explains the location and architecture of core complexes
(Buck, 1991). Continental rifts (like the East African Rift)
form in relatively cold and strong lower crust. In hot,
weak, low-viscosity lower crust, core complexes develop
(Gessner et al., 2007). It is a general observation that there
is a lack of a topographic low above-core complexes and
that the crust-mantle boundary is flat in a number of core
complexes (Gans, 1987). Both observations suggest that
the lower crust is extremely weak and fluid (Buck,
1988). It is assumed that the hot and weak lower crust
enables viscous flow to accommodate horizontal pressure
gradients laterally across detachments, and this lower
crustal flow may be sufficient to accommodate crustal
thickness contrasts, thus leaving the crust-mantle boundary flat across the extended lithosphere as observed, for
example, in the Basin and Range province (Block and
Royden, 1990). In other words, as extension thins the
crust, the fluid lower crust is being drawn in from the surrounding areas keeping topographic and crustal thickness
gradients small. Because of these processes a hot crust is
not thinning significantly during core complex formation,
which makes it hard to break it apart to form a new plate
boundary.
Examples of regions of extended lithosphere
The classical example of a region that has undergone
large-scale continental extension is the Basin and Range
province in western North America (Anderson, 1971;
Armstrong, 1972, 1982). The Basin and Range records
>100 % of extension that has in part been accommodated
by the formation of numerous individual core complexes
(Coney, 1980). Another well-studied example of largescale continental extension is the Aegean Sea extensional
province in the eastern Mediterranean (Jolivet and Brun
2010; Ring et al., 2010). In both settings, extension by
core complex formation was followed by rift-type extension along high-angle normal faulting. The rift-type extension records a stage of much more limited horizontal
extension at a later stage of continental extension.
Radiometric dating of extension-related structures
shows that there is a pause in tectonic activity between
the core complex and the rift stage. It appears that this lull
records the time for the hot, formerly overthickened crust
to cool down. Core complex formation acts as a mechanism (valve) of how the overheated crust can get rid of
its excess energy (heat). The pronounced lower crustal
ductile flow causes advective transport of low-viscosity
material toward the Earth surface and thereby cools
the crust, preconditioning it for rift faulting and
eventual continental breakup. This lull in tectonic activity
leads to a “messy” split of hot continents. Examples from
the Liguro-Provençal basin (Italy/France) and the West
Coast of New Zealand show that it took about 25–30
Myr from initial core complex formation to sea floor
spreading (Rosenbaum and Lister, 2004; Schulte et al.,
2014).
Summary and conclusions
Metamorphic core complexes are spectacular examples of
large-scale continental extension, usually juxtaposing
metamorphic lower crust against upper crustal rocks. They
are bounded by low-angle normal faults that accommodate 10’s of kilometers of displacement. Metamorphic
core complexes form when the strength contrast
between ductile lower crust and brittle upper crust is high,
which means when the thermal gradient is relatively high
and therefore the brittle-ductile transition shallow
( 10 km depths). These hot conditions of the ductile
crust make it hard for the lithosphere to fully break apart
to form a passive continental margin. Therefore, the lower
crust needs to cool down before continental breakup can
occur.
Bibliography
Anderson, R. E., 1971. Thin-skin distension in Tertiary rocks of
southeastern Nevada. Geological Society of America Bulletin,
82, 43–58.
Armstrong, R. L., 1972. Low-angle (denudation) faults, Hinterland
of the Sevier Orogenic Belt, Eastern Nevada and Western Utah.
Geological Society of America Bulletin, 83, 1729–1754.
Armstrong, R. L., 1982. Cordilleran metamorphic core
complexes—from Arizona to southern Canada. Annual Reviews
of Earth Sciences, 10, 129–154.
Axen, G. J., 2007. Research focus: significance of largedisplacement, low-angle normal faults. Geology, 35, 287–288.
Axen, G. J., Bartley, J. M., and Selverstone, J., 1995. Structural
expression of a rolling hinge in the footwall of the Brenner Line
normal fault, eastern Alps. Tectonics, 14, 1380–1392.
Block, L., and Royden, L. H., 1990. Core complex geometries and
regional scale flow in the lower crust. Tectonics, 9, 557–567.
Brichau, S., Ring, U., Carter, A., Ketcham, R. A., Brunel, M., and
Stockli, D., 2006. Constraining the long-term evolution of the
slip rate for a major extensional fault system using
thermochronology. Earth and Planetary Science Letters, 241,
293–306.
Brichau, S., Ring, U., Carter, A., Monié, P., Bolhar, R., Stockli, D.,
and Brunel, M., 2007. Extensional faulting on Tinos Island,
Aegean Sea, Greece: how many detachments? Tectonics, 26(4),
19, doi: TC4009, 10.1029/2006TC001969.
Buck, W. R., 1988. Flexural rotation of normal faults. Tectonics, 7,
959–973.
Buck, W. R., 1991. Modes of continental lithospheric extension.
Journal of Geophysical Research, 96, 20161–20178.
Collettini, C., and Holdsworth, R. E., 2004. Fault zone weakening
and character of slip along low-angle normal faults: insights
from the Zuccale fault, Elba, Italy. Journal of the Geological
Society, 161, 1039–1051.
Coney, P. J., 1980. Cordilleran metamorphic core complexes: An
overview. In Crittenden, M. D., Jr., Coney, P. J., and Davis,
G. H. (eds.), Cordilleran metamorphic core complexes. Boulder:
Geological Society of America. Geological Society of America
Memoir 153, pp. 7–31.
Cowan, D. S., Claddouhos, T. T., and Morgan, J., 2003. Structural
geology and kinematic history of rocks formed along
low-angle normal faults, Death Valley, California. Geological
Society of America Bulletin, 115, 1230–1248.
Crittenden, M. D., Coney, P. J., and Davis, G. H. (eds.), 1980. Tectonic significance of metamorphic core complexes of the North
American Cordillera. Memoir Geological Society of America,
153.
494
METAMORPHIC CORE COMPLEXES
(Buck, 1991). Continental rifts (like the East African Rift)
form in relatively cold and strong lower crust. In hot,
weak, low-viscosity lower crust, core complexes develop
(Gessner et al., 2007). It is a general observation that there
is a lack of a topographic low above-core complexes and
that the crust-mantle boundary is flat in a number of core
complexes (Gans, 1987). Both observations suggest that
the lower crust is extremely weak and fluid (Buck,
1988). It is assumed that the hot and weak lower crust
enables viscous flow to accommodate horizontal pressure
gradients laterally across detachments, and this lower
crustal flow may be sufficient to accommodate crustal
thickness contrasts, thus leaving the crust-mantle boundary flat across the extended lithosphere as observed, for
example, in the Basin and Range province (Block and
Royden, 1990). In other words, as extension thins the
crust, the fluid lower crust is being drawn in from the surrounding areas keeping topographic and crustal thickness
gradients small. Because of these processes a hot crust is
not thinning significantly during core complex formation,
which makes it hard to break it apart to form a new plate
boundary.
Examples of regions of extended lithosphere
The classical example of a region that has undergone
large-scale continental extension is the Basin and Range
province in western North America (Anderson, 1971;
Armstrong, 1972, 1982). The Basin and Range records
>100 % of extension that has in part been accommodated
by the formation of numerous individual core complexes
(Coney, 1980). Another well-studied example of largescale continental extension is the Aegean Sea extensional
province in the eastern Mediterranean (Jolivet and Brun
2010; Ring et al., 2010). In both settings, extension by
core complex formation was followed by rift-type extension along high-angle normal faulting. The rift-type extension records a stage of much more limited horizontal
extension at a later stage of continental extension.
Radiometric dating of extension-related structures
shows that there is a pause in tectonic activity between
the core complex and the rift stage. It appears that this lull
records the time for the hot, formerly overthickened crust
to cool down. Core complex formation acts as a mechanism (valve) of how the overheated crust can get rid of
its excess energy (heat). The pronounced lower crustal
ductile flow causes advective transport of low-viscosity
material toward the Earth surface and thereby cools
the crust, preconditioning it for rift faulting and
eventual continental breakup. This lull in tectonic activity
leads to a “messy” split of hot continents. Examples from
the Liguro-Provençal basin (Italy/France) and the West
Coast of New Zealand show that it took about 25–30
Myr from initial core complex formation to sea floor
spreading (Rosenbaum and Lister, 2004; Schulte et al.,
2014).
Summary and conclusions
Metamorphic core complexes are spectacular examples of
large-scale continental extension, usually juxtaposing
metamorphic lower crust against upper crustal rocks. They
are bounded by low-angle normal faults that accommodate 10’s of kilometers of displacement. Metamorphic
core complexes form when the strength contrast
between ductile lower crust and brittle upper crust is high,
which means when the thermal gradient is relatively high
and therefore the brittle-ductile transition shallow
( 10 km depths). These hot conditions of the ductile
crust make it hard for the lithosphere to fully break apart
to form a passive continental margin. Therefore, the lower
crust needs to cool down before continental breakup can
occur.
Bibliography
Anderson, R. E., 1971. Thin-skin distension in Tertiary rocks of
southeastern Nevada. Geological Society of America Bulletin,
82, 43–58.
Armstrong, R. L., 1972. Low-angle (denudation) faults, Hinterland
of the Sevier Orogenic Belt, Eastern Nevada and Western Utah.
Geological Society of America Bulletin, 83, 1729–1754.
Armstrong, R. L., 1982. Cordilleran metamorphic core
complexes—from Arizona to southern Canada. Annual Reviews
of Earth Sciences, 10, 129–154.
Axen, G. J., 2007. Research focus: significance of largedisplacement, low-angle normal faults. Geology, 35, 287–288.
Axen, G. J., Bartley, J. M., and Selverstone, J., 1995. Structural
expression of a rolling hinge in the footwall of the Brenner Line
normal fault, eastern Alps. Tectonics, 14, 1380–1392.
Block, L., and Royden, L. H., 1990. Core complex geometries and
regional scale flow in the lower crust. Tectonics, 9, 557–567.
Brichau, S., Ring, U., Carter, A., Ketcham, R. A., Brunel, M., and
Stockli, D., 2006. Constraining the long-term evolution of the
slip rate for a major extensional fault system using
thermochronology. Earth and Planetary Science Letters, 241,
293–306.
Brichau, S., Ring, U., Carter, A., Monié, P., Bolhar, R., Stockli, D.,
and Brunel, M., 2007. Extensional faulting on Tinos Island,
Aegean Sea, Greece: how many detachments? Tectonics, 26(4),
19, doi: TC4009, 10.1029/2006TC001969.
Buck, W. R., 1988. Flexural rotation of normal faults. Tectonics, 7,
959–973.
Buck, W. R., 1991. Modes of continental lithospheric extension.
Journal of Geophysical Research, 96, 20161–20178.
Collettini, C., and Holdsworth, R. E., 2004. Fault zone weakening
and character of slip along low-angle normal faults: insights
from the Zuccale fault, Elba, Italy. Journal of the Geological
Society, 161, 1039–1051.
Coney, P. J., 1980. Cordilleran metamorphic core complexes: An
overview. In Crittenden, M. D., Jr., Coney, P. J., and Davis,
G. H. (eds.), Cordilleran metamorphic core complexes. Boulder:
Geological Society of America. Geological Society of America
Memoir 153, pp. 7–31.
Cowan, D. S., Claddouhos, T. T., and Morgan, J., 2003. Structural
geology and kinematic history of rocks formed along
low-angle normal faults, Death Valley, California. Geological
Society of America Bulletin, 115, 1230–1248.
Crittenden, M. D., Coney, P. J., and Davis, G. H. (eds.), 1980. Tectonic significance of metamorphic core complexes of the North
American Cordillera. Memoir Geological Society of America,
153.
494
METAMORPHIC CORE COMPLEXES