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Paleomagnetic constraints on the tectonic evolution of the Sakarya Continent, northwestern Anatolia-Reply

Ian Evans and S.A. Hall

Department of Geosciences, University of Houston, Houston, TX 77204-5503, USA

(Received May 3, 1991; revised version accepted May 27, 1991)

After reading the comments of Saribudak, we can see no reason to modify the conclusions reached in Evans and Hall (1990). We have two major arguments with the comments of Saribu- dak. The first one relates to terminology, specifi- cally his use of the term “Pontides”, and secondly the nature and precision of paleomagnetic re- sults.

In order to appreciate the first point of con- tention, a brief explanation is in order. The work of Evans and Hall (1990) is an attempt to bring paleomagnetic research results to bear on con- straining a geologic model proposed to explain the tectonic evolution of the Anatolian Tethy- sides with particular focus on the region of north- western Anatolia where the closure of the Neo- Tethys is marked by the generation of the Pon- tide Orogenic belt (Sengijr and Yilmaz, 1981; Sengiir et al., 1984). In northwestern Anatolia, this orogen is believed to be the product of a Paleocene collision between the southern margin of Eurasia and a large continental fragment, re- ferred to as the Sakarya Continent, resulting in the generation of the Intra-Pontide Suture. This situation is made more structurally complex when the Anatolide-Tauride Platform then collides with the Sakarya Continent producing the Izmir-Ankara Suture. It is our contention that our paleomagnetic results do not contradict the tectonic model presented by SengGr and Yilmaz (1981). The major thrust of the comment by Saribudak is that the Sakarya Continent did not collide with Eurasia in the Paleocene but that it instead collided with some entity, identified as the “Pontides”, some 8-10” south of the

Eurasian margin during the Paleocene. Saribudak does not provide the outline of a model that shows the presence of a significant tectonic unit that lay between Sakarya and Eurasia in the latest Cretaceous, nor does he provide the geo- logic evidence that describes the fate of the B-10 o of presumably oceanic crustal material that ex- isted between the Sakarya Continent/ “Pontides” and the southern margin of Eurasia. A close examination of fig. 1 of Saribudak shows that the Intra-Pontide Suture resides next to the southern margin of Eurasia (the Rhodope-Pontide frag- ment of SengGr, 1984; see also a paleomagnetic analysis of this margin in Evans et al., 1990) with no evidence of a suture that would mark the post Paleocene/ Eocene closure of an approximately lOOO-km wide ocean. There is no credible geo- logic evidence to suggest that the Sakarya Conti- nent did not collide with Eurasia in the Pale- ocene. The available paleomagnetic evidence does not preclude the possibility that the collision be- tween Sakarya and Eurasia occurred at the lower latitude (c. 20 ’ N) suggested by Saribudak as the error bars for both the southern margin of Eura- sia and of Sakarya encompass this lower latitude, albeit close to the southerly limits of the error bars in question. This then brings us to the role of paleomagnetic evidence in resolving this issue. In his contention that the “Pontides” were clearly “at about 20 o N paleolatitude during Eocene time”, Saribudak places too much faith in the mean paleolatitudes determined from his study (Saribudak, 1989a, b) and that of Kissel et al. (1989). In failing to display the error bars for these sites in his fig. 2, Saribudak gives the mis-

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PALEOMAGNETIC CONSTRAINTS ON TECTONIC EVOLUTION OF SAKARYA-DISCUSSION 321

60 PALEOLATITUDE vs AGE (NW ANATOLIA SITES)

Age (Ma)

Fig. 1. Variation in paleolatitude of Africa and Eurasia during Late Cretaceous and Early Tertiary. Hatched area represents the

swath of possible paleolatitudes (at the 95% confidence limit) for a site at 41.5 “N, 30.5”E attached to the Eurasian plate. The

central line represents the mean paleolatitude based upon the Eurasian poles of Westphal et al. (1986). Dotted area shows the

corresponding swath of possible paleolatitudes for a site at/Alexandria, Egypt attached to the African plate and represents the

maximum southerly limit of any crustal block located between Africa and Eurasia. Open squares, filled square and circle are the

mean paleolatitudes of Late Cretaceous and Eocene sites (Saribudak, 1989a,b), Eocene sites (Kissel et a., 1989), and Site 12

(Paleocene) of Evans and Hall (19901, respectively. Also shown as a triangle is the mean paleolatitude of Sites 11 and 13

(Paleocene) of Evans and Hall (1990). Error bars represent the uncertainties in paleolatitude based upon the statistical parameters

of each study. Note: Eocene sites have been slightly offset to clearly show their respective uncertainties.

leading impression that the paleolatitudes are that it is not possible to use the present paleo-

precisely known. This of course is not the case magnetic results to make any precise or unequiv- and there are substantial uncertainties in the ocal statement about the position of the Sakarya paleomagnetic directions. In addition, the paleo- Continent, the “Pontides” or for that matter the latitudes determined from the apparent polar southern edge of the Eurasian plate during the wander paths of Eurasia and Africa (Westphal et late Cretaceous and early Tertiary. The Eocene al., 1986) for the site (41S”N, 30.5” E) on the results of Saribudak (1989b) and Kissel et al. southern margin of Eurasia and that of Alexan- (1989) plot south of the latitude predicted for dria, Egypt are also associated with significant Alexandria (see Saribudak’s, fig. 2 and Fig. 1 of uncertainties related to the accuracy of the poles. this reply) suggesting that these sites were not When the corresponding error bars for each of only part of the African plate at that time but these paleolatitudes are added (Fig. 11, it is clear also lay on the African continent. This is clearly

TABLE 1

Comparison of Paleocene-Eocene paleomagnetic poles for Africa, Eurasia, Sakarya and the Pontides

Crustal block Age Paleopoles A,< Reference

lat. (” NJ long. f o E) (0,

Eurasia Paleocene 78.9 166.8 4.8 Jowett et al. (1987)

Africa Paleocene 76.0 199.0 7.0 Westphal et al. (1986)

Sakarya Paleocene 64.3 139.9 13.2 a Hall (pers. commun. to Saribudak, 1990)

Eurasia Eocene 76.6 166.1 4.5 Jowett et al. (1987)

Africa Eocene 78.0 178.0 7.0 Westphal et al. (1986)

Pontides Eocene 70.0 239.0 8.1 Saribudak (1989a)

a This is an cYs,-value and not A,,.

322 I t.VANS ANt) X A tl,\l.l

not the case and suggests that these results should be re-examined. For example, in-situ magnetic directions for Eocene sites in the western Pon- tides reported by Saribudak (1989b) give more northerly latitudes (approximately 35 ’ N) that more nearly coincide with the predicted latitude for the southern margin of Eurasia in the Eocene. Is there compelling evidence for a pre-deforma- tion stable magnetization at these Eocene sites?

In his table 1, Saribudak erroneously reports the Paleocene pole position obtained for the Sakarya Continent from our Site 12. The correct pole lies at 64.3” N, 139.9“ E and overlaps the Paleocene poles for both Africa and Eurasia (Ta- ble 1). Saribudak, in his table 1, again fails to give any measure of the uncertainty in these poles. When appropriate uncertainties are included, it is again clear that the overlap of the poles is considerable. Although a plot of these poles shows that Saribudak’s Eocene pole differs from those of Eurasia and Africa, a modest declination “anomaly” produced by a small rotation (ap- proximately 20 a clockwise) about a vertical axis through the site (caused possibly by collision or strike-slip motion of the North Anatolian Fault) is sufficient to bring this pole into agreement with those of both Africa and Eurasia within the un- certainties associated with each pole.

For these reasons, we suggest that the paleo- latitudes of the “Pontides” determined from Saribudak (1989a, b) and Kissel et al. (1989) are not associated with sufficient certainty to permit Saribudak to claim that they constitute “evidence that they (i.e. the “Pontides”) were not part of Eurasia during Cretaceous and Eocene time (see Fig. 1). Similarly, it is not possible, with the available paleomagnetic data, to say unequivo- cally as Saribudak does that Site 12 of Evans and Hall (1990) is north of the Pontides in the Pale- ocene. Each of these statements requires consid- erably smaller uncertainties in the paleomagnetic directions (< 5 o > than are available from the present results.

One area of agreement between us and Saribudak lies in the interpretation of the paleo- latitudes of Sites 11 and 13. Even when the error bars are considered, it is clear that if the paleolat- itudes determined are correct, these sites must

have been located considerably farther south in the Paleocene, quite possibly south of Alexan- dria! As these sites are currently located some 8 ” north of Alexandria and the tectonic history of the region suggests this distance has decreased rather than increased since the Paleocene, it be- comes evident that the paleolatitudes determined for Sites 11 and 13 are impossibly low and must therefore reflect some mechanism that has modi- fied the magnetic signature. Although there may be other explanations, we still believe that shal- lowing of the remanent inclination because of the coarse grain size of the sedimentary rocks repre- sents a plausible and generally accepted explana- tion. The implication of this explanation for the paleolatitude of Site 12 is that if there has been any shallowing of the inclination due to a similar mechanism, then the true paleolatitude of Site 12 will be farther north than shown by Evans and Hall (1990). A more northerly latitude would agree better with the latitude predicted for Eura- sia and would slightly strengthen our interpreta- tion. It is difficult to understand how Saribudak can on the one hand agree that Sites 11 and 13 paleolatitudes are not correct and are much too low and yet on the other hand believe that the Site 12 paleolatitude is too high. Certainly in his remarks, Saribudak does not address this appar- ent lack of consistency.

Finally, in his comments regarding the inter- pretation of our paleomagnetic results from northwestern Anatolia, Saribudak takes us to task for not using all the available paleomagnetic data. The implication being that we have been selective in our treatment and purposefully ignored re- sults. We used all data available in published literature at the time of manuscript preparation. While the subsequent publication of results by Saribudak (1989a, b) and Kissel et al., (1989) post-date submission of our manuscript, it is nev- ertheless very encouraging to note that these more recent results in no way conflict with our original interpretation.

References

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the tectonic evolution of the Sakarya Continent, north-

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