plate tectonics, geophysics, geodynamics,
volcanismandoceanography,thisistheidealintroductiontoakeyglobalphenomenon.
(cid:2)
Supportsstudentsandprofessionalsnewtotechnicalaspectsorgeographicareaswitha
fullglossaryandextensivedirectoryoffeaturenames.
(cid:2)
Avoidsjargonandfullyintroducesanddefinestechnicalconceptsandterms.
(cid:2)
Richlyillustrated,includingcolourfiguresandcomprehensivedatatables.
(cid:2)
Extensive references provide detailed starting points for further study, and a valuable
resourceforprofessionalresearchersfrommanydifferentfields.
RogerSearle is Emeritus Professor of Geophysics at Durham University. He has spent
40 years studying mid-ocean ridges, and was a pioneer in the use of side-scan sonar to
studytheirgeodynamic,tectonicandvolcanicprocesses.Inhisresearchhealsousestopo-
graphicanalysisandgravityandmagneticmodellingtounderstandridgestructures.Hewas
awardedtheRoyalAstronomicalSociety’sPriceMedalin2011andelectedaFellowofthe
AmericanGeophysicalUnionin2012.ProfessorSearlehasworkedinmanyoftheworld’s
major oceanographic institutions, participated in 37 research cruises and led 18. He was
firstfullchairmanoftheinternationalresearchorganisationInterRidge,andhasservedon
nationalandinternationalcommittees,includingchairingtheInternationalOceanDrilling
Program’sSiteSurveyPanel.
‘Thisvolumeprovidesacomprehensive,up-to-dateandauthoritativeaccount,extensively
illustratedandreferenced,ofthegeology,themorphology,thetectonicsandthechemistry
oftheridges,relatingthesetotheunderlyingmantlemovements.Italsodescribesindetail
thetechniquesusedinthesestudies.ProfessorSearlehasbeenattheforefrontofresearch
onthemid-oceanridgesthroughouthiscareer,andhasproducedanidealtextbookbothfor
studentsandthosecurrentlyresearchingthegeologyoftheoceanfloor.’
–SirAnthonyLaughton,FRS,formerlyDirectoroftheInstituteofOceanographic
Sciences,UK
‘ProfessorSearlehasdoneasuperbjobofsummarizingandanalyzingthehistoryof,and
the latest insights into, mid-ocean ridges, ranging from ultra-slow to fast spreading rates
andincludingthetectonics,geophysics,geochemistry,volcanismandhydrothermalactivity
ofthis“longestmountainrangeintheworld.”Thisisanessentialvolumeforanystudent
or researcher studying mid-ocean ridges, both those in the Earth sciences and those with
backgroundsinmarinebiology,chemistryoceanography,physicaloceanographyandother
relatedfields.’
–KenC.Macdonald,EmeritusProfessorofMarineGeophysics,Universityof
CaliforniaatSantaBarbara
Mid-Ocean Ridges
ROGER SEARLE
EmeritusProfessor,DepartmentofEarthSciences,
DurhamUniversity
UniversityPrintingHouse,CambridgeCB28BS,UnitedKingdom
PublishedintheUnitedStatesofAmericabyCambridgeUniversityPress,NewYork
CambridgeUniversityPressispartoftheUniversityofCambridge.
ItfurtherstheUniversity’smissionbydisseminatingknowledgeinthepursuitof
education,learningandresearchatthehighestinternationallevelsofexcellence.
www.cambridge.org
Informationonthistitle:www.cambridge.org/9781107017528
(cid:2)C RogerSearle2013
Thispublicationisincopyright.Subjecttostatutoryexception
andtotheprovisionsofrelevantcollectivelicensingagreements,
noreproductionofanypartmaytakeplacewithoutthewritten
permissionofCambridgeUniversityPress.
Firstpublished2013
PrintedintheUnitedKingdombyTJInternationalLtd.PadstowCornwall
AcataloguerecordforthispublicationisavailablefromtheBritishLibrary
LibraryofCongressCataloguinginPublicationdata
Searle,Roger,1944–
Mid-oceanridges/RogerSearle,EmeritusProfessor,DepartmentofEarthSciences,DurhamUniversity.
pages cm
Includesbibliographicalreferencesandindex.
ISBN978-1-107-01752-8(hardback)
1.Mid-oceanridges. 2.Platetectonics. 3.Seafloorspreading. I.Title.
QE511.7.S45 2013
551.1(cid:3)36–dc23 2013017281
ISBN978-1-107-01752-8Hardback
CambridgeUniversityPresshasnoresponsibilityforthepersistenceor
accuracyofURLsforexternalorthird-partyinternetwebsitesreferredto
inthispublication,anddoesnotguaranteethatanycontentonsuch
websitesis,orwillremain,accurateorappropriate.
Tomyfamily.
‘CouldthewatersoftheAtlanticbedrawnoffsoastoexposetoviewthisgreatsea-
gashwhichseparatescontinents,andextendsfromtheArctictotheAntarctic,itwould
presentascenethemostrugged,grandandimposing.Theveryribsofthesolidearth,
withthefoundationsofthesea,wouldbebroughttolight...’
MatthewFontaineMaury(1860)

Contents
Preface pagexi
1 Introduction 1
1.1 Theglobalmid-oceanridgesystem 1
1.2 ThediscoveryofMORs 3
1.3 Seafloorspreadingandplatetectonics 5
1.4 Oceanographicinstitutions 8
1.5 DedicatedMORresearchprogrammes 9
1.6 Outlineofthisbook 9
2 TechniquesofMORstudy:abriefhistoricalreview 11
2.1 Introduction 11
2.2 Depthmeasurement 11
2.3 Magneticfield 14
2.4 Gravity 18
2.5 Heatflow 21
2.6 Earthquakeseismology 22
2.7 Seismicrefraction 24
2.8 Seismicreflection 26
2.9 Compliance 29
2.10 Side-scansonar 29
2.11 Electricalmethods 31
2.12 Visualimaging 33
2.13 Sampling 35
2.14 Shipsandotherplatforms 36
2.15 Navigation 39
2.16 Summary 42
3 Theoceaniclithosphere 44
3.1 Crust,mantle,lithosphereandasthenosphere 44
3.2 Oceanicheatflowandthethermalstructureofthelithosphere 45
3.3 Thicknessoftheoceaniclithosphere 50
3.4 Flexureandelasticthickness 53
3.5 GravityoverMORs 54
3.6 Isostaticcompensation 57
3.7 Summary 59
vii
viii Contents
4 Ridgesasplateboundaries 61
4.1 Ridgesandplatekinematics 61
4.2 Seismicityandfocalmechanisms 63
4.3 Spreadingcentres 64
4.4 Transformfaultsandfracturezones 69
4.5 Ridgesegmentation 75
4.6 Thehierarchyofridgeaxisdiscontinuities 77
4.7 Triplejunctions 82
4.8 Propagatingrifts 84
4.9 Oceanicmicroplates 87
4.10 Summary 91
5 Crustalstructureandcomposition 92
5.1 Introduction 92
5.2 Crustalthickness 92
5.3 Seismologyandthelayeredmodel 96
5.4 Meltdistributionandmagmachambers 106
5.5 Shallowcrustalsampling 112
5.6 Deepsampling:oceandrilling 114
5.7 Ophiolites 119
5.8 Departuresfromthelayeredcrustmodel 120
5.9 Crustalmagnetisation 123
5.10 Summary 127
6 Volcanism 129
6.1 Introduction 129
6.2 Mantlemelting 129
6.3 Meltdeliverytothecrust 135
6.4 Lavamorphologies 135
6.5 Fast-spreadingridges 139
6.6 Intermediate-spreadingridges 145
6.7 Slow-spreadingridges 150
6.8 Ultra-slow-spreadingridges 158
6.9 Summary 162
7 Tectonism 163
7.1 Introduction 163
7.2 Fissures 163
7.3 Normalfaults 167
7.4 Detachmentfaultsandoceaniccorecomplexes 185
7.5 Ultra-slowspreading 192
7.6 Transformandstrike-slipfaults 194
7.7 Modellingfaulting 196
7.8 Summary 197