The Lower Cretaceous King Lear Formation, northwest Nevada: Implications for Mesozoic orogenesis in the western US Cordillera

Cretaceous paleogeography of the U.S. Cordillera west of the Sevier fold-and-thrust belt is poorly known due to the scarcity of Cretaceous supracrustal rocks in the arc and backarc regions. The Lower Cretaceous King Lear Formation, exposed in the Jackson Mountains and the Krum Hills of northwest Nevada, provides a rare and important record of Early Cretaceous tectonism and paleogeography. Our work shows that the King Lear Formation everywhere overlies deformed and metamorphosed Triassic-Jurassic rocks across a major unconformity. The King Lear Formation is dominated by conglomerates and sandstones deposited in subaerial alluvial-fan and gravelly braided river systems, and it can be divided into three new members based on differences in clast provenance: a lower member derived from local arc and backarc rocks; a middle member dominated by externally derived quartzite and chert clasts; and an upper member derived largely from an intrabasinal volcanic complex. Sedimentary features and provenance analysis indicate that clasts in the middle member were derived from nearby exposures of Paleozoic rocks such as the Roberts Mountains and Golconda allochthons, which must have formed a topographically elevated area in central Nevada during the Early Cretaceous. The stratigraphic record provides evidence of deposition in a tectonically active basin. In contrast with some prior studies inferring that deposition was synchronous with contractional deformation, our new field observations, structural data, and a shallow seismic-reflection profile confirm that the King Lear Formation was deposited in an active half-graben in the Jackson Mountains. These results provide new upper age constraints on the timing of shortening deformation in the arc and backarc regions: shortening must have been complete not only before deposition of the King Lear Formation in the middle Early Cretaceous, but also prior to several kilometers of exhumation of its depositional basement. Possible driving forces for extension leading to development of the King Lear basin include relaxation of thick crust in the hinterland of thrust belts to the east, stresses associated with regional tilting and development of dynamic topography, and stresses related to dextral strike-slip deformation to the west.