Seabeam Survey at the Southern End of the Manila Trench

Tectonophjuics, 146 (1988) 261-278 261 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Seabeam survey at the southern end of the Manila trench. ’ Transition between subduction and collision processes,

I offshore Mindoro Island, Philippines i‘ li

’ CNRS, UA 215 Département de Géotectonique, Université Pierre et Marie Curie, 4, Place Jussieu-Tour 26, 75252 Paris Cede-x O5 (France) ’CNRS, GIS, Océanologie et Géodvnaniie, Brest (France) ’BED, Manila (The Philippines) BRGM, Département de Géologie Marine, Orléuns (France) -‘National Taiwan Uniuersity (Taiwan) ‘Orstoni Centre de Noumea, Nouvelle Calédonie (New Caledonia) ’IFREMER Centre de Brest (France) Uniaersité de Savoie, Chambély (France)

(Received May 11,1987; accepted May 28, 1987)

Abstract

Rangin, C., Stephan, J.F., Blanchet, R., Baladad, D., Bouysee, P., Chen, Min Pen, Chotin, P., Collot, J.Y., Daniel, J., Drouhot, J.M., Marchadier, Y., Marsset, B., Pelletier, B., Richard, M. and Tardy, M., 1988. Seabeam survey at the southern end of the Manila trench. Transition between subduction and collision processes, offshore Mindoro Island, Philippines. In: F.-C. Wezel (Editor), The Origin and Evolution of Arcs. Tectonopliysics, 146:261-278.

The morphological and structural study conducted at the southern tip of the Manila trench, reveals that convergence between the South China Sea basin and Luzon is accommodated differently depending on the nature of the subducted slab. When the oceanic crust is subducted, a simple accretionary prism-fore arc basin pattern is developed. Conversely, where the continental margin of this basin is subducted, intraplate deformation is randomly distributed across the major part of the fore arc area which is fragmented into various crustal microblocks. Results of seabeam mapping, and detailed geophysical surveying conducted at this subduction-collision transition zone, during the POP2 cruise, with R.V. “Jean Charcot” are presented and discussed here, and allow us a new insight into the mechanism of such a subduction-collision transition zone.

! Introduction island arc which can be considered as part of the , ?$ Philippine Sea plate (Fig. 1). This island arc ex- t p7 The subduction front of the Manila trench is tends from Taiwan in the north to Luzon and v..’I the emergence of an important plate boundary Mindoro in the south (Wolfe, 1981; Stephan et al., between the South China Sea basin and its margins 1986). which belong to the Eurasian plate, and the Luzon The seismicity along this plate boundary attests

0040-1951/88/$03.50 Q 1988 Elsevier Science Publishers B.V. ORSTOM Fonds Documentaire

5 Fig. 1. Structural geolt.gy \ketch map of the Philippines

used was the seabeam coupled with single channel seismic, magnetic and gravimetric recording. The fine structure of the first 100 m of sediments was studied using 3.5 kHz echo sounding. The seabeani lines were reset on the R.V. “Jean Charcot”, and the bathymetric maps redrawn with a maximum overlapping at the line intersections. The identified structures interpreted on seismic profiles were plotted on the respective bathymetric profiles, al- lowing a three-dimensional picture of the studied area.

The lower plate morphostructure

Fig. 2. General structural geology sketch imp of the southem The downgoing plate is formed by the South tip of the Manila trench. I = South China Sea oceanic crust; China Sea oceanic crust and its southern passive 2 = Late Miocene-Present clastic sedimentary basins; 3 = margin which is formed of continental crust (Hol- Luzon arc volcanoes; 4 = North Palawan-San JOSE Mindoro loway, 1982). platform; 5 = Eocene-Lower Miocene clastic sediments; 6 = Middle Oligocene Mindoro ophiolites; 7 = Cretaceous ophio- According to Taylor and Hayes (1980, 1983), lites: 8 = Mindoro metamorphic Paleozoic basement. spreading of this oceanic basin occurred between 32 and 17 Ma, but the opening history of this basin is characterized by two probable episodes of trending Lubang ridge, connecting to the east with spreading with two distinct extensional trends the Verde Passage suture, a speculative left-lateral (Pautot et al., 1986). The axis of the basin clearly

strike-slip fault zone. South of the Lubang ridge, shows a N60 O E structural trend, outlined by nor- the fore arc area is poorly known offshore, and mal faults, but the deeper part of the oceanic only scarce industrial seismic lines are located basin is apparently outlined with poorly docu-

close to the Mindoro coast by Sarewitz and Karig mented N80 O E-trending faults. (1986). The southem margin of this basin displays a

The trench shows an important change in trend clear structural pattern trending N60 O E, crosscut

approaching the Mindoro coast, towards an area by N130 O E-trending fault scarps, interpreted as of important on shore active thrust faults (PNOC possible strike-slip faults. (Hinz and Schluter, unpublished seismic profiles). Here, both the up- 1985). However, Fricaud (1984), has revealed that

per plate and the lower plate are formed of the the N60 O E-trending normal fault pattern post- same sedimentary units, both belonging to the dates the N80 o E one, thus documenting various continental margin of the South China Sea (Fig. 2) episodes of normal faulting in this area. and outlining the intracontinental setting of this In the surveyed area, the seismic profiles show convergent zone. a clearly kinked lower plate, dipping steeply to- The objective of the survey carried out off ward the trench axis and affected by small scarce Mindoro during the autumn of 1984, with R.V. E-facing fault scarps (Fig. 4). North of 13”40’N,

“Jean Charcot”, was to document the intersection the faults trend N10 O W-N30 O W, while south of of the Manila trench with the South China Sea 13”20’N they trend N130”E. continental margin, and to compare the fore arc There is no clear evidence of reactivated nor- I geomorphological situation along this subduc- mal faults on the seismic profiles (as is docu- tion-collision transition zone. In the studied area, mented in the Middle America trench (Aubouin et CJ about 3300 nautical miles have been covered dur- al., 1986) or along the Nankai and Japan trenches ing the POP 2 cruise off southem Luzon and (Le Pichon et al., 1986) that could suggest that the Mindoro Island (Fig. 3). The principal equipment major part of the observed minor faults are the result of the &>tic bending of the mhducting notabl>- differs from its extension to the north. lithosphere a5 it ~ippmachesthe trench (\Y:itts and Here. the continental slope rise4 rapidly to :i shal- Talnani. 1974). However. senbeam datil collected low depth wuth of the Calamian Islsndr. This north of this :ireLi. hugest that the inherited fahric slope is ctiverecl by more than 7.5 s (TWT) of of the suhducted plate is clearly re:ictivated close sediments and the mustic basement is poor1.i to the trench xis. (Rangin and Pautot. in prep.). defined (in the beismic profiles. It contrasts ivith The continent:il-nceIin¡c crurt hnundarl is still the relati\eIy thin edinient:iry cover identified to poorll defined in the South China Sea basin. The the north xhch civerlies the oceanic crust. (ln the southern margin cif ths basin is characteriLed hy ;i hisof these &ita ancl a-ith the lack of a\ailahle quiet magnetic zone xvith high-frequency and loa information concerning the deep cru\tal structure length anomalies: the first identified ;rnom;ily of the crust. \ve suggest that in the zurvejrd ;irea. trending E-W enter4 the trench at ahout 13’20‘N the cnntinental-o~eaniccrust bound:tr) is Incated (Taylor and H~iyes. 19%). Farther south of hetwrn 13*2(:)’:ind 13°30’N. 13”2U’N. the structure cif the dmvngning plste NE

sw NE Manila Trench

Fig. 4. Single channel seismic profile across the Manila trench, close to the continent-ocean boundary of the South China Sea. Vertical exaggeration is 1.5. 'hh

The niarphologj of the trench sistently N-S. north of 14'00". and it curves progressively southeastward. approaching the The trench axis displays important morphologi- hlindorn coast. A detailed bathymetric map of cal varisticins betwwn 12'40' and 15'0O'N. It off-shore Mindoro reveals that the outer \\-all is estends to SOO0 ni \vest d hlanils Bay. hut its characterized by a stair step morphology :i:,soci- depth decreases rapidly southward (Fig. 6). It is ated Lvith N130' E-trending vertical faults. hlulti- characterized hy a gravity anomaly ( - 75 niGa1s) channel seismic data collected in this area by between 14"Otl' and IS'OCI'N. where the oceanic BGR. meal that some of these fault:, have iì crust is undergoing subduction hut no clew noticeahle strike-slip component and some :ire identified to the south where the important Lvrench faults (Schluter. pers. commun.. trench intersects with the continental margin. The 1986). nail\ of the trench become prngressively .;teeper to Together. theze d:ita suggest that approaching the south (Fig. 5). and its width also decreases in the hlindoro coast. Xvhere the trench intersect\ the same direction. The turbidites of the trench fill isith the continental margin. compressix defor- cle¿irly ni-erlap the tilted sediments of the Inn-er mation is acting either on the inner \va11 or nm the plate a11 along the base of the outer uall (Fig. 4). outer wnll of the trench. Consequentl>. the rela- These trench-fill sediments :ire not deformed tively efficient decoupling procehs which tahr:, southwest of the trench-fill axis. except at the place to the north between the tii-ci contergent .;oiithernmmt tip of the trench bvhere niincrr fold.; plateb. dis:ippe;irs to the south. into the in- haw heen identified at the buse of the outer wall. trmmtinrntal deformation zone. and the stress In thiz area (Fig. h). thrust faults are present a11 regime i.; accommodiited hy thrusting and folding along the trench. on both sides of the trench a\ir cm hoth convergent plates. nlicli rapidly emerges to ;i sh:illo\i depth to the ,soutlieaxt. The trcnch-fill :isis. trends qui te cnn- The fore arc region

-/ The Lubang ridge (Fig. 7) reprexents an important hound:ìry betu-een tu.o distinct morpho- -1 atruc.tural units of the h1anil:i trench t'cire nrc :irea. In the ivest and north of ths tran.s\erse structure, the ;ìccretinnar>.prism ;i:, \vel1 as the fore arc h;isin can be ea>ily identified. In contr:i:t in the south. the fore ;ìTc region reveals a x;er) complex morphological and structural frame\vork. com- pletely diztinct from that in the north.

In this ;irea. both the accretionary prim and the fore arc basin ;ire very distinct. Onbhore. the Luzm idand ;ìTc is largel>- dr\elopc.ci along the western coast of Luzon. two cif the mnst signifi- cint volcanoes being Taal and L:igun:ì de Briy.

The :iccretion;ir-, prism i4 evenly developed \rith ;in LtverLige n-idth nf 61) lini. the inner n:ì11 dipping gentl! tavards the trench. Upslope. folds atei1 ~ithrevene or thrust faults ciin hi. traced for ;i long di.;t;incz along the trench axis. The single 261

S N

118

S N

VE -1.5

1 Fig. 6. Single channel seismic profiles across the southemmost tip of the Manila trench. The interpreted profiles reveal the same 2 structural fabric as those present to the north where oceanic subduction is taking place. N channel seismic profiles obtained during the cruise do not provide a good iniage of the internal strut- ture of the accretionary \vel@ that w:is de\cribed in detail by Hayes and Lewis (1984) using multi- The Luzon volcanic arc is moderatelq-. but Sig- channel seismic profiles but our seabenni data do nificantly. exposed south cif Luzon where small outline the great linearity of the folds xhich are active volcanoes extend along the eiiwrn coast of nel1 espressrd morphologically. and are 311 paral- hlindoro (e.g.. the Calapan. Naujan and Cumpo lel to the trench asis. volcanoes: Wolfe. 19811. The data presented by Hayes ;ind Lemis (1984). The arc-trench gap is ver>- comples here. reveal an ohvious convergence of the thrust fsults (inshore ;i> well ;is offshore. and includes the miijor at depth. into a possible pnrting plane located at part of llindoro. The structural framework of thi\ the interface betiteen the pelqic and hemipelagic idand is comples and has heen widely debated. diments overlying the subducting oceanic crust Snrewitz lind brig (1986) regard this NW-SE- and the accreted turbidites earlier deposited in the trending orogenic belt as the result of a comples trench. There is no clear evidence for accreted coktge of exotic hlocks. largely htitudennllj dis- oceanic h:rsenient slices in the prism. except for it placed along major left-lateral strike-slip fault,. debatable duplication cif the top of the downgoing On the other hand. Rangin et al. (1985). favour plate at depth. ohsenable on a single seismic SW-verging thruxt faults. lvhich began in hliddle profile (Hayes and Leivis. 1984. p. (1185). Horv- Xiiocene times. Recent \vork on this island has ever. \ve cm reasonably consider the seismic:ilIj revealed that large thrust faults are present11 au- imaged Manila accretionary prism as being formed tive in the southwestern part. and ohlique conver- only of sediments. ivithout the involvement of the gence is prehent elsavhere in Mindoro. oceanic basement. A relatively good decoupling Offshore. the beabeam-derived map ( Fig. 7) re- along a decollement could esist at the boundary veals that the submerged part of the arc-trench between the two convergent plates. Such n situa- gap is structurally comples. Two distinct morpho- tion does not exist south of the Lubang ridge. structural zones can he differentiated: The fore :irc basin has been described in detail (1) At the hase of the inner wall, the iìccretion- hl- Lewis and Hayes (1984). revealing that it i4 ary prism. nhich is parallel to the trench. is n.ell- mainly filled with up to 1600 m of turbidites developed as far :is 119"50'E but it then thins coming from the Luzon trough suhmarine canyons rapidly southeastward. However, :ìctive folds :ìrid ahich cut into the continental slope. Rapid whsi- thrust faults could be clearly identified at the base dence of this fore arc is \vel1 documented. Lind the of the inner wll :is far as the southern boundarl- filling of the basin is :i relatir-el) stead\ \tate of the surveyed area. process only disrupted h\; short periods of excess (1)The upper part of the inner \wll ii char- turbidire mass supply. not ha1;inct.d hj- the \ubsi- acterized bj- structures which are oblique to the cience of the hasin. gener:il trend of the deformation front. most of :\long its eastern bound:iry the baxin is sinunux. the folds trending N-S. These folds. associnted and does not reveal the pre5enc.e of major sctive with E-dipping reverse faults (Fig. 8). curve south- strike-slip faults. bordering the continental slope east\\-ard downslope. and merge Lvith the struc- of Luzon 3s suggested by Kxig (1983).Therefore. tures prehent at the hase of the inner vd.parallel we ciin consider that north of the Lubang ridge to the trench axis. the convergence bet\veen the South China Sra Some of these tramverse structures are also plate and the Philippine Sen plate ix mainly uhsractrrized hj N-S-trending scsrps \shich face :icconimod:ited by simple subduction of an oce- west\v:ìrd and hound smsll basins filled I\ ith :mic plate. inducing a classical xcrrtionarj flat-lying turbidites. This is particularly evident \vedge-fnre xubusin couplet. hetu-ern 130°10' and l?Oo70'E (Fig. 9). These steep ~sa1l.scould represent the emergence of W- verging thruht fault\ or reverfe fault.;. pp. 269-272 LEGEND

VERTICAL FAULT

NORMAL FAULT

ANTICLINE AXIS i )c’ 0Y SYNCLINE AXIS

Fig. 7. Structural geology map of the southern part of the Manila trench, drawn from bathymetric data collected with seabeam, and structures observed on seismic profiles. The dashed structures extrapolated between two seabeam tracks. 213 a 119'40

THRUST

ANTI CLINE

SYNCLINE

13'30

55 [I VE:1.5 I I TWT Fig. 8. Vergence of the thrust and folds towards the fore arc area south of Lubang basin. a. Simplified bathymetric map. b. Interpreted single channel profile.

These N-S-trending structures are active across (1983), this ridge is bordered to the north by the the entire offshore fore arc area from 119'40' to Verde Passage suture, a poorly defined fault zone 120 '20'E (Fig. 7) as is documented by the emer- separating the Zambales ophiolite terrane from gence of the thrust faults on the sea bed and the Mindoro metamorphic complex and extending folding of the sediments up to the sea floor. into Lubang Island. The southern flank of this ridge is bordered by a prominent fault scarp, 3000 The Lubang ridge m high, which fringes a small trough-the Lubang

T basin (Fig. 7-10). \ The Lubang, Golo and Ambil islands represent The fault scarp, probably outlined by vertical the emerged parts of a ridge elongated N60°W, faults not observable on seismic profiles, anas- -+! which disappears westward into the West Luzon tomoses westward. One of these faults slightly basin and rapidly decreases in size eastward along curves N80OW and probably dies out in the the North Mindoro coast. According to Karig Manila trench accretionary prism, while the other B

SE Lubang basin

is trending N50 * W ;ind probably link> n-ith the files. the Lubang hasin ii B structure filled with 1.5 u-edge-fore arc basin hnunditry (Fig. 7). Ea.;t- s (TWT)of sediments. fht-ljing :ìt the base of the xvard. this fault sc:q uldenl\- strikes tt:) the south fsiilt s.xtrp to the north. and progrrssivrlj more and appirently enclow the co:izt of the folded moving south :ìtras> the hasin (Fig. 10). nnrthn.extern tip of Alindori:, (Calvvite Cape). We suggext that the Lubmg >carp is prdxiblj where reterse f;iiilts are in evidence. At the junc- an important >trike-slip fault zi)ne with sonie tion of these tao seth iif fuult,. xteep submarine vertical motion. connecting \tit11 the Cnlavite re- c:inpon\ trend a.e\t\v:trd and may he the trace of verse faults to the east and the compresiive strut- pwihle E-R'-trending minor vertical t'ault>. al- ture> of the :iccretionar) prim in the \vest. It c:tn readi reported hy karig (19S3). On zei\niic pro- he interpreted ;is an inipnrtant trmstcn

105

Fig. 10. The Lubang basin lying southward of the Lubang fault scarp which is interpreted as a transtensional fault. Note the slightly deformed basin-fill sediments along its southwestern margin.

with a left-lateral motion. The Lubang ridge itself vertical faults which connect with reverse faults dies out rapidly northwestward along a regular and thrust faults. slope into the fore arc basin. At the base of the slope, the sediments are folded with a N130OE Structural arrangement of the subduction-collision trend, curving northward and trending to the west. transition zone: An interpretation of the data Here, potential thrust faults connect with a NllO "-trending vertical fault that could be the The morphostructural study conducted at the western extension of the Verde Passage suture of southern tip of the Manila trench reveals that Karig (1983). convergence between the South China Sea and d c, Consequently, the Lubang ridge and associated Luzon is accommodated differently, according to basin, is the site of a complex left-lateral wrench where the lower plate is oceanic or continental. Y fault system, which is difficult to image on seismic We have previously described in this paper the profiles and seabeam data. Thin slices of con- very simple tectonic pattern demonstrated in the tinental material are apparently creeping along case of oceanic subduction, while a complex pat- 27h tern \vith vnrious structural trends is documented spaced active thrust fault> in the San Jose tire;ì in the case of continental subduction. ( PNOC unpublished seismic profiles: hlarchadier, Concerning ths collision, the first iniportant in prep.). result is the presence of :i wide zone of intraplate The East hlindoro block (EMB), bounded to deformation. compared to the subduction setting the west by the Igsoso-Calavite thrust. and to the to the north. where a narrow zone of deformation east bl- the Verdc Passnge fault lvhich links farther testifies to a relatively good decoupling between to the east with the East hfindoro fault. is a the upper and the lower plates. Furthermore. im- NW-elongated block including the major part of portant thrust fault and strike-slip fault zones Mindoro and Lubang Island. This crustal block is delineate distinct crustal microblocks prisent in presenti! colliding along its northLvestern tip with the fore arc area. nhich are jammed betueen the the West Luzon basin (Fig. 11 ). North Palawan block in the southxvest and the These niicroblocks are also affected by intern:ì1 Luzon block in the EaSt. deform:ìtion. It is evident in the \$'ME3 where The West híindoro block (WhlB). is bounded numerou\ N-S-trending folds ;ire docuniented to the \vest by the hlanila trench. and to the Ecist mith of the Luhang basin. attesting the nonrigid hy the active thrusts located offshore lgsoso and behn\.-iour of this block. The same observations Calavite capes in hfindoro whch link to the north are made onshore in the EMB. which is char- aith the Lubang fault. the latter being interpreted acterized hy ;ì complex structure resulting from a5 a strike-slip fault (Fig. Il). These two micro- wperimpozed Tertiar) tectonic events (Rangin et block boundarieh nearly coalesce in Southern al.. 1985: Sarwitz and Karig. 1986). hiindoro. Lvhere they are identified ;IS closel) - The seixniicity registered at the southern tip of the hlanila trench demonstrates the nccurrence of Conclusion several processes (Cardwell et al.. 1980: Hnm- burger et al.. 1983). This study reveals a complex structural (1) The intermediate and deep seismicit) indi- arrangement in the Mindoro collision zone south cate the vertical dip of the Benioff zone approach- of 14ON Lvhich can he opposed to the simple ing the Mindoro collision zone. situation to the north where oceanic crust is sub- (2)The Verde Passage fault can he traced by ducted. 1 plotting the shallow seismicity. and several focal In the collision arca the Ivhole arc-trench gap mechanisms attest to a left-lateral motion along is fragmented into various blocks with distinct . this fault (Seno and Kurita 1978). apparent absolute motion. Their boundary and I (3) The intense and randomly distrihuted re- kinematics is poorly controlled and u'its tenta- gional microseismicity attests to ;ì quite important tively deduced from the geometry of the superfi- internal deformation nithm the WhIB and the cial structures. In hlindoro Island the Middle northern part of the EhfB (Marchsdier in prep.). h-fiocene houndary between the North Psil 1amxn The direction of motion of the distinct micro- block (Eurasian platel and the hlindoro hlock to blocks can be tentatively deduced from the geoni- the east. is outlined by ohducted Middle Oligo- etry of the structures (Fig. 11). If a-e consider the cene ophiolites (Rangin et al.. 1985). interpreted folds affecting the most recent sediments up to the as a jammed fragment of the South China Sea sea floor ab actiw and representative of the pre- oceanic crust hetween the North Pnlau.an and sent state of stress. and if we accept that the Luzon convergent terranes. This suture zone is

principal vector (T,is perpendicular to these active presently inactive attesting to a rapid rearrange- folds. the stress trajectory of IS, can be draa.n for nient of the suture zones within the collision area. these microblocks. These trajectories. shoan in The observed structural arrangement of this colli- Fig. 11 are rotated counterclockwise upslope. down sion zone is prohably unstahle and may be again twvards the trench in the Wh4B for instance. The rapidly rearranged in another ~vay.The unsteady stress trqjectories are fan-shaped. and distributed nature of the sutures nithin collision zones can he at the nose of the two distinct niicroblocks. These easily confronted to the relatively stable and w-ell- protrusions can be interpreted as large-scale. defined decoupling surface of the subduction zone. sheath-like folds, whch are premmed not to he exclusively present in the deep deformation level References of the crust (Hihh:trd and Ksrig. 1986). We speculate here that the symmetrical axis of these 4uhouin. J.. Stephan. J.F.. Roump. .I.,Rend. V.. 1986. The sheath-like folds could indicate the absolute mo- hiiddle America Trench 2s an example of the subduction zone. Tectonophjsic,. Sh: 113-132. tion of the related niicroblock. If this hypothesis is Ciirdn.el1. R.K. et al.. 1WL The spatial diatnbutwn of earth- \.alid. the WMB has an absolute motion tmwd yuahes. focs1 mechanism solutions. and subducted lithn- the west ~vhilethe EMB is moving to.ssrds the sphere in the Philippineb and Northeabtern Indoneaian northwest. This could indicate that the WMB ia Island,. Am. Geophy. Union Geophbs. Ser.. 23: 1-35. heing expelled left-laterally during a northLvest- Dichinson. R.K. and Seely, D.R., 1979. Struclure and btratigra- sv:ird motion of the EMB. The divergent tra-jectory phl cif fore arc regions. Am. .4ssnc. Pet. Geol. Bull.. 63: 2-31. of these blocks is compensated bl the opening of Fricaud. L.. 1984. Étude giologiqur et structurale de la Skirgr a small basin. the Lubang basin. with an ap- Ouebt Palawan i Mer de Chine hl&idionale). These cycle prcxirnately NE-directed extension. The obsen-ed 32me. Univ. d0ra;iy. 2Ih pp. vertical faults (the Lubang and Verde Pasbage Hanhurger, hf.N. et al.. 1'183. Srismotectonics of the Northern faults). interpreted as left-lateral faults uith a Phìllppinea island arc. Geophyz;. hlonogr.. Am. Genphjz;. Union, 27: 1-35. relatively important component are consistent with Hayes. D.E. and Letsis. S.D.. 1984. A geophysical stud! of the , the proposed motion of the EMB. Additionally. hfanila trench. Luzon, Phdippines. 1 : Crustal wucture. during the northtvard migration of the Luzon ter- gravity and regiíin.il tectonic r\oltition. J. tieophis. Rex.. rane along the Eurasian plate houndary. various XQR: 917I-ql9S microblocks :ire created. 278

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