Pacific: crustal recycling in an oceanic setting. London: Geological Society. Special publication, 219, pp. 99–118.
Stern, R. J., 2002. Subduction zones. Reviews of Geophysics, 40,
doi:10.1029/2001RG000108
Stern, R.J. 2010. The anatomy and ontogeny of modern intraoceanic arc systems. In: Kusky, T. M., Zhai, M.-G., and Xiao,
W. (eds.), The Evolving Continents: Understanding Processes
of Continental Growth. London: Geological Society. Special
publication, 338, pp. 7–34.
Stern, R. J., and Dickinson, W. R., 2010. The Gulf of Mexico is a
Jurassic back-arc basin. Geosphere, 6, 739–754.
Stern, R. J., Reagan, M., Ishizuka, O., Ohara, Y., and Whattam, S.,
2012. To understand subduction initiation, study forearc crust;
to understand forearc crust, study ophiolites. Lithosphere, 4,
469–483.
Taylor, B. (ed.), 1995. Backarc Basins: Tectonics and Magmatism.
New York: Springer. 524 p.
Timm, C., de Ronde, C. E. J., Leybourne, M. I., Layton-Matthews,
D., and Graham, I. J., 2012. Sources of chalcophile and
siderophile elements in Kermadec arc lavas. Economic Geology,
107, 1527–1538.
Yang, K., and Scott, S. D., 1996. Possible contribution of a metalrich magmatic fluid to a sea-floor hydrothermal system. Nature,
383, 420–423.
Cross-references
Accretionary Wedges
Active Continental Margins
Black and White Smokers
Chemosynthetic Life
Explosive Volcanism in the Deep Sea
Gabbro
High-pressure, Low-temperature Metamorphism
Hydrothermal Plumes
Hydrothermalism
Intraoceanic Subduction Zone
Lithosphere: Structure and Composition
Marginal Seas
Marine Mineral Resources
Mohorovičić Discontinuity (Moho)
Oceanic Spreading Centers
Ophiolites
Paired Metamorphic Belts
Peridotites
Seamounts
Serpentinization
Subduction
Subduction Erosion
Volcanogenic Massive Sulfides
Wadati-Benioff-Zone
MAGNETOSTRATIGRAPHY OF JURASSIC ROCKS
María Paula Iglesia Llanos
IGEBA, National Scientific and Research Council,
University of Buenos Aires, Buenos Aires, Argentina
Rocks can retain a magnetic imprint acquired in the geomagnetic field that existed during their formation. The
main goal of magnetic polarity stratigraphy is to document
and calibrate the global geomagnetic polarity sequence in
stratified rocks and to apply this geomagnetic polarity
scale for high-resolution correlation of marine magnetic
anomalies and of polarity zones in other sections.
Magnetic polarity scales younger than 160 Ma are
derived from seafloor magnetic anomalies. For older
times, such scales must be derived from polarity successions obtained in the continents yielding a precise age control such as that provided by ammonites, i.e., 1 myr.
In this study, emphasis is paid to the Jurassic period.
Five ammonite-bearing sections located to the north and
center of the Neuquén basin, west-central Argentina, have
been sampled for paleomagnetic purposes. These sections
are made up of ammonite-bearing sedimentary and, subordinately, volcanic rocks of Lower Jurassic age.
The paleomagnetic study showed that there are two
magnetic components, one soft that coincides with the
dipolar field of the region and another one harder that is
dissimilar from any younger magnetization. According
to the field tests for paleomagnetic stability and petrographical studies, the latter component has been acquired
during or shortly after the deposition (cooling) of sedimentary (volcanic) rocks. Thus, the hard component is
interpreted as the original Jurassic.
With polarities isolated in all five sections, the first
Lower Jurassic magnetostratigraphic scale of the Southern
Hemisphere scale has been constructed. To anchor the
polarities to the international geomagnetic scale, we used
the ammonite zonation and correlation from Riccardi
(2008), and the international geomagnetic polarity scale
from Ogg (2004) has been considered. Thus, the resultant
magnetostratigraphic scale is made up of 11 dominant
reverse (JR 1 to JR 11 ) and 12 dominantly normal (JN 1 to
JN 12 ) polarities, included in 19 ammonite zones. They comprise the Andean P. rectocostatum which is correlated with
the Tethyan P. planorbis zone to the Andean Phymatoceras
that is correlated with the Tethyan G. thouarsense.
With respect to the isolated polarities, a good correlation was achieved with the international scale that helped
to constrain some issues that were still at loose ends. In
the first place, in the Hettangian-Sinemurian interval,
polarities comprised within the international Liasicus
and Semicostatum biozones should be displaced down
until the dominant normal polarity coincides with the
regional JN 3 . In the second place, during the
Pliensbachian and Toarcian, the frequency of reversals
increases, and yet, correlations between the scales are
more solid. Results allowed assigning precise ages to
those levels devoid of diagnostic fossils. This is the case
of the Sinemurian-Pliensbachian boundary as well as the
Toarcian in the north and the Pliensbachian-Toarcian in
the center of the basin.
Bibliography
Ogg, J., 2004. The jurassic period. In Gradstein, F., Ogg, J., and
Smith, A. (eds.), A Geologic Time Scale. Cambridge: Cambridge
University Press, pp. 307–343.
Riccardi, A. C., 2008. The marine Jurassic of Argentina: a biostratigraphic framework. Episodes, 31, 326–335.
MAGNETOSTRATIGRAPHY OF JURASSIC ROCKS
407
Stern, R. J., 2002. Subduction zones. Reviews of Geophysics, 40,
doi:10.1029/2001RG000108
Stern, R.J. 2010. The anatomy and ontogeny of modern intraoceanic arc systems. In: Kusky, T. M., Zhai, M.-G., and Xiao,
W. (eds.), The Evolving Continents: Understanding Processes
of Continental Growth. London: Geological Society. Special
publication, 338, pp. 7–34.
Stern, R. J., and Dickinson, W. R., 2010. The Gulf of Mexico is a
Jurassic back-arc basin. Geosphere, 6, 739–754.
Stern, R. J., Reagan, M., Ishizuka, O., Ohara, Y., and Whattam, S.,
2012. To understand subduction initiation, study forearc crust;
to understand forearc crust, study ophiolites. Lithosphere, 4,
469–483.
Taylor, B. (ed.), 1995. Backarc Basins: Tectonics and Magmatism.
New York: Springer. 524 p.
Timm, C., de Ronde, C. E. J., Leybourne, M. I., Layton-Matthews,
D., and Graham, I. J., 2012. Sources of chalcophile and
siderophile elements in Kermadec arc lavas. Economic Geology,
107, 1527–1538.
Yang, K., and Scott, S. D., 1996. Possible contribution of a metalrich magmatic fluid to a sea-floor hydrothermal system. Nature,
383, 420–423.
Cross-references
Accretionary Wedges
Active Continental Margins
Black and White Smokers
Chemosynthetic Life
Explosive Volcanism in the Deep Sea
Gabbro
High-pressure, Low-temperature Metamorphism
Hydrothermal Plumes
Hydrothermalism
Intraoceanic Subduction Zone
Lithosphere: Structure and Composition
Marginal Seas
Marine Mineral Resources
Mohorovičić Discontinuity (Moho)
Oceanic Spreading Centers
Ophiolites
Paired Metamorphic Belts
Peridotites
Seamounts
Serpentinization
Subduction
Subduction Erosion
Volcanogenic Massive Sulfides
Wadati-Benioff-Zone
MAGNETOSTRATIGRAPHY OF JURASSIC ROCKS
María Paula Iglesia Llanos
IGEBA, National Scientific and Research Council,
University of Buenos Aires, Buenos Aires, Argentina
Rocks can retain a magnetic imprint acquired in the geomagnetic field that existed during their formation. The
main goal of magnetic polarity stratigraphy is to document
and calibrate the global geomagnetic polarity sequence in
stratified rocks and to apply this geomagnetic polarity
scale for high-resolution correlation of marine magnetic
anomalies and of polarity zones in other sections.
Magnetic polarity scales younger than 160 Ma are
derived from seafloor magnetic anomalies. For older
times, such scales must be derived from polarity successions obtained in the continents yielding a precise age control such as that provided by ammonites, i.e., 1 myr.
In this study, emphasis is paid to the Jurassic period.
Five ammonite-bearing sections located to the north and
center of the Neuquén basin, west-central Argentina, have
been sampled for paleomagnetic purposes. These sections
are made up of ammonite-bearing sedimentary and, subordinately, volcanic rocks of Lower Jurassic age.
The paleomagnetic study showed that there are two
magnetic components, one soft that coincides with the
dipolar field of the region and another one harder that is
dissimilar from any younger magnetization. According
to the field tests for paleomagnetic stability and petrographical studies, the latter component has been acquired
during or shortly after the deposition (cooling) of sedimentary (volcanic) rocks. Thus, the hard component is
interpreted as the original Jurassic.
With polarities isolated in all five sections, the first
Lower Jurassic magnetostratigraphic scale of the Southern
Hemisphere scale has been constructed. To anchor the
polarities to the international geomagnetic scale, we used
the ammonite zonation and correlation from Riccardi
(2008), and the international geomagnetic polarity scale
from Ogg (2004) has been considered. Thus, the resultant
magnetostratigraphic scale is made up of 11 dominant
reverse (JR 1 to JR 11 ) and 12 dominantly normal (JN 1 to
JN 12 ) polarities, included in 19 ammonite zones. They comprise the Andean P. rectocostatum which is correlated with
the Tethyan P. planorbis zone to the Andean Phymatoceras
that is correlated with the Tethyan G. thouarsense.
With respect to the isolated polarities, a good correlation was achieved with the international scale that helped
to constrain some issues that were still at loose ends. In
the first place, in the Hettangian-Sinemurian interval,
polarities comprised within the international Liasicus
and Semicostatum biozones should be displaced down
until the dominant normal polarity coincides with the
regional JN 3 . In the second place, during the
Pliensbachian and Toarcian, the frequency of reversals
increases, and yet, correlations between the scales are
more solid. Results allowed assigning precise ages to
those levels devoid of diagnostic fossils. This is the case
of the Sinemurian-Pliensbachian boundary as well as the
Toarcian in the north and the Pliensbachian-Toarcian in
the center of the basin.
Bibliography
Ogg, J., 2004. The jurassic period. In Gradstein, F., Ogg, J., and
Smith, A. (eds.), A Geologic Time Scale. Cambridge: Cambridge
University Press, pp. 307–343.
Riccardi, A. C., 2008. The marine Jurassic of Argentina: a biostratigraphic framework. Episodes, 31, 326–335.
MAGNETOSTRATIGRAPHY OF JURASSIC ROCKS
407