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'
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. The areas (Fig. 4). Volcanic activity as reflected by basait flows margins of the Atlantic Ocean are dominated by rifted a nd layers of pyroclastiç muteriaJ has also been associated segments, however, sheared segments occur locally. The with the late phase of rifting and the early phase of sea floor margin across the Agulhas Fracture Zone south of South spreading. The observations imply a relatively complicated Africa and off the Barents Sea in the Norwegian Sea are sequence of break-up. volcanie activit y and sea floor Iypical examples of sheared margins. A third combined spreading during the rirst stage of formation of a Jeep sheared-rifted margin may oçcur when the pole of relative ocean. moti on has changed in such li way that the original trans form fault later became the locus of a new spreading axis. The most characteriSlic region lies between northeastern " Greenland and western Svalbard. ~~rç;y 1 '0 ..: ... \-==-~ ~._ ...... U -_0___ zo...... ,... 0 "ô;;o ...... -!.. • • • • Fi.llure" Crustal ser/ion aaoss /h t 'lOr/hem Biscay marC!" (dt' Charpal • CI al.. /978) •
A region of contaminated crust underlying the margins has • someti mes been proposed assum ing that dyke injection • and/or thermal alteration is responsibJe for the contllmina s :::,:::'7~ , . - """,,,. ,.... tion. - ""90°'" On the other hand, a relutively sharp transition bctween the ...... "'0<> two kinds of basement does not rule OUt a modified deeper Figure J crust and upper mantJe which have graduall y been transfor· med by mechanisms which have not yet been full y under Sktll'h Qf rif/cd and sheared st't(mems along a passi"e marRi". stood. The apparent isostatic equitibrium across mOSI pas· sive margins suggests that a pronounced modification in the physical properties of the lower crust and adjacent upper mantle must have occurred contemporaneously with the Pre-r ift geology process of maturation a nd subsidence of the continental margin. Several surveys have reveaJed that the structural style of Below the Jower part of many continental slopes there arc the margin is to some eX lent governed by the continental areas of reJ ati vely elevated basement. These structures geology prior to the rihing. One end member of this type have been called marginal highs or outer highs. The margi represents break-up of the continent in;1 craton with linle or nal highs are senerally of volcaniç na!Ure, where;.s the outer no sediment cover, the other bei ng break·up within a major highs have been proposed to reflect an elevated oUler edse shallow sedimentary basin of for ex,imple, the North Sca of the continental crus!. II is not known whether thcse highs type. Surprisingly linle attention has yel becn brought indeed represent a similar stage in the margin evoJuti on and towards the pre·rift georogic configuration, allhough this is if they exist along most margins. A classificati on and li parameter which may turn out (0 be fundamcntal in understanding o f these structures must be of high priority defining important events during the later margin develop for the research during the next JeClide. ment. We stated earlier that it appears difficult to develop typical crustal models across a passive marsin. However, if one uses both the structural framework and the pre-rift geologi cal configuration as main methods of class ifying the various types of passive margins we believe thal a series of type sections may represent the evolutionary hi slory of vllrious The amounl of overburden is in man y regions closely margin types. Wc stress that a better understanding of the associated wi th the pre.rift geology. Areas whieh ure history of subsidence is crucial to developing such type represenlati ve of little or no sediment cover represent the secti ons. Today relatively simple models explaining the Slarved margins, whereas regions exhibiting thick sequen· subsidence hi slory of these arcas have becn proposed. On
9 O. ELOH OLM, l. MONTAOERT the other hand, drilling results often show pronounced local REFERENces variati ons in subsidence history along the strike of the margin. De ChBrpal O., Guennoc P., Montadert L. , RoberU D. G. , 1978. Rifling, crustal anenuation and subsidence in the Bay of Biscay, ln the future , one needs for areas representing different No/ure. 175, 706-711 . stages o f development and structural styles. a careful Curray J . R., 1980. The IPOD programme o n passive conti nental definition of the structural framework of the pre- and margins. Philos. Trall s. R. Soc. LondOIl. A294 , 17·23. post-rift history of evolution and of the contine nt-ocean Emuy K. O., 1980. ConlÎnental margins - classificalÎon and boundary. petrole um prospects. Am. Auac. Pe/ rol. Glui. Bull.. 64, 297-31.'1.
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