Aksu et al. (2000) present a geometric model for the Marmara Sea that features northeast-elongate fault-bounded ridges and basins bound by two east-west branches of the North Anatolian right-lateral transform. They state: (1) "The basins and ridges are rotating counterclockwise...," and (2) "...the entire negative flower structure is in a state of wholesale crustal extension." We are not in a position to evaluate the mapping, but if their fault geometry is correct, we suggest that it is consistent with the clockwise block rotation expected in a right-lateral transform (i.e., like books rotating on a shelf; McKenzie and Jackson, 1986; Lamb, 1994), not counterclockwise as stated in (1). Further, shortening (not extension) is expected across the northeast-striking faults in a right-lateral transform system for the given geometry (for angle f > 90°; inset to Fig. 1) unless east-west divergence between the rotating blocks is permitted; and extension is expected across the east-west faults. To demonstrate the expected zones of shortening (stippled areas in Fig. 1) and extension (overlap zones of Fig. 1) for the Aksu et al. (2000) geometry, we restore the blocks to their 1 Ma positions, assuming an Anatolia-Eurasia relative velocity as determined by Global Positioning System (GPS) data at GPS station ERDE (Fig. 1; McClusky et al., 2000). Seismic reflection profiles seem to support our dextral kinematic model as Okay et al. (2000) interpret young short-wavelength compressional folding along the southeast flank of the Central Marmara Ridge ( d in Fig. 1) and long-wavelength anticlines across a and d (Fig. 1). These structures resemble anticlines in California above (oblique) thrust-reactivated blind faults (Seeber and Sorlien, 2000). Shortening structures and thrust earthquakes are also associated with clockwise block rotation and termination of strike-slip faults within the pull-apart Salton Trough of California (Armbruster et al., 1998). Thus, the fault geometry proposed by Aksu et al. (2000) is consistent with shortening synchronous with extension within Marmara Sea if it is interpreted as a clockwise rotation system.

References Cited
Aksu, A.E., Calon, T.J., Hiscott, R.N., and Yasar, D., 2000, Anatomy of the North Anatolian fault zone in the Marmara Sea, western Turkey: GSA Today, v. 10, no. 6, p. 3-7.

Armbruster, J.G., Seeber, L., Sorlien, C.C., and Steckler, M.S., 1998, Rotation vs. rifting in an extensional jog: Salton trough, California: EOS, (Transactions, American Geophysical Union), v. 79, p. F565.

Lamb, S.H., 1994, Behavior of the brittle crust in wide plate boundary zones: Journal of Geophysical Research, v. 99, p. 4457-4483.

McClusky, S., and 27 coauthors, 2000, Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus: Journal of Geophysical Research, v. 105, p. 5695-5719.

McKenzie, D.P., and Jackson, J.A., 1986, A block model of distributed deformation by faulting: Geological Society [London] Journal, v. 143, p. 349-353.

Okay, A.I., Kaslilar-Ozcan, A., Imren, C., Boztepe-Guney, A., Dermirbag, E., and Kuscu, I., 2000, Active faults and evolving strike-slip basins in the Marmara Sea, northwest Turkey: a multichannel seismic reflection study: Tectonophysics, v. 321, p. 189-218.

Seeber, L., and Sorlien, C.C., 2000, Listric thrusts in the western Transverse Ranges, California, Geological Society of America Bulletin, v. 112, p. 1067-1079.

Acknowledgments
Institute for Crustal Studies contribution 372-102TC. Partial funding for Seeber was from the Southern California Earthquake Center (SCEC), a Science and Technology Center of the National Science Foundation, funded in part by the U. S. Geological Survey. SCEC contribution 533.

Christopher C. Sorlien
Institute for Crustal Studies
University of California
Santa Barbara, CA 93106
chris@quake.crustal.ucsb.edu

Leonardo Seeber
Lamont-Doherty Earth Observatory
Columbia University
Palisades, NY 10964


Aksu et al. reply:
Sorlien and Seeber raise two issues: (1) rotation sense within the principal deformation zone (the Anatolian transform), and (2) whether the ridges are compressional or extensional. Their restoration assumes dextral shear between two bounding strike-slip faults, leading to clockwise rotation of intervening rigid blocks. Given that f is obtuse in the Anatolian system, these parameters would indeed yield northwest-southeast contraction and northeast-southwest extension in their model, compatible with the kinematics of a releasing bend. However, several lines of evidence refute northwest-southeast contraction anywhere within the principal deformation zone. First, central graben in the basins are oriented east-northeast (Aksu et al., 2000, Fig. 1) and because the bounding faults of the ridges are at less than 25º from the trend of these graben, the ridges must also be under extension. Extension (not contraction) along northeast faults is also confirmed by normal-sense drag on closely spaced faults in the ridges, the strong subsidence of the basins and ridges (to form the Marmara Sea), and the absence of compressional fault-plane solutions in the principal deformation zone. Why are the modeling results not compatible with our observations? First, the principal deformation zone is totally detached from both the Eurasian and Anatolian plates, riding above a single buried stem fault (unlike the geometry assumed by Sorlien and Seeber and discussed by Lamb, 1994, and Armbruster et al., 1998). In such a detached configuration, individual crustal elements will rotate according to local displacement fields, which can be sufficiently at odds with the regional kinematic framework to allow counterclockwise rotations, especially in a regime of tectonic escape. Second, material in the principal deformation zone is mechanically weak due to pervasive faulting, so cannot be modeled as rotating rigid blocks (e.g., "bookshelf" system). Thus, we maintain that the Marmara-type escape basin (Aksu et al., 2000, Fig. 3C) does not conform with existing models for strike-slip basin formation, particularly the classic pull-aparts that Sorlien and Seeber use as a template.

References Cited
See above listing.

Ali E. Aksu, aaksu@sparky2.esd.mun.ca
Tom J. Calon
Richard N. Hiscott
Memorial University of Newfoundland
St. John's, Newfoundland A1B 3X5

Dogan Yasar
Dokuz Eylül University
Haydar Aliyev Caddesi No.10 Inciralti, Izmir, Turkey 35340

Published in the Geological Society of America's GSA Today, October 2000, vol. 10, no. 10. Copyright (c)2000, The Geological Society of America (GSA), All rights reserved.