JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B9, PAGES 14,577-14,594, AUGUST 10, 1991 Mechanisms for the Formation of Domal and Basinal Detachment Faults: A Three-Dimensional Analysis AN YIN Department of Earth and Space Sciences, University of California, Los Angeles Detachment faults(i.e., regional low-angle normal faults) in metamorphic core complexes of the North American Cordillera commonly exhibit doubly plunging antiformal(domal) and synformal (basinal) geometries.These geometries have been previously attributed to the superposition of two processes: (1) the antiformsand synforms with axes parallel to the extension direction originated asprimary fault undulations or corrugations developed coevally with fault sllp, and (2) the antiforms and synforms with axes perpendicular to the extension direction were produced by isostatic uplift due to tectonic denudation. However, the coaxial relationship between undulations of detachment faults and the sedimentary beds and metamorphic foliations in both the upper and lower plates of some detachment fault systems in the U.S. Cordillera suggests that the synforms and antiforms with axes parallel to the extension direction may have formed as folds, and that the domal and basinal geometry of detachment faults may have formed synchronously by a single process. A three-dimensional, elastic thin plate model is developed to investigate possible mechanisms for the formation of domal and basinal detachment faults. This model explores the interactions among the vertical, horizontal, and basal sheafing forces during the formation of warped detachment faults. The model considers the role of compression perpendicular to the extension direction and stress reduction parallel to the extension direction. The results of the model suggest that the stress reduction itself in the extension direction is insufficient to cause buckling of a thin elastic crust. Two mechanisms are most likely to explain the formation of domal and basinal geometries of detachment faults: (1) an upward pushing by undulatorycrustalroots or buoyant synextensional plutons beneath the extensional belt, and (2) buckling causedby a compressionperpendicular to the extension direction. Both mechanisms are consistent with the geologic constraints during the developmentof Cordilleran core complexes. In addition to isostatic uplift due to tectonic denudation, upward warping of detachment faults can also be caused by buckling, basal shearing, and buoyant forces. INTRODUCTION Genera/Background Cordilleran metamorphic core complexes are distinctive structural associations that consist of detachment faults (i.e., regional low-angle normal faults)juxtaposing brittlely extended rocks in an upper plate with ductilely stretched and sheared metamorphic and igneous rocks in a lower plate [Coney,1980]. The detachment faults are commonly warped into doubly plungingantiforms(domes)and syn- forms (basins) with their major and minor axes parallel and perpendicular to the regional extension direction. The do- mal and basinal geometries of the detachment faults strongly control the distribution and exposures of the midcrustal rocks in the North American Cordillera that were uplifted during Cenozoic extension along detachment faults[Gritten- den et M., 1980; Frost and Martin, 1982; Armstrong, 1982; Wernicke, 1985;Daviset M., 1986]. The origin of Cordilleran metamorphic core complexes is controversial. Coney [1980], Dickinson [1981], and Arm- strong [1982], among others,proposed that the formation of the core complexes was directly related to interaction between the North American and Pacific plates. In con- trast, Coneyand Harms [1984],Sonder et al. [1987],and Wernicke eta/. [1987] emphasized that instability of the overthickened crust formed during the Sevier-Laramide oro- Copyright 1991 by the American Geophysical Union. Paper number9lIB01113. 0148-0227/91/91JB-01113505.00 genieswas the major causefor the development of core com- plexes and Late Cenozoic extension in the North American Cordillera. These workers consider that the thick continen- tal crust formed by the Mesozoicthrusting was gravitation- ally unstable and spread outward under its own weight. The initiation of spreading is attributed to either the reduction of viscosity by a mantle-derived heatingevent during mid- Tertiary time [Coney, 1987] or the thermalrelaxation of the overthickened crust [Sonder eta/., 1987]. On the basis of an elasticmodel, Yin [1989a] suggested that the combined boundary and basalshearing forces due to plate interactions and the spreading of the weak middle and lower crust can explain the formation of regionallow-anglenormal faults in Cordilleran core complexes. The initial dip of detachment faults in the Cordilleran core complexes has been hotly debated in the past few years. Ob- servations of normal faulting in regions of active continen- tal extensionsuggest that large seismogenic normal faults are restricted to dipangles of around 300-600 [Jackson and McKenzie, 1983; Jackson, 1987; Jackson and White, 1989]. However,a low-anglenormal faulting event with a dip an- gle of possibly as low as200 was reported by Doser [1987] for the 1964 Ancash, Peru, earthquakein the high Andes of northern Peru. Contrary to the usual geometry of presently active normal faults in the world, field relationships in the North American Cordillera stronglysuggest that most Ceno- zoic detachments are primary low-angle normal faults that initiatedat dipsof less than 30 o and root into the crust [Burchfiel eta/., 1989; Davis and Lister, 1988; Lister and Davis, 1989; John, 1987; Reynolds and Spencer, 1985; Wer- nickeeta/., 1985;Allmendinger eta/., 1983]. 14,577