in the Sunda Arc. Therefore, the deep sea trench along the
northwestern part of this island-arc system is mostly
masked by the high sedimentation rate, while the outer
ridge, built by the tectonic stacking of fan-supplied sediments, is emerged several hundred meters above sea-level.
It forms islands such as the Andaman and Nicobar
Islands and the Mentawai Ridge in front of Sumatra
(Figs. 3 and 4). Farther southwest away from the influence
of the Bengal fan and adjacent to Java, the trench is
distinctive with depths of almost 7500 m. Accordingly,
the outer ridge is not well developed and lies mostly below
a water depth of 2000 m.
Along Sumatra, the slope of the accretionary wedge
falling from the outer ridge to the deep sea trench typically
displays distinctive subdivisions. Here active thrusts produce elongated flat areas and depressions – the so-called
slope basins (Figure 4). They are common along the
Mentawai Ridge where some are rising above sea-level.
During their complex history, these basins served as sediment traps recording the uplift history of the outer ridge.
Analyses of the microfauna indicate uplift from deep to
shallow water conditions. The youngest sediments contain
reefs having formed in very shallow water before uplifting
above sea-level (Moore et al., 1980).
Bibliography
Dominguez, S., Malavieille, J., and Lallemand, S. E., 2000. Deformation of accretionary wedges in response to seamount subduction: insights from sandbox experiments. Tectonics, 19,
182–196.
Fisher, D. M., 1996. Fabrics and veins in the forearc: a record of
cyclic fluid flow at depths of <15 km. In Bebout, G. E., Schell,
D. W., Kirby, S. H., and Platt, J. P. (eds.), Subduction Top to Bottom. Washington, DC: American Geophysical Union. Geophysics Monograph, Vol. 96, pp. 75–89.
Frisch, W., Meschede, M., and Blakey, R. C., 2011. Plate Tectonics.
Heidelberg/Dordrecht/London/New York: Springer, 212 pp.
Gulick, S.P.S., Bangs, N.L.B., Shipley T.H., Nakamura, Y., Moore,
G., and Kuramoto, S., 2004. Three-dimensional architecture of
the Nankai accretionary prism’s imbricate thrust zone off Cape
Muroto, Japan: Prism reconstruction via en echelon thrust propagation. Journal of Geophysical. Research, 109(B02105).
doi:10.1029/2003JB002654.
Gutscher, M. A., Kukowski, N., Malavielle, J., and Lallemand, S.,
1996. Cyclical behavior of thrust wedges: insights from highbasal friction sandbox experiments. Geology, 24, 135–138.
Kukowski, N., Schillhorn, T., Huhn, K., von Rad, U., Husen, S., and
Flueh, E. R., 2001. Morphotectonics and mechanics of the central Makran accretionary wedge off Pakistan. Marine Geology,
173, 1–19.
Meschede, M., and Pelletier, B., 1994. Structural style of the accretionary wedge in front of the North d'Entrecasteaux Ridge (ODP
Leg 134). Proceedings of the Ocean Drilling Program, Scientific
Results, 134, 417–429.
Moore, G. F., Billman, H. G., Hehanussa, P. E., and Karig, D. E.,
1980. Sedimentology and paleobathymetry of trench-slope
deposits, Nias Island, Indonesia. Journal of Geology, 88,
161–180.
Scholl, D. W., von Huene, R., Vallier, T. L., and Howell, D. G.,
1980. Sedimentary masses and concepts about tectonic processes at underthrust ocean margins. Geology, 8, 564–568.
Silver, E., and Reed, E., 1988. Backthrusting in accretionary
wedges. Journal of Geophysical Research, 93(B4), 3116–3126.
von Huene, R., and Scholl, D. W., 1991. Observations at convergent
margins concerning sediment subduction, subduction erosion,
and the growth of continental crust. Review of Geophysics, 29,
279–316.
Westbrook, G. K., Ladd, J. W., Buhl, P., Bangs, N., and Tiley, G. J.,
1988. Cross section of an accretionary wedge: Barbados Ridge
complex. Geology, 16, 631–635.
Cross-references
Earthquakes
Island Arc Volcanism, Volcanic Arcs
Lithosphere: Structure and Composition
Morphology Across Convergent Plate Boundaries
Ophiolites
Seismogenic Zone
Subduction
Subduction Erosion
ACTIVE CONTINENTAL MARGINS
Serge Lallemand
Geosciences Montpellier, University of Montpellier,
Montpellier, France
Synonyms
Convergent boundary; Convergent margin; Destructive
margin; Ocean-continent subduction; Oceanic subduction
zone; Subduction zone
Definition
An active continental margin refers to the submerged edge
of a continent overriding an oceanic lithosphere at
a convergent plate boundary by opposition with
a passive continental margin which is the remaining scar
at the edge of a continent following continental breakup. The term “active” stresses the importance of the tectonic activity (seismicity, volcanism, mountain building)
associated with plate convergence along that boundary.
Today, people typically refer to a “subduction zone” rather
than an “active margin.”
Generalities
Active continental margins, i.e., when an oceanic plate
subducts beneath a continent, represent about two-thirds
of the modern convergent margins. Their cumulated
length has been estimated to 45,000 km (Lallemand et al.,
2005). Most of them are located in the circum-Pacific
(Japan, Kurils, Aleutians, and North, Middle, and South
America), Southeast Asia (Ryukyus, Philippines, New
Guinea), Indian Ocean (Java, Sumatra, Andaman,
Makran), Mediterranean region (Aegea, Calabria), or
Antilles. They are generally “active” over tens (Tonga,
Mariana) or hundreds (Japan, South America) of millions
ACTIVE CONTINENTAL MARGINS
9
northwestern part of this island-arc system is mostly
masked by the high sedimentation rate, while the outer
ridge, built by the tectonic stacking of fan-supplied sediments, is emerged several hundred meters above sea-level.
It forms islands such as the Andaman and Nicobar
Islands and the Mentawai Ridge in front of Sumatra
(Figs. 3 and 4). Farther southwest away from the influence
of the Bengal fan and adjacent to Java, the trench is
distinctive with depths of almost 7500 m. Accordingly,
the outer ridge is not well developed and lies mostly below
a water depth of 2000 m.
Along Sumatra, the slope of the accretionary wedge
falling from the outer ridge to the deep sea trench typically
displays distinctive subdivisions. Here active thrusts produce elongated flat areas and depressions – the so-called
slope basins (Figure 4). They are common along the
Mentawai Ridge where some are rising above sea-level.
During their complex history, these basins served as sediment traps recording the uplift history of the outer ridge.
Analyses of the microfauna indicate uplift from deep to
shallow water conditions. The youngest sediments contain
reefs having formed in very shallow water before uplifting
above sea-level (Moore et al., 1980).
Bibliography
Dominguez, S., Malavieille, J., and Lallemand, S. E., 2000. Deformation of accretionary wedges in response to seamount subduction: insights from sandbox experiments. Tectonics, 19,
182–196.
Fisher, D. M., 1996. Fabrics and veins in the forearc: a record of
cyclic fluid flow at depths of <15 km. In Bebout, G. E., Schell,
D. W., Kirby, S. H., and Platt, J. P. (eds.), Subduction Top to Bottom. Washington, DC: American Geophysical Union. Geophysics Monograph, Vol. 96, pp. 75–89.
Frisch, W., Meschede, M., and Blakey, R. C., 2011. Plate Tectonics.
Heidelberg/Dordrecht/London/New York: Springer, 212 pp.
Gulick, S.P.S., Bangs, N.L.B., Shipley T.H., Nakamura, Y., Moore,
G., and Kuramoto, S., 2004. Three-dimensional architecture of
the Nankai accretionary prism’s imbricate thrust zone off Cape
Muroto, Japan: Prism reconstruction via en echelon thrust propagation. Journal of Geophysical. Research, 109(B02105).
doi:10.1029/2003JB002654.
Gutscher, M. A., Kukowski, N., Malavielle, J., and Lallemand, S.,
1996. Cyclical behavior of thrust wedges: insights from highbasal friction sandbox experiments. Geology, 24, 135–138.
Kukowski, N., Schillhorn, T., Huhn, K., von Rad, U., Husen, S., and
Flueh, E. R., 2001. Morphotectonics and mechanics of the central Makran accretionary wedge off Pakistan. Marine Geology,
173, 1–19.
Meschede, M., and Pelletier, B., 1994. Structural style of the accretionary wedge in front of the North d'Entrecasteaux Ridge (ODP
Leg 134). Proceedings of the Ocean Drilling Program, Scientific
Results, 134, 417–429.
Moore, G. F., Billman, H. G., Hehanussa, P. E., and Karig, D. E.,
1980. Sedimentology and paleobathymetry of trench-slope
deposits, Nias Island, Indonesia. Journal of Geology, 88,
161–180.
Scholl, D. W., von Huene, R., Vallier, T. L., and Howell, D. G.,
1980. Sedimentary masses and concepts about tectonic processes at underthrust ocean margins. Geology, 8, 564–568.
Silver, E., and Reed, E., 1988. Backthrusting in accretionary
wedges. Journal of Geophysical Research, 93(B4), 3116–3126.
von Huene, R., and Scholl, D. W., 1991. Observations at convergent
margins concerning sediment subduction, subduction erosion,
and the growth of continental crust. Review of Geophysics, 29,
279–316.
Westbrook, G. K., Ladd, J. W., Buhl, P., Bangs, N., and Tiley, G. J.,
1988. Cross section of an accretionary wedge: Barbados Ridge
complex. Geology, 16, 631–635.
Cross-references
Earthquakes
Island Arc Volcanism, Volcanic Arcs
Lithosphere: Structure and Composition
Morphology Across Convergent Plate Boundaries
Ophiolites
Seismogenic Zone
Subduction
Subduction Erosion
ACTIVE CONTINENTAL MARGINS
Serge Lallemand
Geosciences Montpellier, University of Montpellier,
Montpellier, France
Synonyms
Convergent boundary; Convergent margin; Destructive
margin; Ocean-continent subduction; Oceanic subduction
zone; Subduction zone
Definition
An active continental margin refers to the submerged edge
of a continent overriding an oceanic lithosphere at
a convergent plate boundary by opposition with
a passive continental margin which is the remaining scar
at the edge of a continent following continental breakup. The term “active” stresses the importance of the tectonic activity (seismicity, volcanism, mountain building)
associated with plate convergence along that boundary.
Today, people typically refer to a “subduction zone” rather
than an “active margin.”
Generalities
Active continental margins, i.e., when an oceanic plate
subducts beneath a continent, represent about two-thirds
of the modern convergent margins. Their cumulated
length has been estimated to 45,000 km (Lallemand et al.,
2005). Most of them are located in the circum-Pacific
(Japan, Kurils, Aleutians, and North, Middle, and South
America), Southeast Asia (Ryukyus, Philippines, New
Guinea), Indian Ocean (Java, Sumatra, Andaman,
Makran), Mediterranean region (Aegea, Calabria), or
Antilles. They are generally “active” over tens (Tonga,
Mariana) or hundreds (Japan, South America) of millions
ACTIVE CONTINENTAL MARGINS
9