WELSH, JOHN E., SR., Natural Resources Geologist, Holladay, UT

ABSTRACT: Late Jurassic Ductile Shearing and Tectonic Breccias in the Eastern Great Basin of Nevada and Utah

The Paleozoic sedimentary prism was dismembered in the Late Jurassic by shearing initiated at stratigraphic positions in the following list: (1) base of Meade Peak Shale, (2) base of Summit Springs Evaporites, (3) top of contact Chert Member of the Rib Hill Formation, (4) base of Unit G of the Ely Formation, (5) base of the Ely Limestone, (6) base of the Diamond Peak Sandstone, (7) base of Chainman Shale, (8) base of the Joana Limestone, (9) base of the Pilot Formation, (10) base of Lower Guilmette Limestone, (11) in siltstone at the top of Ely Springs (Hanson Creek) Formation, (12) top and base of the Eureka Quartzite, (13) base of the Dunderberg Shale, and (14) base of Pioche Shale and top of Prospect Mountain Quartzite. The largest are regional decollements, e.g., Summit Springs, C ainman, Blue Eagle, Silver Island, and Snake Range. The lesser shears vary in position and extent with lithofacies distribution.

Shales at all levels behaved ductilely. Carbonates locally behaved ductilely up to the base of the Pilot Formation, but more commonly below the Dunderberg Shale.

Brittle deformation in the carbonates produced megatectonic breccias below the shears. These regional breccias became reservoirs for the pore fluids and clay-held fluids expelled from primary source rocks and reservoirs by the dynamic pressures.

Igneous plutons injected from 150-30 Ma crosscut and dilated the shear zones, creating hydrothermal systems within the tectonic reservoirs.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.