Abstract

Healed microfractures in quartz grains in 14 to 169 Ma granites from the Basin and Range province in western Utah and eastern Nevada record a history of paleostress. The microfracture data are in good agreement with the orientations of paleostresses determined for this same region by using orientations of dikes emplaced into only slightly older igneous host rocks. Samples generally have two or three steeply dipping microfracture sets. One set is related to the tectonic stress field at the time of magma emplacement. It strikes northeast-southwest in 31-169 Ma samples and east-northeast-west-southwest in 25-30 Ma samples, rotates to east-west by about 22 Ma, and strikes northwest-southeast in the youngest samples (about 14 Ma). Plutons younger than about 14 Ma are not exposed at the surface in western Utah and eastern Nevada. Other crack sets in the samples could have formed during cooling of the pluton, during a post cooling regional thermal or tectonic event, or during some local stress event. A strong north-south trend in these other crack sets may suggest a formation during Basin and Range extension.

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