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Center 1 Ectonic Evolution of the Cocos-Nazca Spreading

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Center 1 Ectonic Evolution of the Cocos-Nazca Spreading ,, s11bs1i11ae the.se pages for pages J.10../-/../(}(¡ 111 tÍle Ocwber 1977 Bulletin. 1•. 88. 110. JO. Giorgio DE LA TORRE Mora1e~ Teniente de Fragata-SU 1 ectonic evolution of the Cocos-Nazca spreading center RICHARD HEY ''· Department of Geolog1cal and Geophysical Sciences, Prmceton Unwersity, Prmceto11, New }asey 08 5-1 11 ABSTRACT zone of compression suggesr~(ferron and Heirrzler ( 1967) an( Raff (1968) is ºAAfecSbl quirement rhat rwo or pos_sibh Magnetic and bathymetric data from the easrern Pacific have three of rhe rirn'~u s r sprea o liquely, because "ar triple 1unc been analyzed and a model for the evolution of the Galapagos re­ tions, slafloor spreading cannot be perpendicular to al i rhree of th1 gion developed. The Farallon piare appears to have broken apart rifts" ( to~~:o . ;ipd Dietz, 1972, p. 267). Jn' fact. spreading may b, along a pre-existing Pacific-Farallon fracture zone, possibly the perpen ifulaF· tb-'a~l ·~ r!fts wirhour resulting zones of compres- Marquesas fracture zone, ar about 25 m.y. B.P. to fonn the Cocos sion an · ho.uwcio!a~ 0Í-ffte-rig;d-plate hypothesis. and Nazca plates. This break is marked on the Nazca piare topo­ Van Andel and others (197 1) proposed rhar the aseismic Coco~ graphically by the Grij alva scarp and magnetically by a rough­ and Carnegie Ridges were formed by rifring apart of a pre-exisrint smooth boundary coincident with the scarp. The oldest Cocos­ ancestral ridge. This hypothesis has since undergone severa! mod­ Nazca magnetic anomalies parallel this boundary, implying that ificarions (Malfair and Dinkelman, 1972; Heath and van Andel. the early Cocos-Nazca spreading center trended east-northeast. 1973; Rea and Malfait, 1974). This system soon reorganized into an approximately east-west Holden and Dietz (1972) outlined a solution based on the hot rise-north-south transform configuration, which has persisted until spot hypothesis, in which the Cocos and Carnegie Ridges are re­ the present, and the Pacific-Cocos-Nazca triple junction has since garded as hotspot traces fonned as the Cocos and Nazca piares migrated norrh from its original location near lar 5°$. Jf correct, the moved away from the Galapagos hotspot. This attempt. while in­ combination of these simple geometric constraints produced the structive and partially successful conceprually, failed in detail for "enigmatic" east-trending anomalies south of the Camegie Ridge. severa! reasons. Holden and Dietz misquoted Le Pichon (1968), The axes of the Cocos-Nazca spreading center and the Camegie confusing his America-Pacific pole with his Antarctica-Pacific pole Ridge are essentially parallel; this can lead to paradoxical conclu­ and using that as a Nazca-Pacific pole. Using this invalid pole and sions about interpretation of the Cocos and Cam egie Ridges as an unreliable spreading-rate datum from Heirtzler and others hotspot tracks. Hey and others (1977) have shown that recent (1968) (since reinterpreted) on the Pacific-Nazca rise, they calcu­ accretion on the Cocos-Nazca spreading center has been asymmet­ lared a spreading rate for the East Pacific (Pacific-Cocos) Rise north ric, resulting at least in part from small discrete jumps of the rise of the Pacific-Cocos-Nazca triple junction. This tecimique would axis. 1 show here that the geometric objections to both the "hot­ only be valid if rhe Cocos-Nazca spreading center and triple junc­ spot" and "ancestral-ridge" hypotheses on the origin of the Cocos tion irself did not exist. Solving for the Cocos-Nazca rotation rare, and Camegie Ridges can be resolved with an asymmetric-accretion they predicted ages in the Panama basin rhat were so high they were model. However, ali fonns of the ancestral-ridge hypo thesis en­ used by Heath and van Andel (1973) to discredit the hotspot counter more severe geometric difficulties, and these results support hypothesis. the hotspot hypothesis. After further elaboration of the hotspot hypothesis by Johnson and Lowrie (1972) and Hey and others (1973), Sclater and Klitgord (1973) examined both rhe horspot and ancestral-ridge hypotheses INTRODUCTION anci decided that borh should be rejected, concluding that the Cocos and Carnegie Ridges "are not tecronically related" (p. The "instantaneous" plate motions in the east Pacific are appar­ 6973). ently well known. Different people, working with different biases The difficulty in the Galapagos area arises primarily because the and often different data sets, have derived models quite consistent older magnetic anomalies have proven extremely difficult to corre-· with each other, as well as being intemally consistent (Hey and late, surprisingly so considering the high data density and the ease others, 1972, 1977; Herron, 1972; Morgan, 1973; Minster and with which the very young anomalies are correlated. An important others, 1974). These models predict both magnitudes and direc­ problem is the reason for this difficulty in correlating older tions of relative piare motions and have been tested in many ways anomalies - either clearly recognizable anomalies were never (Hey and others, 1972; Herron, 1972; Forsyth, 1972; Stover, formed here, or sorne mechanism has acted in this area to destroy 1973; Rea and others, 1973). Thus, on a gross scale, we appear to them after they were formed. understand the present-day tectonic configuration. However, at­ On rhe basis of all available evidence, including new data pres­ tempts to understand the details of the evolution of the present sys­ ented here, 1 conclude that a model based on the hotspot tem have been less successful. The most obtrusive and controversia! hypothesis, with the modification of asymmetric accretion resulting of the problem areas remains the Galapagos region, despite rela­ ar least in part from discrete jumps of the rise axis as discussed by tively high data density. Hey and others (1973) and d emonstrated by Hey and others · Herron and Heirtzler (1967) made rhe first guess at an evolu­ (1977), successfully meets rhe objection of Sclater and Klirgord tionary scheme for the Galapagos a rea, a guess that violated the (1973) and allows us to ourline rhe history of rhe a rea from the rigid-plate hypothesis. Holden and Dietz (1972) pointed out this break-up of the Farallon piare and birth of the Cocos-Nazca violation, but their reasoning was incorrect. They stated rhat the spreading center to the present. The " instantaneous" (a term Hey and others, 1977, have examined) configurarion of piare bound­ • Present address: Hawaii lnstiture of Geophysics, Universiry of Hawaii, aries and motions (Fig. 1) has generared a wedge of crusr spread Honolulu, Hawaii 96822. from the Cocos-Nazca spreading center, w hich is characterized by Geolog1cal Soc1ery of America Bullerin, v. 88, p. 1404-1420, 9 figs., Ocrober 1977, Doc. no. 7 1003 . 1404 /' ,// ºº /' // Cocos Plate /,/ _,,,,,,, Cocos -- !'> 6J ªo 069º 71 mm /yr -------------- 5º ~o o Paclflc Piafe ºº'' ""'!."'~'! _)!i}"_,-- 8~ - "l" <o:!~;!~----- _____ ,, 6-....__d 6--1--6 2 7 8 o ...-:.-:-'-'-~~f-./--*,_~-:a~~r-+: 4 4 4 -----,¡-; 71 mm/y'~<1 ~0.........vq,,.. --- 2~ ''-s1'110011¡ -- ----bound__ _ ory ----­--------- --... ,~ª 1a pagos Oº ,,1 slonds Nazca Plate ' ' ' .......... ' ' ' " p 102° 71 mm/yr ' ' ' ' ' ' ' ' ' ' '" -- o ' ........ '100 ' ' .«fo 5º p 1 1 105ºW 100º 95º 90° a ,, , ab0 ~~ 'iW ab0 " , ' 77§",J Figure 1. Diagra~ summarizing "known" aspccts of Galapagos arca. Bathymctry in fathoms lincs are is~hrons datcd in million years beforc present; heavier lines are acrive-rise axes. Iso­ from Chase and others (1971), van Andel and othecs (1971), Mammcrickx and others (1974), chron uncenainty increascs with age. Circled nuinbers are DSDP ages from van Andel and othcrs and Johnson and othcrs (1975). Dashed lines are rough-smooth boundaries. Light subparallel (1973 ). Plate motions are relativc to triple junction. - - .. - ·. · · - - . · ---- . - · ; 1 ' ;;'_~~e ,J 14()6 R. HEY a slow spreadi ng rate. rough topography, and ~ tron g magnetic tors of the sides of the velociry triangle for the special case of syrn. 1 anomalies. This wedge, rermed the Galapagos gore by Holdcn and metric, nonoblique spreading (McKenz1e and Morgan, 1969), Oietz ( 1972) and discussed in detail by Hey and others ( 1977), 1s wh1ch is probably occurring on none of these rises at presenr, surrounded by crust spread from rhe Pacifi c-Cocos and Pacific­ although 1 have constructed the frames of reference as rhough ir Nazca spreadmg cemers which has the smooth morphology com­ were. Mo re generally, frames of reference must be constructed mon to fasr-spreading rises and low-amplirude magnetic anomalies, parallel to their respective piare boundaries through poinrs on the as borh these segments of the East Pacific Rise are oriented nearly relative-motion vectors determined by the percentage of asymmer. 1 parallel ro the Earth's magnetic field vector. M y model explains rhe ric spreading. The junction is stable only if these dashed lines inter. location and o rientation of the magnetic and bathymetric rough­ seer in a point, which shows the instantaneous velociry of the triple smooth boundaries that thus bound the gore and implies that th ere junction. The Cocos-Nazca spreading center away from rhe junc. are rwo generically different magnetic and bathymetric boundaries tion is probably now spreading slightly asyrnrnetrically or jumping, ' in the area. with a maximum obliqueness angle, measured betw~en thc 1 transform-fault azimuths and local perpendiculars to the rise axis, EYOLUTION OF PACIFIC-COCOS-NAZCA of about 10º. If this is also true at the junctiO!J, then in order for this TRIPLE JUNCTION junction to be continuously stable, the Pacific-Cocos and (or ) thc Pacific-Nazca spreading center inust be accreting asyrnmetrically McKenzie and Morgan (1969) assumed for simpliciry that ali and (or) obliquely in a highly constrained geometry (although from rises spread symmetrically and nonobliquely. They were then able our measurements this exact geometry is poorly defined).
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