_________________________________________O_ C_E_A_N_O_L_O_G_'C_A__ A_CT _A_,_'_9_6_' ,_N_O_S_P__ ~e------- ' POO Convergenl margÎns A summary of results Japan Mariana Middle America IPOD from the IPOD transects Marges convergentes Japon MarÎannes across the Japan, Mariana, Amérique Centrale and Middle-America convergent margins R. von Huene · , S. Uyeda b • us GeOlogie;!] S urvey, MaÎISlo p 9'J, 345. Middleficld Road . Mellio Park. California 94025, USA. b Earthquake Research Institute. Unive rsity of Tokyo, Bunkyo-ku , Tokyo 113. Japan. ABSTRA CT Investigations of convergent ma rgins along the IPOD transects support the concept of occan floor s preading in bad·arc basins and the concept of tcclonically accreled sediment al the front of convergenl margins. However. not ail convergent margins have large accreted complexes, and other Jess frequcntl y used concepts arc required in the interpretations of the ~ e convergent margins . Ir the pre ~e nt rates of plate convergence are accepted, then much sediment that entered the trenches studied is presumably subd ucted rather than 3ccreted. In some instances, the continental framework i ~ truncated and somehow removed by tectonic erosion. Some convergent margins have s ubsided significantly during subduction. The crus t above the Be ni off zone appears to have been thinned by subcrustal erosion or the configura ti on of the Wadati-Benioff has changed. However. since only pari of the subsidence can be explained by erosion, the change in thermal structure res ulting from e hanging rates of s ubduction may be the major cause. To diHerentiate s ubduction-related processes eommonly requires time·stratigraphie information from drill samples to ma ke a kinematic reconstruction of. the margin. Ocemrol. A cta, 1981 . Proceedings 26,n International Geological Congress. Geology of continental margins symposium. Paris, July 7·17, 1980, 233·239, RÉSUMÉ Résumé des résultats des forages IPOD à travers les marges convergentes du Japon, des Mariannes et de l'Amérique centrale. Le s recherches s ur les ma rges convergentes le long des transects IPOD confirment tes concepts d 'expansion océanique dans les bassins d'arrière-arc. et d'accréti on tectonique de sédiments a u front de ces marges. Cependant, toutes les marges convergentes n'ont p'IS de grands complexes accrétés et l'on doit utiliser d' autres concepts moins classiques pour interpréter ces dernières. Si les taux actuels de convergence de plaques sont acceptés, on doit alors considérer que la majeure partie des sédiments arrivant dans les fosses élUdiées est probablement subd uctée a u lieu de s'accréter. Dans quelques cas, le bâti continental est tronqué et q uelque pe u enlevé pa r l'érosion tectonique. Quelques marges convergentes ont connu une importante subsidence durant la s ubd uction. La croûte a u·dessus du plan de Benioff semble a voir été amincie par érosion so us·crustale. à moins que la confi guration de la zone de Wadati· Be nioff ait changé. Cependa nt . on ne pe ut expliquer qu'une partie de la s ubsidence par l'érosion : le cha ngement dans la structure thermique rés ultant des taux variables de s ubducti on peut en être la cause majeure. Po ur reconnaître les processus dus à ln subduction, il fa ut des données chronologiques et s tratigraphi ques obtenues à partir des forages, afi n de faire une reconstruction cinématique de la marge. Ocecmo/. Ac/a, 1981. Actes 26· Congrès International de Géologie, colloque Géologie des marges continentales, Paris, 7- 17 lu il. 1980, 233·239. 233 A. VON HUENE, S. UYEDA INTRODUCTION The staff of Leg 59, working farther south, concluded from a sequence of basement ages that back-arc spreading in the The IPOO transecls of geophysical data and drill cores south part of the west Philippine Basin may have been across the Japan, Mariana and middle-America col'lvergent simullaneous with volcanism along the Palau-Kyushu margins el!:pand knowledge of active margins into areas not Ridge. Initial spreading and arc volcanism were followed by touched by geophysical data alone. The eombined modern subsequent episodes of spreading and volcanism along the geophysics and drill data provide a time-stratigraphie sec­ Mariana Ridge and Arc. The staffs of Legs 59 and 60 tion from which the rates of tectonÎC and sedimentary documented more thoroughly than before that new volcanic processes ean be esti mated . Samples from beneath the arcs developed sequentially. leaving behind a series of ocean floor indieate the depth of past depositional environ­ rem nant volcanie ridges and assocÎated back·arc basins. ments and Ihus, by comparing their past and present Although the back-arc basin crust appears 10 develop position, sorne sense of vertical tectonism can be obtained simultaneous with island-arc volcanism during long periods . in a geologic sening dominated by the horizontal motion of there is sorne indication that for short periods one process two convergent lithospheric plates. occurs without the other. Therefore it is slili not clear how mechanisms generating back-arc magma and the associated The information provided by IPOD drilling on active tensional forces are assocÎated with converging lithospheric margins supports many of the general concepts that the plates. program was designed 10 tesi. The accretion of sediment at the front of the trenen slope was persuasively supported by dala from the Oaxaca transect (Moore et al.. 1979), but accreted deposils were not ro und everywhere. T he develop­ MAG MATIC ROC KS IN THE FORE-A RC REGION ment of back-arc basins by crustal spread ing was convincin­ gly confirmed (Klein el al. , 1978 ; Kroenke, Scon, 1978 ; Along the Japan and Mariana tra nsects. arc-relaled volcanic Hussong et 01., 1978), and sorne insight was obtained as to and intrusive rocks were recovered at sites near the trench the complexity of this proeess. Allhough new discoveries slope, far seaward of the presenll y active arc (Fig. 1). Off were el!:pected, few anti ci pated their proportions. There is Japan, the recovered island-arc roc ks are of rhyolitic to evidence along ail transects Ihat a large amount of sediment dacitic composition and from intrusive and el!: lrusive envi­ is subducted rather than accreted. that continental li thos­ ronments (Fujioka, 1980). Refraction data (Nagumo el al., phere may be subducted, and thal lithofacies of the treneh 1980) indicate that another igneous body occurs nearby, and floor , treneh slope. anç! shelf cannot be predicted wit h only other plUlons are suggested by low-amplitude magnetic simple models of sediment distribution. T he information anomalies (Oshima et 01. , 1975). The crystallization age of from each leg has been summarized at this colloquium by these rocks determined by isotopic dating is 22-24 MY , the scientific staffs of each of the [POO active-margin legs. whieh may be about the same age as the initial volcanism ln this summary we can refer only to the preliminary results along the present arc (Yanagasawa el al.. 1980). Whe re of the legs because many of the Initial Reports of the Deep these rocks occur. the upper surface of the subduclina Sea Drilling Project on IPOD active margin legs have nOI yet ocean lithosphere is shallow, and if the present temperature been published. We have el!:tracted the main resul ts from gradient measured downhole is eXlended to the base of the the IPO D active margins program in this summary of the upper plate. the temperature is about one-quarter of that first of thTee colloquia on active margins at the needed 10 produce island-arc magma (Fig. 1). 26'h International Geological Congress. ln the Mariana transect. arc-related volcanic and plutonie rock was recovered by dredging from as far seaward as the lower slope o f the Irench (Dietrich el al. , 1978). The dredged and drilled Eocene and Oligocene rocks are sorne of the THE DEVELOPMENT OF BACK-ARC BASINS BY oldest of the Mariana Arc (Hussong et al. , 1978). The SEA-FLOOR SPREA DI NG altered state and their low melting temperatures indicate Karig (1971) proposed Ihat back-arc basÎns in the Philippine Sea (for el!:a mple. the Mariana Trough) formed by spread ing of ocean crust, and the results of Qlomar Cha/lengerdri1ling . ... ........ 1 ...:~: :;"". ~ on Leg 31 are ge nerally consistent with the spreading hypothesis (KarÎg. 1975). There has always been a question '''- _.. ..... , of how back-arc spreading was related to plate convergence " --- and subduction. ..: 1~~="~'~~~~~~~'~"~"~'~'~"~"~'~"~"i"~~;;~~~~~~~ ln the complex Oiato Ridge and Basin province of the Contln.nt.1 Cru" c.,~., northern Philippine Sea. back-arc spreading SÎmultancous " '''nll. with arc magmatism was confirmed by the studies of the -_... - scientific staff on Leg 58 (Klein et al. , 1980). Some fo rmer .. ~=:~~~~O~'~.. ._~__ __________ ~~:::::..._.~::::.... ."::.~ magmatic ridges are now subsided remnant arcs ; the basins ! presently contain volcanic rock formed during arc and Figure 1 post-arc vo[canÎsm. The evolution of Ihis complex are<l can Composite section along lhe lopan Tren ch transect based on now be interpreled within the eonstraints of a broad seismic-refltclion and refTa c/ion dola. land ...ard part of ref/tc/ion mul tidisciplinary set of data. Il includes about 35° nor­ data ofter Ishi ...ada and Ogawa (1976). refraclion data from thward migration and 55° rotation of a back-arc basin and Murauchi and Ludwig (in press). The angular unconformity fonned subsequent periods when paris of the basin were subducted. by sabaeria/ erosion in the late Paleogene and Neogene ;s indicated bl' a \l'a"ey line. The acereled complex is inferred from seismic The remaÎning que stio ns concern the symmetry of the dat(l ; on/)' Ol'er/ying slope sediment II·I.IJ' drilled. Da cite an d spreading that formed the west Philippine Basin and the associa/ed islund-arc rock ll'a s rtcO l'tred al site 439 ; numbers refer ultimate age of ilS crust.