Abstract

On Saddle Island a detachment fault of Tertiary age separates two structurally different plates. In the lower plate, Precambrian amphibolite and gneiss are strongly overprinted by a subhorizontal mylonitic foliation that may have formed during east-directed overthrusting of Sevier age. The mylonitic foliation is in turn overprinted by a later brittle deformation expressed by the detachment fault and by two gently north-dipping zones of anastomosing faults that outline crude lenslike masses of unfractured rock. These fault zones are probably an expression of the brittle deformation that dominates during a detachment event and are themselves cut by north-south–trending, high-angle normal faults related to a second brittle deformation. Our field measurements of the orientation of fault planes and striae on these fault planes suggest that the orientation of the least principal stress direction (σ3) changed during the brittle faulting event from north-northwest to northwest-southeast. Neither of these directions of extension is compatible with the strain field associated with the earlier ductile shearing responsible for the development of the mylonitic foliation.

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