Tertiary tectonic evolution of the NW Sabah continental margin

Transcription

1 Gelgical Sciety 0/ Ma/4yJi. - Circum-Pacific Cuncil/r Energy an} Mineral /OU'CtJ TtdJJnic Frt1Tewrk an} Energy /OU'CtJ 0/ tbe tjtun Margin 0/ tbe Pacific Buin NlJJtTJer 27 - Dtamber Ku.a4 Lump/Uj Maa)J4i. Tertiary tectnic evlutin f the N Sabah cntinental margin HANs P. HAzEBROEK AND DENS N.K. TAN Sabah Shell Petrleum Cmpany Ltd. Lcked Bag N.1, Miri, Sarawak, Malaysia Abstract: The N Sabah cntinental margin can be sub-divided int 6 tectn-stratigraphic prvinces n the basis f differences in structural styles and sedimentatin histries: (1) Rajang Grup Fld-Thrust Belt, (2) nbard Belt, (3) Baram Delta, (4) Outbard Belt, (5) N Sabah Trugh and (6) N Sabah Platfrm. The Tertiary sedimentary sequence was depsited during tw main phases f basin develpment: (1) A pre-early Middle Micene phase f generally deep-marine clastic sedimentatin. (2) A pst-early Middle Micene phase f clastic she1ti'slpe depsitin, which prgraded nrthwestward ver the underlying sediment wedge and is separated frm the latter by a majr reginal uncnfrmity. Evidence fr Palaegene subductin f Suth China Sea ceanic crust beneath N Sabah is incmplete. The subductin hypthesis (Haile, 1973; Hamiltn, 1979) is based n the fllwing main elements: (1) The Rajang Grup Fld-Thrust Belt f turbidites and assciated philites, interpreted t represent an accretinary prism. (2) The sub-linear N Sabah Trugh, interpreted t represent the sea-bed expressin f a subductin trench. A majr missing element in this hypthesis is a vlcanic arc f the crrect age. The N Sabah Trugh can be seen n seismic data t represent the dwnfaulted SE margin f the N Sabah Platfrm. An additinal imprtant element in the mrphlgy f the Trugh is the Baram Delta te thrust zne, bunding the Trugh t the SE, which was emplaced pst Middle Micene. Furthermre, the Trugh terminates abruptly against the Lucnia Blck. These factrs suggest that the present N Sabah Trugh is a relatively yung feature, and that if an lder, Palaegene, trench is present, it wuld ccur landward f, and with a different rientatin t the N Sabah Trugh. The Palaegene trench wuld n lnger have a surface expressin. Hamiltn's (1979) gedynamic interpretatin is supprted with the prpsed mdificatin f a Palaegene subductin system bradly parallel t the structural strike f the Rajang Grup Fld-Thrust Belt. The nbard Belt wrench tectnics trend parallel t the pstulated trench (cf. the Semangk fault f the Sumatra subductin system) as a result f blique subductin. Cunter-clckwise rtatin f Brne frm Ecene t Middle Micene resulted in increasingly blique subductin. The Baram Delta sediment prism masks the Palaegene trench. The fllwing three-stage mdel fr the tectnic evlutin f the N Sabah cntinental margin is prpsed: (1) Late Ecene t early Middle Micene blique subductin f Suth China Sea ceanic crust beneath N Sabah with depsitin and subsequent imbricatin f deep-marine sediments int an accretinary prism. (2) Diachrnus cllisin f the Suth China Sea attenuated cntinental crust (N Sabah Platfrm) with Sabah, and assciated cessatin f cean-flr spreading in the early Middle Micene, led t reginal uplift and ersin f the accretinary prism, resulting in a majr reginal uncnfrnlity (the Deep Reginal Uncnfrmity). This was fllwed by nrthwestward prgradatin f clastic sediments ver the nbard Belt frm Middle Micene t early Late Micene. (3) Resumptin f cnvergent frces between Brne and the N Sabah Platfrm in middle Late Micene was accmpanied by majr tectnic activity. The nbard Belt was subjected t strng cmpressinal defrmatin assciated with majr N-S wrench znes, resulting in the frmatin f a lcalised t semi-reginal uncnfrmity (the Shallw Reginal Uncnfrmity). Tw new depcentres were frmed seaward f the nbard Belt: the Baram Delta and the Outbard Belt. A thick prgrading clastic wedge built ut twards N in bth depcentres, whilst the nbard Belt was cntinuusly erded. A Late Plicene/Pleistcene phase f lcally develped defrmatin affected the nbard Belt, Outbard Belt and Baram Delta. GeL. Sc. MaiaYJia, BuLletin 33, NOllemher 1993; pp

2 196 HANS P. HAZEBROEK AND DENNS N.K. TAN NTRODUCTON This present paper represents a revisin and update f an earlier paper n the same area (Tan and Lamy, 1990). t cmprises newly drawn maps and diagrams. A set f reginal cmpsite seismic lines acrss the N Sabah cntinental margin, frm the Rajang Grup Fld-Thrust Belt t the N Sabah Trugh is als included. PLATE TECTONC SETTNG (FG. 1) The N Sabah cntinental margin ccupies a central psitin in the area f the junctin between the Eurasian, nd-australian, Pacific and Philippines plates. A msaic f smaller plates, sme f which frm marginal seas, characterizes this area, the evlutin f which is still far frm being fully reslved, thus attracting investigatin by numerus wrkers since the early 1970s (Hamiltn, 1979; Tappnier et al., 1982; Letuzey et al., 1988; Hutchisn, 1989; Rangin et al., 1990a and 1990b). The N Sabah cntinental margin is lcated at the sutheastern bundary f the Suth China Sea basin, which is ne f these marginal seas. Oceanic crust, featuring magnetic lineaments, has been recgnised in the centre f this basin, with the ldest and mst recent magnetic anmalies being dated as 32 and 17 Ma respectively (Taylr and Hayes, 1980). An extinct spreading ridge is interpreted t trend E- in the centre f this basin, and an lder ridge trends pssibly NE-S in the western part f the basin (Fig. 1). Hwever the elements f submarine gemrphlgy d nt trend E-but rather NE-S thrughut the whle basin (see discussin in Hutchisn, 1992a). The remaining part f the Suth China Sea basin is interpreted t cnsist f attenuated cntinental crust (Hllway, 1981, Fig. 1), and in the vicinity f Sabah this is referred t as the N Sabah Platfrm. Large parts f the Eurasian plate and nd Australian plate cnsist f pre-meszic cntinental crust. On the Eurasian plate this is referred t as 'Sundaland' r the Sunda Shield. The eastern part f Sundaland, which ccupies almst the entire 140' E Basement f Pra-MelOzlc cntinental crust r mature sland arc Attenuated cntinental crust,..., Oceanic r yung sland arc crust, L-..J cvered n pisces by Late Cretaceus r yunger sediments + T nactive] Spl'laadlng' = Active +.., - inactive -.,.-- Actve ] SUbductin trench -Fault -=+ Directin f relative plate mvement, 500km Figure 1. Plate tectnic elements f Suth East Asia (mdified after Hutchisn, 1989 and Rangin et al., 1990a).

3 TERTARY TECTONC EVOLUTON OF THE N SABAH CONTNENTAL MARGN 197 suthern part f Brne island, is, hwever, nncratnic and may represent terrain accreted t Sundaland during the late Meszic (Hutchisn, 1992a). The remaining part f Brne is interpreted t be largely underlain by ceanic crust (Hutchisn, 1989). N SABAH REGONAL GEOLOGCAL SETTNG (FG. 2) Main elements f the Sabah/Sarawak gelgical framewrk are: The Sunda Shield Occupying the suthwesternmst part f Sarawak, the Shield cnsists f crystalline schists f likely pre-mscvian age, and sedimentary rcks f Late Carbniferus (and pssibly lder) t Cretaceus age. t is assciated with large utcrps f andesitic and basaltic vlcanic rcks f Triassic age. Large granitic intrusins, sme f which are knwn t be f Carbniferus t Triassic age, are als present (Tan, 1986; Liechti et al., 1960). The Rajang Grup Fld-Thrust Belt This arcuate belt f Palaegene deep-marine sediments extends ver a distance in excess f 1,000 km alng strike, and reaches a width f ver 250 km. Assciated with these sediments are brken frmatins and philite depsits (see 'Tectnstratigraphic Prvinces' belw). The Oligcene-Micene Basins These include the nbard Belt (see 'Tectnstratigraphic prvinces' belw) as well as several circular r saucer-shaped basins in central and eastern Sabah and in suthwestern Sarawak. The latter cnsist f fluvi-deltaic sediments. The Oligcene-Micene Basins ccur alng either side fthe Fld-Thrust Belt, and the sediments that make up these basins are the ersin prducts resulting frm the uplift f the Fld-Thrust Belt. The Micene-Plicene Basins These include the Outbard Belt and Baram Delta (see 'Tectn-stratigraphic prvinces' belw) and ccur westward f the nbard Belt. The sediments making up these basins are derived frm uplift f the Fld-Thrust Belt as well as the nbard Belt. The sediment prism frmed by the nbard Belt and Baram Delta exceeds 12 km ttal thickness. The N Sabah Platfrm This platfrm is part f the Suth China Sea basin and is interpreted t cnsist f attenuated cntinental crust. Spradic seismic data indicate NAC TVE SUBDUCTON ZONE RENCH FAULT OPH OLTE OUTCRO P TEC T QNSE D OP HOL TE EXTRUS VE / NT RUSV E MELANGE CLASTC SEDMENTS : E3 MOCENE - PLOCENE E]J OLGOCENE - MDC E N E CJ PALAEOGENE CR E TA CEOU S / JURASS C GNEOUS ROCKS Figure 2. N Brne reginal gelgical setting.

4 198 HANS P. HAZEBROEKAND DENNS N.K. TAN that NE-S trending hrsts and half-grabens characterize the dminant structural style. The tilted fault blcks are prbably Meszic (Triassic t Cretaceus) sediments resting n a cntinental basement, similar t thse dredged frm the Reed Bank and Dangerus Grund areas (Kudrass et al., 1986). The half-graben fill mst prbably cnsists f clastic sediments f early Tertiary age (palecene t Early Micene). The end f the main extensin and cncmitant blck rtatin phase is marked by a strng reflectin which crrespnds t relatively thin, shallw water platfrm carbnates f Late Oligcene-Early Micene age, as in the Palawan area (Hinz and Schluter, 1985; Kudrass et al., 1986). The carbnate is verlain by well-bedded, hemipelagic t pelagic sediments which have been nly mildly defrmed by epeirgenic mvements. On reginal seismic lines this Olig-Micene carbnate ccurs at depths f ca secnds in the N Sabah Platfrm, plunges beneath the N Sabah Trugh t depths f between secnds and can be traced fr sme distance beneath the Micene-Plicene Basins.. The Lucnia Blck This Blck has been interpreted as a cntinental fragment riginally attached t the Suth China cntmental margin, that has been transprted by the pening f the Suth China Sea Basin (Hllway, 1981; James, 1984). This stable platfrm is characterized by a large number f carbnate reef buildups, sme f them cntaining majr gas accumulatins (Scherer, 1980). The Lucnia Blck is bunded by majr N-SE and N-ESE trending faults (S Lucnia-Mukah Line, est Baram Line, Fig. 2) which are pssibly trans-tensinal features related t deep-seated NNE-SS strike-slip faults (Hllway, 1981). Defrmatin, characterized by wrench-induced flding and thrusting, is present alng the suthern margin f the Lucnia Blck: the Balingian Belt. This Belt has been interpreted as a cllisinal margin where the Lucnia Blck has impinged upn the Rajang Grup Fld-Thrust Belt (James, 1984). Hwever, many features characteristic f sinistral wrenching are present. The.East Sabah Ophilite/Melange Terrain Much f East Sabah cnsists f melange and brken frmatins. ithin this terrain, utcrps f a relatively cmplete philite cmplex frm an arcuate belt frm Darvel Bay in sutheastern Sabah t the Kudat Peninsula in the nrth (Fig. 2). The belt appears t cntinue int Palawan. This philite belt marks the sharp inflectin f the Rajang Grup Fld-Thrust Belt (N Brne Trend t Sulu Trend). The philite is cnsidered t frm the ceanic basement t the sediments f Sabah (Hutchisn, 1989). Chert and ther pelagic sediments verlying the philite are Late Cretaceus t Ecene in age (Kirk, 1968), but the ceanic lithsphere itself may be lder. herever they have been restudied, HF-disslved ut radilaria in the cherts cnsistently give a Lwer Cretaceus age. Therefre the cherts are lder than dcumented by Kirk (Basir, 1992). The clay matrix f the melange has been dated as Early t Middle Micene. ithin this matrix are clasts f varius lithlgies f Late Cretaceus t Palaegene age. The clasts vary in size frm centimetres t hill massifs (Clennell, 1991). Varius mdels have been prpsed t explain the cmplex gelgy f East Sabah (Hamiltn, 1979; Hutchisn, 1989, 1992b; Clennell, 1991). n Hutchisn's (1992b) mdel, East Sabah represents an utcrpping extensin f the SE Sulu Sea marginal basin. The philitic cmplex f Sabah is inferred t crrelate with the cean flr f the Early Cretaceus (r lder) Sulu sea. A vlcanic arc was built upn this cean flr and subsequently this arc was rifted in the Early Micene, giving rise t the present Sulu Sea marginal basin. TECTONO-STRATGRAPHC PROVNCES On the basis f differences in structural style and sedimentatin histry, the N Sabah cntinental margin can be subdivided int six tectn-stratigraphic prvinces: The Rajang Grup Fld-Thrust Belt (Figs. 3, 4 and 5b) n Sabah, this Belt cnsists f Ecene t Oligcene turbidites, hemipelagics, and assciated brken frmatins (Crcker, Temburng, Trusmadi and ther frmatins) that have been defrmed int a thrusted, steeply-dipping sequence. The sequence becmes yunger in a seaward directin (nrthwestward) but bedding tps face sutheastward, indicating that the structure must be intricately imbricate (Hamiltn, 1979). On seismic (Fig. 5b) the Fld-Thrust Belt can be seen t extend in the ffshre area at least up t the Bunbury-St. Jseph Ridge (that is, beneath the nbard Belt), where it is sharply bunded t the west by a majr wrench zne. The uplifted, expsed part f the Fld-Thrust Belt prvided the main surce f sediments fr the nbard Belt and subsequently fr the Baram Delta and Outbard Belt depcentres.

5 .. '".J EUSTATC CURVES u,. (Haq et ) EPOCH i=l':h z u :!! - :!! r5 U 0 c.!) a: «!;t a: « !O i'i: ::() «ZUJ muj(!) «(1):;::(1) () LOSLUMP OM -!O SCARS ill 271 ::> n ' A DRU 32 Figure 3. N SABAH CHRONOSTRATGRAPHY RAJANG (SOUTHERN AREA) 1.;[" THRUST BARAM DELTA :+: NBOARD BELT.. BELT Samarang Sabah Ridges Klias Peninsula UNCONFORMTY (SRU) -:-:-:-:-:-:-:-:-:-:e': DEEP REGONAL --:::::::::::::::::::::-UNCONFORMTY (DRU) Cffrlillw --:::::::::::::::::::::::::::::::::::::::::::r-._ ::-:-:-:-:-:-:::?-'" _ =:-:-:-l-'!jg z :-:..::-:-:-::;-=.--::::f;;:;;-=.jjmtumqi(ii_.._'-_cr"otr<er-: --:-:-:p,.rbldl,esaii".':'ormaflq.:":-:;.:labuan) ::- -_-_-_- -: ,---,--ZiiZiiiiiiiiiiiii[: _ :-::-J i a 1l--:A:J:t:5:ihT _.. - QO::b :'P''P'' i :..: _ _ Tectnic/stratigraphic scheme and eustatic curves: N Sabah. REGONAL TECTONC EVENTS li!.j,'".j <nw j::l! 0:, J:. " z: O F d«0 00: le D ::l!ll -0 i f/, = ffi-.: 1\ NORTH 1'/ : '1 i, SA BAH.J, 8 i 1\ SOUTH SABAH 9 KNABALU '0 ajron " 12 EPOCH 3i i! ipl )STO -CENE Z lei: U Q a:!:t U 0 :E :E a: «wlt;r Z U c.!) 0,>- a: «-i m =:j s;: -i is m ( 5 Z "Tl m Z :E :J: (') Z m :;::» :D G) Z... CD CD

6 -; v v,_ /' _...\...' 1 ",/ // /// )" /1...-'" /1/// J / / / /1 /./,.. / / ( / / ",., -_/ / AROASER,,/,,/,.J!- ""..,/ BANK,.../ / J l ( C, #' ",,/ / 1 /, \ ,{4"-/,/ "..., _-;/ pt// / \ /'" - - -,' / / / //" #// /. ( 1/ /r/ " LOER TERTARY / (-:"::> /! \ NORTHEST SABAH PLATFORM -;.--:...",J /, / / \ ( UPPER TERT ARV COVER SEC) _ \ -:'),/ _,,'.( 0# ) /",.., // " (/1 i ( 1""1 " / / '-_, /< \ ' -",) L/.\ \ it! / '-... / '- / // /1 " l' \ / \../ </./ / / t,,;/ /' / _ 't4l " 1/ // / / -J / /,..6;.1' / /... ".> '< O v A.. -- """'..,...'< (...:::. " v -..qv.:;,...,,0 S { " 1/", -- 8ALAaAC /./ \ BASN./,,,/'''' \ l'r! \, /J \ t' / BA»CAUAN ( BASN J \ / rv :r: :t> Z en :-0 :r: m CD :0 m '" :t> Z m z en z? ;:t z Y!... e ' ii j... ;:! "'.:!!! 1::' r S" Figure 4. Tectn-stratigraphic prvinces f N Sabah.

7 TERTARY TECTONC EVOLUTON OF THE N SABAH CONTNENTAL MARGN 201 Me SECTON 1 : THRUST SHEET --, BARAM DELTA,. NR :H TO EZUST -f l OER TERTARY THRUST SHEET C A;_--L me i-- L L OFFSET BY 3 KM Figure 5a. Reginal seismic sectin 1 (N Part). Fr lcatin, see Figure 4. VXM /7/92 OUTBOARD AND NBOARD BELTS OUTBOARD BELT NBOARD BElT PTiduud Syncli n Dunbuf).' - 5 1_ J OS':'ph Ridg.. - CLAY TECTONCS DAPRSM --\ EXTU SO _--- COMPRESSON RENCH NG SE ==y' L AAJANG GP. FOLD-THROST BELT (OR THRUST SHEET) ' " VXM /7192 Figure 5b. Reginal seismic sectin 1 (SE Part). Fr lcatin, see Figure J(M

8 202 HANS P. HAZEBROEKAND DENNS N.K. TAN The nbard Belt (Figs. 3, 4, 5b, and 7b) This Belt trends bradly parallel t the Fld Thrust Belt, and extends frm ffshre N Sabah n t the nshre in a suthward directin. Structurally, the Belt cnsists f bradly NNE SS riented, tight, brken anticlines (Sabah Ridges), separated by wide synclines. The anticlines shw a variety f structural features indicative f wrench-related cmpressin, such as flwer structures, and reverse faults steepening with depth (Bl and van Hrn, 1980). n the nrthern part f the nbard Belt, the rientatin f the anticlines deviates t NE, E aid SE. The bending f this trend ccurs near the axial plane f the sharp inflectin as bserved in the Rajang Grup Fld-Thrust Belt (N Brne Trend t Sulu Trend). Timing f defrmatin in the nbard Belt is based n well and seismic data, indicating a majr Late Micene/Early Plicene phase and a mre lcally develped Late PlicenEVPleistcene phase. An indicatin fr the sense f strike-slip mvement and the magnitude f ffset has been derived frm the ffset in the pattern f Middle Micene shelf margins (dated n pllen; Fig. 9). Sinistral strike-slip mvement with a cumulative ffset f sme 100 km has been estimated fr the suthern part f the nbard Belt. The sedimentatin histry in the Suthern and Central parts f the Belt cnsists f an early Middle Micene regressin (Stage VA), a late Middle Micene transgressin (mainly Stage VB) and a Late Micene t Plicene regressin (Stages VC and VF/G; Stages VD and VE are thin r absent). Further t the nrth, the main difference in sedimentatin histry is a prlnged Late Micene t Plicene regressin marked by well-develped Stages VC, VD and VE sediments. The Outbard Belt (Figs. 4, Sa, 5b and 6b) This Belt ccurs west f the nrthern part f the nbard Belt. The Outbard Belt is structurally cmplex, and shws features indicating bth extensinal and cmpressinal phases f defrmatin. The frmer defrmatin is marked by large, NE-S trending dwn-t-basin nrmal fault systems, whereas the latter is indicated by wrenchinduced features in the Tembung and Kinarut areas and by clay diapirism alng the western margin f this Belt. The prvince represents an elngated Late Micene-Plicene depcentre cmprising Stages VD, E, F and G sediments prgrading nrthwestward frm shallw t deep marine. The Baram Delta (Figs. 3, 4, 7a, and 7b) This prvince is characterised by typical delta tectnics, and can be subdivided int a prximal part, dminated by extensinal grwth-faulting, a transitinal area frmed by the delta slpe, and a distal part, dminated by verthrusted anticlines representing the te-thrust zne f the delta. n the prximal part f the delta large dwn-tbasin grwth-faults and crrespnding cunterreginal faults define the NNE-SS trending Champin-Padas megastructure. n Sabah, this megastructure is made up f several macrstructures, each bunded by its main grwthfault n the landward side. (Timbalai, Samarang and Padas macrstructures). The tpsets f the prgrading wedge f deltaic sediments are largely cnf'med t the Champin-Padas megastructure. These macrstructures merge dwndip int a large expanding flank, bunded n the seaward side by an imprtant cunter-reginal fault system, which trends bradly parallel t the present castline. The te-thrust zne f the delta is 50 t 80 km in width in the main part f the delta, but narrws twards the nrthern fringe f the delta. Up t six, NE-S trending elngated, bradly parallel, verthrusted anticlines are present in this zne. The anticlines frm ridges n the sea flr, that increase in amplitude twards the delta te. Between the anticlines are mini-basins, that have been partly r entirely filled with draping hemipelagics, and t a minr extent with nlapping turbidites. The fill f the mini-basins is less cmplete twards the delta te. These bservatins strngly suggest recent tectnic activity f the delta te. Such te-thrust features may be regarded as an integral part f gravitatinal delta tectnics, and are analgus t thse bserved in the Niger Delta, which has a passive margin setting (Fig. 10). On the delta slpe (as well as clser ashre), lder anticlines becme assciated with clay-diapiric ridges. These clay ridges may represent an advanced stage f develpment f the thrusted ticlines, with the thrust-planes steepening as the delta builds ut. The 'Thru$t Sheet' (Figs. 4, Sa, 6a and 8) Hinz et al. (1989) prpsed the term 'Thrust Sheet' fr a blck f chatic seismic facies, which is bunded t the N and S by steep thrust faults and t the SE by nrmal faults, separating it frm the Outbard Belt. The.chatic seismic facies is verlain by gently flded sediments f pssibly early Middle Micene age. Beneath the chatic seismic facies a cherent reflectin can be seen dipping in

9 TERTARY TECTONC EVOLUTON OF THE N SABAH CONTNENTAL MARGN 203 SECTON 2 BAR AM DELTA,. _ _.. _...,.- - "" - troush --. TOE THRUST ZONE, LOER TERTARY THRUST S ============================== z..., 1 EARLY PLOCENE L. DEFORMED TURSrrl)EAHD HfillPEL..,w:fC DEPOSTS? OLGO - "'OC -- EHE CARBONATES? ,.. VXM /7/92 Figure 6a. Reginal seismic sectin 2 (N Part). Fr lcatin, see Figure 4. THRUST SHEET AND OUTBOARD BELT \ ru OUTBOARD BELT ,1 -+ NBOARD BELT 1:,10 S tl U!! L DEFORMED TURBDTE AHD HEMPELAGC DEPSrTS 4..J T"U'" 'HlT 10 KM OFFSET BY 14 KM VXM /7/92 Figure 6b. Reginal seismic sectin 2 (SE Part). Fr lcatin, see Figure 4.

10 204 HANS P. HAZEBROEK AND DENNS NK TAN SECTON 3 BARAM DELTA 1 3 A ==== ,- N H SABAH TOE THRUST lone.trough======;===============================t===== 3A MHBAS,"S MN B A SNS 'r-:;;;;;;;;;;;;;;;2 ;;;;:---:::::, H",eL..C S L :., - ::::""'1 OLGO - MOCENE CAROHA -... _- - - J Figure 7a. Reginal seismic sectin 3 (N Part). Fr latin, see Figure 4. AND NBOARD BELT DELTA < NBOARD BELT-- CLAY DAPRSM \ DEFORMEO TURBJDTE AHD HEMPELAGC DEPOSrTS 5 10 K M " ln V'.' 11M Figure 7h. Reginal seismic sectin 3 (SE Part). Fr lcatin, see Figure 4.

11 TERTARY TECTONC EVOLUTON OF THE N SABAH CONTNENTAL MARGN 205 a landward directin, which is inferred t crrelate t the Olig-Micene carbnates dredged frm the Dangerus Grunds area (Kudrass et al., 1986). The tentatively interpreted allchtnus mass making up the 'Thrust Sheet' defines the nrtheastern margin f the N Sabah Trugh, and exceeds 40 km in width. t prtrudes up t 150 km ffshre and is lcated at the axial plane f the sharp inflectin f the Fld-Thrust Belt (N Brne Trend/Sulu Trend). The 'Thrust Sheet' has a similar seismic respnse as the deep marine sediments f the Fld-Thrust Belt (Fig. 5a). The 'Thrust Sheet' may therefre represent a nappe, cnsisting f Rajang Grup rcks, that resulted frm gravity sliding assciated with the uplift f the Fld-Thrust Belt. The N Sabah Trugh (Fig. 4) This NE-S linear bathymetic feature with a water depth f up t 2800 m extends ver a length f ver 300 km, and reaches an average width f sme 80 km. T the S, the Trugh terminates abruptly against the Lucnia Blck. The Trugh becmes less well expressed nrthwards twards Palawan island, where it is referred t as the Palawan Trugh. The Trugh is bserved n seismic t be a dwn-faulted part f the N Sabah Platfrm which extends beneath the Baram Delta frnt. An Upper Tertiary fill f sme 2 secnds seismic twway time thickness verlies the Lwer Tertiary platfrm within the Trugh. This fill cnsists largely f pelagic clays with sme turbidite intercalatins. The Baram Delta te thrust zne, which is Late Micene t Recent in age, is an imprtant element in the mrphlgy f the Trugh, bunding it t SE. These factrs suggest that the present N Sabah Trugh is a relatively yung feature, and that if an lder, Palaegene, trench is present, it wuld ccur landward f, and with a different rientatin t the N Sabah Trugh. The Palaegene trench wuld n lnger have a surface expressin. TECTONC EVOLUTON Figure 3 cmbines a simplified stratigraphic scheme fr the suthern part f the N Sabah ffshre area with eustatic sea-level curves (Haq et al., 1988) and with reginal tectnic and magmatic events. t is clear frm this scheme that the Tertiary sedimentatin histry was in this area dminated by tectnic events, whilst eustatic sea-level lwering caused verall regressin frm the early Middle Micene nwards. Eustatic lw stands can nly ccasinally be crrelated with uncnfrmities (late Middle Micene and Late Plicene/Pleistcene uncnfrmities) and shelf-edge slump scars. The Tertiary sedimentary sequence can be subdivided int tw main cmpnents: 1. A pre-early Middle Micene phase f generally deep-marine clastic sedimentatin (Stages, and ). Defrmatin f this clastic wedge in the ffshre area is revealed n seismic in the nbard Belt and Baram Delta te thrust zne. 2. A pst-early Middle Micene phase f clastic shelf/slpe depsitin (Stage V), which prgraded nrthwestward ver the underlying sediment wedge and is separated frm the latter by the Deep Reginal Uncnfrmity. This sequence is characterised, in the nbard Belt, by cmplex tectnics attributable t an verall regime f cmpressive strike-slip faulting alng pre-existing NNE-SS shear znes. The syndepsitinal defrmatin ccurred in a series f tectnic pulses, which resulted in repeated perids f uplift and flding. The lcus f this defrmatin prgressively shifted basinward, in a N directin, thrugh time. The result is a series f uncnfrmities which are mappable in the nbard Belt, but becme cnfrmable hrizns further. The uncnfrmities playa principal rle in subdividing the pst-early Middle Micene sequence int a series f Stages (VA t VG) (Levell and Kasumajaya, 1985 and Levell, 1987). The hypthesis f Palaegene subductin alng the N Sabah cntinental margin was prpsed by Haile (1973) and expanded upn by Hamiltn (1979). The main elements f this hypthesis are: 1. The Rajang Grup Fld-Thrust Belt, which is interpreted as an accretinary prism. 2. The N Sabah Trugh, which is interpreted as the seabed expressin f a subductin trench. A majr missing element in the hypthesis is the lack f a magmatic arc f crrespnding age. The fllwing key bservatins bear n the N Sabah tectnir/magmatic histry (Fig. 3): 1. True sea-flr spreading, frm 32 t 17 Ma, dcumented in magnetic anmaly patterns in the ceanic crust f the central Suth China Sea (Taylr and Hayes, 1983). 2. Cunter-clckwise rtatin f Brne, ver sme 45 frm Ecene t Middle Micene and pssibly later (Schmidtke et al., 1990). This is accepted by mst authrities and based n a series f measurements in varius lcalities and n different rck-types (Hutchisn, 1991). 3. mbricatin and thrusing f the Crcker Frmatin (part f the Rajang Grup) in Oligcene t Middle Micene times. Deep marine sedimentatin ceased in early Middle Micene and the Crcker Frmatin was subsequently uplifted and defrmed. This event is represented by the Deep Reginal Uncnfrmity.

12 206 HANS P. HAZEBROEK AND DENNS N.K. TAN 4. ntrusin f the Mt. Kinabalu bathlith. Preliminary ptassium-argn ages range frm 5 t 12 Ma. Jacbsn (1970) cnsiders 9 Ma the mst likely age f intrusin. Further wrk n dating is required. 5. Vlcanism is widespread alng the strike f the Rajang Grup Fld-Thrust Belt, ranging frm the Hse muntains and Usun Apau plateau in Sarawak t Kalimantan and Sabah (Semprna vlcanic arc). Available ages range frm Late Cretaceus t Plicene (Hutchisn, 1989). Mst ages are yunger than Middle Micene and hence pstdate the uplift f the Rajang Grup rcks, which is cnfirmed by the uncnfrmable relatinship between Rajang Grup and verlying vlcanics (Liechti et al., 1960). 6. Large-scale strike-slip mvements are dcumented in the N Sabah ffshre, and can be traced suthward n land (Fig. 9). On seismic, typical wrench-phenmena such as flwer-structures, define trends that rughly parallel the strike f the Rajang Grup Fld Thrust Belt (Fig 4, see nbard Belt abve). f Palaegene subductin tk place, then the rientatin f the trench may have been nncnfrmable with the present-day N Sabah trugh, as this bathymetric feature is partly made up f an element that pstdates Palaegene subductin: the Baram Delta te thrust zne. A pssible Palaegene trench is likely t have been riented cnfrmable t the strike f the accretinary prism, and t have been cntinuus alng the extent f the prism. The N Sabah Trugh, hwever, is nn-cnfrmable with the strike f the Fld-Thrust Belt, and terminates abruptly against the Lucnia Blck. Oblique subductin wuld have resulted in extensive strike-slip mvements, als riented apprximately parallel t the strike f the accretinary prism (cf. the Semangk fault f the Sumatra subductin zne). The data presently available prvide insufficient evidence t substantiate the hypthesis f Palaegene subductin alng the N Sabah cntinental margin. Nevertheless the fllwing speculative tectnic evlutin is prpsed, which draws n wrk frm Hamiltn (1979), Hllway (1981) and Hutchisn (1989), and is shwn in Figure. Late Cretaceus Oceanic crust is being subducted beneath the Sunda Shield, pssibly driven by cunter-clckwise rtatin f the shield (Hllway, 1982). The philites expsed alng the Lupar line are interpreted as a Late Cretaceus suture. A majr river system, pssibly the prt-mekng (Hutchisn, SECTON 4 : LOER TERTARY THRUST SHEET L :, ;;-\ )m l n" l f.. :t Figure 8. Reginal seismic sectin 4. Fr lcatin, see Figure 4.

13 TERTARY TECTONC EVOLUTON OF THE N SABAH CONTNENTAL MARGN km [Ma] 3.ll '... :... 7 se:»)... :::J. 2/' 1_... -_ Figure 9. nterpretatin f the ffset in the pattern f Middle Micene shelfmargins as due t sinistral strike slip within a Upper Micene t Plicene (mega-) wrench system thrughut Nrthwest Sabah.

14 208 HANS P. HAZEBROEK AND DENNS N.K. TAN Figure 10. Niger Delta Te Thrust. 1989) r prt-kapuas is feeding abundant clastics frm the Sunda Shield nt Brne's ceanic basement, frming majr turbidite fan systems as part f the Rajang Grup. Late Palecene t Early Ecene Relative mvement f Suth China Sea ceanic crust twards and beneath the Sunda Shield is nw driven by attenuatin f cntinental crust n the N margin f the Suth China Sea (Hllway, 1981). The philites alng the Bukit Mersing Line in Sarawak are interpreted as a suture welded during this perid. Gemetric relatinships require majr transfrm fault systems t be active alng the eastern margin f the Sunda Shield. Depsitin f Rajan..g Grup turbidites cntinues. Late Ecene t Oligcene The evlutin f the N Sabah cntinental margin can nly be traced back t the Late Ecene. The lcatin f the subductin zne shifts further twards the lcatin f the present day cast line, as the accretinary turbidite prism builds further ut. True seaflr spreading by frmatin f new ceanic crust begins in the central part f the Suth China Sea. Cntinental fragments, such as the Lucnia Blck, and attenuated cntinental crust apprach the subductin zne. Early t Middle Micene This is a perid in which many events take place. Oblique subductin cntinues, and cllisin f cntinental fragments (Lucnia Blck) and attenuated cntinental crust begins frm the suth, prpagating gradually nrthwestward. Ceval with the cllisin is the diachrnus uplift f the Rajang Grup, being earliest in est Sarawak and latest in Sabah. Ersin prducts f the Rajang Grup Fld-Thrust Belt are rewrked int the nbard Belt depcentre, as well as int the prximal parts f the Baram Delta and Outbard Belt areas. As a result f the gradual lcking-up f the blique subductin zne, majr strike-slip znes were initiated, which trend parallel t the strike f the Rajang Grup Fld-Thrust Belt and subductin trench (cf. Semangk Fault system, Sumatra, which is parallel t the subductin trench). During uplift fthe nrthern part fthe Rajang

15 TERTARY TECTONC EVOLUTON OF THE N SABAH CONTNENTAL MARGN 209 / OCEANC CRUST (N Spreadng) \ \ SUNDA \ SHELD OCEANC CRUST H 1 '" \ \ _ EARLY TO AND MDDLE EROSON OF CROCKER FM. MOCENE LEGEND' -r -r NACTVE } " ACTVE SUBDUCTON TRENCH ====:;> CLASTC SEDMENTS DEPOCENTRE FAULT e ntrusive ---- SUTURE <0 Extrusive :;;;... ===... EXTENSVE RENCH - NDUCED COMPRESSON \tnvn COLLSON OF MCROPLATE -'-- THRUSTNG LATE DRECTON OF RELATVE PLATE MOVEMENT UU' MOCENE- LLLULLLLA- t9 (j) 4- HOLOCENE RVER DRANAGE Figure 11. Tentative Tertiary Evlutin f the N Sabah cntinental margin (partly based n Hutchisn, 1989; Hamiltn, 1979; and Hllway, 1981). Grup Crcker Frmatin, majr thrusting and nappe frmatin may result in emplacement f thrust sheets up t 50 km frm the axis f uplift. Late Micene-Hlcene Large-scale strike-slip mvements with cumulative hrizntal displacements f up t 100 km result frm resumptin f blique cnvergent frces alng the lcked-up subductin zne. Transpressin causes uplift and ersin f the nbard Belt (Shallw Reginal Uncnfrmity), feeding new depcentres t the and N: the Baram Delta and Outbard Belt. ntrusin f the Mt. Kinabalu bathlith and widespread vlcanism take place alng the Rajang Grup Fld-Thrust Belt. Lcal reactivatin f transpressin ccurs during Late Plicene/Pleistcene. The N Sabah margin is at present seismically fairly quiet with a ttal f 48 minr earthquakes recrded in the nrthern part f Sabah in the last 94 years. The mst severe earthquake was measured as 5.0 n the Richter scale in May, The epicentre was lcated sme 75 km ffshre N Sabah and may represent mvement alng the abve-mentined strike-slip znes. ACKNOLEDGEMENTS This paper is based n the results f extensive wrk ver many years by numerus SSBjSSPC

16 210 HANS P. HAZEBROEK AND DENNS N.K. TAN gelgists. Permissin t' publish this paper was kindly granted by Shell nternatinal Petrleum Maatschappij (The Hague), and PETRONAS (Kuala Lumpur). REFERENCES BASR JASN, Significance f radilarian cherts frm the Chert-Spilite Frmatin, Telupid, Sabah. Gel. Sc. Malaysia Bull. 31, BL, AJ. AND VAN HOORN, B., Structural Styles in estern Sabah ffshre. GeSc.MalaysiaBull. 12.,1-16. CLENNELL, B., The rigin and tectnic significance f melanges in Eastern Sabah, Malaysia. Jurnal f Sutheast Asian Earth Sciences, 6(3/4), HAn.E, N.S., The recgnitin f frmer subductin znes in Sutheast Asia. n: Tarling, D.H. and Runcrn, S.K. (Eds.), mplicatins f Cntinental Drift t the Earth Sciences, 2. Academic Press, Lndn, HAMLTON,., Tectnics f the ndnesian Regin. U.S. Gel. Surv. Prf. Paper 1078, 345p. HAQ, B.U., HARDENBOL, J. AND VAL, PR, Meszic and Cenzic Chrnstratigraphy and Cycles f Sea-Level change. n: ilgus e.k. et al. (Eds.), Sea-Level changes - An ntegrated Apprach. SEPM Spec. Publ. 42, HNz,K.,FRTSCH,J.,<EMPTER.E.HK.,MHAMMAD,AM.,MEYER, J., MOHAMED, D., VOSBERG, H., EBER, J., AND BBNA VDEZ, J., Thrust tectnics alng the Nrth-western Cntinental Margin f Sabah/Brne. Gelgische Rundschau, Band 78, Heft 3. HNz,K.,ANDScHuLTER,H.V.,1985. Gelgy f the Dangerus Grund, Suth China Sea and the Cntinental Margin. ff S Palawan: Results f SONNE Cruise and Energy, 10, HOLLOAY,N.H.,1981. NrthPalawanBlck,Philippines-its relatin t Asian mainland and rle in evlutin f Suth China Sea. Gel. Sc. Malaysia Bull. 14, HUTClDSON, e.s., Gelgical Evlutin f Suth-East Asia. Clarendn Press, Oxfrd, 368p. HUTClDSON, e.s., (Ed.), Studies in East Asian tectnics and resurces (SEA TAR), Crustal transect Vll: Jawa-Kalimantan Sarawak-Suth China Sea. OC, Paris, and CCOP, Bankk, CCOP/TP26. HUTCHSON, e.s., 1992a. The Ecene Uncnfrmity n SutheastandEastSundaland. Gel. Sc. Malaysia, Bulletin 32, HUTClDSON, e.s., 1992b. The Sutheast Sulu Sea, a Negene Marginal basin with utcrpping extensins in Sabah. Gel. Sc. Malaysia, Bulletin 32, JACOBSON, G., Gunung Kinabalu, Sabah, Malaysia. Gel. Surv. Malaysia Rept. 8, 111p. JAMES, D.M.D. (Ed.), The gelgy and hydrcarbn resurces fnegara Brunei Darussalam. Brunei Museum and Brunei Shell Petrleum C. Knuc,H.J.e.,1968. The igneus rcks f Sarawak and Sabah. Gel. Survey f Brne regin, Malaysia Bulletin 5. KUORASSHR., EDCKE, M. CEPEK, P.<REuZER, H. AND MULLER, P., Meszic and Cainzic rcks dredged frm the Suth China Sea (Reed Bank area) and Sulu Sea and their significance fr the plate-tectnic re-cnstructins. Marine and Petrl Gelgy, 3, LErUZEY, J., SAGE, L. AND MULLER, e., 198. Gelgical and Structural Map f Eastern Asia with ntrductry Ntes. m. Assc. Petrl. Gel LEvELL, B.K., The nature and significance f reginal uncnfrmities in the hydrcarbn-bearing Negene sequences ffshre est Sabah. Gel. Sc. Malaysia Bull. 21, LEVELL, B.K. AND <AsuMAJAYA, A, Slumping f the Late Micene shelf edge ffshre est Sabah. A review f a turbidite basin edge. Gel. Sc. Malaysia Bull. 18,1-29. Lmm, P., ROE, F.. AND HAn.E, N.S., The gelgy f Sarawak, Brunei and the western part f Nrth Brne. Gelgical Survey Department British Territries in Brne Bulletin 3. RANGN, e., BELLON, H, Bm-ARO, F., LEruZEY, J., MULLER, e. AND SANUDN, T., 1990a. Negene arc-cntinent cllisin in Sabah, N. Brne (Malaysia). Tectnphysics 183, RANGN, e., JOUVET, L., PuBELLER, M., AND THE TElHYS PACFC ORKNG GROUP, 1990b. A simple mdel fr the tectnic evlutin f Sutheast Asia and ndnesia regin fr the past 43 m.y. Bull. Sc. Gel. France, t. V, 6, Sci-mRER,F.e.,1980. ExplratininEastMalaysiaverthelast decade. n: Halbuty,M.T. (Ed.), Giant il and gas fields f the decade Am. Assc. Petrl. Gel. Mem. 30, SaiMnm<E, E.,FULLER,M.,ANDHAsrN,R., Palemagnetic data frm Sarawak, Malaysian Brne and the Late Meszic and cenzic tectnics fsundaland. Tectnics 9, TAN, D.N.K., Palaegegraphic develpment f est Sarawak. Gel. Sc. f Malaysia Bulletin 19, TAN, D.N.K. AND lamy, J.M., Tectnic evlutin f the N Sabah cntinental margin since Late Ecene. Gel. Sc. f Malaysia Bulletin 27, TAPPONNER, P., PELTZER, G., 1E DAN, AY., ARMJ, R. AND CBBOLD, P., Prpagating extrusin tectnics in Asia: new insights frm simple experiments with plasticine. Gelgy, 10, TAYLOR, B. AND HAYES, D.E., Origin and Histry f the Suth China Sea Basin. n: D.E. Hayes (Ed.), The Tectnic and Gelgic Evlutin f SE Asian Seas and Basins Part 2. AGU Gephysical Mngraph 27, GD Manuscript received 22 April 1993