OC EAN OLOGICA ACTA. 1981. N° SP E:al---- The mam• types • • of passive marglns • an introduction O. Eldholm ' , L. Montader! b Department of Geology, Uni versity of Oslo, Norway. b Instit ut Français du Pétrole, Rueil-Malmaison, France. The knowledge about the spatial and temporal evoJution of a series of events associated with the break-up of the conti­ passive continental margin has increased considerably ne ntal crust and how the original structures are bei ng during the Ja st few yeaTs refleeling a progressive]y intensi· preserved as the margin matures. fied rcsearch effort. This activity has been encouraged both Reviewing the investigations on the passive margins Il 15 by the deve lopment of new geophysical techniques and the probably fair to state that we are now entering a "second reatisati on thal rock units containing major accumulations generation" of margin research. Prior 10 1975 most investi­ of hydrocarbons might li e beneath the passive margins. gations were of an exploratory nature relying on conventio­ The passive margins have formed as a pari of the fundamen­ nal geophysical techniques and sampling of the near surface tal geodynamÎc cycle whi ch indudes the break-up of the rocks (Fig. 2). The main limitation was the shallow penetra­ continental crust and the subsequent formation of a deep tion obtained by the o ne-channel seismic profiler. However . ocean. These major evoluti onary stages are oh en referred these surveys have yielded crucial information both in terms to by the key words rifting and drihing representing phases of the structural and depositio nal framework of many of continental extensio n and sea floor spreading respecti­ margins. Particularly , detailed surveys combined with an vely. Relati vely simple tectonic models have normatly been integrated approach of Interpretation of the vario us kinds of chosen to depict the passive margin development (Fig. 1). geophysical data have proved most useful. The typical tectonic sequence consists of : extension, rift At present, ne w technology coupled with a series of formation, iniliati on of sea floor spreading and maturation experiments designed to develop geological models descri­ by subsidence and sedimentation. Recent results, however, bing the present and past history of the ma rgîn characterize indicate that the margins oft en exhibit complex structural what we consider the "second generation" of research. A patterns revealing a great variety in structural style as weil consequence of the more sophisticated techniques is that as depositional regime. Thus, at the present state of the art it the surveying and data processing have become increasingly appears difficuJt to synthesize the available information in to expensive requir ing skillful design and execution of the one or a few type sections. Sorne of the fundamental experiments. questions relate to how the change from continental to oceanic crust takes place along the margin. A major objective is Iherefore 10 obtain a better unôerslanding of the 1910 M"". ·.. .. ,on .... " .... " -. ..'"..... '1 M.'... ' ...... , , ........ ,.. ,... ,.. ~ ..... " ."... .. --y ...-.... f loo:::::~ ....'" :: :' '' '''''-f~~===~~~..~' ~'''~'"~~~·"'1 1 ~ ~.. ~ ~ :~: :: 1 1 -'... ..... "'''''''.' }_. "1 • • ' .... - Figure t 1980 Schema/ic mode/ of el·olu/ion of Il ril/t!d p (.l ssi~e con/inen/al Figure 2 margi" (Curray. 1980). Melhods of pussive murgin i n ~ts/igtl/ions. 7 o ElDHOlM. l. MONTADEAT Some of the new experimenh are (Fig. 2) : dri1Jing program with riser capability designed 10 dnll deep _ mappi"g of the sea floor by the "sea beam" wide cone holes at the continental mllrgin~ is ptanned to Slart in 1983. ft echosounder. allows mapping of a swath of ocean floor ln order 10 develop type sections and compil e the multitude usinK a wide cone signal and computerised dam reduction. of observatio ns wc ncecl to develop a method of pa!>sive Many continental !>Iopes are melnsec ted by deep canyons. margin classification. The dissimilarity and com plellÎIY of Detailed bathymetric mapping as a precursor to sampling . the margins has lead 10 dirrercnl schemes of classificatiOns for example by submersibles. may be li valu:lble comple­ (Table). Here . we pre~enl classifications that are uscfuJ ment 10 drilling ; when IInalyzing Ihe geologic history in space and time . _ the multi-channel seismic rdleClion lechnique originally de'<eloped by the petroleum industry is now actively used lilso by the research inslitutions. Ils main advantaKc is increased penetration although a reliable definition o f the Physiography enlire basement surface i5 slill iacking acro!>s mo!>t ma rKin!> . On the other hand , the large penetration Ki"'e<; im portant The bathymetric map of the world rc veals pronounced constrainh on magnelic. Kravimetric and thermal modeling. differences in the physiogmph ic character of the vllrious We expeçt tha! the depth resolulion will gradually improve passive margins. The depth and wid th of the shelf. the as one introduces bigger energy ~ourçes .md gains expc­ Sleepness a nd width of the slope and Ihe existence of a rienee in processing and dis play of the data from deep and continental fi se are ail refltures that Kive each ma rgin it s intermediale water depths ; characteristic physiographie signature. As the two end - relatively little is known about the nature of Ihe deepe ~t members there are the narrow margin with a typical slcep part of the Ihick sedimentary sequence al mosl margins. slopc: and the wide marKin wilh Il gcntle slope. A separate Furthermore , the understanding of how the physical proper­ classification is applied where there is a marginal plateau. In ties of the underlying crust and upper manlle change Ilcross man y areas there is evidence that the physiography directly the margin is also limitecl . However. new refraction and reflects thc structural development. The margin off sou­ wide-.mgle refleçlion expc:ri menls are now being developc:d the rn Sri Lanka and thc Bay of Bengal are probably the to oblain additional information. Experiments of this kind mOS I weil defined examples of the two types of contrasting mOS I often require IWO, even three. ships. Particularly physiography. whereas Ihe Blake Plateau and the Vming promisi ng are ellperiments deploying long-range sonobuoys Plateau are typical marginal platcau~ . and ocean bottom seismometers together with expancl ing spread profiling and constant orfsel profiling. Expanded spread profiling is a Iwo-ship technique which increases Ihe crreclive array lenglh o f the common-depth-point melhod A,. enabling increased preciSion in determining a velocily-depth curve. whcreas conSlant orrsel profiling >lllow~ m:.ppinlo( of The. ::lige. classificaTion in The Taille. relate~ to Ihe ai:e of the deep interfaces by recording arrivaIs at the critical di!>­ oldest oceanic crUSI next 10 the margin. In other words_ the lances; lime when sea floor spreacling starded in the young imma­ _ The increased accuracy in na vigation has m~lde il pos~ i ­ ture ocean. Genetically. the margin may be considered older ble to carry out reHable closely-spaced aeromagnetic sur­ because the sta r! of sea floor spreadinK is il relntively late veys covering lurge arcliS . By fl yi ng Olt very low altitudes stage in the geologic cycle forming a margin. For simplicity. the data are almost comparable wilh shipborne dat:l. Air­ the margins arc classified as young. intcrmedilltc (lnd borne measurements of the gravit y fiel d arc abo being ma ture. The Red Sell ancl Gulf of California are type devetoped although liule actual §urveying has yet been examples of a young marKin. the margin off southern carned out. Mea!>urcmcnb of thL) kind will he panicularly Auslralia and those in the northern North Atlantic ,Ire important over high latitude ice-covered margins ; intermediate and the Easl Coast of the United States typifies an old margin. _ since 1968 the Deep Sea Drilling Project has col1ected core samples from many margins. Due to !>afety con)idenl­ The age classification is not 10 be confu5ed with Ihal of lions and li mited drilling capability ail drill site) have Emery (1980) who has used cmUÎmwus UÎsmic profiles to lerminatecl at retatively !>hallow depths. Deeper holes 3re form a geneti c classification which applies 10 ail of Ihe needed to crfectively correlale the geophysical dala. A new world -!> margins. Table S.·hrmes of pass;"e margill c/aslffkalioll. Co nljn ou~ ~ti~mic Clas5iticaTion Ph )'slography A,. rdletl101\ STru cturai O.. erburden Pre-rift parllmeler profile. frllmework. Io:eolol)' (Emer)'_ 19801 .. , Narrow Young Ini liul Ritled Slan'cd Craton member (s teep slope) M:ugin Marginal Intermediate Young Shearo:-:d·rifted Type plateau Maluro:-: .. , Wide Malure Dtd age Shearcd Sedimen1 t>asin Major b:')1Il member (gtntlt stope) 8 THE MAIN TYPES OF PA SSIVE MARGlNS : AN INTRODUCTION Structural frllmework ces of sediments may be classified as sediment basi n margins. This classification is based on the structural evolution and ln a discussion of the various margi n Iypes one has to make rerers basicalJy to the geomelry of the original plate assumptions about the nature of the ocean·eontinent boun­ boundary al the lime of initiation of sea fl oor spreading. dury. This boundary has been visualized both as a sharp Consequently, the transfonn taults or offselS at the plate crustal boundary and a transitional area between the Iypica! boundary will be reflecled in the structural fundament of the oeeanic and continental crust. Recent investigations imply margin and the margin may be divided into segments that although there mi ght be a \/ariety in slyles of the represenli ng shear and eXlensional movements, the sheared continent-ocean transition it appears that the change in the and rifted margin segments respectively (Fig. 3). In Ihe case two type s of basemen!, that is the surface or the uppermost of microcontinents a system of rifted and sheared margins kil ometers of Ihe basement, occurs quite rapidly in many will derine the extent of the continental fragment.