Gneiss domes and gneiss dome systems
Published:January 01, 2004
Various mechanisms have been proposed for the dynamic cause and kinematic development of gneiss domes. They include (1) diapiric flow induced by density inversion, (2) buckling under horizontal constriction (i.e., extension perpendicular to compression), (3) coeval orthogonal contraction or superposition of multiple phases of folding in different orientations, (4) instability induced by vertical variation of viscosity, (5) arching of corrugated detachment faults by extension-induced isostatic rebound, and (6) formation of doubly plunging antiforms induced by thrust-duplex development. Despite proliferation of models for gneiss-dome formation, a diagnostic link between the observed geological setting of a gneiss dome and the associated deformational processes remains poorly understood. This is because gneiss domes reflect finite-strain patterns that can be reached through different strain paths or superposition of multiple mechanisms.
To better differentiate the competing mechanisms and to assist the clarity of future discussion, a classification scheme of individual gneiss domes and gneiss-dome systems is proposed. The scheme expands the traditional definition of mantled gneiss domes, which emphasizes the spatial association with synkinematic migmatite and a supracrustal cover, by including those associated with faults. To illustrate possible kinematic interactions between faulting and gneiss-dome development, major geologic properties of two end-member fault-related gneiss domes are discussed: one produced by the development of North American Cordilleran-style extensional detachment faults and the other by passive-roof thrusts in crustal-scale fault-bend folds. Distinguishing the two has become a critical issue in the Himalayan orogen and the western U.S. Cordilleran where emplacement and exhumation of gneiss domes have been variably interpreted to be detachment or thrust related. A systematic examination of gneiss domes related to contractional versus extensional faults indicates that a detachment-related gneiss dome is characterized by the presence of a breakaway system in the footwall, rapid footwall denudation by normal faulting, and coeval development of supradetachment basins in the hanging wall. In contrast, a gneiss dome related to passive-roof faulting is commonly associated with rapid denudation of both hanging-wall and footwall rocks by erosion and the lack of coeval supradetachment basins. The most important aspect of a gneiss-dome system is the spacing between individual domes. The evenly spaced gneiss-dome systems tend to be associated with instabilities induced by density inversion, vertical viscosity variation, or horizontal contraction in a laterally homogenous medium. In contrast, unevenly spaced gneiss-dome systems may be associated with fault development, superposition of multiple folding events, or laterally inhomogeneous properties of rocks comprising the gneiss-dome systems. In nature, gneiss domes are often produced by superposition of several dome-forming mechanisms. This has made determination of the dynamic cause of individual domes and dome systems exceedingly challenging.