Abstract

The Newfoundland Mountains of northwestern Utah preserve an unusually complete record of Mesozoic deformation in the hinterland of the Sevier orogenic belt. Structural relations of Paleozoic miogeoclinal strata with the Newfoundland stock, which has biotite and hornblende K/Ar ages of 153.2 ± 4.6 and 147.7 ± 4.4 m.y., respectively, and dated quartz monzonite dikes (with K/Ar biotite ages of 150.7 ± 4.5 and 143.7 ± 4.3 m.y. and a muscovite age of 149.1 ± 4.5 m.y.) provide the basis for dating Mesozoic deformation in the range. Geologic mapping at 1:24,000 has identified four geometric classes of faults with known or probable Mesozoic ages: (1) older-over-younger thrust faults (including the Desert Peak thrust, named herein); (2) inward-facing folds in the contact aureole of the stock; (3) low-angle normal faults at a high angle to bedding, intruded by undeformed Mesozoic dikes; and (4) younger-over-older faults oriented at a low angle to bedding. Microfabric analysis of recrystallized rocks in the contact aureole and unrecrystallized country rocks shows that most of the intracrystalline strain is pre-latest Jurassic in age and is characterized by layer-parallel shortening. Post-latest Jurassic intracrystalline strain is minor and records shortening originally oriented nearly vertical. The map geometries, geochronology, and microfabric analysis suggest the following structural history (from oldest to youngest): (1) pre-latest Jurassic regional shortening and thrusting and (2) syn- or post-latest Jurassic horizontal extension during the Mesozoic. In the Cretaceous and early Tertiary, whereas strata in the Idaho-Wyoming-northern Utah thrust belt to the east were shortened by 140–150 km, the rocks in the Newfoundland Mountains experienced little internal strain or were being extended, even though they were also probably being translated eastward above the deep-seated westward extension of the thrust belt décollement.

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