The Gabbro-Glacier model

The gross structure and the distribution of hypersolidus deformations are best explained by the gabbro-glacier model (Quick and Denlinger, 1992, 1993; Quick et al., 1992a, 1992b). Quick et al. (1992a, 1994) noted that well studied analogs for the arcuate structure and synmagmatic deformation of the Mafic Complex are found in ophiolitic gabbro (for review, see: Nicolas et al., 1989, 1992; Quick and Denlinger, 1992, 1993). In addition, both the Mafic Complex and ophiolitic gabbro display similar strain gradients with increasing strain downward in the section. Numerical modeling (Quick and Denlinger, 1992, 1993; Phipps Morgan and Chen, 1993; Henstock et al., 1993) demonstrates that these characteristics can be produced by large-scale necking of a thick section of partially molten cumulates beneath a small magma chamber as the crust moves away from a spreading center.

Figure 7. Evolution of the Mafic Complex

Evolution of the Mafic Complex

Schematic diagram summarizing the Permian evolution of the Mafic Complex in the context of a progressively extending crust under simple shear. Location of the Mafic Complex in the crust is followed by enlargements to show details of internal structure of the Mafic Complex. Red, magma chambers; green, cumulates, which contain variable amounts of interstitial melt; orange, dioritic and granitic rocks; blue, pre-underplating crustal rocks. Large, open arrows indicate direction of crustal movements. Small arrows in Mafic Complex indicate trajectories of cumulates and septa during ductile deformation. Foliation indicated by dashes.

According to this model, a huge volume of gabbroic crystal mush is created from a relatively small and continuously-fed magma chamber, as crystallizing cumulates are continuously transposed outwards and downwards from the chamber. Consistently, the best preserved igneous textures, like swarms of mafic enclaves mingled in diorite (Fig. 6), are close to the roof in the vicinity of Varallo, at the core of the arcuate structure, while most feeders are annealed, as were largely transposed at low angle or parallel into the foliation.