can occasionally exceed 1,500 km from the point of failure
and deposition occurs as a result from the transformation
of a decelerating turbidity current (Talling et al., 2007).
On glaciated continental margins, fans or delta-like protrusions occur in front of many glacial troughs or channels
crossing the continental shelves. Vorren et al. (1989) proposed naming these features “trough mouth fans” (TMF).
Debris flows are the main building blocks of TMFs
(Vorren et al., 1998). They originated on the upper slope,
emplaced during periods when the grounding lines of ice
streams were located near or at the shelf breaks at the
trough mouths, probably resulting in very high sediment
input. This situation caused oversteepened slopes and/or
excess pore-fluid pressures in the accumulated sediments,
creating instabilities and mass movement.
Bibliography
Kvalstad, T. J., Andresen, L., Forsberg, C. F., Berg, K., Bryn, P., and
Wangen, M., 2005. The Storegga slide: evaluation of triggering
sources and slide mechanics. Marine and Petroleum Geology,
22, 245–256.
Mohrig, D., Whipple, K. X., Hondzo, M., Ellis, C., and Parker, G.,
1998. Hydroplaning of subaqueous debris flows. Geological
Society of America Bulletin, 110, 387–394.
Talling, P. J., Wynn, R. B., Masson, D. G., Frenz, M., Cronin, B. T.,
Schiebel, R., Akhmetzhanov, A. M., Dallmeier-Tiessen, S.,
Benetti, S., Weaver, P. P. E., Georgiopoulou, A., Zühlsdorff,
C., and Amy, L. A., 2007. Onset of submarine debris flow deposition far from original giant landslide. Nature, 450, 541–544,
doi:10.1038/nature06313.
Vorren, T. O., Lebesbye, E., Andreassen, K., and Larsen, K.-B.,
1989. Glacigenic sediments on a passive continental margin as
exemplified by the Barents Sea. Marine Geology, 85, 251–272.
Vorren, T. O., Laberg, J. S., Blaume, F., Dowdeswell, J. A., Kenyon,
N. H., Mienert, J., Rumohr, J., and Werner, F., 1998. The
Norwegian-Greenland sea continental margins: morphology
and late quaternary sedimentary processes and environment.
Quaternary Science Reviews, 17, 273–302.
Cross-references
Geohazards: Coastal Disasters
Ocean Margin Systems
Tsunamis
METAMORPHIC CORE COMPLEXES
Uwe Ring
Department of Geological Sciences, Stockholm
University, Stockholm, Sweden
Definition
Metamorphic core complexes result from horizontal lithospheric extension and form in low-viscosity lower crust
when extension occurs at high rates and deformation
within the upper crust becomes localized in detachment
faults. They are oval shaped usually updomed structures
in which mid-crustal basement rocks of higher
metamorphic grade have been tectonically juxtaposed
against low-grade upper crustal rocks.
Introduction
Extension of Earth’s lithosphere is one of the most fundamental processes that shape the face of our planet. Extension and breakup of continental lithosphere is key to
understand the evolution of continents, the origin of sedimentary basins, and their hydrocarbon potential, as well as
the thermohaline circulation in the oceans and thus global
climate.
The most spectacular form of extension tectonics is the
formation of metamorphic core complexes. Metamorphic
core complexes mainly develop in continental crust, especially where it has been previously thickened by collisional processes. These processes heated up the
thickened crust mainly by radioactive decay thereby
weakening it and ultimately causing its failure. In oceans,
metamorphic core complexes may form as well near
mid-ocean ridges when magma supply is not efficient
enough to accommodate extension (e.g., North Atlantic;
Tucholke et al., 1998). This entry covers continental core
complexes only.
What is a core complex?
Metamorphic core complexes are usually oval-shaped
bodies in map view their long axis is typically some
20–50 km long. They result from localized extension,
which drags out the middle or lower crust (metamorphic
core) from beneath fracturing and extending upper crustal
rocks and exposed beneath shallow-dipping extensional
faults (detachments) of large areal extent (hundreds to
more than one thousand square kilometers) (Lister and
Davis, 1989) (Figure 1). As the size of a fault is linked
to its displacement, the large areal extent of the detachments indicates considerable tectonic transport along
those faults. Displacement along core complex detachments is typically of the order of 20–80 km but may
exceed 100 km (Foster and John, 1999; Ring et al.,
2001; Brichau et al., 2006) and ultimately juxtaposes rock
types with radically different geological histories
(Figure 2). The large displacements show that deformation
during lithospheric extension is strongly localized during
core complex formation.
The detachments separating the metamorphic core from
the upper crustal rocks are usually underlain by broader
brittle-ductile to ductile extensional shear zones. The
deformation in both is kinematically coordinated. In general, the detachments (and shear zones as well) dip at
low angles (<30
). There has been some debate in the past
whether these low-dip angles represent original values or
whether the faults have been rotated from a steep orientation (%60
) into their present low-angle position.
Although it has been demonstrated that some low-angle
detachments have been rotated from a higher-angle position (Axen et al., 1995; Gessner et al., 2001), there is
incontrovertible evidence that many low-angle faults
METAMORPHIC CORE COMPLEXES
491
and deposition occurs as a result from the transformation
of a decelerating turbidity current (Talling et al., 2007).
On glaciated continental margins, fans or delta-like protrusions occur in front of many glacial troughs or channels
crossing the continental shelves. Vorren et al. (1989) proposed naming these features “trough mouth fans” (TMF).
Debris flows are the main building blocks of TMFs
(Vorren et al., 1998). They originated on the upper slope,
emplaced during periods when the grounding lines of ice
streams were located near or at the shelf breaks at the
trough mouths, probably resulting in very high sediment
input. This situation caused oversteepened slopes and/or
excess pore-fluid pressures in the accumulated sediments,
creating instabilities and mass movement.
Bibliography
Kvalstad, T. J., Andresen, L., Forsberg, C. F., Berg, K., Bryn, P., and
Wangen, M., 2005. The Storegga slide: evaluation of triggering
sources and slide mechanics. Marine and Petroleum Geology,
22, 245–256.
Mohrig, D., Whipple, K. X., Hondzo, M., Ellis, C., and Parker, G.,
1998. Hydroplaning of subaqueous debris flows. Geological
Society of America Bulletin, 110, 387–394.
Talling, P. J., Wynn, R. B., Masson, D. G., Frenz, M., Cronin, B. T.,
Schiebel, R., Akhmetzhanov, A. M., Dallmeier-Tiessen, S.,
Benetti, S., Weaver, P. P. E., Georgiopoulou, A., Zühlsdorff,
C., and Amy, L. A., 2007. Onset of submarine debris flow deposition far from original giant landslide. Nature, 450, 541–544,
doi:10.1038/nature06313.
Vorren, T. O., Lebesbye, E., Andreassen, K., and Larsen, K.-B.,
1989. Glacigenic sediments on a passive continental margin as
exemplified by the Barents Sea. Marine Geology, 85, 251–272.
Vorren, T. O., Laberg, J. S., Blaume, F., Dowdeswell, J. A., Kenyon,
N. H., Mienert, J., Rumohr, J., and Werner, F., 1998. The
Norwegian-Greenland sea continental margins: morphology
and late quaternary sedimentary processes and environment.
Quaternary Science Reviews, 17, 273–302.
Cross-references
Geohazards: Coastal Disasters
Ocean Margin Systems
Tsunamis
METAMORPHIC CORE COMPLEXES
Uwe Ring
Department of Geological Sciences, Stockholm
University, Stockholm, Sweden
Definition
Metamorphic core complexes result from horizontal lithospheric extension and form in low-viscosity lower crust
when extension occurs at high rates and deformation
within the upper crust becomes localized in detachment
faults. They are oval shaped usually updomed structures
in which mid-crustal basement rocks of higher
metamorphic grade have been tectonically juxtaposed
against low-grade upper crustal rocks.
Introduction
Extension of Earth’s lithosphere is one of the most fundamental processes that shape the face of our planet. Extension and breakup of continental lithosphere is key to
understand the evolution of continents, the origin of sedimentary basins, and their hydrocarbon potential, as well as
the thermohaline circulation in the oceans and thus global
climate.
The most spectacular form of extension tectonics is the
formation of metamorphic core complexes. Metamorphic
core complexes mainly develop in continental crust, especially where it has been previously thickened by collisional processes. These processes heated up the
thickened crust mainly by radioactive decay thereby
weakening it and ultimately causing its failure. In oceans,
metamorphic core complexes may form as well near
mid-ocean ridges when magma supply is not efficient
enough to accommodate extension (e.g., North Atlantic;
Tucholke et al., 1998). This entry covers continental core
complexes only.
What is a core complex?
Metamorphic core complexes are usually oval-shaped
bodies in map view their long axis is typically some
20–50 km long. They result from localized extension,
which drags out the middle or lower crust (metamorphic
core) from beneath fracturing and extending upper crustal
rocks and exposed beneath shallow-dipping extensional
faults (detachments) of large areal extent (hundreds to
more than one thousand square kilometers) (Lister and
Davis, 1989) (Figure 1). As the size of a fault is linked
to its displacement, the large areal extent of the detachments indicates considerable tectonic transport along
those faults. Displacement along core complex detachments is typically of the order of 20–80 km but may
exceed 100 km (Foster and John, 1999; Ring et al.,
2001; Brichau et al., 2006) and ultimately juxtaposes rock
types with radically different geological histories
(Figure 2). The large displacements show that deformation
during lithospheric extension is strongly localized during
core complex formation.
The detachments separating the metamorphic core from
the upper crustal rocks are usually underlain by broader
brittle-ductile to ductile extensional shear zones. The
deformation in both is kinematically coordinated. In general, the detachments (and shear zones as well) dip at
low angles (<30
). There has been some debate in the past
whether these low-dip angles represent original values or
whether the faults have been rotated from a steep orientation (%60
) into their present low-angle position.
Although it has been demonstrated that some low-angle
detachments have been rotated from a higher-angle position (Axen et al., 1995; Gessner et al., 2001), there is
incontrovertible evidence that many low-angle faults
METAMORPHIC CORE COMPLEXES
491