A SUPERIMPOSED HALF-GRABEN MODEL FOR METAMORPHIC CORE COMPLEXES

Prevailing interpretations of Cordilleran metamorphic core complexes have difficulties with structural balance. I propose a new model that views extensional tectonics in terms of multiple half grabens that variably overlap in space and develop sequentially in time.

Each half graben has a master normal fault, with subparallel faults or synthetic splays. Grabens formed during rapid extension have more numerous, closely spaced normal faults. The faults are curviplanar to slightly listric (flattening 1-3°/km), initiating with dips of ~60° and rotating to lower angles as they move. The master fault changes with depth from a brittle fault to a moderately dipping mylonitic shear zone, ultimately merging into a zone of ductile stretching, mid-crustal flow, and intrusion. This model explicitly precludes any underlying faults, such as an archetypical gently dipping detachment fault or a subhorizontal decollement at the brittle-ductile transition zone.

As the half graben initiates, it defines a narrow rift. With time, the rift widens and the crust thins. After faults rotate from 60° to ~20-30°, the normal faults of the first half graben fail to accommodate further extension, so a new half graben forms if regional extension continues. The new half graben need not have an orientation similar to the older one. Even if the new master fault has a similar strike to the older one, faults associated with the younger half graben may dip in the opposite direction. Where half grabens overlap spatially, faults associated with a younger half graben cut cleanly across older faults.

Extreme net extension is achieved by superposition of numerous half grabens. Structural reconstructions indicate that the components of a classic metamorphic core complex are composite, contributed by multiple generations of half grabens. The brittle faults in the upper plate formed during early half grabens. The “detachment” fault is associated with an intermediate-stage half graben; it is a master fault that initiated at ~60° and cuts older brittle faults, placing them on a footwall of mylonite related to the toes of older normal faults. Movement on faults that cut the “detachment” fault, associated with youngest half grabens, is responsible for the final geometry of “detachment” faults, which have displacements of kilometers, not tens of kilometers.