An expedition to the Cordillera Huayhuash in the north-central Andes of Peru has shown the range consists of complexly folded and thrust-faulted Cretaceous sedimentary rocks, and less deformed early to middle Tertiary(?) andesitic volcanic rocks, all intruded by granitic plutons thought to be as young as 9 m. y. B.P. A younger sequence of felsic ash flows has escaped folding and intrusion. Unlike the Cordillera Blanca to the north (a batholith), the major 6,000 m summits of the Cordillera Huayhuash are carved from a complex synclinorium of Upper Cretaceous Jumasha Formation carbonates.

The region has suffered two deformations. The first deformation was the most severe, producing tight flexural-slip and flexural-flow folds and east-directed thrust faults between Late Cretaceous and middle(?) Tertiary. During deformation, detachment of the Cretaceous prism from some unexposed "basement" occurred at the horizon of basal Cretaceous Oyon Formation shale. Less severe middle(?) Tertiary deformation warped andesitic volcanic rocks into broad gentle folds, and may have either reactivated or initiated thrust faults.

Field data and hypsometric (area altitude) analysis suggest the middle to late Tertiary Puna erosion surface is represented in the Cordillera Huayhuash by accordant flanking ridge-tops and a landmass concentration below 5,000 m. The 6,000 m axial ridge stood above this surface as a residual mass around which ash flows were deposited. Latest Tertiary to Quaternary orogenic uplift, perhaps during the last 6 m.y., incised drainage and represents the most recent deformation of the region.

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