Metamorphic Core Complexes

Complied by V. L. Rystrom

Schematic Cartoon (Davis and Reynolds, 1996)

General Overview

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).


    Prior to the 1960's these complexes were thought to be exposures of pre-Phanerozoic crystalline basement or granitic intrusive bodies. It was not until the 1960's that the fabric was noted to have been deformed and metamorphosed in the Mesozoic or Cenozoic. Originally these structures where thought to be local phenomenon, each with its own history. Then the linking together of their similar structures accelerated the understanding and examination of the complexes as a group.
    Examining the North American MCCs as a group is valid in their supposed geometry, however it is suggested that the MCCs from California to Canada were polygeneticly formed in the Mesozoic and were overprinted by crustal extension in the Cenozoic. The Arizona Complexes are almost all monogenetic, solely the result of Cenozoic extension   (Armstrong, 1982).

Arizona Core Complexes

Theories of Core Complex Development

    Many theories exist on how the MCCs formed. Years earlier in the development of these theories there existed quit a few end members. Today, there are many combinations of these end members and therefore a cursory look into the literature may be confusing.


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