Rocky Mountain Section - 68th Annual Meeting - 2016

Paper No. 22-3
Presentation Time: 8:00 AM-5:00 PM

POST-EARLY CRETACEOUS SHORTENING IN THE JACKSON MOUNTAINS, NORTHWEST NEVADA


COLBY, Thomas A., Department of Geosciences, Boise State University, 1910 University Dr., Boise, ID 83725, NORTHRUP, Clyde J., Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725, SNYDER, Walter S., Department of Geosciences, Boise State Univ, 1910 University Drive, Boise, ID 83725 and SCHMITZ, Mark D., Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725-1535, tcolby12@gmail.com

New mapping and structural observations coupled with existing geochronology provide strong evidence for post-Early Cretaceous, W-directed shortening in the Jackson Mountains. In early reconnaissance studies of the Black Rock Desert region, Willden (1963) noted that the Happy Creek Igneous Complex (assigned a Permian (?) or older age) overrode the Early Cretaceous King Lear Formation along the E-dipping Deer Creek thrust. While later studies redefined the age of the Happy Creek as a Triassic-Jurassic unit, the same older over younger relationship remained. More recent authors reinterpreted the structural architecture and argued for a steeply W-dipping normal fault rather than an E-dipping thrust to explain the lithologic juxtaposition. Based on that reinterpretation, they rejected the idea of any post-Cretaceous contractional deformation in the rock record of the Jackson Mountains. This study re-examines and documents the Happy Creek-King Lear contact geometry to test these contrasting interpretations. New detailed mapping of the Happy Creek-King Lear contact geometry in the northern and central part of the Jackson Mountains consistently indicates a shallow easterly-dipping contact that places the older Happy Creek Igneous Complex over the King Lear Formation. Three-point analysis of this contact reveals a surface with an orientation of ~N22E, 19SE. Additionally, this study documented pervasive reverse-sense slickenside surfaces on minor shear fractures in the King Lear Formation near the contact, with an average orientation of N44E, 35SE and down-dip slicks raking at ~84SE consistent with the expected orientation of P-shears in the Riedel Shear Model (Riedel, 1929). These new observations suggest a shallowly E-dipping thrust fault that places the Happy Creek Igneous Complex over the King Lear Formation. The presence of Early Cretaceous freshwater gastropods (Willden, 1963) and a 125 ± 1 Ma tuff in the King Lear Formation (Quinn et al., 1997) constrain the age of this thrust to post-Early Cretaceous. Given the general timing and geographic location of the Jackson Mountains, this deformation could be associated with internal shortening of the Sevier hinterland or late-stage Luning-Fencemaker deformation possibly linked to changes in the Farallon-Pacific-North American plate velocity structure.