Tectonic evolution of the easternmost Panama basin: Some new data and inferences

Journal of South American Earth Sciences, Vol. 4, No. 3, pp. 261-269, 1991 Printed in Great Britain

0895-9811/91 $3.00 + 0.00 © 1991 Pergamon Press plc & Earth Sciences & Resources Institute

T e c t o n i c e v o l u t i o n of the e a s t e r n m o s t P a n a m a Basin: S o m e n e w d a t a a n d i n f e r e n c e s N. C. HARDY* School of Earth Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT England, UK (Received October 1990; Accepted February 1991) Abstract---A scheme for the evolution of the easternmost Panama Basin is described, based largely on the interpretation of magnetic data from the area, the most recent of which was collected aboard RRS Charles Darwin in July 1989. A previously unknown extinct oceanic spreading center the "Buenaventura" rifl~, with an extinction date of about 12 Ma, has been identified passing beneath the Colombian accretionary complex at 3°40'N. The extinction of this rift is attributed to the attempted subduction of a scarp in the oceanic basement, marking the northern boundary of the Galapagos gore, beneath what is today Panama. Transtensional sections along the sinuous Cocos-Nazca plate boundary, which existed between 12 and 8 Ma, are invoked to explain the formation of the Yaquina graben. It is further proposed that transpressive sections along the same boundary may have been partly responsible for the "locking up" of the transform, ultimately contributing to the cessation of spreading along the Malpelo rift at 8 Ma. The southern boundary of a possible "Coiba" microplate is tentatively linked with a line of abandoned rifting at 5"N. A rit~jump at 16-17 Ma led to initial abandonment, but subsequent rejuvenation of spreading activity, caused by divergent slab pull forces at the trench, may have led to the formation of a roughly V-shaped rifted zone. R e s u m e n - - L a evoluci6n de la parte mas oriental de la Cuenca de Panamfi se describe mediante un esquema basado pr/tcticamente en la interpretaci6n de los dates magn~ticos del firea, entre los cuales los m#is recientes fueron temados abordo del barco de investigaci6n Charles Darwin en julio de 1989. Ha sido identificado un centro de expansi6n oce#tnico antes desconocido, extinto hace alrededor de 12 Ma denominado: El ripe de Buenaventura, el cual se encuentra pasando bajo el complejo de acreci6n colombiano a 3"40'N. La extinci6n de dicho ritt se atribuye a la subducci6n fallida de tin escarpe en el basamento oce~mico, el cual demarcaba el limite notre del gore de Galapagos debajo de lo que hoy es Panam#i. Se explica la formaci6n del graben de Yaquina partiendo de secciones transtensionales, de edad comprendida entre 12-8 Ma, a lo largo del limite sinuoso de placas Cocos-Nazca. Luego, se sugiere que las secciones transpresivas a lo largo del mismo limite pueden haber sido en parte responsables del bloqueo en el desplazamiento transformacional, contribuyendo finalmente a la suspensi6n de la expansibn a lo largo del rift de Malpelo de 8 Ma. El limite sur de una posible microplaca de "Coiba" coincide tentativamente con la linea de un rift abandonado a 5°N. El salto del rift hace 16-17 Ma conllev6 al abandono inicial del mismo, pero la reactivaci6n subsecuente debida al esfuerzo extensional en el bloque subducente por divergencia en la fosa, puede haber conducido a la formaci6n de una zona de rift en forma de "V."

INTRODUCTION THE EAST PANAMA BASIN h a s b e e n d e f i n e d a s t h a t a r e a e n c l o s e d b y t h e P a n a m a f r a c t u r e zone, t h e C a r negie Ridge and the continental margins of Colombia and Panama. It lies within the Galapagos gore, a region of oceanic crust formed at the Cocos-Nazca spreading center rather than at the East Pacific Rise ( F i g . 1; H o l d e n a n d D i e t z , 1972). T h e e a s t e r n b o u n d a r y o f t h e Cocos p l a t e w i t h t h e Nazca plate lies along the slightly splayed Panama f r a c t u r e z o n e a t 8 2 ° 4 0 T ¢ ( A d a m e k et al., 1988). O v e r a period of several million years, this boundary has jumped westward in steps as a result of successive rift extinctions in the eastern Panama Basin. It has even b e e n p r o p o s e d t h a t t h e p r e s e n t - d a y b o u n d a r y is in t h e p r o c e s s o f a f u r t h e r j u m p to t h e t r a n s f o r m a t 8 4 ° 3 0 ' W ( v a n A n d e l et al., 1971).

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D u r i n g J u l y 1989, a p p r o x i m a t e l y 3 5 0 0 k m o f a d ditional magnetic profiles were collected simultaneously with GLORIA side-scan sonar and seismic reflection data, aboard RRS Charles Darwin (Cruise CD40). T h i s d a t a s e t c o v e r s t h e a r e a d i r e c t l y a d j a c e n t

*Present address: Unocal U K Ltd, 32 Cadbury Road, Sunburyon-Thames, Middlesex T W 1 6 7 L U England, U K (tel: [44] (932) 785600; fax: [44] (932l 780159; telex: 928348 U N O C A L G).

261 SAMES 4/3~H

The pattern of fossil transforms and rifts, and their spreading histories, in the eastern Panama B a s i n ( F i g . 2) h a v e p r e v i o u s l y b e e n e x a m i n e d i n s o m e d e t a i l b y L o n s d a l e a n d K l i t g o r d (1978) u s i n g m a g netic and seismic reflection data. They established that spreading ceased along the Malpelo rift at 8 Ma, with a 3 m.y. period of reduced spreading rate directly preceding this. The oldest crust positively identified w i t h i n t h e e a s t e r n P a n a m a B a s i n w a s o f a n o m a l y 6C a g e (25-26 Ma); t h i s w a s f o u n d o n t h e n o r t h e r n f l a n k o f t h e M a l p e l o rift. D a t a f r o m e a s t o f t h e Y a q u i n a g r a b e n , h o w e v e r , w e r e t o o s p a r s e to a l l o w t h e s e i n v e s t i g a t o r s to d r a w d e t a i l e d c o n c l u s i o n s a b o u t t h e evolution of this easternmost part of the basin.

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to the continental margin of Colombia. About half of the magnetic anomaly profiles are suborthogonal to the oceanic spreading pattern. Where these are of suitable quality, synthetic a n o m a l i e s have been generated from ocean floor spreading models using the magnetic reversal time scale of H a r l a n d et al. (1990). A magnetization of approximately 3 M m has been a s s u m e d for a m a g n e t i z e d l a y e r of 1 km thickness. Approximate depths to basement have been derived from single channel seismic data. The detailed relief of the oceanic basement has not been included in the models, as this is small compared to the overall depth to basement. A magnetic inclination of 0 ° has been assumed because the oceanic crust responsible for the magnetic anomalies was generated close to the equator (Hey, 1977). Observed magnetic anomalies were obtained by subtracting the theoretical earth field, computed from the measured values using the 1985 International Geomagnetic Reference Field (IGRF 85). No correction has been made for diurnal variation. All the magnetic profiles shown in figures are projected onto straight lines orthogonal to spreading fabric. The most o c e a n w a r d profile runs along the Colombian trench, just to the west of the accretionary complex deformation front (Fig. 3; Hardy, in prep.). It is on this profile that the sequence of m a g n e t i c reversals is best resolved. For profiles closer to the coast, the amplitude and clarity of the magnetic anomaly is much reduced. On these inner profiles, the subducting oceanic crust dips beneath an increasing thickness of accreted and fore-arc basin sediment. The increase in depth to the magnetized basement

results in the weaker anomaly observed. In addition a high geothermal gradient, indicated by an unusually shallow gas hydrate reflector visible on the multi-channel seismic reflection data, coupled with the increased depth of burial, may cause a reduction in remanent magnetization. A similar quenching of the oceanic magnetic anomaly, as it approaches a line approximately 100 km landward of the trench, has also been observed along the Aleutian and Japanese convergent margins (Heirtzler, 1985) and east of the Lesser Antilles arc (Westbrook, 1984).

DATA M O D E L L I N G A N D I N T E R P R E T A T I O N Between 2°N and 4°30'N, three parallel profiles show the magnetic lineations to have a strike of 110 ° + 5° (Fig. 3). The generation of a synthetic anomaly (Fig. 4) reveals that an extinct rift (hereafter referred to as the B u e n a v e n t u r a rift) passes b e n e a t h the accretionary complex deformation front at 3°40'N. There is no bathymetric expression of this rift, however, because it is buried by trench sediment. My estimate of the time of rift extinction is approximately 12 Ma, a l t h o u g h to a c h i e v e a b e t t e r fit between the s y n t h e t i c and o b s e r v e d a n o m a l i e s , spreading has been continued on the southern flank alone until 11.8 Ma. Whether this genuinely reflects a short period of one-sided accretion is debatable, although it is considered possible (see for example, Morgan, 1972). The full average spreading rate, 65 ram/year over the period 17 Ma to 12 Ma, is comparable to that

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determined by Lonsdale and Klitgord (1978) across the Malpelo rift (55 mm/year). The most notable difference is that the sense of asymmetry is reversed; the B u e n a v e n t u r a rift is i n t e r p r e t e d as having slightly greater average half spreading rates on the southern flank. Asymmetry of this type has been identified for crust, between 14 Ma and the present, along the Costa Rica rift between 83°W and 84°W (Hey et al., 1977). On the Malpelo rift, the slowdown in spreading was equal on both flanks, whereas on the Buenaventura rift the reduction in spreading rate was confined solely to the northern flank. A magnetic profile collected by Scripps Institute of Oceanography aboard R / V Melville in May 1977 (Cruise FD774) is complementary to the CD40 data set. The spreading rates and timing of the extinction along the Buenaventura rift are largely confirmed by the use of this additional profile (Fig. 5). It is clear from both profiles that, on the rift's northern flank, there is no fit with the synthetic profile for anomalies older than anomaly 5B/5C. The same is found to be true for the southern flank, although if 1-2 m.y. of the m a g n e t i c time scale a r o u n d a n o m a l y 5C/5D is assumed to be absent, on this flank, then, fits for anomalies 5E, 6, and 6A can be obtained (Fig. 6). The CD40 profiles continue north of 5°N (Fig. 7), but the magnetic data collected are of greatly reduced value, either because of the thickness of sediment above basement or because the ship's track is at too great an angle to the strike of the magnetic linea-

Fig. 7. The pattern of magnetic anomalies recognized offshore northern Colombia, using data collected during cruises PANBSN (NOAA, 1970}, PLDS01 (Scripps, 1976), FD774 (Scripps, 1977) and CD40 (University of Birmingham, 1989).

tions. Only a small section close to the Colombian accretionary complex deformation front, between 5°N and 6°N, has been considered at all s u i t a b l e for modelling and, even here, the anomaly is strongly disrupted by the presence of a series of b a s e m e n t ridges, which can be seen on GLORIA and seismic reflection data. The assumption that oceanic basement relief only contributes second order effects breaks down in this case - - probably resulting in the poor fit with the synthetic anomaly (Fig. 8). Of most use in the northern area are two lines collected by Scripps Institute of Oceanography, one in 1976 (Cruise PLDS01) and one in 1977 ( C r u i s e FD774), and a short third line collected by NOAA in 1970 (Cruise PANBSN) (Fig. 7). The synthetic anomaly pattern generated to match these data requires that the half spreading rate be reduced from 42 mm/year to 14 mm/year between 24 Ma and 17 Ma (Fig. 8). This is not inconsistent with the half rate of 40 mndyear between 25 Ma and 20 Ma, which was determined by Lonsdale and Klitgord (1978) along several profiles just to the west of a transform that they inferred to exist at 79°30~V. In slight contrast to the situation in the south, the strike of the lineations is virtually east-west. The poor correlation between the synthetic and observed profiles after anomaly 6C3 is explained by the fact that the ship's track crosses here from crust generated at an oceanic s p r e a d i n g

266

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center, to an allochthonous basement high (Hardy et north of the zone of rifting abandoned by the rift jump, a transform at approximately 78°10Tq is tenal., 1990) with a quite different origin. A curious aspect of the synthetic model for the tatively inferred from an offset in the observed anonorthern section is that anomalies 5B and 5C are malies (Fig. 7). An apparent rotation of the northern apparently required to provide even an approximate part of the basin relative to the south m a k e s it fit with the observed profiles (Fig. 8). These ano- difficult to determine whether this was originally a malies, however, have already been used on what is continuation of the southern t r a n s f o r m described considered to be the s o u t h e r n e x t e n s i o n of this above. northern flank (Fig. 5). Considering the data from both north and south BASIN EVOLUTION of 5°N, it is not possible, by inference of transforms alone, to develop a model that accounts for the abApplying these new inferences on the location, sence of anomalies 5C and 5D on the southern flank (Fig. 6) and the apparent multiple occurrence of spreading rate, and time of extinction of the various anomalies on the northern flank. A more straight- rift sections, the following is proposed as an account forward solution is that the site of active spreading, of the tectonic evolution of the easternmost P a n a m a on the Buenaventura rift, jumped southward about Basin (Fig. 10). The Farallon plate broke apart between 25 and 26 16 Ma removing 1.5-2.0 m.y. of oceanic crust from the southern flank and accreting it to the northern flank Ma, with the birth of the Cocos-Nazca spreading (Fig. 9). Such southward ridge jumps are well docu- center, along a pre-existing 060 ° fracture zone (Hey, mented in the western Panama Basin (Hey, 1977). It 1977). A scar dating from this time is preserved on must be conceded, however, that attempts to incor- the Nazca plate as the Grijalva scarp (see Fig. 2, 4°S). porate the northern zone of abandoned rifting into the This is a prominent bathymetric feature with up to spreading model have not provided a satisfactory 1500 meters of relief. match between synthetic and observed anomalies. In the eastern part of the basin, the s p r e a d i n g This suggests further complications to the situation center rapidly reorganized into a series of short eastwest sections that included the Malpelo and Buenathat are discussed below. A discrepancy in the extent of the rift jump, seen ventura rifts Spreading from 25 Ma to 21 Ma was on the southern flank between profiles CD401 and probably asymmetric on both rifts, with half rates of FD7741 (Fig. 6), can be explained by a single trans- 40 mngyear to the north and 25 mndyear to the south form of limited offset (Fig. 3). On the northern flank, According to Lonsdale and K l i t g o r d (1978), the Galapagos hot spot first b e g a n to g e n e r a t e the thickened crust of the Carnegie and Malpelo ridges sometime between 22 Ma and 17 Ma. It should be noted that hot spot ridge building did not occur along S N the B u e n a v e n t u r a rift, although t h i c k e n e d ridge 1 0 0 k m ,__~ crust was formed at, or close to, the Malpelo rift. PANeS__NN The sense of asymmetry in spreading rate began to reverse on the Buenaventura rift between 21 Ma /I ^ i I and 20 Ma. A relatively constant half rate of 35-40 Iv.//V? W/fV I;flll ~ mm/year was maintained on the s o u t h e r n flank, whereas the spreading half rate on the n o r t h e r n FDT74 2

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Tectonic evolution of the easternmost Panama Basin Some new data and inferences

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N.C. HARDY

flank, between 21 and 17 Ma, reduced in steps to 14 mmJyear. This high level of asymmetry culminated in a southward jump of the active rift at about 16 Ma (Fig. 9). The Buenaventura rift was split by a transform (see Fig. 3) during this episode. From this time onward, however, these two slightly offset Buenaventura rift sections developed with largely similar spreading rates. From 16 Ma to 13 Ma, near-symmetric spreading (on average slightly slower to the north) continued along both sections of the Buenaventura rift, with average h a l f rates of approximately 35 mm/year. However, spreading at this time along the Malpelo rift was more asymmetric, with h a l f r a t e s of 20 mm/year to the south and 35 mm/year to the north. At about 13 Ma, a n o r t h e r n equivalent of the Grijalva scarp (which formed the northern boundary of the Galapagos gore) contacted an active subduction zone south of what is today the Panamanian isthmus. The broadside arrival of such a topographically pronounced and relatively buoyant ridge was sufficient to seriously disrupt the ongoing subduction process (Lonsdale and Klitgord, 1978). In response to this collision, the half rate on the northern flank of the Buenaventura rift was reduced to just 14 mm]year. Significantly, spreading on the southern flank continued unaffected at the same rate as before. Finally, at 12 Ma, the P a n a m a n i a n subduction zone was completely plugged by the underthrust scarp and, in response, spreading ceased along the Buenaventura rift. The rather sinuous n a t u r e of the right-lateral transform boundary now occupied by the Yaquina graben, between the Buenaventura and Malpelo rifts, became of some importance at this time. By 11 Ma, the transform had started to lock up, which resulted in the reduction in spreading rate along the Malpelo rift. Taking the northern rift flank as an example and viewing it from the south, if the transform was offset to the west, it would have become transpressive

N

S

a)

1

C)

1 /I L

1

L

PULL APART BASINS

AN EAST STEPPING RIGHT LATERAL

SMALL PULL APART BASINS

COALESCE TO FORM

TRANSFORM BOUNDARY IS INITIATED

FORM WHERE

THE YAQUINA GRABEN

THE TRANSFORM STEPS TO THE EAST

Fig. 11. Three stages in the development of the Yaquina graben.

- - leading to the locking referred to above. This may have been the case in the northernmost part of the basin. Conversely, where the transform was offset to the east - - as is the case today for the fossil transform between 1°40'N and 4°N (see Fig. 2), it became transtensional and small pull-apart basins opened (Fig. 11). These coalesced to form the Yaquina graben, a pronounced b a t h y m e t r i c f e a t u r e t h a t even t o d a y shows relief of greater than 2000 meters. The absence of a terrestrial sediment source has m e a n t that the graben has never filled. Spreading finally ceased along the Malpelo rift at 8 Ma, probably when the scarp on this section of the northern gore boundary entered the P a n a m a n i a n trench. In contrast to the Buenaventura rift section, it seems that the scarp did not penetrate significantly into the subduction zone. Hence, the full series of anomalies out to 25-26 Ma is preserved. It is not clear whether the active Cocos-Nazca plate boundary jumped directly at this time to its present position along the P a n a m a fracture zone, or whether there was an i n t e r m e d i a t e b o u n d a r y a t 82°W. The e a s t e r n P a n a m a basin a p p e a r s to h a v e remained a tectonically active region since 8 Ma (von Herzen, 1971). A "Coiba" microplate may exist to the north of the Malpelo ridge (Hey, 1977; Adamek et al., 1988), which moves independently of the Nazca plate. The existence of such a microplate could explain the differing strike of the magnetic lineations mapped in the northern and southern parts of the basin. The southern boundary of such a microplate may coincide with a line of inherent weakness associated with the zone of abandoned rifting at 5°N. It is possible that the abandoned rift has at times been rejuvenated by tension resulting from slab pull forces a c t i n g in d i f f e r e n t d i r e c t i o n s due to the curvature of the Colombian trench (see Fig. 7). This is believed to have been the mechanism o r i g i n a l l y responsible for the o p e n i n g of the Cocos-Nazca spreading center (Wortel and Cloetingh, 1981). A similar process today is thought to be the cause of the Mendana fracture zone, situated off Peru, opening in a "V" shape with its apex pointing a w a y from the trench (Huchon and Bourgois, 1990). As a result of this geometry, the strikes of older magnetic lineations on either side of the Mendana fracture zone are no longer parallel. Perhaps this is an explanation for the variation in strike of magnetic lineations observed in the eastern Panama Basin (see Fig. 7). The possibility of north-south oceanic spreading within the eastern Panama Basin has been the subject of speculation by recent authors (de Boer et al., 1988; Silver et al., 1990). This speculation has been prompted by evidence for active convergence along the southern boundary of the P a n a m a block. Recent seismic reflection and side-scan sonar data, however, have shown that the active plate boundary south of the Gulf of Panama is formed by a left-lateral transform (Hardy et al., 1990), with evidence for convergence being confined to more westerly longitudes. From this evidence it can be inferred t h a t e i t h e r

Tectonic evolution of the easternmost Panama Basin: Some new data and inferences spreading is not currently occurring in the easternmost P a n a m a Basin, or that material generated at any active rift is accommodated without contributing a northward component to relative motion between the oceanic plate and the Panama block.

CONCLUSIONS

1.

In addition to the extinct rift-transform pattern described by Lonsdale and Klitgord (1978), an extinct "Buenaventura" rift has been identified that passes beneath the deformation front of the Colombian accretionary complex at approximately 3°40'N. 2. The time of extinction on this rift is about 12 Ma. This supports the argument that the Cocos-Nazca plate boundary has progressively jumped westward relative to a fixed point on the Nazca plate. 3. The sense of asymmetry and associated spreading rates on the Buenaventura rift just prior to extinction suggest that resistance to spreading was encountered on the northern flank. This is attributed to the arrival and attempted subduction, at the P a n a m a n i a n trench, of a northern equivalent of the Grijalva scarp t h a t formed part of the Galapagos gore boundary. 4. The sinuous nature of the Yaquina transform, which acted as the Cocos-Nazca plate boundary between 12 and 8 Ma, may explain both its gradual "locking up" (along transpressive sections) and the formation of the Yaquina graben (from a series of pull-apart structures along transtensional sections). 5. The southern boundary of a possible "Coiba" microplate is tentatively linked with an extinct zone of rifting at 5°N. This was abandoned between 16 and 17 Ma during a rift jump episode. Subsequent rejuvenation of spreading activity between the time of original extinction and the present is considered a possible explanation for the differing strikes of magnetic lineations, as observed in the northern and southern parts of the basin. The lineation pattern is consistent with the growth of a V-shaped rifted zone similar to that found along the Mendana fracture zone offPeru. Acknowledgements--The bulk of the previously unpublished data presented in this paper was acquired during cruise CD40 of RRS Charles Darwin (July 1989). Special thanks must go to Graham Westbrook who both proposed the cruise and acted as principal scientist. I would also like to thank all Research Vessel Services and University of Birmingham participants for their assistance before, during, and after the voyage. Rob Latter, Alex Cunningham, and Richard Woollett, among others, from the British Antarctic Survey very generously provided both the magnetic modelling package nMagsynth" and a great deal of advice. Bill Owens, Tim Minshull, and Graham Westbrook gave much needed constructive criticism of earlier manuscripts and offered encouragement. I am also indebted to Miguel Gazon for helping with the translation of the abstract into Spanish. Finally, I would like to thank the Colombian and Panamanian governments for permission to carry out scientific work in their waters. This research was supported by the Natural Environment Research Council of the UK through provision of ship time and research grant GR3/6663.

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