Abstract

At Crater Island, northern Silver Island Mountains, northwestern Utah, an unbroken Tertiary fault block within the Basin and Range province exhibits Jurassic or older structures that are virtually unmodified by subsequent tectonism, providing an opportunity to examine the tectonics of the Jurassic back arc of the Cordillera. Widespread high-angle faults, mainly striking north and northeast, offset the moderately west-dipping strata down to the west, thereby extending the strata parallel to bedding by 10% to 20%. These faults apparently belong to a single kinematic event of west-northwest oriented extension subparallel to bedding. The normal faults merge with a north-northwest dextral strike-slip fault system. The two fault systems are kinematically compatible, suggesting that they may have been contemporaneous. A low-angle (thrust?) fault nearly parallel to bedding within Ordovician strata is cut by the high-angle faults. Intrusive relations with about 160 Ma granitoid rocks show that all of these faults are Late Jurassic or older, with a reasonable lower age limit of Early Jurassic established on the basis of paleogeographic reconstructions using sedimentary deposits in the region. Jurassic dikes indicate a minimum horizontal compression direction that is parallel to the extension direction indicated by the normal faults, thus supporting the inference that normal and strike-slip faulting took place during the late Middle or early Late Jurassic.

We interpret these data to indicate that minor thrusting, probably during the Jurassic, was followed by extensional faulting within a strike-slip fault system, probably close in time to intrusion. These relations are similar to those reported previously in the nearby Newfoundland Mountains. Other mountains of northern Utah and adjacent Nevada also have examples of Jurassic normal faults that contrast with the more generally observed thrust faults and tectonite fabrics. These data point to a possible regional extensional tectonic event affecting the crust far inland of the Jurassic magmatic arc, an event probably associated with back-arc magmatism.

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