Analysis of late Jurassic-recent paleomagnetic data from active plate margins of South America

Journal of South American Earth Sciences, Vol. I, No. 1, pp. 39-52, 1988 Printed in Great Britain

0895-9811/88 $3.00+0.00 Pergamon Journals Ltd

Analysis of Late Jurassic-Recent paleomagnetic data from active plate margins of South America MYRL

E. B E C K , JR.

Department of Geology, Western Washington University, Bellingham, W A

98225 U S A

(Received for publication May 1987) Abstra©t--Paleomagnetic resultsfor rocks of late Mesozoic and Cenozoic age from South America are analyzed and interpreted. Emphasis is placed on the active margins of the continent. Some important conclusions are reached, with varying degrees of certainty: (1) The reference A P W path for stable South America is fairlywell defined for the Late Jurassic and Cretaceous, and is not much differentfrom the present rotationaxis. (2) The so-called"Bolivian orocline"involved counterclockwise rotationsin Peru and northernmost Chile and clockwise rotations in Chile south of about latitude 18.5S. These rotations probably are a resultof in situ small-block rotationsin response to shear, not actual oroclinalbending. (3) The Bonaire block of northern Venezuela and Colombia has been rotated clockwise relativeto the stable interiorof South America by about 90*. Italso seems to have driftednorthward relativeto the craton by as much as 1600 km. It probably represents a true accreted terrane, one of very few recognized in South America so far. (4) The "Magellanes orocline~ at the southern tip of South America apparently involves some counterclockwise rotationof paleomagnetic vectors,but this too is probably the resultof distributed shear. (5) Tectonic processeshave very thoroughly "rearranged ~ the rocks making up the activemargins of South America. "Rearrange ~ here denotes displacement of crustal blocks relativeto their surroundings, without significantinternaldeformation By analogy with North America "rearrangement ~ might entailin situ block rotation,translation of crustal blocks along the continental margin, and accretion of exotic "tectonostratigraphicterranes.~ However, in western South America "rearrangement ~ seems to have consisted dominantly of block rotations that were essentiallyin situ. For the parts of the Andes investigated so far,late Mesozoic and Cenozoic accretionof exoticterranes and large-scaletranslationof crustal slivers,as found in western North America, seem to be absent. Resumen--Los resultadospaleomagndticos de focus del Mesozoico superior y el Cenozoico de Sudam~rica hun sido analizados e interpretados,poniendo dnfasisprincipalmente en el margen activodel continente. A partir de estos resultados, varias conclusiones importantes han sido deducidas, con ratios grados de incertidumbre: (I)La curva de desplazamiento polar de referenciapara la parte establede Sudam6rica esth bastante bien definida pars el Jur~sico tardloy el Crethcico,y no difieremucho del presente eje de rotaci6n. (2) El llamado "Oroclinal Boliviano" presenta rotaciones antihorarias tanto en Pert~como en el extremo norte de Chile,y rotacioneshorarias en Chile,al sur del paralelo 18.5°. Estas rotacionesson probablemente un resultado de movimientos rotacionales in situ de pequefios bloques como respnesta al cizalley no al doblamiento oroclinal actual. (3) El bloque de Bonaire del norte de Venezuela ha sido rotado aproximadamente 90 ° en sentido horario con respecto al interiorestable de Sudam~irica. Posiblemente, este bloque se ha movido por lo menos 1600 kms. hacia el norte en relaciSn al cratSn, representando uno de los pocos terrenos acrecionados reconocidoshasta la fecha en SudamSrica. (4) El "Oroclinal"de Magallanes en el extremo austral de Sudamdrica aparentemente incluye algunas rotacionesantihorarias de direcciones paleomagn~ticas, pero tambi~n puede set respuesta al cizalle. (5) Los procesos tect~nicoshun "reorganizado" enteramente las rocas,construyendo asi el margen activo de Sudam~irica. "Reorganizar" significa aqui desplazamiento de bloques corticalesen relacibna sus alrededores,sin deformaci6n interna significativa. En analogia con Norteamerica "reorganizar" podrla incluir rotaciones in situ de bloques, traslaciones de bloques a lo largo de[ margen continental y acrecibn de terrenos tectonoestratigrhficos ex6ticos. Sin embargo, en el oeste sudamericano, "reorganizar" parece haber consistido primordialmente en rotaciones de bloques esencialmente in situ. La acrecidn de terrenos exdticos y la translacidn a gran escala de fragrnentos corticalesdurante el Mesozoico tardio al Cenozoico, t~l como se encuentra en Norteam~rica, parecerian estar ausentes en losAndes investigadoshasta la fecha.

INTRODUCTION

DATA SELECTION

PALEOMAGNETIC data have been extremely important in the study of tectonic processes along active plate margins in western North America, southern Europe, and elsewhere. Although few paleomagnetic studies have been done in active orogenic belts of South America, enough data have accumulated to demonstrate several interesting patterns. This brief review summarizes available data and attempts some simple interpretations. As the Andean orogeny is the focus of this study, the compilation only extends to rocks of late Mesozoic and younger age.

In order to ensure that useful paleomagnetic "signal" (valid, well determined directions or poles) is not obscured by paleomagnetic "noise," some simple reliability criteria have been applied in this study. By action of these criteria, results of nearly all of the studies summarized in this study have been recalculated and many published paleomagnetic poles have been discarded. Studies were accepted if 1) they were based on three or more determinations of independent directions of the ancient geomagnetic field. 39

40

M

E. BECK, ~ .

T a b l e 1. M e s o z o i c a n d C e n o z o i c p a l e o m a g n e t i c p o l e s f r o m a c t i v e m a r g i n s o f S o u t h America,, Pole

Unit

Age

x '~

ee,°E

N

K

A95,'

Reference

~2

5

104.0

~:5

MacDonai¢. (1980)

CO1

Corcovado andesite

~

5~,

KO13

La Teta lavas

143

10.4

194.6

8

16.1

14,2

Macdonaio a n d Opdyke (1972)

KO12

Felsic dikes, Aruba

< 85

18,2

208.8

3

40.8

19.6

S t e a r n s et ~l (1982)

KOll

A r u b a batholith

70-85?

68.6

208.4

13

100.6

2.0

S t e a r n s et al (1982)

KO10

W a s h i k e m b a Fm, Bonaire

~65

6.3

212.7

3

63.6

15.6

S t e a r n s et aL (1982)

KO9

E1 Chaco ultramafic comples

< 100

10.6

193.4

4

41.1

14.5

Skerlec and Hargraves (1980)

KO8

C a s m a Grp, Huarmey

100?

74.9

4.4

4

122.4

8.3

Heki et ai. (1984)

K07

C a s m a Grp, Ancon

100?

66.2

1.4

6

74.8

7.8

Heki et al. (1984)

KO6

P u e n t e Piedra Fm

90

73.3

359.2

12

174.3

3.3

May and Butler (1985)

KO5

Atajana Fm

~ 135

74.6

37.0

7

206.4

4.2

Heki et a I (1983a)

KO4

Arica dike s w a r m

K?

77.2

352.4

19

120.1

3.3

Heki et al. (1983a)

KO3

Coloso F m

Ke?

65.0

193.4

3

87.0

13,3

T u r n e r et ai (1984)

KO2

La S e r e n a volcanics

79

79.6

214.6

17

35.2

6.1

P a l m e r eta[, (1980a )

KO1

San F e r n a n d o tufts

100

76.3

192.5

22

20.0

7.1

Beck ct aL (] 986a

JO3

Camaraca Fm

~ 165

86.0

352.0

5

96.3

7.8

t t e k i e t aL (1983b)

JO2

Camaraca Fm

157

69.7

5.3

16

25.6

7.4

P a l m e r et al. (1980b)

JO1

C u y a dike s w a r m

< 165

72.4

66.2

10

19.6

11,2

Heki et at, (1983b)

Age in Ma except where only s t r a t i g r a p h i c age cited in original; K, A95 as defined by Fisher (1953); h, ¢ are south latitude a n d e a s t longitude of pole; N, n u m b e r ofsite VGP. These poles are discussed f u r t h e r in the Appendix.

2) they were based on laboratory d e m a g n e t i z e d directions, either alternating field or thermal. 3) they had 95% confidence intervals of 20 ° or less. Site-mean directions or remanent magnetization were regarded as independent estimates of directions of the ancient geomagnetic field. However, a sitemean direction was accepted only if it was based on 3 or more separately oriented samples and if its confidence circle had a radius of 20 ° or less. In some cases, groups of isolated hand samples or "sites" that were obviously not independent were combined. All calculations of mean pole positions were made using virtual geomagnetic poles (VGP) - - site-mean directions of magnetization were first converted to VGP and then a mean VGP calculated. Thus, all confidence intervals about the poles listed in Tables 1 and 2 are circles and estimates of dispersion (K) apply to VGP, not directions. The "confidence filter" thus described is entirely arbitrary and not particularly exacting, but it should insure that most of the data used in the subsequent analysis are reliable. (That is, one may be reasonably confident that they mean what they purport to mean.) Unfortunately, it also

has the effect of excluding from the analysis some very interesting results, several of which nevertheless are alluded to below. Despite the confidence filter, some cautionary remarks are needed. First, three sites are certainly not many upon which to base a paleomagnetic pole. For that matter, ten sites or even more can give a poor estimate of an ancient pole position, if the sites all happen to have acquired their magnetism within a single brief interval of time. For proper averaging of geomagnetic secular variation at least 104 years are required; up to 106 years is better. Thus, studies which show too little dispersion (say K > 100 or so) are automatically suspect because low dispsersion may mean incomplete a v e r a g i n g of the s e c u l a r variation. However, such studies may yield perfectly valid pole positions. Several studies summarized in this review show suspiciously low values of dispersion. Other problems encountered include doubts about the time of acquisition of magnetization and questions about pest-magnetization s t r u c t u r a l history. Also, some of the units studied have complex magnetizations that were difficult to isolate and may

Late J u r a s s i c - R e c e n t paleomagnetic data from active plate m a r g i n s of South A m e r i c a

41

Table 2. Mesozoic reference poles for South America. Pole

Unit

Age

L°S

~,°E

N

K

A95, °

Reference

KFll

Andacolo series

69

73.6

64.7

(1)

(1)

(1)

Vilas and

KC10

Pocosde Caldas complex

75

84.5

298.9

6

23.6

14.1

Opdykeand MacDonald (1973)

KC9

Volcanichills, central Argentina Las Curtiembres Fm

75.5

65.1

76.3

3

92.3

12.9

77

86.8

85.4

5

23.7

16.0

Valencioet al. (1983b) Valencioet al. (1977)

92

87.6

315.1

9

106

69.4

237.4

4

K.F8 KC7 KF6

Cabo de Santo Agostinho magrnatic rocks La Yesera Fm

127 (2) 59.1

4.5 12.0

Valencio (1978)

Schult and Guerreiro (1980) Valencioet al. (1977)

KC5

MaranhaoBasin basalt

118

83.6

80.8

18

289 (2)

2.0

Schult and Guerreiro (1979)

KC4

Vulcanitas Rumipalla Vulcanitas Cerro Colorado

120

85.1

65.2

4

104.4

9.0

Vilas(1976)

120

78.3

8.5

9

17.2

12.6

E1SaltoAlmufuerta lavas Serra Geral basalts Porto Franco volcanics

123

66.5

20.7

10

17.5

11.9

128

83.8

94.6

54

32.7

3.4

160

85.3

262.5

15

31.8

6.9

JC3 JC2

Chon Aike Fm Guacamayas volcanics

164 195

86.2 70.1

106.3 301.5

14 5

43.1 41.2

6.1 12.1

Vilas(1974) MacDonald and Opdyke (1974)

JC1

Bolivardikes

198

66.9

246.0

5

83.9

8.4

MacDonald and Opdyke (1974)

KC3 KC2 KC1 JC4

Valencio

(1972); Vilas (1976) Mendia(1978) Bellieniet al. (1983) Schult and Guerreiro (1979)

Age in Ma except where only stratigraphic age cited in original; K, A95 as defined by Fisher (1953); k, ¢ are south latitude and east longitude of pole; N, number of site VGP. These poles are discussed further in the Appendix. Notes: 1) remagnetized, not possible to calculate true dispersion; 2) note low dispersion, secular variation possibly not averaged. be erroneous. Some of these problems are mentioned in the Appendix, but there are enough complications in the d a t a set so t h a t a n y serious student of South A m e r i c a n p a l e o m a g n e t i s m should refer to the originals in all cases. W i t h t h e s e d i s c l a i m e r s , it is obvious t h a t a n y tectonic interpretation based on a single pole m u s t be r e g a r d e d with suspicion, whether or not the pole in question has s u r v i v e d a "confidence" filter. H o w e v e r , w h e n s e v e r a l c o m p l e t e l y i n d e p e n d e n t poles support the same tectonic interp r e t a t i o n , the evidence is m u c h m o r e c o n v i n c i n g . Some i n t e r e s t i n g p a t t e r n s are present in the South A m e r i c a n d a t a set.

orogenic South A m e r i c a are compiled in Table 2. The confidence filter also has been applied to these reference poles. The d a t a in both tables are p l o t t e d in Figs. 1-4, all of which are e q u a l - a r e a s o u t h e r n h e m i sphere projections. S a m p l i n g a r e a s are shown on the same plots. Studies from the f o l d - a n d - t h r u s t belt on the interior side of the Andes were t r e a t e d as p a r t of the "stable South A m e r i c a " d a t a set, b u t a r e designated by separate symbols on the plots. Cenozoic studies from stable South A m e r i c a h a v e not been s u m m a r i z e d because it was felt t h a t the p r e s e n t axial dipole provides an a d e q u a t e reference for rocks this young. C r e t a c e o u s reference p o l e s f r o m s t a b l e S o u t h A m e r i c a

PALEOMAGNETIC DATA FOR SOUTH AMERICA Reliable p a l e o m a g n e t i c poles for active orogenic belts of South A m e r i c a are s u m m a r i z e d in Table 1. Most of these poles have been recalculated as described in the preceding section. For c o m p a r i s o n a n d reference, J u r a s s i c and y o u n g e r poles from e x t r a -

Figure l a shows C r e t a c e o u s poles for the S o u t h A m e r i c a n c r a t o n (and i n t e r i o r A n d e a n foothills). Poles are n u m b e r e d in a p p r o x i m a t e c h r o n o l o g i c a l order, from oldest (1) to y o u n g e s t (11). A 95% confidence "envelope" also is shown. It is clear from Fig. l a t h a t Cretaceous poles from stable S o u t h A m e r i c a are d i s t r i b u t e d in a h i g h l y e l o n g a t e p a t t e r n (are

42

M, E. BECK, J l~.

"streaked"), with the direction of elongation lying approximately normal to the paleomeridian Taken at face value Fig. la suggests that the pole swung back and forth several times during the Cretaceous, in the process describing an arc nearly 40 ° in length The problem of the origin and significance of this Cretaceous "streak" is one of the most important in South American paleomagnetism. Some possible explanations, with discussions, follow: l) The streak might represent genuine a p p a r e n t polar wander (APW). This is the explanation preferred by Valencio et al. (1983a). This explanation would imply that South America wobbled back and forth on a local axis ("fishtailed") d u r i n g spreading in the South Atlantic. Plate reconstructions for the Cretaceous are hampered by a lack of magnetic anomalies during the Cretaceous long normal interval. However, if South America did "fishtail" during its flight from Africa, a record should be preserved in the form of changes in trend in South Atlantic fracture zones. These appear to me to be smooth to a first approximation, and so wobble of the entire continent as a principal cause of the streak appears to me to be unlikely, 2) The Cretaceous streak also might represent intracratonal deformation. Substantial amounts of rotation about a vertical axes affecting several of the sampling areas (especially those, such as 2, 6, and 9, with poles that lie near the ends of the streak) would produce the observed elongate distribution. Despite the fact that most of the sampling areas shown in Fig. l a lie in regions that were teetonically stable during Andean deformation, local deformation cannot be ruled out entirely, because rotation about a vertical axis can be very difficult to detect geologically. However, geography pro-

0,8

v

vides a strong argument. For instance, poies KC2, 3, and 4 apparently were derived from a restricted area in southern C6rdoba province, Argentina !t~ appears from the originals that the sampling areas for the three studies are only about 20 km apart, and KC3 and KC4 seem in fact to be part of a single stratigraphic sequence. Yet KC2 is on the extreme edge of the streak, KC3 is slightly dis.placed from the middle of the distribution, and KC4 is at its center. It would seem Lo be impossible to displace pole KC2 by local block rotation without affecting KC4. Similarly, poles KF6 and 8 are from a single sequence in the Andean {both ills, yet KF6 defines the Pacific end of the Cretaceous streak whereas KF8 lies at its exact c e n t e r Thtls, unrecognized local deformation also does not seem to be an important contributor to the elongate distribution of Cretaceous poles. 3) Geomagnetic explanations also can be dismissed The collection of poles shown in Fig. 1a spans the entire Cretaceous, and thus together should represent far more than enough time to average non-axial geomagnetic elements to zero The light., circular distribution of poles tbr the Cretaceous of North America (e.g., Mankinen, 1978) demonstrates that there was nothing geomagnetically peculiar about the Cretaceous. Thereibre, peculiarities of the geomagnetic field also cannot be to blame. Figure lb shows results of an effort to b e t t e r understand the South American APW record. The dots in Fig. lb are North American mean poles for 20 m.y. intervals throughout the Cretaceous (taken from Irving and Irving, 1982), rotated into a South American reference frame. The APW path for North America is particularly well defined for this time

S, 4

, S?O

I

I

I

• ,o

,

i!

_

1110

i

.

/

1{IO

Fig. 1, Cretaceous reference poles, a) Poles from South American interior, keyed to Table 2; triangles indicate poles from eastern foothills of the Andes; envelope of 95% confidence shown, b) Cretaceous reference poles for North America {taken from Irving and Irving, 1982), rotated into South American reference frame; 95% envelope from {a) shown to facilitate comparison.

Late Jurassic-Recent paleomagnetic data from active plate margins of South America interval. The stage poles used to perform the rotation are from Engebretson et al. (1985). The North American poles shown in Fig. lb seem to be displaced a short distance toward South America, relative to the collection of South American poles for the same time period (note confidence circles from Fig. la, included for reference). This probably represents a slight problem with the plate reconstructions. The significant thing shown by Fig. lb, however, is that the distribution of North American poles is not elongate. This suggests, but of course does not prove, that the South American reference pole for the Cretaceous should be similarly unstreaked. If this is correct, then it seems likely that the main Cretaceous pole for stable South America should be based only on those poles near the center of the streak that form a circular cluster w specifically KC1, 3, 4, 5, 7, and 10, and KFS. It is impossible to know at present whether this choice of reference is correct, but it appears to be the best alternative available. The mean pole (Table 3) calculated from these data is used below to compute tectonic displacements for Cretaceous rocks from South American orogenic belts. If the central cluster of South American Cretaceous poles represents the correct APW path, how can one account for outliers KC2 and 9 and KF6 and 11 ? K C2 has a notably elongate distribution of VGP, and its mean is biased into low latitudes by two highly discordant sites. KC9 is based on only three sites, located in what appear to be volcanic plugs. This suggests the possibility t h a t the m a g n e t i c directions have been affected by undetected tilting, although a g r e e m e n t in direction between widely spaced localities makes tilting seem unlikely. KF6 is based on only four sites but appears valid otherwise. Finally, KF11 is a pole position for ashflow tufts of Carboniferous age that have been remagnetized in the Cretaceous. Because the rocks are remagnetized it could be difficult to ascertain the correct paleohorizontal, so perhaps this pole also is affected by undetected tilting. Thus the "outliers" can mostly be accounted for with ad hoc arguments w several of which, however, are not entirely convincing. This explanation views the streaked distribution of Cretaceous reference poles as a coincidence, which may be true but is not particularly satisfying.

43

using data from Engebretson et al. (1985). Two North American mean poles are shown for each time interval in the Jurassic, representing a l t e r n a t i v e methods of data analysis. Solid squares (Irving and Irving, 1982) are 30 m.y. running averages of selected paleomagnetic data, whereas the open squares were calculated by the so-called " p a l e o m a g n e t i c Euler pole" (PEP) technique (Gordon et al., 1984). The two methods give results that are not significantly different for the Cretaceous, but differences are as high as 9 ° for the Jurassic. The South American reference pole is well defined for the Late Jurassic, but becomes less so for earlier times. The slight offset of rotated North American poles from equivalent South American poles that was noted for the Cretaceous plots is present for the Late Jurassic as well. Because poles JC3 and JC4 are nearly equivalent in age and agree well in position, despite being from sampling sites nearly 5000 km apart, it seems acceptable to calculate a mean South American Late Jurassic reference pole based on these two studies. Results are given below.

Cenozoic poles from orogenic belts Surprisingly few paleomagnetic studies have been performed on rocks of Cenozoic age f r o m S o u t h America. Only one study from the orogens survived the confidence filter, and that one involves a small hypabyssal intrusion forming part of a strongly tectonized zone in northwestern Colombia (CO1, Table 1). Additional data from the same area confirm that the region (Cauca depression, between Romeral and Cauca fault zones; MacDonald, 1980) has been subjected to intense shearing since about 8 Ma. Both counterclockwise rotation about a vertical axis and tilting seem called for. Another important study on rocks of Cenozoic age (Ocros dike swarm; Heki et al., 1985) deals with magnetic directions in supposedly postectonic rocks near Ayacucho, Peru. Unfortunately, after recalculation the study did not survive the confidence filter and therefore is not included in Table 1. However, because it has a direct bearing on the problem of the Bolivian orocline, the Ocros result is discussed further below.

Cretaceous poles from orogenic belts Jurassic poles for stable South America Recalculated reference poles for the Jurassic of South America are shown in Fig. 2a. Also shown (Fig. 2b) are mean North American Jurassic poles rotated into the South American reference frame,

Cretaceous units from orogenic belts of South America are relatively well studied; data are summarized in Table 1 and Fig. 3, and individual poles are discussed in the Appendix. Three obvious groups are represented in Fig. 3, one each from the Carib-

Table 3. Mean Cretaceous poles, Andes and craton. Data

A,°S

~,°E

N

K

North of Aries deflection

K04 - K08

74.7

3.0

48

89.5

2.2

South of Aries deflection Reference, craton

KO1 - K 0 3

77.0

199.8

42

24.9

4.5

KC1, 3, 4, 5, 7,10, K F 8

85.5

73.5

105

35.1

2.4

Mean Pole

A95, °

•

'\~

/Ol

,°'

t

\

Fig. 2. Jurassic references poles, a) Poles from South American interior, keyed to Table 2, with associated 95% confidence circles. b) North American Jurassic reference poles, rotated into South American frame. Filled squares from Irving and Irving (1982); open squares from Gordon et al. (1984).

bean margin of South America, the Andes of Peru and northernmost Chile, and coastal Chile south of about latitude 18.5S. Data available for the extreme southern tip of South America (Magellanes-Tierra del Fuego region) indicate that a fourth group might have been present had studies from that area survived the confidence filter, but none did. Nevertheless, information available for southernmost Chile and Argentina is very interesting and is discussed briefly below. Of the three groups shown on Fig. 3, that from the Caribbean is clearly discordant with respect to Cretaceous poles from the craton. The two Andean groups also appear to be discordant, but they overlap the confidence envelope of craton poles and thus need to be tested statistically. Jurassic poles from orogenic belts Figure 4 shows results of three studies on rocks of Jurassic age from the Andes. All three sampling areas are located immediately south of Arica, in northern Chile. All of these studies involve serious complications (see Appendix), but the fact that they tend to agree with one another is reassuring. Note that poles from Arica agree with Cretaceous poles from the same area (compare Figs. 3 and 4).

TECTONIC IMPLICATIONS OF P A L E O M A G N E T I C DATA The data summarized in Table 1 can be interpreted by comparing groups of poles from different parts of the several orogenic zones with one another or with reference poles from the craton. Such comparisons are easily quantified using methods described in Beck (1980), Demarest (1983), and Beck et al.

(1986b). Discussions below are organized by topic (e.g., the Bolivian orocline) rather than by age of pole, age of tectonic event, or mechanism of deformation. In general, there is not enough evidence available to constrain speculations about timing {such evidence as does exist is mostly non-paleomagnetic), and such tectonic models as might be advanced have to be viewed with caution. One important general observation follows from a quick inspection of the illustrations presented so far (compare especially Figs. la and 3). Crustal blocks making up the active margins of South America seem to have been very thoroughly "rearranged," presumably by tectonic processes, since their constituent rocks were formed. This follows from the fact that poles or groups of poles of the same age from different parts of the margin rarely ffever agree with one another, or with poles from coeval rocks on the craton. By analogy with North America, tectonic "rearrangement" might involve accretion of exotic crustal blocks, in situ rotation about vertical axes, coastwise translation of detached crustal slivers, or various combinations of the three processes. As suggested below, in situ rotation is particularly common in western South America, and the type of deformation encountered seems to reflect the type of plate interaction that the margin has undergone. This is not a surprising result, in view of tectonic processes uncovered by recent paleomagnetic studies of other active plate margins. The Bolivian orocline Carey (1955) was the first to suggest that the abrupt change in tectonic and topographic trends at about the Peru-Chile border was a secondary strain (an orocline), and not a primary feature inherited

Late Jurassic-Recent paleomagnetic data from active plate margins of South America

/

~11, t l:l

II 4, 6

i;aO 5 2?0

I

l

"~

I

I

OA

2e

eI 11Aea_

/

L~ 12

180

Fig. 3. Cretaceous poles from the orogenic belts, keyed to Table 1. Circles denote Andean rocks; triangles denote rocks from the Caribbean margin. Poles with open symbols are plotted in the upper (northern) hemisphere.

from a non-linear, pre-Andean configuration of the continental margin. This suggestion can be tested paleomagnetically, as was first noted by Palmer et al. (1980b). Recently, Heki et al. (1983a,b, 1984), Kono, et al. (1985), and May and Butler (1985) have interpreted northwesterly paleomagnetic declinations in Cretaceous rocks from Peru and northern Chile as support for the orocline hypothesis. The nature of the evidence for post-Cretaceous oroclinal bending is obvious from Fig. 3, which shows a clear separation between groups of Cretaceous poles from north and south of the Arica deflection. Heki et al. (1985) also presented results from the Ocros dike swarm which they believe indicate that much of the oroclinal bending took place during Neogene time. Several questions need to be answered about the Bolivian orocline: First, are the two groups of Cretaceous poles (those north and those south of the supposed hinge) different statistically, as required to support the orocline hypothesis? If so, does the amount of paleomagnetic bending (deflection of declination vectors) agree with the amount of bending suggested by the change in topographic and tectonic trends? If both questions are answered in the affirmative, then the Bolivian orocline is on firm footing. The next step in the analysis should be to ask if it were Peru, or Chile, or both, that rotated u measured relative to stable South America. Answering this question entails comparing both groups of poles (north and south of the Arica deflection) with a reference pole for the craton. Finally, does the scanty data available for Jurassic and Cenozoic time permit us to say anything about the timing of oroclinal bending? Table 3 gives some composite Cretaceous poles for South America that can be used to answer some of

45

these questions. The entries in Table 3 are Fisher means (Fisher, 1953) of the individual paleomagnetic poles indicated, allowing unit weight to each site VGP. This method of calculation has the effect of increasing N (the number of independent estimates of the ancient geomagnetic field direction), and thus yields smaller confidence limits than does the customary method of awarding unit weight to formation means. The technique used here is justified and preferable if each site VGP is truly an independent measurement of the paleogeomagnetic field and if no appreciable APW occurred relative to South America during the Cretaceous. To the extent that these assumptions are incorrect, conclusions based on Table 3 are weakened u but only slightly. Applying the method of Beck et al. (1986b) to the data of Table 3 and using the correction factor of Demarest (1983), the following rotations can be calculated for the approximate location of Arica (clockwise rotations positive): "north" group vs *'south" group "north" group vs craton mean pole "south" group vs craton mean pole

+ 4.0 ° 13.0 + 3.5 ° 16.5 + 5.5 °

-29.5 -

The bend in topographic and geologic trends at Arica is roughly 50 to 60 °. Thus, if the deflection in paleomagnetic declinations is the result of oroclinal bending, only about half can have occurred since the Cretaceous (see Fig. 5). Furthermore, bending must have consisted of about equal parts clockwise rotation of Chile and counterclockwise rotation of Peru (Fig. 6). The timing of supposed oroclinal bending can be tested further by comparing a mean upper Jurassic pole for the Arica area (data in Fig. 4) with a mean upper Jurassic pole for the craton (Fig. 2a, poles JC3, JC4). Again the calculation was made using site VGP as input, although in this case the method is on

~J f I

s

/ a

I?O

I

I

I

I

180

Fig. 4. Jurassic poles from orogenic belts, keyed to Table I.

46

M [~';. BECK, ,Ill.

R1 ~ post Cretaceous rotation predicted by paleomagnetic directions

R 2 ~ "oroclinal" rotation suggested by topographic and tectonic trends

direction and assuming as a reference direction ttae present axial dipole field (with arbitrary confidence circle of 3°), the Ocros region appears to have rotated 14° counterclockwise, but with a confidence interval of 26 °. This result certainly is not significant statistically, but it is interesting in that it is almost identicaI to rotations calculated above tbr the northern limb of the "Bolivian orocline" using much better determined directions for the Cretaceous and Jurassic. Thus, for whatever these results are worth, if oroclinal bending has taken place in Peru, i t, would seem to have occurred in two phases (pre-Late Jurassic and post-Miocene), separated by a hiatus of nearly 150 m.y.

Fig. 5. Cartoon contrasting relative rotation of the ~Bolivian orocline" suggested by paleomagnetic measurements, with rotations suggested by topographic and tectonictrends.

shaky ground because of doubt about the age of the Cuya dike swarm (pole JO1). Results are as follows: mean craton pole 89.0S, 217.1E, A95, 4.6 ° mean Arica pole 75.3S, 24.3E, A95, 6.0 ° rotation of Arica relative to the craton 16.5 +_6.5 ° Two things are noteworthy about this calculation. First, Jurassic rocks at and immediately south of Arica seem to have been rotated counterclockwise relative to the craton. This appears to be contrary to the evidence of topography, trend of shoreline, and gross pattern of geology, which would place the Arica area in the southern (clockwise rotated) part of the orocline. Second, the counterclockwise rotation of Jurassic rocks from Arica almost exactly matches the amount of rotation calculated for Cretaceous rocks near Arica and northwestward into l'eru. If the deflection of paleomagnetic declination is oroclinal and if the northern limb of the orocline was reasonably rigid, then no rotation took place from about 160 to perhaps 90 m.y.B.P. Heki et al. (1985) studied a small set of dikes (Ocros dike swarm) that crop out near Ayacucho, Peru. They interpreted their results as evidence for about 14° of counterclockwise rotation of the Ayacucho region since the end of Miocene time, tlowever, it is obvious from their Fig. 3 that all 32 Ocros dike directions are not valid, independent "samples" of the ancient geomagnetic field. It appears that, at most, six independent directions have been sampled and that one of these is transitional. A recalculation of the Oeros data based on this assumption gives the following mean direction: D, 345.1°; I, -37.3°; a95, 24.9 ° . Because of the large 095, this Ocros dike swarm result did not survive the confidence filter and is omitted from Table 1. However, using this mean

There seem to be several reasons to doubt that the deflected declinations in Peru and n o r t h e r n Chile actually are the result of a rigid-body, oroclinal block rotation. Perhaps the most important is the observation made above that the difference between polar groups from Peru and n o r t h e r n Chile on the one hand, and the rest of Chile on the other, is due to approximately equal a m o u n t s of rotation of both groups, measured with respect to the craton. It is difficult to imagine true oroclinal bending of coastal Chile that would leave poles from Argentina sensibly unaffected (compare poles from A r g e n t i n a and Brazil, Fig. la). If coastal Chile has rotated 16.5 ° clockwise relative to Argentina, as indicated by the paleomagnetic results, and if the pivot is located near Arica, as indicated by the topography and geologic trends, then an extensional structure is required between the two regions. The structure m i g h t be relatively insignificant at the latitude of Arica, but it should reach a width in excess of 1000 km near the southern end of the continent. No such feature appears to exist. Even if the rotated block extended no f a r t h e r south t h a n San F e r n a n d o , C h i l e (the

\

P~ 13°c c wise rotation of Peru, relative to the craton

C

R2~17~ clockwise rotation of Chile, relative to the craton

I', \\

/'"'\\N 0

Ra ~25°apparent orocllnal bend not accounted for in postCretaceous peleomagnetic directions

Fig. 6. If the Arica deflectionis an orocline, both Peru and Chile must have moved. See text for discussion.

Late Jurassic-Recent paleomagnetic data from active plate margins of South America

South America

1

l

47

through a range of angles (although all in the same direction). It would even permit some coastal blocks to remain essentially unrotated (e.g., Jurassic directions for the Chilean coast range found by J. Irwin, written communication, 1985). It does not, however, easily account for the agreement in total rotation between Jurassic and Cretaceous (and perhaps even Pliocene!) rock units. One might expect older rock units to show more rotation than younger rock units, simply because they have been in the continental-margin shear zone for a longer time. Perhaps this implies that all the rotation is postMiocene. In situ block rotation also does not account for the fact that rocks immediately south of Arica show counterclockwise, not clockwise, rotation. Perhaps this indicates a small amount of southeastward tectonic transport. This sort of speculation probably is futile, and will continue to be until m u c h more information can be brought to bear on the problem The Bonaire block

Fig. 7. Cartoon illustrating developmentof dextral shear in Chile and sinistral shear in Peru by subduction of a single plate. "Buttress" denotes zone of compression past which detached crustal slivers cannot be translated. See text for discussion. southernmost Chilean area yet investigated paleomagnetically), the e x t e n s i o n a l f e a t u r e r e q u i r e d would be 500 km wide. Thus, rigid block rotation of Chile relative to the craton appears to be very unlikely. A second r e a s o n for d o u b t i n g the oroclinalbending model is the timing of rotation, also discussed above. If the paleomagnetic evidence is to be believed, most of the oroclinal bending must have taken place before Late J u r a s s i c time - - that is, before the breakup of southwestern Gondwana. The remainder, according to the paleomagnetic evidence, must have occurred during the Neogene. While not impossible, this schedule makes rotation difficult to account for with any sort of plausible tectonic model.

Poles K09-13 (Fig. 3) represent available paleomagnetic data for the Caribbean c o a s t of S o u t h America. With the exception of K011 (the A r u b a batholith), these poles call for very large rotations relative to the craton, accompanied by s u b s t a n t i a l amounts of relative (north) poleward transport. The opinion of many workers in the area is t h a t the sampling localities of K09-13 are situated in an allochthonous microplate, the Bonaire block of Silver et al. (1975). Rotation of the Bonaire block through an angle of roughly 90 ° is called for by the paleomagnetic directions. The sense of rotation is indeterminant because the polarity of the measured magnetizations are not known. However, because the Bonaire

A n alternative to the orocline model

It seems to me that the facts summarized above better fit a model emphasizing in situ rotation of small crustal blocks caused by regional shear. Regional shear, in turn, might arise b e c a u s e of o b l i q u e i n t e r a c t i o n of the west coast of South America with subducting oceanic plates. Figure 7 is a cartoon illustrating this mechanism. This model regards the Arica deflection as an ancient feature pre-dating the earliest Andean rocks yet studied paleomagnetically. Because of the deflection, shear is left-lateral north of Arica but is right-lateral farther south. Coastwise t r a n s l a t i o n of coastal slivers is prevented by the presence of a buttress (Beck, 1986). This model accounts for the noticeable "fanning" of paleomagnetic declinations seen in the r e s u l t s of H e k i et al. (1983, 1984a, 1984b) by supposing that blocks only a few dozens of kilometers in lateral dimensions have rotated independently

Fig. 8. VGP from the Bonaire block, rotated so that their mean lies at the center of the projection. Circles, data from Stearns et al. (1982); squares, data from MacDonald and Opdyke (1982); triangles, data from Skerlec and Hargraves (1980).

48

M E BE(~, ,!~<

block is located in a zone of dextral shear (e.g., l';i Pilar, San Sebastian, and Oea faults; Silver" el aI.~ 1975), most investigators seem to regard the sense of rotation as clockwise. Two important questions thai can be addressed with existing paleomagnetic dat~ are the following: Was rotation of the Bonaire block a piecemeal process involving many small independent blocks, or was it a rigid-body rotation of the entire microplate? Should the Bonaire block be regarded as an exotic, aeereted, perhaps far-traveled crustal fragment, or has it always been part of South America? Figure. 8 indicates that, although some internal relative rotation of crustal fragments making up the Bonaire block may have occurred, the bulk of the displacement was probably a rigid body rotation. Figure 8 is a southern hemisphere equal area projection of all Cretaceous site-VGP from the Bonaire block (the Aruba batholith excluded) that satisfy the confidence filter. However, in Fig. 8 the mean VGP (center of the group) has been placed at the center of projection to eliminate distortion of the shape of the distribution. If the Bonaire block is as a collection of small crustal fragments that rotated independently, then (unless the amount of rotation of each fragment happened to be remarkably identical) the VGP in Fig. 8 should be streaked out into a recognizable pattern. The dashed line in Fig. 8 represents the trace of a small circle drawn about 12N, 70W, which is approximately the center of the Bonaire block. If there has been much internal in situ block rotation, then the distribution of VGP should be elongate along this line. There is a slight tendency for the group to be streaked in this general direction, but it is not very strong. It appears from Fig. 8 that there may have been some internal deformation within the Bonaire block, but that the bulk of its displacement conforms to a rigid-rotation model. If the Bonaire block behaved as a rigid microplate, where did it come from? Paleomagnetism can detect north-south transport (in the ancient geographic framework - - that is, toward or away from the paleo pole), but it cannot detect E-W motions. Data on (north) poleward transport for paleomagnetic poles from the Bonaire block are given in Table 4. All four Bonaire poles call for northward transport of the block relative to South America, although only two of the four d e t e r m i n a t i o n s are s i g n i f i c a n t at 95% confidence. Entry M, ]'able 4, gives north-poleward transport calculated using the mean of VGP shown in Fig. 8 (3N, 22E, A95=8.7°). This is probably the best estimate of northward transport for the block. The implied distance & j u s t over 1600 km seems excessive and can be accommodated only by placing the Bonaire block somewhere in the Pacific basin off Ecuador or northern Peru during the Cretaceous. Relative northward transport of the Bonaire block also has been proposed by Stearns et al. (1982) and MacDonald and Opdyke (1972)

As mentioned in the Appendix and ~J~scusse~l a!~ length by Stearns et al. (1982), the Aruba batholith (pole K O l l ) does not fit this tectonic model~ The reason for its peculiar pole position is u n c l e a r T h e MagelIanes orocline

The e a s t w a r d - c u r v i n g s o u t h e r n tip of S o u t h America also was designated an orocline by Carey (1958). Considerable work has been done in the area in an attempt to test the orocline hypothesis by paleomagnetic measurements; results are summarized in Burns el al. (1980), and Dalziel et al. (1973). No doubt owing to the difficulty of collecting a large suite of samples in this remote and inhospitable region, none of this work provided enough data on specific geological units to pass the confidence filter. However, there does seem to be a general tendency for Mesozoic paleomagnetic vectors in the area to be deflected around the bend of the continent, although the tendency cannot be said to be strong. Counterclockwise oroclinal bending of an originally more linear southern Andes can explain the paleomagnetic directions. However, sinistral shear between the Antarctic (or Scotia) plate and an originally bent southern tip of South America can account for the results equally well. More paleomagnetic data obviously are needed.

SOME FINAL O B S E R V A T I O N S With the exception of the Bonaire block, there does not seem to be any compelling paleomagnetic evidence for large latitudinal shifts of Mesozoic or Tertiary terranes in South America. Much of the continent remains unstudied, but there are e n o u g h reliable paleomagnetic d e t e r m i n a t i o n s to suggest that the pattern of accretion and coastwise transport of allochthonous crustal fragments found in North America does not exist in western South America. Given the generally n o r t h w a r d sweep of p l a t e s within the western Pacific basin during the Mesozoic and Tertiary (Engebretson et al., 1985), perhaps it should not be too surprising if exotic crustal fragments turn out to be rare in western South America. This is because relative motions of the larger plates are likely to have swept most candidates for "exotic Table 4. North-poleward translations implied by paleomagnetic results from the Bonaire block. Pole

P +_ AP °

KO13

6

±

KO12

25

+-

16

KO10

27

±

13

9

-+

12

KO9 M

14.5 +-

12

7.8

P, poleward translation; AP, 9 5 % confidence limit on P; M, m e a n of V G P from Fig. 7.

Late Jurassic-Recent paleomagnetic data from active plate margins of South America terranes" (island arcs, oceanic p l a t e a u s , among others) northeastward to North America, bypassing South America for the most part. Tectonic transport of slivers of western South America north or south along the continental margin also seems to be rare (see Scott, 1978, for a possible Triassic exception), and this is harder to explain. Plate models (e.g., Pilger, 1983) show that the direction of convergence between South America and the plate immediately to its west (Nazca) was more nearly normal to the coastline than was the direction of relative motion between North America and its oceanic neighbors (Farallon, Kula, Pacific plates), at least for Cenozoic and late Mesozoic time. Thus "coastwise transport" of continental slivers should be more common in N o r t h A m e r i c a than in South America, which appears to be the case. However, given the changes in trend of the western edge of the South American plate, it is difficult to believe that no instance of highly oblique subduction under South American ever occurred. More likely, as argued above, oblique subduction was common and is responsible for the pattern of block rotations seen in the paleomagnetic data. Extensive translation of crustal slivers may not have occurred in the South A m e r i c a n case because of the buttress problem - - the shape of the South A m e r i c a n coastline and the direction of convergence of the Nasca plate together created a b u t t r e s s at the Arica deflection beyond which potential allochthonous material could not move (see Fig. 7). However, no such restraint should apply to the coast of Chile south of the Nasca-Antarctic-south America triple junction, or to Ecuador and western Colombia north of the H u a n c a b a m b a deflection (approximately 5S, 79W). More paleomagnetic data from these areas would be e x t r e m e l y valuable. Geological evidence apparently suggests that large portions of coastal Ecuador and western Colombia are allochthonous (McCourt et al., 1984; Aspden and McCourt, 1986; Feininger, 1987), and pre-Mesozoic terranes also may exist in the Andes of Chile and Argentina (Rapalini et al., 1985; Ramos el al., 1986). Paleomagnetism does indicate that a considerable amount of rotation has taken place along the active margins of South America. Although oroclinal explanations are possible for some of these situations, small-block/n situ rotation also can explain the evidence, and in some instances does so with models that (it seems to me) provoke far less astonishment than do the oroclinal models. It is possible to state (although perhaps not too convincingly, given the sparse evidence available) that the leading edge of South America has been fairly t h o r o u g h l y "rearranged" by rotations and that the sense of rotation is what would be predicted from the sense of shear arising from the plate interactions. This same pattern has been found in at least two other active orogenic belts (Van der Voo and Zijderveld, 1969; Beck, 1976), and probably will prove to be very common worldwide.

I/I--D

49

Acknowledgements--R. Drake first convinced me to work in South America, for which I am very grateful. I have learned what little I know about South American geology and tectonics mainly from Drake, F. Here6, and F. Munizaga. I especially thank the latter for taking care of me as I have blundered about in Chile doing field work. This manuscript benefited from conversations with Drake, Herv6 and Munizaga, as well a~ R. Burmester, R. Butler, D. Engebretson, W. McDonald, D. V~lencio, and ot~.ers too numerous to mention. Butler provided an extcemely useful review. My research in South America has been supported by bISF grant EAR 8312821. Most of the paper was written while I was on sabbatical leave at the University of Arizona. This paper was prepared for presentation at the IGCP Project 120 symposium on magmatie evolution of the Andes, held in Santiago, 18-24 November 1985. Contribution to IGCP Project 202.

REFERENCES Aspden, J. A., and McCourt, W.J. 1986. Mesozoic oceanic terrane in the central Andes of Colombia. Geology 14,415-418. Beck, M. E., Jr. 1976. Discordant paleomagnetic pole positions as evidence of regional shear in the western Cordillera of North America. American Journal of Science 276,694-712. Beck, M. E., Jr. 1980. Paleomagnetic record of plate-margin tectonic processes along the western margin of North America. Journal of Geophysical R esearch 84,7115-7131. Beck, M. E., Jr. 1986. Model for late Mesozoic-early Tertiary tectonics of coastal California and western Mexico, and speculations on the origin of the San Andreas Fault. Tectonics 5, 49-64. Beck, M. E., Jr., Drake, R. E., and Butler, R. F. 1986a. Paleomagnetism of Cretaceous volcanic rocks from central Chile, and implications for the tectonics of the Andes. Geology 14, 132136. Beck, M. E., Jr., Burmester, R. F., Craig, D. E., Gromme, C. S., and Wells, R. E. 1986b. Paleomagnetism of middle Tertiary volcanic rocks from the Western Cascade Series, northern California. Journal of Geophysical Research 91,8219-8230. Bellieni, G., Brotzu, P., Comin-Chairamonti, P., Ernesto, M., Melfi, A. F., Pacca, I. G., Picirillo, E. M., and Stolfa, D. 1983. Petrological and paleomagnetic data on the plateau basalt and rhyolite sequences of the southern Parana basin (Brazil). Anais de Academia Brasileria de Ciencias 55,355-383. Burns, K. L., Rickard, M. J., Belbia, L., and Chamalaun, F. 1980. Further paleomagnetic confirmation of the Magellanes orocline. Tectonophysics, 63, 75-90. Carey, S. W. 1955. The orocline concept in geotectonics. Proceedings of the Royal Society, Tasmania 89,255-288. Carey, S.W. 1958. The tectonic approach to continental drift. In: Continental Drift -- A Symposium tEdited by Carey, S. W.), pp. 177-358. University of Tasmania, Hobart. Dalziel, I. W. D., Kligfield, R., Lowrie, W., and Opdyke, N.D. 1973. Paleomagnetic data from the southernmost Andes and the Antarctandes. In: Implications of Continental Drift to the Earth Sciences, Vol. 1 (Edited by Tarling, D. H., and Runcorn, S. K.), pp. 37-101. Academic Press, London. Demarest, H. H., Jr. 1983. Error analysis for the determination of tectonic rotation from paleomagnetic data. Journal of Geophysical Research 88,4321-4328. Engebretson, D. C., Gordon, R. G., and Cox, A. 1985. Relative Motions Between Oceanic and Continental Plates in the Pacific Basin. Geological Society of America, Special Paper 206, 59 p. Feininger, T. 1987. Allochthonous terranes in the Andes of Ecuador and northwestern Peru. Canadian Journal of Earth Sciences 24,266-278. Fisher, R. A. 1953. Dispersion on a sphere. Proceedings of the Royal Society, London A217, 275-305. Gordon, R. G., Cox, A., and O'Hare, S. 1984. Paleomagnetic Euler poles and the apparent polar wander and absolute motion of North America since the Carboniferous. Tectonics 3,499-538.

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Heki, K., Hamano, Y., Kono, M., Nomura, K., Morikawa, N., and Kinoshita, H. 1983b. Paleomagnetic study of Jurassic sedimen tary and volcanic rocks in northern Chile. Rock Mag. Paleo geophys. 10,120-128.

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tteki, K., Hamano, Y., KonoshiLa, ]L, Taira, A., and Kono, M. 1984. Paleomagnetic study of Cretaceous rocks of Peru, South America: Evidence for rotation of the Andes. Tectonophysics 108, 267-28~ ifeki, K., Hamano, Y., and Kono M., and Ui, T. 1985. Paleomagnetism of Neogene Ocros dike swarm, the Peruvian Andes: Implication for the Bolivian orocline. Geophysics Journal, Royal A strono mical Society, London 80,527-534. Irving, E., and Irving, G. A. 1982. Apparent polar wander paths, Carboniferous through Cenozoic, and the assembly of Gondwana. Geophysical Surveys 5, 14 l-188. Kono, M., tieki, K., and Hamano, Y. 1985. Paleomagnetic study of the entral Andes: Counterclockwise rotation of the Peruvian block. JourrralofGeodynamics 2,193-209. McCourt, W. J., Aspden, d. A., and Brook, M. 1984. New geological and geochronological data from the Colombian Andes: Continental growth by multiple accreLion. Journal of the Geological Society, London 141,831-845. MacDonald, W. D. 1980. Anomalous paleomagnetic directions in late Tertiary andesitic intrusions of the Cauca depression, Colombian Andes. Tectonophysics68,339-348. MacDonald, W. D., and Opdyke, N. D. 1972. Tectonic rotations suggested by paleomagnetic results from northern Colombia, South America. Journal of Geophysical Research 77, 5720-5730. MacDonald, W. D., and Opdyke, N. D. 1974. Triassic paleomagnetism of northern South America. American Association of Petroleum Geologists Bulletin 58, 208-215. Mankinen, E. A., 1978. Paleomagnetic evidence for a Late Cretaceous deformation of the Great Valley sequence, Sacramento Valley, California. J. Res. U.S. Geol. Surv. 6,383-390. May, S. R., and Butler, R. F. 1985. Paleomagnetism of the Puente Piedra Formation, central Peru. Earth and Planetary Science Letters 72,205-218. Mendia, J. E., 1978. Paleomagnetic study of alkaline vulcanites from Almatuerte, province of Cordoba, Argentina. Geophysics

Journal, Royal A stronomical Society, London 54,539-546. Opdyke, N. D., and MacDonald, W. D. 1973. Paleomagnetism of the Late Cretaceous Pocos de Caldas Alkaline Complex, southern Brazil. Earth and Planetary Science Letters 18, 37-44. Pahner, H. C., Hayatsu, A., and MacDonald, W .D. 1980a. Paleomagnetic and K-Ar age studies of a 6 km-thick Cretaceous section from the Chilean Andes. Geophysics Journal, Royal Astronom ieal Society,London 62, 133-153. Palmer, tt. C., Hayatsu, A., and MacDonald, W. D. 1980b. The middle Jurassic Camaraca Formation, Arica, Chile: Paleomagnetism, K-Ar dating and tectonic implications. Geophysics Journal, Royal Astronomical Society, London 62, 155-172. Pilger, R. H., Jr. 1983. Kinematics of the South American subduction zone from global plate reconstructions. In: Geodynamics of the Eastern Pacific Region, Caribbean and Scotia Arcs. (Edited by Cabre, R.I, pp. 113-126. Geodynamic Series 9, American Geophysical Union and Geological Society of America. Ramos, V. A., Jordan, T. E., Allmendinger, R. W., Mpodozis, C., Kay, S. M., Cortes, J. M., and Palma, M. 1986. Paleozoic terranes of the central Argentine-Chilean Andes. Tectonics 5,855-880. Rapalini, A., Vilas, d., and Valencio, D. 1985. New evidence for an allochthonous plate in southwestern Argentina. Comunieaciones, Departamento de Geologia, Unioersiclad de Chile, 35,195-196. Sehult, A., and Guerreiro, S. D.C. 1979. Paleomagnetism of Mesozoic igneous rocks from the Maranhao Basin, Brazil, and the time of opening of the South Atlantic. Earth andPlanetary Science Letters 42,427-436.

Silver, E. A., Case, J. E., and MacGill~,:ry, L . J . 1975. Geophysical study of the Venezuelan borderlan(~ Geological Society of A merica Bulletin 86, 213-226. Skerlec, G. M., and Hargraves, R. B., 1980. Tectonic significance of paleomagnetic data from northern Venezuela. Journal of Geophysical Research 85, 5303-5315. Stearns, C., Mauk, F. J., and Vail der Voo, P. 1982. Late Cretaceous-early Tertiary paleomagnetism of Aruba and Bonaire (Netherlands Leeward Antilles}. Journal of Geophysical Research 87,112%1141. Turner, P., Clemmey, It, and Fhnt, S. 1984. Paleomagnetic studies of a Cretaceous molasse sequence in the central Andes (Coloso Formation, northern Chile). Journal of the Geological Society, London 141,869-876. Valencio, D.A. 1972. Paleomagnetism of the Imwer Cretaceous Vulcanitas Cerro Colorado Formation of the Sierra de los Cbndores Group, Province of C6rdoba, Argentina. Earth and Planetary Science Letters ] 6,370-378. Valencio, D. A., Mendia, J. E., Guidici, A., and Gascon, J. A. 1977. Paleomagnetism of the Cretaceous Pirgua Subgroup (Argentina) and the age of the opening of the South Atlantic. Geophysics Journal, Royal Astronomical Society, London 51, 47-58. Valencio, D. A., Vilas, J. F., and Pacca, 1. G. 1983a. The significance of the paleomagnetism of Jurassic-Cretaceous rocks from South America: Predrift movements, hairpins and magnetostratigraphy. Geophysics Journal, Royal Astronomical Society, London 73, 135-151. Valencio, D. A., Vilas, d. F., Sola, P., and Lopez, M.G. 1983b. Paleomagnetism of Upper Cretaceous-lower Tertiary igneous rocks from central Argentina. Geophysics Journal, Royal Astronomical Society, London 73, 129-! 34. Van der Voo, P. and Zijderveld, J. 1969. Paleomagnetism in the western Mediterranean area. Geologic Mijnbouw 26,121-138. Vilas, J. F .A. 1974. Paleomagnetism of some igneous rocks of the Middle Jurassic Chon-Aike Formation from Estancia La Reconquista, Province of Santa Cruz, Argentina. Geophysics Journal, Royal A stronomical Society, London 39, 51-522. Vilas, J. F. A. 1976. Paleomagnetism of the Lower Cretaceous Sierra de los Condores Group, Cordoba province, Argentina. Geophysics Journal, Royal Astronomical Society, London 46, 295305. Vilas, J. F. A., and Valencio, D. A. 1978. Paleomagnetism and KAr dating of the Carboniferous Andacollo Series (Argentina) and the age of the hydrothermal overprinting. Earth and Planetary Science Letters 40, 101-106.

APPENDIX PALEOMAGNETIC P O L E S FROM C O A S T A L SOUTH A M E R I C A

Chile south of the Arica deflection (K01-3 ) These appear to be rotated slightly clockwise. KO1 is from a moderately dipping homoclinal sequence of fresh ashflow tufts. A K-Ar age of 103 Ma was obtained from three analyses of biotite and plagioclase separates. The sampling area is located near 34.6S, 71.0W, in the Central Valley of Chile. KO2 gives results for flows and ashflow tufts from nearly 6000 meters of volcanic rocks located near 29.8S,

Late Jurassic-Recent paleomagnetic data from active plate margins of South America 70.8W (northern Chile). The base of the section has a paleontological age of Early Cretaceous, and questionable mid-Cretaceous fossils appear higher up. However, K-Ar results are scattered and young (59 to 92 Ma), and this, combined with evidence of low grade metamorphism, implies that the magnetization may have been re-set. Polarities are almost all normal, suggesting m a g n e t i z a t i o n d u r i n g the Cretaceous long normal interval (roughly 120 to 85 Ma). A positive fold test indicates that the magnetization is pre-folding. KO3 is a study of finegrained interbeds in a conglomeratic molasse sequence cropping out along the coast in northern Chile (24S, 70W). Thermal and alternating field demagnetization of individual specimens indicates that two components of magnetization are present; these are of opposite polarity and are nearly antiparallel. The sampling scheme and method of reporting of results make calculation of a mean direction by the methods of this paper difficult. The entry in Table 1 takes the conservative view that only three valid sites are represented, two of normal polarity and one reversed. This method of calculating the mean is entirely arbitrary and probably produces an exaggerated confidence interval about the pole. The unit is dated as earliest Cretaceous on the basis of its stratigraphic position; the presence of two polarities argues that the magnetization is preAptian. Cretaceous poles from Peru and northernmost Chile (K04-8) These poles appear to be rotated counterclockwise. K04 represents results from a swarm of basaltic and andesitic dikes from near Arica in northern Chile (18.5S, 70.3W). The dikes intrude Neocomian sedimentary rocks, but apparently are not otherwise dated. All 19 dikes have normal polarity, which might suggest that they were magnetized within the Cretaceous long normal interval. However, unusually low dispersion (K=120.1) points to the possibility that the dikes were intruded and magnetized within a very short interval of time. If so, they could have become magnetized within any brief normal interval. The dikes intrude dipping sedimentary rocks, but no structural correction was made, apparently because the dike contacts are vertical (?). Until better age and structural control are acquired, this pole should be used with caution. K05 represents paleomagnetic results from the Neocomian sedimentary unit intruded by the dikes of pole K04; the sampling site is located nearby (18.5S, 70.3W). Seven horizons were sampled at a single locality. Again, all samples were of normal polarity and, again, dispersion is extremely low (K = 206.4). KO6 summarizes results from 15 sites in Lower Cretaceous volcanic and sedimentary rocks located near Lima (11.9S, 77W). Magnetization in the rocks is carried predominantly by pyrrhotite, probably introduced during intrusion of the nearby Coastal Batholith at about 90 Ma. All polarities are normal and

51

dispersion is unusually low (K= 174.3). KO7 and KO8 both represent volcanics of the Casma Group, arbitrarily separated here into two studies on the grounds that the sampling localities are far apart (Ancfn, near 11.8S, 77.2W; Huarmey, near 10.3S, 78.0W). The rocks are dated as early Albian on paleontological evidence. Lavas, pyreelasti¢ flows, and dikes were sampled at both localities. Apparently no problems with geological structure were encountered. As with all Cretaceous poles in this group, dispersion for both K O 7 and K O 8 is surprisingly low. Cretaceous poles from the Caribbean coast of South A me rica (K09 -13) The units studied in this area all fall within the socalled Bonaire block (Silver et al., 1975). The Bonaire block appears to have been rotated about 90 ° clockwise relative to stable South America and to have been translated relatively northward by as much as 1600 km. Some of these studies report additional paleomagnetic data that are not summarized here. K09 gives results from an ultramafic complex in the Caribbean Mountains of northern Venezuela (9.5N, 67.5W). The rocks sampled are part of the Villa de Cura belt, a fault-bounded terrane thought by some to be allochthonous. Although the Villa de Cura belt is mostly composed of glaucophane and lawsonite-bearing metamorphic rocks, the rocks sampled (El Chaco Ultramafic Complex) are unmetamorphosed. Paleohorizontal control appears to be lacking. Dating is by means of indirect, stratigraphic arguments and is not very precise. KO10-12 are studic~ of Cretaceous units on the islands of Aruba (12.5N, 70W) and Bonaire (12.3N, 68.5W). KO10 (Bonaire) reports d a t a from a submarine volcanic sequence of mid- to Late Cretaceous age that may have been remagnetized in Late Cretaceous or earliest Tertiary time. Both polarities are present, in some cases within a single sample. KOll and 12 are both from Aruba. K O l l is a well-defined pole for the Aruba batholith (having, however, suspiciously low dispersion), and KO12 is a pole based on only three felsic dikes that cut the batholith. The failure of KOll to agree with other poles from the Bonaire block poses a serious problem in interpretation. One possible explanation is that the age of magnetization in the batholith is considerably younger than its radiometric age. KO13 is from the Guajira Peninsula of Colombia (11.8N, 71.8W). It is based on results for 8 sites in flat-lying felsic lava flows scattered over some few hundreds of kilometers. Unlike other rocks from the Bonaire block, these rocks seem to be magnetically stable and uncomplicated. Age is based on stratigraphic position and radiometric dating, and is a minimum. Jurassic poles from northernmost Chile (J01-3) These poles are less well grouped than Cretaceous poles from the same area (above). Compared with

52

M.E. BECK, J R

craton poles JC3 and 4, they suggest about 15 ¢ of counterclockwise rotation of northern Chile relative to the craton. J O l is a study of a swarm of thin andesitic dikes cropping out along a traverse --1 km in length near the Pan American highway south of Arica (19.1S, 70.2W). Within-dike dispersion is surprisingly high, and only I0 of 25 dikes were used in this study. Between-site dispersion also is high, The magnetic properties of these dike samples suggest some sort of alteration. Dating is by stratigraphic evidence, and the age of the magnetization is poorly constrained. The dikes intrude dipping sedi-

mentary rocks, but apparently no tilt correction was made. J 0 2 and J 0 3 report results for a sequence of andesites and interbedded marine shales. The pale~ ontological age of these rocks is mid-Jurassic, and JO2 reports a K-Ar isochron of 157 Ma. The rocks were sampled from Arica south to Caleta Vitor, northwestern Chile, a d i s t a n c e of a b o u t 25 k m (roughly 18.5S, 70.3W). J o g reports some problem with secondary alteration. Both polarities are present, and the means of N and R groups are antL parallel. The dispersion found in stud), JO3 is s u s pieiously low,