Metamorphic Core Complexes (MCCs) are unique geologic structures
present in the North American Cordillera. The
basic structure of a MCC consists of a metamorphic basement terrane and
an unmetamorphosed cover. Between these structures is a discontinuity,
or decollement, which consists
of mylonitic fabric. These complexes
show evidence of tectonic denudation
along low-angle faults with brittle over-prints onto already ductily deformed
rock (Armstrong, 1982). The structures are asymmetrically dome like, and
usually form the highest topography in the region (Coney, 1980). They are
usually formed in a region of thick crust which is gravitationally unstable,
and occur in areas of synextensional magmatism. This magmatism is thought
to have heated the upper crust and "lubricated" faults (Foster et al, 1993).
The major fold axis is parallel to the regional extension and the minor
axis is perpendicular (Yin, 1991).
Distribution and general regional tectonic
setting of Cordilleran metamorphic core complexes are represented in the
figure to the left (Figure after Coney, 1980). Their arrangement in the
Basin and Range Province is shown in its three divisible regions.
The Northern Basin and Range is outlined in Red, Central in Blue, and Southern
Basin and Range in Green (boundaries from Sonders and Jones, 1999).
References
Armstrong, R.L., 1982, Cordilleran metamorphic core complexes - from
Arizona to
southern Canada. Annu. Rev. Earth Planet. Sci., 10, 129-154.
Block, L. & Royden, L.H., 1990, Core complex geometries and regional
scale flow in
the lower crust. Tectonics, 9, 557-567.
Buck, W.R., 1988, Flexural rotation of normal faults. Tectonics, 7, 959-973.
Coney, P.J., 1980, Cordilleran metamorphic core complexes. In: Crittenden,
M.D.,
Coney, P.J., and Davis, G.H., (Eds.), Cordilleran Metamorphic Core
Complexes,
GSA Memoir 153, Geological Society of America, Boulder, 7-34.
Coney, P.J. & Harms, T.A., 1984, Cordilleran metamorphic core complexes:
Cenozoic
extensional relics of Mesozoic compression. Geology, 12, 550-554.
Davis, George H., 1987, Saguao National Monument, Arizona: Outstanding
display
of the structural characteristics of metamorphic core complexes,
in
Hill, Mason L. ed,
Cordilleran Section of the Geological Society of America, Centennial
Field Guide No. 1,
pg. 35-40.
Foster, D.A., A.J.W. Gleadow, S.J. Reynolds, and P.G. Fitzgerald, 1993,
The
denudation of metamorphic core complexes and the reconstruction of
the Transition
Zone, west-central Arizona: constraints from apatite fission-track
thermochronology:
Journal of Geophysical Research, v. 98, p. 2167-2185.
Gans, P.B., 1987, An open-system, two-layer crustal stretching model
for the eastern
Great Basin. Tectonics, 6, 1-12.
Hauser, E.C., Gephart, T., Latham, T., Bown, L., Kaufman, S., Oliver,
J., and Lucchitta, I.,
1987, COCORP Arizona transect: Strong crustal reflection data: GSA
Bulletin, 99, 833-844.
Holt, W.E., Chase, C.G. & Wallace, T.C., 1986, Crustal structure
from
three-dimensional gravity modeling of a metamorphic core complex: A
model for
uplift, Santa Catalina-Rincon mountains, Arizona. Geology, 14, 927-930.
Lister, G.S. & Davis, G.A., 1989, The origin of metamorphic core
complexes and
detachment faults formed during Tertiary continental extension in the
northern
Colorado River region, U.S.A. J. Struct. Geol., 11, 65-94.
Myers, S.C. & Beck, S.L., 1994, Evidence for a local crustal root
beneath the Santa
Catalina metamorphic core complex, Arizona. Geology, 22, 223-226.
Nations, Dale and Edmund Stump, 1981, The Geology of Arizona;
Kendall/Hunt
Publishing, Dubuque, Iowa, 221p.
Rehrig, William A., and Stephen J. Reynolds, 1980, Mid-Tertiary
plutonism and
mylonitization, Southern Mountains, central Arizona, in Crittenden,
et al., eds.,
Cordilleran Metamorphic Core Complexes, GSA Memoir 153, Geological
Society
of America, Boulder, 159-175.
Reynolds, Stephen J., and William A. Rehrig, 1980, Geologic and geochronologic
reconnaissance of a northwest-trending zone of metamorphic core complexes
in southern
and western Arizona, in Crittenden, et al., eds., Cordilleran
Metamorphic Core
Complexes, GSA Memoir 153, Geological Society of America, Boulder,
131-158.
Sonders, Leslie J. , and Craig H. Jones, 1999, Western United States
Extension:
How the West was Widened, Annu. Rev Earth Planet. Sci, 27, pg. 417-62.
Spencer, J.E. 1984. Role of tectonic denudation in warping and uplift
of low-angle
normal faults. Geology, 12, 95-98.
Thompson, G.A. & McCarthy, J. 1990. A gravity constraint on the
origin of highly
extended terranes. Tectonophysics, 174, 197-206.
Wernicke, B. 1985. Theory of large-scale, uniform-sense normal simple
shear of the
continental lithosphere. Canada J. Earth Sci., 22, 108-125.
Wernicke, B. & Axen, G.J., 1988. On the role of isostasy in the
evolution of normal
fault systems. Geology, 16, 848-851.
Yin, A. 1991. Mechanisms for the formation of domal and basinal detachment
faults: a
three-dimensional analysis. J. Geophys. Res., 96, 14,577-14,594.
V.L. Rystrom | Dept. of Geological Sciences |Univ. of Colorado at Boulder