Abstract

Field, microstructural, and geochronologic evidence indicates that the Late Cretaceous McDoogle pluton, located near the eastern margin of the Sierra Nevada batholith, was emplaced as a subvertically sheeted complex into a steep reverse-sense shear zone. Evidence of internal subvertical sheeting includes abundant concordant wall-rock inclusions and screens that separate the pluton from adjacent Jurassic plutons, preservation of a ghost tectonostratigraphy in the distribution of the inclusions, and rare interphase contacts. The solid-state tectonic fabric in the wall rocks and the magmatic, submagmatic, and weak solid-state fabrics in the McDoogle pluton all are concordant and record northeast-southwest horizontal shortening, subvertical extension, and a significant component of northeast-side-up simple shear. Intrusive contacts generally are concordant with the fabrics but, where discordances occur, the wall-rock fabric invariably is truncated by the contact. However, late synkinematic emplacement of the McDoogle pluton is indicated by synintrusive boudinage of apophyses of the pluton and by the overall concordance of pluton fabrics including magmatic lineation.

Zircon U-Pb isotope dates were obtained from the quartz monzodiorite central phase of the McDoogle pluton (94 ± 4 Ma), the mafic granodiorite border phase (94.8 ± 0.6 Ma), and an older hornblende granodiorite included in the central phase (97.6 ± 0.4 Ma). Granodiorite orthogneiss in the wall rock yielded a U-Pb zircon date of 164 +2.1/–6.8 Ma, which we interpret to be the crystallization age and which is indistinguishable from a new age of 164.5 ± 2.3 Ma for the Twin Lakes pluton. A U-Pb titanite date of 94.4 ± 0.9 Ma from the 164 Ma orthogneiss sample may reflect thermal effects of the intruding McDoogle pluton, synkinematic growth of titanite in the shear zone, or perhaps both.

The age of the Sawmill Lake shear zone that hosts the McDoogle pluton is bracketed between 148 Ma, the age of the Independence dike swarm, and 92 Ma, the age of the Lamarck pluton that crosscuts both the McDoogle pluton and the shear-zone fabrics. Regional evidence suggests a shear-zone age younger than 110 Ma, but older deformation is possible. The predominance of horizontal shortening over transcurrent motion and an age older than 90 Ma exclude a direct relationship to the dextral Sierra Crest shear system, which has been proposed to pass near the study area. The location, age constraints, and kinematics of the shear zone are consistent with both the later stages of movement in the East Sierran thrust system and isostatic sinking of the arc magmatic complex into its substrate.

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