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

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.