lDynamique des bassins, Universit⢠Paul Sabatier, Toulouse, France. 2Centre de Brest, Institut Fran9ais deRecherche pour l'Exploitation de la Mer, Plouzanâ¢, ...
TECTONICS,VOL. 18,NO. 3, PAGES546-558,JUNE 1999
Partition between collisionand subductionaccretionary prisms alongan inherited transcurrentfault zone' New insightson the Taiwan fold and thrust belt St6phan½ Brussct, • Pierre Souquct, • Joachim Dbramond, • Jean-Claude Sibuct, 2 Shu-Kun Hsu, 3Benoit D½ffontain½s, 4andHao-Tsu Chu• Abstract. A new geotectonic frameworkof the Taiwan orogen is presented in accordance with the hypothesis of an obliquearcarccollision.The collidingLuzonarc is physicallyconnected to the eastern Coastal Range in which a subductioncomplex remnantis preservedand backthrustwith intra-arcsedimentsin
subductionzone to the north and the eastdippingLuzon subduction zoneto the south[Angelier,1986]. It is considered as the result of the collision of the intra-oceanic Luzon arc with the
passivemarginof the Asian continent[Suppe,1984] or with an extinctpart of the Ryukyuarc borderingthis Asian continent a small retroforeland basin. A southern and extinct extension of [SibuetandHsu, 1997].The plate sutureis recognized alongthe the Ryukyuarc is characterized in westernTaiwan. It displaysa LongitudinalValley Fault on the limit of an easternunit, the duplex structure(Tananao and Backbonehorses and Lishan CoastalRangewhich is connected to the Luzonarc (Figure 1). trianglezone)betweena buried floor thrustlocatedin the arc Our purposeis to showthat the westernunits constitutean arccrustand a roof thrustdevelopedin the arc cover(Hsfiehshan arc collisionprism, involving Ryukyu arc materialsin the Rangeand SouthBackboneRange).Westwardthe basal thrust northernpart of the belt and a subductionaccretionary prism climbs in the sedimentaryseriesof the westernproforeland overlyingthe Eurasiaplate margin which includesthe extinct (Foothillsand HengchunPeninsula)anddiesout in a buriedtip former Ryukyuarc in the southernpart. We also discussthe line. The northernpart of the orogen,includingall the Tananao relationshipsbetween these two tectonic wedges and the arc core, is shown as an intra-oceanic-continentalarc-arc relationships betweenthemandthe CoastalRange. collisionbelt characterizedby an unroofedduplex culmination The newly proposedgeotectonicschemeagreeswith the above a leading floor thrust and both proforeland and recent hypothesisof an arc-arc collision by Sibuet and Hsu retroforelandbasins.The southernpart, which displaysa roof [1997]. The schemetakesinto accountsatellite-derived gravity thrust sequenceabove a buried duplex, is shown as an anomalies[SibuetandHsu, 1997] andseismicprofiles[Reedet accretionaryprism built in a transitionzonebetweencontinent al., 1992] from the southernTaiwan offshore. The schemeis and oceanicsubduction(transitionfrom the Asian continental basedon a thrustinterpretationaccordingto the conceptsof crust, includingthe former Ryukyu arc, to the oceanicOld Boyer and Elliott [1982] and on a stratigraphicalsynthesis PhilippineSeacrust).The partitionis believedto be inducedby (Figure2) comingfromHo [1988], bothcompleted by regional a deepintracontinental transcurrent fault zoneable to influence studiesquotedin the text and by our own field investigations. the differencein shortening,duplexpattern,and leadingthrust We attemptto explainthe geometryand the progression of the depth.The evolutionwas controlledby the Ryukyusubduction deformation frontusinga model[Spechtet al.; 1991;D•ramond (backarcextension,arc magmatismextinction,and coolingand et al.; 1993] alreadyappliedin the TaiwanFoothills[Ddramond intra-arccollapse)until the early middle Miocene(around15 et al., 1996] that links the thrustpropagationand the growthof Ma) and then it was controlledby the Luzon arc progression the associated anticline to the sedimentation in a frontal (continental subduction,collision, indentation, and hinterland synclinaldepression. uplift andfrontalthrustpropagation). _
1.
2. Present-DayConfiguration:A Revised
Introduction
Geotectonic
Framework
The Taiwan mountain belt is located along the EurasiaPhilippineSeaplateboundarybetweenthe northdippingRyukyu
The Taiwanmountainbelt is dividedinto N-S trendingunits [Ho, 1988].Theseare, from Eastto West (Figure 1), the Coastal Range(CR) east of the LongitudinalValley Fault (LVF); the Tananao Schist (TS), the Backbone Range (BR), and the lDynamique desbassins, Universit• PaulSabatier, Toulouse, France. Hsfiehshan Range(HR) groupedinto the CentralRange;andthe 2Centre deBrest, Institut Fran9ais deRecherche pour l'Exploitation WesternFoothills(WFH). The CoastalRangeis connected to the de la Mer, Plouzan•, France. 3Institute ofOceanography, National Taiwan University, Taipei, Taiwan Luzonarc.We intendto showthatthe otherunitsbelongto two 4D•partement deG•otectonique, URA1759,Universit6 Pierre etMarie different tectonicwedges controlledby two distinct basal Curie, Paris,France.
5Central Geological Survey, Ministry ofEconomic Affairs, Taipei, Taiwan.
d6collement surfaces.
2.1. Structural and Stratigraphical Setting
Copyright 1999bytheAmerican Geophysical Union.
2.1.1. Coastal Range. The Coastal Range representsthe Papernumbe• 1999TC900005.
collided northern extension of the Luzon arc now accreted to the
0278-7407/99/1999TC900005512.00
easternTaiwan(Figure1). It is boundedwestwardby the Lon546
BRUSSET ET AL.: COLLISION AND SUBDUCTION PRISMS IN TAIWAN 120 ø
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Figure1. Simplified structural mapof the'Taiwan orogen. CR,Coastal Range; SBR,Southern Backbone Range; TS,Tananao Schist; BR,Backbone Range; WFH,Western Foothills; HP,Hengchun Peninsula; LVF,Longitudinal ValleyFault;SLT,Southern Longitudinal Trough; OPSP, OldPhilippine Seaplate;HR,Hsiiehshan Range [modified afterStephan etal., 1986;Ho, 1988;Biq, 1992;Huanget al., 1995;Di,ramondetal., 1996;SibuetandHsu,1997].
postcollisionalepisutural basin Bmning from the onshore LongitudinalValley to the offshore SouthernLongitudinal Trough.It encompasses, fromeastto west[Huanget al., 1995]; the Luzon arc basement(volcanicislands), a forearc basin corresponding to an earlyMioceneto Plioceneforearcsequence (North Luzon Trough), and a subductioncomplex (Huatuag (or intra-arcsequence) [Huanget al., 1995]of volcaniclasticsRidge).This subductioncomplexrepresentsfor us the offshore (Tuluanshan Formation) overlying an igneous complex(Luzon extensionof the Lichi m•lange. Seismicprofiles [Reedet al., arc)andto a synchronous subduction complex weidentifyin the 1992;Lundberget aL, 1997] showa strongdeformationby eastasdescribedabovein the CoastalRange. Lichisubruction m61ange (Figure2). Owingto theN-S progres- vergentbackthrusts 2.1.2. Northern are-are collisionalprism. The four units sionof thecollision[Suppe,1984],thism61ange is preserved in the southern endof theLongitudinal ValleyFaultandelsewhere developedwest of the LongitudinalValley Fault (Figure 1) eroded.Syncollisional orogenic sediments arerepresented by the constitutea westvergenttectonicwedge (Figure 2 and 3) we
gitudinal ValleyFault(present-day platesuture), whichdisplays a sinistralstrike-slip component, andwe interpretit asa former west-vergent thrust(Figure3 and4). The CoastalRangecomprisesrocks of the precollisionalLuzon arc-trenchsystem
overlyingPlio-Pleistocene sequence of terrigenous and intra- relate to a former Ryukyuarc system. basinalgravityflow sediments (Takangkou-Shuilien-Chimei2.1.2.1. Tananao Schist:The TananaoSchistis unit is a precomplexoriginatedin the uaderthrust Formation) whichfill several smallen6chelon basins. We inter- Tertiarymetamorphic
pret. these basins, which exhibit a southeastward youaging trend and includeTaiwan be}t-deriveddetritus[Huanget aL, 1995],
Asiancontinentandincorporated in the mouataincore.It has
beeninterpreted as a remnantof a passive continental margin [Huanget al., 1997]or of a magmatic arcsystem[Biq, 1981; Chen, 1991]. The latter interpretation is supported by the petrotectonic polarity,whichled us to consider the Tananao
asmigrating depocenters in a retroforeland basingenerated by an east-vergent backthrust sequence and thendeformedby a sinistralstrike-slip faulting(Figure3 and4). The CoastalRangeextendssouthward to the northernsub- Schistas the southernextensionof the Ryukyuarc [Kisald, marine portion oftheLuzon arcsystem (Figure 1).Thiscolliding 1986;SibuetandHsu, 1997].TheTananaoSchistincludesthen of a magmatic arc(westernTailukobelt: system develops eastof theplatebouadary fault,markedby a [Ho, 1988]remnants
548
BRUSSET ET AL.: COLLISION
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