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The Nonacho Group is a sequence of talus, alluvial-fan. braided-stream. pond, beach, fan-delta, and lacustrine rocks. Deposition occurred over an area 200 km by 60 km. in rhomb, wedge, and rectangular sub-basins. The sub-basins were separated during sedimentation by basement uplifts, as indicated by changes in clast composition, unit thickness, and facies that occur across fault-bounded basement inliers. Sinistral strike slip along near-vertical, north-northeast-striking faults that border the western margin of the Nonacho Basin is regarded as having controlled basin evolution. Syn-depositional strike slip is suggested by an extreme longitudinal cumulative thickness (>40 km) in conjunction with a uniform, lower greenschist facies metamorphic grade. A sinistral sense is indicated by (1) the location of the basin where there is a left-hand stepover in the regional fault system; (2) the probable southward sequential development of the basin as the basin floor moved, like a conveyor belt, northwards along faults concentrated on the western margin; and (3) the inferred tectonic emplacement from south of the basin of source rocks for conglomerates stratigraphically high in the sequence. The western margin of the basin was bordered by high-gradient alluvial fans and braid-plains, which in turn were located on the edge of a relatively deep lake. The eastern margin was characterized by a low-gradient alluvial plain with isolated ponds. This inferred paleotopography is consistent with active faulting along the western margin during sedimentation. Post-depositional sinistral faulting is indicated by traces of a regionally penetrative cleavage (and axial surfaces of related folds) that trend at an angle of 20°-40° to the average strike of the faults. Cleavage traces define sigmoidal patterns of heterogeneous sinistral shear. Zones of high strain on the western side of the basin display a lower fabric-to-fault angle than zones of low strain in the central and eastern parts of the basin. A gently plunging displacement vector is confirmed by near-horizontal stretching lineations (pebbles and quartz fibers), which reflect the finite extension direction adjacent to shear zones.

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